From AAS NOVA: “Climates of Distant Terrestrial Worlds”

From AAS NOVA

27 November 2019
Susanna Kohler

Artist’s illustration of an M-dwarf star surrounded by three planets. [NASA/JPL-Caltech]

What determines the climate of an Earth-like planet orbiting its host star? And how is that climate affected by the type of star the planet orbits? A new study explores how distant terrestrial worlds are shaped by their hosts.

Radiation In, Radiation Out

The climate for a planet like Earth is largely set by the delicate balance between incoming radiation from the planet’s star, and outgoing radiation in the form of heat emitted into space. The amount of energy absorbed, reflected, and emitted by a planet’s surface and atmosphere dictate how this balance plays out.

Diagram describing the annual mean energy budget for a planet orbiting a G dwarf star.[Adapted from Shields et al. 2019]

The pathways that govern this global energy budget for our own planet have been worked out through many decades of modeling and analysis of observations — to the point where we can identify sources of imbalance in the Earth’s system, like those currently caused by anthropogenic CO2 emissions.

But these climate models don’t apply directly to other planets, because the factors that determine a planet’s global energy budget all depend on the wavelength distribution of incoming light. Since stars of different temperatures emit varying amounts of radiation at different wavelengths, models that describe the energy budget for a planet around a Sun-like G dwarf won’t accurately describe a planet around a cooler M dwarf or hotter F dwarf.

So how do the climates of distant, Earth-like worlds change when orbiting a different type of host star? A team of scientists led by Aomawa Shields (University of California, Irvine) has now used detailed 3D global climate models to find out.

A Difference of Hosts

Shields and collaborators’ models of terrestrial planets take into account details like the interaction between the incoming host star’s radiation and gases like CO2 and H2O in the planet’s atmosphere, as well as with icy and snowy surfaces on the ground.

Plot of the global mean surface temperature as a function of the amount of incoming stellar radiation at the top of the planet’s atmosphere, shown for a planet orbiting an F dwarf (blue triangles), a G dwarf (black plus symobls), and an M dwarf (red x symbols). [Adapted from Shields et al. 2019]

The authors show that M-dwarf planets absorb more of their hosts’ radiation, both in their atmospheres and their surfaces, whereas F-dwarf planets absorb less. As a result, a planet can have a climate similar to that of modern-day Earth if it’s receiving current solar amounts of incoming radiation from a G-dwarf star — but to achieve the same climate around an M-dwarf star, it would need to receive 12% less incoming radiation. Around an F-dwarf star, it would need to receive 8% more.

What about rotation? The above models assumed that the planets all had 24-hour rotation rates, but Shields and collaborators also test how this compares to a tidally locked planet that always shows the same face to its host. For an M-dwarf host, a tidally locked planet has lower minimum and maximum dayside temperatures when compared with a planet with a 24-hour rotation period; the average dayside temperature is around 37 K colder on the tidally locked planet.

As we continue to discover more planets around a variety of stars, a constant question is whether these distant worlds have the potential to support life. Understanding how these planets’ global climates are shaped by their host stars is an important part of this exploration!

Citation

“Energy Budgets for Terrestrial Extrasolar Planets,” Aomawa L. Shields et al 2019 ApJL 884 L2.
https://iopscience.iop.org/article/10.3847/2041-8213/ab44ce

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: “Fantastically Fast Transients and How They Happen”

From AAS NOVA

22 November 2019
Tarini Konchady

An example of the appearance (left) and disappearance (right) of a fast transient — in this case, the optical counterpart of a binary neutron star merger. A new study explores a similarly rapid fast transient that may have a very different origin. [Soares-Santos et al. 2017]

A supernova-like transient was observed to decline stupendously fast. What could have caused it?

Fast Transients

“Fast transients” are objects whose brightness rises and then falls drastically, usually on the order of weeks. They are not regularly varying objects; they have more in common with supernovae, which brighten once and then fade. However, fast transients change more rapidly than supernovae do, suggesting they have different explosive progenitors.

With the advent of large astronomical surveys, fast transients are spotted more often now than ever before. The “fastest” optical fast transient is kilonova AT2017gfo — the result of the first observed binary neutron star merger. A recent study by Owen McBrien (Queen’s University Belfast) and collaborators discusses a transient that’s right on the heels of AT2017gfo in terms of the speed of its variation: a supernova-like object named SN2018kzr.

