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Two Relatively Near Brown Dwarfs

Two brown dwarfs relatively near to the Sun may be just the first such objects we’ll soon identify with data from the WISE (Wide-field Infrared Survey Explorer) satellite. Ralf-Dieter Scholz (Leibniz-Institut für Astrophysik, Potsdam) and colleagues have gone to work on a search for brown dwarfs with high proper motion, looking for brown dwarfs in the immediate solar neighborhood using not just the preliminary WISE data release but the previous near-infrared (2MASS) and deep optical (SDSS) surveys. The search has already begun to pay off.

The two brown dwarf discoveries — WISE J0254+0223 and WISE J1741+2553 — are at estimated distances of 15 and 18 light years respectively. Their strong infrared signature and their extremely faint appearance at visible wavelengths attracted the team’s attention, and both show the high proper motion across the sky that flags nearby stellar objects. The team was able to use the Large Binocular Telescope (LBT) in Arizona to determine spectral type and distance more accurately. Interestingly, both objects fit into the category of T-type brown dwarfs, at the boundary of the still not well defined class of Y-type brown dwarfs.

Image: The (un)known Solar neighbors. The stars are shown with symbols of different sizes and colours, roughly corresponding to their real sizes and spectral types. Most stars in the Solar neighborhood are red dwarf stars of spectral type M (in the middle of the figure) with surface temperatures of slightly more than 2000 Kelvin. Proxima, our nearest known neighbor, also belongs to this class. The number of brown dwarf discoveries (almost all with spectral types L and T, and surface temperatures below 2000 K) is already higher than the number of white dwarfs (shown as small white dots at the top). The two nearest brown dwarfs, epsilon Indi Ba and Bb, the discovery of which was reported by the AIP in 2003 and 2004, and the newly found objects are marked. (Credit: AIP).

Do brown dwarfs, hitherto undetected, surround us in large numbers? We certainly can’t rule out the possibility, and we can expect much more data mining from the riches WISE has accumulated. And yes, the case for a brown dwarf closer than the Alpha Centauri stars is still open, making the brown dwarf hunt of unusual interest for identifying potential targets for future probes. But the two brown dwarfs in question could prove useful in many ways in their own right, as the paper on this work notes:

While WISE J0254+0223 and WISE J1741+2553 are likely similar to the few other T8-T10 brown dwarfs known, they are the first ultracool brown dwarfs detected in both 2MASS and SDSS. With their relatively bright magnitudes they are excellent targets for detailed spectroscopic investigations and for high resolution imaging in search of possible binarity. They may become important laboratory sources at the boundary between the T-type and the suggested Y-type (Kirkpatrick et al. 1999) classes of brown dwarfs.

Image: False-colour images of the two brown dwarf discoveries WISE J0254+0223 and WISE J1741+2553 (composite of three images taken by the Wide-field Infrared Survey Explorer (WISE) with different filters in the infrared). In the WISE colours, the extremely cool brown dwarfs appear as yellow-green objects. The positions of the objects as observed by a previous near-infrared sky survey about ten years before the WISE observations are also marked. Every image covers a sky field about 200 times smaller than the full moon. After 700 and 1200 years, respectively, the proper motions of the two objects lead to a shift in their position as large as the full moon diameter. (Credit: AIP, NASA/IPAC Infrared Science Archive).

The paper is Scholz et al., “Two very nearby (d ~ 5 pc) ultracool brown dwarfs detected by their large proper motions from WISE, 2MASS, and SDSS data” (preprint).

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Comments on this entry are closed.

  • ljk July 15, 2011, 13:50

    In addition to brown dwarfs, I bet there are also a lot of dead stars and those rogue exoplanets nearby, probably even closer than Alpha Centauri.

    This is further evidence just how big our Universe is, when even billions of objects in a relatively tight area like our Milky Way galaxy can drift about undetected by us and also fail to collide with our planet, at least on a regular basis. The latter of which is a good thing.

    All the more reason why we need more and more powerful instruments on Earth and in space searching our cosmic neighborhood for all kinds of things.

  • kurt9 July 15, 2011, 14:06

    Just two? I thought these brown dwarfs were supposed to be as common as red dwarfs.

