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13 December 2016

Catching Galactic Recycling In The Act

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting 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: ALMA Reveals Potential Localized Dust Enrichment from Massive Star Clusters in II Zw 40
Authors: S. Michelle Consiglio, Jean L. Turner, Sara Beck, David S. Meier
First Author’s Institution: University of California, Los Angeles
Status: Published in ApJL, open access

Galaxies are recycling centers for gas. Dense gas collapses under gravity to form stars. When massive stars form, they quickly impact their surroundings with intense radiation and mass loss. This enriches the galaxy with heavy elements forged inside the massive stars and becomes the raw material for future star and planet formation. The evolution of the galaxy is determined by this cycle of gas to stars and back.

To measure the lifecycle of gas in galaxies, astronomers use radio telescopes that are sensitive to the three main phases of gas. First, the dense gas where stars form emits light with millimeter-size wavelengths from carbon monoxide (CO). Next, gas ionized by massive stars gives off radio waves when particles collide, in a process called free-free emission. Finally, heavy elements lost by massive stars, which will fuel the next generation of star formation, coalesce to form grains of dust that emit light in the submillimeter regime. With an unmatched angular resolution, the Atacama Large Millimeter/submillimeter Array (ALMA) is the tool of choice for studying the cycle of gas in galaxies.

The authors of today’s paper used ALMA to measure the gas and dust in the nearby galaxy II Zw 40. This small galaxy is forming stars at a prodigious rate. The starlight in the galaxy is dominated by massive stars. The authors investigate the effect of these stars on the gas and dust in the galaxy.

Observations of Gas and Dust

With ALMA, the authors observed three different components of II Zw 40. At a wavelength of 3 mm, the dominant source is free-free emission from ionized gas around the massive star cluster. At 870 µm, after accounting for free-free emission, most of the light is emitted by dust grains. These two components are shown in Figure 1. The peaks of ionized and dust emission are distinct, and the dust emission is localized in several clumps around the cluster.

Figure 1: The galaxy as seen at 3 mm and 870 µm wavelengths. The 3 mm map (upper right and blue contours) shows the extent of free-free emission from ionized gas around massive stars. Removing the free-free contribution from the total 870 µm emission (upper left) gives a map of dust emission (lower left and red contours). [Consiglio et al. 2016]

Figure 2 shows the dense gas compared to dust emission in II Zw 40. Dense gas is traced by emission from CO molecules. The CO emission is offset from dust emission, with areas of dense gas devoid of dust and vice versa. The authors convert the intensity of CO emission to gas mass using the so-called ‘X-factor’. Dividing the mass in gas and dust, the authors find that the gas-to-dust ratio varies from ~70 to 270 across the galaxy. Not only does the ratio vary, but it’s also low overall — there is more dust per unit gas than the authors expect for a galaxy of this type.

Figure 2: Dense gas and dust emission in the galaxy. Dense gas is traced by emission from CO molecules (blue map). Dust emission is traced by 870 µm emission (orange contours, same as lower left panel in Figure 1). Note the discrepancy between the peaks of dense gas and dust. [Consiglio et al. 2016]

Clumps of Stardust

To explain the low, variable gas-to-dust ratio and the clumpy structure of the dust emission, the authors propose that massive stars enrich nearby clouds with heavy elements and dust. Because this enrichment is ongoing, more dust is joining the dense gas, lowering the total gas-to-dust ratio. The dust is clumpy because it has not yet mixed with the rest of the galaxy. This enrichment model suggests that dust without associated dense gas came from an older star cluster that is not visible in free-free emission (Figure 1) because it no longer ionizes gas.

Pushing Dust With Light

Take another look at Figure 1, and notice how the peaks of dust emission are all slightly offset from the peak of the ionized gas. In addition to ionizing gas, the intense radiation from massive stars can actually push on dust grains. Acting like a multitude of tiny billiard balls, the photons from bright stars exert radiation pressure on the dust. The dust then drifts relative to the gas, which may explain the offset between gas and dust peaks.

ALMA has revolutionized the study of star formation in galaxies. This paper shows that the cycle from dense gas (traced by CO) to massive stars (3 mm) to dust-rich gas (870 µm) is complex. The massive stars in the galaxy have enriched parts of the galaxy, while other areas remain relatively dust-free. The dust-rich products of stellar evolution are pushed by radiation pressure but have not yet mixed into the galaxy. Future studies of the galactic recycling plant will explain the origin and dispersal of the ingredients needed for planets and (perhaps) life. Cue Carl Sagan.

About the author, Jesse Feddersen:

I am a 4th-year graduate student at Yale, where I work with Héctor Arce on the effect of stellar feedback on nearby molecular clouds. I’m a proud Indiana native (ask me what a Hoosier is), and received my B.S. in astrophysics from Indiana University. If I’m not working, you’ll probably find me on a trail or at a concert somewhere.

6 December 2016

X-Ray Beasts and Magnetic Fields

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting 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: Revisiting the structure and spectrum of the magnetic-reconnection-heated corona in luminous AGNs
Authors: J.Y. Liu, E.L Qiao, and B.F. Liu
First Author’s Institutions: National Astronomical Observatories, Chinese Academy of Sciences
Status: Accepted to ApJ

Deep in the fathomless centers of galaxies, there lurk fantastic beasts of incredible energy and power. Legend has it that our ancestors have long since been aware of their presence (e.g. Carl Seyfert in 1943 and Maarten Schmidt in 1963), but separated by our great divide and limited by our technologies, little has been known about them. Appearing in various shapes and sizes, some beasts have been spotted to spurt out jets, while others appear more docile. How to find these fantastic beasts, you ask? Well, most give away their presence in the optical and UV, while others show up in the radio. These shape shifters are unified by one thing: the source of their power through the accretion of matter onto supermassive black holes. Astronomers call them active galactic nuclei, or AGNs (shhh, here is a secret guide on their different shape-shifting abilities).

Although AGNs are predominantly discovered in the optical/UV and radio, they actually emit all the way up to X-rays, which is true with most highly energetic astrophysical phenomena. Their spectral energy distributions (SEDs; an example is shown in the figure below), are characterized by different components that arise from different parts of their structures. First, we have a blackbody-like bump — the “big blue bump” — in the optical and UV, which is thought to originate from their accretion disks. The big blue bump is well-fit by the standard accretion disk model, which is geometrically thin and opaque to radiation (“optically thick”). Then, we have some emission in soft X-ray (< 5 keV) whose origin is still unclear. Finally, we have hard X-ray emission (> 5 keV) well-described by a power law. The origin of the hard X-ray emission is not completely solved, but astronomers believe that the up-scattering, or inverse Compton scattering, of accretion disk photons by plasma of hot electrons in a corona surrounding the disk produces these hard X-ray emissions.

