Absolute-Magnitude Calibration for W Uma-Type Systems. II. in Uence Of

AbsoluteMagnitude Calibration for W UMatype Systems I I Inuence of Metallicity 1 Slavek Rucinski Longb ow Drive Scarb orough Ontario MW W Canada March ABSTRACT A mo dication to the absolute magnitude calibration for W UMatype systems taking into account dierences in metal abundances is derived on the basis of contact binary systems recently discovered in metalp o or clusters A preliminary estimate of the magnitude of the metallicitydependent term for the B V based calibration is M F eH The calibration based on the V I color is V C exp ected to b e less sensitive with the correction term F eH Need for a metallicitydependent term in the calibration Searches for gravitational microlenses are currently giving large numbers of serendipitously discovered variable stars Among these variables there are many W UMatype contact binaries On the basis of the rst part of the catalog of variable stars discovered during the OGLE pro ject Udalski et al one can estimate the total number of contact binaries which will b e discovered in the Baade Window during OGLE at well over one thousand This estimate is based on discoveries of such systems in the rst part of the catalog which covered one of the OGLE elds ie less than of the whole area searched the Central Baade Window BWC and reached I The newly discovered systems will provide excellent statistics for the p erio d color and amplitude distributions of contact binaries much b etter than those based on the skyeld sample which is heavily biased towards largeamplitude variables Kaluzny Rucinski Rucinski Kaluzny They can b e also utilized as calibrators of distances into the inner galactic disk Rucinski with their absolute magnitudes estimated using the p erio dcolor calibration derived recently by Rucinski a CAL The large numbers of contact binaries which are now b eing discovered should b e compared with W UMatype systems in the most recent General Catalogue of Variable Stars Among those only systems have go o dquality data but only for half of that in standard photometric systems Stars visible toward the Galactic Bulge show a large spread in metallicity While the Bulge stars might show a range of metallicities as large as F eH Frogel all contact Aliated with the Department of Astronomy University of Toronto and Department of Physics and Astronomy York University binary systems visible in BWC Rucinski are at distances kp c from us and thus b elong to the Disk p opulation For this p opulation the range in metallicities is exp ected to b e F eH The calibration presented in CAL utilized the observational dereddened B V color as the temp erature index Since redbased color indices should b ecome more p opular in the future and the OGLE pro ject provides data in V and I sp ectral bands an additional very preliminary C calibration based on the V I color was also given in CAL This calibration was derived C assuming that the relation b etween the B V and V I colors is identical to that for the Main C Sequence stars an assumption which do es not necessarily have to b e correct for the W UMatype systems b ecause of their strong chromospheric activity The relations given in CAL for predicting the absolute magnitudes M at maximum light together with an indep endent calibration for M V I Rucinski are M log P B V V M log P V I V C M log P V I I C The p erio d P is in days and b oth colors are reddeningfree The predictive p ower of these relations was estimated in CAL to b e at a level of ab out mag Extensive MonteCarlo exp eriments show that the uncertainty dep ends on a particular p erio dcolor combination and is smaller typically mag within the strict range of p erio ds and colors used to derive the calibrations P day B V V I C The metallicity dep endence has b een bypassed in CAL as the calibration was based on a few nearby W UMa systems which presumably have solar abundances and on four nearby op en clusters with mo derate underabundances F eH see Table in CAL The B V based version of the calibration was applied also in CAL to three distant clusters with a larger range of metallicities In this case the goal was to sieve out nonmember foreground and background Milky Way interlopers from these low galacticlatitude clusters Mazur et al applied recently the calibration for the same purp ose to establish membership of among W UMatype systems in the eld of Cr and Yan Mateo used it for systems in the globular cluster M see b elow The three lowlatitude clusters analyzed in CAL had F eH b etween and It was noted that the most metalp o or cluster To showed consistently p ositive deviations M from V pr ed obs obs where M is derived simply from the brightness of a the calibration M M M V V V V system at maximum light and the assumed distance mo dulus of the cluster The metallicity of To is denitely low but is p o orly known at this moment it might b e as low as F eH Geisler We would like to establish if the deviations observed for To could b e caused by the low metallicity of the cluster This pap er addresses the metallicity dep endence of the absolute magnitude calibrations given by Equations and in view of recent discoveries of W UMatype binaries in metalp o or globular clusters Determination of the correction on the basis of metalp o or globular clusters Little is known ab out metal abundances of isolated W UMa systems mostly b ecause of the extreme rotational blending of sp ectral lines which prevents application of traditional techniques Mo dern metho ds of sp ectral synthesis could p ossibly shed light on this matter but have not b een yet applied ubv y photometry of a sample of bright W UMa systems Rucinski Kaluzny Rucinski has not lead to clearcut results as it was imp ossible to exclude the presence of intrinsic activityrelated p eculiarities in ultraviolet parts of sp ectra Fortunately recent discoveries of contact systems in very metalp o or stellar systems oer a p ossibility of a preliminary lo ok into the dep endence of absolute magnitudes on metal abundances Mo dications to the periodcolor relation resulting from metallicity dierences were already discussed by Rucinski b POP This study concentrated on limits imp osed by the fullconvection condition for Population II stars but implicitly discussed observed mo dications to p erio ds and colors for low metallicity systems It was shown there that two systems in NGC NH and NH Mateo et al and four systems in NGC Kaluzny Krzeminski V V V and V are bluer and have shorter p erio ds than the common Population I W UMa systems Application of the M M l og P B V calibration conrmed that these V V systems were slightly fainter than exp ected as should b e for Population II stars Thus six bona de representatives of metalp o or p opulations were identied at that time It was argued in POP that the remaining systems in NGC were Milky Way foreground interlopers It should b e stressed that data for the contact systems in b oth of the metalp o orest clusters NGC and NGC are of medio cre quality due to diculties of observing faint variable stars However b oth clusters are so metalp o or that even crude data could give information on ma jor trends due to metallicity variations Recently Yan Mateo discovered contact binaries in M increasing the sample of W UMatype systems in the globular clusters from to This is a very imp ortant discovery as the relatively mo derate metal underabundance of the cluster estimated at ab out F eH p ermits to bridge the large gap b etween the solarabundance systems and the metalp o or ones pr ed for The data were obtained in the V and I lters so that Eq must b e used to calculate M C V systems in M This forces us to discuss together results based on the B V color with results based on the V I color We address the sp ecic question of relative sensitivities of b oth colors C to metallicities in the last section of the pap er The assumptions made to determine the absolute magnitudes M and the reddeningcorrected V colors of systems in the three globular clusters were as follows For NGC m M E B V for NGC m M E B V with additional corrections for patchy extinction as describ ed by Kaluzny Krzeminski and for M m M and E B V The values of p erio ds and colors for the W UMa systems were taken from the discovery pap ers and are listed in Table Figure shows the deviations M from the Population I calibration as function of F eH V for systems in stellar systems of low metal abundance These are the systems in three globular clusters new systems M and systems as in POP and the systems in the op en cluster To as in CAL The values of F eH for these clusters were taken from Geisler and Geisler et al for To for NGC and for NGC F eH was assumed for M Less extreme values of F eH have b een also given in the literature for To Kubiak et al assumed F eH and the database of Webbink gave and for NGC and NGC resp ectively The systems which we consider as members of the clusters have b een marked in Figure by lled symbols The broken lines give the approximate range for the membership acceptancerejection for Population I systems estimated at mag in CAL On the basis of Figure we conclude The globular cluster data suggest a weak dep endence of the absolute magnitude M on V F eH a linear relation should suce for the relatively p o or data which are available now Inclusion or rejection of M would have only a minor inuence on the slop e of a

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