Variability of long-period pulsating stars. III. Changes in the parameters of humps at the ascending branch

The properties of such phenomena as the appearance of humps at the ascending branch or double peaks in the light curve of long-period pulsating stars are examined. The characteristics of humps and the average light curves are obtained for a group of Mira-type stars and semiregular (SRa) variable stars. Correlations between the parameters of the light curves and the characteristics of the humps for these stars are discussed. The results obtained here are compared with published theoretical models. __________ Translated from Astrofizika, Vol. 50, No. 1, pp. 99–105 (February 2007).     

Fast optical variability of SS 433

We study the variability of the optical flux from the peculiar Galactic source SS 433 based on observations with the Russian-Turkish 1.5-m telescope. We describe in detail the technique of highquality photometric measurements with a time resolution of 0.3–1 s using an ordinary CCD. Through test observations of nonvariable stars, we show that atmospheric turbulence introduces no significant distortions into the light curves. Therefore, such data are well suited for studying the aperiodic variability of various objects.     

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.     

On the possibility of systematizing classical novae by light curve type. I. Type criteria

Nova light curves, plotted on “outburst amplitude-logarithm of main shell radius” axes, have differences that can be systematized. As a result of a survey of over 90 novae, nova groups that are typical in light curve shape have been distinguished: DQ Her, CP Pup, RR Pic, PU Vul, V1974 Cyg, CP Lac, and GQ Mus. The light curve criterion is supplemented by other characteristics. The DQ Her group includes only novae having dust formation in the ejected shell. The GQ Mus group combines certain novae with smooth light curves that are separated into several subgroups with respect to outburst amplitude and the duration of the light curve section before the transitional stage, but for which the linear section of the light curve has the same slope. Translated from Astrofizika, Vol. 42, No. 1, pp. 61–74, January–March, 1999.     

Infrared Study of T Tauri Stars Based on IRAS and 2MASS Observations

The IRAS and 2MASS associations for 193 T Tauri stars are identified in this paper. From the color–color diagrams and spectral index, it is found that the IR excesses for most samples are due to thermal emission from the circumstellar material, as suggested previously. It is also found that the IR excesses at IRAS region for few T Tauri stars and the near-IR excesses for some T Tauri stars are likely attributed to free-free emission or free-bound emission from the circumstellar ionized gas. Moreover, It is found in deredened J–H versus H–K color–color diagram that there is a slight separation in different spectral groups. The T Tauri stars locus equation in J–H versus H–K color–color diagram for our sample is also presented.     

UV Piscium and the Radiative Flux Scale

Studies of eclipsing binaries with Hipparcos parallaxes are found to define a radiative flux relation for main sequence stars in the spectral range from B6 – F0which is about as well as to the stars derived from angular diameters. At lower temperatures the fluxes of the components fall below this curve which is caused due to the large intrinsic variations and starspots. From the present analysis it is found that the secondary component of UV Piscium, a late type eclipsing binary, is a normal K3V star and it fits the radiative flux-colour relation quite satisfactorily. This is explained due to the reliable values of the fluxes and colours derived from the clean light curves (light curves obtained after removing the effect of the distortion wave). However, the primary component which is also responsible for the intrinsic variations and starspots continue to deviate from this curve. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variability of long-period pulsating stars. II. Additional parameters for classifying stars

Results are presented from an analysis of observations in a sample of 53 long-period pulsating stars (44 of which are Mira-type and 9 are semiregular variables). A catalog of the characteristics of the individual pulsation cycles of the light curves is obtained for these stars. A correlation analysis of the characteristics of the individual pulsation cycles is carried out for each star. A number if diagrams relating the variability of the characteristics of the individual cycles to the average parameters of the variability of the stars and to their spectral classes. New criteria for classifying long-period variables are proposed on the basis of these diagrams. __________ Translated from Astrofizika, Vol. 49, No. 4, pp. 595–612 (November 2006).     

Bimodal brightness oscillations in models of young binary systems

We consider a model for the cyclic activity of young binary stars that accrete matter from the remnants of a protostellar cloud. If the orbit of such a binary system is inclined at a small angle to the line of sight, then the streams of matter and the density waves excited in the circumbinary disk can screen the primary component of the binary from the observer. To study these phenomena by the SPH (smoothed particle hydrodynamics) method, we have computed grids of hydrodynamic models for binary systems based on which we have constructed the light curves as a function of the orbital phase. The main emphasis is on investigating the properties of the brightness oscillations. Therefore, the model parameters were varied within the following ranges: the component mass ratio q = M ₂: M ₁ = 0.2–0.5 and the eccentricity = 0–0.7. The parameter that defined the binary viscosity was also varied. We adopted optical grain characteristics typical of circumstellar dust. Our computations have shown that bimodal oscillations are excited in binaries with eccentric orbits, provided that the binary components do not differ too much in mass. In this case, the ratios of the periods and amplitudes of the bimodal oscillations and their shape depend strongly on the inclination of the binary plane and its orientation relative to the observer. Our analysis shows that the computed light curves can be used in interpreting the cyclic activity of UX Ori stars.     

