Photopolarimetric activity of the Ae Herbig star SV Cep

We present the results of simultaneous UBVRI photometric and polarimetric observations of the Ae Herbig star SV Cep made in 1987–1998. Over these 11 years, only a single deep (ΔV>1^m) brightness minimum was observed. Near this minimum, the brightness decrease was accompanied by an increase of the linear polarization, as is typical of young UX Ori stars. The photometric observations of SV Cep indicate reversals of the color tracks in brightness minima, as is common for stars of this type, as well as variations of the slopes of the color tracks during and after minima. This provides evidence that the circumstellar dust clouds screening the star differ in their sizes and masses, and also in the optical properties of their dust particles. A Fourier analysis of the brightness variations of SV Cep (including data from the literature) confirms the presence of previously suspected activity cycles with periods P ₁=4000^d and P ₂=670^d The polarimetric observations indicate that, along with the inverse correlation between the degree of linear polarization and brightness, the polarization parameters vary on characteristic time scales of 4000 and 1000 days. This suggests the existence of large-scale inhomogeneities in the circumstellar dust disk rotating about the star.     

Photometric and polarimetric activity of the Herbig Ae star VX Cas

We present the results of our simultaneous photometric and polarimetric observations of the Herbig Ae/Be star VX Cas acquired in 1987–2001. The star belongs to the UX Ori subtype of young variable stars and exhibits a rather low level of photometric activity: only six Algol-like minima with amplitudes ΔV>1^m were recorded in 15 years of observations. Two of these minima, in 1998 and 2001, were the deepest in the history of the star’s photometric studies, with V amplitudes of about 2^m. In each case, the dimming was accompanied by an increase in the linear polarization in agreement with the law expected for variable circumstellar extinction. The highest V polarization was about 5%. Observations of VX Cas in the deep minima revealed a turnover of the color tracks, typical of stars of this type and due to an increased contribution from radiation scattered in the circumstellar disk. We separated the observed polarization of VX Cas into interstellar (P_is) and intrinsic (P_in) components. Their position angles differ by approximately 60°, with P_is dominating in the bright state and P_in dominating during the deep minima. The competition of these two polarization components leads to changes in both the degree and position angle of the polarization during the star’s brightness variations. Generally speaking, in terms of the behavior of the brightness, color indices, and linear polarization, VX Cas is similar to other UX Ori stars studied by us earlier. A number of episodes of photometric and polarimetric activity suggest that, in their motion along highly eccentric orbits, circumstellar gas and dust clouds can enter the close vicinity of the star (and be disrupted there).     

The accretion activity of the Ae Herbig star CQ Tau

Results of spectral observations of the isolated Ae Herbig star CQ Tau obtained in 1995–1998 in the H_α line and near the sodium resonance doublet are presented, together with simultaneous photometric monitoring of the star. CQ Tau is a member of the family of young UX Ori stars with nonperiodic Algol-like brightness decreases. The star is surrounded by an accretion disk, in which its emission-line spectrum and part of its absorption-line spectrum are formed. The strong variability of the H_α, D NaI, and HeI 5876 Å lines testifies that the gaseous disk is appreciably inhomogeneous in both the radial and azimuthal directions. This inhomogeneity probably results from uneven feeding by the circumstellar material from the peripheral regions of the circumstellar disk. Over the four years of observations, we observed the star in deep minima three times (ΔV～2). On these nights, an increase in the H_α equivalent width followed the decrease in radiation flux. In the two deepest minima, the normally two-component line profile had only a single component with a nearly symmetrical profile. This behavior of the H_α line is in good agreement with the results of numerical modeling of Algol-like minima and can be used to estimate the parameters of the dust clouds eclipsing the star and inner accretion disk. These estimates suggest that the circumstellar dust clouds can approach very close to the star and be sublimated there.     

The Herbig Ae/Be star HD 200775 as a spectroscopic binary

New spectroscopic observations obtained in 1998–2001 and published radial velocities of neutral helium absorption lines suggest that the Herbig Ae/Be star HD 200775 is a spectroscopic binary with a 1180±60 day period. We have determined the elements of the spectroscopic orbit. The presence of both rapid and long-time-scale spectral variability is demonstrated. The rapid variability is apparently due to a nonuniform circumstellar environment and modulation by the axial rotation of the system’s primary. Using data from the literature, we plot the V light curve of the system, which shows some brightening near the time of the minimum radial velocity, with its duration not exceeding 0.2 of the orbital cycle. We suggest that the system’s secondary is a low-mass (≥ 0.5M_⊙) protostar, and that a semitransparent extended circumstellar envelope gives rise to eclipses in the system.     

