Robert H. Koch

The sdB+M Eclipsing System HW Virginis and its Circumbinary Planets

For the very short period subdwarf B eclipsing binary HW Vir, we present new CCD photometry made from 2000 through 2008. In order to obtain consistency of the binary parameters, our new light curves, showing sharp eclipses and a striking reflection effect, were analyzed simultaneously with previously published radial velocity data. The secondary star parameters of M 2 = 0.14 M ?, R 2 = 0.18 R ?, and T 2 = 3084 K are consistent with those of an M6-7 main-sequence star. A credibility issue regarding bolometric corrections is emphasized. More than 250 times of minimum light, including our 41 timings and spanning more than 24 yr, were used for a period study. From a detailed analysis of the O ? C diagram, it emerged that the orbital period of HW Vir has varied as a combination of a downward-opening parabola and two sinusoidal variations, with cycle lengths of P 3 = 15.8 yr and P 4 = 9.1 yr and semiamplitudes of K 3 = 77 s and K 4 = 23 s, respectively. The continuous period decrease with a rate of ?8.28 ? 10?9 days yr?1 may be produced by angular momentum loss due to magnetic stellar wind braking but not by gravitational radiation. Of the possible causes of the cyclical components of the period change, apsidal motion and magnetic period modulation can be ruled out. The most reasonable explanation of both cyclical variations is a pair of light-travel-time effects driven by the presence of two substellar companions with projected masses of M 3sin i 3 = 19.2 M Jup and M 4sin i 4 = 8.5 M Jup. The two objects are the first circumbinary planets known to have been formed in a protoplanetary disk as well the first ones discovered by using the eclipse-timing method. The detection implies that planets could be common around binary stars just as are planets around single stars and demonstrates that planetary systems formed in a circumbinary disk can survive over long timescales. Depending on the thermal inertia of their massive atmospheres, the hemispheres of the planets turned toward the stars can experience substantial reciprocating temperature changes during the minutes-long primary eclipse intervals.

THE PERIOD VARIATION OF AND A SPOT MODEL FOR THE ECLIPSING BINARY AR BOOTIS

New CCD photometric observations of the eclipsing system AR Boo were obtained from 2006 February to 2008 April. The stars photometric properties are derived from detailed studies of the period variability and of all available light curves. We find that over about 56 yr the orbital period of the system has varied due to a combination of an upward parabola and a sinusoid rather than in a monotonic fashion. Mass transfer from the less massive primary to the more massive secondary component is likely responsible for at least a significant part of the secular period change. The cyclical variation with a period of 7.57 yr and a semi-amplitude of 0.0015 d can be produced either by a light-travel-time effect due to an unseen companion with a scaled mass of M 3sin i 3 = 0.081 M ☉ or by a magnetic period modulation in the secondary star. Historical light curves of AR Boo, as well as our own, display season-to-season light variability, which are best modeled by including both a cool spot and a hot one on the secondary star. We think that the spots express magnetic dynamo-related activity and offer limited support for preferring the magnetic interpretation of the 7.57 yr cycle over the third-body interpretation. Our solutions confirm that AR Boo belongs to the W-subtype contact binary class, consisting of a hotter, less massive primary star with a spectral type of G9 and a companion of spectral type K1.

Photometric Studies of the Triple Star ER Orionis

A total of 1637 BVR CCD observations of the overcontact binary ER Ori are used as the basis for new light curves, including the first red one. From these measurements, new timings of minimum light have been calculated. The complex period variation can be sorted into a linear period improvement, a secular period increase of considerable magnitude, and a 50 year cyclical component. This latter is discussed in terms of a light-time contribution from a bound third star, which finds some support from Hipparcos results. The new light curves were synthesized using the Wilson-Divinney code. It was found useful to model a single modest spot in order to minimize the residuals from the observations. Our model is found to agree with previous light curves.

