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 .

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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: “Tracking Gas in Star-Forming Galaxies”

From AAS NOVA

14 October 2019
Susanna Kohler

The spiral galaxy NGC 1559, pictured in this Hubble image, is an example of a local star-forming galaxy. How does the amount of fuel for this galaxy’s star formation compare to that for galaxies earlier in our universe? [NASA/ESA/Hubble]

NASA/ESA Hubble Telescope

How has galaxy evolution changed over our universe’s history? To understand this, we need to track galaxies’ stars and gas over time. Stars are relatively easy: they’re bright and can be observed with deep optical and infrared observations. But gas? That’s a little trickier.

This image of local galaxy M81 reveals the extent of its atomic hydrogen gas — measured using the 21-cm emission line — in blue. [NASA Spitzer Space Telescope / NRAO VLA]

NASA/Spitzer Infrared Telescope

NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

Atomic Challenges

Reservoirs of molecular gas — like carbon monoxide — have become progressively better studied in recent years, even in galaxies that lie at huge distances. But atomic gas is a real challenge to observe beyond our local universe.

Atomic hydrogen (H I) is of particular interest: this neutral gas provides the primary fuel reservoir for star formation. But H I doesn’t have any bright emission lines, making it hard to spot. In fact, the main way to detect H I is via what’s known as the 21-centimeter line, a spectral line produced by a rare change in the energy state of the hydrogen atom. This line has such a low transition probability that you need vast amounts of hydrogen to detect it.

Since the 21-cm line is so weak, observing the neutral hydrogen from most individual galaxies beyond our local universe is out of reach until telescope technology improves. But a team of scientists led by Apurba Bera (National Centre for Radio Astrophysics, India) has used an alternative approach: stacking the observations of many distant galaxies.

The uGMRT 1.2 GHz image of the Extended Groth Strip. Red circles indicate the locations of the 445 galaxies in the authors’ sample. [Bera et al. 2019]

Amplifying a Weak Signal

Bera and collaborators used the upgraded Giant Metrewave Radio Telescope (uGMRT) in India to conduct deep radio observations of a region of the sky known as the Extended Groth Strip.

Giant Metrewave Radio Telescope, an array of thirty telecopes, located near Pune in India

With 117 hours of observations, the team gathered detailed data for a sample of 445 blue, star-forming galaxies that lie at redshifts of 0.2 < z < 0.4. These redshifts represent galaxies from when the universe was roughly 2.5 to 4.3 billion years younger than it is now.

By stacking the spectra for these 445 galaxies, the authors were able to make a statistically significant detection of the total H I 21-cm emission — from which they could measure the average mass of H I gas in these intermediate-redshift galaxies.

The stacked H I 21-cm emission spectrum of the 445 galaxies of the authors’ sample, which allows the authors to measure the average H I mass for their galaxies. [Adapted from Bera et al. 2019]

How Star Factories Evolve

Bera and collaborators found that the galaxies in their sample had an average H I mass of roughly 4.9 billion solar masses; for comparison, that’s around 1.2 times their average stellar mass. Based on their average star formation rate, these distant star factories should use up the fuel of their H I reservoirs in roughly 9 billion years.

How do these numbers compare to values in both the current, local universe and the distant, much older universe? The average H I mass and depletion time are consistent with the values measured in the local universe. In contrast, higher-redshift galaxies (z ~ 1.3) have been measured to have an average H I depletion time of less than a billion years.

These results therefore suggest that the efficiency of star formation evolved drastically from our universe’s early stages up to a few billion years ago, but it has held roughly steady since then. More deep radio observations like these should help us to further explore this evolution!

Citation

“Atomic Hydrogen in Star-forming Galaxies at Intermediate Redshifts,” Apurba Bera et al 2019 ApJL 882 L7.
https://iopscience.iop.org/article/10.3847/2041-8213/ab3656

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: “Possibly Impossible Planets”

From AAS NOVA

11 October 2019
Tarini Konchady

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]

Knowing which planets can form will help us understand how planets form. So as we discover more and more exoplanets, one must wonder: what if there are certain types of planets that just don’t exist?

