arXiv subjects
Publications and source records attributed to Floris van der Tak.
We discuss some implications of our recent detection of extragalactic H3O+: the location of the gas in M82, the origin of energetic radiation in M82, and the possible feedback effects of star formation on the cosmic ray flux in galaxies.
This review discusses recent results on the astrochemistry of (mostly high-mass) star-forming regions. After an introduction on the use of chemistry in astrophysics and some basic concepts of astrochemistry, specific results are presented. Highlighted areas are the use of chemistry in the search for massive circumstellar disks, the interaction of molecular clouds with cosmic rays, and the feedback effects of protostellar irradiation on the parent molecular cloud. The review concludes with a discussion of future observational opportunities.
The H3O+ molecule probes the oxygen chemistry and the ionization rate of dense circumnuclear gas in galaxies. In particular, recent H3O+ observations show variations in the cosmic-ray ionization rate by factors of $>$10 within our Galaxy. Using the JCMT, we have observed the 364 GHz line of p-H3O+ in the centers of M82 and Arp 220. In Arp 220, the line profile suggests that the emission originates in the Western nucleus. In M82, both the eastern molecular peak and the circumnuclear region contribute to the emission. The derived column densities, abundances, and H3O+ / H2O ratios indicate ionization rates similar to or even exceeding that in the Galactic Center. Model calculations of the chemistry of irradiated molecular gas indicate a likely origin of this high ionization rate in the extended, evolved starburst of M82. In contrast, irradiation by X-rays from the AGN disk is the most likely model for Arp 220.
The large quantity and high quality of modern radio and infrared line observations require efficient modeling techniques to infer physical and chemical parameters such as temperature, density, and molecular abundances. We present a computer program to calculate the intensities of atomic and molecular lines produced in a uniform medium, based on statistical equilibrium calculations involving collisional and radiative processes and including radiation from background sources. Optical depth effects are treated with an escape probability method. The program is available on the World Wide Web at http://www.sron.rug.nl/~vdtak/radex/index.shtml . The program makes use of molecular data files maintained in the Leiden Atomic and Molecular Database (LAMDA), which will continue to be improved and expanded. The performance of the program is compared with more approximate and with more sophisticated methods. An Appendix provides diagnostic plots to estimate physical parameters from line intensity ratios of commonly observed molecules. This program should form an important tool in analyzing observations from current and future radio and infrared telescopes.
This paper presents an overview of several modeling tools for analyzing molecular line observations at submillimeter wavelengths. These tools are already proving to be very valuable for the interpretation of data from current telescopes, and will be indispensable for data obtained with ALMA. The tools are: (1) the Leiden Atomic and Molecular DAtabase (LAMDA), a collection of spectroscopic data and collisional excitation rates; (2) RADEX, an on-line and off-line program to calculate non-LTE excitation and emission from a homogeneous medium, based on the escape probability approximation; (3) RATRAN, an accelerated Monte Carlo program to solve molecular excitation and radiative transfer in spherical and cylindrical symmetry. The paper presents examples of how to use these tools in conjunction with existing data reduction packages to quantitatively interpret submillimeter single-dish and interferometric observations. The described tools are publically available at http://www.strw.leidenuniv.nl/~moldata . The paper concludes with a discussion of future needs in the fields of molecular data and radiative transfer.
Single-dish spectra and interferometric maps of (sub)mm lines of H2O-18 and HDO are used to study the chemistry of water in eight regions of high-mass star formation. The spectra indicate HDO excitation temperatures of ~110 K and column densities in an 11'' beam of ~2x10^14 cm^-2 for HDO and ~2x10^17 cm^-2 for H2O, with the N(HDO)/N(H2O) ratio increasing with decreasing temperature. Simultaneous observations of CH3OH and SO2 indicate that 20-50% of the single-dish line flux arises in the molecular outflows of these objects. The outflow contribution to the H2O-18 and HDO emission is estimated to be 10-20%. Radiative transfer models indicate that the water abundance is low (~10^-6) outside a critical radius corresponding to a temperature in the protostellar envelope of ~100 K, and `jumps' to H2O/H2 ~ 10^-4 inside this radius. This value corresponds to the observed abundance of solid water and together with the derived HDO/H2O abundance ratios of ~1/1000 suggests that the origin of the observed water is evaporation of grain mantles. This idea is confirmed in the case of AFGL 2591 by interferometer observations of HDO, H2O-18 and SO2 lines, which reveal compact (~800 AU) emission with a systematic velocity gradient. This size is similar to that of the 1.3 mm continuum towards AFGL 2591, from which we estimate a mass of ~0.8 M0, or ~5% of the mass of the central star. We speculate that we may be observing a circumstellar disk in an almost face-on orientation.
This paper reports 43 GHz imaging of the high-mass protostars W33A, AFGL 2591 and NGC 7538 IRS9 at 0.04'' and 0.6'' resolution. In each case, weak (~mJy), compact (~100 AU) emission is detected, which has an elongated shape (axis ratio ~3). For AFGL 2591 and NGC 7538 IRS9, the emission is single-peaked, while for the highest luminosity source, W33A, a `mini-cluster' of three sources is detected. The derived sizes, flux densities, and broad-band radio spectra of the sources support recent models where the initial expansion of HII regions around very young O-type stars is prevented by stellar gravity. In these models, accretion flows onto high-mass stars originate in large-scale molecular envelopes and become ionized close to the star. These models reproduce our observations of ionized gas as well as the structure of the molecular envelopes of these sources on 10^3--10^4 AU scales derived previously from single-dish submillimeter continuum and line mapping. For AFGL 2591, the 43 GHz flux density is also consistent with dust emission from a disk seen in near-infrared `speckle' images. However, the alignment of the 43 GHz emission with the large-scale molecular outflow argues against an origin in a disk for AFGL 2591 and NGC 7538 IRS9. In contrast, the outflow from W33A does not appear to be collimated. Together with previously presented case studies of W3 IRS5 and AFGL 2136, our results indicate that the formation of stars and stellar clusters with luminosities up to ~10^5 L0 proceeds through accretion and produces collimated outflows as in the solar-type case, with the `additional feature' that the accretion flow becomes ionized close to the star. Above ~10^5 L0, clusters of HII regions appear, and outflows are no longer collimated, possibly as the result of mergers of protostars or pre-stellar cores.
This paper explores the sensitivity of line profiles of H2D+, HCO+ and N2H+, observed towards the center of L1544, to various kinematic and chemical parameters. The total width of the H2D+ line can be matched by a static model and by models invoking ambipolar diffusion and gravitational collapse. The derived turbulent line width is b=0.15 km/s for the static case and <~ 0.05 km/s for the collapse case. However, line profiles of HC18O+ and N2H+ rule out the static solution. The double-peaked H2D+ line shape requires either infall speeds in the center that are much higher than predicted by ambipolar diffusion models, or a shell-type distribution of H2D+, as is the case for HCO+ and N2H+. At an offset of ~20 arcsec from the dust peak, the H2D+ abundance drops by a factor of ~5.