Polarimetric observations of 16 long-period variables and three red giants

We give the results of polarimetric observations of 16 long-period variable stars of Mira Ceti type and three irregular variable giants of late spectral classes. Four of the 19 program stars exhibited light polarization. Analysis of the observational data shows that the light polarization for these stars probably arises in the initial phase of the increase in brightness.Translated fromAstrofizika, Vol. 38, No. 3, 1995.     

Polarimetric and photometric observations of long-period variables

Observations of 35 long-period variables have been done mainly at the Byurakan Astrophysical Observatory during the last years. A polarimetric monitoring of 34 stars has been carried out with simultaneous photometric measurements for some of them. Photometric data in the UBVR bands have been obtained for the star Y Ori, which is the faintest one. The results show that the degree of light polarization is correlated with the period and brightness of these stars. This indicates that the degree of light polarization is connected with the mass loss as well. Finally the photoelectric observations of the stars Y Ori, T Cep, R Gem, and R Boo show a short-term increase of their brightness in the decreasing branch of their light curves. The measured (U-B) or (B-V) colors of detected rapid variations are bluer than the colors of the star measured before these events. It is supposed that the change of polarization with time, the relatively blue colors of miras near their minimum brightness, and the colors of detected rapid variations may be explained by the influence of a invisible blue companion, as in the case of the star R Aqr. Published in Astrofizika, Vol. 42, No. 3, pp. 341–350, July–September, 1999.     

Observations of variable stars with the R Coronae Borealis phenomenon and other unique objects

The results of photometry and polarimetry of the R Coronae Borealis type stars and other interesting objects are given. The observation of the former objects are obtained at the light maximum or at a brightness lower by 2-3 mag. The polarization of R CrB stars at light maximum has interstellar origin. The absolute magnitude of V 854 Cen is estimated to Ms_vw = −3^m, and for Y Mus it is M_v > −3^m.7. ρ Cas has a variable polarization and is probably a giant (M_v ≅ 0^m) rather than a supergiant. Many early stars in its surroundings are photometrically and polarimetrically variable. The protoplanetary star BD −0°3679 has a polarization with the Rayleigh component.     

The UBVRI photometry and polarimetry of the young binary system V628 Cas

The results of multicolor photopolarimetric observations of the Ae/Be Herbig star V628 Cas are presented. It has been shown that the observed changes of the linear polarization correlate with the phase of the orbital period. The system demonstrated changes in brightness, the amplitude of which decreases from 0.4 to 0.1m when moving from the U band to the I band. No dependence of the polarization degree on the brightness is observed.     

Secular variations of the photometric parameters of Mira Ceti variables and semiregular variables

The photometric properties of long-period variable stars and problems related to their traditional classification are analyzed. A general review of secular variations of such light-curve parameters studied in our previous papers as the period, amplitude, and mean brightness is given. Several types of Mira Ceti stars that differ in the secular period variation pattern are identified. A connection between the secular amplitude variations of certain variables with the effect of multiperiodicity is revealed. It is shown that the pattern of these variations and their numerical properties may be used to classify these variable stars.     

Polarimetric observations of pulsating variable stars

Results are presented for polarimetric observations of 17 red giants and supergiants, of which nine are long-period Mira variables, five are semiregular variables (SR), and three are slowly fluctuating variables (Lb). Light polarization is detected for eight stars, seven of them for the first time.Translated fromAstrofizika, Vol. 39, No. 3, pp. 385–391, July–September, 1996.     

A flare in the mira X oph

Before the observation of the 1974 U Ori eruption, it was considered that the Mira stars had only some regular OH variations. With this eruption, we realized that sometimes flares can occur in this type of star. In the course of an OH Mira star monitoring programme with the Nançay radio telescope, we have discovered a new eruptive type of OH maser emission in several sources. Especially, in early 1992, we observed a quickly rising 1665 Mhz emission in the Mira X Oph. The main characteristics of this flare were: large flux variations independent of the light curve; large degree of circular polarization; radial velocity emission close to the stellar velocity.     

Classification of the light curves of Mira variables

Based on an analysis of light curves of 223 long-period variables of the Mira Ceti type, recorded using the HIPPARCOS space telescope, it is shown that all the light curves of these stars can be divided by outward form into two groups: stars exhibiting simple light curves of sinusoidal shape and stars with complicated light curves, with hump-shaped formations on the ascending branch of the curve. Some observational parameters of the investigated stars display a tendency to separation into groups of stars with complicated and simple light curves. Stars with complicated light curves have longer periods, they are brighter in absolute bolometric magnitude, and there is a greater probability of detecting polarization in their light. Translated from Astrofizika, Vol. 42, No. 4, pp. 541–554, October–December, 1999.     

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.     