  • Mike July 15, 2011, 16:26

    kurt9, That’s the question alright. Could the WISE astronomers be sitting on some big announcement about huge numbers of brown dwarfs? More common then M-dwarfs?
    I have my doubts. WISE is a very powerful instrument and if the brown dwarfs are there in the many dozens in nearby space it will detect them. I think there would at least be some cautious statements to that effect by now if that is the case. Time will tell.
    Brown dwarfs are a big puzzle. Perhaps there are rare products of some unusual formation process. Not quite stars, not really planets, often not orbiting a star.

  • Astronist July 15, 2011, 17:27

    Dead stars, ljk? My understanding was that the smaller dwarfs haven’t yet had time to run through their life cycle, whereas white dwarfs haven’t yet had time to cool down to resemble red dwarfs, let alone to go cold. Please give more details.

    Stephen
    Oxford, UK

  • Adam July 15, 2011, 18:26

    Larry, that’s the second time you’ve mentioned dead stars. What kind do you mean?

  • Mike July 15, 2011, 21:04

    To ljk. Dead stars = white dwarfs or neutron stars or stellar mass black holes. Or maybe brown dwarfs finished toying with deuterium if you consider brown dwarfs to be truly stars. I’m not sure I do. If you know of any other kind of “dead star” could you please describe it for our benefit?
    I’m certain if any of these previously described “dead stars” existed within 4.3 light years we would know by now thanks to the multiwavelength observational coverage astronomers enjoy nowadays.
    Rogue planets are another kettle of fish. Who knows how many are wandering out there? Are they a hazard to navigation? How many dwarf planets wander interstellar space?

  • ljk July 15, 2011, 23:46

    I meant stars that were totally burned out, just dark cinders of their once glorious selves. I take it now the Universe isn’t old enough to have stars that far gone?

    It’s a good thing I didn’t mention the moon-size space station headed our way….

  • Mike July 16, 2011, 9:55

    Yep, it takes an amazingly long time for a white dwarf to cool to black, none are estimated to have done so yet. Same for neutron stars.

    Independence Day or Star Wars?

  • spaceman July 16, 2011, 11:45

    Between brown dwarfs and rogue exoplanets, yes, there probably is one or more of these large objects approximately as close or closer to us than the Alpha Centauri system.

    Isn’t there still a large portion of the WISE data that has yet to be released and analyzed??

  • David July 16, 2011, 12:38

    “moon-size space station ?”
    Expecting a visit from the Xeelee ?

  • Paul Gilster July 16, 2011, 14:06

    spaceman asks:

    Isn’t there still a large portion of the WISE data that has yet to be released and analyzed??

    The April 14 preliminary data release includes data from the first 105 days of WISE survey observations, 14 January 2010 to 29 April 2010. The final data release will be in 2012. More here:

    http://wise2.ipac.caltech.edu/docs/release/prelim/expsup/sec1_1.html#prel_rel

  • Eniac July 16, 2011, 15:51

    Looking at the figure, I find it interesting that the sun-like neighbors seem to be clustered at 4, 12, 16 and 20 ly, approximately, while the dwarfs appear to be evenly distributed. The clusters appear too tight to be a coincidence. Are they?

  • Rob Henry July 16, 2011, 16:59

    From ljk’s comments I gather it is possible for a true star (ie not a brown dwarf) to have such a low mass that its eventual remnant is not massive enough for further contraction to be halted by electron degeneracy rather than normal chemical repulsion. If so, this hypothetical remnant seems so neglected that is has no name, and befits ljk to give it one for posterity.

  • Rob Henry July 16, 2011, 17:50

    Ljk, as your dead star’s cool from a plasma state, is it possible that the strength of the rebound of space take up by the reconstituted atomic matter is greater than the energy released by the capture of the electrons, and is it possible, that in a steady state universe, they will go through a phase of actually absorbing heat from the environment? Where can I find out more about them?

