D. C. Lis

The composition of ices in comet C/1995 O1 (Hale-Bopp) from radio spectroscopy - Further results and upper limits on undetected species

From radio spectroscopic observations of comets, more than 22 molecules, radicals and ions, plus several isotopologues, were detected, the majority of them being recently revealed in comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp). Among them, 6 molecules were detected for the first time (Bockelee-Morvan et al. [CITE]) in the course of a spectral survey conducted at radio wavelengths in comet Hale-Bopp with the CSO, the IRAM 30-m telescope and Plateau de Bure interferometer. In addition, many species were searched for unsuccessfully, some of them with stringent upper limits. We present here a review of these observations and further analysis of their results. This include: (i) confirmed detection of acetaldehyde (CH_3CHO); (ii) limits on small molecules such as ketene (H_2CCO) or methanimine (CH_2NH); (iii) limits on the abundance ratios in homologous series such as HC_5N/HC_3N, ethanol/methanol, acetic acid/formic acid; (iv) searches for precursors of key cometary species such as atomic Na and HNC; (v) constraints on more exotic species ranging from water dimer (H_2O)_2 to glycine; (vi) detection of the H_2^(34)S isotopic species and independent observations of HDO and DCN; (vii) limits on several other deuterated species; (viii) limits on several radicals and ions and a tentative detection of the C_2H radical; (ix) the presence of unidentified lines. Typical abundance upper limits of 2–5 x 10^(-4) relative to water are achieved for many species. Better upper limits are obtained for some linear molecules with high dipole moments. But more complex molecules such as dimethyl ether or glycine are poorly constrained. These results should give important clues to the chemical composition of cometary ices, to the formation mechanisms of cometary material, and to the chemical processes which occur in the inner coma.

Carbon Monoxide and Dust Column Densities: The Dust-to-Gas Ratio and Structure of Three Giant Molecular Cloud Cores

We have observed emission in the three lowest rotational transitions of the optically thin species C18O and the dust continuum emission at three millimeter/submillimeter wavelengths. By employing the proper combination of the intensities of the three lowest rotational transitions of C18O, we can obtain the total molecular column density, with relatively little sensitivity to density and temperature variations along the line of sight. We use the line and continuum data to determine column densities of the dust and gas across three giant molecular cloud cores. We find that two of the three sources, M17 and Cepheus A, have the same gas column density-to-dust optical depth ratio, given by log [N(C18O)/τ(790 μm)] = 18.8. In the third source, the Orion molecular cloud, the gas-to-dust ratio is typically a factor of 3 lower than in the other two sources. The gas-to-dust ratio shows only a small (factor ≤ 3) variation across the region of M17 that we have mapped and a comparable reduction at the center of Cepheus A relative to the cloud edge. We have good evidence for the correlation of the continuum emission in different bands for the Orion molecular cloud and find the frequency dependence of the optical depth in the densest regions near the embedded sources to be given by τ ∝ ν1.9. For positions away from the embedded sources, there is a larger scatter in the data points, with a suggestion that the frequency dependence is steeper, such that τ ∝ ν2.4. This may be an indication of a change in the grain properties between less dense and very dense regions and is consistent with the results of grain growth. Using standard values for the fractional abundance of C18O relative to H2, the mean densities of the cloud cores are 3-5 × 104 cm-3 . These regions appear to be close to virial equilibrium. The dense gas [revealed by multiple transition studies of tracers such as CS and HC3N to have n(H2) 106 cm-3] has a volume filling factor of a few percent. Assuming a fractional abundance of C18O equal to 1.7 × 10-7, we find that the 790 μm dust optical depth to mass column density ratio for M17 and Cepheus A is 0.0062 cm2 g-1, while the average value for the Orion molecular cloud is a factor of 3 larger.

Interstellar deuterated ammonia: from NH3 to ND3

We use spectra and maps of NH 2 D, ND 2 H, and ND 3 , obtained with the CSO, IRAM 30 m and Arecibo telescopes, to study deuteration processes in dense cores. The data include the first detection of the hyperfine structure in ND 2 H. The emission of NH 2 D and ND 3 does not seem to peak at the positions of the embedded protostars, but instead at offset positions, where outflow interactions may occur. A constant ammonia fractionation ratio in star-forming regions is generally assumed to he consistent with an origin on dust grains. However, in the pre-stellar cores studied here, the fractionation varies significantly when going from NH 3 to ND 3 . We present a steady state model of the gas-phase chemistry for these sources, which includes passive depletion onto dust grains and multiply saturated deuterated species up to five deuterium atoms (e.g. CD + 5 ). The observed column density ratios of all four ammonia isotopologues are reproduced within a factor of 3 for a gas temperature of 10 K. We also predict that deuterium fractionation remains significant at temperatures up to about 20 K. ND and NHD, which have rotational transitions in the submillimeter domain are predicted to be abundant.

