The evolution of charged particles in a model of contracting cloud

The evolution of the charged particles are followed during contraction of a model of an interstellar cloud, with initial density number of n = 10 cm^–3. The contraction is followed up to density increase by five orders of magnitude. Special care is given to the details of the negative ions. In addition, we have tested the ambipolar diffusion according to the results of the ion density.The results predict the importance of atomic ions in the diffuse regions. H⁺ and C⁺ are distinctly enhanced in the beginning of contraction but decrease as contraction proceeds. Molecular ions enhance as contraction proceeds and becomes important in dense regions. The most enhanced molecular ions are HCO⁺, O₂ ⁺, C₂H₃ ⁺, H₃O⁺ and SO⁺, H₃ ⁺ is less abundant. The atomic ions (except metalic ions) decrease noticeably as density increases. In general the negative ions are of negligible fractional abundances. It has also been found that the time of ambipolar diffusion is shorter than the dynamical time, hence the magnetic field should be weakened in the central core as the central density increases to n = 10⁴ cm^–3.     

Time dependent chemical study of contracting interstellar clouds. III. the charge distribution in interstellar clouds

We have constructed a chemical reaction model in a contracting interstellar cloud including 104 species which are involved in a network of 557 reactions. The chemical kinetic equations were integrated as a function of time by using gear package. The evolution of the system was followed in the density range 10–10⁷ particles cm^-3.The calculated fractional abundances of the charged species are in good agreement with those given by other investigators. The charge density has been followed in diffuse, intermediate and dense regions. The most dominant ionic species are metallic ions, HCO⁺ and H ₃ ⁺ in the shielded regions and atomic ions H⁺, C⁺, Si⁺, He⁺, S⁺ and metal ions in the diffuse and intermediate regions. The abundances of negatively charged ions were found to be negligible. The results of the calculations on the different metallic ions are interpreted.     

Evaporation and Condensation Processes of Giant Molecular Clouds in a Hot Plasma

2D hydrodynamical simulations are performed to examine the evaporation and condensation processes of giant molecular clouds in the hot phase of the interstellar medium. The evolution of cold and dense clouds (T = 1000 K, n _H = 3 cm^-3,M = 6·10⁴ M_⊙) is calculated in the subsonic stream of a hot tenuous plasma (T = 5 ·10⁶ K, n _H = 6·10^-4cm^-3). Our code includes self-gravity, heating and cooling processes and heat conduction by electrons. The thermal conductivity of a fully ionized hydrogen plasma (κ ∝ T^5/2) is applied as well as a saturated heat flux in regions where the mean free path of the electrons is large compared to the temperature scaleheight. Significant differences occur between simulations with and without heat conduction. In the simulations without heat conduction, the clouds outermost regions is stired up by Kelvin-Helmholtz (KH) instability after only a few dynamical times. This prevents an infiltration of a significant amount of hot gas into the cloud before its destruction. In contrast, models including heat conduction evolve less violently. The boundary of the cloud remains nearly unsusceptible to KH instabilities. In this scenario it is possible to mix the formerly hot streaming gas very effectively with the cloud material. This revised version was published online in July 2006 with corrections to the Cover Date.     

The role of a strong far-infrared radiation field in line excitation and cooling of photodissociation regions and molecular clouds

We have theoretically studied the influence of a far-infrared radiation (FIR) field from H_π region on the cooling by C and O atoms, C⁺ ion and CO molecule in a photodissociation region, and a molecular cloud associated with H_π region (hereinafter referred as H_I region) at low temperatures (T _k≤200 K). Comparisons have been made for cooling with and without FIR for two extreme abundances (10⁻⁴ and 10⁻⁷) of the mentioned species for temperatures ranging between 10 and 200K and an hydrogen particle density range 10 cm⁻³≤n _o≤ 10⁷ cm³. The cooling by the species with low line-splitting (C_I, C_π and CO) is significantly influenced by the radiation field for temperaturesT _k < 100 K while the effect of radiation field on cooling by O_I is significant even at higher temperatures (T _k > 100 K). The effect of FIR field on the cooling of CO from low rotational transitions is negligibly small, whereas it is considerable for higher transitions. In general, the cooling terms related to the short-wavelength transitions are more affected by FIR than those related to longer wavelengths. It is also demonstrated here that in the determination of thermal structure of an HI region the dust grains play an important role in the heating of gas only through photoelectron emission following irradiation by far-ultraviolet (FUV) radiation, as the infrared radiation from the dust is too small to have substantial effect on the cooling. It is found that in the H_π /H_I interface the FIR field from grains in the H_π region is not capable of modifying the temperature of the warmest regions but does so in the inner part where the temperature is low enough.     

