astrobites 2018

Find out the latest thinking about our universe.
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Revisiting the Barnard’s Star Debate

Post by bystander » Tue Dec 11, 2018 5:37 pm

Revisiting the Barnard’s Star Debate
Astrobites | 2018 Dec 10
Jessica Roberts wrote:
Sitting at only 6 light years from us, Barnard’s star is the closest single star from us and the second closest M dwarf (Proxima Centauri being the closest). This little 0.16 solar mass star has also been the subject of some serious debate and controversy over the possible existence of a planetary system. In the 1960s, Peter van de Kamp claimed a detection of not one but two planets orbiting at 12 and 20 days, with similar masses to Jupiter. However, multiple groups have been unable to reproduce these results. One study, highlighted in this bite, also ruled out planets with masses greater than 10 Earth masses and periods less than 2 years. If there are planets of smaller masses or on longer orbits, the observations up to this point just haven’t been precise enough to detect them.

But the authors of today’s paper didn’t give up on this tiny star. They observed Barnard’s star every possible night between 2016 and 2017 with CARMENES, HARPS, and HARPS-N instruments. These measurements were then combined with all the other observations of this star over the last 20 years. By using the radial velocity technique, the authors searched for possible “wobbles” in the star’s movement as it is pulled towards and away from us by the undetected planet. And lo and behold, not one, but two signals emerged from the dataset (Figure 1)! One was a repeating or periodic signal at 233 days and another one at >4000 days (or about 11 years). Something (or multiple things) appeared to be playing a gravitational tug-of-war with Barnard’s star. ...

These dusty young stars are changing the rules of planet-building
Nature News | 04 Dec 2018

A Candidate Super-Earth Planet Orbiting near the Snow-Line of Barnard’s Star ~ I. Ribas et al
  • Nature 563(7731):365 (15 Nov 2018) DOI: 10.1038/s41586-018-0677-y
    arXiv.org > astro-ph > arXiv:1811.05955 > 14 Nov 2018 (v1), 23 Nov 2018 (v2)

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What is causing these Little Dippers?

Post by bystander » Tue Dec 11, 2018 5:56 pm

What is causing these Little Dippers?
Astrobites | 2018 Dec 11
Emma Foxell wrote:
The now retired Kepler space telescope has done well, discovering many of the nearly 4000 exoplanets. But Kepler can be used to study anything which causes the amount of starlight we see to dip (or increase). The authors of today’s paper were looking for ‘dipper’ stars in the K2 lightcurves and instead found their smaller cousins.

The dipper stars they were looking for are young stars (less than 10 Myr) with spectral types K/M, which have deep (>~10%) dips in light lasting ∼0.5–2 days, reoccurring semi-regularly or episodically (meaning they exhibit several events over a short period of time then none for a longer period of time). These dips are believed to be caused by structures in protoplanetary disks, allowing us to probe disk structure and dynamics during planet formation.

Instead they found two objects with lightcurves like dipper stars but with dips 10-100 times shallower. Figure 1 shows EPIC 205718330, a mid-K dwarf with two single dips and another two with multiple dips. Dips are generally symmetric, with depths 0.5-1.5% and last 0.5-1 day. EPIC 235240266 in Figure 2 is a late F dwarf with six dips lasting a similar amount of time but shallower (~0.1%) and asymmetric. ...

The Little Dippers: Transits of Star-Grazing Exocomets? ~ Megan Ansdell et al
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The Properties of Simulated Jellyfish

Post by bystander » Sat Dec 15, 2018 5:32 pm

The Properties of Simulated Jellyfish
Astrobites | 2018 Dec 12
Caitlin Doughty wrote:
When an astronomer glances through a telescope at the night sky, the last thing they expect to see is a sea creature. However, it is difficult to deny that a fairly large number of galaxies bear a remarkable resemblance to that most noble of cnidarians, the jellyfish. These so-called “jellyfish galaxies” have peculiar shapes that visually resemble jellyfish tentacles trailing behind the main body of the galaxy and are often found inside galaxy clusters, acting as a satellite of the cluster’s central galaxy. But although it is known that they appear in clusters, we still don’t know how they take shape. The authors of today’s paper use cosmological simulations to create simulated jellyfish galaxies and learn more about how these intriguing structures are formed. ...

Jellyfish galaxies with the IllustrisTNG simulations:
I. Gas-stripping phenomena in the full cosmological context
~ Kiyun Yun et al
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viewtopic.php?t=37482
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Living a (Solar System) Lifetime in Color

Post by bystander » Sat Dec 15, 2018 5:46 pm

Living a (Solar System) Lifetime in Color
Astrobites | 2018 Dec 13
Stephanie Hamilton wrote:
The outer reaches of our own Solar System remain a mystery. Astronomers are only just beginning to shine (colored) light on the distant region of our Solar System called the Kuiper Belt. But this region has a lot to tell us about the history of the Solar System — N-body simulations predict the types of objects we should find and what their orbits should look like. Are they locked into an orbital resonance with Neptune? Have they been flung out of the plane of the Solar System by a past interaction with another object? Other studies also predict the types of molecules we should see based on where the objects formed and how they have been flung around the Solar System.

