Features RSS

M-dwarf planets

M-dwarf stars are excellent targets for planet searches because the signal of an orbiting planet is relatively larger (and therefore easier to detect!) around small, dim M dwarfs, compared to Sun-like stars. But are there better or worse stars to target within this category when searching for habitable, Earth-like planets?

Confusing the Signal

Radial velocity campaigns search for planets by looking for signatures in a star’s spectra that indicate the star is “wobbling” due to the gravitational pull of an orbiting planet. Unfortunately, stellar activity can mimic the signal of an orbiting planet in a star’s spectrum — something that is particularly problematic for M dwarfs, which can remain magnetically active for billions of years. To successfully detect planets that orbit in their stars’ habitable zones, we have to account for this problem.

In a recent study led by Elisabeth Newton (Harvard-Smithsonian Center for Astrophysics), the authors use literature measurements to examine the rotation periods for main-sequence, M-type stars. They focus on three factors that are important for detecting and characterizing habitable planets around M dwarfs:

  1. Whether the habitable-zone orbital periods coincide with the stellar rotation
    False planet detections caused by stellar activity often appear as a “planet” with an orbital period that’s a multiple of the stellar rotation period. If a star’s rotation period coincides with the range of orbital periods corresponding to its habitable zone, it’s therefore possible to obtain false detections of habitable planets.
  2. How long stellar activity and rapid rotation last in the star
    All stars become less magnetically active and rotate more slowly as they age, but the rate of this decay depends on their mass: lower-mass stars stay magnetically active for longer and take longer to spin down.
  3. Whether detailed atmospheric characterization will be possible
    It’s ideal to be able to follow up on potentially habitable exoplanets, and search for biosignatures such as oxygen in the planetary atmosphere. This type of detection will only be feasible for low-mass dwarfs, however, due to the relative size of the star and the planet.

An Ideal Range

rotation period vs. mass

Stellar rotation period as a function of stellar mass. The blue shaded region shows the habitable zone as a function of stellar mass. For M dwarfs between ~0.25 and ~0.5 solar mass, the habitable-zone period overlaps with the stellar rotation period. [Newton et al. 2016]

Newton and collaborators find that stars in the mass range of 0.25 to 0.5 solar mass (stellar class M1V-M4V) are non-ideal targets, because their stellar rotation periods (or a multiple thereof) coincide with the orbital periods of their habitable zones. In addition, atmospheric characterization will only be feasible in the near future for stars with mass less than ~0.25 solar mass.

On the other hand, dwarfs with mass less than ~0.1 solar masses (stellar classes later than M6V) will retain their stellar activity and faster rotation rates throughout most of their lifetimes, making them non-ideal targets as well.

When searching for habitable exoplanets, the best targets are therefore the mid M dwarfs in the mass range of 0.1 to 0.25 solar mass (stellar class M4V-M6V). Building a sample focused on these stars will reduce the likelihood that planets found in the stars’ habitable zones are false detections. This will hopefully produce a catalog of potentially habitable exoplanets that we can eventually follow up with atmospheric observations.

Citation

Elisabeth R. Newton et al 2016 ApJ 821 L19. doi:10.3847/2041-8205/821/1/L19

Arp 220

Recent reanalysis of data from the Fermi Gamma-ray Space Telescope has resulted in the first detection of high-energy gamma rays emitted from a nearby galaxy. This discovery reveals more about how supernovae interact with their environments.

Colliding Supernova Remnant

After a stellar explosion, the supernova’s ejecta expand, eventually encountering the ambient interstellar medium. According to models, this generates a strong shock, and a fraction of the kinetic energy of the ejecta is transferred into cosmic rays — high-energy radiation composed primarily of protons and atomic nuclei. Much is still unknown about this process, however. One open question is: what fraction of the supernova’s explosion power goes into accelerating these cosmic rays?

In theory, one way to answer this is by looking for gamma rays. In a starburst galaxy, the collision of the supernova-accelerated cosmic rays with the dense interstellar medium is predicted to produce high-energy gamma rays. That radiation should then escape the galaxy and be visible to us.

Pass 8 to the Rescue

Observational tests of this model, however, have been stumped by Arp 220. This nearby ultraluminous infrared galaxy is the product of a galaxy merger ~700 million years ago that fueled a frenzy of starbirth. Due to its dusty interior and extreme levels of star formation, Arp 220 has long been predicted to emit the gamma rays produced by supernova-accelerated cosmic rays. But though we’ve looked, gamma-ray emission has never been detected from this galaxy … until now.

