arXiv subjects

K. Z. Stanek

Publications and source records attributed to K. Z. Stanek.

At least 289 records · Page 16Linked to original sources

arXiv1999

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. V. Variables in the Field M31F

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33 -- using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, DEBs provide us with the potential to determine these distances with an accuracy better than 5%. The extensive photometry obtained in order to detect DEBs provides us with good light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries, the distance estimates will be free of any intermediate steps. As a first step in the DIRECT project, between September 1996 and October 1997 we obtained 95 full/partial nights on the F. L. Whipple Observatory 1.2 m telescope and 36 full nights on the Michigan-Dartmouth-MIT 1.3 m telescope to search for DEBs and new Cepheids in the M31 and M33 galaxies. In this paper, fifth in the series, we present the catalog of variable stars found in the field M31F $[(α,δ)= (10.\arcdeg10, 40.\arcdeg72), J2000.0]$. We have found 64 variable stars: 4 eclipsing binaries, 52 Cepheids and 8 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, is available via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

astro-ph↗

arXiv1999

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. IV. Variables in the Field M31D

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33 -- using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, DEBs provide us with the potential to determine these distances with an accuracy better than 5%. The extensive photometry obtained in order to detect DEBs provides us with good light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries, the distance estimates will be free of any intermediate steps. As a first step in the DIRECT project, between September 1996 and October 1997 we obtained 95 full/partial nights on the F. L. Whipple Observatory 1.2 m telescope and 36 full nights on the Michigan-Dartmouth-MIT 1.3 m telescope to search for DEBs and new Cepheids in the M31 and M33 galaxies. In this paper, fourth in the series, we present the catalog of variable stars, most of them newly detected, found in the field M31D $[(α,δ)= (11.\arcdeg03, 41.\arcdeg27), J2000.0]$. We have found 71 variable stars: 5 eclipsing binaries, 38 Cepheids and 28 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, is available via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

astro-ph↗

arXiv1999

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. III. Variables in the Field M31C

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33 -- using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, DEBs provide us with the potential to determine these distances with an accuracy better than 5%. The extensive photometry obtained in order to detect DEBs provides us with good light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries, the distance estimates will be free of any intermediate steps. As a first step in the DIRECT project, between September 1996 and October 1997 we obtained 95 full/partial nights on the F. L. Whipple Observatory 1.2 m telescope and 36 full nights on the Michigan-Dartmouth-MIT 1.3 m telescope to search for DEBs and new Cepheids in the M31 and M33 galaxies. In this paper, third in the series, we present the catalog of variable stars, most of them newly detected, found in the field M31C [(alpha,delta)=(11.10, 41.42) deg, J2000.0}]. We have found 115 variable stars: 12 eclipsing binaries, 35 Cepheids and 68 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, is available via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

astro-ph↗

arXiv1998

BVI CCD Photometry of the Globular Cluster 47 Tuc

We present $BVI$ photometry for about 16000 stars from a 220 arcmin$^{2}$ 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 $I_{0}=13.09\pm 0.005 $mag. This in turn implies distance modulus of the cluster $(m-M)_{0,47Tuc}=13.32\pm 0.03\pm 0.036 $mag (statistical plus systematic error), if we adopt $M_{I,m}=-0.23 \pm 0.03 $mag for the average absolute luminosity of {\em Hipparcos}-calibrated clump giants, following Paczyński & Stanek and Stanek & 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.

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arXiv1998

Identification and photometry of globular clusters in M31 and M33 galaxies

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.

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arXiv1998

A ``Short'' Distance to the Large Magellanic Cloud With the Hipparcos Calibrated Red Clump Stars

Following an approach developed by Paczyński & Stanek, we derive a distance to the Large Magellanic Cloud (LMC) by comparing red clump stars from the Hipparcos catalog with the red clump stars observed in two fields in the LMC that were selected from the ongoing photometric survey of the Magellanic Clouds to lie in low extinction regions. The use of red clump stars allows a single step determination of the distance modulus to the LMC, $μ_{0,LMC} = 18.065\pm 0.031\pm 0.09 $mag (statistical plus systematic error), and the corresponding distance, $R_{LMC}= 41.02\pm 0.59\pm 1.74 kpc$. This measurement is in excellent agreement with the recent determination by Udalski et al., also based on the red clump stars, but is $\sim 0.4 $mag smaller than the generally accepted value of $μ_{0,LMC} = 18.50\pm 0.15 $mag. We discuss possible reasons for this discrepancy and how it can be resolved.

