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TESS

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Precision Light Curves from TESS Full-Frame Images: A Difference Imaging Approach
Author: Ryan J. Oelkers and Keivan G. Stassun
First Author’s Institution: Vanderbilt University
Status: Submitted to AJ

Disclaimer by Tarini: The first author of this paper was my supervisor during an REU, and while I was briefly involved with TESS as an undergraduate I did not participate in this work.

Figure 1: Left: field of view of the four TESS cameras. Middle: the twenty-six observation sectors TESS will cover across the northern and southern hemispheres of the sky. Right: the duration for which TESS will observe different parts of the sky, with the James Webb Space Telescope (JWST) continuous viewing zone. [NASA/TESS]

The Transiting Exoplanet Survey Satellite (TESS) will be launching very soon (in about a month, according to NASA’s launch countdown clock), and it promises to yield exciting results with a near all-sky survey for exoplanets around bright, nearby stars. Over two years, TESS will observe over 400 million stars by systematically scanning sectors of the sky for 27 days at a time (see Figure 1). About 400,000 of these stars will be studied closely and will have light curves associated with them in data releases. The rest of the stars can be studied through full-frame images (FFIs), which have a 30-minute cadence and consist of TESS’s entire field of view. To clarify, this doesn’t mean that an observation is taken every thirty minutes. The TESS cameras take images every two seconds. To save on storage and transmission time those images are stacked to create new images with effective exposure times of thirty minutes.

Because TESS is designed to conduct a wide survey, its pixels span a large part of the sky — 21 arcseconds per pixel, to be exact. For comparison, the Wide Field Camera 3 on the Hubble Space Telescope spans 0.04 to 0.13 arcseconds per pixel, depending on the detector. The large arcsecond-per-pixel scale means that the stars in TESS FFIs are more likely to be distorted and blurred into each other, making it difficult to measure changes in brightness (see Figure 2). How can one get around this? The authors of this paper offer up an image-processing pipeline that does the trick.

Figure 2: Comparing the pipeline’s output for three different data sets with different arcsec/pixel scales. The x-axis records magnitude and the y-axis records the deviation from the average magnitude of a given object. The inset images show how stars appear in the typical image from that data set. From left to right, the scales are 6.4”/pix, 15”/pix, 21”/pix (TESS). [Oelkers & Stassun 2018]

Stack, Subtract, Extract

One way to measure how an object’s brightness changes is to count the number of photons it emits over time. This is easy to do when a star is in a sparsely-populated part of an image; it’s safe to assume that any light from that part of the image is coming from the star. In crowded fields, things get complicated. Stars can appear to overlap, making it difficult to tell where the light is coming from.

A way around this is Difference Imaging Analysis (DIA). To do this, you need at least two images of the same part of the sky. Usually, one has a higher signal-to-noise ratio (SNR) than the other. Blur the image with the higher SNR to match the image with the lower SNR, and subtract one from the other. This should eliminate any objects that have a constant brightness and leave behind — you guessed it — the objects that do change brightness!

To determine a DIA approach to TESS FFIs, the authors used NASA’s “End-to-End 6” (ETE-6) simulated FFIs for TESS. To create the higher-SNR image, all 1,348 ETE-6 images were aligned and stacked to create a master frame. Aside from creating a frame to subtract from, stacking several images also ensures that transient objects like asteroids are removed from the master image. To be sure that subtraction had occurred properly, the authors checked the distribution of the values of background pixels in the differenced image. If only variable objects were left behind, then the rest of the pixels ought to be around zero. This is indeed the case with the differenced images that are produced (see Figure 3).

Figure 3: Left: a typical science image, with the colors inverted and an arrow pointing towards a candidate variable object. Middle: the differenced image, with an arrow still pointing at the much more distinct variable object. Right: the distribution of background pixel values from the differenced image. The distribution follows a Gaussian centered at zero, which means that the subtraction occurred properly. [Oelkers & Stassun 2018]

Through the Light Curves, and What the Pipeline Found There

The exact number of variable events (transits and variable stars) injected into the simulated images is unknown, but there should be several hundred of each type. The authors recovered 2,275 stars that showed variability (see Figure 4), which is good news for the pipeline. With their current resources, the authors estimate that they should be able to extract light curves from TESS FFIs a few weeks after data releases. The pipeline is publicly available and can be adapted for other data sets.

Figure 4: Examples of the variable-object light curves extracted from the ETE-6 FFIs. Left: likely simulated periodic variable star. Middle: likely simulated transit or binary candidate. Right: simulated large-amplitude variable star. [Oelkers & Stassun 2018]

All in all, it seems that TESS is going to uncover loads of interesting objects. It’s expected to recover approximately 1,700 planets from its target stars — including 70 or so Earth-like planets — and variable objects by the thousands from its FFIs. And DIA will definitely make a difference.

About the author, Tarini Konchady:

I’m a first year graduate student at Texas A&M University. Currently I’m looking for variable stars to better calibrate the distance ladder. I’m also looking for somewhere to hide my excess yarn (I’m told I may have a problem).

SL2S0217

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: A Window on the Earliest Star Formation: Extreme Photoionization Conditions of a High-Ionization, Low-Metallicity Lensed Galaxy at z∼2∗
Author: Danielle A. Berg, Dawn K. Erb, Matthew W. Auger, Max Pettini, Gabriel B. Brammer
First Author’s Institution: University of Wisconsin Milwaukee
Status: Submitted to ApJ

A crucial component of understanding how galaxies evolve is figuring out what their earliest years are like. However, this process presents a problem to astronomers, since most early galaxies are believed to have been small and faint and are quite distant to boot. One simple workaround is to study nearby galaxies that we believe have similar properties to the progenitors of older galaxies like the Milky Way or Andromeda. Today’s paper covers the analysis of one such galaxy.

Previous observations of the galaxy SL2S0217 using the Hubble Space Telescope have revealed that it is young, small, metal-poor, and forming about 23 solar masses of new stars per year. Small galaxies like SL2S0217 can sometimes be hard to observe at large distances since they tend to be faint. However, this galaxy is serendipitously located behind a much larger galaxy, which gravitationally lenses it and magnifies its light by a factor of 17, simultaneously smearing it into an arc-like shape (see the cover image above). Using a model of the lens, astronomers have been able to reconstruct the true appearance of the galaxy, revealing a clumpy and irregular shape.

In today’s paper, the authors used spectra of this galaxy obtained from the Keck I telescope to study its strong UV emission lines (Figure 2), which likely come from a combination of nebular gas around hot stars and from the interstellar medium (ISM). These features are highly unusual for redshift z = 2 galaxies, which generally don’t show signatures of strong emission.

spectrum of emission line galaxy overlaid with stacked spectra from "normal" galaxies

Figure 2:  The spectrum of SL2SJ0217 is shown in black, while the orange line shows the stacked spectrum of 1,000 redshift z = 2 galaxies to serve as a comparison. The blue dotted lines and purple dashed lines show nebular and ISM features, respectively. The gray bars show regions of the spectrum that may be contaminated in observations by spectral features of the Earth’s atmosphere. [Adapted from Berg et al. 2018]

In order to figure out what the conditions in the real galaxy are like, the authors use the program CLOUDY17 to model the chemistry and photoionization occurring in nebular gas and the ISM, generating a simulated spectrum.  By varying the input parameters to the model and comparing the simulated and observed spectra, the authors find the best-fit model to the observed spectrum — confirming the previous finding that the nebular gas of SL2S0217 is metal-poor and embedded in a hard radiation field. This hard radiation indicates that there is an abundance of high energy photons in the galaxy.

