WISE finds fewer brown dwarfs than expected

[kzb]

WISE Finds Few Brown Dwarfs Close to Home

http://www.jpl.nasa.gov/news/news.cf...&msource=12164


Quote:
...about 33 brown dwarfs reside within 26 light-years of sun. There are 211 stars within this same volume of space, so that means there are about six stars for every brown dwarf.

So that's slightly disappointing, but probably not unexpected by now.

On the plus side, nearby free-floating Jupiters are not excluded by the WISE data processing to date. It's also said to be likely that further BDs will be found later, but not in large numbers.

[m74z00219]

WISE Finds Few Brown Dwarfs Close to Home

http://www.jpl.nasa.gov/news/news.cf...&msource=12164


Quote:
...about 33 brown dwarfs reside within 26 light-years of sun. There are 211 stars within this same volume of space, so that means there are about six stars for every brown dwarf.

So that's slightly disappointing, but probably not unexpected by now.

On the plus side, nearby free-floating Jupiters are not excluded by the WISE data processing to date. It's also said to be likely that further BDs will be found later, but not in large numbers.

If you were hoping for more brown dwarfs, I wouldn't quite despair just yet. The way I see it, much ambiguity remains.

The interesting points to consider:

1) WISE is not sensitive to hypothetical brown dwarfs below 5 Jupiter-Masses and effective temps less than 200 K (as you pointed out).

2) Burrows 2003 models indicate that a 10 Jupiter-Mass (or greater) brown dwarf has not had enough time to cool below 200 K, even if formed with the Milky Way. Such objects known to be about 10 Jupiter masses may in fact be brown dwarfs.

3) The old rule of thumb of ~13 Jupiter Masses being the lower mass cutoff for brown dwarfs is operational but not conclusive: we can use it to move forward, but the reality is that many objects maintain an ambiguous nature.

"We find that, while 13MJ is generally a reasonable rule of thumb, the deuterium fusion mass depends on the helium abundance, the initial deuterium abundance, the metallicity of the model, and on what fraction of an object’s initial deuterium abundance must combust in order for the object to qualify as having burned deuterium" (Spiegel 2011: arXiv:1008.5150v2).

Leading to a range of ~11 Jupiter-Masses to ~16 Jupiter-Masses. That is, the true threshold is not univariate, but multivariate.

If the range does really extend below 5 jupiter masses, as per Burrows 2003, then we'll have to wait and see. This also makes me wonder if any of the planets discovered so far are actually brown dwarfs. Particularly for such objects that are about ~10 Jupiter Masses with nothing else known about them.

There is also a philosophical issue: if we wish to use the ability to fuse deuterium as the boundary between planets and low mass brown dwarfs (this is the original notion that led to the ~13 Jupiter mass threshold), then where shall we draw the line? Should it be 50% of initial deuterium fused? -- 63.2%(1 - 1/e)? -- 90%?

Second philosophical issue: if brown dwarf, stellar, and planetary formation theories predict overlapping observational parameters, then should some objects be classified as "brown dwarf or planet" and "brown dwarf or low mass star"?

What of late-M dwarfs and L dwarfs?: according to models, objects of these classes may be brown dwarfs and they may be low-mass stars. We cannot determine dynamical masses for field objects, so determining whether a known late-M or L dwarf is a brown dwarf or low mass star is problematic.

A question: why do some astronomers insist on using "combust" and "burning" when speaking of deuterium and hydrogen fusion? As a physicist doing astronomy work, I find it grating.

M74

[SagittariusAStar]

Ken Croswell did a story on this back in May; not everyone accepts the new result, as you can see at http://KenCroswell.com/BrownDwarfNumbers.html .

[Kullat Nunu]

Not so surprising result considering how complicated the formation of brown dwarfs seems to be. Still, one would imagine the numbers increase as the mass decreases.

Had no idea how rare class L dwarfs actually are.

[kzb]

m74z00219 wrote

If you were hoping for more brown dwarfs, I wouldn't quite despair just yet. The way I see it, much ambiguity remains.
Can you explain a bit more? Most of your post appears to be about taxonomy or nomenclature, but the key point of the WISE press release is that substellar objects, whatever they are called, are rarer than some hoped for.

