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molecules in space

What do methylidyne, cyanamide, vinyl alcohol, and rugbyballene all have in common? They’re all molecules that have been detected in space — and they’re all included in a recent census of our universe’s chemical makeup.

molecule detections over time

Cumulative number of known interstellar molecules over time. Commissioning dates of major contributing facilities are noted with arrows. [McGuire 2018]

Looking For Complexity

Since the first detection of methylidyne (CH) in the interstellar medium in the 1930s, scientists have been on the lookout for the many molecules — groups of two or more atoms held together by chemical bonds — they know must exist beyond our own planet.

Observations of molecules can help us to understand the chemical evolution of the interstellar medium, the formation of planets, and the physical conditions and processes of the universe around us. But molecules produce complex spectral features that are difficult to correctly attribute, making definitive observations of specific molecules challenging — which means that we’re still only just beginning to understand the chemical composition of our universe.

In a recent publication, scientist Brett McGuire (Hubble Fellow of the National Radio Astronomy Observatory, Harvard-Smithsonian Center for Astrophysics) provides an overall summary of observed interstellar, circumstellar, extragalactic, protoplanetary-disk, and exoplanetary molecules. This publication marks the first “living paper” published in AAS journals — a paper that will continue to be updated over years to come as our observations amass and our understanding of the universe around us grows.

Location, Location, Location

molecule sources

Percentage of known molecules that were detected for the first time in carbon stars, dark clouds, LOS clouds, and SFRs. [McGuire 2018]

McGuire’s census, which includes observations from dozens of facilities across the electromagnetic spectrum, identifies the molecules that have been discovered in various locations.

  1. Interstellar and circumstellar molecules
    All of the molecules that we’ve detected beyond Earth have been spotted in the interstellar or circumstellar medium in our galaxy. In total, 204 different molecules have been identified, ranging in size from two atoms (like methylidyne) to 70 atoms (like rugbyballene, C70).
  2. Extragalactic molecules
    67 of the known interstellar and circumstellar molecules (33%) have also been detected in observations of external galaxies.
  3. Protoplanetary-disk molecules
    Only 36 of the known interstellar and circumstellar molecules have been found in protoplanetary disks, in part due to the harsh physical environment around young stars and the challenge of maintaining gas-phase molecules under these conditions.
  4. Exoplanetary molecules
    Just five molecules — CO, TiO, H2O, CO2, and CH4 — have been found in exoplanetary atmospheres thus far.
atoms that make up molecules

Periodic table of the elements color-coded by number of detected species containing each element. [McGuire 2018]

Analyzing Detections

What can we learn from this census? It’s interesting to note that the entirety of the known molecular inventory is constructed from just 16 of the 118 known elements. As for where they form, more than 90% of the detected molecules were made in a carbon star, a dark cloud, a diffuse/translucent/dense cloud that lies between us and a background source, or a star-forming region.

McGuire points out that our detection of new molecules has progressed at a fairly constant rate since the 1960s. Nonetheless, there are many prospects for future advances — such as the upcoming James Webb Space Telescope’s ability to study exoplanet atmospheres in greater detail. Be sure to check back with this living paper in the future to see how our knowledge of the universe’s chemistry changes!

Citation

“2018 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules,” Brett A. McGuire 2018 ApJS 239 17. doi:10.3847/1538-4365/aae5d2

Blue stragglers in NGC 6362

As stars age, they gradually lose angular momentum and spin more slowly. This process occurs so predictably for normal, solar-type stars that we can treat them as cosmic clocks using a technique called gyrochronology. But could the same strategy be applied to an unusual type of main-sequence star called blue stragglers?

M55 color-magnitude diagram

The blue stragglers in globular cluster M55 are easily identified in a color-magnitude diagram (cyan circle). [Adapted from B.J. Mochejska, J. Kaluzny (CAMK), 1-m Swope Telescope]

Stars That Linger

Based on their mass and age, we would expect blue-straggler stars to have exhausted their core hydrogen and evolved off the main sequence already. Instead, these oddball objects have managed to loiter long past their time by gaining mass — either by siphoning it from a binary companion star or by consuming another star altogether through a collision.

