From AAS NOVA: “A Nearby Stellar Stream Gets Carded”

From AAS NOVA

14 August 2019
Susanna Kohler

This image of the night sky, centered on the South Galactic Pole, shows the locations of the members of the Pisces–Eridanus stellar stream as red dots that span the southern galactic hemisphere. A new study suggests this stream may be much younger than originally thought. [Stefan Meingast, ESA/Gaia/DPAC]

Pisces–Eridanus may try to pass itself off as a billion years old, but scientists are calling its bluff. The Transiting Exoplanet Survey Satellite (TESS) has now carded this nearby stream of stars, revealing that it’s actually a relative baby!

NASA/MIT TESS replaced Kepler in search for exoplanets

Looking for Age

Stellar streams are faint associations of stars that were born together and move together, but they’ve been stretched into long tails across the sky. Some stellar streams likely originated as dense, compact clusters of stars that were pulled into streams by tidal interactions; others may have formed in a decentralized fashion and been spread further apart with time.

To understand the evolution of stars in streams and clusters, we use benchmarks: sample star clusters of different ages that we’ve explored in high detail. Unfortunately, most star clusters and associations that we can observe closely are young. Known older clusters all lie at larger distances — the 1-Gyr-old benchmark cluster NGC 6811, for example, is 3,600 light-years away — which limits what we can learn from them.

Artist’s impression of a stellar stream arcing high in the Milky Way’s halo. The Pisces-Eridanus stream has been discovered much closer to Earth than those illustrated here. [NASA]

Plot of distance vs. age for a selection of benchmark open star clusters. Pisces–Eridanus was originally identified as being 1 Gyr old (Meingast et al. 2019 red marker on the plot), which would make it the oldest cluster within 300 pc (~1,000 light-years). [Curtis et al. 2019]

Can I See Your ID?

It’s for this reason that the recent discovery of the Pisces–Eridanus stream — a faint stellar stream that spans 120° in the sky, is located just 260–740 light-years away, and was originally aged at 1 billion years — was met with a warm welcome. This unexpectedly close stream could prove to be a critical new 1-Gyr-old benchmark that would help us better understand stellar evolution.

Acting as bouncers for the 1 Gyr+ club, however, is a team of astronomers led by Jason Curtis (NSF Astronomy and Astrophysics Postdoctoral Fellow at Columbia University). They’ve set out to check that Pisces–Eridanus is as old as it initially led us to believe — and it turns out we’ve been deceived.

Revealing Rotation

Curtis and collaborators used TESS light curves of more than 100 members of the Pisces-Eridanus stream to identify how rapidly the stars are spinning. In a process called gyrochronology, the authors used the stars’ measured rotation rates to determine the age of the stream by comparing the distribution of rotation periods to the distributions for benchmark clusters with known ages.

Rotation period distributions for Pisces–Eridanus (red) and three benchmark clusters: 120 Myr Pleiades (blue), 670 Myr Praesepe (cyan), and 1 Gyr NGC 6811 (orange). Pisces–Eridanus stars clearly overlap with the Pleiades stars, indicating the two clusters have the same age. [Adapted from Curtis et al. 2019]

They found that Pisces-Eridanus’s distribution precisely overlapped the distribution for the stars of the Pleiades, indicating that these two groups are the same age: a mere 120 million years old!

Curtis and collaborators then used Gaia data combined with past radial-velocity measurements to hunt for new members of the Pisces-Eridanus stream. They identified 34 new high-mass candidate members — and the colors and brightnesses of these stars also support a young age of around 120 million years.

A Target for Planet-Hunting

Does the Pisces-Eridanus stream’s newly revealed youth mean that it’s no good to us after all? Not at all, according to Curtis and collaborators. One particular value of this stream is as an exploration ground in the hunt for exoplanets; planet discoveries here will allow us to learn about planet formation in a unique, diffuse environment.

What else have we learned? This study marks the first gyrochronology study conducted using TESS data — demonstrating the valuable role TESS has to play in the future as we continue to work to understand stellar and planetary birth and evolution.

Citation

“TESS Reveals that the Nearby Pisces–Eridanus Stellar Stream is only 120 Myr Old,” Jason L. Curtis et al 2019 AJ 158 77.

https://iopscience.iop.org/article/10.3847/1538-3881/ab2899

See the full article here .

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Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “Spotting Molecular Gas in the Distant Universe”

From AAS NOVA

9 August 2019
Susanna Kohler

This eerie dark object, Barnard 68, is an example of a molecular cloud: a cloud of molecular gas that provides the fuel for star formation. A new study has found molecular gas in the very distant universe. [ESO]

Let’s be honest, the universe has an awful lot of gas. But the gas discovered in a new study isn’t your run-of-the-mill atomic gas! We’ve now found dense, star-formation-enabling molecular gas further out than ever before.