The Origin of Diffuse X-Ray and γ-Ray Emission from the Galactic Center Region: Cosmic-Ray Particles

The inner couple of hundred parsecs of our Galaxy are characterized by a significant amount of synchrotron-emitting gas. Many of the best studied sources in this region exhibit a mixture of 6.4 keV Fe Kα emission, molecular line emission, and nonthermal radio continuum radiation. The spatial correlation between fluorescent Fe Kα line emission at 6.4 keV and molecular line emission from Galactic center molecular clouds has been explained as reflected X-rays from a past outburst of Sgr A*. Here we present a multiwavelength study of this region and find a correlation between the nonthermal radio filaments and the X-ray features. This correlation, when combined with the distribution of molecular gas, suggests against the irradiation model. Instead, we account for this distribution in terms of the impact of the relativistic particles from local (nonthermal filaments) and extended sources with diffuse neutral gas producing both nonthermal bremsstrahlung X-ray continuum emission and diffuse 6.4 keV line emission. The production rate of Fe Kα photons associated with the injection of electrons into a cloud as a function of column density is calculated. The required energy density of low-energy cosmic rays associated with the synchrotron-emitting radio filaments or extended features is estimated to be in the range between 20 and ~103 eV cm-3 for Sgr C, Sgr B1, Sgr B2, and the 45 and -30 km s-1 clouds. We also generalize this idea to explain the cosmic-ray heating of molecular gas, the interstellar cosmic-ray ionization, the pervasive production of the diffuse Kα line, and TeV emission from the Galactic center molecular clouds. In particular, we suggest that inverse Compton scattering of the submillimeter radiation from dust by relativistic electrons may contribute substantially to the large-scale diffuse TeV emission observed toward the central regions of the Galaxy.

Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate

In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen-bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (ζH) and molecular hydrogen fraction (f_H_2). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines. From the molecular abundances we compute f_H_2 in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042 ± 0.018. This confirms previous findings that OH+ and H2O+ primarily reside in gas with low H2 fractions. We also infer ζH throughout our sample, and find a lognormal distribution with mean log (ζH) = –15.75 (ζH = 1.78 × 10–16 s–1) and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H_3^+ observations. Ionization rates in the Galactic center tend to be 10-100 times larger than found in the Galactic disk, also in accord with prior studies.

Infrared Space Observatory Long Wavelength Spectrometer Observations of a Cold Giant Molecular Cloud Core near the Galactic Center

We have used the Long Wavelength Spectrometer aboard the Infrared Space Observatory in the grating mode to map the far-infrared continuum emission (45-175 ?m) toward the massive giant molecular cloud core GCM 0.25+0.11 located near the Galactic center. Graybody models of the observed far-infrared spectral energy distribution indicate that the bulk of the dust in the diffuse component along the line of sight toward GCM 0.25+0.11 has a mean temperature of ~26 K and a 100 ?m optical depth of ~0.17. GCM 0.25+0.11 is observed in emission at far-infrared (FIR) wavelengths (100 ?m). However at midinfrared wavelengths (70 ?m) the core is seen in absorption against the general Galactic center background. This indicates that GCM 0.25+0.11 is located in front of the bulk of the dust responsible for the diffuse FIR emission, most likely a few hundred parsecs from the Galactic center. By subtracting the spectrum of the diffuse component from the spectrum observed toward GCM 0.25+0.11, we have been able to extract the intrinsic spectrum of this GMC core. Graybody fits to the resulting far-infrared spectrum combined with our previous submillimeter measurements (350-800 ?m) give a low temperature ~18 K for the bulk of the dust in the GCM 0.25+0.11 core. In addition, the grain emissivity is a very steep function of frequency (?2.8). The high grain emissivity exponent is consistent with the presence of dust grains covered with thick ice mantles. We have complemented our ISO data with CO (2-1) and HCO+ (3-2) observations carried out with the Caltech Submillimeter Observatory. The molecular emission shows a large velocity gradient across the southern part of the core indicative of streaming motions of the gas or of the presence of multiple, spatially overlapping velocity components. The observed gas kinematics may indicate that GCM 0.25+0.11 is in the process of being disrupted by the strong tidal forces caused by the high mass concentration in the Galactic center region. This might explain why there is no evidence for ongoing high-mass star formation associated with this core, in spite of its large molecular mass. However, the mean H2 density of GCM 0.25+0.11 is well above the tidal stability limit for a Galactocentric distance of a few hundred parsecs implied by our observations. An alternative explanation is that we are witnessing the very early stage of a cloud-cloud collision that may result in a future star formation episode.

