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planet formation

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 Boundary Between Gas-rich and Gas-poor Planets
Author: Eve J. Lee
First Author’s Institution: California Institute of Technology
Status: Accepted to ApJ

Astronomers often compare exoplanets to the planets in our own Solar System — Jupiters, Neptunes, super-Earths, etc. — because they are familiar. But the distinction can be made even simpler: planets that are gas-rich, and those that are not. Where does the boundary between the two fall, and how does it arise? Today’s paper addresses that very question.

An Excess of Sub-Saturn Planets

Figure 1. In the core accretion model of planetary formation, rocky cores form within the gas disk around the star, accrete gas as they cool, and, if they formed massive and early enough, experience runaway accretion to become gas giants. [jupiter.plymouth.edu]

The most successful theory of planet formation to date is that of core accretion (Figure 1). In this theory, planets first form as rocky cores embedded within the star’s gas disk. As the core cools, the decreased thermal pressure allows more and more gas to accrete onto the core. The outward thermal pressure of the atmosphere supports additional accreted gas in hydrostatic equilibrium until the mass of the gas envelope approaches the core mass. After this critical point, the system experiences runaway accretion and the planet becomes a gas-rich giant planet. Critically, runaway accretion occurs only if the core and atmosphere become massive enough before the end of the typical 10-million-year lifespan of the gas disk. More massive cores will accrete gas faster and therefore be more likely to trigger runaway accretion before the dissipation of the gas disk.

The core-accretion story of planet formation results in a binary picture of planets: those with large gaseous envelopes relative to their cores, and those with small envelopes. But what about the planets in the middle? The core-accretion model suggests that we should expect to find a lot of Jupiters (planets sized 8–24 R⨁, where R⨁ is Earth’s radius) and a lot of Neptunes or rocky planets (<1–4 R⨁), but not much in between. Defying theory, such in-between “sub-Saturns,” which are on the verge of runaway accretion with gas-to-core mass ratios (GCRs) of ~0.1–1.0, are observed at the same rate as gas giants!

Gassy … or Not?

The fact that sub-Saturns are observed as often as gas giants suggests that the story is a bit more complicated. The cooling of the core is not the only process that must be considered when simulating the formation of planets in a gas disk. Complex interactions between the gas in the planet’s atmosphere and the gas remaining in the disk can play a large role in a planet’s ultimate fate.

To quantify the effects of these additional processes, Lee ran a series of planetary formation simulations. She first determined the best-fit core mass distribution through comparison with observations. Notably, this paper is the first time a single core mass distribution reproduced both the observed plethora of sub-Neptunes and the similar numbers of gas giants and sub-Saturns (see Equation 5 in the paper). Considering planets with orbital periods between 10–300 days, Lee generated a range of planetary cores with masses from 0.1–100 M⨁ (where M⨁ is Earth’s mass) from the best-fit core mass model. These cores were placed in a gas disk at uniform times between 0 to 12 million years and evolved until the end of the 12 million years. The bottom line is perhaps unsurprising: the planet’s fate depended both on the initial core mass and when during the disk’s lifetime the planet formed.

More interestingly, by taking into account processes beyond cooling, Lee’s simulations resolved the discrepancy between the expected and observed number of sub-Saturns. The simulations also revealed four distinct core mass ranges that ultimately result in different planet types (see Figure 2):

  1. Core masses <0.4 M⨁ can only accrete a small amount of gas through cooling and remain sub-Neptunes and super-Earths.
  2. Core masses between 0.4–10 M⨁ accrete gas through cooling until the gas disk dissipates, while interactions between the atmosphere and gas disk decrease the amount of gas that falls onto the core. These planets do not reach runaway accretion and so remain sub-Saturns.
  3. Core masses between 10–40 M⨁ experience runaway accretion but growth is ultimately stymied by fluid interactions between the planet’s atmosphere and the gas disk. These planets become Jupiters.
  4. Core masses >40 M⨁ accrete gas so quickly that they carve deep gaps in the disk and ultimately deprive themselves of further accretion. These planets are massive Jupiters.

Figure 2. The resulting GCR given an initial core mass and time available for accretion. Each point is one planet formation simulation, and darker colors indicate that the core formed later in the disk’s lifetime. The regions A,B,C,D are described in the text. [Lee et al. 2019]

Figure 2 shows the wide variety of planets that can be formed given an initial core mass and time available for gas accretion. In particular, more massive cores can span the full GCR range depending on when they formed, becoming gas-rich or gas-poor planets. Conversely, low-mass cores will only ever become gas-poor planets. This provides a potential explanation for why metal-rich solar systems with more massive elements appear to host a wider variety of planets.

The Gassy Conclusion

Today’s paper is the first study that is consistent with observations across all core mass ranges. Furthermore, Lee shows the importance of including the fluid interactions between the planet’s atmosphere and the gas disk, resolving the discrepancy between the expected and observed number of sub-Saturns. As both observational and computational techniques improve, we will move closer to a comprehensive and complete description of planet formation.

About the author, Stephanie Hamilton:

Stephanie is a physics graduate student and NSF graduate fellow at the University of Michigan. For her research, she studies the orbits of the small bodies beyond Neptune in order learn more about our solar system’s formation and evolution. As an additional perk, she gets to discover many more of these small bodies using a fancy new camera developed by the Dark Energy Survey Collaboration. When she gets a spare minute in the midst of hectic grad school life, she likes to read sci-fi books, binge TV shows, write about her travels or new science results, or force her cat to cuddle with her.

circumgalactic region

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 detectability of visible-wavelength line emission from the local circumgalactic and intergalactic medium
Author: Deborah Lokhorst, Roberto Abraham, Pieter van Dokkum, Nastasha Wijers, Joop Schaye
First Author’s Institution: Lockheed University of Toronto, Canada
Status: Published in ApJ

Generally when we look at a picture of a galaxy, we see the bright central region occupied by many stars and brightly glowing gas. This can include shining arms in spiral galaxies or the clumpy splotches of light scattered around irregular galaxies. However, there is much more to a galaxy than what is brightly glowing: every galaxy is surrounded by a thin, cool, and difficult to observe cloud of gas called the circumgalactic medium (CGM). Even farther away there is a yet thinner distribution of gas, called the intergalactic medium (IGM). These structures have such low densities that the gas doesn’t emit enough light to be visible to normal telescopes. Consequently, astronomers only have a vague picture of the geometry, composition, and conditions of these components of the universe. It’s important to understand the CGM and IGM though, because they contain the majority of the baryonic matter (i.e. normal, non-dark matter) in the universe and are crucial for regulating the flow of gas onto galaxies, which allows for things like the formation of stars. Further, the CGM is currently observed mostly with absorption-line studies, which are restricted to directions in the sky where a background light source — such as a quasar — is available. This means that observations are often quite limited in number, making it difficult to get a comprehensive view of what is happening.