A Hubble view of a molecular cloud, roughly two light-years long, that has broken off of the Carina Nebula. [NASA/ESA, N. Smith (University of California, Berkeley)/The Hubble Heritage Team (STScI/AURA)]

NASA/ESA Hubble Telescope

A Crucial Ingredient

Interstellar gas fills galaxies, lingering in the space between stars. Most of this material is in atomic form — primarily low-density ionized hydrogen and helium. But in some regions, conditions are right for atoms to join together into molecules, forming reservoirs of molecular gas. Less than 1% of the Milky Way’s interstellar medium is molecular gas by volume — yet this gas is critical to the galaxy’s development.

You can’t get star formation without molecular gas; this cold, dense material forms the fuel that can eventually collapse into hydrogen-fusing cores. This means that hunting molecular gas can give us insight into how galaxies build up and form their stellar populations: molecular gas reservoirs actively feed violently starbursting galaxies throughout the universe.

Molecular gas is also often associated with the host galaxies of distant quasars, supermassive black holes accreting vast amounts of matter and shining brightly. By studying the properties of this molecular gas, we can learn more about how supermassive black holes evolve with their host galaxies.

Intensity maps of CO line emission show two locations of molecular gas: PSO145+19 and PSO145+19N. The blue cross marks the location of the known quasar.[Koptelova & Hwang 2019]

Looking Back in Time

Because galaxy formation and evolution is very much a big-picture question, we might wonder how molecular gas was different in the early universe. Did early star-forming galaxies contain more molecular gas than today’s galaxies? What were the properties of the gas? How did early galaxies form and evolve, creating young stars and feeding their central black holes?

To answer these questions, we need to hunt for large reservoirs of molecular gas at high redshifts. But this is challenging! The most common component of molecular gas, molecular hydrogen, isn’t easily detectable. For this reason, we turn to carbon monoxide (CO) as a tracer of molecular gas reservoirs.

So far, the most distant detections we’ve made of molecular gas using CO emission are at redshifts of z = 6–6.9. But now a pair of scientists from National Central University in Taiwan have looked even further.

Drama of a Distant Interaction

Using observations of CO emission lines, Ekaterina Koptelova and Chorng-Yuan Hwang have discovered two sources containing molecular gas at a redshift of z = 7.09. That’s 13 billion light-years away, or from a time when the universe was just ~700,000 years old!

ALMA spectra of PSO145+19 (top panels) and PSO145+19N (bottom panels) reveal spectral lines corresponding to CO emission and water emission. [Koptelova & Hwang 2019]

Koptelova and Hwang estimate the two molecular-gas sources to be roughly 27,000 and 41,000 light-years across. One of the two sources is coincident with a previously discovered quasar, and the other is located about 68,000 light-years away.

The properties of the sources lead the authors to suggest that the gas may be tracing two or more star-forming galaxies that are interacting in the early universe. These colliding monsters contain reservoirs of molecular gas to fuel their star formation, as well as at least one quasar.

Future observations will hopefully confirm this picture and help us to better understand the role that molecular gas plays in the dramatic formation and evolution of galaxies in the early universe.

Citation

“A Luminous Molecular Gas Pair beyond Redshift 7,” Ekaterina Koptelova and Chorng-Yuan Hwang 2019 ApJL 880 L19.
https://iopscience.iop.org/article/10.3847/2041-8213/ab2ed9

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “An ‘Impossible’ White Dwarf Identified in Kepler Data”

From AAS NOVA

7 August 2019
Susanna Kohler

In this artist’s illustration of self-lensing, a white dwarf transits in front of a companion star, gravitationally lensing the star behind it. [NASA/JPL-Caltech]

Gravitational Lensing NASA/ESA

Gravitational microlensing, S. Liebes, Physical Review B, 133 (1964): 835

Though Kepler’s primary mission ended years ago, the resulting dataset remains a vast playground in which astronomers continue to discover new surprises in stellar light curves. The latest? Evidence of a white dwarf that defies all expectations.

Forming a Lightweight

White dwarfs come in a range of different sizes. A typical white dwarf might be around 0.6 solar masses and arise when an isolated star of perhaps a few times the mass of the Sun expands into a red giant, exhausts its fuel supply, and puffs off its outer layers, leaving behind its cold, dense core.

But some observed white dwarfs have much lower masses — say, between 0.15 and 0.3 solar masses. To produce such a small remnant mass, the mass of the initial progenitor star would also have to be very low. But this poses a problem: smaller stars take longer to evolve, so a star of such low mass would need longer than the age of the universe to exhaust its fuel supply!