  • jkittle July 16, 2011, 20:03

    *Planet, Planet, burning bright*
    There is a lot of difference between being able to detect an object and being able to “discover it” WISE, for example, can detect an object the size of Jupiter out to about light year, even a Neptune-sized object should generate enough heat to be detected out to about 700 AU. But.. there is a big caveat. It is one thing to be able see a needle if you know where to look and quite another to be able to pick it out of a astronomical haystack. If you look at the pictures WISE has produced ( or Spitzer for that matter) you see that the sky is full of warm objects including distant nebula that form a background for the search. Sure you can distinguish these from planet-sized objects or form Brown Dwarfs, given enough observations and using different wavelengths. But all that takes time, computation and the especially followup time on other big instruments. For most parts of space, WISE only had one day or so to see these objects at all four wavelengths, ( for 8 or more overlapping frames) and a second epoch 6 months later, ( when it had only two operational wavelengths. This only gives one noisy chance to detect apparent motion. It is possible to compare the images to the 2 micron surveys but these are not going to be nearly as sensitive to cool objects. Soo00. While the the WISE team and others keep looking through available data with ever improved number crunching routines, it still takes time to find things that are interesting enough to get time on a big ‘scope to follow up, (as they did in this case). Remember they are also spending effort to look at other issues in cosmology and astronomy as well, doing the hard science of astronomy-this is not just a hunt for Tyche.
    Here is something you have heard before.. we still have WISE up there in orbit, dormant but operational. We might be able to collect another data set that is separated by a year form the original observation runs-,plenty of time for motion to be observed even on faint objects. In total we would then have 3 or 4 “detections” of interesting objects which is FAR better than just one or 2. With that data, we might be able to much more quickly identify nearby cool objects. The cost would be about 2% of the cost of a new mission, and new missions like this are not likely to be funded THIS DECADE ( to bad for the JWST- did you write your congressman in support? I did!)
    Let me be clear. I make a living as an objective scientist, but in this matter I have a definite preferences. A “nearby” Brown dwarf or an even closer planet-sized object beyond the “Kuiper Cliff” might reignite space exploration. We could reach out 500 AU now with probes, and maybe set sail for a brown dwarf half a light year away in a few decades. If any of these are out there, I want to know about it, and am very impatient to learn!
    In the mean time I think few people in general society appreciate the quality of the work the WISE team is doing – so much value at so little cost!

  • Michael Simmons July 16, 2011, 21:20

    >A “nearby” Brown dwarf or an even closer planet-sized object
    >beyond the “Kuiper Cliff” might reignite space exploration.
    My thoughts. Especially if it had planets/moons.

    The Wise Final Release will include the second pass of the sky taken 6 months later. However not all of it will be fully processed.
    From an email
    “The WISE final data release next year will include the post-cryogenic mission data. However, only the single-exposure images and extracted source lists produced from the first-pass processing of those data will be released.”

    It’s a pity they couldn’t have included enough coolant to do another full pass of the sky. They could have hibernated it after the first pass. Waited a couple of years, then woken it up and done another pass.
    However their priority was NEO’s and not close brown dwarfs and planets.

    Another option might have been to send it much further from the sun where coolent would not have been needed. But then I’d imagine it would need a larger, longer life power source , to transmit from a much greater distance.

    My hope is that their is another WISE mission in 10yrs time.

  • ljk July 17, 2011, 15:33

    Why thank you for the honor! I could go the typical astronomer route and call it the Really Dead White Dwarf. Or how about Deadar, keeping in line with pulsar and quasar (and collapsar, what black holes should really be called). Or maybe Cindar. I kinda really like that last one.

  • Paul Gilster July 17, 2011, 17:38

    Larry, think Kuiper Belt. In other words, name this object after yourself!

    Thus: The really dead white dwarf becomes: The Klaesar… You have to admit, Larry, it’s perfect.

  • ljk July 18, 2011, 0:56

    Paul, I did not want to get egotistical, but you’re right, it IS perfect! The name, please note, not that a star becomes really dead. Thank you! Now, where do I go to register this new name?

  • Adam July 18, 2011, 7:06

    Lorenzo Iorio has computed the distance range for different sized objects in the outer solar system based on observed anomalies in the orbits of the planets. A brown dwarf would be at least 2,000 AU away, though it might swing closer.

  • Paul Gilster July 18, 2011, 8:13

    ljk writes:

    Paul, I did not want to get egotistical, but you’re right, it IS perfect! The name, please note, not that a star becomes really dead. Thank you! Now, where do I go to register this new name?

    Tricky question. Your job is to a) get scientists to write about klaesars in the first place and 2) to get the name ‘klaesar’ in front of them before they do. I’m open to suggestions, and maybe some of our resident astronomers can help ;-)

  • kzb July 18, 2011, 8:17

    jkittle; excellent post, I think of this the same way.

    Michael Simmons: where does it say NEO’s are a higher priority than BD’s? Everything I have read says BD’s are a major mission objective. Do you have anything to back this up?

