astrobites 2017

[BDanielMayfield]

Could a non-constant dark energy "constant" explain the disagreements?

That does indeed appear to be the direction that the evidence is converging toward: a non-constant Hubble factor with universal expansion rates changing over time. It looks like it is time to stop calling it a “constant”, since it ain’t ‘cuz it ain’t one.

Or is it possible that the more we learn about the Universe, the lesser the need for a mysterious dark energy contributor becomes? Or is that, too, wishful thinking?

Ann

Possible but unlikely, I think. Although studies looking across different epochs are yielding various expansion rates, they all show expansion. This variable effect must be caused by something that is variable over universal time, IMHO.

Bruce

[Chris Peterson]

Could a non-constant dark energy "constant" explain the disagreements?

That does indeed appear to be the direction that the evidence is converging toward: a non-constant Hubble factor with universal expansion rates changing over time. It looks like it is time to stop calling it a “constant”, since it ain’t ‘cuz it ain’t one.

It doesn't seem unreasonable that as a finite amount of energy gets stretched over an increasingly large volume, the physical properties of the Universe might change. That leads to the interesting possibility that other "constants" might change with time, as well. While I don't see that changing our big picture of cosmology, it could prove very interesting in answering some of the lingering questions that remain in our theories.

[bystander]

Multi-Messenger Observations of a Binary Neutron Star Merger
astrobites | 2017 Oct 16

On August 17, 2017, ripples traveling along the fabric of spacetime passed through a small planet after more than a 100 million year journey, gently stretching and squeezing the pale blue dot by fraction of an atom. Moments later, a split-second burst of high-energy gamma rays finished their journey to our little speck of dust in the Milky Way, with a rainbow of light across the electromagnetic spectrum in its wake. This flurry of information was the long-sought-after holy grail of multi-messenger astronomy. ...

http://asterisk.apod.com/viewtopic.php?t=37665

[bystander]

Lucky planets?
astrobites | 2017 Oct 17

We might have mentioned this before once or twice (or three or four or five times…) but dynamically hot Jupiters (exoplanets the size of Jupiter, but extremely close to their host stars) are really really weird. They shouldn’t be able to form so close to their host stars (or maybe they should?) but there’s a fair few of them floating around, so something has to be shunting them extremely close to their stars – and their orbits are often observed to be at very high eccentricities. If that doesn’t surprise you, minute physics have a great video explaining why all our solar system planets are in a plane, and why we’d expect the same for exoplanet systems: the stellar spin and any planetary orbits and spins should all be aligned.

However, in stellar binary systems with a star much further away than the hot Jupiter, it might be possible for an outer stellar companion to force a planet into a tight orbit with high eccentricity via the Lidov-Kozai mechanism. Quanitifying the fraction of hot Jupiter host stars that have wide separation stellar companions would help us to understand this formation mechanism, and shed more light on the weird and wonderful hot worlds that are hot Jupiters.

This is something that’s been studied before, and has even been discussed here on astrobites. Following the trend, today’s paper searches 97 hot Jupiter host stars for stellar binary companions, using a particularly interesting technique known as Lucky imaging. ...

High-resolution Imaging of Transiting Extrasolar Planetary systems (HITEP).
II. Lucky Imaging results from 2015 and 2016 - D. F. Evans et al

• Astronomy & Astrophysics (accepted 21 Sep 2017) DOI: 10.1051/0004-6361/201731855
  arXiv.org > astro-ph > arXiv:1709.07476 > 21 Sep 2017 (v1), 13 Oct 2017 (v2)

High-resolution Imaging of Transiting Extrasolar Planetary systems (HITEP).
I. Lucky imaging observations of 101 systems in the southern hemisphere - D. F. Evans et al

• Astronomy & Astrophysics 589:A58 (May 2016) DOI: 10.1051/0004-6361/201527970
  arXiv.org > astro-ph > arXiv:1603.03274 > 10 Mar 2016 (v1), 15 Mar 2016 (v2)

[bystander]

Observing a Strange Pulsar in X-ray and Radio
astrobites | 2017 Oct 18

What’s more interesting than a rapidly spinning neutron star that emits electromagnetic radiation parallel to it’s magnetic poles? One that doesn’t exactly behave as expected of course. This weirdly acting pulsar, PSR J1023+0038 is a transitional millisecond pulsar (tMSP) which is fancy speak for a pulsar with a millisecond or so rotational period that switches between radio and x-ray emission on a several year timescale. However emitting in both x-ray and radio on these longer timescales isn’t what piques the interest of astronomers in the case of this astrobite. ...

