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photograph of a stream of material looping off of the sun's surface

The habitability of distant exoplanets is dependent upon many factors — one of which is the activity of their host stars. To learn about which stars are most likely to flare, a recent study examines tens of thousands of stellar flares observed by Kepler.

Need for a Broader Sample

flaring dwarf star

Artist’s rendering of a flaring dwarf star. [NASA’s Goddard Space Flight Center/S. Wiessinger]

Most of our understanding of what causes a star to flare is based on observations of the only star near enough to examine in detail — the Sun. But in learning from a sample size of one, a challenge arises: we must determine which conclusions are unique to the Sun (or Sun-like stars), and which apply to other stellar types as well.

Based on observations and modeling, astronomers think that stellar flares result from the reconnection of magnetic field lines in a star’s outer atmosphere, the corona. The magnetic activity is thought to be driven by a dynamo caused by motions in the star’s convective zone.

Kepler flaring stars

HR diagram of the Kepler stars, with flaring main-sequence (yellow), giant (red) and A-star (green) stars in the authors’ sample indicated. [Van Doorsselaere et al. 2017]

To test whether these ideas are true generally, we need to understand what types of stars exhibit flares, and what stellar properties correlate with flaring activity. A team of scientists led by Tom Van Doorsselaere (KU Leuven, Belgium) has now used an enormous sample of flares observed by Kepler to explore these statistics.

Intriguing Trends

Van Doorsselaere and collaborators used a new automated flare detection and characterization algorithm to search through the raw light curves from Quarter 15 of the Kepler mission, building a sample of 16,850 flares on 6,662 stars. They then used these to study the dependence of the flare occurrence rate, duration, energy, and amplitude on the stellar spectral type and rotation period.

This large statistical study led the authors to several interesting conclusions, including:

  1. rotation influence

    Flare star incidence rate as a a function of Rossby number, which traces stellar rotation. Higher rotation rates correspond to lower Rossby numbers, so these data indicate that more rapidly rotating stars are more likely to exhibit flares. [Van Doorsselaere et al. 2017]

    Roughly 3.5% of Kepler stars in this sample are flaring stars.
  2. 24 new A stars are found to show flaring activity. This is interesting because A stars aren’t thought to have an outer convective zone, which should prevent a magnetic dynamo from operating. Yet these flaring-star detections add to the body of evidence that at least some A stars do show magnetic activity.
  3. Most flaring stars in the sample are main-sequence stars, but 653 giants were found to have flaring activity. As with A stars, it’s unexpected that giant stars would have strong magnetic fields — their increase in size and gradual spin-down over time should result in weakening of the surface fields. Nevertheless, it seems that the flare incidence of giant stars is similar to that of F or G main-sequence stars.
  4. All stellar types appear to have a small fraction of “flare stars” — stars with an especially high rate of flare occurrence.
  5. Rapidly rotating stars are more likely to flare, tend to flare more often, and tend to have stronger flares than slowly rotating stars.

As a next step, the authors plan to apply their flare detection algorithm to the larger sample of all Kepler data. In the meantime, this study has both deepened a few mysteries and moved us a step closer in our understanding of which stars flare — and why.

Citation

Tom Van Doorsselaere et al 2017 ApJS 232 26. doi:10.3847/1538-4365/aa8f9a

ESO 243-49

What is the structure of the Milky Way’s disk, and how did it form? A new study uses giant stars to explore these questions.

A View from the Inside

Schematic showing an edge-on, not-to-scale view of what we think the Milky Way’s structure looks like. The thick disk is shown in yellow and the thin disk is shown in green. [Gaba p]

Spiral galaxies like ours are often observed to have disks consisting of two components: a thin disk that lies close to the galactic midplane, and a thick disk that extends above and below this. Past studies have suggested that the Milky Way’s disk hosts the same structure, but our position embedded in the Milky Way makes this difficult to confirm.

