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Brown Dwarf Observations and Speculations

It’s tantalizing to speculate that there might be a brown dwarf system nearer to us than the Alpha Centauri stars. The odds seem long, but the discovery of a pair of brown dwarfs that are each no more than a millionth as bright as the Sun makes for exciting reading. The objects were originally cataloged by the Two Micron All Sky Survey (2MASS) as a single brown dwarf identified as 2MASS J09393548-2448279, but Adam Burgasser (Massachusetts Institute of Technology) has been able to show that the ‘object’ is actually a pair of the faint dwarfs.

Here the Spitzer Space Telescope was the instrument of choice, showing that 2M 0939’s brightness was twice what would have been expected from its temperature, which was determined to be in the range of 565 to 635 Kelvin (560 to 680 degrees Fahrenheit). The implication was that this is a brown dwarf binary, two dwarfs each with a mass some thirty to forty times that of Jupiter. And while the objects are a million times fainter than the Sun in total light, they’re a billion times fainter in visible light alone.

2M 0939 becomes the fifth closest known brown dwarf, but what gets my attention is Burgasser’s comment that this discovery may be the tip of the iceberg. In an MIT news release, he puts it this way:

“These brown dwarfs are the lowest power stellar light bulbs in the sky that we know of. In this regime [of faintness] we expect to find the bulk of the brown dwarfs that have formed over the lifetime of the galaxy. So in that sense these objects are the first of these ‘most common’ brown dwarfs, which haven’t been found yet because they are simply really faint.”

Faint enough for there to be examples closer to Earth than this one, and indeed closer than the Centauri stars? It seems unlikely, but it’s a stimulating thought, and the work on 2M 0939 shows just how difficult it may be to be sure. After all, Burgasser’s team spent three years studying the object with data from the Anglo-Australian Observatory before they could come up with a definitive read on its distance. That work was crucial, when combined with the Spitzer infrared observations, in measuring the object’s brightness, and thus deducing its true nature as a binary from the contrast with observed temperatures.

The vastness of interstellar space naturally staggers the imagination, but it’s striking that as we learn more, we realize how many objects are out there that we know little about. A true stellar census of the Milky Way, for example, would need to include brown dwarfs, but we aren’t prepared at this point to say how common they are. We do know that the larger and hotter M-dwarfs comprise more than 70 percent of all stars in the galaxy, and we also believe there must be planets ejected from their solar systems wandering through the interstellar depths. Get us the technology to make it to the Oort Cloud and we may find that the next truly interesting destination isn’t necessarily four light years away.

The paper is Burgasser et al., “2MASS J09393548−2448279: The Coldest and Least Luminous Brown Dwarf Binary Known?” Astrophysical Journal Letters 689 (December 10 2008), pp. L53–L56 (abstract).

Comments on this entry are closed.

  • James M. Essig December 12, 2008, 12:01

    Hi Paul;

    This is a really cool finding.

    Brown dwarfs according to some theorists may be able to keep a planet warm for billions of years. As I mentioned in a previous Tau Zero thread, I wonder if long half-lived radionuclides could be introduced into the braown dwarfs to reheat them.

    At 560 to 680 degrees Fahrenheit, and a biosphere containing planet in orbit at a distance from a parent brown dwarf such that the angular diamter of the brown dwarf with respect to the planet surface is between about 45 degrees and say 30 degrees, the black body radiated heat intensity on the planet surface would be simmilar to that on a human standand about 3 to 6 feet away from a wood stove. Granted that the above analogy is not precise, but it is a good approximation that can give some intuitive grounding as to how warm planets could be in the presense of a brown dwarf.

    I wonder if there might not be 10s of billions of these dwarfs within the Milky Way just waiting for human and or ETI civilizations to set up habitats around then. We would have to adjust our vision to near visible IR frequencies but perhaps that could be accomplished by evolution or by the use of a night vision IR type of eye glasses.

    Regardless, if we found a brown dwarf within say 1/2 light year of Earth, that would be an excellent target for an interstellar mission.

    A really interesting senario would entail a planet that became dislodged from solar orbit or from another star that is freely roaming interstellar space but which has an entire subterranian civilization of highly advanced ETI. With potentially Earth size planets or planetary bodies located in the Oort clould, I am interested in what we will find once we travel out to an explore any such large Oort Cloud objects.

    Thanks;

    Jim

  • kurt9 December 12, 2008, 15:20

    There seems to be a trend that as stars get smaller, the more numerous they become. Is there any reason to believe that this trend stops at the smallest M-type stars? It seems to me that there would be even more sub-stars (brown dwarfs or whatever you call them) than stars, then more planets, not tied to any star than sub-stars and so on until you get down to the size of dust particles. Is this indeed the case?