In today’s paper, the authors wrangled two tempestuous creatures: the EAGLE cosmological simulation and the Dragonfly Telephoto Array. EAGLE is a numerical code that creates a simulated chunk of the universe, and Dragonfly is a 48-lensed instrument specially designed to observe emission from very dim objects. The idea is that the authors can simulate CGM and IGM around galaxies using EAGLE and predict what their emission should look like. Then, knowing the parameters of the Dragonfly array, they can determine whether this emission should be observable by such an instrument. The authors use the capabilities of a (now in-progress) upgrade to Dragonfly, which will add the capacity to use narrow-band filters.

Modeling Emission in EAGLE

The authors are interested in one emission line in particular, the Hα feature. Hα emission is formed by a process called recombination that occurs in the bubbles of ionized hydrogen called H II (pronounced “H 2”) regions that surround young, massive stars. This process occurs so commonly in the gas around young stars that it can be very bright, making it one of the most likely emission lines to be observed.

Figure 1: Maps of simulated surface brightness emission in Hα from a small subset of the EAGLE simulation volume. Yellow indicates a higher surface brightness (SB in the label). [Lokhorst et al. 2019]

Information about the state of the gas particles in the simulation, such as their density, temperature, and metallicity, are used to model the gas emission and calculate the surface brightness, or the amount of light per square arcsecond coming from the gas (today’s paper uses units of photons cm-2 s-1 sr-1). The only remaining question: Is the signal bright enough to be seen with the upcoming iteration of Dragonfly?

Does Dragonfly Need Better Eyes?

Extracting the emission of Hα around galaxies within the EAGLE volume, the authors calculate the surface brightness averaged in rings centered on each galaxy, creating radial profiles of the Hα’s glow. Splitting the galaxies into categories based on mass, they find that the inner edge of the CGM (corresponding to the red circle in Figure 2) should be visible for galaxies with stellar masses above about 1010 solar masses. This means that in the search for CGM emission, astronomers shouldn’t need to target rare, outrageously massive galaxies to find a signal. A similar analysis is done by creating a false Dragonfly observation of a single test galaxy, with noise and an instrumental spread function applied to the emission map (Figure 2). The finding is similar: portions of the inner edge of the CGM should be easily visible with only ~10 hour exposures by Dragonfly, without even the need for radial averaging used in the previous test. To push farther into the CGM, however, such averaging would appear to be necessary, since the Hα surface brightness drops as the distance from the galaxy increases.

Figure 2: Left panel shows Hα emission of a test galaxy, with the inner edge of the CGM indicated by the red circle. Superimposed in red and white is a real galaxy, NGC 300, to indicate the relative size of the bright portion of the galaxy. Each panel to the right shows the false observation with noise applied, integrated for 10, 100, and 1,000 hours. [Lokhorst et al. 2019]

The IGM has a much lower density than the CGM, so it should be intrinsically fainter in emission. Using a similar false-observation method, the authors isolate an IGM filament from the simulation (Figure 3) and determine whether the signal would be observable in a reasonable amount of time by Dragonfly. They find that even the brightest emission of the IGM, coming from dense clumps, reaches only about 1 photon cm-2 s-1 sr-1. With such a low surface brightness, it would take over 1,000 hours (almost 6 weeks!) to obtain a signal that outshines the noise. Unfortunately, this means that the Dragonfly instrument upgrade plan would need to incorporate additional lenses to observe the IGM in this way.

Figure 3: The sample IGM filament selected for creation of the false observation, where the yellow color indicates a brighter surface brightness in Hα. [Lokhorst et al. 2019]

Although it is disappointing that the upcoming Dragonfly upgrade likely won’t be able to observe the IGM, the ground it could gain on studies of the CGM are fundamental to studies of galaxies. Compared to the severe limitations on absorption-line studies, observations of the CGM in emission may reveal more about its structure, how it is affected by inflows and outflows, and its interaction with the galaxy proper.

About the author, Caitlin Doughty:

I am a fourth year graduate student at New Mexico State University. I use cosmological simulations to study galaxy evolution during the epoch of reionization, with a focus on metal absorption in the circumgalactic medium.

coronal loops

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: New Evidence that Magnetoconvection Drives Solar-Stellar Coronal Heating
Author: Sanjiv K. Tiwari, Julia K. Thalmann, Navdeep K. Panesar, Ronald L. Moore, Amy R. Winebarger
First Author’s Institution: Lockheed Martin Solar and Astrophysics Laboratory; Bay Area Environmental Research Institute
Status: Published in ApJL

The coronal heating problem is one of the biggest unsolved mysteries in solar physics. The solar corona is the region of the Sun’s atmosphere that extends past the surface — or photosphere — of the Sun. It is a diffuse cloud of plasma that is heated to temperatures several hundred times that of the photosphere, which isn’t what you’d expect as you move further away from a hot object. The coronal heating problem has been hotly debated since the 1940s and is thought to be related to the Sun’s magnetic field. However, no theory has yet been able to explain why the corona is so much hotter than the photosphere, and the possibility remains that multiple processes may be at work.  

Figure 1. Image of a sunspot showing the umbra and penumbra region. [SpaceWeatherLive]

To study coronal heating, solar physicists use various structures that operate on smaller scales. The authors of today’s paper focus on the heating processes of coronal loops, which occur when plasma in the corona flows along the solar magnetic field (see the cover image above). Coronal loops are rooted in strong concentrations of magnetic field such as sunspots — dark patches in the solar photosphere. Sunspots are composed of two regions, a dark umbra and a surrounding penumbra (Figure 1), that are surrounded by a bright region called a plage. Understanding how coronal loops are linked to their sunspot footprints and the surrounding magnetic field is critical to determining the heating mechanisms at work.

Using data from the Solar Dynamics Observatory (SDO), the authors selected two regions with both sunspots and coronal loops, known as active regions. Although we can see coronal plasma trace magnetic field lines in coronal loops, there is still no way to directly measure the coronal magnetic field. The authors used simulations to reconstruct and determine the strength of the coronal magnetic field based on the magnetic field at the base of the loops (which can be directly measured using instruments like SDO). One of the example active regions, along with its simulated coronal magnetic field, is shown in Figure 2.