Since isolated stellar evolution can’t explain extremely low-mass white dwarfs, astronomers have settled on another explanation: binary interactions. In this scenario, the close orbit of two stars in a binary results in material being stripped away from the progenitor star, accelerating its mass loss and allowing it to evolve into a very low-mass white dwarf.

So far, this explanation has fit our observations. But now, the discovery of a new low-mass white dwarf is challenging our understanding.

Example diagram of a different self-lensing binary system, KOI-3278. When the white dwarf passes between us and the primary star, gravitational magnification causes a brightening in the light curve that we detect. For KIC 8145411, we do not observe an occultation, because the light from the white dwarf is too faint to detect directly. [Eric Agol]

Self-Lensing Surprise

In a new publication, a team of scientists led by Kento Masuda (NASA Sagan Fellow at Princeton University) present the discovery of the binary system KIC 8145411 from Kepler data. This unique binary is one of only five known self-lensing systems: one object in the binary gravitationally lenses the light of the other as it passes in front once per orbit.

Masuda and collaborators use follow-up observations from the Fred Lawrence Whipple Observatory in Arizona and the Subaru telescope in Hawaii to pin down the properties of the system, confirming that we’re looking at a 0.2 solar-mass white dwarf orbiting a Sun-like star in an edge-on, eclipsing orbit.

But here’s the catch: KIC 8145411’s orbit is quite wide, at 1.28 AU (a period of ~450 days) — ten times too wide for the primary and the white-dwarf progenitor to have interacted in the way we’d expect. How, then, did this “impossible” white dwarf come to exist?

Masses of known white dwarfs in binaries and their orbital periods. The KIC 8145411 system is a clear outlier, having both a low mass and a very wide orbit. Click to enlarge. [Masuda et al. 2019]

Tip of the Iceberg

Masuda and collaborators discuss a few proposed formation mechanisms — like interactions with a since-ejected or swallowed tertiary object — but none of them are especially satisfying.

So what’s next? The authors point out that we had only a 1 in 200 chance of detecting this particular system, due to its edge-on orientation — which likely means that KIC 8145411 is just the tip of the iceberg. Now that we know what we’re looking for, dedicated searches may turn up many more of these systems in the future — hopefully helping us to explain why this white dwarf is possible after all!

Citation

“Self-lensing Discovery of a 0.2 M⊙ White Dwarf in an Unusually Wide Orbit around a Sun-like Star,” Kento Masuda et al 2019 ApJL 881 L3.
https://iopscience.iop.org/article/10.3847/2041-8213/ab321b

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “Using Mergers to Understand Neutron Stars”

From AAS NOVA

5 August 2019
Susanna Kohler

Artist’s impression of a black hole tearing apart and devouring a neutron star during a merger. [Dana Berry/NASA]

There’s still a lot we don’t know about the internal structure and behavior of neutron stars, the compact remnants of giant, collapsed stars. Can the mergers of neutron stars with another type of exotic object, black holes, reveal important information?

Uncharted Remnants

Since neutron stars were first theorized, we’ve observed about 2,000 of them in the Milky Way and the Magellanic Clouds. Due to the exotic high-density conditions inside neutron stars, however, we still don’t well understand their interior structure and behavior.

A star’s internal properties are characterized by its “equation of state”. To better constrain the equation of state for neutron stars, we need accurate measurements of their masses and radii. But though we know that neutron stars are typically have the mass of a couple of Suns packed into a sphere of order 10 km in radius, it’s challenging to get precise enough radius measurements to constrain the equation of state for these dense, distant objects.

Can we do a better job of measuring neutron-star radii using future observations of merging neutron-star–black-hole binaries? A team of scientists led by Stefano Ascenzi (Tor Vergata University of Rome, INAF Rome Observatory, and Sapienza University of Rome, Italy) thinks the answer is yes — and in a new study, the team outlines how.

A Well-Marked Collision

It’s expected that the merger of a neutron star with a black hole would result in the destruction of the neutron star, the brief formation of a torus around the black hole as the neutron-star matter rains back down, and the rapid accretion of the torus.

This process would produce a pair of observable signals:

a gravitational-wave chirp from the merger, visible to detectors like the Laser Interferometer Gravitational-wave Observatory (LIGO) and its European counterpart Virgo, and
a short gamma-ray burst (GRB) caused by the accretion of the torus.

