V440 Per: the longest-period overtone Cepheid

V440 Per is a Population I Cepheid with a period of 7.57 d and low-amplitude, almost sinusoidal light and radial velocity curves. With no reliable data on the first harmonic, its pulsation mode identification remained controversial. We obtained a radial velocity curve of V440 Per with our new high-precision and high-throughput Poznań Spectroscopic Telescope. Our data reach an accuracy of 130 m s⁻¹ per individual measurement and yield a secure detection of the first harmonic with an amplitude of   A ₂= 140 ± 15 m s⁻¹  . The velocity Fourier phase φ₂₁ of V440 Per is inconsistent at the 7.25σ level with those of fundamental-mode Cepheids, implying that the star must be an overtone Cepheid, as originally proposed by Kienzle et al. Thus, V440 Per becomes the longest-period Cepheid with securely established overtone pulsations. We show that a convective non-linear pulsation hydrocode can reproduce the Fourier parameters of V440 Per very well. The requirement to match the observed properties of V440 Per constrains the free parameters of the dynamical convection model used in the pulsation calculations, in particular the radiative loss parameter.     

On the orbital periods of the AM CVn stars HP Librae and V803 Centauri

We analyse high time resolution spectroscopy of the AM CVn stars HP Librae and V803 Centauri, taken with the New Technology Telescope (NTT) and the Very Large Telescope (VLT) of the European Southern Observatory, Chile.
We present evidence that the literature value for V803 Cen's orbital period is incorrect, based on an observed ' S -wave' in the binary's spectrogram. We measure a spectroscopic period   P _V803 Cen= 1596.4 ± 1.2 s  of the S -wave feature, which is significantly shorter than the 1611-s periods found in previous photometric studies. We conclude that the latter period likely represents a 'superhump'. If one assumes that our S -wave period is the orbital period, V803 Cen's mass ratio can be expected to be much less extreme than previously thought, at   q ∼ 0.07  rather than   q ∼ 0.016  . This relaxes the constraints on the masses of the components considerably: the donor star then does not need to be fully degenerate, and the mass of the accreting white dwarf no longer has to be very close to the Chandrasekhar limit.
For HP Lib, we similarly measure a spectroscopic period   P _HP Lib= 1102.8 ± 0.2 s  . This supports the identification of HP Lib's photometric periods found in the literature, and the constraints upon the masses derived from them.     

Validation of LAMOST stellar parameters with the PASTEL catalog

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST) published its first data release(DRl)in 2013,which is currently the largest dataset of stellar spectra in the world.We combine the PASTEL catalog and SIMBAD radial velocities as a testing standard to validate stellar parameters(effective temperature T_(eff),surface gravity log g,metallicity[Fe/H]and radial velocity V_r)derived from DR1.Through cross-identification of the DR1 catalogs and the PASTEL catalog,we obtain a preliminary sample of 422 stars.After removal of stellar parameter measurements from problematic spectra and applying effective temperature constraints to the sample,we compare the stellar parameters from DR1 with those from PASTEL and SIMBAD to demonstrate that the DR1 results are reliable in restricted ranges of T_(eff).We derive standard deviations of 110 K,0.19 dex and 0.11 dex for T_(eff),log g and[Fe/H]respectively when T_(eff)8000 K,and 4.91 km S~(-1) for V_r when T_(eff)10 000 K.Systematic errors are negligible except for those of V_r.In addition,metallicities in DR1 are systematically higher than those in PASTEL,in the range of PASTEL[Fe/H]-1.5.     

The rapidly pulsating sdO star, SDSS J160043.6+074802.9

A spectroscopic analysis of Sloan Digital Sky Survey (SDSS) J160043.6+074802.9, a binary system containing a pulsating subdwarf-O (sdO) star with a late-type companion, yields   T _eff= 70 000 ± 5000 K  and  log  g = 5.25 ± 0.30  , together with a most likely type of K3 V for the secondary star. We compare our results with atmospheric parameters derived by Fontaine et al. and in the context of existing evolution models for sdO stars. New and more extensive photometry is also presented which recovers most, but not all, frequencies found in an earlier paper. Therefore, it seems probable that some pulsation modes have variable amplitudes. A non-adiabatic pulsation analysis of uniform metallicity sdO models show those having  log  g > 5.3  to be more likely to be unstable and capable of driving pulsation in the observed frequency range.     

