Photodestruction of Hydrogen Molecules in H I Regions

Stecher, T. P.; Williams, D. A.

doi:10.1086/180047

Cited by 244 publications

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“…Molecular hydrogen, in its ground state, is photo-dissociated by the two-step Solomon process (Stecher & Williams 1967) where absorption of LW photons, leads to electronically and vibrationally excited states that subsequently decay into the ground state. The rate constant can be derived by summing over all the bands, the product of the strength of the oscillations in a given band (Allison & Dalgarno 1970) and the probability of decay (Dalgarno & Stephens 1970).…”

Section: H2mentioning

confidence: 99%

Revised rate coefficients for H2 and H− destruction by realistic stellar spectra

Agarwal

Khochfar

2014

Monthly Notices of the Royal Astronomical Society

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Understanding the processes that can destroy H 2 and H − species is quintessential in governing the formation of the first stars, black holes and galaxies. In this study we compute the reaction rate coefficients for H 2 photo-dissociation by Lyman-Werner photons (11.2 − 13.6 eV), and H − photo-detachment by 0.76 eV photons emanating from self-consistent stellar populations that we model using publicly available stellar synthesis codes. So far studies that include chemical networks for the formation of molecular hydrogen take these processes into account by assuming that the source spectra can be approximated by a power-law dependency or a black-body spectrum at 10 4 or 10 5 K. We show that using spectra generated from realistic stellar population models can alter the reaction rates for photo-dissociation, k di , and photo-detachment, k de , significantly. In particular, k de can be up to ∼ 2 − 4 orders of magnitude lower in the case of realistic stellar spectra suggesting that previous calculations have over-estimated the impact that radiation has on lowering H 2 abundances. In contrast to burst modes of star formation, we find that models with continuous star formation predict increasing k de and k di , which makes it necessary to include the star formation history of sources to derive self-consistent reaction rates, and that it is not enough to just calculate J 21 for the background. For models with constant star formation rate the change in shape of the spectral energy distribution leads to a non-negligible late-time contribution to k de and k di , and we present self-consistently derived cosmological reaction rates based on star formation rates consistent with observations of the high redshift Universe.

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Section: H2mentioning

confidence: 99%

Revised rate coefficients for H2 and H− destruction by realistic stellar spectra

Agarwal

Khochfar

2014

Monthly Notices of the Royal Astronomical Society

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“…Photodissociation and self-shielding of H 2 have been studied by Field et al (1966), Stecher and Williams (1967), ), Jura (1974, Black and Dalgarno (1977), Shull (1978), Federman et al (1979), van Dishoeck and Black (1986, Abgrall et al (1992), Heck et al (1992), LeBourlot et al (1995), and Lee et al (1996); the field has been reviewed by and recently discussed in detail by Draine and Bertoldi (1996). H 2 absorbs farultraviolet photons via Lyman and Werner electronic transitions in the 912-1100-Å range.…”

Section: H 2 Formation Destruction and Heatingmentioning

confidence: 99%

Photodissociation regions in the interstellar medium of galaxies

Hollenbach

Tielens²

1999

Rev. Mod. Phys.

737

712

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The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eVϽh Ͻ13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the interstellar medium in which the heating and chemistry are regulated by far ultraviolet photons are termed PhotoDissociation Regions (PDRs). These regions are the origin of most of the non-stellar infrared (IR) and the millimeter and submillimeter CO emission from galaxies. The importance of PDRs has become increasingly apparent with advances in IR and submillimeter astronomy. The IR emission from PDRs includes fine structure lines of C, C ϩ , and O; rovibrational lines of H 2 ; rotational lines of CO; broad mid-IR features of polycyclic aromatic hydrocarbons; and a luminous underlying IR continuum from interstellar dust. The transition of H to H 2 and C ϩ to CO occurs within PDRs. Comparison of observations with theoretical models of PDRs enables one to determine the density and temperature structure, the elemental abundances, the level of ionization, and the radiation field. PDR models have been applied to interstellar clouds near massive stars, planetary nebulae, red giant outflows, photoevaporating planetary disks around newly formed stars, diffuse clouds, the neutral intercloud medium, and molecular clouds in the interstellar radiation field-in summary, much of the interstellar medium in galaxies. Theoretical PDR models explain the observed correlations of the [CII] 158 m with the CO Jϭ1 -0 emission, the CO Jϭ1 -0 luminosity with the interstellar molecular mass, and the [CII] 158 m plus [OI] 63 m luminosity with the IR continuum luminosity. On a more global scale, PDR models predict the existence of two stable neutral phases of the interstellar medium, elucidate the formation and destruction of star-forming molecular clouds, and suggest radiation-induced feedback mechanisms that may regulate star formation rates and the column density of gas through giant molecular clouds. [S0034-6861(99)01001-6] CONTENTS

