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exoplanet atmosphere

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: Redox Evolution via Gravitational Differentiation on Low Mass Planets: Implications for Biosignatures, Water Loss and Habitability
Authors: R. Wordsworth, L. Schaefer, R. Fischer
First Author’s Institution: Harvard University
Status: Submitted to ApJ, open access

Looking for Life

If you’ve been tuning into astronomy news lately, you’ve probably heard about a number of cool new exoplanet discoveries, like those in the TRAPPIST-1 system, continuously rolling in from our telescopes hard at work. But no matter how, when, and where a new exoplanet is discovered, there’s always that question burning at the back of our minds: could this exoplanet have Earth-like life?

Figure 1: A screenshot from the open-universe space exploration video game called No Man’s Sky. Even with Earth-like constraints, the sky’s the limit in terms of the many living worlds we can imagine are out there. [No Man’s Sky].

This question is certainly not an easy one to answer. For an exoplanet to house life (as we understand life so far), there’s a long checklist of requirements (like those discussed here and also here) that we need the exoplanet to fulfill. For example, life as we know it survives and thrives on liquid water, so we require that the exoplanet has the ability to hold liquid water.

Today’s astrobite focuses on another important requirement: the exoplanet’s atmospheric composition. Here on Earth, for instance, we have a lot of wonderful plant-based and plant-like creatures (like trees) that produce oxygen through photosynthesis. Then other creatures here (like humans) use that oxygen to survive and thrive. So for an exoplanet to have Earth-like life, we expect it to have a buildup of oxygen.

You might imagine, then, that we should be looking for oxygen on these exoplanets, as a sign of Earth-like life — and you’d be right, mostly! But unfortunately it’s not clear that a buildup of oxygen will “always” be a sign of Earth-like life. Instead, an exoplanet could possibly accumulate oxygen from purely chemical, completely not-organic-or-life-related (aka, abiotic) processes. This means that if we find an exoplanet that has a buildup of oxygen, we need to be cautious and somehow make sure that we haven’t just discovered a false positive (i.e., a case where what we think to be true is actually very false).

Today’s authors present a nifty theoretical framework for thinking about and modeling the atmospheres of exoplanets, which can help both characterize and predict the atmospheres of the many new exoplanets we’ve observed and continue to observe. The authors did many, many other cool things in their paper, but here we will focus on understanding the backbone of their theoretical framework.

A Little Chemistry

The authors use the concepts of ‘redox’ as the main variable of their model. ‘Redox’ is an abbreviation for ‘oxidation-reduction reaction‘, which is a fancy term for a reaction involving the exchange of electrons between two chemical species. The atom, molecule, or ion gaining an electron(s) is ‘reduced’, while the atom, molecule, or ion giving up an electron(s) is ‘oxidized’. Combustion is a common example of a redox. When you burn firewood at, say, a campfire, the carbon (C) from the wood reacts with the oxygen (O2) in the air, producing carbon dioxide (CO2). In this case, the carbon gives up four electrons and is oxidized, while the oxygen collectively gains four electrons and is reduced.

If a planet has an atmosphere that is very oxidizing, that means there is, in a sense, a net demand for electrons. That means we would expect O2, which wants to take electrons, to abiotically build up in an oxidizing atmosphere, because there would be no net supply of chemical species floating around for the O2 to react with and take electrons from. So to evaluate if an exoplanet would likely abiotically build up O2, we want to check if and when its atmosphere is likely to be oxidizing — and that depends on the planet’s chemical composition.

Figure 2 shows different elements from the periodic table as a function of electronegativity (which is basically how oxidizing versus reducing the elements are) plotted against atomic mass and abundance. With Earth as our example, we expect that the less massive elements like hydrogen (H), carbon (C), nitrogen (N), and oxygen (O) largely hang out in the volatile layer (which includes the atmosphere) and upwards towards space; magnesium (Mg), silicon (Si), and sulfur (S) largely get caught up in the planetary crust and mantle; and the heavy element iron (Fe) is largely trapped down in the core.

Figure 2: Electronegativity (x-axis) versus atomic mass (y-axis) for the most abundant elements in our solar system. Each circle corresponds to an element, which are labeled according to their periodic table abbreviations (H is hydrogen, for example). The sizes of the circles reflect the relative logarithmic abundances of these elements in our solar system. The faint gray asterisks in the background label elements less than 10% of the abundance of silicon (Si). Reducing elements are towards the left of the plot, while oxidizing elements are towards the right. And overall, the less massive elements rise towards space, while the more massive elements fall towards the planet’s core. [Wordsworth et al. 2017]

Studies have shown that, just like Earth, planets of around 1 to 10 Earth masses tend to split into these three major layers: volatile, mantle, and core. So to understand how oxidizing the atmosphere of a planet in this mass range is, we need to understand how these three planetary layers interact and exchange these different elements.

Putting It All Together

The authors quantify how oxidizing a planetary layer is in terms of the layer’s total oxidizing power using the following equation (Equation 1 in the paper):

Where N is the total oxidizing power, Ni is the number of atoms of some element ‘i’ (like carbon or oxygen), and pi is the oxidizing potential of element ‘i’. An atom of oxygen, for example, takes two electrons, so it has an oxidizing potential of +2. An atom of hydrogen, on the other hand, gives away one electron, so it has a negative oxidizing potential of -1. Finally, the Σ in the equation says that we want to sum over all elements ‘i’ in the layer. The authors drew out a model of the three-layer planetary system, as shown in Figure 3, and assigned a total oxidizing power N for each layer.