    I do have this feeling, that if the nearest stellar system to the sun has been found by WISE, it’d be impossible to keep it quiet for long. This makes me feel it has not been found. We should remember that the possibilities were ALREADY strongly constrained by existing surveys, long before WISE. It is only relatively small BD’s, free-floating planets, and “Tyche” that are allowed. Of these, Tyche is tightly constrained to a certain mass range and orbit.

    Something else that I’ve been thinking is in relation to a volume of a sphere. When you think about it, statistically, bodies at a larger distance are more common than those closer. That’s simply because the volume of a shell between say 3 and 4 light years radius is larger than a shell between 2 and 3 light years in radius.

  • Mike July 18, 2011, 10:34

    I can’t think of any good reason why dark matter can’t be called Klaesarite.
    Problem is I can’t think of any good reason it should be either. Other then that Klaesarite sounds cooler then dark matter.

  • amphiox July 18, 2011, 12:23

    From ljk’s comments I gather it is possible for a true star (ie not a brown dwarf) to have such a low mass that its eventual remnant is not massive enough for further contraction to be halted by electron degeneracy rather than normal chemical repulsion. If so, this hypothetical remnant seems so neglected that is has no name, and befits ljk to give it one for posterity.

    Of course, such very low mass stars (ie the smallest of all the red dwarfs) would have lifespans in the trillions of years. So such remnants, even if they could exist, would not exist right now.

  • jkittle July 18, 2011, 14:54

    you make a good point kzb.
    “Something else that I’ve been thinking is in relation to a volume of a sphere. When you think about it, statistically, bodies at a larger distance are more common than those closer.”

    I have thought about the issue of relative volumes but in fact space is not flat. We have a bit of a gravity well out to about 1 light year, so that warps ( pun intended) the probabilities a bit. It is also possible that some bodies may have formed in the same collapsing protostar cloud as our sun then migrated outward. There are some pretty strong constraints on the brightness and locations ( and even mass) of objects in the outer solar system. – nothing as bid a s earth up close for example. The darker, cooler objects at at are not constrained are also hard(er) to discover. I am pretty confident that anything that I would identify as “interesting ” by my non-objective criteria within the sphere of the Oort cloud has already been detected in images but not identified. Personally I think the guys and gals at Pan Starrs will take the stage after the release of the WISE data next year. A very faint slow moving objects may take a few years to tease out of the background, and like to Wise IR signature of interest. Again is is not the case of not enough data, it is the case of not enough sitings linked together to form a track across the sky. It is tricky to leap from one data set to another . Once you use a data set to find something interesting then you can “precover” the object in any of several quality surveys. Also- the LBT and other big telescopes can make short work of any candidate they focus on, once they know where to look.
    KZB- the logical outcome of your point is that space is pretty empty. However the two dimensional image we call the sky is very crowded and gets more so the closer you look!

  • ljk July 18, 2011, 15:16

    Mike, just so long as the name sounds cool, that is all that really (dark) matters.

    Amphiox, it does not matter if they do not exist right now. The point is they will some day and they will need a name to go along with them.

  • philw1776 July 18, 2011, 20:35

    @jkittle: great posts! Agree that “nearby” objects of reachable interest would be stimulating. Hadn’t realized that WISE could be rejuvenated so inexpensively. Zombie time! Extending observations from 2x to 3-4x really bumps up the sigma. Any semi-billionaires out there wanting to fund this? :)

  • kzb July 19, 2011, 7:41

    @jkittle
    Another thing that concerns me is “Type 2” error, that is, falsely deciding that something is not there when it is.

    First, in these searches, there are millions of objects to process. Therefore you have to set your identification algorithm criteria very strictly or you will end up with thousands of hits. Which you might then have to wade through manually.

    Second, you don’t want to embarrass yourself by publishing discoveries that are later rubbished by your peers. That would be reputationally bad. So you only claim your most iron-clad discoveries in the press.

    These two “market forces” I think must inevitably lead to valid possibilities being consigned to the reject pile. Hence the danger of a lot of Type 2 error.

  • coolstar July 19, 2011, 10:43

    @jkittle I’ll join in with a thank you for the very informative posts. I think you’re right about Pan Starrs being the next big discovery engine for interesting things nearby (and then LSST, if it ever gets finished).

