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Issue A&A
Volume 397, Number 2, January II 2003
Page(s) 623 - 634
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021489



A&A 397, 623-634 (2003)
DOI: 10.1051/0004-6361:20021489

$\mathsf{H_2}$ infrared line emission across the bright side of the $\rho$ Ophiuchi main cloud

E. Habart1, F. Boulanger1, L. Verstraete1, G. Pineau des Forêts1, E. Falgarone2 and A. Abergel1

1  Institut d'Astrophysique Spatiale, Université Paris Sud, Bât. 121, 91405 Orsay Cedex, France
2  LERMA, École Normale Supérieure and Observatoire de Paris, France

(Received 20 July 2001 / Accepted 9 October 2002 )

Abstract
We present imaging and spectroscopic observations of dust and gas (H 2) emission, obtained with ISO, from the western edge of the $\rho$ Ophiuchi molecular cloud illuminated by the B2 star HD147889 ( $\chi \sim 400$). This photodissociation region (PDR) is one of the nearest PDRs to the Sun ( $d = 135 \pm 15$ pc from the stellar parallax) and the layer of UV light penetration and of H 2 emission is spatially resolved. It is therefore an ideal target to test the prediction of models on the integrated fluxes but also on the spatial distribution. The emission from dust heated by the external UV radiation, from collisionally excited and fluorescent H 2 are observed to coincide spatially. The spectroscopic data, obtained with ISO-SWS, allows us to estimate the gas temperature to be 300-345 K in the H 2 emitting layer, in which the ortho-to-para H 2 ratio is about 1 or significantly smaller than the equilibrium ratio (~3 at that temperature). We interpret this data with an equilibrium PDR model. In this low excitation PDR, the gas heat budget is dominated by the contribution of the photoelectric heating from very small grains and polycyclic aromatic hydrocarbons (PAHs). With the standard PAH abundance ( $[{\rm C/H}]_{\rm PAH}\simeq5 \times 10^{-5}$), we find that the H 2 formation rate $R_{\rm f}$ must be high in warm gas (~6 times the rate derived by Jura, 1975), in order to account for the observed H 2 emission. This result and the spatial coincidence between the PAHs and H 2 emission suggest that H 2 forms efficiently by chemisorption on the PAHs surface. If the latter interpretation is correct, the enhancement in $R_{\rm f}$ may also result from an increased PAH abundance: assuming that $R_{\rm f}$ scales with the PAH abundance, the observed H 2 excitation is well explained with $R_{\rm f}\simeq 1\times 10^{-16}$  cm 3 s -1 at $T_{\rm gas}=330$ K (~3 times the rate derived by Jura 1975) and $[{\rm C/H}]_{\rm PAH}\simeq 7.5 \times 10^{-5}$.


Key words: ISM: clouds -- ISM: dust, extinction -- atomic processes -- molecular processes -- radiative transfer

Offprint request: E. Habart, emilie.habart@ias.u-psud.fr

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