Building on this result, their best fit is obtained when including the effects of binary-star evolution, indicating that this galaxy contains a high fraction of binary stars (which is not generally assumed when modeling stellar populations). However, one characteristic of the spectrum could not be reproduced using stellar photoionization alone: a particularly strong and narrow He II emission line (Figure 3). To reproduce this line, the authors consider additional ionization sources for the gas, separately incorporating the effects of shock and active galactic nuclei (AGN) heating into the photoionization code to see if these could potentially cause the high He II emission.  Varying the shock velocity and the radiation field of the AGN, they find the inclusion of strong ionization from shocks or AGN into the model increases the predicted He II emission, but it does so at the expense of overpredicting the emission in the other observed lines. Thus, this line remains unexplained.

spectrum of galaxy showing strong emission lines along with a simulated model fit

Figure 3:  The observed spectrum of SL2SJ0217 shown in black, while the best-fit model spectrum is shown in orange. Most emission features are well matched with the exception of the He II line, which is underpredicted. [Berg et al. 2018]

Another item of interest in the spectrum of SL2SJ0217 is its double-peaked Lyman-alpha emission (Figure 4).  Lyman-alpha emission arises when an electron in the n = 2 orbital of a neutral hydrogen atom drops into the n = 1 orbital, emitting a photon with a wavelength of 1216 Angstroms. It isn’t uncommon to see Lyman-alpha emission in a galaxy, but the double-peaked structure is less typical. Luckily, simple models have shown that a double-peaked structure can arise in Lyman-alpha emission when the photons scatter through a spherical or shell-like cloud of gas around a region of star formation. The unequal peak heights can then be attributed an inflow of gas into the cloud. Thus, SL2SJ0217 appears to be hosting an inflow, which could also explain its star formation activity.

double-peaked, red-suppressed lyman alpha emission profile

Figure 4: The double-peaked emission of SL2SJ0217 in Lyman alpha. The red dashed lines show the average velocity range of absorption features in the spectrum (not shown), the black dashed line shows the systemic velocity of the galaxy, while the solid blue line shows the blended Gaussian profiles used to fit the emission. [Berg et al. 2018]

Overall, it is clear that this galaxy has many unique characteristics distinguishing it from others at its cosmic epoch. However, the strong He II emission is unexplained and the authors speculate that better models of massive, metal-poor stars are necessary in order to fully explain the observed nebular emission. If SL2SJ0217 is indeed representative of the very first generation of galaxies, there is still some work do be done before we can understand them completely.

About the author, Caitlin Doughty:

I’m a third year graduate student at New Mexico State University working with Dr. Kristian Finlator. I use numerical simulations to study galaxy evolution during the epoch of reionization, with a focus on metal absorption in the circumgalactic medium.

brown dwarf

Editor’s note: This article, written by AAS Media Fellow Kerry Hensley, was originally published on Astrobites.

Figure 1. A comparison of the sizes of Sun-like and low-mass stars to brown dwarfs, gas giants, and terrestrial planets. Though brown dwarfs have only slightly larger radii than Jupiter, they contain more than ten times the mass. [NASA/JPL-Caltech/UCB]

Stars dutifully fuse hydrogen into helium throughout their main-sequence lifetimes, while planets quietly fuse nothing at all. In between these two extremes — large and hot enough to fuse deuterium but too small and cool to process its lighter cousin, hydrogen — lie brown dwarfs (see Figure 1). Like giant planets, they have cloudy atmospheres and sport polar aurorae. Like stars, they are powered by nuclear fusion, but unlike stars, they cool as they age, which could have interesting implications for the development of life on planets orbiting around them.

Astronomers have discovered over a thousand brown dwarfs, ranging in spectral type from the barely-sub-stellar late M dwarfs to the ultra-cool Y dwarfs, but questions about their formation, interior goings-on, and early lives remain. Of particular interest is the lower end of the mass range: where do we draw the line between brown dwarfs and planets? And where do the transitions between brown-dwarf spectral types lie?

A Curious Brown Dwarf in AB Doradus

Figure 2. The spectral energy distribution of 2M1324+6358 (black line) compared to two other T2 dwarfs. 2M1324+6358 is much brighter at long wavelengths than either of the other T2 dwarfs, which could mean that it’s an unresolved binary. [Gagné et al. 2018]

In this paper, the authors investigate an object that has defied past classification attempts: 2MASS J13243553+6358281, or 2M1324+6358 for short. Other than being the top baby name for 2018, this unwieldy name tells us where to find the object in the sky and that it was cataloged by the Two Micron All-Sky Survey. Previous observations of this object (see Figure 2) indicated that it might be a single, very young brown dwarf or an unresolved binary system composed of two brown dwarf flavors: one L-dwarf and one T dwarf.

In order to learn more about 2M1324+6358, the authors first determine whether or not it belongs to AB Doradus, a young (~150 million years old), nearby (~65 light-years away) moving group. A moving group is a collection of stars, traveling together through the Galaxy, that formed at the same time from the same cloud of gas and dust. It’s much easier to figure out the age of a group of stars than an individual star, and since all stars in a moving group formed at the same time, figuring out if an object belongs to a moving group tells us its approximate age. Combining luminosity and color measurements with distance and age gives modelers the information they need to determine the brown dwarf’s radius, temperature, and surface gravity—critical information for exploring the muddy waters between small stars and giant planets.

First, the authors use parallax to determine the distance to 2M1324+6358. The parallax measurements hint that 2M1324+6358 belongs to the moving group because it’s at the same distance from the Earth. It’s not enough to just be at the right distance, though; stars are constantly in motion, and it’s common for a star to escape its natal cluster and mosey through neighboring clusters. However, a star that’s just passing through will tend to have a different velocity from stars that belong to the cluster, so if 2M1324+6358’s distance and velocity both match AB Doradus’, it’s very likely to belong. The authors pass the object’s velocity and location to a Bayesian statistical framework and find a cluster membership probability of 98% — bingo!

2M1324+6358: One Brown Dwarf or Two?

Figure 3 shows that 2M1324+6358 is fainter than other objects of similar spectral type, which means it’s unlikely to be a binary system. As a member of the AB Doradus moving group, it must also be young — just about 150 million years old. Young brown dwarfs are thought to be highly variable, due to both stellar activity and clouds drifting through their atmospheres, which could explain the unusual spectral features that led past studies to conclude it was a binary.

