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illustration of dust and gas swirling around a bright, newly forming star.

The dusty disks around young stars make the news regularly due to their appeal as the birthplace of early exoplanets. But how do disks like these first form and evolve around their newly born protostars? New observations from the Atacama Large Millimeter/submillimeter Array (ALMA) are helping us to better understand this process.

Formation from Collapse

Stars are born from the gravitational collapse of a dense cloud of molecular gas. Long before they start fusing hydrogen at their centers — when they are still just hot overdensities in the process of contracting — we call them protostars. These low-mass cores are hidden at the hearts of the clouds of molecular gas from which they are born.

ALMA

Aerial image of the Atacama Large Millimeter/submillimeter Array. [EFE/Ariel Marinkovic]

During this contraction phase, before a protostar transitions to a pre-main-sequence star (which it does by blowing away its outer gas envelope, halting the star’s growth), much of the collapsing material will spin into a centrifugally supported Keplerian disk that surrounds the young protostar. Later, these circumstellar disks will become the birthplace for young planets — something for which we’ve seen observational evidence in recent years.

But how do these Keplerian disks — which eventually have scales of hundreds of AU — first form and grow around protostars? We need observations of these disks in their early stages of formation to understand their birth and evolution — a challenging prospect, given the obscuring molecular gas that hides them at these stages. ALMA, however, is up to the task: it can peer through to the center of the gas clouds to see the emission from protostellar cores and their surroundings.

ALMA observations of the protostar Lupus 3 MMS. The molecular outflows from the protostar are shown in panel a. Panel b shows the continuum emission, which has a compact component that likely traces a disk surrounding the protostar. [Adapted from Yen et al. 2017]

ALMA observations of the protostar Lupus 3 MMS. The molecular outflows from the protostar are shown in panel a. Panel b shows the continuum emission, which has a compact component that likely traces a disk surrounding the protostar. [Adapted from Yen et al. 2017]

New Disks Revealed?

In a recent publication led by Hsi-Wei Yen (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan), a team of scientists presents results from ALMA’s observations of three very early-stage protostars: Lupus 3 MMS, IRAS 15398–3559, and IRAS 15398–2429. ALMA’s spectacular resolution allowed Yen and collaborators to infer the presence of a 100-AU Keplerian disk around Lupus 3 MMS, and signatures of infall on scales of <30 AU around the other two sources.

The authors construct models of the sources and show that the observations are consistent with the presence of disks around all three sources: a 100-AU disk around a 0.3 solar-mass protostar in the Lupus system, a 20-AU disk around a 0.01 solar-mass protostar in IRAS 15398–3559, and 6-AU disk around a 0.03 solar-mass protostar in IRAS 15398–2429.

By comparing their observations to those of other early-stage protostars, the authors conclude that in the earliest protostar stage, known as the Class 0 stage, the protostar’s disk grows rapidly in radius. As the protostar ages and enters the Class I stage, the disk growth stagnates, changing only very slowly after this.

These observations mark an important step in our ability to study the gas motions on such small scales at early stages of stellar birth. Additional future studies will hopefully allow us to continue to build this picture!

Citation

Hsi-Wei Yen et al 2017 ApJ 834 178. doi:10.3847/1538-4357/834/2/178

SN 2014C

For the first time ever, we’ve been able to watch the complete metamorphosis of an unusual explosion from one type of supernova to another. What do our observations of SN 2014C mean for our understanding of how massive stars end their lives?

Categorizing Explosions

Supernovae — the explosions that mark the end of massive stellar lifetimes — are broadly categorized into two types: Type I supernovae, which do not show evidence of hydrogen in their spectra, and Type II supernovae, which do.

The majority of supernovae in both categories have the same cause: the fuel in the star’s core is exhausted, and the core subsequently collapses under its own gravity. But a supernova will appear as a Type I or Type II depending upon whether or not the star had already lost its outer hydrogen envelope long before the explosion.

SN 2014C

This plot summarizes key observational features of SN 2014C across the spectrum. The X-ray and radio evolution of SN 2014C is shown compared to other Type Ib/c supernovae (center panel). SN 2014C shows an uncommon rise in luminosity at late times, as well as the onset of Hα emission (bottom panel) when its shock plows into the hydrogen shell. [Margutti et al. 2017]

Supernova Surprise

SN 2014C has muddied this categorization, however. When this unusual explosion was first observed in December 2014, it looked like a Type Ib supernova: there was no sign of hydrogen in its spectrum. But over a timescale of roughly a year — during which it was continuously monitored across all wavelengths — SN 2014C transformed into a Type IIn supernova.

