Ages and masses of delta scuti stars in the praesepe cluster

Physical parameters and radial pulsation modes are determined for three Delta Scuti stars in the Praesepe cluster: HD 73345, HD 73712, and HD 73746. Their agest and evolutionary massesM _eare interpolated in the track systems of Iben (1967) and Paczyski (1970). By combining these age estimates with our previous results for nine other Delta Scuti-stars in the Praesepe cluster (Tsvetkov, 1989), the following weighted mean age estimates of this cluster are obtained: (14.2 ± 1.3) × 10⁸ yr (Iben) and (5.4 ± 0.4) × 10⁸ yr (Paczyski). Age and evolutionary mass estimates of the twelve cluster variables are also obtained in the modern track system of Maeder and Meynet (1988), in which the weighted mean cluster age is (15.3 ± 1.0) × 10⁸ yr.Pulsation massesM _Qcalculated from the fitting formulae of Faulkner (1977) and Fitch (1981) as well as massesM _g=gR ² / G were evaluated for the twelve cluster variables by Tsvetkov (1990) or in the present paper. In most cases there is a satisfactory agreement between our estimates of massesM _e,M _g, andM _Q.Due to a large disagreement between the long period and low luminosity of the star HD 73746, its radial pulsation mode and pulsation massM _Qcannot be evaluated.     

Trapping of test particles in static and spherical space-time geometries

Estimates of the pulsation massesM _Q and of massesM _g=gR²/G (g is the surface gravity;R, the mean stellar radius; andG, the gravitational constant) of 89 Delta Scuti-variables have been obtained. An intercomparison of three kinds of masses —M _e, M_g, andM _Q — is performed; the evolutionary massesM _e of the same variables were estimated in a previous paper (Tsvetkov, 1986). It is pointed out that within the limits of the accuracy of determination, the three kinds of masses agree for the majority of the stars under study. For several varaibles, however, there is a considerable discrepancy between the estimates of the pulsation massesM _Q and the estimates of the massesM _e andM _g. Arguments are represented, which may alleviate or even remove this mass discrepancy for some of these stars. It is shown that the results from the comparison between the three kinds of masses of the considered Delta Scuti-stars are not very sensitive to the choice of the system of evolutionary tracks and the adopted chemical composition as well as to the evolutionary phases of the variables (core or shell hydrogen burning). The variations of the chemical composition in most of the stars under study are probably not very large.     

A search for wide visual companions of exoplanet host stars: The Calar Alto Survey

We have carried out a search for co‐moving stellar and substellar companions around 18 exoplanet host stars with the infrared camera MAGIC at the 2.2 m Calar Alto telescope, by comparing our images with images from the all sky surveys 2MASS, POSS I and II. Four stars of the sample namely HD80606, 55 Cnc, HD46375 and BD–10°3166, are listed as binaries in the Washington Visual Double Star Catalogue (WDS). The binary nature of HD80606, 55 Cnc, and HD46375 is confirmed with both astrometry as well as photometry, thereby the proper motion of the companion of HD46375 was determined here for the first time.We derived the companion masses as well as the longterm stability regions for additional companions in these three binary systems. We can rule out further stellar companions around all stars in the sample with projected separations between 270AU and 2500AU, being sensitive to substellar companions with masses down to ∼60 M_Jup (S /N = 3). Furthermore we present evidence that the two components of the WDS binary BD–10°3166 are unrelated stars, i.e this system is a visual pair. The spectrophotometric distance of the primary (a K0 dwarf) is ∼67 pc, whereas the presumable secondary BD–10°3166B (a M4 to M5 dwarf) is located at a distance of 13 pc in the foreground. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Abundance analysis of the γ Doradus–δ Scuti hybrid metallic line (Am) star HD 8801

