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neutron star binary

In case you missed the news in January: the Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected its second merger of two neutron stars — probably. In a recent publication, the collaboration details the interesting uncertainties and implications of this find.

GWB190425

Artist’s illustration of a binary neutron star merger. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

What We Saw and Why It’s Weird

On April 25, 2019, the LIGO detector in Livingston, Louisiana, spotted a gravitational-wave signal from a merger roughly 520 million light-years away. This single-detector observation — LIGO Hanford was offline at the time, and the Virgo detector in Europe didn’t spot it — was nonetheless strong enough to qualify as a definite detection of a merger.

Analysis of the GW190425 signal indicates that we saw the collision of a binary with a total mass of 3.3–3.7 times the mass of the Sun. While the estimated masses of the merging objects — between 1.1 and 2.5 solar masses — are consistent with the expected masses of neutron stars, that total mass measurement is much larger than any neutron star binary we’ve observed in our galaxy. We know of 17 galactic neutron star pairs with measured total masses, and these masses range from just 2.5 to 2.9 times that of the Sun. Why is GW190425 so heavy?

What It Suggests For Formation Channels

BNS total masses

Blue and orange curves show the estimated total mass of GW190425 under different spin assumptions. In either case, the estimated mass is dramatically different from the total masses for the known galactic population of binary neutron stars, indicated with the grey histogram bars and the dashed line. [Abbott et al. 2020]

GW190425’s unusual mass may indicate that it formed differently from known galactic neutron star binaries.

Theory suggests that massive, fast-merging neutron-star pairs like GW190425 could potentially result from especially low-metallicity stars evolving in close binary systems. Under the right conditions, the energetic kicks caused by supernova explosions might be suppressed, allowing the objects to stay together in the close binary even after their evolution into neutron stars.

If this is the case, GW190425 could represent a population of binary neutron stars that we haven’t observed before. These binaries have remained invisible due to their ultra-tight orbits with sub-hour periods; the rapid accelerations of these objects would obscure their signals in pulsar surveys. The shortest-period neutron star binary we’ve detected with pulsar surveys has a period of 1.88 hours, and it won’t merge for another 46 million years. GW190425 could represent a very different binary neutron star population that’s just as common as the galactic population we know.

What If It’s Not Neutron Stars?

Unfortunately, the single-detector observation of GW190425 means we couldn’t pin down the gravitational-wave source’s location well — so follow-up observations haven’t yet spotted an electromagnetic counterpart like the one we had for GW170817, the first binary neutron star merger LIGO observed.

GW190425 localization

GW190425’s signal was localized to an unfortunately large area of ~16% of the sky, providing a challenge for electromagnetic and neutrino observatories hoping to discover counterparts. [Abbott et al. 2020]

This means we’re missing outside information confirming that this was a neutron star binary; it’s therefore possible that one or both of the merging objects was actually a black hole. If so, this would be smaller than any black holes we’ve detected so far, and we would need to significantly revamp our models of black hole binary formation.

There are clearly still a lot of open questions, but it’s early days yet! With the many recent upgrades to the LIGO and Virgo detectors, we can hope for more binary neutron star detections soon — and every new signal brings us a wealth of information in this rapidly developing field.

Citation

“GW190425: Observation of a Compact Binary Coalescence with Total Mass ~ 3.4 M⊙,” B. P. Abbott et al 2020 ApJL 892 L3. doi:10.3847/2041-8213/ab75f5

exoplanet K2-18b

One of our goals with the soon-to-launch James Webb Space Telescope (JWST) is to better characterize the atmospheres of exoplanets. But will clouds get in the way of our chances?

The Hunt for Water

transmission spectroscopy

As a star’s light filters through a planet’s atmosphere on its way to Earth, the atmosphere absorbs certain wavelengths depending on its composition. [European Southern Observatory]

When an exoplanet transits across the face of its host star, it presents us with a golden opportunity: with a sensitive enough telescope — like the upcoming JWST, scheduled to launch a year from now — we can explore the atmosphere of the planet as it filters the light from its host. Through this transmission spectroscopy, we can look for spectral features that indicate the presence of specific atoms and molecules in the planet’s atmospheric gas.

