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1

Sounding Interiors of Four Pulsating Subdwarf B Stars with Stellar Pulsations

Baran A., Sanjayan S.

Acta Astronomica

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2023

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Vol. 73, No. 1

21--34

EN

We present the results of asteroseismic modeling of four pulsating sdB stars KIC 2438324, KIC 2569576, KIC 2991403 and KIC 11159657. We used fully evolutionary models and this is the first attempt of seismic modeling of sdB stars using the MESA models and GYRE. We adopted an already reported list of periods identified with dipole modes. Additionally, we used two spectroscopic parameters, i.e., Teff and log g, and, only for KIC 2438324 and KIC 2569576, we used cluster parameters, i.e., total age and metallicity, to further filter the outcome of matching observed and theoretical pulsation periods. In the case of a spectroscopically constrained approach we derived a unique solution only for KIC 2438324. For two stars we derived unique input parameters except for the hydrogen and progenitor mass, respectively for KIC 2569576 and KIC 2991403. All input parameters except for the convective core and progenitor masses are unique for KIC 11179657. Except for KIC 11179657, our fits, measured by a relative period difference (ΔP/P), are well within 1%. These results sound very promising and more asteroseismic modeling of sdB stars should be done in future.

2

Roche Modeling and Evolutionary History of Six Low Mass Contact Binary Systems

Alton K. B., Stępień K.

Acta Astronomica

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2021

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Vol. 71, No. 2

123--161

EN

We describe six eclipsing binary systems that were first detected by the ROTSE-I telescope and ultimately shown to be low mass contact binaries (LMCB). New photometric data (B, V and Ic) acquired from V473 Cam, PV Com, EI CVn, V2790 Ori, V2802 Ori and PS Vir were used to calculate an orbital period for each system along with updated ephemerides. All systems were subject to secular changes in orbital period with evidence for a third gravitationally bound body in V473 Cam, EI CVn, V2790 Ori, and PS Vir. Since a total eclipse is observed in all six systems, Roche modeling based on the Wilson-Devinney (WD) code yielded in each case a well-constrained photometric value for the mass ratio. Potential progenitors of these LMCBs were evaluated using an evolutionary model of cool close binaries. The best fits were obtained when the initial detached binaries have orbital periods ranging from 2.70 d to 3.71 d, total masses between 1.36 M and 1.53 M with mass ratios ranging from 2.34 to 4.04. The model progenitors require between 7.75-10.9 Gyr to attain the presently observed orbital period and physical attributes. Collectively, the models show that about half of orbital periods will increase in the future leading to a common envelope resulting from the evolutionary expansion of the accretor whereas the other half will decrease resulting in the overflow of the outer critical Roche surface and an ultimate merger.

3

A Comprehensive Study of the Eclipsing Binaries V1241 Tau and GQ Dra

Ulaş B., Gazeas K., Liakos A., Ulusoy C., Stateva I., Erkan N., Napetova M., Iliev I. K.

Acta Astronomica

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2020

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Vol. 70, No. 3

219--240

EN

We present new photometric and spectroscopic observations and analyses for the eclipsing binary systems V1241 Tau and GQ Dra. Our photometric light and radial velocity curve analyses combined with the TESS light curves show that both are conventional semi-detached binary systems. Their absolute parameters are also derived. We present the O-C analyses of the systems and we propose the most likely orbital period modulating mechanisms. Furthermore, Fourier analyses are applied to the photometric residual data of the systems to check for the pulsational behavior of the components. We conclude that the primary component of the system GQ Dra is a δ Sct type pulsator with a dominant pulsation frequency of 18.58 d-1 based on our B filter residual light curve although it can not be verified by 30-minute cadence TESS data. No satisfactory evidence of pulsational behavior for V1241 Tau was verified. Finally, the evolutionary tracks of the components of both systems are calculated. Their locations within evolutionary diagrams are compared with other Algol-type systems.

4

Hen 3-160 - the First Symbiotic Binary with Mira Variable S Star

Gałan C., Mikołajewska J., Monard B., Iłkiewicz K., Pieńkowski D., Gromadzki M.