In a recent study, a team of scientists led by Fang-Kun Peng (Nanjing University) reprocessed 7.5 years of Fermi observations using the new Pass 8 analysis software. The resulting increase in resolution revealed the first detection of GeV emission from Arp 220!

Acceleration Efficiency

scaling relation

Gamma-ray luminosity vs. total infrared luminosity for LAT-detected star-forming galaxies and Seyferts. Arp 220’s luminosities are consistent with the scaling relation. [Peng et al. 2016]

Peng and collaborators argue that this emission is due solely to cosmic-ray interactions with interstellar gas. This picture is supported by the lack of variability in the emission, and the fact that Arp 220’s gamma-ray luminosity is consistent with the scaling relation between gamma-ray and infrared luminosity for star-forming galaxies. The authors also argue that, due to Arp 220’s high gas density, all cosmic rays will interact with the gas before escaping.

Under these two assumptions, Peng and collaborators use the gamma-ray luminosity and the known supernova rate in Arp 220 to estimate how efficiently cosmic rays are accelerated by supernova remnants in the galaxy. They determine that 4.2 ± 2.6% of the supernova remnant’s kinetic energy is used to accelerate cosmic rays above 1 GeV.

This is the first time such a rate has been measured directly from gamma-ray emission, but it’s consistent with estimates of 3-10% efficiency in the Milky Way. Future analysis of other ultraluminous infrared galaxies like Arp 220 with Fermi (and Pass 8!) will hopefully reveal more about these recent-merger, starburst environments.

Citation

Fang-Kun Peng et al 2016 ApJ 821 L20. doi:10.3847/2041-8205/821/2/L20

47 Tuc

Observations of globular clusters — gravitationally-bound, spherical clusters of stars that orbit galaxies as satellites — are critical to studies of galactic and stellar evolution. What type of galaxies host the largest total number of globular clusters in today’s universe? A recent study answers this question.

GCs per host gal luminosity

Total number of globular clusters vs. host galaxy luminosity for a catalog of ~400 galaxies of all types. [Harris 2016]

Globular Favoritism

Globular clusters can be found in the halos of all galaxies above a critical brightness of about 107 solar luminosities (in practice, all but the smallest of dwarfs). The number of globulars a galaxy hosts is related to its luminosity: the Milky Way is host to ~150 globulars, the slightly brighter Andromeda galaxy may have several hundred globulars, and the extremely bright giant elliptical galaxy M87 likely has over ten thousand.

But the number of galaxies is not evenly distributed in luminosity; tiny dwarf galaxies are extremely numerous in the universe, whereas giant ellipticals are far less common. So are most of the universe’s globulars found around dwarfs, simply because there are more dwarfs to host them? Or are the majority of globular clusters orbiting large galaxies? A scientist at McMaster University in Canada, William Harris, has done some calculations to find the answer.

Finding the Peak

Harris combines two components in his estimates:

  1. The Schechter function, a function that describes the relative number of galaxies per unit luminosity. This function drops off near a characteristic luminosity roughly that of our galaxy.
  2. Empirical data from ~400 galaxies that describe the average number of globulars per galaxy as a function of galaxy luminosity.
Where are the GCs?

Relative number of globular clusters in all galaxies at a given luminosity, for metal-poor globulars only (blue), metal-rich globulars only (red), and all globulars (black). The curves peak around the Schechter characteristic luminosity, and metal-poor globulars outnumber metal-rich ones 4 to 1. [Harris 2016]

He finds that globular clusters are most commonly found in galaxies within a surprisingly narrow range around the characteristic luminosity of the Schechter function. This means that, at the current time, the collection of galaxies similar in brightness to the Milky Way or Andromeda host the largest total number of globulars in the universe.

Metal-Poor Dominance

Harris extends these calculations by examining two subpopulations of globulars: blue (metal-poor) and red (metal-rich). Metal-poor globular clusters are found in all galaxies, but metal-rich ones reside preferentially in massive, bright galaxies. Strikingly, Harris finds that this preference results in metal-poor globulars making up almost 80% of all globular clusters in the universe, outnumbering the metal-rich ones by nearly 4 to 1.