In a new study led by Raffaella Margutti (CIERA, Northwestern University), data is presented from close monitoring of SN 2014C over this entire transition period. The intriguing outcome? After about a year, SN 2014C’s expanding shock appears to have plowed into a substantial shell of hydrogen gas encasing the supernova.

Implications of an Unexpected Collision

How did this shell of hydrogen get there? Margutti and collaborators show that it must have been thrown off of the dying star in an eruptive episode decades to centuries before the supernova explosion.

This presents an interesting problem, because theoretical models of core-collapse supernovae typically assume that when a star loses its hydrogen envelope, this happens gradually over a long period of time, via a mechanism like strong stellar winds. If some stars are instead eruptively throwing off their outer layers long before they explode as supernova, we may need to make some serious changes in how we model stellar evolution and deaths in the future.

supernovae with late-time re-brightenings

A few Type Ib/c supernovae that display late-time radio re-brightenings with similarities to SN 2014C. [Margutti et al. 2017]

Is SN 2014C Alone?

Margutti and collaborators suggest that SN 2014C might be a type of bridge supernova. In this picture, the difference between ordinary Type Ib/c supernovae and Type IIn supernovae (which show signs of interaction with a dense medium right away) lies only in when the star’s hydrogen envelope is thrown off: shortly before explosion for Type IIn supernovae, or decades to centuries beforehand for Type Ib/c supernovae.

When the authors search a sample of 183 Type Ib/c supernovae, they find that roughly 10% of the supernovae in their sample show signs of late-time interactions similar to SN 2014C, lending support to this picture.

We can certainly hope to gather observations of other supernovae behaving like SN 2014C in the future! In the meantime, our full look at SN 2014C’s transformation has given us plenty of new information to consider.

Citation

Raffaella Margutti et al 2017 ApJ 835 140. doi:10.3847/1538-4357/835/2/140

Centaurus A

We believe that supermassive black holes evolve in tandem with their host galaxies — but how do the two communicate? Observations from the Atacama Large Millimeter/submillimeter Array (ALMA) have revealed new clues about how a monster black hole talks to its galaxy.

Central galaxy in Phoenix cluster

A Hubble image of the central galaxy in the Phoenix cluster. [Adapted from Russell et al. 2017]

Observing Feedback

Active galactic nuclei (AGN), the highly luminous centers of some galaxies, are thought to radiate due to active accretion onto the supermassive black hole at their center.

It’s long been suspected that the radiation and outflowing material — which often takes the form of enormous bipolar radio jets emitted into the surroundings — influence the AGN’s host galaxy, affecting star formation rates and the evolution of the galaxy. This “AGN feedback” has been alternately suggested to trigger star formation, quench it, and truncate the growth of massive galaxies.

The details of this feedback process, however, have yet to be thoroughly understood — in part because it’s difficult to obtain detailed observations of how AGN outflows interact with the galactic gas surrounding them. Now, a team of scientists led by Helen Russell (Institute of Astronomy in Cambridge, UK) has published the results of a new, high-resolution look at the gas in a massive galaxy in the center of the Phoenix cluster.

Many Uses for Fuel

The Phoenix cluster, a nearby (z = 0.596) group of star-forming galaxies, is the most luminous X-ray cluster known. The central galaxy in the cluster is especially active: it hosts a starburst of 500–800 solar masses per year, the largest starburst found in any galaxy below a redshift of z = 1.

The star formation in this galaxy is sustained by an enormous reservoir of cold molecular gas — roughly 20 billion solar masses’ worth. This reservoir also powers the galaxy’s central black hole, fueling powerful radio jets that extend into the hot atmosphere of the galaxy and blow a giant bubble into the hot gas at each pole.

molecular gas by ALMA

ALMA observations of the molecular gas in the central galaxy of the Phoenix cluster. The bubbles blown by the radio jets are indicated by the dashed white contours. Extended filaments of molecular gas can be seen to wrap around these cavities. [Adapted from Russell et al. 2017]

ALMA Spots Filaments

ALMA’s observations of this reservoir show that extended filaments of molecular gas wrap around the peripheries of the radio bubbles. These filaments span 10–20 kpc (~30–60 thousand light-years) and have a mass of several billion solar masses. The velocity gradients along them are smooth, suggesting that the gas is moving in an ordered flow around the bubble.

