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27 July 2016

Water Clouds in the Atmosphere of a Jupiter-Like Brown Dwarf

Lying a mere 7.2 light-years away, WISE 0855 is the nearest known planetary-mass object. This brown dwarf, a failed star just slightly more massive than Jupiter, is also the coldest known compact body outside of our solar system — and new observations have now provided us with a first look at its atmosphere.

Temperature–pressure profiles of Jupiter, WISE 0855, and what was previously the coldest extrasolar object with a 5-μm spectrum, Gl 570D. Thicker lines show the location of each object’s 5-μm photospheres. WISE 0855’s and Jupiter’s photospheres are near the point where water starts to condense out into clouds (dashed line). [Skemer et al. 2016]

Challenging Observations

With a chilly temperature of 250 K, the brown dwarf WISE 0855 is the closest thing we’ve been able to observe to a body resembling Jupiter’s ~130 K. WISE 0855 therefore presents an intriguing opportunity to directly study the atmosphere of an object whose physical characteristics are similar to our own gas giants.

But studying the atmospheric characteristics of such a body is tricky. WISE 0855 is too cold and faint to be able to obtain traditional optical or near-infrared (< 2.5 µm) spectroscopy of it. Luckily, like Jupiter, the opacity of its gas allows thermal emission from its deep atmosphere to escape through an atmospheric window around ~5 µm.

A team of scientists led by Andrew Skemer (UC Santa Cruz) set out to observe WISE 0855 in this window with the Gemini-North telescope and the Gemini Near-Infrared Spectrograph. Though WISE 0855 is five times fainter than the faintest object previously detected with ground-based 5-µm spectroscopy, the dry air of Mauna Kea (and a lot of patience!) allowed the team to obtain unprecedented spectra of this object.

WISE 0855’s spectrum shows absorption features consistent with water vapor, and it’s best fit by a cloudy brown-dwarf model. [Skemer et al. 2016]

Water Clouds Found

Exoplanets and brown dwarfs cooler than ~350 K are expected to form water ice clouds in upper atmosphere — and these clouds should be thick enough to alter the emergent spectrum that we observe. Does WISE 0855 fit this picture?

Yes! By modeling the spectrum of WISE 0855, Skemer and collaborators demonstrate that it’s completely dominated by water absorption lines. This represents the first evidence of water clouds in a body outside of our solar system.

Atmospheric Turbulence

WISE 0855’s water absorption profile bears a striking resemblance to Jupiter’s. Where the spectra differ, however, is in the lower-wavelength end of observations: Jupiter also shows absorption by a molecule called phosphine, whereas WISE 0855 doesn’t.

Jupiter’s spectrum is strikingly similar to WISE 0855’s from 4.8 to 5.2 μm, where both objects are dominated by water absorption. But from 4.5 to 4.8 μm, Jupiter’s spectrum is dominated by phosphine absorption, indicating a turbulent atmosphere, while WISE 0855’s is not. [Skemer et al. 2016]

Interestingly, if the bodies were both in equilibrium, neither WISE 0855 nor Jupiter should contain detectable phosphine in their photospheres. The reason Jupiter does is because there’s a significant amount of turbulent mixing in its atmosphere that dredges up phosphine from the planet’s hot interior. The fact that WISE 0855 has no sign of phosphine suggests its atmosphere may be much less turbulent than Jupiter’s.

These observations represent an important step as we attempt to understand the atmospheres of extrasolar bodies that are similar to our own gas-giant planets. Observations of other such bodies in the future — especially using new technology like the James Webb Space Telescope — will allow us to learn more about the dynamical and chemical processes that occur in cold atmospheres.

Citation

Andrew J. Skemer et al 2016 ApJ 826 L17. doi:10.3847/2041-8205/826/2/L17

25 July 2016

A Pulsar and a Disk

Recent, unusual X-ray observations from our galactic neighbor, the Small Magellanic Cloud, have led to an interesting model for SXP 214, a pulsar in a binary star system.

Artist’s illustration of the magnetic field lines of a pulsar, a highly magnetized, rotating neutron star. [NASA]

An Intriguing Binary

An X-ray pulsar is a magnetized, rotating neutron star in a binary system with a stellar companion. Material is fed from the companion onto the neutron star, channeled by the object’s magnetic fields onto a “hotspot” that’s millions of degrees. This hotspot rotating past our line of sight is what produces the pulsations that we observe from X-ray pulsars.