Infrared measurements of the chemical composition of C/2006 P1 McNaught

C/2006 P1 McNaught is a dynamically new comet from the Oort cloud that passed very close to the Sun, driving overall volatile production rates up to about 10³¹ molecules s⁻¹. Post-perihelion observations were obtained in a target-of-opportunity campaign using the CSHELL instrument at the NASA Infrared Telescope Facility atop Mauna Kea, Hawaii, on UT 2007 January 27 and 28. Eight parent volatiles (H₂O, CH₄, C₂H₂, C₂H₆, HCN, CO, NH₃, H₂CO) and two daughter fragments (OH and NH₂) were detected, enabling the determination of a rotational temperature and production rate for H₂O on UT January 27 and absolute and relative production rates for all the detected parent species on UT January 28. The chemical composition measured in the coma suggests that this close perihelion passage stripped off processed outer surface layers, likely exposing relatively fresh primordial material during these observations. The post-perihelion abundances we measure for CO and CH₄ (relative to H₂O) are slightly depleted while C₂H₂, NH₂ and possibly NH₃ are enhanced when compared to the overall comet population. Measured abundances for other detected molecular species were within the range typically observed in comets.     

Injection of molecules from the surfaces of grains into the gas phase chemistry of an expanding cloud

We present model results for the chemistry in an expanding cloud or clump in which molecules are injected into the gas phase from grain surfaces when the clump reaches a certain visual extinction A _v during the expansion. We consider separately injection at two different values of A _v, and include a representative large hydrocarbon, C₆H, and sulphur-bearing molecule, H₂SO₄, as well as H₂O and CO. We examine the abundances of certain molecules which have been observed in diffuse and translucent clouds, and compare the results obtained for these abundances with and without an injection during expansion. We also compare our results withpublished observations, and conclude that in most clouds an injection of molecules has occurred.     

Physical parameters of the diffuse molecular cloud envelope at G180.9+4.1, next to S241

Methylidyne (CH) line observations were obtained at Algonquin from the diffuse molecular cloud envelope at G180.9+4.1, sandwiched between the optical H II region S241 and the molecular cloud core at G180.8+4.0. An analysis of these observations yields several of the envelope parameters, notably a CH column density of 2×10¹³ cm^–2, a microturbulent velocity of 2.6 km s^–1, and a total space density of 40 cm^–3.     

Interstellar catalysis

Since gas-phase reactions alone cannot account for the observed abundances of H₂ in the typical interstellar cloud, one or more surface reactions are probably involved. Of the three possible candidates, only the catalytic production of H₂ on transition metal grains is supported by laboratory evidence. Using the rate equations developed in a previous paper for this process, the steady-state equilibrium abundances of H, H₂,e ^–, H⁺, H^–, H₂ ⁺, and H₃ ⁺ are calculated for large (r>10 pcs;M10² M ), tenuous (n=10²–10⁴ cm^–3) hydrogen dust clouds under a wide variety of conditions. In addition to the four rate equations involved in the catalytic reactions, 18 gas-phase and one additional surface reaction—the physical adsorption of H-atoms on cold, dielectric surfaces and their subsequent recombination and desorption as H₂ molecules—are included in the calculations. It is found that metal grains can produce as much interstellar H₂ as the best physical adsorption mechanism under optimum conditions if the extinction in the visible is less than 5^m.0. The three critical parameters for efficient catalysis (activation energy of desorption, grain temperature, and the number density of available sites) are examined, and it is shown that catalytic reactions are efficient producers of H₂ under all but the most unfavorable conditions.     