The host galaxy of SN2018kzr, as seen more than two months after the transient appeared. The image was constructed using data taken by the ESO (European Southern Observatory) Faint Object Spectrograph and Camera on the New Technology Telescope. [Adapted from McBrien et al. 2019]

ESO Faint Object Spectrograph and Camera 2 (EFOSC2)
on the NTT

ESO/NTT at Cerro La Silla, Chile, at an altitude of 2400 metres

Magnetars and Nickel

SN2018kzr was discovered independently by the Zwicky Transient Factory and the Asteroid Terrestrial-impact Last Alert System.

Zwicky Transient Facility (ZTF) instrument installed on the 1.2m diameter Samuel Oschin Telescope at Palomar Observatory in California. Courtesy Caltech Optical Observatories

ATLAS is an asteroid impact early warning system of two telescopes being developed by the University of Hawaii and funded by NASA

Observers were tipped off by its rapid brightening, which took place over hours. SN2018kzr was then observed extensively over the next two weeks by multiple observatories, yielding a wealth of photometric and spectroscopic data. It began declining in brightness the same night it was first detected.

To explain SN2018kzr’s rise and fall, McBrien and collaborators consider mechanisms that have previously been used for fast transients. They start with the reasonable assumption that nickel-56 is involved. The isotope nickel-56 can form in the explosions associated with fast transients and supernovae, and its radioactive decay can contribute greatly to a transient’s brightness. However, SN2018kzr dims too rapidly for it to be explained by nickel-56 alone, and the decay of other radioactive isotopes don’t explain observations either.

One solution is to tweak the progenitor scenario to include a massive remnant: a rotating neutron star with a strong magnetic field, known as a magnetar. A magnetar can contribute to the energy put out by SN2018kzr by slowing its own rotation. When coupled with nickel-56 decay, a magnetar’s spin-down could explain the shape of SN2018kzr’s light curve.

Fits to the light curve of SN2018kzr assuming different progenitor scenarios. The red points are associated with SN2018kzr and the white diamonds are associated with another fast transient, SN2005ek. SN2005ek is better fit by the He star model, while SN2018kzr is better fit by the nickel-56–magnetar scenario. [Adapted from McBrien et al. 2019]

Assuming that nickel-56 and a magnetar are involved in the progenitor of SN2018kzr, the authors present three possible scenarios: the core-collapse of a helium-rich star, the collapse of a white dwarf that’s accreted too much matter (accretion induced collapse, or AIC), and the merger of a white dwarf and a neutron star.

The first scenario isn’t favored since any remnant it produces wouldn’t rotate fast enough to explain SN2018kzr’s rapid decline. The authors favor the other scenarios, though the AIC model is on shaky ground based on previous studies.

The more fast transients we discover, the better our understanding becomes of how they form. Stay tuned!
Citation

“SN2018kzr: a rapidly declining transient from the destruction of a white dwarf,” Owen R. McBrien et al 2019 ApJL 885 L23.
https://iopscience.iop.org/article/10.3847/2041-8213/ab4dae

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: “Hunting for a Dark Matter Wake”

From AAS NOVA

13 November 2019
Susanna Kohler

The Large Magellanic Cloud is the Milky Way’s most massive satellite. What evidence has this galaxy left behind as it plows through the Milky Way’s dark matter halo? [ESO/VMC Survey]

As the Large Magellanic Cloud plows through the Milky Way’s dark matter halo, it may leave telltale signs of its passage. A recent study explores whether we’ll be able to spot this evidence — and what it can tell us about our galaxy and the nature of dark matter.

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

The Milky Way’s Large Companion

The Milky Way is far from lonely. Dozens of smaller satellite-galaxy companions orbit around our galaxy, charging through its larger dark matter halo. The most massive of these is the Large Magellanic Cloud (LMC), a galaxy of perhaps 10 or 100 billion solar masses that’s about 14,000 light-years across.

Studies suggest that the LMC is on its first pass around the Milky Way, traveling on a highly eccentric orbit; it likely only first got close to our galaxy (within about 200 kpc, or 650,000 light-years) about two billion years ago.

There are still many uncertainties about this satellite and its travels, however. How massive, exactly, is the LMC? What does its past orbit look like? And how has it interacted with our galaxy’s dark matter halo, which it’s passing through?