Simultaneous Chandra and VLA Observations of the Transitional Millisecond Pulsar
PSR J1023+0038: Anti-correlated X-ray and Radio Variability - Slavko Bogdanov et al

• arXiv.org > astro-ph > arXiv:1709.08574 > 25 Sep 2017

viewtopic.php?t=32203

[bystander]

Proof that standard sirens work!
astrobites | 2017 Oct 19

By now, you’ve almost certainly heard about this week’s big scientific announcement: a binary neutron star merger resulting in the observation of both gravitational waves and counterparts all over the electromagnetic spectrum (you can read Astrobites’ coverage here). Astronomers are very excited about this event; as the first of its type to be observed, it ushers in a new era of multi-messenger astronomy as well as confirms that neutron star mergers are a source of short gamma-ray bursts.

However, there is a lot of other interesting science we can get out of this event as well, which implications for many subfields of astrophysics. Just scan this list of 81 (at last count) papers about the event for an idea of the magnitude of physics we can do here. Today’s paper goes all the way into the field of cosmology and uses the event to derive the Hubble Constant completely independently of existing calculations. ...

A gravitational-wave standard siren measurement of the Hubble constant - LIGO Scientific Collaboration, Virgo Collaboration et al

• Nature (online 16 Oct 2017) DOI: 10.1038/nature24471
  arXiv.org > astro-ph > arXiv:1710.05835 > 16 Oct 2017

http://asterisk.apod.com/viewtopic.php?t=37665

[bystander]

Energy transport in white dwarfs: what about magnetic fields?
astrobites | 2017 Oct 20

The main issue with estimating the temperature of magnetic white dwarfs is that we have theoretical evidence that the magnetic field can suppress convection in the photosphere of magnetic white dwarfs. As the white dwarfs cool down, they reach temperatures where radiation is no longer effective in transporting energy, so convection should kick in. In other words, the photons alone cannot effectively transport energy via radiation, so the matter itself starts moving up and down in convective cells to help with the process. However, the magnetic field can prevent that, as found in magnetohydrodynamics simulations (a big name for simulations of an electrically conducting magnetic fluid). Despite this evidence, the predictions had not been confirmed, until today’s paper.

The authors studied four white dwarfs – one magnetic, and three non-magnetic. They chose a magnetic white dwarf with a very weak field, of only tens of kG, so that the lines are not so much affected by the Zeeman effect and spectral analysis can still be used. They then compared the agreement between effective temperatures and surface gravities derived from fits to optical and UV spectra, both with radiative and convective models. ...

Can magnetic fields suppress convection in the atmosphere of cool white dwarfs?
A case study on WD2105−820 - Nicola Pietro Gentile Fusillo et al

• Monthly Notices of the RAS (online 05 Oct 2017) DOI: 10.1093/mnras/stx2584
  arXiv.org > astro-ph > arXiv:1710.02151 > 05 Oct 2017

[bystander]

Modeling Limitless Skies
astrobites | 2017 Oct 23

If you’ve been tuning into astronomy news lately, you’ve probably heard about a number of the cool new exoplanet discoveries, like those in the TRAPPIST-1 system, continuously rolling in from our telescopes hard at work. But no matter how, when, and where a new exoplanet is discovered, there’s always that question burning at the back of our minds: could this exoplanet have Earth-like life?

This question is certainly not an easy one to answer. For an exoplanet to house life (as we understand life so far), there’s a long checklist of requirements (like those discussed here and also here) that we need the exoplanet to fulfill. For example, life as we know it survives and thrives on liquid water, so we require that the exoplanet has the ability to hold liquid water.

Today’s astrobite focuses on another important requirement: the exoplanet’s atmospheric composition. Here on Earth, for instance, we have a lot of wonderful plant-based and plant-like creatures (like trees) that produce oxygen through photosynthesis. Then other creatures here (like humans) use that oxygen to survive and thrive. So for an exoplanet to have Earth-like life, we expect it to have a buildup of oxygen. ...

