On the internal structure of starless cores - I. Physical conditions and the distribution of CO, CS, N$\mathsf{_2}$H$\mathsf{^+}$, and NH$\mathsf{_3}$ in L1498 and L1517B

We have characterized the physical structure and chemical composition of two close-to-round starless cores in Taurus-Auriga, L1498 and L1517B. Our analysis is based on high angular resolution observations in at least two transitions of NH3 ,N 2H + ,C S, C 34 S, C 18 O, and C 17 O, together with maps of the 1.2 mm continuum. For both cores, we derive radial profiles of constant temperature and constant turbulence, together with density distributions close to those of non-singular isothermal spheres. Using these physical conditions and a Monte Carlo radiative transfer model, we derive abundance profiles for all species and model the strong chemical differentiation of the core interiors. According to our models, the NH3 abundance increases toward the core centers by a factor of several (≈5) while N2H + has a constant abundance over most of the cores. In contrast, both C 18 O and CS (and isotopomers) are strongly depleted in the core interiors, most likely due to their freeze out onto grains at densities of a few 10 4 cm −3 . Concerning the kinematics of the dense gas, we find (in addition to constant turbulence) a pattern of internal motions at the level of 0.1 km s −1 . These motions seem correlated with asymmetries in the pattern of molecular depletion, and we interpret them as residuals of core contraction. Their distribution and size suggest that core formation occurs in a rather irregular manner and with a time scale of a Myr. A comparison of our derived core properties with those predicted by supersonic turbulence models of core formation shows that our Taurus cores are much more quiescent than representative predictions from these models. In two appendices at the end of the paper we present a simple and accurate approximation to the density profile of an isothermal (Bonnor-Ebert) sphere, and a Monte Carlo-calibrated method to derive gas kinetic temperatures using NH3 data.

[1]  A. Tielens.  The Physics and Chemistry of the Interstellar Medium , 2005 .

[2]  Zhi-Yun Li,et al.  A Coupled Dynamical and Chemical Model of Starless Cores of Magnetized Molecular Clouds. II. Chemical Differentiation , 2003 .

[3]  E. Herbst,et al.  Molecular Evolution in Collapsing Prestellar Cores. II. The Effect of Grain-Surface Reactions , 2003, astro-ph/0305054.

[4]  P. Thaddeus,et al.  Precise Millimeter-Wave Laboratory Frequencies for CS and C34S , 2003 .

[5]  R. Klessen,et al.  Dynamic Cores in Hydrostatic Disguise , 2003, astro-ph/0304008.

[6]  R. Saykally,et al.  An enhanced cosmic-ray flux towards ζ Persei inferred from a laboratory study of the H3+–e- recombination rate , 2003, Nature.

[7]  P. Caselli,et al.  Dust emissivity in the submm/mm SCUBA and SIMBA observations of Barnard 68 , 2003 .

[8]  J. Alves,et al.  The Dynamical State of Barnard 68: A Thermally Supported, Pulsating Dark Cloud , 2002, astro-ph/0211507.

[9]  M. Juvela,et al.  The kinetic temperature of Barnard 68 , 2002, astro-ph/0209577.

[10]  M. Walmsley,et al.  The structure and stability of molecular cloud cores in external radiation fields , 2002, astro-ph/0208416.

[11]  C. Puzzarini,et al.  Millimeter- and submillimeter-wave spectrum of C17O. Rotational hyperfine structure analyzed using the Lamb-dip technique , 2002 .

[12]  Shapes of Molecular Cloud Cores and the Filamentary Mode of Star Formation , 2002, astro-ph/0206311.

[13]  C. Ceccarelli,et al.  The degree of CO depletion in pre-stellar cores ? , 2002, astro-ph/0205154.

[14]  J. Alves,et al.  N2H+ and C18O Depletion in a Cold Dark Cloud , 2002, astro-ph/0204016.