Russell and collaborators suggest that these observations indicate that the clouds of molecular gas were either lifted by the radio bubbles as they inflated, or they formed in place via instabilities caused by the inflating bubbles.

Either way, the data provide clear confirmation that the jets from the black hole affect the location and motion of the cold gas in the surrounding galaxy. This is a beautiful piece of direct evidence showing how supermassive black holes might be communicating with their galaxies.

Citation

H. R. Russell et al 2017 ApJ 836 130. doi:10.3847/1538-4357/836/1/130

hot Jupiter

A tiny telescope has discovered a scalding hot world orbiting its star 1,300 light-years from us. KELT-16b may only be around for a few more hundreds of thousands of years, however.

Don’t Underestimate Tiny Telescopes

KELT-North

The KELT-North telescope in Arizona. This tiny telescope was responsible for the discovery of KELT-16b. [Vanderbilt University]

In an era of ever larger observatories, you might think that there’s no longer a place for small-aperture ground-based telescopes. But small ground-based telescopes have been responsible for the discovery and characterization of around 250 exoplanets so far — and these are the targets that are especially useful for exoplanet science, as they are more easily followed up than the faint discoveries made by telescopes like Kepler.

The Kilogree Extremely Little Telescope (KELT) consists of two telescopes — one in Arizona and one in South Africa — that each have a 4.2-centimeter aperture. In total, KELT observes roughly 70% of the entire sky searching for planets transiting bright hosts. And it’s recently found quite an interesting one: KELT-16b. In a publication led by Thomas Oberst (Westminster College in Pennsylvania), a team of scientists presents their find.

KELT-16b light curves

Combined follow-up light curves obtained for KELT-16b from 19 transits. The best-fit period is just under a day. [Oberst et al. 2017]

A Hot World

KELT-16b is what’s known as a hot Jupiter. Using the KELT data and follow-up observations of 19 transits, Oberst and collaborators estimate KELT-16b’s radius at roughly 1.4 times that of Jupiter and its mass at 2.75 times Jupiter’s. Its equilibrium temperature is a scalding 2453 K — caused by the fact that it orbits so close to its host star that it completes each orbit in a mere 0.97 days!

This short period is extremely unusual: there are only five other known transiting exoplanets with periods shorter than a day. KELT-16b is orbiting very close to its host, making it subject to extreme irradiation and strong tidal forces.

Based on KELT-16b’s orbit, Oberst and collaborators estimate that the planet began a runaway inspiral by the age of 1 billion years. Now, at ~3.1 billion years old, KELT-16b is orbiting at a radius of just over 3 stellar radii above its host’s surface. The authors estimate that KELT-16b’s continuing inward spiral could end in the planet’s destruction by tidal forces in as little as another 550,000 years.

What We Can Learn from KELT-16b

transiting exoplanets

KELT-16b in context with other transiting-exoplanet discoveries on a diagram of planet radius vs. period. Only five other planets have been found with periods shorter than a day. [Oberst et al. 2017]

This highly irradiated world makes for an especially useful target due to its short period (which means we can observe many transits) and bright host (which means follow-up observations are more convenient and have a large signal-to-noise ratio).

In particular, with followup observations of KELT-16b from missions like Hubble, Spitzer, and eventually the James Webb Space Telescope, we can learn more about open questions in exoplanet atmospheric processes — like how heat is transferred vertically through the atmosphere, or what happens at the day-to-night terminator line on such a highly irradiated planet.

In addition, by studying KELT-16b, we can hope to gain overall insight into hot Jupiter formation and migration. The ease of observing this planet and the wealth of information it can provide will likely make it one of the top-studied exoplanets. KELT-16b has a lot to teach us before it’s torn apart!

Citation

Thomas E. Oberst et al 2017 AJ 153 97. doi:10.3847/1538-3881/153/3/97

LIGO detection

Could dark matter be made of intermediate-mass black holes formed in the beginning of the universe? A recent study takes a renewed look at this question.