This result implies that the earliest stages of hierarchical galaxy mergers — when most of the available gas was low-metallicity — provided the most favorable conditions for the formation of dense, massive star clusters. This early environment birthed the majority of the globular clusters we see today.

Citation

William E. Harris 2016 AJ 151 102. doi:10.3847/0004-6256/151/4/102

J1211

The recent discovery of a hyper-velocity binary star system in the halo of the Milky Way poses a mystery: how was this system accelerated to its high speed?

Accelerating Stars

Unlike the uniform motion in the Galactic disk, stars in the Milky Way’s halo exhibit a huge diversity of orbits that are usually tilted relative to the disk and have a variety of speeds. One type of halo star, so-called hyper-velocity stars, travel with speeds that can approach the escape velocity of the Galaxy.

How do these hyper-velocity stars come about? Assuming they form in the Galactic disk, there are multiple proposed scenarios through which they could be accelerated and injected into the halo, such as:

  1. Ejection after a close encounter with the supermassive black hole at the Galactic center
  2. Ejection due to a nearby supernova explosion
  3. Ejection as the result of a dynamical interaction in a dense stellar population.

Further observations of hyper-velocity stars are necessary to identify the mechanism responsible for their acceleration.

J1211’s Surprise

J1211 orbit

Models of J1211’s orbit show it did not originate from the Galactic center (black dot). The solar symbol shows the position of the Sun and the star shows the current position of J1211. The bottom two panels show two depictions (x-y plane and r-z plane) of estimated orbits of J1211 over the past 10 Gyr. [Németh et al. 2016]

To this end, a team of scientists led by Péter Németh (Friedrich Alexander University, Erlangen-Nürnberg) recently studied the candidate halo hyper-velocity star SDSS J121150.27+143716.2. The scientists obtained spectroscopy of J1211 using spectrographs at the Keck Telescope in Hawaii and ESO’s Very Large Telescope in Chile. To their surprise, they discovered the signature of a companion in the spectra: J1211 is actually a binary!

Németh and collaborators found that J1211, located roughly 18,000 light-years away, is moving at a rapid ~570 km/s relative to the galactic rest frame. The binary system consists of a hot (30,600 K) subdwarf and a cool (4,800 K) companion star in a wide orbit, likely separated by several AU.

An Unknown Past and Future

Why are these new observations of J1211 such a big deal? Because all the acceleration scenarios for a star originating in the Galactic disk fail in the case of J1211. The authors find by modeling J1211’s motion that the system can’t have originated in the Galactic center, so interactions with the supermassive black hole are out. And supernova explosions or dynamical interactions would tear the wide binary apart in the process of accelerating it. Németh and collaborators suggest instead that J1211 was either born in the halo population or accreted later from the debris of a destroyed satellite galaxy.

J1211’s speed is so extreme that its orbit could be either bound or unbound. Interestingly, when the authors model the binary’s orbit, they find that the assumed mass of the Milky Way’s dark-matter halo determines whether J1211’s orbit is bound. This means that future observations of J1211 may provide a new way to probe the Galactic potential and determine the mass of the dark matter halo, in addition to revealing unexpected origins of high-velocity halo stars.

Citation

Péter Németh et al 2016 ApJ 821 L13. doi:10.3847/2041-8205/821/1/L13

supernova

Supernovae — enormous explosions associated with the end of a star’s life — come in a variety of types with different origins. A new study has examined how the brightest supernovae in the Universe are produced, and what limits might be set on their brightness.

Ultra-Luminous Observations

Recent observations have revealed many ultra-luminous supernovae, which have energies that challenge our abilities to explain them using current supernova models. An especially extreme example is the 2015 discovery of the supernova ASASSN-15lh, which shone with a peak luminosity of ~2*1045 erg/s, nearly a trillion times brighter than the Sun. ASASSN-15lh radiated a whopping ~2*1052 erg in the first four months after its detection.

How could a supernova that bright be produced? To explore the answer to that question, Tuguldur Sukhbold and Stan Woosley at University of California, Santa Cruz, have examined the different sources that could produce supernovae and calculated upper limits on the potential luminosities of each of these supernova varieties.