Results of Many-year Photometry of the Herbig Ae Star AB Aur in the Near Infrared

Astronomy Reports - The results of many-year infrared observations of the Herbig AeBe star AB Aur at 1-5 μm (JHKLM bands) are presented. The duration of the photometric series, together with...     

Modeling of Spectroscopic and Interferometric Observations of the Herbig Star VV Ser with Hybrid Models

Modeling of hydrogen emission lines is a powerful tool to study physical processes in the nearest vicinity of young stars because spectral lines carry information on the kinematics and physical conditions of the gas. One of the lines that probe emitting regions closest to the star is the Br$$\gamma $$ line. We consider different types of hybrid models to reproduce both interferometric VLTI-AMBER observations and LBT-LUCIFER spectroscopic observations of the single-peak profile of the Br$$\gamma $$ line of the Herbig AeBe star (HAEBE) VV Ser, a member of the UX Ori type subclass. We computed models of a magneto-centrifugal disk wind, a magnetospheric accretion region (magnetosphere), Cranmer’s polar wind, and scattered light from circumstellar polar dust. Furthermore, we calculated hybrid two-component models consisting of a disk wind and one of the aforementioned models. We computed visibilities and line profiles for all types of models and compared them with the available interferometric observations to constrain model parameters. We conclude that for the inclinations reported for this star (60°–70°), the disk wind alone cannot explain the Br$$\gamma $$ line profile although it may be a dominant contributor to the hydrogen line radiation. However, magneto-centrifugal disk wind in combination with aforementioned emitting regions (magnetosphere, polar wind, or scattered light from polar dust) may be able to reproduce the observations.     

UBVRI photometry and polarimetry of the spotted red dwarf MS Ser

UBVRI photometry of the poorly studied spotted star HD 143313 (MS Ser) was carried out in 1991–1997, supplemented with polarization observations in 1996–1997. These observations confirmed the presence of BY Dra-type variability and reliably detected the star’s intrinsic polarization, which is variable in the U band. A model for the star’s spottedness that is consistent with the polarimetry data was constructed based on the collected photometric data. The model calculations suggest that the spotted areas cover up to 15% of the total stellar surface and are about 1500 K cooler than the surrounding photosphere.     

A search for Keplerian periods in the brightness variations of T Tauri and Ae Herbig stars

Long-term, uniform series of U BV R observations of T Tauri and Ae Herbig stars obtained over 20 yr at the Maidanak Observatory as part of the ROTOR program are analyzed. We find a linear relationship between the characteristic variability time scale and the bolometric luminosity of the star+disk system: the higher the luminosity, the slower the brightness variations. This dependence is valid over a wide range of masses and luminosities, from T Tauri stars to Ae Herbig stars. On average, the variability time scale is one-quarter the Keplerian period at the dust-sublimation radius, which is known from interferometric observations. Some T Tauri stars have periods from 25 to 120 days, which are preserved over several observing seasons. These periods correspond to Keplerian orbits with semi-major axes from 0.14 to 0.52 AU. The results obtained provide indirect evidence for the existence of protoplanets in the gas-dust disks of stars in early stages of their evolution toward the main sequence.     

Photometric activity of the Herbig Be star MWC 297 over 25 years

The photometric behavior of the hot, young Herbig Be starMWC 297 on various time scales is studied using published data, as well as new observations. The series of photometric observations covers about 25 years. Over this time, the star showed low-amplitude (ΔV ≈ 0.3 ^m ) irregular variabilitymodulated by large-scale cyclic variabilitywith an amplitude close to 0.2 ^m and a period (or quasi-period) of 5.4±0.1 yr. A detailed seasonal analysis of the data shows that the light curve of MWC 297 displays two types of photometric features: low-amplitude Algol-like fading with an amplitude close to 0.2 ^m and low-amplitude flares resembling the flares of UV Ceti stars, but being more powerful and having longer durations. The variations of the stellar brightness are accompanied by variations of the B-V and V -R colors: when the brightness decreases, B-V decreases, while V -R increases (the star reddens). The reddening law is close to the standard interstellar reddening law. Although the character of the brightness variability ofMWC 297 resembles the photometric activity of UX Ori type stars, which is due to variations of their circumstellar extinction, its scale is very far from the scales observed for UX Ori stars. It is difficult to reconcile the level of photometric activity with the idea that MWC 297 is observed through its own gas-dust disk viewed almost edge-on, as has been suggested in several studies.     