Low frequency oscillation, typical for γ Doradus g‐mode type stellar core sensitive pulsation, as well as higher frequency δ Scuti type pulsation typical for p ‐modes, sensitive to the envelope, make HD 8801 a remarkable hybrid pulsator with the potential to probe a stellar structure over a wide range of radius. In addition HD 8801 is a rare pulsating metallic line (Am) star. We determined the astro‐physical fundamental parameters to locate HD 8801 in the H‐R diagram. We analyzed the element abundances, paying close attention to the errors involved, and confirm the nature of HD 8801 as a metallic line (Am) star. We also determined an upper limit on the magnetic field strength. Our abundance analysis is based on classical techniques, but uses for the final step a model atmosphere calculated with the abundances determined by us. We also discuss spectropolarimetric observations obtained for HD 8801. This object is remarkable in several respects. It is a nonmagnetic metallic line (Am) star, pulsating simultaneously in p‐ and g‐modes, but also shows oscillations with periods in between these two domains, whose excitation requires explanation. Overall, the pulsational incidence in unevolved classical Am stars is believed to be quite low; HD 8801 does not conform to this picture. Finally, about 75 % of Am stars are located in short‐period binaries, but there is no evidence that HD 8801 has a companion. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Delta Scuti stars in the Praesepe cluster observed by the MOST satellite

The Praesepe cluster contains a number of δ Sct and γ Dor pulsators. Asteroseismology of cluster stars is simplified by the common distance, age and stellar abundances. Since asteroseismology requires a large number of known frequencies, the small pulsation amplitudes of these stars require space satellite campaigns. The present study utilizes photometric MOST satellite measurements in order to determine the pulsation frequencies of two evolved (EP Cnc, BT Cnc) and two main‐sequence (BS Cnc, HD 73872) δ Sct stars in the Praesepe cluster. The frequency analysis of the 2008 and 2009 data detected up to 34 frequencies per star with most amplitudes in the submillimag range. In BS Cnc, two modes showed strong amplitude variability between 2008 and 2009. The frequencies ranged from 0.76 to 41.7 cd^–1. After considering the different evolutionary states and mean stellar densities of these four stars, the differences and large ranges in frequency remain (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lithium in chemically peculiar CP stars with magnetic fields

The problem of lithium in chemically peculiar Ap-CP stars has been the subject of debate for many years. The main reason for this is a lack of spectral observations of Ap stars in the neighborhood of the lithium resonance doublet Li I 6708 Å. An international cooperation project on “Lithium in cool CP stars with magnetic fields” was started in 1996. Systematic observations of CP stars in spectral regions of the 6708 Å and 6103 Å lines at the ZTSh (CrAO), CAT (ESO), Feros (ESO), and the 74″ telescope of the Mount Stromlo Observatory (Australia) have been used to analyze spectra of several CP stars studied by the way the 6708 Å lithium line varies with the stars’ rotational phase. Monitoring of the spectra of the oscillating CP stars (group I) HD 83368, HD 60435, and HD 3980, for which significant Doppler shifts of the Li I 6708 Å line are observed led to the discovery of “lithium spots” on the surface of these stars whose positions are related to the magnetic field structure. Models of the surfaces of these stars with the special program “ROTATE” based on the profiles of the Li I 6708 Å line are used to estimate the size of the spots, their positions on the stars’ surface, and the lithium abundances in these spots. A detailed analysis and modelling of the spectra of slowly rotating oscillating CP stars with strong, invariant lithium 6708 Å emission, including blending with lines of the rare earth elements, reveals an enhanced lithium abundance, with the abundance determined from the lithium 6103 Å line being higher than that determined from the 6708 Å line for all the stars. This may indicate vertical stratification of lithium in the atmospheres of CP stars with an anomalous isotopic composition (⁶Li/⁷Li = 0.2–0.5). HD 101065, an ultraslow rotator (vsini ≈ 1.5) visible from the poles and with powerful oscillations which cause pulsating line broadening in its spectrum, is unique among these stars. The amount of lithium in the atmosphere of HD 101065 logN(Li) = 3.1 on a scale of logN(H) = 12.0 and the isotope ratio ⁶Li/⁷Li ≈ 0.3. The high estimates of ⁶Li/⁷Li may be explained by the production of lithium in spallation reactions and the preservation of surface ⁶Li and ⁷Li by strong magnetic fields in the upper layers of the atmosphere near the magnetic poles. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 463–492 (August 2007).     

