Sulfur appears to be depleted by an order of magnitude or more from its elemental abundance in star-forming regions. In the last few years, numerous observations and experiments have been performed in order to to understand the reasons behind this de
pletion without providing a satisfactory explanation of the sulfur chemistry towards high-mass star-forming cores. Several sulfur-bearing molecules have been observed in these regions, and yet none are abundant enough to make up the gas-phase deficit. Where, then, does this hidden sulfur reside? This paper represents a step forward in our understanding of the interactions among the various S-bearing species. We have incorporated recent experimental and theoretical data into a chemical model of a hot molecular core in order to see whether they give any indication of the identity of the sulfur sink in these dense regions. Despite our model producing reasonable agreement with both solid-phase and gas-phase abundances of many sulfur-bearing species, we find that the sulfur residue detected in recent experiments takes up only ~6 per cent of the available sulfur in our simulations, rather than dominating the sulfur budget.
We report observations of molecular oxygen (O$_2$) rotational transitions at 487 GHz, 774 GHz, and 1121 GHz toward Orion Peak A. The O2 lines at 487 GHz and 774 GHz are detected at velocities of 10-12 km/s with line widths 3 km/s; however, the transi
tion at 1121 GHz is not detected. The observed line characteristics, combined with the results of earlier observations, suggest that the region responsible for the O$_2$ emission is 9 (6e16 cm) in size, and is located close to the H2 Peak 1position (where vibrationally-excited H$_2$ emission peaks), and not at Peak A, 23 away. The peak O2 column density is 1.1e18/cm2. The line velocity is close to that of 621 GHz water maser emission found in this portion of the Orion Molecular Cloud, and having a shock with velocity vector lying nearly in the plane of the sky is consistent with producing maximum maser gain along the line-of-sight. The enhanced O$_2$ abundance compared to that generally found in dense interstellar clouds can be explained by passage of a low-velocity C-shock through a clump with preshock density 2e4/cm3, if a reasonable flux of UV radiation is present. The postshock O$_2$ can explain the emission from the source if its line of sight dimension is ~10 times larger than its size on the plane of the sky. The special geometry and conditions required may explain why O$_2$ emission has not been detected in the cores of other massive star-forming molecular clouds.
Within a few parsecs around the central Black Hole Sgr A*, chemistry in the dense molecular cloud material of the circumnuclear disk (CND) can be affected by many energetic phenomena such as high UV-flux from the massive central star cluster, X-rays
from Sgr A*, shock waves, and an enhanced cosmic-ray flux. Recently, spectroscopic surveys with the IRAM 30 meter and the APEX 12 meter telescopes of substantial parts of the 80--500 GHz frequency range were made toward selected positions in and near the CND. These datasets contain lines from the molecules HCN, HCO$^+$, HNC, CS, SO, SiO, CN, H$_2$CO, HC$_3$N, N$_2$H$^+$, H$_3$O$^+$ and others. We conduct Large Velocity Gradient analyses to obtain column densities and total hydrogen densities, $n$, for each species in molecular clouds located in the southwest lobe of CND. The data for the above mentioned molecules indicate 10$^5,$cm$^{-3} lesssim n <10^6,$cm$^{-3}$, which shows that the CND is tidally unstable. The derived chemical composition is compared with a chemical model calculated using the UCL_CHEM code that includes gas and grain reactions, and the effects of shock waves. Models are run for varying shock velocities, cosmic-ray ionization rates, and number densities. The resulting chemical composition is fitted best to an extremely high value of cosmic-ray ionization rate $zeta sim 10^{-14},$s$^{-1}$, 3 orders of magnitude higher than the value in regular Galactic molecular clouds, if the pre-shock density is $n=10^5,$cm$^{-3}$.
We use 3D-PDR, a three-dimensional astrochemistry code for modeling photodissociation regions (PDRs), to post-process hydrodynamic simulations of turbulent, star-forming clouds. We focus on the transition from atomic to molecular gas, with specific a
ttention to the formation and distribution of H, C+, C, H2 and CO. First, we demonstrate that the details of the cloud chemistry and our conclusions are insensitive to the simulation spatial resolution, to the resolution at the cloud edge, and to the ray angular resolution. We then investigate the effect of geometry and simulation parameters on chemical abundances and find weak dependence on cloud morphology as dictated by gravity and turbulent Mach number. For a uniform external radiation field, we find similar distributions to those derived using a one-dimensional PDR code. However, we demonstrate that a three-dimensional treatment is necessary for a spatially varying external field, and we caution against using one-dimensional treatments for non-symmetric problems. We compare our results with the work of Glover et al. (2010), who self-consistently followed the time evolution of molecule formation in hydrodynamic simulations using a reduced chemical network. In general, we find good agreement with this in situ approach for C and CO abundances. However, the temperature and H2 abundances are discrepant in the boundary regions (Av < 5), which is due to the different number of rays used by the two approaches.