Galactic Lurkers

The nature of dark matter has long been questioned, but the recent discovery of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) has renewed interest in the possibility that dark matter could consist of primordial black holes in the mass range of 10–1000 solar masses.

dark matter

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

According to this model, the extreme density of matter present during the universe’s early expansion led to the formation of a large number of intermediate-mass black holes. These black holes now hide in the halos of galaxies, constituting the mass that we’ve measured dynamically but remains unseen.

LIGO’s first gravitational-wave detection revealed the merger of two black holes that were both tens of solar masses in size. If primordial black holes are indeed a major constituent of dark matter, then LIGO’s detection is consistent with what we would expect to find: occasional mergers of the intermediate-mass black holes that formed in the early universe and now lurk in galactic halos.

Quasar Microlensing

There’s a catch, however. If there truly were a large number of intermediate-mass primordial black holes hiding in galactic halos, they wouldn’t go completely unnoticed: we would see signs of their presence in the gravitational microlensing of background quasars. Unseen primordial black holes in a foreground galaxy could cause an image of a background quasar to briefly brighten — which would provide us with clear evidence of such black holes despite our not being able to detect them directly.

quasar microlensing

A depiction of quasar microlensing (click for a closer look!). The microlensing object in the foreground galaxy could be a star (as depicted), a primordial black hole, or any other compact object. [NASA/Jason Cowan (Astronomy Technology Center)]

A team of scientists led by Evencio Mediavilla (Institute of Astrophysics of the Canaries, University of La Laguna) has now used our observations of quasar microlensing to place constraints on the amount of dark matter that could be made up of intermediate-mass primordial black holes.

Poor Outlook for Primordial Black Holes

Mediavilla and collaborators used simulations to estimate the effects of a distribution of masses on light from distant quasars, and they then compared their results to microlensing magnification measurements from 24 gravitationally lensed quasars. In this way, they were able to determine both the abundance and masses of possible objects causing the quasar microlensing effects we see.

The authors find that the observations constrain the mass of the possible microlensing objects to be between 0.05 and 0.45 solar masses — not at all the intermediate-mass black holes postulated. What’s more, they find that the lensing objects make up ~20% of the total matter, which is barely more than expected for normal stellar matter. This suggests that normal stars are doing the majority of the quasar microlensing, not a large population of intermediate-mass black holes.

What does this mean for primordial black holes as dark matter? Black holes in the range of 10–200 stellar masses are unlikely to account for much (if any) dark matter, Mediavilla and collaborators conclude — which means that LIGO’s detection of gravitational waves likely came from two black holes collapsed from stars, not primordial black holes.

Citation

E. Mediavilla et al 2017 ApJL 836 L18. doi:10.3847/2041-8213/aa5dab

Big Bang

How did our universe come into being? The Big Bang theory is a widely accepted and highly successful cosmological model of the universe, but it does introduce one puzzle: the “cosmological lithium problem.” Have scientists now found a solution?

Too Much Lithium

In the Big Bang theory, the universe expanded rapidly from a very high-density and high-temperature state dominated by radiation. This theory has been validated again and again: the discovery of the cosmic microwave background radiation and observations of the large-scale structure of the universe both beautifully support the Big Bang theory, for instance. But one pesky trouble-spot remains: the abundance of lithium.

primary reactions

The arrows show the primary reactions involved in Big Bang nucleosynthesis, and their flux ratios, as predicted by the authors’ model, are given on the right. Synthesizing primordial elements is complicated! [Hou et al. 2017]

According to Big Bang nucleosynthesis theory, primordial nucleosynthesis ran wild during the first half hour of the universe’s existence. This produced most of the universe’s helium and small amounts of other light nuclides, including deuterium and lithium.

But while predictions match the observed primordial deuterium and helium abundances, Big Bang nucleosynthesis theory overpredicts the abundance of primordial lithium by about a factor of three. This inconsistency is known as the “cosmological lithium problem” — and attempts to resolve it using conventional astrophysics and nuclear physics over the past few decades have not been successful.

In a recent publication led by Suqing Hou (Institute of Modern Physics, Chinese Academy of Sciences) and advisor Jianjun He (Institute of Modern Physics & National Astronomical Observatories, Chinese Academy of Sciences), however, a team of scientists has proposed an elegant solution to this problem.

primordial abundance evolution

Time and temperature evolution of the abundances of primordial light elements during the beginning of the universe. The authors’ model (dotted lines) successfully predicts a lower abundance of the beryllium isotope — which eventually decays into lithium — relative to the classical Maxwell-Boltzmann distribution (solid lines), without changing the predicted abundances of deuterium or helium. [Hou et al. 2017]

Questioning Statistics

Hou and collaborators questioned a key assumption in Big Bang nucleosynthesis theory: that the nuclei involved in the process are all in thermodynamic equilibrium, and their velocities — which determine the thermonuclear reaction rates — are described by the classical Maxwell-Boltzmann distribution.