Figure 3: A three-layer model for a planet of about 1 to 10 Earth masses after formation. The N at the bottom-left of each layer represents that layer’s total oxidizing power. The k terms represent the exchange between these layers over time. And the E at the top represents the escape of the less-heavy elements, especially the reducing element hydrogen, out into space. [Wordsworth et al. 2017]

To calculate how oxidizing the atmosphere is with this model, we must track the flow of elements between these three layers over time. The authors discuss a lot of the cool processes that can allow transport of material between these layers at different times in the planet’s evolution. For example, when the planet is young, its mantle layer can be partially or fully ultra-hot and molten, like an ocean of magma. During this phase, the liquidy mantle layer and the atmosphere-containing volatile layer can interact and exchange materials much more easily. But once the planet cools down and the mantle layer hardens, the interactions get more complicated. Depending on the planet’s geodynamic structure (such as any churning plate tectonics and gas-spewing volcanoes), the mantle layer can shift around and still allow material to flow between these planetary layers. And over time the much-less massive elements, especially the reducing element of hydrogen, can leak out of a planet’s atmosphere and escape to space (that’s the ‘E’ term in Figure 2), also changing how oxidizing the atmosphere is.

Once we have a good handle on how these conditions and processes play out on other planets — such as answers to the tough question of how geodynamics evolve on planets other than Earth — we can apply this framework to dig out any planetary scenarios where compounds like O2 could build up abiotically over time. And for those seemingly habitable exoplanets that appear very unlikely to abiotically build up O2, we can be cautiously optimistic that any oxygen buildup we do see might come from a biotic source — exactly what we’ve been searching for all along.

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!

PSR J1023+0038

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: Simultaneous Chandra and VLA Observations of the Transitional Millisecond Pulsar PSR J1023+0038: Anti-correlated X-ray and Radio Variability
Authors: Slavko Bogdanov, Adam T. Deller, James C. A. Miller-Jones, et al.
First Author’s Institution: Columbia University
Status: Submitted to ApJ, open access

What’s more interesting than a rapidly spinning neutron star that emits electromagnetic radiation parallel to its magnetic poles? One that doesn’t exactly behave as expected, of course. One such weirdly acting pulsar, PSR J1023+0038, is a transitional millisecond pulsar (tMSP) — which is fancy speak for a pulsar with a millisecond rotational period that switches between radio and X-ray emission on a several-year timescale. The fact that this pulsar emits in both X-ray and radio on these longer timescales isn’t what piques the interest of astronomers, however, in the case of the study in this astrobite.

Weird Pulsar Behavior

Figure 1: Radio emissions (black) and x-ray emissions (blue) recorded by the VLA and Chandra respectively over time. This shows that when radio emissions drop off, X-ray emissions pick up.

Pulsars can typically fall into one of the following categories: radio pulsars are powered by exchanging rotational energy from the spinning neutron star into emitting radiation. This means that their rotation slows and their pulse length increases. Meanwhile, X-ray pulsars are accretion powered, meaning they turn heated infalling matter into X-ray emission. What distinguishes PSR J1023+0038 from the background of pulsars that switch between accretion-powered X-ray and rotation-powered radio pulsars is that it has a simultaneous anti-correlated X-ray and radio emission. The authors looked at about 5 hours of overlapping and concurrent observations from the Chandra X-ray Observatory and the Very Large Array (VLA) to try and understand this weird relationship between the X-ray and radio emissions. This is very clearly shown in Fig. 1 where we can see a tiny sample of time of overlapping X-ray and radio flux measurements. The anti-correlation is quite strong, meaning that when the X-ray emissions are weakest, the radio emission is strongest.

Figure 2: (Top) Chandra x-ray observations over a period of 5 hours.The 3 x-ray modes can be seen as the one large peak (~12.7 hrs), the low (minima), and high (steady maxima).    (Middle) The simultaneous observation as seen from the VLA. (Bottom) The overlapped top and middle observations show the anti-correlation between x-ray and radio emissions.

But wait…there’s more! When we zoom out on the flux/time series observations (Fig. 2) we can not only see that the anti-correlation is persistent, but we can also see that the X-ray emission has at least 3 unique modes of operation. The authors classify these X-ray emission modes as (1) sporadic flaring (~12.7 hrs in Fig. 2), (2) high, and (3) low modes. The difference between high and low in this context is the magnitude of the luminosity.

Trying to Explain Away the Strangeness

This complex and weird behavior unfortunately does not come with an easy or readily available answer. What we know about pulsars and how we can model pulsar accretion doesn’t shed any new light on the situation. The authors do suggest that the switching between high and low modes might occur due to a changing unstable magnetosphere. They also propose that the increase in radio emission can be explained by an outflow of plasma that emits synchrotron radiation as it travels. Additionally, when comparing PSR J1023+0038 to a low-mass X-ray-emitting binary black hole (BH LMXB) (Fig. 3), we can see that the low mode of this tMSP falls into the binary BH region. This is unusual because there is a pretty clear separation between the X-ray/radio luminosity relationship of neutron stars and BHs. Knowing this now, it may call into question whether some BH LMXBs could have been misidentified.

Figure 3: PSR J1023+0038 low (red diamond) and high (red pentagon) X-ray modes closely follow the accreting low-mass X-ray binary black hole systems, making it indistinguishable in the low mode.

Now you may be asking, “so what did we actually discover?”, which is a completely valid question. Well for one, we learned that there in fact do exist strange and unique pulsars that exhibit odd behavior. But the more exciting result is that we may not have a great understanding of pulsars in general. This is exciting because it can spur new astrophysical theories and models; ones that can more generally explain even the weirdest behaviors. Like most of astronomy (and science in general) however, before we can fully claim any specific mechanism for causing the anti-correlated X-ray and radio emissions and the switching between emission modes, we’ll probably need more observations.

About the author, Joshua Kerrigan:

I’m a 3rd Year PhD student at Brown University studying the early universe through the 21cm neutral hydrogen emission. I do this by using radio interferometer arrays such as the Precision Array for Probing the Epoch of Reionization (PAPER) and the Hydrogen Epoch of Reionization Array (HERA).

