Abstract. We present detailed thermal and gas-phase chemical models for the envelope of the massive star-forming region AFGL 2591. By considering both time-and space-dependent chemistry, these models are used to study both the physical structure proposed by van der Tak et al. (1999, as well as the chemical evolution of this region. The model predictions are compared with observed abundances and column densities for 29 species. The observational data cover a wide range of physical conditions within the source, but significantly probe the inner regions where interesting high-temperature chemistry may be occurring. Taking appropriate care when comparing models with both emission and absorption measurements, we find that the majority of the chemical structure can be well-explained. In particular, we find that the nitrogen and hydrocarbon chemistry can be significantly affected by temperature, with the possibility of high-temperature pathways to HCN. While we cannot determine the sulphur reservoir, the observations can be explained by models with the majority of the sulphur in CS in the cold gas, SO2 in the warm gas, and atomic sulphur in the warmest gas. Because the model overpredicts CO2 by a factor of 40, various high-temperature destruction mechanisms are explored, including impulsive heating events. The observed abundances of ions such as HCO + and N2H + and the cold gas-phase production of HCN constrain the cosmic-ray ionization rate to ∼5.6 × 10 −17 s −1 , to within a factor of three. Finally, we find that the model and observations can simultaneously agree at a reasonable level and often to within a factor of three for 7 × 10 3 ≤ t(yrs) ≤ 5 × 10 4 , with a strong preference for t ∼ 3 × 10 4 yrs since the collapse and formation of the central luminosity source.

Abstract. The sulphur chemistry in nine regions in the earliest stages of high-mass star formation is studied through singledish submillimeter spectroscopy. The line profiles indicate that 10-50% of the SO and SO 2 emission arises in high-velocity gas, either infalling or outflowing. For the low-velocity gas, excitation temperatures are 25 K for H 2 S, 50 K for SO, H 2 CS, NS and HCS + , and 100 K for OCS and SO 2 , indicating that most observed emission traces the outer parts (T < 100 K) of the molecular envelopes, except high-excitation OCS and SO 2 lines. Abundances in the outer envelopes, calculated with a Monte Carlo program, using the physical structures of the sources derived from previous submillimeter continuum and CS line data, are ∼10 −8 for OCS, ∼10 −9 for H 2 S, H 2 CS, SO and SO 2 , and ∼10 −10 for HCS + and NS. In the inner envelopes (T > 100 K) of six sources, the SO 2 abundance is enhanced by a factor of ∼100-1000. This region of hot, abundant SO 2 has been seen before in infrared absorption, and must be small, < ∼ 0. 2 (180 AU radius). The derived abundance profiles are consistent with models of envelope chemistry which invoke ice evaporation at T ∼ 100 K. Shock chemistry is unlikely to contribute. A major sulphur carrier in the ices is probably OCS, not H 2 S as most models assume. The source-to-source abundance variations of most molecules by factors of ∼10 do not correlate with previous systematic tracers of envelope heating. Without observations of H 2 S and SO lines probing warm ( > ∼ 100 K) gas, sulphur-bearing molecules cannot be used as evolutionary tracers during star formation.

Abstract. Cornerstone molecules (CO, H 2 CO, CH 3 OH, HCN, HNC, CN, CS, SO) were observed toward seven sub-millimeter bright sources in the Orion molecular cloud in order to quantify the range of conditions for which individual molecular line tracers provide physical and chemical information. Five of the sources observed were protostellar, ranging in energetics from 1−500 L , while the other two sources were located at a shock front and within a photodissociation region (PDR). Statistical equilibrium calculations were used to deduce from the measured line strengths the physical conditions within each source and the abundance of each molecule. In all cases except the shock and the PDR, the abundance of CO with respect to H 2 appears significantly below (factor of ten) the general molecular cloud value of 10 −4 . Formaldehyde measurements were used to estimate a mean temperature and density for the gas in each source. Evidence was found for trends between the derived abundance of CO, H 2 CO, CH 3 OH, and CS and the energetics of the source, with hotter sources having higher abundances. Determining whether this is due to a linear progression of abundance with temperature or sharp jumps at particular temperatures will require more detailed modeling. The observed methanol transitions require high temperatures (T > 50 K), and thus energetic sources, within all but one of the observed protostellar sources. The same conclusion is obtained from observations of the CS 7-6 transition. Analysis of the HCN and HNC 4-3 transitions provides further support for high densities n > 10 7 cm −3 in all the protostellar sources. The shape of the CO 3-2 line profile provides evidence for internal energetic events (outflows) in all but one of the protostellar sources, and shows an extreme kinematic signature in the shock region. In general, the CO line and its isotopes do not significantly contaminate the 850 µm broadband flux (less than 10%); however, in the shock region the CO lines alone account for more than two thirds of the measured sub-millimeter flux. In the energetic sources, the combined flux from all other measured molecular lines provides up to an additional few percent of line contamination.

