GCNEWS
A Newsletter for Galactic Center Research
This Volume was edited by Angela Cotera & Heino Falcke
email: gcnews@aoc.nrao.edu
GCNEWS
Wolfgang Duschl discussed the radio spectrum of Sgr A*, presenting a two-component fit of his "monoenergetic electron model" to the spectrum, and pointed out that the Narayan model falls short of the data by a factor of 10 at lower radio frequencies. Heino Falcke, however, was quick to point out that the recent, simultaneous radio spectrum taken of Sgr A* required at least a three component fit (which will vary with time). He suggested that, if you keep adding components, a stratified medium, such as in a jet, would eventually be a simpler explanation. Unfortunately, interstellar broadening hides the intrinsic structure of Sgr A* as was shown by K.Y. Lo, who presented multi-wavelength VLBA observations of Sgr A* which showed the (in)famous lambda2-law and little else. Finally, Mark Reid reported on his attempts to measure the trigonometric parallax to Sgr A* with the VLBA in order to measure the distance to the GC. While the motion of the sun around the GC is easily visible in his maps, the motion of the earth around the sun was not yet seen in the available maps (giving at least a lower limit to the distance), but with some further improvement in the observational technique a reliable distance estimate for the most important dimension of the Galaxy (from our point of view of course) may be within reach.
The formation of stars and the stellar population were addressed by a number of speakers. Gene Serabyn suggested the apparent dichotomy of an aging stellar bulge and the known hot young stars in the central regions was not in fact contradictory. He argued that our Galaxy's center r-2 cluster is an intermediate age population resulting from long-term star formation and not simply the innermost part of the bulge. Dieter Lutz, however, presented ISO data which he suggested would look more like a starburst region if the beam where larger, such as would be the case in external galaxies. Tetsuya Nagata presented ISO results on the Quintuplet and Arches clusters which showed, in addition to [Ar II.] 6.99 micron from the Pistol itself, CO (4.3 micron and 4.7 micron) absorption from the stellar clusters. Don Figer presented data on OB and WR stars in the Quintuplet Cluster, in addition to data on the LBV candidate, in a talk which wins our "most information presented in the time allotted" award. He suggested that the material which forms the Pistol is the result of a previous episode of mass loss from the LBV candidate, the Pistol Star (subject of a recent HST press release).
Studying the CS J=1->0 transition with the Nobeyama 45-m telescope, Masato Tsuboi identified dozens of shell-like structures which he had identified within the central molecular zone. Tetsuo Hasegawa found, with 12CO and 13CO images, over two hundred shell- or arc-like structures in the same region. Although the exact source of these structures was not identified, the possibility of their being formed by supernovae or being wind swept bubbles around massive stars was discussed. Yoshiaki Sofue presented observations in the continuum and CO-line emissions and derived the total central mass using a newly determine conversion factor from the CO line intensity to the H2 molecular mass. He found the mass to be lower, by a factor of three, than previous results. He also suggested that within a well defined molecular ring at 120 pc, the star formation efficiency is low. Not always, however, do you need to justify observation of molecular clouds with your desire to understand stars and star formation, sometimes stars are just a useful foreground screen to learn more about the position and structure of molecular clouds. This was done by Robert Zylka who used star counts along various line of sights in his NIR images to place GC clouds in the third dimension.
In the MIR, Eric Becklin presented 12.5 micron Keck observations of the central parsec. Although they see the MIR source identified by Stolovy et al. (1996, ApJL, 470, 45), their astrometry places Sgr A* southeast of this source (this was consistent with the recent results of Cotera in 8.7 micron images taken from the Hale 200-inch telescope). Dan Gezari presented data on the central parsecs at several wavelengths and suggested that there is considerable variation in the positions of the MIR sources, which he attributed (in his characteristic tender way) to wrong scaling and rotation in other MIR observers images.
Finally, more is being carried out and has been presented on the non-thermal filaments in the GC. While Cornelia Lang & Mark Morris showed some very detailed VLA studies of isolated non-thermal filaments, Johannes Staguhn presented the results of a combined effort to study some new regions where magnetic field lines might interact with molecular clouds in a scenario very similar to the Qunituplet/Pistol/Sickle complex - a field which has developed quite rapidly in recent years.