  • jkittle July 19, 2011, 11:00

    Emails with Ned Wright confirm Wise is still functional and added that there are no problems with consumables to maintain attitude control. It is just not collecting and transmitting that right now.
    To KZB’s point there are many faint objects that lead to many low probability detections of apparent motion -none for these are reported until they are confirmed by ground telescopes or Spitzer for example, ( heck ,you can go in the data and find your own candidates , a process that will be even more enticing when the next set is released). It is just a matter of how stringent you want to set the detection criteria. Another data round would really help (if you set the stringency to allow the probability of 1 in 10,000 false hits, that yields a lot of possibilities to follow up on- when applied to >100 million objects,,, however, another data set to reduce false detection by another factor of 10,000, then you are getting down to manageable number of tracks to follow up on, even for faint objects.) The computation time is pretty steep, but again can be prioritized to the highest-sigma objects first..

  • Eniac July 19, 2011, 20:51

    Is nobody else intrigued by the title figure which appears to show that sun-like stars come in distinct shells at 4, 12, 16, and 20 ly, approximately? Why? Am I missing something? Is this well-known? A coincidence?

    Even the dwarfs appear to have density peaks at those same spots.

  • kzb July 20, 2011, 8:07

    Eniac that is an interesting point! I wonder if it is something to do with distance units in parsecs being transcribed into a plot with the unit in light years?

    Other than that, Poisson statistics with small numbers is known to produce clusters by random chance. You see this effect with disease occurence plotted on maps, hence the apparent childhood leukemia clusters.

    Someone who had the time could probably do a statistical analysis on the likelyhood of this pattern occuring by chance, my guess is that it is higher than you might think.

  • Holger July 20, 2011, 8:23

    @Eniac: The coincidence is greatly reduced if you treat all binary systems (which figure in the image as two dots in exactly the same vertical line) as a single star (which, for “coincidence purposes”, they are): E.g. there is only a single sun-like star system at 4 ly, namely Alpha Centauri A/B.
    And I would consider the remaining pattern as a real coincidence due to the small sample size (only some 16 systems containing “yellow” stars). And I don’t really see a similar pattern for the red dwarfs.

  • kzb July 20, 2011, 13:06

    That’s a good point Holger about the binary systems.
    There’s no apparent clustering with the red dwarfs, but they DO get more common as you get further away- that will be the greater volume of a shell of larger radius effect.

    That’s also apparent, if less obvious with the brown dwarfs. However I can’t believe there are none to be found in the great empty space in the diagram in the bottom left quadrant.

    In theory the bottom half of the figure should approximately mirror the top half when the census is complete.

  • jkittle July 20, 2011, 18:03

    One could do a power law distribution and likely find that because of gravitation, stars end to cluster even when embedded in the gravity field of the galaxy. On a larger scale this is clearly seen in globular clusters and in the density waves that comprise the visible arms of the galaxy. Superimpose this three-dimensional clustering on a one dimensional log scale and I think you get the gaps and peaks. On the other had trying to figure out what is going on from a small number of discrete objects is not very rigorous.. ( Daddy I see a bunny rabbit- do you?- no darlin’ it looks more like an alligator to me!)
    If there is clustering of gravitationally unbound objects,then the likelihood of undetected brown dwarfs nearby is enhanced somewhat. I do think the number of BD’s on the diagram indicates the search is far from complete – the volume is ~33,000 cubic light years and the number does not go up as the cubeof the distance. we should have a good census of Red dwarfs etc.
    And so we wait.
    The Europeans are going to launch another 1.2 meter survey scope- Gaia, it has a detector just shy of a gigabit and thus will capture a really great survey in its planned 5 year mission. Not an IR scope though, but will see red dwarfs very clearly at 1,000 nm. ( nasa pitched the US version of this over the side to keep the shuttle flying. )

  • kzb July 21, 2011, 8:10

    According to the RECONS site, there are a lot of red dwarfs within 25pc still awaiting discovery:

    http://www.recons.org/published25.pdf

    Quote:
    <>

    Assuming the density of BD’s is the same as red dwarfs (c. 0.1/cu pc) gives us say 100 in 33,000 cu LY. I count 14 on the diagram.

    The apparent clustering: I don’t think there is clustering on such local scales in the galactic disk. The disk stellar population is modelled something like an ideal gas.

    There ARE of course star clusters, but they are special features. Disk stars are either thick disk or thin disk, the thick disk stars (e.g. Barnards star) having a higher velocity as they pass through the disk plane.

    A snapshot in time of a localised (i.e. small extent compared to disk scale height) stellar population ought to look much like a Poisson distribution. Yes there will be non-ideal gas behaviour caused by gravity between the particles (stars), but as long as the velocities are high compared to the gravity they experience I don’t know how important that is.