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“…Fluorescence follows excitation, producing a highly structured emission in the 912Y1650 8 bandpass, as electrons fall back into the excited vibrational levels of X 1 AE þ g . Approximately 11%Y15% of the time, the fluorescent pumping process leaves the molecule in the vibrational continuum of the ground state (X 1 AE þ g ), located 4.48 eVabove the (J 00 , v 00 ) = (0, 0) level, 10 resulting in its spontaneous dissociation into H(1s) þ H(1s) (Stecher & Williams 1967;Dalgarno & Stephens 1970;Draine & Bertoldi 1996). Electrons ending up below the vibrational continuum cascade toward the ground vibrational state v 00 ¼ 0 via slow quadrupole transitions with radiative lifetimes $a few days and longer (Wolniewicz et al 1998), producing the infrared fluorescence spectrum (Black & Dalgarno 1976).…”

Section: H 2 Excitation Processesmentioning

confidence: 99%

Molecular and Atomic Excitation Stratification in the Outflow of the Planetary Nebula M27

McCandliss¹,

France²,

Lupu³

et al. 2007

ApJ

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High-resolution spectroscopy with FUSE and HST STIS of atomic and molecular velocity stratification in the nebular outflow of M27 challenge models of the abundance kinematics in planetary nebulae. The simple picture of a very high speed ($1000 km s À1 ), high-ionization, radiation-driven stellar wind surrounded by a slower ($10 km s À1 ) mostly molecular outflow, with low-ionization and neutral atomic species residing at the wind interaction interface, is not supported. Instead, we find vibrationally excited H 2 intermixed with mostly neutral atomic species at a transition velocity (33 km s À1 ) between a fast (33Y65 km s À1 ) low-ionization zone and a slow (P33 km s À1 ) high-ionization zone. Ly fluorescence of H 2 has been detected, but far-UV continuum fluorescence has not. The diffuse nebular medium is inhospitable to molecules and dust. Maintaining a modest equilibrium abundance of H 2 [N (H 2 )/N (H i)T1] in the diffuse nebular medium requires a source of H 2 , mostly likely the clumpy nebular medium. The stellar SED shows no sign of reddening [E(B À V ) < 0:01], but paradoxically H /H indicates E(B À V ) $ 0:1. The enhancement of H /H in the absence of dust may result from a two-step process of H 2 ionization by Lyman continuum photons followed by dissociative recombination [ H 2 + ! H þ 2 + e À ! H(1s) + H(nl )], which ultimately produces fluorescence of H and Ly . In the optically thin limit at the inferred radius of the velocity transition, we find that dissociation of H 2 by stellar Lyman continuum photons is an order of magnitude more efficient than spontaneous dissociation by far-UV photons. The importance of this H 2 destruction process in H ii regions has been overlooked.

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Photodestruction of Hydrogen Molecules in H I Regions

Cited by 244 publications

References 0 publications

Revised rate coefficients for H2 and H− destruction by realistic stellar spectra

Revised rate coefficients for H2 and H− destruction by realistic stellar spectra

Photodissociation regions in the interstellar medium of galaxies

Molecular and Atomic Excitation Stratification in the Outflow of the Planetary Nebula M27

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