Hyades cluster

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 repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: K2-NnnA b: A Binary System In The Hyades Cluster Hosting A Neptune-Sized Planet
Authors: David R. Ciardi, Ian J. M. Crossfield, Adina D. Feinstein et al.
First Author’s Institution: Caltech/IPAC-NASA Exoplanet Science Institute
Status: Submitted to AJ, open access

The authors of today’s paper report the discovery of a Neptune-sized planet in the nearby Hyades Cluster orbiting within a binary star system. A binary consists of two stars mutually orbiting their center of gravity; the larger star is called the primary and the smaller is the secondary. In a close binary where the stars are separated only by a short distance, each of the two stars affects the evolution and size of the other.

The binary system in today’s paper is referred to as EPIC 247589423 (also called LP 358-348), but the primary and secondary stars within this system are called K2-NnnA and K2-NnnB, respectively. Since the planet discovered in this system orbits just the primary star, the authors of this paper refer to it as K2-NnnA b: exoplanets are generally named after their host star and then have another letter added to the end. However, this planet does not yet officially have a name, so there is hope that it will be called something that rolls off the tongue a bit more easily.

Significance to Planetary Formation and Evolution

Planets in binaries form under the influence of two stars — an extreme environment for planet formation, particularly if the two stars are close. The system EPIC 247589423 has a very close separation of 40 AU, which is roughly the distance from Pluto to the Sun. Using our solar system as an example, if we replaced Pluto with a star, it is easy to see that the rotation and generally everything about the planets would be drastically affected.

Studying binary-orbiting planets like K2-NnnA b allows us to test the robustness of planet formation, since the planets have to survive in an extreme environment for a long time. These systems also let us test how often planets are retained by their host star rather than being destroyed by the changing gravitational field. What’s more, by finding and studying planets in star clusters, we may begin to understand how planetary systems form and evolve and find out the timescale for such events.

K2 and Follow-up Observations

K2, or as I like to call it, Zombie Kepler, is the current mission for the Kepler Space Telescope. The Kepler Space Telescope was launched in 2009 with the mission of finding exoplanets through transit photometry. The method of transit photometry means recording the brightness of the star for an extended period of time, then looking for any small periodic ‘dips’ in the brightness that occur when an exoplanet eclipses a small part of the star. This data is generally referred to as a light curve, since there is a rounded dip where the exoplanet eclipses the star. After a mechanical mishap in 2013 with the Kepler Space Telescope, the scientists working on this mission still found a way to use the telescope to look for transiting exoplanets. Instead of observing one single part of the sky for an extended period of time as before, K2 now observes smaller patches of the sky for shorter amounts of time. Figure 1 shows several stages of the light curve analysis from K2.

Figure 1: This shows the light curve of EPIC 247589423 in various stages of analysis from K2. The topmost panel shows the light curve with the telescope rotation removed. The second panel shows the binned version of the top panel. The third shows the data with the stellar variability removed, and the lowest panel shows the folded and binned result for the planet transit. [Ciardi et al. 2017]

Scientists discovered the short-period exoplanet in the EPIC 247589423 system by examining the star’s K2-obtained light curve. Since K2-NnnA b orbits a binary system, the researchers had to first remove the stellar variability inherent in the data. The two stars within the binary already eclipse each other and cause dips in the light curve, which can make it difficult to find a much smaller dip from the planet. After removing the eclipse variability, they found the planet and solved for its period and radius.

After the initial discovery with K2, the researchers followed up their detection with new observations and archival data to confirm the planet’s presence. They used the archival data from 1950 Palomar Observatory Sky Survey to rule out any other object that could be causing the dip in the light curve — for example, another eclipsing binary behind the system, which could create variation in the light curve that could be mistaken for a planet — and then made additional observations of the system using the Keck I telescope.

K2-nnnA b: Planet Properties

The EPIC 247589423 binary system is located in the nearby Hyades cluster — a cluster roughly 750 Myr old and the nearest star cluster to the Sun. The two stars are separated by about 40 AU. The planet orbits the star K2-NnnA with a period of 17.3 days, and its transit lasts roughly 3 hours.

K2-NnnA b is one of the first Neptune-sized planets that has been observed orbiting in a binary system within an open cluster. The discovery of this planet can provide us with a better understanding of the planet population in stellar clusters and allow us to place more limits on planetary formation and evolution.

The authors of this paper say planets discovered in nearby star clusters ‘provide snapshots in time and represent the first steps in mapping out [planetary]evolution,’ and I wholeheartedly agree. The discovery of K2-NnnA b brings with it new understanding of planet formation in star clusters and in binary systems. The possibilities of planet formation and evolution are certainly not limitless, and with more and more discoveries like K2-NnnA b, we can hopefully find the extremes of planetary systems.

About the author, Mara Zimmerman:

Mara is working on her PhD in astronomy at the University of Wyoming. She has done research with Heartbeat binary stars and currently works on modeling debris disks.

S0-2

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 repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Investigating the Binarity of S0-2: Implications for its Origins and Robustness as a Probe of the Laws of Gravity around a Supermassive Black Hole
Authors: D. S. Chu, T. Do, A. Hees, A. Ghez, et al.
First Author’s Institution: University of California, Los Angeles
Status: Submitted to ApJ, open access

The most exciting discoveries in astronomy all have something in common: they let us marvel at the fact that nature obeys laws of physics. The star S0-2 is one of these exciting discoveries. S0-2 (also known as S2) is a fast-moving star that has been observed to follow a full elliptical, 16-year orbit around the Milky Way’s central supermassive black hole, precisely according to Kepler’s laws of planetary motion. Serving as a test-particle probe of the gravitational potential, S0-2 provides some of the best constraints on the black hole’s mass and distance yet. S0-2 is the brightest of the S-stars, a group of young main-sequence stars concentrated within the inner 1” (0.13 ly) of the nuclear star cluster.

The next time S0-2 reaches its closest approach to the black hole, in 2018, there will exist a unique opportunity to detect a deviation from Keplerian motion — namely the relativistic redshift of S0-2’s radial (line-of-sight) velocity — in a direct measurement. In anticipation of this event, the authors of today’s paper investigate possible consequences of S0-2 being not a single star, but a spectroscopic binary, which would complicate this measurement.

