Low-Mass Star Formation and the Initial Mass Function in the Rho Ophiuchi Cloud Core

We have obtained moderate-resolution (R=800-1200) K-band spectra for ~100 stars within and surrounding the cloud core of rho Oph. We have measured spectral types and continuum veilings and have combined this information with results from new deep imaging. The IMF peaks at about 0.4 M_sun and slowly declines to the hydrogen burning limit with a slope of ~-0.5 in logarithmic units (Salpeter is +1.35). Our lower limits on the numbers of substellar objects demonstrate that the IMF probably does not fall more steeply below the hydrogen burning limit, at least down to ~0.02 M_sun. We then make the first comparison of mass functions of stars and pre-stellar clumps (Motte, Andre, & Neri) measured in the same region. The similar behavior of the two mass functions in rho Oph supports the suggestion of Motte et al. and Testi & Sargent that the stellar mass function in young clusters is a direct product of the process of cloud fragmentation. After considering the effect of extinction on the SED classifications of the sample, we find that ~17% of the rho Oph stars are Class I, implying ~0.1 Myr for the lifetime of this stage. In spectra separated by two years, we observe simultaneous variability in the Br gamma emission and K-band continuum veiling for two stars, where the hydrogen emission is brighter in the more heavily veiled data. This behavior indicates that the disk may contribute significantly to continuous K-band emission, in contrast to the proposal that the infalling envelope always dominates. Our detection of strong 2 micron veiling (r_K=1-4) in several Class II and III stars, which should have disks but little envelope material, further supports this proposition.Comment: 35 pages, 14 figures, accepted to Ap

An L-type substellar object in Orion: reaching the mass boundary between brown dwarfs and giant planets

We present J-band photometry and low-resolution optical spectroscopy (600-1000 nm) for one of the faintest substellar member candidates in the young sigma Ori cluster, SOri 47 (I=20.53, Bejar et al. 1999). Its very red (I-J)=3.3+/-0.1 color and its optical spectrum allow us to classify SOri 47 as an L1.5-type object which fits the low-luminosity end of the cluster photometric and spectroscopic sequences. It also displays atmospheric features indicative of low gravity such as weak alkaline lines and hydride and oxide bands, consistent with the expectation for a very young object still undergoing gravitational collapse. Our data lead us to conclude that SOri 47 is a true substellar member of the sigma Ori cluster. Additionally, we present the detection of LiI in its atmosphere which provides an independent confirmation of youth and substellarity. Using current theoretical evolutionary tracks and adopting an age interval of 1-5 Myr for the sigma Ori cluster, we estimate the mass of SOri 47 at 0.015+/-0.005 Msun, i.e. at the minimum mass for deuterium burning, which has been proposed as a definition for the boundary between brown dwarfs and giant planets. SOri 47 could well be the result of a natural extension of the process of cloud fragmentation down to the deuterium burning mass limit; a less likely alternative is that it has originated from a protoplanetary disc around a more massive cluster member and later ejected from its orbit due to interacting effects within this rather sparse (~12 objects/pc^3) young cluster.Comment: 9 pages, 3 figures, accepted for publication in ApJ Letter

Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud

We present the ROSAT All-Sky Survey data in a 126 deg^2 area in and around the CrA star forming region. With low-resolution spectroscopy of unidentified ROSAT sources we could find 19 new pre-main sequence stars, two of which are classical T Tauri stars, the others being weak-lined. The spectral types of these new T Tauri stars range from F7 to M6. The two new classical T Tauri stars are located towards two small cloud-lets outside of the main CrA cloud. They appear to be ~10 Myrs old, by comparing their location in the H-R diagram with isochrones for an assumed distance of 130 pc, the distance of the main CrA dark cloud. The new off-cloud weak-line T Tauri stars may have formed in similar cloud-lets, which have dispersed recently. High-resolution spectra of our new T Tauri stars show that they have significantly more lithium absorption than zero-age main-sequence stars of the same spectral type, so that they are indeed young. From those spectra we also obtained rotational and radial velocities. For some stars we found the proper motion in published catalogs. The direction and velocity of the 3D space motion - south relative to the galatic plane - of the CrA T Tauri stars is consistent with the dark cloud being formed originally by a high-velocity cloud impact onto the galactic plane, which triggered the star formation in CrA. We also present VRIJHK photometry for most of the new T Tauri stars to derive their luminosities, ages, and masses.Comment: A&A Suppl. in pres

A Survey for Circumstellar Disks Around Young Substellar Objects

(Abridged) We have completed the first systematic survey for disks around spectroscopically identified young brown dwarfs and very low mass stars. We have obtained L'-band (3.8 um) imaging for 38 very cool objects in IC 348 and Taurus. Our targets span spectral types from M6 to M9.5 (~100 to ~15 Mjup). Using the objects' measured spectral types and extinctions, we find that most of our sample (77%+/-15%) possess intrinsic IR excesses, indicative of disks. Because the excesses are modest, conventional analyses using only IR colors would have missed most of the sources with excesses. The observed IR excesses are correlated with Halpha emission, consistent with a common accretion disk origin. The excesses can be explained by disk reprocessing of starlight alone; the implied accretion rates are at least an order of magnitude below typical values for classical T Tauri stars. The observed distribution of IR excesses suggests the presence of inner disk holes. The disk frequency appears to be independent of the mass and age. In the same star-forming regions, disks around brown dwarfs are at least as long-lived (~3 Myr) as disks around the T Tauri stars. Altogether, the frequency and properties of young circumstellar disks appear to be similar from the stellar regime down to the substellar and planetary-mass regime. This provides prima facie evidence of a common origin for most stars and brown dwarfs.Comment: ApJ, in press, 28 pages. Minor change to the online, abridged version of the abstract. No change to the actual pape

