Pulsations and long-term photometric variability of three candidates for protoplanetary nebulae

We present new photometric data and analyze long-term UBV observations of three candidates for protoplanetary nebulae—F supergiants with infrared excesses at high Galactic latitudes—IRAS 18095+2704, IRAS 19386+0155, and IRAS 19500-1709. All these stars exhibit quasi-periodic low-amplitude variations caused by pulsations against the background of long-term brightness trends. For IRAS 18095+2704=V887 Her, we have found a pulsation period of 109 days and revealed a linear brightness trend—the star brightens at constant (within the limits of the measurement errors) yearly mean color indices. The light curve of IRAS 19386+0155=V1648 Aql in 2000–2008 is represented by a wave with a fundamental period of 102 days whose modulation with a close period of 98 days leads to variations with a variable amplitude. V1648 Aql also shows a systematic rise in V brightness along with a reddening. IRAS 19500–1709=V5112 Sgr exhibits irregular pulsations with periods of 39 and 47 days. The long-term variability component of V5112 Sgr may indicate that the star is binary.     

Spectral variability of LBV star V 532 (Romano’s star)

We present the results of studying the spectral and photometric variability of the luminous blue variable star V532 in M33. The photometric variations are traced from 1960 to 2010, spectral variations—from 1992 to 2009. The star has revealed an absolute maximum of visual brightness (1992–1994, high/cold state) and an absolute minimum (2007–2008, low/hot state) with a brightness difference of ΔB ≈ 2.3 ^m . The temperature estimates in the absolute maximum and absolute minimum were found to be T ∼ 22000 K and T ∼ 42000 K, respectively. The variability of the spectrum of V532 is fully consistent with the temperature variations in its photosphere, while both permitted and forbidden lines are formed in an extended stellar atmosphere. Broad components of the brightest lines were found, the broadening of these components is due to electron scattering in the wind parts closest to the photosphere. We measured the wind velocity as a difference between the emission and absorption peaks in the PCyg type profiles. The wind velocity clearly depends on the size of the stellar photosphere or on the visual brightness, when brightness declines, the wind velocity increases. In the absolute minimum a kinematic profile of the V532 atmosphere was detected. The wind velocity increases and its temperature declines with distance from the star. In the low/hot state, the spectral type of the star corresponds to WN8.5h, in the high/cold state—to WN11. We studied the evolution of V532 along with the evolution of AGCar and the massive WR binary HD5980 in SMC. During their visual minima, all the three stars perfectly fit with the WNL star sequence by Crowther and Smith (1997). However, when visual brightness increases, all the three stars form a separate sequence. It is possible that this reflects a new property of LBV stars, namely, in the high/cold states they do not pertain to the bona fide WNL stars.     

Photometric study of the FUor star V 1735 Cyg (Elias 1-12)

Results from optical photometric observations of the PMS star V 1735 Cyg are reported. The star is located in the IC 5146 dark cloud complex—a region of active star formation. On the basis of observed outburst and spectral properties, V 1735 Cyg was classified as a FUor object. We present data from BVRI CCD photometric observations of the star, collected from March 2003 to January 2009. Plates from the Rozhen Schmidt telescope archive were scanned for a brightness estimation of the star. A sequence of sixteen comparison stars in the field of V 1735 Cyg was calibrated in BVRI bands. The data from photographic observations made from 1986 to 1992 show a strong light variability (ΔV=1_⋅^m2). In contrast, the recent photometric data obtained from 2003 to 2009 show only small amplitude variations (ΔI=0_⋅^m3). The analysis of existing photometric data shows a very slow decrease in star brightness—1_⋅^m8 (R) for a 44 year period. The possibilities for future photometric investigations of V 1735 Cyg using the photographical plate archives is discussed briefly.     