If we can measure the properties of a broad sample of distant tracer stars and use this to better understand the construction of the Milky Way’s disk, then we can start to ask additional questions — like, how did the disk components form? Formation pictures for the thick disk generally fall into two categories:

  1. Stars in the thick disk formed within the Milky Way — either in situ or by migrating to their current locations.
  2. Stars in the thick disk formed in satellite galaxies around the Milky Way and then accreted when the satellites were disrupted.

Scientists Chengdong Li and Gang Zhao (NAO Chinese Academy of Sciences, University of Chinese Academy of Sciences) have now used observations of giant stars — which can be detected out to great distances due to their brightness — to trace the properties of the Milky Way’s thick disk and address the question of its origin.

metallicity gradients

Best fits for the radial (top) and vertical (bottom) metallicity gradients of the thick-disk stars. [Adapted from Li & Zhao 2017]

Probing Origins

Li and Zhao used data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) in China to examine a sample of 35,000 giant stars. The authors sorted these stars into different disk components — halo, thin disk, and thick disk — based on their kinematic properties, and then explored how the orbital and chemical properties of these stars differed in the different components.

Li and Zhao found that the scale length for the thick disk is roughly the same as that of the thin disk (~3 kpc). The scale height found for the thick disk is ~1 kpc, compared to the thin disk’s few hundred parsecs or so.

The metallicity of the thick-disk stars is roughly constant with radius; this could be a consequence of radial migration of the stars within the disk, which blurs any metallicity distribution that might have once been there. The metallicity of the stars decreases with distance above or below the galactic midplane, however — a result consistent with formation of the thick disk via heating or radial migration of stars formed within the galaxy.

Orbital eccentricity distribution for the thick-disk stars. [Li & Zhao 2017]

Further supporting these formation scenarios, the orbital eccentricities of the stars in the authors’ thick-disk sample indicate that they were born in the Milky Way, not accreted from disrupted satellites.

The authors acknowledge that the findings in this study may still be influenced by selection effects resulting from our viewpoint within our galaxy. Nonetheless, this is interesting new data to add to our understanding of the structure and origins of the Milky Way’s disk.

Note: This post was edited to remove a statement implying that the authors’ results indicate the thin and thick disks have the same radial extent.

Citation

Chengdong Li and Gang Zhao 2017 ApJ 850 25. doi:10.3847/1538-4357/aa93f4

jet-cloud interaction simulation

What happens when the highly energetic jet from the center of an active galaxy rams into surrounding clouds of gas and dust? A new study explores whether this might be a way to form stars.

simulation results

The authors’ simulations at an intermediate (top) and final (bottom) stage show the compression in the gas cloud as a jet (red) enters from the left. Undisturbed cloud material is shown in blue, whereas green corresponds to cold, compressed gas actively forming stars. [Fragile et al. 2017]

Impacts of Feedback

Correlation between properties of supermassive black holes and their host galaxies suggest that there is some means of communication between them. For this reason, we suspect that feedback from an active galactic nucleus (AGN) — in the form of jets, for instance — controls the size of the galaxy by influencing star formation. But how does this process work?

AGN feedback can be either negative or positive. In negative feedback, the gas necessary for forming stars is heated or dispersed by the jet, curbing or halting star formation. In positive feedback, jets propagate through the surrounding gas with energies high enough to create compression in the gas, but not so high that they heat it. The increased density can cause the gas to collapse, thereby triggering star formation.

In a recent study, a team of scientists led by Chris Fragile (College of Charleston) modeled what happens when an enormous AGN jet slams into a dwarf-galaxy-sized, inactive cloud of gas. In particular, the team explored the possibility of star-forming positive feedback — with the goal of reproducing recent observations of something called Minkowski’s Object, a stellar nursery located at the endpoint of a radio jet emitted from the active galaxy NGC 541.

star formation rate

The star formation rate in the simulated cloud increases dramatically as a result of the jet’s impact, reaching the rate currently observed for Minkowski’s Object’s within 20 million years. [Fragile et al. 2017]

Triggering Stellar Birth

Fragile and collaborators used a computational astrophysics code called Cosmos++ to produce three-dimensional hydrodynamic simulations of an AGN jet colliding with a spherical intergalactic cloud. They show that the collision triggers a series shocks that move through and around the cloud, condensing the gas and triggering runaway cooling instabilities that can lead to cloud clumps collapsing to form stars.