Brightness oscillations in models of young binary systems with low-mass secondary components

We consider a model for the cyclic brightness variations of a young star with a low-mass companion that accretes matter from the remnants of a protostellar cloud. At small inclinations of the binary orbit to the line of sight, the streams of matter and the density waves excited in the circumbinary disk can screen the primary component of the binary from the observer. To study these phenomena, we have computed grids of hydrodynamic models for binary systems by the SPH method based on which we have calculated the phase light curves for the different orientations of the orbit. The model parameters were varied within the following ranges: the component mass ratio q = 0.01–0.1 and the eccentricity e = 0–0.5. We adopted optical grain characteristics typical of circumstellar dust. Our computations have shown that the brightness oscillations with orbital phase can have a complex structure. The amplitudes and shapes of the light curves depend strongly on the inclination of the binary orbit and its orientation relative to the observer and on the accretion rate. The results of our computations are used to analyze the cyclic activity of UX Ori stars.     

Large-scale photometric activity of UX Ori type stars

Radical readjustments in the photometric activity of young stars owing to sudden changes in the circumstellar extinction are discussed using the light curves of two UX Ori type stars, CQ Tau and V1184 Tau, as examples. Changes of this sort can be caused by large deviations from axial symmetry in the distribution of circumstellar dust, as well as by large variations in the mass accretion rate in circumstellar disks. A large amount of dust may also appear in the vicinity of a young star owing to collisions of planetesimals. __________ Translated from Astrofizika, Vol. 51, No. 1, pp. 5–13 (February 2008).     

X-ray nova candidates among old classical novae

A review of light curves of known x-ray novae made it possible to identify criteria by which x-ray nova candidates were selected among old novae: amplitude of optical outburst 7–10 ^m, shape of light curve during the outburst with a temporary fading by 2–3^m lasting up to four days and an abrupt final fading from the 6^m level (relative to the quiet state). We identified LS And, AL Com, V592 Her, and HV Vir as x-ray nova candidates. Recurrent outbursts should be expected for the first and third stars. Less reliable candidates are V341 Nor, V787 Sgr, and V719 Sco. A possible recurrent nova candidate may be V1330 Cyg. Translated from Astrofizika, Vol. 42, No. 3, pp. 359–364, July–September, 1999.     

Photometric variability of five candidates for protoplanetary nebulae

We analyze long-term UBV observations and ASAS-3 photometry for five candidates for protoplanetary nebulae—F and G supergiants with infrared excesses at highG alactic latitudes—V340 Ser, IRAS 05113+1347, V552 Pup, V448 Lac, and RV Col. These stars exhibit quasi-periodic multifrequency light variations caused by pulsations with characteristic time scales from 83 to 139 days, depending on the stellar temperature. Cooler stars undergo variations with larger amplitudes and periods. The variations at close frequencies with a period ratio of 1.03–1.09 are responsible for the amplitude modulation revealed for most program stars.     

Preliminary results for RR Lyrae stars and Classical Cepheids from the Vista Magellanic Cloud (VMC) survey

The Vista Magellanic Cloud (VMC, PI M.R. Cioni) survey is collecting K _S -band time series photometry of the system formed by the two Magellanic Clouds (MC) and the “bridge” that connects them. These data are used to build K _S -band light curves of the MC RR Lyrae stars and Classical Cepheids and determine absolute distances and the 3D geometry of the whole system using the K-band period luminosity (PLK _S ), the period–luminosity–color (PLC) and the Wesenhiet relations applicable to these types of variables. As an example of the survey potential we present results from the VMC observations of two fields centered respectively on the South Ecliptic Pole and the 30 Doradus star forming region of the Large Magellanic Cloud. The VMC K _S -band light curves of the RR Lyrae stars in these two regions have very good photometric quality with typical errors for the individual data points in the range of ∼0.02 to 0.05 mag. The Cepheids have excellent light curves (typical errors of ∼0.01 mag). The average K _S magnitudes derived for both types of variables were used to derive PLK _S relations that are in general good agreement within the errors with the literature data, and show a smaller scatter than previous studies.     