In the search for life beyond our solar system, surface liquid water is generally considered a necessary ingredient for a habitable world — so signatures of water vapor in planet atmospheres are a prime target for transmission spectroscopy. But any planet with abundant surface water is likely to have something else, too: clouds of liquid and ice condensing in its atmosphere.

A new study led by Thaddeus Komacek (The University of Chicago) explores whether these clouds will foil our chances of detecting water vapor in the atmospheres of terrestrial exoplanets.

transmission spectra

Transmission spectra for planets with clouds (open markers) and without clouds (closed markers), for rotation periods of 8 (orange) and 16 (blue) days. Clouds significantly mute the spectral features, especially for long-period planets. JWST’s expected noise floor is ~20 ppm. [Komacek et al. 2020]

An Obstructed View

Komacek and collaborators examine the results of three-dimensional general circulation models of tidally locked planets orbiting M-dwarf stars. The authors generate simulated transit spectra for planets with different rotation rates, incoming starlight, surface pressure, radius, and more. They then explore whether the presence of clouds in the atmospheres of these planets will impede JWST’s ability to detect the water vapor features that arise from lower in the atmosphere.

The result? Bad news. The authors find that the presence of clouds significantly mutes spectral features; when clouds are present, JWST would typically need to observe 10–100 times more transits of the planet to be able to detect the water vapor features in its atmosphere.

This impact is especially strong for slower-rotating planets. The climate models show that planets with periods longer than about 12 days form significantly more cloud cover on their daysides, due to more water vapor being carried to high altitudes. This leads to even stronger muting of these planets’ spectral features.

Hope for the Future

JWST

An artist’s illustration of the James Webb Space Telescope, set to launch in March 2021. [NASA/JWST]

The authors summarize with the disappointing conclusion that, with JWST, we’re going to have a tough time using atmospheric transmission spectroscopy to spot signs of surface water from the tantalizing targets of terrestrial planets orbiting in M-dwarf habitable zones.

There is still some hope, however. An extended mission lifetime for JWST, lowered signal-to-noise threshold for detection, or the discovery of a habitable-zone planet that JWST can monitor continuously could all push the spectral features above the telescope’s noise limit.

What’s more, since only the features from the atmosphere below the clouds will be affected, species that are well-mixed above the cloud deck may still be detectable. Even on cloudy worlds, we’re sure to have plenty to learn with JWST!

Citation

“Clouds will Likely Prevent the Detection of Water Vapor in JWST Transmission Spectra of Terrestrial Exoplanets,” Thaddeus D. Komacek et al 2020 ApJL 888 L20. doi:10.3847/2041-8213/ab6200

galaxy VCC 848

What happens when the large-scale drama of a violent galaxy merger plays out on small scales for a pair of dwarf galaxies? New observations document the scene of a recent dwarf-galaxy collision.

Dramatic Encounters

Antennae Galaxies

The Antennae Galaxies are an example of a starburst galaxy with rapid star-formation activity driven by a recent merger. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration]

When two galaxies merge, the collision can have dramatic consequences — particularly if the galaxies are rich in gas. The gravitational interaction of galaxies oscillating during a merger drives shock waves through their gas. This can trigger bursts of star formation, launch jets from active galactic nuclei, and result in the eventual formation of a new galaxy with drastically different morphology than the original merging pair.

We’ve seen this drama play out on large scales between giant galaxies, but we know a lot less about what happens when dwarf galaxies collide. Dwarf galaxies are the most abundant type of galaxy in the universe, but they’re also very small and faint. This poses a significant challenge to finding and studying dwarfs — which means there’s a lot we don’t know about how the mergers of dwarf galaxies impact overall star formation and the shape of the new galaxy that forms in the collision.

image of VCC 848

This g-band image of VCC 848 better shows the galaxy’s three extended shell-like structures (outlined with red arcs) surrounding the central body of stars. [Adapted from Zhang et al. 2020]

Fortunately, we may now have an opportunity to learn more. In a recent publication led by Hong-Xin Zhang (University of Science and Technology of China), a team of scientists reports on the discovery of a small, compact galaxy formed by the collision of two dwarfs.

Feeling Shell-Shocked

VCC 848 is what’s known as a blue compact dwarf galaxy — a small galaxy that’s actively undergoing a burst of star formation. Located in the outskirts of the Virgo Cluster some 65 million light-years away, this little dwarf shows telltale signs of a recent merger: careful analysis reveals a complex set of three extended shell-like structures of stars around the bright stellar main body.