Or are you saying there is still hope because the data has not been processed in the correct way so far?

why do some astronomers insist on using "combust" and "burning" when speaking of deuterium and hydrogen fusion?

I guess it's similar to all elements heavier than helium being metals ! All scientific fields have their own dialects, but I agree this particular example is not helpful in explaining how stars get there energy to a non-scientific audience. It confuses them.

SagittariusAStar wrote:

Ken Croswell did a story on this back in May; not everyone accepts the new result

Yes but the trouble is, the astronomer who is quoted doubts the data the other way round, i.e he thinks the WISE BDs could be more distant than the WISE team say, and the space density could be as low as 1:20.

ANyhow, I believe the WISE team are given a head start on using the data. Perhaps even now, independent teams are processing the data using different cuts and criteria, and more stuff will turn up. Is this realistic?

[Kullat Nunu]

In the paper the authors suggest the number of cold brown dwarfs could be much higher than estimated if many of them formed in the early universe 10 billion years ago, but according to them there is no reason to think so.

That the results agree with young cluster frequencies strongly supports the observed frequency. But then again, the cluster frequency could be biased...

Anyway, somewhat disappointing result. A BD closer to Proxima Centauri (or nearly so) could have been really cool. If only there were an efficient way of detecting rogue planets...

[m74z00219]

m74z00219 wrote

If you were hoping for more brown dwarfs, I wouldn't quite despair just yet. The way I see it, much ambiguity remains.
Can you explain a bit more? Most of your post appears to be about taxonomy or nomenclature, but the key point of the WISE press release is that substellar objects, whatever they are called, are rarer than some hoped for.

Or are you saying there is still hope because the data has not been processed in the correct way so far?

Taxonomy is highly relevant. If theory is incorrect or unsettled or if you must deal with overlapping observables, then you are confronted with ambiguity. That ambiguity makes classification difficult and can invariably lead to incorrect classification.

I'm mainly saying that if we cannot in some situations say whether or not an object is a brown dwarf or a star or if in other cases an object is a brown dwarf or a planet then perhaps it's a bit premature to make definitive statements about the proportion of brown dwarfs to stars in the local volume.

I am in no way making a judgement of correctness, just pointing out relevant practical and philosophical issues.


M74

[EDG]

Do they determine and present the trigonometric parallaxes for these nearby BDs in the paper (along with RA and Dec)? I'm not interested in photometrically-derived parallaxes, those are rather inaccurate by comparison.

When they say there are 33 BDs within 26 ly of the sun, are these solo BDs or do they include ones that are companions of other stars (e.g. Epsilon Indi's distant BD pair)?

And is the paper available online for free anywhere (e.g. Arxiv?)

[m74z00219]

Do they determine and present the trigonometric parallaxes for these nearby BDs in the paper (along with RA and Dec)? I'm not interested in photometrically-derived parallaxes, those are rather inaccurate by comparison.

When they say there are 33 BDs within 26 ly of the sun, are these solo BDs or do they include ones that are companions of other stars (e.g. Epsilon Indi's distant BD pair)?

And is the paper available online for free anywhere (e.g. Arxiv?)

Hi EDG, I don't really have time to say anything intelligent, but here is a link to the paper in question: http://arxiv.org/abs/1205.2122

He is only including T and Y class dwarfs in his figure of 33 brown dwarfs, so I should think that this includes eps Indi Ba,Bb (T1,T6). Eps Indi (or GJ 845) is well within 8 parsecs (~26 ly) at 3.63 parsecs.

From page 17:

"L dwarfs are especially rare – only three examples are known within 8 pc – and represent a mix of old stars at the low-mass end of the stellar mass function and old brown dwarfs at the high-mass end of the substellar mass function13. The numbers of brown dwarfs then rises at later types – 22 known T dwarfs and 8 known Y dwarfs are thought to lie within this 8-pc volume."



M74

[Kullat Nunu]

The paper is available here.

[Kullat Nunu]

Do they determine and present the trigonometric parallaxes for these nearby BDs in the paper (along with RA and Dec)? I'm not interested in photometrically-derived parallaxes, those are rather inaccurate by comparison.

List of class T and Y BDs with trigonometric parallaxes are listed on pp. 52--53.

