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Ophiuchus stream's orbit

Dwarf galaxies or globular clusters orbiting the Milky Way can be pulled apart by tidal forces, leaving behind a trail of stars known as a “stellar stream.” One such trail, the Ophiuchus stream, has posed a serious dynamical puzzle since its discovery. But a recent study has identified four stars that might help resolve this stream’s mystery.

Conflicting Timescales

The stellar stream Ophiuchus was discovered around our galaxy in 2014. Based on its length, which appears to be 1.6 kpc, we can calculate the time that has passed since its progenitor was disrupted and the stream was created: ~250 Myr. But the stars within it are ~12 Gyr old, and the stream orbits the galaxy with a period of ~350 Myr.

Given these numbers, we can assume that Ophiuchus’s progenitor completed many orbits of the Milky Way in its lifetime. So why would it only have been disrupted 250 million years ago?

Fanning Stream

Led by Branimir Sesar (Max Planck Institute for Astronomy), a team of scientists has proposed an idea that might help solve this puzzle. If the Ophiuchus stellar stream is on a chaotic orbit — common in triaxial potentials, which the Milky Way’s may be — then the stream ends can fan out, with stars spreading in position and velocity.

The fanned part of the stream, however, would be difficult to detect because of its low surface brightness. As a result, the Ophiuchus stellar stream could actually be longer than originally measured, implying that it was disrupted longer ago than was believed.

Search for Fan Stars

To test this idea, Sesar and collaborators performed a search around the ends of the stream, looking for stars that

  1. are of the right type to match the stream,
  2. are at the predicted distance of the stream,
  3. are located near the stream ends, and
  4. have velocities that match the stream and don’t match the background halo stars.
Target star velocities

Histogram of the heliocentric velocities of the 43 target stars. Six stars have velocities matching the stream velocity. Two of these are located in the main stream; the other four may be part of a fan at the end of the stream. [Sesar et al. 2016]

Of the 43 targets for which the authors obtained spectra, four stars met these criteria and are located beyond the main extent of the stream, possibly comprising a fan at the stream’s end. Including these stars as part of the Ophiuchus stream, its length becomes 3 kpc, implying that its time of disruption was closer to 400 million years ago. This relieves the timescale tension but does not resolve it.

That said, the mere evidence of a fan in the Ophiuchus stream suggests that its progenitor may have been on a chaotic orbit. If this is the case, it’s entirely possible that the progenitor could have survived for ~11 Gyr, only to have been disrupted within the last 0.5 Gyr. Detailed modeling and further identification of potential fan stars in the Ophiuchus stream will help to test this idea and resolve the puzzle of this stream.

Citation

Branimir Sesar et al 2016 ApJ 816 L4. doi:10.3847/2041-8205/816/1/L4

Planet Nine

The recent discovery that the orbits of some Kuiper belt objects (KBOs) share properties has proved puzzling. A pair of scientists have now proposed a bold explanation: there may be a planet-sized object yet undetected in our solar system.

Mysterious Clustering

KBOs, the population of mainly small objects beyond Neptune, have proven an especially interesting subject of study in the last decade as many small, distant bodies (such as Eris, the object that led to the demotion of Pluto to dwarf planet) have been discovered.

Previous studies have recently discovered that some especially distant KBOs — those that orbit with semimajor axes of a > 150 AU, nearly four times that of Pluto — all cross the ecliptic at a similar phase in their elliptical trajectories. This is unexpected, since gravitational tugs from the giant planets should have randomized this parameter over our solar system’s multi-billion-year lifespan.

Physical alignment of the orbits of Kuiper belt objects with a > 250 AU (and two objects with a > 150 AU that are dynamically stable). [Batygin & Brown 2016]

Physical alignment of the orbits of Kuiper belt objects with a > 250 AU (and two objects with a > 150 AU that are dynamically stable). [Batygin & Brown 2016]

Two scientists at California Institute of Technology, Konstantin Batygin and Michael Brown (you might recognize Brown as the man who “killed Pluto”) have now increased the mystery. In a recently published a study, they demonstrate that for KBOs that have orbits with a > 250 AU, the orbits are actually physically aligned.

To explain this unexpected alignment — which Batygin and Brown calculate has only a 0.007% probability of having occurred by chance — the authors ask an exciting question: could this be caused by the presence of an unseen, large, perturbing body further out in the solar system?

Simulating a Ninth Planet

The authors test this hypothesis by carrying out both analytical calculations and numerical N-body simulations designed to determine if the gravitational influence of a distant, planetary-mass companion can explain the behavior we observe from the large-orbit KBOs.

