Coronal gas in the galactic halo: ORFEUS obser vations of NGC 346 #1

Abstract

We present high resolution ( λ δλ = 3000 ) observations of the Small Magellanic Cloud (SMC) star NGC 346 #1 near the O vi resonance doublet (1031.9, 1037.6 Å). The data were collected with the Berkeley extreme/far ultraviolet (EUV/FUV) spectrometer in the ORFEUS telescope aboard the space platform Astro-SPAS during the mission of space shuttle Discovery flown in September 1993. The O vi feature shows broad absorption centered near ν LSR ≈ +140 km/sec. This is almost certainly associated with the OB cluster NGC 346 itself, which also shows deep absorption at Si iv and C iv at comparable velocities. An absorption feature corresponding to O vi near ν LSR ≈ −230 km/sec is present in our data, but this feature is caused by contamination from a second star, probably an early B-type main sequence star at lower local standard of rest (LSR) velocity, which is also present in the 20″ circular aperture. Between these velocity extrema lies one broad feature ( b ≈ 70 km/sec) or multiple narrower features, with a combined column density of about 1.4 × 10 14 cm −2. Assuming an exponential distribution of O vi with the in-plane density of Jenkins /1/, we infer a scale height, H 0, of no more than ≈1.2 kpc toward NGC 346 #1. Our O vi column density is about a factor of 4 below the predictions of cooling fountain models, but our low scale height is consistent with more recent predictions /2/.