Disks around Young Brown Dwarfs

A Survey for Circumstellar Disks around Young Brown Dwarfs

Michael Liu (IfA/Hawaii)
Joan Najita (NOAO)
Alan Tokunaga (IfA/Hawaii)

Question

How do brown dwarfs form? While brown dwarfs are now being found in abundance, their origin remains a mystery. One important clue is whether these objects possess circumstellar disks when they are young. There is ample observational evidence for disks around young (few Myr old) Sun-like stars. Also from a theoretical perspective, disks are believed to be central to the formation and early evolution of solar-mass stars. However, much less is known at lower masses.

Survey

We have completed the first systematic survey for disks around young brown dwarfs and very low mass stars. We use IR observations (L'-band = 3.8 um) to search for the thermal emission from dust grains in the disk. Observing at long wavelengths is important since brown dwarf disks are expected to be cooler and less luminous than disks around T Tauri stars. However, our data are still sensitive enough to detect objects without any disks (i.e. bare substellar photospheres), and hence for the first time the disk frequency can be measured in an unbiased fashion.

Objects

Our targets reside in the well-known Taurus and IC 348 star-forming regions, which have estimated ages of a few (1-3) Myr. All of our targets have been spectroscopically confirmed to be very cool, with spectral types of M6 to M9.5. This corresponds to estimated masses of ~100 to ~15 Mjup.

Results

	We find that most (~75%) of the young brown dwarfs in our sample have disks. This plot shows the intrinsic (i.e. dereddened) IR colors as a function of spectral type (mass). Most of the objects show emission in excess of that from their bare photospheres, indicative of the presence of disks. Note that the T Tauri stars in these regions have a comparably high disk fraction.
	As expected from intuition and simple disk models, brown dwarf disks only produce modest IR excesses, and many would have been missed in conventional IR color-color analyses. Such analyses will be less reliable for young brown dwarfs than for T Tauri stars.
	Disk frequency appears to be independent of central mass. This plot is a histogram of IR excesses as a function of spectral type (i.e. mass). There is no significant difference the three largest mass bins. The exception is the lowest mass bin, in which the excesses are small/nonexistent.
	We find evidence that brown dwarf disks seem to have inner holes. This plot compares the observed IR excess distribution with two sets of disk models, one without inner holes and one with holes of ~2 R*. Disks w/o holes would produce more sources with large excess than are observed. (You have to read the paper to really understand this plot!)

Implications

The observed IR excesses can be explained by disk reprocessing of starlight alone. The implied accretion rates are at least an order of magnitude below that of T Tauri stars.

In the same star-forming regions, brown dwarf disks are at least as long-lived as stellar disks. This finding and the aforementioned high disk frequency are difficult to explain in formation scenarios involving dynamical interactions, e.g. disk-disk collisions or premature ejection.

Altogether the frequency and properties of circumstellar disks are similar from the stellar regime down to the brown dwarf and planetary-mass regime. This provides prima facie evidence of a common origin for most stars and brown dwarfs.

Learn more

You can retrieve our refereed paper from the astro-ph. This contains a full description of our observations, methods, analysis, examination of potential errors, and implications for brown dwarf formation.

Brief summaries are available as conference proceedings. These distill some of the principal findings, without much discussion of methods or implications. One proceedings discusses finding disks around young brown dwarfs. The other describes some of our results on the properties of circum(sub)stellar accretion disks.

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Last modified on October 24, 2002.