Shell structures — which, previously, had only been detected in larger galaxies — are known to be a signature of a recent minor or major galaxy merger; they are formed as the merger sends ripples through the galaxy and disrupts its structure. The detection of these shells in such a small galaxy provides evidence that we’re looking at the recent merger of two dwarfs.

A Flurry of Activity

Zhang and collaborators use their observations of VCC 848 — made with the MegaCam instrument on the Canada–France–Hawaii Telescope in Hawaii — to analyze the stars of the galaxy and learn more about its history.

cluster formation rate

The average formation rate per billion years of star clusters above a certain birth mass, for the age intervals of 0–1 Gyr and 1–13 Gyr. This indicates that star formation was significantly more active in the last billion years. [Adapted from Zhang et al. 2020]

They determine that the two dwarfs that collided were likely similar in mass to within a factor of a few, and the merger triggered a burst of star formation over the past billion years that was ~7–10 times higher than normal. This enhancement in star formation peaked near the center of the galaxy a few hundred million years ago, and it’s since declined; current star formation activity is primarily in VCC 848’s outer regions.

VCC 848 is just one of several blue compact dwarfs with hints of tidal shells that the authors uncovered in their survey, so there’s more data on the way! We have a lot more to learn about what happens when tiny galaxies collide.

Citation

“The Blue Compact Dwarf Galaxy VCC 848 Formed by Dwarf–Dwarf Merging,” Hong-Xin Zhang et al 2020 ApJL 891 L23. doi:10.3847/2041-8213/ab7825

CHIME radio telescope

In October of 2018, we wrote about a new project to study fast radio bursts (FRBs) — brief, energetic flashes of light from beyond our galaxy. At the time, we knew of about 30 FRB sources; the new project by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope in British Columbia promised to dramatically increase that number.

fast radio burst

Artist’s impression of a fast radio burst detection. [CSIRO/Andrew Howells]

Now, a year and a half later, we can see the impressive progress made: CHIME has already detected around 700 bursts from FRB sources! Included among those is the collaboration’s latest announcement: nine new repeating sources.

A Question of Repetition

FRBs were first discovered more than a decade ago. These bright, short (around a millisecond) flashes of radio emission are a million times brighter than the brightest pulses from galactic pulsars, and they carry the signature of being produced at a great distance — something that has been further confirmed by the localization of several FRBs to faraway galaxies.

Despite all we’ve learned about FRBs, we still don’t know how they’re produced — though the list of theories has now grown large enough that there’s actually a living catalog of them. One particular puzzlement is that some FRBs have been observed to repeat, whereas others have produced only one detected flash.

burst profiles

Burst profiles for some of the new fast radio bursts detected by CHIME. Click to enlarge. [Fonseca et al. 2020]

Does this mean that the two types of FRBs — repeating and non-repeating — are produced in two different ways? Or in two different environments? Or is there another explanation for why some repeat and others don’t?

Clues from New Flashes

To answer these questions, our best bet is to find enough FRBs to be able to make statistical inferences — and CHIME is helping to build a large sample. In a new publication led by Emmanuel Fonseca (McGill University, Canada), the CHIME collaboration presents a collection of bursts from nine new repeating FRB sources, bringing the total number of known repeaters to 20.

What does this new sample tell us? So far, it’s confirmed previous assessments of the two populations of repeaters and non-repeaters:

pulse widths

Pulse widths for repeating (orange) vs. nonrepeating (blue) fast radio bursts show a distinct difference between the two populations. [Adapted from Fonseca et al. 2020]

  1. The dispersion measures — a measure of the matter the signals travel through to reach us — for repeaters have the same distribution as those for non-repeaters, suggesting the two populations originate in similar local environments and have similar distributions in space.
  2. The pulse widths are larger for repeaters than for non-repeaters, meaning that repeating sources have slightly longer-duration bursts. This may point to different emission mechanisms for the two types of bursts.
  3. The Faraday rotation measures — a measure of the magnetized environment around the burst source — were obtained for two of the new repeaters, and they are lower than the surprisingly high rotation measure of FRB 121102, the first known repeater. We don’t have enough measurements to tell for certain yet, but it’s starting to look like FRB 121102 is an anomaly, and both repeaters and non-repeaters typically originate from more modestly magnetized environments.

