7 research outputs found
The photometric-amplitude and mass-ratio distributions of contact binary stars
The distribution of the light-variation amplitudes, A(a), in addition to
determining the number of undiscovered contact binary systems falling below
photometric detection thresholds and thus lost to statistics, can serve as a
tool in determination of the mass-ratio distribution, Q(q), which is very
important for understanding of the evolution of contact binaries. Calculations
of the expected A(a) show that it tends to converge to a mass-ratio dependent
constant value for a->0. Strong dependence of A(a) on Q(q) can be used to
determine the latter distribution, but the technique is limited by the presence
of unresolved visual companions and by blending in crowded areas of the sky.
The bright-star sample to 7.5 magnitude is too small for an application of the
technique while the the Baade's Window sample from the OGLE project may suffer
stronger blending; thus the present results are preliminary and illustrative
only. Estimates based on the Baade's Window data from the OGLE project, for
amplitudes a>0.3 mag. where the statistics appear to be complete allowing
determination of Q(q) over 0.12<q<1, suggest a steep increase of Q(q) with
q->0. The mass-ratio distribution can be approximated by a power law, either
Q(q)~(1-q)^a1 with a1=6+/-2 or Q(q)~q^b1, with b1=-2+/-0.5, with a slight
preference for the former form. Both forms must be modified by the
theoretically expected cut-off caused by a tidal instability at about q_min
0.07-0.1. An expected maximum in Q(q), is expected to be mapped into a local
maximum in A(a) around 0.2-0.25 mag.Comment: AASTeX5, 12 figures, 5 tables, accepted by AJ, Aug.200
The Frequency of Binary Stars in the Core of 47 Tucanae
Differential time series photometry has been derived for 46422 main-sequence
stars in the core of 47 Tucanae. The observations consisted of near-continuous
160-s exposures alternating between the F555W and F814W filters for 8.3 days in
1999 July with WFPC2 on the Hubble Space Telescope. Using Fourier and other
search methods, eleven detached eclipsing binaries and fifteen W UMa stars have
been discovered, plus an additional ten contact or near-contact non-eclipsing
systems. After correction for non-uniform area coverage of the survey, the
observed frequencies of detached eclipsing binaries and W UMa's within 90
arcseconds of the cluster center are 0.022% and 0.031% respectively. The
observed detached eclipsing binary frequency, the assumptions of a flat binary
distribution with log period and that the eclipsing binaries with periods
longer than about 4 days have essentially their primordial periods, imply an
overall binary frequency of 13 +/- 6 %. The observed W UMa frequency and the
additional assumptions that W UMa's have been brought to contact according to
tidal circularization and angular momentum loss theory and that the contact
binary lifetime is 10^{9} years, imply an overall binary frequency of 14 +/- 4
%. An additional 71 variables with periods from 0.4 - 10 days have been found
which are likely to be BY Draconis stars in binary systems. The radial
distribution of these stars is the same as that of the eclipsing binaries and W
UMa stars and is more centrally concentrated than average stars, but less so
than the blue straggler stars. A distinct subset of six of these stars fall in
an unexpected domain of the CMD, comprising what we propose to call red
stragglers.Comment: Accepted for publication in the Astrophysical Journal, 65 pages
including 26 figure