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arXiv1998

Distance to M31 With the HST and Hipparcos Red Clump Stars

Following an approach by Paczyński & Stanek we compare red clump stars with parallaxes known to better than 10% in the Hipparcos catalog with the red clump stars observed in three fields in M31 using the HST. There are $\sim 600$ and $\sim 6,300$ such stars in the two data sets, respectively. The local red clump luminosity function is well represented by a Gaussian with the peak at $M_{I,m}=-0.23$, and the dispersion $σ_{RC}\approx0.2 $mag. This allows a single step determination of the distance modulus to M31 $μ_{0,M31} = 24.471\pm 0.035 \pm 0.045 $mag (statistical plus systematic error) and the corresponding distance $R_{M31}= 784\pm 13\pm 17 kpc$. The number of red clump stars is large enough that the formal statistical error in the distance is only $\lesssim 2$%. We also correct the treatment of the local interstellar extinction by Paczyński & Stanek and we obtain the Galactocentric distance modulus $μ_{0,GC}=14.57 \pm0.04 \pm 0.04 $mag (statistical plus systematic error), and the corresponding Galactocentric distance $R_0=8.2 \pm0.15 \pm 0.15 kpc$.

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arXiv1998

Using the DIRBE/IRAS All-Sky Reddening Map To Select Low-Reddening Windows Near the Galactic Plane

Recently Schlegel, Finkbeiner & Davis published an all-sky reddening map based on the COBE/DIRBE and IRAS/ISSA infrared sky surveys. Using the reddening map of Baade's Window and sample of 19 low-latitude ($|b|<5°$) Galactic globular clusters I find that the DIRBE/IRAS reddening map overestimates $E(B-V)$ at low galactic latitudes by a factor of $\sim 1.35$. I also demonstrate the usefulness of this high resolution map for selecting low-reddening windows near the Galactic plane.

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arXiv1998

Testing the COBE/IRAS All-Sky Reddening Map Using the Galactic Globular Clusters

We live in a dusty Universe, and correcting for the dust extinction and reddening affects almost all aspects of the optical astronomy. Recently Schlegel, Finkbeiner & Davis published an all-sky reddening map based on the COBE/DIRBE and IRAS/ISSA infrared sky surveys. Their map is intended to supersede the older Burstein & Heiles reddening estimates. In this paper I test this new reddening map by comparing the reddening values for a sample of 110 $|b|>5°$ Galactic globular clusters selected from compilation of Harris. I find a good agreement for globular clusters with galactic latitude $|b|>20°$ and fair overall agreement for globular clusters with $20>|b|>5°$, but with several significant deviations. I discuss four individual clusters with largest deviations, NGC 6144, Terzan 3, NGC 6355 and IC 1276, in order to investigate the reasons for these large deviations. It seems that the new reddening map overestimates the reddening in some large extinction regions. However, with its high spatial resolution the new reddening map can be used to estimate the relative variation of the reddening on scales $\lesssim 10'$.

astro-ph↗

arXiv1997

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. II. Variables in the Field M31A

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33, using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, detached eclipsing binaries provide us with the potential to determine these distances with an accuracy better than 5%. The massive photometry obtained in order to detect DEBs provides us with the light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries the distance estimates will be free of any intermediate steps. As a first step of the DIRECT project, between September 1996 and January 1997 we have obtained 36 full nights on the Michigan-Dartmouth-MIT (MDM) 1.3-meter telescope and 45 full/partial nights on the F. L. Whipple Observatory (FLWO) 1.2-meter telescope to search for detached eclipsing binaries and new Cepheids in the M31 and the M33 galaxies. In this paper, second in the series, we present the catalog of variable stars, most of them newly detected, found in the field M31A ($α_{2000.0},δ_{2000}=11.34°,41.73°$). We have found 75 variable stars: 15 eclipsing binaries, 43 Cepheids and 17 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, are available using the anonymous ftp service and the WWW. The CCD frames are available on request.