Blue stragglers are easy to pick out in a star cluster, where they are bluer and brighter than the main-sequence turnoff point on a color–magnitude diagram. Post-mass-transfer stars like blue stragglers also exist outside of clusters, where they can be identified by abnormal chemical abundances or the presence of a white-dwarf companion.

To better understand post-mass-transfer stars like blue stragglers, we would like to know how long ago they accreted mass from their companions. We know that these stars experience a jump in spin rate immediately after mass accretion — but what happens after that point? Do they undergo predictable spin-down like normal, solar-type stars, allowing us to use gyrochronology to determine their post-mass-transfer ages?

Going for a Spin

To explore this question, a team led by Emily Leiner (Northwestern University) studied the rotation-rate slowdown of blue-straggler and other post-mass-transfer stars. Leiner and collaborators compiled a sample of post-mass-transfer binaries of varying ages by selecting stars with spectral types F, G, and K with white-dwarf companions in close orbits. Here, age doesn’t refer to time since the star formed, but rather time since the mass transfer took place.

The very young systems were selected by direct detection of the white-dwarf companion in the extreme ultraviolet. In older systems, the white-dwarf companion is too cool to be visible but can be detected by gravitational microlensing.

Leiner and collaborators combined the age estimates from white-dwarf cooling models with rotation periods derived from photometric or spectral measurements. The authors found that the stars spin faster after the mass transfer, then steadily slow down after about 100 million years since the mass transfer have passed.

Leiner et al. 2018 Fig. 1

Ages and rotation periods for this sample of post-mass-transfer systems. The purple and gold lines are single-star models, while the red and cyan lines are collisional-product models. Click to enlarge. [Leiner et al. 2018]

A Model for Spin-down

To understand the physics of post-mass-transfer star spin-down, the authors compared the observed spin-down to models for single solar-type stars and stellar collision products. They found that the models for the stellar collision products showed distinctly different behavior; the collision products maintained their rapid rotation rates far longer than the single stars or post-mass-transfer stars.

Leiner and collaborators attributed this to the possibility that the collision products don’t form normal stellar magnetic fields and can’t lose angular momentum through magnetic braking the way single main-sequence stars do.

On the other hand, the models for spin-down of single solar-type stars matched the blue-straggler observations well. This suggests that blue stragglers and other post-mass-transfer stars have a promising future as gyrochronometers!

Citation

“Observations of Spin-down in Post-mass-transfer Stars and the Possibility for Blue Straggler Gyrochronology,” Emily Leiner, Robert D. Mathieu, Natalie M. Gosnell, and Alison Sills 2018 ApJL 869 L29. doi:10.3847/2041-8213/aaf4ed

magnetar

Could the mysterious fast-radio-burst signal FRB 121102 be emitted from a flaring, strongly magnetic neutron star? In a new study, two scientists explore the evidence.

Mysterious Signals

More than a decade ago, a powerful burst of coherent radio emission lasting only a few milliseconds mystified astronomers. The dispersion of the signal — the delay of its component frequencies by different amounts of time, depending on the wavelength — indicated that this pulse came from beyond our galaxy. But what was it?

fast radio burst

Artist’s impression of a fast radio burst observed by the Parkes Radio Telescope. [Swinburne Astronomy Productions]

Today, we’ve detected many dozens of these odd fast radio bursts (FRBs), including two sources that appear to repeat. The repetition has allowed scientists to learn more about the best studied of these, FRB 121102: this burst has been localized to a star-forming dwarf galaxy that lies three billion light-years from Earth. Upon closer inspection of the region, scientists found that in addition to FRB 121102’s repeating bursts, a dim and steady source of radio emission lies nearby.

These accumulating clues all address a broad mystery: what object could be responsible for the bursting and steady emission we observe? What is the source of an FRB?