Acta Astronomica

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2019

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Vol. 69, No. 1

25--44

EN

Hen 3-160 is reported in Belczyński et al. catalog as a symbiotic binary system with M7 giant donor. Using V- and I-band photometry collected over 20 years we have found that the giant is a Mira variable pulsating with 242.5-day period. The period-luminosity relation locates Hen 3-160 at the distance of about 9.4 kpc, and its Galactic coordinates (l=267.°7, b=-7.°9) place it ≈1.3 kpc above the disk. This position combined with relatively high proper motions (μαcosδ=-1.5 mas/yr, μ_δ=+2.9 mas/yr, Gaia DR2) indicates that Hen 3-160 has to be a Galactic extended thick-disk object. Our red optical and infrared spectra show the presence of ZrO and YO molecular bands that appear relatively strong compared to the TiO bands. Here we propose that the giant in this system is intrinsic S star, enriched in products of slow neutron capture processes occurring in its interior during an AGB phase which would make Hen 3-160 the first symbiotic system with Mira variable S star.

5

CCD Photometry, Light Curve Deconvolution, Period Analysis and Evolutionary Status of the HADS Variable V524 And

Alton K. B., Stępień K.

Acta Astronomica

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2019

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Vol. 69, No. 3

283--304

EN

New multi-color BVIc CCD photometric data of V524 And, a pulsating variable long considered to be an SX Phe-type subdwarf system were acquired during 2018. Deconvolution of precise time-series light curve data was accomplished using discrete Fourier transformation and revealed a mean fundamental mode f0 of oscillation at 10.583 d-1 along with four other harmonics from 2f0 to 5f0. Following successive pre-whitening of each residual signal, no other statistically significant frequency shared by all band-passes was resolved. Potential period changes over time were evaluated using four new times-of-maximum light derived from the present study along with other values reported in the literature. Photometric data collected during the ROTSE-I (1999) and SuperWASP (2007-2008) surveys combined with CCD-derived V-mag data acquired from the AAVSO archives produced an additional sixty-six times-of-maximum measurements. Corresponding residuals from the observed minus calculated values indicate very little change in the primary pulsation period since 1999. However, a far more interesting finding from this secular analysis suggests an underlying sinusoidal-like variation of PB=6.41 yr consistent with a putative binary partner that may be a brown dwarf (M<0.07 M☉). An investigation with PARSEC models for generating stellar tracks and isochrones provided further insight into the evolutionary status and physical character of V524 And. This intrinsic variable associated with the Galactic disk has an estimated mass around 2 M☉ with near-solar metallicity. These characteristics point to V524 And being a Population I HADS with an age approaching 1 Gyr rather than an SX Phe-type variable.

6

Roche Modeling and Evolutionary History of Multiple Low Mass Contact Binary Systems

Alton K. B., Stępień K.

Acta Astronomica

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2018

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Vol. 68, No. 4

449--482

EN

The Northern Sky Variable Survey (NSVS) has been a rich source of variable stars many of which have only been studied to a limited extent. Herein we describe five eclipsing binary systems that were first detected by the ROTSE-I telescope and ultimately shown to be low mass contact binaries. Photometric data (B, V and Ic) acquired from MU Cnc (NSVS 10133793, TYC 1397-1030-1), V596 Peg (NSVS 6225359, GSC 2765-0348), TYC 1597-2327-1 (NSVS 11082150), GSC 2723-2376 (NSVS 8849526) and GSC 4946-0765 (NSVS 13176410) were used to calculate an orbital period for each system along with new linear ephemerides. Except for MU Cnc, the limited number of time-of-minimum values precluded the characterization of any long-term period change. Since a total eclipse is observed with all five systems, Roche modeling based on the Wilson-Devinney W-D code yielded in each instance a well-constrained photometric value for the mass ratio. Using the evolutionary model of cool close binaries we searched for possible progenitors of the investigated binaries. The best fit is obtained if the initial binaries have orbital periods from the interval between 2.4–3.4 d, total masses between 6% and 9% higher than presently observed and mass ratios between 2.2 and 3.3. The model progenitors need between 7.5–10 Gyr to attain the presently observed parameters of the variables. The present periods of three models slowly increase with simultaneous mass ratio decrease, which suggests the formation of the common envelope in the future. However, in four other models the period decreases and the model computations predict the ultimate overflow of the outer Roche lobe.

7

P13 an ULX that is a Potential Progenitor of Merging BH-NS System

Beldycki B., Belczynski K.

Acta Astronomica

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2016

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Vol. 66, No. 3

347--355

EN

We have studied the future evolution of a recently discovered ULX source P13 in NGC 7793. This source was shown to contain a 5-15 M⊙ black hole and a massive 18-23 M⊙ B9Ia companion on a 64 day orbit. For low black hole mass (5-10 M⊙) and high companion mass ≳20 M⊙ the binary is predicted to initiate a common envelope evolution in near future and significantly decrease the orbital separation (6 hr orbit). This leads to a high probability (≈70%) of the system surviving a supernova explosion that will form a neutron star out of the companion. About one third of the surviving BH-NS systems will merge within Hubble time and be a source of high frequency gravitational radiation. We estimate that the chances of detection of BH-NS systems with Advanced LIGO/Virgo that form via P13-evolutionary channel are at the level of 0.1 yr-1 with wide range of allowed probability 0÷0.6 yr-1. This is the fourth empirical estimate of BH-NS merger rate.

