Starship Asterisk*

Ali Crisp wrote:
The distance astronomers can “see” is limited by the age of the universe, in addition to being limited by current technology. Despite seeming instantaneous, light travels at a finite speed – specifically 3×10­­^₈ m/s, or 186,282 miles per second for fans of the imperial system. In our daily lives, this isn’t a big deal because the distances the light needs to travel are negligible compared to the speed of light. However, since space is “vastly, hugely, mind-bogglingly big,” the distance is no longer negligible, and astronomers can only observe objects whose light has had enough time to reach us. For example, our Sun is about 150 million km away (or 93.5 million miles), and the light we receive from it takes 8 minutes to travel from the Sun to the Earth and 5.5 hours to reach Pluto. On a larger scale, since the universe is around 13 billion years old, light from the beginning of the universe has been traveling for around 13 billion years. That means the furthest things we can observe are the oldest, and we see them as they were around 13 billion years ago rather than how they are now. This makes them good indicators of what the early universe was like.

Using spectroscopic studies, some of these early objects have been classified as Lyman-α Emitters (LAEs). LAEs, perhaps unsurprisingly, are galaxies that emit “Lyman-α radiation” from hydrogen, but what does that even mean? Well, spectroscopy in general is the study of the electromagnetic spectrum that different objects give off, specifically the emission and absorption of photons when electrons transition between energy levels in an atom. The Lyman series – consisting of Lyman-α, Lyman-β, Lyman-γ, etc. – is a special set of ultraviolet emission lines that result from the transition of electrons from higher energy levels to the ground state in the hydrogen atom. Lyman-α specifically is the emission line from the electron transitioning from the first excited state to the ground state. Lyman-α Emitters are generally extremely distant and extremely young, and can be used to study the Epoch of Reionization and early galaxy formation. Usually, Lyman-α is so energetic that it is absorbed by neutral hydrogen, which was predominant in the early universe, after expansion and cooling. That means that LAEs must have formed later, just as the universe began to reionize, because Lyman-α radiation can make it through ionized hydrogen. This period of reionization corresponds to when star and galaxy formation really kicked in, so the increased formation rates are thought to be the source of the reionization. So, if we look at an LAE, we’re looking at potential progenitors for reionization and galaxy formation.

Amongst these LAEs is the galaxy Cosmos Redshift 7 (CR7), named as a nod to Portuguese footballer Cristiano Ronaldo (see, some astronomers do care about sports!). Discovered in 2015 using the Very Large Telescope (VLT), CR7 is one of brightest known LAEs at a redshift of z > 6.0 and was initially thought to show some indicators of both Population III stars – the first generation of stars, containing very few “metals” – and of a direct collapse black hole. CR7 has three distinct UV regions (Figure 2), two of which are made up of older The authors of today’s paper use the Multi-Unit Spectroscopic Explorer (MUSE) instrument on the VLT combined with near-infrared archival data from Hubble’s Wide Field Camera 3 (WFC3) and ESO’s UltraVISTA to do an in-depth study of CR7’s morphology. ...

• Monthly Notices of the RAS (online 22 Aug 2020) DOI: 10.1093/mnras/staa2550
• arXiv.org > astro-ph > arXiv:2008.01731 > 04 Aug 2020 (v1), 21 Aug 2020 (v2)

Abygail Waggoner wrote:
Let’s be honest. Space dust is cool (literally, it’s about 5 K in the interstellar medium [ISM]). When a molecular cloud collapses to form a star and protoplanetary disk, dust grains eventually collide and stick together to form planets. But, before dust grains become very big planets, first they’re very small chemical factories. In the ISM, dust grains are typically about 0.5 microns in size; for reference, a strand of human hair has a diameter ranging between 17 to 181 microns.

As gas particles collide with dust grains, the particles can stick to them, creating a new icy coating on the grain surface. This layer of ice effectively becomes the smallest operating chemical factory in the Universe, where the majority of molecules are formed in space. Ice chemistry is driven by irradiation from cosmic rays, ultra-violet and infra-red photons present in the ISM. Irradiation can break apart or excite molecules in the ice, thus enabling the “factory” production of even more complex molecules, including molecules significant to the origins of life as we know it (Figure 1).