But do nuclei still obey this classical distribution in the extremely complex, fast-expanding Big Bang hot plasma? Hou and collaborators propose that the lithium nuclei don’t, and that they must instead be described by a slightly modified version of the classical distribution, accounted for using what’s known as “non-extensive statistics”.

The authors show that using the modified velocity distributions described by these statistics, they can successfully predict the observed primordial abundances of deuterium, helium, and lithium simultaneously. If this solution to the cosmological lithium problem is correct, the Big Bang theory is now one step closer to fully describing the formation of our universe.

Citation

S. Q. Hou et al 2017 ApJ 834 165. doi:10.3847/1538-4357/834/2/165

M32

How do ultra-compact dwarf galaxies (UCDs) — galaxies that are especially small and dense — form and evolve? Scientists have recently examined distant galaxy clusters, searching for more UCDs to help us answer this question.

Origins of Dwarfs

In recent years we have discovered a growing sample of small, very dense galaxies. Galaxies that are tens to hundreds of light-years across, with masses between a million and a billion solar masses, fall into category of “ultra-compact dwarfs”.

UCD candidate

An example of an unresolved compact object from the authors’ survey that is likely an ultra-compact dwarf galaxy. [Adapted from Zhang & Bell 2017]

How do these dense and compact galaxies form? Two possibilities are commonly suggested:

  1. An initially larger galaxy was tidally stripped during interactions with other galaxies in a cluster, leaving behind only its small, dense core as a UCD.
  2. UCDs formed as compact galaxies at very early cosmic times. The ones living in a massive dark matter halo may have been able to remain compact over time, evolving into the objects we see today.

To better understand which of these formation scenarios applies to which galaxies, we need a larger sample size! Our census of UCDs is fairly limited — and because they are small and dim, most of the ones we’ve discovered are in the nearby universe. To build a good sample, we need to find UCDs at higher redshifts as well.

A New Sample

In a recent study, two scientists from University of Michigan have demonstrated how we might find more UCDs. Yuanyuan Zhang (also affiliated with Fermilab) and Eric Bell used the Cluster Lensing and Supernova Survey with Hubble (CLASH) to search 17 galaxy clusters at intermediate redshifts of 0.2 < z < 0.6, looking for unresolved objects that might be UCDs.

UCD distribution

The mass and size distributions of the UCD candidates reported in this study, in the context of previously known nuclear star clusters, globular clusters (GCs), UCDs, compact elliptical galaxies (cEs), and dwarf galaxies. [Zhang & Bell 2017]

Zhang and Bell discovered a sample of compact objects — grouped around the central galaxies of the clusters — that are consistent with ultra-compact galaxies. The inferred sizes (many around 600 light-years in radius) and masses (roughly one billion solar masses) of these objects suggest that this sample may contain some of the densest UCDs discovered to date.

The properties of this new set of UCD candidates aren’t enough to distinguish between formation scenarios yet, but the authors argue that if we find more such galaxies, we will be able to use the statistics of their spatial and color distributions to determine how they were formed.

Zhang and Bell estimate that the 17 CLASH clusters studied in this work each contain an average of 2.7 of these objects in the central million light-years of the cluster. The authors’ work here suggests that searching wide-field survey data for similar discoveries is a plausible way to increase our sample of UCDs. This will allow us to statistically characterize these dense, compact galaxies and better understand their origins.

Citation

Yuanyuan Zhang and Eric F. Bell 2017 ApJL 835 L2. doi:10.3847/2041-8213/835/1/L2

Coronal jet simulation

jet formation

Formation of a coronal jet from twisted field lines that have reconnected with the ambient field. The colors show the radial velocity of the plasma. [Adapted from Szente et al. 2017]

How do jets emitted from the Sun’s surface contribute to its corona and to the solar wind? In a recent study, a team of scientists performed complex three-dimensional simulations of coronal jets to answer these questions.