    If you were to go forward or backwards millions of years and take a snapshot, you’d probably visually discern clusters, but they’d have swapped stars from the current era and would be in different places.

    Bear in mind we also do not know the provenance of this chart. If the yellow star distances were entered in integer units of parsecs, plus the multiple stellar system issue noted above, that might explain it without going any further!

  • Rob Henry July 21, 2011, 21:29

    I often find unstated premises the most interesting part of the discussion. It seems that we have learnt something like “at our distance from the galactic centre average proper motions and densities are such that, clustering due to stellar gravitational well considerations are significant for stellar mass stars, but not for half stellar mass stars”. If not, much of the above does not make sense.

  • kzb July 25, 2011, 7:41

    Rob Henry: stars plotted at the same distance from the sun could be on opposite sides of the sun and not near each other at all.

    The plot appears to show SHELLS not clusters, which is difficult to explain, and certainly not by the mutual gravity theory.

  • Michael Simmons July 26, 2011, 1:34

    kbz: sorry I was talking about the second pass.
    Funding for this was done under the NEOWISE project. Check Wikipedia.

    “They could have hibernated it after the first pass. Waited a couple of years, then woken it up and done another pass.”
    I realised after I posted this that they probably could not have shut-down the coolant use and hence had to run the second pass straight away.
    That said I wonder how long they could hibernate WISE for and then wake it up and do another warm pass.
    Based on the images above I’m guessing 5 – 10 yrs is needed to make the proper motion really visible in the Wise images.

    BTW you can see the brown dwarfs mentioned in the paper in the Wise Data release.
    Goto http://irsa.ipac.caltech.edu/applications/wise/
    type in 17h41m24.25s +25d53m19.5s into the Name or Position text box and hit search down the bottom. Once the page has loaded, click the 3 colour tab.
    The other one is at 02h54m09.45s +02d23m59.1s

  • Michael Simmons July 26, 2011, 8:20

    From “A Search for High Proper Motion T Dwarfs with Pan-STARRS1 + 2MASS + WISE”
    http://arxiv.org/abs/1107.4608
    “The modest number of candidates from our search suggests that the immediate (~10 pc) solar neighborhood does not contain a large reservoir of undiscovered T dwarfs earlier than about T8. ‘

  • kzb July 26, 2011, 13:31

    @Michael Simmons: thanks for the link. Disappointingly it’s becoming probable that there are certainly not many, and quite possibly no, BD’s nearer than Alpha Cent.

    Still, that paper is based very much on proper motion rather than WISE spectral recognition. WISE data is very much secondary in the discovery process here, as I read it. Perhaps more will turn up using spectral analysis of the WISE images.

  • Rob Henry July 26, 2011, 20:05

    Kzb, your point about gravity being able to explain clustering, not shells looked to me to be a killer argument, then suddenly I realised that if two unbound stars fell into each others gravitational well to any degree that could cause a distortion in their statistical distribution, this might be caused by them speeding through any “close” approach. This could cause a statistical apparent ring of exclusion around solar mass stars that might explain the first shell as non-random; though trying to explain the second shell by extending this hand waving further would be pushing it.

  • kzb July 27, 2011, 12:59

    Rob Henry (and others)
    If you look at say Atlas of the Universe website, and go to the “Within 12.5 LY radius” scale, I don’t think you will discern any clustering by eye.

    To estimate the magnitude of any clustering effect by your mechanism, you need to find the average relative velocity of stars in the galactic neighbourhood (from memory this is say 100km/s) and see how much that is affected by mutual gravity at the average separation between systems (about 7 LY).

    My guess is not much. If the effect were significant all stars would collect together, and on local scales they do not. Also, stars are formed in clusters, but those clusters do not stay together for long. They get broken up by the galactic tide, spread out and their positions randomised.

    Papers modelling the stellar disk structure frequently use the language of gas physics, even using the words “heated” and “isothermal”. The stars are treated like molecules of a gas.

  • kzb July 27, 2011, 13:10

    Michael Simmons
    I read that paper last night. I keep coming back to my point about “Type 2 Error”. There are so many filters to the data in that piece of work that I think it is inevitable that a lot of BD’s were missed.
    ALL spectral type L would be filtered out.
    Bodies with low (or very high) proper motion would be filtered out. This includes bodies moving along our line of sight, rather than tangentially.