A Magnetized Solution

Two scientists at Columbia University, former graduate student Ben Margalit (now a NASA Einstein Postdoctoral Fellow at UC Berkeley) and advisor Brian Metzger, recently proposed an explanation for FRB 121102: perhaps this source is a young, flaring, highly magnetized neutron star that is embedded in a decades-old supernova remnant.

Neutron stars are dense cores left behind after a star’s spectacular death in a supernova or a gamma-ray burst. In particular, a magnetar is a type of neutron star with an extremely powerful magnetic field that causes flares and bursts early in the object’s life. Such flares from a distant young magnetar, Margalit and Metzger argue, could explain the FRB signals we observe.

flaring magnetar concordance model

Schematic of the authors’ model, in which a young, flaring magnetar is embedded in a magnetized nebula trapped behind the shell of supernova ejecta. Electrons in the magnetized nebula emit the persistent radio radiation, and the nebula leaves an imprint on the burst emission — which originates from the magnetar — as well. [Margalit & Metzger 2018]

In addition, the newly-formed magnetar may rest in the center of a compact, magnetized nebula that’s trapped behind the expanding shell of supernova ejecta created when the magnetar was born. This magnetized nebula could power persistent radio emission like what we observed near FRB 121102.

As a final piece of the puzzle, the authors point out that the identified home for FRB 121102 is consistent with the type of galaxy in which magnetars often form. Such small galaxies with high specific star formation rates are known to preferentially host long gamma-ray bursts and superluminous supernovae, events in which magnetars are born.

Predicting the Future

To test their theory, Margalit and Metzger develop a detailed time-dependent model of an expanding, magnetized electron-ion nebula inflated by a flaring, young magnetar. They then show that the energetics of their model beautifully match the properties of both the bursting and persistent radio emission from FRB 121102.

Does this mean the mystery’s solved? We can’t say for sure yet — but the authors make specific predictions for future observations of FRB 121102 that will provide a robust test of their model. In addition, the very recent discovery of a second repeating burst, FRB 180814.J0422+73, will hopefully allow us to further explore these mysterious sources and confirm their origin.

Citation

“A Concordance Picture of FRB 121102 as a Flaring Magnetar Embedded in a Magnetized Ion–Electron Wind Nebula,” Ben Margalit and Brian D. Metzger 2018 ApJL 868 L4. doi:10.3847/2041-8213/aaedad

Ultima Thule

What did you do on New Year’s Eve this year? Whatever it was, it probably wasn’t quite as extreme as what the New Horizons spacecraft was doing: passing by 2014 MU69 in the most distant flyby of any object in our solar system.

Today, we’ll get our first detailed look at 2014 MU69 — nicknamed Ultima Thule — from high-resolution data arriving from New Horizons. But while we wait, we can take a moment to explore what we’ve already learned about this small body in our outer solar system.

A Distant Target

New Horizons view of Ultima Thule

A first, low-resolution image of Ultima Thule from New Horizons, taken on 31 Dec 2018, just before the spacecraft’s closest approach. The right inset shows an artist’s sketch of Ultima Thule’s possible shape. [NASA/JHUAPL/SwRI; James Tuttle Keane]

Ultima Thule is a trans-Neptunian object located in the Kuiper belt, the icy disk in the outer solar system that contains leftover material from when the Sun was born. After the New Horizons spacecraft arrived at Pluto, scientists chose Ultima Thule — a body of perhaps a few tens of kilometers in size — as New Horizons’s next target for visitation.

Why fly by Ultima Thule? This target offered a rare opportunity to learn more about the geology and morphology of objects in our outer solar system. In particular, scientists hoped to learn about its surface composition, its structure, and whether it hosts moonlets, a coma, or rings.

But flyby data from New Horizons is not the only means we have of observing such distant bodies — and almost as soon as the target was selected, stalking of Ultima Thule began. Through what are known as occultation observations, scientists have already learned quite a bit about the previously unknown 2014 MU69.

