Energetic radiation and the sulfur chemistry of protostellar envelopes: Submillimeter interferometry of AFGL 2591

09/2007; DOI: 10.1051/0004-6361:20078032
Source: arXiv

ABSTRACT CONTEXT: The chemistry in the inner few thousand AU of accreting envelopes around young stellar objects is predicted to vary greatly with far-UV and X-ray irradiation by the central star. Aim We search for molecular tracers of high-energy irradiation by the protostar in the hot inner envelope. METHODS: The Submillimeter Array (SMA) has observed the high-mass star forming region AFGL 2591 in lines of CS, SO, HCN, HCN(v2=1), and HC15N with 0.6" resolution at 350 GHz probing radial scales of 600-3500 AU for an assumed distance of 1 kpc. The SMA observations are compared with the predictions of a chemical model fitted to previous single-dish observations. RESULTS: The CS and SO main peaks are extended in space at the FWHM level, as predicted in the model assuming protostellar X-rays. However, the main peak sizes are found smaller than modeled by nearly a factor of 2. On the other hand, the lines of CS, HCN, and HC15N, but not SO and HCN(v2=1), show pedestal emissions at radii of about 3500 AU that are not predicted. All lines except SO show a secondary peak within the approaching outflow cone. A dip or null in the visibilities caused by a sharp decrease in abundance with increasing radius is not observed in CS and only tentatively in SO. CONCLUSIONS: The emission of protostellar X-rays is supported by the good fit of the modeled SO and CS amplitude visibilities including an extended main peak in CS. The broad pedestals can be interpreted by far-UV irradiation in a spherically non-symmetric geometry, possibly comprising outflow walls on scales of 3500 -- 7000 AU. The extended CS and SO main peaks suggest sulfur evaporation near the 100 K temperature radius. Comment: Astronomy and Astrophysics, in press

Download full-text


Available from: Pascal Staeuber, May 02, 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Context. Cygnus OB2 is one of the richest OB associations in the local Galaxy, and is located in a vast complex containing several other associations, clusters, molecular clouds, and HII regions. However, the stellar content of Cygnus OB2 and its surroundings remains rather poorly known largely due to the considerable reddening in its direction at visible wavelength.Aims. We investigate the possible existence of an extended halo of early-type stars around Cygnus OB2, which is hinted at by near-infrared color–color diagrams, and its relationship to Cygnus OB2 itself, as well as to the nearby association Cygnus OB9 and to the star forming regions in the Cygnus X North complex.Methods. Candidate selection is made with photometry in the 2MASS all-sky point source catalog. The early-type nature of the selected candidates is confirmed or discarded through our infrared spectroscopy at low resolution. In addition, spectral classifications in the visible are presented for many lightly-reddened stars.Results. A total of 96 early-type stars are identified in the targeted region, which amounts to nearly half of the observed sample. Most of them have featureless near-infrared spectra as expected from OB stars at the available resolution. Another 18 stars that display Brackett emission lines can be divided between evolved massive stars (most likely Be stars) and Herbig Ae/Be stars based on their infrared excesses. A component associated with Cygnus OB9/NGC 6910 is clearly identified, as well as an enhancement in the surface density of early-type stars at Cygnus X North. We also find a field population, consisting largely of early B giants and supergiants, which is probably the same as identified in recent studies of the inner $1^\circ$ circle around Cygnus OB2. The age and large extension of this population discards a direct relationship with Cygnus OB2 or any other particular association.Conclusions. Earlier claims of the possible large extent of Cygnus OB2 beyond its central, very massive aggregate seem to be dismissed by our findings. The existence of a nearly ubiquitous population of evolved stars with massive precursors suggests a massive star formation history in Cygnus having started long before the formation of the currently observed OB associations in the region.
    Astronomy and Astrophysics 08/2008; 486(2). DOI:10.1051/0004-6361:200809917
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Upcoming facilities such as the Herschel Space Observatory or Atacama Large Millimeter Array will deliver a wealth of molecular line observations of young stellar objects (YSOs). Based on line fluxes, chemical abundances can then be estimated by radiative transfer calculations. To derive physical properties from abundances, the chemical network needs to be modeled and fitted to the observations. This modeling process is however computationally exceedingly demanding, particularly if in addition to density and temperature, far-UV (FUV) irradiation, X-rays, and multi-dimensional geometry have to be considered. We develop a fast tool, suitable for various applications of chemical modeling in YSOs. A grid of the chemical composition of the gas having a density, temperature, FUV irradiation and X-ray flux is pre-calculated as a function of time. A specific interpolation approach is developed to reduce the database to a feasible size. Published models of AFGL 2591 are used to verify the accuracy of the method. A second benchmark test is carried out for FUV sensitive molecules. The novel method for chemical modeling is more than 250,000 times faster than direct modeling and agrees within a mean factor of 1.35. The tool is distributed for public use. Main applications are (1) fitting physical parameters to observed molecular line fluxes and (2) deriving chemical abundances for two- and three-dimensional models. They will be presented in two future publications of this series. In the course of developing the method, the chemical evolution is explored: we find that X-ray chemistry in envelopes of YSOs can be reproduced by means of an enhanced cosmic-ray ionization rate with deviations less than 25%, having the observational consequence that molecular tracers for X-rays are hard to distinguish from cosmic-ray ionization tracers. We provide the detailed prescription to implement this total ionization rate approach in any chemical model. We further find that the abundance of CH+ in low-density gas with high ionization can be enhanced by the recombination of doubly ionized carbon (C++) and suggest a new value for the initial abundance of the main sulfur carrier in the hot core.
    The Astrophysical Journal Supplement Series 07/2009; 183(2):179. DOI:10.1088/0067-0049/183/2/179
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Context. Outflow cavities in envelopes of young stellar objects (YSOs) have been pr edicted to allow far-UV (FUV) photons to escape far from the central source, with significant observable e ffects, especially if the protostar is a forming high-mass star suspected of emitting a copious amount of FUV radiation. Indirect evidence of this picture has been provided by models and unresolved single-dish observations, but direct high-resolution data are necessary for con firmation. Previous chemical modeling has suggested that CS and HCN are good probes of the local FUV field, so make good target specie s. Aims. We directly probe the physical conditions of the material in the outflow walls to te st this prediction. Methods. Interferometric observations of the CS(7-6) and HCN(4-3) rotational lines in the high-mass star-forming region AFGL 2591 are carried out in the compact and extended configuration of the SubMillim eter Array (SMA). The velocity structure was analyzed, and integrated maps compared to K-band near-IR observations. A chemical model predicts abundances of CS and HCN for a gas under protostellar X-ray and FUV irradiation, and was used in conjunction with the data to distinguish between physical scenarios. Results. CS and HCN emission was found in spatial coincidence in extended sources displaced up to 7'' from the position of the young star. Their line widths are small, excluding major shocks. Chemical model calculations predict an enhanced abundance of the two molecules in warm, dense, and FUV irradiated gas. Hot dust observed between the molecular emission and the outflow accounts for the necessary attenuation to prevent photodissociation of the molecule s. Conclusions. The SMA data suggest that the outflow walls are heated and chemically altere d by the FUV emission of the central high-mass object, providing the best direct evidence yet of large-sca le direct irradiation of outflow walls.
    Astronomy and Astrophysics 08/2009; 506(1). DOI:10.1051/0004-6361/200912620