Redox evolution via gravitational differentiation on low mass planets:
Implications for biosignatures, water loss and habitability - R. Wordsworth, L. Schaefer, R. Fischer

• arXiv.org > astro-ph > arXiv:1710.00345 > 01 Oct 2017

[bystander]

Dark Energy and the Future
astrobites | 2017 Oct 24

The future is constantly, unremittingly, and obtrusively here. It can’t stop. It’s happening all the time.

It’s happening to you right now.

Go outside and take a look.

Good, now come back.

Are you back?

Please tell me your back.

(I didn’t really think this through.)

And yet, for all its perennial presence, most scientists have a studied disinterest in the future. A good experiment has a predictable conclusion, observable and testable, but we wouldn’t exactly call a litmus test prescient.

The future is the domain of weathermen and climatologists. (Not to say these jobs aren’t important—two of my biggest priorities are staying dry, and not systematically destroying the capability to sufficiently food, house and support the entire human species.)

Astronomy, perhaps more than any other science, is a study of the past. Brief and excitingtransient events can be predicted and observed, but beyond that, we’re lost.

When you went outside (are you’re back yet?), how much did the universe change? If you say anything other than “not at all” you’re very self-centered.

We study a static picture of a moving universe, and even Sisyphus would lack the patience to watch the cosmic clock tick on.

But whilst our vantage point is beached on one brief moment of time, our tools are not.

Maths, much like tides in harbour or bad plumbing, runs both forwards and backwards.

So today’s authors do a rare thing: they let the game run on, and study the future of the universe. ...

The impact of dark energy on galaxy formation.
What does the future of our Universe hold? - Jaime Salcido et al

• arXiv.org > astro-ph > arXiv:1710.06861 > 18 Oct 2017

[bystander]

On the Insides of Giants: Heat Transfer in Hot-Start, Core-Accreting Gas Giants
astrobites | 2017 Oct 25

If you’ve ever searched for pictures of Jupiter on the web – and if you haven’t, you definitely should! – you’ve probably come across gorgeous images of the giant planet’s swirling surface, and the iconic, mystifying, stormy red spot.

But there’s something that these breathtakingly beautiful pictures don’t show: what’s going on beneath the surface?

It’s still a mystery exactly what goes on within the cores of gas giants like Jupiter. But the authors of today’s astrobite set out to place another piece in the puzzle of gas giant formation. They considered heat transfer (literally the transfer of heat) within gas giants – specifically those that form by the core accretion model with a “hot start”. Core accretion refers to the process of some object pulling in surrounding material and growing from it, kind of like a snowball getting larger as it rolls down a hill. And a growing gas giant that has a “hot start” has a very hot, luminous outer boundary. ...

Hot Start Giant Planets Form with Radiative Interiors - David Berardo, Andrew Cumming

• Astrophysical Journal Letters 846(2):L17 (2017 Sep 10) DOI: 10.3847/2041-8213/aa81c0
  arXiv.org > astro-ph > arXiv:1707.00740 > 03 Jul 2017 (v1), 20 Jul 2017 (v2)

[bystander]

Feeding black holes through galactic bars
astrobites | 2017 Oct 26

When it comes to picking their host galaxies, active galactic nuclei (or AGN) are rather promiscuous. They reside in all types of galaxies: ellipticals, irregulars, and spirals. AGNs of the same feather tend to flock together — the more luminous and radio-loud ones are found in elliptical galaxies while the lower luminosity ones are more often found in spiral galaxies. This is a manifestation of the black hole mass-host galaxy luminosity correlation, where spiral galaxies like our Milky Way tend to have less massive black holes than elliptical galaxies. Besides spiral arms, spiral galaxies sometimes also boast of having bars, if the right mood strikes. How are bars related to their AGNs? Could they trigger the central black holes to light up as AGNs?

