Features RSS

Milky Way Center

What model of dark matter best describes our universe? A new study uses a unique region in our own galaxy to constrain one particular model: that of fuzzy dark matter.

A Matter of Modeling

dark matter

There are many models describing the composition and behavior of dark matter, and how its evolution has affected the structure of our universe. [AMNH]

Observations of our universe tell us that only 15% of the universe’s matter is the ordinary baryonic matter that we’re able to see. The remaining 85% is dark matter — mysterious material that has shaped the structure and evolution of our universe via its gravitational interactions, but that doesn’t give off any light.

Because we can’t directly observe it, dark matter is still a relative unknown — and there are many different hypothesized models that describe its nature. Is dark matter hot? Cold? Composed of subatomic particles? Or macroscopic objects like primordial black holes? There’s a model for all of these options, and the best way to test them is to compare their predictions to the actual structure that we observe.

nuclear bulge only

Plot of gas surface density from a simulation showing the formation of the CMZ — seen as the high-density gas ring at the heart of the plot — in the center of the Milky Way. This simulation included a nuclear bulge only, with no dark-matter core from the fuzzy dark matter model. [Li et al. 2020]

Constraints from an Odd Structure

One such constraining structure is a unique region in our own galaxy: the Central Molecular Zone, or CMZ. This extremely dense, rich collection of orbiting molecular gas lies in the very center of the Milky Way and spans just a few hundred light-years in diameter. Observations suggest that the molecular gas clouds orbit in a ring or a disk with a twisted 3D shape, but the thick dust that shrouds the galactic center limits what we can learn about the CMZ directly.

The CMZ’s shape is not its only mystery, however: we also don’t fully understand what caused this odd structure to develop. Past studies of the birth of our galaxy’s structure from a thin disk suggest that formation of the CMZ relies on a combination of the Milky Way’s barred gravitational potential and an especially dense nuclear region.

In a new publication led by Zhi Li (Shanghai Jiao Tong University, China), a team of scientists has now used this picture to constrain a dark matter model that relies on light dark-matter particles concentrated at the center of the galaxy.

Adding Fuzziness to the Milky Way

nuclear bulge + soliton core

Zoomed-in plot of gas surface density from a simulation showing the formation of the CMZ in the center of the Milky Way. This simulation included both a nuclear bulge and a dark-matter core from the fuzzy dark matter model. [Adapted from Li et al. 2020]

Li and collaborators conduct a series of cosmological simulations that model the formation of the Milky Way from a thin disk in a realistic gravitational potential. In some of these simulations, the authors include only a dense nuclear bulge at the center of the galaxy. In others, they also add a galaxy core consistent with the predictions of fuzzy dark matter, a model that describes the universe’s dark matter as very light bosons that exhibit wave behavior on some scales.

The authors show that the structure and dynamics of the CMZ can be reproduced well with only an exceedingly compact nuclear bulge. But the combination of a smaller nuclear bulge and a fuzzy-dark-matter core also neatly reproduces observations, leaving the door open for this dark-matter model.

So is our dark matter fuzzy or not? We can’t tell yet, but Li and collaborators outline some future observations — like pinning down the mass-to-light ratio in the galactic center — that will help us answer this question and better understand what’s going on with that invisible 85% of our universe’s matter.

Citation

“Testing the Prediction of Fuzzy Dark Matter Theory in the Milky Way Center,” Zhi Li et al 2020 ApJ 889 88. doi:10.3847/1538-4357/ab6598

TESS

This week, scientists are celebrating the recent conclusion of the primary mission for the Transiting Exoplanet Survey Satellite (TESS). On Monday, we talked about TESS’s ongoing contributions to exoplanet science. But what else has this spacecraft been up to? Today we’re exploring its addition to the mystery of a puzzling white dwarf.

Puzzling Pollution

G29-38 Debris disc

GD 394 was one of the earliest metal-rich white dwarfs discovered, though others with varying explanations have been detected more recently. In the illustration above, a disintegrating planetesimal accretes onto a white dwarf. [NASA/JPL-Caltech]

During TESS’s primary two-year mission, the spacecraft monitored 200,000 stars, discovering more than 2,000 planet candidates. But other targets also fell under the telescope’s scrutiny — including GD 394, an unusual white dwarf located less than 200 light-years away.

