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Molecular hydrogen, methane, water vapor and tritium concentrations near stratopause from air samples collected on Aerobee flight with liquid hydrogen cooled cryocondenser
The photodestruction of molecular hydrogen is reexamined in the context of recent observational data. Improvements in the theory of radiation transfer are made by using a more accurate self-shielding integral, treating anisotropic scattering by dust grains, and including photodissociation from excited rotational states. The total column density of hydrogen required to obtain 10% conversion decreases with a parameter, epsilon, the ratio of the formation rate to the destruction rate, as epsilon to the -m power, with m in the range 1.4-1.6. Satisfactory agreement with the data for thicker diffuse clouds is obtained for epsilon of about 0.00006.
This grant has supported the acquisition, reduction and analysis of data targeting the structure and excitation of molecular hydrogen in the reflection nebula IC 63 and in particular the fluorescent emission seen in the UV. In addition to manpower for analyzing the FUSE data, the grant supported the (attempted) acquisition of supporting ground-based data. We proposed for and received observing time for two sets of ground based, data; narrow band imaging ([S II], [O III) at KPNO (July 2002; Observer: Burgh) and imaging spectro-photometry of several of the near-infrared rotation-vibration lines of H2 at the IRTF (October 2003; Observer: Andersson). Unfortunately, both of these runs were failures, primarily because of bad weather, and did not result in any useful data. We combined the FUSE observations with rocket borne observations of the star responsible for exciting the H2 fluorescence in IC 63: gamma Cas, and with archival HUT observations of IC 63, covering the long-wavelength part of the molecular hydrogen fluorescence.
High-angular resolution molecular hydrogen (H2) v = 1-0 S(1) line images of several well-known molecular outflow sources (DR 21, NGC 2071, HH 7-11, and HH 12) are presented. It is found that all of the outflow sources possess highly collimated jets of shock-excited gas that are extremely clumpy and often consist of bright knots that are placed periodically along the jet axis. The H2 emission-line jets are significantly more collimated than their corresponding CO outflows and remain collimated to large distances from their driving source. In DR 21 and NGC 2071, the H2 jets appear to become more collimated with increasing distance from the central source, suggesting that they are pressure-confined by the ambient cloud medium. In HH 7-11 and HH 12 the shock-excited ionized and molecular components are spatially coincident and appear to be of comparable shape and size; this suggests that the shocked gas is highly mixed on scales significantly smaller than the current spatial resolution.
We have constructed three-dimensional models for the equilibrium abundance of molecular hydrogen in diffuse interstellar clouds of arbitrary geometry that are illuminated by ultraviolet radiation. The position-dependent photodissociation rate of H2 in such clouds was computed with a 26 ray approximation to model the attenuation of the incident ultraviolet radiation field by dust and by H2 line absorption. We have applied our modeling technique to the isolated diffuse cloud G236+39, assuming that the cloud has a constant density and that the thickness of the cloud along the line of sight is at every point proportional to the 100 micron continuum intensity measured by IRAS. We find that our model can successfully account for observed variations in the ratio of 100 micron continuum intensity to H I column density, with larger values of that ratio occurring along lines of sight in which the molecular hydrogen fraction is expected to be the largest. Using a standard chi square analysis to assess the goodness of fit of our models, we find (at the 60 a level) that a three-dimensional model is more successful in matching the observational data than a one-dimensional model in which the geometrical extent of the cloud along the line of sight is assumed to be much smaller than its extent in the plane of the sky. If D is the distance to G236 + 39, and given standard assumptions about the rate of grain-catalyzed H2 formation, we find that the cloud has an extent along the line of sight that is 0.9 +/- 0.1 times its mean extent projected onto the plane of the sky and a gas density of 53 +/- 8 (100 pc/D) H nuclei/cc and is illuminated by a radiation field of 1.1 +/- 0.2 (100 pc/D) times the mean interstellar radiation field. The derived 100 micron emissivity per nucleon is 1.13 +/- 0.06 x 10(exp -20) MJy/sr sq cm.
Physical mechanisms for producing vibrationally excited molecular hydrogen, such as has recently been detected toward the Orion Nebula, are discussed. The most likely mechanisms are collisional excitation behind a shock moving into a molecular cloud and near-ultraviolet pumping in the H2 Lyman and Werner bands and subsequent cascade. The absolute intensities of the Orion lines require either a 10-km/s shock moving into a cloud with a density of 300,000 per cu cm or an incident near-UV flux 1 million times the mean interstellar value. The shock model is favored because it matches the observed relative line intensities and because the near-UV source, Theta-1 Ori C, may be too weak to provide the required flux. Intensities of other H2 lines in the shock model are predicted as a further observational discriminant.
