Microvariability and fast variability of stars. I. standard stars

This is a study of observations of the photometric quantities Vt and Bt, reported in the Hipparcos catalog, for 15 standard stars in order to search for microvariability and rapid variability in their emission. The microvariability is found to be characterized mainly by smooth fluctuations in the brightness. Changes in the magnitudes of the stars HD 28355 and HD 130109 with a period of ~150 min and an amplitude <0^m.02 appear to be caused by vertical shifts in their photospheres. Changes in the magnitudes with an amplitude of ~0^m.01 and a period of 11.4^d in the star e Eri were related to the rotation of spots. A similar microvariability period and amplitude in the star 51 Peg most likely originates in the influence of a planet.     

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)     

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)     

HD 52721 - A close binary system among the Ae/Be Herbig stars

Photometric and high resolution spectroscopic data for the Herbig star HD 52721 obtained from October 2009 through October 2010 are presented. We confirm the cyclical character of the photometric variability of HD 52721 discovered previously by others. The variability shows up as minima in the light curve of this star which repeat with a period P = 0^d.805 and are typical of eclipsing binary systems. ASAS data covering an observation period from September 2003 through December 2009 show that two neighboring photometric minima differ from one another, and the actual orbital period of the system must be twice as high (P = 1^d.610). This is confirmed by the spectroscopic data. These show that the center of gravity of the emission Hα line and the profile of the HeI 6678 line with a distinct, bright circumstellar component have a clear correlation with a period of 1^d.610. To all appearances, HD 52721 is a close binary system consisting of two class B2 stars with quite similar parameters. We propose that the observed cyclical variations in the circumstellar parameters may be related to the existence of a global azimuthal inhomogeneity within the shell which rotates synchronously with the orbital motion of the system components.     

Lichtelektrische Untersuchungen der vier Ap-Sterne HD 9531, HD 10221, HD 27309 und HD 184905

The lightcurves in the UBV system are given for the four Ap stars HD 9531, HD 10221, HD 27309 and HD 184905 from 1968 to 1971. The measurements are represented by the periods o^d.6684, 3^d.16, 2^d.7098 and 1^d.845031 respectively.     

Search for variability in hydrogen-poor stars

The evolutionary status of hydrogen-poor stars is analyzed.Photoelectric observations of six objects are reported as a first step of a long-term project devoted to search for variability of a large sample of hydrogen-poor stars.The observed stars show phenomena of microvariability with an amplitude of the order of 0^m. 1 or less.Two extreme helium stars have been examined: a period in the range of 0^d.162-0^d.164 has been found for BD+ 10⁰2179, andP=0^d.1079962 for BD+13⁰3324.The mass-losing O subdwarf (sdO) BD+37⁰ 443 presents short-term fluctuations with a time-scale of several minutes and long-term variations on a scale of months. The standard sdO BD+75⁰325 is probably non-variable, although light variations of very small amplitude (m0.03) with a time-scale of about 1 hr might be present. The high gravity sdO BD+25⁰4655, which is very close to the white dwarf stage, also presents variability on a time-scale of about 13 minutes, and might be an analog of the recently discovered pulsating sdO, or hot white dwarf, PG 1159-035.The variability of the intermediate helium star HD 37776 is finally confirmed.     

Magnetic field of CP stars in the Ori OB1 association. II. HD36540, HD36668, HD36916, HD37058

We present the results of magnetic field measurements of four chemically peculiar (CP) stars with helium abundance anomalies which are the members of the Orion stellar association OB1. The stars under study were classified as magnetic by other authors earlier. The present paper contains the results of the extensive study of the stars. Magnetic field measurements allowed us to conclude that HD36540 has a weak field and the longitudinal component B _e does not exceed 500 G. The longitudinal field of HD36668 varies with the period P = 2_. ^d 11884 and the amplitude from ?2 to +2 kG. The magnetic field of HD36916 has mainly negative polarity and varies within the range from 0 to ?1 kG with the period P = 1.d 565238. HD37058 is a magnetic star, the longitudinal field of which varies from ?1.2 to +0.8 kG with the period P = 14_. ^d 659. The B _e field variability pattern for the stars HD36916 and HD37058 is of a simple harmonic type. The longitudinal field of HD36668 is best described with two combined harmonic functions (“a doublewave”). The variability period of HD36540 is still undetermined. For all the stars from this paper, we measured radial velocities V _r, axial rotation rates v _e sin i, and determined basic parameters of atmospheres (effective temperatures T _eff and gravity acceleration log g). We also estimated masses M, luminosities L, and radii R of the stars.     