In recent years, more and more complex molecules have been detected in space, such as benzonitrile. Benzonitrile (Figure 2) is a 6-carbon ring and tied directly to an entire group of molecules known as polycyclic aromatic molecules. Unfortunately, we still don’t know exactly how carbon-based molecules, such as benzonitrile and others, affect the evolution of our icy chemical factories. Today’s paper seeks to determine exactly that: what happens if we irradiate benzonitrile ice? ...

• arXiv.org > astro-ph > arXiv:2008.10011 > 23 Aug 2020

Gloria Fonseca Alvarez wrote:
X-ray binaries are systems with a primary that is a compact object like a black hole or neutron star. The compact object accretes material from the companion (the donor star), emitting brightly in X-rays. Outside of our galaxy, some of these X-ray sources have been observed to have luminosities exceeding the Eddington Limit for a stellar-mass black hole (L ~ 10^₃₉ ergs/s), we call these sources Ultraluminous X-ray sources (ULXs). There are a few possibilities for the origin of such high luminosities. One explanation is geometry – the Eddington limit assumes that emission occurs isotropically — if emission is actually beamed, then the luminosity is overestimated. Another possibility is if the black hole is actually an intermediate mass black hole accreting below the Eddington limit for its mass (> 100 M_☉). In order to understand the nature of these objects, it’s necessary to know their mass.

One way to study these objects is to look at their companion stars. These companions can be anything from a low-mass star to a high-mass star with strong stellar winds. Radial velocity studies of the donor star would result in dynamical mass measurements of the compact object, which is the most reliable way to measure mass.

Many ULXs are found near young star clusters and massive stars evolve fast, so it’s possible that some ULXs have companion stars that have evolved into Red Supergiants (RSGs), making them detectable in the near-infrared (NIR). Today’s paper is the third installment in a series of papers that aim to systematically identify NIR counterparts to ULXs. In this paper, the authors use a sample of 23 previously observed ULXs to look for their companions using NIR imaging, as well as perform spectroscopic follow-up to previously identified NIR counterparts. ...

• Monthly Notices of the RAS 497(1):917 (Sep 2020) DOI: 10.1093/mnras/staa1920
• arXiv.org > astro-ph > arXiv:2006.02795 > 04 Jun 2020

Briley Lewis wrote:
There are lots of stars out there in the Universe, and a large chunk of those are M dwarfs. These are the smallest and reddest stars, coming last in the sequence of spectral types (O, B, A, F, G, K, and last but not least: M). Bonus: since they’re so small and dim, it actually makes it easier to find smaller, terrestrial planets around them! Given that they’re so plentiful and we have a good shot at peering into their habitable zones, it makes sense that we’d want to think about what life on a planet around a M dwarf would be like.

But, there’s a catch. M dwarfs are also known to be very active stars, flaring and giving off a lot of ultraviolet light and X-rays that are bad news for biological life. This stellar activity is so strong that it drives atmospheric escape, stripping these rocky planets of their atmospheres, which are critical for habitability. Extreme ultraviolet light (known as EUV or XUV) is particularly good at stripping away an atmosphere, and young M dwarfs give off more of this since they spend a longer time in their pre-main sequence evolution phase. So, the beginning of these stars’ lives are extreme, ruining chances for a planet to be habitable. What about older M dwarfs? Planets around M dwarfs could have a do-over on their atmosphere, gaining a “secondary atmosphere” created by gases released through impacts or volcanos. Do they mellow with age, quieting down all that radiation, making it possible for this secondary atmosphere to stick around long enough for life to arise?

Today’s paper seeks to answer these questions by observing a nearby old M dwarf for it’s UV and X-ray activity, then computing what would happen to the atmosphere of an Earth-like planet in its habitable zone. ...

• arXiv.org > astro-ph > arXiv:2009.01259 > 02 Sep 2020

bystander wrote: ↑Thu Sep 17, 2020 6:04 pm Life finds a way (even on M dwarfs?)
astrobites | Daily Paper Summaries | 2020 Sep 17

Briley Lewis wrote:
There are lots of stars out there in the Universe, and a large chunk of those are M dwarfs. These are the smallest and reddest stars, coming last in the sequence of spectral types (O, B, A, F, G, K, and last but not least: M). Bonus: since they’re so small and dim, it actually makes it easier to find smaller, terrestrial planets around them! Given that they’re so plentiful and we have a good shot at peering into their habitable zones, it makes sense that we’d want to think about what life on a planet around a M dwarf would be like.