Good Form

We know of over 4,000 confirmed exoplanets and nearly as many exoplanet candidates. With a population of this size, we can look for trends in exoplanet characteristics much more easily than we could a decade ago.

The most helpful quantities in this regard are planet radius, planet mass, and orbital period. Plotting these quantities against each other has revealed an abundance of certain planets — like Jupiter-sized planets with short orbits — and a dearth of others. Direct analogs to Neptune appear to be missing, though Neptune-sized planets with shorter periods are common.

Earth-like planets discovered by Kepler, pictured next to the Earth. [NASA/JPL-Caltech]

Some sorts of planets may genuinely be unable to form, but we also have to consider observational bias. For instance, the transit method (utilized by the Kepler spacecraft and now by the Transiting Exoplanet Survey Satellite (TESS)) is more sensitive to larger planets with shorter orbital periods. So, any gaps and trends that emerge while plotting exoplanet characteristics have to be tested for authenticity. In a recent study, David Armstrong and collaborators focus on a gap that emerges while considering short-period planets less massive than 20 Earths.

Planet transit. NASA/Ames

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

NASA/MIT TESS replaced Kepler in search for exoplanets

Mind the Gap!

Armstrong and collaborators looked at a sample of planets with mass under 25 Earth-masses and orbital periods of less than 20 days. Plotting mass versus orbital period yielded a gap that split their sample into two. The gap was more distinct for planets whose inclinations — and, by extension, planet radii — were known.

Given how many observational biases were involved, the authors chose to investigate the presence of the gap rather than its properties. The authors were especially concerned about whether different methods of mass determination all still produced the gap. A statistical test showed that it persisted regardless of method.

Planet mass in Earth-masses versus orbital period in days for the sample for which inclination was known. The gap is illustrated with the dashed gray line. The color of the points relates to planetary radius. The abundance of small, less massive planets below the gap suggests that the gap could be attributed to photoevaporation. [Armstrong et al. 2019]

To better define the existence of the gap, the authors fit models to their mass–period data that allowed for multiple underlying distributions. The best fit turned out to be a model that assumed two distinct distributions, supporting the gap’s presence.

In light of this, what could be causing the gap?

Why the Gap?

The authors injected other properties of planetary systems into their mass–period plots. The gap appears impartial to nearly everything, showing trends only with planetary radius. This mild relationship could be due to photoevaporation, the process by which radiation from the host star strips gas off nearby objects. In the case of exoplanets, photoevaporation could shrink a gas giant planet — possibly to its rocky core.

In multi-planet systems, the most stable orbits may be those that don’t put planets in the gap. Another possibility is that stable regions in a system shift as it evolves.

The only way to better understand the gap is to find more exoplanets, which is just what TESS and other efforts are doing. Stay tuned!

Citation ​

“A Gap in the Mass Distribution for Warm Neptune and Terrestrial Planets,” David J. Armstrong ​et al ​ 2019 ​ApJL 880​ L1.
https://iopscience.iop.org/article/10.3847/2041-8213/ab2ba2

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: “Should We Blame Pulsars for Too Much Antimatter?”

From AAS NOVA

9 October 2019
Susanna Kohler

Artist’s illustration of Geminga, a nearby pulsar that has been proposed to be the source of excess positrons measured at Earth. [Nahks Tr’Ehnl]

The Earth is constantly being bombarded by cosmic rays — high energy protons and atomic nuclei that speed through space at nearly the speed of light. Where do these energetic particles come from? A new study examines whether pulsars are the source of one particular cosmic-ray conundrum.

