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System Evolution: Delving into Brown Dwarf Disks

We’ve seen circumstellar disks around numerous stars, significant because it is from such disks that planets are formed, and we would like to know a good deal more about how this process works. Now we have word of planet-forming disks around several low-mass objects that fit into the brown dwarf range, and one small star about a tenth the mass of the Sun. With the brown dwarfs, we’re working with objects small enough to be at the boundary between planet and star.

The work is led by Anne Boucher (Université de Montréal), whose team drew photometric data from the Two-Micron All-Sky Survey (2MASS) and the Wide-field Infrared Survey Explorer (WISE) mission, allowing the detection of the objects at infrared wavelengths. Boucher notes the strong attraction such objects hold for astronomers:

“Finding disks in low-mass systems is really interesting to us, because objects that exist at the lower limit of what defines a star and that still have disks that indicate planet formation can tell us a lot about both stellar and planetary evolution.”

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Image: An artist’s conception of a planet-forming disk around a brown dwarf. Credit: Robin Dienel.

But let’s pause for a moment on the nature of the disks themselves. Dusty debris disks are considered to be signs of past planet formation, while gas-rich primordial disks mark the presence of active planet formation and accretion into the host star. Because disks can be detected by the presence of excess infrared, beyond what we would expect from the star by itself, we can use infrared observations to measure the differences in thermal emission from one infant system to another, showing us a range of disks at various stages of their evolution.

Thus we get massive primordial disks that are gas-rich and dense around younger systems, while dusty debris disks are colder and more depleted in gas, marking the remnants of planetary system formation even as they are replenished by the collisions of small objects in the system. These can last up to 100 million years; they’re colder and observable in the mid-infrared (primordial disks produce a strong signature in the near-infrared). We also see so-called ‘transition’ disks with an inner region depleted of dust, and an outer region still rich in it.

What Boucher and colleagues have been looking at are three objects ranging from 13 to 19 Jupiter masses, and a fourth of between 101 and 138 Jupiter masses. The paper makes the case that all four disk candidate stars are members of stellar associations — the TW Hydra, Columba, and Tucana-Horologium associations — allowing the researchers to draw conclusions about their age. A stellar association is a large grouping of stars similar in spectral type and origin, grouped more loosely than open or globular-type star clusters. These are young stars that have not had time to move any great distance from their place of formation.

The four disks are all thought to be in the planet-forming phase, none of them having attained the age of a dusty debris disk. Even so, two of these objects appear older — at between 42 and 45 million years — than we would normally associate with an active disk system. From the paper:

Disk temperatures and fractional luminosities were determined from this analysis. Their values indicate that the new disks are likely transitional or primordial. The spectral types of these new disk hosts are late (M4.5 to L0δ), and correspond to low-mass BDs (13 − 19 and 101 − 138MJup) of young ages (∼ 7 − 13 and ∼ 38 − 49Myr). These four systems join a relatively small sample of low-mass objects known to harbor a circumstellar disk. There is still a lot to learn about primordial and (pre-)transitional disks around low-mass stars, and these four new candidates could play an important role to shed light on the formation and evolution processes of stars and planetary systems.

Also in question is the fraction of brown dwarfs that have developed planetary systems. Finding debris disks around young objects like these is one way into the problem, for we can watch the process of planet formation unfold around objects of different ages. Bear in mind as well that their lower luminosity means objects like these could be interesting targets for exoplanet searches using direct imaging methods. All four of the new disk candidates should likewise be prime candidates, the paper suggests, for the Atacama Large Millimeter Array.

The paper is Boucher et al., “BANYAN. VIII. New Low-Mass Stars and Brown Dwarfs with Candidate Circumstellar Disks,” accepted at The Astrophysical Journal (preprint).

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Comments on this entry are closed.

  • FrankH October 4, 2016, 15:57

    It will be interesting to see if brown dwarf planets (moons?) follow the Canup-Ward mass ratio of approx. 10E-4, or if the ratio breaks down above a certain mass.

  • andy October 4, 2016, 16:38

    On a similar subject: the latest study of the transiting ring system at 1SWASP J1407 suggests that the ring is in a retrograde orbit around a 60–100 Jupiter mass object (either a massive brown dwarf or a low-mass red dwarf star).

    Rieder & Kenworthy (arXiv:1609.08485 [astro-ph.EP]) “Constraints on the size and dynamics of the J1407b ring system

    • Paul Gilster October 4, 2016, 20:35

      Thanks for this, Andy. Had completely escaped my attention. Very interesting!

  • ljk October 5, 2016, 8:50

    3 October 2016

    Giant hidden Jupiters may explain lonely planet systems

    By Marcus Woo

    Lonely planets can blame big, pushy bullies. Giant planets may bump off most of their smaller brethren, partly explaining why the Kepler space telescope has seen so many single-planet systems.

    Of the thousands of planetary systems Kepler has discovered, about 80 per cent appear as single planets passing in front of their stars. The rest feature as many as seven planets – a distinction dubbed the Kepler dichotomy.

    Recent studies suggest even starker differences. While multiple-planet systems tend to have circular orbits that all lie in the same plane – like our solar system – the orbits of singletons tend to be more elliptical and are often misaligned with the spins of their stars.

    Now, a pair of computer simulations suggest that hidden giants may lurk in these single systems. We wouldn’t be able to see them; big, Jupiter-like planets in wide orbits would take too long for Kepler to catch, and they may not have orbits that cause them to pass in front of their stars in our line of sight. But if these unseen bullies are there, they may have removed many of the smaller planets in closer orbits, leaving behind the solitary worlds that Kepler sees.

    The simulations show that gravitational interactions involving giants in outer orbits can eject smaller planets from the system, nudge them into their stars or send them crashing into each other.

