Search

We present the results of a single-pointing survey of 207 dense cores embedded in Planck Galactic Cold Clumps distributed in five different environments ($\lambda$ Orionis, Orion A, B, Galactic plane, and high latitudes) to identify dense cores on the verge of star formation for the study of the initial conditions of star formation. We observed these cores in eight molecular lines at 76-94 GHz using the Nobeyama 45-m telescope. We find that early-type molecules (e.g., CCS) have low detection rates and that late-type molecules (e.g., N$_2$H$^+$, c-C$_3$H$_2$) and deuterated molecules (e.g., N$_2$D$^+$, DNC) have high detection rates, suggesting that most of the cores are chemically evolved. The deuterium fraction (D/H) is found to decrease with increasing distance, indicating that it suffers from differential beam dilution between the D/H pair of lines for distant cores ($>$1 kpc). For $\lambda$ Orionis, Orion A, and B located at similar distances, D/H is not significantly different, suggesting that there is no systematic difference in the observed chemical properties among these three regions. We identify at least eight high D/H cores in the Orion region and two at high latitudes, which are most likely to be close to the onset of star formation. There is no clear evidence of the evolutionary change in turbulence during the starless phase, suggesting that the dissipation of turbulence is not a major mechanism for the beginning of star formation as judged from observations with a beam size of 0.04 pc., Comment: 55 pages, 12 Figures, 8 Tables, accepted for publication in ApJS

Aims. We present a survey of the ortho-H2D+ (1(1,0)-1(1,1)) line toward a sample of 10 starless cores and 6 protostellar cores, carried out at the Caltech Submillimeter Observatory. The high diagnostic power of this line is revealed for the study of the chemistry, and the evolutionary and dynamical status of low-mass dense cores. Methods. The derived ortho-H2D+ column densities (N(ortho-H2D+)) are compared with predictions from simple chemical models of centrally concentrated cloud cores. Results. The line is detected in 7 starless cores and in 4 protostellar cores. N(ortho-H2D+) ranges between 2 and 40 x 10(12) cm(-2) in starless cores and between 2 and 9 x 10(12) cm(-2) in protostellar cores. The brightest lines are detected toward the densest and most centrally concentrated starless cores, where the CO depletion factor and the deuterium fractionation are also largest. The large scatter observed in plots of N(ortho-H2D+) vs. the observed deuterium fractionation and vs. the CO depletion factor is likely to be due to variations in the ortho-to-para (o/p) ratio of H2D+ from > 0.5 for T-kin

This paper describes a James Clerk Maxwell Telescope (JCMT) legacy survey that has been awarded roughly 500 hr of observing time to be carried out from 2007 to 2009. In this survey, we will map with SCUBA-2 (Submillimetre Common-User Bolometer Array 2) almost all of the well-known low-mass and intermediate-mass star-forming regions within 0.5 kpc that are accessible from the JCMT. Most of these locations are associated with the Gould Belt. From these observations, we will produce a flux-limited snapshot of star formation near the Sun, providing a legacy of images, as well as point-source and extended-source catalogs, over almost 700 deg(2) of sky. The resulting images will yield the first catalog of prestellar and protostellar sources selected by submillimeter continuum emission, and should increase the number of known sources by more than an order of magnitude. We will also obtain with the array receiver HARP (Heterodyne Array Receiver Program) CO maps, in three CO isotopologues, of a large typical sample of prestellar and protostellar sources. We will then map the brightest hundred sources with the SCUBA-2 polarimeter (POL-2), producing the first statistically significant set of polarization maps in the submillimeter. The images and source catalogs will be a powerful reference set for astronomers, providing a detailed legacy archive for future telescopes, including ALMA, Herschel, and JWST.

Context. The presence of H2D+ in dense cloud cores underlies ion-molecule reactions that strongly enhance the deuterium fractionation of many molecular species. Aims. We determine the H2D+ abundance in one starless core, Barnard 68, that has a particularly well established physical, chemical, and dynamical structure. Methods. We observed the ortho-H2D+ ground-state line 1(10)-1(11), the N2H+ J = 4-3 line, and the (HCO+)-C-13 4-3 line with the APEX telescope. Results. We report the probable detection of the o-H2D+ line at an intensity T-mb = 0.22 +/- 0.08 K and exclusively thermal line width, and find only upper limits to the N2H+ 4-3 and (HCO+)-C-13 4-3 intensities. Conclusions. Within the uncertainties in the chemical reaction rates and the collisional excitation rates, chemical model calculations and excitation simulations reproduce the observed intensities and that of o-H2D+ in particular.

