Abstract.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 AFGL 2591 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 H 2 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 H + 3 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 ≈200−300 AU, the region enhanced by X-rays is 1000 AU. We find that N 2 O, HNO, SO, SO + , HCO + , CO + , OH + , N 2 H + , SH + and HSO + (among others) are more enhanced by X-rays than by FUV photons even for X-ray luminosities as low as L X ≈ 10 30 erg s −1 . CO 2 abundances are reduced in the gas-phase through X-ray induced FUV photons. For temperatures T 230 K, H 2 O is destroyed by X-rays with luminosities L X 10 30 erg s −1 . Best-fit models for AFGL 2591 predict an X-ray luminosity L X 10 31 erg s −1 with a hard X-ray spectrum T X 3 × 10 7 K. 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 L X /L bol ≈ 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 L X 10 33 erg s −1 . 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.

Abstract.We have studied the influence of far ultraviolet (UV) radiation (6 < hν < 13.6 eV) from a massive young stellar object (YSO) on the chemistry of its own envelope by extending the models of Doty et al. (2002) to include a central source of UV radiation. The models are applied to the massive star-forming region AFGL 2591 for different inner UV field strengths. Depth-dependent abundance profiles for several molecules are presented and discussed. We predict enhanced column densities for more than 30 species, especially radicals and ions. Comparison between observations and models is improved with a moderate UV field incident on the inner envelope, corresponding to an enhancement factor G 0 ≈ 10-100 at 200 AU from the star with an optical depth τ ≈ 15-17. The chemical networks of various species are explored. Subtle differences are found compared with traditional models of Photon Dominated Regions (PDRs) because of the higher temperatures and higher gasphase H 2 O abundance caused by evaporation of ices in the inner region. In particular, the CN/HCN ratio is not a sensitive tracer of the inner UV field, in contrast with the situation for normal PDRs: for low UV fields, the extra CN reacts with H 2 in the inner dense and warm region and produces more HCN. It is found that the CH + abundance is strongly enhanced and grows steadily with increasing UV field. In addition, the ratio CH + /CH is increased by a factor of 10 3 -10 5 depending on the inner UV flux. High-J lines of molecules like CN and HCN are most sensitive to the inner dense region where UV radiation plays a role. Thus, even though the total column density affected by UV photons is small, comparison of high-J and low-J lines can selectively trace and distinguish the inner UV field from the outer one. In addition, future Herschel-HIFI observations of hydrides can sensitively probe the inner UV field.

The Herschel Space Observatory enables observations in the far-infrared at high spectral and spatial resolution. A particular class of molecules will be directly observable: light diatomic hydrides and their ions (CH, OH, SH, NH, CH + , OH + , SH + , NH + ). These simple constituents are important both for the chemical evolution of the region and as tracers of high-energy radiation. If outflows of a forming star erode cavities in the envelope, protostellar far-UV (FUV; 6 < E γ < 13.6 eV) radiation may escape through such low-density regions. Depending on the shape of the cavity, the FUV radiation then irradiates the quiescent envelope in the walls along the outflow. The chemical composition in these outflow walls is altered by photoreactions and heating via FUV photons in a manner similar to photo-dominated regions. In this work, we study the effect of cavity shapes, outflow density, and of a disk with the two-dimensional chemical model of a high-mass young stellar object introduced in the second paper in this series. The model has been extended with a self-consistent calculation of the dust temperature and a multi-zone escape probability method for the calculation of the molecular excitation and the prediction of line fluxes. We find that the shape of the cavity is particularly important in the innermost part of the envelope, where the dust temperatures are high enough ( 100 K) for water ice to evaporate. If the cavity shape allows FUV radiation to penetrate this hot-core region, the abundance of FUV-destroyed species (e.g., water) is decreased. On larger scales, the shape of the cavity is less important for the chemistry in the outflow wall. In particular, diatomic hydrides and their ions CH + , OH + , and NH + are enhanced by many orders of magnitude in the outflow walls due to the combination of high gas temperatures and rapid photodissociation of more saturated species. The enhancement of these diatomic hydrides is sufficient for a detection using the HIFI and PACS instruments on board Herschel. The effect of X-ray ionization on the chemistry is found to be small, due to the much larger luminosity in FUV bands compared to X-rays.