MIT /Caltech Advanced aLigo

Example of how the various curves of mass vs. radius described by different neutron-star equations of state (colored curves) could be eliminated based on mass and radius measurements of a neutron star during a merger. In this test run, the authors’ injected neutron star properties are shown by the blue dot and the recovered properties are shown by the red dot. The red dashed lines show the 68% and 90% credible regions for the recovered properties — which, in this example, eliminate several of the possible equations of state. [Adapted from Ascenzi et al. 2019]

According to Ascenzi and collaborators, we can use these two signals in tandem to obtain information about the neutron star. Fitting the gravitational-wave signal of such a merger will reveal the neutron-star and black-hole masses, as well as the spin of the black hole. Measurements of the short GRB energy will tell us how much mass was in the torus that accreted onto the black hole.

The combination of this information allows us to infer the radius of the neutron star in the initial binary. Ascenzi and collaborators show that this method would let us estimate a neutron star radius to within 20% accuracy for a gravitational-wave detection with a signal-to-noise ratio of 10. For comparison, the signal-to-noise ratio for GW 170817, the first observed binary neutron star merger, was more than 30.

Looking for a New Type of Merger

How lucky do we have to get to observe a neutron-star–black-hole merger simultaneously in gravitational waves and electromagnetic radiation? Theory predicts these joint detections will perhaps occur at rates of 0.1 to 10 per year with current technology, and future telescopes and gravitational-wave detectors should increase our odds.

Here’s hoping these spectacular explosions will reveal more about neutron-star interiors soon!

Citation

“Constraining the Neutron Star Radius with Joint Gravitational-wave and Short Gamma- Ray Burst Observations of Neutron Star–Black Hole Coalescing Binaries,” Stefano Ascenzi et al 2019 ApJ 877 94.
https://iopscience.iop.org/article/10.3847/1538-4357/ab1b15

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “A Speedier Check of Models”

From AAS NOVA

24 July 2019
Susanna Kohler

An output from Meraxes, a galaxy formation model. A new study explores how scientists can more efficiently analyze increasingly complex models. [Meraxes/Simon Mutch]

Astronomy is driven forward by a combination of novel observations and complex, inventive modeling. How can astronomers better analyze their models? A new study presents a tool for the job — and is also the first article published under a new partnership between the American Astronomical Society (AAS) and the Journal of Open Source Software.

Exploring a Complex Universe

Modeling complicated astronomical systems is an important part of how we work to understand the universe. As technology advances, models have become increasingly more complex, encompassing more and more parameters. Complex models can do a better job of describing the astronomical systems we observe, but they’re also more challenging — and time-consuming — to analyze to see how well they might fit data.

Reconstruction of a multi-Gaussian model with 12 parameters using two techniques: normal MCMC parameter sampling (right column) and hybrid PRISM+MCMC parameter sampling (left column). The reconstruction is shown as a blue solid line, and the true model is shown as a dashed black line. The different rows represent increasing iterations. The hybrid reconstruction fits the known data better after fewer iterations — effectively analyzing the model 16 times faster than the normal MCMC approach. [van der Velden et al. 2019]

One approach astronomers often use to analyze many-parameter models is Markov Chain Monte Carlo (MCMC) methods. With MCMC methods, instead of fully evaluating a model throughout parameter space, you randomly sample the model in a variety of places. This provides a general idea of the behavior of the model without requiring the time and computing investment of a full analysis.

While MCMC methods are generally robust, they can be quite slow — you might spend a lot of time sampling uninteresting parts of parameter space rather than focusing on the ones that are most likely to describe your system. To address this problem, a team of scientists led by Ellert van der Velden (Swinburne University of Technology and ARC Centre of Excellence in All Sky Astrophysics, Australia) have developed a new tool: a software package they call Probabilistic Regression Instrument for Simulating Models, or PRISM.

Let’s Speed Up the Process

How does PRISM work? When given a model to analyze, PRISM uses clever statistical methods to create an approximation of the model and iteratively predict which regions of parameter space aren’t of interest. This allows the user to home in on the interesting regions and explore the general behavior of a model very quickly. This algorithm can be used either alone or in conjunction with MCMC methods to analyze models more efficiently.

PRISM’s approach isn’t new: these techniques have previously been used to analyze models in a variety of scientific disciplines, including the study of whales, oil reservoirs, galaxy formation, disease, and biological systems. But PRISM takes the techniques and neatly bundles them up into a python software package that anyone can use to analyze their models — a valuable tool for astronomers and other scientists alike!

A New Partnership for Software

Want more assurance about this software? You’ve got it! Van der Velden and collaborators’ article on PRISM, published in ApJS, is just one of a pair of publications; the second is the scrutinized software itself.

Under a new agreement between the AAS and the Journal of Open Source Software (JOSS), scientists submitting articles about astronomical software to AAS journals may choose not only to have their article reviewed, but also to have the software itself reviewed at JOSS in parallel. When both review processes are complete, the reviewed software is linked with the paper describing it in AAS journals.