Located in the Small Magellanic Cloud, SXP 214 is a transient X-ray pulsar in a binary with a Be-type star. This star is spinning so quickly that material is thrown off of it to form a circumstellar disk.

Recently, a team of authors led by JaeSub Hong (Harvard-Smithsonian Center for Astrophysics) have presented new Chandra X-ray observations of SXP 214, tracking it for 50 ks (~14 hours) in January 2013. These observations reveal some very unexpected behavior for this pulsar.

X-ray Puzzle

The energy distribution of the X-ray emission from SXP 214 over time. Dark shades or blue colors indicate high counts, and light shades or yellow colors indicate low counts. Lower-energy X-ray emission appeared only later, after about 20 ks. [Hong et al. 2016]

Three interesting pieces of information came from the Chandra observations:

SXP 214’s rotation period was measured to be 211.5 s — an increase in the spin rate since the discovery measurement of a 214-second period. Pulsars usually spin down as they lose angular momentum over time … so what caused this one to spin up?
Its overall X-ray luminosity steadily increased over the 50 ks of observations.
Its spectrum became gradually softer (lower energy) over time; in the first 20 ks, the spectrum only consisted of hard X-ray photons above 3 keV, but after 20 ks, softer X-ray photons below 2 keV appeared.

Hong and collaborators were then left with the task of piecing together this strange behavior into a picture of what was happening with this binary system.

The authors’ proposed model for SXP 214. Here the binary has a ~30-day orbit tilted at 15° to the circumstellar disk. The pulsar passes through the circumstellar disk of its companion once per orbit. The interval marked “A” (orange line) is suggested as the period of time corresponding to the Chandra observations in this study: just as the neutron star is emerging from the disk after passing through it. [Hong et al. 2016]

Passing Through a Disk

In the model the authors propose, the pulsar is on a ~30-day eccentric orbit that takes it through the circumstellar disk of its companion once per orbit.

In this picture, the authors’ Chandra detections must have been made just as the pulsar was emerging from the circumstellar disk. The disk had initially hidden the soft X-ray emission from the pulsar, but as the pulsar emerged, that component became brighter, causing both the overall rise in X-ray counts and the shift in the spectrum to lower energies.

Since the pulsar’s accretion is fueled by material picked up as it passes through the circumstellar disk, the accretion from a recent passage through the disk likely also caused the observed spin-up to the shorter period.

If the authors’ model is correct, this series of observations of the pulsar as it emerges from the disk provides a rare opportunity to examine what happens to X-ray emission during this passage. More observations of this intriguing system can help us learn about the properties of the disk and the emission geometry of the neutron star surface.

Citation

JaeSub Hong et al 2016 ApJ 826 4. doi:10.3847/0004-637X/826/1/4

22 July 2016

The Hunt for a Counterpart to GW150914

On 14 September 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) — in a pre-operative testing state at the time — detected its first sign of gravitational-waves. The LIGO team sprang into action, performing data-quality checks on this unexpected signal. Within two days, they had sent a notification to 63 observing teams at observatories representing the entire electromagnetic spectrum, from radio to gamma-ray wavelengths.

Illustration of a binary neutron star merger. The neutron stars 1) inspiral, 2) can produce a short gamma-ray burst, 3) can fling out hot, radioactive material in the form of a “kilonova”, and 4) form a massive neutron star or black hole with a possible remnant debris disk around it. [NASA/ESA/A. Feild (STScI)]

Thus began the very first hunt for an electromagnetic counterpart to a detected gravitational wave signal.

What were they looking for?

As two compact objects in a binary system merge, the system is expected to emit energy in the form of gravitational waves. If both of the compact objects are black holes, we’re unlikely to see any electromagnetic radiation in the process, unless the merger is occurring in an (improbable) environment filled with gas and dust.

But if one or both of the two compact objects is a neutron star, then there are a number of electromagnetic signatures that could occur due to energetic outflows. If a relativistic jet forms, we could see a short gamma-ray burst and X-ray, optical, and radio afterglows. Sub-relativistic outflows could produce optical and near-infrared signals, or a radio blast wave.