Cosmic rays produced by high-energy astrophysics sources (ASPERA collaboration – AStroParticle ERAnet)

An Excess of Positrons

In 2008, our efforts to understand the origin of cosmic rays hit a snag: data from a detector called PAMELA showed that more high-energy positrons were reaching Earth in cosmic rays than theory predicted.

INFN PAMELA spacecraft

INFN PAMELA Schematic

Positrons — the antimatter counterpart to electrons — are thought to be primarily produced by high-energy protons scattering off of particles within our galaxy. These interactions should produce decreasing numbers of positrons at higher energies — yet the data from PAMELA and other experiments show that positron numbers instead go up with increasing energy.

Something must be producing these extra high-energy positrons — but what?

Clues from Gamma-rays

One of the leading theories is that the excess positrons are produced by nearby pulsars — rapidly rotating, magnetized neutron stars. We know that pulsars gradually spin slower and slower over time, losing power as they spew a stream of high-energy electrons and positrons into the surrounding interstellar medium. If the pulsar is close enough to us, positrons produced in and around pulsars might make it to Earth before losing energy to interactions as they travel.

Women in STEM – Dame Susan Jocelyn Bell Burnell

Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

Dame Susan Jocelyn Bell Burnell at work on first plusar chart 1967 pictured working at the Four Acre Array in 1967. Image courtesy of Mullard Radio Astronomy Observatory.

Dame Susan Jocelyn Bell Burnell 2009

Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

Observations from the High-Altitude Water Čerenkov (HAWC) Gamma-Ray Observatory show TeV nebulae around pulsars Geminga and PSR B0656+14. But do these sources also have extended GeV nebulae that would provide more direct constraints on positron density? [John Pretz]

HAWC High Altitude Čerenkov Experiment, located on the flanks of the Sierra Negra volcano in the Mexican state of Puebla at an altitude of 4100 meters(13,500ft), at WikiMiniAtlas 18°59′41″N 97°18′30.6″W. searches for cosmic rays

Could nearby pulsars produce enough positrons — and could they diffuse out from the pulsars efficiently enough — to account for the high-energy excess we observe here at Earth? A team of scientists now addresses these questions in a new publication led by Shao-Qiang Xi (Nanjing University and Chinese Academy of Sciences).

To test whether pulsars are responsible for the positrons we see, Xi and collaborators argue that we should look for GeV emission around candidate sources. As the pulsar-produced positrons diffuse outward, they should scatter off of infrared and optical background photons in the surrounding region. This would create a nebula of high-energy emission around the pulsars that glows at 10–500 GeV — detectable by observatories like the Fermi Gamma-ray Space Telescope.

NASA/Fermi LAT

NASA/Fermi Gamma Ray Space Telescope

Fermi LAT gamma-ray count map (top) and residuals after the background is subtracted (bottom) for the region containing Geminga and PSR B0656+14. [Adapted from Xi et al. 2019]

Two Pulsars Get an Alibi

Xi and collaborators carefully analyze 10 years of Fermi LAT observations for two nearby pulsars that have been identified as likely candidates for the positron excess: Geminga and PSR B0656+14, located roughly 800 and 900 light-years away from us.

The result? They find no evidence of extended GeV emission around these sources. The authors’ upper limits on emission from Geminga and PSR B0656+14 give these objects an alibi, suggesting that pulsars can likely account for only a small fraction of the positron excess we observe.

So where does this leave us? If pulsars are cleared, we will need to look to other candidate sources of high-energy positrons: either other nearby cosmic accelerators like supernova remnants, or more exotic explanations, like the annihilation or decay of high-energy dark matter.