An example spectral energy distribution (SED) of an AGN. [Elvis et al. 1994]

The formation of the hot corona and its heating mechanism are still unclear. Leading models posit that it is formed by the evaporation of material from the underlying cool disk and then continuously heated by the disk’s magnetic field. More specifically, magnetic loops produced by the disk emerge and reconnect with other magnetic loops in the corona in a phenomenon known as magnetic reconnection, thereby releasing magnetic energy to maintain the corona at high temperatures. The authors of today’s paper investigate the effect of the magnetic field on the structure and spectrum of the magnetic-reconnection-heated disk-corona model, particularly focusing on luminous AGNs.

The authors encapsulate the effect of magnetic field in their model in a parameter they call the magnetic parameter β₀ = (P_gas + P_rad)/P_B, where magnetic pressure is assumed to be proportional to the sum of the gas pressure and radiation pressure in the disk. Larger β₀ corresponds to weaker magnetic field strength and vice versa. By tuning β₀ and the accretion rate, the authors solve for the disk structure and derive the emergent SED. The figure below shows the simulated SEDs for different β₀ and accretion rates. When β₀ is small, strong magnetic fields are produced, which gives rise to similar-looking spectra regardless of accretion rates. As the magnetic field gets weaker (going down the panel), the model with higher accretion rate (dashed line) produces less and less hard X-ray emission. Eventually, the spectrum is dominated by the disk’s “big blue bump”. This agrees with observational trend that hard X-ray spectra of luminous AGNs become softer at higher accretion rates.

Simulated SEDs of AGN at various β₀ (different panels) and accretion rates (different lines). [Liu et al. 2016]

To compare their models with observations, the authors plotted the observable L_bol/L_x as a function of accretion rate (ṁ) for different magnetic field strengths (shown below). The red crosses are observational samples of luminous AGNs with measured black-hole masses from a different work. The model with β₀= 200 more or less agrees with observations for ṁ < 0.2, but at higher accretion rates this model underpredicts L_x — the corona becomes too wimpy to produce enough X-rays. At ṁ > 0.2, the model with β₀= 100 at slightly higher magnetic field agrees with the last data point, although it fails to match observations at lower accretion rates. Although the models are far from a perfect fit, those with higher β₀ (weaker magnetic fields) agree with observational results at large. As with all things research-related, there remains more work to be done before we can piece together a complete picture of those fantastic beasts called AGNs.

L_bol/L_x as a function of accretion rate for various β₀, overlaid with observational data (red crosses). The β₀= 100 and 200 lines agree with the data while the β₀= 10 and 50 lines do not. [Liu et al. 2016]

About the author, Suk Sien Tie:

I am a third year PhD student at the Department of Astronomy at The Ohio State University. I am currently working on quantitative analyses of various quasar selection methods using the Dark Energy Survey (DES) and quasar variability via microlensing.

29 November 2016

It Turns Out the Sun Is More Chill Than We Previously Thought

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting 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: The Mount Wilson Observatory S-index of the Sun
Authors: Ricky Egeland, Willie Soon, Sallie Baliunas, Jeffrey C. Hall, Alexei A. Pevtsov, and Luca Bertello
First Author’s Institutions: High Altitude Observatory; Montana State University
Status: Accepted to ApJ

One of the few certainties that we have as humans is that the Sun always comes and goes, always in intervals of 24 hours, and will continue to do so for the next 5 billion years or so. Owing to this familiar cycle, we sometimes take for granted that our host star is not itself completely stable. It is active, and this activity shapes the evolution the solar system and life on Earth. Because we use the Sun as a reference for other stars, it is thus crucial that we measure its activity as accurately and precisely as possible.

Blame Magnetic Fields

Mount Wilson Observatory’s 100-inch telescope is one of the telescopes that was used to make solar observations for the HK project. [Ken Spencer]

Stars are big balls of hot gas with lots of moving parts. The ones that are similar to the Sun (i.e., solar-type stars) have large convective atmospheres, which act just like boiling water inside a cooking pot. The convective circulation of plasma generates magnetic fields, and the stellar rotation, in turn, makes field lines wrap around the star, creating a stellar dynamo. When the magnetic field lines concentrate, they produce dark spots in the stellar surface and spectacular mass ejections; the activity of a star is measured by the strength of these episodes.

One precise and accurate way of assessing stellar activity uses the features in stellar spectrum known as the Ca II H & K lines. Their strength can be easily measured with spectrometers, and are then translated into a ratio called the S-index (the higher it is, the more active the star). The most famous survey of stellar activity is the HK Project at Mount Wilson Observatory (MWO), which consisted of assessing the S-index of many stars in the sky and ended up becoming the standard calibration for current studies on activity. The problem is that not all observations are carried out with the same instrument, and hence systematic errors start to become a serious problem.

Context Is Key

In order to understand the role of activity in the physics of stellar and planetary evolution, it is important to place the Sun, the one star we know best, in the same context as the others. In today’s paper, the authors aim to precisely and accurately measure its activity using spectroscopic observations of the Moon — which reflects sunlight — obtained with the same instrument employed at the MWO.

Sun-like stars have activity cycles with periods of the order of a few years. The solar cycle has an 11-year period, encompassing a minimum and a maximum. The authors directly measured the minimum, maximum and mean S-index of the Solar Cycle 23 (1996–2007, although the MWO data goes only up to 2003), and found that they were significantly lower than previous estimates of the same cycle. This result shows that, when using different instruments, systematic errors plague the measurements of activity — but the good news is that now we can correct them by applying a better calibration with results from today’s paper.

The solar activity during Cycle 23. The MWO data are the red dots, and the red curve is a fit to the data. Most of the activity cycle measured by previous studies (dashed curves) are visibly shifted to higher values. [Egeland et al. 2016]

Well, now that we have dealt with accuracy, what about precision? As it turns out, there is no lack of solar activity data in the literature, which are now correctly calibrated. The authors used them to constrain the solar activity minimum, maximum and mean values within less than 1% for all indices.

