Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a....94..238r&link_type=abstract
Astronomy and Astrophysics, Vol. 94, P. 238, 1981
Astronomy and Astrophysics
Astrophysics
38
Scientific paper
A two-degree field including the H II region NGC 281 has been mapped with a synthesis interferometer both in continuum and H I-line emission at 21cm wavelength. The resolution is 2'.0 × 2'.4 by 066 km s-1 in radial velocity. Low order spatial components measured with a single telescope have been added to the maps.
The thermal continuum emission extends over 40' diameter and comprises a total flux density of 21 Jy. At the presumed distance of 2200 pc the total mass of H II is 2200 Msun. Two dense concentrations of gas are apparent from two peaks of emission on either side of the 0 6.5 V exciting star, HD 5005.
A broad region of enhanced H I emission partly surrounds the nebula and is most intense on the southeast side, adjacent to the sharpest decline in thermal H II emission. The H I spectral profiles near the edge of the ionized gas are almost twice as wide in velocity as profiles remote from the nebula. They are well fitted by a model in which the atomic shell has a radial extent of ˜8 pc, a maximum density of 20 cm-3 and an expansion velocity of ˜6 km s-1. The 3500 Msun of H I which makes up this broad expanding shell is likely to be derived from H2 directly dissociated by ultraviolet emission from the exciting star. The cause of the expansion is not clear. A dense molecular concentration in the southwest quadrant, detected in CO emission and coincident with dense optical obscuration, has no direct counterpart in H I emission but can be seen to preclude expansion of H I in the approach direction.
An interesting H I feature is a 25 M0 cloud coincident with the east thermal continuum peak which is receding at 10-18 km relative to the bulk of H I emission. The centroid of emission from this partially ionized cloud varies with velocity in a sense that implies movement outwards from the exciting star. Both the ionized component (˜60 Msun) and HD 5005 itself are receding from the main H I concentration. We suggest that the star and associated gas may have had av origin in the southwest foreground molecular cloud which contains an H2O maser source.
New theories of the evolution of H II regions in molecular clouds must be adapted to quantitatively account for the dynamics and properties of the peripheral dissociated hydrogen.
Pedlar Alan
Roger Robert S.
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