Photometric Investigation of the Eclipsing Binary Star KN Per

We used the Behlen observatory 0.76 m telescope and the CCD photometer to secure 689 observation of the eclipsing binary star KN Per. The observations were made on 8 nights during 1993 and 1994 with V and R bandpass filters. From 7 determinations of eclipse timings of minimum (V&R together), we have determined a new epoch and an orbital period of 0.8664604 days. The published spectral classification is A9. The 1993 version of the Wilson-Devinney model gave the photometric solutions. The adopted solution indicates that KN Per is a W UMa type contact binary. The mass ratio, q = (m₂/m₁, where star 1 eclipses at the primary minimum) = 0.23 suggests that KN Per is a W UMa system with A-type configuration. The secondary minimum shows a total eclipse. The asymmetry in the light curve is fitted with a cold spot on the secondary component of the system. The luminosity difference between the components is very large KN Per therefore, is most likely a single line spectroscopic binary. We recommend spectroscopic study of this system. Generally contact systems of spectral type A9 have periods ranging from 0.4 to 0.6 days. KN Per has considerably longer period and thus appears to be an evolved contact system with case B mass transfer. This revised version was published online in July 2006 with corrections to the Cover Date.     

The determination of white dwarf parameters in dwarf novae by optical spectral modeling

This paper presents the results of a model analysis of optical spectra and determination of the parameters of three individual SU UMa and WZ Sge type dwarf novae.The moderate resolution spectra of TY Psc,FL Psc and V455 And were obtained at the 6-m BTA of the SAO RAS in the low state of these systems with the determination of white dwarf radiation.The theoretical spectra were calculated using the grid models of hydrogen dwarf atmospheres of white dwarfs by varying the parameters(T_(eff) and log g) to reach the best agreement with the observed ones.We highlight different effects of the parameters on the shape and intensity of the HI lines.Therefore,it is possible to unambiguously determine T_(eff) and log g from the analysis of observations.The fundamental parameters of white dwarfs(M and R) were found by comparing the parameters of atmospheres with theoretical models of the internal structure.The obtained parameters of the primaries of TY Psc,FL Psc and V455 And are consistent with the average values for SU UMa and WZ Sge systems.As a result,we demonstrate the efficiency of the method for determining the parameters of such systems based on the analysis of a limited set of observed optical spectra.     

Non-adiabatic linear pulsation models for low-mass helium stars

Non-adiabatic linear pulsation models have been calculated for low-mass stars with effective temperatures between 16 000 and 35 000 K, and with surface gravities in the range 3⊙, X =0.00, Z =0.02. It is shown that the Z -bump instability persists to low masses ( M ∼0.4 M_⊙) but is suppressed either by a reduction in metallicity Z or by a selective enhancement of the carbon abundance. An unexpected result is the discovery that Z -bump instability persists at hydrogen abundances X >0.3, although the position of the red edge is sensitive to X . We have found that non-radial pulsations are also excited in the same instability region as radial pulsations.
The implications of these results for individual low-mass helium stars are discussed. It is concluded that Z -bump driven pulsations (radial and/or non-radial) may be excited in some helium-rich subdwarf B stars, representing a possible major extension to the class of variable stars represented by the prototype V652 Her.     

A photometric study of a weak-contact binary: V873 Per

We present a photometric study of a weak-contact binary V873 Per. New observations in BVR filter bands showed asymmetric light curves to be a negative type of the O’Connell effect, which can be described by magnetic activity of a cool spot on the more massive component. Our photometric solutions showed that V873 Per is a W-type with a mass ratio of q = 2.504(±0.0029), confirming the results of Samec et al. (2009). The derived contact degree was found to be f = 18.10%(±1.36%). Moreover, our analysis found the cyclic variation with the period of about 4 yr that could be due to existence of the third companion in the system or the mechanism of magnetic activity cycle in the binary. While available data indicated that the long-term orbital period tends to be stable rather than decreasing.     

Photoelectric and spectroscopic observations of the binary star VV8=V471 Per over the period 1971–2007

We present our photoelectric observations of the binary star VV8=V471 Per over the period 1971–2007. A long-term photometric variability with a period of about 17 yr in the V and B bands has been confirmed. A systematic rise in brightness was recorded in the U band, suggesting that the gas ionization increases in the binary system. Our spectroscopic observations from 1995 to 2007 have shown that the emission line fluxes, on average, did not change compared to the data of other authors obtained in the previous decades. We have shown that the cool giant is a G5III star and is probably enriched with nucleosynthesis products from the evolved former primary component of the binary. We provide arguments for the hypothesis that the hot component in V471 Per is the massive nucleus of a young planetary nebula that rapidly evolved to a temperature of 65 000–75 000 K, while the surrounding nebula is very dense and optically thick and has not yet been entirely ionized.     

