The Closest Dwarf

by Paul Gilster on August 24, 2009

A conference like the recent on in Aosta offers plenty of opportunity to listen in on fascinating conversations, one of which had to do with what would happen if we found a brown dwarf closer to the Earth than the Centauri stars. The general consensus was that such a find would be a powerful stimulus to the public imagination and would probably result in renewed interest in getting to and exploring such a place. A boon, in short, for all our interstellar efforts, an awakening to a new set of possibilities.

But if there were a brown dwarf that close, wouldn’t we have other signs of it? One figure I heard mentioned at Aosta was three light years. Here I have to do some checking, because I don’t recall who dropped that figure or what paper he was referring to, but the upshot was that someone has argued that even a small brown dwarf closer to the Sun than three light years would leave an unmistakable signature in the orbits of our Solar System’s planets. I’ll see if I can track down the original reference (see note below). In any case, we didn’t have any astronomers in our number at Aosta to check the figure against.

A brown dwarf out there waiting to be discovered may not be unknown for long. In fact, we may well be no more than a few years away from finding it. The Wide-field Infrared Survey Explorer (WISE) mission is set for a November 1 launch from Vandenberg Air Force Base and should be able to put the matter to rest. Among the many activities of this observatory will be to find cold, dim stars. WISE should track down about a thousand brown dwarfs, among them those closest to our Solar System. That’s quite an exciting thought, as Peter Eisenhardt (JPL) has opined:

“We’ve been learning that brown dwarfs may have planets, so it’s possible we’ll find the closest planetary systems. We should also find many hundreds of brown dwarfs colder than 480 degrees Celsius (900 degrees Fahrenheit), a group that as of now has only nine known members.”

iso_eagle_med

That figure of nine may be off by now, as Eisenhardt made this comment back in early June. But whatever the number, the infrared detectors on WISE are going to revise our understanding of the nearby brown dwarf population. Unlike the Spitzer Space Telescope and ESA’s Herschel Space Observatory, WISE will survey the entire sky in an effort to build target lists for current and future observatories like the James Webb Space Telescope. Bear in mind, as noted in this news release, that right now we’re still using the catalog produced by the Infrared Astronomical Satellite (IRAS), a fine set of data but drawn from a mission that flew in 1983.

Image: An infrared image of M16, the Eagle Nebula, taken by the ESA/ISO satellite. The false-color image was constructed from a 7.7 micron infrared exposure (shown as blue), and a 14.5 micron infrared exposure (shown as red). This nebula is the site of active star formation in the Milky Way Galaxy. WISE will observe the region in similar wavelengths of light to see the dust that often enshrouds star forming regions. Credit: European Space Agency.

The actual WISE mission is relatively short. The full-sky mapping will take six months (after a one-month checkout of the system), to be followed by a second, probably partial scan that will depend upon the health of the spacecraft’s hydrogen coolant, necessary to cool the infrared detectors. I don’t want to underplay WISE’s role in identifying both near-Earth objects and main belt asteroids, either, nor its ability to find more distant planetary systems in formation. This is going to be one significant mission.

Ponder for a moment where we are, a point in history where we’re about to answer some questions that have preoccupied scientists for decades. Does Alpha Centauri have planets around either of its main stars? Debra Fischer or Michel Mayor’s Geneva team, working separately, may well have an answer within a few short years (and, perhaps, even months). Are there other terrestrial worlds out there, and in what number? Both Kepler and COROT are sending us data that will help us make a preliminary call, again within just a few years. And now WISE, which may soon be able to tell us whether there is indeed a brown dwarf closer than Centauri. Has there ever been a time of astronomical discovery more packed with excitement than this one?

A Quick Administrative Note: I’m unexpectedly in a situation where I’m having to write these entries without a working Internet connection. I then have to shoot the entry up to the site the next time I’m in my office, making Net time difficult and keeping me away from many resources. If you happen to know the reference re the three light year limit on brown dwarfs, please let me know. I’m not quite sure when things are going to get back to normal, so posting may be a bit erratic here, at least for a while, and when I can post, it will probably be in the afternoon rather than the morning.

tzf_img_post

{ 20 comments }

kurt9 August 24, 2009 at 15:03

Nemesis?