Using spectroscopy to learn about the compositions of Kuiper Belt objects (KBOs) has typically been impossible because these objects are generally so faint (due to their great distances from Earth of 30-40 astronomical units or more). One solution to this problem is to take “broad-band” filter measurements — instead of separating the object’s light into individual wavelengths as in spectroscopy, astronomers take images using filters that select wider ranges of wavelengths (see Figure 1). While this technique sacrifices detailed wavelength information, it increases photon counts, making even faint KBOs measurable. The difference in flux between images in two filters gives a “color.” Unfortunately, many KBO surveys have taken images in only one filter. Thus, many of the ~2,100 currently known KBOs don’t have associated colors. Today’s paper details results from ongoing Col-OSSOS, or the Colors of the Outer Solar System Origins Survey, which uses the Gemini-North telescope in Hawaii to measure the colors of select objects discovered by OSSOS. Today’s authors compiled the largest KBO sample to date with well-measured colors — 229 KBOs in three different filters.

Today’s authors were specifically interested in how an object’s color correlates with its orbital inclination. Since color and orbital properties like inclination can tell us about an object’s dynamical history, considering both together could potentially place stronger constraints on the Solar System’s complicated dynamical past than either alone. ...

Col-OSSOS: Color and Inclination are Correlated Throughout the Kuiper Belt ~ Michael Marsset et al
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Understanding Dwarf Galaxies with a Heart of Steel

Post by bystander » Mon Dec 24, 2018 1:42 am

Understanding Dwarf Galaxies with a Heart of Steel
Astrobites | 2018 Dec 17
Tomer Yavetz wrote:
Dwarf galaxies are trickier than they appear. As their name suggests, they are basically small versions of regular galaxies. Their masses range between several million to several billion solar masses (for comparison, the Milky Way, which is a regular-sized galaxy, is thought to come in at approximately a trillion solar masses). They come in many shapes, including elliptical, spheroidal, and irregular, and they can be found either on their own (aka “field galaxies”), or orbiting larger galaxies or galaxy clusters (aka “satellite galaxies”; see Figure 1 for some examples of the Milky Way’s satellite dwarf galaxies). ...

Today’s paper explores yet another dwarf galaxy riddle: many of the observed dwarf galaxies (including two of the examples in Figure 1) have two distinct populations of stars, which can be easily separated in terms of both their spatial distribution and their metallicities (in astronomy, the word ‘metal’ refers to any element that is heavier than Hydrogen or Helium; typically, older stars are more “metal-poor” and younger stars are more “metal-rich”). One of the observed stellar populations in dwarf galaxies is both metal-rich (and therefore likely younger) and highly concentrated in the center of the galaxy, whilst the other is metal-poor (so probably older) and extends much further out than the metal-rich population. The riddle is, of course, what causes this segregation? ...

The Distinct Stellar Metallicity Populations of Simulated Local Group Dwarfs ~ Anna Genina et al
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Traits of Accreting Galaxies

Post by bystander » Mon Dec 24, 2018 1:50 am

Traits of Accreting Galaxies
Astrobites | 2018 Dec 18
Caitlin Doughty wrote:
Looking at a galaxy, the first thing we are likely to notice is its stars. All stars, whether they are massive, bright, short-lived blue stars or small, dim, long-lived red stars, form in regions called stellar nurseries, which are pockets of cold, dense molecular gas. Given the correct conditions, this gas will collapse and begin to form stars. Consequently, the abundance of gas within a galaxy can be treated as a measure of its ability to form stars. A problem arises though: given the rate of star formation observed in the nearby Universe (0.3-1 solar masses per year), most galaxies should run out of gas quite quickly and, once this fuel is depleted, should stop forming stars completely. Obviously these galaxies are somehow acquiring gas, but it is very difficult to observe accretion, the gravitationally induced collection of gas by a galaxy (or other objects). Thus, there are many remaining questions regarding how galaxies accrete gas and at what rate. The authors of today’s paper examine galaxies for signs of recent gas accretion and examine correlations with galaxy properties and apparent instigators of accretion. ...