The authors are able to find a model in which the dramatic increase in the star formation rate matches that measured for Minkowski’s Object very well. In particular, the increased star formation occurs upstream of the bulk of the available H I gas, which is consistent with observations of Minkowski’s Object and implicates the jet’s interaction with the cloud as the cause.

star properties

The spatial distribution of particles tracing stars that formed as a result of the jet entering from the left, after 40 million years. Color tracks the particle age (in Myr) in the top panel and particle velocity (in km/s) in the bottom. [Adapted from Fragile et al. 2017]

An intriguing result of the authors’ simulations is a look at the spatial distribution of the velocities of stars that form when triggered by the jet. Because the propagation speed of the star-formation front gradually slows, the fastest-moving stars are those that were formed first, and they are found furthest downstream. This provides an interesting testable prediction — we can look to see if a similar distribution is visible in Minkowski’s Object.

Fragile and collaborators plan further refinements to their simulations, but they argue that the success of their model to reproduce observations of Minkowski’s Object are very promising. Positive feedback from AGN jets indeed appears to have an important impact on the surrounding environment.

Citation

P. Chris Fragile et al 2017 ApJ 850 171. doi:10.3847/1538-4357/aa95c6

cosmic rays

The Sun plays an important role in protecting us from cosmic rays, energetic particles that pelt us from outside our solar system. But can we predict when and how it will provide the most protection, and use this to minimize the damage to both piloted and robotic space missions?

The Challenge of Cosmic Rays

spacecraft cosmic rays

Spacecraft outside of Earth’s atmosphere and magnetic field are at risk of damage from cosmic rays. [ESA]

Galactic cosmic rays are high-energy, charged particles that originate from astrophysical processes — like supernovae or even distant active galactic nuclei — outside of our solar system.

One reason to care about the cosmic rays arriving near Earth is because these particles can provide a significant challenge for space missions traveling above Earth’s protective atmosphere and magnetic field. Since impacts from cosmic rays can damage human DNA, this risk poses a major barrier to plans for interplanetary travel by crewed spacecraft. And robotic missions aren’t safe either: cosmic rays can flip bits, wreaking havoc on spacecraft electronics as well.

heliosphere

The magnetic field carried by the solar wind provides a protective shield, deflecting galactic cosmic rays from our solar system. [Walt Feimer/NASA GSFC’s Conceptual Image Lab]

Shielded by the Sun

Conveniently, we do have some broader protection against galactic cosmic rays: a built-in shield provided by the Sun. The interplanetary magnetic field, which is embedded in the solar wind, deflects low-energy cosmic rays from us at the outer reaches of our solar system, decreasing the flux of these cosmic rays that reach us at Earth.

This shield, however, isn’t stationary; instead, it moves and changes as the strength and direction of the solar wind moves and changes. This results in a much lower cosmic-ray flux at Earth when solar activity is high — i.e., at the peak of the 11-year solar cycle — than when solar activity is low. This visible change in local cosmic-ray flux with solar activity is known as “solar modulation” of the cosmic ray flux at Earth.

In a new study, a team of scientists led by Nicola Tomassetti (University of Perugia, Italy) has modeled this solar modulation to better understand the process by which the Sun’s changing activity influences the cosmic ray flux that reaches us at Earth.

Modeling a Lag

Tomassetti and collaborators’ model uses two solar-activity observables as inputs: the number of sunspots and the tilt angle of the heliospheric current sheet. By modeling basic transport processes in the heliosphere, the authors then track the impact that the changing solar properties have on incoming galactic cosmic rays. In particular, the team explores the time lag between when solar activity changes and when we see the responding change in the cosmic-ray flux.