Timeline of observations of GW150914, separated by wavelength band, and relative to the time of the gravitational-wave trigger. The top row shows LIGO information releases. The bottom four rows show high-energy, optical, near-infrared, and radio observations, respectively. Click for a closer look! [Abbott et al. 2016]

Surprise Signal

Since LIGO and Virgo (LIGO’s European counterpart), were primarily expecting to detect binaries involving neutron stars, they set up a notification system to be able to quickly alert electromagnetic observatories of a gravitational-wave detection. Those observatories would then be able to follow up on the gravitational-wave detectors’ rough localization, with the goal of detecting the source by its electromagnetic signature.

Given that LIGO had only just come online for testing when GW150914 was detected, it’s impressive that the pipeline was ready and there were observatories able to follow up so quickly! When the alert went out, 25 teams responded, mobilizing satellites and ground-based telescopes spanning 19 orders of magnitude in electromagnetic wavelength.

The Search Party

The only information the teams were initially given was the localization of the signal to roughly 600 square degrees on the sky. With this starting point, over the next three months, these 25 facilities carefully observed the entirety of the estimated localization area.

Footprints of observations in comparison with the initial LIGO localization of GW150914 (black contours). Shown are radio fields (red), optical/infrared fields (green), and X-ray fields (blue circles); not shown are the all-sky Fermi GBM, LAT, INTEGRAL SPI-ACS, and MAXI observations. [Abbott et al. 2016]

Some high-energy observatories, like Fermi and INTEGRAL, covered the whole sky. Many optical facilities used a tiling strategy, together covering about 900 square degrees. Still other observatories used a targeted approach, specifically looking at fields that contained a high density of nearby galaxies, in the hopes of detecting signs of a neutron-star merger or a core-collapse supernova.

For the transient sources that were found, follow-up spectroscopy and further photometry was performed, to determine if the transient could have been the source of the detected gravitational waves.

What Was the Outcome?

No electromagnetic counterpart to GW150914 was found. It turns out this isn’t surprising; GW150914 was later determined to have been the merger of two black holes, which should not generate an electromagnetic signature.

So why report on this? In the publication prepared jointly by LIGO, Virgo, and these 25 teams (with one of the longer author lists you’re likely to encounter!), the authors emphasize not the conclusion, but the process leading to it.

In spite of the fact that LIGO had not yet even begun its first observing run, the alert system worked, and the community mobilized to cover the entire 600 square degrees of sky with observations and follow-up characterization of candidate sources. If all this can be accomplished for an unexpected signal, imagine how well the system will work for future detections during actual science runs! With any luck, we’ll be identifying the electromagnetic counterparts to gravitational-wave sources soon.

Citation

B. P. Abbott et al 2016 ApJ 826 L13. doi:10.3847/2041-8205/826/1/L13

20 July 2016

Do Galactic Bars Buckle to Form Bulges?

The Milky Way is one of many galaxies that has a peanut-shaped bulge at its center. A new study has now caught two galaxies in the process of forming similar bulges, yielding insight into how ours was created.

Unstable Buckling

Artist’s illustration of the Milky Way, including the galactic bar at its center. [NASA/JPL-Caltech/ESO/R. Hurt]

Roughly 60-70% of disk galaxies in the local universe have stellar bars at the centers of their disks. Many of these — including our own galaxy — are vertically thickened in their inner regions, giving their bulges a boxy or peanut-shaped appearance in an edge-on view. We call these “B/P bulges”.

What causes B/P bulges? Twenty years of simulations of galaxy formation and evolution have pointed to an answer: galactic bars in simulations can buckle, due to a vertical instability that can occur in the bar shortly after its formation. When this asymmetric buckling eventually ends, the inner part of the bar settles into a vertically symmetric structure again: the B/P bulge.

But despite the fact that simulations predict this formation mechanism, we’ve yet to confirm it observationally. Though we’ve observed many examples in the universe of galaxies with boxy bulges that match the outcomes of the simulations, we’ve never yet caught a galactic bar in the act of buckling … until now.