Figure 1: Top: Radial velocity measurements of S0-2 over time. Bottom: Residual velocities after subtraction of the best-fit model for the orbital motion. [Chu et al. 2017]

To search for any periodicity in S0-2’s radial velocity curve that would indicate the presence of a companion star, the authors combine their most recent velocity measurements with previous ones obtained as part of monitoring programs carried out at both the WMKO in Hawaii and the VLT in Chile. The resulting data set consists of 87 measurements in total, which are spread over 17 years of observations and have a typical uncertainty of a few 10 km/s (Figure 1, top panel). When S0-2 passes the black hole, the relativistic redshift of its radial velocity is predicted to amount to roughly 200 km/s at closest approach, while the radial velocity is expected to change from +4000 to -2000 km/s. S0-2’s actual speed at this time will be close to 8000 km/s, about 2.7% of the speed of light.

Figure 2: Lomb-Scargle periodogram of S0-2’s residual radial velocity curve (see Fig. 1). No peak reaches the 95% confidence level, which implies that no statistically significant signature of a periodic variation is found in the observations. [Chu et al. 2017]

After having accounted for the long-term radial velocity variation due to the orbital motion of S0-2 (Figure 1, bottom panel), the authors create a Lomb-Scargle periodogram to search for short-term periodic signatures in the velocity residuals. A companion of S0-2 would need to have an orbital period shorter than 120 days at maximum, or the binary system would be too wide to remain stable against tidal forces so close to the black hole. The minimum orbital period could be no less than a few days, or the two stars would come into contact. Yet even in between these limits, the measured periodogram shows no statistically significant peak at any particular period (Figure 2).

However, this non-detection of a periodic signal places an upper limit on the radial velocity variations that could be caused by a possible companion of S0-2, which can be converted into a mass limit. For instance, at a period of 100 days, velocity changes larger than about 12 km/s would have been confidently detected. This implies a companion mass smaller than about 1.7 solar masses, assuming a reasonable total mass of the binary in the range of 14.1 to 20 solar masses.

To estimate the effect of such a companion on the prospective measurement of the relativistic redshift, the authors simulate observations of S0-2 extending into 2018, using a relativistic orbit model and assuming that S0-2 is in fact a binary. These data sets are then fit in the same way as the real data would normally be, using a model in which S0-2 is assumed to be a single star and the strength of the expected relativistic effect is described by a free parameter. The authors conclude that even if S0-2 is a binary, the relativistic redshift could still be detected with high statistical significance in 2018, although the measurement could come out slightly biased, depending on the specific configuration of the binary system (Figure 3).

Figure 3: The bias in estimating the parameter describing the strength of the relativistic redshift, if S0-2 is a binary but assumed to be a single star, for different realizations of plausible binary configurations. Shown are deviations from the expected value of this parameter in general relativity, which is 1, where for a Keplerian orbit it is 0. [Chu et al. 2017]

A continued monitoring beyond 2018 will provide further opportunities to detect relativistic effects on the on-sky motion of S0-2 as well, and it remains to be studied how a possible binarity would influence those particular measurements. The authors also note that if, in time, the search for spectroscopic binaries could be extended to the fainter S-stars too, a comprehensive study of their binary fraction should be able to distinguish between different proposed scenarios for their formation. So stay tuned!

About the author, Philipp Plewa:

I am currently a graduate student at the Max-Planck-Institute for Extraterrestrial Physics in Germany. My main interest is in developing new tools for high-precision infrared astrometry, with the aim of learning more about the supermassive black hole at the center of the Milky Way and the stars in its vicinity.

Illustris project

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 repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Log-Normal Star Formation Histories in Simulated and Observed Galaxies
Authors: Benedikt Diemer, Martin Sparre, Louis E. Abramson, and Paul Torrey
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics
Status: Published in ApJ, open access

Every galaxy has a story to tell. And every story has a few common plot devices: violent supernovae from dying young stars, bursts of activity from central supermassive black holes, mergers with other galaxies, and other dramatic astrophysical events that can change the course of a galaxy’s evolution. One galaxy property in particular can be severely impacted by these events, and that’s how many stars it is forming at a given time. The star formation rate (SFR) throughout the lifetime of a galaxy is known as its star formation history (SFH), and it is of interest to physicists trying to understand the lives of all galaxies.

Unfortunately, we can only see the stars that are in the galaxy at any given time; to infer the past history of star formation we must look for evidence of previous star-forming events. For nearby galaxies, we can view enough detail to see separate populations of stars and determine their individual ages. But for galaxies further away we can only observe the combined light of all the stars mixed together. Then we have to disentangle each component in order to get the underlying history of star formation.

However, in simulations of galaxies we can explicitly see the SFH in high resolution (see these ‘bites for previous examples). The authors of today’s paper looked at galaxies in the state-of-the-art Illustris simulation. They found that, whilst SFHs tend to be noisy, on average they show similar shapes over time. This shape is known as the log-normal distribution, and it typically exhibits a sharp rise to a peak, then a gradual fall.

Example Star Formation Histories

Figure 1: Top panels show the SFHs of two different galaxies in Illustris, measured in solar masses per year. The bottom panels show the cumulative SFR in solar masses. Galaxy (a) is a massive central galaxy, with a thousand billion Suns’ worth of stars, whereas galaxy (b) is a smaller satellite galaxy whose gas gets stripped by its host, ending up with only a billion solar masses of stars.

Figure 2 shows another two galaxies that are also very different. Both are still forming stars today, and galaxy (d) actually has an increasing SFR. Despite having very different forms for their SFH, the log-normal still provides a good fit.

Star Formation Histories

Figure 2: As for figure 1, but for two late-forming galaxies. Galaxy (c) is a galaxy that is still forming stars today, and (d) is a galaxy whose SFR is actually rising.