8

CCD Photometry, Roche Modeling and Evolutionary History of the W UMa-type Eclipsing Binary TYC 01664-0110-1

Alton K. B., Stępień K.

Acta Astronomica

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2016

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Vol. 66, No. 3

357--379

EN

TYC 01664-0110-1 (ASAS J212915+1604.9), a W UMa-type variable system (P=0.282962 d), was first detected over 17 years ago by the ROTSE-I telescope. Photometric data (B, V and Ic) collected at UnderOak Observatory (UO) resulted in five new times-of-minima for this variable star which were used to establish a revised linear ephemeris. No published radial velocity (RV) data are available for this system. However, since this W UMa binary undergoes a total eclipse, Roche modeling based on the Wilson-Devinney (W-D) code yielded a well-constrained photometric value for M2/M1 (q=0.356±0.001). There is a suggestion from ROTSE-I (1999) and ASAS survey data (2003, 2005, and 2008) that the secondary maximum is more variable than the primary one probably due to the so-called O'Connell effect. However, peak asymmetry in light curves (LC) from 2015 was barely evident during quadrature. Therefore, W-D model fits of these most recent data did not yield any substantive improvement with the addition of spot(s). Using the evolutionary model of cool close binaries we searched for a possible progenitor of TYC 01664-0110-1. The best fit is obtained if the initial binary has an orbital period between 3.3-3.8 d and component masses between 1.0-1.1 M⊙ and 0.30-0.35 M⊙. The model progenitor needs about 10 Gyr to attain the presently observed parameters of the variable. Its period slowly increases and the mass ratio decreases. According to the model predictions TYC 01664-0110-1 will go through the common envelope (CE) phase in the future, followed by merging of both components or formation of a double degenerate. Due to its apparent brightness (mV,max≈10.9 mag) and unique properties, the star is an excellent target for spectroscopic investigation of any possible deviations from a simple static model of a contact binary.

9

Optical and Near Infrared Study of the Open Cluster Czernik 17

Sujatha S., Krishna K. K., Komala S., Babu G. S. D.

Acta Astronomica

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2015

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Vol. 65, No. 3

283--295

EN

In this paper we present the fundamental physical parameters of the poorly studied open cluster Czernik 17. The parameters have been derived from the UBVRI CCD photometric observations obtained with the 2-m Himalayan Chandra Telescope as well as using data taken from the Two-Micron All Sky Survey. This cluster is located in the direction of Camelopardalis constellation with a radial extent of 4' and is estimated to be at a distance of 7 ± 0.5 kpc. The interstellar extinction in the line of sight of the cluster is found to be E(B-V) = 0.72 ± 0.05 mag. Based on the evolutionary stage of the evolved stars the log(age) of the cluster is found to be 8.05 to 8.1 making it a relatively young cluster which can be considered as a spiral arm tracer of the outer arm. We have constructed the luminosity function for the stars within the adopted cluster radius which shows the common trend of the number of stars increasing toward the fainter magnitudes. We also find the presence of Böhm-Vitense gaps in the main-sequence branch of the color-magnitude diagrams of this cluster which may be attributed to surface convection which has the effect of making the stars redder although the luminosity remains unchanged.

10

Evolution of Cool Close Binaries - Rapid Mass Transfer and Near Contact Binaries

Stępień K., Kiraga M.

Acta Astronomica

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2013

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Vol. 63, No. 2

239--266

EN

We test the evolutionary model of cool close binaries developed by one of us (KS) on the observed properties of near contact binaries (NCBs). These are binaries with one component filling the inner critical Roche lobe and the other almost filling it. Those with a more massive component filling the Roche lobe are SD1 binaries whereas in SD2 binaries the Roche lobe filling component is less massive. Our evolutionary model assumes that, following the Roche lobe overflow by the more massive component (donor), mass transfer occurs until mass ratio reversal. A binary in an initial phase of mass transfer, before mass equalization, is identified with SD1 binary. We show that the transferred mass forms an equatorial bulge around the less massive component (accretor). Its presence slows down the mass transfer rate to the value determined by the thermal time scale of the accretor, once the bulge sticks out above the Roche lobe. It means, that in a binary with a (typical) mass ratio of 0.5 the SD1 phase lasts at least 10 times longer than resulting from the standard evolutionary computations neglecting this effect. This is why we observe so many SD1 binaries. Our explanation is in contradiction to predictions identifying the SD1 phase with a broken contact phase of the Thermal Relaxation Oscillations model. The continued mass transfer, past mass equalization, results in mass ratio reversed. SD2 binaries are identified with this phase. Our model predicts that the time scales of SD1 and SD2 phases are comparable to one another. Analysis of the observations of 22 SD1 binaries, 27 SD2 binaries and 110 contact binaries (CBs) shows that relative number of both types of NCBs favors similar time scales of both phases of mass transfer. Total masses, orbital angular momenta and orbital periods of SD1 and SD2 binaries are indistinguishable from each other whereas they differ substantially from the corresponding parameters of CBs. We conclude that the results of the analysis fully support the model presented in this paper.