Citation

“GeV Observations of the Extended Pulsar Wind Nebulae Constrain the Pulsar Interpretations of the Cosmic-Ray Positron Excess,” Shao-Qiang Xi et al 2019 ApJ 878 104.
https://iopscience.iop.org/article/10.3847/1538-4357/ab20c9

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: “Watching Stars Evolve in Real Time”

From AAS NOVA

2 October 2019
Susanna Kohler

Artist’s illustration of a star ejecting matter at the end of its lifetime. A recent study suggests we’re seeing the evolution of a star in real time as it experiences its final stages of life. [JAXA]

Sometimes slow and steady wins the race — like in the long-term, continuous monitoring of a variable star. A collection of more than 100 years of data has now given us a rare opportunity to watch, in real time, as a star evolves.

Flashes Late in Life

This H-R diagram for the globular cluster M5 shows where AGB stars lie: they are represented by blue markers here. The AGB is one of the final stages in a low- to intermediate-mass star’s lifetime. [Lithopsian]

Low- to intermediate-mass stars (~0.5-8.0 solar masses) live for billions of years, so stellar evolution ordinarily occurs on timescales that are far too long to observe over our brief human lifespans.

But one particular stage of stellar evolution happens on timescales where we could see something happen, if we’re patient and watch for long enough: the end of the asymptotic giant branch (AGB). This is a stage at the end of a star’s life after it’s exhausted all of the hydrogen and helium in its core.

Low- to intermediate-mass stars aren’t able to ignite fusion of heavier elements in their cores, so at this stage, fusion proceeds only in a shell of hydrogen outside of the core. As the hydrogen shell burns, it piles up a thin layer of helium below it. But that quiet helium is deceptive: when enough of it piles up, it will ignite in a sudden flash called a thermal pulse, rapidly burning until the helium is depleted and the pile-up begins anew.

Rapidly, that is, on stellar evolution timescales — a typical thermal pulse lasts maybe a few hundred years, and these pulses occur only every 10,000 or 100,000 years. Still very difficult to observe on human timescales!

Example plot showing the radius change over time in a modeled star undergoing thermal pulses. Based on observational constraints of T UMi’s radius, the region of interest in this model for T UMi’s current evolutionary stage is marked in red. [Molnár, Joyce, & Kiss 2019]

But there are some clues that we might be able to spot when a thermal pulse is just getting started — and a team of scientists led by László Molnár (Konkoly Observatory, MTA CSFK, Hungary) and Meridith Joyce (Australian National University) think that the star T Ursa Minoris (T UMi) is exhibiting those signs now.

Taking the Pulse of a Star

At the start of a pulse, the flash of igniting helium causes the inner regions of the star to expand. This, in turn, causes the outer parts of the star to cool and contract — so the star rapidly shrinks in radius and decreases in luminosity. If it’s a variable star — a star with periodic oscillations in brightness — the sudden drop in radius causes the period of its variability to plummet as the oscillating stellar envelope shrinks.

More than a century of data from variable star T UMi are shown in this AAVSO visual light curve. Changes in its period and amplitude are most evident in the bottom panel. Click to enlarge. [Molnár, Joyce, & Kiss 2019]

The variable star T UMi presents a golden opportunity to spot this process. Molnár and collaborators have compiled a dataset for this star reaching all the way back to 1904. From this, we can see that T UMi’s period started plummeting roughly 40 years ago, shortening by more than 3 days per year — and at the same time, the star’s brightness started dropping.

Using evolutionary and pulsation models, Molnár and collaborators confirmed that we’re indeed watching the onset of a thermal pulse in an AGB star. From their models, the authors predict that T UMi’s period will continue to drop for another several decades before the star begins to expand again — so be sure to check back in 50 years or so, to see if predictions pan out from this unique opportunity to watch a star evolve!

Citation

“Stellar Evolution in Real Time: Models Consistent with the Direct Observation of a Thermal Pulse in T Ursae Minoris,” László Molnár et al 2019 ApJ 879 62.
https://iopscience.iop.org/article/10.3847/1538-4357/ab22a5

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: “Two Eyes to Hunt Stray Planet Masses”

From AAS NOVA

Artist’s impression of a gas-giant exoplanet that has been ejected from its star system and now has no host. [NASA/Caltech]

How can we measure the masses of free-floating planets wandering around our galaxy? A new study identifies one approach that combines the power of two upcoming missions.