Production and condensation of organic gases in the atmosphere of Titan

The rates and altitudes for the dissociation of atmospheric constituents of Titan are calculated for solar UV, solar wind protons, interplanetary electrons, Saturn magnetospheric particles, and cosmic rays. The resulting integrated synthesis rates of organic products range from 10²–10³ g cm^?2 over 4.5 × 10⁹ years for high-energy particle sources to 1.3 × 10⁴ g cm^?2 for UV at $\text{λ < 1550}\text{A}\text{?}$, and to 5.0 × 10⁵ g cm^?2 if $\text{λ > 1550}\text{A}\text{?}$ (acting primarily on C₂H₂, C₂H₄, and C₄H₂) is included. The production rate curves show no localized maxima corresponding to observed altitudes of Titan's hazes and clouds. For simple to moderately complex organic gases in the Titanian atmosphere, condensation occurs below the top of the main cloud deck at 2825 km. Such condensates comprise the principal cloud mass, with molecules of greater complexity condensing at higher altitudes. The scattering optical depths of the condensates of molecules produced in the Titanian mesosphere are as great as ～ 10²/(particulate radius, μm) if column densities of condensed and gas phases are comparable. Visible condensation hazes of more complex organic compounds may occur at altitudes up to ～ 3060 km provided only that the abundance of organic products declines with molecular mass no faster than laboratory experiments indicate. Typical organics condensing at 2900 km have molecular masses = 100–150 Da. At current rates of production the integrated depth of precipitated organic liquids, ices, and tholins produced over 4.5 × 10⁹ years ranges from a minimum ～ 100 m to kilometers if UV at $\text{λ > 1550}\text{A}\text{?}$ is important. The organic nitrogen content of this layer is expected to be ～ 10^?1?10^?3 by mass.     

Observations and modeling of infrared and millimeter molecular lines towards GL2591

We have observed C₂H₂ and HCN rovibrational transitions near 13µm in absorption against GL2591. We also have observed rotational transitions at 0.6-3 mm of CS, HCN, H₂CO, and HCO⁺. Analysis of the rotational lines, which arise in the extended cloud around the source, shows that no single density model can explain all the data. Models with density and temperature gradients do much better; in particular models withn(r) r ^–1.5 can reproduce the observed pattern of emission line strengths. The abundances show significant depletion compared to models of gas-phase chemistry. The rovibrational data were analyzed in comparison to the absorption line analysis of CO by Mitchellet al. (1989). Our data are consistent with the C₂H₂ and HCN absorption arising in the same warm (200 K) and hot (1010 K) components seen in CO, but we see little evidence for the cold (38 K) component seen in CO. The rovibrational lines from higher states (J 21) indicate that the hot HCN deviates from LTE, leading to a density of about 3 × 10⁷ cm^–3. Comparison of the two data sets shows that the rovibrational absorption of HCN, rather than arising in the extended envelope, must come from a region with a small angular extent. A model in which early-time gas phase abundances are preserved on grain mantles and released at high temperature can explain the data.     

Venus cloud properties: Infrared opacity and mass mixing ratio

By using the Mariner 5 temperature profile and a homogeneous cloud model, and assuming that CO₂ and cloud particles are the only opacity sources, the wavelength dependence of the Venus cloud opacity is infrared from the infrared spectrum of the planet between 450 and 1250 cm^?1. Justification for applying the homogeneous cloud model is found in the fact that numerous polarization and infrared data are mutually consistent within the framework of such a model; on the other hand, dense cloud models are not satisfactory.Volume extinction coefficients varying from 0.5 × 10^?5 to 1.5 × 10^?5 cm^?1, depending on the wavelength, are determined at the tropopause level of 6110 km. By using all available data, a cloud mass mixing ratio of approximately 5 × 10^?6 and a particle concentration of about 900 particles cm^?3 at this level are also inferred. The derived cloud opacity compares favorably with that expected for a haze of droplets of a 75% aqueous solution of sulfuric acid.     