    My understanding is, that WISE can find BD’s solely from spectral data. Yes these have to be confirmed later, but the inititial filter is solely on colour. This paper uses proper motion as the initial filter.

  • Michael Simmons July 27, 2011, 23:30

    kzb
    I agree that we need to wait for the full wise data release. However I do think that their argument hints that a lot of BD’s won’t be found.

    >My understanding is, that WISE can find BD’s solely from spectral data.
    Well kinda… It samples in four bands. It doesn’t have a full spectrometer of any kind that I know of.

    This is my limited and possibly incorrect understanding. Take a black body radiation diagram and draw four lines across it. The different amplitudes represent the different brightnesses in the different WISE bands.
    In the case of a T dwarf, the peak is somewhere between WISE band 2 and band 3.
    So you go looking for objects who are brightest in those bands and fainter in bands 1 and 4.
    I guess you could try and do an exact brightness fit between black body curves of interest and the 4 bands.
    This would work much better if their where more bands, say 6 or 8.

  • Rob Henry July 28, 2011, 6:16

    Kzb, as I already stated your argument that the above diagram would only be powerful at finding shells not clusters cannot be faulted. If anything has been found it must thus be a shell not a cluster. I must also take issue with using the total speed of a star, rather than its typical velocity relative to is neighbours. This second speed should be much closer to 10 km/s than 100km/s.

    The gravitational-well of one star could only have any great effect on its nearest neighbours, and this will produce two effects that might seem contradictory, but are not. These are, that when a random vacant position is compared to a star filled one, we will see
    1) that a star will more frequently have a “very close” (but unbound) stellar neighbour than expected,
    2) that the average number of “close” neighbours is lower than expected.
    Here we hope that the second effect contains real significance only for sun-like stars.

    Each effect is easy to prove (though how significant they are is hard to prove, your proposal only being able to give suggestive results). I suggest forgoing formal proof and instead renormalising a chart of the closest sun-like stars to us around each of our closest neighbour, and see if one, or more shells also turns up in one of these.

  • kzb July 28, 2011, 13:11

    Rob Henry, yes you are absolutely right about the velocity, “100” was a misprint, I meant to put 10 km/s.

    Wouldn’t spherical shells around the sun put us in a “special position” ?

    Michael Simmons -I vacillate between thinking BD’s are unfortunately fairly rare, and then thinking about the shortcomings of detection methods to date.

    Somewhere I have a recent paper where detections that COULD be cool Y-dwarfs, but were not proven to be Y-dwarfs, were reported. If these detections are in fact Y-dwarfs, the space density would be higher than for “proper” stars. Don’t forget we also had the microlensing survey reporting lone planets twice as common as stars. So I still live in hope.

  • Michael Simmons July 28, 2011, 16:09

    >Wouldn’t spherical shells around the sun put us in a “special position” ?

    If it was a perfect spherical shell yes.
    If stars tended to be spaced out by a common distance (say 4ly) then wouldn’t you see more stars at multiples of that distance.
    You would need some kind of MOND (sinc function component?) or hidden repulsive dark matter between stars to have that…
    Gaia mission would reveal this.

    This site has a pretty cool 3d view of the nearby stars
    http://www.solstation.com/47ly-ns.htm

  • kzb July 29, 2011, 11:14

    That site does not convince me we are looking at anything other than a Poisson distribution.

    If you use the buttons to change the viewpoints, your brain discerns patterns from some angles that disappear when you switch view. The brain is hard-wired to try and detect patterns, which makes us detect patterns that are not really there, as we know.

  • Eniac July 30, 2011, 0:00

    Molecules in a liquid have shells in their distance distribution. The first shell is obvious, since no two molecules can be closer than their radius. But there are also second and third shells, because the first shell excludes the second, etc, etc. And no molecule is in a special position, despite the apparent “spherical” shells.

    Of course, stars are very different, and there is no obvious exclusion radius, but I must say even discounting binaries the distribution seems significantly off random, by eye. Rob’s suggestion that stars closer than 4 ly would accelerate, pass each other quickly, and thus cause a statistical exclusion is interesting, but probably will not survive closer scrutiny. 4 ly seems too large to have such an effect from gravity alone.

    The matter could readily be settled the way Rob proposes, by doing a distance distribution analysis across many stars, not just ours. It seems simple and worth the trouble, if it is not already done.