Cosmic-ray flux data and model

Cosmic-ray flux observations are best fit by the authors’ model when an 8-month lag is included (red bold line). A comparison model with no lag (black dashed line) is included. [Tomassetti et al. 2017]

By comparing their model outputs to the large collection of time-dependent observations of cosmic-ray fluxes, Tomassetti and collaborators show that the best fit to data occurs with an ~8-month lag between changing solar activity and local cosmic-ray flux modulation.

This is an important outcome for studying the processes that affect the cosmic-ray flux that reaches Earth. But there’s an additional intriguing consequence of this result: knowledge of the current solar activity could allow us to predict the modulation that will occur for cosmic rays near Earth an entire 8 months from now! If this model is correct, it brings us one step closer to being able to plan safer space missions for the future.

Citation

Nicola Tomassetti et al 2017 ApJL 849 L32. doi:10.3847/2041-8213/aa9373

quasar

SDSS J102009.99+104002.7

A cloud of gas surrounds the distant quasar SDSS J102009.99+104002.7 in this image from ESO’s Very Large Telescope. The name “quasar” is a shortening of “quasi-stellar radio source”, though we now know that only a small fraction of quasars are radio-loud. [ESO/Arrigoni Battaia et al.]

Some distant active galaxies are louder in radio wavelengths than others. A new study explores whether this difference could be due to how quickly the supermassive black holes at their centers are spinning.

Loud and Quiet Quasars

Quasars, the most luminous type of active galactic nuclei, are powered by the accretion of material onto the supermassive black holes located at the centers of the galaxies. These distant beasts tend to fall into two general categories:

  1. radio-loud quasars, which host powerful relativistic radio jets and make up roughly 10% of the quasar population, and
  2. radio-quiet quasars, which feature only weak core radio emission and make up the remaining 90% of quasars.

What causes this distinction in jet behavior? Many theories have been put forward, but today we’ll explore one potential factor in particular: the spin of the black hole.

Radio-loud vs. radio-quiet

Histogram of the [O III] equivalent width for radio-loud (solid red) vs. radio-quiet (dashed blue) quasars, for three different definitions of radio-loudness. [Adapted from Schulze et al. 2017]

In the spin paradigm, it’s postulated that the angular momentum from a black hole’s spin — which can be retrograde, prograde, or nonexistent — is what allows (or doesn’t allow) for the launch of relativistic jets. In this picture, radio-loud quasars should have rapidly spinning supermassive black holes at their centers, whereas radio-quiet quasars should host low-spin black holes.

A Tricky Measurement

Past studies examining the spin paradigm suggest that it doesn’t hold up — several radio-quiet quasars were found to host black holes with apparently high spin. But measuring black-hole spins is notoriously tricky, with each method relying on a number of inferences. It’s possible that the method used to infer the high spins of these radio-quiet quasars might not have yielded accurate results.

A team of scientists led by Andreas Schulze (National Astronomical Observatory of Japan) has now proposed an alternative approach to test the spin paradigm. Schulze and collaborators suggest using the strength of a particular emission line, [O III], to indirectly measure the black holes’ average radiative efficiency — i.e., how much of the energy of the mass accreting onto the black holes is converted into radiation. If the average efficiency for a sample of radio-loud quasars is different than that for a sample of radio-quiet quasars, this would mean a difference in black-hole spins for the two samples.

Counting Spin Back In

Using a sample of nearly 8,000 quasars identified in the Sloan Digital Sky Survey, the authors find that the [O III] line strength is enhanced by a factor of at least 1.5 in a radio-loud sample, compared to a radio-quiet sample matched in redshift, black-hole mass, and accretion rate.

EW[O III] vs. redshift

[O III] equivalent width for the radio-loud (solid red) and radio-quiet (dashed blue) samples as a function of redshift. [Schulze et al. 2017]

Schulze and collaborators argue that this suggests the black-hole spins of the radio-loud quasar population are systematically higher than those of the radio-quiet population.

The authors caution that, like other tactics used to learn about black-hole spins, their approach relies on a number of key assumptions — and their results certainly don’t mean that spin must be the only factor differentiating between radio-loud and radio-quiet quasars. The results do suggest, however, that we shouldn’t count spin out of the game: it may play an important role in determining the loudness of these distant accreting monsters.