HD 115781 and HD 116204—two RS CVn binaries

HD 115781 and HD 116204 (BL CVn and BM CVn) are shown to be RS CVn binaries with periods near 20 days. HD 115781 is double-lined; the primary type is about K1III, while the secondary is probably a late-type subgiant. The masses of the two components are equal within observational error. There is substantial photometric variability with a period half the orbital period; it is attributed to ellipsoidal variation. HD 116204 is also of type K1III. It shows exceptionally strong Ca II H and K emission, together with an emission-line spectrum typical of RS CVn stars in theIUE ultraviolet region, but Hα is an absorption line. The secondary star in the HD 116204 system has not been detected. The primary shows photometric variations, presumably due to starspots, with a period 5 per cent longer than the orbital period.     

Search for and study of photometric variability in magnetic white dwarfs

We report the results of photometric observations of a number of magnetic white dwarfs in order to search for photometric variability in these stars. These V-band observations revealed significant variability in the classical highly magnetized white dwarf GRW+70?8247 with a likely period from several days to several dozen days and a half-amplitude of about 0_. ^m 04. Our observations also revealed the variability of the well-known white dwarf GD229. The half amplitude of its photometric variability is equal to about 0_. ^m 005, and the likely period of this degenerate star lies in the 10–20 day interval. This variability is most likely due to the rotation of the stars considered.We also discuss the peculiarities of the photometric variability in a number of other white dwarfs. We present the updated “magnetic field–rotation period” diagram for the white dwarfs.     

New photometric investigation of the Herbig Ae/Be star HD 52721, a close binary system: Evidence for the existence of large-scale azimuthal inhomogeneities

We present new results of our photometry for the Herbig Be star HD 52721 obtained from January 16 to March 25, 2013. A new data reduction technique is used. Using this technique, we have also reanalyzed the previous results of our photometry for this object pertaining to the period from March 7 to March 28, 2010. The Be star HD 52721 is known as an eclipsing variable with the period P = 1^d. 610. Two photometric minima observed during one period are a peculiarity of its photometric variability. They are separated in phase of the period P by 0.5 and differ from one another in depth by 0 ^m . 04. We have also detected additional minima observed at the phases of maximum brightness. We hypothesize that they can be associated with the existence of local azimuthal inhomogeneities rotating synchronously with the orbital motion of the binary component stars in the circumstellar envelope. When processing our CCD frames, we have applied an efficient CCD-frame rejection method that has allowed the accuracy of observations to be increased considerably. The CCD frames have been further processed using the Apex II software package, which is a universal software platform for astronomical image processing. We justify the need for additional photometric observations of HD 52721 in various color bands to confirm the hypothesis about the existence of azimuthal inhomogeneities in the program binary system and to analyze their physical properties.     

The pole of (51) Nemausa

The complex lightcurves make (51) Nemausa a good case for the study of general methods for pole determination. From six lightcurves the pole is determined to 20^h24^m; +53° (1950); the rotation is retrograde with period 7^h.782936 ± 0^h.000005. Presence of nongeometric scattering is proved by a significant 0.008 mag amplitude. Formulae and photometric elements are given for predictions of the shapes of lightcurves in future oppositions. The precision of the Fourier coefficients may be reduced below the present ±0.003 mag level by avoiding the systematic errors in the observations due to phase factor variations and discontinuities when changing comparison stars.     

Discovery of magnetic fields in three He variable Bp stars with He and Si spots

It is essential for the understanding of stellar structure models of high mass stars to explain why constant stars, nonpulsating chemically peculiar hot Bp stars and pulsating stars co‐exist in the slowly pulsating B stars and β Cephei instability strips. We have conducted a search for magnetic fields in the four Bp stars HD55522, HD105382, HD131120, and HD138769 which previously have been wrongly identified as slowly pulsating B stars. A recent study of these stars using the Doppler Imaging technique revealed that the elements He and Si are inhomogeneously distributed on the stellar surface, causing the periodic variability. Using FORS 1 in spectropolarimetric mode at the VLT, we have acquired circular polarisation spectra to test the presence of a magnetic field in these stars. A variable magnetic field is clearly detected in HD55522 and HD105382, but no evidence for the existence of a magnetic field was found in HD131120. The presence of a magnetic field in HD138769 is suggested by one measurement at 3σ level. We discuss the occurrence of magnetic B stars among the confirmed pulsating B stars and find strong magnetic fields of order kG and oscillations to be mutually exclusive. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Butterfly diagrams of strongly spotted late type stars