Figure 3. Color-magnitude diagram showing 2M1324+6358 (J-K ~ 1.6) in relation to other likely AB Doradus moving group members and field stars. 2M1324+6358 is slightly fainter in J-band than other T dwarfs. [Gagné et al. 2018]

With the potential binary reduced to a single object, it’s also possible to estimate its radius and mass: just 20% larger than Jupiter and 11–12 times as massive, making 2M1324+6358 one of the nearest known planetary-mass brown dwarfs! While there is still much we don’t know about young brown dwarfs, studying nearby objects like 2M1324+6358 can help us understand what fills the gap between small stars and large planets.

Citation

Jonathan Gagné et al 2018 ApJL 854 L27. doi:10.3847/2041-8213/aaacfd

Pluto and Charon

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: On the Early In Situ Formation of Pluto’s Small Satellites
Author: Man Yin Woo and Man Hoi Lee
First Author’s Institution: The University of Hong Kong
Status: Accepted to AJ

Many Moons, Many Mysteries

Pluto’s moons. Top: Styx (left) and Kerberos (right). Middle: Nix (left) and Hydra (right). Bottom: Charon. [NASA/JHUAPL/SwRI]

Planet or otherwise, Pluto’s not some lonely chunk of rock lurking around the edges of our solar system — as previous astrobites and gorgeous pictures from the New Horizons spacecraft have shown. Pluto has not one, not two, but five satellites (natural satellites, not artificial ones) making up its complex moon system. The largest of the moons is Charon, which is half the length of Pluto and one-eighth of Pluto’s mass. The other four moons, in order from closest to farthest from Pluto, are Styx, Nix, Kerberos, and Hydra. All together, these moons make up quite a peculiar system; all five of them, for example, have orbits that are almost perfectly circular and nearly coplanar. Moreover, the smaller moons’ orbital periods relative to Charon fall near some very neat intervals of 1:3:4:5:6 — meaning that the orbital periods of Styx, Nix, Kerberos, and Hydra are about 3, 4, 5, and 6 times the orbital period of Charon, respectively.

So what astrophysical phenomenon led to such a nicely oriented orbiting system? The answer is still unclear. Scientists do think they have a pretty good explanation for how Charon formed, however. Scientists discovered Charon back in the 1970’s, and today the most widely accepted idea for Charon’s formation is the intact capture scenario. This scenario says that Charon formed back when the Kuiper Belt was a lot more crowded; at some point, Pluto collided with a Kuiper Belt object and captured the impactor — Charon — into orbit. After the collision, tidal evolution (which would have slowed down Pluto’s spin and pushed Charon’s orbit outward) helped bring Pluto and Charon into the orbital system that the two have today.

Pluto’s four smaller moons, on the other hand, are recent, 21st-century discoveries, and their strange orbits have yet to be explained. There are a number of proposed formation scenarios out there, but scientists are still trying to find a scenario that completely and consistently explains the complex moon system that we observe today.

The authors of today’s paper, Man Yin Woo and Man Hoi Lee, joined this very search. They focused on one scenario in particular: the early in-situ formation scenario. This scenario follows right along with the Charon-forming intact capture scenario described earlier: it states that the impact from the collision that formed Charon also produced a ring of debris at about 20 RP, where RP is the radius of Pluto. This ring spread outward over time, due to angular momentum transfer from the Pluto-Charon system. Pluto’s smaller moons then formed from the debris ring near their modern orbital distances, in orbits that were already nearly coplanar and circular as a result of the ring.

But the outward spread of the ring would have happened quickly, reaching today’s distances all while Charon was still close to Pluto (aka, before Charon’s tidal evolution). Woo and Lee point out that the story couldn’t just end there; Charon would then move its orbit outward due to tidal evolution — and this outward movement of Charon (as a very massive body) might have disrupted the smaller moons’ orbits. To investigate this possibility, the authors set out to simulate Charon’s tidal evolution after the early in-situ formation scenario, to explore how Charon’s movement outward might have affected the smaller moons orbiting around Pluto as well.

No Body by N-Body

The authors used N-Body simulations to play out the scene of Charon’s tidal evolution, after the four small moons had already formed from a debris disk. They used the Charon-forming intact capture scenario to help inform the large moon’s initial conditions, such as for choosing Charon’s initial eccentricity (which describes how circular an orbit is) and the moon’s starting distance from Pluto before tidal evolution. They also varied certain initial conditions, including Charon’s initial eccentricity and the form and speed of Charon’s tidal evolution, to help cover a range over how the intact capture scenario might have played out in practice.

To simulate small satellites, the authors used massless test particles, which they placed randomly between about 35 and 60 RP from the Pluto-Charon system. This range covered the orbital distance range that Pluto’s four small moons orbit in today. The authors based the test particles’ initial conditions, such as their coplanar orbits and initial eccentricities, on theory as to what satellite orbits would look like after forming from a debris disk.

The authors started with 200 test particles for each simulation run with different initial conditions. Then they let Charon tidally evolve. They kept track of how many test particles survived after the tidal evolution, as well as of surviving particles’ final eccentricities and orbital periods relative to Charon.

pluto system simulations

Results from the simulation run that most closely explains the moon system we observe around Pluto today. The x-axis gives the final mean distance to the Pluto-Charon system in units of Pluto radii. The y-axis represents the final eccentricity in log scale, with smaller log values indicating orbits closer to circular in shape. The scattered points show the test particles that survived after tidal evolution. Green points stand for test particles not affected by resonance with Charon during Charon’s tidal evolution; black points are those particles that were affected by resonance but were not trapped by it; red points are those particles that were trapped by resonance but escaped; and blue points are those particles still trapped in resonance with Charon at the end of the simulation run. The dotted black lines guide the eyes along the distances where Pluto’s moons orbit today. The blue points in this case seem to have a preference for the 4:1 and 5:1 orbital ratios of Nix and Kerberos, since they are so thickly concentrated there and nowhere else. But no points have such a significant preference for the 3:1 and 6:1 orbital ratios of Styx and Hydra. [Woo and Lee 2018]

In most of the simulation runs, the final system after evolution looked quite different compared to the moon system we see around Pluto today. In some cases, for example, the majority of the test particles were ejected from the system by Charon during the tidal evolution phase, while in other cases the orbital periods of the surviving test particles didn’t end up near the special 1:3:4:5:6 values. The most promising simulation run, as shown in the figure above, featured test particles that survived through tidal evolution and had orbital period ratios concentrated around 4:1 and 5:1. However, the only particles that reached the 6:1 distance of Hydra seemed to have no preference in doing so, and hardly any particles reached the 3:1 distance of Styx.

In short, none of the simulation runs led to the complex moon system we see around Pluto today. The authors did acknowledge that they made some crucial assumptions and simplifications for their simulations, such as their treatment of satellites as massless, and that relaxing these characteristics might change the simulation results. But overall they noted that in order to explain how Pluto’s moons came to be, we need to find a scenario that accounts for all of the complexities in the orbiting system.

And so the search continues! We can use the results from today’s paper to inform new ideas and new scenarios for how Pluto’s moons might have formed. Hopefully, we’ll be able to one day witness a simulation that reproduces every characteristic of Pluto’s orbiting moon system — and in doing so, unravel more of Pluto’s mysteries as a whole. Because even if Pluto isn’t considered a planet anymore, it’s still a beloved part of the solar system — and its origin story forms an important piece in the puzzle of how the solar system we live in today came to be.

