MR Cygni revisited

New analysis tools and additional unanalyzed observations justify a reanalysis of MR Cygni. The reanalysis applied successively more restrictive physical models, each with an optimization program. The final model assigned separate first and second order limb darkening coefficients, from model atmospheres, to individual grid points. Proper operation of the optimization procedure was tested on simulated observational data, produced by light synthesis with assigned system parameters, and modulated by simulated observational error. The iterative solution converged to a weakly-determined mass ratio of 0.75. Assuming the B3 primary component is on the main sequence, the HR diagram location of the secondary from the light ratio (ordinate) and adjusted T sub eff (abscissa) was calculated. The derived mass ratio, together with a main-sequence mass for the B3 component, implies a main-sequence secondary spectral type of B4. The photometrically-determined secondary radii agree with this spectral type, in marginal disagreement with the B7 type from the HR diagram analysis. The individual masses, derived from the radial velocity curve of the primary component, the photometrically-determined i, and alternative values of derived mass ratio are seriously discrepant with main sequence objects. The imputed physical status of the system is in disagreement with representations that have appeared in the literature

Evaluating GAIA performances on eclipsing binaries. I. Orbits and stellar parameters for V505 Per, V570 Per and OO Peg

The orbits and physical parameters of three detached, double-lined A-F eclipsing binaries have been derived combining H_P, V_T, B_T photometry from the Hipparcos/Tycho mission with 8500-8750 Ang ground-based spectroscopy, mimicking the photometric+spectroscopic observations that should be obtained by GAIA, the approved Cornerstone 6 mission by ESA. This study has two main objectives, namely (a) to derive reasonable orbits for a number of new eclipsing binaries and (b) to evaluate the expected performances by GAIA on eclipsing binaries and the accuracy achievable on the determination of fundamental stellar parameters like masses and radii. It is shown that a 1% precision in the basic stellar parameters can be achieved by GAIA on well observed detached eclipsing binaries provided that the spectroscopic observations are performed at high enough resolution. Other types of eclipsing binaries (including semi-detached and contact types) and different spectral types will be investigated in following papers along this series.Comment: A&A, 11 pages, 5 figures, 5 table

GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor

The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly continuous monitoring of its early optical afterglow decay, as well as high-quality optical spectra that determined a redshift of z3=2.328 for its host galaxy, an active starburst galaxy with strong Lyman-alpha emission and several absorption lines. Spectral observations show multiple absorbers at z3A=2.323, z3B= 2.317, and z3C= 2.293 blueshifted by 450, 990, and 3,155 km/s respectively relative to the host galaxy Lyman-alpha emission. We argue that these correspond to a fragmented shell nebula that has been radiatively accelerated by the gamma-ray burst (GRB) afterglow at a distance greater than 0.3 pc from a Wolf-Rayet star progenitor. The chemical abundance ratios indicate that the nebula is overabundant in carbon and silicon. The high level of carbon and silicon is consistent with a swept-up shell nebula gradually enriched by a WCL progenitor wind over the lifetime of the nebula prior to the GRB onset. The detection of statistically significant fluctuations and color changes about the jet-like optical decay further supports this interpretation since fluctuations must be present at some level due to inhomogeneities in a clumpy stellar wind medium or if the progenitor has undergone massive ejection prior to the GRB onset. This evidence suggests that the mass-loss process in a Wolf-Rayet star might lead naturally to an iron-core collapse with sufficient angular momentum that could serve as a suitable GRB progenitor.Comment: Replaced with version accepted by ApJ; 40 pages, 9 figure

Evaluating GAIA performances on eclipsing binaries. II. Orbits and stellar parameters for V781 Tau, UV Leo and GK Dra

The orbits and physical parameters of three close, double-lined G0 eclipsing binaries have been derived combining H_P, V_T, B_T photometry from the Hipparcos/Tycho mission with 8480-8740 \AA ground-based spectroscopy. The setup is mimicking the photometric and spectroscopic observations that should be obtained by GAIA. The binaries considered here are all of G0 spectral type, but each with its own complications: V781 Tau is an overcontact system with components of unequal temperature, UV Leo shows occasional surface spots and GK Dra contains a delta-Scuti variable. Such peculiarities will be common among binaries to be discovered by GAIA. We find that the values of masses, radii and temperatures for such stars can be derived with a 1-2% accuracy using the adopted GAIA-like observing mode.Comment: accepted for publication in Astronomy and Astrophysics, contains 5 Tables and 3 Figures; correction of an important typ