But, there’s a catch. M dwarfs are also known to be very active stars, flaring and giving off a lot of ultraviolet light and X-rays that are bad news for biological life. This stellar activity is so strong that it drives atmospheric escape, stripping these rocky planets of their atmospheres, which are critical for habitability. Extreme ultraviolet light (known as EUV or XUV) is particularly good at stripping away an atmosphere, and young M dwarfs give off more of this since they spend a longer time in their pre-main sequence evolution phase. So, the beginning of these stars’ lives are extreme, ruining chances for a planet to be habitable. What about older M dwarfs? Planets around M dwarfs could have a do-over on their atmosphere, gaining a “secondary atmosphere” created by gases released through impacts or volcanos. Do they mellow with age, quieting down all that radiation, making it possible for this secondary atmosphere to stick around long enough for life to arise?

Today’s paper seeks to answer these questions by observing a nearby old M dwarf for it’s UV and X-ray activity, then computing what would happen to the atmosphere of an Earth-like planet in its habitable zone. ...

The High-Energy Radiation Environment around a 10 Gyr M Dwarf: Habitable at Last? ~ Kevin France et al

• arXiv.org > astro-ph > arXiv:2009.01259 > 02 Sep 2020

Although they’re less active, this paper has shown that even old M dwarfs can lose a lot of atmosphere, particularly due to flares. We still need to learn more about the flare cycles, since that seems to be a key parameter in atmospheric retention and M dwarf habitability!

Ann wrote: ↑Fri Sep 18, 2020 4:21 am The title sure sounds like wishful thinking to me.

We have no idea how habitable planets orbiting M dwarfs really are. Oh sure, some of them may be habitable. But we don't know of a single M-dwarf planet that is truly habitable.

Life may find a way, would have been a much better title in my opinion.

Ann

Spencer Wallace wrote:
To become a terrestrial planet, you’ve gotta start small. Terrestrial worlds, like those found in the inner solar system, are thought to have been constructed via the gradual accumulation of tiny (micron-sized) dust grains. Over hundreds of thousands of years, these grains bump into each other, stick together and build progressively larger bodies. First come pebbles (cm-sized), then planetesimals (meter-sized), embryos (Moon-sized) and finally planets.

For this picture to work, however, a number of potential growth barriers must be somehow overcome. Laboratory experiments show that mm-sized objects don’t like to stick together when they collide, and if they do manage to stick, they are rather easy to break apart. Furthermore, the headwind experienced by slightly larger (meter-sized) bodies as they plow through the gas in a protoplanetary disk tends to force planet-building material to fall onto the star in only a few thousand years (much shorter than the time it takes to build a planet)! ...

The authors of today’s paper examine yet another (commonly overlooked) barrier to growth, known as the aeolian-erosion barrier. Here, aeolian-erosion simply refers to the breakdown and transport of solid materials by wind. This mechanism, which is also used to explain how sand dunes move and evolve on worlds like Mars and Titan, is likely to be at play as planet-forming material orbits through a much slower moving disk of gas. As pebbles and planetesimals cut through the dense primordial gas, aeolian-erosion acts to eat away at the outer layers of these bodies. ...

• Monthly Notices of the RAS 496(4):4827 (Aug 2020) DOI: 10.1093/mnras/staa1864
• arXiv.org > astro-ph > arXiv:1910.02941 > 07 Oct 2019 (v1), 24 Jun 2020 (v3)

Bryanne McDonough wrote:
When astronomers look at galaxies in our universe, they found that they were usually either red or blue. Only a handful of galaxies lay somewhere in between, the so-called “green valley.” (If you’re wondering why it isn’t the purple valley, remember that green is between red and blue on a light spectrum.) Figure 1 illustrates this bimodal distribution in terms of star formation rate.