Figure 2. Image of one example active region (NOAA 12108) plotted with the simulated coronal magnetic field. The line color indicates the height of the magnetic field line above the surface of the Sun. [Tiwari et al. 2017]

From these observations, the authors found that the loops present in their sample active regions are rooted in both sunspots and plage, and that the brightest loops have one foot at the edge of a sunspot umbra and another foot in a plage or a sunspot penumbra. Although they find no visible loops with both footprints in sunspot umbrae, the simulated coronal magnetic field shows field lines that connect sunspot umbrae. This indicates that the coronal loops that connect sunspot umbrae are too cool to be visible in extreme ultraviolet wavelengths (the type of light observed by SDO). The relationship between loop brightness and footprint location is shown in Figure 3.

Figure 3. Schematic drawing of an active region showing the dependence of coronal loop brightness on footpoint location. Brighter colors indicate brighter emission in SDO images. [Tiwari et al. 2017]

A key property of sunspots is that they suppress convection (or the ‘boiling’ behavior in the solar photosphere indicated by the presence of granules surrounding the sunspot in Figure 1), with sunspot umbrae suppressing convection the most. The authors find that since the brightest loops are partially rooted in non-umbral regions (i.e. the regions with more convective activity), convection is a major driving force of coronal loop heating. However, if a loop has both footprints in non-umbral regions, they will not be as bright. Therefore the strong magnetic field present in the umbral footprint, along with enhanced convection in the non-umbral footprint, is necessary for generating the brightest (and hottest) loops.

Through observations and simulations of two active regions, today’s paper shows that both convection and a strong magnetic field in the footprints of coronal loops are necessary for heating to occur. This provides yet another clue to solving the decades-old coronal heating problem. With several new instruments like the ground-based solar telescope, DKIST, and the Parker Solar Probe coming online soon, this mystery may be solved sooner than you think.

About the author, Ellis Avallone:

I am a first-year graduate student at the University of Hawaii at Manoa Institute for Astronomy, where I study the Sun. My current research focuses on how the solar magnetic field triggers eruptions that can affect us here on Earth. In my free time I enjoy rock climbing, painting, and eating copious amounts of mac and cheese.

M33

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: Measuring Star-Formation Histories, Distances, and Metallicities with Pixel Color-Magnitude Diagrams I: Model Definition and Mock Tests
Author: B. A. Cook, Charlie Conroy, Pieter van Dokkum, & Joshua S. Speagle
First Author’s Institution: Harvard-Smithosonian Center for Astrophysics
Status: Submitted to ApJ

The wealth of information we have gathered about the lives of galaxies within our universe is due in part to increasingly larger and more sophisticated surveys of the night sky. Better resolution, precipitated by rapid advancement in imaging technology and telescope design, has enabled detailed studies ranging from our local group to the edges of the cosmos.

color–magnitude diagram

Figure 1: Example stars plotted on a color–magnitude diagram. [ESO]

Meanwhile, the color–magnitude diagram (CMD) has remained one of the most called upon diagnostic figures in an astronomer’s tool belt (see Figure 1). The combination of color and apparent magnitude alone is enough to map out a stellar population without much clutter, allowing for a clean-cut track of main-sequence stars burning away their hydrogen cores as well as potentially more massive stars in later stages of their lives that have moved off the main sequence. Traditionally, the CMD has enabled astronomers to obtain estimates of the ages of star clusters by determining their main-sequence turn-off point. More useful, perhaps, is the ability to measure dust as a shift in color (due to reddening) and magnitude (due to extinction), as well as to estimate distances by leveraging the statistical advantage of having tens to hundreds of stars to compare observed magnitudes with expected magnitudes based on theoretical CMD tracks.

While many of these techniques have been known and readily applied in the past and met with great success within our Galaxy, the authors of today’s astrobite demonstrate that this old dog can be taught new tricks.

The Usefulness of Barely Resolved Galaxies

Our ability to understand the universe is limited in part by angular resolution. For the few galaxies in the nearby universe for which individual stars can be discerned, we can rely on traditional techniques to understand their stellar populations, dust, and metal content in exquisite detail. The opposite is true for extremely distant galaxies, captured in only a handful of pixels with no hope of resolving any stars whatsoever. In this latter regime, we must rely on difficult-to-calibrate theoretical models of galaxy spectra to estimate the integrated properties of these remote systems.

In between these two extremes lie the semi-resolved galaxies, and they exhibit an interesting observational quirk. Although individual stars cannot be resolved, there are enough pixels such that, for any one pixel, there may be surface brightness fluctuations caused by rare but bright stars. The fluctuations trace the number of bright stars visible due to Poisson sampling and have been exploited in the past. The authors of this paper go further by leveraging these surface brightness fluctuations to construct a pixelized analog of a color–magnitude diagram for a semi-resolved system, termed a pixelized CMD (pCMD). The traditional diagnostics to estimate star, dust, and metal content are hence accessible, albeit with more complex implementation.

Figure 2: Summary of the pCMD method. A) Metallicity and a star-formation history model dictate the stellar evolution tracks. B) Stars are randomly sampled per pixel. C) Stars are modified for dust and distance. D) The simulated image is made by summing the fluxes of all the stars drawn into that pixel, as shown in the top left corner. E) The models are convolved with models of the instrument response. F) Pixel fluxes are converted to apparent magnitude, with the original stellar evolution track shown beneath for reference. [Cook et al. 2019]

The Pixelized Color-Magnitude Diagram

The pCMD contains information not only about the galaxy, but also effects relating to the instrumentation — including the point-spread function (PSF) and photometric noise — that must be accounted for. The authors opt for a forward modeling approach: from a model of stellar photometry and knowledge of the instrument, they construct a simulated pCMD and compare it to observations, as described in Figure 2. By constructing a grid of simulated pCMDs varying in stellar content, dust, and metallicity, the authors can then determine the best-fit parameters by searching through the grid of models to find the closest match.

In the current implementation of the pCMD method (PCMDPy — available on GitHub), there are four components of the model inherent to the galaxy:

  • Stellar populations: The current stellar populations observed are the direct result of star formation dating back to the formation of that galaxy. They should trace the assembly of stellar mass over time — known as the star-formation history (SFH) — and vice versa. Hence, the distribution of stellar masses formed (the inital mass function, or IMF) will govern not only the distribution of stellar masses, but also the conversion between the number of stars and the mass of stars within a pixel. Having confident estimates the SFH of a galaxy can enable definitive studies of the assembly of galaxies in the universe, which is currently a major line of research.
  • Metallicity: The metallicity of each pixel is modeled independently of the star-formation history in the current implementation. This is a simplification, as interstellar metallicity is driven by enrichment from dying stars and supernovae.
  • Dust: Dust will have the effect of both reddening and dimming the light from a galaxy. In this implementation, the dust is modeled as a single thin screen per pixel. Although this, too, is a simplification, it is necessary, as dust geometries are virtually impossible to obtain.
  • Distance: As a given galaxy is seen at larger distances, the number of stars per pixel increases. Although the surface brightness remains constant, the rare Poisson fluctuations due to particularly bright stars decreases and the average luminosity per pixel increases. As shown in Figure 3, this increases the overall brightness level in the pCMD but decreases the scatter. Hence, pCMDs can simultaneously recover distances and stellar populations for galaxies in the semi-resolved regime.