Finding Invisible Planets

Most exoplanets we’ve found so far have relied on measurements of their host stars, either via dips in the host star’s light as the planet passes in front (transit detections), or via wiggling of lines in the host star’s spectra caused by the planet’s gravitational tug (radial velocity detections).

Planet transit. NASA/Ames

Radial velocity Image via SuperWasp http:// http://www.superwasp.org/exoplanets.htm

Radial Velocity Method-Las Cumbres Observatory

But free-floating planets have no hosts and are therefore effectively invisible, since they don’t give off much light of their own. To find these rogues, we rely on another method: gravitational microlensing.

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

In microlensing, the mass of a passing foreground planet — either free-floating or bound to a host star — can act as a lens, briefly gravitationally focusing the light of a background star behind it.

A diagram of how planets are detected via gravitational microlensing. In this case, the planet is in orbit around a foreground lens star, but this same diagram can also apply to a free-floating planet acting alone as the lens. [NASA]

As a result, the background star temporarily brightens (on timescales of perhaps seconds to years) in our observations. Though we never directly see the foreground planet, we can infer its presence from the spike in the background star’s brightness.

Masses from Parallax

By itself, a microlensing observation usually can’t tell us about the mass of a free-floating planet; this is because the timescale of a brightening event depends on both the mass of the lens and on the relative proper motion between the background source and the foreground lensing planet.

But if we could simultaneously observe a microlensing event from two different locations, separated by a large enough distance? Then the parallax would allow us to break that degeneracy: the differences in peak brightness and its timing at the two locations would allow us to calculate both the speed of lens relative to the source and the planet mass.

Vantage Points in Space

Where do we find two sensitive eyes located far enough apart to make this work? In space, of course!

NASA’s Wide Field Infrared Survey Telescope (WFIRST) is set for launch in the mid-2020s, and one of its primary mission objectives is to perform wide-field imaging that may allow for the detection of hundreds of free-floating planets — and many additional bound planets — via microlensing.

NASA/WFIRST

As for the second eye, scientists Etienne Bachelet (Las Cumbres Observatory) and Matthew Penny (The Ohio State University) propose that ESA’s upcoming Euclid mission is exactly what we need.

ESA/Euclid spacecraft depiction

Euclid, launching in 2022, will have similar wide-field imaging capabilities to WFIRST, and it will be able to make complementary microlensing parallax measurements as long as the two satellites are 100,000 km or more apart.

Making Use of Gaps

Though Euclid’s primary science goal is to study dark energy and dark matter, Bachelet and Penny demonstrate that a modest investment of Euclid observing time — approximately 60 days during its primary mission, and another 60 days during its extended mission — during scheduling gaps would be enough to obtain the masses for 20 free-floating planets and many more bound planets.

So what are we waiting for? Let’s go learn more about the rogue planets sneaking through our galaxy!

Citation

“WFIRST and EUCLID: Enabling the Microlensing Parallax Measurement from Space,” Etienne Bachelet and Matthew Penny 2019 ApJL 880 L32.
https://iopscience.iop.org/article/10.3847/2041-8213/ab2da5

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: “Constraining Collisions of Dark Matter”

From AAS NOVA

11 September 2019
Susanna Kohler

The Milky Way hosts many small satellite galaxies — similarly to the Andromeda galaxy, pictured here. Could these satellites be a key to constraining the nature of dark matter? [NASA/ESA/Digitized Sky Survey 2/Davide De Martin)]

Large Magellanic Cloud. Adrian Pingstone December 2003

Small Magellanic Cloud. NASA/ESA Hubble and ESO/Digitized Sky Survey 2

Though dark matter appears to be common in the universe, there’s still a lot we don’t know about it. A new study has now shed some light on this mysterious topic using faint satellite galaxies around the Milky Way.