We aimed to study the molecular composition of the interstellar medium (ISM) surrounding an Active Galactic Nucleus (AGN), by making an inventory of molecular species and their abundances, as well as to establish a chemical differentiation between st
arburst galaxies and AGN. We used the IRAM-30 m telescope to observe the central 1.5-2 kpc region of NGC1068, covering the frequencies between 86.2 GHz and 115.6 GHz. Using Boltzmann diagrams, we calculated the column densities of the detected molecules. We used a chemical model to reproduce the abundances found in the AGN, to determine the origin of each detected species, and to test the influence of UV fields, cosmic rays, and shocks on the ISM. We identified 24 different molecular species and isotopologues, among which HC3N, SO, N2H+, CH3CN, NS, 13CN, and HN13C are detected for the first time in NGC1068. We obtained the upper limits to the isotopic ratios 12C/13C=49, 16O/18O=177 and 32S/34S=5. Our chemical models suggest that the chemistry in the nucleus of NGC1068 is strongly influenced by cosmic rays, although high values of both cosmic rays and far ultraviolet (FUV) radiation fields also explain well the observations. The gas in the nucleus of NGC1068 has a different chemical composition as compared to starburst galaxies. The distinct physical processes dominating galaxy nuclei (e.g. C-shocks, UV fields, X-rays, cosmic rays) leave clear imprints in the chemistry of the gas, which allow to characterise the nucleus activity by its molecular abundances.
We surveyed the circumnuclear disk of the Seyfert galaxy NGC1068 between the frequencies 86.2 GHz and 115.6 GHz, and identified 17 different molecules. Using a time and depth dependent chemical model we reproduced the observational results, and show
that the column densities of most of the species are better reproduced if the molecular gas is heavily pervaded by a high cosmic ray ionization rate of about 1000 times that of the Milky Way. We discuss how molecules in the NGC1068 nucleus may be influenced by this external radiation, as well as by UV radiation fields.
We aimed to study the chemistry of the circumnuclear molecular gas of NGC1068, and to compare it with those of the starburst galaxies M82 and NGC253. Using the IRAM-30m telescope, we observed the inner 2 kpc of NGC1068 between 86.2 GHz and 115.6 GHz.
We identified 35 spectral features, corresponding to 24 different molecular species. Among them, HC3N, SO, N2H+, CH3CN, NS, 13CN, and HN13C are detected for the first time in NGC1068. Assuming local thermodynamic equilibrium (LTE), we calculated the column densities of the detected molecules, as well as the upper limits to the column densities of some undetected species. The comparison among the chemistries of NGC1068, M82, and NGC253, suggests that, apart from X-rays, shocks also determine the chemistry of NGC1068. We propose the column density ratio between CH3CCH and HC3N as a prime indicator of the imprints of starburst and AGN environments in the circumnuclear interstellar medium. This ratio is, at least, 64 times larger in M82 than in NGC1068, and, at least, 4 times larger in NGC253 than in NGC1068. Finally, we used the UCL_CHEM and UCL_PDR chemical codes to constrain the origin of the species, as well as to test the influence of UV radiation fields and cosmic rays on the observed abundances.
We investigate the physical and chemical conditions necessary for low-mass star formation in extragalactic environments by calculating various characteristic timescales associated with star formation for a range of initial conditions. The balance of
these timescales indicates whether low-mass star formation is enhanced or inhibited under certain physical conditions. In this study, we consider timescales for free-fall, cooling, freeze-out, desorption, chemistry and ambipolar diffusion and their variations with changes in the gas density, metallicity, cosmic ray ionisation rate and FUV radiation field strength. We find that extragalactic systems with high FUV radiation field strengths and high cosmic ray fluxes considered at a range of metallicities, are likely to have enhanced low-mass star formation unless the magnetic pressure is sufficient to halt collapse. Our results indicate that this is only likely to be the case for high-redshift galaxies approaching solar metallicities. Unless this is true for all high-redshift sources, this study finds little evidence for a high-mass biased IMF at high redshifts.
We present high spatial resolution maps, obtained with the Plateau de Bure Interferometer, of the blue lobe of the L1157 outflow. We observed four lines at 3 mm, namely CH3OH (2_K-1_K), HC3N (11-10), HCN (1-0) and OCS (7-6). Moreover, the bright B1 c
lump has also been observed at better spatial resolution in CS (2-1), CH3OH (2_1-1_1)A-, and 34SO (3_2-2_1). These high spatial resolution observations show a very rich structure in all the tracers, revealing a clumpy structure of the gas superimposed to an extended emission. In fact, the three clumps detected by previous IRAM-30m single dish observations have been resolved into several sub-clumps and new clumps have been detected in the outflow. The clumps are associated with the two cavities created by two shock episodes driven by the precessing jet. In particular, the clumps nearest the protostar are located at the walls of the younger cavity with a clear arch-shape form while the farthest clumps have slightly different observational characteristics indicating that they are associated to the older shock episode. The emission of the observed species peaks in different part of the lobe: the east clumps are brighter in HC3N (11-10), HCN (1-0) and CS (2-1) while the west clumps are brighter in CH3OH(2_K-1_K), OCS (7-6) and 34SO (3_2-2_1). This peak displacement in the line emission suggests a variation of the physical conditions and/or the chemical composition along the lobe of the outflow at small scale, likely related to the shock activity and the precession of the outflow. In particular, we observe the decoupling of the silicon monoxide and methanol emission, common shock tracers, in the B1 clump located at the apex of the bow shock produced by the second shock episode.