A photometric and polarimetric study of the moon's surface

From the results of photometric and polarimetric observations in three colors of small areas of the Moon's illuminated surface, taking stars as the photometric standards, it is concluded that the Moon's surface brightness often fluctuates from the luminosity calculated by Hapke's formula. It seems that the Moon's light is composed of a part which is completely ruled by Hapke's law and has a uniform polarization degree over the entire illuminated surface, and a part which is quite nonpolarized. The brightness fluctuation is due to the variation of the latter part. The enhancement of the brightness is more remarkable in red color than in yellow and blue colors.     

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.     

Polarimetric and Photometric Observations of the Star CH Cygni

The results of photometric and polarimetric observations of the star CH Cygni in the B, V, and R filters are presented. Variations of brightness and color indices are given as functions of the time of observation. A fairly irregular character was found, both in the brightness variations and in the variations of color indices. The polarization observations showed that the light of CH Cygni is intrinsically polarized.     

New M-Type Variables in Galactic Dark Clouds

The results of the spectral classification of newly discovered M-type variables obtained as part of an H-alpha survey of Galactic dark cloud regions are presented. The survey was carried out with the 40 Schmidt camera of Byurakan Observatory, and the spectra of the variable stars for the classification were obtained with the Observatory's 2.6-m telescope. The observational material allowed register of late M-type variables as well as their brightness variations. Among the 97 newly discovered M-type stars, 22 show brightness variations. The results of the spectral classification of these new variables show that 21 out of 22 are red giants, and in all probability they are Mira Ceti type long period variables.     

Astronomical Searches for Nonbarionic Dark Matter

Astronomical methods of searching for light Goldstone bosons (axions and arions), which are candidates for dark matter, are briefly discussed. Three processes for the coupling between axions and photons are considered: (a) the decay of axions into two photons; (b) the conversion of photons into axions in the magnetic fields of stars and interstellar and intergalactic media; (c) the inverse transformation of axions generated inside the cores of stars. The intergalactic light of clusters of galaxies and the brightness of the night sky are good candidates for searches for an emission line arising from the radiative decay of axions. The results of observations made with the 6 m telescope of the Special Astrophysical Observatory do not provide any evidence for the existence of the hadronic axion decay line. The best upper limit for the photon-axion coupling constant is derived from linear polarization observations of magnetic Ap stars.     

Appearance of a nebula near Y Ori

The results of photographic and photoelectric observations for the star Y Ori are presented. During these observations a red nebulosity appeared around the star Y Ori near the maximum of brightness. The optical sizes of the nebulosity are comparable with the OH sizes of long period variables and OH/IR stars. A real brightness increase on the decreasing branch of the light curve was detected in U spectral band.Published in Astrofizika, Vol. 38, No. 4, pp. 555–558, October–December, 1995.     

<Emphasis Type="Italic">JHKLM</Emphasis> photometry for carbon stars

We discuss our JHKLM photometry for nine carbon Mira stars, eighteen carbon semiregular variables, and two oxygen Mira stars. For fourteen carbon stars, we present and analyze their infrared light and color curves. For all of the observed objects, we have estimated the optical depths of the circumstellar dust envelopes, the angular diameters of the stars, and their temperatures.     

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.     

Period update of 40 Mira stars

Current periods and new epochs are presented for 40 very poorly studied Mira variables. The average periods of these stars were also determined using earlier epochs found in the literature. The current period differs significantly from the average period value in 4 cases. The importance of the period variation study of Mira stars is also briefly discussed.     

Testing the bivariate series of Mira light curve rise and fall times for changes

The American Association of Variable Star Observers supplies the astronomical community with a large data base of times of light maxima and minima of Mira (long-period pulsating) stars. Period change studies using these data invariably use either times between maxima, or those between minima. A statistical analysis based on the two-component time series of light curve rise and fall times is developed. The results, which enable one to detect changes in the shapes of light curves, are applied to observations of seven long-period variables.     

Automated classification of variable stars for All-Sky Automated Survey 1–2 data

With the advent of surveys generating multi-epoch photometry and the discovery of large numbers of variable stars, the classification of these stars has to be automatic. We have developed such a classification procedure for about 1700 stars from the variable star catalogue of the All-Sky Automated Survey 1–2 (ASAS 1–2) by selecting the periodic stars and by applying an unsupervised Bayesian classifier using parameters obtained through a Fourier decomposition of the light curve. For irregular light curves we used the period and moments of the magnitude distribution for the classification. In the case of ASAS 1–2, 83 per cent of variable objects are red giants. A general relation between the period and amplitude is found for a large fraction of those stars. The selection led to 302 periodic and 1429 semiperiodic stars, which are classified in six major groups: eclipsing binaries, 'sinusoidal curves', Cepheids, small amplitude red variables, SR and Mira stars. The type classification error level is estimated to be about 7 per cent.