Composite time series of the S-index of the Sun, using various data sets (colored symbols), which are now correctly calibrated. The colored curves are fits to the data. [Egeland et al. 2016]

Effects on Future Studies

So, the Sun is slightly less active than we previously measured, and this impacts our understanding of solar-type stars the most. By correctly placing our star in the context of others, we can better assess how common it is, which helps us answer questions about the conditions necessary for life to emerge and how it evolves along with the star. These results also rectify some inconsistencies previously observed in the activity of solar-type stars, and again reminds us of a critical aspect of science: systematics matter. The article itself will serve as a guide on measuring stellar activity, paving the way towards better practices in the field.

Bonus if you read to the end:

The Sun from September to November 2016. Larger GIF available here.[ESA/NASA/SOHO]

About the author, Leonardo dos Santos:

Leonardo is a graduate student of Astronomy at University of São Paulo, Brazil, and a visiting student at University of Chicago. His current research consists of studying physical and chemical properties of stars similar to the Sun, hunting for exoplanets and software development for astronomical instrumentation. When not sitting in front of a computer, he enjoys doing night sky photography, cooking, riding his mountain bike and failing miserably at pretty much all these tasks.

22 November 2016

The True Colours of a Blue Straggler

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting 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: Formation and Evolution of Blue Stragglers in 47 Tucanae
Authors: J. Parada, H. Richer, J. Heyl, J. Kalirai, R. Goldsbury
First Author’s Institution: Department of Physics & Astronomy, University of British Columbia, Canada
Status: Published in ApJ

Just by looking at a star’s colour, astronomers can learn a few things about its properties. The most direct one is its temperature: red stars like Betelgeuse are cool, blue stars like Rigel are hot. Another property we can estimate by eye is the apparent magnitude, a concept idealised by the Greek Hipparchus almost 2000 years ago. He ranked stars from brightest to faintest in a scale of one to six, one denoting the twenty brightest stars he could see, and six denoting those he could barely spot with the naked eye. This scheme was adapted into a logarithmic scale by N. R. Pogson in 1856, and is still in use nowadays. The apparent magnitude depends on the star’s luminosity and distance, so if we know the latter, we can estimate the former using the measured magnitude. Knowing the luminosity and the temperature we’ve estimated from the star’s colour, we can place it in a Hertzprung-Russell (HR) diagram and infer other properties, like evolutionary stage, mass, and radius. Unfortunately, distance is a very hard thing to measure accurately in astronomy. However, there’s a way to overcome that: studying stars in clusters. In a star cluster, all stars are basically at the same distance from us, so we can ignore the effect of the distance and compare the apparent magnitude of these stars directly. We can then build a colour-magnitude diagram (CMD), the observational version of the HR diagram. Lots of our knowledge about stellar evolution came from comparing theoretical evolutionary models to this kind of diagram.

Blue stragglers in the open star cluster NGC 3766. [ESO]

With the development of high resolution imaging, CMDs revealed the presence of some unexpected stellar populations. A prominent population is blue straggler stars (BSS). These stars appear as a blue extension of the main sequence (MS) in globular clusters, right above the so-called turnoff point, where stars are about to leave the main sequence. They are unexpected because, given the cluster’s age and metallicity, stars with their properties should already have evolved off the main sequence. Blue stragglers are still there though, like they were too lazy to leave the MS. How they are formed and where they go when they finally evolve are two questions still up for debate (bite1, bite2). The authors of today’s paper studied the blue stragglers in the cluster 47 Tucanae with the aim of shedding some light on the subject.

Blue stragglers: what we know so far

The only way to explain the blue stragglers being brighter and bluer than the turnoff is if they went through some rejuvenating process. There are different ways to do that, but two main conditions must be followed: i) one MS star must be involved, and ii) one of the stars involved gained mass to become rejuvenated. This mass gain gives the star more fuel, extending its main sequence lifetime. There are two possible ways for this to occur: mass transfer or merger. The authors considered three different scenarios: i) direct collision of stars, ii) evolution of primordial binaries, and iii) hierarchical evolution of triple systems. The resulting blue straggler can have different characteristics depending on the mechanism that caused it. The populations we observe are a combination of all these mechanisms, but one of them can prevail over the others depending on the environment.

Combining theory and observations: what 47 Tucanae can teach us

In order to try to identify the origins and possible evolution of the blue stragglers in 47 Tucanae, the authors first separated the stars into different stellar populations. First, based on the stars’ location on the CMD, the cluster was split into two blue straggler populations, bright (bBSS) and faint (fBSS), each containing half of the blue straggler sample, main sequence (MS), binary main sequence (MSBn), and red giant branch (RGB). This is illustrated in Fig. 1. The authors then estimated the cumulative distribution of each of these populations along the radius of the cluster, also shown in Fig. 1. The first thing to notice is that the bBSS and the fBSS samples are actually different: the bBSS is more concentrated in the inner regions than the fBSS is. Moreover, the bBSS cumulative radial distribution does not resemble any of the other populations, making it difficult to link its formation to any specific group. The fBSS, on the other hand, show a distribution very similar to the MSBn, pointing to a binary origin for the fBSS. The estimated masses of these two populations are also similar, with the fBSS being slightly smaller, as would be expected for a final product of binary evolution. The mass of the bBSS is in contrast higher, suggesting an origin involving triple or multiple stellar systems.

Figure 1: The left panel shows a colour magnitude diagram using Hubble's filters F255W and F336W. The stellar populations identified are indicated. On the right panel, the cumulative distribution for each population is shown. The numbers of stars in each panel are slightly different because a completeness correction was done on the right panel, taking into account stars that aren't detected because of limitations in the method.

Figure 1: The left panel shows a CMD using Hubble’s filters F255W and F336W. The stellar populations identified are indicated. On the right panel, the cumulative distribution for each population is shown. The numbers of stars in each panel are slightly different because a completeness correction was done on the right panel, taking into account stars that aren’t detected because of limitations in the method.