Density perturbations caused by the LMC’s motion for one of the authors’ Milky Way models. The Milky Way’s disk is in the x–y plane; the black curve traces the LMC’s past orbital path and the red star indicates its current position. Three primary overdense/underdense features are visible as signatures of the LMC’s wake. [Adapted from Garavito-Camargo et al. 2019]

A Telltale Trail

A team of scientists led by Nicolas Garavito-Camargo (Steward Observatory, University of Arizona) thinks there may be evidence we can use to answer these questions.

Like a boat, the LMC should generate a wake as it plows through the Milky Way’s dark matter halo. This wake is caused by gravitational interactions between the satellite and dark matter particles that drag at the LMC, causing the galaxy to lose angular momentum as it orbits.

The perturbations that make up this wake — overdensities and underdensities in the dark matter and stellar distribution in the halo — are signatures that we can predict and hunt for. In a new study, Garavito-Camargo and collaborators use high-resolution N-body simulations to explore the motion of the LMC through the Milky Way’s halo and examine the perturbations caused by this charging satellite.

Spotting the Evidence of Passage

The authors find that the LMC’s motion produces a pronounced dark matter wake that can be decomposed into three parts:

Transient response, a trailing wake of overdensity behind the satellite that traces its orbital history
Global underdensity, a large underdense region south of the transient response
Collective response, an extended overdensity leading the LMC in the galactic north

These features in the dark-matter distribution are echoed in how stars are distributed in the regions, and the stars should also show distinctive kinematic signatures.

Observing strategies for identifying the LMC’s wake using stellar densities. To avoid confusion with the Sagittarius stellar stream (the prominent yellow, orange, and red points indicated), the authors identify several regions for observation (colored rectangles) away from the stream where the wake should be detectable. [Garavito-Camargo et al. 2019]

Garavito-Camargo and collaborators outline an observing strategy to spot the predicted overdensities and underdensities of the wake, and they show that the detection of just 20–30 stars in specific regions could provide useful confirmation of their models. The measurements needed should be achievable with current and upcoming stellar surveys.

What can we learn from these observations? The detection of the LMC’s wake will track its past orbit, which will provide an indirect measure of our own galaxy’s mass. The specifics of the LMC’s motion will also better constrain the satellite’s mass, as well as provide clues as to the nature of the dark-matter particles that drag on it.

Citation

“Hunting for the Dark Matter Wake Induced by the Large Magellanic Cloud,” Nicolas Garavito-Camargo et al 2019 ApJ 884 51.

https://iopscience.iop.org/article/10.3847/1538-4357/ab32eb

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 New Measurement of Turbulence”

From AAS NOVA

11 November 2019
Susanna Kohler

Many astrophysical plasmas demonstrate turbulence, like the gas of the Crab Nebula, pictured here. A new study has brought us one step closer to understanding this complex physical process. [NASA, ESA, J. Hester and A. Loll (Arizona State University)]

The same physical phenomenon that causes bumpy airplane rides also pervades our universe, jumbling stellar atmospheres, interstellar clouds, and even the magnetized sheath surrounding the Earth. Now, a new study brings us a little closer to understanding turbulence.

This image captures the transition between laminar and turbulent flow in the convection plume above a candle flame.

A Complex Phenomenon

Have you ever watched the entrancing wisps of smoke rising above a candle flame? What you’re looking at is turbulence — and despite this phenomenon’s prevalence throughout the universe, a complete description of turbulence remains one of the unsolved problems in physics.

The difficulty is that turbulent motion — characterized by rapid and chaotic fluctuations of fluid properties — is incredibly complex. Turbulence begins when energy is injected on large scales, causing field-level fluctuations. This energy then cascades down to smaller and smaller scales, creating chaotic motions all the way down to microscales. When the energy reaches small enough scales, it can dissipate, accelerating individual particles and converting into heat.

But scientists don’t fully understand the physical mechanisms at work in turbulence that inject the energy, transfer it to smaller scales, and eventually dissipate it. Worse yet, these processes take a different form when we’re no longer talking about fluids, but instead about astrophysical plasmas.

Plasmas, Plasmas Everywhere

Astrophysical plasmas are soups of ionized gas found everywhere from supernova remnants to the compressed solar wind surrounding the Earth in its magnetosheath — and in these plasmas, energy could be dissipated through a variety of mechanisms related to interactions between particles and waves.

This diagram of the Earth’s magnetosphere shows the location of the magnetosheath, the region behind the bow shock where the compressed solar wind detours around the Earth. [NASA/Goddard/Aaron Kaase]

How can we tell which mechanisms are at work? The key is to explore the rate at which turbulence in a plasma is dissipated across different length scales. In a recent study, a team of scientists led by Jiansen He (Peking University, China) has now developed a new approach to examine this spectrum and applied it within the Earth’s magnetosheath.