Understanding the chemical complexity in Circumstellar Envelopes of C-Rich AGB stars: the case of IRC +10216

The circumstellar envelopes of carbon-rich AGB stars show a chemical complexity that is exemplified by the prototypical object IRC +10216, in which about 60 different molecules have been detected to date. Most of these species are carbon chains of the type C _n H, C _n H₂, C _n N, HC _n N. We present the detection of new species (CH₂CHCN, CH₂CN, H₂CS, CH₃CCH and C₃O) achieved thanks to the systematic observation of the full 3 mm window with the IRAM 30m telescope plus some ARO 12m observations. All these species, known to exist in the interstellar medium, are detected for the first time in a circumstellar envelope around an AGB star. These five molecules are most likely formed in the outer expanding envelope rather than in the stellar photosphere. A pure gas phase chemical model of the circumstellar envelope is reasonably successful in explaining the derived abundances, and additionally allows to elucidate the chemical formation routes and to predict the spatial distribution of the detected species.     

On the interstellar extinction law toward young stars

We have determined the atomic hydrogen column density N _HI toward all of the young stars from the Taurus-Auriga-Perseus star-forming complex for which the corresponding spectra are available in the Hubble Space Telescope archive (nine stars) by analyzing the Lyα line profile. We show that the stars studied, except DR Tau, lie not far from the edge of the gaseous cloud of the star-forming region closest to us or, more precisely, inside the outer H I shell of the cloud. This shell with a column density of N _HI ? 6 × 10²⁰ cm^?2 surrounds the molecular gas of the cloud composed of a diffuse component (the so-called diffuse screen) in which dense, compact TMC-1 cores are embedded. The properties of the dust grains toward the stars that lie at the front edge of the cloud most likely differ only slightly from those of the interstellar dust outside star-forming regions. This casts doubt on the validity of the hypothesis that the extinction curve toward young stars has an anomalously low amplitude of the 2175 Å bump—such an extinction curve is observed for the field stars HD 29647 and HD 283809 toward which the line of sight passes through the TMC-1 core.     

Global and temporal variations in hydrocarbons and nitriles in Titan's stratosphere for northern winter observed by Cassini/CIRS

Mid-infrared spectra measured by Cassini's Composite InfraRed Spectrometer (CIRS) between July 2004 and January 2007 (Ls=293°-328°) have been used to determine stratospheric temperature and abundances of C₂H₂, C₃H₄, C₄H₂, HCN, and HC₃N. Over 65,000 nadir spectra with spectral resolutions of 0.5 and 2.5 cm⁻¹ were used to probe spatial and temporal composition variations in Titan's stratosphere. Cassini's 180° orbital transfer in mid-2006 allowed low emission angle observations of the north polar region for the first time in the mission and allowed us to probe the full latitude range. We present the first measurements of composition variations within the polar vortex, which display increasing abundances right up to 90° N. The lack of a homogeneous abundance-latitude variation within the vortex indicates limited horizontal mixing and suggests that subsidence is greatest at the vortex core. Contrary to numerical model predictions and tropospheric cloud observations, we do not see any evidence for a secondary circulation cell near the south pole, which suggests a single Hadley-type circulation in the stratosphere at this epoch. This difference can be reconciled if the secondary cell is restricted to altitudes below 100 km, where there is no sensitivity in our data. Temporal variations in composition were observed in the south, with volatile species becoming less abundant as the season progressed. The observed variations are compared to numerical model predictions and observations from Voyager.     

