Other
Scientific paper
Apr 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992pasp..104..309l&link_type=abstract
Publications of the Astronomical Society of the Pacific, v.104, p.309
Other
Scientific paper
A Galactic gaseous halo formed through the interstellar disk-halo connection is simulated by means of a two-dimensional axisymmetric hydrodynamic code based upon the chimney model of the interstellar medium, a new version of the galactic fountain. Galactic rotation, heating processes by diffuse UV flux, and radiative cooling processes are taken into account. The resulting gaseous halo can be divided into three categories, i.e., wind-type halo, bound-type halo, and cooled-type halo. In this way, we try to reproduce the column densities of C IV, N V, O VI, and Si IV in the observed absorption lines of halo stars. Assuming that the radiatively cooled halo gas condenses into clouds due to thermal instabilities, we can calculate their distribution and ballistic motions in the Galactic gravitational field. These correspond to the high- and intermediate-velocity clouds observed at high Galactic latitudes. We find that a cooled-type halo with a gas temperature between 5 X 10^5 and 10^6 K and a density between 10^-3 and 10^-2 cm^-3 at the disk-halo interface can reproduce the observational facts about our Galaxy. Supposing that the metal-absorption-line systems of QSOs arise from the halos of intervening galaxies formed by similar processes, we calculate features of the Ca II, Mg II, C IV, and Si IV absorption lines in various stages of galactic evolution. We conclude that C IV systems which are greater than 50 kpc in size correspond to the wind-type halo. On the other hand, Mg II and Ca II systems can only be detected in a very restricted region ( Metaxa, SMALL FAINT CLUSTERS IN THE LMC This is a short review of the main results of my Ph.D. thesis concerning some important problems on the dynamical properties of the LMC star clusters. The topic of this thesis was to find and study the dynamical paramters (tidal radius r_t core radius r_c concentration parameters log (r_t/r_c), and total mass M) for a large sample of small LMC clusters and to define their location in the parent galaxy. In particular we studies 70 small clusters (M < 10^4 solar mass) (Metaxa et al. 1988) which are located throughout the whole periphery of this galaxy. In addition, 93 small clusters (Kontizas et al. 1988) were detectd and cataloged in the central LMC area in order to establish if all small LMC cluster form one system. The examined sample represents 4% of the total LMC star-cluster population with diameters greater than 6 pc (Kontizas et al. 1990). The dynamical parameters were obtained by means of star counts performed on high-quality plates (I, J, R) taken with the 1.2-m Schmidt telescope for the remote clusters, whereas the central crowded LMC regions were examined on plates (U,B,V) taken with the 3.6-m ESO telescope. The main results are as follows: (1) An important criterion for classifying clusters is their density profile. Eighty percent of the studied LMC star clusters were found to have profiles which fit King (1966) radial-density distributions. The remaining 20% of the star clusters give profiles which are divided into categories which characterize each group of the discrepant clusters. What must be noticed is that one group of these clusters does not reach a flat central density, and at radii larger than the core radius will show various deviations from the theoretical profiles. This has been attributed to warm initial conditions. Another star-cluster group exhibits a ``bump'' in their density profiles (Kontizas et al. 1987). This can be explained as either due to the weak-gravitational attraction of the parent galaxy during the clusters' dynamical evolution, or that these clusters are in the stage of core oscillation (Heggie 1989). (2) The majority of small LMC star clusters of our sample (80%) seem to be relaxed, most likely by ``two-body'' encounters, considering that the age of their stellar content is old enough to allow this mechanism to be effective. (3) We cataloged all small clusters in three LMC central regions where 93 new clusters have been detected. Coordinates, apparent sizes, and colors were given for the first time (Kontizas et al. 1988). These clusters were found to have small masses and sizes (10^3 solar mass < M < 10^4 solar mass, 14 pc < r_t < 37 pc). (4) Small star clusters with similar dynamical parameters were also found situated at distances larger than 4.5 from the rotational center of the LMC. Their total dynamical masses are found to give a very good extrapolation to the LMC rotational curve. These lead us to the assumption of an extended disk in the LMC. Therefore the small intermediate-age (1.4 x 10^9 yr) clusters of the LMC occupy the extended disk of the LMC where the old stellar population is located. (5) The age of the stellar population for four star clusters was estimated from their HR diagrams and found to be 1.4 X 10^8-2 X 10^9 yr, confirming that the age range of the LMC small clusters is narrower than that of our own Galaxy since we did not find very young small clusters. (6)We have studied dynamically for the first time LMC clusters with masses smaller than 10^4 solar mass and found that the mass spectrum of the small LMC star clusters coincides partly with the mass spectrum of the galactic open clusters, but extends to larger values. (7) Finally we have compared the frequency distribution of the derived dynamical parameters of all studied clusters with those of the open clusters of our Galaxy. We found that faint and old globular LMC clusters form one disk system, which is similar to the open-cluster system of our Galaxy. The LMC clusters' disruptive mechanism must be weak and does not seem to affect the small- or intermediate-mass star clusters. Further progress in understanding the formation and early evolution of the LMC clusters requires a detailed study of the conditions prevailing in the inner regions of the molecular clouds from which they are probably formed. (SECTION: Dissertation Abstracts)
No associations
LandOfFree
Formation of a Giant Galactic Gaseous Halo: Metal-Absorption Lines and High-Velocity Clouds does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Formation of a Giant Galactic Gaseous Halo: Metal-Absorption Lines and High-Velocity Clouds, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Formation of a Giant Galactic Gaseous Halo: Metal-Absorption Lines and High-Velocity Clouds will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1739670