Measuring a Fluctuating Environment

The authors’ approach takes advantage of unprecedented, high-quality measurements made by the Magnetospheric Multiscale mission, a constellation of four spacecraft exploring the plasma environment around the Earth. As these spacecraft — separated by a distance of about 10 km — pass through magnetosheath plasma, they make measurements of the three-dimensional electric and magnetic fields, tracking the field fluctuations caused by turbulence.

Artist depiction of the Magnetospheric Multiscale Mission spacecraft. [NASA/GSFC]

He and collaborators present a method that uses these measured fluctuations to investigate how the dissipation rate is distributed across various length scales within the plasma. This spectrum of dissipation rates can then tell us which physical processes are most likely at play, driving the dissipation.

While we still have a lot to learn, He and collaborators’ work indicates that ion cyclotron waves — waves generated when ions oscillate in a magnetized plasma — play an important role in dissipating turbulent energy in the Earth’s magnetosheath.

More importantly, the authors’ approach for measuring the dissipation rates at different scales can be widely applied to different space plasma environments — so we can hope for more insight into turbulence in space in the future!

Citation

“Direct Measurement of the Dissipation Rate Spectrum around Ion Kinetic Scales in Space Plasma Turbulence,” Jiansen He et al 2019 ApJ 880 121.
https://iopscience.iop.org/article/10.3847/1538-4357/ab2a79

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 Standard of Black Hole Mergers”

From AAS NOVA

8 November 2019
Tarini Konchady

A simulated image of two black holes merging. [SXS]

Being able to make precise measurements of distances and redshifts will help us understand how the universe is evolving. With the advent of gravitational wave observatories, we can make these measurements by using black holes in a very different way than before.

MIT /Caltech Advanced aLigo

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Standard Sirens

To measure how the universe is expanding, we need to simultaneously obtain the distances and redshifts to sources. When it comes to measuring large distances in space, astronomers have typically leaned on “standard candles” — objects whose intrinsic brightness is known. The dimmer a standard candle appears, the farther away it is.

Merging binary black holes (BBHs) can serve as standard candles, in a way. When compact objects like black holes merge, they produce gravitational waves, which can be picked up by observatories like the Laser Interferometer Gravitational-Wave Observatory (LIGO). The emitted gravitational waves have a characteristic energy, meaning that these mergers could be used to measure distances as “standard sirens”.

The trouble comes when trying to simultaneously measure the redshift of these sources. The gravitational wave detection on Earth gives us a mass measurement for the black holes that’s a combination of their redshift and their true masses in the source frame. If we know the true masses, we can disentangle these variables and determine the redshift. To achieve this, Will Farr (Stony Brook University and Flatiron Institute) and collaborators propose using a particular constraint on the masses of BBHs.

True black hole mass (not measured mass) versus redshift as obtained from one year of simulated BBH merger observations. The blue line indicates the maximum mass of black holes as set by PISNs, with the dark and light bars showing the confidence intervals. [Farr et al. 2019]

Capping Masses

When we model the population of merging black holes we’ve detected via gravitational wave observations, we see a drop-off in black hole mass above 45 solar masses. Farr and collaborators suggest this upper limit could be tied to one specific route of black hole creation: pair instability supernovae (PISNs).

Only massive stars can die as PISNs. In these events, the core of a star gets hot enough to allow electron–positron pairs to pop into existence, which lowers the star’s internal pressure enough for gravity to trigger the trademark explosion of a supernova. The remnants left behind by PISNs peak in mass around 45 solar masses.

By taking advantage of the mass scale imprinted on the population of BBH mergers by the PISN process, Farr and collaborators argue, we can extract redshifts from our detector measurements. Simulating 5 years of detections, the authors show that we could potentially constrain the Hubble parameter — our measurement of the expansion of the universe — at a specific redshift to within an impressive 2.9%.

Distributions of the Hubble parameter at a redshift of 0.8 as estimated by one year of observations (blue) and five years of observations (orange). The true value of the Hubble parameter at that redshift is indicated by the black vertical line. [Adapted from Farr et al. 2019]

Paring Down Parameters

The authors find that BBHs are most useful for constraining the Hubble parameter at a redshift of z = 0.8 (redshifts that can be explored with the current capabilities of gravitational wave observatories are between z = 0 and 1.5). This is because at that redshift the models peg the uncertainty on the Hubble parameter at a minimum. Additionally, the uncertainty is halved when going from one year of observations to five years.