Explosive Models

Sukhbold and Woosley explore multiple different models for core-collapse supernova explosions, including:

  1. Prompt explosion
    A star’s core collapses and immediately explodes.
  2. Pair instability
    Electron/positron pair production at a massive star’s center leads to core collapse. For high masses, radioactivity can contribute to delayed energy output.
  3. Colliding shells
    Previously expelled shells of material around a star collide after the initial explosion, providing additional energy release.
  4. Magnetar
    The collapsing star forms a magnetar — a rapidly rotating neutron star with an incredibly strong magnetic field — at its core, which then dumps energy into the supernova ejecta, further brightening the explosion.

They then apply these models to different types of stars.

Setting the Limit

ASASSN-15lh

The authors show that the light curve of ASASSN-15lh (plotted in orange) can be described by a model (black curve) in which a magnetar with an initial spin period of 0.7 ms and a magnetic field of 2*1013 Gauss deposits energy into ~12 solar masses of ejecta. Click for a closer look! [Adapted from Sukhbold&Woosley 2016]

The authors find that the maximum luminosity that can be produced by these different supernova models ranges between 5*1043 and 2*1046 erg/s, with total radiated energies of 3*1050 to 4*1052 erg. This places the upper limit on the brightness of a supernova at about 5 trillion times the luminosity of the Sun.

The calculations performed by Sukhbold and Woosley confirm that, of the options they explore, the least luminous events are produced by prompt explosions. The brightest events possible are powered by the rotational energy of a newly born magnetar at the heart of the explosion.

The energies of observed ultra-luminous supernovae are (just barely) contained within the bounds of the mechanisms explored here. This is even true of the extreme ASASSN-15lh — which, based on the authors’ calculations, was almost certainly powered by an embedded magnetar. If we were to observe a supernova more than twice as bright as ASASSN-15lh, however, it would be nearly impossible to explain with current models.

Citation

Tuguldur Sukhbold and S. E. Woosley 2016 ApJ 820 L38. doi:10.3847/2041-8205/820/2/L38

Super-Earth

Short-period super-Earths — planets larger than Earth, but smaller than gas giants — have been found orbiting ~60% of Sun-like stars around us. But how do these planets form? A new study proposes a mechanism for birthing these contradictory planets, as well as their rarer counterparts, “super-puffs”.

Two Formation Puzzles

Super-Earths are exoplanets with radii of 1–4 Earth radii and masses of 2–20 Earth masses. These numbers pose a puzzle: this mass range includes cores massive enough that they should trigger runaway accretion and result in the formation of gas giants. Yet super-Earths don’t accumulate that much gas: their atmospheres are only 1-10% the mass of their core. How do these planets manage to avoid runaway accretion?

Super-Earths may often have too little gas for their large core masses, but the flip side of this puzzle is that of super-puffs. Super-puffs are a rare class of short-period Kepler planets with the opposite problem: they have too much gas relative to their core mass. Super-puffs have large radii of 4–10 Earth radii, but small masses of 2–6 Earth masses. Their atmosphere-to-core ratios are > 20%.

atmosphere-to-core mass ratio

The final planet atmosphere-to-core mass ratio for models with different factors of gas depletion in the inner disk. The ratio is too high (like a gas giant) if the disk is not depleted (black curve), but it’s in the range of 1-10% if the disk is depleted by a factor of 100 to 10,000,000 (colored curves). [Lee&Chiang 2016]

In a recent study, two scientists at University of California Berkeley, Eve Lee and Eugene Chiang, investigated the possible scenarios that could lead to the formation of these two types of planets.

A Late Birth

Lee and Chiang found that super-Earths are able to form without running away and becoming Jupiter-like — if they aren’t born until late in the game! The inner regions of the gas disk surrounding the host star will gradually clear out over time. Planetary cores that assemble and accrete gas at the tail end of the inner disk’s lifetime — when most of the gas is already depleted — will be able to grow only small atmospheres. The limit is primarily set by the timescale: these planets have only 1 Myr to form, rather than 10 Myr, so they run out of time before running away.

The minimal gas in the inner disk in this scenario means there’s also little friction to cause migration of the planetary core. The authors demonstrate that this supports in situ formation of these super-Earths close to the host star.

Schematic of a transitional disk, in which the inner regions have been cleared of gas. [Catherine Espaillat]

Schematic of a transitional disk, in which the inner regions have been cleared of gas. [Catherine Espaillat]

Formation at a Distance

As for super-puffs, the authors’ calculations show that these planets can build their thick atmospheres further out in the disk (at distances beyond ~1 AU), where the nebular gas is colder and less dense. The rapid cooling of their dust-free atmospheres allows them to amass gas much more quickly. After building their atmospheres, the super-puffs then migrate inwards to their currently observed orbits < 0.1 AU from their host stars.

