Summary: © 1998 RAS
Mon. Not. R. Astron. Soc. 294, 338­346 (1998)
Molecular hydrogen line emission from the reflection nebula
Parsamyan 18
S. D. Ryder, L. E. Allen, M. G. Burton, M. C. B. Ashley and J. W. V. Storey
School of Physics, University of New South Wales, Sydney 2052, Australia
Accepted 1997 October 7. Received 1997 August 19; in original form 1997 June 10
ABSTR ACT
The newly commissioned University of New South Wales Infrared Fabry­Perot
(UNSWIRF) has been used to image molecular hydrogen emission at 2.12 and
2.25 m in the reflection nebula Parsamyan 18. P 18 is known to exhibit low values
of the (1­0)/(2­1) S(1) ratio, suggestive of UV-pumped fluorescence rather than
thermal excitation by shocks. Our line ratio mapping reveals the full extent of this
fluorescent emission from extended arc-like features, as well as a more concentrated
thermal component in regions closer to the central exciting star. We show that the
emission morphology, line fluxes and gas density are consistent with the predictions
of photodissociation region (PDR) theory. Those regions with the highest intrinsic
1­0 S(1) intensities also tend to show the highest (1­0)/(2­1) S(1) line ratios.
Furthermore, variations in the line ratio can be attributed to intrinsic fluctuations in
the incident radiation field and/or the gas density, through the self-shielding action

Source: Ashley, Michael C. B. - School of Physics, University of New South Wales

Collections: Physics