Next the authors relied on evolutionary models calculated with the code MESA (Modules for Experiments in Stellar Astrophysics) to identify the region occupied by evolved blue straggler stars (see Fig. 2). Calculating the cumulative radial distribution, they noted that what they call evolved blue stragglers (eBSS) follow a similar distribution to the blue stragglers, suggesting they are indeed linked. Moreover, they found an excess of stars in the RGB and the horizontal branch (HB) when compared to the expected number considering only single evolution. According to their estimates, this excess can be explained by stars evolving from blue stragglers into these regions. So it appears that the blue stragglers have a post-MS evolution comparable to that of a normal star of the same mass. There’s still some disagreement between the lifetimes the authors estimated and others found in the literature, indicating a more detailed study of individual blue straggler properties is in order to better constrain these values. Future studies using high quality spectra may help with that.

Figure 2: The left panel shows again the cluster’s CMD, now with some MESA models overplotted. The sequences have initial masses of f 0.9, 1.1, 1.4, and 1.8 solar masses, from bottom to top. The right panel shows the cumulative distribution for the samples selected on the right panel. There’s again a completeness correction that makes the number of stars slightly different in the plots.

In short, the authors verified that different mechanisms leading to blue stragglers can in fact be identified within a cluster. Interactions in multiple systems seem to dominate in the central regions, while binary evolution seems to be the dominant mechanism in the cluster outskirts. The former leads to more massive, brighter objects, while fainter blue stragglers are explained by the latter. The evolution of the blue stragglers seems to be similar to simple MS stars with same mass, which makes it easier to model their evolution. However, this still has some discrepancies with other results, so more detailed studies, focused on individual objects, are needed. The blue stragglers have not shown all their true colours yet.

About the author, Ingrid Pelisoli:

I am a second year PhD student at Universidade Federal do Rio Grande do Sul, in Brazil. 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.

15 November 2016

Investigating One of the Strongest Starbursts in the Universe

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting 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: SPT0346-52: Negligible AGN Activity in a Compact, Hyper-starburst Galaxy at z = 5.7
Authors: J. Ma, et al.
First Author’s Institution: University of Florida
Status: Accepted to ApJ

In recent years, the South Pole Telescope has discovered an intriguing population of gravitationally lensed dusty star-forming galaxies. These galaxies, some of the most distant and faint in Cosmos, are able to be studied because their light is magnified by a foreground gravitational lens. Using this phenomenon, astronomers can learn more about the stars, gas and dust — not the kind of dust under your bed that you should probably vacuum up some day, but instead the type that blocks, absorbs and scatters light — in the very distant universe.

One galaxy in particular drew the researchers’ attention as it stood out in the 2500 square degree (that’s about 6% of the sky; for comparison: the full Moon subtends about 0.2 square degrees) survey undertaken by the South Pole Telescope: its name is SPT-S J034640-5204.9, though we shall call it SPT0346-52.

A 20″x20″ cutout of SPT-3462-52 showing HST (grey), Spitzer (blue contours), and ALMA observations (red contours). [Ma et al. 2016]

Galaxy SPT0346-52

This galaxy, the main protagonist of today’s astrobite paper, is located at a redshift of z=5.656. That means that, since its light can only travel at a finite speed, we are observing it at a time when the universe itself was a mere 1 billion years old. This makes SPT0346-52 part of the first population of galaxies to have formed in the universe, and one of the most distant of the type of galaxy known as “dusty star-forming galaxies” to have been observed thus far. Because of this it has been studied intensely using a wide range of wavelengths to probe its physical properties: so far it has been observed with HST, Spitzer, Herschel, ALMA, APEX and the VLT!

From ALMA observations it could be determined that the foreground lensing galaxy (a “mere” 8 billion light-years from Earth) was magnifying the light from SPT0346-52 by a factor of about 5.6, and that the intrinsic infrared luminosity of SPT0346-52 was a factor of 1000 higher than that of our nearest neighbour, the Andromeda galaxy. The star formation rate of SPT0346-52 was calculated to be about 4500 solar masses per year — compare that to the Milky Way’s “meagre” rate of 1 solar mass per year! In addition, given that with a radius of about 0.6 kpc it’s about 25 times smaller than our own galaxy, this indicates that SPT0346-52 has one of the highest star formation rate densities of any known galaxy!

While much has been learned about this fascinating galaxy from previous multi-wavelength observations, one intriguing question remains:

What causes its high luminosity?

Does SPT0346-52’s high infrared luminosity arise solely from intense star formation, or does it harbour an obscured active galactic nucleus (AGN)? This is the question explored in today’s paper.

Several indicators appear to favour the presence of an AGN: the dust temperature in SPT0346-52 is found to be about 52 K, making it warmer that that of typical dusty star-forming galaxies. The luminosity ratio of some of its emission lines is consistent with a quasar (the most energetic and distant type of active galactic nucleus), and it shows strong H2O emission lines which are similar to those found in distant quasars. However, SPT0346-52 does not show any indications of an AGN in deep optical spectroscopy.

In order to shed more light on this mystery, the authors of today’s paper undertook two further observations, probing the extremes of the electromagnetic spectrum. They observed SPT0346-52 in X-rays using the Chandra satellite. A clear detection of X-ray photons would be indicative of the signature of an accreting black hole powering an AGN, while a purely star-formation-driven galaxy would emit a lot less X-ray flux. Additionally, the authors undertook radio observations with the Australian Telescope Compact Array (ATCA). Similarly to X-rays, radio waves can be used to distinguish star-forming galaxies from radio-loud active galactic nuclei.

Spectral Energy Distribution of SPT0346-52

The best-fitting spectral energy distribution for the galaxy SPT0346-52. It uses input flux measurements from a range of wavelengths in order to find the best-fitting spectrum given a range of model spectra. Since a galaxy’s spectrum changes with age, mass, dust content and other factors, the best-fitting model can tell us a lot about the physical properties within the galaxy. In this best-fitting model the authors found that AGN likely contribute less than 5% to the total infrared luminosity of this galaxy. [Ma et al. 2016]

Results and What They Can Tell Us

In almost 14 hours of X-ray observations, just over 3 photons were detected, and over one hour of radio observations yielded no detection at the source position…

However, together with the previous multi-wavelength observations, it was possible to use the upper limits from the X-ray and radio part of the spectrum to constrain the shape of the spectrum of SPT0346-52. This indicated that a pure starburst spectrum of the galaxy is the best fit to the observations. Similarly, the upper limits of the ATCA radio observations are consistent with a star-forming galaxy, and would have to be a factor of at least 2 higher for a radio-loud AGN.