Determination of Distances to Galaxies of the NGC 1023 Group. The Hubble Constant

On the basis of photographs from the 6 m Large Azimuthal Telescope and the Hubble Space Telescope, VRI photometry of stars in 11 galaxies in the NGC 1023 group has been carried out. The distances to these galaxies were determined by the method of brightest stars. The distances to NGC 925 and NGC 1023 were determined from the position of the top of the red giant branch (the TRGB method). From the calculated average and root-mean-square distances to the NGC 1023 group (10.3 ± 2.2 Mpc and 9.7 ± 0.5 Mpc) the Hubble constant in this direction was determined: H _0R = 75 ± 8 km·sec^-1·Mpc^-1 and H _0M = 81 ± 5 km ·sec^-1·Mpc^-1.     

The Cepheid impostor HD 18391 and its anonymous parent cluster

New and existing photometry for the G0 Ia supergiant HD 18391 is analyzed in order to confirm the nature of the variability previously detected in the star, which lies off the hot edge of the Cepheid instability strip. Small‐amplitude variability at a level of δV = 0.016 ± 0.002 is indicated, with a period of P = 123^d.04 ± 0^d.06. A weaker second signal may be present at P = 177^d.84 ± 0^d.18 with δV = 0.007 ± 0.002, likely corresponding to fundamental mode pulsation if the primary signal represents overtone pulsation (123.04/177.84 = 0.69). The star, with a spectroscopic reddening of E_B–V = 1.02 ± 0.003, is associated with heavily‐reddened B‐type stars in its immediate vicinity that appear to be outlying members of an anonymous young cluster centered ∼10′ to the west and 1661 ± 73 pc distant. The cluster has nuclear and coronal radii of r_n = 3.5′ and R_c = 14′, respectively, while the parameters for HD 18391 derived from membership in the cluster with its outlying B stars are consistent with those implied by its Cepheid‐like pulsation, provided that it follows the semi‐period‐luminosity relation expected of such objects. Its inferred luminosity as a cluster member is M_V = –7.76 ± 0.10, its age (9 ± 1) × 10⁶ years, and its evolutionary mass ∼19 M_⊙. HD 18391 is not a classical Cepheid, yet it follows the Cepheid period‐luminosity relation closely, much like another Cepheid impostor, V810 Cen (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Identification of absorption lines of rare-earth elements in the region of the spectrum λλ 610.25–610.57 nm roAp stars,HD 101065, HD 134214, HD 137949, and HD 24712

In the spectral region λλ = 610.25−610.57 nm of the roAp star HD 101065, we performed the identification of the rare-earth element absorption lines absent from the atomic spectral line databases VALD and DREAM. The identified lines were used for calculating the synthetic spectra of the roAp stars HD 137949, HD 134214, and HD 24712. The upper limit of the identified line estimates was determined. The rotation/pulsation parameter νsini and magnetic field modulus were determined using the lines Nd III 669.083 nm and Ca I 616.217 nm. The element abundance in the roAp stars HD 134214 and HD 24712 was determined for the first time using Nd III lines. Calculations were carried out with the help of the code SynthM developed by S.A. Khan.     

Speckle interferometry of magnetic stars with the BTA. I. First results

We present the results of speckle interferometry of a sample of 117 chemically peculiar stars with global magnetic fields. The observations were made in December 2009 at the BTA with a spatial resolution of about 0.02″ in the visual spectral region. Twenty-nine stars were resolved into individual components, 14 of them for the first time (HD965, HD5797, HD8855, HD10783, HD16605, HD21699, HD35502, HD51418, HD64486, HD79158, HD103498, HD108651, HD213918, HD293764). In twelve cases a companion turned out to be 2–4 ^m fainter than the main component—a magnetic star. Young hot Bp stars HD35502 and HD213918 are exceptions, since their companions are fainter by about 1 ^m . In all cases, the linear distance from a star to its companion at the epoch of observations in the picture plane exceeded 10⁹ km. Eighty-eight magnetic CP stars revealed no secondary components within our study. Thus, the fraction of speckle interferometric binaries in our sample amounts to 25%.     

Photographicuvby photometry of the open cluster M52

Photographicuvby photometry of 15 stars in the open cluster M52 (NGC 7654) has been obtained. The distance modulusV _o–M _v=11.3±0.1 and mean cluster reddeningE(B–V)=0.57 are determined. Some basic physical characteristics of 30 stars with observeduvby values are obtained as well as a numerical estimation of the open cluster age 9.6×10⁷ yr is made.     