Small Explosions

Coronal jets are relatively small eruptions from the Sun’s surface, with heights of roughly 100 to 10,000 km, speeds of 10 to 1,000 km/s, and lifetimes of a few minutes to around ten hours. These jets are constantly present — they’re emitted even from the quiet Sun, when activity is otherwise low — and we’ve observed them with a fleet of Sun-watching space telescopes spanning the visible, extreme ultraviolet (EUV), and X-ray wavelength bands.

comparison 1

A comparison of simulated observations based on the authors’ model (left panels) to actual EUV and X-ray observations of jets (right panels). [Szente et al. 2017]

Due to their ubiquity, we speculate that these jets might contribute to heating the global solar corona (which is significantly hotter than the surface below it, a curiosity known as the “coronal heating problem”). We can also wonder what role these jets might play in driving the overall solar wind.

Launching a Jet

Led by Judit Szente (University of Michigan), a team of scientists has explored the impact of coronal jets on the global corona and solar wind with a series of numerical simulations. Szente and collaborators used three-dimensional, magnetohydrodynamic simulations that provide realistic treatment of the solar atmosphere, the solar wind acceleration, and the complexities of heat transfer throughout the corona.

In the authors’ simulations, a jet is initiated as a magnetic dipole rotates at the solar surface, winding up field lines. Magnetic reconnection between the twisted lines and the background field then launches the jet from the dense and hot solar chromosphere, and erupting plasma is released outward into the solar corona.

comparison 2

A second comparison of simulated observations based on the authors’ model (left panels) to actual EUV observations of jets (right panels). [Szente et al. 2017]

Global Influences

After demonstrating that their models could successfully lead to jet production and propagation, Szente and collaborators compared their results to actual observations of solar jets. The authors constructed simulated EUV and X-ray observations of their modeled events, and they verified that the behavior and structures in these simulated observations were very similar to real observations of coronal jet events from telescopes like SDO/AIA and Hinode.

With this confirmed, the authors then used their models to determine how the jets influence the global solar corona and the solar wind. They found that the large-scale corona is significantly affected by the plasma waves from the jet, which travel across 40° in latitude and out to 24 solar radii. In spite of this, the simulated jets contributed only a few percent to the steady-state solar-wind energy outflow.

These simulations represent an important step in realistic modeling of the quiet Sun. Because the models make specific predictions about temperature and density gradients within the corona, we can look forward to testing them with upcoming missions like Solar Probe Plus, which should be able to explore the Sun all the way down to nine solar radii.

Citation

J. Szente et al 2017 ApJ 834 123. doi:10.3847/1538-4357/834/2/123

climate model

Having a giant planet like Jupiter next door can really wreak havoc on your orbit! A new study examines what such a bad neighbor might mean for the long-term climate of an Earth-like planet.

Influence of a Bad Neighbor

The presence of a Jupiter-like giant planet in a nearby orbit can significantly affect how terrestrial planets evolve dynamically, causing elements like the planets’ orbital eccentricities and axial tilts to change over time. Earth is saved this inconvenience — Jupiter isn’t close enough to significantly influence us, and our large moon stabilizes our orbit against Jupiter’s tugs.

outcomes

Top panels: Authors’ simulation outcomes for Case 1, in which the planet’s eccentricity varies from 0 to 0.283 over 6500 years. Bottom panels: Outcomes for Case 2, in which the planet’s eccentricity varies from 0 to 0.066 over 4500 years. The higher eccentricities reached in Case 1 causes the climate parameters to vary more widely. Click for a better look! [Way & Georgakarakos 2017]

Mars, on the other hand, isn’t as lucky: it’s possible that Jupiter’s gravitational pull causes Mars’s axial tilt, for instance, to evolve through a range as large as 0 to 60 degrees on timescales of millions of years! Mars’s orbital eccentricity is similarly thought to vary due to Jupiter’s influence, and both of these factors play a major role in determining Mars’s climate.

As exoplanet missions discover more planets — many of which are Earth-like — we must carefully consider which among these are most likely to be capable of sustaining life. If having a nearby neighbor like a Jupiter can tug an Earth-like world into an orbit with varying eccentricity, how does this affect the planet’s climate? Will the planet remain temperate? Or will it develop a runaway heating or cooling effect as it orbits, rendering it uninhabitable?

