We present the results of a search for companions to young brown dwarfs in the Taurus and Chamaeleon I star forming regions (1/2-3 Myr). We have used WFPC2 on board HST to obtain F791W and F850LP images of 47 members of these regions that have spectr
al types of M6-L0 (0.01-0.1 Msun). An additional late-type member of Taurus, FU Tau (M7.25+M9.25), was also observed with adaptive optics at Keck Observatory. We have applied PSF subtraction to the primaries and have searched the resulting images for objects that have colors and magnitudes that are indicative of young low-mass objects. Through this process, we have identified promising candidate companions to 2MASS J04414489+2301513 (rho=0.105/15 AU), 2MASS J04221332+1934392 (rho=0.05/7 AU), and ISO 217 (rho=0.03/5 AU). We reported the discovery of the first candidate in a previous study, showing that it has a similar proper motion as the primary through a comparison of astrometry measured with WFPC2 and Gemini adaptive optics. We have collected an additional epoch of data with Gemini that further supports that result. By combining our survey with previous high-resolution imaging in Taurus, Chamaeleon, and Upper Sco (10 Myr), we measure binary fractions of 14/93 = 0.15+0.05/-0.03 for M4-M6 (0.1-0.3 Msun) and 4/108 = 0.04+0.03/-0.01 for >M6 (<0.1 Msun) at separations of >10 AU. Given the youth and low density of these three regions, the lower binary fraction at later types is probably primordial rather than due to dynamical interactions among association members. The widest low-mass binaries (>100 AU) also appear to be more common in Taurus and Chamaeleon than in the field, which suggests that the widest low-mass binaries are disrupted by dynamical interactions at >10 Myr, or that field brown dwarfs have been born predominantly in denser clusters where wide systems are disrupted or inhibited from forming.
We have performed a census of disks around brown dwarfs in the Sigma Ori cluster using all available images from IRAC onboard the Spitzer Space Telescope. To search for new low-mass cluster members with disks, we have measured photometry for all sour
ces in the Spitzer images and have identified the ones that have red colors that are indicative of disks. We present 5 promising candidates, which may consist of 2 brown dwarfs, 2 stars with edge-on disks, and a low-mass protostar if they are bona fide members. Spectroscopy is needed to verify the nature of these sources. We have also used the Spitzer data to determine which of the previously known probable members of Sigma Ori are likely to have disks. By doing so, we measure disk fractions of ~40% and ~60% for low-mass stars and brown dwarfs, respectively. These results are similar to previous estimates of disk fractions in IC 348 and Cha I, which have roughly the same median ages as Sigma Ori (3 Myr). Finally, we note that our photometric measurements and the sources that we identify as having disks differ significantly from those of other recent studies that analyzed the same Spitzer images. For instance, previous work has suggested that the T dwarf S Ori 70 is redder than typical field dwarfs, which has been cited as possible evidence of youth and cluster membership. However, we find that this object is only slightly redder than the reddest field dwarfs in [3.6]-[4.5] (1.56+/-0.07 vs. 0.93-1.46). We measure a larger excess in [3.6]-[5.8] (1.75+/-0.21 vs. 0.87-1.19), but the flux at 5.8um may be overestimated because of the low signal-to-noise ratio of the detection. Thus, the Spitzer data do not offer strong evidence of youth and membership for this object, which is the faintest and coolest candidate member of Sigma Ori that has been identified to date.
Forty new low mass members with spectral types ranging from M4-M9 have been confirmed in the Orion Molecular Cloud 2/3 region. Through deep, I, z, J, H, K photometry of a 20 x 20 field in OMC 2/3, we selected brown dwarf candidates for follow-up spec
troscopy. Low resolution far-red and near-infrared spectra were obtained for the candidates, and 19 young brown dwarfs in the OMC 2/3 region are confirmed. They exhibit spectral types of M6.5-M9, corresponding to approximate masses of 0.075-0.015 M_solar using the evolutionary models of Baraffe et al. (1998). At least one of these bona fide young brown dwarfs has strong Halpha emission, indicating that it is actively accreting. In addition, we confirm 21 new low mass members with spectral types of M4-M6, corresponding to approximate masses of 0.35-0.10 M_solar in OMC 2/3. By comparing pre-main sequence tracks to the positions of the members in the H-R diagram, we find that most of the brown dwarfs are less than 1 Myr, but find a number of low mass stars with inferred ages greater than 3 Myr. The discrepancy in the stellar and substellar ages is due to our selection of only low luminosity sources; however, the presence of such objects implies the presence of an age spread in the OMC 2/3 region. We discuss possible reasons for this apparent age spread.
We present observations of a circumstellar disk that is inclined close to edge-on around a young brown dwarf in the Taurus star-forming region. Using data obtained with SpeX at the NASA Infrared Telescope Facility, we find that the slope of the 0.8-2
.5 um spectrum of the brown dwarf 2MASS J04381486+2611399 cannot be reproduced with a photosphere reddened by normal extinction. Instead, the slope is consistent with scattered light, indicating that circumstellar material is occulting the brown dwarf. By combining the SpeX data with mid-IR photometry and spectroscopy from the Spitzer Space Telescope and previously published millimeter data from Scholz and coworkers, we construct the spectral energy distribution for 2MASS J04381486+2611399 and model it in terms of a young brown dwarf surrounded by an irradiated accretion disk. The presence of both silicate absorption at 10 um and silicate emission at 11 um constrains the inclination of the disk to be ~70 deg, i.e. ~20 deg from edge-on. Additional evidence of the high inclination of this disk is provided by our detection of asymmetric bipolar extended emission surrounding 2MASS J04381486+2611399 in high-resolution optical images obtained with the Hubble Space Telescope. According to our modeling for the SED and images of this system, the disk contains a large inner hole that is indicative of a transition disk (R_in~58 R_star~0.275 AU) and is somewhat larger than expected from embryo ejection models (R_out=20-40 AU vs. R_out<10-20 AU).
We present a Spitzer based census of the IC 348 nebula and embedded star cluster. Our Spitzer census supplemented by ground based spectra has added 42 class II T-Tauri sources to the cluster membership and identified ~20 class 0/I protostars. The pop
ulation of IC 348 likely exceeds 400 sources after accounting statistically for unidentified diskless members. Our Spitzer census of IC 348 reveals a population of protostars that is anti-correlated spatially with the T-Tauri members, which comprise the centrally condensed cluster around a B star. The protostars are instead found mostly at the cluster periphery about 1 pc from the B star and spread out along a filamentary ridge. We find that the star formation rate in this protostellar ridge is consistent with that rate which built the exposed cluster while the presence of fifteen cold, starless, millimeter cores intermingled with this protostellar population indicates that the IC 348 nebula has yet to finish forming stars. We show that the IC 348 cluster is of order 3-5 crossing times old, and, as evidenced by its smooth radial profile and confirmed mass segregation, is likely relaxed. While it seems apparent that the current cluster configuration is the result of dynamical evolution and its primordial structure has been erased, our findings support a model where embedded clusters are built up from numerous smaller sub-clusters. Finally, the results of our Spitzer census indicate that the supposition that star formation must progress rapidly in a dark cloud should not preclude these observations that show it can be relatively long lived.