Simulations vs. real galaxies after buckling

Top panel: N-body simulations showing the result after a galactic bar buckles. Bottom panels: two examples of real galaxies (NGC 3185 and NGC 3627) with B/P bulges matching simulations. [Adapted from Erwin & Debattista 2016]

Matching Observation to Simulation

Scientists Peter Erwin (Max Planck Institute for Extraterrestrial Physics, Germany) and Victor Debattista (University of Central Lancashire, UK) searched through barred disk galaxies with the Spitzer Space Telescope, looking for buckling galactic bars. Their search was successful: two galaxies, NGC 4569 and NGC 3227, have the central characteristics of buckling bars!

The authors made this identification by comparing their observations of galaxies to simulated galaxies that were undergoing bar buckling. Several characteristics — like trapezoidal bulge structure and spurs that extend symmetrically off of the long end of the trapezoid — are specifically characteristic of bars that are in the process of buckling. NGC 4569 and NGC 3227 both nicely match these morphological predictions from simulations.

Examining the stellar motions in the center of NGC 4569, Erwin and Debattista additionally find that the stellar kinematics match the specific predictions from simulations of a buckling bar as well.

Top panel: N-body simulations showing the result during the buckling of a galactic bar. Bottom panels: the two galaxies discovered in this study (NGC 4569 and NGC 3227), which show characteristics of buckling bars matching simulations. [Adapted from Erwin & Debattista 2016]

A Common Structure

Erwin and Debattista’s overall survey results indicate that B/P bulges are extremely common in high-stellar-mass galaxies: they are present in ~80% of the 44 high-stellar-mass barred-disk galaxies they examined. Based on these observations, the fraction of high-mass barred galaxies with bars in the process of buckling is estimated to be ~4.5% in the local universe.

In contrast, the authors calculate that the fraction of galaxies with buckling bars should be much higher in the earlier universe — the buckling fraction peaks at ~40% at a redshift of z = 0.7. The James Webb Space Telescope should be up to the task of detecting these galaxies, so future observations will provide a useful test of the authors’ model for B/P bulge formation.

Citation

Peter Erwin and Victor P. Debattista 2016 ApJ 825 L30. doi:10.3847/2041-8205/825/2/L30

15 July 2016

New Objects Beyond the Kuiper Belt

What has the search for the hypothetical Planet Nine led to? In the case of this study, the discovery of a collection of new — and puzzling — objects located in the outer reaches of our solar system.

Illustration of the orbits of outer-solar system bodies (with the perihelia co-located on the left for easy comparison). Includes low-eccentricity classical Kuiper belt objects (blue), moderate-eccentricity resonant Kuiper belt objects (green), and high-eccentricity, high-perihelia scattered objects (black). The yellow circle represents Neptune’s orbit. [Created 2006 using the Minor Planet Center Orbit database]

Characterizing the Outer Solar System

The Kuiper belt is a collection of small icy bodies that lies just beyond the orbit of Neptune — but it turns out that Neptune is still a major factor in the shaping of this belt.

Objects in the Kuiper belt fall broadly into two categories: those that orbit between the resonances of Neptune, and those that have been captured into those resonances, likely during Neptune’s outward migration in the past. All of these objects have low or moderate eccentricities and semimajor axes within ~48 AU, making this distance the approximate “edge” of the outer Kuiper belt.

Beyond this distance, objects tend to have much more interesting orbits. These objects have very eccentric or inclined orbits with large semimajor axes and high perihelia (> 40 AU) — and they were likely scattered into these orbits by encounters with Neptune in their past.

Perihelion vs. eccentricity for objects in the outer solar system. Red circles are objects discovered by the authors in this survey; large red circles are the objects specifically discussed in this article. These objects, which have high perihelia beyond the Kuiper Belt edge at ~48 AU but only moderate eccentricity, are likely created by a combination of resonances. [Sheppard et al. 2016]

Recently, a team of scientists made an interesting discovery while searching for new distant solar system objects (including Planet Nine): a collection of objects that don’t fit into any of these categories.

Distant Discoveries

The team, led by Scott Sheppard (Carnegie Institution for Science), conducted a series of surveys of the outer solar system with new wide-field cameras on the Subaru and Cerro Tololo Inter-American Observatory (CTIO) telescopes.