Hotter, younger stars give off more blue light, while cooler, older stars give off more red light. Galaxies that are still actively star-forming have younger stars and thus are bluer, while galaxies that have mostly shut-down star formation are redder. Galaxies that have shut down star formation are referred to as “quenched.” Stars form from the intergalactic gas, and if the quenching process were simply a matter of using up the gas over time, we would expect to see many more galaxies in the green valley.

Instead, some process must be quickly shutting down star formation, making the transition from blue to red relatively short on a cosmological timescale. This process has been identified as feedback from active galactic nuclei (AGN) and supernovae. These violent processes can create winds of material, heat up the surrounding medium and cause star formation to cease. However, the specifics of how quenching occurs still need to be hammered out, and the authors of today’s paper used a large survey of galaxies to gain some insight. ...

• Monthly Notices of the RAS (online 15 Sep 2020) DOI: 10.1093/mnras/staa2806
• arXiv.org > astro-ph > arXiv:2009.05341 > 11 Sep 2020

Jenny Calahan wrote:
In the summer of 2017, the James Clerk Maxwell Telescope (JCMT) in Hawaii turned towards our sister planet, Venus. The goal of that night was to observe the molecule phosphine, or PH₃. Why PH₃? It is a promising sign of life, if detected on another planet. Phosphine is found in Earth’s atmosphere, and its origin is due to human activity or microbes. PH₃ reacts easily and effectively with oxygen to create phosphorous acid, destroying the phosphine. Without life on Earth, PH₃ would most likely not exist in our atmosphere because of how oxygen-rich the atmosphere is. Thus, an abundance of PH₃ in a rocky planet’s atmosphere could very well be suggestive of life. PH₃ has been detected in Jupiter and Saturn, in trace amounts, however the mechanism that creates PH₃ in these planets occurs deep within the planet where there are very high temperatures and pressures, and then brought up to the surface of those gas giants via convection. Rocky planets have similar environments deep within the interior of the planet, however the rocky surfaces on Mercury, Venus, Earth, and Mars block the PH₃ from moving up into the atmosphere where it can be observed.

Venus in particular, also has a very oxygen-rich atmosphere, so any PH₃ that could be created via known chemistry would quickly be made into H₃PO₃. Based on what this science team knew about Venus and phosphine chemistry, they did not expect a detection of phosphine. Thus, when phosphine was detected in 2017, the team decided to do a follow-up observation with the Atacama Large Millimeter/submillimeter Array (ALMA). This telescope would provide a higher sensitivity, as well as better spatial resolution. They could then confirm that another telescope at another time would also detect the presence of phosphine, and map out the location of phosphine in the Venusian atmosphere.

Sure enough, when ALMA gazed towards Venus in 2019, phosphine was yet again detected. The detections from JCMT and ALMA agreed with each other, telling us that not only that this must be a real detection, but over the course of two years the amount of PH₃ didn’t seem to change. This team went to great lengths to make absolutely sure that this was a real PH₃ detection. They ruled out the possibility of another molecule contaminating the observed line and they had multiple people run the data reduction on the observations. This most certainly means that PH₃ has been detected on Venus! Using the spatial resolution that ALMA provides, they also determined that the PH₃ must exist at or higher than ~53-61 km into the Venusian atmosphere, and that the distribution of phosphine must be somewhere between completely uniform across the surface and in small distinct patches. ...

• Nature Astronomy (online 14 Sep 2020) DOI: 10.1038/s41550-020-1174-4
• arXiv.org > astro-ph > arXiv:2009.06593 > 14 Sep 2020

Haley Wahl wrote:
One field of astronomy that has exploded in the past ~30 years is the study of exoplanets, or planets outside of our solar system. Studying planets around other stars will not only help scientists answer the question of whether or not our little blue marble is unique in the universe, but it can also help astronomers figure out how solar systems are formed and how planets themselves are created. Since the first discovery of an exoplanet around a pulsar in 1992, astronomers have been on the hunt for new worlds, and have found thousands. Characteristics of the exoplanets that have been discovered thus far span the imagination: from the diamond planet orbiting pulsar J1719-1438 to the planet that experiences a triple sunset every day, exoplanets have been found to come in all shapes and sizes. Most of these planets have been found orbiting stars like our Sun, but the authors of today’s paper have discovered the first exoplanet in orbit around a white dwarf (WD). ...