Citation

Andreas Schulze et al 2017 ApJ 849 4. doi:10.3847/1538-4357/aa9181

NGC 2718

A hunt for merging dwarf galaxies has yielded an intriguing result: 180 million light-years away, a galaxy very similar to the Milky Way — with two dwarf-galaxy satellites just like our own Magellanic clouds.

Unusual Satellites

LMC and SMC

The Large and Small Magellanic clouds, as observed from Earth. [ESO/S. Brunier]

The Large and Small Magellanic clouds (LMC and SMC), the only bright and star-forming satellite galaxies around the Milky Way, have proven unusual in the universe: satellite pairs of LMC–SMC mass are neither common in observation nor  typically produced in numerical simulations of galaxy formation and evolution.

Since the probability of having such an interacting pair of satellites in a massive halo is so low, this raises questions about how our system came about. Did the Magellanic clouds form independently around the Milky Way and then interact? Were they more recently captured as an already-merging pair of dwarf galaxies? Or is there some other explanation?

If we could find other systems that look like the LMC–SMC–Milky-Way system, we might be able to learn more about pairs of dwarf galaxies and how they interact near the halos of large galaxies like the Milky Way. Conveniently, two researchers from Yonsei University in South Korea, Sanjaya Paudel and Chandreyee Sengupta, have now identified exactly such a system.

UGC 4703

The UGC 4703 pair of dwarf galaxies show a stellar bridge connecting them — a sign of their past interaction, when tidal forces stripped material from them as they passed each other. [Adapted from Paudel & Sengupta 2017]

An Interacting Pair

Hunting for merging dwarf galaxies in various environments, Paudel and Sengupta found UGC 4703, an interacting pair of dwarf galaxies that are located near the isolated spiral galaxy NGC 2718. This pair of satellites around the massive spiral bear a striking resemblance to the LMC–SMC system around the Milky-Way.

The authors performed a multi-wavelength study of the system using archival images from the Sloan Digital Sky Survey, The Galaxy Evolutionary Explorer spacecraft, and the Spitzer Space Telescope. They also gather new observations of the H I gas distribution in the system using the Giant Metrewave Radio Telescope in India.

Paudel and Sengupta find that NGC 2718 and the Milky Way have similar stellar masses, and the stellar mass ratio of the UGC 4703 interacting pair is around 5:1, similar to the mass ratio of the LMC to the SMC. The separation of the UGC 4703 pair is also roughly the same as that of the LMC and SMC: ~70,000 light-years.

Similarities and Differences

H I gas in UGC 4703

The H I gas distribution in UGC 4703 reveals both similarities and differences between this system and the LMC–SMC system. [Paudel & Sengupta 2017]

The stellar bridge connecting the components of the UGC 4703 system are a sign of their past interaction, but a comparison of the optical and H I morphology between the UGC 4703 pair and the LMC–SMC pair suggests that the UGC 4703 galaxies are either interacting more slowly than the Magellanic clouds or that the interaction is at a more advanced stage than we see with the LMC–SMC.

Understanding these similarities and differences between the LMC–SMC–Milky-Way system and this analog are an important first step to studying dwarf galaxy pairs as they interact near the massive halos of their large spiral hosts. In the future, further observations of UGC 4703 and detailed modeling of the system may help continue to puzzle out how our own Magellanic clouds came about.

Citation

Sanjaya Paudel and C. Sengupta 2017 ApJL 849 L28. doi:10.3847/2041-8213/aa95bf

Kepler multiplanet system

After 8.5 years of observations with the Kepler space observatory, we’ve discovered a large number of close-in, tightly-spaced, multiple-planet systems orbiting distant stars. In the process, we’ve learned a lot about the properties about these systems — and discovered some unexpected behavior. A new study explores one of the properties that have surprised us: planets of the same size tend to live together.

