Photometric Studies of Twelve Deep, Low-mass Ratio Overcontact Binary Systems

The formations of the blue straggler stars and the FK Com-type stars are unsolved problems in stellar astrophysics. One of the possibilities for their formations is from the coalescence of W UMa-type overcontact binary systems. Therefore, deep (f > 50%), low-mass ratio (q < 0.25) overcontact binary stars are a very important source to understand the phenomena of Blue Straggler/FK Com-type stars. Recently, 12 W UMa-type binary stars, FG Hya, GR Vir, IK Per, TV Mus, CU Tau, V857 Her, V410 Aur, XY Boo, SX CrV, QX And, GSC 619-232, and AH Cnc, were investigated photometrically. Apart from TV Mus, XY boo, and GSC 619-232, new observations of the other 9 binaries were obtained. Complete light curves of the 10 systems, FG Hya, GR Vir, IK Per, TV Mus, CU Tau, V857 Her, GSC 619-232, V410 Aur, XY Boo, and AH Cnc, were analyzed with the 2003 version of the W-D code. It is shown that all of those systems are deep (f > 50%), low-mass ratio (q < 0.25) overcontact binary stars. We found that the system GSC 619-232 has the highest degree of overcontact (f = 93.4%). The derived photometric mass ratio of V857 Her, q = 0.0653, indicates that it is the lowest-mass ratio system among W UMa-type binaries.Of the 12 sample stars, long-term period changes of 11 systems were found. About 58% (seven) of the sample binaries show cyclic period oscillation. No cyclic period changes were discovered for the other 5 systems, which may be caused by the short observational time interval or by insufficient observations. Therefore, we think that all W UMa-type binary stars may contain cyclic period variations. By considering the long-term period changes (both increase and decrease) of those binary stars, we proposed two evolutionary scenarios evolving from deep, low-mass ratio overcontact binaries into Blue Straggler/FK Com-type stars.     

Photoelektrische Untersuchungen des Magnetfeldes und der Helligkeit der Am-Sterne 15 Vul und 68 Tau

The Am stars 15 Vul and 68 Tau have been observed photoelectrically in the UBV system and for determination of the longitudinal magnetic field component. Both the stars show periodical variations of the magnitude and the magnetic field. The periods are determined to 14.0 days for 15 Vul and 57.25 days for 68 Tau. The variations of these Am stars are the same as those of typical Ap stars. The star 68 Tau was found to be a spectroscopical binary. The period of the orbital motion is 57.25 days, too. The rotation of the components is synchronous.     

O i line emission in cool stars: calculations using partial redistribution

Radiative transfer calculations have been performed for five cool stars: α Tau, β Gem, Procyon, ε Eri and the Sun, for the purpose of investigating the behaviour of the O  i emission over a wide range of stellar types, and its dependence on coherent photon scattering. These stars span a range of spectral types from F5 iv – v to K5 iii and surface gravities  1.25 < log  g * < 4.75  . Particular attention has been paid to the calculation of the flux in the resonance triplet around 1305 Å which is pumped by H Lyβ, including the effects of partial redistribution (PRD) and cross-redistribution of photons. These are the first calculations for the resonance triplet in giant stars using a full PRD treatment. Calculations of the predominantly collisionally excited intersystem doublet at 1355, 1358 Å are included, and it is found that the ratio of these fluxes shows the effects of opacity. The flux in the forbidden line at 1641 Å is calculated for the giant stars and the effects of coherent scattering on this line are investigated. The discrepancy between the calculated and observed fluxes in the O  i lines is used to infer the inadequacy of single-component chromospheric models.     

A study of X-ray flares – I. Active late-type dwarfs

We present temporal and spectral characteristics of X-ray flares observed from six late-type G–K active dwarfs (V368 Cep, XI Boo, IM Vir, V471 Tau, CC Eri and EP Eri) using data from observations with the XMM–Newton observatory. All the stars were found to be flaring frequently and altogether a total of 17 flares were detected above the 'quiescent' state X-ray emission which varied from 0.5 to  8.3 × 10²⁹ erg s⁻¹  . The largest flare was observed in a low-activity dwarf XI Boo with a decay time of 10 ks and ratio of peak flare luminosity to 'quiescent' state luminosity of 2. We have studied the spectral changes during the flares by using colour–colour diagram and by detailed spectral analysis during the temporal evolution of the flares. The exponential decay of the X-ray light curves, and time evolution of the plasma temperature and emission measure are similar to those observed in compact solar flares. We have derived the semiloop lengths of flares based on the hydrodynamic flare model. The size of the flaring loops is found to be less than the stellar radius. The hydrodynamic flare decay analysis indicates the presence of sustained heating during the decay of most flares.     

