The 7.5 Magnitude Limit Sample of Bright Short‐Period Binary Stars. I. How Many Contact Binaries Are There?

Abstract

A sample of bright contact binary stars (W UMa-type or EW, and related: with β Lyr light curves, EB, and ellipsoidal, ELL -in effect, all but the detached, EA), to the limit of V max = 7.5 magnitude is deemed to include all discoverable short-period (P < 1 days) binaries with photometric variation larger than about 0.05 magnitude. Of the 32 systems in the final sample, 11 systems have been discovered by the Hipparcos satellite. The combined spatial density is evaluated at (1.02 ± 0.24) × 10 −5 pc −3 . The Relative Frequency of Occurrence (RFO), defined in relation to the Main Sequence stars, depends on the luminosity. An assumption of RFO ≃ 1/500 for M V > +1.5 is consistent with the data, although the number statistics is poor with the resulting uncertainty in the spatial density and the RFO by a factor of about two. The RFO rapidly decreases for brighter binaries to a level of 1/5,000 for M V < +1.5 and to 1/30,000 for M V < +0.5. The high RFO of 1/130, previously determined from the deep OGLE-I sample of Disk Population W UMa-type systems towards Baade's Window, is inconsistent with and unconfirmed by the new results. Possible reasons for the large discrepancy are discussed. They include several observational effects, but also a possibility of a genuine increase in the contact-binary density in the central parts of the Galaxy.

Contact binary stars of the W UMa-type as distance tracers

“…The estimate based on the OGLE-I pencil-beam search volume gave the density as high as 1/130 among late-A to early-K MS stars, a number consistent with some old open clusters (Rucinski, 1998). However, this high estimate is not confirmed by a rigorous analysis of the Hipparcos, solar-neighbourhood sample limited to stars brighter than 7.5 magnitude (Rucinski, 2002); this analysis suggests the relative density of about 1/500 (this corresponds to the spatial density of 1.0 × 10 −5 pc −3 ). It is at present not clear, if the high density obtained from the OGLE-I sample was due to the improperly accounted image blending or to a genuine increase of the contact-binary density in the central parts of the Thick Disk of our Galaxy.…”

“…While Rucinski (1994Rucinski ( , 1998 were based mostly on the open cluster data (EW systems appear at a relative frequency of some 1/500 -1/1000 in old open clusters), RD97 was based on the Hipparcos data. The Hipparcos database remains the best source and has recently been analyzed in great detail for the complete sample of systems with V < 7.5 mag (Rucinski, 2002). The currently best calibrations utilizing de-reddened B − V and V − I colour indices are:…”

“…On one hand, it complicates the matters by introducing an internal correlation between the quantities in the M V calibration so that they are not really "orthogonal". On the other hand, it sometimes helps in weeding out variable stars which are of entirely different type in that the data points cannot appear above and to the left of a short-period blue envelope; they can only shift down to redder colour indices and to the right due to the evolution (Rucinski, 2002).…”

A CCD Photometric Study of Close Binary V445 Cep

“…First we normalized magnitude difference to unit by adding 2. m 686, -2. m 442, and -2. m 240 to BVR light curves, respectively and converted magnitude scale to intensity scale for using 2005 version of Wilson et al (1971) differential correction method. We used mode 3 of the Wilson and Devinney code because V445 Cep has been already known as a contact binary of W UMa type (Duerbeck 1997, Eker et al 2009, Rucinski 2002. As reported by the spectroscopic observations of Grenier …”

“…The amplitude of Hipparcos light curve is in the magnitude range of 0.03. Duerbeck (1997) and Rucinski (2002) reported that the light variation of V445 Cep may be caused by the pulsation because the color index of V445 Cep is abnormally bluer compared to the period-color relation of contact binaries. However, Pych et al (2004) reported that V445 Cep is an eclipsing binary with very low mass ratio (q = 0.17 ± 0.01) and mass function of ([M1 + M2] sin³ i = 0.134 ± 0.006M⊙) based on their radial velocity curve.…”