[X] My Assistant

[Guest_Richard Trigaux_*]

I'm barely familiar with the literature, but it's out there. The main point is that low mass M and brown dwarf stars are much less abundant than simple extrapolation of brighter star abundances used to suggest.

I hope so, as the curves I already seen were increasing sharply with lower masses. If so, there would be such an abundance of brown dwarves that it could explain the dark matter. But searches of MACHOS (objects like brown, red or white dwarves) found essentially objects with half of the sun masses, probably red or white dwarves, and in much less quantity than expected to explain the dark matter. So objects like lone brown dwarves or lone planets (free in space) must be not so common than stars. But the theory predicted too that the IMF could go at such low masses than Earth. Also lonely planets could result from interaction between forming stars, or later of star encounters (especially in close clusters).

So we can still envision systems with small planets as primaries.

[ngunn]

Even if the number of low mass systems is much less than indicated by simple extrapolation that doesn't mean that the number actually decreases with decreasing mass. As we have several orders of magnitude of mass range to consider that still leaves the possibility of relatively abundant low-mass systems, even without considering the secondary formation processes such as ejection or collision that Richard mentions.

[Guest_Myran_*]

ngunn wrote: Even if the number of low mass systems is much less than indicated by simple extrapolation that doesn't mean that the number actually decreases with decreasing mass.

No there doesnt seem to be any decrease in number and I dont think it was edstrick's idea to suggest anything such either.
But there appear to be somewhat like a cutoff where the curve of even increasing numbers for lower mass objects flatten out. I say 'appear' since brown dwarfs are very hard to find. Even so, with out current knowledge it appears that brown dwarfs might not be as plentiful as some theoreticans first proposed.

[ngunn]

O.K. let's make the very conservative assumption that the distribution is completely flat - in other words for each decreasing order of magnitude there are the same number of systems. All the nearby visible stars fall within just 2 orders of magnitude, 0.1 to 10 Msun approx. From there downward we have 5 orders of magnitude to get to terrestrial planet sizes, so on the above assumption low mass systems of terrestrial mass or greater would still outnumber visible stars by 5:2. My guess is that this is a gross underestimate and I predict that over the next couple of decades we will start finding numerous low mass systems closer than Alpha Centauri - if we look for them.

[edstrick]

My *VAGUE* recollections of abstracts I've skimmed and PDF papers I've downloaded and glanced contains a phrase something like the "brown dwarf desert", where we're seeing very few BD's as planet-like companions of stars. I just don't know if the total abundance of BD's is less than the equivalent mass range of red dwarfs or what.

Xarchive.org has had many and various papers float by on relevent topics float by. Have at'm! (I currently just don't have enough time and "effort")

[ngunn]

Thnks for that phrase edstrick:
http://www.aas.org/publications/baas/v36n4/dps2004/163.htm
I wish I could see that histogram, but I suspect the word desert is really a bit of an overstatement -it's only a relative thing.

[ljk4-1]

Thnks for that phrase edstrick:
http://www.aas.org/publications/baas/v36n4/dps2004/163.htm
I wish I could see that histogram, but I suspect the word desert is really a bit of an overstatement -it's only a relative thing.

Here is the actual paper online:

http://arxiv.org/abs/astro-ph/0412356

[ngunn]

Here is the actual paper online:

http://arxiv.org/abs/astro-ph/0412356

EXCELLENT, thanks.

So: The 'brown dwarf desert' refers to objects in the range one tenth to one hundredth Msun, but applies only to the population of fairly close stellar companions with periods under 5 years. More generally the IMF (initial mass function) derived from cluster studies shows only a modest decrease in the continuing rise in numbers of objects at lower masses, and this decrease in the rate of increase could be due to observational selection anyhow. dN/d(log M) comes nowhere near to changing sign, as would be required to make the population of smaller objects lower than the population of visible stars. These observations do not constrain the number density of independent low-mass systems in the solar neighbourhood at all. In fact there could be thousands of them within 10 parsecs.

[Guest_Richard Trigaux_*]

Googling "Initial Mass functions" I found several articles of interest (among many others)

curve:
http://webast.ast.obs-mip.fr/hyperz/hyperz...ual1/node7.html
theory:
http://www.astro.caltech.edu/~george/ay20/Ay20-Lec17x.pdf.

From these it appears that the IMF is a power law. So that the number of zero mass stars would be infinite... in reality ther would be a cutoff somewhere, but it is rather a matter of conjecture where this cutoff is. What happens in the 0.1-0.5 solar mass range is already uncertain (some flattening seems to appear), below it is purely speculative. See the curves on the second link, p16, according to various hypothesis.


So theoricians still don't know today where is the baryonic mass, in red dwarves or into brown dwarves sytems, which then would be very numerous, down to some Jupiter size, or even smaller (interstellar KBOs??

So we cannot rule out the discovery of some brown dwarves systems even closer than Proxima Centauri. But in the same time we cfannot yet state that this must happen.