Anomalously Low Metallicity Regions in MaNGA Star-Forming Galaxies:
Accretion Caught in Action?
~ Hsiang-Chih Hwang et al
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Hunting for Variable White Dwarfs in the GALEX Archives

Post by bystander » Mon Dec 24, 2018 2:00 am

Hunting for Variable White Dwarfs in the GALEX Archives
Astrobites | 2018 Dec 19
Matthew Green wrote:
White dwarfs are stars in their silver years. 97% of all stars will end their lives as white dwarfs. These stars have stopped all fusion in their cores and are powered by left-over heat from their younger lives. It’s easy to think of white dwarfs as ‘dead stars’, doing nothing but hanging in space while they slowly cool. However, these stars have plenty of activity left in them. Some white dwarfs pulsate: instabilities in their atmospheres cause them to shrink and stretch in size. Studying these pulsations can tell us about the internal structure of a white dwarf. Other white dwarfs are in binary systems and undergo eclipses when their companion star blocks their light from reaching us. Studying these eclipses can help us accurately measure the masses and radii of the two stars. And some white dwarfs have their own planetary systems, which collide, rip themselves apart and shower the white dwarf with debris. These can help us study the composition of planets and give us an insight into planetary systems in the late stages of their lives.

All of these types of activity cause a white dwarf’s apparent brightness to change. There have been numerous searches for this type of variability by observing these stars in optical light. However, white dwarfs are very blue objects and most are brightest when observed at ultraviolet wavelengths. The satellite GALEX observed in the ultraviolet for nine years, between 2003 and 2012. It observed patches of sky for about half an hour each, and over its lifetime looked at around 77% of the sky. In today’s paper, our authors have looked back through archival GALEX data to search for white dwarfs showing different kinds of variability. ...

Detections and Constraints on White Dwarf Variability from Time-Series GALEX Observations ~ Dominick M. Rowan et al
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Seeing Double: Binary Stars in Dwarf Galaxies

Post by bystander » Mon Dec 24, 2018 2:11 am

Seeing Double: Binary Stars in Dwarf Galaxies
Astrobites | 2018 Dec 20
Mia de los Reyes wrote:
Stars aren’t usually only children. In fact, we think most stars are born in binary or multiple systems. But just how many binary systems are out there?

Understanding the fraction of binary stars is important in studying galaxies. For example, the number of binaries can affect some estimates of global galaxy parameters like star formation rates, which depend heavily on models of stellar populations. Binary stars can also lead to events like Type Ia supernovae (the thermonuclear explosions of some white dwarf stars with binary companions), so knowing the fraction of binary stars can help us figure out the rates of these events.

Binary stars might be even more important in the smallest and faintest of galaxies, called “ultra-faint dwarf galaxies” (UFDs). UFDs are strange systems. They seem to be hybrids between globular clusters and dwarf galaxies, but they’re mostly classified as “galaxies” instead of stellar clusters. This classification is, in part, because UFDs (like other galaxies) appear to be dominated by dark matter based on observations of the velocities of their stars.

How does this work? The velocity dispersion (a measure of how much the stars’ velocities differ from the average motion of the galaxy) is high for a UFD. This suggests that there’s a lot of mass in the galaxy, making the stars orbit quickly around the galaxy’s center of mass. The velocity dispersions are even high enough to imply that there’s more matter in UFDs than just the visible matter: hence, dark matter! This could make UFDs promising targets to probe the physics of dark matter.

But binary stars could mess this all up. As the stars in binaries move around their companions, they can increase the velocity dispersion of a galaxy and make it seem like the galaxy has more mass than it really does. If UFDs have high fractions of binary stars, they might not have as much dark matter as we think! ...

The Binary Fraction of Stars in Dwarf Galaxies: the Cases of Draco and Ursa Minor ~ Meghin Spencer et al
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Stepping into a Supernova

Post by bystander » Mon Dec 24, 2018 2:22 am

Stepping into a Supernova
Astrobites | 2018 Dec 21
Emma Foxell wrote:
How many of us have dreamed of flying among the stars? While interstellar travel is still a long way off, virtual reality can allow us to immerse ourselves in space and get up close to the astrophysical phenomena we point our telescopes at. The authors of today’s paper adapted a 3D model of a supernova remnant from Delaney and team into virtual reality. ...

Walking Through an Exploded Star: Rendering Supernova Remnant Cassiopeia A into Virtual Reality ~ Kimberly K. Arcand et al
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Vary, Vary, Little Star…Or Don’t, If You’re the Sun

Post by bystander » Sun Dec 30, 2018 7:36 pm

Vary, Vary, Little Star…Or Don’t, If You’re the Sun
Astrobites | 2018 Dec 24
Daniel Berke wrote:
Have you stopped to ponder lately about just how incredibly stable our Sun is? Sure, it occasionally lets off powerful flares that can cause problems for us here on Earth, but compared to most other stars out there our Sun’s average energy output is remarkably—even anomalously—constant. A Sun that varied in brightness by even just a few percent over the years could have significant, potentially disastrous consequences on our climate or ability to grow crops. My previous post on Thanksgiving ended up with a title sounding like a leftover from Halloween, so it’s only fitting that my Christmas Eve post touches on being thankful for the extreme stability of our Sun. Today’s paper sheds some light on the Sun’s stability by using simulations to investigate the effects of changing various parameters in Sun-like stars. ...

