Article - GCNEWS, Vol. 3, November 1996


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

Volume 3, November 1996 - ARTICLE

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Conference Report: Highlights of the QPS Workshop

Angela Cotera (IPAC/Caltech)

A mini-conference ``QPS: A Rosetta Stone for Understanding Large-Scale MHD Activity near the Galactic Center'' was held at UCLA on September 19 & 20th. There were twelve talks, and much discussion, on work being done at almost all wavelengths at which the Quintupulet/Pistol/Sickle (QPS) have been observed (with the exception of high energy observations), including the first ISO images of the Quintuplet (see the abstract by Nagata et al. in this volume of GCNEWS)

Don Figer (UCLA) presented results on the Quintuplet cluster which contains approximately 10 Wolf-Rayet (WR) stars, including four probable WCL (late-type WR stars which show carbon emission lines), 14 OB supergiants, at least one red supergiant and a possible LBV candidate. The ratio of WN/WC stars is >0.5, which would suggest a higher than solar metallicity, although the ratio still represents small number statistics. The Qunituplet cluster extends well towards the Sickle, with at least one WN6 star located on top of the southwestern extension of the Sickle. Figer suggested that the hot stars in the Quintuplet cluster, which are moving at roughly the Galactic rotation speed, produced an expanding bubble which is now pushing into a slower-moving molecular cloud, the surface of which is being ionized to produce the Sickle. This was supported by work on the NIR spectra of the Sickle and Pistol as presented by Angela Cotera (JPL/Caltech), who showed that the extinction in the Br-gamma line is consistent with the stellar population and therefore concluded that the ionized gas is not embedded in the molecular cloud. In addition no stellar sources are coincident with any of the peaks in the Br-gamma emission. The ionization within the Sickle therefore appears from this evidence to be the fortunate happenstance of a wayward molecular cloud and a cluster of hot stars. Figer also pointed out the location of the cluster within a larger apparent bubble of ionized gas that can be seen in radio continuum maps, but most strikingly in recent MSX images (presented at the meeting by Janet Simpson).

Munetaka Ueno (University of Tokoyo) presented ISO images of the Quintuplet at 7 microns which clearly show the Quintuplet and another faint peak of emission to the northeast of the Quintuplet. In the same presentation, high spatial resolution NIR images of the quintuplet were shown. Although at lower resolution the Quintuplet stars appear to be extended, they appear point like in the new images. It was suggested by Figer that the Quintuplet stars are dusty WCL stars, although the probability of five stars in a single cluster being in such a shortlived evolutionary state was the subject of some disscusion.

To discuss the ramifications on the ISM by the presence of WR stars, Tony Marston (Drake Univerisity) showed that there are often large shells around WR stars, in fact ~40% of all WR stars exhibit such shells. The tendency is to get an evacuated cavity from the stellar winds and only afterward the ejecta from the stars. Such a cavity is seen around the Quintuplet cluster in both the radio and in the MSX images. The Quintuplet cluster is offset from the center of the shell, but such an offset is common in other WR systems where the bubble is subject to a drag force produced by its interaction with the local ISM. How such a shell could possibly retain such a nearly perfect spherical shape under the considerably disturbed conditions in the Galactic Center is, however, problematic. Marston also pointed out that the Pistol appears to be morphologically similar to NGC 6888, an HII region made of circumstellar ejecta from a WR star; this supports the suggestion by Figer that the Pistol is ejecta from the LBV candidate.

The FIR observations of the QPS region presented by Janet Simpson (Ames Research Center), also indicate that the Sickle is caused by stellar ionization with some variations indicating that the ionization source is likely to be the Quintuplet stars. The Pistol however, does not look like a typical galactic H II. region, which would be consistent with the pistol being formed from the outflow of a previous mass loss episode of a Luminous Blue Variable (LBV) star. Cornelia Lang (UCLA), presented VLA radio data on the velocity of the ionized gas as seen in across the Sickle and Pistol. In the Sickle, there is a definite velocity gradient and several of the spectra show a double peaked profile, indicating a more complex structure at smaller scales. In agreement with previous results, the Sickle has an average velocity of 34 km/s and the Pistol 110 km/s. The results for the Pistol are also in agreement with the Quintuplet stars which Figer has found, using super-resolution techniquesm to have an LSR velocity of 125 km/s.

Doug Roberts (Univ. Illinois), observed the H92-alpha line toward the pistol and sickle with the VLA in its lowest resolution configuration (D-array). In general, the results are identical to the higher resolution observations presented by Lang et al. However, more prominent in the low resolution data is a line at the forbidden negative velocity of -40 km/s which peaks at the location of a nonthermal filament. He also noted a high positive velocity (+150 km/s) line from a position near the ``wake'' north of the same nonthermal filament. This gas is likely from the sickle yet the velocity is higher than that of the pistol.

Tom Pauls (NRL) reported on new VLA observations of the non-thermal filaments at 22 GHz. A mosaic image roughly centered on the Sickle has been produced from four VLA pointings. The filaments are clearly seen in this image, and a comparison with 5 GHz observations indicates that the spectral index of the non-thermal filaments is roughly flat up to 22 GHz. Observations with the Nobeyama Array at 43 GHz did not detect the filaments, which implies that the spectral index must turn over somewhere between 22 and 43 GHz.

BIMA results from a mosaic of six fields around the Sickle were presented by Ray Plante (Univ. Illinois). The higher resolution CS 2-1 observations showed that the molecular filament stretches out from the main molecular cloud along the nonthermal filmaments, increasing the evidence for an interaction between two features and strengthening the case for the spatial coexistence of the ionized and molecular gas.

The generation of the non-thermal filaments was suggested by Gene Serabyn (Caltech) to be the result of the interaction of a molecular cloud ionized by stars, which then reconnects to the magnetic field and streams outward from the Sickle region. Serabyn showed that the Pistol is depolarized and therefore suggests that the Pistol, and thus the Quintuplet cluster, is behind the molecular gas, rather than in front as suggested by others. Mark Morris (UCLA) presented radio polarization studies of synchrotron emission from the nonthermal filaments (NTF's), which confirm that the magnetic field is oriented parallel to the filaments: perpendicular to the galactic plane. Far-IR measurements made on the KAO with the Univ. of Chicago array polarimeter, STOKES, show that the thermal emission from dust in or near the ionized interface with the molecular cloud underlying the sickle is strongly polarized (up to 10-12\%), with the electric field vectors being very uniform, and oriented perpendicular to the Galactic plane. The highest polarization in the region was shown by Roger Hildebrand (Univ. of Chicago) to be coincident with both the Arched Filaments and the Sickle. The polarization in these regions is a factor of 3 higher than in Orion. The implied magnetic field in the molecular cloud is thus parallel to the galactic plane-perpendicular to the field in the NTF's just outside the clouds. The interaction of the two perpendicular field systems at the ionized cloud interface will give rise to field line recombination, which can accelerate electrons to relativistic energies and account for the synchrotron ``illumination'' of the NTF's.

As to be expected, no hard conclusions were reached, but some questions, such as the ionization source of the Sickle, seem to have been answered. Also, as expected, many intriguing questions were raised. How can the geometries of the cluster of ionizing stars, the multiple velocity gases and the non-thermal filaments possibly be reconciled? What is the extent of the interaction of the thermal Sickle and the straight non-thermal filaments? What does this imply for the other regions of thermally ionized gas and non-thermal threads? We look forward to learning the answers to these and other such questions at future conferences and workshops!

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