In their search, they discovered a collection of strange new objects that not only have high perihelia (q > 40 AU), but have also surprisingly low or moderate semimajor axes (50 < a < 100 AU) and eccentricities (e < 0.3). The most extreme of these objects have some of the highest perihelia of objects known in our solar system — and yet they’re not especially distant, unlike similarly high-perihelia objects like Sedna. How did they achieve these unusual orbits?

Semimajor axis vs. eccentricity for objects in the outer solar system. The dashed lines show strong mean-motion resonances with Neptune. [Sheppard et al. 2016]

Resonant Shaping

Sheppard and collaborators use the bodies’ orbital properties to speculate on their dynamical origins. The authors demonstrate that most of these bodies could have arrived at their current orbits due to a combination of two types of resonances: mean-motion resonances with Neptune (in which the objects are driven into orbits with periods that are integer multiples of Neptune’s), and Kozai resonances (in which the objects can be perturbed into higher-inclination orbits).

Based on the discovery of these new high-perihelia objects, the authors argue that a significant population of these objects likely exist. By studying their orbits, we can expect to learn more about Neptune’s history of interactions with smaller bodies, helping us to understand how this giant planet shaped the outer reaches of our solar system.

Citation

Scott S. Sheppard et al 2016 ApJ 825 L13. doi:10.3847/2041-8205/825/1/L13

13 July 2016

A Preferred Home for Disrupted Stars

Observed “burps” from the shredding of stars by supermassive black holes suggest that this behavior is more common in an unusual type of galaxy. A new study has examined NGC 3156, an example from this galaxy type, to better understand what causes this preference.

Stellar Betrayal

An artist’s illustration of a tidal disruption event, in which a star is sent on a plunging orbit near a supermassive black hole and is subsequently torn apart by the black hole’s tidal forces. [NASA/CXC/M.Weiss]

Tidal disruption events (TDEs) are events where a star plunges too close to a supermassive black hole and is torn apart by the black hole’s tidal forces. We’ve observed roughly a dozen of these violent events in the last five years, and we expect to finds hundreds to thousands more with future surveys.

TDEs are triggered when a star is sent on a plunging orbit close to a supermassive black hole. But what sends the star into harm’s way? One possible culprit is a dynamical mechanism known as two-body relaxation. In this process, stars orbiting a black hole undergo individual star–star interactions that, with a close enough encounter, can send them on plunging orbits.

Choosing an Unusual Host

One puzzle with TDEs is that they tend to be preferentially found in rather unusual galaxies: galaxies that recently exhibited a lot of star formation but are now quiescent. In particular, several of the TDEs have been discovered in what are known as “E+A galaxies,” a rare subtype of elliptical galaxy that has recently undergone a major starburst.

Since this subtype makes up only ~0.1% of all galaxies, it’s surprising that we’ve found so many TDEs in E+A galaxies so far. So why the preference?

In an effort to answer this question, two scientists, Nicholas Stone (Einstein Fellow at Columbia University) and Sjoert van Velzen (Hubble Fellow at Johns Hopkins University), have teamed up to examine a nearby E+A galaxy, NGC 3156.

Tidal disruption rates as a function of central supermassive-black-hole mass. The blue curve shows the authors’ model for NGC 3156 (assuming different masses for its central black hole), and the black curve shows the power-law best fit for a large galaxy sample. NGC 3156’s predicted TDE rate is an order of magnitude higher that of a typical galaxy. [Stone & van Velzen 2016]

Collisions in a Crowded Nucleus

By analyzing Hubble Space Telescope photometry of NGC 3156, Stone and van Velzen determine that there is an overdensity of stars in the central region of the galaxy — which is expected to be the case for all E+A galaxies, due to their starburst history. The authors next use their measurements and arguments of stellar density and dynamics to calculate a predicted rate of two-body star–star interactions that lead to TDEs.

Stone and van Velzen predict that TDEs from two-body interactions should occur at a rate of ~10^-3per year in NGC 3156. This is an order of magnitude larger than the rate that the same calculations would predict for a typical galaxy (10^-4per year).

The authors’ observations and analysis of NGC 3156 strongly support the idea that E+A galaxies overproduce TDEs because their very dense centers — created by past starbursts — provide a highly collisional environment, allowing more star–star interactions. These interactions can then lead to stellar orbits that plunge near the supermassive black hole at the galactic center, producing TDEs.