The authors note that a change of 1–2 solar masses in their maximum black hole mass does not change their results drastically. Their method would also work with a different maximum mass — so long as there is some mass scale, BBH mergers can be used to measure distances.

New gravitational-wave detectors will extend our sample of BBH mergers enormously. With a larger sample and a better understanding of the utility of black holes, we will be closer to pinning down the fate of the universe.

Citation

“A Future Percent-level Measurement of the Hubble Expansion at Redshift 0.8 with Advanced LIGO,” Will M. Farr et al 2019 ApJL 883 L42.

https://iopscience.iop.org/article/10.3847/2041-8213/ab4284

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: “Lessons from a Quartet of Newborn Planets”

From AAS NOVA

6 November 2019
Susanna Kohler

Artist’s impression of an extrasolar planet system. [R. Hurt (IPAC)/NASA/JPL-Caltech]

Though we’ve discovered thousands of planets beyond our solar system, we still have a lot to learn about how these bodies form and evolve. Now, a newly discovered baby planetary system may provide some insight.

An illustration of some of the planetary systems discovered by the Kepler spacecraft. The stars at the centers of these systems are not pictured. [NASA Ames/UC Santa Cruz]

Multiplanet Wealth

The Kepler mission has been instrumental in our exploration of worlds beyond our solar system, helping us to discover nearly 5,000 confirmed and candidate exoplanets.

NASA/Kepler Telescope, and K2 March 7, 2009 until November 15, 2018

In particular, Kepler’s gaze has revealed a wealth of compact, multiplanet systems that share both intriguing similarities and striking differences with our own solar system.

Kepler multiplanet systems tend to be coplanar, with nearly circular orbits and low obliquities. There is often a high degree of intrasystem uniformity — planets of similar sizes, masses, and orbital spacing are more likely to be found together in the same system. And, intriguingly, most Kepler compact multiplanet systems tend to consist of small planets that have radii of less than 3 Earth radii.

Could these systems’ traits point to how they form and evolve? Might these planets once have had larger sizes, before they cooled and contracted or lost some of their atmospheres to photoevaporation? In order to answer these questions, we need to explore multiplanet systems much earlier in their lifetimes.

Phase-folded transits for each of the four V1298 Tau planet candidates. [Adapted from David et al. 2019]

Planet transit. NASA/Ames

New Baby Planets Found

Now, a team of scientists led by Trevor David (Jet Propulsion Laboratory, California Institute of Technology) has identified a system of planets that might be exactly what we’re looking for.

David and collaborators re-analyzed Kepler data from 2015 to identify three new planets transiting the young solar analog V1298 Tau, which was already known to host one Jupiter-sized planet on a 24 day orbit. The newly discovered planets have periods of 8.25 days, 12.46 days, and somewhere between 36 and 223 days (we only have one transit for this last one, so its orbit isn’t yet well-constrained).

Critically, V1298 Tau is a very young star, at just 23 million years old — so we’re examining this planetary system early in its formation.

Young transiting exoplanets in the period–radius plane. The new planets discovered around V1298 Tau are indicated by yellow stars, and they occupy sparsely populated regions of the plane. [David et al. 2019]

A Valuable Laboratory

What makes the planets of the V1298 Tau system interesting is their unusually large sizes: though their masses are low, these planets are Neptune-to-Saturn-sized, clocking in at 5.6, 6.4, and 8.7 Earth radii. V1298 Tau’s planets are therefore significantly larger than the planets found in the vast majority of Kepler multiplanet systems.

The authors speculate that these planets may still be radiatively cooling and contracting, and perhaps losing atmosphere. The V1298 Tau system could, in fact, be the precursor to the compact multiplanet systems Kepler has found throughout the galaxy.

V1298 Tau provides a valuable laboratory to explore a stellar system in the early stages of its evolution. By following up with additional observations — such as planet mass measurements and atmospheric characterization — we stand to learn much more about how this baby planetary system and others like it formed and evolved.