One of the Most Extreme Starbursting Galaxies in the Universe

Also, while the presence of an AGN cannot be completely ruled out, the authors conclude that star-formation — and not AGN activity — is the dominant driver behind the far-infrared emission observed. The authors plotted SPT0346-52 on a diagram showing the distribution of X-ray luminosity vs far-infrared luminosity for both star-forming and AGN-dominated galaxies (see Fig. 3). This shows that SPT0346-52 lies left of the dividing line between star-formation and AGN, making it likely that it is indeed powered by very high star formation rates.

One question, however, remains:

Fig. 3: This plot shows the positions of a range of galaxies on a far-infrared vs. X-ray luminosity diagram. SPT0346-52 is the red arrow, while star-forming galaxies are marked as blue S, and AGN as yellow A. Using the dividing lines between AGN and star-formation driven luminosities indicated on the plot suggests that SPT0346-52 does not contain an AGN. Hence it appears as though SPT0346-52’s extreme brightness is powered by star-formation.

What Could Power Such High Star Formation Rates?

The authors offer several possible explanations for the finding that SPT0346-52 has one of the highest star formation rate densities (a measure of how many stars form each year in a given volume) in the universe.

They suggest that SPT0346-52 may have an especially high star formation efficiency; probably higher than about 40%. This means that SPT0346-52 converts 40% of its available hydrogen gas into stars; a more typical values for galaxies like the Milky Way is around 5%. Additionally, the gas fraction — the amount of gas available for this star formation within the galaxy — is also exceptionally high at above 40%. The authors suggest that it might be possible to explain the very high star formation rate in SPT0346-52 with such a high gas fraction, even without a very high star formation efficiency.

Hence, despite the many observations of SPT0346-52, several mysteries still remain unsolved. The authors suggest that this would make SPT0346-52 an ideal candidate for follow-up observations with the JWST, which is due to launch in 2018…

About the author, Steph Greis:

I’m a third (out of four) year PhD student at the University of Warwick, UK, where I study local analogues to redshift z~5 Lyman break galaxies (LBGs) which are some of the earliest galaxies in the Universe. When I’m not thinking about galaxies far, far away, I enjoy reading, cooking, and geocaching.

8 November 2016

Modest Chaos in the Early Solar System

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting 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: The Asteroid Belt as a Relic From a Chaotic Early Solar System
Authors: Andre Izidoro, Sean N. Raymond, Arnaud Pierens, Alessandro Morbidelli, Othon C. Winter, David Nesvorny
First Author’s Institution: Laboratoire d’astrophysique de Bordeaux, Pessac, France
Status: Accepted to ApJ

The puzzling architecture of the Solar system has long been a headache for planetary dynamicists. We can sort of divide its structure (cover image, not to scale) into several zones. First, the terrestrial planets Mercury, Venus, Earth and Mars, which are divided from the gas and ice giant planets by the asteroid belt. Beyond the ice giants there is the Kuiper belt, which spans out to very large distances from the Sun. One longstanding conundrum in this ordering is the relatively small mass of Mars and existence of the asteroid belt in between Mars and Jupiter. It shouldn’t be there, and Mars should be way bigger. In fact, Mars is only 10% of the mass of Earth and therefore seems to have never accreted enough material to become a fully fledged planet. In planet formation, Mars-sized objects are usually termed “planetary embryos”, as we think this is the intermediate stage of a planet’s growth.

Sailing adolescent Jupiter to blame?

To explain the apparent dip in the mass-distance relation of our solar system (increasing mass with planetary distance from the Sun until Jupiter, and then decreasing again), planetary scientists developed a model deeply rooted in dynamical principles: the “Grand Tack”. The idea of the model is that Jupiter, directly after its formation out of the early Solar nebula at ~3.5 AU (AU = distance Sun–Earth), migrated toward the Sun, became “tacked” on at ~1.5 AU and then migrated outward again to its current positon at 5.2 AU. By doing so it depleted the mass concentrated at the locations of nowadays Mars and the asteroid belt, and thus Mars was never able to grow bigger. It also successfully explains some other issues, such as the inclinations and excitations of asteroids and the transition from water-poor to water-rich asteroids in the middle of the belt.

Figure 2: Chaos in the giant planets's orbits and their long-term stability. During the gas disk phase the giant planets migrated inward toward the Sun and are eventually locked in so-called mean-motion resonances. After the gas dissipation Saturn and Jupiter orbit in chaotic, but stable orbits. (Fig. 8 from the paper)

Chaos in the giant planets’ orbits and their long-term stability. During the gas-disk phase the giant planets migrated inward toward the Sun and are eventually locked in so-called mean-motion resonances. After the gas dissipation, Saturn and Jupiter orbit in chaotic but stable orbits. [Izidoro et al. 2016]

A more “primordial” origin

Alternatively, the mass-depletion at Mars’ location could have very early roots, even before (or during) the formation of Jupiter, caused by microphysics in the disk, as was explained in this Astrobite about the pile-up and evolution of dust. Here, the initial small dust grains in the disks did not accumulate everywhere in the disk and start to form planets. Instead, they end up only in specific regions and thus planet formation is concentrated in some narrow zones. In our case, this could be in two locations: one in the inner region, where the terrestrial planets reside nowadays, and one in the outer region, possibly around the region of Jupiter (more details given in this recent review paper). Such a scenario however poses another problem. The Grand Tack scenario from above easily explains the current relatively high inclinations and excitations of the asteroids in the belt: Jupiter drops by and gives everyone a huge gravitational swing due to its enormous mass. In the other scenario this isn’t the case — so we would nominally expect the asteroids to remain in relatively calm and low-excitation orbits.

But Andé Izidoro and collaborators think differently. They propose that instead of migrating (like in the Grand Tack model), Jupiter and Saturn were on chaotic but stable orbits. This scenario is illustrated in the figure above, which shows how the gas and ice giants evolved over time in such a configuration. Essentially what happens is that all the planets roughly formed at approximately where they are today. However, as our early solar system was subject to dynamic evolution (we possibly lost some planetary embryos early on), the orbits of Jupiter and Saturn moved continously and chaotically through the disk (though not nearly as dramatically as in the Grand Tack scenario). Thus, Jupiter came close to the asteroid belt again and its gravity excited the asteroids. You’ll probably ask yourself now where the great difference lies. This is highlighted in the figure below, where the deviations between the Grand Tack and the “chaotic motion” pictures become readily apparent.

