Current Star Formation in the Ophiuchus and Perseus Molecular Clouds: Constraints and Comparisons from Unbiased Submillimeter and Mid-Infrared Surveys. II

We present a census of the population of deeply embedded young stellar objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination of Spitzer Space Telescope mid-infrared data from the "Cores to Disks" (c2d) legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have applied a method developed for identifying embedded protostars in Perseus to these datasets and in this way construct a relatively unbiased sample of 27 candidate embedded protostars with envelopes more massive than our sensitivity limit (about 0.1 M_sun). Embedded YSOs are found in 35% of the SCUBA cores - less than in Perseus (58%). On the other hand the mid-infrared sources in Ophiuchus have less red mid-infrared colors, possibly indicating that they are less embedded. We apply a nearest neighbor surface density algorithm to define the substructure in each of the clouds and calculate characteristic numbers for each subregion - including masses, star formation efficiencies, fraction of embedded sources etc. Generally the main clusters in Ophiuchus and Perseus (L1688, NGC1333 and IC348) are found to have higher star formation efficiencies than small groups such as B1, L1455 and L1448, which on the other hand are completely dominated by deeply embedded protostars. We discuss possible explanations for the differences between the regions in Perseus and Ophiuchus, such as different evolutionary timescales for the YSOs or differences, e.g., in the accretion in the two clouds.Comment: Accepted for publication in ApJ (56 pages, 13 figures; abstract abridged). Version with full-resolution figures available at http://www.astro.uni-bonn.de/~jes/paper120.pd

Mid-infrared Imaging of a Circumstellar Disk Around HR 4796: Mapping the Debris of Planetary Formation

We report the discovery of a circumstellar disk around the young A0 star, HR 4796, in thermal infrared imaging carried out at the W.M. Keck Observatory. By fitting a model of the emission from a flat dusty disk to an image at lambda=20.8 microns, we derive a disk inclination, i = 72 +6/-9 deg from face on, with the long axis of emission at PA 28 +/-6 deg. The intensity of emission does not decrease with radius as expected for circumstellar disks but increases outward from the star, peaking near both ends of the elongated structure. We simulate this appearance by varying the inner radius in our model and find an inner hole in the disk with radius R_in = 55+/-15 AU. This value corresponds to the radial distance of our own Kuiper belt and may suggest a source of dust in the collision of cometesimals. By contrast with the appearance at 20.8 microns, excess emission at lambda = 12.5 microns is faint and concentrated at the stellar position. Similar emission is also detected at 20.8 microns in residual subtraction of the best-fit model from the image. The intensity and ratio of flux densities at the two wavelengths could be accounted for by a tenuous dust component that is confined within a few AU of the star with mean temperature of a few hundred degrees K, similar to that of zodiacal dust in our own solar system. The morphology of dust emission from HR 4796 (age 10 Myr) suggests that its disk is in a transitional planet-forming stage, between that of massive gaseous proto-stellar disks and more tenuous debris disks such as the one detected around Vega.Comment: 9 pages, 4 figures as LaTex manuscript and postscript files in gzipped tar file. Accepted for publication in Astrophysical Journal Letters. http://upenn5.hep.upenn.edu/~davidk/hr4796.htm

A Survey for Low-mass Stars and Brown Dwarfs in the Upper-Scorpius OB Association

The Upper-Scorpius association is the OB association nearest to the Sun (145 pc). Its young age (5 Myr) makes it an ideal place to search for low-mass stars and brown dwarfs, as these objects should be relatively bright. We have performed a photometric search for the low-mass members of the association, using the R, I, and Z filters. The completeness limit is I=18.5 and the saturation limit is I=13. We obtain 138 candidate members, covering nearly the entire M spectral type range. We find an excess of brown dwarf candidates over the number predicted by a Miller-Scalo Initial Mass Function. In addition, we have performed infrared imaging and low resolution optical spectroscopy of selected candidates. We find that the infrared observations confirm the spectral types obtained with the optical photometry. Furthermore, we find H_alpha in emission in 20 of the 22 objects observed spectroscopically. As H_alpha is an indicator of youth, we believe that these 20 objects may belong to the association. One of them, UScoCTIO 128 has a very strong and constant H_alpha line (equivalent width: -130 A), and its position in the color magnitude diagram suggests that it is a brown dwarf with mass equal to 0.02 msun. Confirmation of this and the other candidates will have to wait for higher resolution observations that can reveal spectroscopic mass indicators like Li I and gravity indicators, such as K I and the subordinate lines of Na I.Comment: 20 pages, 7 figures, 3 tables, accepted in the Astronomical Journa

The Wide Brown Dwarf Binary Oph 1622-2405 and Discovery of A Wide, Low Mass Binary in Ophiuchus (Oph 1623-2402): A New Class of Young Evaporating Wide Binaries?