High-Frequency Measurements of the Spectrum of Sagittarius A*

We report near-simultaneous interferometric measurements of the spectrum of Sagittarius A* over the 5-354 GHz range and single-dish observations that have yielded the first detection of Sgr A* at 850 GHz. We confirm that Sgr A*s spectrum rises more steeply at short millimeter wavelengths than at centimeter wavelengths, leading to a near-millimeter/submillimeter excess that dominates its luminosity. Below 900 GHz, Sgr A*s observed luminosity is 70 ± 30 L. A new upper limit to Sgr A*s 24.3 μm flux, together with a compilation of other extant IR data, imply a far-infrared spectral turnover, which can result from either an intrinsic synchrotron cutoff or excess extinction near Sgr A*. If the former applies, Sgr A*s total synchrotron luminosity is <103 L, while in the latter case it is <3 × 104 L if spherical symmetry also applies.

Ubiquitous argonium (ArH+) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas

Aims. We describe the assignment of a previously unidentified interstellar absorption line to ArH + and discuss its relevance in the context of hydride absorption in di use gas with a low H2 fraction. The confidence of the assignment to ArH + is discussed, and the column densities are determined toward several lines of sight. The results are then discussed in the framework of chemical models, with the aim of explaining the observed column densities. Methods. We fitted the spectral lines with multiple velocity components, and determined column densities from the line-to-continuum ratio. The column densities of ArH + were compared to those of other species, tracing interstellar medium (ISM) components with di erent H2 abundances. We constructed chemical models that take UV radiation and cosmic ray ionization into account.

Statistical Properties of Line Centroid Velocities and Centroid Velocity Increments in Compressible Turbulence

We have calculated probability density functions (PDFs) of centroid velociti and centroid velocity increments of line profiles computed from the output of a 512 simulation of compressible turbulence. The PDFs of centroid velocities calculated over the whole data cube are roughly Gaussian. On a smaller scale, non-Gaussian PDFs are observed in some cases. However, this is far from being the rule, and most of the distributions show relatively minor deviations from a Gaussian. By contrast, PDFs of centroid velocity increments clearly show non-Gaussian wings that are associated with regions of increased vorticity in the flow and thus appear related to the phenomenon of intermittency. Investigations of PDFs of centroid velocity increments in non-star4orming regions thus seem a promising avenue for studying the intermittency in the interstellar medium, as an alternative to the line-shape approach. Subject headings: hydrodynamics ISM: clouds ISM: kinematics and dynamics turbulence

Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN, and HNC isotopologues

Context. The 15 N isotopologue abundance ratio measured today in different bodies of the solar system is thought to be connected to 15 N-fractionation effects that would have occurred in the protosolar nebula. Aims. The present study aims at putting constraints on the degree of 15 N-fractionation that occurs during the prestellar phase, through observations of D, 13 C, and 15 N-substituted isotopologues towards B1b. Molecules both from the nitrogen hydride family, i.e. N2H + , and NH3, and from the nitrile family, i.e. HCN, HNC, and CN, are considered in the analysis. Methods. As a first step, we modelled the continuum emission in order to derive the physical structure of the cloud, i.e. gas temperature and H2 density. These parameters were subsequently used as input in a non-local radiative transfer model to infer the radial abundance profiles of the various molecules. Results. Our modelling shows that all the molecules are affected by depletion onto dust grains in the region that encompasses the B1-bS and B1-bN cores. While high levels of deuterium fractionation are derived, we conclude that no fractionation occurs in the case of the nitrogen chemistry. Independently of the chemical family, the molecular abundances are consistent with 14 N/ 15 N ∼ 300, a value representative of the elemental atomic abundances of the parental gas. Conclusions. The inefficiency of the 15 N-fractionation effects in the B1b region can be linked to the relatively high gas temperature ∼17 K, which is representative of the innermost part of the cloud. Since this region shows signs of depletion onto dust grains, we cannot exclude the possibility that the molecules were previously enriched in 15 N, earlier in the B1b history and that such an enrichment could have been incorporated into the ice mantles. It is thus necessary to repeat this kind of study in colder sources to test such a possibility.