Figure 3. Effect of surface number density and distance, where each row has the same average flux. Increasing the surface number density Npix raises the CMD and decreases the scatter due to fewer fluctuations, but it does not change its color. Changing distance has a similar effect, but it does not affect the scatter. [Cook et al. 2019]

As the distance to a galaxy increases, the fluctuations lessen, as there are more stars per pixel. At about 10 Mpc, the power of these fluctuations to constrain the stellar populations declines as the uncertainty rises sharply. However, with Hubble resolution, the authors report that mock properties can still be recovered within 68% confidence out to 100 Mpc, highlighting the utility of this newly revitalized method in providing additional constraints on galaxies previously only characterized by their integrated stellar light.

About the author, John Weaver:

I am a first year PhD student at the Cosmic Dawn Center at the University of Copenhagen, where I study the formation and evolution of galaxies across cosmic time with incredibly deep observations in the optical and infrared. I got my start at a little planetarium, and I’ve been doing lots of public outreach and citizen science ever since.

binary black hole merger

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: First measurement of the Hubble constant from a dark standard siren using the Dark Energy Survey galaxies and the LIGO/Virgo binary–black–hole merger GW170814
Author: Marcelle Soares-Santos, Antonella Palmese, et al. (DES, LIGO, and Virgo collaborations)
First Author’s Institution: Brandeis University (M. S.-S.), Fermi National Accelerator Laboratory (A. P.)
Status: Published in ApJL

Disclaimer: The author of this Astrobites post is a member of the Dark Energy Survey but researches a different topic and did not take part in this analysis.

Nearly all of the galaxies we observe in the night sky are rushing away from us. Only the Andromeda galaxy is moving toward us — we are trapped in a gravitational dance that will end in a major collision about 4.6 billion years from now. The remainder of galaxies are receding due to the expansion of the universe. But how fast are the rest of the galaxies flying away from us? This is actually a difficult question to answer, partly because it is difficult to accurately measure distances across the universe. Today’s paper details a new method to measure how quickly the universe is expanding using the gravitational-wave (GW) signals from binary black hole collisions.

The Sounds of the Universe

Gravitational waves are ripples of spacetime itself, analogous to sound waves traveling through the air. They are generated in violent collisions between compact objects like neutron stars and black holes. The LIGO and Virgo collaborations have detected 11 such collisions, ten of which have been the collisions of two black holes (see Figure 1). The frequency and amplitude of the GWs, or the pitch and volume of the “sound,” encode information about the mass of the merging system and how far away it is. Exactly how the signal evolves tells us everything we need to know about the gravitational “brightness,” or luminosity, of the event. By comparing the measured amplitude to the calculated amplitude, we get a precise distance to the source. The ability to do this with GW signals has earned their sources the name “standard sirens.”

stellar graveyard

Figure 1. Our current knowledge of the end states of massive stars, namely black holes and neutron stars. Because the GW signals are so precisely tied to the properties of the system, we can determine the masses of the initial objects before merger in addition to the mass of the final object after merger. [LIGO-Virgo/Frank Elavsky/Northwestern U.]

What does this have to do with measuring the expansion rate of the universe? Hubble’s Law tells us that the velocity v at which an object at redshift z recedes away from us depends on its distance from us: v(z) = H0d, where H0 is the expansion rate of the universe. Previous measurements of this parameter, called the Hubble constant, have used electromagnetic radiation from either the cosmic microwave background (CMB) or Type Ia supernovae. These measurements currently conflict with one another, suggesting there might be some missing physics in our understanding of the universe. Further, measuring distances in the universe is tricky. CMB and Type Ia supernovae measurements rely on the cosmic distance ladder, so errors from one rung will propagate to the next.

On the other hand, standard sirens with electromagnetic counterparts don’t rely on the cosmic distance ladder and so offer an independent way to measure H0. In this case, the electromagnetic signal pins down the host galaxy’s location, which identifies the redshift of the signal and thus its velocity. At the same time, the GW signal gives the precise distance to the source. In fact, this has already been done using the binary neutron star merger GW170817. But we need many more than one binary neutron star event to truly pin down H0 (Figure 3), and ten of the 11 LIGO/Virgo events have not had accompanying EM signals. Today’s paper shows that there is still a way to calculate H0 from these events!

Listening in the Dark

The authors of today’s paper report the first measurement of H0 using the “dark” siren GW170814, a GW signal from two colliding black holes with no accompanying electromagnetic radiation. Recall from Hubble’s Law that we need a distance and a redshift to calculate H0 — but to determine a redshift, we need to know what galaxy the source was located in. That’s a hard thing to determine with no electromagnetic counterpart signal. The probability maps produced by LIGO/Virgo for the on-sky location of the GW signal can encompass a large area containing tens of thousands of galaxies at as many different redshifts!

GW170814 happened to fall smack in the middle of the Dark Energy Survey (DES, see Figure 2). DES has produced exquisite galaxy maps of a quarter of the Southern Hemisphere sky, complete with estimated redshifts calculated from the coarse “spectrum” of DES’s five wavelength filters. Soares-Santos, Palmese, et al. devised a statistical analysis that selected potential host galaxies from DES’s galaxy maps using the LIGO/Virgo maps and calculated what H0 would be for each in turn.

Figure 2. The LIGO/Virgo highest probability region for where GW170814 originated from, overlaid on the DES survey area. [Dark Energy Survey Collaboration]

After analyzing 77,000 galaxies, the authors calculate that H0 = 75.2 +(-) 39.5(32.4) km s-1 Mpc-1. Figure 3 shows how this value compares to previous measurements using the CMB and Type-Ia supernovae. While the uncertainties are quite large using only one GW event, the authors estimate that uncertainties comparable to the CMB and supernovae measurements are possible with ~100 GW events. Improvements to the LIGO detectors were recently completed and the observatory’s third run (O3) started on April 1st. There could potentially be a dark siren event every week, meaning we might only have to wait a couple of years to measure H0 to sufficient precision using GW events!

