Astronomy and Astrophysics – Astronomy
Scientific paper
May 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...192.7704s&link_type=abstract
American Astronomical Society, 192nd AAS Meeting, #77.04; Bulletin of the American Astronomical Society, Vol. 30, p.932
Astronomy and Astrophysics
Astronomy
Scientific paper
I present the results of a GHRS archival study of the interstellar absorption lines in the line-of-sight to the H i-rich, starburst dwarf galaxy NGC 1705 in the 1170 to 1740 Angstroms \ range at ~ 120 km s(-1) resolution. In an earlier study using the same data, Sahu & Blades (1997) identified the interstellar component at --20 km s(-1) (LSR velocities) with Milky Way disk/halo gas and the 260 km s(-1) component with a small, isolated high-velocity cloud HVC 487, located ~ 10(deg) from the H i gas which envelops the Magellanic Clouds and the Magellanic Stream (MS). The 540 km s(-1) absorption component seen in the GHRS spectra was identified with the front side of a kpc-scale expanding, ionized supershell centred on the central superstar cluster NGC 1705-1 and has a low Fe/Al abundance ratio. I report a low N/Si abundance ratio for the 540 km s(-1) component. Massive stars (> 10 M_sun) evolve on time scales <= 2 x 10(7) years and return elements like Si into the ISM through Type II SNe while the main contributors to nitrogen are intermediate stars (1 to 10 M_sun) through He shell burning. The N/Si ratio of the supershell component of NGC 1705 is a good indicator of the age of the dwarf galaxy: the N/Si is expected to decrease because of the increase in Si through Type II SNe due to the massive stars, until the onset of He shell burning in intermediate mass stars which occurs on time scales of 10(8) -- 10(9) years. The data point to nucleosythetic enrichment of the supershell component by the first generation of massive stars in NGC 1705-1 over a time scale of < 10(8) years. This time scale for the supershell component is consistent the age estimate of 10 -- 20 x 10(6) years for NGC 1705-1 (Heckman and Leitherer, 1997) and with the low Fe/Al abundance reported by Sahu and Blades.
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