At present, long-term (over 30 years) multicolor photometric observations give the possibility to determine general properties of spotted areas on late-type stars. Star-spot modeling from broadband photometric data has been carried out by Alekseev and Gershberg since 1996 under the assumption that spots are situated in two latitudinal zones. Here we propose a new analysis of their results for several G and K dwarf stars with high irregular activity. On these stars, EK Dra, VY Ari, V775 Her, and V833 Tau, two spot belts exist separately and do not merge into a single equatorial active region, as occurs on cooler red-dwarf stars. The zonal spottedness models allow us to fit simultaneously both rotational modulation and long-term variability of stellar brightness. These models give evidence for an equatorward drift of the lower latitude boundary of the spotted region, φ₀, during the rising phase of activity, beyond any possible errors concerned with our methodology. In order to evaluate the drift rate we introduce the concept of `effective' spot belt, whose width is independent of longitude. This permits us to construct butterfly diagrams for stellar spots. The equatorward drift rates of the lower boundary of the spotted region D=dφ_low/dt are (− 1)–(− 2) deg year⁻¹ in the years of increasing spottedness. These values are less than the analogous solar one D≈−4 deg year⁻¹ for the rising phase of the cycle. Thus, cyclic activity can be revealed from butterfly diagrams and derived drifts of starspots prior to a possible detection from the spectral analysis of photometric variability. Finally, we briefly discuss a possible explanation of high-latitude activity and surface drifts of starspots in the framework of the current state of dynamo theory.     

Apsidal motion in the eclipsing binary system of BW AQR

The first photoelectric light curve of the eclipsing binary system BW Aqr (F71V+F81V;P=6^d.7;V=10 ^m .31), discovered by Miss Leavitt at the beginning of the century, was obtained. The photometric elements were detemined. The components of this system are considerably evolved stars: the age of the system is about 2×10⁹ yr. It follows from the photometric data that the secondary component should have greater mass than the primary one The zero-age spectral classes of components were F2V and F1V. The system has an elliptical orbit with the eccentricitye=0.18. The angular rate of the apsidal motion (_obs = 0.070 deg yr^–1) and the corresponding value of the apsidal parameterk ₂=0.0090 (the relativistic term included) were found. The derived valuek ₂ exceeds by more than a factor of 2 the theoretical coefficient obtained from the modern theory of internal structure of stars with moderate masses .     

Stars outside the Hipparcos list closely encountering the Solar system

Based on currently available kinematic data, we have searched for stars outside the Hipparcos list that either closely encountered in the past or will encounter in the future the Solar system within several parsecs. For the first time, we have identified two single stars, GJ 3379 (G 099-049) and GJ 3323 (LHS 1723), as candidate for a close encounter with the solar orbit. The star GJ 3379 could encounter the Sunmore closely to aminimumdistance d _min = 1.32±0.03 pc at time t _min = −163 ± 3 thousand years. We have found two potential candidates for a close encounter that have only photometrical distances: the white dwarf SSSPM J1549-3544 without any data on its radial velocity and the L-dwarf SDSS J1416+1348. The probabilities of their penetration into the Oort cloud region are 0.09 (at a model radial velocity <V _r < = 50 km s⁻¹) and 0.05, respectively.     

Rotational Brightness Modulation and Starspots on the RS CVn-type Stars IN Com, IL Com, UX Ari, and V711 Tau

UBVRI photometric observations and models of spotting are presented for four noneclipsing RS CVn systems: IN Com (G5III/IV), IL Com (F8V+F8V), UX Ari (K01V+G5V), and V711 Tau (K1IV+G5V). A low amplitude variability caused by cold (T=1700K)spots which can occupy up to 19% of the star's surface is confirmed for the little-studied star IL Com. Long-term light curves are constructed and the stellar magnitudes and color indices of the unspotted photospheres are estimated for IN com, UX Ari, and V711 Tau. It is shown that UX Ari becomes bluer with decreasing brightness, so its variability cannot be fully explained in terms of cold spots. Models of spotting on In Com and V711 Tau are constructed from the full set of available photometric observations. The spots on both of these variables lie at middle latitudes and occupy up to 22% (In Com) and 33% (V711 Tau) of the stars' surfaces. Both stars manifest a tendency for the width of the spots to decrease as their area increases. This is a crude analog of the Maunder butterflies. These stars experience cyclical spot activity and have a differential rotation of the type found on the sun.     