Simulation of Planet Nine's effects

Simulation of the effect of a distant planet (M = 10 M⊕, a = 700 AU, and e = 0.6) on KBOs; click for a better look! The perihelion position of KBOs with a > 250 AU clusters around 180° from the perihelion position of the perturbing planet. More-transparent points are less observable. [Batygin & Brown 2016]

The result? It turns out that such a distant planet can cause the orbits of KBOs with a > 250 AU to all align in the opposite direction of the orbit of the planet. What’s more, the gravitational pull of this planet can also explain other unresolved puzzles about the Kuiper belt, such as the presence of high-perihelion Sedna-like objects, as well as a population of KBOs we’ve observed that have misaligned orbits.

Unfortunately, Batygin and Brown found it isn’t possible to exactly determine the properties of the possible planet, since multiple combinations of its mass, eccentricity, and semimajor axis can create the same observational results. That said, they believe the distant perturber’s orbit is highly eccentric, its orbital inclination is low, and it’s fairly massive (since anything less than an Earth-mass won’t create the observed clustering of KBO orbits within the age of the solar system).

As an example, one possible set of parameters that approximately reproduces the observed KBO orbits is the following:

  • planet mass of 10 Earth-masses
  • semi-major axis of a = 700 AU
  • eccentricity of e = 0.6

This would correspond to a perihelion distance of 280 AU and an aphelion distance of 1,120 AU.

The authors speculate such a planet might have been formed closer in to the Sun, but it was ejected later on during our solar system’s evolution. Interactions with the Sun’s birth cluster could have then caused the planet to be retained in a bound orbit.

Future Tests

Solar system

Our solar system on a logarithmic scale (click for the full view). KBOs with a semimajor axis of a > 250 AU may be being aligned by a planetary-mass body with an even more distant orbit. [NASA]

How can we test this hypothesis of a ninth planet? Obviously, directly observing the planet would confirm its presence. But the authors’ model has an additional testable hypothesis: if it’s correct, there should be a population of high-perihelion Kuiper belt objects that don’t exhibit the same alignment of their orbits as the KBOs we know about, but instead have opposite-aligned orbits. If we discover such a collection of objects, that would be an excellent confirmation of this model.

The authors caution that their work is preliminary, and additional investigation will be required to better understand the possibilities presented here. But with any luck, future theoretical work, as well as observational tests of this model’s predictions, will help us determine whether there might be a distant ninth planet in our solar system!

Bonus

Check out this video (created with WWT!), which walks us first through a view of the six aligned KBO orbits, then shows a possible orbit for the hypothesized planet, and then shows an additional population of already-discovered objects (also predicted by the model) that have orbits perpendicular both to the plane of the solar system and to the planet’s orbit. [Caltech/Robert Hurt]

Citation

Konstantin Batygin and Michael E. Brown 2016 AJ 151 22. doi:10.3847/0004-6256/151/2/22

Located a mere 176 light-years away, TW Hydrae is an 8-million-year-old star surrounded by a nearly face-on disk of gas and dust. Recent observations have confirmed the existence of a gap within that disk — a particularly intriguing find, since gaps can sometimes signal the presence of a planet.

Gaps and Planets

Numerical simulations have shown that newly-formed planets orbiting within dusty disks can clear the gas and dust out of their paths. This process results in pressure gradients that can be seen in the density structure of the disk, in the form of visible gaps, rings, or spirals.

For this reason, finding a gap in a protoplanetary disk can be an exciting discovery. Previous observations of the disk around TW Hydrae had indicated that there might be a gap present, but they were limited in their resolution; despite TW Hydrae’s relative nearness, attempting to observe the dim light scattered off dust particles in a disk surrounding a distant, bright star is difficult!

But a team led by Valerie Rapson (Rochester Institute of Technology, Dudley Observatory) recently set out to follow up on this discovery using a powerful tool: the Gemini Planet Imager (GPI).

New Observations

models v. observation

Comparison of the actual image of TW Hydrae’s disk from GPI (right) to a simulated scattered-light image from a model of a ~0.2 Jupiter-mass planet orbiting in the disk at ~21 AU (left) in two different bands (top: J, bottom: K1).[Adapted from Rapson et al. 2015]

GPI is an instrument on the Gemini South Telescope in Chile. Its near-infrared imagers, equipped with extreme adaptive optics, allowed it to probe the disk from ~80 AU all the way in to ~10 AU from the central star, with an unprecedented resolution of ~1.5 AU.

These observations from GPI allowed Rapson and collaborators to unambiguously confirm the presence of a gap in TW Hydrae’s disk. The gap lies at a distance of ~23 AU from the central star (roughly the same distance as Uranus to the Sun), and it’s ~5 AU wide.