About the author, Jamila Pegues:

Hi there! I’m a 2nd-year grad student at Harvard. I focus on the evolution of protoplanetary disks and extra-solar systems. I like using chemical/structural modeling and theory to explain what we see in observations. I’m also interested in artificial intelligence; I like trying to model processes of decision-making and utility with equations and algorithms. Outside of research, I enjoy running, cooking, reading stuff, and playing board/video games with friends. Fun Fact: I write trashy sci-fi novels! Stay tuned — maybe I’ll actually publish one someday!

Titan

Editor’s note: This article, written by AAS Media Fellow Kerry Hensley, was originally published on Astrobites.

By now, exoplanet enthusiasts will be familiar with hot Jupiters, super-Earths, mini Neptunes, and even exo-Venuses, to name just a few. As the search continues, astronomers are finding colder and smaller planets, making possible the discovery of more analogs to our solar system. In today’s paper, Lora et al. consider exoplanets similar to one of the most tantalizingly Earth-like yet alien bodies in our solar system: Titan.

Figure 1. False-color RADAR image of Titan’s north pole from the Cassini spacecraft. Click here for a larger version. [Gazetteer of Planetary Nomenclature]

Aptly named, Titan is the second largest moon in our solar system, outdone only by Jupiter’s moon Ganymede. Larger (though less massive) than Mercury, if Titan were to orbit the Sun rather than Saturn, it would be a planet in its own right. Titan’s main claim to fame is being the only known solar-system object other than the Earth to have stable liquid on its surface — in lakes and seas, as Figure 1 shows — beneath a thick, hazy atmosphere.

While Earth is amenable to hosting water in liquid, solid, and gaseous states simultaneously, Titan appears to be similarly welcoming — but to methane. Because of this similarity, it’s fun to imagine Titan as Earth’s chilly alter-ego, a cool and hazy home for life — but not life as we know it. The ample hydrocarbons and nitrogen compounds might assemble into lurking carbon-based creatures crawling beneath the haze and swimming in the hydrocarbon seas … but before we get too deep into science-fiction speculations, let’s get back to the science.

Modeling Titanic Planets

In today’s paper, Lora et al. use theoretical models to investigate the atmospheres of Titan-like exoplanets orbiting Sun-like stars as well as K and M dwarfs, which are smaller and redder than the Sun. Thanks to Cassini, we already have an idea of what Titan might look like masquerading as an exoplanet around a Sun-like star, but Titan takes about 30 years to orbit the Sun; that’s a long time to wait to detect and confirm an exoplanet! Around an M dwarf, a planet with the same effective temperature as Titan (~80 K) takes only about 2 years to complete an orbit — much easier and quicker to detect. Titan-like exoplanets don’t need to crowd M dwarfs as closely as Earth-like planets do, so they are less susceptible to the notoriously nasty space weather of cool stars and they are less likely to be tidally locked. Though neither issue immediately disqualifies M-dwarf planets from the habitability contest, considering a cooler planet appears to solve both problems at once.

For this investigation, the authors pair a general circulation model with a photochemical model. General circulation models are widely used in planetary and Earth science to investigate atmospheric dynamics and the driving forces behind climate. While general circulation models can help us understand the bulk motion of the atmosphere, a photochemical model can tell us what atoms and molecules will be present, and in what proportion. Photochemistry is driven by starlight as photons trickle through the upper atmosphere to the surface, providing energy to nudge chemical reactions along. In the case of Titan, the atmosphere is largely made up of nitrogen and methane, with trace amounts of light hydrocarbons like ethane, acetylene, and hydrogen cyanide.

The authors pass the model parameters back and forth between the general circulation model and the photochemical model until, roughly 2,000 Earth-years later in model-time, the atmosphere reaches equilibrium. Model-generated maps of equilibrium temperature are shown in Figure 2. The slight differences between stellar types arise due to the presence of atmospheric haze; the haze tends to absorb short wavelengths and transmit long wavelengths, so redder stars have a higher proportion of starlight making it to the surface. The amount of haze in each case appears to be about the same, which could mean that haze is a persistent feature of Titan-like exoplanets, regardless of what type of star they orbit.

Figure 2. Temperature maps for a Titan-like exoplanet orbiting (from left to right) a G-, K-, and M-type star. The cooler the star, the cooler the stratosphere (top, red and orange) and the warmer the troposphere (bottom, blue and green). [Lora et al. 2018]

What observational features would these atmospheres have? Using the model atmospheres, the authors generate emission spectra for each of the three host-star spectral types, which are shown in Figure 3.

Figure 3. Globally averaged emission spectra for a Titan-like planet orbiting the Sun (blue), a K dwarf (orange), and an M dwarf (red). In some cases, the strength of the emission feature is determined by the atmospheric temperature (such as the methane feature on the far right), while in other cases it reflects how much of the compound is present (such as the ethane feature around 800/cm). [Lora et al. 2018]

Changing the host star’s spectral type changes the temperature and abundance of each chemical species, which in turn changes the strength of the emission features. This work considered only the effect of host-star spectral type on Titan-size and Titan-mass planets, but future modeling will consider the effects that changing surface gravity, axial tilt, and rotation rate have on the emission spectra, preparing astronomers to investigate the full diversity of Titan-like exoplanets. With highly sensitive space observatories on the horizon (e.g. WFIRST, LUVOIR, HabEx, OST), models like these will be valuable for atmospheric characterization of cold, hazy planets.

Citation

Juan M. Lora et al 2018 ApJ 853 58. doi:10.3847/1538-4357/aaa132

hot Jupiter

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: H− Opacity and Water Dissociation in the Dayside Atmosphere of the Very Hot Gas Giant WASP-18 b
Author: Jacob Arcangeli, Jean-Michel Desert, Michael R. Line, et al.
First Author’s Institution: University of Amsterdam, the Netherlands
Status: Accepted to ApJ

Disclaimer: Vatsal Panwar works in the same department as the lead author, but he did not have any scientific involvement in this project.

Hot Jupiters — one of the first types of exoplanets to be detected — have continued to challenge our understanding of planetary systems since their discovery. Their relative size, mass, and proximity to the host star make them the easiest exoplanets for detection and atmospheric characterization, especially from ground-based instruments; it’s no wonder that recent wide-angle surveys from the ground (WASP, KELT, and MASCARA, to name a few) have been quite successful in finding these gas giants.

WASP-18b

Artist’s illustration of WASP-18b. Insets show the optical and X-ray views of the system. [X-ray: NASA/CXC/SAO/I.Pillitteri et al; Optical: DSS]

Atmospheres of hot Jupiters orbiting early-type bright stars are blasted with significant radiation on their day side. Observations using the technique of transmission spectroscopy have revealed some surprising features of the vertical structure and chemical composition of these atmospheres. Today’s paper focuses on the atmospheric properties of one such gas giant, WASP-18b, which is in a tight, 0.94-day orbit around its host star and has a scorching equilibrium temperature of 2,700 K.