We still have a lot to figure out, but as we build up FRB statistics with samples like these, we can start to rule out some of the many origin theories for fast radio bursts. It’s exciting to watch this field as it rapidly evolves!

Citation

“Nine New Repeating Fast Radio Burst Sources from CHIME/FRB,” E. Fonseca et al 2020 ApJL 891 L6. doi:10.3847/2041-8213/ab7208

X-ray binary

An X-ray binary consists of a dense compact object that strips material off its stellar companion, producing X-rays in the process. These binaries are surrounded by radiating accretion disks of infalling material, but they also sometimes fling matter out in powerful relativistic jets. What can their infrared emission tell us about the speed of these jets?

Looking for Lorentz Factors

Schematic of black hole X-ray binary

A schematic of a black hole X-ray binary highlighting the black hole, the accretion disk, and the jets. The expected IR emissions at different inclinations are also explained. Click to enlarge. [Saikia et al. 2019]

Black hole X-ray binaries (BHXBs) are X-ray binaries where the accreting compact object is a black hole. The jets in BHXBs can approach the speed of light, and they can even give the false appearance of moving faster than light. This relativistic illusion is characterized by something called a Lorentz factor, Γ, which quantifies the distortions that come from moving at near light-speed. Unfortunately, the Lorentz factors of BHXB jets haven’t yet been well measured — which limits our understanding of how these speedy outflows may be launched, accelerated, and collimated as they are flung from the black holes.

Luckily we may have a new way of measuring these Lorentz factors: by looking at the BHXBs in infrared (IR) light. In a recent study, a group of scientists led by Payaswini Saikia (New York University Abu Dhabi, UAE) explored what a BHXB’s IR emission says about its structure and its jet’s Lorentz factor.
Light curves of black hole X-ray binary

Transitions between the “on” (“hard”) to “off” (“soft”) states for the black hole X-ray binary GX 339–4. Multiple light curves are shown to emphasize the repeated transitions between the “on” and “off” states. Top: transition from “on” to “off” (“flux drop”), bottom: transition from “off” to “on” (“flux recovery”). [Saikia et al. 2019]

Eyes on IR Emissions

The IR emission from a BHXB can be largely attributed to two things: the accretion disk and synchrotron emission from the jets. Saikia and collaborators explored the infrared emission from 14 BHXBs, gauging how it changed when the BHXB jets turned “on”, emitting highly energetic X-rays, and “off”, emitting less energetic X-rays. Saika and collaborators argue that when the jets were “off”, any observed IR emission could be attributed to the disk; when they were “on”, the excess IR emission was due to the jets. This framework for looking at the BHXBs allowed the authors to isolate the jet emission and characterize the Lorentz factors for some of these outflows.

Inclined to Model

To determine the Lorentz factors, Saikia and collaborators used the IR flux ratio between the states when the jets were “on” and “off”. Here disk inclination comes into play: due to a combination of disk geometry and relativistic beaming of the jet, at high and low disk inclinations, the ratio between the “on” and “off” states ought to be high. For intermediate inclinations, the ratio should be low.

The modeled flux ratios between the “on” and “off” states versus disk inclination for different Lorentz factors, with observed BHXBs overplotted. [Saikia et al. 2019]

Using observed ratios and disk inclinations, Saikia and collaborators were able to model and constrain the Lorentz factors for nine BHXBs for the first time, finding a range of Γ = 1.3–3.5 — which means the jet bulk flows are moving at 64–96% of the speed of light. In addition, the authors put limits on the underlying distribution of BHXB Lorentz factors and could confidently attribute the variations in excess IR emission to disk inclination and jet direction.

With more observations of BHXBs across the spectrum, the techniques in this work should be more widely applicable and could help us better understand these highly energetic objects.

Citation

“Lorentz Factors of Compact Jets in Black Hole X-Ray Binaries,” Payaswini Saikia et al 2019 ApJ 887 21. https://doi.org/10.3847/1538-4357/ab4a09

Protoplanetary disks

Some of the most spectacular images to come out of observatories like the Atacama Large Millimeter/submillimeter Array (ALMA) or the Very Large Telescope (VLT) are detailed views of protoplanetary disks. These disks of gas and dust around young stars aren’t just smooth and featureless; instead, they exhibit arcs, rings, gaps, and spirals. What causes this impressive array of structure? 