Citation

Nicholas C. Stone and Sjoert van Velzen 2016 ApJ 825 L14. doi:10.3847/2041-8205/825/1/L14

11 July 2016

A History of Collisions Between the Magellanic Clouds

Recent deep observations of the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way, have revealed a faint arc of stars extending from its northern outskirts. Was this stream created by the gravitational pull of the Milky Way? Or could it have a more violent source?

The area surrounding the LMC. The stellar arc discovered with the Dark Energy Survey is shown in the region labeled A. The current study discovered additional asymmetric substructure in the region labeled C. [Besla et al. 2016]

Searching for Spiral Structure

When deep optical imaging by the Dark Energy Survey discovered this faint stream of stars extending eastward from the northern periphery of the LMC, scientists’ assumption was that this arm was created by the tidal pull of the Milky Way.

But a team of authors led by Gurtina Besla (University of Arizona) argue for an alternate theory: what if this stellar stream was instead caused by repeated interactions between the LMC and the Small Magellanic Cloud (SMC)?

One way to test these models is to look for a symmetrically corresponding arm in the south of the LMC extending west; such an arm would be expected if tidal forces from the Milky Way were acting globally on the LMC to create the northeast arm.

The Dark Energy Survey’s footprint doesn’t cover the southern regions of the LMC’s disk, but Besla and collaborators have an alternative: they performed their own wide-field survey using small robotic telescopes, which provide long exposures at low cost.

Modeling Past and Future Interactions

The simulated interaction history of the LMC and SMC in isolation (i.e., without the Milky Way). The top left panel shows the SMC–LMC separation as a function of time; the remaining panels show the system at different stages of the simulation. Only particles associated with the LMC are shown here; the SMC’s position is indicated by a blue star. [Besla et al. 2016]

The team’s deep optical observations of the LMC and SMC fields confirmed the presence of asymmetric stellar arc structures in the northern outskirts of the LMC — and they didn’t find any corresponding structures in the southern region. This strongly supports the idea that the structures were caused by interactions between the LMC and the SMC, rather than by galactic tides.

To further test this model, Besla and collaborators ran a series of simulations of interactions between LMC and SMC, first in isolation and then with the added tidal forces from the Milky Way.

The simulations supported the conclusions drawn from the observations: while Milky Way tides may influence the final distribution of structures in the LMC’s outskirts, close interactions between the LMC and the SMC appear to be the primary cause responsible for the asymmetric spiral structure found.

As is shown in the authors’ simulations, the complete model of LMC/SMC interactions predicts that the two dwarfs will continue to interact until they eventually merge. Comparison of detailed simulations with future high-resolution observations of the LMC should help us further understand the interaction history of the LMC and SMC, thereby allowing us to better predict their eventual fate.

Bonus

Check out the gif below, cut from a video of the authors’ simulations. In these simulations, the SMC interacts with the LMC over the span of ~9 Gyr, passing through it several times before the LMC completely cannibalizes the SMC. You can visit the authors’ article to view the original video.

Citation

Gurtina Besla et al 2016 ApJ 825 20. doi:10.3847/0004-637X/825/1/20

8 July 2016

Forming Planets in the Hostile Carina Nebula

Can protoplanetary disks form and be maintained around low-mass stars in the harsh environment of a highly active, star-forming nebula? A recent study examines the Carina nebula to answer this question.

Crowded Clusters

Stars are often born in clusters that contain both massive and low-mass stars. The most massive stars in these clusters emit far-ultraviolet and extreme-ultraviolet light that irradiates the region around them, turning the surrounding area into a hostile environment for potential planet formation.

Planet formation from protoplanetary disks typically requires timescales of at least 1–2 million years. Could the harsh radiation from massive stars destroy the protoplanetary disks around low-mass stars by photoevaporation before planets even have a chance to form?

Artist’s impression of a protoplanetary disk. Such disks can be photoevaporated by harsh ultraviolet light from nearby massive stars, causing the disk to be destroyed before planets have a chance to form within them. [ESO/L. Calçada]

Turning ALMA Toward Carina

A perfect case study for exploring hostile environments is the Carina nebula, located about 7500 lightyears away — and home to nearly 100 O-type stars as well as tens of thousands of lower-mass young stars. The Carina population is ~1–4 Myr old: old enough to form planets within protoplanetary disks, but also old enough that photoevaporation could already have wreaked havoc on those disks.