NS August 24, 2009 at 16:24

I had the same thought as kurt9. Sorry, it’s too technical for me though.

http://arxiv.org/abs/0904.1562

ROCA August 24, 2009 at 16:55

There is a lower limit proposed by Lorenzo Iorio (It) regarding the minimal distance between a Brown Dwarf and the Sun: 4,334 − 5,170 AU [ 0,069 - 0,082 ly ].

Constraints on planet X/Nemesis from Solar System’s inner dynamics
http://arxiv.org/abs/0904.1562

“We put full 3D constraints on a putative planet X by using the dynamics of the inner planets of the solar system. In particular, we compute the mimium distance of X as a function of its heliocentric latitude and longitude for different values of its mass. ”

ROCA

Administrator August 24, 2009 at 18:13

ROCA, thanks. I remember the name Iorio and sure enough, wrote him up a while ago:

http://www.centauri-dreams.org/?p=7243

I notice his numbers are much lower than the 3 light years I mentioned.

Carl August 24, 2009 at 19:36

Isaac Asimov had a collected edition of Soviet Science Fiction. In it was a story, “Infra Draconis”. It portrayed the long flight to an object, closer than the Centauri system, which resembles the modern notion of a brown dwarf. It was a dark gaseous body which generated enough thermal energy to sustain an unique biosphere. The protagonist takes a craft down and discovers a civilisation therein… a remarkable tale which seems to presage… with artistic license… later discoveries.

Administrator August 24, 2009 at 20:47

Interesting! Had never heard of this one, Carl. Thanks.

D.Rose August 24, 2009 at 20:51

Pardon my dimness (and the obvious pun) on the subject, but how can a brown dwarf have planets? Aren’t they essentially super-massive gas giants (and do we call them planets if we find them gravitationally unassociated with any star?) that just fell short in mass from becoming true stars? Shouldn’t any orbiting bodies be called moons not planets, even if they’re gas giants themselves? I get the quibbling over Pluto’s planet/not-planetiness and why technically defining what is and is not a planet right now might be a tad premature, but it seems pretty clear in my mind that if a body fails to ignite in the first place, it isn’t a star and thus orbiting bodies cannot technically be called planets. An argument might be made about the exhausted remains of old stars, but that is an own goal: They were STARS to begin with and thus remain so, though worn-out, while brown dwarfs never were.

Be gentle with this pleb… ;-)

Kenneth Harmon August 25, 2009 at 1:44

Carl,

Dr Michao Kaku in his recent book Physics of the Impossible indicated that if we discover a viable material that allows room temperature super-conductivity we might be able to rapidly develop a form of quasi anti-gravity propulsion that exploits the magnetic fields of Earth and various other bodies in space to allow for a type of “Field Propulsion”. If this was the case, and we did this perhaps as early as 2020, and we find a Brown Dwarf at just the right location between Earth/Sol and the Alpha Centauri System might it not be possible to skip our way across our Solar System, get to the far reaches of the Ort Cloud, pick up velocity around the Brown Dwarf and then leap frog across the Alpha Centauri System to our desired destination. Admittedly, these are all big ifs, but if there is a Brown Dwarf in the right location at approximately 3 light years then it may be argued that Interstellar Travel at least in our local neighborhood (5 LY radius) starts to look very viable by the end of the 21st Century following a stepping stone approach. It may not be elegant and it may be limited to our immediate “traversable zone”, but it would still be a form of limited Interstellar travel. Comments, thoughts!!!

KRH

keith August 25, 2009 at 7:46

Yes Iorio has been mentioned on this very site. However I think some of us found his calculation somewhat suspect.

I did post a link on here sometime back about WISE and expectation numbers of brown dwarves. It’s clear that at least some scientists involved with WISE expect BDs to be at least as numerous as stars, and they also expect to find the closest “star” (actually a cool BD) with this mission. It looks like there is a strong possibility of several BDs closer than the Centauri system. I’ll try and find the link when I’ve more time later.

Administrator August 25, 2009 at 8:10

D. Rose writes:

Pardon my dimness (and the obvious pun) on the subject, but how can a brown dwarf have planets? Aren’t they essentially super-massive gas giants (and do we call them planets if we find them gravitationally unassociated with any star?) that just fell short in mass from becoming true stars?