SOFIA occultation observations

As 2014 MU69 — Ultima Thule — passes in front of a background star, this occultation can be observed from Earth with carefully placed lines of ground telescopes or with SOFIA, an airborne observatory. [NASA]

Stalking Occultations

In a recently published study led by Eliot Young (Southwest Research Institute), a team of scientists detailed their search for evidence of rings around Ultima Thule using occultation observations.

Young and collaborators explored data obtained on three dates in the summer of 2017: on 3 June, from South Africa and Argentina, on 10 June, from the airborne observatory SOFIA, and on 17 July, from Argentina. On these days, telescopes were assembled with the goal of catching 2014 MU69 as it crossed in front of a background star, briefly blocking the star’s light.

The light curves produced by these occultations allowed the team to explore whether Ultima Thule is encircled by additionally light-blocking rings.

Ruling Out Rings

occultations rule out rings

An example of candidate rings (red ellipses) ruled out by occultation observations on 17 July 2018 (yellow lines and map in the right panel). [Young et al. 2018]

Young and collaborators produced a set of 62 million different models for rings around Ultima Thule. The authors then compared these predicted light curves to actual light curves captured during Ultima Thule’s occultations.

The result? Ultima Thule seems highly unlikely to host any rings: rings with radii up to 1,000 km and radial widths of ~720 meters are inconsistent with the occultation light curves, and any ring larger than 1,000 km in radius would produce enough light to have been detected in Hubble Space Telescope imaging.

So far, this apparent lack of rings is consistent with the low-resolution images we’ve received from New Horizons’s flyby. Today — and in the months to come — we’ll find out what is revealed in higher resolution images and data. As always, it’s exciting to watch science in action!

Citation

“Limits on a Ring System at 2014 MU69 from Recent Stellar Occultations,” Eliot F. Young et al 2018 Res. Notes AAS 2 224. doi:10.3847/2515-5172/aaf574

Sgr A

Editor’s note: In these last two weeks of 2018, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Detection of Intrinsic Source Structure at ~3 Schwarzschild Radii with Millimeter-VLBI Observations of SAGITTARIUS A*

Published May 2018

Main takeaway:

Radio observations of the supermassive black hole at the center of the Milky Way have reached their highest resolution yet, teasing out details of the structure of Sagittarius (Sgr) A* on a scale of just ~30 microarcseconds. This corresponds to a size of only ~3 times that of the black hole’s event horizon (the distance at which not even light can escape).

Why it’s interesting:

By observing our galaxy’s ~4-million-solar-mass black hole, Sgr A*, in unprecedented detail at radio wavelengths, astronomers hope to explore the structure of a supermassive black hole’s event horizon and learn about the physical processes that occur there. The observations presented in this study, led by Ru-Sen Lu (Max Planck Institute for Radio Astronomy, Germany; MIT Haystack Observatory), represent important advancement toward that goal, and they provide an tantalizing glimpse of Sgr A* that is thus far consistent with the picture of ring-like structure that is predicted for the black hole’s event horizon.

What we can expect ahead:

Event Horizon Telescope

The participating telescopes of the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA). A subset of these telescopes were used to make the observations in the Lu et al. 2018 study presented here. [ESO/O. Furtak]

The observations of Sgr A* were made by six stations for very long baseline interferometry (VLBI) located in Hawaii, California, Arizona, and Chile. These telescopes are linked together as part of the Event Horizon Telescope, a growing project that aims to assemble a VLBI network of millimeter wavelength dishes to image the event horizon of the Milky Way’s supermassive black hole. By combining observations from geographically distributed telescopes across the globe, the Event Horizon Telescope will behave like a telescope that is effectively the size of the Earth — which should provide an unprecedented view of Sgr A*’s shadow in the near future.