Scientists have primarily focused on two explanations:

  1. The structures are caused by the perturbations of massive baby planets interacting with the disk as they orbit.
  2. The structures are generated by various instabilities within the disk that cause the gas and dust to clump.
HL Tau

This ALMA image of the protoplanetary disk surrounding the star HL Tauri reveals the detailed substructure of the disk. [ALMA (ESO/NAOJ/NRAO)]

A new study has now put forward an alternative explanation: the structures are the result of catastrophic, destructive collisions of planetesimals within the disk. Scientists Tatiana Demidova (Crimean Astrophysical Observatory) and Vladimir Grinin (Pulkovo Observatory of the Russian Academy of Sciences; St. Petersburg University, Russia) lay out their scenario of destruction in a recent publication.

Outcome of a Crash

Collisions of large bodies — planetesimals and planetary embryos — are likely common during the formation of planetary systems around young stars. Some gentle collisions may help build up the mass of these bodies as they grow into planets. But objects that smash together at high enough velocities will be completely destroyed in the process, generating an expanding cloud of many smaller bodies and particles.

This cloud won’t remain stationary, however; instead, it will continue to orbit within the protoplanetary disk. Due to the different speeds of the various particles, the initial debris clump should be sheared out into arced structures that might persist for multiple disk orbits.

Could this process faithfully reproduce the disk structures that we’ve observed with ALMA or the VLT? Demidova and Grinin conduct simulations to find out.

Dragging Debris in a Disk

By modeling an expanding debris cloud within a disk that starts at a distance of 30 AU from its solar-mass star, the authors show how the dust and gas will evolve over several disk orbits. They then produce simulated observations of the results at a wavelength of 1.3 mm.

simulated disk observations

Simulated 1.3-mm observations of the evolution of an expanding debris cloud over 40 orbital periods of the cloud center. Time steps advance from top left to bottom right panel. Click to enlarge. [Demidova & Grinin 2019]

The result? Demidova and Grinin find that as the dust cloud stretches, it successively reproduces all three structures we’ve seen in protoplanetary disks — first, it shapes into an arc, then a tightly wound spiral, and eventually into a ring. The simulated observations at 1.3 mm look very similar to various disk images we’ve captured.

There are still many open questions about the structure of the disks around young stars, but this work shows that there are also many potential answers. As planetary systems form, collisions may both grow planetary embryos and destroy them, possibly causing some of the disk features that we’ve observed. One thing is for certain: the environment around young stars is certainly dramatic!

Citation

“Catastrophic Events in Protoplanetary Disks and Their Observational Manifestations,” Tatiana V. Demidova and Vladimir P. Grinin 2019 ApJL 887 L15. doi:10.3847/2041-8213/ab59e0

Exoplanet K2-18b

Whether or not a planet lies in its star’s habitable zone is commonly used to gauge its ability to host life. But what about non-habitable-zone planets that have sources of heat besides starlight?

Warming Up in the Zone

A star’s traditional habitable zone marks the range of distances at which an orbiting planet receives enough heat from its star to host liquid water on its surface. Since water (or another liquid) is generally considered a necessary ingredient for life to arise and survive, stellar habitable zones represent convenient boundaries within which to search for life beyond our solar system.

Habitable zones

Schematic showing how the traditional habitable zone’s location and width changes around different types of stars. [NASA]

But we already know that habitable zones don’t tell the whole story. Plenty of planets that lie within their stars’ habitable zones aren’t livable — perhaps because they’re inhospitable gas giants, or because they have the wrong type of atmosphere, or because they’re routinely blasted by energetic stellar flares from their host.

Could the opposite be true as well, however? Could planets outside of a star’s habitable zone be capable of supporting life? A new study by scientists Manasvi Lingam (Harvard University; Florida Institute of Technology) and Abraham Loeb (Harvard University) explores this possibility. 

