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On the accretion process in a high-mass star forming region - A multitransitional THz Herschel-HIFI study of ammonia toward G34.26+0.15

Mitra Hajigholi (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; Carina M. Persson (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; Eva Wirström (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; John H. Black (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; Per Bergman (Institutionen för rymd- och geovetenskap, Onsala rymdobservatorium) ; Henrik Olofsson (Institutionen för rymd- och geovetenskap, Onsala rymdobservatorium) ; Michael Olberg (Institutionen för rymd- och geovetenskap, Onsala rymdobservatorium) ; Friedrich Wyrowski ; Audrey Coutens ; Åke Hjalmarsson (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; Karl Menten
Astronomy and Astrophysics (0004-6361). Vol. 585 (2016), A158, p. 20.
[Artikel, refereegranskad vetenskaplig]

Aims. Our aim is to explore the gas dynamics and the accretion process in the early phase of high-mass star formation. Methods. The inward motion of molecular gas in the massive star forming region G34.26+0.15 is investigated by using high-resolution profiles of seven transitions of ammonia at THz frequencies observed with Herschel-HIFI. The shapes and intensities of these lines are interpreted in terms of radiative transfer models of a spherical, collapsing molecular envelope. An accelerated Lambda Iteration (ALI) method is used to compute the models. Results. The seven ammonia lines show mixed absorption and emission with inverse P-Cygni-type profiles that suggest infall onto the central source. A trend toward absorption at increasingly higher velocities for higher excitation transitions is clearly seen in the line profiles. The J=3←2 lines show only very weak emission, so these absorption profiles can be used directly to analyze the inward motion of the gas. This is the first time a multitransitional study of spectrally resolved rotational ammonia lines has been used for this purpose. Broad emission is, in addition, mixed with the absorption in the 1_0−0_0 ortho-NH_3 line, possibly tracing a molecular outflow from the star forming region. The best-fitting ALI model reproduces the continuum fluxes and line profiles, but slightly underpredicts the emission and absorption depth in the ground-state ortho line 1_0−0_0. An ammonia abundance on the order of 10^{−9} relative to H_2 is needed to fit the profiles. The derived ortho-to-para ratio is approximately 0.5 throughout the infalling cloud core similar to recent findings for translucent clouds in sight lines toward W31C and W49N. We find evidence of two gas components moving inwards toward the central region with constant velocities: 2.7 and 5.3 km s^{−1}, relative to the source systemic velocity. Attempts to model the inward motion with a single gas cloud in free-fall collapse did not succeed. The inferred mass accretion rates derived rises from 4.1×10^{−3} to 4.5×10^{−2} M⊙ yr^{−1}, which is sufficient to overcome the expected radiation pressure from G34.26+0.15.

Nyckelord: H-Ii-Regions, Ultracompact Hii-Regions, Multilevel Radiative-Transfer, Molecular Line Observations, Lambda Iteration Method, Interstellar Ammonia, Nitrogen Hydrides, Gravitational Collapse. Competitive Accretion, Propensity Rules

Denna post skapades 2015-10-20. Senast ändrad 2016-03-16.
CPL Pubid: 224513