With these scenarios, the authors are able to reconcile the puzzles of both super-Earths and super-puffs with the model of planet formation via nebular accretion. Intriguingly, their proposed picture of super-Earths — forming in a gas-poor inner disk that’s fed by gas bleeding inward from a massive outer disk — is consistent with observations of transitional disks that have their inner regions cleared of gas. Future observations of such disks may help to confirm this formation model.

Citation

Eve J. Lee and Eugene Chiang 2016 ApJ 817 90. doi:10.3847/0004-637X/817/2/90

disk galaxies

Pure disk galaxies — thin disk galaxies that don’t have a central bulge — are a puzzling presence in our universe. How were these galaxies able to escape the effects that normally generate bulges? A new study has examined the properties of pure disk galaxies over the last 8 billion years in an effort to learn more.

Challenging the Model

pure disk galaxy

Example of a pure disk galaxy at z=0.86. Its brightness profile, in which surface brightness is plotted against semi-major axis, exhibits a pure exponential decay from the center of the galaxy to its edge. [Sachdeva&Saha 2016]

According to the commonly accepted picture of galaxy formation, galaxies grow hierarchically via major and/or minor mergers. These mergers ultimately scramble the galaxies’ preexisting disks, thicken the disk structures, and cause the formation of classical bulges at the disk centers.

The fact that we also observe pure disk galaxies without central bulges challenges this picture. If all galaxy formation is driven by the hierarchical model, then how have pure disk galaxies manage to escape the effects of merger activity?

In a new study, Sonali Sachdeva and Kanak Saha (Inter-University Centre for Astronomy and Astrophysics, India) examine the population of pure disk galaxies out to a redshift of z~1. Their goal is to better understand the properties of this strange category of galaxies throughout the last 8 billion years.

Disks Past and Present

Sachdeva and Saha examine the light profiles of ~570 galaxies from the Hubble Deep Field and the Sloan Digital Sky Survey. They categorize as pure disk galaxies those that can be described well by a single exponential function from the center of the galaxy out to its outer edges. Galaxies requiring an additional functional component to model the excess light in the center are classified as galaxies with bulges.

Using this categorization, the authors find that 94 of the 570 galaxies are pure disk galaxies. When they bins these galaxies into three redshift bins between z~1 and z~0, pure disk galaxies account for 15–18% of the total galaxies in each bin. This tells us that the fraction of pure disk galaxies hasn’t altered much in the last 8 billion years.

B/T ratio

Distribution of the bulge-to-total light ratio for the pure disk galaxies (blue) and the other disk galaxies (red) in the sample. [Sachdeva&Saha 2016]

Non-Merger Growth

Further examining the brightness profiles for these pure disk galaxies, Sachdeva and Saha find that both the average central surface brightness and the average scale length are the same across different redshift bins — which means that gas isn’t being fed into the interior parts of these galaxies over time. Yet in spite of this, the total stellar mass and the size of these galaxies grows substantially from z~1 to the present day — by 40% and 60%, respectively.

How could these galaxies be growing without changing their profiles (as would happen if their growth were caused by mergers)? The authors propose that these galaxies may be isolated and protected from mergers, and they grow through smooth accretion via cosmic filaments of cold gas onto their outskirts. Additional study of this unique category of galaxies may provide further insight into different mechanisms of galaxy evolution.

Citation

Sonali Sachdeva and Kanak Saha 2016 ApJ 820 L4. doi:10.3847/2041-8205/820/1/L4

pulsar timing array

Though the recent discovery of GW150914 is a thrilling success in the field of gravitational-wave astronomy, LIGO is only one tool the scientific community is using to hunt for these elusive signals. After 10 years of unsuccessful searching, how likely is it that pulsar-timing-array projects will make their own first detection soon?

GW frequencies

Frequency ranges for gravitational waves produced by different astrophysical sources. Pulsar timing arrays such as the EPTA and IPTA are used to detect low-frequency gravitational waves generated by the stochastic background and supermassive black hole binaries. [Christopher Moore, Robert Cole and Christopher Berry]

Supermassive Background

Ground-based laser interferometers like LIGO are ideal for probing ripples in space-time caused by the merger of stellar-mass black holes; these mergers cause chirps in the frequency range of tens to thousands of hertz. But how do we pick up the extremely low-frequency, nanohertz background signal caused by the orbits of pairs of supermassive black holes? For that, we need pulsar timing arrays.