    If so, there should be lots of things closer to us than Alpha Centauri.

  • James M. Essig December 12, 2008, 16:53

    Hi kurt9;

    That is a very interesting question. If 70 percent of the stars within the Milky Way are M class dwarfs. One might consider the possibility that the number of brown dwarfs could be about 1 trillion given that there are estimated to be about 200 billion stars within the Milky Way.

    If such is the case, then potentially huge reserves of nuclear fusion fuels exist for future human and ETI civilizations to mine that are of a relatively easy to access, localized form. I wonder just how many free roaming gas giant planets there are within the Milky Way. The possibilities boggle the mind.

    Thanks;

    Jim

  • Adam December 12, 2008, 20:18

    Hi kurt9 & all

    There is reason to think there is a minimum mass to regular star formation. The standard stellar mass-number power-law if extrapolated to low mass predicts about 10 times as many brown dwarfs (down to 13 Jupiter masses) as regular stars (over 80 Jupiter masses.) This isn’t found observationally in new star-forming regions, and the actual Initial Mass Function observed has a peak at ~0.2 solar masses with a steep decline as you go lower. There might be as many brown dwarfs as stars, but not multiples more.

    An even more incredible find would be a binary pair of brown dwarfs made of antimatter – Joe Haldeman has such a discovery in a couple of his stories (“Worlds Apart”, “Tricentennial” etc.), to provide antimatter fuel for starships. Would be a massive incentive to pay them a visit then.

  • djlactin December 12, 2008, 22:22

    “Nemesis.”

  • Tibor December 13, 2008, 9:14

    …the next truly interesting destination isn’t necessarily four light years away.

    …and if this destination turns out to be a star – when is a brown dwarf a star? see Brown dwarfs -; well, this will be good news for our Long Bet 395.

    Tibor

  • kurt9 December 13, 2008, 15:14

    James and Adam,

    This is interesting. As James pointed out, this has implications for interstellar migration and travel. If the observation are real, then the peak really is 0.2 solar mass objects (red dwarfs) and interstellar space is largely empty. This is bad for a nearby resource base but is good for interstellar travel to other stars (lower risk of collision while traveling at relativistic velocities). If there is no lower limit, the interstellar space should be full up of stuff and one does not want to go careening through it at 0.9c.

    Any explanation for a minimum limit on mass formation?

  • andy December 13, 2008, 18:57

    Any explanation for a minimum limit on mass formation?

    At some point you start hitting a regime where it is less and less likely that there is enough mass to trigger gravitational collapse.

  • Adam December 15, 2008, 9:34

    Hi kurt9 & andy

    What andy said pretty much covers it. Study of dust and extinction of light has shown there’s about 30 jupiter masses of material not in stars per cubic parsec in the main part of the Galaxy. Sounds like a lot, but really it’s not. About half a solar mass extra is between the stars, similar to what’s in them. If it was all hydrogen it’d be about one atom per cc. About 1% is dust – ices, silicates, fluffy clusters about 100 nano-metres in size. For comparison, the half a trillion Oort Clouds comets, each about 10 km in size, would mass about 1/10 of the Interstellar Medium (ISM) density if evenly spread out.

    Some times hot plasma pushes the ISM around, gathers it together, and causes over-densities that collapse under their own gravity – stars. But there’s a minimum mass for any given density – the higher the density, the lower the minimum mass.

    Turbulence and radiation effects really complicate any simplistic picture, and so the processes need heavy number-crunching simulation to extract meaningful understanding from. Seems about ~3 Jupiter masses is as low as the process will go directly. Smaller objects require more complex scenarios, like massive disks around solar-mass stars or collisions between disks around stars to create dense enough conditions for direct collapse. Such events are probably sufficient to create all the free-floating planet-like objects seen in young star-forming regions. To get smaller solid objects requires either fragmentation of the larger objects via collision, or a planetesimal-forming nebula around a star.

  • James M. Essig December 18, 2008, 15:34

    Hi kurt9 and Adam;

    With the potentially huge number of free roaming planets and planetoids in interstellar space, and the potentially huge number of comets in the Oort Cloud, one definately feels confident regarding the future availablity of fusion fuel to power fusion rocket space craft, and for the production of electromagnetic energy beaming stations to power light sail driven space craft and also for the purpuse of producing antimatter fuels as a compact energy source to power antimatter rockets to potentially high gamma factors.