Mass Flows in Cometary UCHII Regions

High spectral and spatial resolution, mid-infrared fine structure line observations toward two ultracompact HII (UCHII) regions (G29.96 -0.02 and Mon R2) allow us to study the structure and kinematics of cometary UCHII regions. In our earlier study of Mon R2, we showed that highly organized mass motions accounted for most of the velocity structure in that UCHII region. In this work, we show that the kinematics in both Mon R2 and G29.96 are consistent with motion along an approximately paraboloidal shell. We model the velocity structure seen in our mapping data and test the stellar wind bow shock model for such paraboloidal like flows. The observations and the simulation indicate that the ram pressures of the stellar wind and ambient interstellar medium cause the accumulated mass in the bow shock to flow along the surface of the shock. A relaxation code reproduces the mass flow's velocity structure as derived by the analytical solution. It further predicts that the pressure gradient along the flow can accelerate ionized gas to a speed higher than that of the moving star. In the original bow shock model, the star speed relative to the ambient medium was considered to be the exit speed of ionized gas in the shell.Comment: 34 pages, including 14 figures and 1 table, to be published in ApJ, September 200

First Detection of Millimeter Dust Emission from Brown Dwarf Disks

We report results from the first deep millimeter continuum survey targeting Brown Dwarfs (BDs). The survey led to the first detection of cold dust in the disks around two young BDs (CFHT-BD-Tau 4 and IC348 613), with deep JCMT and IRAM observations reaching flux levels of a few mJy. The dust masses are estimated to be a few Earth masses assuming the same dust opacities as usually applied to TTauri stars.Comment: 5 pages, accepted for ApJ

The Abundance of Interstellar Nitrogen

Using the HST Goddard High Resolution Spectrograph (GHRS), we have obtained high S/N echelle observations of the weak interstellar N I 1160, 1161 A absorption doublet toward the stars Gamma Cas, Lambda Ori, Iota Ori, Kappa Ori, Delta Sco, and Kappa Sco. In combination with a previous GHRS measurement of N I toward Zeta Oph, these new observations yield a mean interstellar gas phase nitrogen abundance (per 10$^6$ H atoms) of 10$^6$ N/H = 75 +/- 4. There are no statistically significant variations in the measured N abundances from sightline to sightline and no evidence of density-dependent depletion from the gas-phase. Since N is not expected to be depleted much into dust grains in these diffuse sightlines, its gas-phase abundance should reflect the total interstellar abundance. Consequently, the GHRS observations imply that the abundance of interstellar nitrogen (gas plus grains) in the local Milky Way is about 80% of the solar system value of 10$^6$ N/H = 93 +/- 16. Although this interstellar abundance deficit is somewhat less than that recently found for oxygen and krypton with GHRS, the solar N abundance and the N I oscillator strengths are too uncertain to definitively rule out either a solar ISM N abundance or a 2/3 solar ISM N abundance similar to that of O and Kr.Comment: 14 pages, LaTeX, 2 Postscript figures; ApJ Letters, in pres

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. XI. Lupus Observed With IRAC and MIPS

We present c2d Spitzer/IRAC observations of the Lupus I, III and IV dark clouds and discuss them in combination with optical and near-infrared and c2d MIPS data. With the Spitzer data, the new sample contains 159 stars, 4 times larger than the previous one. It is dominated by low- and very-low mass stars and it is complete down to M $\approx$ 0.1M$_\odot$. We find 30-40 % binaries with separations between 100 to 2000 AU with no apparent effect in the disk properties of the members. A large majority of the objects are Class II or Class III objects, with only 20 (12%) of Class I or Flat spectrum sources. The disk sample is complete down to ``debris''-like systems in stars as small as M $\approx$ 0.2 M$_\odot$ and includes sub-stellar objects with larger IR excesses. The disk fraction in Lupus is 70 -- 80%, consistent with an age of 1 -- 2 Myr. However, the young population contains 20% optically thick accretion disks and 40% relatively less flared disks. A growing variety of inner disk structures is found for larger inner disk clearings for equal disk masses. Lupus III is the most centrally populated and rich, followed by Lupus I with a filamentary structure and by Lupus IV, where a very high density core with little star-formation activity has been found. We estimate star formation rates in Lupus of 2 -- 10 M$_\odot$ Myr$^{-1}$ and star formation efficiencies of a few percent, apparently correlated with the associated cloud masses.Comment: Accepted for publication in the ApJS. Contains 101 pages, 23 figures, and 13 tables. A version with full resolution figures can be found at http://peggysue.as.utexas.edu/SIRTF/PAPERS/pap102.pub.pd

An Interaction of a Magellanic Leading Arm High Velocity Cloud with the Milky Way Disk

The Leading Arm of the Magellanic System is a tidally formed HI feature extending \sim 60\arcdeg from the Magellanic Clouds ahead of their direction of motion. Using atomic hydrogen (HI) data from the Galactic All Sky-Survey (GASS), supplemented with data from the Australia Telescope Compact Array, we have found evidence for an interaction between a cloud in the Leading Arm and the Galactic disk where the Leading Arm crosses the Galactic plane. The interaction occurs at velocities permitted by Galactic rotation, which allows us to derive a kinematic distance to the cloud of 21 kpc, suggesting that the Leading Arm crosses the Galactic Plane at a Galactic radius of $R\approx 17$ kpc.Comment: 14 pages, 5 figures, accepted to Astrophysical Journal Letters. Full resolution version available at ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/LeadingArm_apjl.pd