Pulsar timing arrays are sets of pulsars whose signals are analyzed to look for correlations in the pulse arrival time. As the space-time between us and a pulsar is stretched and then compressed by a passing gravitational wave, the pulsar’s pulses should arrive a little late and then a little early. Comparing these timing residuals in an array of pulsars could theoretically allow for the detection of the gravitational waves causing them.

Globally, there are currently four pulsar timing array projects actively searching for this signal, with a fifth planned for the future. Now a team of scientists led by Stephen Taylor (NASA-JPL/Caltech) has estimated the likelihood that these projects will successfully detect gravitational waves in the future.

Probability for Success

detection probabilities

Expected detection probability of the gravitational-wave background as a function of observing time, for five different pulsar timing arrays. Optimistic and conservative assumptions are made for merger rates (blue and red lines, respectively) and environmental conditions (solid and dashed lines, respectively). [Taylor et al. 2016]

Taylor and collaborators statistically analyzed the detection probability for each of the projects as a function of their observing time, based on the projects’ estimated sensitivities and both conservative and optimistic assumptions about merger rates and environmental influences.

First the bad news: based on the authors’ estimates, small arrays — which contain only a few pulsars that each have minimal timing noise — will not be likely to detect gravitational waves within the next two decades. These arrays are more useful for setting upper limits on the amplitude of the gravitational-wave background.

On the other hand, large pulsar timing arrays have far more promising detection probabilities. These include the Parkes Pulsar Timing Array, the European Pulsar Timing Array, and NANOGrav — which each target tens of pulsars, with the intent to add more in the future — as well as the International Pulsar Timing Array, which combines the efforts of all three of these projects. There is an 80% chance that, within the next decade, these projects will successfully detect the gravitational-wave background created by orbiting supermassive black holes.

Based on this study, the outlook for these large arrays remains optimistic even in non-ideal conditions (such as if supermassive-black-hole merger rates are lower than we thought). So, though we may still have to wait a few years, the possibility of probing an otherwise inaccessible range of frequencies continues to make pulsar timing arrays a promising avenue of study for gravitational waves.

Citation

S. R. Taylor et al 2016 ApJ 819 L6. doi:10.3847/2041-8205/819/1/L6

Hot Jupiter

Weather variations in the atmosphere of a planet on a highly eccentric orbit are naturally expected to be extreme. Now, a study has directly measured the wild changes in the atmosphere of a highly eccentric hot Jupiter as it passes close to its host star.

HD 80606 system

Diagram of the HD 80606 system. The inset images labeled A–H show the temperature distribution of the planet at different stages as it swings around its star. [de Wit et al. 2016]

Eccentric Opportunity

For a hot Jupiter — a gas giant that orbits close to its host star — the exoplanet HD 80606 b exhibits a fairly unusual path. Rather than having a circularized orbit, HD 80606 b travels on an extremely elliptic 111-day orbit, with an eccentricity of e ~ 0.93. Since the amount of flux HD 80606 b receives from its host varies by a factor of ~850 over the course of its orbit, it stands to reason that this planet must have extreme weather swings!

Now a team of scientists led by Julien de Wit (Massachusetts Institute of Technology) has reanalyzed old observations of HD 80606 and obtained new ones using the Spitzer Space Telescope. The longer observing time and new data analysis techniques allowed the team to gain new insights into how the exoplanet’s atmosphere responds to changes in the stellar flux it receives during its orbit.

Extreme Variations

By measuring the infrared light coming from HD 80606, de Wit and collaborators modeled the planet’s temperature during 80 hours of its closest approach to its host star. This period of time included the ~20 hours in which most of the planet’s temperature change is expected to occur, as it approaches to a distance a mere 6 stellar radii from its host.

The authors find that the layer of the atmosphere probed by Spitzer heats rapidly from <500K to 1400K (that’s ~440°F to a scalding 2000+°F!) as the planet approaches periastron.The atmosphere then cools similarly quickly as the planet heads away from the star once more.