Multiplanet orbital architecture

Orbital architectures for 25 of the authors’ multiplanet systems. The dots are sized according to the planets’ relative radii and colored according to mass. Planets of similar sizes and masses tend to live together in the same system. [Millholland et al. 2017]

Ordering of Systems

From Kepler’s observations of extrasolar multiplanet systems, we have seen that the sizes of planets in a given system aren’t completely random. Systems that contain a large planet, for example, are more likely to contain additional large planets rather than additional planets of random size. So though there is a large spread in the radii we’ve observed for transiting exoplanets, the spread within any given multiplanet system tends to be much smaller.

This odd behavior has led us to ask whether this clustering occurs not just for radius, but also for mass. Since the multiplanet systems discovered by Kepler most often contain super-Earths and mini-Neptunes, which have an extremely large spread in densities, the fact that two such planets have similar radii does not guarantee that they have similar masses.

If planets don’t cluster in mass within a system, this would raise the question of why planets coordinate only their radii within a given system. If they do cluster in mass, it implies that planets within the same system tend to have similar densities, potentially allowing us to predict the sizes and masses of planets we might find in a given system.

Insight into Masses

Led by NSF graduate research fellow Sarah Millholland, a team of scientists at Yale University used recently determined masses for planets in 37 Kepler multiplanet systems to explore this question of whether exoplanets in a multiplanet system are more likely to have similar masses rather than random ones.

Millholland and collaborators find that the masses do show the same clustering trend as radii in multiplanet systems — i.e., sibling planets in the same system tend to have both masses and radii that are more similar than if the system were randomly assembled from the total population of planets we’ve observed. Furthermore, the masses and radii tend to be ordered within a system when the planets are ranked by their periods.

median planetary radius vs host star metallicity

The host star’s metallicity is correlated with the median planetary radius for a system. [Adapted from Millholland et al. 2017]

The authors note two important implications of these results:

  1. The scatter in the relation between mass and radius of observed exoplanets is primarily due to system-to-system variability, rather than the variability within each system.
  2. Knowing the properties of a star and its primordial protoplanetary disk might allow us to predict the outcome of the planet formation process for the system.

Following up on the second point, the authors test whether certain properties of the host star correlate with properties of the planets. They find that the stellar mass and metallicity have a significant effect on the planet properties and the structure of the system.

Continuing to explore multiplanet systems like these appears to be an excellent path forward for understanding the hidden order in the broad variety of exoplanets we’ve observed.

Citation

Sarah Millholland et al 2017 ApJL 849 L33. doi:10.3847/2041-8213/aa9714

black hole merger

Wednesday evening the Laser Interferometer Gravitational-wave Observatory (LIGO) collaboration quietly mentioned that they’d found gravitational waves from yet another black-hole binary back in June. This casual announcement reveals what is so far the lightest pair of black holes we’ve watched merge — opening the door for comparisons to the black holes we’ve detected by electromagnetic means.

A Routine Detection

GW170608

The chirp signal of GW170608 detected by LIGO Hanford and LIGO Livingston. [LIGO collaboration 2017]

After the fanfare of the previous four black-hole-binary merger announcements over the past year and a half — as well as the announcement of the one neutron-star binary merger in August — GW170608 marks our entry into the era in which gravitational-wave detections are officially “routine”.

GW170608, a gravitational-wave signal from the merger of two black holes roughly a billion light-years away, was detected in June of this year. This detection occurred after we’d already found gravitational waves from several black-hole binaries with the two LIGO detectors in the U.S., but before the Virgo interferometer came online in Europe and increased the joint ability of the detectors to localize sources.

GW170608 component masses

Mass estimates for the two components of GW170608 using different models. [LIGO collaboration 2017]

Overall, GW170608 is fairly unremarkable: it was detected by both LIGO Hanford and LIGO Livingston some 7 ms apart, and the signal looks not unlike those of the previous LIGO detections. But because we’re still in the early days of gravitational-wave astronomy, every discovery is still remarkable in some way! GW170608 stands out as being the lightest pair of black holes we’ve yet to see merge, with component masses before the merger estimated at ~12 and ~7 times the mass of the Sun.