Citation

Ru-Sen Lu et al 2018 ApJ 859 60. doi:10.3847/1538-4357/aabe2e

Kepler-90

Editor’s note: In these last two weeks of 2018, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Identifying Exoplanets with Deep Learning: A Five-Planet Resonant Chain around Kepler-80 and an Eighth Planet around Kepler-90

Published January 2018

Main takeaway:

Using machine learning, Google Brain engineer Christopher Shallue and NASA Sagan Postdoctoral Fellow Andrew Vanderburg (The University of Texas at Austin, Harvard-Smithsonian CfA) have discovered two new planets within previously known Kepler multi-planet systems.

Why it’s interesting:

training light curves

Examples of light curves used to train the authors’ neural network models. [Shallue & Vanderburg 2018]

In today’s field of exoplanet astronomy, observatories like Kepler and TESS have guaranteed us plenty of data. But the transit signal of an Earth-sized planet around a Sun-like star remains at the edge of detectability, and our best bet for reliably picking such signals out of the noise is automation. Shallue and Vanderburg’s study demonstrates the power of training a deep convolutional neural network to identify planet signals in data like Kepler’s.

What was found:

Shalle and Vanderburg’s models identified two signals from among data from previously known Kepler multi-planet systems: one planet that is part of a five-planet resonant chain around Kepler-80, and one planet orbiting Kepler-90. Kepler-90 was previously known to host seven planets, so this discovery of an eighth has brought Kepler-90 to a tie with our own Sun for the star known to host the largest number of planets.

Citation

Christopher J. Shallue and Andrew Vanderburg 2018 AJ 155 94. doi:10.3847/1538-3881/aa9e09

universe expansion

Editor’s note: In these last two weeks of 2018, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant

Published July 2018

Main takeaway:

Recent Gaia-measured parallaxes and Hubble photometry of 50 Milky-Way Cepheid variable stars — pulsating stars used as yardsticks to measure cosmic distances — have provided the most precise measurement yet of the local rate of expansion of our universe.

Why it’s interesting:

Since astronomers first discovered the universe is expanding, there has been tension between the observed local rate of expansion (which is measured by tracking the distances to and recession speeds of objects around us) and the expansion rate inferred for the early universe (which is derived from observations of the cosmic microwave background). The new and more precise local measurements, made by a team of astronomers led by Adam Riess (Space Telescope Science Institute and Johns Hopkins University), increases that tension further.

Possible explanations for the tension:

expanding universe

A schematic illustrating one model for the expansion of our universe. Click to enlarge. [NASA/WMAP Science Team]

Why might the local and early-universe expansion rates be different? One possibility is that the universe’s expansion is accelerating over time — dark energy might drive space apart more quickly now than the expansion rate early in the universe’s history. Other possibilities include unexpected physics that render our models — and, therefore, inferences of the early-universe expansion rate — incorrect, like the existence of previously unknown subatomic particles. Further high-precision measurements like those from Gaia will help us to better understand this mystery.

Citation

Adam G. Riess et al 2018 ApJ 861 126. doi:10.3847/1538-4357/aac82e

Laser from Earth

Editor’s note: In these last two weeks of 2018, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Optical Detection of Lasers with Near-Term Technology at Interstellar Distances

Published November 2018

Main takeaway:

Could we communicate with distant extraterrestrial intelligence using lasers? Two scientists from the Massachusetts Institute of Technology, James Clark and Kerri Cahoy, have determined that we could produce a detectable laser signal out to 20,000 light-years using current or near-term technology.

Why it’s interesting:

The challenges of communicating with hypothesized life beyond our solar system are numerous. One of the most fundamental questions is whether we are technologically capable of producing a strong signal that could be easily detected at large distances. In their feasibility study, Clark and Cahoy show that we can — and, moreover, that such a signal could have a broad enough beam that we could target nearby exoplanets with uncertain orbits (like the planet Proxima Centauri b) or the entire habitable zones of more distant systems (like the TRAPPIST-1 system).