We imaged five objects near the star forming clouds of Ophiuchus with the Keck Laser Guide Star AO system. We resolved Allers et al. (2006)'s #11 (Oph 16222-2405) and #16 (Oph 16233-2402) into binary systems. The #11 object is resolved into a 243 AU binary, the widest known for a very low mass (VLM) binary. The binary nature of #11 was discovered first by Allers (2005) and independently here during which we obtained the first spatially resolved R~2000 near-infrared (J & K) spectra, mid-IR photometry, and orbital motion estimates. We estimate for 11A and 11B gravities (log(g)>3.75), ages (5+/-2 Myr), luminosities (log(L/Lsun)=-2.77+/-0.10 and -2.96+/-0.10), and temperatures (Teff=2375+/-175 and 2175+/-175 K). We find self-consistent DUSTY evolutionary model (Chabrier et al. 2000) masses of 17+4-5 MJup and 14+6-5 MJup, for 11A and 11B respectively. Our masses are higher than those previously reported (13-15 MJup and 7-8 MJup) by Jayawardhana & Ivanov (2006b). Hence, we find the system is unlikely a ``planetary mass binary'', (in agreement with Luhman et al. 2007) but it has the second lowest mass and lowest binding energy of any known binary. Oph #11 and Oph #16 belong to a newly recognized population of wide (>100 AU), young (<10 Myr), roughly equal mass, VLM stellar and brown dwarf binaries. We deduce that ~6+/-3% of young (<10 Myr) VLM objects are in such wide systems. However, only 0.3+/-0.1% of old field VLM objects are found in such wide systems. Thus, young, wide, VLM binary populations may be evaporating, due to stellar encounters in their natal clusters, leading to a field population depleted in wide VLM systems.Comment: Accepted version V2. Now 13 pages longer (45 total) due to a new discussion of the stability of the wide brown dwarf binary population, new summary Figure 17 now included, Astrophysical Journal 2007 in pres

Pulsar Parallaxes at 5 GHz with the Very Long Baseline Array

We present the first pulsar parallaxes measured with phase-referenced pulsar VLBI observations at 5 GHz. Due to the steep spectra of pulsars, previous astrometric measurements have been at lower frequencies. However, the strongest pulsars can be observed at 5 GHz, offering the benefit of lower combined ionospheric and tropospheric phase errors, which usually limit VLBI astrometric accuracy. The pulsars B0329+54, B0355+54 and B1929+10 were observed for 7 epochs spread evenly over 2 years. For B0329+54, large systematic errors lead to only an upper limit on the parallax (pi < 1.5 mas). A new proper motion and parallax were measured for B0355+54 (pi = 0.91 +- 0.16 mas), implying a distance of 1.04+0.21-0.16 kpc and a transverse velocity of 61+12-9 km/s. The parallax and proper motion for B1929+10 were significantly improved (pi = 2.77 +- 0.07 mas), yielding a distance of 361+10-8 pc and a transverse velocity of 177+4-5 km/s. We demonstrate that the astrometric errors are correlated with the angular separation between the phase reference calibrator and the target source, with significantly lower errors at 5 GHz compared to 1.6 GHz. Finally, based on our new distance determinations for B1929+10 and B0355+54, we derive or constrain the luminosities of each pulsar at high energies. We show that, for thermal emission models, the emitting area for X-rays from PSR B1929+10 is roughly consistent with the canonical size for a heated polar cap, and that the conversion of spin-down power to gamma-ray luminosity in B0355+54 must be low. The new proper motion for B1929+10 also implies that its progenitor is unlikely to have been the binary companion of the runaway O-star zeta-Ophiuchi.Comment: 8 pages, including 3 figures and 3 tables; emulateapj; ApJ submitte