We present a survey of the ortho-H2D+(1_{1,0}-1_{1,1}) line toward a sample of 10 starless cores and 6 protostellar cores, carried out at the Caltech Submillimeter Observatory. The high diagnostic power of this line is revealed for the study of the chemistry, and the evolutionary and dynamical status of low-mass dense cores. The line is detected in 7 starless cores and in 4 protostellar cores. N(ortho-H2D+) ranges between 2 and 40x10^{12} cm^{-2} in starless cores and between 2 and 9x10^{12} cm^{-2} in protostellar cores. The brightest lines are detected toward the densest and most centrally concentrated starless cores, where the CO depletion factor and the deuterium fractionation are also largest. The large scatter observed in plots of N(ortho-H2D+) vs. the observed deuterium fractionation and vs. the CO depletion factor is likely to be due to variations in the ortho-to-para (o/p) ratio of H2D+ from >0.5 for T_{kin} < 10 K gas in pre-stellar cores to ~0.03 (consistent with T_{kin} ~15 K for protostellar cores). The two Ophiuchus cores in our sample also require a relatively low o/p ratio (~0.3). Other parameters, including the cosmic-ray ionization rate, the CO depletion factor (or, more in general, the depletion factor of neutral species), the volume density, the fraction of dust grains and PAHs also largely affect the ortho-H2D+ abundance. The most deuterated and H2D+-rich objects (L429, L1544, L694-2 and L183) are reproduced by chemical models of centrally concentrated (central densties ~10^{6} cm^{-3}) cores with chemical ages between 10^4 and 10^6 yr. Upper limits of the para-H3O+ (1_1- -2_1+) and para-D2H+ (1_{1,0}-1_{0,1}) lines are also given. (Abridged), 29 pages, 6 figures. To appear in A&A

This paper describes a James Clerk Maxwell Telescope (JCMT) legacy survey that has been awarded roughly 500 hrs of observing time to be carried out from 2007 to 2009. In this survey we will map with SCUBA-2 (Submillimetre Common User Bolometer Array 2) almost all of the well-known low-mass and intermediate-mass star-forming regions within 0.5 kpc that are accessible from the JCMT. Most of these locations are associated with the Gould Belt. From these observations we will produce a flux-limited snapshot of star formation near the Sun, providing a legacy of images, as well as point-source and extended-source catalogues, over almost 700 square degrees of sky. The resulting images will yield the first catalogue of prestellar and protostellar sources selected by submillimetre continuum emission, and should increase the number of known sources by more than an order of magnitude. We will also obtain CO maps with the array receiver HARP (Heterodyne Array Receiver Programme), in three CO isotopologues, of a large typical sample of prestellar and protostellar sources. We will then map the brightest hundred sources with the SCUBA-2 polarimeter (POL-2), producing the first statistically significant set of polarization maps in the submillimetre. The images and source catalogues will be a powerful reference set for astronomers, providing a detailed legacy archive for future telescopes, including ALMA, Herschel and JWST., 60 page manuscript, inc. 18 figures. Accepted by PASP

The presence of H2D+ in dense cloud cores underlies ion-molecule reactions that strongly enhance the deuterium fractionation of many molecular species. We determine the H2D+ abundance in one starless core, Barnard 68, that has a particularly well established physical, chemical, and dynamical structure. We observed the ortho-H2D+ ground-state line 1_10-1_11, the N2H+ J=4-3 line, and the H13CO+ 4-3 line with the APEX telescope. We report the probable detection of the o-H2D+ line at an intensity Tmb=0.22 +- 0.08 K and exclusively thermal line width, and find only upper limits to the N2H+ 4-3 and H13CO+ 4-3 intensities. Within the uncertainties in the chemical reaction rates and the collisional excitation rates, chemical model calculations and excitation simulations reproduce the observed intensities and that of o-H2D+ in particular., APEX A&A special issue, accepted

SWAS and Odin provided stringent upper limits on the gas phase water abundance of dark clouds (x(H2O) < 7x10^-9). We investigate the chemistry of water vapor in starless cores beyond the previous upper limits using the highly improved angular resolution and sensitivity of Herschel and measure the abundance of water vapor during evolutionary stages just preceding star formation. High spectral resolution observations of the fundamental ortho water (o-H2O) transition (557 GHz) were carried out with Herschel HIFI toward two starless cores: B68, a Bok globule, and L1544, a prestellar core embedded in the Taurus molecular cloud complex. The rms in the brightness temperature measured for the B68 and L1544 spectra is 2.0 and 2.2 mK, respectively, in a velocity bin of 0.59 km s^-1. The continuum level is 3.5+/-0.2 mK in B68 and 11.4+/-0.4 mK in L1544. No significant feature is detected in B68 and the 3 sigma upper limit is consistent with a column density of o-H2O N(o-H2O) < 2.5x10^13 cm^-2, or a fractional abundance x(o-H2O) < 1.3x10^-9, more than an order of magnitude lower than the SWAS upper limit on this source. The L1544 spectrum shows an absorption feature at a 5 sigma level from which we obtain the first value of the o-H2O column density ever measured in dark clouds: N(o-H2O) = (8+/-4)x10^12 cm^-2. The corresponding fractional abundance is x(o-H2O) ~ 5x10^-9 at radii > 7000 AU and ~2x10^-10 toward the center. The radiative transfer analysis shows that this is consistent with a x(o-H2O) profile peaking at ~10^-8, 0.1 pc away from the core center, where both freeze-out and photodissociation are negligible. Herschel has provided the first measurement of water vapor in dark regions. Prestellar cores such as L1544 (with their high central densities, strong continuum, and large envelopes) are very promising tools to finally shed light on the solid/vapor balance of water in molecular clouds., Accepted for publication in Astronomy and Astrophysics (HIFI first results issue)