In the Grand Tack model, Jupiter is formed within a sea of primordial planetesimals/asteroids (which are divided into S- and C-types: water-poor and water-rich). It migrates inwards and then outwards, and scatters some of the outer asteroids to their current locations. In the “low-mass asteroid belt”/”chaotic motion” model the S- and C-type deviation and the mass-depletion at around Mars’ current location is primordial. Jupiter and Saturn form later and evolve in chaotic but stable orbits, which scatters some of the outer asteroids inside. [Morbidelli & Raymond, 2016, arXiv:1610.07202]

What now?

Instead of one we have now two models, both of which can give us an answer to why our solar system’s architecture looks like it does today. Fortunately, the two models make different predictions about how the dynamics evolved in the disks. In the coming years we may thus be able to distinguish between them by testing their differences, and we may be able to get deeper insights into how our home in the universe evolved into how it looks today.

About the author, Tim Lichtenberg:

I am a graduate student at ETH Zurich in planetary astrophysics. By combining astro- and geophysical numerical approaches I try to understand the influence of star-forming environments on the formation of terrestrial planets. Occasionally, when I am not at the bus swinging back and forth between the institutes, I enjoy doing sports or become involved in science outreach projects.

1 November 2016

A Long Time Ago Around a Star Far, Far Away…

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting 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: OGLE-2012-BLG-0950Lb: The First Planet Mass Measurement from Only Microlens Parallax and Lens Flux
Authors: N. Koshimoto, A. Udalski, J.P. Beaulieu et al.
First Author’s Institution: Department of Earth and Space Sciences, Osaka University, Japan
Status: Accepted in AJ

In today’s paper, we’re looking at a planet discovery. The planet in question is called OGLE-2012-BLG-0950Lb. That’s a bit of a mouthful, so for the purposes of this article I’m going to nickname it Oggy.

Oggy is an unusual planet. It’s intrinsically fairly normal: a planet roughly the size of Neptune, orbiting an M-dwarf star. But Oggy is 3 kiloparsecs, or 9,800 light years, away, making it one of the most distant planets ever discovered. That’s because it was discovered via the microlensing technique.

A Quick Microlensing Primer:

Microlensing is a bit of a forgotten relative of observational exoplanet science — we hear about transit and radial velocity planets all the time, and direct imaging is intuitively simple and makes pretty pictures, so it gets lots of press time too. The microlensing technique, meanwhile, has been quietly churning out planet detections since 2004. And yet, if you asked your average exoplanet scientist how microlensing works, they’d probably panic and mutter something about general relativity before running away as soon as possible.

Microlensing works like this: your system consists of a lens star, which magnifies light from a source star. The source star is moving relative to the lens, and the closer it gets to the lens, the more the light rays are bent, and the more the lens magnifies the source. One could plot contours of equal magnification — they would be circles, traced around the lens star, as I’ve tried to demonstrate in Figure 1. The closer to the lens, the more a source would be magnified. The background source will move across the diagram following the red arrow, and as the flux increases and then decreases a flux peak is observed.

Figure 1: On the left, the blue contours show regions of equal magnification — increasing towards the centre where the star is positioned. On the right is the sort of flux curve you’d observe as the source passes behind the lens, following the red arrow. [Astrobites]

Now, if the lens star also has a planet, this can warp the light rays even more. In fact, the interference between the two gravitational sources can even create places where the magnification of the source is infinite. Alternatively, depending on the system geometry, it can reduce the magnification of the lens for a very short time. As the source moves across the sky, there’s an added peak or trough, on top of the peak from the lens magnifying the source. Figure 2 shows a schematic of this setup.

Figure 2: Three lens diagrams for the three possible scenarios. Left: a source is magnified by a lens star. Centre and right: the lens star now hosts a planet, which also warps the light: either enhancing or reducing the lens effect for a short period of time, and creating an additional bump or dip on top of the lensing curve. [Astrobites]

Confused yet?

Applying this to Oggy:

During the summer of 2012, two different microlensing surveys — one called OGLE and one called MOA — both independently identified a flux ramp-up around the same star. A source was just beginning its journey behind a lens. They monitored the event, and I’ve shown their data in Figure 3. This matches the situation in Figure 2: there’s a wide peak as the source passes behind the star, and a teensy dip at day 6149, where the planet briefly dims the source — a zoom-in of this dip is shown in the bottom panel.

Figure 3: The microlensing event where planet Oggy was identified. Top: the whole event as the source star passes behind the lens. Bottom left: a close up of the effect of the planet on the observed flux. Bottom right: a close up of the final dimming of the event as the source passes out from behind the star. [Koshimoto et al. 2016]

All of these data can be fitted to gain an understanding of the lens and planet system, and work out what’s actually been seen by the team. However, there are a lot of free parameters in the system, and only a few free parameters for your curve, so the fit is degenerate. In other words, the curve isn’t enough to constrain all the parameters, and there are multiple different good fits. There are various ways to solve this problem: for example, the length of time that the source is magnified might tell you about the source diameter (since it will be magnified for slightly longer if it is slightly bigger). However, in the case of Oggy, the source size is consistent with zero — so the errors are too big for the magnification duration to give us any information.

However, the total passage of the star is a fairly long event, with the star being magnified for a total of ~120 days. This means that the orbital motion of the Earth during the microlensing event is actually detectable! The models that include the so-called ‘microlens parallax’ are shown in Figure 3, and they fit better than those that ignore the Earth’s motion. For completeness, the authors also use a third model, one they call the xallarap model, where the orbital motion of the lens is considered (kind of like a backwards parallax). However, this is found to be a less good fit than the parallax model.

By determining the microlens parallax, and with some additional Keck telescope observations that confirm the flux of the lens star, the authors calculate a mass for the planet and for the star. It’s 35 times the mass of the Earth, orbiting an M-dwarf host star, with a projected distance of three times the Earth–Sun distance. Oggy is the first planet for which a mass has been determined with just the parallax and the lens flux.

So, why should you care?

Microlensing is exciting as it’s the only method that can find these extremely far away planets. This allows us to figure out whether our part of the galaxy is representative: does the galactic bulge have as many planets as we do out here in a spiral arm? Are the planets in the galactic bulge generally bigger or smaller than ours? For now, microlensing is the only technique that can answer this question.