The authors find a correlation between the time of the peak and the width of the distribution: earlier peaks tend to be narrower, whilst later peaks tend to be much wider. In other words, galaxies that form early assemble quickly, whereas galaxies that form later take their time, leisurely building up mass over a longer period. You can see this in the examples in figures 1 and 2; the galaxies that form most of their stars early have narrow distributions, whereas those that are still forming stars have much wider distributions.

Log-normals are not without their limitations. Interactions between galaxies, such as mergers, can lead to bursts followed by sudden shutdowns of star formation that log-normals struggle to fit. But for most massive galaxies, log-normals describe their story arc in terms of star formation very, very well, helping physicists to understand every galaxy’s story, from start to finish.

About the author, Christopher Lovell:

I’m a 2nd year postgrad at the University of Sussex. I model high redshift galaxies using hydrodynamical simulations. When I’m not reading for work I read for pleasure, mostly science fiction and history, and when I’m not reading I enjoy dodging London traffic on my bike.

white dwarf

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 repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Astrophysical Implications of a New Dynamical Mass for the Nearby White Dwarf 40 Eridani B
Authors: Howard E. Bond, P. Bergeron, A. Bedard
First Author’s Institution: Pennsylvania State University
Status: Accepted to ApJ, open access

About a hundred years ago, one particular star made many astronomers scratch their heads. The star in question is 40 Eridani B, which is part of triple stellar system in the Sun’s neighbourhood, only 5.0 parsec away from us. It was classified as an A-type star by Williamina Fleming, one of the women computers working at the Harvard College Observatory. Fleming devised a system to classify stars according to the relative amount of hydrogen observed in their spectra, which was later improved by Annie Jump Cannon and is the basis of the system still in use. A-type, in particular, means that the star shows hydrogen as its most abundant element. This is usually a consequence of the star being so hot that molecules can’t stick together and metals are mostly ionised. As luminosity depends on temperature, this type of star is also very bright. 40 Eridani B, however, was too faint for a typical A star. A few years later, another faint star was also found to be of A-type: Sirius B, the companion to Sirius A, the brightest star in the sky. This suggested the existence of a new class of underluminous stars with spectra dominated by hydrogen.

You might have already guessed what they are, but it took astronomers back in the day a while to figure it out. Astronomer Willem Luyten was the first to refer to them as “white dwarfs”, back in 1922. These stars have already retired from the job of synthesising new elements, so there’s nothing to prevent gravity from acting and compressing them to a very small (about the size of Earth), extremely dense (a teaspoon of its material would weigh a tonne!) object. Unlike the typical A stars, the reason we can only detect hydrogen in their atmospheres is not their temperature, but the fact that their gravity is so strong that it pulls all the heavier elements to the core. As a result, they can be dominated by hydrogen and still be faint; thus, the mystery of the faint star was solved.

The Mystery of Core Composition

As our methods of measurement improved, astronomers were able to model the orbit of the stars in the system and use Kepler’s laws to constrain their mass. In 1974, astronomer W. D. Heintz obtained a mass of 0.43 times the mass of the Sun for 40 Eridani B. At the time astronomers already knew, however, that the lowest possible mass of a white dwarf that can be formed through single-star evolution — considering the present age of the Universe — is about 0.5 times the mass of the Sun. Any star that would form white dwarfs lighter than that should still be in the main sequence, calmly burning hydrogen into helium. So assuming the mass for 40 Eridani B was correct, it would have to be a helium-core white dwarf resulting from a binary-star merger. In this situation, the stars release energy when spiralling into each other, and this energy carries away material from the outer layers, allowing the formation of a lower mass white dwarf with a helium core.

A new mystery surrounding 40 Eridani B arose in 1998 when J. L. Provencal and collaborators estimated the radius of the star using parallax measurements. With the independent estimates of mass and radius for this and other stars, they proceeded to test the mass-radius relationship for white dwarfs (you can read more about it in this astrobite). They found that the combination of mass and radius of 40 Eridani B would be better explained by a core composed of a combination of magnesium and iron. So our theory of stellar evolution suggested the star had a helium core, while our theory of white-dwarf structure indicated a core composed of much heavier elements.

The Mystery of Heavier Mass

The plot thickened in 2012 when N. Giammichele and collaborators estimated the mass of 40 Eridani B using a model atmosphere analysis. This consists of comparing the spectrum of the star to model spectra derived from atmospheric models to obtain temperature and gravity, and subsequently estimating the mass of the star adopting a theoretical mass-radius relationship. This is by far the most widely used method to estimate the mass of white dwarfs, so we expect it to be reliable. Hence astronomers were very surprised to find that this method yielded a mass of 0.59 solar masses for 40 Eridani B, inconsistent with the mass derived from the orbital analysis. Now our theory of stellar evolution seemed to be clashing with our theory of stellar atmospheres as well.

Which theories should be revised? The authors of today’s paper have the answer.

More Data!

The authors were very surprised to discover that the orbit of 40 Eridani B had not been updated with new observations over the more than four decades since the work of Heintz. They contacted the binary star group at the United States Naval Observatory (USNO), which in turn assembled all the data they could find containing this star. With these new measurements, the group found a smaller orbital period, which led them to obtain a much higher mass for 40 Eridani B than Heintz: 0.573 solar masses.

This new mass agrees within uncertainties with the value derived by Giammichele and collaborators. To double check, the authors also fitted all the available spectra of 40 Eridani B to obtain the average gravity and, using a radius derived from parallax measurements, calculated a new mass. They found a mass of 0.565 solar masses, which, taking the uncertainties into account, also agrees with the new value derived from the orbit. We can thus rest assured that our spectral analysis method for deriving masses indeed works! The mystery of the heavier mass is solved.
Finally, the authors compared the new independent estimates of mass and radius to theoretical mass-radius relationships. As you can see in Fig. 1, the mystery of the core composition is also solved. As the mass was revised above the limit for single-star evolution, we would expect 40 Eridani B to be a carbon-oxygen core white dwarf. Those two elements are the heaviest mosts stars can synthesise, so they form the core of most white dwarfs we know. This indeed turns out to be the case for 40 Eridani B. The only catch is that the authors have to assume a thin hydrogen atmosphere, while our modern theories of stellar evolution mostly predict white dwarfs to have thick hydrogen layers.