Distribution of B stars in the directions of some southern cepheids

The distribution of B stars in the vicinity of nine southern, long-period (P > 11 days) Cepheids is studied on the basis of U, B, V data. The parameters Q, the normal color indices (B — V)₀ and (U — B)₀, the color excesses E(B — V), the spectral types, and the distance moduli of these stars are determined. Translated from Astrofizika, Vol. 40. No. 4, pp. 573–580, October-December, 1997.     

VLA observations of extreme hydrogen deficient stars

VLA observations at 2 and 6 cm have been obtained for six hydrogen-deficient stars R CrB, HD 160641, BD — 9°4395, V348 Sgr, MV Sgr and Sgrv Upper limits to the massloss rates have been estimated for some of these using the upper limits to the radio flux density. National Radio Astronomy Observatory’s Very Large Array is operated by Associated Universities Inc. under contract with National Science Foundation, USA     

Infrared photometry for three RV Tau variable stars

We have analyzed our long-term (1995–2008) JHKLM photometry for three variable RV Tau supergiants. It follows from this analysis that: the amplitude of the JHK brightness variations is approximately the same, 0 ^m . 9–1 ^m , for the three stars; a negative linear trend is noticeable in the variations of the mean J brightness for AC Her; and the mean J brightness of R Sct and V Vul remained constant. The observed J brightness fluctuations in the supergiants are shown to be consistent either only with the temperature pulsations (AC Her and V Vul) or with the temperature and radial pulsations (R Sct). We have detected long-term pulsations in the J brightness and J-H color variations of the supergiants with the following characteristic time scales: ∼1500 days (AC Her), ∼3500 days (R Sct), and ∼800 and ∼1500 days (V Vul). Our search for a periodic component in the infrared brightness variations of the three stars has revealed periods that differ from those obtained from optical observations. We show that relatively hot dust shells (T _d ∼ 700–800 K) exist around the supergiants under study; the densest and most massive shell was observed for V Vul. The ranges of optical-depth variations for the dust shells were τ(1.25 μm) ≈ 0.04–0.10 (AC Her), ≈0.02–0.12 (R Sct), and ≈0.18–0.32 (V Vul). The spectral types in the infrared varied over narrower ranges than those that followed from optical observations of these stars.     

A general catalogue of ephemerides and apparent orbits for visual binaries

Summary Informations on 736 pairs of visual binaries are given in the form of the Tables. The General Catalogue gives ephemerides (t,θ°,ρ″) in 20 years (1984–2003) for each pair with the figures of its apparent elliptical orbit where the positions of secondary component relative to the primary one at different epochs are indicated.The General Catalogue contains four parts: Part one — Source and Grading of Orbit; Part two — Ephemérides and Atlas of Apparent Orbits; Part three — Classifications of Visual Binary Stars; Part four — Indexes of Visual Binary Stars. The principle of calculation and the statistical data are presented in this paper. There are seven statistical tables, giving the elements distribution of true and apparent orbits, grading distribution of orbits, number distribution with different physical property of component of binary star. The number of binary stars in anyone constellation, the number of binary stars brighter than 6^m.5. The number of binary stars nearer than 25 parsec.     

Specificities of magnetic field structures in chemically peculiar stars

We continue our program of the study of large-scale structures of magnetic fields in chemically peculiar stars. In this paper we analyze eight stars, out of which three stars have the structure of a central dipole, three—the structure of a dipole shifted along the axis, and two—of a dipole shifted across the axis. High-precision measurements (with σ = 50 and 80 G) are available for two stars (HD62140 and HD71866, respectively). The model phase dependences agree with the measurements within the errors. This result shows that the hypothesis about the dipole structure of the magnetic fields of CP stars is well founded.     