The article presenting PRISM is the first of these simultaneous reviews to be conducted and published, and we expect many more to come! Software plays such an integral role in the study of astronomy today, and AAS publishing is pleased to help ensure that these valuable tools are shared.

Citation

“Model Dispersion with PRISM: An Alternative to MCMC for Rapid Analysis of Models,” Ellert van der Velden et al 2019 ApJS 242 22.
https://iopscience.iop.org/article/10.3847/1538-4365/ab1f7d

“Model dispersion with PRISM; an alternative to MCMC for rapid analysis of models,” van der Velden 2019 Journal of Open Source Software, 4(38) 1229.
http://joss.theoj.org/papers/10.21105/joss.01229

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “Five Years Watching Volcanoes on Another World” Jupiter’s moon Io

From AAS NOVA

22 July 2019
Susanna Kohler

Image of Jupiter’s volcanic moon Io, taken by the Galileo spacecraft in 1997. [NASA/JPL/University of Arizona]

NASA/Galileo 1989-2003

For all that space telescopes are powerful tools for exploring our universe, we can achieve some remarkable science using ground-based observations! A new study explores the lessons learned from five years of monitoring Jupiter’s volcanic moon Io from the ground.

This set of images from Keck, all taken within 30 minutes of each other, demonstrates the range of filters used to observe Io during this campaign. [de Kleer et al. 2019]

Keck Observatory, operated by Caltech and the University of California, Maunakea Hawaii USA, 4,207 m (13,802 ft)

A Dramatic Landscape

Jupiter’s innermost moon, Io, is a dramatic, roiling world of heated activity. The moon’s not-quite-circular orbit means that it receives a varying gravitational tug from Jupiter, generating friction and warming up the moon’s interior. This heat then escapes from Io’s surface in the form of active volcanic vents, tremendous explosions, and scalding lava flows.

Continuous monitoring of all of these activities — Io’s hotspots, or locations of thermal emission — is essential to understand how heat is dissipated in this violently active moon. We’ve had the opportunity to explore Io’s volcanism up close as the Voyager, Galileo, Cassini, and New Horizons missions have each passed by the moon, revealing more than 150 active volcanoes on Io’s surface. But these brief flybys don’t provide the important long-term, high-cadence observations of Io’s hotspots needed to truly track its activity.

Luckily, space-based astronomy is not the only solution!

View from the Ground

Over the last five years, scientists have carefully monitored Io’s thermal emission using the Keck and Gemini North telescopes located in Hawaii.

NOAO Gemini North on MaunaKea, Hawaii, USA, Altitude 4,213 m (13,822 ft)

Think their observations couldn’t possibly be as useful as the up-close data from space telescopes? Think again! The powerful adaptive optics on Keck and Gemini North allowed the team to resolve down to distances of 100–500 km on Io’s surface in infrared— a scale not far from the resolution attained by the Near-Infrared Mapping Spectrometer on Galileo during its flybys.

Keck Adaptive Optics

Gemini North Adaptive Optics

What’s more, the flexible scheduling of Gemini North and a dedicated observing program at Keck made it possible for the team to gather 271 nights of observations of Io over 5 years. In a new study led by Katherine de Kleer (California Institute of Technology), the team now details what they’ve learned from this campaign.

Lessons from Hotspots

Five years of observing have produced a grand total of 980 detections of more than 75 unique hotspots. A few points of interest from these observations:

The brightest eruptions are generally short-lived (lasting only a few days) and very hot (above 800 K, or nearly 1,000°F). They also almost all cluster in Io’s trailing hemisphere — the side of the moon located away from its direction of motion. This trend remains unexplained.
A number of new hotspots have only been detected in the past three years. Some of these likely existed before but only emit sporadically; others may have arisen more recently.
113 detections of the extremely active Loki Patera hint at a periodicity to this volcano of ~470 days — behavior that could be tied to Io’s orbital properties.

The authors have made all of their hotspot data available for public download and invite the astronomy community to extend their work. Between future analysis of these data and further observations of Io, we can certainly look forward to more insights into this heated, dynamic world.

Citation

“Io’s Volcanic Activity from Time Domain Adaptive Optics Observations: 2013–2018,” Katherine de Kleer et al 2019 AJ 158 29.
https://iopscience.iop.org/article/10.3847/1538-3881/ab2380

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “Heating Up a Solar Flare”

From AAS NOVA

19 July 2019
Kerry Hensley

NASA’s Solar Dynamics Observatory captures a solar flare in the act. [NASA/SDO]

NASA/SDO

Powerful solar flares are dazzlingly bright in ultraviolet and X-ray images of the Sun. Despite their demands for attention, there’s still a lot that we don’t know about these unpredictable eruptions.