Recent studies of some very-hot hot Jupiters suggest the presence of thermal inversion in their atmospheres, akin to what happens in the Earth’s stratosphere due to the presence of ozone. In the case of hot Jupiters, energy from stellar irradiation is absorbed by gas-phase TiO and VO. While thermal inversion is not totally unexpected in these atmospheres, the anomalously high suggested values of metallicity and C/O ratio (frequently used indicators of chemistry and abundances in exoplanet atmospheres) indicate that there is more to the atmospheres of very hot Jupiters than meets the eye. Today’s paper tries to resolve this issue by taking a cue from the conditions in stellar photospheres with effective temperatures similar to those of very hot gas-giant exoplanets.

Who’s Drinking All the Water?

Water is one of the most prominent sources of opacity taken into account by the theoretical models of hot Jupiters’ emission spectra in the wavelength range probed by today’s paper (see Figure 1). Despite the low resolution in Spitzer/IRAC bands, there is good indication of the presence of some spectral features around 4.5 μm. An emission feature in a band where the atmosphere is optically thick (due to the presence of opacity sources — including, in this case, water — absorbing relatively more in that band) can typically be explained by the presence of a thermal inversion. The absence of corresponding water features expected around 1.4 μm doesn’t quite fit this picture, though. A high C/O ratio could be invoked in this case, as that would drive the chemistry of the atmosphere to deplete water and its features, while still allowing an inverted atmospheric profile. However, the authors suggest that water at the high-temperature and low-pressure conditions of very hot Jupiters should instead undergo thermal dissociation. For comparison, stellar photospheres with similar effective temperatures have higher pressures (due to higher surface gravities) that prevents water from thermal dissociation and makes it show up in their emission spectrum.

Another key factor used by the authors to explain the absence of water features is the presence of H− ions whose opacities become important in the temperature range of 2500–8000 K. The effect of H− opacities has been included in atmospheric models for brown dwarfs and hot Jupiters in past, but it has not been considered for retrieving the properties of very hot gas giants. Generation of H− ions in the dayside atmospheres of very-hot hot Jupiters can occur due to thermal dissociation of molecular hydrogen and the presence of ample free electrons from metal ionization at high temperatures.

emission spectrum of WASP-18b

Figure 1: The emission spectrum of WASP-18b (shown by the black points) is obtained by observing the secondary eclipses (before and after the planet is going behind the star in the line of sight). This allows us to measure the flux emitted by the day side of the planet (shown on the y-axis). The excess in flux around 4.5 μm indicates the presence of emission features. However, the spectrum is featureless in the band probed by HST/WFC3 instrument, which is explained as the combined effect of H− opacity and water depletion due to thermal dissociation. This can be seen from the opacity cross sections of H− and water (shown by red and blue curves, values on right-hand y-axis) around the HST/WFC3 bandpass. [Arcangeli et al. 2018]

Inversion Could Indeed Be a Trend

After inclusion of both thermal dissociation of water and H− opacity contribution in the theoretical models, the retrieved values for metallicity and C/O ratio for the atmosphere of WASP-18b drop to solar values. The best fit temperature structure, in this case, is also inverted due to the presence of high-altitude absorbers like TiO and VO. However, their features in the emission spectrum are damped by competing absorption due to H− ions (see Figure 1). This is in contrast to the earlier retrieved results for WASP-18b that suggested super-solar values for its metallicity and ℅ ratio — but these new results may be more plausible given the expected formation history for planets in this mass range (see Figure 2).

planet metallicities

Figure 2: A comparison of metallicities of planets with respect to their host stars. Massive gas giants like WASP-18b are not expected to follow the same trend as that for less massive planets; their metallicities should instead closely resemble those of their host stars. This is reconfirmed from the observations in this paper. [Arcangeli et al. 2018]

The authors’ results in this study emphasize the importance of considering H− opacities and depletion of atmospheric species due to thermal dissociation when looking at the atmospheres of exoplanets with high equilibrium dayside temperatures. They also strengthen the trend of thermal inversion which has been observed for most very-hot gas giants. It would be interesting to see how this plays out in future in the context of other very-hot gas giants.

About the author, Vatsal Panwar:

I am a PhD student at the Anton Pannekoek Institute for Astronomy, University of Amsterdam. I work on characterization of exoplanet atmospheres in order to understand more about the diversity and origins of planetary systems. I also enjoy yoga, exploring world cinema, and pushing my culinary boundaries every weekend.

NASA Earth-observing satellites

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Possible Photometric Signatures of Moderately Advanced Civilizations: The Clarke Exobelt
Author: Hector Socas-Navarro
First Author’s Institution: University of La Laguna, Spain
Status: Accepted to ApJ

The Search for Extraterrestrial Intelligence

The detection of extraterrestrial intelligence is a quest that has fascinated astronomers since we first realized that there were worlds beyond our own.

Listening projects, like SETI@home and Breakthrough Listen, look for possible signals from other civilizations. Such a discovery would unequivocally prove that we are not alone. However, in order to efficiently transmit a message across interstellar space, a civilization would likely beam it as tightly as possible towards its destination. That means that we wouldn’t be able to eavesdrop if we weren’t in line with the beam.

Another possible detection method is analyzing an exoplanet’s atmosphere with spectroscopy. As a planet passes in front of its star, a portion of the light from that star will be absorbed by the planet’s atmosphere. Analyzing the way the observed spectrum changes as the planet transits can tell us the planet’s atmospheric composition. A similar method may be applied by using light reflected off the planet’s atmosphere. Once we know what molecules are present in the planet’s atmosphere, we could search for chemical signs of life. Certain species of gas, like oxygen, could be the result of life on the planet’s surface. Some, such as chlorofluorocarbons (CFCs), would be highly indicative of the presence of an industrial civilization. While such a discovery would not be a direct detection of alien life, it would provide strong evidence for its presence.

A New Detection Strategy

In this paper, Socas-Navarro presents a new potential way to detect moderately advanced civilizations. A civilization that has reached or surpassed our own level of technology may have a great number of satellites in orbit around its planet. In particular, a good place to look for these satellites is in geosynchronous orbit. In geosynchronous orbit, a satellite has an orbital period that matches the rotation period of the planet. Thus, it remains directly over one spot on the planet. These orbits are useful because they allow a satellite to maintain contact with its base station at all times. For example, many of the Earth’s communications and navigations satellites are in geosynchronous orbit. If other civilizations use satellites for similar purposes, they would likely also make use of geosynchronous orbits.

In order to remain in geosynchronous orbit, a satellite must stay at a particular distance from its planet. If it comes closer, it will orbit faster than the Earth rotates, and if it is too far away, it will go slower than the Earth rotates. However, these orbits can have some slight inclinations. If the orbit is inclined, the satellite will not remain directly overhead, but will trace out a small analemma over a sidereal day. The author coins the term “Clarke exobelt” (CEB) to describe the satellites in this region of space around a planet.