Why Size Matters

With the exception of GW151226, the gravitational-wave signal discovered on Boxing Day last year, all of the black holes that have been discovered by LIGO/Virgo have been quite large: the masses of the components have all been estimated at 20 solar masses or more. This has made it difficult to compare these black holes to those detected by electromagnetic means — which are mostly under 10 solar masses in size.

Compact object masses

GW170608 is the lowest-mass of the LIGO/Virgo black-hole mergers shown in blue. The primary mass is comparable to the masses of black holes we have measured by electromagnetic means (purple detections). [LIGO-Virgo/Frank Elavsky/Northwestern]

One type of electromagnetically detected black hole are those in low-mass X-ray binaries (LMXBs). LMXBs consist of a black hole and a non-compact companion: a low-mass donor star that overflows its Roche lobe, feeding material onto the black hole. It is thought that these black holes form without significant spin, and are later spun up as a result of the mass accretion. Before LIGO, however, we didn’t have any non-accreting black holes of this size to observe for comparison.

Now, detections like GW170608 and the Boxing Day event (which was also on the low end of the mass scale) are allowing us to start exploring spin distributions of non-accreting black holes to determine if we’re right in our understanding of black-hole spins. We don’t yet have a large enough comparison sample to make a definitive statement, but GW170608 is indicative of a wealth of more discoveries we can hope to find in LIGO’s next observing run, after a series of further design upgrades scheduled to conclude in 2018. The future of gravitational wave astronomy continues to look promising!

Citation

“GW170608: Observation of a 19 Solar-mass Binary Black Hole Coalescence,” B. P. Abbott et al 2017 ApJL 851 L35. doi:10.3847/2041-8213/aa9f0c

Milky Way

Studying the large-scale structure of the Milky Way is difficult given that we’re stuck in its interior — which means we can’t step back for a broad overview of our home. Instead, a recent study uses distant variable stars to map out a picture of what’s happening in the outskirts of our galaxy.

Mapping with Tracers

RR Lyrae light curves

Phase-folded light curve for two of the RR Lyrae stars in the authors’ sample, each with hundreds of observations over 7 years. [Cohen et al. 2017]

Since observing the Milky Way from the outside isn’t an option, we have to take creative approaches to mapping its outer regions and measuring its total mass and dark matter content. One tool used by astronomers is tracers: easily identifiable stars that can be treated as massless markers moving only as a result of the galactic potential. Mapping the locations and motions of tracers allows us to measure the larger properties of the galaxy.

RR Lyrae stars are low-mass, variable stars that make especially good tracers. They pulsate predictably on timescales of less than a day, creating distinctive light curves that can easily be distinguished and tracked in wide-field optical imaging surveys over long periods of time. Their brightness makes them detectable out to large distances, and their blue color helps to separate them from contaminating stars in the foreground.

Best of all, RR Lyrae stars are very nearly standard candles: their distances can be determined precisely with only knowledge of their measured light curves.

Sample locations

Locations on the sky of the several hundred outer-halo RR Lyrae stars in the authors’ original sample. The red curve shows the location of the Sagittarius stream, an ordered structure the authors avoided so as to only have unassociated stars in their sample. [Cohen et al. 2017]

Distant Variables

In a new study led by Judith Cohen (California Institute of Technology), the signals of hundreds of distant RR Lyrae stars were identified in observations of transient objects made with the Palomar Transient Factory (PTF) survey. Cohen and collaborators then followed up with the Keck II telescope in Hawaii to obtain spectra for a narrower sample of 122 RR Lyrae stars.

The stars in the sample lie at whopping distances of ~150,000–350,000 light-years from us. For comparison, we’re about 25,000 light-years from the center of the galaxy, and the stellar disk of the galaxy is only thought to be perhaps 100,000 light-years across — so these variable stars lie firmly in the Milky Way’s outer halo. The spectra of the stars reveal their radial velocity, providing us with precise measurements of how objects in the outer halo move.