Modeled Possibilities

There are a number of other potential explanations for this gap — for instance, the inner disk could be casting a shadow on the outer disk, or the gap could be a natural consequence of how grains fragment and evolve within the disk.

Nevertheless, an orbiting planet embedded in the disk may well be the cause. When Rapson and collaborators ran numerical simulations of a planet orbiting within a disk like TW Hydrae’s, they found that a planet of 0.16 Jupiter masses, orbiting at a distance of 21 AU, reproduces the observations well.

With any luck, we’ll be able to learn more with additional observations in the future. Deeper images may reveal additional features that point to a planet shaping the disk structure. And if the planet is actively accreting gas in the disk, we may even be able to directly image the planet!

Citation

Valerie A. Rapson et al 2015 ApJ 815 L26. doi:10.1088/2041-8205/815/2/L26

Sagittarius Dwarf Galaxy

Dwarf galaxies are typically very faint, and are therefore hard to find. Given that, what are our chances of finding their distant ancestors, located billions of light-years away? A recent study aims to find out.

Ancient Counterparts

Dwarf galaxies are a hot topic right now, especially as we discover more and more of them nearby. Besides being great places to investigate a variety of astrophysical processes, local group dwarf galaxies are also representative of the most common type of galaxy in the universe. For many of these dwarf galaxies, their low masses and typically old stellar populations suggest that most of their stars were formed early in the universe’s history, and further star formation was suppressed when the universe was reionized at redshifts of z ~ 6–10. If this is true, most dwarf galaxies are essentially fossils: they’ve evolved little since that point.

To test this theory, we’d like to find counterparts to our local group dwarf galaxies at these higher redshifts of z = 6 or 7. But dwarf galaxies, since they don’t exhibit lots of active star formation, have very low surface brightnesses — making them very difficult to detect. What are the chances that current or future telescope sensitivities will allow us to detect these? That’s the question Anna Patej and Abraham Loeb, two theorists at Harvard University, have addressed in a recent study.

Entering a New Regime

SB vs. size

The surface brightness vs. size for 73 local dwarf galaxies scaled back to redshifts of z=6 (top) and z=7 (bottom). So far we’ve been able to observe high-redshift galaxies within the boxed region of the parameter space. JWST will open the shaded region of the parameter space, which includes some of the dwarf galaxies. [Patej & Loeb 2015]

Starting from observational data for 87 Local-Group dwarf galaxies, Patej and Loeb used a stellar population synthesis code to evolve the galaxies backward in time to redshifts of z = 6 and 7. Next, they narrowed this sample to only those dwarfs for which most star formation had already occurred by this time.

Finally, the authors compared the properties of these 73 scaled-back dwarfs to those of high-redshift galaxies that we have already detected with the Hubble and Spitzer Space Telescopes, as well as to the detection limits of the upcoming James Webb Space Telescope (JWST) mission launching in 2018.

Patej and Loeb find that, when scaled back to redshifts of z = 6 or 7, the dwarf galaxies would be too faint to detect with current telescopes — despite being roughly the same size as high-redshift galaxies we’ve already detected. But the capabilities of JWST will push into this regime: according to Patej and Loeb’s calculations, JWST would be able to detect 13 of the 73 galaxies in the sample at a redshift of z = 6, and 9/73 at a redshift of z = 7.

Furthermore, the fraction of detectable galaxies would increase if these ancient dwarfs contained large numbers of Population-III-like, massive, bright stars. But even without such a boost, the hunt for the ancestors of local dwarf galaxies appears to be well within JWST’s capabilities!

Citation

Anna Patej and Abraham Loeb 2015 ApJ 815 L28. doi:10.1088/2041-8205/815/2/L28

M101

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

The Changing Fractions of Type Ia Supernova NUV–Optical Subclasses with Redshift

Published April 2015

 

Main takeaway:

A team of scientists led by Peter Milne (University of Arizona) used ultraviolet observations from the Swift spacecraft to determine that type Ia supernovae, stellar explosions previously thought to all belong in the same class, actually fall into two subgroups: those that are slightly redder in NUV wavelengths and those that are slightly bluer.

Supernovae fraction

Plot of the percentage of supernovae that are NUV-blue (rather than NUV-red), as a function of redshift. NUV-blue supernovae dominate at higher redshifts. [Milne et al. 2015]

Why it’s interesting:

It turns out that the fraction of supernovae in each of these two groups is redshift-dependent. At low redshifts (i.e., nearby), the population of type Ia supernovae is dominated by NUV-red supernovae. At high redshifts (i.e., far away), the population is dominated by NUV-blue supernovae. Since cosmological distances are measured using Type Ia supernovae as standard candles, the fact that we’ve been modeling these supernovae all the same way (rather than treating them as two separate subclasses) means we may have been systematically misinterpreting distances.

