Roland Gredel

Cosmic-ray-induced photodissociation and photoionization rates of interstellar molecules

In the Prasad-Tarafdar mechanism, ultraviolet photons are created in the interior of dense interstellar clouds by the impact excitation of molecular hydrogen by secondary electrons generated by cosmic-ray ionization. Detailed calculations of the emission spectrum are described, and the resulting photodissociation and photoionization rates of a wide range of interstellar molecules are calculated. 84 refs.

C/CO ratio in dense interstellar clouds

The cosmic-ray-induced photodestruction (CRIP) of CO in dense interstellar clouds is investigated by means of numerical simulations to determine the C/CO abundance ratio. The model takes the dependence of C/CO on the rotational temperature, abundance, and velocity distribution of the CO into account. The results are presented in a table and briefly characterized. A uniform 30-K cloud with CO/H/sub 2/ = 0.00015 is found to have steady-state C/CO = 0.005 when CRIP is included, about 1000 times the ratio without CRIP. The unusually high C/CO of 0.1 seen in some clouds (Keene et al., 1985) is explained in terms of a C depletion factor of 0.001 relative to O and a velocity difference through the cloud of about 10 km/s. 22 references.

Infrared Response of H2 to X-Rays in Dense Clouds

The excitation by X-rays and cosmic rays of molecular hydrogen in interstellar clouds is analyzed. We carried out detailed calculations of entry efficiencies in rovibrational levels of H2 following impact with fast electrons produced by X-ray ionization of the gas. The competing effect of collisional excitation, and quenching, by the ambient gas is examined in detail. Up to date values for H-H2 collisional rate coefficients are adopted, and some derivations of H2-H2 rovibrational rate coefficients from existing literature data are proposed. Several models as a function of temperature, density, and ionization rate are presented. We found that H2 infrared emission in X-ray dominated regions (XDR) is potentially observable for temperatures and ionization rates lower than certain critical values (typically T < 1000 K and ζ/nH < 10-15 cm3 s-1, where ζ is the ionization rate). At higher temperatures, collisional excitation by the ambient gas dominates the population of low vibrational levels, and at higher values of ζ/nH the abundance of H2 is negligible. If such conditions are satisfied, the resulting infrared emission spectrum can be used as a diagnostic of nearby X-ray sources such as in cooling flows in galaxy clusters, quasars, Seyfert galaxies and supernova remnants. The intensity ratio of the 2-1S(1) and 1-0S(1) lines measured for the Seyfert galaxy NGC 1275 is consistent with X-ray pumping.

Spectral Analysis Using the Wavelet Transform

We introduce a new signal processing technique to analyze noisy spectra. The method is based on the wavelet transform and employs the a trous algorithm. Noise determination and detection criteria are discussed in detail, together with pitfalls related to the use of wavelets in the analysis of spectra. Simulations are presented to demonstrate the power and the shortcomings of our method. We apply our technique to the case of continuum sources that show superposed interstellar or circumstellar absorption or emission bands that are shallow and broad. In particular, we analyze an L-band spectrum of the Herbig-Haro energy source HH 100 IRS. The analysis indicates the presence of a shallow emission band near 3.51 μm that is tentatively assigned to arise from aliphatic (CH2) vibrations.

Near-infrared observations of the HH 111 region

We have obtained near-infrared K-band images of the HH 111 region, and detected, besides the HH 111 jet itself, a new optically invisible bipolar jet here called HH 121, from the source region of HH 111, but at large angles to the HH 111 flow axis. Near-infrared long-slit spectra of both jets confirm their emission-line nature. Radial velocities derived from H 2 lines show that the northern lobe of HH 121 is approaching and the southern is receding, but the velocities are so low that the system is likely to lie almost in the plane of the sky. There is a considerable angle between the two lobes of HH 121

Performance of the ESO AO system, ADONIS, at the La Silla 3.6-m telescope

We report on the calibration and field test results obtained with the ESO/Meudon Adaptive Optics System, Adonis. This 7x7, 50cm subaperture system is now routinely in use at the ESO La Silla 3.6m telescope, offered to the astronomers together with two infrared cameras covering the 1-5 µm wavelength range. The calibration activity summarized here has investigated the field performances achieved so far with Natural Guide Stars (NGS) using the either the Reticon or the EBCCD wavefront sensors.

