The interstellar space is not empty but is filled with gases(such as molecular and atomic hydrogen), dust, and the grains of various species. These interstellar materials are the active ingredients of the Milky Way in that they exchange energy and matter with stars. For example, the main-sequence stars, spending most of their life time in this stage, radiate intense ultraviolet photons that excite, ionize, and dissociate the gases and molecules in the interstellar medium. As the main-sequence stars come to the end of their life at the last stage of evolution, these stars return their matter and energy to the interstellar medium by supernova explosion or forming planetery nebulae. Meanwhile, the gases inside the interstellar molecular cloud are condensed due to the self-gravity to become an egg for proto-stars. Hence, the interstellar medium is a kind of mother earth of the universe, from which the stars are born and to which they go back when they die.
Interstellar medium can take various forms according to their physical states: $H_Ⅱ$ region where atomic hydrogen is ionized by strong radiation from the ambient hot young O, B stars ($T = 10^4 K$, $n = 5\times10^{-3}cm^{-3}$); $H _Ⅰ$/low temperature, low density neutral hydrogen cloud (T = 100 K, $n = 50 cm^{-3}$); molecular cloud composed of mostly molecular hydrogens (T = 10 K, $n = 10^3 cm^{-3}$); interclouds medium with relatively hot neutral hydrogen (T = 5000-10000 K, $n = 0.3 cm^{-3}$); and very hot and diffuse coronal gas region ($T = 10^6 K$, $n ＜ 10^{-2}cm^{-3}$). Among these various structures of the interstellar medium, molecular clouds are one of the extensively studied objects since they are believed to be the place where stars are born.
While atomic hydrogen is easily detectable through its 21 cm radio emission, molecular hydrogen, which composes most of the molecular clouds, is difficult to observe in general, because of its spectroscopic characteristics. Molecular hydrogen emits only flourescent lines in the FUV and IR region, while its ro-vibrational absorption lines can only be observed in the FUV region when there exists a hot background star along the line of sight. Hence, we have only a small amount of observations accomplished in the FUV region, compared to the ground-based visual or radio observations and even compared to the space observations in the X-ray or IR band.
In this study, we analyze the spectra of 56 hot young stars in the Milky Way and the Magellanic Clouds observed by ORFEUS to obtain the column density of each rotational level of $H_2$ and the kinetic temperature of molecular clouds toward these stars. The result shows that most of these molecular clouds have $H_2$ total column densities between $10^{15}$ $cm^{-2}$ and $10^{21}$ $cm^{-2}$, and kinetic temperatures from 21 K to 232 K, with average of 89 K, consistent with the result of Copernicus (Savage et al. 1977). It is also seen that disk stars (z ＜ 500 pc) have higher $H_2$ column densities than halo stars, which implies that molecular hydrogen is more or less concentrated in the disk in our Galaxy. The kinetic temperature of the clouds tends to decrease as the $H_2$ total column density increases.
We also perform the correlation study for the $H_2$ column density N($H_2$) and other physical parameters, such as the column density of neutral hydrogen $N(H_Ⅰ$), color excess E(B-V), and the fraction of molecular hydrogen f = 2N($H_2$) / N(H). The results are in good agreement with those of Copernicus (Savage et al. 1977), in spite that the target stars of Copernicus observations are mainly the disk stars near the Sun while ORFEUS observations include distant stars, even in the Magellanic Clouds. We investigate the derived parameters in relation with the distance and height from the galactic disk.
In addition, we analyze in detail the three O-type stars in the Carina Nebula, HD 93129a, HD 93250, and HD 303308, located in the giant molecular cloud. The derived $H_2$ column densities and kinetic temperatures reveal the hot environment of the molecular clouds in the nebula. The radiation intensity in the FUV wavelength at the surface of the molecular cloud toward each star turns out to be correlated with an inverse-square law with the distance from η Car, the central star in the nebula, which makes it possible to derive the actual distance between the molecular clouds and η Car. The result agrees well with the actual angular distance of three stars from η Car. We also derive the CO-to-$H_2$ conversion factor of the molecular clouds in the Carina Nebula using the cited results of CO radio observation. It is generally believed that CO is a good tracer for $H_2$ since CO is most abundant in the molecular cloud next to $H_2$ molecule and emits in millimeter wavelength.
It is necessary to observe emission lines to understand the general morphology of the molecular clouds. Although $H_2$ composes most of the cloud, it does not emit in the cold state. Hence, CO is used as a tracer for $H_2$ but in this case, it is important to know the precise CO-to-$H_2$ conversion factor to derive the total mass of molecular cloud. Though there have been many observational and theoretical studies regarding the conversion factor, the results are full of ambiguities and they are different case by case. We analyze the CO absorption line in the spectrum of HD 37903 in the southern part of Orion B GMC, instead of the conventional radio emission, to obtain the direct conversion factor of the molecular cloud. The result gives [CO]/[$H_2$] = $2.2×10^{-4}$. We discuss the result in comparison with the observation of CO radio emission.
Finally, future FUV mission on the KAISTSAT-4 is briefly mentioned. The first Korean scientific micro-satellite, KAISTSAT-4, is planned for launch in 2002 with the main payload Far-ultarviolet IMaging Spectrograph(FIMS) to perform interstellar observations. FIMS is specialized with a wide field f vw d medium spectl resolution in the FUV region to study interstellar diffuse plasma, such as coronal gases, supernova remnants, fluorescent emission lines from $H_2$ molecules, etc. Evolution models of hot interstellar plasma will also be tested with the results of FIMS sky survey.