Scanner observations of the classical cepheids RT Aur and T Vul

The effective temperatures of the classical Cepheids RT Aur and T Vul have been determined by a comparison of their spectral scans with appropriate model atmospheres. The radii of the stars have been determined through the Wesselink method. Using these temperatures and the Wesselink radii, the luminosities of the stars have been determined. These radii estimates, including the radii of SU Cas (Joshi & Rautela 1980) andζ Gem (unpublished) fit better in the theoretical period-radius relationship given by Cogan (1978), as compared to earlier determinations of Wesselink radii. The pulsation masses and evolutionary masses of the stars have been calculated. The pulsation to evolutionary mass ratio is derived to be 0.85. Based on the effective temperatures obtained by us at different phases of the stars aθ _c ? (B-V)₀ relationship is found of the form, \(\begin{gathered} \theta _e = 0.274 (B - V)_0 + 0.637 \\ \pm 0.011 \pm 0.007 \\ \end{gathered} \)      

The atmospheres of helium-deficient Bp stars

Based on spectra taken with a 6-m telescope, we analyzed the abundances of chemical elements in the He-weak stars HD 21699 and HD 217833, estimated their surface magnetic fields (B_s = 4000 and 4500 G, respectively) from the magnetic intensification of spectral lines, and determined their microturbulences (V _t = 0.80 and 0.75 km s⁻¹, respectively). The low values of V _t show that the stellar atmospheres are stabilized by a magnetic field, which explains the presence of diffusion processes that lead to chemical anomalies. Helium is strongly underabundant, and its deficiency is −1.50 and −1.81 dex in HD 21699 and HD 217833, respectively. We used model atmospheres to determine the effective temperatures, T _eff = 16 000 and 15 450 K, and surface gravities, log g = 4.15 and 3.88, for the stars from the Hδ line, implying that they lie on the main sequence near the stars of luminosity class V.     

Masses and ages of delta scuti stars in eclipsing binary systems

By using data mainly from Frolovet al. (1982) for four Delta Scuti stars in eclipsing binary systems, AB Cas, Y Cam, RS Cha, and AI Hya, their physical parameters, distances, and radial pulsation modes are determined. The evolutionary track systems of Iben (1967), Paczyski (1970), and Maeder and Meynet (1988) are interpolated, in order to estimate evolutionary massesM _eand agest of these variables. Their pulsation massesM _Qare estimated from the fitting formulae of Faulkner (1977) and Fitch (1981). Our estimates of evolutionary massesM _eand pulsation massesM _Qare close to the massesM determined by Frolovet al. (1982) from the star binarity. The only exception is AB Cas, for which there is no agreement between certain star parameters.Another, independent approach is also applied to the stars RS Cha and AI Hya: by using their photometric indicesb — y andc ₁ from the catalogue of López de Cocaet al. (1990) and appropriate photometric calibrations, other sets of physical parameters, distances, modes, ages, evolutionary and pulsation masses of both variables are obtained.     

Detection of accreting circumstellar gas around weak emission-line Herbig Ae/Be stars

We present archival and recent IUE high dispersion spectra of late B stars which reveal the presence of accreting gas with velocities as high as 350 km s^–1, collisional ionization of the accreting gas to temperatures above the stellar T_eff, and column densities intermediate between those observed toward classical Herbig Ae/Be stars and the nearby proto-planetary system Pictoris. One of the stars, HD 176386, while lacking obvious optical signatures of youth, is a member of the R CrA star formation region, and with an inferred age of 2.8 Myr has not yet arrived on the zero-age main sequence (ZAMS). The other object, an isolated, field B star with pronounced IR excess due to warm, circumstellar dust, 51 Oph, exhibits only modest h emission. The combination of high velocity, accreting gas in systems with IR excesses due to circumstellar dust suggests that not only are these objects candidate proto-planetary systems, but that they may represent an extension to higher stellar masses of the weak-emission pre-main sequence (PMS) stars.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Comprehensive study of the magnetic stars HD 5797 and HD 40711 with large chromium and iron overabundances

We present the results of a comprehensive study of the chemically peculiar stars HD 5797 and HD 40711. The stars have the same effective^temperature, T _eff = 8900 K, and a similar chemical composition with large iron (+1.5 dex) and chromium (+3 dex) overabundances compared to the Sun. The overabundance of rare-earth elements typically reaches +3 dex. We have measured the magnetic field of HD 5797. The longitudinal field component B _e has been found to vary sinusoidally between −100 and +1000 G with a period of 69 days. Our estimate of the evolutionary status of the stars suggests that HD 5797 and HD 40711, old objects with an age t ≈ 5 × 10⁸ yr, are near the end of the core hydrogen burning phase.     