Analysis of the HST ultraviolet spectra for T Tauri stars: RY Tau and HD 115043

The ultraviolet spectra of the stars RY Tau and HD 115043 from the Hubble Space Telescope are analyzed. RY Tau belongs to the classical T Tauri stars, while HD 115043 is a young (t~3×10⁸ years), chromospherically active star. The most intense emission lines were identified, and their fluxes were measured. Low-resolution spectra of RY Tau and HD 115043 in the wavelength range 1160–1760 Å exhibit almost the same set of emission lines. However, first, the luminosity of RY Tau in these lines is approximately a factor of 300 higher than that of HD 115043, and, second, the relative line intensities differ greatly. The intensity ratio of the C IV λ1550, Si IV λ1400, and NV λ1240 doublet components is close to 1: 2 in the spectra of both stars. Judging by the continuum energy distribution, the spectral type of RY Tau is later than that of HD 115043. Synchronous flux variability in the C IV λ1550 and He II λ1640 lines in a time of ~20 min was detected in RY Tau. The flux rise in these lines was accompanied by a redshift of the intensity peak in the profiles by~50 km s^?1. Intermediate-resolution spectra are used to study line profiles in the spectrum of RY Tau. In particular, the profiles of (optically thin) Si III]λ1892 and C III]λ1909 lines were found to be asymmetric and about 300 km s^?1 in width. The (optically thick) C IV λ1550 doublet lines have similar profiles. The Mg II λ2800 doublet lines are also asymmetric, but their shape is different: they consist of a broad (?750 km s^?1 at the base) emission component on which an interstellar absorption line shifted from the line symmetry center by about 20 km s^?1 is superimposed. The intensity ratio of the Mg II λ2800 doublet components is?1.4. Whether there are molecular hydrogen lines in the spectrum of RY Tau is still an open question. It is shown that the emission lines in the ultraviolet spectrum of RY Tau cannot originate in a hydrostatically equilibrium chromosphere. It is argued that quasi-steady accretion of circumstellar matter is responsible for the emission.     

UBV photometry of the eclipsing binary RZ Eridani

U, B, andV observations of RZ Eri have been reported. Colours of comparison stars BD-10°994, BD-10°996 and of the variable RZ Eri have been presented. The observations of RZ Eri do not indicate the distortion wave in the light curve beyond observational scatter.     

Peculiarities of the UV continuum energy distribution for T Tauri stars

In the UV spectra of BP Tau, GW Ori, T Tau, and RY Tau obtained with the Hubble Space Telescope, we detected an inflection near 2000 Å in the F _λ ^c (λ) curve that describes the continuum energy distribution. The inflection probably stems from the fact that the UV continuum in these stars consists of two components: the emission from an optically thick gas with T<8000 K and the emission from a gas with a much higher temperature. The total luminosity of the hot component is much lower than that of the cool component, but the hot-gas radiation dominates at λ<1800 Å. Previously, other authors have drawn a similar conclusion for several young stars from low-resolution IUE spectra. However, we show that the short-wavelength continuum is determined from these spectra with large errors. We also show that, for three of the stars studied (BP Tau, GW Ori, and T Tau), the accretion-shock radiation cannot account for the observed dependence F _λ ^c (λ) in the ultraviolet. We argue that more than 90% of the emission continuum in BP Tau at λ>2000 Å originates not in the accretion shock but in the inner accretion disk. Previously, a similar conclusion was reached for six more classical T Tau stars. Therefore, we believe that the high-temperature continuum can be associated with the radiation from the disk chromosphere. However, it may well be that the stellar chromosphere is its source.     

Spectral Variability of T Tauri Stars in the Optical

We present the results of time variability analysis of the Hα, He I (λ5876 Å) and Na D (λ5890, 5896 Å) lines in two active T Tauri stars, DL Tau and DR Tau. The aim is the study of time behavior of these lines, during one week, as well as possible correlations among them. The observed profiles of some lines are very complex, often showing characteristics of outflow and accretion.     

Spectrophotometric study of the suspected UU Herculis-star HR 4912

We have obtained high-dispersion blue spectograms and BV photometry of the high-latitude F-supergiant HR 4912=HD 112374. As a result of our curve-of-growth analysis we conclude that it appears to have the parameters of a F-supergiant with slight metal deficiency ([Fe/H]=–0.7). The latter depends much upon the derived temperature of HR 4912, hence we pay special attention to this problem. Our blue spectra and new BV photometry made it possible to obtain four independent estimates of the temperature-better than in the previous two studies of HR 4912 in the UV and red regions.Despite all recent efforts in a wide spectral region the status and origin of HR 4912 remains as yet not established, although it seems to stand much closer to the stars of the UU Herculis type.     