Figure 3. Comparison of values of H0 calculated from the CMB (Planck, dark blue), supernovae (ShoES, light blue), the binary neutron star event (GW170817, grey), and the dark siren (DES GW170814, red). With ~100 GW events, we will approach the sensitivity of the traditional electromagnetic measurements, giving an independent measurement of H0. [Soares-Santos et al. 2019]

Having a new way to measure H0 is a big deal for potentially resolving the tension between the CMB and supernovae measurements. If the dark/standard siren methods, which probe the late-time universe, end up being consistent with the early-universe CMB results, that might imply that something is wrong with our cosmic distance ladder and the late-universe supernovae measurements. On the other hand, if the GW measurements are consistent with the supernovae results, we might need to add new physics to our current understanding of the universe to explain why H0 would evolve with time. Either way, the next few years will be a very exciting time in precision cosmology!

About the author, Stephanie Hamilton:

Stephanie is a physics graduate student and NSF graduate fellow at the University of Michigan. For her research, she studies the orbits of the small bodies beyond Neptune in order learn more about the Solar System’s formation and evolution. As an additional perk, she gets to discover many more of these small bodies using a fancy new camera developed by the Dark Energy Survey Collaboration. When she gets a spare minute in the midst of hectic grad school life, she likes to read sci-fi books, binge TV shows, write about her travels or new science results, or force her cat to cuddle with her.

protoplanetary disks

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: Protoplanetary Disk Rings and Gaps Across Ages and Luminosities
Author: Nienke van der Marel, Ruobing Dong, James di Francesco, Jonathan P. Williams, John Tobin
First Author’s Institution: Herzberg Astronomy & Astrophysics Programs, National Research Council of Canada
Status: Published in ApJ

If you’re well-versed in exoplanets (or even the formation of your own planet), you may be familiar with the term protoplanetary disk. These objects are disks of gas and dust surrounding a fairly newborn star, although newborn here means up to several million years old. Interestingly, images of protoplanetary disks captured by astronomers reveal gaps in the disks — or rather, separate rings of material, depending on your perspective. These types of disks were once expected to be completely smooth, so why are we seeing gap-like features in essentially all resolved images of them?

disk images

Figure 1: Left: Continuum emission for the disk sample. The beam size is shown in the lower left corner with the spectral type of each star in the right. HL Tau, TW Hya, and V1247 Ori, which had higher resolution, have been reduced to reflect the 20-AU beam size of the other images in order to make them comparable. Right: Enhanced image representations for each disk, which were used in the analyses. Gaps have been made easier to see using a variety of unsharp masking techniques. Transition disks are RXJ 1615, AA Tau, DM Tau, V 1247 Ori, HD 97048, HD 100546, TW Hya, HD 169142, and HD 135344B. [Adapted from van der Marel et al. 2019]

The authors of today’s paper use ALMA data to explore what could be causing these gaps. They examine images of 16 different protoplanetary and transition disks (disks where the material closest to the star has been cleared out; see Figure 1). The disks in the sample surround stars of various spectral types, and each exhibits multiple gap features. Since these gaps are present throughout the sample, some correlation between them should reveal the responsible mechanism, right? After all, we would expect that these features evolve in similar ways for most disk systems.

Before they can answer this question, the authors first determine each star + disk’s luminosity and use this information to age each star with model evolutionary tracks. The resulting age range gave them a way to classify their disks as older or younger. They also determined approximate gap locations and sizes via a type of intensity profile fitting, which essentially models the light coming from the star + disk in each image (where there is a gap, there is less light detected, etc.).

Searching for Answers

gap properties

Figure 2: Gap properties for each disk. Each red dot represents a gap in the corresponding disk, with the disks sorted by increasing age. The gap center radius is raised to the 3/2 power, to mimic orbital resonance ratios. [Adapted from van der Marel et al. 2019]

With this information, the authors searched for trends in
the data — any correlations between the stars’ properties and the disk properties that may point to an origin story. Figures 2 and 3 show these chosen parameters … as well as the obvious dearth of trends. The only visible trend seems to be the decrease in outermost ring radius for the four oldest stars when compared to the rest of the sample (see Figure 3, bottom). This sure does make it hard to imagine a common mechanism responsible for the gaps.

Two of the most common theories for gaps within disks are 1) planets and 2) stuff freezing. Certain compounds are present in the disk, and there should be a point away from the star where the temperature becomes just right for those compounds to freeze into ice crystals. This is called the snow (or frost) line. At that snow line, it is thought that as these crystals all form together, they may stick to each other and the disk’s dust, which could clear out some space and turn into the gaps we see.

The authors looked to 5 different molecules in order to test this hypothesis: N2, CO, CH4, CO2, and NH3. This is the “stuff” we were talking about freezing. They determine the location of each respective snow line for each star and find that these locations don’t seem to have anything to do with the gaps — there are a few instances where the gap and snow line overlap, but it does not seem to be a systematic trend. So it seems the snow line scenario is a no-go.

Planetary formation is another story. It is thought that as planetesimals accrete surrounding disk material, a gap forms as it carves out its orbit. The authors state that they simply don’t have enough information to disprove or support this theory. Neptune-mass planets are currently undetectable and only one disk (HD 163296) in the sample has been suspected of having a planet. They do note however that one model of planet formation (cold-start model) would allow for giant-planet formation at the location of many of the observed gaps. So, planet formation is still a possible culprit.

gap properties

Figure 3: Additional gap properties across ages and luminosities, with each red dot representing a gap. [Adapted from van der Marel et al. 2019]

In case you were wondering, authors also ponder the case of graviational instability. Gravitational instabilities in the disks could potentially cause the gaps, but this depends on the gas-to-dust mass ratio. Unfortunately this is very much uncertain and hard to measure, so the trail stops there.

So What Do We Know?

The only solid correlation present in the sample is that the outer disk ring is much closer in for the four oldest stars — i.e., the older disks are smaller in diameter. In that case, it seems we are missing some outer rings. So maybe the outer rings dissipate faster than the inner rings (due to drag forces or radiation pressure). Either that, or these outer dust grains accrete to become planetesimal sized and are therefore undetectable at the observed wavelengths.

Although the authors didn’t find the origin story they were looking for, they can say a few things with certainty. The lack of trends in their data show that disk gaps are diverse and their presence is largely independent of stellar properties, like spectral type or age.  They also found that snow lines don’t have anything to do with the gaps we observe, but planets very well might. And last but not least, transition disks seem to host these features in the same manner as the truly protoplanetary disks, implying that they evolve in the same way, even if we don’t know what that way is. This is actually quite a big step in the right direction. These clues get astronomers one step closer to to closing the gap on … gaps.

About the author, Lauren Sgro:

I am a PhD student at the University of Georgia and, as boring as it may sound, I study dust. This includes debris disk stars and other types of strange, dusty star systems. Despite the all-consuming nature of graduate school, I enjoy doing yoga and occasionally hiking up a mountain.

