Gas and dust in the star-forming region ρ Oph A ∗, ∗∗, ∗∗∗: The dust opacity exponent β and the gas-to-dust mass ratio g2d
Journal article, 2015

© ESO, 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 N 2 H+, which is believed to be much less sensitive to freeze-out than CO and its isotopologues. Radiative transfer modelling of the N 2 H+ (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(H 2 ), 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 N 2 H+ is frozen onto the grains.

ISM: individual objects: rho Oph A

Dust extinction

ISM: abundances

Stars: formation

ISM: general

Author

René Liseau

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

B. Larsson

AlbaNova University Center

Tuomas Lunttila

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

Michael Olberg

Chalmers, Earth and Space Sciences, Onsala Space Observatory

Gustaf Rydbeck

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

Per Bergman

Chalmers, Earth and Space Sciences, Onsala Space Observatory

Kay Justtanont

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

G. Olofsson

AlbaNova University Center

B. L. de Vries

AlbaNova University Center

Stockholm University

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 578 A131

Subject Categories

Fusion, Plasma and Space Physics

DOI

10.1051/0004-6361/201525641

More information

Latest update

4/10/2019