Galactic bars are thought to contribute to the dynamical evolution of their host galaxies. Numerical studies show that they can funnel in gas from the outskirts to the central regions of the galaxies, triggering star formation and possibly AGN activity. It is still unclear whether bars actually help trigger AGNs, as previous studies have produced conflicting results and tend to suffer from small number statistics and biased AGN diagnostics. In today’s paper, the authors bring better tools to bear on the problem, by utilizing the large wealth of information from the SDSS Galaxy Zoo citizen science project and X-ray stacking analyses. ...

Galaxy-scale Bars in Late-type Sloan Digital Sky Survey Galaxies Do Not
Influence the Average Accretion Rates of Supermassive Black Holes - Andy D. Goulding et al

• Astrophysical Journal 843(2):135 (10 July 2017) DOI: 10.3847/1538-4357/aa755b
  arXiv.org > astro-ph > arXiv:1705.08895 > 24 May 2017

[bystander]

Can we detect auroral emission from Proxima b?
astrobites | 2017 Oct 30

Dazzling, auroral displays are not uncommon in our solar system. In fact, when the Sun sends highly-energetic particles out in to the solar system, any planets with a substantial magnetic field will interact with the particles, resulting in the emission of radio waves. Detecting such emission allows for many interesting planetary properties to be determined, such as: orbital parameters, habitability, plate tectonics and atmospheric compositions. Yet we have not observed any auroral activity from planets which lie outside our solar system (we have, however, detected radio auroral emission on a brown dwarf star!).

Today’s bite investigates our nearest stellar neighbour, Proxima Centauri, to answer one very important question – can we detect auroral emissions from its exoplanet, Proxima b? ...

The Detectability of Radio Auroral Emission from Proxima b - Blakesley Burkhart, Abraham Loeb

• Astrophysical Journal Letters 849(1):L10 (2017 Nov 01) DOI: 10.3847/2041-8213/aa9112
  arXiv.org > astro-ph > arXiv:1706.07038 > 21 Jun 2017 (v1), 29 Jun 2017 (v2)

[bystander]

Radio Images of a Nearby, Ghostly Globule
astrobites | 2017 Oct 31

It’s Halloween, and you’ve opted out of dressing up as your favorite scientist in favor of visiting an old, abandoned, (and possibly haunted) mansion. Your heart races wildly as you explore the house; after all, your astrophysics expertise won’t help you escape the clutches of a monster. At the end of a long corridor, you spot a translucent figure passing in front of a lantern. The light is muddled and dimmed in such a way that you can’t make out the ghost responsible for the occultation. You are terrified. However, when the candlelight comes back into focus and the figure has vanished, you regret not having snapped a picture of the scene.

J1819+3845: Our Candle in a Haunted Mansion

Eerie apparitions similar to your Halloween spook occur in space, too. In today’s astrobite, we discuss the case of the quasar J1819+3845: an extragalactic radio source that has long been a source of interest to astronomers due to its extreme and rapid changes in brightness. The authors of our featured paper managed to capture radio images of the ghostly globule that obscures J1819+3845, enhancing our understanding of its structure. ...

A Dense Plasma Globule in the Solar Neighborhood - H. K. Vedantham, A. G. de Bruyn, J.-P. Macquart

• Astrophysical Journal Letters 849(1):L3 (2017 Nov 01) DOI: 10.3847/2041-8213/aa8f92
  arXiv.org > astro-ph > arXiv:1710.07652 > 20 Oct 2017

[bystander]

Supernovae as near-infrared Standard Candles: Measuring H0
astrobites | 2017 Nov 02

...
The field of precision cosmology is at a fascinating juncture, as elaborated in Astrobites’ reporting of the subject in the last few months. Various measurements of H0 – the expansion rate of the universe – are at disagreements with each other. In the tale of performing H0 measurements with a variety of probes, today’s bite touches upon the local and ‘direct’ measurements of H0 from supernovae i.e. exploding stars! ...

Measuring the Hubble constant with Type Ia supernovae
as near-infrared standard candles - Suhail Dhawan, Saurabh W. Jha, Bruno Leibundgut

• arXiv.org > astro-ph > arXiv:1707.00715 > 03 Jul 2017

[bystander]

Teaching Computers to Find Non-Transiting Hot Jupiters
astrobites | 2017 Nov 03

When NASA’s Kepler Mission brought the transit method’s prospects for detecting an immense amount of planets in other star systems to fruition, it also brought a wondrous joyful feeling into the hearts and minds of astronomers and ordinary people who had long been dying to know if there were other planets out there and what they would look like. With that joy, however, comes the torment of knowing that most exoplanets do not transit their stars, leaving us to wonder if many of the stars still thought to be barren actually harbor planets.

