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Astron. Astrophys. 338, L1-L4 (1998)

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1. Introduction

A hot galactic corona was first postulated by Spitzer (1956) as a medium to confine the high velocity clouds discovered by Münch (1952, 1957). Münch found absorption in the Ca II lines at `high' velocities in spectra of stars at high galactic latitude.

Almost all information on the hot gas distribution within or surrounding the Galactic disk gathered since then derives from three different kinds of observation: the soft X-ray background at low and medium energies, the high-stage ion populations (C IV , N V and O VI ) observed in absorption in the UV and Far UV, and on the detected FUV and EUV emission line backgrounds. Among the FUV absorption lines the O VI ion contributes the most important information to the understanding of the (hot) Galactic halo. It samples rather high temperature gas ([FORMULA] K) with little contamination expected from photoionised gas and it has a line strength large enough that even nearby stars normally have detectable column densities. With the Copernicus satellite the first detection of O VI absorption lines (Rogerson et al. 1973) at 1031.92 Å ([FORMULA]) and 1037.61 Å ([FORMULA]) was made 25 years ago. These data were analyzed and summarized in a series of papers by Jenkins (1978a, b, c). Excluding a few lines of sight on the grounds of atypically high N(O VI ), Jenkins found an average midplane density [FORMULA] of about 2.8 [FORMULA] cm-3 and a scale height [FORMULA] of [FORMULA] pc.

Since the wavelength region of the O VI resonance doublet is inaccessible for IUE and HST the last available observations with reasonable wavelength resolution were made with the Berkeley spectrometer in Sept. 1993 during the ORFEUS I mission launched on the Space Shuttle Discovery. The ORFEUS-SPAS mission is discussed in detail in Grewing et al. (1991). Hurwitz et al. (1995) present part of their spectrum of the SMC star NGC 346 No.1 near the O VI resonance line and a revisit of the ORFEUS spectrum of PKS 2155-304. They derived from this sparse data an upper limit for N(O VI ) in the galactic halo of 2.0 [FORMULA] cm-2 toward NGC 346 No.1 and 2.2 [FORMULA] cm-2 towards PKS 2155-304. Hurwitz & Boywer (1996b) analyzed 14 early type halo stars (obtained with the Berkeley spectrometer) during the first mission. They derived a scale height for O VI between about 80 pc and 600 pc if the midplane density is between 1.5 and 5 [FORMULA] cm-3. These results were inconsistent with the comparativeley high column densities of N V reported by Sembach & Savage (1992) and the O VI /N V ratios measured in disk stars and predicted by various theories (see Spitzer 1996). New observations, performed with the Berkeley spectrograph aboard the ORFEUS II mission in Nov./Dec. 1996, yield to a N(O VI ) of [FORMULA] cm-2 toward the quasi-stellar object 3C 273 (Hurwitz et al. 1998).

Here we present first results from ORFEUS echelle spectra obtained during the mission of Nov./Dec. 1996. The strength of the absorption by the O VI line at 1031.92 Å has been determined and column densities have been calculated. The ensemble of data allows us to determine a substantially larger scale height of O VI in the galactic halo.

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
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