Cosmic Ray Production Rates in Supernova Remnants

Supernova Remnants (SNRs) are the most likely sources of the galactic cosmic rays up to energies of about 10¹⁵ eV/nuc. The large scale shock waves of SNRs are almost ideal sites to accelerate particles up to these highly non-thermal energies by a first order Fermi mechanism which operates through scattering of the particles at magnetic irregularities. In order to get an estimate on the total amount of the explosion energy E _SNconverted into high energy particles the evolution of a SNR has to be followed up to the final merging with the interstellar medium. This can only be done by numerical simulations since the non-linear modifications of the shock wave due to particle acceleration as well as radiative cooling processes at later SNR stages have to be considered in such investigations. Based on a large sample of numerical evolution calculations performed for different ambient densities n _ext, SN explosion energies, magnetic fields etc. we discuss the final ‘yields’ of cosmic rays at the final SNR stage where the Mach number of the shock waves drops below 2. At these times the cosmic rays start to diffuse out of the remnant. In the range of external densities of10^-2 ≤ n _ext/[cm^-3] ≤ 30 we find a the total acceleration efficiency of about 0.15 E _SN with an increase up to 0.24 E _SN at maximum for an external density of n _ext = 10 cm^-3. Since for the larger ambient densities radiative cooling can reduce significantly the total thermal energy content of the remnant dissipation of Alfvén waves can provide an important heating mechanism for the gas at these later stages. From the collisions of the cosmic rays with the thermal plasma neutral pions are generated which decay subsequently into observable γ-rays above 100 MeV. Hence, we calculate these γ-ray luminosities of SNRs and compare them with current upper limits of ground based γ-raytelescopes. The development of dense shells due to cooling of the thermal plasma increases the γ-ray luminosities and e.g. an external density of n _ext = 10 cm^-3 with E _SN = 10⁵¹ erg can lead to a γ-ray flux above 10^-6 ph cm^-2 s^-1 for a remnant located at a distance of 1 kpc. This revised version was published online in July 2006 with corrections to the Cover Date.     

The composition of Titan's stratosphere from Cassini/CIRS mid-infrared spectra

We have analyzed data recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft during the Titan flybys T0-T10 (July 2004-January 2006). The spectra characterize various regions on Titan from 70° S to 70° N with a variety of emission angles. We study the molecular signatures observed in the mid-infrared CIRS detector arrays (FP3 and FP4, covering roughly the 600-1500 cm⁻¹ spectral range with apodized resolutions of 2.54 or 0.53 cm⁻¹). The composite spectrum shows several molecular signatures: hydrocarbons, nitriles and CO₂. A firm detection of benzene (C₆H₆) is provided by CIRS at levels of about 3.5×¹⁰⁻⁹ around 70° N. We have used temperature profiles retrieved from the inversion of the emission observed in the methane ν₄ band at 1304 cm⁻¹ and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. No longitudinal variations were found for these gases. Little or no change is observed generally in their abundances from the south to the equator. On the other hand, meridional variations retrieved for these trace constituents from the equator to the North ranged from almost zero (no or very little meridional variations) for C₂H₂, C₂H₆, C₃H₈, C₂H₄ and CO₂ to a significant enhancement at high northern (early winter) latitudes for HCN, HC₃N, C₄H₂, C₃H₄ and C₆H₆. For the more important increases in the northern latitudes, the transition occurs roughly between 30 and 50 degrees north latitude, depending on the molecule. Note however that the very high-northern latitude results from tours TB-T10 bear large uncertainties due to few available data and problems with latitude smearing effects. The observed variations are consistent with some, but not all, of the predictions from dynamical-photochemical models. Constraints are set on the vertical distribution of C₂H₂, found to be compatible with 2-D equatorial predictions by global circulation models. The D/H ratio in the methane on Titan has been determined from the CH₃D band at 1156 cm⁻¹ and found to be . Implications of this deuterium enrichment, with respect to the protosolar abundance on the origin of Titan, are discussed. We compare our results with values retrieved by Voyager IRIS observations taken in 1980, as well as with more recent (1997) disk-averaged Infrared Space Observatory (ISO) results and with the latest Cassini-Huygens inferences from other instruments in an attempt to better comprehend the physical phenomena on Titan.     