In today’s paper, Sarah Millholland and Greg Laughlin attempt to alleviate some of that pain by searching over 140,000 of Kepler’s transit-less light curves for missing exoplanets that hopefully leave a different type of signature. When planets transit their stars, they block a small amount of light – typically about 1% for a moderately-sized planet that is not too far away, just enough for us to be able to see it happen. Can planets that do not transit also alter the amount of light coming their star system? ...

Supervised Learning Detection of Sixty Non-transiting Hot Jupiter Candidates - Sarah Millholland, Gregory Laughlin

• Astronomical Journal 154(3):83 (2017 Sep) DOI: 10.3847/1538-3881/aa7a0f
  arXiv.org > astro-ph > arXiv:1706.06602 > 20 Jun 2017

[bystander]

Looking for Structure in Dark Matter with Gravitational Lensing
astrobites | 2017 Nov 06

The nature of dark matter remains a mystery, but scientists are not giving up hope. Instead, they are coming up with new techniques to learn everything they can about this mysterious substance. Previous astrobites have covered the search for gamma-rays created in the annihilation of dark matter particles and the study of holes torn through streams of stars by invisible clumps of dark matter. Today we will learn about one more method for hunting down these invisible particles, which takes advantage of the awesome phenomenon of gravitational lensing. ...

Through a Smoother Lens: An expected absence of LCDM substructure detections
from hydrodynamic and dark matter only simulations - Andrew S. Graus et al

• arXiv.org > astro-ph > arXiv:1710.11148 > 30 Oct 2017

[bystander]

Challenges with Testing the No-Hair Theorem
astrobites | 2017 Nov 07

Black holes are believed to be incredibly simple astrophysical objects, described completely by the two parameters mass and spin. This idea is called the “no-hair theorem”; bites discussing this theorem can be found here and here. With the advent of gravitational wave astronomy, we are in a position to thoroughly test the no-hair theorem by probing the gravitational “ringing” of the black hole left after a black hole merger. However, there are some challenges that must be overcome when using gravitational waves to test the no-hair theorem, which the authors of today’s paper investigate. ...

Challenges testing the no-hair theorem with gravitational waves - Eric Thrane, Paul Lasky, Yuri Levin

• arXiv.org > gr-qc > arXiv:1706.05152 > 16 Jun 2017 (v1), 23 Oct 2017 (v2)

[bystander]

More on GW170817: Black Hole Popcorn and the Gravitational Wave Background
astrobites | 2017 Nov 08

On August 17, 2017, a cosmic event was observed for the first time ever via both gravitational and electromagnetic waves! The event – named GW170817 – was produced by the cataclysmic collision of two neutron stars. There is certainly no shortage of papers written about this historic multi-messenger detection. Here is the list of 67 preprints released on the day of the announcement of the binary neutron star coalescence. In this bite, we will be discussing the implications of GW170817 for the stochastic gravitational wave background, which is the random gravitational wave signal generated by an abundance of weak, unresolved sources. ...

GW170817: Implications for the Stochastic Gravitational-Wave Background
from Compact Binary Coalescences - LIGO Scientific Collaboration, Virgo Collaboration

• arXiv.org > gr-qc > arXiv:1710.05837 > 16 Oct 2017

http://asterisk.apod.com/viewtopic.php?t=37665

http://asterisk.apod.com/viewtopic.php?p=276035#p276035
http://asterisk.apod.com/viewtopic.php?p=276036#p276036
http://asterisk.apod.com/viewtopic.php?p=276039#p276039

[bystander]

Cloudy with a chance of coronal mass ejections
astrobites | 2017 Nov 09

Coronal mass ejections (CMEs) are immense eruptions of solar plasma and magnetic fields. When a CME strikes a planet, it can have huge effects; over billions of years, CMEs can strip away a planet’s atmosphere. In the short term, CMEs wreak havoc at Earth by causing dangerous and costly geomagnetic storms. ...