Figure 1: The position of 40 Eridani B in a mass-radius plane given the new derived values for mass and radius is marked by the blue circle. The orange line shows the mass-radius relationship for an iron core, which was needed to explain previous estimates of mass and radius, but is very far from the new ones. The blue lines show the relationship assuming a carbon-oxygen core. The dash-dot line is for a thick hydrogen atmosphere, while the solid line, which agrees better with the data, assumes a thin hydrogen atmosphere. [Bond et al. 2017]

You usually read on Astrobites that we need more data to reach conclusions. Today’s paper is a nice example of how this actually works. Analysing more data, the authors proved that both the mass and the core composition of 40 Eridani B are well explained by our stellar evolution theory. They have also confirmed that the popular method of combining spectral analysis and theoretical mass-radius relationship to estimate stellar mass works quite well. None of those needs to be revised thus far. But luckily our work doesn’t end there: we have now to scratch our heads to figure out why the atmosphere of 40 Eridani B is thinner than our theory predicts. As often happens in science, solving one mystery ended up raising another.

About the author, Ingrid Pelisoli:

I am a third year PhD student at Universidade Federal do Rio Grande do Sul, in Brazil, and currently a visiting academic at the University of Warwick, UK. I study white dwarf stars and (try to) use what we learn about them to understand more about the structure and evolution of our galaxy. When I am not sciencing, I like to binge-watch sci-fi and fantasy series, eat pizza, and drink beer.

TRAPPIST-1

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 repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Temporal evolution of the high-energy irradiation and water-content of TRAPPIST-1 exoplanets
Authors: V. Bourrier, J. de Wit, E. Bolmont, et al.
First Author’s Institution: Geneva Observatory, Switzerland
Status: Published in AJ, open access

In early 2017, people quickly turned their attentions to TRAPPIST-1, a seemingly run-of-the-mill M-dwarf star in the constellation of Aquarius, owing to the discovery that it hosts seven exoplanets, of which several are inside its habitability zone (HZ). After a careful look at the ultraviolet (UV) spectra of the star during a planetary transit and out of transits, V. Bourrier and collaborators discovered a variability that could be traced to the history of water escaping from the TRAPPIST-1 planets.

Come for an Experiment, Stay for the Puzzle

transit of TRAPPIST-1c

Figure 1. Lyman ɑ emission line in Visit 4, containing the transit of TRAPPIST-1c. The variation in this feature indicates an evolving line shape, which could be due to a system-wide cloud of neutral hydrogen around TRAPPIST-1. The hatched region contains contaminating emission from Earth and is ignored. [Bourrier et al. 2017]

To judge the potential habitability of a world, we need to take into account several factors — such as the distance from the host star, and the presence and composition of an atmosphere. It is likely that planets that orbit M-dwarf stars have their atmospheres eroded by stellar winds or UV radiation, and this can be a game-changer for habitability.

The escape of exoplanetary atmospheres has already been observed in gas giants orbiting close to their host stars by looking at the Lyman-ɑ emission line at 1215.67 Å (in the UV region of the spectrum). When a planet transits its host star, it is possible that it will display an increased absorption feature in this line that persists after the transit, indicating the presence of a hydrogen cloud trailing the planet like a comet tail. In today’s bite, the authors report the observation of the transit of TRAPPIST-1c with the Hubble Space Telescope (HST), which was used to look for such a signature of an extended hydrogen atmosphere.

The observations consisted of four parts: Visits 1 through 3 were out-of-transit, while Visit 4 was performed over 5 HST orbits during the transit of TRAPPIST-1c. In Figure 1 to the right, corresponding to Visit 4, we see that the flux on the redder side of the spectrum (positive velocities) is systematically higher than the reference flux (grey spectrum, a combination of Visits 1, 2 and 3), while the bluer side below -160 km/s is also higher than the reference. Furthermore, this feature is variable, which suggests that the shape of the line evolved from Visits 1 through 4.

cumulative hydrogen loss

Figure 2. Upper limits for the cumulative amount of hydrogen lost (measured in Earth Oceans) for each planet in TRAPPIST-1 during the lifetime of the system. Left and right panels represent two different hypotheses for the high-energy luminosity of the host star. [Bourrier et al. 2017]

Planets Enshrouded in the Remnants of a Long-Gone Past

The authors did not find evidence for an extended atmosphere around the planet TRAPPIST-1c. On the other hand, one explanation for the variability in the Lyman-ɑ line is the presence of a complex system-wide cloud of neutral hydrogen, which likely originated from the escaping atmosphere of several planets.

Since the TRAPPIST-1 planets have very compact orbits, they experience significant UV irradiation from the host star; this radiation causes water molecules to break down, producing hydrogen and oxygen. Therefore, the presence of a system-wide cloud of hydrogen could indicate a history of water loss from the planets. In fact, the authors point out that the outer planets (d to h), which are inside the HZ, could have already been through the most intense phases of water loss, while planets b and c could be in a runaway greenhouse state, depending on whether they formed with enough water to do so.

Estimating how much water was removed from these planets is complicated because of uncertainties in their masses, the age of the system, and the amount of water present during its formation. Taking into account the efficiency of water break down and the outgassing of water from the planetary mantles, Bourrier and collaborators estimate that planets b through g have lost more than 20 Earth oceans of water due to hydrodynamical escape (see Figure 2). However, they reinforce that this is an upper limit, owing to the aforementioned uncertainties and limitations of the estimates.