    However, like you said, we may find a minimum likely size for free roaming planets and other such bodies in interstellar space due to the kinematics of gas cloud contractions and limited gas cloud densities.

    If large quantities of interstellar dust particles with sizes on the order of 100 nanometers or larger would otherwise limit interstellar travel to only very mildly to mildly relativistic velocities, perhaps the usual mantra of using laser beams, particle beams, and other systems to vaporize and/or ionize the particles wherein the plasma could perhaps be captured and used as an energy source and/or a reaction mass to propell highly relativistic manned interstellar space craft would apply.

    Another option might be to devise some sort of narrow beam electic and/or magnetic field to somehow shove the particles aside in a direction close to or fully perpendicalar to the crafts direction of travel. I am not sure how such a directed E-field or B-field could be generated, but perhaps some sort of high gain electromagnetic beaming mechanism on board the craft could be used to produce a super position of electromagnetic waves that temporally superpose in such a manner such that the E-field or B-field vector components of the individual waves super-pose over a long enough temporal duration and with a net time averaged directionality commensurate with the production of a transverse repulsive and/or attractive tractor beam like field.

    B-fields of high enough intensity will magnetize any types of molecular or atomic bulk matter by introducing dipole moments in the materials in such a manner that they will become magnetic. Note that a living frog was suspended in a B-field as it was magnetized by a B-field of very high intensity. Interestingly, if I am not mistaken, the frog was suspended for several hours without any harm done to the frog. When I first read of this story, my immeadiate reaction was “How Freaky!”.

    Regardless, a cataloguing and carteographic mapping of interstellar objects and debris will be important for safe future interstellar manned missions.

    Thanks;

    Jim

  • ljk December 23, 2008, 1:48

    arXiv:0812.4094

    Date: Mon, 22 Dec 2008 05:46:40 GMT (16kb)

    Title: HD 91669b: A New Brown Dwarf Candidate from the McDonald Observatory Planet Search

    Authors: Robert A. Wittenmyer, Michael Endl, William D. Cochran, Ivan Ramirez, Sabine Reffert, Phillip J. MacQueen, Matthew Shetrone

    Categories: astro-ph

    Comments: Accepted for publication in AJ

    We report the detection of a candidate brown dwarf orbiting the metal-rich K dwarf HD 91669, based on radial-velocity data from the McDonald Observatory Planet Search.

    HD 91669b is a substellar object in an eccentric orbit (e=0.45) at a separation of 1.2 AU. The minimum mass of 30.6 Jupiter masses places this object firmly within the brown dwarf desert for inclinations i>23 degrees. This is the second rare close-in brown dwarf candidate discovered by the McDonald planet search program.

    http://arxiv.org/abs/0812.4094 , 16kb

  • ljk June 24, 2009, 15:48

    Photometric Variability of the T2.5 Brown Dwarf SIMP J013656.5+093347; Evidence for Evolving Weather Patterns

    Authors: Étienne Artigau, Sandie Bouchard, René Doyon, David Lafrenière

    (Submitted on 18 Jun 2009 (v1), last revised 22 Jun 2009 (this version, v2))

    Abstract: We report the discovery of a photometric variability in the bright T2.5 brown dwarf SIMP J013656.5+093347. Continuous J-band photometry has been obtained for several hours on four different nights. The light curves show a periodic modulation with a period of ~2.4 hours, a peak-to-peak amplitude of ~50 mmag and significant night-to-night evolution.

    We suggest that the light curve modulation is due to the brown dwarf’s rotation and that the longer term variations come from surface features evolution and/or differential rotation. We obtained complementary observations over a single night in the J and Ks bands; the object displays correlated photometric variability in both bands, albeit with smaller Ks-band amplitude. The ratio of the Ks and J variability amplitudes puts strong constraints on the physical mechanisms at play.

    Based on theoretical models of brown dwarf atmospheres, our results suggest that the atmosphere of SIMP0136 is comprised of both grain-free and colder (by ~100 K) grain-bearing cloudy regions. This discovery, and its interpretation, provide a natural explanation of the so-called J-band brightening.

    Comments: Accepted for publication in ApJ. Modification in the replacement : One author missing in the astro-ph metadata

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0906.3514v2 [astro-ph.SR]

    Submission history

    From: David Lafrenière [view email]

    [v1] Thu, 18 Jun 2009 20:00:04 GMT (123kb)

    [v2] Mon, 22 Jun 2009 21:50:09 GMT (123kb)

    http://arxiv.org/abs/0906.3514