Interstellar CH+ — an Outstanding Enigma

Optical absorption line observations of interstellar CH+ and CH towards the southern OB associations CMa OB1, NGC 2439, Vela OB1, NGC 4755, and Cen OB1, demonstrate that the CH+ column density N(CH+) is correlated with the visual extinction A V of the background star, and that N(CH+) is correlated with N(CH). This indicates that the CH+ column density is correlated with the optical depth of a cloud. It is also found that the CH+ and CH radial velocities agree within the measurement uncertainties. These findings are difficult to reconcile with a scenario where the CH+ production is driven by shocks, or where the sites of CH+ formation are constrained to the surface of molecular clouds. The observations support ideas which involve turbulence as a major CH+ production mechanism.

A DUST MODEL FOR THE FROSTY NEBULA HH 100 IRS

Comprehensive model calculations are presented in order to interpret the observed spectral energy distribution of HH 100 IRS as a function of dust parameters such as the grain size, the ice volume fraction, and the fluffiness of the particles. The radiative transfer calculations treat the spectroscopic signatures of various ice bands in detail. The observed infrared spectrum together with the strength of the water ice band of HH 100 IRS is successfully reproduced if an upper size limit of the grains below 1 μm is used. Large, comet-like grains, with sizes above 1 μm, result in a poor fit to the observations of HH 100 IRS. Contributions from scattering by the grains to the long-wavelength wing of the 3.08 μm H2O absorption are thus excluded. Our best-fit dust model includes grains with sizes a ≤ 1 μm, a vacuum volume fraction of fvac = 0.5, and a mass ratio of ice to refractive materials of Mice/Mref = 0.5. The modeled dust cloud toward HH 100 IRS is characterized by a dust density distribution of ρ(r) ∝ r-1, a total gas mass of 0.3 M☉, an inner ice evaporation zone with extension of r ≤ 23 AU, and an outer boundary of 2300 AU.

Molecular abundances in translucent clouds

Sensitive searches for 13CO and C18O J=1 → 0 emission are presented for a sample of translucent molecular clouds previously studied by optical absorption lines towards background stars. C18O has been detected in only 5 clouds, most of which have extinctions well in excess of 2 mag. The inferred 13CO/C18O abundance ratios range from 7–25, and the lower limits from ≳13 to ≳35. These values are as much as five times larger than the overall interstellar ([13C]⋅[16O])/([12C]⋅[18O]) ratio, suggesting that isotope–selective photodissociation plays a role in at least some of the clouds. Searches for other molecules at millimeter wavelengths have been made for a few of the best characterized clouds. Surprisingly, no emission was detected from the C2H or C3H2 molecules, even though the abundances of diatomic C2 and CH are quite large. On the other hand, the abundance of HCO+ appears comparable to that found in denser clouds, and abundance of HCN may be up to an order of magnitude larger than the prediction of model...

The Excitation of Molecular Hydrogen in XDRS

X-rays generate energetic secondary electrons which in a molecular cloud slow down by exciting and ionizing H2. Impact excitation of electronic states of H2 is followed by cascading to the ro-vibration levels of the X \( {}^1\sum {_g^{ + }} \) ground state. The lowest vibration levels in the ground state are directly excited by the secondary electrons as well. Electric quadrupole radiation produces an emission spectrum that peaks in the near-infrared wavelength region. The H2 spectrum is calculated as a function of various physical parameters such as the gas temperature T, the gas density nH, and the ionization rate ς-rays dominate the excitation of the strongest vibrational lines if T 10−21 s−1 cm−3. In this parameter regime, the resulting infrared spectrum can be used as a diagnostic of X-ray dominated regions.