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Water vapor toward starless cores: The Herschel view

Caselli, P. and Blake, G. A. and Lis, D. (2010) Water vapor toward starless cores: The Herschel view. Astronomy and Astrophysics, 521 . Art. No. L29. ISSN 0004-6361.

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Aims. Previous studies by the satellites SWAS and Odin provided stringent upper limits on the gas phase water abundance of dark clouds (x(H_2O) < 7 × 10^(−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. Methods. High spectral resolution observations of the fundamental ortho water (o-H_2O) transition (557 GHz) were carried out with the Heterodyne Instrument for the Far Infrared onboard Herschel toward two starless cores: Barnard 68 (hereafter B68), a Bok globule, and LDN 1544 (L1544), a prestellar core embedded in the Taurus molecular cloud complex. Detailed radiative transfer and chemical codes were used to analyze the data. Results. 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 3σ upper limit is consistent with a column density of o-H_2O N(o-H_2O) < 2.5 × 10^(13) cm^(−2), or a fractional abundance x(o-H_2O) < 1.3 × 10^(−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 5σ level from which we obtain the first value of the o-H_2O column density ever measured in dark clouds: N(o-H_2O) = (8 ± 4) × 10^(12) cm^(−2). The corresponding fractional abundance is x(o-H_2O) ≃ 5 × 10^(−9) at radii >7000 AU and ≃ 2 × 10−10 toward the center. The radiative transfer analysis shows that this is consistent with a x(o-H_2O) profile peaking at ≃ 10^(−8) 0.1 pc away from the core center, where both freeze-out and photodissociation are negligible. Conclusions. Herschel has provided the first measurement of water vapor in dark regions. Column densities of o-H_2O are low, but prestellar cores such as L1544 (with their high central densities, strong continuum, and large envelopes) appear to be very promising tools to finally shed light on the solid/vapor balance of water in molecular clouds and oxygen chemistry in the earliest stages of star formation.

Item Type:Article
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Caselli, P.0000-0003-1481-7911
Blake, G. A.0000-0003-0787-1610
Lis, D.0000-0002-0500-4700
Additional Information:© 2010 ESO. Received 31 May 2010, Accepted 18 June 2010, Published online 01 October 2010. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U. Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiologá (CSIC-INTA). Sweden: Chalmers University of Technology – MC2, RSS & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University – Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC. HCSS/HSpot/HIPE are joint developments by the Herschel Science Ground Segment Consortium, consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS and SPIRE consortia.
Subject Keywords:astrochemistry – line: formation – molecular processes – radiative transfer – stars: formation – ISM: clouds
Record Number:CaltechAUTHORS:20101221-105628509
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Official Citation:Water vapor toward starless cores: The Herschel view P. Caselli, E. Keto, L. Pagani, Y. Aikawa, U. A. Yıldız, F. F. S. van der Tak, M. Tafalla, E. A. Bergin, B. Nisini, C. Codella, E. F. van Dishoeck, R. Bachiller, A. Baudry, M. Benedettini, A. O. Benz, P. Bjerkeli, G. A. Blake, S. Bontemps, J. Braine, S. Bruderer, J. Cernicharo, F. Daniel, A. M. di Giorgio, C. Dominik, S. D. Doty, P. Encrenaz, M. Fich, A. Fuente, T. Gaier, T. Giannini, J. R. Goicoechea, Th. de Graauw, F. Helmich, G. J. Herczeg, F. Herpin, M. R. Hogerheijde, B. Jackson, T. Jacq, H. Javadi, D. Johnstone, J. K. Jørgensen, D. Kester, L. E. Kristensen, W. Laauwen, B. Larsson, D. Lis, R. Liseau, W. Luinge, M. Marseille, C. McCoey, A. Megej, G. Melnick, D. Neufeld, M. Olberg, B. Parise, J. C. Pearson, R. Plume, C. Risacher, J. Santiago-García, P. Saraceno, R. Shipman, P. Siegel, T. A. van Kempen, R. Visser, S. F. Wampfler and F. Wyrowski A&A 521 L29 (2010) DOI: 10.1051/0004-6361/201015097
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:21481
Deposited By: Benjamin Perez
Deposited On:21 Dec 2010 19:33
Last Modified:09 Mar 2020 13:18

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