A Period Study and Light Synthesis for the W Ursae Majoris Type Binary SS Arietis

Numerous new CCD measures of the cool overcontact binary SS Arietis are compiled into six well-covered light curves. The confusing history of the binarys period variability is laid out in order to consider the diverse interpretations that have been presented previously. It is concluded that the period activity can be ascribed to no fewer than three separate causes. There is some little support for a magnetic cycle functioning in phase with one of the components of the period variability. The three light curves from 1996 have responded satisfactorily to a modern synthesis code, and the characterization of the binary characterization is better constrained than was the case from earlier light-curve studies. Light curves from 1999 have not been so satisfactorily represented. Conditioning the light-curve results with published radial velocities has very little effect on the light-curve parameters. It does, however, reveal a curious distribution of the velocity residuals, and we offer some possible interpretations. Low-level spot activity is also modeled. There is no evidence of a third star contributing significant light to the system.

Photometric Studies of the Near‐Contact Binary AX Draconis

ABSTRACT We have obtained CCD photometric observations of the near‐contact binary AX Dra during two observing seasons from 2001 March through May and from 2003 April and May. From these measurements, the seasonal light curves of AX Dra for 2001 and 2003 were compiled, and a total of 13 new timings of minimum light were calculated. An analysis of the resulting \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

IUE observations of V Sagittae

O-C

ON THE LINEAR POLARIZATION OF CLOSE BINARIES.

\end{document} diagram reveals evidence for a periodic change with a semiamplitude of 0.0061 days and a period of 56.1 ...

LONG-TERM PHOTOMETRIC BEHAVIOR OF THE ECLIPSING BINARY GW CEPHEI

Observations of the nova-like variable V Sge are discussed, and the ground-based light curve is analyzed, compensating for the component interaction effect. Low-dispersion IUE spectra indicate that interstellar reddening is smaller than suggested from visible-band observations. The UV and visible continuum are modeled by a He-rich model and by a simple accretion disk, and the UV spectrum emission lines are used to determine ionization temperatures and a lower N(e) limit. The present data are consistent with the picture of the companion to the He star being embedded in an accretion disk, with the disk being large (of dimension comparable to that of the embedding Roche lobe as a result of vigorous mass transfer) at the time of the ground-based data, and with the disk having been diminished to a radius of the order of 200 times of the neutron star, at the time of the IUE spectra.

A Long-term Photometric Study of the Near-contact Binary RU Ursae Minoris

There now exist thousands of filtered and unfiltered linear polarization measures distributed among dozens of close binaries. This assemblage of data is studied for the incidence of polarization variability among the binary systems. With the exception of U Oph, unevolved binaries do not show intrinsic polarization. It is relatively easy to detect polarization variability among evolved pairs whose photospheres are separated by at least 10 solar radii. For evolved pairs closer together than this threshold value, an assortment of factors is described which makes it difficult to detect the systemic polarization unless mass transfer is proceeding vigorously. The cause of the binary polarization is commonly scattering from circumstellar material, but in some systems the same mechanisms which cause polarization variability in single cool giants may be operating.

THE FIRST COMPREHENSIVE PHOTOMETRIC STUDY OF THE ALGOL-TYPE SYSTEM CL AURIGAE

New CCD photometry over four successive years from 2005 is presented for the eclipsing binary GW Cep, together with reasonable explanations for the light and period variations. All historical light curves, obtained over a 30 yr interval, display striking light changes, and are best modeled by the simultaneous existence of a cool spot and a hot spot on the more massive cool component star. The facts that the system is magnetically active and that the hot spot has consistently existed on the inner hemisphere of the star indicate that the two spots are formed by (1) magnetic dynamo-related activity on the cool star and (2) mass transfer from the primary to the secondary component. Based on 38 light-curve timings from the Wilson-Devinney code and all other minimum epochs, a period study of GW Cep reveals that the orbital period has experienced a sinusoidal variation with a period and semi-amplitude of 32.6 yr and 0.009 days, respectively. In principle, these may be produced either by a light-travel-time effect due to a third body or by an active magnetic cycle of at least one component star. Because we failed to find any connection between luminosity variability and the period change, that change most likely arises from the existence of an unseen third companion star with a minimum mass of 0.22 M ☉ gravitationally bound to the eclipsing pair.