Determination of two groups of high luminosity red variable stars

Based on analysis of photometric, colorimetric, and polarimetric data, we show that red variable stars of high luminosity are divided with respect to their period of brightness variation into two groups: I) binary stars with periods larger than 480 days; II) single stars with periods less than 480 days. The binary stars exhibit: a) large correlation coefficients between the V-band brightness variation and the U-B, B-V colors; b) comparatively smaller U-B colors (< 1. ^m 5);c)strong variability of polarization parameters. Based on this division, we suspect that the star PZ Cas is a binary.Translated from Astrofizika, Vol. 37, No. 2, pp. 235–244, April–June, 1994.In conclusion, the author wishes to express deep gratitude to professor L. V. Mirzoyan for valuable comments.     

HARPS spectropolarimetry of Herbig Ae/Be stars

Our knowledge of the presence and the strength of magnetic fields in intermediate‐mass pre‐main‐sequence stars remains very poor. We present new magnetic field measurements in six Herbig Ae/Be stars observed with HARPS in spectropolarimetric mode. We downloaded from the European Southern Observatory (ESO) archive the publically available HARPS spectra for six Herbig Ae/Be stars. Wavelength shifts between right‐ and left‐hand side circularly polarised spectra were interpreted in terms of a longitudinal magnetic field 〈B_z〉, using the moment technique introduced by Mathys. The application of the moment technique to the HARPS spectra allowed us in addition to study the presence of the crossover effect and quadratic magnetic fields. Our search for longitudinal magnetic fields resulted in first detections of weak magnetic fields in the Herbig Ae/Be stars HD 58647 and HD 98922. Further, we confirm the previous tentative detection of a weak magnetic field in HD 104237 by Donati et al. and confirm the previous detection of a magnetic field in the Herbig Ae star HD 190073. Surprisingly, the measured longitudinal magnetic field of HD 190073, 〈B_z〉 = 91 ± 18 G at a significance level of 5σ is not in agreement with the measurement results of Alecian et al. (2013), 〈B_z〉 = –10 ± 20 G, who applied the LSD method to exactly the same data. No crossover effect was detected for any star in the sample. Only for HD 98922 the crossover effect was found tobe close to 3σ with a measured value of –4228 ± 1443 km s^–1 G. A quadratic magnetic field of the order of 10 kG was detected in HD 98922, and of ∼3.5 kG in HD 104237. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic,chemically peculiar (CP2) stars in the SuperWASP survey

The magnetic chemically peculiar (CP2) stars of the upper main sequence are well‐suited for investigating the impact of magnetic fields on the surface layers of stars, which leads to abundance inhomogeneities (spots) resulting in photometric variability. The light changes are explained in terms of the oblique rotator model; the derived photometric periods thus correlate with the rotational periods of the stars. CP2 stars exhibiting this kind of variability are classified as α² Canum Venaticorum (ACV) variables. We have analysed around 3850000 individual photometric WASP measurements of magnetic chemically peculiar (CP2) stars and candidates selected from the catalogue of Ap, HgMn, and Am stars, with the ultimate goal of detecting new ACV variables. In total, we found 80 variables, from which 74 are reported here for the first time. The data allowed us to establish variability for 23 stars which had been reported as probably constant in the literature before. Light curve parameters were obtained for all stars by a least‐squares fit with the fundamental sine wave and its first harmonic. Because of the scarcity of Strömgren uvbyβ measurements and the lack of parallax measurements with an accuracy better than 20%, we are not able to give reliable astro‐physical parameters for the investigated objects. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

VR c photometry of dwarf cepheids (I)

In this paper, we present VRc photometric observations of four dwarf cepheids: YZ Boo (P = 0 _. ^d 104, V = 0 _. ^m 5), AD CMi (P = 0 _. ^d 123, V = 0 _. ^m 5), XX Cyg (P = 0 _. ^d 135, V = 0 _. ^m 5), EH Lib (P = 0 _. ^d 088, V = 0 _. ^m 7). The light curves were obtained at West Mountain Observatory, Provo, Utah on 14 nights from 1983 through 1986 and contain 589 data points in each of theV andR bands in the Cousin photometric system. A detailed study of these stars, based on the present light curves, will be published separately.