Figure 1: An illustration of the Clarke exobelt. Each small dot represents a satellite in geosynchronous orbit around the planet, the brown sphere. The yellow sphere on the edge represents the star. Here, the size and density of satellites have been greatly exaggerated. The opacity, χ, increases from the face (χo) towards the edge (χmax). [Socas-Navarro 2018]

As can be seen in Figure 1, a high density of satellites in geosynchronous orbit will create a thick band which could block out light from the star. The more satellites, the more light they will block. This opacity will block more light from the star than the planet alone, resulting in a noticeable difference in the transiting planet’s light curve. An example of what a light curve affected by Earth’s CEB might look like is shown in Figure 2. As the planet moves between us and its star, the light we receive from the star will decrease. If the planet does not have a CEB, the light curve will exhibit a relatively flat decline. However, if there is a thick band of satellites around it, these will begin to obscure the light from the star before the planet is fully in front. This will result in a small dip right before the main transit happens. It will also cause more light to be blocked overall, resulting in a deeper light curve.

Figure 2: An example light curve for an Earth-like planet transiting a Sun-like star. The orange dashed line represents what the light curve will look like during a transit if there is no CEB. The blue line is a simulation of what the light curve would look like with a large number of satellites in geosynchronous orbit. Note how the light curve with the CEB is deeper, and the corners are rounded at the beginning and end of the transit. [Socas-Navarro 2018]

Based on publicly available databases, the author identifies 1,738 satellites in geosynchronous orbit around the Earth. This is a low estimate, because these databases do not include decommissioned satellites, space junk, or classified satellites. For the past 15 years, the number of satellites in geosynchronous orbit has been increasing exponentially. The author predicts that at this rate, within 200 years, our CEB will be thick enough that it could be observed from nearby stars by civilizations with the same detection capabilities as us.

The author puts forth several other simulated light curves for systems of interest, including TRAPPIST-1. In each case, the simulations use the telescope specifications of the Kepler mission, making these possible to be detected using our own technology.

In order to determine if the shape of a light curve is from a CEB, and not caused by some other effect, it is important to know how far from the planet the CEB would be. This would allow researchers to model what the light curve should look like with and without a CEB.

The orbital radius (rC) of the CEB depends on the mass (M) and rotational period (T) of the planet as rC3 = (GMT2)/(4π2}. An estimation of the planet’s mass can be made by using the transit photometry to find its size. Assuming the planet has a density similar to the Earth, we can calculate its mass. The rotational period, on the other hand, is more difficult to find. Future observations may be able to create surface maps of planets to determine rotational periods. While this has not yet been done, studies show that this could be possible for planets up to 5 parsecs away. If the planet is tidally locked to its star, the rotation would be straightforward to find. Tidal locking may be very common for planets orbiting in their star’s habitable zone, making it possible for astronomers to at least approximate the planet’s rotational period.

Figure 3: Light curves for the Proxima b and TRAPPIST-1 systems. Like in Figure 2, these show the light curve of a planetary transit without a CEB (dashed line) and with one (solid line). [Socas-Navarro 2018]

Finally, the author examines whether or not such a light curve could occur naturally. A planetary ring system could produce a similar light curve pattern. However, as the author points out, evidence from our own Solar System suggests that rings only form outside of the frost line. This is beyond the star’s habitable zone, making it unlikely that life like ours would develop out there. There is also no reason a ring system would prefer to form in geosynchronous orbit instead of any other inclination. While this orbital band is of high use to a civilization, there is no natural preference for it. Ring systems also tend to be very flat. Objects in the rings are spread out over the radial direction, but do not have much thickness in inclination. Geosynchronous orbits, on the other hand, are very thin radially, but have a thicker inclination. This will create a slightly different signal, which could be detected.

Are They Out There?

Many of the ideas presented in this paper are still speculation. These are extrapolations of what we know about civilization here on Earth. Aliens might not use geosynchronous orbits enough to create a thick band of satellites. They may be significantly more advanced than us, and have no need for a large number of satellites. Or they could be much less advanced than us, without the capability to get into orbit. The nearest civilization to us might even be too far away for us to be able to see its CEB. When discussing alien civilizations, it is important to keep in mind that we have very little data to base our ideas on, and we must make many assumptions.

Speculative though it may be, the method presented in this paper is another tool in our kit we can use on our search for extraterrestrial intelligence. This technique represents a way we can search for alien civilizations with our current detection capabilities. It relies only on technologies that we know are possible. Upcoming missions like the James Webb Space Telescope and TESS could apply this method to search for an alien civilization.

About the author, Peter Sinclair:

I’m a graduate student at the University of New Mexico. My hobbies include reading, cooking, and playing board and video games. You can find me on Twitter @Phiteros and on my blog, themodernpolymath.com.

HD 100453

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The Orbit of the Companion to HD 100453A: Binary-Driven Spiral Arms in a Protoplanetary Disk
Author: Kevin Wagner, Ruobing Dong, Patrick Sheehan, et al.
First Author’s Institution: Steward Observatory, University of Arizona
Status: Published in ApJ

Today’s paper combines a wide range of data sets — spanning the radio to near-infrared — and analysis techniques — orbit fitting and hydrodynamic simulations — to connect a binary companion to intriguing features seen in the protoplanetary disk around the primary star.

Using the Spectro-Polarimetric High Contrast Exoplanet Research instrument (SPHERE) in 2015, astronomers discovered a two-armed spiral structure in the disk around HD 100453 A (see the cover image). This structure is very different from the gaps seen in images of protoplanetary disks from the Atacama Large Millimeter/submillimeter Array (ALMA) such as HL Tau and TW Hya. The spiral arms seen in the disk around HD 100453 A and two other disks (SAO 206462 and MWC 758) could be caused by a massive companion (planet or star) orbiting outside the disk or processes within the disk such as self-gravity or dead zones. The HD 100453 system is unique in that it has a known M-dwarf companion of about 0.2 solar masses (HD 100453 B). The authors of this paper show that this companion is the cause of the spiral arms seen in the disk, without invoking other driving mechanisms.

The first step in connecting the companion star to the spiral arms of the disk was to determine the companion’s orbit. The authors used six observations with SPHERE and the Nasmyth Adaptive Optics System and Near-Infrared Imager (NACO) cameras on the Very Large Telescope and the Magellan Adaptive Optics system taken over a span of 14 years. The authors took care to minimize systematic errors in the astrometry which could be introduced by errors in the plate scale, orientation of the telescope (which direction is north on the camera) and using a coronagraph. With six pairs of separations and position angles, the authors were able to fit the orbital parameters of the companion M-dwarf. Most important for determining the origin of the spiral arms are the semi-major axis (109 ± 9 au), eccentricity (0.17 ± 0.07), and inclination (32.5 ± 6.5 degrees). This semi-major axis and eccentricity are consistent with the companion truncating the disk at 40 AU, much smaller than a typical disk around a single star.

Since the mutual inclination between the companion and disk has a significant effect on the evolution of the system, the authors needed to determine the inclination of the protoplanetary disk. They used publicly available ALMA observations of carbon monoxide in the disk. Fitting a simple smooth disk profile to the Keplerian orbits of the gas gave a disk inclination of 28 degrees, consistent within 1σ with the inclination of the companion.

