On the accretion process in a high-mass star forming region - A multitransitional THz Herschel-HIFI study of ammonia toward G34.26+0.15
Artikel i vetenskaplig tidskrift, 2016

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.

Gravitational Collapse. Competitive Accretion

Molecular Line Observations

Interstellar Ammonia

Lambda Iteration Method

H-Ii-Regions

Ultracompact Hii-Regions

Nitrogen Hydrides

Multilevel Radiative-Transfer

Propensity Rules

Författare

Mitra Hajigholi

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

Carina Persson

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

Eva Wirström

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

John H Black

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

Per Bergman

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

Henrik Olofsson

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

Michael Olberg

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

F. Wyrowski

Max Planck-institutet

A. Coutens

Niels Bohr Institute

Åke Hjalmarsson

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

K. Menten

Max Planck-institutet

Astronomy and Astrophysics

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

Vol. 585 A158 20- A158

Ämneskategorier

Astronomi, astrofysik och kosmologi

Fundament

Grundläggande vetenskaper

Infrastruktur

Onsala rymdobservatorium

DOI

10.1051/0004-6361/201526451

Mer information

Senast uppdaterat

2018-02-21