Kepler Cycle 1 Observations of Low Mass Stars: New Eclipsing Binaries, Single Star Rotation Rates, and the Nature and Frequency of Starspots

We have analyzed Kepler light curves for 849 stars with T_eff < 5200 K from our Cycle 1 Guest Observer program. We identify six new eclipsing binaries, one of which has an orbital period of 29.91 d, and two of which are probably W UMa variables. In addition, we identify a candidate "warm Jupiter" exoplanet. We further examine a subset of 670 sources for variability. Of these objects, 265 stars clearly show periodic variability that we assign to rotation of the low-mass star. At the photometric precision level provided by Kepler, 251 of our objects showed no evidence for variability. We were unable to determine periods for 154 variable objects. We find that 79% of stars with T_eff < 5200 K are variable. The rotation periods we derive for the periodic variables span the range 0.31 < P_rot < 126.5 d. A considerable number of stars with rotation periods similar to the solar value show activity levels that are 100 times higher than the Sun. This is consistent with results for solar-like field stars. As has been found in previous studies, stars with shorter rotation periods generally exhibit larger modulations. This trend flattens beyond P_rot = 25 d, demonstrating that even long period binaries may still have components with high levels of activity and investigating whether the masses and radii of the stellar components in these systems are consistent with stellar models could remain problematic. Surprisingly, our modeling of the light curves suggests that the active regions on these cool stars are either preferentially located near the rotational poles, or that there are two spot groups located at lower latitudes, but in opposing hemispheres.Comment: 48 pages, 11 figure

Evaluating Gaia performances on eclipsing binaries. IV. Orbits and stellar parameters for SV Cam, BS Dra and HP Dra

This is the fourth in a series of papers that aim both to provide reasonable orbits for a number of eclipsing binaries and to evaluate the expected performance of Gaia of these objects and the accuracy that is achievable in the determination of such fundamental stellar parameters as mass and radius. In this paper, we attempt to derive the orbits and physical parameters for three eclipsing binaries in the mid-F to mid-G spectral range. As for previous papers, only the Hp, Vt, Bt photometry from the Hipparcos/Tycho mission and ground-based radial velocities from spectroscopy in the region 8480-8740 A are used in the analyses. These data sets simulate the photometric and spectroscopic data that are expected to be obtained by Gaia, the approved ESA Cornerstone mission to be launched in 2011. The systems targeted in this paper are SV Cam, BS Dra and HP Dra. SV Cam and BS Dra have been studied previously, allowing comparisons of the derived parameters with those from full scale and devoted ground-based investigations. HP Dra has no published orbital solution. SV Cam has a beta Lyrae type light curve and the others have Algol-like light curves. SV Cam has the complication of light curve anomalies, usually attributed to spots; BS Dra has non-solar metallicity, and HP Dra appears to have a small eccentricity and a sizeable time derivative in the argument of the periastron. Thus all three provide interesting and different test cases.Comment: 10 pages, 4 figures, 5 tables, A&A accepte

Light Curve Analysis of Hipparcos Data for the Massive O-type Eclipsing Binary UW CMa

Hipparcos photometric data for the massive O-type binary UW CMa were analysed within the framework of the Roche model. Photometric solutions were obtained for five mass ratios in the $q=M_2/M_1=0.5-1.5$ range. The system is found to be in a contact configuration. Independently of $q$, the best-fitting model solutions correspond to the orbital inclination $i \sim 71^\circ$ and the temperature of the secondary component $T_2 \sim 33500\,K$, at the fixed temperature of the primary $T_1=33750 K$. Considering that the spectrum of the secondary is very weak, photometric solutions corresponding to the contact configuration favor the mass ratio $q$ smaller than unity (in which case the luminosity of the secondary is smaller than that of the primary). The absolute parameters of the system are estimated for different values of the mass ratio.Comment: 18 pages, 6 figure

New Observations and Analysis of the Bright Semi-Detached Eclipsing Binary mu1 Sco

Using new and published photometric observations of mu1 Sco (HR 6247), spanning 70 years, a period of 1.4462700(5) days was determined. It was found that the epoch of primary minimum suggested by Shobbrook at HJD 2449534.178 requires an adjustment to HJD 2449534.17700(9) to align all the available photometric datasets. Using the resulting combined-data light-curve, radial velocities derived from IUE data and the modelling software PHOEBE, a new system solution for this binary was obtained. It appears that the secondary is close to, or just filling, its Roche-lobe.Comment: 4 figures, 6 tables, 9 pages, uses mn2e.sty, to be published in MNRA