GD 394 has challenged our expectations for white dwarfs ever since its discovery in the 1960s. A white dwarf — a hot, dense stellar remnant — has such a strong gravitational pull that heavier elements are expected to rapidly sink to the white dwarf’s center, leaving only lightweight hydrogen, helium, and sometimes carbon and oxygen for us to identify in its atmosphere.

But early observations of GD 394 revealed an object unexpectedly polluted with heavier metals like silicon and iron in its atmosphere. To explain this, GD 394 would need to be actively and continuously accreting fresh, metal-rich material from some external source that we couldn’t detect.

EUV light curve

This phase-folded EUV light curve — constructed from observations of GD 394 in the mid-1990s made with the Extreme Ultraviolet Explorer satellite — show the strong dip in EUV flux over a 1.15-day period. [Adapted from Wilson et al. 2020]

An Unsteady Source

But GD 394’s metals were only part of its mystery. In the mid-1990s, observations showed that this white dwarf’s extreme ultraviolet (EUV) emission wasn’t steady — it varied in intensity by about 25% with a rapid period of about 1.15 days.

What could cause this variability? The answer remains unclear, as efforts to test various hypotheses have all come up short. A recent study of Hubble observations of GD 394, for instance, found the star’s far-ultraviolet light curve to be steady to within 1%. Had the variability’s source vanished in the last several decades? Or is the variability caused only in EUV light, and not at longer wavelengths?

In a recent publication, scientist David Wilson (McDonald Observatory, University of Texas at Austin) and collaborators now have presented new TESS observations at optical wavelengths that have further deepened the mystery of GD 394.

More Clues, But No Answers Yet

optical light curve

The TESS light curve for GD 394, folded onto the fitted period, shows the subtle sinusoidal variability at optical wavelengths. [Adapted from Wilson et al. 2020]

Wilson and collaborators monitored GD 394 for 52 days across two TESS sectors. From these observations, they identify clear evidence of a ~1.15-day period in the dwarf’s optical light curve — the first evidence of variability for GD 394 outside of EUV wavelengths. Compared to the EUV variability, however, the optical variability is tiny, at a level of just 0.12%! Only TESS’s high sensitivity allowed us to detect this subtle signal.

So what’s causing the long-lived variation of this star? Leading theories that could fit both sets of observations include metal spots caused by channeled accretion, occultation by an outflow from an orbiting (but not transiting) planet, and a magnetically induced hot spot.

Future simultaneous observing with an EUV telescope and a sensitive optical observatory like TESS could help us to differentiate between these models. In the meantime, TESS has cheerfully begun its extended mission — hunting for more planets, characterizing more stars, and exploring more cosmic mysteries.

Citation

“Optical Detection of the 1.1 day Variability at the White Dwarf GD 394 with TESS,” David J. Wilson et al 2020 ApJL 897 L31. doi:10.3847/2041-8213/ab9d7b

gas giant exoplanet

Over the past 25 years, we’ve found thousands of worlds beyond our solar system. Nonetheless, some categories of exoplanets remain elusive — for instance, planets that orbit their hosts on long, slow paths. A new study shows how we might hunt these worlds down.

Observational Limits

gas-giant transit

Artist’s impression of a hot Jupiter transiting across the face of its host star. [ESA/C. Carreau]

Since the first exoplanet discovery a quarter century ago, we’ve found more than 4,000 confirmed planets orbiting other stars. A large number of these discoveries are planets that transit across the face of their host star — most identified by the Kepler Space Telescope or, more recently, by the Transiting Exoplanet Survey Satellite (TESS). These exoplanets are valuable targets because we can use the transits to measure properties like their radii, densities, bulk compositions, and even their atmospheres.

Unfortunately, due to the nature of transit detections, our observations are inherently biased: it’s easier to detect and confirm short-period, large planets, which means we know a lot about hot Jupiters, but relatively little about wide-orbit, cooler planets.

Because the TESS spacecraft observes a typical region for less than a month, planets on wide orbits longer than 30 days will register — at most — a single transit in TESS data before the telescope moves on to the next section of sky. But can we somehow leverage these single transits to learn more about slow, wide-orbit planets?

Avoiding Overbooking

A planet candidate with only one transit detection could be confirmed with radial-velocity measurements of its host star. But high-precision radial-velocity instruments are in high demand! Without precise knowledge of a planet’s period, confirming that planet’s existence and measuring its properties would require a huge observational time investment from already overbooked radial-velocity instruments.