• Nature 585(7825):363 (17 Sep 2020) DOI: 10.1038/s41586-020-2713-y
• arXiv.org > astro-ph > arXiv:2009.07282 > 15 Sep 2020

Keir Birchall wrote:
To butcher an apocryphal quote about cars, galaxies can be any colour, as long at its red or blue. If you were to plot the magnitude and colour of a large sample of galaxies you would see that they fall into two groups: one is actively star-forming and filled with young blue stars, the other has long since finished making new stars so is left with only the older, redder populations. Between these two monolithic groups, however, there is an elusive class of galaxies. Post-starburst galaxies (PSBs) are objects which are thought to have had large amounts of star-formation shut off very rapidly in a quenching event. This short time period means that PSB samples are generally quite small, so the origins of quenching are still quite uncertain. However, studying PSBs is still thought to be the best route to understanding what causes galaxies to transition from blue to red.

Today’s authors are looking inside the galaxy for their quenching trigger. They focus on the galaxy’s central supermassive black hole. Emission from active galactic nuclei (AGN) are thought to inject huge amounts of energy back into their host galaxy. Such huge injections are believed to either create strong winds that eject star-forming material from the host galaxy or heat the gas so much as to prevent it cooling and collapsing to form new stars. Today’s paper searches for signs of nuclear activity in a sample of PSBs to see if AGN could be responsible for their quenching. ...

• Astrophysical Journal Letters 899(1):L9 (2020 Aug 10) DOI: 10.3847/2041-8213/aba534
• arXiv.org > astro-ph > arXiv:2007.02967 > 06 Jul 2020

Jamie Sullivan wrote:
When studying something gets too hard, sometimes the answer is to study nothing. Modeling the nonlinear dynamics of halos and galaxies to improve our current understanding of cosmology is really hard. To a-void this problem, the authors of today’s paper use an unusual kind of (almost) nothing, cosmic voids, to learn about cosmology instead. They find some of the tightest cosmological constraints using voids so far, and they can do it without any model calibration!

What do voids have to do with cosmology? Don’t cosmologists care about galaxies and halos? What is cosmological model calibration? Let’s find out! ...

• arXiv.org > astro-ph > arXiv:2007.07895 > 15 Jul 2020

Alex Pizzuto wrote:
If you ever find yourself in the Southern Hemisphere on a clear night, look up to the night sky. If you’re lucky, you should be able to see what looks like two luminous clouds of stars, nearly resembling two severed pieces of the Milky Way.

These smears in the sky are the Magellanic Clouds, a pair of satellite galaxies orbiting our own galaxy. This dyad of dwarf galaxies is one of the Milky Way’s nearest neighbors, clocking in at a measly 150,000 light years away (for some other recent coverage of the Magellanic Clouds, check out this bite). Less visible to the naked eye is a stream of high-velocity clouds of gas that inhibits a swath of sky over 100 degrees in extent, and which we believe accounts the history of a 6 Gigayear waltz between these two celestial bodies.

This structure is the “Magellanic Stream,” and while it’s known to be the result of constant interplay of tidal forces and ram-pressure stripping between the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and the Milky Way, theorists have long struggled to explain exactly how the Stream came to be as massive and as high-velocity as it actually is. That is, they struggled to explain its formation until now.

The solution to this decades-old mystery comes from one long-neglected element of the Magellanic bodies — the Magellanic Corona. ...

• Nature 585(7824):203 (10 Sep 2020) DOI: 10.1038/s41586-020-2663-4
• arXiv.org > astro-ph > arXiv:2009.04368 > 09 Sep 2020

Laila Linke wrote:
When astronomers conduct a giant survey, they collect overwhelming amounts of data. Of course, this is a good thing: more data equals more information and, therefore, new insights into our universe. But how can we check whether the data is correct? Well, the authors of today’s paper try to use a mathematical curiosity generally used for detecting tax fraud: Benford’s law. ...

• arXiv.org > astro-ph > arXiv:2008.12271 > 27 Aug 2020

Ashley Piccone wrote:
This past month has been huge for our neighboring planets. Phosphine was detected on Venus, and it may point towards microbial life. Now, liquid water has been confirmed and mapped underneath the surface of Mars! The water was first discovered in 2018 by the Mars Express spacecraft underneath an ice-sheet near the planet’s South Pole. Today’s paper used the same instrument, the Mars Advanced Radar for Subsurface & Ionosphere Sounding (MARSIS), to look at the region in more detail.