[SagittariusAStar]

SagittariusAStar wrote:

Ken Croswell did a story on this back in May; not everyone accepts the new result

Yes but the trouble is, the astronomer who is quoted doubts the data the other way round, i.e he thinks the WISE BDs could be more distant than the WISE team say, and the space density could be as low as 1:20.

That astronomer is an expert on the nearby stars and has seen numerous cases where stars were initially claimed to be much closer to the Sun than they really were. He suspects the same for many of these newfound brown dwarfs.

[EDG]

The paper is available here.

Thanks! Annoyingly though there's no RA and Dec values that I can see; there's "J2000 coordinates" which are shown as "xxxx+yyyy" - how does one convert that to normal RA and Dec? I guess I can probably find the coords on SIMBAD or something?

[m74z00219]

Thanks! Annoyingly though there's no RA and Dec values that I can see; there's "J2000 coordinates" which are shown as "xxxx+yyyy" - how does one convert that to normal RA and Dec? I guess I can probably find the coords on SIMBAD or something?

Actually, that is Ra and Dec, albeit abbreviated: hhmm+ddmm.


Most of the 8 pc objects are in Simbad, but not necessarily with the most up to date data. Wholly newly discovered objects may not be in SIMBAD or simply not matched up with corresponding identifiers from 2MASS, DENIS, or other.

WISEPA J182831.08+265037.8 for instance does not appear to be on Simbad. In any case, try Harvard's Simbad mirror. CDS has taken Simbad down for the day to, I think, replace or update their power supply.

http://simbak.cfa.harvard.edu/simbad/

M74

[Kullat Nunu]

Yeah, the full designation already includes the coordinates to sub-second level: for example, the coordinates for WISEP J182831.08+265037.8 are RA = 18h28m31.08s, Dec = +26°50'37.8" (2000 epoch).

[EDG]

Oh right... that makes sense (I suspected the numbers in the name might be coordinates) . Cool, so I can use this to update my near sol maps, maybe this will fill some of the gaps!

[kzb]

Not so surprising result considering how complicated the formation of brown dwarfs seems to be. Still, one would imagine the numbers increase as the mass decreases.

Had no idea how rare class L dwarfs actually are.

But, taking on board what m74z00219 says, is this actually an artifact of the taxonomy? In the sense of, maybe L is a narrow window along the mass and/or time dimensions? So rather than this dip in frequency at class L, if we plotted mass alone, maybe we'd see a smooth distribution instead?

[kzb]

I managed to half-read the paper last night. They actually have 534 candidates within 20pc, of which only a minority have been followed up with ground-based observations. To get on the candidate list, an object has to get past a considerable series of hurdles.

Also, an area of sky 6x20 degrees towards the galactic centre has been excluded. However I make this only 0.29% of the sky (is this correct?), so is unlikely to affect the result.

Once again I am struck by the care with which astronomers exclude false-positives, and wonder about the possibility of "Type 2" error, in other words, false-negatives.

Does anyone else think about this? Please be aware I realise it has to be like this: the last thing an astronomer wants is his or her claims of identifying an object being subject to later ridicule. I fully understand this.

[EDG]

But, taking on board what m74z00219 says, is this actually an artifact of the taxonomy? In the sense of, maybe L is a narrow window along the mass and/or time dimensions? So rather than this dip in frequency at class L, if we plotted mass alone, maybe we'd see a smooth distribution instead?

I'm not too surprised, because from what I understand the L dwarfs are going to be rarer because they represent the hotter brown dwarfs, which are by necessity also young (under a billion years old). IIRC the most massive BDs could even be mistaken for late M V stars if they're really young and still fusing deuterium (and lithium?) in their cores. I would expect that if we take a random sample of BDs then most would be older than a billion years, and therefore cooled down into the T type.

[whimsyfree]

Not so surprising result considering how complicated the formation of brown dwarfs seems to be.

AFAIK it isn't definitely resolved.

Still, one would imagine the numbers increase as the mass decreases.

That's the general way of the universe, but over some mass ranges there may be reversals. If BDs are the left tail of the stellar mass distribution, then there should be fewer BDs than regular stars. If BDs are the right tail of the planetary mass distribution, then there should be fewer BDs than planets.