We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the time-dependent gas-phase chemistry of zero-dimensional and one-dimensional interstellar sources.

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A KInetic Database for Astrochemistry (KIDA)

We present a far-ultraviolet (PDR) and an X-ray dominated region (XDR) code. We include and discuss thermal and chemical processes that pertain to irradiated gas. An elaborate chemical network is used and a careful treatment of PAHs and H(2) formation, destruction and excitation is included. For both codes we calculate four depth-dependent models for different densities and radiation fields, relevant to conditions in starburst galaxies and active galactic nuclei. A detailed comparison between PDR and XDR physics is made for total gas column densities between similar to 10(20) and similar to 10(25) cm(-2). We show cumulative line intensities for a number of fine-structure lines (e.g., [CII], [OI], [CI], [SiII], [FeII]), as well as cumulative column densities and column density ratios for a number of species ( e. g., CO/H(2), CO/C, HCO(+)/HCN, HNC/HCN). The comparison between the results for the PDRs and XDRs shows that column density ratios are almost constant up to N(H) = 1022 cm(-2) for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to NH = 10(22) cm(-2). The CO/C and CO/H(2) ratios are lower in XDRs at low column densities and rise at NH > 1023 cm-2. At most column densities NH > 10(21.5) cm-2, HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher N(H).

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Diagnostics of irradiated gas in galaxy nuclei. I. A far-ultraviolet and X-ray dominated region code

Context Low-energy cosmic rays are a fundamental source of ionization for molecular clouds, influencing their chemical, thermal, and dynamical evolution Aims The purpose of this work is to explore the possibility that a low-energy component of cosmic rays, not directly measurable from the Earth, can account for the discrepancy between the ionization rate measured in diffuse and dense interstellar clouds Methods We collected the most recent experimental and theoretical data on the cross sections for the production of H + and He + by electron and proton impact and discuss the available constraints on the cosmic-ray fluxes in the local interstellar medium Starting from different extrapolations at low energies of the demodulated cosmic-ray proton and electron spectra, we computed the propagated spectra in molecular clouds in the continuous slowing-down approximation taking all the relevant energy loss processes into account Results The theoretical value of the cosmic-ray ionization rate as a function of the column density of traversed matter agrees with the observational data only if the flux of either cosmic-ray electrons or of protons increases at low energies The most successful models are characterized by a significant (or even dominant) contribution of the electron component to the ionization rate, in agreement with previous suggestions However, the large spread of cosmic-ray ionization rates inferred from chemical models of molecular cloud cores remains to be explained Conclusions Available data combined with simple propagation models support the existence of a low-energy component (below ∼100 MeV) of cosmic-ray electrons or protons responsible for the ionization of molecular cloud cores and dense protostellar envelopes

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Cosmic-ray ionization of molecular clouds

We calculate the thermal-chemical structure of the gaseous atmospheres of the inner disks of T Tauri stars, starting from the density and dust temperature distributions derived by D’Alessio and coworkers in 1999. As a result of processes such as X-ray irradiation or mechanical heating of the surface layers, the gas temperature at the very top of the disk atmosphere in the neighborhood of 1 AU is of the order of 5000 K. Deep down, it drops rapidly into the range of the dust temperature, i.e., several hundred degrees kelvin. In between these upper hot and lower cool layers, there is a transition zone with gas temperatures in the range 500–2000 K. The thickness and location of this warm region depend on the strength of the surface heating. This region also manifests the basic chemical transitions of H to H2 and C + and C to CO. It is remarkable that even though the H2 transition begins first (higher up), it does not go to completion until after CO does. Consequently, there is a reasonably thick layer of warm CO that is predominantly atomic H. This thermal-chemical structure is favorable to the excitation of the fundamental and overtone bands of CO because of the large rate coefficients for vibrational excitation in H+CO as opposed to H2+CO collisions. This conclusion is supported by the recent observations of the fundamental band transitions in most T Tauri stars. We also argue that layered atmospheres of inner T Tauri disks may play an important role in understanding the observations of H2 UV fluorescence pumped from excited vibrational levels of that molecule. Possible candidates for surface heating include the interaction of a wind with the upper layers of the disk and dissipation of hydromagnetic waves generated by mechanical disturbances close to the midplane, e.g., by the Balbus-Hawley instability. Detailed modeling of the observations has the potential to reveal the nature of the mechanical surface heating that we model phenomenologically in these calculations and to help explain the nature of the gas in protoplanetary disks.

Heating Protoplanetary Disk Atmospheres

Cosmic-ray astrophysics has advanced rapidly in recent years, and its impact on other astronomical disciplines has broadened. Many new experiments measuring these particles, both directly in the atmosphere or space and indirectly via γ rays and synchrotron radiation, have widened the range and quality of the information available on their origin, propagation, and interactions. The impact of low-energy cosmic rays on interstellar chemistry is a fast-developing topic, including the propagation of these particles into the clouds in which the chemistry occurs. Cosmic rays, via their γ-ray production, also provide a powerful way to probe the gas content of the interstellar medium. Substantial advances have been made in the observations and modelling of the interplay between cosmic rays and the interstellar medium. Focusing on energies up to 1 TeV, these interrelating aspects are covered at various levels of detail, giving a guide to the state of the subject.

http://publications.lib.chalmers.se/records/fulltext/222464/local_222464.pdf

The Nine Lives of Cosmic Rays in Galaxies

A detailed discussion is presented of the slowing down of fast electrons in a partly ionized gas mixture of H, H2, and He with a varying ratio of H to H2 and a fixed fraction of He. The mean energies and yields for ionization, electronic excitation, dissociation, and vibrational excitation and the heating efficiencies are calculated for fractional ionizations from 0 to 0.1. Analytical formulas are obtained for many of the parameters.

Weihong Liu

Papers

Electron Energy Deposition in a Gas Mixture of Atomic and Molecular Hydrogen and Helium