Systematic Molecular Differentiation in Starless Cores

Abstract

(Abridged) We present evidence that low-mass starless cores, the simplest units of star formation, are systematically differentiated in their chemical composition. Molecules including CO and CS almost vanish near the core centers, where the abundance decreases by one or two orders of magnitude. At the same time, N2H+ has a constant abundance, and the fraction of NH3 increases toward the core center. Our conclusions are based on a study of 5 mostly-round starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and (2,2), and the 1.2 mm continuum. For each core we have built a model that fits simultaneously the radial profile of all observed emission and the central spectrum for the molecular lines. The observed abundance drops of CO and CS are naturally explained by the depletion of these molecules onto dust grains at densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to densities of several 10^5, while the NH3 abundance may be enhanced by reactions triggered by the disappearance of CO from the gas phase. With the help of our models, we show that chemical differentiation automatically explains the discrepancy between the sizes of CS and NH3 maps, a problem which has remained unexplained for more than a decade. Our models, in addition, show that a combination of radiative transfer effects can give rise to the previously observed discrepancy in the linewidth of these two tracers. Although this discrepancy has been traditionally interpreted as resulting from a systematic increase of the turbulent linewidth with radius, our models show that it can arise in conditions of constant gas turbulence.

25 pages, 9 figures, accepted by ApJ

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• Probing the structure of a birthplace of intermediate-mass stars: Ammonia cores in Lynds 1340
• Chemical and Physical Conditions in Molecular Cloud Core DC 000.4-19.5 (SL42) in Corona Australis
• The molecular emission of the irradiated star forming core ahead of HH 80N
• Star formation timescale in the molecular filament WB 673
• Multi log-normal density structure in Cygnus-X molecular clouds: A fitting for N-PDF without power-law
• Differences in chemical evolution between isolated and embedded prestellar cores
• Dust properties of the dark cloud IC5146 - Submillimeter and NIR imaging
• A spectral line survey of the starless and proto-stellar cores detected by BLAST toward the Vela-D molecular cloud
• Toward a robust physical and chemical characterization of heterogeneous lines of sight: The case of the Horsehead nebula
• Line emission from filaments in molecular clouds
• Studies of Dense Cores with ALMA
• Deuterium fractionation in cold dense cores in the low-mass star forming region L1688
• ALMA Datacubes and Continuum Maps of the Irradiated Western Wall in Carina
• A New Recipe for Obtaining Central Volume Densities of Prestellar Cores from Size Measurements
• Deuterium fractionation of the starless core L 1498
• KAgoshima Galactic Object survey with Nobeyama 45-metre telescope by Mapping in Ammonia lines (KAGONMA): Discovery of parsec-scale CO depletion in the Canis Major star-forming region
• Role of NH3 Binding Energy in the Early Evolution of Protostellar Cores
• Methanol: a diagnostic tool for high-mass star-forming regions
• Why do some cores remain starless ?
• KAgoshima Galactic Object survey with Nobeyama 45-metre telescope by Mapping in Ammonia lines (KAGONMA): Star formation feedback on dense molecular gas in the W33 complex
• The role of radiative torques in the molecular cloud core L43
• Astrochemistry and Observations
• Synthetic observations using POLARIS: an application to simulations of massive prestellar cores