Naturally, a more complete understanding of the habitability of TRAPPIST-1 and other systems will depend on further observational efforts to pin down the masses of the planets, monitor the high-energy spectra of the host star, and look for water signatures in the lower atmospheres of the planets using infrared spectra. Further theoretical studies are also needed to evaluate the impact of geophysical processes and the role of magnetic fields in the escape of planetary atmospheres.

Disclaimer: In late September 2017 I will start collaborating with two or more of the authors of today’s paper, but I am not in any form involved with this particular publication.

About the author, Leonardo dos Santos:

Leo is an exoplanet scientist and PhD candidate at the Geneva Observatory. His current research consists of studying the atmospheres of exoplanets, physical and chemical properties of stars similar to the Sun and developing astronomical software. Not to be confused with the constellation.

circumstellar 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 repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Probing the Cold Dust Emission in the AB Aur Disk: A Dust Trap in a Decaying Vortex?
Authors: Asunción Fuente, Clément Baruteau, Roberto Neri, et al.
First Author’s Institution: Spanish National Observatory (OAN, IGN)
Status: Published in ApJL, open access

Planet formation still holds many mysteries for today’s astronomers. One of the biggest unknown is how pebbles and dust clump together to form planetesimals rather than drifting in towards the star due to its strong gravitational pull. A possible solution is dust traps — in a circumstellar disk, the inward drift of the pebbles and particles can be stopped by high pressure in the gaseous disk. A dust trap like this would be an ideal place to form planetesimals. The authors of today’s paper look at a circumstellar disks around AB Aur. They create hydrodynamical simulations of the dust and gas to see if there is a dust trap with the possible formation of small planets.

Circumstellar disks are the link between bunches of dust around a star and fully formed planetesimals. There are a few types of circumstellar disks — protoplanetary, transition, and debris. A transition disk is exactly what it sounds like — a transition from protoplanetary to debris, which means that planets would likely be forming in a transition disk. About 450 light-years away from us, AB Aur hosts an asymmetric transition disk. The unevenness of the disk could indicate a dust trap or gas vortex. At about 120 AU from the central star, the cold dust emission disk creates an asymmetric ring in the disk.

AB Aur

Figure 1: Observations of the continuum emission of AB Aur’s disk from NOEMA. [Fuente et al. 2017]

The authors use NOEMA (NOrthern Extended Millimeter Array) to observe the dust’s continuum emission at 1.12 and 2.22 mm in the AB Aur system. Their observations, shown in Figure 1, reveal that the intensity varies throughout the emission ring, and the intensity decreases at higher wavlengths. The intensity variations that the authors observed are smaller at 2.22 mm than at 1.12 mm, which is the opposite of what purely theoretical dust-trapping/gas-vortex models predict.

AB Aur simulations

Figure 2: The simulations of gas created by the authors. The four images on the left show the distribution and density of gas, and the four images on the right show the continuum emission reproduced from the simulated gas distributions. [Fuente et al. 2017]

The authors then create their own model and use their hydrodynamical simulations for both gas and dust particles in the ring. From this, they are able to they reproduce the emission observed in the disk. Their results from the simulations can be seen in Figure 2. Their simulations show that the lopsided nature of the disk can be explained if the gas vortex has started to decay due to turbulent diffusion, and dust particles are thus losing the azimuthal trapping on different timescales that depend on the particles’ sizes. The authors constrain the size distribution from the comparison between the observations and simulations. They find that there is a mass of about 30 Earth masses in the dust trap, which would be more than enough to form rocky planets.

The authors’ gas and dust simulations show a dust-trapping feature in the disk at about 96 AU. The vortex would need to be decaying as the planet formed in order to fully reproduce their observations, including the intensity varations along the ring. More observations and work are needed to fully understand the disk, but so far the authors’ simulations show that planets and planetesimals are likely currently forming in a dust trap around AB Aur.

About the author, Mara Zimmerman:

Mara is working on her PhD in Astronomy at the University of Wyoming. She has done research with binary stars, including Heartbeat stars, and currently works on modeling debris disks.

Boyajian's star

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 repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Where Is the Flux Going? The Long-Term Photometric Variability of Boyajian’s Star
Authors: Joshua D. Simon, Benjamin J. Shappee, G. Pojmański, et al.
First Author’s Institution: Observatories of the Carnegie Institution for Science
Status: Submitted to ApJ, open access

Boyajian’s star loves the limelight. It splashed onto the astronomical scene in 2015 by dimming dramatically and messily, much more messily than can be explained by a transiting planet. In 2016, astronomers realized that this sensational debut was only the latest act in a century-long fade, confirmed by exquisite observations from the Kepler mission. And the drama keeps building — just this summer, Boyajian’s star has undergone (and is still undergoing!) three more dimmings, similar in shape to the dimmings reported in 2015.

Figure 1. The situation, basically.

This week, Boyajian’s star is back in the news again, the subject of two new studies. The first confirms that the slow fade observed by Kepler is happening across a broad range of colors. The second, which we’ll explore in today’s bite, points out that this consistent fade might not be so consistent after all.

When an interesting astronomical object like Boyajian’s star is discovered, the first thing astronomers do is go comb through older observations taken of the same piece of sky. They hope that the object will serendipitously appear in this older data, and that they can compare what it looked like then (before it started behaving strangely) and now. When a supernova goes off, for example, astronomers rush to the archives to try to see what it looked like before it exploded — what type of star was it, and were there any clues to hint at its spectacular demise?

Today’s authors analyze over ten years of observations taken by the two robotic telescopes of the All Sky Automated Survey, which have been scanning the northern and southern hemisphere skies in tandem since 2006. They’ve been looking for changes in stellar brightness, and Boyajian’s star certainly fits that bill. Below is a graph of what they’ve found: the brightness of Boyajian’s star over the past ten years.

Figure 2. The brightness of Boyajian’s star over time. Black points come from the All Sky Automated Survey. Red points come from the Kepler mission. Blue points come from the All-Sky Automated Survey for Supernovae (no relation). According to the black points, Boyajian’s star hasn’t just been getting straightforwardly fainter — it’s had brightening spells, too!