What this means for the universe’s expansion:

This seemingly simple discovery carries hefty repercussions — in fact, our estimates of the expansion rate of the universe may be incorrect! The authors believe that if we correct for this error, we’ll find that the universe is not expanding as quickly as we thought.

Citation

Peter A. Milne et al 2015 ApJ 803 20. doi:10.1088/0004-637X/803/1/20

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

An Ancient Extrasolar System with Five Sub-Earth-Size Planets

Published January 2015

 

Main takeaway:

transit light curves

Transit light curves for the five planets orbiting Kepler-444. [Campante et al. 2015]

A team led by Tiago Campante (University of Birmingham, Aarhus University) reported Kepler spacecraft observations of Kepler-444, a system of five transiting exoplanets around a metal-poor, Sun-like star. All five planets are sub-Earth-sized. Furthermore, the system is measured to be over 11 billion years old — making this the oldest known system of terrestrial-size planets.

Why it’s interesting:

While gas-giant planets show a preference for forming around metal-rich stars, smaller planets appear to be less picky. This suggests that Earth-size planets may have been able to form at earlier times in the universe’s history, when metals were scarcer. The determination that Kepler-444 is 11.2 billion years old confirms that terrestrial-size planets have been able to form throughout most of the universe’s 13.8 billion year history.

Awesome technical achievement:

The age of the Kepler-444 system was determined from asteroseismology of the host star. The fact that we can measure oscillations in the interior of this ancient star located 116 light-years away — and use this to determine its age to a precision of 9%! — is a remarkable achievement made possible by 4 years of continuous, high-quality observations of the system.

Citation

T. L. Campante et al 2015 ApJ 799 170. doi:10.1088/0004-637X/799/2/170

VFTS 352

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Discovery of the Massive Overcontact Binary VFTS 352: Evidence for Enhanced Internal Mixing

Published October 2015

 

Main takeaway:

A team led by Leonardo Almeida (Johns Hopkins University and University of São Paulo, Brazil) discovered the binary star system VFTS 352 in the Large Magellanic Cloud. This pair of O-type stars is an “overcontact binary” — the two stars are orbiting each other so closely that they’re actually touching each other.

Why it’s interesting:

Snapshots of VFTS 352 at a few orbital phases, the system’s light curves, and its radial velocity curves. [Almeida et al. 2015]

Snapshots of VFTS 352 at a few orbital phases, the system’s light curves, and its radial velocity curves. [Almeida et al. 2015]

We know little about the overcontact stage that occurs when two massive stars coalesce — primarily because it’s typically short-lived, so we have few observations of stars in this stage. VFTS 352 is the most massive and earliest spectral type overcontact system known to date. It’s especially interesting because the observations suggest that the strong tidal forces in this system may have caused enhanced internal mixing between the stars’ centers and envelopes. These stars’ interiors may therefore be much more homogenous than is typical.

What to expect:

Ultimately, this pair of stars will likely share one of two fates. In the classical scenario, they’ll expand and eventually merge to produce a single rapidly rotating, massive star. If their internal mixing is large enough, however, they could remain compact rather than expanding. In that case, they would progress to the end of their main-sequence lifetimes without ever merging, potentially evolving to become a black-hole binary system.

Citation

L. A. Almeida et al 2015 ApJ 812 102. doi:10.1088/0004-637X/812/2/102

EGSY8p7

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Lyα Emission from a Luminous z = 8.68 Galaxy: Implications for Galaxies as Tracers of Cosmic Reionization

Published August 2015

 

Main takeaway:

A team led by Adi Zitrin (Hubble Fellow at California Institute of Technology) detected Lyα emission in the bright galaxy EGSY8p7 using the MOSFIRE spectrograph at Keck Observatory. From this emission line, they calculated that the galaxy has an astonishing redshift of z=8.68.

Why it’s interesting:

This spectroscopic confirmation crowned EGSY8p7 as the record-holder for the farthest-known (and therefore oldest) galaxy. Its redshift shattered the previous record, a galaxy at z=7.73.

Why it’s even more interesting than that:

spectroscopic confirmation

Spectroscopic detection of emission in EGSY8p7 with MOSFIRE. The black line is the raw data; the red line shows the best-fit model to the data. [Zitrin et al. 2015]

Based on our understanding of how the universe evolved, the detection of Lyα emission from this galaxy came as a surprise. At EGSY8p7’s redshift of 8.68, the universe was still full of clouds of neutral hydrogen that should have absorbed the galaxy’s Lyα emission long before it reached us. So what does it mean that we do see Lyα emission from EGSY8p7? The reionization of the universe — through which the neutral hydrogen clouds were made transparent — may have been a patchy process. In particular, EGSY8p7 might have emitted an unusual amount of ionizing radiation, creating an early ionized bubble around it that allowed the Lyα emission to escape.