In today’s paper, the authors use simulations to determine the magnetic fields of CMEs at Earth based on the properties of the active region from which they erupt. They begin this process by determining the location of the CME eruption on the solar disk. Then, they extract the magnetic field properties of the eruption zone using magnetograms—maps of the magnetic field strength and direction across the solar surface. The solar magnetic field becomes an input parameter for Forecasting a CME’s Altered Trajectory (ForeCAT), which models the propagation of a CME through the solar corona. The CME may rotate, expand, or be deflected as it travels out, depending on how the Sun’s magnetic field is structured. The ForeCAT results are passed to the ForeCAT In situ Data Observer (FIDO), which simulates what two Earth-orbiting spacecraft, ACE and Wind, would observe as a CME passes over them. ...

Using the Coronal Evolution to Successfully Forward Model CMEs' In Situ Magnetic Profiles - C. Kay, N. Gopalswamy

• Journal of Geophysical Research – Space Physics (accepted 05 Oct 2017) DOI: 10.1002/2017JA024541
  arXiv.org > physics > arXiv:1710.03825 > 10 Oct 2017

[Ann]

More on GW170817: Black Hole Popcorn and the Gravitational Wave Background
astrobites | 2017 Nov 08

On August 17, 2017, a cosmic event was observed for the first time ever via both gravitational and electromagnetic waves! The event – named GW170817 – was produced by the cataclysmic collision of two neutron stars. There is certainly no shortage of papers written about this historic multi-messenger detection. Here is the list of 67 preprints released on the day of the announcement of the binary neutron star coalescence. In this bite, we will be discussing the implications of GW170817 for the stochastic gravitational wave background, which is the random gravitational wave signal generated by an abundance of weak, unresolved sources. ...

Click to view full size image

Popping popcorn, possibly in space.
Source: https://www.natgeocreative.com/photography/1374320

So the stochastic gravitational background, which is the random gravitational wave signal generated by an abundance of weak, unresolved sources, is really black hole popcorn?

Ann

[bystander]

GRB Afterglows: Coming out of a Cocoon, and Doing Just Fine?
astrobites | 2017 Nov 13

Today’s paper uses X-ray observations to study gamma ray bursts (GRBs) with strange components in their spectra.

For once, astronomers didn’t pick a catchy-but-ultimately-confusing name for an astrophysical phenomenon! Gamma ray bursts are exactly what they sound like: bursts of gamma rays. To be fair, they’re a bit more than mere bursts. GRBs are tremendously energetic; a typical GRB could release as much energy as the entire mass of our Sun converted into radiation (try plugging a solar mass into E=mc² to get a sense of how much energy that is!).

What’s more, all that energy is released in just a short period of time. As this previous Astrobite explains, GRBs are classified by their duration. Short GRBs, which are thought to be produced by neutron star mergers (like the one recently observed by LIGO), last for just a few seconds. Long GRBs last longer and are thought to be produced by core-collapse supernovae. ...

Thermal Components in the Early X-ray Afterglows of GRBs:
Likely Cocoon Emission and Constraints on the Progenitors - Vlasta Valan, Josefin Larsson, Björn Ahlgren

• Monthly Notices of the RAS (online 14 Nov 2017) DOI: 10.1093/mnras/stx2920
  arXiv.org > astro-ph > arXiv:1711.02948 > 08 Nov 2017

[bystander]

Beacons of Life
astrobites | 2017 Nov 14

Once upon a time, the detection of abundant molecular oxygen in an exoplanet’s atmosphere was considered to be a “slam-dunk” indication of a biosphere. O₂ is an exhaust gas from the production of glucose, life’s molecular Clif Bar. The heavy, disequilibrium presence of oxygen in the Earth’s atmosphere only exists because lots of biomass keeps pumping it in.

However, as observing capabilities have advanced to the point where we can actually start studying exoplanet atmospheres, the widening theoretical picture has become much more interesting. Would molecular oxygen indicate the presence of life? Well, maybe… and maybe not. It could be generated without life when UV insolation breaks up H₂O or CO₂ molecules. This is not significant in the Earth’s atmosphere, but today’s paper notes that it could be significant in dry, CO₂-dominated atmospheres of planets around M-dwarfs.

