# 2005A&A...440..949S

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 Query : 2005A&A...440..949S

2005A&A...440..949S - Astronomy and Astrophysics, volume 440, 949-966 (2005/9-4)

X-ray chemistry in the envelopes around young stellar objects.

STAEUBER P., DOTY S.D., VAN DISHOECK E.F. and BENZ A.O.

Abstract (from CDS):

We present chemical models of the envelope of a young stellar object (YSO) exposed to a central X-ray source. The models are applied to the massive star-forming region AFGL2591 for different X-ray fluxes. Model results for this region show that the X-ray ionization rate with and without the effects of Compton scattering differs by only a few percent and the influence of Compton scattering on the chemistry is negligible. The total X-ray ionization rate is dominated by the secondary'' ionization rate of H2 resulting from fast electrons. The abundance profiles of several molecular and atomic species are shown to depend on the X-ray luminosity and on the distance from the source. The carbon, sulphur and nitrogen chemistries are discussed. It is found that He+ and H3+ are enhanced and trigger a peculiar chemistry. Several molecular X-ray tracers are found and compared to tracers of the far ultraviolet (FUV) field. Like ultraviolet radiation fields, X-rays enhance simple hydrides, ions and radicals. In contrast to ultraviolet photons, X-rays can penetrate deep into the envelope and affect the chemistry even at large distances from the source. Whereas the FUV enhanced species cover a region of ≃200-300AU, the region enhanced by X-rays is >1000AU. We find that N2O, HNO, SO, SO+, HCO+, CO+, OH+, N2H+, SH+ and HSO+ (among others) are more enhanced by X-rays than by FUV photons even for X-ray luminosities as low as LX≃1030erg/s. CO2 abundances are reduced in the gas-phase through X-ray induced FUV photons. For temperatures T≲230K, H2O is destroyed by X-rays with luminosities LX>1030erg/s. Best-fit models for AFGL2591 predict an X-ray luminosity LX>1031erg/s with a hard X-ray spectrum TX>3x107K. This is the first time that the X-ray flux of a highly obscured source has been estimated by its envelope chemistry. Furthermore, we find LX/Lbol≃10–6. The chemistry of the bulk of the envelope mass is dominated by cosmic-ray induced reactions rather than by X-ray induced ionization for X-ray luminosities LX≲1033erg/s. The calculated line intensities of HCO+ and HCS+ show that high-J lines are more affected than lower J lines by the presence of X-rays due to their higher critical densities, and that such differences are detectable even with large aperture single-dish telescopes. Future instruments such as Herschel-HIFI or SOFIA will be able to observe X-ray enhanced hydrides whereas the sensitivity and spatial resolution of ALMA is well-suited to measure the size and geometry of the region affected by X-rays.

Journal keyword(s): stars: formation - stars: individual: AFGL 2591 - ISM: molecules - X-rays: ISM

Full paper

 Number of rows : 3
N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2022
#notes
1 NAME ORI MOL CLOUD MoC 05 56 -01.8           ~ 955 1
2 IRAS 16293-2422 cor 16 32 22.56 -24 28 31.8           ~ 1158 1
3 RAFGL 2591 Y*O 20 29 24.86688 +40 11 19.4100           ~ 595 0

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2022.05.19-23:03:23