The relative amounts of the different constituents of the universe. Dark matter makes up roughly 27%. [ESA/Planck]

Prolific Yet Unseen

Our universe is composed almost entirely of dark energy, dark matter, and ordinary matter. While ordinary matter makes up a scant 5% of the universe, dark matter appears to be more common, accounting for 27%. But while dark matter reveals itself through its gravitational effects — adding bulk to galaxy halos that helps hold galaxies together and changes how they move, for instance — we’ve yet to detect it directly.

This challenge means that we’re still working to understand the nature of this unseen substance. Is dark matter made up of primordial black holes? An as-yet undiscovered subatomic particle? Or something else entirely?

Strong gravitational lensing like that observed in this image of Abell 1689 provides evidence for dark matter, but we still don’t understand its nature. [NASA/N. Benitez/T. Broadhurst/H. Ford/M. Clampin/G. Hartig/G. Illingworth/the ACS Science Team/ESA]

The Hunt for the Right Model

Based on our observations and models of our universe, the standard picture of dark matter is the ΛCDM model, in which dark matter is described as cold (it moves slowly, forming structures only gradually) and collisionless (it doesn’t scatter off of ordinary matter, instead effectively passing through it).

Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex Mittelmann Cold creation

Inflationary Universe. NASA/WMAP

The cold, collisionless dark-matter model has held up to a number of tests, and it neatly explains the large-scale structure of our universe. But some challenges to the model exist, and astronomers are still considering a number of alternative pictures.

In a new study led by Ethan Nadler (Kavli Institute for Particle Astrophysics and Cosmology, Stanford University), a team of scientists has now tested alternative theories by asking whether dark matter might not be collisionless, but instead interacts with ordinary matter.

Suppressing Structure

Nadler and collaborators point out that alternative models that treat dark matter as a collisional fluid come with a catch: in this picture, as dark matter scatters off of particles in the early universe, heat and momentum are transferred. This transfer smooths out perturbations in the distribution of matter, suppressing the very glitches that would later grow to become small-scale structure in the universe today.

In effect, the more that dark matter collides with baryons, the less small-scale structure there should be today — limiting the number of low-mass dark-matter halos in our galactic neighborhood and constraining how many small, faint galaxies reside within them.

Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016

Upper limits on the velocity-independent dark-matter–proton scattering cross section as a function of dark-matter particle mass. The different shaded regions show values excluded by various observations. The blue shaded exclusion region is the new constraint placed by observations of Milky Way satellites in the current study. [Nadler et al. 2019]

Collisions Limited

So what do observations tell us? By combining the observed population of classical and Sloan Digital Sky Survey (SDSS)-discovered Milky-Way satellite galaxies with some clever probabilistic modeling of the population, Nadler and collaborators were able to place strict limits on the scattering cross sections for different-sized dark-matter particles, thereby constraining just how “collisional” dark matter can be.

The authors’ work continues to support the standard, collisionless picture of dark matter — but there’s plenty of room for deeper constraints. As data arrives from upcoming imaging programs like the Large Synoptic Survey Telescope (LSST), we’re sure to learn more about the small-scale structure of our surroundings and what it means for the nature of mysterious dark matter.

Citation

“Constraints on Dark Matter Microphysics from the Milky Way Satellite Population,” Ethan O. Nadler et al 2019 ApJL 878 L32.
https://iopscience.iop.org/article/10.3847/2041-8213/ab1eb2

More About Dark Matter

Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http:// palomarskies.blogspot.com

Coma cluster via NASA/ESA Hubble

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

The LSST, or Large Synoptic Survey Telescope is to be named the Vera C. Rubin Observatory by an act of the U.S. Congress.

LSST telescope, currently under construction at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes, altitude 2,663 m (8,737 ft),

Dark Matter Research

Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

Scientists studying the cosmic microwave background hope to learn about more than just how the universe grew—it could also offer insight into dark matter, dark energy and the mass of the neutrino.

Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al

Dark Matter Particle Explorer China

DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB deep in Sudbury’s Creighton Mine

LBNL LZ Dark Matter project at SURF, Lead, SD, USA

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: “Planetary History Written in Saturn’s Rings”

From AAS NOVA

30 August 2019
Kerry Hensley

This natural-color image from the Cassini spacecraft reveals Saturn’s famous rings in detail. [NASA/JPL-Caltech/Space Science Institute]

NASA/ESA/ASI Cassini-Huygens Spacecraft

Saturn may appear calm and motionless from afar, but the immense planet is subtly pulsing and oscillating — and those oscillations impose a pattern on the planet’s rings that could tell us about Saturn’s history.

A Planet in Motion

This extreme close-up of Saturn’s rings from Cassini shows the alternating dark and light bands of spiral density waves. [NASA/JPL-Caltech/Space Science Institute]

As the Cassini spacecraft orbited Saturn, it watched light trickle through the planet’s icy rings as they passed in front of distant stars. The flickering starlight revealed density waves — alternating stripes of compacted and loose material. Those density waves tell us much more than just what’s going on in the rings — they also tell us about the motions of Saturn’s surface.

Yanqin Wu (University of Toronto, Canada) and Yoram Lithwick (Northwestern University) combined observations and theory to study Saturn’s surface oscillations. They found that impacts from small objects were the most likely cause of the oscillations, with convection and atmospheric storms playing a minor role. Each of those impacts caused Saturn to “ring” like a bell, and the volume of the “sound” that we hear now depends on how hard it was struck, how many times, how long ago, and how quickly it fades.

Energies associated with different oscillation modes as derived from Cassini observations (black squares) and theory (colored circles and grey dashed line). While the impact theory matches the observations well for high l-values, it’s several orders of magnitude too low at low l-values. Alternative explanations, shown in the right-hand plot, match the data more closely at those low l-values. [Wu & Lithwick 2019]

Ringing Like a Bell

Saturn’s oscillations diminish as energy is carried away by the density waves in its rings, a process that can take up to 20 million years. By considering the expected frequency and size of impacts over that time period, the authors find that collisions in the distant past could have imparted enough energy to set Saturn ringing in the way we see today — with the exception of a few oscillation modes.

The authors explored several possibilities to explain the mismatch. Saturn could have experienced a once-in-a-million-year impact within the past 40,000 years — a so-called “lucky” strike. It’s also possible that some oscillation modes fade away more quickly than others or that energy is transferred between modes.

Another intriguing possibility is that those missing modes are excited not by impacts but by something more exotic: rock storms. These massive storms might begin deep within Saturn, where the atmospheric pressure is roughly ten thousand times higher than the pressure at Earth’s surface. Since it’s still not clear whether these massive storms actually exist, the authors acknowledge that the theory can’t yet be proved or disproved.

Simulations of two potentially observable signatures of the impact of a 150-km object: gravitational moments (left) and radial velocity (right). [Wu & Lithwick 2019]

From One Gas Giant to Another

Could oscillations be used to learn about the impact history of other planets? Since Jupiter lacks an extensive ring system to act as a dampener, any impact-induced oscillations would last far longer — potentially as long as billions of years — and we may be able to spot them.

To show this, Wu and Lithwick estimated how Jupiter would respond to a collision with a 150-km body a billion years ago. They found that the resulting changes in Jupiter’s gravitational field and surface velocity should be detectable by Juno and ground-based spectroscopy, respectively. With further study, we may be able to read the oscillations of Saturn and Jupiter to look back in time.

Citation

“Memoirs of a Giant Planet,” Yanqin Wu and Yoram Lithwick 2019 ApJ 881 142.
https://iopscience.iop.org/article/10.3847/1538-4357/ab2892

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 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 .

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: “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