Citation

“Four Newborn Planets Transiting the Young Solar Analog V1298 Tau,” Trevor J. David et al 2019 ApJL 885 L12.
https://iopscience.iop.org/article/10.3847/2041-8213/ab4c99

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: “Life on the Red Edge”

From AAS NOVA

28 October 2019
Tarini Konchady

An artist’s illustration of the Kepler-186 system, highlighting the planet Kepler-186f. Kepler-186f is an Earth-sized planet orbiting in the habitable zone of its star. [NASA Ames/SETI Institute/JPL-Caltech/T. Pyle]

How can we identify life on other planets? The Earth might be able to help with that — specifically with something called the red edge.

The Green Light for Life

One of the most exciting prospects of exoplanet science is discovering another planet that can harbor life. However, this necessitates us knowing how to identify life at a distance, which is quite a challenge!

The light reflected by an exoplanet is one of the most useful observations to have on this quest. The reflected light is dependent on the surface and atmospheric conditions of the planet, and it may hold key features that point to the existence of life. What those features are, however, is another question.

This is where the Earth comes in handy. With our intimate understanding of the Earth, we can simulate it as an exoplanet fairly accurately. Those simulations can help us better pick out Earth-like planets from reflected-light observations.

The Earth’s reflected-light spectrum contains a unique feature: something called the red edge, a region of rapid change in the near-infrared part of the spectrum. The red edge is caused by chlorophyll in the Earth’s organisms, which has the quirk of strongly reflecting red light. Could this red edge be used to identify chlorophyll-containing life on other planets? In a recent study, Jack O’Malley-James and Lisa Kaltenegger (Carl Sagan Institute, Cornell University) considered the effects of various organisms on the red edge and what that means for the red edge’s detectability.

Exploring Scenarios

A surprising number of organisms contain chlorophyll, but at present, land-based vegetation is most responsible for the red edge. Aside from trees (a catchall term for land-based vegetation), O’Malley-James and Kaltenegger considered other organisms like cyanobacteria, algae, and lichens.

Initially, the authors used a simplified model of the Earth to understand the impact of each organism. They assumed that the entirety of the planet was covered by just one organism and determined how the red edge would appear for an atmosphere that was still like the Earth’s. They then tried the same scenario with a more realistic Earth, which had a surface that was 30% land and 70% ocean.

The authors also considered the effect of clouds. They tried two cases for each planet scenario, one with clear skies and the other with 60% cloud cover. The difference in cloud cover was more significant for the realistic planet model.

The fraction of light reflected at different wavelengths by different chlorophyll-containing organisms (corals; trees; elysia viridis, a photosynthetic sea slug; lichens; algae; cyanobacteria). The gray area shows the wavelength range in which the red peak appears. [O’Malley-James and Kaltenegger 2019]

Earths at Different Times

The red edge likely evolved with life on the Earth. Trees are relative newcomers, having only established themselves ~500–725 million years ago. Algae and lichens are much older — ~1 billion years old — and cyanobacteria likely appeared at least 2 billion years ago. These staggered arrival times imply that planets that are similar to the early Earth could still produce red edges.

There may not be a lot of planets exactly like the present-day Earth, but O’Malley-James and Kaltenegger suggest that this isn’t a setback. We could potentially identify planets that have just begun to harbor life — and that’s a big step in the right direction.

Citation

“Expandinghttps://iopscience.iop.org/article/10.3847/2041-8213/ab2769 the Timeline for Earth’s Photosynthetic Red Edge Biosignature,” Jack T. O’Malley-James and Lisa Kaltenegger 2019 ​ApJL​ ​879​ L20.
https://iopscience.iop.org/article/10.3847/2041-8213/ab2769

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: “When Neutron Stars Merge”

From AAS NOVA

25 October 2019
Tarini Konchady

Merging neutron stars. Image Credit: Mark Garlick, University of Warwick.

To predict how often binary neutron star mergers occur, we need to know when binary neutron stars are born and how long it takes them to merge. An avenue for understanding this is to carefully study their host galaxies.

Where To Look?

Neutron stars are the seemingly anticlimactic remnants of supernovae. However, aside from containing fascinating states of matter, they may also be responsible for creating some of the elements that can’t be created in the cores of normal stars. This would happen when a pair of neutron stars — binary neutron stars (BNS) — merge, emanating characteristic gravitational waves.

Proving this hypothesis of element formation requires an understanding of where and when BNS form and collide. This is where Delay Time Distributions come in. The Delay Time Distribution for binary neutron stars predicts how long after a binary birth two neutron stars will spend spiraling around each other before they finally merge. If we obtained a well-constrained Delay Time Distribution for BNS, we would have a more complete idea of how often BNS form and merge.