Fortunately, a team of scientists led by Samuel Gill (University of Warwick, UK) has now demonstrated a more efficient way of pinning down a planet after just one TESS transit.

NGTS-11 light curve

Light curve showing the best fit to the transit photometry of NGTS-11 from TESS and NGTS. [Gill et al. 2020]

Two Transits, Fewer Options

Gill and collaborators followed up on a planet candidate that had registered a single TESS transit in September 2018. The team conducted an intense photometric monitoring campaign of the candidate’s host star using the economical telescopes of the Next-Generation Transit Survey (NGTS) facility in Chile — and with 79 nights of observations, they detected a second transit of the candidate 390 days after the initial TESS transit.

From the combined photometric observations, the authors were able to narrow down the possible periods for the planet to just 13 options. Using these constraints, they could then wrap up with brief and efficient radial-velocity measurements to identify the properties of the planet.

A Cool Discovery

NGTS

The array of twelve 0.2-meter robotic telescopes of the NGTS facility in Chile. Only one of these economical telescopes was needed to identify a second transit of NGTS-11 b. [ESO/R. West]

The result? NGTS-11 b (or TOI-1847 b), as the confirmed planet is now named, is a Saturn-mass planet on a wide, ~35-day orbit. With an equilibrium temperature of just 435 K, NGTS-11 b is one of the coolest known transiting gas giants and a valuable target for future transmission spectroscopy to explore its atmosphere.

Gill and collaborators’ identification of NGTS-11 b from just a single TESS transit shows the power of using ground-based photometry to pin down wide-orbit planets. This approach could help us to dramatically expand our understanding of the slower, long-period worlds beyond our solar system.

Citation

“NGTS-11 b (TOI-1847 b): A Transiting Warm Saturn Recovered from a TESS Single-transit Event,” Samuel Gill et al 2020 ApJL 898 L11. doi:10.3847/2041-8213/ab9eb9

starburst galaxy MCG+07-33-027

The flow of gas into and out of a galaxy regulates many of its features, such as its rate of star formation and chemical content. Computer simulations have allowed us to probe these gas flows in greater detail, but there still remains much to uncover. For example, on what scales does gas flow out of actively star-forming galaxies?

Outlining Galactic Outflows

Currently, computer simulations model the flows of gas leaving a galaxy, called galactic outflows, very generally. They are typically informed either by observations or large-scale physics. However, galactic outflows need to be modeled with much finer detail to grasp their full impact on galaxies, which means that observations of outflows need to probe much smaller scales than they currently do.

galaxy J1157

A near-ultraviolet image of the galaxy J1157 taken by the Hubble Space Telescope’s Cosmic Origins Spectrograph in acquisition mode. The white circle indicates the extent of the spectrograph’s imaging capability. [Wang et al. 2020]

Typically, outflows are explored via absorption spectroscopy, which utilizes the fact that the spectra of light shining through a cloud of material will be imprinted with features unique to the cloud’s material. But this technique is only useful when we’re looking “down the barrel” of an outflow such that the outflow absorbs the light behind it — and this configuration prevents us from usefully probing the outflow size.

To study outflow sizes, we need the flip side of absorption: emission. The material spewed out in galactic outflows fluoresces — it absorbs light at specific wavelengths and reemits it at longer wavelengths. A group of researchers led by Bingjie Wang (Johns Hopkins University) traced this emitted radiation from outflows in nearby starburst galaxies using data taken by the Hubble Space Telescope (HST). They then used this emission to explore the sizes of the outflows.

How to Trace an Outflow

Starburst galaxies have much higher rates of star formation than the average galaxy (for example, the Milky Way). This means that their outflows will also be much more prominent, along with the emission used to trace the outflows.

outflow column density

The ratio between the column density of the material within the radius studied by the Cosmic Origins Spectrograph (COS) and the total column density, plotted versus 𝛂, which quantifies how the density of outflow material depends on radius. The color bar shows the ratio between the radius of COS’s observations and the galaxy radius. The galaxies studied are plotted as well, showing that for this sample, 𝛂 is between 1 and 1.5. This means that the outflow contains more material further away from the galaxy. [Wang et al. 2020]

By studying fluorescence emission lines, Wang and collaborators could focus on the material being swept out of the galaxy in an outflow. Comparing the strength of these fluorescence emission lines to the tracers typically used — resonance absorption lines — allowed them to then make inferences about the structure of the outflows and determine whether we’ve been interpreting them correctly.