M-dwarf flare

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 Surface UV Environment on Planets Orbiting M-Dwarfs: Implications for Prebiotic Chemistry & Need for Experimental Follow-up
Author: Sukrit Ranjan, Robin Wordsworth, & Dimitar D. Sasselov
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics
Status: Published in ApJ

Note: The reference to “life” in this post refers to “life as we know it here on Earth.”

M dwarfs, the smallest and coolest (in temperature and by reputation) of the main sequence stars, are also the most abundant type in our galaxy. As this Astrobite post notes, Earth-sized and super-Earth sized planets are also quite common around these stars, with 0.86 of these types of planets per M dwarf. Also, as M dwarfs are cooler, their habitable zone is located much closer in — and the consequent shorter periods of M dwarfs’ habitable-zone planets makes it easier to observe multiple transits. Therefore, if we want to search for a habitable planet, then we should be looking for it around an M-dwarf. Several habitable-zone planets have already been discovered around M-dwarfs; Trappist-1 has 3 of them!

Unfortunately, just because a planet is in the habitable zone doesn’t necessarily mean that it can support life. M dwarfs produce a lot of ultraviolet (UV) light, especially at the shorter UV wavelengths (extreme UV or EUV). Not only does UV light give you a sunburn, it can wreak havoc on life, from destroying cells to stripping a planet of its water and atmosphere. Most studies agree UV is bad for business and there is a high probability that M-dwarf planets can’t support life as we know it.

The authors of today’s paper shine a new light (pun intended) on the effect of UV on life. They note that previous laboratory studies have discovered that UV light is actually necessary to create the building blocks required for life, such as RNA, amino acids, and sugars. The creation of these building blocks is also known as prebiotic photochemistry. Could M-dwarf UV radiation actually be fueling life instead of destroying it?

Early Earth Around an M Dwarf

Figure 1: Spectra of multiple types of M dwarfs and the approximate spectra of the 3.9-billion-year-old young Sun in black. Top panel is the total amount of flux the stars create at varying wavelengths. At shorter ultraviolet wavelengths, both M dwarfs and the young Sun radiate a comparable amount of flux. Bottom panel is the amount of light that reaches the surface of the early Earth. The cutoff at 200 nm is due to absorption by carbon dioxide in the atmosphere. A significant amount of UV light from the young Sun reaches the planet surface compared to any of the M dwarfs. [Ranjan et al. 2017]

To address this question, the authors first determined what sort of UV environment a pre-life Earth would experience around different types of M dwarfs. They assumed their planet had a simple 1-bar atmosphere composed of 90% nitrogen and 10% carbon dioxide. This pre-life Earth was then hit with a variety of UV radiation levels based on known M-dwarf spectra. The authors found that the carbon dioxide in the atmosphere protected the surface of the planet from the harmful EUV radiation — a plus for life. However, because M dwarfs are smaller, they only produce 1–10% as much near-UV radiation as our young Sun (Figure 1). Using the amount of light that reaches the planet surface, known as surface radiance, the authors determined that the reaction rates for prebiotic photochemistry would take 2–4 times longer on an M-dwarf planet than on early Earth.

Can Life Develop or Not?

This doesn’t necessarily mean that life as we know it couldn’t have developed on a M-dwarf planet, just that the pathways to achieving prebiotic chemistry would take substantially longer (on order of 1010 years or almost the current age of the universe). If this is the case, then there hasn’t been enough time for life on most M-dwarf planets to develop! However, if the reactions take too long, the paper points out that other chemistry might dominate that which is needed for life, though it is unclear what the timescale for its development would be.

But there may be a way around this. M dwarfs are fairly active and are known to flare often. The authors repeat their experiment using the spectrum of a known flare from the M dwarf AD Leo. They find that for a short period of time, the planet experiences UV flux that is 10x that of the Earth for some wavelengths, leading to reaction rates which are up to 10x as fast. A comparison of reaction rates before and after a flare is shown in Figure 2. The Relative Dose Rate corresponds to reaction rates. The different colored boxes in Figure 2 represent different chemical reactions. Before a flare, AD Leo, a fairly active M dwarf, drives reaction rates that are slower than those driven by an early-Sun host star. But during a high-energy flare, its reaction rates jump orders of magnitude!

Figure 2: Reaction rates (Relative Dose Rate) of the young Sun, AD Leo when it is not flaring (i.e., it is quiescent), and AD Leo when it does flare. Each colored box represents different prebiotic photochemical reactions. [Adapted from Ranjan et al. 2017]

With enough flares hitting the planet, it may be possible for prebiotic photochemistry to occur on similar timescales to that of early Earth. Though how many flares are needed, with how much energy, and what this could do to the atmosphere of the planet, are all still questions that need answering both through experiments in the lab and continued M-dwarf observations. The authors argue that we should consider targeting more active or flaring M dwarfs in our search for life.

As to whether UV radiation is good or bad, we keep coming up with the same answer: “we don’t know.” It turns out we continue to prove a very obvious conclusion: life and the development of life is really complicated.

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.

protoplanetary disk

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 Survey of CH3CN and HC3N in Protoplanetary Disks
Author: Jennifer B. Bergner, Viviana G. Guzman, Karin I. Öberg, Ryan A. Loomis, Jamila Pegues
First Author’s Institution: Smithsonian Astrophysical Observatory and Sternberg Astronomical Institute
Status: Published in ApJ

The Prebiotic Importance of Nitrogen-Bearing Molecules

Did you know that you are made out of primordial dust and gas? It’s true! Our solar system formed from a gravitationally collapsing cloud of interstellar dust and hydrogen gas, birthing a proto-Sun in the center of this hot dense material. The various planets are then thought to have formed out of the material in the solar nebula, the disk-shaped cloud of gas and dust left over from the Sun’s formation, which ultimately takes the shape of a rotating protoplanetary disk. As discussed in a previous astrobite, these disks exhibit an intriguing variety of chemistry that is only beginning to be probed with the higher sensitivity of telescopes such as ALMA. Understanding how the early inventory of organic molecules present in this stage of planet formation developed into the vast complexity of biochemistry we see today is key to the study of the origins of life.

Chemists and astronomers alike have been especially interested in nitrile-bearing molecules, which contain a carbon-nitrogen triple bond. These molecules likely play a crucial role in prebiotic chemistry, as recent studies have shown that this particular bond is involved in prebiotic synthesis of RNA and protein precursors. In today’s astrobite, we take a look at these important types of molecules in a recent survey of protoplanetary disks, and we explore the implications for nitrogen-based chemistry in our early solar system.