Photometric variability of four coronally active stars

We present photometric observations of four stars that are optical counterparts of soft X-ray/EUV sources, namely 1ES 0829+15.9, 1ES0920-13.6, 2RE J1 10159+223509 and 1ES 1737+61.2. We have discovered periodic variability in two of the stars, viz., MCC 527 (1ES 0829+15.9; Period = 0 ^d .828 ± 0.0047) and HD 81032 (1ES 0920-13.6; Period = ∼ 57.02 ± 0.560 days). HD 95559 (2RE J1 10159+223509) is found to show a period of 3 ^d . HD 160934 (1ES1737+61.2) also shows photometric variability but needs to be monitored further for finding its period. These stars most likely belong to the class of chromospherically active stars.     

Photometry of the open cluster NGC 6996 in the North America Nebula

Thirty stars in the direction of the open cluster NGC 6996 are measured in the Vilnius photometric system. Photometric spectral types, absolute magnitudes, interstellar reddenings, extinctions, and distances are determined for most of them. Fifteen stars are suspected to be cluster members. Their mean distance is 620±30 pc and mean extinctionA _v is 1.74 mag. The extinction within the cluster is variable. The age of the cluster is of the order of 10⁸ yr and this excludes the possibility that it is evolutionary related with the North America and Pelican Nebulae complex. The distance of this complex 550 pc is confirmed.     

The evidence for clumpy accretion in the Herbig Ae star HR 5999

Analysis of IUE high- and low-dispersion spectra of the young Herbig Ae star HR 5999 (HD 144668) covering 1978–1992 has revealed dramatic changes in the Mg II h and k (2795.5, 2802.7 Å) emission profiles, changes in the column density and distribution in radial velocity of accreting gas, and flux in the Ly, OI and CIV emission lines, which are correlated with the UV excess luminosity. We also observe variability in the spectral type inferred from the UV spectral energy distribution, ranging from A5 IV-III in high state to A7 III in the low state. The trend of earlier inferred spectral type with decreasing wavelength and with increasing UV continuum flux has previously been noted as a signature of accretion disks in lower mass pre-main sequence stars (PMS) and in systems undergoing FU Orionis-type outbursts. Our data represent the first detection of similar phenomena in an intermediate mass (M 2M ) PMS star. Recent IUE spectra show gas accreting toward the star with velocities as high as +300 km s^–1, much as is seen toward Pic, and suggest that we also view this system through the debris disk. The absence of UV lines with the rotational broadening expected given the optical data (A7 IV,v sini = 180 ± 20 km s^–1) for this system also suggests that most of the UV light originates in the disk, even in the low continuum state. The dramatic variability in the column density of accreting gas, consistent with clumpy accretion, such as has been observed toward Pic, is a hallmark of accretion onto young stars, and is not restricted to the clearing phase, since detectable amounts of accretion are present for stars with 0.5 <t _age < 2.8 Myr. The implications for models of Pic and similar systems are briefly discussed.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Multicolour photoelectric photometry of the open cluster NGC 752

In the open cluster NGC 752, 89 stars are observed in the Vilnius seven-colour photometric system, including probable members fainter than observed hitherto. Spectral classification of observed stars are made and individual reddening values are determined. The mean reddening of the cluster stars is equivalentE(B - V) = 0 _. ^m 025. By fitting on colour index-magnitude diagrams the cluster Main Sequence with the Hyades one the distance modulus (m - M)₀ = 8 _. ^m 15 ± 0 _. ^m 15 is found. For 11 red giant members of the cluster absolute magnitudes are derived and compared with ones of the standard calibration. It is obtained that the cluster giants are in mean 0 _. ^m 6 fainter than giants of corresponding spectral class in the solar vicinity.