Optical photometry of GM Cep: evidence for UXor type of variability

Results from optical photometric observations of the pre-main sequence star GM Cep are reported in the paper. The star is located in the field of the young open cluster Trumpler 37—a region of active star formation. GM Cep shows a large amplitude rapid variability interpreted as a possible outburst from EXor type in previous studies. Our data from BVRI CCD photometric observations of the star are collected from June 2008 to February 2011 in Rozhen observatory (Bulgaria) and Skinakas observatory (Crete, Greece). A sequence of sixteen comparison stars in the field of GM Cep was calibrated in the BVRI bands. Our photometric data for a 2.5 years period show a high amplitude variations ($\Delta V \sim2\mbox{$\Delta V \sim2\mbox{) and two deep minimums in brightness are observed. The analysis of collected multicolor photometric data shows the typical of UX Ori variables a color reversal during the minimums in brightness. On the other hand, high amplitude rapid variations in brightness typical for the Classical T Tauri stars also present on the light curve of GM Cep. Comparing our results with results published in the literature, we conclude that changes in brightness are caused by superposition of both: (1) magnetically channeled accretion from the circumstellar disk, and (2) occultation from circumstellar clouds of dust or from features of a circumstellar disk.     

Infrared study of infrared carbon stars based on 2MASS,IRAS and ISO SWS data

For this paper, we collected all infrared carbon stars (IRCSs) known so far from the literature and identified the 2MASS counterparts of all IRCSs. Using 2MASS, IRAS and ISO SWS data, we investigate the infrared properties of IRCSs. We find that the infrared colors and temperatures of IRCSs—not only in the IRAS region but also in the near infrared—are between those for visual carbon stars and extreme carbon stars. The results in this paper strongly support the suggestion that the sequence of visual carbon stars → infrared carbon stars → extreme carbon stars is the evolutionary sequence in the AGB phase for carbon-rich stars. In addition, using the ISO SWS data, we find that an evolutionary sequence also exists within the IRCS stage.     

Non-thermal emission processes in massive binaries

In this paper, I present a general discussion of several astrophysical processes likely to play a role in the production of non-thermal emission in massive stars, with emphasis on massive binaries. Even though the discussion will start in the radio domain where the non-thermal emission was first detected, the census of physical processes involved in the non-thermal emission from massive stars shows that many spectral domains are concerned, from the radio to the very high energies. First, the theoretical aspects of the non-thermal emission from early-type stars will be addressed. The main topics that will be discussed are respectively the physics of individual stellar winds and their interaction in binary systems, the acceleration of relativistic electrons, the magnetic field of massive stars, and finally the non-thermal emission processes relevant to the case of massive stars. Second, this general qualitative discussion will be followed by a more quantitative one, devoted to the most probable scenario where non-thermal radio emitters are massive binaries. I will show how several stellar, wind and orbital parameters can be combined in order to make some semi-quantitative predictions on the high-energy counterpart to the non-thermal emission detected in the radio domain. These theoretical considerations will be followed by a census of results obtained so far, and related to this topic. These results concern the radio, the visible, the X-ray and the γ-ray domains. Prospects for the very high energy γ-ray emission from massive stars will also be addressed. Two particularly interesting examples—one O-type and one Wolf-Rayet binary—will be considered in details. Finally, strategies for future developments in this field will be discussed.     

Asteroseismology: Past, present and future

Asteroseismology studies stars with a wide variety of interior and surface conditions. For two decades asteroseismic techniques have been applied to many pulsating stars across the HR diagram. Asteroseismology is now a booming field of research with stunning new discoveries; I highlight a personal selection of these in this review, many of which are discussed in more detail elsewhere in these proceedings. For many years the Nainital-Cape Survey for northern roAp stars has been running at ARIES, so I emphasise new spectroscopic results for roAp stars and point out the outstanding prospects for the planned ARIES 3-m telescope at Devastai. High precision spectroscopy has revolutionised the asteroseismic study of some types of stars — particularly solar-like oscillators and roAp stars — while photometry is still the best way to study the frequency spectra that are the basic data of asteroseismology. New telescopes, new photometers and space missions are revolutionising asteroseismic photometry. In addition to the ground-based potential of asteroseismic spectroscopy, India has the knowledge and capability for space-based asteroseismic photometry. The future for asteroseismology is bright indeed, especially for Indian astronomers.     