Stars Aren’t Everything

surface temperatures over time

Surface temperature as a function of age in Myr for a world with radioisotope abundances 1,000x that of Earth, for three different planet masses. The blue, green, and brown horizontal lines bound the temperature range in which liquid water, ammonia, and ethane can exist, respectively. [Lingam & Loeb 2020]

External heating from starlight is not the only way to keep a planet warm enough for surface liquids, Lingam and Loeb argue. There are additional processes that can instead heat a planet’s surface from the inside — in particular, radioactive decay and primordial heat from the planet’s formation. How powerful would these processes need to be for a planet to maintain liquid on its surface long enough for life to arise and evolve, even without the added heat from starlight?

To be inclusive of life forms that may be different from Earth’s, Lingam and Loeb choose to explore three different liquids in their models: water, ammonia, and ethane. The authors investigate the radioactive heat flux from both long-lived and short-lived isotopes, as well as the typical heat flux released as a world cools after its formation.

Radioactive Worlds

Lingam and Loeb find that a rocky super-Earth with a tenuous atmosphere would need radioactive isotope abundances roughly 1,000 times higher than that of Earth to host long-lived water oceans without the help of starlight. Long-lived ethane oceans are easier to achieve, requiring only 100 times Earth’s radioisotope abundances.

neutron star merger

Artist’s impression of the collision and merger of two neutron stars. [NSF/LIGO/Sonoma State University/A. Simonnet]

Are these high concentrations feasible? Worlds in the dense inner regions of the galactic bulge (where radioisotope-producing neutron-star mergers are more common) or in gas-poor environments are expected to exhibit higher radioisotope abundances. These higher concentrations may be enough to generate the heat needed to sustain liquid on the planets’ surfaces.

Since the number of planets outside of stellar habitable zones is likely orders of magnitude larger than the number inside them, the chance for life on non-habitable-zone worlds opens a wealth of possibilities. Keep an eye out in the future — the James Webb Space Telescope may be able to detect the infrared signatures of some of these internally heated worlds!

Citation

“On the Habitable Lifetime of Terrestrial Worlds with High Radionuclide Abundances,” Manasvi Lingam and Abraham Loeb 2020 ApJL 889 L20. doi:10.3847/2041-8213/ab68e5

TESS

We can only really see what’s going on at the surface of a star. However, the motions within stellar interiors show themselves as subtle variations in the star’s brightness, and with the dense observations of planet-finding missions, we can pick up these variations at very fine levels.

Brightness Changes

stellar oscillations

Asteroseismology uses different oscillation modes of a star to probe its internal structure and properties. [Tosaka]

Several things can be responsible for a star’s changes in brightness, such as planets or companion stars, but stars can also brighten and dim intrinsically. The outer layers of a star can oscillate as the star cools and heats up, while convective processes can cause different oscillations in the body and at the surface of a star. By studying these oscillations — in a process called asteroseismology — we can learn more about stellar properties and internal structures.

Asteroseismology studies require stellar observations taken regularly over a long time. Luckily for astronomers, those are the same sort of observations required for the transit planet-finding method! Over the last decade, missions like Kepler and CoRoT have provided data that’s been used both to find planets and to characterize the insides of stars.

HR diagram with frequency for stars in sample

A Hertzspring-Russell (HR) diagram showing stars’ maximum oscillation frequency (instead of brightness) versus their temperature. The gray dots are all Hipparcos stars brighter than sixth magnitude, while the red dots are the red giant stars used in this study. The Sun is shown at the lower-left corner of the diagram. The tracks show evolutionary models for stars with the masses indicated (in solar-masses) [Aguirre et al. 2020].

The Transiting Exoplanet Survey Satellite (TESS) is carrying out an all-sky survey searching for planets. It covers the sky in sectors, taking images of its entire field of view every thirty minutes. The thirty-minute cadence is well-suited to characterize red giant stars’ oscillations, and TESS is expected to turn up roughly 500,000 red giant stars whose oscillations can be studied.

So it makes sense to set up a framework to analyse TESS data in the context of stellar oscillations. Motivated by this, a group of scientists led by Víctor Silva Aguirre (Aarhus University, Denmark) carried out the first analysis of red giant star oscillations using TESS data.

TESS-ting for Frequencies

Aguirre and collaborators considered stars observed in TESS sectors 1 and 2 (not necessarily both) for their sample. They then used the Hipparcos astrometry mission catalog to identify red giant stars whose oscillations could be seen in TESS data. The maximum frequency of oscillations is set by a star’s temperature, mass, and brightness.

