A far UV study of interstellar gas towards HD 34078: High excitation H₂ and small scale structure^*

¹ Institut d'Astrophysique de Paris, 75014 Paris, France
² LERMA/ENS, France
³ LUTH, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France
⁴ Onsala Space Observatory, 439 92 Onsala, Sweden
⁵ Institute of Astronomy, Cambridge, UK
⁶ IAS, Université d'Orsay, 91405 Orsay Cedex, France
⁷ Johns Hopkins University, Baltimore, USA
⁸ ESA, Vilspa, Spain
⁹ Laboratoire d'Astrophysique de Marseille, 13376 Marseille, France
¹⁰ LESIA, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France

Received: 24 January 2004
Accepted: 12 July 2004

Abstract

To investigate the presence of small scale structure in the spatial distribution of H₂ molecules we have undertaken repeated FUSE UV observations of the runaway O9.5V star, HD 34078. In this paper we present five spectra obtained between January 2000 and October 2002. These observations reveal an unexpectedly large amount of highly excited H₂. Column densities for H₂ levels from (, ) up to (, ) and for several and  levels are determined. These results are interpreted in the frame of a model involving essentially two components: i) a foreground cloud (unaffected by HD 34078) responsible for the H₂ (, 1), CI, CH, CH⁺ and CO absorptions; ii) a dense layer of gas ( cm^-3) close to the O star and strongly illuminated by its UV flux which accounts for the presence of highly excited H₂. Our model successfully reproduces the H₂ excitation, the CI fine-structure level populations as well as the CH, CH⁺ and CO column densities. We also examine the time variability of H₂ absorption lines tracing each of these two components. From the stability of the , 1 and 2 damped H₂ profiles we infer a 3σ upper limit on column density variations (H₂)/N(H₂) of 5% over scales ranging from 5 to 50 AU. This result clearly rules out any pronounced ubiquitous small scale density structure of the kind apparently seen in HI. The lines from highly excited gas are also quite stable (equivalent to %) indicating i) that the ambient gas through which HD 34078 is moving is relatively uniform and ii) that the gas flow along the shocked layer is not subject to marked instabilities.