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Red Dwarfs: Oldest Known Circumstellar Disk

Determining the age of a star is not easy, but one way of proceeding with at least some degree of confidence is to identify the star as a member of a stellar association. Here we’re talking about a loose cluster of stars of a common origin. Over time, the stars have begun to separate, but they still move together through space. It was the Armenian astronomer Viktor Ambartsumian, the founder of the Byurakan Observatory, who discovered the nature of these associations and demonstrated that they were composed of relatively young groups of stars.

Stellar associations, or young moving groups (YMGs), provide an outstanding place to study the evolution of protoplanetary disks around young stars, for all associated stars have a similar age. Indeed, their galactic motion can be traced back to their place of origin. Another benefit: Exoplanets in such infant systems are often still hot, well within the capabilities of our near-infrared direct imaging techniques. Many direct imaging and disk evolution surveys in recent years have focused on the members of young moving groups.

Without Ambartsumian’s discovery of these groups, we would be hard pressed to come up with the age of the interesting red dwarf tagged AWI0005x3s. We’ve just learned, through a team led by Steven Silverberg (University of Oklahoma), working with a group of citizen scientists using the Disk Detective site, that this star has a warm circumstellar disk, an interesting find in its own right because we haven’t found many disks around red dwarfs.

But AWI0005x3s is found in the Carina association, on the order of 200 light years away in the Carina nebula, and appears to be moving with it. That pegs the young red dwarf as highly unusual, as Silverberg explains:

“Most disks of this kind fade away in less than 30 million years. This particular red dwarf is a candidate member of the Carina stellar association, which would make it around 45 million years old [like the rest of the stars in that group]. It’s the oldest red dwarf system with a disk we’ve seen in one of these associations.”

awi0005x3s-cropped

Image: Artist’s concept of the newly discovered disk. Credit: Jonathan Holden.

Looking through the paper, I learned that the intriguing AWI0005x3s disk would be the oldest ever observed around an M-dwarf, assuming the star can be confirmed as a member of Carina (the authors argue that the star has a probability of over 90 percent of being part of the association). Astronomers have found a disk frequency of about 6 percent around M-dwarfs less than 40 million years old, dropping to 1.3 percent around older members of this stellar class.

So where does AWI0005x3s fit in? The paper contrasts what we see in M-dwarfs with other types of star:

…debris disks are detected around 32 ± 5% of young A stars with Spitzer/MIPS (Su et al. 2006), and around 1−6% of old (∼ 670 Myr) Sun-like (F5-K9) stars with Spitzer/MIPS (Urban et al. 2012). Survival models predict that M dwarf debris disks occur at a similar frequency as disks around Sun-like stars, and that the dearth of detections to date is either due to systems having blackbody-like dust close to their central star, or due to systems having a smaller amount of dust distributed over a larger orbital separation (Heng & Malik 2013).

But other possibilities are still in play, including accelerated disk dissipation through interactions with a young stellar wind. Its age places AWI0005x3s in a potentially useful place in relation to other M-dwarfs, as the paper makes clear:

Our new M dwarf debris disk would bridge the gap between YMG and field M dwarf disks. Given their common spectral type (both M5.5V), this system could be a young analog for the Proxima Centauri system (Anglada-Escudé et al. 2016), as well.

The authors believe that AWI0005x3s is a potential target for study via adaptive optics on large telescopes and should be within range for high-contrast imaging, which could allow us to resolve the structure of the disk and potentially identify exoplanet candidates.

As for Disk Detective, it’s well worth a look. In fact, some 30,000 people have gotten involved in viewing short videos from surveys like the Wide-field Infrared Survey Explorer mission (WISE) and Two-Micron All Sky Survey (2MASS) projects, with some two million classifications of celestial objects now achieved. Eight of the citizen scientists involved are listed as co-authors on the AWI0005x3s paper, which is now available online.

The paper is Silverberg et al., “A New M Dwarf Debris Disk Candidate in a Young Moving Group Discovered with Disk Detective,” Astrophysical Journal Letters Vol. 830, No. 2 (14 October 2016). Abstract / preprint.

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  • ljk October 25, 2016, 9:06

    https://arxiv.org/abs/1610.03460

    Formation and composition of planets around very low mass stars

    Yann Alibert, Willy Benz

    (Submitted on 11 Oct 2016)

    The recent detection of planets around very low mass stars raises the question of the formation, composition and potential habitability of these objects. We use planetary system formation models to infer the properties, in particular their radius distribution and water content, of planets that may form around stars ten times less massive than the Sun.

    Our planetary system formation and composition models take into account the structure and evolution of the protoplanetary disk, the planetary mass growth by accretion of solids and gas, as well as planet-planet, planet-star and planet-disk interactions. We show that planets can form at small orbital period in orbit about low mass stars. We show that the radius of the planets is peaked at about 1 rearth and that they are, in general, volatile rich especially if proto-planetary discs orbiting this type of stars are long-lived.

    Close-in planets orbiting low-mass stars similar in terms of mass and radius to the ones recently detected can be formed within the framework of the core accretion paradigm as modeled here. The properties of protoplanetary disks, and their correlation with the stellar type, are key to understand their composition.

    Comments: to appear in Astronomy and Astrophysics Letters

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1610.03460 [astro-ph.EP]
    (or arXiv:1610.03460v1 [astro-ph.EP] for this version)

    Submission history

    From: Yann Alibert [view email]

    [v1] Tue, 11 Oct 2016 18:42:38 GMT (391kb,D)

    https://arxiv.org/pdf/1610.03460v1.pdf

  • Robert Feyerharm October 25, 2016, 11:23

    BTW – astronomers using the Microwave and Oscillations of Stars Telescope conducted a search for Proxima Centauri b transits. Beating the odds, MOST has detected evidence of an Earth sized planet transiting Proxima Centauri b every 11.2 days! Alas, the evidence isn’t statistically significant – the sample size is small (43.5 days observation time) and there’s too much noise in Proxima’s luminosity. Follow up observations will nail down whether the signal was spurious or real.

    http://www.slate.com/blogs/bad_astronomy/2016/10/19/how_big_is_proxima_centauri_b.html

    https://arxiv.org/abs/1609.08718v1

    • Paul Gilster October 25, 2016, 11:32

      The authors regard the signal as dubious at best. This is from the abstract:

      “We estimate a false-positive rate of at least a few percent and a much higher false-negative rate of 20-40%, likely caused by the very high flare rate of Proxima Centauri. Comparing our candidate signal to HATSouth ground-based photometry reveals that the signal is somewhat, but not conclusively, disfavored (1-2 sigmas) leading us to argue that the signal is most likely spurious. We expect that infrared photometric follow-up could more conclusively test the existence of this candidate signal, owing to the suppression of flare activity and the impressive infrared brightness of the parent star.”