Clues from 121.6 nm

These Solar Dynamics Observatory images of the Sun show a solar flare in three extreme ultraviolet wavelengths. From left to right: 17.1, 30.4, and 13.1 nanometers. [NASA/GSFC/SDO]

Solar flares shoot energetic particles and photons from across the electromagnetic spectrum into interplanetary space. In order to understand how energy is released in solar flares, we need to first know how energy is injected.

To explore where flares get their energy, a team led by Jie Hong (Nanjing University, China) focused on a familiar feature of the ultraviolet solar spectrum: the 121.6-nm hydrogen Lyman-α emission line, produced by the roiling, turbulent hydrogen gas in the Sun’s atmosphere. The shape and behavior of the Lyman-α emission line can be used to learn about many different types of activity in the Sun’s chromosphere and corona — including solar flares.

Evolution of Lyman-α profiles over time. The top row shows the time evolution of the profiles in the non-thermal heating case. The asymmetry of the peaks transitions from long to short wavelengths as time and energy increase. The bottom row shows the thermal heating case (left) and the thermal heating plus a soft electron beam case (right). In the thermal heating case, the double peak morphs into a single peak.[Hong et al. 2019]

Modeling Flare Emission

Hong and collaborators used radiative hydrodynamics to model solar flares heated by different mechanisms. Their goal was to explore how the type of heating might change the shape of the Lyman-α line we observe.

In particular, they examined two means of heating the flares: a thermal mechanism where the energy comes from conduction from nearby plasma, and a non-thermal mechanism where the heat is provided by a beam of energetic electrons generated by magnetic reconnection. In the non-thermal case, they also varied the strength of the heating by an order of magnitude.

After allowing the modeled flares to evolve for eight or ten seconds, the researchers looked for subtle changes in the shape of the Lyman-α profile that could be linked to the underlying heating mechanism.

The asymmetries and peaks of the modeled emission lines showed distinctive patterns and behavior over time — fingerprints, Hong and collaborators argue, that could help identify the source of heat for an observed flare.

Flare Photography

Hong and collaborators note that their modeling efforts will complement future solar observations, helping to clarify the complex picture of flare evolution.

In particular, they look forward to the joint NASA-ESA Solar Orbiter mission, set to launch in 2020, which will be the first spacecraft to snap extreme-ultraviolet pictures of the Sun from out of the ecliptic plane, and China’s Advanced Space-Based Solar Observatory (ASO-S), which is scheduled for launch in 2022. ASO-S will carry a dedicated Lyman-α imager.

ESA/NASA Solar Orbiter depiction

After decades of observations, it looks like the field of flare research is still heating up!

Citation

“The Response of the Lyα Line in Different Flare Heating Models,” Jie Hong et al 2019 ApJ 879 128.
https://iopscience.iop.org/article/10.3847/1538-4357/ab262e

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “Exploring an Odd Stellar Death”

From AAS NOVA

17 July 2019
Susanna Kohler

Artist’s impression of a supernova explosion. A new study explores whether the merger of two massive stars could lead to a unique kind of supernova. [ESO/M. Kornmesser]

Massive stars can die in a lot of different ways! A new study explores one possible channel in more detail.

Detectives Are on the Case

Artist’s illustration of a star exploding in a supernova at the end of its lifetime. [NASA/CXC/M. Weiss]

NASA/Chandra X-ray Telescope

Studying supernovae is a little like being a detective in an odd sort of murder mystery. You’ve witnessed the death of a massive star — and from this evidence, you must determine what type of star died, how it died, and even what interactions it had before its death.

As we enter the era of ever more expansive sky surveys, we can expect to amass not just evidence of typical stellar deaths, but also some more unusual ones. In the process, piecing together the evidence to solve each mystery becomes progressively more challenging — but also more intriguing!

In a recent study, a team of scientists led by Alejandro Vigna-Gómez (U. of Birmingham, UK; Monash U., Australia; U. of Copenhagen, Denmark) have explored one particular oddball type of theorized stellar death: pulsational pair-instability supernovae (PISNe).

Gravity (Usually) Wins

According to theory, PISNe occur when a very massive (hundreds of solar masses) star gets hot enough to start producing pairs of electrons and positions. This process saps the star’s internal energy, leading to its sudden collapse as the force of gravity triumphs.

This collapse can end in the dramatic explosion of a PISN, or it may lead to a smaller eruption that only sheds some of the star’s mass. In the latter case, the star may go through multiple rounds of smaller eruptions before eventually running out of nuclear fuel and undergoing a final explosion — as a pulsational PISN.