Estimating the interstellar extinction and the contribution from an accretion shock to the formation of emission continuum in DS Tau and DG Tau

We analyze the UV spectra of the young stars DG Tau and DS Tau taken with the STIS spectrograph from the Hubble Space Telescope. For these stars, we found the upper limits of the interstellar extinction A _V . Their values proved to be lower than those obtained by other authors from optical observations. For DS Tau, DG Tau, and T W Hya, we also determined the ratio of theflux in the C IV 1550 doublet lines to the excess continuum flux. It proved to be an order of magnitude lower than its value predicted by the accretion-shock (AS) models of Lamzin (1998) and Calvet and Gullbring (1998). It thus follows that for these stars, the emission continuum originates mainly in the accretion disk and/or the boundary layer rather than in the AS, as has been thought previously. Since a similar conclusion has previously been reached for DR Tau, T Tau, and RY Tau, we may assume that the disks around most young stars reach the stellar surface and accretion mainly proceeds through the boundary layer.     

The early-type variables

We review the observational status of several different kinds of intrinsic variables among the early-type stars and attempt to interpret the variations in terms of our current understanding of stellar pulsation. Four distinct types of intrinsic variable can be defined: the Cep, 53 Per, Oph and Eri stars. A simple observational classification scheme, which is readily interpreted in terms of pulsation properties, is proposed. The limits of the instability strip and pulsation constants for the Cep and 53 Per stars is discussed. Problems with the interpretation of Eri stars in terms of pulsation are pointed out. The observations are consistent with rotational modulation. A problem with mode identification in Eri stars is discussed.     

On the period variations of several low declination high amplitude delta Scuti variables

A 16-inch Schmidt-Cassegrain telescope on the campus of Naresuan University of Thailand and several similar-size telescopes in China equipped with CCD cameras were used to observe 14 high amplitude delta Scuti stars: GP And, CY Aqr, BS Aqr, YZ Boo, AD CMi, VZ Cnc, EH Lib, DY Her, V927 Her, KZ Hya, BE Lyn, V1162 Ori, DY Peg, and CW Ser, between the years 1999 and 2010. Data were also collected from scientific journals and sources on the Internet for these variable stars. Times of light maximum of these delta Scuti stars were then either determined from the observations or obtained from the literature to analyze the pulsation period variations. For the 14 delta Scuti stars we concluded that 7 stars (BS Aqr, CY Aqr, AD CMi, EH Lib, KZ Hya, BE Lyn and DY Peg) are binary or multiple systems. 10 delta Scuti stars are found with periods increasing with rates between 5.86×10⁻⁹ and 2.34×10⁻⁶ per year and the other 4 stars (BS Aqr, DY Her, BE Lyn and DY Peg) show periods decreasing with rates of about 10⁻⁹ to 10⁻⁸ per year.     

New Hα stars. Cyg OB7 region. I.

Preliminary results from a search for emission stars in the neighborhood of Cyg OB7 are reported. A new region of size 14×14 arc min lies 12 arc min to the south of HH 448. 17 new objects have been found, of which one is related to a nebula with an interesting shape and, apparently, the characteristics of a cometary nebula. Inside the nebula there are 4 faint, compact formations with very high proper motions. The spectrum of a star shows that it is of spectral class K9-M0 and is a T Tau type star at a distance of 220 pc. Most of the discovered stars with a strong or intermediate Ha emission line intensity are presumably also T Tau stars. This region appears to be a continuation of the T association around HH 448.     

Supergiant variables: Recent observational results

Recent observations of several types of supergiant variable stars are reviewed: massive blue, yellow and red supergiants; classical and population II Cepheids; RV Tauri stars; yellow semi-regular (SRd) variables, including UU Herculis stars; and R Coronae Borealis stars. The emphasis is on non-linear aspects such as: amplitude and shape of the light and velocity curves; multiperiodicity, irregularity and chaos; long-term changes in period and amplitude; episodic and continuous mass loss.     