One Fellow’s View of the “Royal Road”

The paper attempts to discern the place and direction of the “Royal Road” of astronomical usage, with the particular perspective that the late Prof. Zdeněk Kopal may have had of it. In seeking further insights into the Kopalian royal road map, we take note of his own “guide stars.” Referring then more closely to the royal road of eclipses, the information content of photometric light curves is discussed, noting in particular, their Fourier representation and the relationship of that to generally believed causes of the light variation.     

Mechanisms for eclipsing in exotic eclipsing systems

The stars ε Aur, KH 15D, and H 187 are utterly different in their fundamental characteristics. However, they share two important features: (1) prolonged eclipses are observed in all three and (2) there are no spectroscopic signs of a second component. The light curves of these objects are substantially similar, which may indicate a uniform mechanism for the eclipses. This article analyzes models for eclipsing in these objects proposed by different authors and discusses the problems associated with their application. __________ Translated from Astrofizika, Vol. 49, No. 4, pp. 553–571 (November 2006).     

Circumstellar envelopes and Asymptotic Giant Branch stars

Summary. Red giants are sometimes surrounded by envelopes, the result of the ejection of stellar matter at a large rate (/yr) and at a low velocity (10 km/s). In this review the envelopes are discussed and the relation between stars and envelope: what stars combine with what envelopes? The envelope emits radiation by various processes and has been detected at all wavelengths between the visual and the microwave range. I review the observations of continuum radiation emitted by dust particles and of rotational transitions of molecules, where these molecules have been excited by thermal or by non–thermal (“maser”) processes. I discuss mainly the oxygen–rich stars, those of spectral type M, and only briefly the closely related carbon–rich stars. By and large the density in the envelope is well described by spherically symmetric outflow at a constant velocity; on the time scale needed to flow from stellar surface to the outermost layers, i.e. yr, the loss of mass is sometimes interrupted suddenly after which the envelope becomes “detached” from the star. The temperature decreases when moving outward; heat input is by friction between dust particles and gas and cooling occurs by line radiation by various molecules, especially by HO. The molecular composition is determined by formation in an equilibrium process deep in the atmosphere and by destruction in the outer parts of the outflow by interstellar UV radiation (H, CO, HO) or by depletion due to condensation on dust grains (SiO); dust particles of silicate material solidify where the radiation temperature is decreased to about 1000 K, and this is at a few stellar radii. The various continuum spectra produced by the dust particles in different stars are well modelled by a simple model of the density and dust temperature distribution plus the assumption that the particles consist of “dirty silicate”, i.e. silicate with Fe and Al ions added. A large range of optical depths, , is observed: from 0.01 to 10. In envelopes with large optical depth the star itself can no longer be detected directly. Model calculations also show that the momentum in the outflow, i.e. is provided by radiation pressure on the dust particles followed by the complete transfer of this momentum to the gas. The mass–loss rate itself, , is not determined by radiation pressure but by dynamic processes in the region below the dust condensation layer. When is sufficiently large its measurement, that of the stellar luminosity, and that of the outflow velocity, , permit the determination of , i.e. the total outflow rate, without making assumptions about the abundance of the dust particles or of the molecular gases. Detached envelopes have been seen in a few cases. Thermal molecular radiation is faint compared to the maser emission but has been measured in distant stars, e.g. in stars near the galactic center. Different molecules outline different “spheres” around the star. The largest sphere (a radius of 0.1 pc) is outlined by an emission line belonging to the CO() transition. Higher rotational transitions of CO give smaller diameters. A comparison of CO () and () fluxes in stars with very thick envelopes leads to the conclusion that an abrupt decrease in the mass–loss rate occurred some ten thousand years ago. Three molecules produce each several maser lines: SiO, HO and OH. Several new HO lines have recently been discovered; their exploration has hardly been started. The high intensity of the maser lines makes interferometry possible and hence detailed mapping. The SiO lines are formed deep in the envelope, below the dust condensation layer. OH maser lines are produced farthest out, HO lines in between. The excitation mechanisms for most maser lines is understood globally, but detailed models are lacking, largely because the problem is non–linear and the solution of the radiative transfer equation requires a highly anisotropic geometry. The geometrical and kinematical properties of the 1612 MHz OH maser, which in many objects is very strong, are explained by a thin shell of OH; because the angular diameter of the shell can be measured directly and the linear diameter can be determined from the difference in the time of maximum flux of blue and red maser peaks, the distance of the shell and of the star can be measured. The presence or absence of individual maser lines appears to depend on the value of and is well described by a sequence called “Lewis' chronology”. The central star is a long–period variable with a period of 300 days or longer and with a large luminosity amplitude (). Evidence is given that each star has the maximum luminosity it will reach during its evolution and that it is a thermally–pulsing Asymptotic–Giant–Branch star (TP–AGB) with a main–sequence mass between 1 and 6 . Stars of the same main–sequence mass, , have different mass–loss rates, in some cases by a factor of 10. The mass–loss rate probably increases with time, and the highest mass–loss rates are reached toward the end of the evolution. Stars with higher ultimately reach higher mass–loss rates. The calibration of the main–sequence mass is reviewed. Most Mira variables with mass loss have a mass between 1.0 and 1.2 . OH/IR stars with periods over 1000 days have no counterparts among the carbon stars and thus have . Stars as discussed in this review have been found only in the thin galactic disk and in the bulge. Finally I review several recently proposed scenarios for TP–AGB evolution in which mass loss is taken into account. These scenarios represent the observations quite well; their major short–coming is the lack of an explanation why the central stars are always large–amplitude, long–period variables and why such stars are the ones with high mass–loss rates. Received: 10 January 1996     