The search for water on Mars has been a long one that has mostly resulted in finding lots of ice. The North Pole makes up a lot of that, whereas the South Pole is smaller and actually contains a lot of solid carbon dioxide. Scientists have also found evidence of water formerly flowing on the planet, and even an entire missing ocean. But the main idea is this: Mars once had water, and now it has a lot less, possibly because of an extreme global warming event. Finding the liquid water that Mars might have is really important because it’s our best chance at finding signs of life on the red planet.

To find water underneath the surface of Mars, today’s authors used the same technique that is used to find subsurface lakes on Earth: radio-echo sounding. Basically, MARSIS bounced radio waves off of Mars and measured what was reflected back. Figure 1 explains how radio-echo sounding works. The time it takes for a signal to return to the satellite depends on how thick the layer of ice is, and the intensity of the returning signal depends on how reflective the surface underneath the ice is. By measuring these properties, today’s authors were able to determine the structures underneath a region of ice on Mars’ south pole. ...

• Nature Astronomy (online 28 Sep 2020) DOI: 10.1038/s41550-020-1200-6
• arXiv.org > astro-ph > arXiv:2010.00870 > 02 Oct 2020

Jason Hinkle wrote:
In our own Galaxy and in many galaxies throughout the universe, stars are actively being born. However this process is complicated, requiring large reservoirs of molecular gas. When dense portions of large molecular clouds reach a critical mass or size, they are unstable to small perturbations. If such a perturbation occurs, they begin to collapse on themselves and eventually will form a star.

One spectacular way to perturb gas into conditions ripe for making stars is the collision of clouds of gas and dust. When this happens, the gas becomes compressed into dense filaments and cores. These dense cores accrete material from the surrounding clouds and collapse into protostars, the precursors of the stars we see in the night sky. In today’s paper, the authors study a particular system, G133.50+9.01 (hereafter G133), and the possibility that it is the result of such a cosmic collision. ...

• arXiv.org > astro-ph > arXiv:2009.13731 > 29 Sep 2020

Spencer Wallace wrote:
Despite their destructive nature, hurricanes (more generally referred to as cyclones), are thought to play an integral role in regulating the Earth’s global climate. Hurricanes are one of the main mechanisms that act to move heat energy away from the equator and up toward cooler latitudes. The effectiveness of this heat transport depends sensitively on the location in which hurricanes form and the resulting paths that they follow. The detailed behavior of these storms may be a key factor in what makes the Earth suitable for life. ...

The authors of today’s paper seek to understand what conditions are most favorable for hurricane genesis on ocean-covered planets orbiting M stars. In particular, the authors want to determine whether there is a ‘sweet spot’ in terms of planetary orbital period where hurricanes are most likely to form. To do so, the authors model an exoplanetary atmosphere using a general circulation model (GCM), vary the parameters of the planet-star configuration, and measure a number of well-established meteorological metrics to quantify the potential for the formation of an exo-hurricane. ...

• Astrophysical Journal 898(2):115 (2020 Aug 01) DOI: 10.3847/1538-4357/aba0b9
• arXiv.org > astro-ph > arXiv:2006.15120 > 26 Jun 2020

Will Saunders wrote:
A fresh snowfall can be a beautiful sight. A week-old, dirty, salty slush puddle, not so much. Beauty is relative, they tell me, and so is a brine, because that same dirty, salty slush puddle on Mars could be the key to life on the Red Planet.

An exceptional recent Astrobite outlined the latest in the saga of water discovery on Mars: the 2018 detection of liquid water deep under the South Pole was confirmed by the MARSIS instrument aboard the Mars Express spacecraft. Armed with new, high-resolution maps of the water, the authors of that paper concluded brines were responsible for liquid water formation, not recent magma flows. The authors of today’s paper step things up a level—literally—to see if stable brines on the surface of Mars could form and support life. ...