astro-ph↗

arXiv1997

Galactocentric Distance With the OGLE and Hipparcos Red Clump Stars

We compare red clump stars with parallaxes known to better than 10% in the Hipparcos catalog and corrected for the interstellar extinction, with the OGLE red clump stars in Baade's Window also corrected for the interstellar extinction. There are $\sim 600$ and $\sim 10,000$ such stars in the two data sets, respectively. We find empirically that the average I-band magnitude of red clump stars does not depend on their intrinsic color in the range $ 0.8 < (V-I)_0 < 1.4 $. The red clump luminosity function is well represented by a gaussian with the peak at $ M_{I_0,m} = -0.26 $, and the dispersion $ σ_{RC} \approx 0.2 $ mag. This allows a single step determination of the distance to the galactic center and gives $ R_0 = 8.4 \pm 0.4 $ kpc. The number of red clump stars is so large that a formal statistical error is only $\sim 1%$. The local stars are relatively blue and have a small color dispersion: $<(V-I) > = 1.01, σ_{(V-I)} = 0.08$, while for the bulge stars $<(V-I)_0 > = 1.22, σ_{(V-I)_0} = 0.14$. Presumably, the bulge population has a broader range and a higher average metallicity than the local disk population.

astro-ph↗

arXiv1997

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. I. Variables in the Field M31B

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33, using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, detached eclipsing binaries provide us with the potential to determine these distances with an accuracy better than 5%. The massive photometry obtained in order to detect DEBs provides us with good light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries the distance estimates will be free of any intermediate steps. As a first step of the DIRECT project, between September 1996 and January 1997 we have obtained 36 full nights on the Michigan-Dartmouth-MIT (MDM) 1.3-meter telescope and 45 full/partial nights on the F. L. Whipple Observatory (FLWO) 1.2-meter telescope to search for detached eclipsing binaries and new Cepheids in the M31 and the M33 galaxies. In this paper, first in the series, we present the catalog of variable stars, most of them newly detected, found in the field M31B ($α_{2000.0},δ_{2000}=11.20°,41.59°$). We have found 85 variable stars: 12 eclipsing binaries, 38 Cepheids and 35 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, are available using the anonymous ftp service and the WWW.

astro-ph↗

arXiv1997

Two Confirmed Cataclysmic Variables in the Old Stellar Cluster NGC 6791

Based on new CCD photometry and spectroscopy we confirm the presence of two cataclysmic variables (CVs) in the very old open cluster NGC 6791. One of these variables, known as B8, was observed by Rucinski, Kaluzny & Hilditch (1996) to undergo a large magnitude outburst in 1995. The spectrum of this star outside the outburst, obtained by us with MMT, shows clearly the emission lines characteristic for dwarf novae. We observed the second star, known as B7, to undergo large ($\sim3 mag$) drop in its brightness over $\sim 10 days$. The spectrum of B7 obtained in the high state resembles spectra of nova-like CVs. This star a likely member of UX~UMa subtype of CVs. Variables B7 and B8 represent only the second and the third cataclysmic variables known in open clusters. Variable B7 has observational characteristics which would make it difficult to identify as a CV with some of the methods being currently used in surveys for CVs in globular clusters.

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arXiv1997

The Optical Gravitational Lensing Experiment. The Catalog of Periodic Variable Stars in the Galactic Bulge. V. Periodic Variables in Fields: MM5-A, MM5-B, MM7-A and MM7-B

We present the fifth part of the OGLE Catalog of Periodic Variable Stars in the Galactic bulge. 574 variable stars: 44 pulsating, 369 eclipsing and 161 miscellaneous type were detected in four fields located symmetrically in galactic lattitude around the Galactic center: MM5-A, MM5-B, MM7-A and MM7-B. The Catalog and individual observations are available in digital form from the OGLE Internet archive.