Line, Please!

Wang and collaborators found that the fluorescence lines are systematically weaker than the resonance absorption lines. This suggests that the fluorescence lines produced within the HST’s view are more often associated with the galaxy’s stationary interstellar medium or the slow-moving central parts of outflows, rather than tracing the fast-moving material in the outer regions of the outflows.

So where are the strong fluorescence lines? Wang and collaborators suggest that since outflows contain more material the further they are from the galaxy, the fluorescence would also appear further away from a galaxy’s center. However, the HST instrument used for this study is not equipped to observe very large areas, meaning that the regions with strong fluorescence likely lie outside of the instrument’s field of view and are being excluded from the data.

This result suggests that outflow rates for distant galaxies are significantly underestimated with the technique we’ve been using to measure them, and they do not account for changing strength with distance from the galaxy. One thing’s for certain: we’re going to need a bigger telescope.

Citation:

“A Systematic Study of Galactic Outflows via Fluorescence Emission: Implications for Their Size and Structure,” Bingjie Wang et al 2020 ApJ 894 149. doi:10.3847/1538-4357/ab88b4

protoplanetary disk illustration

How did our solar system’s planets first form within the swirling disk of gas and dust that surrounded the newborn Sun? One of the best ways to answer this question is watch other solar systems as they form — and the Atacama Large Millimeter/submillimeter Array (ALMA) continues to help us do so.

A History of Large Targets

full DSHARP gallery

Gallery of 240 GHz (1.25 mm) continuum emission images for the disks in the DSHARP sample. The scale bars in the lower right of each image indicate 10 au. Click to enlarge. [Andrews et al. 2018]

When ALMA revealed early observations of the disks of gas and dust around young stars, we were stunned by the exquisitely detailed look this interferometer provided into newborn solar systems. Since then, again and again, ALMA has produced remarkable images of gaps, rings, and spiral arms in disks, all of which hint at how planets might be forming.

Thus far, however, we’ve mostly focused on imaging the especially large disks that give us the best look at disk substructure. As an example, the Disk Substructures at High Angular Resolution Project (DSHARP) survey used ALMA to image twenty large, bright disks with effective radii — the radius that encompasses 68% of the light from the dusty disk — of ~50 au on average.

But the vast majority of disks are faint, and their dusty disks are much more compact, with effective radii of less than 20 au. Do these more typical disks show the same wealth of substructures that we’ve spotted in larger disks? And what can this tell us about planet formation?

GQ Lup disk

ALMA continuum image of the GQ Lup disk. The scale bar at the lower right indicates 5 au. [Adapted from Long et al. 2020]

Stopping the Migration

In a new study led by Deryl Long (University of Michigan), a team of scientists has used ALMA to explore one of these more typical systems: a compact disk with effective radius of 19 au around the star GQ Lup A. The high angular resolution of ALMA’s observations allow the team to resolve the dust emission even in this small disk, revealing a wealth of substructures very similar to those spotted in larger disks.

What do these substructures tell us? One challenge to planet formation theories is that larger dust grains should migrate inward through the disk in a process called radial drift, accreting onto the star before they can clump together to form planetesimals.

Long and collaborators’ observations of the GQ Lup system suggest that pebble-sized dust grains can be trapped by variations in pressure in the disk, halting the grains’ drift and giving them a chance to clump. This means that small disks may have the same opportunity as large disks to form young planets.

The Birth of a Solar System

disk size v. luminosity

Size-luminosity relationship for millimeter continuum sources with disk properties. DSHARP disks are marked with green circles. The new observations of GQ Lup (red diamond) move us toward a discovery space of smaller disks. [Long et al. 2020]

Long and collaborators caution that there could be other explanations for the disk substructures we see in GQ Lup. But if they are indicators of planet formation, this disk provides us with a unique opportunity to learn more about how our own solar system formed.

One of the disk gaps identified by the authors lies at ~10 au, which is roughly the distance of Saturn from our Sun. Studying GQ Lup could therefore reveal how planets like Saturn develop from a dusty disk. What’s more, these observations of GQ Lup indicate that there may be a rich population of young solar-system analogs out there, just awaiting discovery and exploration.

