B. J. Sams

An Unusual Brightening of the Eclipsing Binary Star AKO 9 in the Globular Cluster 47 Tucanae Observed with the Hubble Space Telescope

Fifteen sequential images of the core of the globular cluster 47 Tucanae taken in ultraviolet light with the Hubble Space Telescope have revealed one star that increased in brightness by more than 2 mag in less than an hour. By the end of our observations, this star was the brightest object in the core of the cluster in our bandpass. Auriere et al. first cataloged this star as a blue object (AKO 9), considering it as a potential visible counterpart of the then still single X-ray source. Edmonds et al. found it to be an eclipsing binary, located in the color-magnitude diagram in the vicinity of the main-sequence turnoff. Possible causes for such a brightening are (1) a very large flare on a magnetically active star (RS CVn), (2) an increase in the brightness due to an accretion disk instability in a cataclysmic variable or (3) in a soft X-ray transient, or (4) a nova. There are arguments against every one of these possibilities, and more observations will be needed to understand this system.

Diffraction Limited Near-Infrared Imaging of the Galactic Center

We present deep 1.6 (H) and 2.2 µm (K) images of the central parsec of the Galaxy at a resolution of 0.15″. The images were obtained using a combination of the simple shift-and-add algorithm and the Lucy-Richardson algorithm for “cleaning”. We give a detailed description of the procedure that led to a mosaic covering an area of about 20″ × 20″.

Diffraction Limited Near Infrared Imaging of the Central Parsec of the Galaxy

We present deep 1.6 and 2.2 pμ images of the central parsec of the Galaxy at a resolution of 0.15”. Most of the flux in earlier seeing limited images comes from about 340 unresolved stellar sources with K≤14. The IRS 16 and 13 complexes are resolved into about two dozen and half a dozen sources, a number of which are probably luminous hot stars. We confirm the presence of a blue near infrared object (K ≈ 13) at the position of the compact radio source Sgr A*. The spatial centroid of the source number distribution is consistent with the position of Sgr A’ but not with a position in the IRS 16 complex. The stellar surface density in the central 10” is very well fitted by an isothermal cluster model with a well defined core radius. The derived core radius of all 340 sources is 0.15±0.05 pc. The central stellar density is a few times 107M⊙pc-3. Buildup of massive stars by merging of lower mass stars and collisional disruption of giant atmospheres are very probable processes in the central 0.2 pc.

The Nucleus of NGC 253: The Role of Dust

The galactic nucleus coincides with an extinction peak of A V ≈ 24 corresponding to cloud of mass ≈ 6 × 104 M ⊙ and lies ≈ 2.2” NW of the NIR intensity peak. The radio point sources are most likely HIIregions. The “Hot Spots” are not hot.