Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005hia....13..586l&link_type=abstract
In: Highlights of Astronomy, Vol. 13, as presented at the XXVth General Assembly of the IAU - 2003 [Sydney, Australia, 13 - 26 J
Astronomy and Astrophysics
Astronomy
Stellar Abundances
Scientific paper
We present observations of a 10-Gyr-old molecular cloud at the outer edge of the Galactic disk (28 kpc). We detected CO 13CO 18CO CS CN SO HCN HNC HCO+ CH3OH HCS+ H2CO C2H C3H2 and NH3 but we did not detect CO+ N2H+ DCN HC3N 34CS SiO SiS 17CO or SO2. The NH3 H2CO and CS abundances indicate that T = 20 K and n = 5x10(3) cm(-3). The N-containing molecules were weak and we did not detect the usually strong N2H+ or HC3N lines. Using our 5300 chemical reaction model we calculate that the N is depleted in this cloud by about 3x and this cloud has a lower metallicity (similar to dwarf irregular galaxies or damped Lyman alpha systems) and a lower cosmic-ray ionization rate possibly resulting from the infall of halo gas enriched in O C and S from a burst of massive star formation in the Galactic halo shortly after the Milky Way was formed. This activity would have produced both O and S which are produced by massive stars; C which is produced by massive and intermediate mass stars; but less N abundance because the secondary element N is produced primarily from low mass stars.
Brammer Gabriel
Henkel Carsten
Lubowich Donald A.
Millar Thomas J.
Pasachoff Jay. M.
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