High resolution spectra of the Spitzer Space Telescope show vibration-rotation absorption bands of gaseous C 2 H 2 , HCN, and CO 2 molecules toward a sample of deeply obscured (U)LIRG nuclei. The observed bands reveal the presence of dense (n 10 7 cm −3 ), warm (T ex = 200 − 700 K) molecular gas with high column densities of these molecules ranging from a few 10 15 − 10 17 cm −2 . Abundances relative to H 2 , inferred from the silicate optical depth, range from ∼ 10 −7 to 10 −6 and show no correlation with temperature. Theoretical studies show that the high abundances of both C 2 H 2 and HCN exclude a X-ray dominated region (XDR) associated with the toroid surrounding an AGN as the origin of this dense warm molecular gas. Galactic massive protostars in the so-called Hot Core phase have similar physical characteristics with comparable high abundances of C 2 H 2 , HCN, and CO 2 in the hot phase. However, the abundances of C 2 H 2 and HCN and the C 2 H 2 /CO 2 and HCN/CO 2 ratios are much higher toward the (U)LIRGs in the cooler (T ex 400 K) phase. We suggest that the -2warm dense molecular gas revealed by the mid-IR absorption lines is associated with a phase of deeply embedded star formation where the extreme pressures and densities of the nuclear starburst environment have inhibited the expansion of HII regions and the global disruption of the star forming molecular cloud cores, and 'trapped' the star formation process in an 'extended' Hot Core phase.Subject headings: infrared: ISM -ISM: evolution -ISM: galaxies -ISM: molecules -galaxies: nuclei 11 The IRS-SH slit width is 4.7", equal to the size of the PSF at 19.5µm.

Aims. The aim is to probe high energy radiation emitted by deeply embedded protostars. Methods. Submillimeter lines of CN, NO, CO+ and SO + , and upper limits on SH + and N 2 O are observed with the James Clerk Maxwell Telescope in two high-mass and up to nine low-mass young stellar objects and compared with chemical models.Results. Constant fractional abundances derived from radiative transfer modeling of the line strengths are x(CN) ≈ a few ×10 −11 -10 −8 , x(NO) ≈ 10 −9 -10 −8 and x(CO + ) ≈ 10 −12 -10 −10 . SO + has abundances of a few ×10 −11 in the high-mass objects and upper limits of ≈10 −12 -10 −11 in the low-mass sources. All abundances are up to 1-2 orders of magnitude higher if the molecular emission is assumed to originate mainly from the inner region ( 1000 AU) of the envelope. For high-mass sources, the CN, SO + and CO + abundances and abundance ratios are best explained by an enhanced far-ultraviolet (FUV) field impacting gas at temperatures of a few hundred K. The observed column densities require that this region of enhanced FUV has scales comparable to the observing beam, such as in a geometry in which the enhanced FUV irradiates outflow walls. For low-mass sources, the required temperatures within the FUV models of T 300 K are much higher than found in models, so that an X-ray enhanced region close to the protostar (r 500 AU) is more plausible. Gas-phase chemical models produce more NO than observed, suggesting an additional reduction mechanism not included in current models. Conclusions. The observed CN, CO+ and SO + abundances can be explained with either enhanced X-rays or FUV fields from the central source. High-mass sources likely have low opacity regions that allow the FUV photons to reach large distances from the central source. X-rays are suggested to be more effective than FUV fields in the low-mass sources. The observed abundances imply X-ray fluxes for the Class 0 objects of L X ≈ 10 29 -10 31 erg s −1 , comparable to those observed from low-mass Class I protostars. Spatially resolved data are needed to clearly distinguish the effects of FUV and X-rays for individual species.