Citation

“Hints of a Population of Solar System Analog Planets from ALMA,” Deryl E. Long et al 2020 ApJL 895 L46. doi:10.3847/2041-8213/ab94a8

quasar

Our universe is filled with distant supermassive black holes that feed on surrounding gas and dust, emitting bright gamma-ray radiation. A new study explores how many of these show periodic patterns in the variations of their high-energy light.

Putting on a Show

At the center of every galaxy lies a supermassive black hole of millions to billions of solar masses. Many of these — like the Milky Way’s own Sagittarius A* — are quiet, largely invisible lurkers. But some are considerably more attention-seeking, actively accreting material, spewing out winds and jets, and emitting radiation that spans the electromagnetic spectrum.

These active galactic nuclei (AGNs) show variability in their light curves on many different timescales — and a few have been caught flickering in a regular pattern.

Why the Patterns?

SMBH binary

Artist’s impression of supermassive black holes that have formed a binary as they’ve sunk to the center of their merged galaxies. [NAOJ]

There are lots of potential explanations for periodic variability in an AGN’s light curve. The black hole may host powerful jets that precess about an axis, driving a lighthouse effect in their emission. The flow of accreting material onto the black hole might wax and wane periodically. And in some cases, the emission we observe might even be coming from a pair of black holes: a supermassive black hole binary whose orbital motion causes periodic patterns.

To better understand the nature of these distant, dramatic sources, we need to study the AGN that show regular variability. But, though a number of studies have explored individual variable AGN, we haven’t yet conducted a large, systematic search for periodic patterns in high-energy AGN light curves. That is, until now.

A Sample of Regular Feeders

A team of scientists led by Pablo Peñil (Complutense University of Madrid, Spain) has leveraged a powerful tool: the Fermi Large Area Telescope (LAT), a space-based gamma-ray detector with a wide field of view that has been scanning the sky consistently for more than a decade. In that time, Fermi LAT has tracked the gamma-ray emission from thousands of AGN, providing Peñil and collaborators with a wealth of data they use to systematically hunt for patterns.

periodic AGN sky map

Sky map showing the locations of the 11 sources with periodic emission (filled symbols) and 13 sources with lower significance (open symbols). Click to enlarge. [Peñil et al. 2020]

By applying ten different algorithms that identify periodicity, the authors discover 11 AGN that robustly show patterns in their variability that repeat on roughly 1- to 5-year timescales, as well as another 13 AGN that show strong hints of periodicity but can’t be confirmed yet with the data we have. The authors’ 11 robust detections include two AGN with previously detected variability and nine new identifications.

Peñil and collaborators’ non-detections are also interesting. There were a number of AGN that, in previous individual studies, were reported to show periodic variability — yet they did not show statistically significant periodicity in the authors’ systematic analysis.

What’s Ahead

So now that we’ve identified periodically variable AGN, what can we do with them? The authors’ consistently identified sample can now be studied to better understand what causes the variability of these sources. We can also use the predictable patterns of these AGNs’ high-energy emission to efficiently schedule follow-up observations with telescopes that have more limited fields of view and observing duty cycles.

And this is just the start! Another few years of observation with Fermi LAT should greatly expand the sample of AGN identified with periodic variability, providing even more insight into the feeding behavior of these distant monsters.

Citation

“Systematic Search for γ-Ray Periodicity in Active Galactic Nuclei Detected by the Fermi Large Area Telescope,” P. Peñil et al 2020 ApJ 896 134. doi:10.3847/1538-4357/ab910d

magnetar

Pulsars have historically been classified into different categories — but the distinction between them may be blurrier than we thought. The discovery of the youngest pulsar yet observed is now raising questions about how we classify these extreme objects.

The Source of a Pulsar’s Power

black widow pulsar

Artist’s illustration of an accretion-powered pulsar (left) and its small stellar companion (right), viewed within their orbital plane. [NASA Goddard SFC/Cruz deWilde]

When a massive star explodes as a supernova at the end of its lifetime, an incredibly dense remnant with the mass of one or two Suns — but spanning only 20 km or so in diameter — is left behind. If this resulting neutron star is powerfully magnetized, it can emit a beam of radiation that sweeps across the Earth as the star spins, appearing to us as a pulsar.