Image of the HD 100453 system (top) compared with hydrodynamic and radiative transfer simulation viewed from an inclination of 30 degrees (bottom). [Wagner et al. 2018]

The final step was to run a hydrodynamic simulation of the entire system, including the effects of the companion. The authors evolved an initially smooth disk for 100 orbits of the companion and produced synthetic observations using a radiative transfer code. A sample of simulation results is shown in the figure to the right. The separation of the spiral arms, their pitch angle, and the locations where they sprout from the central ring are all well reproduced by the model. The authors note that the disks in their simulations are ~30% larger than the observed disk, though they suspect this is likely due to the short amount of time for which the simulations were run (100 companion orbits) compared to the age of the system (~12,000 companion orbits). If the computer time were available to run the simulation longer, the authors speculate that the companion would truncate the disk further. The amount of truncation also depends on the scale height and viscosity of the disk which are likely not exactly correct in their models.

The agreement between the inclination of the M-dwarf companion and the disk suggest that the entire system formed from a single cloud rather than the companion later becoming bound to the primary star (and its disk). The likely inclination of HD 100453 A (determined by comparing the observed rotational velocity of the star with stars of similar mass) is also consistent with the disk and companion star. This rules out a possible scenario where the companion formed separately but torqued the disk to share its inclination while leaving the star untouched.

While the spiral arms in the HD 100453 A disk are clearly driven by HD 100453 B, it is hard to make the same conclusion for the other two disks hosting “grand design” spiral arms. This and other studies suggest where a companion could be located with respect to the spiral arms in those systems, but previous searches for such a companion in these systems have found nothing, setting strict limits on companion mass (or brightness). As always, more work is needed to determine the origin of the spiral arms in SAO 206462 and MWC 758.

About the author, Samuel Factor:

Sam Factor is a 3rd year Ph.D. candidate at The University of Texas at Austin studying direct imaging of extrasolar planets and low mass binary stars. He uses an interferometric post processing technique to allow the detection of companions below the diffraction limit of the telescope.

binary black holes

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Merger of Multiple Accreting Black Holes Concordant with Gravitational Wave Events
Author: Hiromichi Tagawa & Masayuki Umemura
First Author’s Institution: Eötvös University, Hungary; National Astronomical Observatory of Japan
Status: Accepted to ApJ

A love story that begins with a chance encounter between strangers might sound romantic, but for black holes, the resulting attachment is often inescapable. Today’s astrobite explores one of the many theory-oriented publications written in the wake of LIGO‘s six gravitational-wave (GW) events. We’ll see how the authors explored the ramifications of throwing several unassociated black hole (BH) “strangers” into the mix (it’s complicated — accretion, three-body interactions, and more are at play in mediating mergers), and what it could mean in the context of recent GW discoveries.

Though the LIGO and Virgo detectors have been on hiatus since last fall (the start of a year-long break between observing runs O2 and O3), the world of astrophysics continues to be bombarded with new GW results informed by O1 and O2 data. In October, for instance, the LIGO-Virgo team announced the detection of GWs from a binary neutron star merger (GW170817) accompanied by a gamma-ray burst (GRB 170817A). The timing couldn’t have been more impeccable: the event, which was glimpsed in LIGO, Virgo, and electromagnetic observations, occurred just days before the conclusion of O2. Just like that, the era of multi-messenger astronomy had finally begun.

GW observations of BH mergers yield some information about the properties of the objects themselves, but the question of how unassociated BHs end up close enough to merge (and what that environment looks like) remains unanswered. In today’s featured paper, the authors go about exploring these issues using N-body simulations of multiple-black-hole systems in gas-dense environments. Their simulations are sophisticated (post-Newtonian), with detailed general relativity and gas dynamics being taken into account.

Usually, BH mergers are simulated with binary evolution in mind; that is, systems with associated BHs are considered. In contrast, this team’s plan was to simulate the behavior of five unassociated accreting BHs in several gas-dense environments (we’ll see later why that helps unassociated holes come together) in order to determine what initial parameters could yield LIGO-like mergers. This means that initial BH masses were comparable to LIGO’s ~30 M component BHs (the authors simulate 20, 25, and 30 M equal-mass systems). Gas number density varied between 102 and 1010 cm-3, though the total amount of gas in the simulation stayed constant at 105 M.

Figure 1: A plot of the masses of the closest BHs (m2 vs. m1) right before they finally merge. Blue points are 20, black are 25, and red are 30 M. The component masses from the three most massive LIGO mergers (along with errors) are shown in the boxes. [Tagawa & Umemura 2018]

Figure 2 shows how various parameters change over the course of one simulation. In the beginning, BHs get closer due to gas dynamical friction: if a massive object is moving through a sea of particles (like a dense gas cloud), the small components get pulled gravitationally into the wake of the larger one, causing it to lose energy. The final binary merger is mediated by loss of energy through GW radiation. Between these two periods, the unassociated BHs become well acquainted, with interloping BHs taking the place of one of the binary components (twice!) and wreaking havoc on the system. The addition of accretion to the model is enlightening, too. Each BH gains ~10 M through accretion near the beginning of the three-body interactions, but that quickly abates before the binary merger (in Figure 2 (c) and (a), a rapid increase in velocity during merging causes a significant drop in accretion rate). This is an interesting detail, as less gas accretion around the merger may cause electromagnetic counterparts to be dimmer than expected.

Figure 2: Several properties of black-hole mergers as a function of time. Panel (a) shows accretion rate, (b) shows mass, (c) shows velocity, and (d) shows distance between the closest two black holes. The simulation ends when the first two BHs merge. [Tagawa & Umemura 2018]

Through a thorough argument, the authors conclude that active galactic nuclei, or AGN, are the most likely environments for LIGO-esque mergers to take place. In short, this required estimating the expected merger time in both an AGN system and a giant molecular cloud (their estimate was between 30 and 100 Myr). Because of the importance of dynamical friction in causing the mergers, these timescales were possible only in environments with gas density > 106 cm-3, according to the simulations. This information, along with estimated event rates informed by LIGO detections, led the authors to conclude that AGN with high gas density provided the most fertile environments for unassociated BH strangers to merge.

The assumption of evenly distributed gas and the lack of a central, massive BH make these simulations imperfect. However, the scientific team’s ability to simulate multiple unassociated BHs is vital in expanding upon the classical model of systems with already-associated binaries. Further LIGO-Virgo detections will help us understand the environments in which the mergers occur in much greater detail. Still, these simulations are already incredible for elucidating the complicated dynamics of mergers with only a few GW event detections.