Secular Perion Changes and Fundamental Parameters of Long–Period Cepheids

Hydrodynamic computations of nonlinear Cepheid pulsation models with periods from 20 to 100 day on the evolutionary stage of core helium burning were carried out. Equations of radiation hydrodynamics and time–dependent convection were solved with initial conditions obtained from selected models of evolutionary sequences of population I stars with initial masses from 8 M_⊙ to 12.5 M_⊙. For each crossing of the instability strip the pulsation period Π and the rate of period change \(\dot \prod \) were derived as a function of evolutionary time. Comparing results of our computations with observational estimates of Π and \(\dot \prod \) we determined fundamental parameters (the age, the mass, the luminosity and the radius) of seven long–period Cepheids. Theoretical estimates of the stellar radius are shown to agree with radius measurements by the Baade–Wesselink technique within 3% for RS Pup and GY Sge whereas for SV Vul the disagreement between theory and observations does not exceed 10%.     

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.     

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.     

Parameterizing Stellar Spectra Using Deep Neural Networks

Large-scale sky surveys are observing massive amounts of stellar spectra.The large number of stellar spectra makes it necessary to automatically parameterize spectral data,which in turn helps in statistically exploring properties related to the atmospheric parameters.This work focuses on designing an automatic scheme to estimate effective temperature(T_(eff)),surface gravity(log g) and metallicity[Fe/H] from stellar spectra.A scheme based on three deep neural networks(DNNs) is proposed.This scheme consists of the following three procedures:first,the configuration of a DNN is initialized using a series of autoencoder neural networks;second,the DNN is fine-tuned using a gradient descent scheme;third,three atmospheric parameters T_(eff),log g and [Fe/H] are estimated using the computed DNNs.The constructed DNN is a neural network with six layers(one input layer,one output layer and four hidden layers),for which the number of nodes in the six layers are 3821,1000,500,100,30 and 1,respectively.This proposed scheme was tested on both real spectra and theoretical spectra from Kurucz's new opacity distribution function models.Test errors are measured with mean absolute errors(MAEs).The errors on real spectra from the Sloan Digital Sky Survey(SDSS) are 0.1477,0.0048 and 0.1129 dex for log g,log T_(eff) and [Fe/H](64.85 K for T_(eff)),respectively.Regarding theoretical spectra from Kurucz's new opacity distribution function models,the MAE of the test errors are 0.0182,0.0011 and 0.0112 dex for log g,log T_(eff) and [Fe/H](14.90 K for T_(eff)),respectively.     

A search for variable stars in the globular cluster M3

We describe here the results of a photometric time-sequence survey of the globular cluster M3 (NGC 5272), in a search for contact and detached eclipsing binary stars. We have discovered only one likely eclipsing binary and one SX Phe-type star in spite of monitoring 4077 stars with V  < 20.0 and observing 25 blue straggler stars (BSS). The newly identified SX Phe star, V237, shows a light curve with a variable amplitude. Variable V238 shows variability either with a period of 0.49 d or with a period of 0.25 d. On the cluster colour–magnitude diagram, the variable occupies a position a few hundredths of a magnitude to the blue of the base of the red giant branch. V238 is a likely descendant of a binary blue straggler.   As a side result we obtained high-quality data for 42 of the previously known RR Lyrae variables, including 33 of Bailey type ab, seven type c and two double-mode pulsators. We used equations that relate the physical properties of RRc stars to their pulsation periods and Fourier parameters in order to derive masses, luminosities, temperatures and helium parameters for five of the RRc stars. We also tested equations that relate the [Fe/H], M _V and temperature of RRab stars to pulsation period and Fourier parameters. We derived [Fe/H]= −1.42 in good agreement with spectroscopic determinations.     

Dust Shells Around Carbon Mira Variables

The spectral energy distributions and mid-infrared spectra of 44 carbon Mira variables are fitted using a dust radiative transfer model. The periods cover the entire range observed for carbon Miras. The luminosities are calculated from a period-luminosity relation. Parameters derived are the distance, the dust mass loss rate and the ratio of silicon carbide to amorphous carbon dust. The total mass loss rate is derived from a modified relation between the photon momentum (L/c) and the momentum in the wind (M υ_∞). Mass loss rates between 1 × 10⁻⁸ and 4 × 10⁻⁵ M_⊙ yr⁻¹ are found. We find good correlations between mass loss rate and pulsation period, and mass loss rate and luminosity. The dust-to-gas ratio appears to be almost constant up to periods of about 500 days, corresponding to about 7900 L_⊙, and then to increase by a factor of 5 towards longer periods and higher luminosities. A comparison is made with radiation-hydrodynamical calculations including dust formation. The mass loss rates predicted by these models are consistent with those derived in this paper. The main discrepancy is in the predicted expansion velocities for models with luminosities below ∼5000 L_⊙. The radiation-hydrodynamical calculations predict expansion velocities which are significantly too large. This is related to the fact that these models need to be calculated with a large C/O ratio to get an outflow in the first place. This is contrary to observations. It indicates that a principle physical ingredient in these radiation-hydrodynamical calculations is still missing. Possibly the winds are ‘clumpy’ which may lead to dust formation on a local scale, or there is an additional outwards directed force, possibly radiation pressure on molecules. This revised version was published online in September 2006 with corrections to the Cover Date.     