Star formation environments and the distribution of binary separations

We have carried out K-band speckle observations of a sample of 114 X-ray selected weak-line T Tauri stars in the nearby Scorpius-Centaurus OB association. We find that for binary T Tauri stars closely associated to the early type stars in Upper Scorpius, the youngest subgroup of the OB association, the peak in the distribution of binary separations is at 90 A.U. For binary T Tauri stars located in the direction of an older subgroup, but not closely associated to early type stars, the peak in the distribution is at 215 A.U. A Kolmogorov-Smirnov test indicates that the two binary populations do not result from the same distibution at a significance level of 98%. Apparently, the same physical conditions which facilitate the formation of massive stars also facilitate the formation of closer binaries among low-mass stars, whereas physical conditions unfavorable for the formation of massive stars lead to the formation of wider binaries among low-mass stars. The outcome of the binary formation process might be related to the internal turbulence and the angular momentum of molecular cloud cores, magnetic field, the initial temperature within a cloud, or - most likely - a combination of all of these. We conclude that the distribution of binary separations is not a universal quantity, and that the broad distribution of binary separations observed among main-sequence stars can be explained by a superposition of more peaked binary distributions resulting from various star forming environments. The overall binary frequency among pre-main-sequence stars in individual star forming regions is not necessarily higher than among main-sequence stars.Comment: 7 pages, Latex, 4 Postscript figures; also available at http://spider.ipac.caltech.edu/staff/brandner/pubs/pubs.html ; accepted for publication in ApJ Letter

Hot Organic Molecules Toward a Young Low-Mass Star: A Look at Inner Disk Chemistry

Spitzer Space Telescope spectra of the low mass young stellar object (YSO) IRS 46 (L_bol ~ 0.6 L_sun) in Ophiuchus reveal strong vibration-rotation absorption bands of gaseous C2H2, HCN, and CO2. This is the only source out of a sample of ~100 YSO's that shows these features and the first time they are seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined with Keck L- and M-band spectra gives excitation temperatures of > 350 K and abundances of 10(-6)-10(-5) with respect to H2, orders of magnitude higher than those found in cold clouds. In spite of this high abundance, the HCN J=4-3 line is barely detected with the James Clerk Maxwell Telescope, indicating a source diameter less than 13 AU. The (sub)millimeter continuum emission and the absence of scattered light in near-infrared images limits the mass and temperature of any remnant collapse envelope to less than 0.01 M_sun and 100 K, respectively. This excludes a hot-core type region as found in high-mass YSO's. The most plausible origin of this hot gas rich in organic molecules is in the inner (<6 AU radius) region of the disk around IRS 46, either the disk itself or a disk wind. A nearly edge-on 2-D disk model fits the spectral energy distribution (SED) and gives a column of dense warm gas along the line of sight that is consistent with the absorption data. These data illustrate the unique potential of high-resolution infrared spectroscopy to probe organic chemistry, gas temperatures and kinematics in the planet-forming zones close to a young star.Comment: 4 pages, 4 figures; To appear in Astrophysical Journal Letter

High Accretion Rate during Class 0 Phase due to External Trigger

Recent observations indicate that some class 0 sources have orders of magnitude higher accretion rates than those of class I. We investigated the conditions for the high accretion rates of some class 0 sources by numerical calculations, modelling an external trigger. For no external trigger, we find that the maximum value of the accretion rate is determined by the ratio $\alpha$ of the gravitational energy to the thermal one within a flat inner region of the cloud core. The accretion rate reaches \sim 10^{-4} M_{\sun} yr^{-1} if the cloud core has $\alpha > 2$. For an external trigger we find that the maximum value of the accretion rate is proportional to the momentum given to the cloud core. The accretion rate reaches > 10^{-4} M_{\sun} yr^{-1} with a momentum of \sim 0.1 M_{\sun} km s^{-1} when the initial central density of the cloud core is $\sim 10^{-18} g cm^{-3}$. A comparison between recent observational results for prestellar cores and our no triggered collapse model indicates that the flat inner regions of typical prestellar cores are not large enough to cause accretion rates of \sim 10^{-4} M_{\sun} yr^{-1}. Our results show that the triggered collapse of the cloud core is more preferable for the origin of the high accretion rates of class 0 sources than no triggered collapse.Comment: 7 pages, 8 figures, accepted for publication in MNRA