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arXiv1996

The Color--Magnitude Diagram in Baade's Window Revisited

We have reanalyzed the OGLE $V$ and $I$ photometry of $\sim 500,000$ stars in Baade's Window, and we confirm the extinction map published by Stanek (1996). However, we find that the interpretation of the OGLE color--magnitude diagram for the disk stars proposed by Paczyński et al. (1994) was incorrect: the dominant disk population in Baade's Window is old, and we find no evidence for a large hole in the inner Galactic disk. We find evidence for a small systematic error in the OGLE photometry for stars below the ``red clump'': the faint stars with $V>18$ have their $(V-I)$ color indices too red by $\sim 0.1\;mag$. We find tentative evidence from the OGLE and HST photometry that the the bulge main sequence turn--off point is brighter by $\sim 0.5 \;mag$ than it is in either 47~Tuc or NGC~6791, indicating that the dominant population of the Galactic bulge is considerably younger than those clusters.

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arXiv1996

Modelling the Galactic Bar Using Red Clump Giants

The color-magnitude diagrams of $\sim 7 \times 10^5$ stars obtained for 12 fields across the Galactic bulge with the OGLE project reveal a well-defined population of bulge red clump giants. We find that the distributions of the apparent magnitudes of the red clump stars are systematically fainter when moving towards lower galactic $l$ fields. The most plausible explanation of this distinct trend is that the Galactic bulge is a bar, whose nearest end lies at positive galactic longitude. We model this Galactic bar by fitting for all fields the observed luminosity functions in the red clump region of the color-magnitude diagram. We find that almost regardless of the analytical function used to describe the 3-D stars distribution of the Galactic bar, the resulting models have the major axis inclined to the line of sight by $20-30°$, with axis ratios corresponding to $x_0:y_0:z_0=3.5:1.5:1$. This puts a strong constraint on the possible range of the Galactic bar models. Gravitational microlensing can provide us with additional constrains on the structure of the Galactic bar.

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arXiv1996

Extinction Map of Baade's Window

Recently Woźniak \& Stanek (1996) proposed a new method to investigate interstellar extinction, based on two band photometry, which uses red clump stars as a means to construct the reddening curve. I apply this method to the color-magnitude diagrams obtained by the Optical Gravitational Lensing Experiment (OGLE) to construct an extinction map of $(40')^2 $ region of Baade's Window, with resolution of $\sim30\;arcsec$. Such a map should be useful for studies of this frequently observed region of the Galactic bulge. The map and software useful for its applications are available via {\tt anonymous ftp}. The total extinction $A_V$ varies from $1.26\;mag$ to $2.79\;mag$ within the $(40')^2 $ field of view centered on $(α_{2000}, δ_{2000}) =$ (18:03:20.9,--30:02:06), i.e. $(l,b)=(1.001,-3.885)$. The ratio $ A_V/E(V-I) = 2.49\pm0.02 $ is determined with this new method.

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arXiv1995

Variations of the Selective Extinction Across the Galactic Bulge - Implications for the Galactic Bar

We propose a new method to investigate the coefficient of the selective extinction, based on two band photometry. This method uses red clump stars as a means to construct the reddening curve. We apply this method to the OGLE color-magnitude diagrams to investigate the variations of the selective extinction towards various parts of the Galactic bulge. We find that $A_{_V}/E_{_{V-I}}$ coefficient is within the errors the same for $l=\pm 5°$ OGLE fields. Therefore, the difference of $\sim 0.37\;mag$ in the extinction adjusted apparent magnitude of the red clump stars in these fields (Stanek et al.~1994, 1995) cannot be assigned to a large-scale gradient of the selective extinction coefficient. This strengthens the implication of this difference as indicator of the presence of the bar in our Galaxy. However using present data we cannot entirely exclude the possibility of $\sim 0.2\;mag/mag$ variations of the selective extinction coefficient on the large scales across the bulge.

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