Due to the dense stellar populations in Carina’s clusters, this is a difficult region to explore, but the Atacama Large Millimeter-submillimeter Array (ALMA) is up to the task. In a recent study, a team of scientists led by Adal Mesa-Delgado (Pontifical Catholic University of Chile) made use of ALMA’s high spatial resolution to image four regions spaced throughout Carina, searching for protoplanetary disks.

Detections and Non-Detections

Two evaporating gas globules in the Carina nebula, 104-593 and 105-600, that each contain a protoplanetary disk. The top panels are Hubble images of the globules; the bottom panels are ALMA images of the disks detected within them. [Mesa-Delgado et al. 2016]

In searching regions outside of the densest, most luminous clusters, the team succeeded in detecting two protoplanetary disks. This region in Carina now marks the most distant massive cluster in which disks have ever been imaged! The discovered disks have radii of roughly 60 AU and masses of 30 and 50 Jupiter masses — and given their ages, it’s entirely plausible that planets are actively forming in these disks.

Equally important: Mesa-Delgado and collaborators failed to detect any indication of disks in the core of Trumpler 14, a cluster in Carina that is home to some of the most massive and luminous stars in the Galaxy. This non-detection suggests that the particularly harsh environment of Trumpler 14 is too brutal for disks within it to survive.

These observations provide new clues as to where we should be looking to study planet formation: less dense regions in star-forming nebulae seem to be locations that can support giant-planet-forming disks, whereas the harsh radiation fields of especially dense subclusters seem to cause the rapid destruction of such disks.

Citation

A. Mesa-Delgado et al 2016 ApJ 825 L16. doi:10.3847/2041-8205/825/1/L16

6 July 2016

A Different Way to Visualize Solar Changes

This time series of SDO images of an active region shows coronal dimming as well as flares. These images can be combined into a minimum-value persistence map (bottom panel) that better reveals the entire dimming region. [Adapted from Thompson & Young 2016]

What if there were a better way to analyze a comet’s tail, the dimming of the Sun’s surface, or the path of material in a bright solar eruption? A recent study examines a new technique for looking at these evolving features.

Mapping Evolving Features

Sometimes interesting advances in astronomy come from simple, creative new approaches to analyzing old data. Such is the case in a new study by Barbara Thompson and Alex Young (NASA Goddard Space Flight Center), which introduces a technique called “persistence mapping” to better examine solar phenomena whose dynamic natures make them difficult to analyze.

What is a persistence map? Suppose you have a set of N images of the same spatial region, with each image taken at a different time. To create a persistence map of these images, you would combine this set of images by retaining only the most extreme (for example, the maximum) value for each pixel, throwing away the remaining N-1 values for each pixel.

Persistence mapping is especially useful for bringing out rare or intermittent phenomena — features that would often be washed out if the images were combined in a sum or average instead. Thompson and Young describe three example cases where persistence mapping brings something new to the table.

Top: Single SDO image of Comet Lovejoy. Center: 17 minutes of SDO images, combined in a persistence map. The structure of the tail is now clearly visible. Bottom: For comparison, the average pixel value for this sequence of images. Click for a closer look! [Thompson & Young 2016]

A Comet’s Tail

As Comet Lovejoy passed through the solar corona in 2011, solar physicists analyzed extreme ultraviolet images of its tail — because the motion of the tail particles reveals information about the local coronal magnetic field.

Past analyses have averaged or summed images of the comet in orbit to examine its tail. But a persistence map of the maximum pixel values far more clearly shows the striations within the tail that reveal the directions of the local magnetic field lines.

Dimming of the Sun

Dimming of the Sun’s corona near active regions tells us about the material that’s evacuated during coronal mass ejections. This process can be complex: regions dim at different times, and flares sometimes hide the dimming, making it difficult to observe. But understanding the entire dimming region is necessary to infer the total mass loss and complete magnetic footprint of a gradual eruption from the Sun’s surface.

SDO and STEREO-A images of a prominence eruption. Tracking the falling material is difficult due to the complex background. [Thompson & Young 2016]

Creating a persistence map of minimum pixel values achieves this — and also neatly sidesteps the problem of flares hiding the dimming regions, since the bright pixels are discarded. In the authors’ example, a persistence map estimates 50% more mass loss for a coronal dimming event than the traditional image analysis method, and it reveals connections between dimming regions that were previously missed.