From Solar to Stellar Brightness Variations: The Effect of Metallicity ~ V. Witzke et al
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Capturing Interstellar Wanderers

Post by bystander » Sun Dec 30, 2018 7:43 pm

Capturing Interstellar Wanderers
Astrobites | 2018 Dec 26
Peter Sinclair wrote:
Last year, our solar system was visited by our first recorded interstellar guest, ʻOumuamua. Based on the extreme trajectory of this asteroid, it is clear that it did not originate from within our solar system. Thanks to ʻOumuamua, we now have a better idea of what interstellar objects may be like. However, its passing has left astronomers wondering if some of the objects in the solar system originate from outside our neighborhood. In today’s paper, Siraj and Loeb model the most likely orbital parameters for a captured interstellar object. Based on these parameters, they identify several asteroids which may have originated from outside our solar system. Closer study of these objects could tell us more about exoplanet systems. ...

Identifying Interstellar Objects Trapped in the Solar System through Their Orbital Parameters ~ Amir Siraj, Abraham Loeb
viewtopic.php?t=37698
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Planetary Cookie Doughs at High Angular Resolution

Post by bystander » Sun Dec 30, 2018 7:54 pm

Planetary Cookie Doughs at High Angular Resolution
Astrobites | 2018 Dec 27
wrote:
If you had a cookie for each of the 3,877 planets confirmed to date,would you be able to figure out the recipe behind them all? I understand if this holiday season’s sugar rush is already preventing you from thinking about cookies (let alone their recipes) anymore, but figuring out cookie recipes from just a bunch of cookies is pretty much the nature of questions that planet formation studies usually deal with. The main goal of planet formation models is to understand how protoplanetary disks evolve and result in the huge diversity in the demographics of observed planet population (in terms of the planetary mass distributions, orbital architectures, and atmospheric compositions). Theoretical studies in this direction have been attempting to form a coherent picture of how over the course of their lifetimes of just a few million years, the dust particles in the protoplanetary disks grow from roughly centimeter and meter-sized particles (often referred to as pebbles) to kilometer-sized planetesimals that then form the seeds for the formation of terrestrial and giant planet cores.

One of the key obstacles to this approach faced by physical models of small particle agglomeration is the uncertainty in the timescales and hence the efficiency of planetesimal growth starting from pebbles over a disk’s typical lifetime of few million years. This is directly related to how the pressure of the gas in the disk varies with respect to radial distance from the star since the local pressure and gravity at any place in the disk affect how efficiently fast-drifting pebbles can slow down due to the viscous drag of surrounding gas and start to grow into larger planetesimals. Observations that can trace the small-scale variations in the concentration of solids and associated gas pressures with respect to the distance from the star, can thus help in tuning the physical models that describe the growth of planetesimals and ultimately solid protoplanet cores in the disks. Think of scrutinizing a bunch of swirl cookie dough samples with the goal of deciphering the master swirl cookie recipe behind all of them! Today’s paper is the first in the series of 10 papers that present high-resolution observations of 20 such planetary-cookie-dough samples taken by the Atacama Large Millimeter Array (ALMA). ...

The Disk Substructures at High Angular Resolution Project (DSHARP):
I: Motivation, Sample, Calibration, and Overview ~ Sean M. Andrews et al
viewtopic.php?t=38971
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Why is there nothing between Mercury and the Sun?

Post by bystander » Sun Dec 30, 2018 8:06 pm

Why is there nothing between Mercury and the Sun?
Astrobites | 2018 Dec 28
Michael Hammer wrote:
When Michel Mayor and Didier Queloz discovered the very first exoplanet, 51 Pegasi b, in 1995, astronomers were shocked to find it was separated from its star by just 0.05 AU, more than seven times closer than the distance between Mercury and our Sun (0.38 AU). In fact, if you were to take the orbits of every exoplanet known today and place them in our solar system, most of them would fit in the empty space in-between Mercury and our Sun. Kepler-11 in particular has five planets that fit inside Mercury’s orbit, while our solar system has none!

If these close-in planets are so common, why doesn’t our solar system have any? Today’s paper by Christopher Spalding blames the Sun itself for the void of planets within Mercury’s orbit! Whereas a previous paper we covered on Astrobites proposed that the building blocks of planets (called planetesimals) may have never formed at less than 0.3 AU, Spalding instead argues that the Sun would have blown away all of the smallest planetesimals in this region with its powerful solar wind. With fewer building blocks left so close to the Sun, it would have been a lot more difficult for planets to form there. ...

The Primordial Solar Wind as a Sculptor of Terrestrial Planet Formation ~ Christopher Spalding
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