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1

The Very Massive and Hot LMC Star VFTS 682: Progenitor of a Future Dark Gamma-Ray Burst?

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Zhang D. , Stanek K. Z.

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tom Vol. 62, No. 1

23--32

EN

VFTS 682, a very massive and very hot Wolf-Rayet (WR) star recently discovered in the Large Magellanic Cloud near the famous star cluster R136, might be providing us with a glimpse of a missing link in our understanding of Long Gamma-Ray Bursts (LGRBs), including dark GRBs. It is likely its properties result from chemically homogeneous evolution (CHE), believed to be a key process for a massive star to become a GRB. It is also heavily obscured by dust extinction, which could make it a dark GRB upon explosion. Using Spitzer data we investigate the properties of interstellar dust in the vicinity of R136, and argue that its high obscuration is not unusual for its environment and that it could indeed be a slow runaway (``walkaway'') from R136. Unfortunately, based on its current mass loss rate, VFTS 682 is unlikely to become a GRB, because it will lose too much angular momentum at its death. If it were to become a GRB, it probably would also not be dark, either escaping or destroying its surrounding dusty region. Nevertheless, it is a very interesting star, deserving further studies, and being one of only three presently identified WR stars (two others in the Small Magellanic Cloud) that seems to be undergoing CHE.

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HAT Discovery of 76 Bright Periodic Variable Stars Toward the Galactic Bulge

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Nataf D. M. , Stanek K. Z. , Bakos G. Á.

Acta Astronomica

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2010

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tom Vol. 60, No. 3

261--282

EN

We report on photometric I-band observations of 147 bright periodic variable stars toward the Galactic bulge including 76 new discoveries. We used one of the HATnet telescopes to obtain 151 exposures spanning 88 nights in 2005 of an 8.4°×8.4° field of view (FOV) approximately centered on (l,b)=(1.73,-4.68). We observed the Galactic bulge in 2005 as part of a microlensing feasibility study, and here we discuss the periodic variable stars we found in our data. Among our discoveries we count 52 new eclipsing binaries and 24 other periodic variable stars.

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Finding the Brightest Galactic Bulge Microlensing Events with a Small Aperture Telescope and Image Subtraction

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Nataf D. M. , Stanek K. Z. , Bakos G. A.

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tom Vol. 59, No. 3

255--272

EN

Following the suggestion of Gould and Depoy (1998) we investigate the feasibility of studying the brightest microlensing events towards the Galactic bulge using a small aperture (≈10 cm) telescope. We used one of the HAT telescopes to obtain 151 exposures spanning 88 nights in 2005 of an 8°4×8°4 FOV centered on (l,b)=(2.85, -5.00). We reduced the data using image subtraction software. We find that such a search method can effectively contribute to monitoring bright microlensing events, as was advocated. Comparing this search method to the existing ones we find a dedicated bulge photometric survey of this nature would fulfill a significant niche at excellent performance and rather low cost. We obtain matches to 7 microlensing events listed in the 2005 OGLE archives. We find several other light curves whose fits closely resemble microlensing events. Unsurprisingly, many periodic variable stars and miscellaneous variable stars are also detected in our data, and we estimate approximately 50% of these are new discoveries. We conclude by briefly proposing Small Aperture Microlensing Survey, which would monitor the Galactic bulge around the clock to provide dense coverage of the highest magnification microlensing events.

4

BVI CCD Photometry of the Globular Cluster 47 Tuc

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Kaluzny J. , Wysocka A. , Stanek K. Z. , Krzemiński W.

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1998

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tom Vol. 48, No. 3

439--453

EN

We present BVI photometry for about 16000 stars from a 220 square arcmin field centered 8 arcmin East of the center of 47 Tuc. We have identified eight likely blue stragglers located in the outer parts of the cluster. Four of these objects are easy targets for spectroscopic studies with ground-based telescopes. Six candidates for blue horizontal branch stars were identified. However, it is possible that all or most of them belong in fact to the SMC halo. One faint blue star being candidate for a cataclysmic variable was found close to the cluster center. The average I-band magnitude for stars forming the red giant branch clump is determined at I0=13.09±0.005 mag. This in turn implies distance modulus of the cluster (m-M)0,47Tuc=13.32±0.03±0.036 mag (statistical plus systematic error), if we adopt MI,m=-0.23±0.03 mag for the average absolute luminosity of Hipparcos-calibrated clump giants, following Paczyński and Stanek and Stanek and Garnavich. This distance modulus of 47 Tuc is lower by 0.2-0.25 mag than its recent estimates based on Hipparcos parallaxes for subdwarfs. We discuss possible reasons for this discrepancy. The photometric data are available through the anonymous ftp service.

5

Protecting Life in the Milky Way: Metals Keep the GRBs Away

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Stanek K. Z. , Gnedin O. Y. , Beacom J. F. , Gould A. P. , Johnson J. A. , Kollmeier J. A. , Modjaz M. , Pinsonneault M. H. , Pogge R. , Weinberg D. H.

Acta Astronomica

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2006

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tom Vol. 56, No. 4

333--345

EN

The host galaxies of the five local, z≤0.25, long-duration gamma-ray bursts (GRBs 980425, 020903, 030329, 031203 and 060218), each of which had a well-documented associated supernova, are all faint and metal-poor compared to the population of local star-forming galaxies. We quantify this statement by using a previous analysis of star-forming galaxies (0.005iso, steeply decreases with increasing host oxygen abundance. This might further indicate that (low) metallicity plays a fundamental physical role in the GRB phenomenon, and suggesting an upper metallicity limit for "cosmological" GRBs at ≈0.15 Z_solar.