It’s a good question because brown dwarfs run between our categories and in some ways blend them together. But do realize that brown dwarfs of a certain mass (above 13 Jupiter masses, I believe) do manage to fuse deuterium, and much larger ones can fuse lithium. I just checked the Wikipedia and thought this snip might be useful:

Some astronomers believe that there is in fact no actual black-and-white line separating light brown dwarfs from heavy planets, and that rather there is a continuum. For example, Jupiter and Saturn are both made out of primarily hydrogen and helium, like the Sun. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giants in our solar system (Jupiter, Saturn, and Neptune) emit more heat than they receive from the Sun. And all four giant planets have their own “planetary systems” — their moons. In addition, it has been found that both planets and brown dwarfs can have eccentric orbits.

Currently, the International Astronomical Union considers objects with masses above the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) to be a brown dwarf, whereas those objects under that mass (and orbiting stars or stellar remnants) are considered planets.

andy August 25, 2009 at 13:10

There is a bit of a distinction between brown dwarfs and planets in that brown dwarfs are thought to represent the low mass end of the stellar mass distribution. Planets form in a different way and represent a different population of objects.

Using deuterium fusion to distinguish between the two groups does make a certain kind of sense: it would lead to the conclusion that planets don’t do fusion, but brown dwarfs do. For a while this made sense since there was a wide gap in the mass distribution between known brown dwarfs and the known planets. Subsequent observations have shown that while there does seem to be a “brown dwarf desert”, it is not totally dry, and there are objects on either side of the deuterium fusion limit that seem to fit in better with the other population. After all, there’s no particularly good reason why the formation processes involved should respect fusion taking place in the object’s interior.

There exist several known systems with objects massive enough to fuse deuterium, but in non-hierarchical orbital configurations with respect to other objects in the system. This is unlike the configurations of star systems in which the stars pair off into a hierarchical arrangement. Calling some of the objects in such systems “brown dwarfs” and others “planets” seems very artificial.

There are also a few known objects below the deuterium fusion limit with high mass ratios relative to their parent stars, the majority at wide separations from their parent stars. These would require unfeasibly massive circumstellar discs around the primary to be part of the planet population, and may well be brown dwarfs (i.e. the very lowest mass tail of the stellar mass distribution).

Judging by the kind of systems that are actually being observed, it seems likely that some planets are large enough that they go through a phase of their evolution where they fuse deuterium in their cores, and some brown dwarfs are small enough that they don’t. This makes distinguishing the two harder than just using a 13 Jupiter mass cutoff, but does seem to better reflect what is actually going on out there.

Erik Anderson August 25, 2009 at 15:25

It would be even more interesting to cross-reference the coordinates of brown dwarfs discovered by WISE with Pan-STARRS astrometry (parallax and proper motions, to be released ~2012) so that it may be discerned what proportion of the local brown dwarf population belong to the Galactic Halo versus the Galactic Disk. It would be prudent to have the parallaxes before “The Closest Dwarf” is proclaimed; photometric distances tend to be very sloppy and are subject to revision.

keith August 26, 2009 at 7:51

Look at the plot on page 4 in the link below. Depending on what mass function you choose, there could be several hundred BDs within 10pc. In fact, in the RECONS survey data, the best fit exponent for the within-10pc stellar population (down to class M) is -1.2. That supports the idea the true number may be actually ABOVE the upper estimate line in the plot:

http://arxiv.org/ftp/arxiv/papers/0902/0902.2604.pdf

(if I’ve read it correctly of course)

ROCA August 26, 2009 at 16:40

Please let us know if you find the article that describes de 3 ly limit for a brown dwarf.

Regards.
ROCA

Administrator August 26, 2009 at 18:08

Will be glad to do so, ROCA. Let me check with some of the participants at the conference.

Steve Pease August 27, 2009 at 18:49

If the BD was at 1 to 3 lightyear’s distance, but projected against a crowded sky background with many stars and a relative high infrared background, could anyone detect it?

ljk September 1, 2009 at 21:29

Detecting Planets Around Very Low Mass Stars with the Radial Velocity Method

Authors: A. Reiners, J.L. Bean, K.F. Huber, S. Dreizler, A. Seifahrt, S. Czesla
(Submitted on 31 Aug 2009)

Abstract: The detection of planets around very low-mass stars with the radial velocity method is hampered by the fact that these stars are very faint at optical wavelengths.