More Space in the Suburbs?

distance histogram

Histogram with distance for the ~450 RR Lyrae stars in the authors’ broader sample. When the authors include their estimates for the completeness of their sample, the best fit scales with distance as r-4, shown by the red line. [Cohen et al. 2017]

After reporting the velocity dispersions that they measure — which can be used to make more precise estimates of the Milky Way’s total mass — Cohen and collaborators discuss the stellar density implied by their sample. They find that the density of stars in the outer halo of the Milky Way scales with their distance as r-4. This is similar to the drop-off in density we’ve measured in the inner halo, and it contradicts some studies that have predicted a much sharper drop in stellar density in the Milky Way’s outermost regions.

The work presented in this study goes a long way toward building our view of the galaxy’s outer halo. Future catalogs like the Pan-STARRS RR Lyrae catalog and upcoming surveys like LSST should also significantly increase the tracer sample size and measurement accuracy, further allowing us to map out the outskirts of the Milky Way.

Citation

Judith G. Cohen et al 2017 ApJ 849 150. doi:10.3847/1538-4357/aa9120

solar corona

The solar corona has a problem: it’s weirdly hot! A new study explores how magnetic waves might solve the mystery of the unusually hot corona by transporting energy to the outer atmosphere of the Sun.

The Problem with the Corona

Solar temperatures

The temperatures of different layers of the Sun. Click for a closer look. [ISAS/JAXA]

The corona, the outer layer of the Sun’s atmosphere, has typical temperatures of 1–3 million K — significantly hotter than the cool 5,800 K of the photosphere, the surface of the Sun far below it. Since temperatures ordinarily drop the further you get from the heat source (in this case, the Sun’s atom-fusing center), this so-called “coronal heating problem” poses a definite puzzle.

As is the case for many astronomical mysteries, the answer may have something to do with magnetic fields. Alfvén waves, magnetohydrodynamic waves that travel through magnetized plasma, could potentially carry energy from the convective zone beneath the Sun’s photosphere up into the solar atmosphere. There, the Alfvén waves could turn into shock waves that dissipate their energy as heat, causing the increased temperature of the corona.

DKIST

The Daniel K. Inouye Solar Telescope, located on the summit of Haleakala in Hawaii, is scheduled to be completed in 2018. [Ekrem Canli]

Predicting Observations

Alfvén waves as a means of delivering heat to the corona makes for a nice picture, but there’s a lot of work to be done before we can be certain that this is the correct model. Observational evidence of Alfvén waves has thus far been limited to specific conditions — and the observations have not yet been enough to convince us that Alfvén waves can deliver enough energy to explain the corona’s temperature.

Lucas Tarr, a scientist at the Naval Research Laboratory, argues that upcoming solar telescopes may make it easier to detect these waves — but first we need to know what to look for! In a recent study, Tarr uses a simplified analytic model to show which frequencies of waves are likely to carry power when magnetic field lines in the corona are pertubed.

A Promising Future

power distribution

The power carried by Alfvén waves as a function of frequency, as a result of an initial perturbation, plotted for several different initial conditions (such as the size of the perturbation or the length of the loop on which it is introduced). [Tarr 2017]

Tarr modeled the effects of a minor perturbation — like a local magnetic reconnection event in the corona — on a coronal arcade, a common structure of magnetic field loops found in the corona. Tarr determined that such a disturbance would peak in power at a low frequency (maybe tens of millihertz, or oscillations on scales of minutes), but a substantial portion of the power is carried by waves of higher frequencies (0.5–4 Hz, or oscillations on scales of seconds).

Tarr’s findings confirm that with the cadence and sensitivity of current instrumentation, we would not expect to be able to detect these Alfvén waves. The results do indicate, however, that high-cadence observations with future telescope technology — like the instrumentation at the upcoming Daniel K. Inouye Solar Telescope, which should be completed in 2018 — may have the ability to reveal the presence of these waves and confirm the model of Alfvén waves as the means by which the Sun achieves its mysteriously hot corona.

Citation

Lucas A. Tarr 2017 ApJ 847 1. doi:10.3847/1538-4357/aa880a

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