Modelling the components of binaries in the Hyades: the dependence of the mixing-length parameter on stellar mass

We present our findings based on a detailed analysis of the binaries of the Hyades, in which the masses of the components are well known. We fit the models of the components of a binary system to observations so as to give the observed total V and B − V of that system and the observed slope of the main sequence in the corresponding parts. According to our findings, there is a very definite relationship between the mixing-length parameter and the stellar mass. The fitting formula for this relationship can be given as  α= 9.19( M /M_⊙− 0.74)^0.053− 6.65  , which is valid for stellar masses greater than  0.77 M_⊙  . While no strict information is gathered for the chemical composition of the cluster, as a result of degeneracy in the colour–magnitude diagram, by adopting   Z = 0.033  and using models for the components of 70 Tau and θ² Tau we find the hydrogen abundance to be   X = 0.676  and the age to be 670 Myr. If we assume that   Z = 0.024  , then   X = 0.718  and the age is 720 Myr. Our findings concerning the mixing-length parameter are valid for both sets of the solution. For both components of the active binary system V818 Tau, the differences between radii of the models with   Z = 0.024  and the observed radii are only about 4 per cent. More generally, the effective temperatures of the models of low-mass stars in the binary systems studied are in good agreement with those determined by spectroscopic methods.     

A photometric and spectroscopic study of the short-period eclipsing binary BV Eridani

UBV observations of the short-period eclipsing binary BV Eri are reported and analyzed. The radial velocity curve is given. The system shows a light curve distortion, making an analysis difficult. The secondary fills its critical Roche lobe. The system seems to be in an interesting evolutionary.     

Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars

We present Hα spectropolarimetry observations of a sample of 10 bright T Tauri stars, supplemented with new Herbig Ae/Be star data. A change in the linear polarization across Hα is detected in most of the T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret in terms of a compact source of line photons that is scattered off a rotating accretion disc. We find consistency between the position angle (PA) of the polarization and those of imaged disc PAs from infrared and millimetre imaging and interferometry studies, probing much larger scales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find the polarization PA to be perpendicular to the imaged disc, which is expected for single scattering. On the other hand, the polarization PA aligns with the outer disc PA for the T Tauri stars DR Tau and SU Aur and FU Ori, conforming to the case of multiple scattering. This difference can be explained if the inner discs of Herbig Ae stars are optically thin, whilst those around our T Tauri stars and FU Ori are optically thick. Furthermore, we develop a novel technique that combines known inclination angles and our recent Monte Carlo models to constrain the inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, we consider the connection of the inner disc structure with the orientation of the magnetic field in the foreground interstellar medium: for FU Ori and DR Tau, we infer an alignment of the stellar axis and the larger magnetic field direction.     

The visible spectra of three B stars: π Cet, 36 Lyn 134 Tau

We present an identification list for the visible spectra of three B stars: π Cet (B7V), 36 Lyn B8 IIIp (?)),and 134 Tau (B9 IV). Equivalent widths have been measured on medium dispersion plates (7.0 and 12.4 Å mm^?1) taken with the 152 cm coudé reflector at the Observatoire de Haute Provence (France). These results are also presented. The photographic plates were digitized by using a PDS 1010A microdensitometer. The spectroscopic data were reduced by means of a dedicated software package and an attempt was made to compute equivalent widths in a homogeneous way. T _eff and logg parameters were estimated by using the computer to compare automatically the spectroscopic data with the value of theoretical models.     

UV survey of δ Scuti stars: Final results and open problems

The results obtained from our IUE observations of some Scuti stars and in particular 71 Tau and 69 Tau are reviewed. 71 Tau is the second brightest X-ray source of the Hyades cluster; on the contrary 69 Tau, very similar to 71 Tau in the optical range, does not show X emissions. The complex and in somewhat controversial scenario obtained from this survey is also discussed. In particular we consider the X-ray emission detected in some Scuti with respect to the chromospheric activity level as measured through the MgII line and to other stellar parameters.     

Analysis of eight binaries in Lyncis constellation: RV Lyn,AA Lyn,AH Lyn,CD Lyn,CF Lyn,DR Lyn,EK Lyn,and FS Lyn

The available photometry mainly from the WASP database was used for the first light curve analysis of eight eclipsing binary systems located in the Lyncis constellation: RV Lyn, AA Lyn, AH Lyn, CD Lyn, CF Lyn, DR Lyn, EK Lyn, and FS Lyn. Most of these eclipsing stars are detached ones, having the orbital periods from 0.54 to 2.3 days. For the systems AA Lyn and CF Lyn a non-negligible third light was detected during the light curve solution. Moreover, 284 new times of minima for these binaries were derived, trying to identify the period variations. For the system CD Lyn a hypothetical third body was detected with the period of about 59 yr.