Circumstellar activity around the Herbig Ae/Be stars MWC 361, MWC 614, AB Aur

A study of the optical spectra of three Herbig Ae/Be stars is presented: MWC 361,MWC 614, AB Aur. Variations of the equivalent widths, radial velocities, and profiles of spectral lines formed in different regions of the circumstellar environments of these stars are analyzed. The most active spectral variations are observed in the Hα, Hβ, and He I 5876 lines. Other emission lines, such as Si II λλ6347, 6371 Åand [O I] λλ6300, 6363 Å, also display variations, but these are much smaller. Interstellar DIBs at λλ5780 and 5797 ?A were detected for the stars MWC 361 and MWC 614 for the first time. The active processes that are underway in the circumstellar environments of these systems can also reach more distant, outer parts of their circumstellar disks.     

Photometric and spectrophotometric observations of the evolution of a strong outburst of the classical symbiotic star YY Herculis

We present spectrophotometric (3400–7500 Å) observations of the evolution of a strong outburst of the classical symbiotic star YY Her in 1993 and photoelectric UBV observations of the star’s eclipse in 1997. The duration of the phase of lowest brightness, when the U brightness had decreased by ～1.3^m, was ～0.17 P _orb (P _orb is the orbital period). If this phase is due to the total eclipse of ～70% of the radiation of the circumstellar envelope, this duration implies that the cool component of YY Her fills its Roche lobe, the bulk of the envelope’s volume emission measure is concentrated around the hot component in a region with rather sharp boundaries r R _giant (R _giant is the giant’s radius), and the line of sight is close to the binary orbital plane. We model fit the spectral energy distribution of YY Her to obtain estimates of the parameters of several structural components of the system. The red giant’s spectral type correlates with its visual brightness and does not correlate with the hot component’s brightness. At minimum brightness, the hot component’s luminosity fluctuates about that of its cool companion (L _{h, bol} /L _{c, bol} ≈0.9), and its temperature is T _h ≈(9–11)×10⁴ K. Activity of the hot component of YY Her is accompanied by increased brightness and reduced temperature (L _{h, bol} ∝T _h ^?5 ), though the relation between these parameters is not unique. At maximum brightness, L _{h, bol} /L _{c, bol} ≈10 and T _h ≈6.0×10⁴ K. If the red giant fills its Roche lobe, the hot component’s luminosity is ～3.3 × 10⁴ L _⊙. The active period of YY Her lasted about 5 years, and the activity decrease was not monotonic.     

A study of the outburst development in the classical symbiotic star Z And within the colliding-winds model

Two-dimensional gas-dynamical modeling of the mass-flow structures in binary systems is used to study the outburst development in the classical symbiotic star Z And. The stage-by-stage rise of the light during the outburst can be explained using a model with colliding winds. We suggest a scenario for the development of the outburst and study the influence of possible changes in the flow structure on the brightness of the system. The model variations of the luminosity due to the formation of a system of shocks are in good agreement with the observed brightness variations.     

Photometric activity of UX orionis stars and related objects in the near infrared and optical: CO Ori,RR Tau,UX Ori,and VV Ser

This paper continues a study of the photometric activity of UX Ori stars in the optical and near-infrared (JHKLM bands) initiated in 2000. For comparison, the list of program stars contains two Herbig Ae stars that are photometrically quiet in the optical: MWC480 andHD179218. Fadings ofUXOri stars in the optical (V band) due to sporadic increases of the circumstellar extinction are also observed in the infrared (IR), but with decreasing amplitude. Two stars, RR Tau and UX Ori, displayed photometric events when V -band fadings were accompanied by an increase in IR fluxes. Among the two Herbig Ae stars that are photometrically quiet in the optical, MWC 480 proved to be fairly active in the IR. Unlike the UX Ori stars, the variation amplitude of MWC 480 increases from the J band to the M band. In the course of the observations, no deep fadings in the IR bands were detected. This indicates that eclipses of the program stars have a local nature, and are due to extinction variations in the innermost regions of the circumstellar disks. The results presented testify to an important role of the alignment of the circumstellar disks relative to the direction towards the observer in determining the observed IR variability of young stars.     