[15]  D. Wesolek,et al.  An Experimental Investigation of Collisions of NH3 with Para-H2 at the Temperatures of Cold Molecular Clouds , 2002 .

[16]  P. Caselli,et al.  Dense Cores in Dark Clouds. XIV. N2H+ (1-0) Maps of Dense Cloud Cores , 2002, astro-ph/0202173.

[17]  Zhi-Yun Li,et al.  A Coupled Dynamical and Chemical Model of Starless Cores of Magnetized Molecular Clouds. I. Formulation and Initial Results , 2002, astro-ph/0201019.

[18]  H Germany,et al.  Systematic Molecular Differentiation in Starless Cores , 2001, astro-ph/0112487.

[19]  S. Basu,et al.  The Intrinsic Shapes of Molecular Cloud Fragments over a Range of Length Scales , 2001, astro-ph/0112463.

[20]  P. Caselli,et al.  Molecular Ions in L1544. II. The Ionization Degree , 2001, astro-ph/0109023.

[21]  P. Andre',et al.  The initial conditions of isolated star formation – V. ISOPHOT imaging and the temperature and energy balance of pre stellar cores , 2001, astro-ph/0109173.

[22]  P. Caselli,et al.  Molecular Ions in L1544. I. Kinematics , 2001, astro-ph/0109021.

[23]  W. Langer,et al.  Preprotostellar Core Properties from Far-Infrared Observations , 2001 .

[24]  L. Hartmann,et al.  Rapid Formation of Molecular Clouds and Stars in the Solar Neighborhood , 2001, astro-ph/0108023.

[25]  W. Langer,et al.  Measuring the Magnetic Field Strength in L1498 with Zeeman-splitting Observations of CCS , 2001 .

[26]  C. Walmsley,et al.  The dust temperature distribution in prestellar cores , 2001, astro-ph/0107055.

[27]  P. Myers,et al.  A Survey for Infall Motions toward Starless Cores. II. CS (2-1) and N2H+ (1-0) Mapping Observations , 2001, astro-ph/0105515.

[28]  E. Herbst,et al.  Molecular Evolution in Collapsing Prestellar Cores , 2001, astro-ph/0202061.

[29]  J. Alves,et al.  Molecular Excitation and Differential Gas-Phase Depletions in the IC 5146 Dark Cloud , 2001, astro-ph/0103521.

[30]  J. Alves,et al.  Internal structure of a cold dark molecular cloud inferred from the extinction of background starlight , 2001, Nature.

[31]  S. Basu,et al.  On the Timescale for the Formation of Protostellar Cores in Magnetic Interstellar Clouds , 2000, astro-ph/0009262.

[32]  D. Ward-Thompson,et al.  The initial conditions of isolated star formation - IV. C18O observations and modelling of the pre-stellar core L1689B , 2000, astro-ph/0012095.

[33]  P. Goldsmith.  Molecular Depletion and Thermal Balance in Dark Cloud Cores , 2000 .

[34]  P. Bastien,et al.  Probing the Magnetic Field with Molecular Ion Spectra , 2000, astro-ph/0311335.

[35]  L. Mundy,et al.  Tracing the Mass during Low-Mass Star Formation. II. Modeling the Submillimeter Emission from Preprotostellar Cores , 2000, astro-ph/0006183.

[36]  R. Klessen,et al.  Control of star formation by supersonic turbulence , 2000, astro-ph/0301093.

[37]  B. Elmegreen.  Star Formation in a Crossing Time , 1999, astro-ph/9911172.

[38]  P. Caselli,et al.  CO Depletion in the Starless Cloud Core L1544 , 1999 .

[39]  Zhi-Yun Li.  A Spherical Model for Starless Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains , 1999, astro-ph/9907358.

[40]  P. Myers,et al.  A Survey of Infall Motions toward Starless Cores. I. CS (2-1) and N2H+ (1-0) Observations , 1999, astro-ph/9906468.

[41]  D. Flower.  Rotational excitation of HCO+ by H2 , 1999 .

