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Register a new userSpectroscopy of Brown Dwarf Candidates in the rho Ophiuchi Molecular Core
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We present an analysis of low resolution infrared spectra for 20 brown dwarf candidates in the core of the $rho$ Ophiuchi molecular cloud. Fifteen of the sources display absorption-line spectra characteristic of late-type stars. By comparing the depths of water vapor absorption bands in our candidate objects with a grid of M dwarf standards, we derive spectral types which are independent of reddening. Optical spectroscopy of one brown dwarf candidate confirms the spectral type derived from the water bands. Combining their spectral types with published near-infrared photometry, effective temperatures and bolometric stellar luminosities are derived enabling us to place our sample on the Hertzsprung-Russell diagram. We compare the positions of the brown dwarf candidates in this diagram with two sets of theoretical models in order to estimate their masses and ages. Considering uncertainties in placing the candidates in the H-R diagram, six objects consistently lie in the brown dwarf regime and another five objects lie in the transition region between stellar and substellar objects. The ages inferred for the sample are consistent with those derived for higher mass association members. Three of the newly identified brown dwarfs display infrared excesses at $lambda$=2.2 $mu$m suggesting that young brown dwarfs can have active accretion disks. Comparing our mass estimates of the brown dwarf candidates with those derived from photometric data alone suggests that spectroscopy is an essential component of investigations of the mass functions of young clusters.
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Molecular oxygen, O2 has been expected historically to be an abundant component of the chemical species in molecular clouds and, as such, an important coolant of the dense interstellar medium. However, a number of attempts from both ground and from space have failed to detect O2 emission. The work described here uses heterodyne spectroscopy from space to search for molecular oxygen in the interstellar medium. The Odin satellite carries a 1.1 m sub-millimeter dish and a dedicated 119 GHz receiver for the ground state line of O2. Starting in 2002, the star forming molecular cloud core rho Oph A was observed with Odin for 34 days during several observing runs. We detect a spectral line at v(LSR) = 3.5 km/s with dv(FWHM) = 1.5 km/s, parameters which are also common to other species associated with rho Ohp A. This feature is identified as the O2 (N_J = 1_1 - 1_0) transition at 118 750.343 MHz. The abundance of molecular oxygen, relative to H2,, is 5E-8 averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.
We have performed deep, wide-field imaging on a ~0.4 deg^2 field in the Pleiades (Melotte 22). The selected field was not yet target of a deep search for low mass stars and brown dwarfs. Our limiting magnitudes are R ~ 22mag and I ~ 20mag, sufficient to detect brown dwarf candidates down to 40MJ. We found 197 objects, whose location in the (I, R - I) color magnitude diagram is consistent with the age and the distance of the Pleiades. Using CTK R and I as well as JHK photometry from our data and the 2MASS survey we were able to identify 7 new brown dwarf candidates. We present our data reduction technique, which enables us to resample, calibrate, and co-add many images by just two steps. We estimate the interstellar extinction and the spectral type from our optical and the NIR data using a two-dimensional chi^22 fitting.
We present near-infrared (1.15-2.50 microns) medium-resolution (R = 1700) spectroscopy of a sample of 23 brown dwarf candidates in the young Upper Sco association. We confirm membership of 21 brown dwarfs based on their spectral shape, comparison with field dwarfs, and presence of weak gravity-sensitive features. Their spectral types range from M8 to L2 with an uncertainty of a subclass, suggesting effective temperatures between 2700 and 1800 K with an uncertainty up to 300 K and masses in the 30-8 Mjup range. Among the non-members, we have uncovered a field L2 dwarf at a distance of 120-140 pc, assuming that it is single. The success rate of our photometric selection based on five photometric passbands and complemented partly by proper motion is over 90%, a very promising result for future studies of the low-mass star and brown dwarf populations in young open clusters by the UKIDSS Galactic Cluster Survey. We observe a large dispersion in the magnitude versus spectral-type relation which is likely the result of the combination of several effects including age dispersion, extent and depth of the association, a high degree of multiplicity and the occurrence of disks.
We present the results of optical spectroscopy of 139 stars obtained with the Hydra multi-object spectrograph. The objects extend over a 1.3 square degree area surrounding the main cloud of the rho Oph complex. The objects were selected from narrowband images to have H alpha in emission. Using the presence of strong H alpha emission, lithium absorption, location in the Hertzsprung-Russell diagram, or previously reported x-ray emission, we were able to identify 88 objects as young stars associated with the cloud. Strong H alpha emission was confirmed in 39 objects with line widths consistent with their origin in magnetospheric accretion columns. Two of the strongest emission-line objects are young, x-ray emitting brown dwarf candidates with M8 spectral types. Comparisons of the bolometric luminosities and effective temperatures with theoretical models suggest a medianage for this population of 2.1 Myr which is signifcantly older than the ages derived for objects in the cloud core. It appears that these stars formed contemporaneously with low mass stars in the Upper Scorpius subgroup, likely triggered by massive stars in the Upper-Centaurus subgroup.
The rho Oph molecular cloud is undergoing intermediate-mass star formation. UV radiation from its hottest young stars heats and dissociates exposed layers, but does not ionize hydrogen. Only faint radiation from the Rayleigh-Jeans tail of ~10-100K dust is expected at wavelengths longwards of 3mm. Yet Cosmic Background Imager (CBI) observations reveal that the rho Oph W photo-dissociation region (PDR) is surprisingly bright at centimetre wavelengths. We searched for interpretations consistent with the WMAP radio spectrum, new ISO-LWS parallel mode images and archival Spitzer data. Dust-related emission mechanisms at 1 cm, as proposed by Draine & Lazarian, are a possibility. But a magnetic enhancement of the grain opacity at 1cm is inconsistent with the morphology of the dust column maps Nd and the lack of detected polarization. Spinning dust, or electric-dipole radiation from spinning very small grains (VSGs), comfortably explains the radio spectrum, although not the conspicuous absence from the CBI data of the infrared circumstellar nebulae around the B-type stars S1 and SR~3. Allowing for VSG depletion can marginally reconcile spinning dust with the data. As an alternative interpretation we consider the continuum from residual charges in rho Oph W, where most of carbon should be photoionised by the close binary HD147889 (B2IV, B3IV). Electron densities of ~100 cm^{-3}, or H-nucleus densities n_H > 1E6 cm^{-3}, are required to interpret rho Oph W as the CII Stromgren sphere of HD147889. However the observed steep and positive low-frequency spectral index would then require optically thick emission from an hitherto unobserved ensemble of dense clumps or sheets with a filling factor ~1E-4 and n_H ~ 1E7 cm^{-3}.
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