We investigate the precision that can be achieved in radial velocity measurements of low mass stars in the near infrared (nIR) Y-, J-, and H-bands, and we compare it to the precision achievable in the optical.

For early-M stars, radial velocity measurements in the nIR offer no or only marginal advantage in comparison to optical measurements. Although they emit more flux in the nIR, the richness of spectral features in the optical outweighs the flux difference.

We find that nIR measurement can be more precise than optical measurements in stars of spectral type ~M3, and from there the nIR offers significant gains in precision towards cooler objects. We studied potential calibration strategies in the nIR finding that a stable spectrograph with a ThAr calibration probably offers the best choice with currently available technology. Furthermore, we simulate the wavelength-dependent influence of activity (cool spots) on radial velocity measurements.

Our spot simulations reveal that the radial velocity jitter does not decrease as dramatically towards longer wavelengths as often thought. The jitter strongly depends on the details of the spots, i.e., on spot temperature and the spectral appearance of the spot. At low temperature contrast (~200K), the jitter shows a decrease towards the nIR up to a factor of ten, but it is substantially smaller with larger temperature contrast.

Forthcoming nIR spectrographs will allow the search for planets with a particular advantage in mid- and late-M stars. Activity will remain an issue, but simultaneous observations at optical and nIR wavelengths can provide strong constraints on spot properties in active stars.

Comments: 19 pages, 12 figures, submitted to ApJ, abstract abridged

Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:0909.0002v1 [astro-ph.SR]

Submission history

From: Ansgar Reiners [view email]

[v1] Mon, 31 Aug 2009 20:00:07 GMT (602kb)

http://arxiv.org/abs/0909.0002

andy September 2, 2009 at 17:40

I guess the limit depends on whether you’re assuming the brown dwarf is coeval with the Sun. If you assume a brown dwarf that is not bound to the Sun and just passing through the neighbourhood, it could be much older than the Sun and therefore fainter than a solar-age brown dwarf.

ljk September 10, 2009 at 13:16

Our stellar destinations out to 65 light years, or 20 parsecs, from Sol:

http://www.deepfly.org/TheNeighborhood/6-NeighborhoodSphere.html

ljk September 14, 2009 at 12:56

A Numerical Study of Brown Dwarf Formation via Encounters of Protostellar Disks

Authors: Sijing Shen (1), James Wadsley (1), Tristen Hayfield (2), Nicholas Ellens (1) ((1) McMaster University, (2) ETH Zürich)

(Submitted on 11 Sep 2009)

Abstract: The formation of brown dwarfs (BDs) due to the fragmentation of proto-stellar disks undergoing pairwise encounters was investigated. High resolution allowed the use of realistic initial disk models where both the vertical structure and the local Jeans mass were resolved.

The results show that objects with masses ranging from giant planets to low mass stars can form during such encounters from initially stable disks. The parameter space of initial spin-orbit orientations and the azimuthal angles for each disk was explored. An upper limit on the initial Toomre Q value of ~2 was found for fragmentation to occur.

Depending on the initial configuration, shocks, tidal-tail structures and mass inflows were responsible for the condensation of disk gas. Retrograde disks were generally more likely to fragment. When the interaction timescale was significantly shorter than the disks’ dynamical timescales, the proto-stellar disks tended to be truncated without forming objects.

The newly-formed objects had masses ranging from 0.9 to 127 Jupiter masses, with the majority in the BD regime. They often resided in star-BD multiples and in some cases also formed hierarchical orbiting systems. Most of them had large angular momenta and highly flattened, disk-like shapes. The objects had radii ranging from 0.1 to 10 AU. The disk gas was assumed to be locally isothermal, appropriate for the short cooling times in extended proto-stellar disks, but not for condensed objects.

An additional case with explicit cooling that reduced to zero for optically thick gas was simulated to test the extremes of cooling effectiveness and it was still possible to form objects in this case. Detailed radiative transfer is expected to lengthen the internal evolution timescale for these objects, but not to alter our basic results.

Comments: 18 pages, 12 figures and 2 tables. Accepted for publication in MNRAS

Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:0909.2044v1 [astro-ph.SR]

Submission history

From: Sijing Shen [view email]

[v1] Fri, 11 Sep 2009 19:55:52 GMT (896kb)

http://arxiv.org/abs/0909.2044

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