The likelihood of different rates of BNS mergers, given Delay Time Distributions with different parameters. The top plot assumes a slow, continuous star formation history and the bottom plot assumes a single burst of star formation. [Adapted from Safarzadeh ​et al. ​2019]

Mohammadtaher Safarzadeh (Harvard-Smithsonian Center for Astrophysics and Arizona State University) and collaborators have studied the Delay Time Distribution for BNS quite extensively over a series of recent publications. Most recently, they examined the star formation history of galaxies that have hosted BNS mergers through simulations, exploring how this could be used to constrain the BNS Delay Time Distribution.

Modeling Star Formation Histories

Past efforts by Safarzadeh and collaborators have previously studied the BNS Delay Time Distribution using the properties of BNS merger hosts — specifically galaxy mass and redshift. Both quantities can be broadly tied to a galaxy’s star formation history, which is key to constraining the Delay Time Distribution. In this work, the authors attempt to more directly examine the star formation histories of the merger hosts.

They start by modeling star formation histories for about 6,000 galaxies that were observed in the Galaxy and Mass Assembly survey. From this modeling, two sorts of histories emerge: one where stars formed quickly and nearly all at once and the other where star formation happened slowly and continuously.

A given star formation history can be used to estimate the number of BNS that are born in a galaxy over time. The authors then use a subset of their galaxy sample with the different star formation histories to simulate several sets of BNS mergers. By comparing these simulations to current and future observations of BNS merger rates, the authors succeed in placing new constraints on the BNS Delay Time Distribution parameters.

In Search of More

Using star formation histories to constrain Delay Time Distributions proves to be an improvement over using galaxy masses. Additionally, the simulations provide a larger sample of BNS host galaxies to work with. However, the best results will be obtained when we eventually build a larger sample of observed BNS mergers that spans a much larger volume of space.

Given that gravitational wave astronomy is in its infancy, our sample of BNS mergers is likely to explode as new observatories come online. Will this tell us more about how binary neutron stars form, collide, and brew the chemical elements that pervade our universe? Likely so!

Citation ​

“Measuring the Delay Time Distribution of Binary Neutron Stars. III. Using the Individual Star Formation Histories of Gravitational-wave Event Host Galaxies in the Local Universe,” Mohammadtaher Safarzadeh ​et al ​2019 ​ApJL 878​ L14.
https://iopscience.iop.org/article/10.3847/2041-8213/ab24e3

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: “Snowball Events for Tidally Locked Planets?”

From AAS NOVA

23 October 2019
Susanna Kohler

Are tidally locked, potentially habitable planets at risk of experiencing “snowball” events that ice over their surfaces? [ESO]

The Earth likely underwent several periods of planet-wide ice coverage in the past, in what’s known as snowball Earth events. A new study explores whether snowball events are also a risk for tidally locked, habitable exoplanets.

Current theory suggests that the Earth underwent several snowball events in its past history. [NASA]

An Icy Fate

Snowball events can arise suddenly on a planet like Earth, driven by a rapid feedback loop. A planet that experiences a sudden drop in stellar light reaching its surface — say, due to a volcanic eruption or asteroid impact — can quickly ice over through a runaway effect: as ice coverage grows, more light is reflected from the planet’s surface. This drops the temperature of the planet, which causes ice coverage to expand even further.

Under some conditions, this runaway snowball effect can lead to a fully icy world that’s no longer able to defrost itself, even if incoming stellar light returns to original levels.

Artist’s impression of a cold, tidally-locked planet. Ice covers much of the planet’s surface, but the point directly facing the planet’s host star remains ice-free. [NASA/JPL-Caltech]

Looking Beyond Our Solar System

The paradigm described above depends on specifics of how heat is transferred in the atmosphere of a rapidly rotating planet like Earth. But in searching for habitable planets beyond our solar system, we might wonder whether other types of worlds also experience snowball events.

In particular, the majority of the potentially habitable planets we’ve discovered lie around dim M-dwarf stars, and many of these planets are tidally locked, meaning the same side of the planet faces its host star at all times. Can worlds like this snowball, too?

To investigate this question, a team of scientists led by Jade Checlair (University of Chicago) used an atmospheric global climate model to conduct simulations of a tidally locked, Earth-sized planet that circles its M-dwarf host on a 50-day orbit. In particular, the team was curious whether heat transfer within a global ocean would affect the outcome — so they covered their simulated planet in a multi-layer ocean that reached a depth of 189 meters.