Surveying CH3CN and HC3N in Planet-Forming Disks

Up to this point, few disks had well-characterized nitrile abundances. More importantly, it was unknown if other disks contain similar nitrile abundances as the solar nebula. Also, it was unclear how robust the nitrile chemistry is in different circumstellar environments. For instance, do differences in the density and temperature around young stars lead to significantly different amounts of nitrogen-bearing molecules, or can they survive in a wide range of physical conditions? The authors of today’s bite sought to obtain a larger sample of observations to answer these questions.

The authors used ALMA to survey six nearby protoplanetary disks at distances ranging from 235 to 466 light-years from Earth. Cyanoacetylene (HC3N) is detected in all but one of the disks, while methyl cyanide (CH3CN) is firmly detected in three of the six disks with additional tentative detections in another two disks. A face-on view of the HC3N and CH3CN gas, along with the observed spectra in half of the observed disks, is shown in Figure 1.

Figure 1: Face-on view of total radio emission from cold dust (left column) and CH3CN and HC3N (middle columns) around the IM Lup, V4046 Sgr, and MWC 480 disks. Images are centered on the peak dust emission (“+”). Darker colors and additional contours represent increasing strength of the signal. The x- and y-axes are in equatorial coordinates in arcseconds and, depending on source distance, they show a total range of 360–805 AU, or nearly 7–16 times the size of our solar system. The light gray circle in the lower left indicates the resolution of the radio observations. The disk-integrated spectra from rotational transitions of each molecule (right column) are shown in terms of velocity and radio intensity. The double-peaked structures indicate that the disks are undergoing Keplerian rotation. [Bergner et al. 2018]

Discovery of a Robust Nitrile Chemistry

In protoplanetary disks, the underlying chemistry is determined by both the presence of gas-phase molecules and dust grains. In the case of these two molecules, astrochemists have discerned that HC3N is only able to form efficiently in the gas phase, while CH3CN forms either in the gas phase or on the surfaces of dust grains. To investigate not only these formation mechanisms, but also to test the chemical dependence on various physical conditions, the disks selected for this study were carefully chosen to span a range of physical conditions, including their age, mass, and luminosity. However, after careful modeling, the authors find no strong trends in nitrile emission strength or abundances with these environmental differences. This result implies that nitrile chemistry is chemically robust and is likely to be found with similar outcomes even in substantially different physical conditions.

Protostellar and Cometary Comparisons

To further investigate this unexpected chemical resilience, the authors compare their results in protoplanetary disks with previous observations of protostellar envelopes, as well as with solar-system comets, which are thought to be relatively pristine records of our own solar nebula. To do so, the authors compare abundance ratios among HC3N, CH3CN, and HCN with those measured in protostars and comets in Figure 2. For each of these disks, the molecule HCN, which is a precursor molecule to HC3N, had previously been observed and thus is also included in their comparisons.

Figure 2: Abundance comparisons between (a) CH3CN/HCN, (b) HC3N/HCN, and (c) CH3CN/HC3N. Due to uncertainties in the determined gas temperatures, the ratios are shown for a range of temperatures from 30–70 K. The top two panels show comparisons against typical cometary fractions, while the bottom panel is compared with the measured fraction in protostellar envelopes from a previous study by the same authors. The authors find that the nitrile chemistry exhibits similar abundance ratios in protoplanetary disks, comets, and protostellar envelopes. [Bergner et al. 2018]

Nitrile abundance ratios are surprisingly consistent across the surveyed disks as well as with comets and protostellar envelopes, despite the inherent evolutionary differences of these objects. This consistency again demonstrates that complex nitrile species should be reliably produced in a variety of different star- and planet-forming environments. These results also suggest that the solar system is not unique in its nitrile chemistry, which means that the raw materials needed for the biochemical beginnings of life may be common around newly forming planets in other solar systems!

About the author, Charles Law:

Hi! I’m a first-year graduate student at Harvard/CfA. I’m interested in observationally studying the chemical complexity found in space, including throughout high-mass star-forming regions and in protoplanetary disks. In my free time, I enjoy hiking, bicycling, and traveling.

accreting black hole

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 Population of Bona Fide Intermediate Mass Black Holes Identified as Low Luminosity Active Galactic Nuclei
Author: Igor V. Chilingarian et al.
First Author’s Institution: Smithsonian Astrophysical Observatory and Sternberg Astronomical Institute
Status: Published in ApJ

Most, if not all, galaxies contain a supermassive black hole (SMBH) at their center. But where did these giants come from? Astronomers know that less massive black holes can form when a star collapses in on itself. These stellar-mass black holes can have the mass of a few dozen Suns, but they do not come close to the millions or billions of solar masses SMBHs have.

There are a few ideas for where SMBHs may have come from. One scenario is that the earliest stars left behind small baby black holes, which over time merged together to form more and more massive grown-up black holes that now reside at the center of galaxies. If this were the case, we’d expect to see some teenage, intermediate-mass black holes (IMBHs) — black holes larger than those born from stars but smaller than those we see in the center of galaxies.

Another possibility is that large gas clouds in the early universe collapsed to form massive black-hole “seeds” the size of hundreds of thousands of stellar masses. We then wouldn’t expect to see the adolescent phase of IMBHs, as SMBHs would start growing from these large seeds instead of small ones left behind by stars.

The authors of today’s paper wanted to see if they could find IMBHs, and thus evidence for the first scenario for SMBH formation.

Finding AGNsty Adolescents

While previous searches for IMBHs looked at only a few, pre-selected galaxies, Chilingarian et al. set up an automated search through 1 million catalogued galaxies to look for signs of SMBHs at their centers that are actively gaining mass — objects known as active galactic nuclei (AGN). AGN show distinct spectral lines, as seen in the bottom left of Figure 1. The authors used the width and strength of some of these lines to calculate the mass of the black hole for each galaxy.

Fig 1: The top row shows a diagram breaking down the components of an AGN. The black hole at the center (far right) has an accretion disk emitting light, while the clouds in the broad- and narrow-line regions (center) absorb and reemit the light, causing the peaks seen in the spectra at the lower left. The lower right plots show the curves used to fit the emission lines. [Chilingarian et al. 2019]

Of the million objects analyzed, 305 had black holes with masses less than that of 200,000 times that of the Sun, categorizing these as potential IMBHs.

By selecting a subset of these candidates for follow-up observations, Chilingarian et al. confirmed the AGN nature of ten of these candidates by detecting them in X-rays. This provided the authors with a sample of ten bona fide AGN with black-hole masses measured between 43,000 and 202,000 solar masses, five of which were previously known. Images of the ten galaxies that host the proposed IMBHs are shown in Figure 2.

