A Photodissociated Region Associated with the Compact H II Region near GGD 12-15

Details A Photodissociated Region Associated with the Compact H II Region near GGD 12-15 A Photodissociated Region Associated with the Compact H II Region near GGD 12-15

Aug 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...503..297g&link_type=abstract

Astrophysical Journal v.503, p.297

Astronomy and Astrophysics

Astronomy

19

Ism: H Ii Regions, Ism: Abundances, Ism: Individual Alphanumeric: Ggd 12-15, Radio Lines: Ism

Scientific paper

We present Very Large Array (VLA) continuum (2, 3.6, and 20 cm) and line (H I 21 cm, C92 alpha , and H92 alpha ) observations toward the obscured cometary-like H II region located near the optical nebulosities GGD 12-15. We find that the H92 alpha recombination line profile is asymmetric, probably due to a superposition of line emission from the H II region and from an underlying partially ionized medium (H0). The observed kinematics of the ionized gas suggests that the H II region is undergoing a champagne flow. The C92 alpha observations show that the C+ emission arises from an extended region of ~20" in size that is closely associated with the H II region. The C+ emission has a line center velocity of 11.9 km s-1, similar to the velocity of the ambient molecular cloud (~11 km s-1). The 21 cm H I spectrum shows emission and absorption line components. The H I line in emission is detected from a region of ~23" in diameter, with a center velocity of 15.4 km s-1 and peaks to the east of the H II region. The absorption feature is unresolved (<=22"), has a line center velocity of 11.7 km s-1, and lies in front of the H II region. Both the C92 alpha and H I emissions are interpreted as arising from a photodissociated region (PDR) around the cometary H II region. An isothermal model has been used to derive the physical parameters of the photodissociated hydrogen gas around the H II region. We derive that the PDR region has an excitation temperature of ~330 K, a hydrogen column density of ~6 x 1021 cm-2, a H I number density of ~1.5 x 104 cm-3, and a H I mass of ~5 M&sun;. The mass in photodissociated hydrogen is about 3 orders of magnitude larger than the mass in ionized hydrogen (~2 x 10-3 M&sun;).

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