Citation

Adi Zitrin et al 2015 ApJ 810 L12. doi:10.1088/2041-8205/810/1/L12

Coma Cluster

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma Cluster

Published January 2015

 

Main takeaway:

Using the Dragonfly Telephoto Array, a team led by Pieter van Dokkum (Yale University) discovered 47 ultra-diffuse galaxies in the Coma galaxy cluster. These galaxies are very large, with half-light (“effective”) radii of 1.5–4.6 kpc, similar to that of the Milky Way’s disk. But their stellar masses are a factor of 1000 lower than the Milky Way’s, and they’re accordingly much dimmer.

ultra-diffuse galaxies

Plot of the effective radius versus the central surface brightness for the ultra-diffuse Coma cluster galaxies (red markers). These galaxies are similar in size to the Milky Way’s disk (blue), but significantly dimmer. [Van Dokkum et al. 2015]

Why it’s interesting:

These galaxies make up an odd population. Why are their stellar masses so low? The authors posit that these objects may be failed galaxies that lost their gas after having formed their first generation of stars. Adding to the intrigue, the authors find that in order for these galaxies to hold themselves together at their current distance from the cluster core, they must have a whopping dark-matter fraction of 98%.

About the discovery:

These ultra-diffuse galaxies were actually discovered entirely by accident. Van Dokkum and collaborators observed the Coma cluster in a project to measure properties of the intra-cluster light and look for streams and tidal features. Surprisingly, their images revealed these faint, uncataloged galaxies.

Citation

Pieter G. van Dokkum et al 2015 ApJ 798 L45. doi:10.1088/2041-8205/798/2/L45

Spiral arms

Young, forming planets can generate immense spiral structures within their protoplanetary disks. A recent study has shown that observations of these spiral structures may allow astronomers to measure the mass of the planets that create them.

Spirals From Waves

disk simulations

Snapshots of the surface density of a protoplanetary disk in a 2D simulation, 3D simulation, and synthesized scattered-light image. Click for a closer look! [Fung & Dong, 2015]

Recent studies have shown that a single planet, if it is massive enough, can excite multiple density waves within a protoplanetary disk as it orbits. These density waves can then interfere to produce a multiple-armed spiral structure in the disk inside of the planet’s orbit — a structure which can potentially be observed in scattered-light images of the disk.

But what do these arms look like, and what factors determine their structure? In a recently published study, Jeffrey Fung and Ruobing Dong, two researchers at the University of California at Berkeley, have modeled the spiral arms in an effort to answer these questions.

Arms Provide Answers

A useful parameter for describing the structure is the azimuthal separation (φsep) between the primary and secondary spiral arms. If you draw a circle within the disk and measure the angle between the two points where the primary and secondary arms cross it, that’s φsep.

scaling relation

Azimuthal separation of the primary and secondary spiral arms, as a function of the planet-to-star mass ratio q. The different curves represent different disk aspect ratios. [Fung & Dong, 2015]

The authors find that φsep stays roughly constant for different radii, but it’s strongly dependent on the planet’s mass: for larger planets, φsep increases. They discover that φsep scales as a power of the planet mass for companions between Neptune mass and 16 Jupiter masses, orbiting around a solar-mass star. For larger, brown-dwarf-size companions, φsep is a constant 180°.

If this new theory is confirmed, it could have very interesting implications for observations of protoplanetary disks: this would give us the ability to measure the mass of a planet in a disk without ever needing to directly observe the planet itself!

Modeling Observations

Fung and Dong confirm their models by additionally running 3D simulations, which yield very similar outcomes. From these simulation results, they then synthesize scattered-light images similar to what we would expect to be able to observe with telescopes like the VLT, Gemini, or Subaru. The authors demonstrate that from these scattered-light images, they can correctly retrieve the planet’s mass to within 30%.

Finally, as a proof-of-concept, the authors apply this modeling to an actual system: SAO 206462, a nearly face-on protoplanetary disk with an observed two-armed spiral within it. From the measured azimuthal separation of the two arms, the authors estimate that it contains a planet of about 6 Jupiter masses.

Citation

Jeffrey Fung (馮澤之) and Ruobing Dong (董若冰) 2015 ApJ 815 L21. doi:10.1088/2041-8205/815/2/L21

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