If you focus on the middle of the data, which overlapped with observations taken by the Kepler space telescope, you’ll see why we thought Boyajian’s star was dimming gradually over time. That’s a pretty straight downward-pointing line. But if you zoom out to the full ten years, you see that Boyajian’s Star has had at least two brightening episodes in the past decade. The first happened from late 2006 through 2008, and the second from late 2013 through early 2015.

So what, you ask? Boyajian’s star is still spending most of its time getting fainter, not brighter, and the end result is that it’s fainter now than it was in 2007. The answer is that we still don’t know why any of this is happening — and any explanation must now account not only for the crazy 20% dips in flux and the century-long net dimming, but also for these intermittent glows. Previous hypotheses — which include the idea that Boyajian’s star is dimming after gulping down a planet some time ago, and that the present-day flux dips are due to leftover orbiting debris — may struggle to explain it all. Stay tuned!

About the author, Emily Sandford:

I’m a PhD student in the Cool Worlds research group at Columbia University. I’m interested in exoplanet transit surveys. For my thesis project, I intend to eat the Kepler space telescope and absorb its strength.

NGC 3199

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 repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Hot Gas in the Wolf-Rayet Nebula NGC 3199
Authors: J.A. Toalá, A.P. Marston, M.A. Guerrero, Y.-H. Chu, R.A. Gruendl
First Author’s Institution: Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA)
Status: Accepted to ApJ, open access

As very massive stars evolve off the main sequence, they sometimes lose more than half their initial mass through dense, slow winds. The stars’ hot cores are laid bare, emitting copious ultraviolet photons that ionize the expelled material, which forms an optically bright, bubble-shaped nebula. The exposed hot core is referred to as a Wolf-Rayet (WR) star, while the discarded outer layers that surround it make up what is known as a Wolf-Rayet nebula. These objects are named after the pair of French astronomers who first discovered them via their unusual spectra, which feature broad emission lines jutting above the continuum. While WR stars are readily detectable (even out to nearby galaxies) because of their unusual emission-line spectra, the origin of the surrounding nebulosity is more difficult to pinpoint because of its close resemblance to other types of emission nebulae — especially planetary nebulae — and the interaction of the ejected material with the interstellar medium.

Theory suggests that WR nebulae should emit X-rays. After a star loses its outer layers and becomes a WR star, its stellar winds accelerate to ~1500 km/s — more than ten times the previous pace. These faster winds collide with the previously ejected material propagating into the interstellar medium. This process should generate a diffuse bubble of shocked material that should emit in the X-ray, but this predicted X-ray emission has proved elusive; only a few WR nebulae have been detected in X-rays. In this paper, the authors use narrow-band optical images and X-ray images and spectra to investigate the properties of the area surrounding WR 18, a Wolf-Rayet star closely associated with emission nebula NGC 3199. The narrow-band optical images in Figure 1 show the bright arc of the emission nebula curving around WR 18.

Figure 1. Left: False-color optical image of NGC 3199. WR 18 is circled in red to the east of the bright arc. Right: A map of the [O III] to H-alpha ratio. Two measurements of the proper motion of the star are indicated by the colored arrows. [Toalá et al. 2017]

Figure 2. False-color X-ray image of NGC 3199. The western (right) side emits significantly more in the 1.1 – 2.5 keV range than the eastern (left) side. [Toalá et al. 2017]

The authors present X-ray observations of the nebula by XMM-Newton and reveal for the first time diffuse X-ray emission suffusing NGC 3199, seemingly bounded by [O III] emission on the southwest edge. (Here, the brackets denote emission via a “forbidden” transition — one that isn’t induced by the most common mechanism and therefore rarely occurs in a laboratory setting.) This discovery cements NGC 3199’s status as a WR nebula — only the fourth such nebula to be observed in X-rays. From the false-color X-ray images in Figure 2 to the right, we can see spatial variations in the X-ray emission. The authors find that the western (right) side emits more in the 1.1–2.5 keV range than the eastern (left) side. They then use spectral fitting to extract the global and local properties of the nebula. Most importantly, they find that while the material nearest the optically bright arc is enriched in metals from the strong stellar winds, the material opposite the arc appears to be a mixture of the stellar outflow and the interstellar medium. This implies that the nebular material of NGC 3199 is being actively enriched by the stellar winds from WR 18.

WR 18: A Homebody Mistaken for a Runaway?

The authors also consider the origins of the optically bright arc shown in Figure 1. It was first thought to be a bow shock formed as WR 18 barrels into the surrounding interstellar medium, which piles up like snow before a plow. This sometimes happens in the case of runaway stars — stars ejected from their natal clusters at high velocities, careening through space with large proper motions; bow shock formation requires that the star be traveling at a very high speed (greater than the speed of sound in the material surrounding the star). However, although the bright optical arc is certainly suggestive of a bow shock, the authors extend an alternative explanation. Because the proper motion of the star is nearly perpendicular to the direction of motion that the bow shock suggests, and because the authors find no evidence that the star’s velocity is especially large, they posit that the morphology of the bright arc results simply from the interaction of the star’s fast stellar wind with a dense region of the interstellar medium.

The morphology of NGC 3199 revealed by X-ray observations (namely, the bubble of diffuse X-ray emission bounded by bright optical [O III] emission) is common to the four WR nebulae observed in X-rays. This result shows that the configuration of the local interstellar medium strongly affects the structure of stellar outflows. Future X-ray observations of WR nebulae can help us understand how massive stars enrich the interstellar medium, and how the interstellar medium can shape the outflows of these stars.

About the author, Kerrin Hensley:

I am a third year graduate student at Boston University, where I study the upper atmospheres and ionospheres of Venus and Mars. I’m especially interested in how the ionospheres of these planets change as the Sun proceeds through its solar activity cycle and what this can tell us about the ionospheres of planets around other stars. Outside of grad school, you can find me rock climbing, drawing, or exploring Boston.

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