An Erupting Prominence

The authors’ final example is of falling prominence material after a solar eruption, seen in absorption against the bright corona. They show that you can construct a persistence map of minimum pixel values over the time the material falls (see the cover image), allowing the material’s paths to be tracked despite the evolving background behind it. Tracing these trajectories provides information about the local magnetic field.

Thompson and Young’s examples indicate that persistence mapping clearly provides new information in some cases of intermittent or slowly evolving solar phenomena. It will be interesting to see where else this technique can be applied!

Citation

B. J. Thompson and C. A. Young 2016 ApJ 825 27. doi:10.3847/0004-637X/825/1/27

1 July 2016

Forming Spirals From Shadows

What causes the large-scale spiral structures found in some protoplanetary disks? Most models assume they’re created by newly-forming planets, but a new study suggests that planets might have nothing to do with it.

Perturbations from Planets?

In some transition disks — protoplanetary disks with gaps in their inner regions — we’ve directly imaged large-scale spiral arms. Many theories currently attribute the formation of these structures to young planets: either the direct perturbations of a planet embedded in the disk cause the spirals, or they’re indirectly caused by the orbit of a planetary body outside of the arms.

Another example of spiral arms detected in a protoplanetary disk, MWC 758. [NASA/ESA/ESO/M. Benisty et al.]

But what if you could get spirals without any planets? A team of scientists led by Matías Montesinos (University of Chile) have recently published a study in which they examine what happens to a shadowed protoplanetary disk.

Casting Shadows with Warps

In the team’s setup, they envision a protoplanetary disk that is warped: the inner region is slightly tilted relative to the outer region. As the central star casts light out over its protoplanetary disk, this disk warping would cause some regions of the disk to be shaded in a way that isn’t axially symmetric — with potentially interesting implications.

Montesinos and collaborators ran 2D hydrodynamics simulations to determine what happens to the motion of particles within the disk when they pass in and out of the shadowed regions. Since the shadowed regions are significantly colder than the illuminated disk, the pressure in these regions is much lower. Particles are therefore accelerated and decelerated as they pass through these regions, and the lack of axial symmetry causes spiral density waves to form in the disk as a result.

Initial profile for the stellar heating rate per unit area for one of the authors’ simulations. The regions shadowed as a result of the disk warp subtend 0.5 radians each (shown on the left and right sides of the disks here). [Montesinos et al. 2016]

Observations of Shadow Spirals

In the authors’ models, two shadowed regions result in the formation of two spiral arms. The arms that develop start at a pitch angle of 15°–22°, and gradually evolve to a shallower 11°–14° pitch at distances of ~65–150 AU.

The more luminous the central star, the more quickly the spiral arms form, due to the greater contrast between illuminated and shadowed disk regions: for a 0.25 solar-mass disk illuminated by a 1 solar-luminosity star, arms start to form after about 2500 orbits. If we increase the star’s brightness to 100 solar luminosities, the arms form after only 150 orbits.

Montesinos and collaborators conclude by testing whether or not such spiral structures would be observable. They use a 3D radiative transfer code to produce scattered-light predictions of what the disk would look like to direct-imaging telescopes. They find that these shadow-induced spirals should be detectable.

This first study clearly demonstrates that large-scale spiral density waves can form in protoplanetary disks without the presence of planets. The authors now plan to add more detailed physics to their models to better understand what we might observe when looking at systems that were shaped in this way.

Density evolution in two shadowed disks. Top row: disk illuminated by a 100 L⊙ star, at 150, 250, and 500 orbits (from left to right). Bottom row: disk illuminated by a 1 L⊙ star, at 2500, 3500, and 4000 orbits. The rightmost top and bottom panels show control simulations (no shadows were present on the disk) after 1000 and 6000 orbits. (A different type of spiral starts to develop in the bottom control simulation as a result of a gravitational instability, but it never extends to the edges of the disk.) [Montesinos et al. 2016]

Citation

Matías Montesinos et al 2016 ApJ 823 L8. doi:10.3847/2041-8205/823/1/L8

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