The pulsars that we’ve observed are classified into three categories based on what we think powers their emission:

  1. Rotation-powered pulsars
    Usually detected from their pulsed radio emission, this is the most commonly observed type of pulsar. These rapidly rotating stars gradually spin down over time. Their lost rotational energy powers the particle acceleration that produces the emission we observe.
  2. Accretion-powered pulsars
    These pulsars occur in binaries and accrete matter from their companion stars. Pulsed X-ray radiation is produced by rotating hot spots caused when the accretion flow strikes the surface of the pulsar.
  3. Magnetically-powered pulsars
    These bodies, known as magnetars, are the most magnetized objects in the universe, sporting magnetic fields of around 1014–1015 Gauss (compare this to Earth’s magnetic field, which is less than one Gauss!). The decay of their unstable magnetic field powers the emission of high-energy radiation, particularly at X-ray and gamma-ray wavelengths.

But what if these pulsar categories aren’t as distinct as we think they are? Observations of a very recently born pulsar, described in a publication led by Paolo Esposito (Scuola Superiore IUSS and INAF, Italy), are now challenging our classifications.

Swift J1818

The source Swift J1818, as observed by the XMM-Newton spacecraft. [Adapted from Esposito et al. 2020]

Neither Here Nor There

The source Swift J1818.0–1607 was first discovered in March 2020 as a flaring outburst of X-ray radiation. Esposito and collaborators present X-ray observations of the source using the Swift Observatory, XMM-Newton, and NuSTAR, all of which paint the picture of an incredibly young — just 240 years, a relative baby on cosmic scales! — magnetar undergoing an outburst.

Swift J1818 radio pulse

Profile of a bright radio pulse from the source Swift J1818, as observed by the Sardinia Radio Telescope. [Adapted from Esposito et al. 2020]

But Swift J1818 has its quirks. Of the roughly 30 magnetars we’ve discovered, Swift J1818 spins faster than any of them, with a period of just 1.36 seconds. Its quiescent luminosity is lower than we’d expect given its young age. And follow-up radio observations with the Sardinia Radio Telescope in Italy reveal that Swift J1818 also exhibits the strong and short radio pulses expected for a rotation-powered pulsar.

Esposito and collaborators’ observations lead them to conclude that Swift J1818 is a peculiar magnetar with properties that straddle those of rotationally and magnetically powered pulsars. This makes this newborn the latest in a small collection of oddball young neutron stars with diverse properties, suggesting that there may still be much we don’t know about the driving forces behind pulsar emission, and how this changes over a pulsar’s lifetime.

Citation

“A Very Young Radio-loud Magnetar,” P. Esposito et al 2020 ApJL 896 L30. doi:10.3847/2041-8213/ab9742

WASP-121b artist impression

With atmospheric temperatures ranging from roughly 3,000 to 6,500 degrees Fahrenheit, ultra-hot Jupiters are ready-made laboratories for extreme planetary science. For instance, any molecules in the atmosphere of an ultra-hot Jupiter will be broken down into their component atoms and ions. So what can be found in the atmosphere of the ultra-hot Jupiter WASP-121 b?

Laboratories for Extreme Science

Kelt-9b orbit

Still from an animation of the ultra-hot Jupiter KELT-9 b orbiting its host. [NASA/JPL-Caltech]

Ultra-hot Jupiters (UHJs) are unlike any planet in our solar system. They are massive, yet they live very close to their host stars. This proximity causes many unusual phenomena, such as chemical variations between the planet’s daysides and nightsides.

The intense heat UHJs experience also leads to their atmospheric components breaking down. Various metal atoms and ions have been identified in the atmospheres of UHJs, including neutral sodium, iron, and magnesium, and ionized titanium and calcium. However, more neutral metals ought to be detected, especially in the lower parts of these planets’ atmospheres.

Knowing which metals to expect in a UHJ would greatly aid observations and the classification of these planets. To this end, a group of researchers led by Maya Ben-Yami (University of Cambridge, UK) attempted to predict what metals could be found in the atmosphere of the UHJ WASP-121 b and then compared their results to observations of the planet.

The model transmission spectra of WASP-121 b based on the contributions from neutral iron, chromium, vanadium, and titanium. Click to enlarge. [Ben-Yami et al. 2020]

Creating a Metal Metric

WASP-121 b has been the subject of many studies over the past few years. It orbits its host star every 1.3 days and is roughly the mass of Jupiter. It’s a good candidate for transmission spectroscopy — looking at light from the host star filtered through a planet’s atmosphere to learn about the atmospheric composition — since it’s large and its host is very bright.