Assuming a mass of 1.2 solar masses for red giant stars (a reasonable assumption), the maximum frequency could be expected to fall between 30 and 220 µHz. From the available stars, the authors selected the twenty-five brightest stars for their oscillation analysis.

We’ve Got the Power (Frequency Spectrum)!

Frequency power spectra for red giant stars

Representative power spectra obtained from the red giant light curves. The left panels have the spectra in log–log space, with the maximum frequency distinguished by the vertical arrow. The right panels show a close-up of the power spectra showing frequencies associated with particular oscillations. The separation between those dominant frequencies is shown using the red vertical lines with the horizontal arrow showing the average separation [Adapted from Aguirre et al. 2020].

From the light curves of each star, Aguirre and collaborators created power frequency spectra, which show the power in the various frequencies that make up a signal. For stars, such power spectra can be used to identify oscillations that could be associated with internal processes.

By combining the oscillation frequency information with astrometry data from the Gaia mission, Aguirre and collaborators showed that they could measure the properties of these red giants to high precision levels — stellar radii can be determined to a precision of a few percent, masses to 5%–10%, and ages to around 20%.

With the incoming flood of TESS and Gaia data, we will almost certainly be able to characterize red giant stars better than we have in the past. And this is to say nothing of what will come out of TESS’s extended mission, beginning July 2020!

Citation

“Detection and Characterization of Oscillating Red Giants: First Results from the TESS Satellite,” Víctor Silva Aguirre et al 2020 ApJL 889 L34. https://doi.org/10.3847/2041-8213/ab6443

Collinder 261

Though stars within the same cluster all typically form around the same time, they don’t all evolve in the same way. A recent study has carefully explored a population of particularly unusual, straggling stars in the old open cluster Collinder 261.

blue stragglers in NGC 6362

This Hubble image of the globular cluster NGC 6362 reveals a number of stars that appear younger and bluer than their companions: so-called blue stragglers. [ESA/Hubble & NASA]

Why So Blue?

A stellar cluster is typically born in a burst of star formation that creates member stars from the same source material. After the stars form, the cluster ages over cosmic time, its individual stars evolving according to their masses. Bright, blue, massive stars have short lifespans, evolving quickly off the main sequence; dim, red, low-mass stars live much longer and evolve slowly. This difference causes clusters to become progressively redder as they age.

For particularly old clusters, we would not expect to see any bright blue stars, as these should have all aged off the main sequence already. And yet, again and again, we find handfuls of these bright blue stars — in the Milky Way’s globular and open clusters, and even in other nearby galaxies. How do these so-called blue stragglers arise?

Oh, to Be Young Again

formation of blue stragglers

Two possible formation channels for blue stragglers: two stars collide (top), or a star gains mass from a binary companion (bottom). [NASA/ESA]

Since blue-straggler stars are more massive and brighter than expected for their host cluster, we think they must have gained that mass more recently. There are two proposed rejuvenation scenarios that could create blue stragglers:

  1. Two stars collide and merge to form one massive star.
  2. A star gains mass from a close-binary companion.

By studying populations of blue stragglers and exploring these possibilities, we have the potential opportunity to learn about about cluster dynamics and histories, and about binary systems. But blue stragglers tend to lie near the very crowded centers of galaxies — which makes it difficult to observe individual stars and be certain of their membership in the cluster.

Led by Maria Rain (University of Padua, Italy), a team of scientists has now met this challenge using the precise stellar measurements of the Gaia mission in conjunction with spectroscopy from an instrument on ESO’s Very Large Telescope in Chile. The team conducted one of the most detailed studies of a blue straggler population, exploring the open cluster Collinder 261.

Stars of a Different Color

proper motions for Collinder 261

Proper motions for Collinder 261. Black circles are cluster members; blue circles are blue straggler candidates, and orange circles are yellow straggler candidates. [Adapted from Rain et al. 2020]

Aged at 7 to 9.3 billion years, Collinder 261 is one of the oldest open clusters of the Milky Way. Rain and collaborators used Gaia data describing the colors and brightnesses, the proper motions, and the parallaxes of stars in Collinder 261’s field to identify 53 blue straggler candidates and one potential yellow straggler — an evolved blue straggler — in the cluster. The authors then followed up 10 of these stars with spectroscopic measurements, determining that at least five of them are members of close binary systems.

