CFHT Information Bulletin, number 38, First Semester 1998

Distribution of the interstellar medium in the outer parts of the Galaxy: a preparation of the FUSE space mission.

Guillaume Hébrard (,) Roger Ferlet and Alfred Vidal-Madjar, Institut d'Astrophysique de Paris.

The distribution and physical conditions of the low density neutral gas within the interstellar medium (ISM) is of paramount importance for understanding the abundance patterns of the most common elements. In particular, the proportion of the hot and very low density regions relative to the warm ISM is still unknown at the galactic scale. The pioneering studies in the far UV with the Copernicus satellite were limited both in spectral resolution, R = / 2×104, and in distance to essentially the first kpc from the sun. Then IUE made it possible to study more distant sight lines, particularly for the highly ionized species (Pettini & West 1982; Savage et al. 1990 and references therein). However, the IUE spectral resolution (R 104) was insufficient to study the cooler gas distribution. This was first accomplished with ground based observations at a much higher spectral resolution (R 105), but again in the same local portion of the Galaxy (see e.g. Hobbs 1978 or Welsh et al. 1991). On the other hand, lower resolution (R 2×104) ground based studies toward more distant stars in the galactic disk revealed the correlation of absorption systems with the galactic spiral structure (Rickard 1974). Only with HST has it been possible to study distant lines of sight in the ISM at (R 105) in the UV, toward stars identified and carefully selected for this purpose on the basis of their ground based spectra (e.g. Spitzer & Fitzpatrick 1995).

Spectra at R 105 are required to fully resolve the patchiness and velocity structure of the ISM. Indeed the mean cloud-to-cloud separation in the velocity space is known to be a few km/s in the vicinity of the Sun. If most of the previous works at this resolution was confined to the nearby ISM, the first large attempt to extend it toward distant galactic stars was performed by Sembach et al. (1993 & 1995). Mainly concentrated in the Galactic center hemisphere, their study led to a better picture of the ISM structure.

In collaboration with C. Gry, M. Lemoine, K.R. Sembach, S.J. Smartt and G. Sonneborn, we extend that study towards the outer parts of the galactic disk, for which the properties of the low density warm ISM relative to galactic rotation and scale height are poorly known. With the first work by Sembach et al., this new study could permit to understand the distribution and properties of the ISM as a function of galactocentric distance, which is essential for understanding and modeling the dynamical structure and chemical evolution of the Galaxy.

We obtained for this program a total of six nights at the CFH Telescope (from December 1996 to October 1997). With the Coudé f/4 Gecko spectrograph, we observed interstellar Na I D (5890Å and 5896Å) and Ca II K (3934Å) lines in direction of distant (d > 1kpc) early type stars. These observations of both Na I and Ca II can give information on the gas distribution, ionization and depletion, whereas the picture provided by only one of these lines is incomplete. We reached the high resolving power R 9×104 which is mandatory to study the velocity structure of the absorbants when only few lines, like in the visible, are observed.

Because the observationnal windows were not allocated in an optimized way and due to the bad weather, we have observed only 50 stars in Na I and 15 of these in Ca II. The analysis is yet in progress and is awaiting for other observations, but already it has allowed us to select target stars for the FUSE space mission.

FUSE (the Far Ultraviolet Spectroscopic Explorer) is a PI-class NASA astronomy mission, developed in collabration with the Canadian Space Agency and the Centre National d'Études Spatiales (France), which will be launched in late 1998. This satellite will obtain high-resolution spectra (R 3×104) in the wavelength range 905-1195Å which is very rich in spectral lines arising from the ISM and hitherto very little explored. In addition to the Guest Investigator time (for which the first announcement was released by NASA in mid-December 1997), a part of the FUSE obsering time is dedicated to the FUSE Science Team programs, which include deuterium abundance measurements (Lyman lines), sudies of hot gas (O VI lines), observations of the molecular hydrogen lines, and other investigations from planetary atmospheres to galactic nuclei.

Our observations at the CFH Telescope have permitted to select targets in the lines of sight of which we will observe the deuterium and hydrogen Lyman lines, in order to measure the D/H ratio, in the context of the FUSE guaranted time. The knowledge of this ratio in different astrophysical environments is a crucial constraint for the Big Bang Nucleosynthesis and the galactic evolution. In the figure we show three lines of sight which present interstellar components with high and negative heliocentric velocities. With respect to the galactic rotation, these high and negative velocities toward stars in the anticenter direction probably indicate a large distance (Rickard 1974); thus these clouds seem to be representative of the outer parts of the Galaxy. Moreover, as the deuterium Lyman lines are shifted by 82 km/s toward the blue from the hydrogen Lyman lines, they won't be blended with the hydrogen lines from the local ISM (occuring between 10 and 20 km/s on the four spectra, see figure). Thus the three stars are examples of good targets for the galactic deuterium FUSE Science Team program.

These first results are very precious, and they illustrate the need for high spectral resolution in both the red and the blue toward targets of magnitude about 10, as it is available with Gecko at the CFH Telescope, for studies of the diffuse interstellar medium.


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Editor: Dr. T. M. C. Abbott,
Copyright © 1998, CFHT Corporation. All rights reserved.