Article - GCNEWS, Vol. 6, November 1997


A Newsletter for Galactic Center Research
This Volume was edited by Angela Cotera & Heino Falcke

Volume 6, November 1997 - ARTICLE

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Highlights of the IAU Symposium 184

Angela Cotera & Heino Falcke

The recently concluded IAU Symposium 184, "The Central Regions of the Galaxy and Galaxies" held in Kyoto, Japan, was as breathtaking in the the range of topics covered as it was in the location. Therefore, we will not attempt to cover, in a cursory review such as this, all the talks at the conference but will highlight those we found to be most interesting (we apologize ahead of time to any of our subscribers who may feel slighted, and wholeheartedly admit to extreme personal bias in this report). In particular, we are going to concentrate on the results for the inner ~500 pc of the Galaxy.

Sgr A*

With that said, certainly the most exciting results were those which have significantly reduced the possibilities that Sgr A* is anything but a black hole. The observational results by Eckart & Genzel, Ghez et al., and the theoretical discussion by Maoz have already been highlighted in the Editorial. There was, however, much more presented about Sgr A* than this. The theory crowd presented their - as usual - differing views. Of course, the ADAF (Advection Dominated Accretion Flow) proponent, represented by Ramesh Narayan, presented their results, once again drawing lively reactions from the few theorists present. An important advantage of the ADAF solution is that it explains what we do not see (e.g. the lack of NIR emission). However, this is also achieved by the Bondi-Hoyle accretion scenario presented by Fulvio Melia, and for which Robert Coker presented new 3D hydrodynamical calculations showing the important effect of distributed wind sources (in contrast to the earlier homogeneous models). The expected accretion rate onto Sgr A* can now be much higher than in the idealized solutions, and inhomogeneous density clumps (or pancakes) can form in the ISM. Melia (with Sera Markoff) also discussed the possibility of Gamma-emission from this accretion process and (to his honest surprise) could not rule out that Sgr A* is indeed responsible for the EGRET source at the GC (see article by Mattox in GCNEWS, Vol. 4) - this is also predicted in the ADAF model.

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.

Elsewhere in the Galactic Center...

Taking the rest of the talks roughly in the order in which they were presented, we start by mentioning the ongoing research by Kris Sellgren on high resolution, R~40,000, spectra of the M giant IRS7, which seems to show that the metallicity of the star, as derived from the iron lines, is approximately solar. Solange Ramirez also presented high resolution spectra of other GC stars and found an enhancement of Mg, Si, Gs, and Ti relative to iron, but also found the iron abundance to be solar in IRS7, IRS11 and IRS12. Why the iron abundances should be solar in a region which has been suggested to have an enhanced supernova rate is quite puzzling, and we look forward to the final results from this data. That there has been a large number of supernovae in the region was suggested by both Tsuboi and Hasegawa (see below) in their presentations of molecular cloud observations made with the Nobeyama 45-m telescope. In that context the morphology of Sgr A East becomes also of interest, since it resembles a supernova-like bubble who's nature and interaction with the molecular gas in the GC was discussed by Kevin Uchida. He found that the cluster of H II. regions found just east of Sgr A East was in an apparent valley of molecular gas.

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.

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