11

Evolution of Low Mass Contact Binaries

Stępień K., Gazeas K.

Acta Astronomica

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2012

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Vol. 62, No. 2

153--177

EN

We present a study on low-mass contact binaries (LMCB) with orbital periods shorter than 0.3 days and total mass lower than about 1.4 MSun. We show that such systems have a long pre-contact phase, which lasts for 8-9 Gyrs, while the contact phase takes only about 0.8 Gyr, which is rather a short fraction of the total life. With low mass transfer rate during contact, moderate mass ratios prevail in LMCBs since they do not have enough time to reach extreme mass ratios often observed in higher mass binaries. During the whole evolution both components of LMCBs remain within the MS band. The evolution of cool contact binaries toward merging is controlled by the interplay between the evolutionary expansion of the less massive component resulting in the mass transfer to the more massive component and the angular momentum loss from the system by the magnetized wind. In LMCB the angular momentum loss prevails. As a result, the orbital period systematically decreases until the binary overflows the outer critical Roche surface and the components merge into a single fast rotating star of a solar type surrounded by a remnant disk carrying excess angular momentum. The disk can be a place of planet formation with the age substantially lower than the age of a host star. The calculated theoretical tracks show good agreement with the physical properties of LMCB from the available observations. Estimates of the frequency of occurrence of LMCB and the merger formation rate indicate that about 40 LMCBs and about 100 low mass merger products is expected to exist within 100 pc from the Sun.

12

Large Magellanic Cloud Cepheids in the ASAS Data

Karczmarek P., Dziembowski W. A., Lenz P., Pietrukowicz P., Pojmański G.

Acta Astronomica

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2011

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Vol. 61, No. 4

303--318

EN

A catalog of Cepheids in the Large Magellanic Cloud (LMC) from the ASAS project is presented. It contains data on 65 fundamental mode pulsators with periods longer than about 8 days. The period-luminosity (PL) relation in the V-band does not significantly differ from the relation determined from the OGLE-III data extended toward longer periods but shows much larger scatter. For objects with periods longer than 40 d there is an evidence for a shallower PL relation. The rates of long-term period variations significant at 3σ level are found only for seven objects. The rates for 25 objects determined with the 1σ significance are confronted with the values derived from stellar evolution models. The models from various sources yield discrepant predictions. Over the whole data range, a good agreement with measurements is found for certain models but not from the same source.

13

Evolution of Cool Close Binaries - Approach to Contact

Stępień K.

Acta Astronomica

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2011

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Vol. 61, No. 2

139--159

EN

As a part of a larger project, a set of 27 evolutionary models of cool close binaries was computed under the assumption that their evolution is influenced by the magnetized winds blowing from both components. Short period binaries with the initial periods of 1.5 d, 2.0 d and 2.5 d were considered. For each period three values of 1.3 Msun, 1.1 Msun and 0.9 Msun were taken as the initial masses of the more massive components. The initial masses of the less massive components were adjusted to avoid extreme mass ratios. Here the results of the computations of the first evolutionary phase are presented, which starts from the initial conditions and ends when the more massive component reaches its critical Roche lobe. In all considered cases this phase lasts for several Gyr. For binaries with the higher total mass and/or longer initial periods this time is equal to, or longer than the main sequence life time of the more massive component. For the remaining binaries it amounts to a substantial fraction of this life time. From the statistical analysis of models, the predicted period distribution of detached binaries with periods shorter than 2 d was obtained and compared to the observed distribution from the ASAS data. An excellent agreement was obtained under the assumption that the period distribution in this range is determined solely by magnetic braking (MB), i.e., the mass and angular momentum loss due to the magnetized winds, as considered in the present paper. This result indicates, in particular, that virtually all cool detached binaries with periods of a few tenths of a day, believed to be the immediate progenitors of W UMa-type stars, were formed from young detached systems with periods around 2-3 d. MB is the dominant formation mechanism of cool contact binaries. It operates on the time scale of several Gyr rendering them rather old, with age of 6-10 Gyr. The results of the present analysis will be used as input data to investigate the subsequent evolution of the binaries, through the mass exchange phase and contact or semi-detached configuration till the ultimate merging of the ultimate merging of the components.