• Nature Astronomy 4(8):756 (Aug 2020) DOI: 10.1038/s41550-020-1080-9

Yuchen Xing wrote:
White Dwarfs (WDs) are the remnants of low-mass and average mass stars. They are the end point of 97% of the stars in our galaxy. WDs can be divided into three main groups based on their core composition: oxygen-neon WDs, carbon-oxygen WDs, and helium WDs. The chemical composition of a WD is based on what type of star it came from and how it evolved. For single WDs and WDs in binary systems without mass-transfer, the core composition can be inferred from the mass of the stars they have evolved from. Those WDs that have evolved from stars with lower mass have cores composed primarily of carbon and oxygen, while those which have evolved from stars with higher mass have cores that contain heavier elements like neon and magnesium.

When it comes to WDs interacting in a binary system where mass transfer is involved, things get more complicated. For a pre-WD whose companion is a red giant star or on the asymptotic giant branch (AGB), the WD may accrete hydrogen/helium-rich material and evolve into a very low mass (< 0.45 M_☉) WD. Those very low mass WDs can have either helium cores or hybrid (composed of both carbon-oxygen and helium) cores. So, for low mass WDs in binary systems, core composition of helium, carbon-oxygen and hybrid are all expected.

In today’s paper, with the discovery of the first known pulsating WD in an eclipsing binary, the authors find a new method in studying the core composition of WDs. ...

• Nature Astronomy 4(7):690 (Jul 2020) DOI: 10.1038/s41550-020-1037-z
• arXiv.org > astro-ph > arXiv:2003.07371 > 16 Mar 2020

Wynn Jacobson-Galan wrote:
On the 13th day of the spookiest month, it’s time for a murder stellar mystery! Today’s whodunit revolves around a new explosion in space and the resulting cosmic hunt to find the star system responsible. Whether it be a massive star burping up some hot gas or a sun-sized star diving into a supermassive black hole, we can (usually) only speculate on the specific star(s) involved prior to the explosion. This is because of a lack of pre-explosion imaging of the exact stars responsible for the transient phenomenon. There are A LOT of star systems out there, so it’s understandable that we can’t image all of them before one of them explodes!

Nevertheless, the science mystery team behind today’s paper did have imaging of the star that caused a violent stellar outburst! Thanks to the Hubble and Spitzer (RIP) space telescopes, the murder explosion site was imaged for almost 2 decades prior to the transient, an event nicknamed AT 2019krl, which occurred last year (Figure 1). The stellar explosion highlighted in today’s paper is a “supernova imposter” whose observed luminosity (~million times brighter than our Sun!) is below that of a typical supernova explosion, yet still bright enough to be detected from Earth. These awesome eruptions do not completely unbind the progenitor star, but may occur from stellar instabilities in massive stars (>10x the mass of our sun) or from the collision of a massive star with its binary companion. Jinkies! ...

• arXiv.org > astro-ph > arXiv:2009.13541 > 28 Sep 2020

Luna Zagorac wrote:
In the space between stars and planets lives a class of astronomical objects known as brown dwarfs. Too massive to be planets (~75 times more massive than Jupiter!) but not massive and hot enough to fuse hydrogen like our Sun, these objects are nevertheless classified like stars with spectral types—in other words, by the molecules we detect in their emission lines. Depending on their spectral type, dwarfs can be classified as one of the subdwarf types, usually marked by a letter, indicating the types and abundances of metals we detect in their atmospheres. These abundances can reveal clues about the process by which brown dwarfs are formed, which in turn informs our understanding of how planets and stars come to be.

In today’s paper, the authors consider a binary system of two brown dwarfs of different spectral subtypes, and in particular the dwarfs’ atmospheres. First discovered by SDSS and lovingly called SDSS J14162408+1348263AB (but we’ll follow the authors in calling it J1416AB), this system is unique because it is the only known binary of subdwarfs with the spectral types L and T (see Figure 1 for some general information about brown dwarf spectral types). This means the two dwarfs have both different temperatures and different chemical composition of their atmospheres while living in the same neighborhood in space. In analyzing this binary system, the authors can compare each of the dwarfs and make predictions about what their atmospheres look like, what molecules are present, and whether they were created and evolved together. In addition, they can test out different atmospheric models on the existing observational data of these objects to find the one best suited to low-metallicity subwarfs’s atmospheres. ...