    Full article here:

    https://www.newscientist.com/article/2107893-giant-hidden-jupiters-may-explain-lonely-planet-systems/

    To quote:

    “We know these configurations have to occur in some fraction of exoplanet systems,” Mustill says.

    But that doesn’t mean they’re universal. “They don’t occur all the time, and this is one reason why you can’t explain the large number of single planets purely through this mechanism,” Mustill says. According to his analysis, bullying giants can only account for about 18 per cent of Kepler’s singles.

    To confirm their proposed mechanism, the researchers must wait until next year for the launch of the Transiting Exoplanet Survey Satellite (TESS), which will target closer and brighter systems – and thus be easier for follow-up observations to uncover the bully planets.

    Journal references: arXiv; arxiv.org/abs/1609.08110, arxiv.org/abs/1609.08058

  • ljk October 5, 2016, 8:59

    Astronomers find a planet through a never-before-used method

    They used pulsation to confirm a long-period planet around a Kepler candidate world.

    By Korey Haynes | Published: Tuesday, October 04, 2016

    Astronomers find most exoplanets from indirect signals, noticing changes in the light of the planet’s host star instead of by seeing the planet itself. But some stars’ light changes all on its own, making these methods tricky at best. KIC 7917485b is the first exoplanet identified around a main sequence A-type star from its orbital motion, and the first found near an A -typestar’s habitable zone.

    A-type stars are bigger and hotter than most stars in the Kepler catalog and tend to be noisy, changing brightness at regular intervals. This dimming and brightening can be hard to untangle from, for instance, a planet transiting and dimming its light. As such, while there’s no reason for A-type stars not to have planets, it’s been difficult for astronomers to identify them. So far, the few exoplanets found around A-type stars are either from direct imaging (which can only, where the planets are very far from their star, or from transits where the planets are very close to the star, where the signal is strong.

    But astronomers came up with a novel idea to use the variability of the star itself as a way to look for exoplanets. The star pulses because of helium changes in its lower layers. It puffs up, cools and dims, shrinks, heats and brightens, and then repeats the process multiple times in a day. In a Kepler light curve, this shows up as a periodic dimming and brightening, like clockwork. But this clock shows a delay. The pulsations appear a little early or late, and by calculating this delay, astronomers can measure that the star is actually moving in a back-and-forth, orbital motion. And this movement is due to the gravitational tug of a nearby planet.

    Full article here:

    http://www.astronomy.com/news/2016/10/astronomers-find-a-planet-through-a-never-before-used-method

    To quote:

    The delays in KIC 7917485’s pulsations revealed a planet about 12 Jupiter masses with a period of 840 days, which is close to the habitable zone of such a hot star. While 12 Jupiter masses makes this planet nearly a brown dwarf, and certainly a gas giant, the study’s authors point out that potential moons keep the question of habitability an intriguing one.

    The pulsation delays are very similar to how astronomers find planets via the radial velocity method, but in this case, no spectrometer is needed. The Kepler light curve provides all the necessary information; the planet doesn’t need to transit to reveal itself. This is key, because most planets on orbits that take hundreds of days— planets in the habitable zone of hot stars — won’t. Having a method that can reveal them anyway is an important tool in the exoplanet-finding kit.

  • ljk October 6, 2016, 9:12

    What Swings a Star Around – Another Star or a Distant Planet?

    October 5, 2016

    An international team of astronomers using the Subaru Telescope and led by a graduate student member of SOKENDAI (The Graduate University of Advanced Studies, Japan) has discovered companions circling “intermediate-mass” stars. These are stars that are heavier than the Sun and the companions were thought to be either planets or possibly small stars. The excellent performance of the Subaru Telescope enabled the detection of faint objects circling around three of six bright stars surveyed.

    The target objects γ Hya, HD 5608, and HD 109272 have companion stars (Figure 1) called γ Hya B, HD 5608 B, and HD 109272 B. The other three stars surveyed did not have them. Comparison of these results with the frequency of companions predicted by planet formation theory is a crucial step in better understanding how planets are formed.

    Full article here:

    http://subarutelescope.org/Pressrelease/2016/09/07/index.html

  • ljk November 11, 2016, 10:53

    JPL News | November 10, 2016

    NASA Space Telescopes Pinpoint Elusive Brown Dwarf

    In a first-of-its-kind collaboration, NASA’s Spitzer and Swift space telescopes joined forces to observe a microlensing event, when a distant star brightens due to the gravitational field of at least one foreground cosmic object. This technique is useful for finding low-mass bodies orbiting stars, such as planets. In this case, the observations revealed a brown dwarf.

    Brown dwarfs are thought to be the missing link between planets and stars, with masses up to 80 times that of Jupiter. But their centers are not hot or dense enough to generate energy through nuclear fusion the way stars do. Curiously, scientists have found that, for stars roughly the mass of our sun, less than 1 percent have a brown dwarf orbiting within 3 AU (1 AU is the distance between Earth and the sun). This phenomenon is called the “brown dwarf desert.”

    The newly discovered brown dwarf, which orbits a host star, may inhabit this desert. Spitzer and Swift observed the microlensing event after being tipped off by ground-based microlensing surveys, including the Optical Gravitational Lensing Experiment (OGLE). The discovery of this brown dwarf, with the unwieldy name OGLE-2015-BLG-1319, marks the first time two space telescopes have collaborated to observe a microlensing event.

    “We want to understand how brown dwarfs form around stars, and why there is a gap in where they are found relative to their host stars,” said Yossi Shvartzvald, a NASA postdoctoral fellow based at NASA’s Jet Propulsion Laboratory, Pasadena, California, and lead author of a study published in the Astrophysical Journal. “It’s possible that the ‘desert’ is not as dry as we think.”

    Full article here:

    http://www.jpl.nasa.gov/news/news.php?feature=6673