About the author, Thankful Cromartie:

I am a graduate student at the University of Virginia and completed my B.S. in Physics at UNC-Chapel Hill. As a member of the NANOGrav collaboration, my research focuses on millisecond pulsars and how we can use them as precise tools for detecting nanohertz-frequency gravitational waves. Additionally, I use the world’s largest radio telescopes to search for new millisecond pulsars. Outside of research, I enjoy video games, exploring the mountains, traveling to music festivals, and yoga.

warm Jupiter

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Models of Warm Jupiter Atmospheres: Observable Signatures of Obliquity
Author: Emily Rauscher
First Author’s Institution: University of Michigan
Status: Published in ApJ

Observing exoplanets is challenging! So how can we ever imagine learning something about their seasons? Over the past decade, astronomers have made extensive progress in understanding the atmospheres of hot Jupiters, including weather detections. But hot Jupiters are tidally locked and therefore experience no seasons. For observations of seasons, we need to push outwards to planets on longer orbits, where tidal interaction with the star is minimal. But a longer orbit means cooler planets. Cooler planets emit less thermal radiation, making them far dimmer than hot Jupiters, with blackbody spectra that peak at longer infrared wavelengths. This population of “warm Jupiters,” or Jupiter-sized planets with temperatures between 500–1000K, are out of reach for current telescopes. But with the James Webb Space Telescope’s 6.5-m mirror and its ability to observe out to longer wavelengths than current telescopes, astronomers will soon be studying the atmospheres of this new population of exoplanets!

How do these warm Jupiters differ from tidally locked hot Jupiters? Gravitational interactions between a star and a planet on a close orbit — like a hot Jupiter — will slow the rotation of the planet to the point where its rotation is the same as its orbital period. As a consequence, one side of the planet always faces its star, while the other side is never illuminated. Tidal locking also circularizes the orbit of a planet and removes any rotational tilt (obliquity). We therefore know the rotational period, eccentricity, and obliquity of a hot Jupiter without any required analysis. Warm Jupiters, on the other hand, are less affected by significant tidal effects, which means we have no intrinsic knowledge of these parameters. The author addresses the obliquity part of this problem in today’s astrobite by posing the following question: can we detect and determine the obliquity of a warm Jupiter and, in doing so, finally observe seasons on an exoplanet?

Wait! Time Out! Obliquities, Rotational Tilt, Seasons?

Figure 1: Earth’s rotational or axis tilt is the reason for our season(s). The hemisphere tilted towards the Sun experiences summer while the opposite hemisphere experiences winter. Spring and Fall occur when neither hemisphere is tilted towards the Sun leading to equal heating. [Golden Guide to Weather from St. Martin’s Press]

Besides being a word that scores you 23 points in Scrabble, the obliquity, or rotational tilt, of a planet controls the length and strength of that planet’s seasons. Figure 1 illustrates how Earth’s obliquity of 23 degrees creates seasonal changes over the course of an orbit. Summer or winter in one hemisphere depends on whether our rotational axis is pointing towards or away from the Sun, respectively. Now imagine Earth with no tilt. With no tilt, we wouldn’t have seasons. But with larger tilt, our seasons would be more extreme.

OK, Got It! Let’s Build a Planet!

The author creates a hypothetical warm Jupiter that has all the same properties of Jupiter, including same radius, mass, and rotational period. But instead of orbiting the Sun once every 5 years, this planet orbits a Sun-like star every 10 days, giving it a temperature of about 900 K. Using a global circulation model (GCM), the author simulates the atmosphere of this warm Jupiter at varying obliquities. What seasons look like on this warm Jupiter is plotted in Figure 2 for obliquities of 30 degrees (top panel), 60 degrees (middle panel), and 90 degrees (bottom panel). The fast rotational period (10 hours) of this planet compared to the orbital period of 10 days causes the atmosphere to smear out most of the day/night temperature contrast, allowing the author to average the temperature over longitude (east–west direction). The larger obliquity correlates with longer and more extreme seasons at higher latitudes (north–south direction). For obliquities greater than 60 degrees, the poles of the warm Jupiter become hotter than the equator, leading to larger temperature contrasts than the 30-degree (Earth-like obliquity) model.

Figure 2: Map of the longitudinally averaged temperature as a function of latitude and time over one orbit. Top panel is a warm Jupiter with 30 degrees obliquity, middle panel is 60 degrees obliquity, and bottom panel has 90 degrees obliquity. The black dashed line represents the location of the subsolar point over time. [Rauscher et al. 2017]

The Planet Now Has Seasons, Let’s “Observe” It

The paper first analyzes the phase curves of these hypothetical warm Jupiters. A phase curve is the light curve of a planet as it orbits around its star. At different points in its orbit, the planet will emit more or less light depending on what fraction of the day side we observe. As the planet has a 10-day orbit, the author notes that this would require continuous observations with JWST for those 10 days. From these phase curve models, the author noticed a degeneracy between the obliquity of a warm Jupiter and its viewing orientation. Figure 3 shows that a planet with the same obliquity can appear very differently depending on from the angle at which we observe it. By summing up the total flux of this planet at different locations in its orbit, we can create phase-curve observations. However, phase curves only provide a 1D total flux map of the planet. Even with the same obliquity, we will observe different amounts of flux simply due to the viewing angle. Phase curves alone don’t provide enough information to measure obliquity and viewing angle independently.

Figure 3: Models of the warm Jupiter at viewed with different orientations. The top panel shows orientations of the planet if we were to observe the planet directly above the equator. The second panel shows the same planet at obliquities 30, 60, and 90 degrees as in the top panel, but twisting the planet towards our line of sight by half of the obliquity value. The bottom panel twists the planet even more towards our line of sight where our viewing angle is equal to that of the planet’s obliquity. For example, the bottom right image twists the planet 90 degrees from its original orientation in the top right image. This adds a complication to the problem, we now have a degeneracy between obliquity and the viewing angle or orientation of the planet to our line of sight. A movie of this figure can be found here. [Rauscher et al. 2017]

In order to break this degeneracy, we will need more than just a measurement of the planet’s total flux. Eclipse mapping might be the solution, as it provides a 2D spatial map of the planet’s dayside. Figure 4 (from Majeau et al. 2012) illustrates the concept behind eclipse mapping. As the planet passes behind its star, slices of the planet are hidden over time corresponding to the shape of the eclipse. Rauscher concludes that by studying the shape of this eclipse, we will gain sufficient information to distinguish between the obliquity and viewing angles of the planet. And JWST should have high enough precision to detect these different shapes. 

Figure 4: The concept of eclipse mapping. As the planet passes behind its star, slices of the planet map to the slopes of the secondary eclipse. Combining this information with the eclipse depth should help observers distinguish the direction from which we are viewing the planet. [Majeau et al. 2012]

That said, this paper explores obliquities on a warm Jupiter assuming a known eccentricity and rotation rate. The reality is that these parameters will be unknown when observing an actual warm Jupiter. How this will affect the presented observations is currently being explored. This paper does stress that these unknowns will not wipe out our ability to measure obliquity; instead they will just make the data a little more “interesting” to analyze. With JWST, the future does appear to be hot, or uh, bright for warm Jupiters and exo-seasons!

About the author, Jessica Roberts:

I am a graduate student at the University of Colorado, Boulder, where I study extra-solar planets. My research is currently focused on understanding the atmospheres of the extremely low-mass low-density super-puffs. Out of the office, you will probably find me running, cross-stitching, or playing with my dog.

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