HD 80606 b light curve

Relative infrared brightness of HD 80606 b at 4.5 and 8 µm. The dip marks where the planet passes behind the star, as viewed from Earth. [de Wit et al. 2016]

Exploring an Atmospheric Layer

Based on the authors’ models, the layer of the planet’s atmosphere probed by Spitzer absorbs ~20% of the radiation incident from the host star. This atmospheric layer has a ~4-hour radiative timescale, much shorter than the ~93-hour rotation period the authors estimate for HD 80606 b — which means that the heat is not transported efficiently from the day side to the night side of the planet.

These measurements are the first of their kind for an exoplanet’s atmosphere, opening a new window into our understanding of hot Jupiters. Applying the methods used here to other eccentric planets should help us to better understand the formation mechanisms and atmospheres of these extreme planets.

Citation

Julien de Wit et al 2016 ApJ 820 L33. doi:10.3847/2041-8205/820/2/L33

Caterpillar halo

The Caterpillar Project is a beautiful series of high-resolution cosmological simulations. The goal of this project is to examine the evolution of dark-matter halos like the Milky Way’s, to learn about how galaxies like ours formed. This immense computational project is still in progress, but the Caterpillar team is already providing a look at some of its first results.

Lessons from Dark-Matter Halos

Why simulate the dark-matter halos of galaxies? Observationally, the formation history of our galaxy is encoded in “galactic fossil record” clues, like the tidal debris from disrupted satellite galaxies in the outer reaches of our galaxy, or chemical abundance patterns throughout our galactic disk and stellar halo.

But to interpret this information in a way that lets us learn about our galaxy’s history, we need to first test galaxy formation and evolution scenarios via cosmological simulations. Then we can compare the end result of these simulations to what we observe today.

This figure illustrates the difference that mass resolution makes. In the left panel, the mass resolution is 1.5*10^7 solar masses per particle. In the right panel, the mass resolution is 3*10^4 solar masses per particle [Griffen et al. 2016]

This figure illustrates the difference that mass resolution makes. In the left panel, the mass resolution is 1.5*10^7 solar masses per particle. In the right panel, the mass resolution is 3*10^4 solar masses per particle [Griffen et al. 2016]

A Computational Challenge

Due to how computationally expensive such simulations are, previous N-body simulations of the growth of Milky-Way-like halos have consisted of only one or a few halos each. But in order to establish a statistical understanding of how galaxy halos form — and find out whether the Milky Way’s halo is typical or unusual! — it is necessary to simulate a larger number of halos.

In addition, in order to accurately follow the formation and evolution of substructure within the dark-matter halos, these simulations must be able to resolve the smallest dwarf galaxies, which are around a million solar masses. This requires an extremely high mass resolution, which adds to the computational expense of the simulation.

First Outcomes

These are the challenges faced by the Caterpillar Project, detailed in a recently published paper led by Brendan Griffen (Massachusetts Institute of Technology). The Caterpillar Project was designed to simulate 70 Milky-Way-size halos (quadrupling the total number of halos that have been simulated in the past!) at a high mass resolution (10,000 solar masses per particle) and time resolution (5 Myr per snapshot). The project is extremely computationally intense, requiring 14 million CPU hours and 700 TB of data storage!

Halo mass evolution

Mass evolution of the first 24 Caterpillar halos (selected to be Milky-Way-size at z=0). The inset panel shows the mass evolution normalized by the halo mass at z=0, demonstrating the highly varied evolution these different halos undergo. [Griffen et al. 2016]

In this first study, the Griffen and collaborators show the end states for the first 24 halos of the project, evolved from a large redshift to today (z=0). They use these initial results to demonstrate the integrity of their data and the utility of their methods, which include new halo-finding techniques that recover more substructure within each halo.

The first results from the Caterpillar Project are already enough to show clear general trends, such as the highly variable paths the different halos take as they merge, accrete, and evolve, as well as how different their ends states can be. Statistically examining the evolution of these halos is an important next step in providing insight into the origin and evolution of the Milky Way, and helping us to understand how our galaxy differs from other galaxies of similar mass. Keep an eye out for future results from this project!

Bonus

Check out this video (make sure to watch in HD!) of how the first 24 Milky-Way-like halos from the Caterpillar simulations form. Seeing these halos evolve simultaneously is an awesome way to identify the similarities and differences between them.

Citation

Brendan F. Griffen et al 2016 ApJ 818 10. doi:10.3847/0004-637X/818/1/10

1 … 85 86 87 88 89 … 96