[alan]

From our construction and analysis of the near-infrared luminosity functions of the NGC 2362, IC 348 and Trapezium clusters, we find that these clusters display remarkably similar underlying mass functions, all forming broad peaks between 0.1 and 0.3 times the mass of the Sun; deeper observations of the latter two clusters reveal an IMF that turns over and declines throughout the brown dwarf regime. Thus, we find that brown dwarfs do not dominate stars either by number or total mass. Lastly, we use a statistically significant sample of candidate brown dwarfs to show that these objects appear as likely to have been born with circumstellar disks as stars. Combining this finding with the continuity of the shape of the initial mass function across numerous environments suggests that a single physical mechanism may dominate the star formation process.

http://www.aas.org/publications/baas/v34n4/aas201/679.htm

Gragh of log(N) vs log(M) in this paper
http://www.astro.psu.edu/~kluhman/BDreview.pdf

I've seen an estimate of brown dwarf population, based on 2MASS observations, as being roughly 2x the population of regular stars.

[edstrick]

OK.... There's an inflection in the log-log curve, not a actual decrease in numbers per fractional delta-mass. That's what I couldn't remember.

The Japanese, wkith their current infrared whole-sky-survey mission in orbit, may be able to dectect near solar-system BD's. They've got much higher resolution than IRAS did except at the very longest wavelengths (70 micrometer is on the long-wavelength side of even primordial BD emission blackbody curves, anyway)

I've hoped and hoped that we do find some BDs or rogue planets nearer than Alpha/Proxima Centauri. The biggest problem in the search is the stellar background we have to search against. Until we can search against the milky way background for high proper motion BD like objects with sensativity and resolution to capture everything more massive than Jupiter out to maybe 15 or so light years, we won't have an essentially complete local census.

[Guest_Richard Trigaux_*]

Thanks alan for your links, especially the second which is very interesting.

It is impressive to see how science advanced since my last post. From your second link, we can conclude:

-the Innitial Mass Function peaks into the red dwarves region, and then decreases into the brown dwarves region. The reason for this is still not clear, but observational evidences seem safe enough.

-The total number of brown dwarves may be great, but not much larger than of stars. The total mass of brown is not greater of that of normal stars.

-Brown dwarves have proven accretion disks qimilar to stars
-They may have planets too (not proven, but likely)


I should add:
-Brown dwarves are not numerous enough to be the dark matter

-brown dwarves are not so numerous that we may find many closer than Proxima Centauri. Discovering one is however far from impossible.

-Free planets lighter than Jupiter seem unlikely (unless they were ejected from a larger solar system. This may be rare in our region, but very common in dense star clusters)

At last some studies (see the thread on Pioneer effect) give narrow limits to the presence of massive objects near the solar system.

[ngunn]

Sorry to disagree Richard but these conclusions are far from secure.
The last paragraph of the SUMMARY AND DISCUSSION section of Grether and Linewaver, May 27 2006 posted by ljk4-1 yesterday clearly refers to "The fact that there is a close orbiting brown dwarf desert but no free floating brown dwarf desert . . ." So at the very least the jury is still out on this. Furthermore, we have ABSOLUTELY NO IDEA what happens to the numbers below 0.01 Msun. I cannot believe that the mass function conveniently terminates just at the point where the objects cease to be visible. So I'm sticking with my hypothesis of many small objects closer than Alpha Centauri for now. However I am not suggesting that they outweigh the stars in terms of total mass.

My case is : Don't rule them out or their continuing non-discovery could become a self-fulfilling prophecy over the next decade or two.

[Guest_Richard Trigaux_*]

I (or rather the study I quote) never said that there are NONE, I (the study) just say that they are much less than we could predict with simply extrapolating the IMF. Progress was made into the determination of the IMF, and the cutoff and decreasing slope was found (thantks to studying recent clusters where Brown dwarves are much more luminous than in older ones). So the more numerous category is the red dwarves category, not the brown dwarves. And, about very low masses (3 Jup or less) we still don't know, we cannot exclude the IMF having a second peak for some reason. But it is not very likely.

To have several brown dwarves closer than Proxima Centauri would require that, statistically, the brown dwarves would be much more numerous than the red dwarves, which seems not the case. Of course we could be lucky and find one, and perhaps several. But the statistical average gives closest brown dwarf rather in the 10- 20 light years. Unless of course you disagree with the study.

[ngunn]

I suppose what I'm saying is this:
1/ The different studies lead to very different conclusions about free floating brown dwarf numbers, although this is not the main focus of either study.
2/ Both agree that brown dwarfs are relatively more common among the free-floating population than they are as close orbiting companions to stars.
3/ Free-floating brown dwarfs are harder to find than brown dwarf companions, so the total observed sample is skewed in the direction of the (relatively scarce) brown dwarf companions.
4/ The smaller they are the harder they are to find so there is a very strong effect of observational selection at the low mass end which is difficult to quantify properly.
5/ There are at least some very loosely bound brown dwarf binaries (30-100 AU)
6/ All the observable brown dwarfs occupy only about one order of magnitude in mass.
7/ I am not saying brown dwarfs outnumber stars - only that low mass systems probably do, spanning at least 4 orders of magnitude below the presently detectable brown dwarfs.
8/ Low mass binary or multiple systems (30-100AU) would be dynamically the easiest places from which the sun could capture an object like Sedna.
9/ We need to be on the lookout, not only for more 'Sednas', but also for dark interstellar low-mass systems as soon as we have the means to search for these very difficult observational targets. If there are more objects in one category there are likely to be more in the other.
10/ We are still literally in the dark about all this but new developments are about to expand our observational powers enormously. That makes it an especially good time to keep an open mind while we look for what is there and what is not.