Schematic showing three possible ways massive stars can die; click to enlarge. Top and bottom panels describe outcomes of single-star evolution, depending on the star’s mass. The center channel depicts the merger of two evolved, massive stars to form an object with a large envelope of hydrogen. This can lead to a hydrogen-rich pulsational PISN. [Vigna-Gómez et al. 2019]

Starting with a Merger

If this weren’t complicated enough, Vigna-Gómez and collaborators propose one further twist on this stellar death scenario: the object exploding in a pulsational PISN needn’t simply be a massive star. Instead, it might be the product of the merger of two massive stars.

Vigna-Gómez and collaborators argue that this type of merger is expected to be common, and it would produce a very massive object with a large outer hydrogen shell. By running a series of simulations using the Modules for Experiments in Stellar Astrophysics (MESA), the authors demonstrate that such a merger product could undergo a pulsational PISN and still retain a significant portion of its hydrogen shell up to the final explosion, leaving the fingerprint of hydrogen in the supernova spectrum.

The light curve of iPTF14hls is extremely unusual, featuring multiple apparent explosions. [Adapted from Las Cumbres Observatory/S. Wilkinson]

LCO_map_2017. Map of the Las Cumbres Observatory global network of robotic telescopes

Explanation for a Zombie Star?

Why does this particular theorized death matter? Stellar detectives are currently working to explain the deaths in a number of especially weird observed supernovae, and this model might match some of them. One example is iPTF14hls, the “zombie star” that’s made headlines for apparently erupting multiple times and defying explanation — in part because of the unexpected hydrogen signatures in its spectra.

We can’t yet say for sure whether iPTF14hls is an example of a stellar-merger-turned-pulsational-PISN — that will require more extensive modeling and analysis of observations — but Vigna-Gómez and collaborators think it’s a good candidate! And while we wait on the verdict of that mystery, we can be sure that transient surveys are busy finding many more examples of stellar deaths for us to puzzle over.

Citation

“Massive Stellar Mergers as Precursors of Hydrogen-rich Pulsational Pair Instability Supernovae,” Alejandro Vigna-Gómez et al 2019 ApJL 876 L29.
https://iopscience.iop.org/article/10.3847/2041-8213/ab1bdf

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “How Venus Reacts when the Sun Strikes”

From AAS NOVA

8 July 2019
Susanna Kohler

Artist’s impression of the Venus Express spacecraft in orbit around Venus. [ESA]

What happens to Venus when an enormous solar eruption slams into the planet? In 2011, the Venus Express spacecraft was on site to find out!

Schematic illustration of the Earth’s global magnetic field. Venus does not have an intrinsic field. [NASA / Peter Reid / The University of Edinburgh]

Benefits of a Field

The Earth’s magnetic field does an excellent job of protecting us from the damaging influence of the solar wind. Energetic particles emitted by the Sun are deflected around our planet and channeled to the poles, where they harmlessly light up the sky in haunting aurorae. Even the danger of sporadic solar eruptions — like flares and coronal mass ejections — is largely mitigated by our protective shield.

But our sister planet, Venus, is less fortunate: though similar to Earth in many ways, Venus lacks its own global magnetic field to protect it from the Sun’s onslaught.

What happens to this clouded planet when the Sun sends an enormous interplanetary coronal mass ejection its way?

The interaction of Venus with the magnetized solar wind produces an induced magnetosphere. [Ruslik0]

With a Little Help from the Sun

Venus has a trick up its sleeve: though it doesn’t carry its own magnetic field, it boasts an induced magnetosphere.

As extreme ultraviolet radiation from the Sun lights up Venus’s dayside, it ionizes the planet’s upper atmosphere, forming a plasma known as the ionosphere. When the solar wind — which carries the Sun’s magnetic field with it — encounters Venus, the thermal pressure of the ionosphere pushes back against the magnetic pressure of the solar wind, causing the field lines to drape around Venus and remain supported there.

This induced magnetosphere has a bow shock on the Sun side and a long, trailing magnetotail on the anti-Sun side. The pile-up of magnetic field between the magnetosphere and Venus’s ionosphere — the magnetic barrier — prevents the solar-wind plasma from penetrating deeper down into Venus’s atmosphere.

Front-Row Seats to Action

So Venus isn’t unprotected — but how well does this shield hold up in the face of powerful solar storms? In 2011, we had a orbiter ready to watch the stormy drama up close: the Venus Express spacecraft.

Venus Express, launched in 2005, orbited around Venus’s poles and studied the global space environment around the planet.

ESA/Venus Express

On 5 November, 2011, an extremely strong interplanetary coronal mass ejection hit Venus while the spacecraft was in orbit — and now, in a publication led by Qi Xu (Macau University of Science and Technology, China), a team of scientists has detailed what the spacecraft learned.

