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1

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

Zhang D., Stanek K. Z.

Acta Astronomica

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2012

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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.

2

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

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

Acta Astronomica

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2010

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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.

3

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

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

Acta Astronomica

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2009

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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

Planets in Stellar Clusters Extensive Search. V. Search for Planets and Identification of 18 new Variable Stars in the Old Open Cluster NGC 188

Mochejska B. J., Stanek K. Z., Sasselov D. D., Szentgyorgyi A. H., Cooper R. L., Hickox R. C., Hradecky V., Marrone D., P., Winn J. N., Schwarzenberg-Czerny A.

Acta Astronomica

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2008

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Vol. 58, No. 3

263--278

EN

We have undertaken a long-term project, Planets in Stellar Clusters Extensive Search (PISCES), to search for transiting planets in open clusters. In this paper we present the results for NGC 188, an old, rather populous cluster. We have monitored the cluster for more than 87 hours, spread over 45 nights. We have not detected any good transiting planet candidates. We have discovered 18 new variable stars in the cluster, bringing the total number of identified variables to 28, and present for them high precision light curves, spanning 15 months.

5

A Study of Stellar Photometric Variability Within the Central 4 pc of the Galactic Center with Infrared Image Subtraction

Peeples M. S., Stanek K. Z., DePoy D. L.

Acta Astronomica

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2007

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Vol. 57, No. 2

173--199

EN

We present a catalog of 110 variable stars within ≈1' of Sgr A* based on image subtraction of near-infrared (H and K) photometry. Our images were obtained over 133 nights from 2000 to 2002 in H-band and over 134 nights from 2001 to 2002 in K-band; the typical FWHM is 1".4. We match the catalog to other near-infrared, X-ray, and radio (i.e., maser) data, and we discuss some of the more interesting objects. The catalog includes 14 sources with measurable periods, several known long-period variables and three new LPV candidates. We associate IRS 10* with OH, SiO, and H2O masers and a bright X-ray point source; this analysis suggests IRS 10* is an AGB star with an accreting companion. Among the ≈90 newly discovered sources are a probable cataclysmic variable, a potential edge-on contact 84 day period eclipsing binary, and a possible 41 day period pulsating variable.

6

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

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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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.

7

UBVI Color-Magnitude Diagrams in Baade's Window: Metallicity Range, Implications for the Red Clump Method, Color "Anomaly" and the Distances to the Galactic Center and the Large Magellanic Cloud

Stanek K. Z., Kaluzny J., Wysocka A., Thompson I.

Acta Astronomica

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2000

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Vol. 50, No. 2

191--210

EN

We analyze the UBVI color-magnitude diagrams towards the Galactic bulge in a relatively low-reddening region of Baade's Window. The dereddened red giant branch is very wide [≈1.0 mag in (U-B)0 and ≈0.4 mag in (B-V) 0 and (V-I) 0], indicating a significant dispersion of stellar metallicities, which by comparison with the theoretical isochrones and data for Galactic clusters we estimate to lie between approximately -0.7<[Fe/H]<+0.3, i.e., spanning about 1 dex in metallicity, in good agreement with earlier spectroscopic studies. We also discuss the metallicity dependence of the red clump I-band brightness and we show that it is between 0.1-0.2 mag/dex. This agrees well with the previous empirical determinations and the models of stellar evolution. The dereddened (V-I)0 color of the red clump in the observed bulge field is ⟨(V-I)0⟩=1.066, σ(V-I)0=0.14, i.e., 0.056 mag redder than the local stars with good parallaxes measured by Hipparcos. It seems that the large "color anomaly" of ≈0.2 mag noticed by Paczyński and Stanek and discussed in many recent papers was mostly due to earlier problems with photometric calibration. When we use our data to re-derive the red clump distance to the Galactic center, we obtain the Galactocentric distance modulus μ0,GC=14.69±0.1 mag (R0=8.67± 0.4 kpc), with error dominated by the systematics of photometric calibration. We then discuss the systematics of the red clump method and how they affect the red clump distance to the Large Magellanic Cloud. We argue that the value of distance modulus μ0,LMC=18.24±0.08 (44.5±1.7 kpc), recently refined by Udalski, is currently the most secure and robust of all LMC distance estimates. This has the effect of increasing any LMC-tied Hubble constant by about 12%, including the recent determinations by the HST Key Project and Sandage et al. The UBVI photometry is available through the anonymous ftp service.

8

Identification and Photometry of Globular Clusters in M31 and M33 Galaxies

Mochejska B. J., Kaluzny J., Krockenberger M., Sasselov D. D., Stanek K. Z.

Acta Astronomica

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1998

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Vol. 48, No. 3

455--488

EN

We have used the data from the DIRECT project to search for new globular cluster candidates in the M31 and M33 galaxies. We have found 67 new objects in M31 and 35 in M33 and confirmed 38 and 16 previously discovered ones. A VI and BVI photometry has been obtained for all the clusters in M31 and M33, respectively. Luminosity functions have been constructed for the clusters in each galaxy and compared with that of the Milky Way.

9

BVI CCD Photometry of the Globular Cluster 47 Tuc

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

Acta Astronomica

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1998

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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.