Titan: Far-infrared and microwave remote sensing of methane clouds and organic haze

It is shown that Titan's surface and plausible atmospheric thermal opacity sources—gaseous N₂, CH₄, and H₂, CH₄ cloud, and organic haze—are sufficient to match available Earth-based and Voyager observations of Titan's thermal emission spectrum. Dominant sources of thermal emission are the surface for wavelenghts λ ? 1 cm, atmospheric N₂ for 1 cm ? λ ? 200 μm,, condensed and gaseous CH₄ for 200 μm ? λ ? 20 μm, and molecular bands and organic haze for λ ? 20 μm. Matching computed spectra to the observed Voyager IRIS spectra at 7.3 and 52.7° emission angles yields the following abundances and locations of opacity sources: CH₄ clouds: 0.1 g cm^? at a planetocentric radius of 2610–2625 km, 0.3 g cm^?2 at 2590–2610 km, total 0.4 ± 0.1 g cm^–2 above 2590 km; organic haze: $\text{4 ± 2 × 10}{}^{\mathrm{?6}}\text{,}\text{g cm}\text{,}{}^{\mathrm{?2}}$ above 2750 km; tropospheric H₂: 0.3 ± 0.1 mol%. This is the first quantitative estimate of the column density of condensed methane (or CH₄/C₂H₆) on Titan. Maximum transparency in the middle to far IR occurs at 19 μm where the atmospheric vertical absorption optical depth is $\text{?0.6}$ A particle radius $\text{r}\text{? 2 μ}\text{m}$ in the upper portion of the CH₄ cloud is indicated by the apparent absence of scattering effects.     

An Observational Study of the Molecular Cloud Core Toward IRAS 23133+6050

This paper reports ¹³CO, C¹⁸O, HCO⁺ (J = 1−0) spectral observations toward IRAS 23133+6050 with the 13.7 m millimeter-wave telescope at Qinghai Station of PMO. Corresponding to the ¹³CO, C¹⁸O, HCO⁺ line emissions, the size of the observed molecular cloud core is 4.0 pc, 2.1 pc and 2.3 pc, the virial mass is 2.7 × 10³ M_⊙, 0.9 × 10³ M_⊙ and 2.3 × 10³ M_⊙, and the volume density of H₂ is 2.7 × 10³ cm⁻³, 5.1 × 10³ cm⁻³ and 4.6 × 10³ cm⁻³, respectively. Using the power-law function n(r) ∼r^−p, the spatial density distribution of the cloud core was analyzed, the obtained exponent p is respectively 1.75, 1.56 and 1.48 for the ¹³CO, C¹⁸O and HCO⁺ cores, and it is found that the density distribution becomes gradually flatter from the outer region to the inner region of the core. The HCO⁺ abundance is 4.6 × 10⁻¹⁰, one order of magnitude less than the value for dark clouds, and slightly less than that for giant molecular clouds. The ¹³CO/C¹⁸O relative abundance ratio is 12.2, comparable with the value 11.8 for dark clouds, and the value 9.0 ∼ 15.6 for giant molecular clouds. A ¹³CO bipolar outflow is found in this region. The IRAS far-infrared luminosity and the virial masses give the luminosity-mass ratios 18.1, 51.1 and 21.2 from the three lines.     

Condensation in Titan's stratosphere during polar winter

In Titan's north polar region stratospheric clouds are expected to form due to a combination of low temperatures and downward motion of volatile-enriched air. Here we investigate possible sources of stratospheric clouds at Titan's pole using data from the Cassini Composite Infrared Spectrometer and a simple condensation model. An upper limit for C₄N₂ gas was determined to be 9×¹⁰⁻⁹, which is less than required to make the C₄N₂ cloud at the Voyager epoch. Hence, the presence of this cloud after equinox remains a mystery. The largest cloud seen in far-infrared spectra has a feature around 220 cm⁻¹ and is located around an altitude of 140 km. The upper limit for propionitrile (C₂H₅CN) gas shows that the feature around 220 cm⁻¹ is probably not due to pure propionitrile ice. Instead, our model calculations show that HCN should cause by far the largest cloud around 140 km. We therefore propose that HCN ice plays an important role in the formation of the massive polar cloud, because of the unavailability of sufficient condensable gas other than HCN to produce a strong enough condensate feature. However, the signature at 220 cm⁻¹ is not consistent with that of pure HCN ice at 172 cm⁻¹ and mixing of HCN ice with other ices, or chemical alteration of HCN ice might mask the HCN ice signature.