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%.     

Investigation of flare stars in the pleiades by the method of stellar tracks. I

Observations of the Pleiades cluster by the method of stellar tracks, carried out on the 40-inch Schmidt telescope of Byurakan Astrophysical Observatory, have resulted in the detection of 49 flares from 38 stars, 17 of which were not previously known to be flare stars. It is shown that for bright stars (U ≤ 16.0) the detection of flares in observations by the method of stellar tracks is at least three times more efficient than for observations by the method of stellar chains. Another advantage of the first method is that one can detect brief flares that last less than 6 min. The visual stellar magnitude at the minimum for the brightest of the flare stars that we found is 11.92. This raised the upper luminosity limit of known flare stars in the Pleiades by 0.21 magnitude. A comparison of the expected number of bright flare stars in the Pleiades with the number of all bright members of the cluster (falling in the range from V ≈ 12.0 toV ≈16.0) suggested that all these stars evidently must be flare stars. Translated from Astrofizika, Vol. 42, No. 3, pp. 351–358, July–September, 1999.     

Key problems in cool-star astrophysics

Selected key problems in cool-star astrophysics are reviewed, with emphasis on the importance of new ultraviolet missions to tackle the unresolved issues.UV spectral signatures are an essential probe of critical physical processes related to the production and transport of magnetic energy in astrophysical plasmas ranging, for example, from stellar coronae, to the magnetospheres of magnetars, and the accretion disks of protostars and Active Galactic Nuclei. From an historical point of view, our comprehension of such processes has been closely tied to our understanding of solar/stellar magnetic activity, which has its origins in a poorly understood convection-powered internal magnetic dynamo. The evolution of the Sun's dynamo, and associated magnetic activity, affected the development of planetary atmospheres in the early solar system, and the conditions in which life arose on the primitive Earth. The gradual fading of magnetic activity as the Sun grows old likewise will have profound consequences for the future heliospheric environment. Beyond the Sun, the magnetic activity of stars can influence their close-in companions, and vice versa.Cool star outer atmospheres thus represent an important laboratory in which magnetic activity phenomena can be studied under a wide variety of conditions, allowing us to gain insight into the fundamental processes involved. The UV range is especially useful for such studies because it contains powerful diagnostics extending from warm (∼ 10⁴ K) chromospheres out to hot (1–10 MK) coronae, and very high-resolution spectroscopy in the UV has been demonstrated by the GHRS and STIS instruments on HST but has not yet been demonstrated in the higher energy EUV and X-ray bands. A recent example is the use of the hydrogen Lyα resonance line—at 110 000 resolution with HST STIS—study, for the first time, coronal winds from cool stars through their interaction with the interstellar gas. These winds cannot be detected from the ground, for lack of suitable diagnostics; or in the X-rays, because the outflowing gas is too thin.A 2m class UV space telescope with high resolution spectroscopy and monitoring capabilities would enable important new discoveries in cool-star astronomy among the stars of the solar neighborhood out to about 150 pc. A larger aperture facility (4–6 m) would reach beyond the 150 pc horizon to fainter objects including young brown dwarfs and pre-main sequence stars in star-forming regions like Orion, and magnetic active stars in distant clusters beyond the Pleiades and α Persei. This would be essential, as well, to characterize the outer atmospheres of stars with planets, that will be discovered by future space missions like COROT, Kepler, and Darwin.Deceased October 23, 2005     

Kyiv meridian axial circle with a new CCD camera

Specifications of a new Apogee Alta U47 CCD camera mounted at the Kyiv meridian axial circle (MAC) are presented. The camera is based on the 1024 × 1024 pxl e2v CCD47-10 with pixel sizes of 13 × 13 μm. Observations are carried out in the scan-drift mode with an effective exposure time of 77 s for equatorial stars. The MAC photometric system answers the standard Johnson V band, the MAC limiting magnitude V is 17 ^m . The test MAC observations of 2009 give positional accuracy and V magnitude errors equal to approximately 0.1″ and 0.09 ^m , respectively, for Tycho-2 stars. The telescope is used for observations of equatorial stars with the purpose of detecting their positions, proper motion, and brightness.     