Ben-Yami and collaborators began their analysis by modeling atomic abundances for WASP-121 b. They then used these abundances to understand how strongly the signature of a metal would appear in a spectrum. After accounting for effects like stellar rotation, the end result is a model transmission spectrum for WASP-121 b.

With a model spectrum in hand, Ben-Yami and collaborators were then able to quantify how likely it was for a metal to show up in an observed spectrum of WASP-121 b. Assuming a reasonable spectrum quality and signal noise, they found that the most likely neutral metals to be observed in WASP-121 b would be iron, titanium, vanadium, and chromium.

Functional Metal Detectors

WASP-121 b metal detection

From left to right and top to bottom, the detections of neutral iron, neutral chromium, neutral vanadium, and ionized iron. The dashed white lines indicate where an applied template picked up the strongest signal, which, if significant enough, would be a detection. Vsys is related to the velocity of the host star. Kp quantifies the effect a planet has on its host star’s motion. [Adapted from Ben-Yami et al. 2020]

The team used observations taken by the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph in Chile to test their predictions. Based on their metric, they searched for the four aforementioned metals along with scandium, yttrium, and zirconium. They were unable to detect neutral titanium, scandium, yttrium, or zirconium, but they did recover previous detections of neutral and ionized iron. Most excitingly, they detected neutral vanadium and chromium for the first time.

These detections and non-detections provide insight into the role of vanadium oxide and titanium oxide in UHJs. Both molecules are believed to cause deviations from the expected relation between altitude and temperature. The detection of neutral vanadium suggests that vanadium oxide gets broken down while the non-detection of neutral titanium suggests the opposite is true for titanium oxide.

Aside from the new detections of neutral vanadium and chromium, this study suggests that it is viable to probe the lower atmospheres of UHJs using model-based metrics and high quality spectra. With WASP-121 b being on the cooler side, there remains a large variety of UHJs left to characterize.

Citation

“Neutral Cr and V in the Atmosphere of Ultra-hot Jupiter WASP-121 b,” Maya Ben-Yami et al 2020 ApJL 897 L5. doi:10.3847/2041-8213/ab94aa

black hole

The black holes we’ve observed in the universe typically fall into two categories: small star-sized black holes, and gargantuan black holes lurking at the centers of galaxies. Now, a new black-hole discovery sheds some light on the gray area between these extremes.

Growing Together

two types of accreting black holes

Illustrations of two types of accreting black holes: a stellar-mass black hole accreting from a binary companion (top) and a supermassive black hole accreting gas in a galaxy’s center (bottom). [Top: ESA/NASA/Felix Mirabel; Bottom: ESO/M. Kornmesser]

Stellar-mass black holes of up to 100 solar masses are scattered by the millions throughout galaxies. At the opposite end of the spectrum, most galaxies are thought to contain just one massive black hole: a black hole of millions to tens of billions of solar masses that lies in the galaxy’s core.

Intriguingly, the mass of these central black holes seems to be inherently tied to that of their host. An empirical relationship known as the M-σ relation shows a correlation between a central black hole’s mass and the spread of star velocities in its host galaxy’s bulge, which acts as a proxy for the bulge mass. The M-σ relation and other, similar relationships show that black holes seem to grow in tandem with their host galaxies throughout the universe.

If the M-σ relation holds across a broad range of masses, then we would expect to find smaller massive black holes at the hearts of especially low-mass galaxies. So far, evidence for these low-mass central black holes has been scarce. But a new study led by Ingyin Zaw (New York University Abu Dhabi, UAE) has now delivered a low-mass massive black hole for us to contemplate.

IC 750

A 4’ x 4’ view of IC 750, a low-mass galaxy that hosts a massive (though less so than expected!) black hole at its center. [Sloan Digital Sky Survey]

Mass Measurement from Masers

Zaw and collaborators used the Very Long Baseline Array to obtain radio observations of the low-mass galaxy IC 750.

At the galaxy’s heart, the authors found emission from water masers, clumps of water molecules that emit light naturally in a process similar to laser emission. Light from the masers shows that they are orbiting in a disk around a compact central mass — a massive black hole — and Zaw and collaborators used their motion to measure the mass enclosed in their orbit, providing an upper limit on the black hole’s mass.

The authors then reduced and analyzed publicly available multiwavelength data to understand the location of the black hole and measure the properties of its host galaxy.