Fig 2: Optical images from the Sloan Digital Sky Survey of the ten IMBH host galaxies. The white line shows the scale; five kiloparsecs (kpc) is approximately 1017 kilometers. The red circle is the location of their observations in X-ray wavelengths. The number at the top is their catalogue designation and the number at the bottom is their mass estimate in solar-mass units. The bottom row contains the galaxies with proposed IMBHs found in previous studies. [Chilingarian et al. 2019]

Results

These results are promising for the scenario where supermassive black holes are grown from stellar-mass seeds. If SMBHs grew from seeds larger than these observed intermediate-mass black holes, then we would not expect to see any IMBHs.

From their original sample of 305 possible IMBH host galaxies, 14 currently have enough observational evidence to test the most stringent observing criteria for IMBHs. Only six of those 14 (or 43%) successfully pass all the tests, suggesting that they host a real IMBH. Thus, the authors estimate a lower limit of 131 galaxies in their sample will have real IMBHs when further follow-up observations are performed.

The 305 possible IMBHs the authors explore in this paper are only those that are actively collecting more material. There may be many more non-accreting IMBHs out there that cannot be detected because they are too far away for our current instruments.

The accretion of matter from a host galaxy is not enough to explain the growth of stellar-mass black holes to IMBHs, so IMBHs must form from the merger of smaller black holes. The authors conclude that at least some of the SMBHs we observe must therefore ultimately be built from mergers of small, stellar-mass seed black holes.

About the author, Bryanne McDonough:

First year graduate student at Boston University where I am currently studying the distribution of dark matter and satellites around galaxies using data from the Illustris simulations. My primary research interests are galactic and extragalactic astrophysics using computational methods. Outside of grad school I enjoy reading and crafting.

WASP-12b

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: Obliquity Tides May Drive WASP-12b’s Rapid Orbital Decay
Author: Sarah Millholland, Gregory Laughlin
First Author’s Institution: Yale University
Status: Published in ApJ

It’s not an easy life being a hot Jupiter. Besides their eternal loss of privacy due to being on the hit list of astronomers since the very moment they are detected, theirs is a tale of intense drama. The extreme radiation and tidal flexing they experience due to their proximity to their host stars make them ideal targets for studying planetary science in extreme physical conditions, particularly since there are no hot-Jupiter analogs in our solar system. One of these weirdos, and also one of the most studied hot Jupiters in recent years, is WASP-12b. In addition to being one of the hottest hot Jupiters around (with equilibrium temperature of ~2500 K) and losing mass at an exceptionally high rate, it is also the only known hot Jupiter inspiralling so rapidly towards its star that we can observe this decay in real time. It has been speculated that sustained tidal interactions between the star and the planet could be responsible for the observed orbital decay of the planet. The authors of today’s paper investigate the case of WASP-12b in this context to understand what might be driving its orbital decay.

Turning with the Tide

Tidal interactions between a star and its planet act together to dissipate the orbital energy of the system in the interiors of the planet and the star. Think of the planet and the star acting like resistors that dissipate the gravitational energy of their orbits into heat due to tidal flexing in their interiors. This tidal dissipation is most effective when a planet has large orbital eccentricity and obliquity (the angle between the orbital plane and spin axes of the planet). However, for a lonely hot Jupiter with no other planetary companion in the system, tidal torques from the host star over time would eventually force the planet to reach an equilibrium state with low orbital eccentricity and obliquity, which would reduce the tidal dissipation the planet experiences. The fact that WASP-12b’s orbit is decaying rapidly indicates that there may be some other force at work helping to drive its sudden inspiral. The authors of today’s paper propose that the presence of another planet in the system (see Figure 1) could sustain WASP-12b’s obliquity, causing the planet to continue to experience high tidal dissipation and preventing its inspiral from stabilizing.

spin-orbit interactions

Figure 1: Schematic of the spin-orbit interactions between WASP-12b and another planet in the system. In the case of spin-orbit resonance (called a Cassini state), the spin angular momentum vector S1 and orbital angular momentum vector L1 precess about a common axis at the same rate. [Millholland & Laughlin 2018]

If there is another planet in the system, it’s likely that WASP-12b could have gotten locked in a state of spin–orbit resonance (known as a Cassini state, also observed in the case of Earth–moon system) which means that the rate of precession of its orbital axis is the same as the spin axis precession rate. This implies that as the orbital precession rate of WASP-12b changes due to dynamical interactions with the perturbing planet in the system, this will drive a change in its spin precession rate as well, increasing WASP-12b’s obliquity.

The authors simulate the evolution of the WASP-12 system using an obliquity tide model that couples two things: the relations governing the secular dynamical interactions between WASP-12b and another planet, and the tidal interaction between the star and WASP-12b. These interactions then combine to cause the inspiral. With very little fine-tuning of the properties of the perturbing planet and the efficiency of tidal dissipation (which is dependent on the unknown interior structure of WASP-12b), they are able to reproduce the inspiral rate that we have observed for the system (Figure 2). An interesting result of their analyses using the obliquity tide model is that even if WASP-12b starts with a very low obliquity (< 1°), it can easily get locked into a Cassini state and attain a high obliquity within the lifetime of the planetary system.

WASP-12b's obliquity evolution

Figure 2: Simulated evolution of WASP-12b’s obliquity (gray) and semi-major axis (purple) with respect to time from the obliquity tide model. Note that tidal dissipation due to obliquity tides is a runaway process: orbital decay (represented by the decreasing semi-major axis) due to tidal dissipation leads to increasing obliquity as the other planet forces WASP-12b to maintain the spin-orbit resonance. This further increases the efficiency of tidal dissipation. [Millholland & Laughlin 2018]

So obliquity tides can cause the observed orbital decay — but how do we confirm this? The good news is that, based on the predictions of their obliquity tide model, the authors conclude that the perturber planet is very likely within the limits of detection for extreme-precision radial velocity instruments coming up in the near future. Tighter constraints on transit duration variations or transit timing variations (a signature of orbital precession) from more precise long-term photometric monitoring of the system by TESS could also help in strengthening the obliquity tide hypothesis. If confirmed, obliquity tides might additionally be able to explain WASP-12b’s extreme radius inflation and unusual features in its thermal phase curves and will provide compelling evidence for in situ formation of hot Jupiters. Since tidal interactions are intimately tied to the interiors of the planets, they could also be an unprecedented tool for X-raying the interior structure, formation histories, and demographics of exoplanets.

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 to understand the diversity and origins of planetary systems. I also enjoy yoga, Lindyhop, and pushing my culinary boundaries every weekend.

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