Not Unflappable

The magnetic field strength (top) and direction (bottom) measured by the Venus Express reveal the rapid flapping motion of the plasma sheet in the magnetotail in response to the interplanetary coronal mass ejection. The red line shows that the Bx component of the magnetic field changed direction 5 times within 1.5 minutes (7:49:30–7:51:00)! [Adapted from Xu et al. 2019]

Venus Express’s data show that the planet’s induced magnetosphere and its ionosphere responded dramatically to the strong solar eruption. Venus’s bow shock was compressed and broadened as the storm hit; the plasma sheet of the magnetotail flapped back and forth rapidly; the magnetic barrier increased in strength; and the ionosphere was excited, jumping to a whopping three times the quiet-Sun plasma density!

Based on their analysis, Xu and collaborators expect that interplanetary coronal mass ejections like this one substantially increase the rate of Venus’s atmospheric loss, violently driving ions from the planet’s gravitational grasp.

We still have a lot to learn about about how our sister planet reacts when solar storms strike, but these observations have shed new light on the dramatic struggle.

Citation

“Observations of the Venus Dramatic Response to an Extremely Strong Interplanetary Coronal Mass Ejection,” Qi Xu et al 2019 ApJ 876 84.
https://iopscience.iop.org/article/10.3847/1538-4357/ab14e1

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

From AAS NOVA: “Exomoon, or No Exomoon?”

From AAS NOVA

Susanna Kohler
24 June 2019

Artist’s impression of the exomoon candidate Kepler-1625b-i and its host planet and star. [NASA/ESA/L. Hustak (STScI)]

Last October, the first discovery of a potential exomoon was announced. But is Kepler-1625b-i an actual moon in another solar system? Or just an artifact of data reduction?

A Tricky Business

Artist’s depiction of an Earth-like exomoon orbiting a gas-giant planet. [NASA/JPL-Caltech]

Moons are a useful diagnostic — they can provide all kinds of information about their host planets, like clues to formation history, evolution, and even whether the planet might be habitable. What’s more, exomoons themselves have been indicated as potential targets in the search for life: while a habitable-zone gas-giant planet might not be an ideal host, for example, such a planet could have moons that are.

Given all we stand to learn from exomoons, it’d be great to find some! But for all that our solar system is chock full of moons (at last count, Jupiter alone hosts 79!), we’ve yet to find any sign of exomoons orbiting planets beyond the solar system.

This may well be because exomoon signals are difficult to spot. Not only would an exomoon’s signal be tiny compared to that of its host planet, but we also would need to separate that signal from the host’s — a tricky business. Throw in some instrument systematics to obscure all the data, and exomoon identification becomes even more of a challenge.

For these reasons, it was a pretty exciting announcement last fall when Columbia University astronomers Alex Teachey and David Kipping presented Kepler-1625b-i, a signal that they argued represented an exomoon around the gas giant Kepler-1625b. But a healthy dose of scientific caution has sent other teams scrambling to explore these data and draw their own conclusions — and one of these groups is calling the exomoon discovery into question.

Waiting for Consensus

Best-fit models for the Kepler 1625 light curve assuming a planet and no moon (top) or moon (bottom). Data as analyzed by Kreidberg et al. are on the left (blue); data as analyzed by Teachey&Kipping are on the right (red). Kreidberg et al. find that the best fit is given by the no-moon model. Click to enlarge. [Kreidberg et al. 2019]

Led by Laura Kreidberg (Harvard-Smithsonian Center for Astrophysics), a team of astronomers has independently analyzed the same Hubble transit data that Teachey and Kipping used to identify their exomoon candidate. Unlike the other group, however, Kreidberg and collaborators found that the data are best fit by a simple planet transit model — the presence of an exomoon isn’t necessary or indicated.

According to Kreidberg and collaborators, the discrepancy between their results and Teachey and Kipping’s is most likely due to differences in data reduction. Teachey and Kipping have responded to this work with additional analysis in a recent paper submitted to AAS journals, but the debate is far from settled.

So is there an exomoon, or isn’t there? We don’t know yet, but that’s okay!

The case of Kepler 1625 is a beautiful illustration of the messy reality of the scientific process: sometimes the data don’t immediately spell out an answer, and it takes more time, more analysis, and likely more observations before the scientific community reaches a consensus. This isn’t a bad thing, though — this is science being done right! Keep an eye on the story of Kepler 1625b-i going forward; we’re bound to continue to learn about this maybe/maybe-not exomoon.

Citation

“No Evidence for Lunar Transit in New Analysis of Hubble Space Telescope Observations of the Kepler-1625 System,” Laura Kreidberg et al 2019 ApJL 877 L15.
https://iopscience.iop.org/article/10.3847/2041-8213/ab20c8

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009