Volume 578, June 2015
|Number of page(s)||21|
|Section||Interstellar and circumstellar matter|
|Published online||16 June 2015|
The dust opacity exponent β and the gas-to-dust mass ratio g2d
1 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
2 AlbaNova University Centre, Stockholm University, Department of Astronomy, 106 91 Stockholm, Sweden
3 Stockholm University Astrobiology Centre, 106 91 Stockholm, Sweden
Received: 12 January 2015
Accepted: 8 April 2015
Aims. We aim at determining the spatial distribution of the gas and dust in star-forming regions and address their relative abundances in quantitative terms. We also examine the dust opacity exponent β for spatial and/or temporal variations.
Methods. Using mapping observations of the very dense ρ Oph A core, we examined standard 1D and non-standard 3D methods to analyse data of far-infrared and submillimetre (submm) continuum radiation. The resulting dust surface density distribution can be compared to that of the gas. The latter was derived from the analysis of accompanying molecular line emission, observed with Herschel from space and with APEX from the ground. As a gas tracer we used N2H+, which is believed to be much less sensitive to freeze-out than CO and its isotopologues. Radiative transfer modelling of the N2H+ (J = 3−2) and (J = 6−5) lines with their hyperfine structure explicitly taken into account provides solutions for the spatial distribution of the column density N(H2), hence the surface density distribution of the gas.
Results. The gas-to-dust mass ratio is varying across the map, with very low values in the central regions around the core SM 1. The global average, = 88, is not far from the canonical value of 100, however. In ρ Oph A, the exponent β of the power-law description for the dust opacity exhibits a clear dependence on time, with high values of 2 for the envelope-dominated emission in starless Class –1 sources to low values close to 0 for the disk-dominated emission in Class III objects. β assumes intermediate values for evolutionary classes in between.
Conclusions. Since β is primarily controlled by grain size, grain growth mostly occurs in circumstellar disks. The spatial segregation of gas and dust, seen in projection toward the core centre, probably implies that, like C18O, also N2H+ is frozen onto the grains.
Key words: ISM: general / ISM: individual objects: rho Oph A / dust, extinction / ISM: abundances / stars: formation
Based on observations with APEX, which is a 12 m diameter submillimetre telescope at 5100 m altitude on Llano Chajnantor in Chile. The telescope is operated by Onsala Space Observatory, Max-Planck-Institut für Radioastronomie (MPIfR), and European Southern Observatory (ESO).
And also based on observations with Herschel which is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
FITS files containing the data of Figs. 2, 11, 12, and 16−18 are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/578/A131
© ESO, 2015
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