Interstellar H2 toward HD 147888

Abstract

The ultraviolet and far-ultraviolet spectra of HD 147888 allows the H 2 vibrational level ν = 0 to be accessed along with higher vibrational levels of the ground H 2 electronic level. The large number of H 2 absorption lines in the HST spectra allows column densities to be determined even from a noisy spectra. We have determined column densities of the H 2 molecule on vibrational levels ν = 0-5 and rotational levels J = 0-6 using the profile fitting method. No variations in the column densities of H 2 on vibrationally excited levels were observed from 2000 through 2009. The ortho to para H 2 ratio (O/P)* for the excited vibrational states ν = 1-4 equals to 1.13. For the lowest vibrational state ν = 0 and rotational level J = 1 the ortho to para H 2 ratio is only 0.15. The temperature of ortho-para thermodynamical equilibrium is T OP = 42 ± 3 K. The measurements of H 2 column densities on excited vibrational levels (from the HST spectra) leads to constraints on the radiation field in photon-dominated region (PDR) models of the interstellar cloud towards HD 147888. The Meudon PDR model locates the cloud 0.62 pc from the star. The modeled hydrogen cloud density (89-336 cm −3 ) agrees with independent density estimations based on the C 2 molecule and the chemical model. The observed (O/P) J = 1 and (O/P)* H 2 ratios cannot be explained by a simple model.

Vibrationally excited molecular hydrogen production from the water photochemistry

“…Vibrationally excited H 2 has been commonly observed in the interstellar circumstances, as in PDRs where the H 2 (v > 0) molecules are considered to be produced by FUV fluorescence, and in shocked gas where the excitation is mainly collisional. Astronomical observations, however, displayed that the H 2 molecules have significant populations in high excitation levels 8,11 . These high-excitation populations may arise from other non-thermal pumping processes.…”

“…Recent astronomical observations have illustrated that conspicuous emissions from vibrationally excited H 2 exist in the photo-dominated regions (PDRs) and shocks 8 . The first ultraviolet detection of vibrational excited interstellar H 2 was performed by Federman et al using the Hubble Space Telescope toward the star ζ Ophiuchi 9 , and then detected by Jensen et al toward HD 38087 and HD 199579 10 , Gnacinski et al towards HD 147888 11 , and Racheford et al near Herschel 36 12 . In particular, over 500 interstellar H 2 absorption lines from excited vibrational levels v = 1-14 were reported by Meyer et al toward HD 37903, one of the hot stars located in the NGC 2023 reflection nebula 13 .…”

Molecular Excitation in the Interstellar Medium: Recent Advances in Collisional, Radiative, and Chemical Processes

“…subsequently reported the rich ultraviolet spectrum of vibrational excited H 2 toward HD 37903 (HD 37903 is a bright UV-emitting star embedded in the L1630 molecular cloud, creating the reflection nebula NGC 2023) and raised the role of ultraviolet radiative pumping induced by the intense radiation field of this star, illuminating the bright reflection nebula NGC2023 in Orion. Finally, let us mention the reanalysis byGnaciński 378,379 of the HD 37903 and HD 147888 (a blue main-sequence star in the Ophiuchus constellation) lines of sight where both FUSE and HST available data allow to have a complete description of H 2 in these environments. It should be recalled that the derivation of accurate values of column densities requires very detailed analysis and is far from obvious.…”

HD 62542: Probing the Bare, Dense Core of a Translucent Interstellar Cloud

“…Fits to the line profiles indicate that the lines are fairly narrow (b ∼ 1.4 km s −1 ) and are found at the velocity (13.7±0.6 km s −1 ) of the main cloud (Fig. 3) -as seems to be the case for the few other sight lines in which H 2 * absorption has been detected in high-resolution UV spectra [HD 37021 (Abel et al 2016); HD 37061 (Gnaciński 2009); HD 37903 (Meyer et al 2001); ρ Oph D (Gnaciński 2013)]. While H 2 * can be produced either by shocks or by radiative pumping (e.g., Shull & Beckwith 1982), the relatively small b value and the lack of velocity offset (relative to the other molecular species) suggest that the H 2 * is excited radiatively in relatively cool gas, rather than by shocks.…”