• arXiv.org > astro-ph > arXiv:2010.01224 > 02 Oct 2020

Wei Vivyan Yan wrote:
As the oldest object, the Universe is 13.8 billion years old. Figure 1 briefly demonstrates the expansion of the Universe, starting from the Big Bang. At the age of 400 thousand years old, the Universe gave birth to the first star and the first galaxy, which ignited the darkness. From there, the Epoch of Reionization (EoR, noted in yellow) begins, as light starts to ionize the neutral hydrogen. This reionization process takes about 1 billion years and ends at redshift (z) around 5.5. After the EoR, the Universe is fully ionized and forms the way we see it today and the place we live in now.

The mysterious EoR leaves behind its products only at very high redshift (z > 6), including quasars, a type of active galactic nuclei (AGNs) containing extremely powerful supermassive black holes. If we consider the EoR as the “good old days”, then those high redshift quasars are among the oldest citizens of the Universe. Their luminous light contains the valuable history of the long-lost era we will never see again.

However, in today’s paper, the authors found a special quasar. At z = 6.3, as far back as just ~1 billion years after the Big Bang, this quasar only has a lifetime of only a few thousand years. More interestingly, unlike the youth of humans with boldness in heart, this young quasar is very weak in the context of emissions. It has very few and weak broad lines. This particular type of quasar is called the weak emission line quasar (WLQ). From the observations of this WLQ at such a high redshift, we can take a peek at the late stage of the EoR. ...

• arXiv.org > astro-ph > arXiv:2009.07784 > 16 Sep 2020

Brent Shapiro-Albert wrote:
This year’s Nobel Prize in Physics was, in part, awarded to Andrea Ghez and Reinhard Genzel for their discovery of the supermassive black hole (SMBH) at the center of the Milky Way. This was an amazing discovery, and has led many astronomers, including the authors of today’s paper, to look for SMBHs in other galaxies. However the authors of today’s paper are looking not for a single SMBH, but for supermassive black hole binary systems (SMBHBs) which may be formed when two galaxies merge. To do this, they utilize multimessenger astrophysics. First they analyze electromagnetic observations to identify candidate SMBHBs. Then they determine the strength of the gravitational waves emitted by the orbits of these SMBHBs, and predict if they will be detectable with current and future pulsar timing arrays.

Some SMBHBs candidates belong to a class called active galactic nuclei , or AGN, which emit massive amounts of energy that we observe as electromagnetic waves. As the SMBHBs orbit each other, they can cause periodic changes in the brightness, or magnitude, of the light emitted over time. This time-variable light emission is known as a light curve. Many large scale surveys such as the SDSS V or the Catalina Real-Time Transient Survey (CRTS) regularly monitor the light from sources across the sky, resulting in light curves from potential SMBHB sources that span years, like that shown in Figure 1.

The authors of today’s paper compile a total of 149 of these SMBHB candidates from various catalogs, and then use the source’s distance and masses to predict the frequency and strength, or strain, of the gravitational waves that may be emitted by the binary. ...

• arXiv.org > astro-ph > arXiv:2009.11865 > 24 Sep 2020

Mitchell Cavanagh wrote:
Cosmology and high performance computing often go hand-in-hand. Modelling the large-scale, filamentary structure of the Universe – and comparing it with observations – requires highly optimised code and power-hungry supercomputers. A key issue lies with running hydrodynamical simulations at a high enough resolution to model galaxy formation, within a volume large enough to encompass the next generation of sky surveys. One way to cut down on the complexity is to run a strictly dark matter-only simulation, which ignores the expensive baryons and instead adds them via analytic post-processing. Yet such methods cannot properly account for phenomena reliant on gas properties, e.g. the Sunyaev-Zel’dovich effect.

The authors of today’s paper introduce a new deep learning method, Lagrangian Deep Learning (LDL), with which to learn and model the physics governing baryonic hydrodynamics in cosmological simulations. By combining a quasi N-body gravity solver with their LDL model, the authors were able to generate stellar maps from the linear density field with worst-case computational costs at an impressive 4 orders of magnitude lower than the reference TNG300-1 hydrodynamic simulation. ...

• arXiv.org > astro-ph > arXiv:2010.02926 > 06 Oct 2020