14

Seismic Diagnostics of Mixing Beyond the Convective Core in Intermediate Mass Main-Sequence Stars

Popielski B. L., Dziembowski W. A.

Acta Astronomica

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2005

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Vol. 55, No. 2

177--193

EN

We study prospects for seismic sounding the layer of a partial mixing above the convective core in main-sequence stars with masses in the 1.2-1.9 Msolar range. There is an initial tendency to increase of the convective core mass in such stars and this leads to ambiguities in modeling. Solar-like oscillations are expected to be excited in such objects. Frequencies of such oscillations provide diagnostics, which are sensitive to the structure of the innermost part of the star and they are known as the small separations. We construct evolutionary models of stars in this mass range assuming various scenarios for element mixing, which includes formation of element abundance jumps, as well as semiconvective and overshooting layers. We find that the three point small separations employing frequencies of radial and dipole modes provide the best probe of the element distribution above the convective core. With expected accuracy of frequency measurement from the space experiments, a discrimination between various scenarios should be possible.

15

On the Nature of Regular Pulsation in Two LBV Stars of NGC 300

Dziembowski W. A., Sławińska J.

Acta Astronomica

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2005

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Vol. 55, No. 2

195--204

EN

Two A-type supergiants in NGC 300 exhibit periodic light variations. The measured periods are 72.5 and 96.1 days. We show that the variability may be explained by the excitation of strange mode driven by the opacity mechanism acting in the layer of the iron opacity bump. The explanation requires that the two stars are significantly undermassive. In sequences of stellar models with decreasing luminosity-to-mass ratio, the unstable strange modes represent continuations of strictly aperiodic thermal modes.

16

Period Changes in Galactic Classical Cepheids. Slow Evolution of Long-Period Cepheids

Pietrukowicz P.

Acta Astronomica

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2003

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Vol. 53, No. 1

63--71

EN

We compared period changes derived from O-C diagrams for 63 classical Cepheids from our Galaxy with model calculations. We found that for Cepheids with log P>1.0 the observed changes are smaller than predicted values, except variable SZ Cas. However some of the first overtone Cepheids, particularly EU Tau and Polaris, change its period much faster than it follows from theory. Summary of the known data on period changes in Cepheids from the Galaxy and from the Magellanic Clouds (previous papers) leads to conclusion that none of the 999 Cepheids is undergoing the first crossing of the instability strip. Also the observed period changes for long-period Cepheids are a few times slower than predicted by the models. These results imply that much larger fraction of helium is burned in the Cepheid stage than it is predicted by models.

17

Period Changes of the SMC Cepheids from the Harvard, OGLE and ASAS Data

Pietrukowicz P.

Acta Astronomica

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2002

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Vol. 52, No. 2

177--187

EN

Comparison of the old observations of Cepheids in the Small Magellanic Cloud from the Harvard data archive, with the recent OGLE and ASAS observations allows an estimate of their period changes. All of matched 557 Cepheids are still pulsating in the same mode. One of the Harvard Cepheid, HV 11289, has been tentatively matched to a star which is now apparently constant. Cepheids with log P>0.8 show significant period changes, positive as well as negative. We found that for many stars these changes are significantly smaller than predicted by recent model calculations. Unfortunately, there are no models available for Cepheids with periods longer than approximately 80 days, while there are observed Cepheids with periods up to 210 days.

18

Period Changes of the LMC Cepheids Determined from the Harvard, OGLE and ASAS Data

Pietrukowicz P.

Acta Astronomica

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2001

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Vol. 51, No. 3

247--257

EN

Observations of Cepheids in the Large Magellanic Cloud, made over the last several decades, allow us to search for evolutionary period changes. None of the Cepheid from our sample of 378 stars stopped pulsating. Also none of them showed a large period change which could indicate mode switching. However for Cepheids with log P>0.9 we found significant period changes, positive as well as negative. A comparison between the observed period changes and theoretical predictions shows moderate agreement with some models (Bono et al. 2000), and a very large disagreement with others (Alibert et al. 1999). The large differences between the models are likely caused by the very high sensitivity of stellar evolution during core helium burning phase to even small changes in the input physics, as discovered by Lauterborn, Refsdal and Weigert (1971).

19

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.