Oceans and Orbits

To examine these questions, two scientists have built the first ever 3D global climate model simulations of an Earth-like world using a fully coupled ocean (necessary for understanding the transport of heat across the planet) with a planetary orbit that evolves over time.

surface air temperature

The surface air temperature variation of a planet with orbital eccentricity of 0.283. The top panel shows the surface temperature when the planet is closest to the star in its orbit (periastron); the bottom when the planet is furthest from the star in its orbit (apoastron). [Way & Georgakarakos 2017]

The scientists, Michael Way (NASA Goddard and Uppsala University, Sweden) and Nikolaos Georgakarakos (New York University Abu Dhabi), focus in this study on the specific effects of a varying orbital eccentricity on an Earth-like planet’s climate, holding the planet’s axial tilt steady at Earth’s 23.5°. They explore two scenarios: one in which the planet’s eccentricity evolves from 0 to 0.283 over 6500 years, and the other in which it evolves from 0 to 0.066 over 4500 years.

Temperate Outcomes

Way and Georgakarakos find that the planet with the more widely varying eccentricity has a greater increase rainfall and humidity as the planet approaches its host star in its orbit. Nonetheless, this effect is not enough to cause a runaway greenhouse scenario in which the planet becomes too warm for habitability. Similarly, the ocean ice fraction remains low enough even at apoastron in high-eccentricity scenarios for the planet to remain temperate.

What does these results imply? Having a changing eccentricity — caused by the gravitational pull of a nearby Jupiter-like neighbor — may make a planet’s climate more variable, but not to the extent where the planet is no longer able to support life. Therefore, as we discover more such planets with current and upcoming exoplanet missions, we know that we needn’t necessarily assume that they aren’t interest for habitability.

Citation

M. J. Way and Nikolaos Georgakarakos 2017 ApJL 835 L1. doi:10.3847/2041-8213/835/1/L1

Circumstellar disk

New observations may help us to learn more about the birth of high-mass star systems. For the first time, scientists have imaged a very young, high-mass binary system and resolved the individual disks that surround each star and the binary.

Massive Multiples

It’s unusually common for high-mass stars to be discovered in multiple-star systems. More than 80% of all O-type stars — which have masses greater than 16 times that of the Sun — are in close multiple systems, compared with a multiplicity fraction of only ~20% for stars of ~3 solar masses, for instance.

IRAS17216-3801

Reconstructed VLTI observations of the two components of the high-mass binary IRAS17216-3801. [Adapted from Kraus et al. 2017]

Why do more massive stars preferentially form in multiple-star systems? Many different models of high-mass star formation have been invoked to explain this observation, but before we can better understand the process, we need better observations. In particular, past observations have placed few constraints on the architecture and disk structure of early high-mass stars.

Conveniently, a team of scientists led by Stefan Kraus (University of Exeter) may have found exactly what we need: a high-mass “protobinary” that is still in the process of forming. Using ESO’s Very Large Telescope Interferometer (VLTI), Kraus and collaborators have captured the first observations of a very young, high-mass binary system in which the circumbinary disk and the two circumstellar dust disks could all be spatially resolved.

Clues from Resolved Disks

The VLTI near-infrared observations reveal that IRAS17216-3801, originally thought to be a single high-mass star, is instead a close binary separated by only ~170 AU. Its two components are both surrounded by disks from which the protostars are actively accreting mass, and both of these circumstellar disks are strongly misaligned with respect to the separation vector of the binary. This confirms that the system is very young, as tidal forces haven’t yet had time to align the disks.

binary model

The authors’ model of the geometry of the binary system, including the orientations of the two circumstellar disks (sketch not to scale). [Adapted from Kraus et al. 2017]

Fitting models to the observations, Kraus and collaborators find the best-fitting description of the system’s geometry and the masses of the components — roughly 18 and 20 solar masses, they estimate.

By tracing the hot gas in their observations of the system, the authors also determine that the secondary, smaller component is accreting at a higher rate than the larger star. This suggests that the secondary disrupts the accretion stream onto the primary star, channeling the infalling material onto its own disk instead — an observation that confirms the prediction of hydrodynamic simulations.

IRAS17216-3801 is roughly three times more massive and five times more compact than other high-mass multiple star systems imaged in infrared, and it is the first system in which resolution of its component disks has been possible. These images present an exciting laboratory for studying star–disk interactions and the formation of high-mass multiple systems.

Citation

S. Kraus et al 2017 ApJL 835 L5. doi:10.3847/2041-8213/835/1/L5

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