Other challenges to communication:

European Extremely Large Telescope

The European Extremely Large Telescope, a proposed upcoming telescope with a 39-meter mirror. A telescope of this size could be used to focus a megawatt laser to communicate with distant intelligence. [Swinburne Astronomy Productions/ESO]

To be spotted by a hypothetical civilization orbiting a distant star, our communicating laser beam must be bright enough to stand out above the background light of our own star, the Sun. If this is possible — which Clark and Cahoy suggest would be with a megawatt-class laser focused by a telescope of tens of meters in diameter — then we still run up against low odds of an actual conversation within human lifetimes due to the long time it would likely take to send and receive signals. Nonetheless, it would be a good start!

Citation

James R. Clark and Kerri Cahoy 2018 ApJ 867 97. doi:10.3847/1538-4357/aae380

star-forming galaxy

Editor’s note: In these last two weeks of 2018, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Dark Galaxy Candidates at Redshift ~3.5 Detected with MUSE

Published May 2018

Main takeaway:

A team of scientists led by Raffaella Marino (ETH Zürich, Switzerland) have used the Multi Unit Spectroscopic Explorer (MUSE) instrument at ESO’s Very Large Telescope to discover six candidate “dark galaxies”, galaxies that contain a large amount of gas but don’t yet contain any stars.

Why it’s interesting:

We still don’t fully understand what the fuel for the first stars in the universe was — how did the diffuse intergalactic medium first come together to trigger star formation in early galaxies? One theory is that there was an epoch in the early phase of galaxy formation during which galaxies were gas-rich but still inefficient at forming stars. By discovering signs of these dark galaxies in the early universe, Marino and collaborators have added supporting evidence to this theory.

Why we haven’t found these dark galaxies before now:

dark galaxy candidates

Example observations of three of the dark galaxy candidates. Left panels show the presence of hydrogen gas; right panels show the lack of detection of stellar continuum emission. [Adapted from Marino et al. 2018]

Since dark galaxies aren’t forming stars yet, they aren’t emitting easily observable starlight. Marino and collaborators instead searched for another kind of emission to identify candidates: fluorescence of the vast reservoirs of hydrogen gas caused by the ultraviolet radiation of nearby quasars, active galactic centers powered by supermassive black holes. Due to the deep imaging made possible by the MUSE instrument, the authors were able to identify six strong candidates for dark galaxies at redshifts of z > 3.5.

Citation

Raffaella Anna Marino et al 2018 ApJ 859 53. doi:10.3847/1538-4357/aab6aa


magnetar outburst

Editor’s note: In these last two weeks of 2018, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Revival of the Magnetar PSR J1622–4950: Observations with MeerKAT, Parkes, XMM-Newton, Swift, Chandra, and NuSTAR

Published April 2018

Main takeaway:

A ultra-magnetized neutron star that has been quiescent for three years has now reawakened, according to a study led by Fernando Camilo (SKA South Africa). New radio and X-ray observations of the magnetar PSR J1622–4950 reveal pulses of radiation from this source for the first time since 2014.

Why it’s interesting:

Unlike pulsars, which are neutron stars with emission powered by the decay of their rotation, magnetars are neutron stars powered by the decay of their extremely strong magnetic fields. Of the nearly two dozen confirmed magnetars, only four have been discovered to exhibit radio pulses in addition to X-rays — and J1622–4950 is one of them. Exploring this source is therefore important for understanding the physics at work, as well as the similarities and differences between magnetars and pulsars.

The additional intrigue of a new telescope:

MeerKAT

The MeerKAT array in South Africa. [SARAO]

The radio observations of J1622–4950 were made in large part by the brand new MeerKAT radio telescope in South Africa, an array of 64 dishes that is now the largest and most sensitive radio telescope in the southern hemisphere. The MeerKAT observations of J1622–4950 were made in April through October 2017, while the telescope was still in the process of being built — only 16 of the 64 dishes were used. Camilo and collaborators’ study mark the first scientific publication based on MeerKAT data … and we can hope for many more in the future!

Citation

F. Camilo et al 2018 ApJ 856 180. doi:10.3847/1538-4357/aab35a

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