Since several microlensing surveys are now being carried out with simultaneous ground and space observations (using the Spitzer and Kepler telescopes), there will be more and more planets identified where the microlens parallax can be very precisely determined — so the method used here will be valuable in calculating masses for these future discoveries. Oggy is a great benchmark, proving that these missions will give valuable scientific output.

About the author, Elisabeth Matthews:

I’m currently a second year PhD student at the University of Exeter, in the south of England, where I’m aiming to detect and characterise extra-solar planets and debris disks via direct imaging. So far this has meant lots of detecting background stars that happen to be near bright, nearby stars and no detecting of actual planets — but hopefully my luck will change soon!

25 October 2016

Evolution in the Stellar Nursery

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting 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: The JCMT Gould Belt Survey: Evidence for Dust Grain Evolution in Perseus Star-forming Clumps
Authors: Michael Chun-Yuan Chen, J. Di Francesco, D. Johnstone, et al.
First Author’s Institution: University of Victoria
Status: Published in ApJ

Deep in the cold and dusty corners of the universe, baby stars are formed. When a molecular cloud (aka an interstellar dust bunny) made out of gas and small dust particles reaches a critical density, it begins to form protostars as the gas in the cloud gravitationally collapses. Scientists can study these stellar nurseries to understand how gas is transformed into giant stars. Unfortunately, molecular hydrogen, which composes ~99% of a molecular cloud, is hard to measure at the low temperatures at which clouds form. Instead, the dust in the clouds is studied, as it behaves in a manner similar to the gas.

Recently, Mike Chen and his collaboration investigated several stellar nurseries in one molecular cloud, Perseus, in order to map the temperature and other parameters that they use to understand the evolution of dust grains in the regions. They look primarily at the regions’ temperatures and a parameter called the dust emissivity spectral index, or beta. This index is critical to estimating the mass and temperature of the star-forming structure and is dependent on the properties of the dust grains, like size and composition.

Using the JCMT sub-millimetre telescope, the group derived maps for temperature, dust emissivity spectral index and optical depth (i.e. how well you can see through it). Plotting known stars and young stellar objects on the map, they were able to see relations between these objects and the dust grain properties. The results from the temperature maps showed distinct warm regions within the clouds — though by warm we only mean about 15 K (-430°F). Most of the areas that had above average temperatures also had a young stellar object, which is likely responsible for the region’s extra warmth. However, not all regions containing young stellar objects were warm. It could be that these regions contain particularly infant objects that haven’t yet had the time to warm their dust blankets.

Temperature maps for two dense regions in the Perseus molecular cloud. Red indicates higher temperatures and blue, colder. Circles, triangles and stars indicate young stellar objects and stars. [Chen et al. 2016]

Examination of the dust emissivity spectral index maps showed significant differences between star-forming clumps in the Perseus molecular cloud. Furthermore, the group noticed smooth structure on small scales which indicates that the dust emissivity spectral index is affected by its local environment. Different clumps in one cloud showed different distributions of the index. This shows that the dust grains are probably growing significantly as the clump itself evolves.

Dust emissivity maps for two dense regions in the Perseus molecular cloud. Yellow indicates regions with higher emissivity index and blue, lower. Circles, triangles and stars indicate young stellar objects and stars. [Chen et al. 2016]

When comparing the dust emissivity spectral index and temperature maps, Chen and his collaborators noticed the colder temperature regions tended to have higher beta indices. This could indicate an evolution of the dust grains. As the temperature drops, ice can begin to grow on the dust grains, giving them a larger size and thus a higher beta index; in warmer regions where stars are forming, this ice sublimates and the index is lower. In other words, the beta index decreases when the grain size increases. A grain with a ice mantle, however, can have a higher beta index compared to a bare grain of the same size. However, it’s a complicated picture and another theory suggests that instead large grains are produced in the densest gas near protostars, which are typically warm. While this work begins to probe the origin of dust grains with low dust emissivity indices, further observations are necessary to see the full picture of grain evolution.

About the author, Mara Johnson-Groh:

Mara is working on her master’s at the University of Victoria, Canada. In a nutshell, her research is taking pretty pictures of the universe. She spends half of her time with the Gemini Planet Imager Exoplanet Survey team trying to directly image new exoplanets, and the other half looking for galaxies responsible for damped Lyman alpha systems in the images of quasars. When she’s not looking at scientifically pretty pictures, she’s capturing earthly beauty with digital photography or trying to stay grounded with rock climbing.

11 October 2016

Why Are Pulsar Planets Rare?

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting 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: Why Are Pulsar Planets Rare?
Authors: Rebecca G. Martin, Mario Livio, and Divya Palaniswamy
First Author’s Institution: University of Nevada Las Vegas
Status: Accepted to ApJ

Pulsar planets were the first type of planet ever discovered beyond the solar system, and this discovery shocked the astronomical world. These were not the planets we expected: solar system-like planets around a Sun-like star. Instead, these planets orbited a pulsar, a rapidly rotating neutron star (the extremely dense core of a massive star that exploded as a supernova). However, since their initial discovery in 1992, only five such pulsar planets have been found, making them quite rare. Fewer than 1% of pulsars have been found to host planets. In this paper, the authors explore how these planets may have formed as a way to explain the rarity of pulsar planets.

Figure 1: The mass and semi-major axis of pulsar companions in colored circles (blue for confirmed planets, red for main-sequence stars, black for low-mass stars and brown dwarfs, green for neutron stars, purple for heavy white dwarfs, and yellow for low-mass white dwarfs). The violet asterisks are our eight solar-system planets, the Moon, and the asteroid-belt dwarf planet Ceres, for reference. The black lines are the detection limits for fast millisecond pulsars (bottom line) and more normal pulsars (top line). [Martin et al. 2016]

Formation scenarios

Planets that survive the supernova: The most obvious formation scenario is that the planets formed simultaneously with the original star just like our own solar system. However, many astronomers believe that stars above three solar masses (3x the mass of the Sun) can’t form planets, and stars that supernova into neutron stars are at least eight solar masses. Even if they could form, the planet would have to avoid being eaten when the star swells up into a red supergiant and then stick around after the supernova explosion removes most of the mass from the system, an unlikely scenario.
Supernova fallback disk: After the supernova, some of the material falls back into a disk where, just like with a protoplanetary disk, it might form planets. However, this “fallback disk” is expected to have little angular momentum, which means the material likely doesn’t have enough rotational speed to avoid falling back directly onto the neutron star. (For example, a rocket shot vertically up would fall back down to Earth. It needs “sideways” velocity to get into orbit and avoid hitting the Earth.)
Destruction of a companion star: A low-mass companion star orbiting a neutron star loses mass through evaporation — which, if strong enough, can entirely destroy the star. The star’s debris can then form a disk orbiting the neutron star with a mass about 10% the mass of Earth.
Evaporation of a companion: An alternative outcome of evaporation from the intense pulsar radiation is that the companion star just loses so much mass that it is reduced down to planetary size.