Detection and verification of biosignatures will be hard, and not only because false positives will try to fool us. The signals are tiny, and will require long integration times with next-generation space telescopes and extremely large telescopes (ELTs). But those telescopes will be heavily oversubscribed, and observation times will have to be kept down to the absolute minimum.

Therefore, it is to our benefit to be clever and find complementary and easier ways of inferring the presence of a biosphere. ...

Atmospheric Beacons of Life from Exoplanets Around G and K Stars - Vladimir S. Airapetian et al

• Scientific Reports 7:14141 (online 02 Nov 2017) DOI: 10.1038/s41598-017-14192-4

[bystander]

Protoplanetary Disks Might Be More Turbulent
astrobites | 2017 Nov 15

We know it happens. We see that protoplanetary disks – the birthplace of planets – spill the gaseous material at their inner edge onto the young stars around which they orbit. This process of accretion persists throughout the disk’s lifetime and within 1 to 5 million years (or 10 Myr in rare cases), a disk will feed all of its gas to its star and completely fade away – leaving behind only the planets that formed along the way and any leftover rocky material that remains.

The rate at which accretion occurs – as well as why accretion occurs – are both driving forces behind determining what types of planets can form quickly enough in a protoplanetary disk’s relatively short lifetime. While we can measure accretion rates onto stars directly, we still do not know why disks accrete! This is one of the most important unsolved problems in planet formation. Until we can solve it, our models for planet formation are incomplete.

There are two leading potential explanations for why accretion occurs: (1) turbulence, and (2) magnetic winds. In the last two years, several attempts have been made to measure turbulence in a nearby disk for the first time using CO (carbon monoxide) spectral lines. These measurements showed that the turbulence in that system is not strong enough to be responsible for accretion, winning favor for the other idea of magnetic winds.

However, today’s paper led by Emma Yu argues that those measurements were not interpreted properly since they did not take into account the relatively rapid rate CO depletes over time (see Figure 1). This paper asks: What can we really learn about turbulence from CO spectral lines if we include CO depletion in our model? ...

The Effects of Protostellar Disk Turbulence on CO Emission Lines: A Comparison
Study of Disks with Constant CO Abundance vs. Chemically Evolving Disks - Mo Yu et al

• arXiv.org > astro-ph > arXiv:1710.03657 > 10 Oct 2017 (v1), 07 Nov 2017 (v2)

[bystander]

Proxima Centauri sports a fancy dust ring
astrobites | 2017 Nov 16

Hiding behind the gleaming light of a pale red dot lies one of the world’s favorite exoplanets, Proxima b, whose orbital motion imprints a barely detectable wobble in its host star, Proxima Centauri. Following the seminal discovery of a planet in its habitability zone, this undistinguished low-mass red dwarf star became a target for tireless examination, despite the challenges involved in observing it. In today’s paper, we will see that one of these searches, however, seems to have paid off… in dust? ...

ALMA Discovery of Dust Belts Around Proxima Centauri - Guillem Anglada et al

• Astrophysical Journal Letters 850(1):L6 (2017 Nov 20) DOI: 10.3847/2041-8213/aa978b
  arXiv.org > astro-ph > arXiv:1711.00578 > 02 Nov 2017

viewtopic.php?t=37725

[bystander]

A Contracting White Dwarf?
astrobites | 2017 Nov 17

Today’s paper takes another look at a binary system, HD 49798, which has been puzzling scientists for some time. HD 49798 is an X-ray binary, a type of binary in which matter is transferred onto a central, compact star (a white dwarf, neutron star or black hole) from a donor star. The authors suggest that many of the unusual attributes of the system could be explained if one of the stars is a young, half-formed white dwarf which is still in the process of contracting. ...

A Young Contracting White Dwarf in the Peculiar Binary HD 49798/RX J0648.0–4418 ? - S.B. Popov et al

• Monthly Notices of the RAS (online 13 Nov 2017) DOI: 10.1093/mnras/stx2910
  arXiv.org > astro-ph > arXiv:1711.02449 > 07 Nov 2017

http://asterisk.apod.com/viewtopic.php?t=30985