While these data are not yet enough to draw firm conclusions about the origin of blue stragglers, it should be possible to spectroscopically follow up the remaining candidate stars to learn more. This study provides a particularly detailed exploration of these odd straggling stars, which we can hope to build on in the near future.

Citation

“A study of the blue straggler population of the old open cluster Collinder 261,” M. J. Rain et al 2020 AJ 159 59. doi:10.3847/1538-3881/ab5f0b

galaxy collision

Thanks to the Laser Interferometer Gravitational-wave Observatory (LIGO), we now know that black holes in our distant universe sometimes find each other in a dramatic inspiral and collision, releasing a burst of gravitational-wave emission that we can detect here on Earth.

But what happened earlier in these black holes’ lives to bring them to this point? A new study explores the possibility that LIGO’s black holes once lay at the centers of very small galaxies — until those galaxies collided.

black hole merger

Simulated image of two merging black holes, viewed face-on. LIGO has announced the detection of ten of these events so far. [SXS Lensing]

Central Lurkers

Since we discovered the first wiggles in spacetime signifying the distant merger of two black holes, LIGO has announced around ten confident detections of gravitational waves from black hole–black hole collisions — with the prospect of many more discoveries in the future.

But how did these black holes find each other? A team of scientists led by Christopher Conselice (University of Nottingham, UK) has proposed a picture that hinges on the central black holes we believe lie at the heart of most, if not all, galaxies.

The team proposes that very low-mass dwarf galaxies contain central black holes of less than 100 solar masses. The mergers of pairs of these tiny galaxies ultimately lead to the inspirals and mergers of their central black holes — possibly accounting for the majority of LIGO’s detections of black hole–black hole collisions. 

Testing Feasibility

Conselice and collaborators test this scenario by breaking it down into multiple steps.

black hole mass vs. galaxy mass

The relationship between black hole mass and host galaxy stellar mass (black solid line; blue dashed lines show uncertainties), extrapolated down to low masses. Red lines indicate the masses of LIGO-detected black holes. [Conselice et al. 2020]

  1. Can you get central black holes of the right mass?
    We’ve observed a relationship between galaxy mass and central black hole mass. By extrapolating this relationship to low masses, we find that ultradwarf galaxies can have central black holes of less than 100 solar masses — consistent with the LIGO-observed black holes of 10–70 solar masses.
  2. Will these ultradwarf galaxies merge frequently enough?
    Mergers of galaxies occurred more frequently in the early universe than they do today. Cosmological models indicate that galaxies don’t merge frequently enough today to reproduce LIGO’s observations — but at a redshift of z ~ 1.5 or higher, ultradwarf galaxies could merge often enough to match LIGO-measured gravitational-wave event rates.
  3. Will the black holes collide fast enough after the galaxies merge?
    If the galaxies merged at a redshift of z > 1.5, the central black holes would have to sink to the middle of the merger, inspiral, and collide on timescales of 6–8 billion years to match LIGO observations. This is feasible if the black holes are embedded in a massive star cluster at the galaxy center.

A Future Hunt for Hosts

Conselice and collaborators’ calculations show that merging ultradwarf galaxies in the distant universe could, conceivably, account for LIGO’s black hole–black hole merger detections.

Fornax dwarf

An example of a dwarf spheroidal galaxy. The smallest dwarfs are far too faint to detect at high redshifts with current technology. [ESO/Digitized Sky Survey 2]

In the future, we can hope to test this theory by better pinpointing the hosts of gravitational-wave events. If we find that the black-hole collisions all originate from bright, massive galaxies, then the ultradwarf-merger theory is out. But if we can’t spot the hosts, this might be because they’re ultradwarfs that are too small and faint to detect.

The field of gravitational-wave astronomy is still only just coming of age, and theoretical work like this study shows how just how much we can hope to learn in the future!

Citation

“LIGO/Virgo Sources from Merging Black Holes in Ultradwarf Galaxies,” Christopher J. Conselice et al 2020 ApJ 890 8. doi:10.3847/1538-4357/ab5dad

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