A statistical model, the local plasma approximation, is considered for the calculation of the logarithmic mean excitation energy for stopping power of chemically bound particles by taking into consideration chemical bonding. This statistical model is applied to molecular hydrogen and leads to results that suggest a value for the logarithmic mean excitation energy of molecular hydrogen that is larger than the accepted experimental and theoretical values.
Model calculations are performed which demonstrate that H II - H I transition zones with densities between 10 and 100 per cu cm may contain molecular-hydrogen column densities of the order of 10 to the 13th power to 10 to the 15th power per sq cm. Observational data for several stars are presented which confirm the theoretical values. It is concluded that all detectable molecular hydrogen is not necessarily located in cold regions of space.
We present a series of molecular hydrogen images of the Herbig-Haro 1-2 system in the 1-0 S(1) transition at 2.121 microns, with a spatial resolution of ~2 inches. The distribution of H 2 is then compared with that of the excitation, given by the [S II] 6717+6731 to H-alpha line ratio. We find that most optical condensations in the HH 1-2 system, including the VLA 1 jet, have H 2 counterparts. H 2 emission is detected in most low excitation knots, as expected for low velocity shocks (50 km s sup-1 <), but also in high excitation regions, like in HH 1F and HH 2A'.
Franck-Condon factors for band systems of molecular hydrogen, computing wave functions for vibrational levels using potential energy function, Part I
Intensity and intensity ratio of principal singlet and triplet lines of molecular hydrogen
Using a coupled rotation-vibration-dissociation model the rotational relaxation times for molecular hydrogen as a function of final temperature (500-5000 K), in a hypothetical scenario of sudden compression, are computed. The theoretical model is based on a master equation solver. The bound-bound and bound-free transition rates have been computed using a quasiclassical trajectory method. A review of the available experimental data on the rotational relaxation of hydrogen is presented, with a critical overview of the method of measurements and data reduction, including the sources of errors. These experimental data are then compared with the computed results.
Franck-Condon factors for band systems of molecular hydrogen, computing wave functions for vibrational levels using potential energy function, Part II
Franck-Condon factors for band systems of molecular hydrogen, computing wave functions for each electronic state by numerical solutions of radial Schroedinger equation
The total shocked and fluorescent molecular hydrogen 1-0 S(1) line luminosities from Orion have been measured to be about 2.5 solar luminosity and about 2.0 solar luminosity, respectively. The implications for using Orion to study the interstellar medium in galaxies is discussed.
The results from a semiclassical calculation of the thermal rate coefficient for the radiative association process H(n = 2) + H(n = 1) - H2 + hv are presented (n is the principal quantum number of the separated hydrogen atoms). The relative importance of this reaction in various environments is briefly discussed. Models of the early universe around the epoch of recombination and protostellar winds have been calculated which include the excited atom process. Not surprisingly, it is shown that the excited atom process will not be important in the general interstellar medium, except possibly in environments where the amount of Ly-alpha photon trapping is large. Examples may be the material surrounding quasars, active galactic nuclei, and bright H II regions. The most likely application of this process might be within rapidly evolving systems where a large transient n = 2 population of neutral hydrogen could result in a burst of molecular hydrogen formation.
Near-infrared spectroscopic observations of the active star-forming region near NGC 7538 IRS 1 and IRS 2 were made. The relative intensities of the v = 1-0 Q(1), Q(3), and Q(5) lines of molecular hydrogen are used to calculate a rotational excitation temperature. Comparison of the measured intensity of the Q(2) transition relative to the intensity of Q(1) and Q(3) permitted the retrieval of the ratio of ortho-to-para hydrogen. It is found that an ortho-to-para ratio of between 1.6 and 2.35 is needed to explain the Q-branch line intensity ratios, depending on the excitation model used. This range in ortho-to-para ratios implies a range of molecular hydrogen formation temperature of approximately 105 K to 140 K.
The detailed photochemistry of methane oxidation has been studied in a coupled chemical/dynamical model of the middle atmosphere. The photochemistry of formaldehyde plays an important role in determining the production of water vapor from methane oxidation. At high latitudes, the production and transport of molecular hydrogen is particularly important in determining the water vapor distribution. It is shown that the ratio of the methane vertical gradient to the water vapor vertical gradient at any particular latitude should not be expected to be precisely 2, due both to photochemical and dynamical effects. Modeled H2O profiles are compared with measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment at various latitudes. Molecular hydrogen is shown to be responsible for the formation of a secondary maximum displayed by the model water vapor profiles in high latitude summer, a feature also found in the LIMS data.