[42]  N. Evans.  Physical conditions in regions of star formation , 1999, astro-ph/9905050.

[43]  P. Myers,et al.  A Catalog of Optically Selected Cores , 1999, astro-ph/9901175.

[44]  J. Alves,et al.  Correlation between Gas and Dust in Molecular Clouds: L977 , 1998, astro-ph/9809027.

[45]  W. Langer,et al.  Dust Emission and Molecular Depletion in L1498 , 1998 .

[46]  P. Caselli,et al.  L1544: A Starless Dense Core with Extended Inward Motions , 1998 .

[47]  A. Goodman,et al.  Coherence in Dense Cores. II. The Transition to Coherence , 1998 .

[48]  A. Goodman,et al.  Coherent Dense Cores. I. NH3 Observations , 1998 .

[49]  Holger S. P. Müller,et al.  THE COLOGNE DATABASE FOR MOLECULAR SPECTROSCOPY, CDMS , 2001 .

[50]  S. Charnley.  Chemical models of interstellar gas-grain processes — III. Molecular depletion in NGC 2024 , 1997 .

[51]  E. Bergin,et al.  Chemical Evolution in Preprotostellar and Protostellar Cores , 1997 .

[52]  P. Goldsmith,et al.  Physical Conditions in Quiescent Dark Cloud Cores Determined from Multitransition Observations of CCS , 1997 .

[53]  D. Lynden-Bell,et al.  An Analytic Approximation to the Isothermal Sphere , 1996, astro-ph/9604084.

[54]  W. Langer,et al.  Evolutionary status of the pre-protostellar core L1498. , 1996, The Astrophysical journal.

[55]  R. Pudritz,et al.  A Model for the Internal Structure of Molecular Cloud Cores , 1996, astro-ph/9605018.

[56]  P. Caselli,et al.  Radio-astronomical Spectroscopy of the Hyperfine Structure of N2H+ , 1995 .

[57]  P. Andre',et al.  A submillimetre continuum survey of pre-protostellar cores , 1994 .

[58]  G. Fuller,et al.  Thermal Material in Dense Cores: A New Narrow-Line Probe and Technique of Temperature Determination , 1993 .

[59]  Alyssa A. Goodman,et al.  Dense cores in dark clouds. VIII - Velocity gradients , 1993 .

[60]  G. Fuller,et al.  A CS survey of low-mass cores and comparison with NH3 observations , 1989 .

[61]  P. Myers,et al.  A survey for dense cores in dark clouds , 1989 .

[62]  P. Schilke,et al.  A recalibration of the interstellar ammonia thermometer , 1988 .

[63]  G. Fuller,et al.  Near-infrared and optical observations of IRAS sources in and near dense cores , 1987 .

[64]  F. Adams,et al.  Star Formation in Molecular Clouds: Observation and Theory , 1987 .

[65]  C. Beichman,et al.  Candidate solar-type protostars in nearby molecular cloud cores , 1986 .

[66]  P. Myers.  Dense cores in dark clouds. III. Subsonic turbulence. , 1983 .

[67]  R. Linke,et al.  Dense cores in dark clouds. I. CO observations and column densities of high-extinction regions , 1983 .

[68]  A. Penzias.  The isotopic abundances of interstellar oxygen. , 1981 .

[69]  B. Elmegreen.  Magnetic diffusion and ionization fractions in dense molecular clouds - The role of charged grains , 1979 .

[70]  C. Hunter.  The collapse of unstable isothermal spheres. , 1977 .

[71]  S. Kukolich.  Measurement of ammonia hyperfine structure with a two-cavity maser , 1967 .

[72]  W. Bonnar,et al.  Boyle's Law and gravitational instability , 1956 .

[73]  S. Chandrasekhar,et al.  The Isothermal Function. , 1949 .

[74]  H. Poincaré,et al.  Les Méthodes nouvelles de la Mécanique céleste and An Introduction to the Study of Stellar Structure , 1958 .