On the possibility of systematizing classical novae by light curve type. II. Observational characteristics of novae and groups

The separation of novae into groups, proposed in Part 1 of this paper, is discussed: it is based on the criterion of the shape of the light curve, plotted on “outburst amplitude-logarithm of shell radius ” scales. The observational characteristics of novae belonging to certain groups are considered. Novae of the GQ Mus group may be characterized by x ray emission. The proposed separation of novae into groups may be related to the structure of the shell ejected in the outburst. Novae with smooth light curves have ellipsoidal shells without pronounced nonuniformities. Novae with temporary fading or with brightness fluctuations in the transitional stage of the outburst have ellipsoidal shells with a nonuniform distribution of matter (equatorial and tropical belts in novae of the DQ Her group, an equatorial belt for the RR Pic group, and a patchy structure for the CP Pup group). Relationships among certain parameters of novae are considered: absolute stellar magnitude, outburst amplitude, shell expansion velocity, etc. From the “depth of temporary fading in the transitional stage of the outburst-shell expansion velocity” dependence it can be concluded that the kinetic energy of the shell may be a constant for novae of the DQ Her group. Translated from Astrofizika, Vol. 42, No. 2, pp. 189–202, April–June, 1999.     

Untersuchungen des Lichtwechsels von zwölf Ap-Sternen in zehn Spektralbereichen

Twelve Ap stars with known periods and fairly large amplitudes were photoelectrically investigated in a ten colour system (λ 3400 to λ 7600 ). The amplitude-wavelength relations following from the light curves cannot be explained by the influence of the backwarming effect. These relations point to complicated structures in the atmospheres of the peculiar regions of the magnetic stars.     

Hypernovae and Gamma-Ray Bursts

The nature of very energetic supernovae (hypernovae) is discussed. They are the explosive death of stars more massive than ~20–25M _⊙, probably linked to the enigmatic Gamma-Ray Bursts. The optical properties of hypernovae indicate that they are significantly aspherical. Synthetic light curves and late-phase spectra of aspherical supernova/hypernova models are presented. These models can account for the optical observations of SNe 1998bw and 2002ap. The abundance patterns of hypernovae are characterized by large ratios (Zn, Co)/Fe and small ratios (Mn, Cr)/Fe, indicating a significant contribution of hypernovae to the early Galactic chemical evolution.     

Evolution of Population II stars

The current knowledge of the evolution of Population II stars, as observed in galactic halos and globular clusters, is outlined. The recent theoretical results provided by an improved physical understanding of the stars are reported, with a particular emphasis upon those stellar evolutionary phases which are the keystones of the interpretation of globular cluster stars. Within the up–to–date theoretical scenario, the luminosity of the turn–off stars and horizontal branch stars, which are of fundamental importance for distance and age determinations, turns out to be fainter and brighter, respectively, in comparison with previous theoretical computations. The predicted absolute magnitude of RR Lyrae stars is consistent with the bright values suggested from the long distance scale (Sandage 1993), but the slope of the relation between and metallicity ([Fe/H]) agrees with that suggested from the short distance scale (see, e.g., Carney et al. 1992, Clementini et al. 1995), at least with [Fe/H]-1.3. As to the globular cluster ages, the new computations provide younger ages ( 10 Gyr–13 Gyr), weakening the conflict with current cosmological estimates. The recent results derived from the fitting of HIPPARCOS parallaxes of field subdwarfs to the fiducial main sequence of globular clusters are consistent with the up–to–date theoretical models. However, the direct determinations of , which are based on the HIPPARCOS proper motions and trigonometric parallaxes of field RR Lyrae stars, give fainter absolute magnitudes than those obtained from the improved evolutionary models. This seems to suggest that the problem of the globular cluster distance and age is still open. Received 6 April 1998