The authors’ results show that sea-ice coverage follows a smooth relationship with stellar irradiation on tidally locked planets: for each level of stellar irradiation, the planet equilibrates to the same final state regardless of where it started. This is not the case on planets with runaway snowball events. [Checlair et al. 2019]

No Snowballs

Checlair and collaborators found that, unlike a rapidly rotating planet, tidally locked planets are stable against runaway snowball events. In their model, as the planet experienced decreasing irradiation, its sea ice extent grew gradually — and it defrosted again as the stellar irradiation was brought back to original levels.

This means that for a tidally locked planet in its star’s habitable zone, snowball states should not be possible for extended periods of time. If a planet were to experience a catastrophic event like a volcanic eruption or asteroid impact, it may ice over briefly. But the stellar radiation concentrated on the side of the planet facing its host would quickly cause the planet to warm back up again and return to its original state.

Good or Bad?

Is the lack of tendency for tidally locked planets to snowball a good thing or a bad thing? Though a global ice age could wipe out preexisting complex life, it’s also possible that snowball events could help drive life to evolve more rapidly, by providing evolutionary pressure to adapt. The jury’s still out on the impact of snowball events, but now we know a bit more about where to expect them!

Citation

“No Snowball on Habitable Tidally Locked Planets with a Dynamic Ocean,” Jade H. Checlair et al 2019 ApJL 884 L46.
https://iopscience.iop.org/article/10.3847/2041-8213/ab487d

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: “The Big Picture of Solar Flares”

From AAS NOVA

16 October 2019
Susanna Kohler

SDO/AIA 9.4 nm image of a solar flare in March 2014. How are the properties of flares like these distributed? [SDO/AIA/Verbeeck et al. 2019]

NASA/SDO

Bright eruptions from the Sun’s surface can influence everything from the Sun’s own atmosphere to the Earth and beyond. The good news: we’ve got decades of detailed observations of solar flares available for study. The bad news: we may be interpreting these data incorrectly.
Distributing Flares

The distributions of properties of solar flares is a topic of great interest to solar physicists. Much of our understanding of how the Sun ejects energy into its surroundings depends on the number of flares emitted by the Sun at different energies and durations — but we’re only able to measure the larger, more energetic end of this distribution.

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

For this reason, scientists build databases of observed solar flares and fit power laws to the distributions of their properties. By extrapolating the power laws from the large end of the flare energy scale (which we can observe) down to the smaller end (which we can’t), scientists attempt to estimate the number of unresolved mini-flares the Sun emits. This could shed light on a number of solar mysteries, like why the Sun’s atmosphere is so much hotter than its surface.

But a team of scientists led by Cis Verbeeck (Royal Observatory of Belgium) cautions against this approach. Instead of just measuring the shape of these power-law fits for flares, they say, we first need to ask: is a power law actually the right fit to the data?

To Power Law or Not to Power Law

Power law distributions correctly describe a wide variety of astrophysical data, but Verbeeck and collaborators think we should test this assumption for solar flares. To this end, the team performed a comprehensive study of nearly 7,000 flares detected in Solar Dynamics Observatory AIA 9.4-nm images between May 2010 and March 2018, conducting statistical analyses to determine the best fit to the peak flare flux distribution.

Sure enough, the authors find that once the flare data has been background-subtracted — meaning that only the flares are included, not the non-flaring solar background — the distribution is not well fit by a power law.

The peak flare intensity distribution is better described by a lognormal fit (green) than by a power law fit (red). [Verbeeck et al. 2019]

Instead, a good fit is provided by a lognormal distribution, the distribution that describes a variable that is normally distributed in log space. Lots of things are naturally described by lognormal distributions — for instance, the length of time that users will probably dwell on this post (congratulations: if you’ve made it this far, you’re likely doing better than average!).

A Lognormal Outlook

So why have we been using the wrong fit? The authors show that raw flare data that hasn’t been background-subtracted does follow a power-law distribution, so it’s possible that past studies just haven’t correctly isolated the flares from everything in the background that isn’t a flare.

Regardless of the reason, it seems clear from the work in this study that power laws are not the right approach going forward. As we continue to work to understand the flares from our nearest star, a careful treatment of the big picture is needed!
Citation

“Solar Flare Distributions: Lognormal Instead of Power Law?,” Cis Verbeeck et al 2019 ApJ 884 50.
https://iopscience.iop.org/article/10.3847/1538-4357/ab3425

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