Abstract. We present infrared spectra of gas-phase CO 2 around 15 µm toward 14 deeply embedded massive protostars obtained with the Short Wavelength Spectrometer on board the Infrared Space Observatory. Gas-phase CO 2 has been detected toward 8 of the sources. The excitation temperature and the gas/solid ratio increase with the temperature of the warm gas. Detailed radiative transfer models show that a jump in the abundance of two orders of magnitude is present in the envelope of AFGL 2591 at T > 300 K. No such jump is seen toward the colder source NGC 7538 IRS9. Together, these data indicate that gas-phase CO 2 shows the same evolutionary trends as CO 2 ice and other species, such as HCN, C 2 H 2 , H 2 O, and CH 3 OH. The gas-phase CO 2 abundance toward cold sources can be explained by gas-phase chemistry and possible freeze-out in the outer envelope. Different chemical scenarios are proposed to explain the gas-phase CO 2 abundance of 1-2 × 10 −6 for T > 300 K and of ∼10 −8for T < 300 K toward AFGL 2591. The best explanation for the low abundance in the warm exterior is provided by destruction of CO 2 caused by the passage of a shock in the past, combined with freeze-out in the coldest part at T < 100 K. The high abundance in the interior at temperatures where all oxygen should be driven into H 2 O is unexpected, but may be explained either by production of OH through X-ray ionization leading to the formation of abundant gas-phase CO 2 , or by incomplete destruction of evaporated CO 2 for T > 300 K.

Abstract. We present models and observations of gas-phase H 2 O lines between 5 and 540 µm toward deeply embedded massive protostars, involving both pure rotational and ro-vibrational transitions. The data have been obtained for 6 sources with both the Short and Long Wavelength Spectrometers (SWS and LWS) on board the Infrared Space Observatory (ISO) and with the Submillimeter Wave Astronomy Satellite (SWAS). For comparison, CO J = 7−6 spectra have been observed with the MPIfR/SRON 800 GHz heterodyne spectrometer at the James Clerk Maxwell Telescope (JCMT). A radiative transfer model in combination with different physical/chemical scenarios has been used to model these H 2 O lines for 4 sources to probe the chemical structure of these massive protostars. The results indicate that pure gas-phase production of H 2 O cannot explain the observed spectra. Ice evaporation in the warm inner envelope and freeze-out in the cold outer part are important for most of our sources and occur at T ∼ 90-110 K. The ISO-SWS data are particularly sensitive to ice evaporation in the inner part whereas the ISO-LWS data are good diagnostics of freeze-out in the outer region. The modeling suggests that the 557 GHz SWAS line includes contributions from both the cold and the warm H 2 O gas. The SWAS line profiles indicate that for some of the sources a fraction of up to 50% of the total flux may originate in the outflow. Shocks do not seem to contribute significantly to the observed emission in other H 2 O lines, however, in contrast with the case for Orion. The results show that three of the observed and modeled H 2 O lines, the 3 03 −2 12 , 2 12 −1 01 , and 1 10 −1 01 lines, are good candidates to observe with the Herschel Space Observatory in order to further investigate the physical and chemical conditions in massive star-forming regions.