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.     

Spectrophotometric studies of CQ Cephei

Spectrophotometric and spectroscopic observations of CQ Cep — the shortest-period binary with WN component — are presented. Excepting the NV λ4603, the fluxes of all other emission lines show enhancement at minima. They can be explained by the Roche surfaces that take into account the strong wind of the WN7 component. Various radial velocity curves for emission and absorption give different orbital solutions with a general positive shift of λ axes. Although N IV λ4058 represents the true motion of the WN7 component, its flux variations are influenced by geometric effects. There is no signature of the companion. The extent of the atmosphere of CQ Cephei appears larger than in V444Cyg, another eclipsing binary with a Wolf-Rayet component.     

Intrinsic properties of carbon stars. II. Spectra,colours, and HR diagram of cool carbon stars

On the basis of the effective temperature scale proposed previously for cool carbon stars (Paper I), other intrinsic properties of them are examined in detail. It is shown that the major spectroscopic properties of cool carbon stars, including those of molecular bands due to polyatomic species (SiC₂, HCN, C₂H₂ etc.), can most consistently be understood on the basis of our new effective temperature scale and the theoretical prediction of chemical equilibrium. Various photometric indices of cool carbon stars also appear to be well correlated with the new effective temperatures. Furthermore, as effective temperatures of some 30 carbon stars are now obtained, the calibration of any photometric index is straightforward, and some examples of such a calibration are given. In general, colour index-effective temperature calibrations for carbon stars are quite different from those for K-M giant stars. It is found that the intrinsic (R —I)₀ colour is nearly the same for N-irregular variables in spite of a considerable spread in effective temperatures, and this fact is used to estimate the interstellar reddening of carbon stars. An observational HR diagram of red giant stars, including carbon stars as well as K-M giant stars, is obtained on the basis of our colour index-effective temperature calibrations and the best estimations of luminosities. It is shown that carbon stars and M giant stars are sharply divided in the HR diagram by a nearly vertical line at aboutT _eff = 3200 K (logT _eff = 3.50) and the carbon stars occupy the upper right region of M giant stars (except for some high luminosity, high temperature J-type stars in the Magellanic Clouds; also Mira variables are not considered). Such an observational HR diagram of red giant stars shows rather a poor agreement with the current stellar evolution models. Especially, a more efficient mixing process in red giant stars, as compared with those ever proposed, is required to explain the formation of carbon stars.     

Spectroscopy of B and Be stars in the diffuse stellar clusters NGC 6871 and NGC 6913

This is a study of the population of B and Be stars in the young, relatively poor, diffuse stellar clusters NGC 6871 and NGC 6913. High resolution spectra are used to study the Hα line of eleven stars in order to detect emission. Emission profiles were found for three stars in the cluster NGC 6871; one of these is a known WR-star and the Be-star BD +35°3956 demonstrates the transition from the B to the Be phase. Spectra of seven of the B stars revealed no traces of emission in the Hα line. During the time of our observations, the Be star V1322 Cyg in the cluster NGC 6913 had a strong emission Hα line profile with substantial variability in intensity and equivalent width. Moderate resolution spectra of seven stars in the cluster NGC 6871 over wavelengths of 4420-4960 ? and ten stars in the cluster NGC 6913 over wavelengths of 4050-5100 ? are used to classify the series of B and Be stars spectrally and to estimate their T _eff and log g. It was found that three of the stars are not members of the clusters NGC 6871 or NGC 6913. Translated from Astrofizika, Vol. 52, No. 2, pp. 257–274 (May 2009).