Artist’s illustration of a binary system in which the left star is exploding as a supernova. [ESA/Justyn R. Maund (University of Cambridge)]

The authors determine that pulsar planets are likely formed only when there is a low-mass companion star to the neutron star. Almost every star with enough mass to become a supernova is born with a companion star, but only 10% of these companions have low enough mass to make pulsar planets a realistic possibility. Of these, only about 10% are able to survive on a gravitationally bound orbit after the massive star goes supernova. This means that only ~1% of neutron-star progenitors (stars that eventually become neutron stars) even have the potential to form pulsar planets.

In the case of a star being disrupted and forming a disk, the disk receives intense radiation from the pulsar that heats and helps evaporate the disk. If the surface density of a disk is very large, dead zones are formed in the disk where material is able to build up to form planets. Disks with a lower surface density are not able to effectively shield the disk to prevent its evaporation and therefore cannot form any planets. Only when the companion star is disrupted into a massive enough disk is there a real possibility of a planet then forming.

Results

Only under a very specific set of circumstances can planets form around pulsars. They require a companion star with a low mass, which only about 10% of neutron star progenitors have. Of these, only 10% can then survive the supernova explosion. Of the survivors, some may be evaporated into a planetary mass, while others may be disrupted by the pulsar. In the case of disruption, the subsequent disk then needs to be dense enough to withstand the intense pulsar radiation long enough to create stars.

Out of the five pulsar planets known, the authors believe that the three planets in the PSR 1257+12 system were formed from the disk of a disrupted star, the planet orbiting PSR J1719-1438 is the core of an evaporated white dwarf, and the planet around PSR B1620-26 was captured along with its white dwarf, with the planet now orbiting both of them as a circumbinary planet.

About the author, Joseph Schmitt:

I’m a 5th year graduate student at Yale University. My main research is on the discovery, characterization, and statistics of exoplanets. I’m also one of the science leads on the citizen science project Planet Hunters, a website where the general public can join the search for exoplanets.

4 October 2016

Gravity-Darkened Seasons on Planets

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting 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: Gravity-Darkened Seasons: Insolation Around Rapid Rotators
Authors: John P. Ahlers
First Author’s Institution: University of Idaho
Status: Accepted to ApJ

On Earth, our seasons come about due to the Earth’s tilted rotational axis relative to its orbital plane (and not due to changes in distance from the Sun, as it is commonly mistaken!) Essentially, this is due to the varying amounts of radiation that the Earth receives from the Sun in each hemisphere. But what would happen if the Sun were to radiate at different temperatures across its surface?

It’s hard to imagine such a scenario, but a phenomenon known as gravity darkening causes rapidly spinning stars to have non-uniform surface temperatures due to their non-spherical shape. As a star spins, its equator bulges outwards as a result of centrifugal forces (specifically, into an oblate spheroid). Since a star is made of gas, this has interesting implications for its temperature. If its equator is bulging outwards, the gas at the equator experiences a lower surface gravity (being slightly further away from the star’s center) a lower density and temperature. The equator of a spinning star is thus considered to be “gravitationally darkened”. The gas at the star’s poles on the other hand, has a slightly higher density and temperature (“gravitational brightening”) since it is closer to the center of the star relative to the gas at the equatorial bulge. Thus, there is a temperature gradient between the poles and equator of a rapidly rotating star.

While this is an interesting phenomenon in itself, the author of today’s paper introduces a new twist: what if there’s a planet orbiting such a star, and what implication does this gravity darkening have on a planet’s seasonal temperature variations? Compared to Earth. exoplanets have potentially more complex factors governing its surface temperature variations. For example, if a planet’s orbit is inclined relative to the star’s equator (see Figure 1), it can preferentially receive radiation from different parts of its star during the course of its orbit.

Fig 1: All the parameters describing a planet’s orbit. In this paper, the author mainly focuses on the inclination i, which is the angle of a planet’s orbital plane relative to the star’s equator. [Wikipedia]

The author claims that this effect can cause a planet’s surface temperature to vary as much as 15% (Figure 2). This essentially doubles the number of seasonal temperature variations a planet can experience over the course of an orbit. However, the author does not attempt to model the complex heat transfer that occurs on the planet’s surface due to the atmosphere and winds.

Fig. 2: Some examples of seasonal temperature changes of a planet for various orbital parameters. The top left figure shows the orientation of the planet’s tilt (precession angle, color-coded to match the plots), and the times corresponding to one orbit around the host star. In each subplot, the author shows the flux a planet would receive for different orbital inclinations (i.e. the angle i in Fig. 1). [Ahlers 2016]

Not only that, but there is also some variation in the type of radiation that a planet receives during the course of its orbit. Since the poles of rotating star are at a higher temperature, it will radiate relatively more ultraviolet (UV) radiation compared to the equatorial regions. The author claims that a planet orbiting in a highly inclined orbit will alternate receiving radiation preferentially from a star’s poles or equator, causing the amount of UV radiation to vary as much as 80%. High levels of UV radiation can cause a planet’s atmosphere to evaporate, as well as other complex photochemical reactions (such as those responsible for the hazy atmosphere on Saturn’s moon Titan).

As we discover new exoplanets over the course of the coming years, we will likely find examples of planets potentially experiencing these gravitationally darkened seasons. This will have interesting implications on how we view the habitability of these other worlds.

About the author, Anson Lam:

I am a graduate student at UCLA, where I am working with Steve Furlanetto on models of galaxy clustering and their applications to the reionization era. My main interests involve high redshift cosmology, dark matter, and structure formation.
Previously, I was an undergraduate at Caltech, where I did my BS in astrophysics. When I’m not doing astronomy, I enjoy engaging in some linear combination of swimming/biking/running.

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