Formation and evolution of Ae and Be stars

Several scenarios for the formation of accretion and decretion disks in single and binary Ae and Be stars are proposed. It is shown that, in order for a rapidly rotating main-sequence Be star to lose mass via a disk, the star’s rotation must be quasi-rigid-body. Estimates show that such rotation can be maintained by the star’s magnetic field, which is probably a relict field. The evolution of single Be main-sequence stars is numerically simulated allowing for mass loss via the stellar wind and rotational mass loss assuming rigid-body rotation. The stellar wind is the factor that determines the maximum mass of Be stars, which is close to 30M _⊙. The evolution of Be stars in close binaries is analyzed in the approximation adopted in our scenario. Long gamma-ray bursts can be obtained as a result of the collapse of rapidly rotating oxygen—neon degenerate dwarfs—the accreting companions of Be stars—into neutron stars.     

Broad-band multicolor photometry and polarimetry of spotted stars

We have confirmed the BY Dra-type variability of the active spotted stars MS Ser, LQ Hya, VY Ari, and EK Dra using simultaneous UBVRI photometric and polarimetric observations. We have also reliably detected the intrinsic linear polarization of their radiation and its rotational modulation in U due to the inhomogeneous distribution of active magnetized regions over the surfaces of the stars. Modeling of the linear polarization based on the Zeeman effect indicates that all the stars display strong local magnetic fields (about 2 kG, similar to those in sunspots), with filling factors of up to 40% of the total stellar surface. The magnetized regions coincide with cool photospheric spots detected in photoelectric observations.     

Photometry and spectroscopy of the system V4641 sagittarii in quiescence

We present the results of our CCD photometric and moderate-dispersion spectroscopic observations of the binary system V4641 Sgr, which contains a black hole of mass ≈9.5M_⊙ and a normal B9III star. The photometric light curve reveals an ellipticity effect with very high amplitudes in V and R, 0.40^m and 0.37^m, and the color curve shows that the surface temperature is nonuniform. All this testifies to tidal distortion of the normal star's surface due to the massive companion and to a high inclination of the orbit to the line of sight. In June and July 2002, during quiescence, we obtained data during three flares with amplitudes up to 0.26^m. In particular, spectroscopic observations were acquired near the time of the black hole's inferior conjunction. One hour before conjunction, a depression by EW=0.5 Å was observed in the red wing of the Hα absorption line, interpreted as absorption by gas flowing in the direction from the observer toward the normal star. This flow is apparently associated with a rarefied gas disk around the black hole, and the conjunction grazes the stellar surface if the orbital inclination is close to 70.7°. The maximum velocity along a circular Keplerian orbit is 650 km/s at a distance of R=0.15–0.20a from the black hole (where a is the component separation). Thus, we find the mass of the black hole to be M _BH =7.1–9.5M_⊙, confirming the model of Orosz et al. (2001).     

Photometry and spectroscopy of CI camelpardalis from 1998–2005

Our long-time monitoring of the B[e] star and transient X-ray source CI Cam during quiescence following the 1998 outburst demonstrates that the complex, stratified circumstellar envelope has tended to stabilize after this structure was perturbed by the passage of a shock wave from the outburst. The star’s U BV R brightness shows slow, possibly cyclic, variations with an amplitude of about 0.2^m. We determined the spectral type of the primary, B4III-V, based on the widths of the absorption wings of high-numbered Balmer lines. A Doppler shift of 460 km/s was detected for the Hell λ4686 Å emission line. The shifts in this line yield an orbital period of 19.41 days, which is also manifested itself in the photometric data as a wave with a V amplitude of 0.034^m. The orbit is elliptical, with an eccentricity of 0.62. It is most likely that the secondary is a white dwarf surrounded by an accretion disk. The primary’s mass exceeds 12 M _⊙ . The system may be at a late stage of its evolution, after the stage of mass exchange.     

The chemical composition of the red giant η Ser

The paper presents the results of an analysis of elemental abundances in the atmosphere of η Ser—a red giant of spectral type K2IIIabCN1, according to Hoffleit and Jaschek [4]. The resulting atmospheric parameters correspond to a K0 giant, and the peculiarities of the star’s chemical composition provide evidence that the assignment of η Ser to the class of CN stars was erroneous.     

Physical parameters and dynamical properties of the multiple star o and

An analysis of the physical characteristics, orbital parameters, and dynamical stability of the multiple Be star HD 217675 (o And) is presented. Observations on the 6-m SAO and 1.5-m RTT telescopes are used to refine the orbital parameters of the subsystems. The spectral types, absolute magnitudes, and masses of the components are estimated. The total mass of the system is 18 ± 2 M _db. It is shown that the configuration of the system is 2 + 2, where one of the subsystems is a spectral binary with a period of 33 d and the outer pair has a period of 117 yr. The parameters of the second inner subsystem are uncertain, making it difficult to draw firm conclusions about the stability of this multiple star.