A Decidedly Low-Mass Monster

M-sigma relation

This plot of black hole mass vs. bulge stellar velocity dispersion (the M-σ relation) shows IC 750 marked with a red star, with upper mass limits indicated by downward-pointing arrows. It falls two orders of magnitude below where we’d expect it to lie on the relation. Click to enlarge. [Zaw et al. 2020]

The result? IC 750’s central massive black hole is a definite lightweight, with an upper limit of 140,000 solar masses — and it may actually be less than a third of that weight. Not only is this remarkably small for a central massive black hole, it’s also unusually light even relative to the mass of its host galaxy: IC 750’s black hole lies two orders of magnitude below where it should sit on the M-σ relation!

What’s going on with this unusual object? There are two possible explanations: either there’s more scatter at the low-mass end of the M-σ relation, or the scaling relationship is simply different for low-mass galaxies. The latter option is supported by some simulations that suggest that black holes don’t grow efficiently in low-mass galaxies.

Though we don’t yet know which explanation is more likely, more observations like those presented here will eventually fill in our picture of these low-mass massive monsters.

Citation

“An Accreting, Anomalously Low-mass Black Hole at the Center of Low-mass Galaxy IC 750,” Ingyin Zaw et al 2020 ApJ 897 111. doi:10.3847/1538-4357/ab9944

UX Tauri A disk

The gaseous, dusty disks surrounding newly born stars can reveal a wealth of information about how distant stellar systems form and evolve. In a new study, scientists have now watched the interaction of two such disks in a stellar flyby.

Spotting Spirals

In the past decade, new instrumentation has led to a dramatic improvement in our views of circumstellar environments. We’ve spotted remarkable structure in the dusty disks that surround newborn stars — including, in many cases, pronounced spiral arms.

MWC 758

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

The presence of these spiral arms has provoked much discussion and debate. Are they caused by gravitational instabilities in the gas and dust? Or are they produced by perturbations from unseen, newborn planets orbiting within the disks? While both of these explanations could be at play in different systems, there’s an additional possibility to consider: the arms could be excited by tidal interactions with another star.

In a new study led by Luis Zapata (UNAM Radio Astronomy and Astrophysics Institute, Mexico), a team of scientists has used the sensitive and high-angular-resolution observations of the Atacama Large Millimeter/submillimeter Array (ALMA), located in Chile, to understand how tidal interactions with an orbiting star might be responsible for spiral arms observed in UX Tauri.

A New Disk Found

Located ~450 light-years away, the UX Tauri system consists of four stars: UX Tau A (the main star), UX Tau B (a binary star), and UX Tau C (a close companion that lies just to UX Tau A’s south). Past observations have revealed a disk of gas and dust around UX Tau A exhibiting distinct spiral arms.

ALMA obs of UX Tau

The intensity (top) and radial velocities (bottom) of the molecular gas observed in UX Tauri reveals a disk around both UX Tau A (top star in both images) and UX Tau C (bottom star), as well as a stream of gas connecting the two. Curves tracing the spiral arms in the disk surrounding UX Tau A are overlaid in the top image. [Adapted from Zapata et al. 2020]

Zapata and collaborators have now followed up with detailed ALMA observations to explore the structure of the molecular gas and dust in UX Tau. In addition to further resolving the disk around UX Tau A, the team was also able to detect — for the first time — molecular gas swirling in a disk around UX Tau C. What’s more, the observations reveal tidal interactions between the two disks that surround these stars.

Drama in UX Tau

What do these findings mean? Zapata and collaborators suggest that we’re witnessing a close flyby of UX Tau C as it progresses on a wide, evolving, and eccentric orbit around the disk of UX Tau A. As UX Tau C plowed through UX Tau A’s circumstellar disk, it captured some of the gas, forming its own disk. Through its motion and this tidal interaction, UX Tau C also excited the observed spiral arms in UX Tau A’s disk.

The drama spotted in UX Tauri represents one of the few cases of binary disk interactions that have been mapped out in molecular gas — but this is likely a common occurrence, since stars often occur in multiple-star systems. Sensitive observations like the ALMA detections presented here will likely reveal more such interactions in the future, shining additional light on the process of star and planet formation.

Citation

“Tidal Interaction between the UX Tauri A/C Disk System Revealed by ALMA,” Luis A. Zapata et al 2020 ApJ 896 132. doi:10.3847/1538-4357/ab8fac

1 … 60 61 62 63 64 … 118