The role of chemical composition in RR Lyrae pulsationalproperties – I. Periods

We analyse the periods of theoretical radial pulsators, covering the range of total masses, luminosities, effective temperatures and chemical compositions expected for RR Lyrae variables in both galactic fields and globular clusters.
We show that for fixed values of the structural parameters (mass, luminosity and effective temperature), the period of fundamental and first-overtone pulsators is independent of the helium content ( Y ), whereas it slightly increases as the amount of metals ( Z ) increases. Furthermore, we find that the period along the blue edge for first-overtone pulsation is a function of mass, luminosity and helium content, with a marginal dependence on Z .
On these grounds, new linear relations connecting the periods to stellar parameters are derived. Such new relations should allow a more accurate interpretation of the RR Lyrae observed periods and, in particular, they should help in ascertaining the calibration of the mean absolute magnitude of RR Lyrae stars in terms of metal content.     

Nonlinear models of pulsation and mass loss

We briefly discuss the current status of our radiation-hydrodynamical models of pulsation in various stellar objects and of dust-driven mass loss in LPVs. We emphasize the importance of a future combined modelling of pulsation and mass loss in AGB stars which has to be based on reliable physical and numerical methods.     

Detection of low-amplitude brightness oscillations in the unique eclipsing system V718 Per (HMW 15, H 187)

We present new results of our visual (V, R, I) and near-infrared (J,H,K) photometry for the unusual young star V718 Per. They show that, in addition to prolonged eclipses following one another with a period of 4.7 yr, the star also exhibits low-amplitude brightness oscillations with a period that is approximately a factor of 8 shorter than the main one. In contrast to the large-scale eclipses accompanied by the star’s reddening, the low-amplitude oscillations are neutral in character and are produced by large particles. Bimodal oscillations of this type can arise in a circumstellar disk divided by a large matter-free gap into two zones—an inner, dense region and an outer, less dense disk. Such configurations emerge in the presence of a fairly massive perturbing body in the disk. In this case, density waves rotating with different angular velocities can be formed in each of these zones. Therefore, when such systems are observed nearly edge-on, two oscillation modes with different periods can be present in the extinction variations. We suggest that such a situation takes place in the case of V718 Per. Since this star exhibits no signatures of spectroscopic binarity, the perturbing body can be either a giant planet or a brown dwarf.     

The semi-detached binary system IU Per and its intrinsic oscillation

We present a long-term time-resolved photometry of the short-period eclipsing binary IU Per. It confirms the intrinsic δ Scuti-like pulsation of the system reported by Kim et al.. With the obtained data, an orbital period study and an eclipsing light curve synthesis based on the Wilson-Devinney method were carried out. The photometric so- lution reveals a semi-detached configuration with the less-massive component filling its own Roche-lobe. By subtracting the eclipsing light changes from the data, we obtained the pure pulsating light curve of the mass-accreting primary component. A Fourier anal- ysis reveals four pulsation modes with confidence larger than 99%. A mode identification based on the results of the photometric solution was made. It suggests that the star may be in radial pulsation with a fundamental period of about 0.0628 d. A brief discussion concerning the evolutionary status and the pulsation nature is finally given.     

Spectral variability of the α Sco AB binary system observed with IUE

We present ultraviolet spectra of the α Sco AB binary system taken by the International Ultraviolet Explorer during the period from 1979 to 1995. An investigation is carried out on the spectral variability of Mg II k and h emission lines arising from the chromosphere of α Sco A (Van der Hucht et al., 1979). There are absorption and emission components on the blue sides of the Mg II k and h lines, which are formed in the cool circumstellar gas shells around two stars (Bernat and Lambert, 1976).This work is based on calculations of line fluxes and line widths for the aforementioned spectral lines. We found that there is spectral variability for these physical parameters with pulsation phase, which we attribute to the changes of density and temperature of the chromosphere of α Sco A as a result of the semi-regular pulsation and the variability of mass loss of the red supergiant (Reimers et al., 2008). The observed values of the k-to-h ratio are approximately one, implying that the k and h emission lines originate from an optically thick atmosphere.