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).     

Photometric activity of UX Ori stars and related objects in the near infrared and visual. BF Ori, CQ Tau, WW Vul, and SV Cep

We have analyzed the activity of four UX Ori stars in the near-IR (JHKL) and visual (V) using the results of long-term photometric observations. For comparison, we also obtained IR (JHKLM) photometric observations of two visually quiet young stars of close spectral types (AB Aur and HD 190073). For the photometrically most active UX Ori stars BF Ori, CQ Tau, and WW Vul, the Algol-like declines of brightness in the visual, which are due to sporadic enhancements of the circumstellar extinction, are also observed (with decreasing amplitude) in the IR bands. A strict correlation between the V and J brightness variations is observed for all the stars except for SV Cep. For some of the UX Ori stars, a strong correlation between the visual and IR activity is observed up to L, where the main contribution to the emission is made by circumstellar dust. In the case of SV Cep, the visual variability is not correlated with the variability of the IR fluxes. On one occasion, a clear anti-correlation was even observed: a shallow, but prolonged decrease of the visual brightness was accompanied by an increase in the IR fluxes. This indicates that circumstellar clouds themselves can become powerful sources of IR emission. Our results provide evidence that the photometric activity of UX Ori stars is a consequence of instability of the deepest layers of their gas-dust accretion disks. In some cases (SV Cep), fluctuations of the density in this region are global, in the sense that they occur along a significant part of the circle marking the inner boundary of the dust disk. It is interesting that AB Aur, which is the quietest in the visual, appeared to be the most active in the IR. In contrast to UX Ori stars, the amplitude of its brightness variations increases from the J to the M band. It follows from analysis of the IR colors of this star that their variability cannot be described by models in which the variable IR emission has a temperature close to the sublimation temperature of silicate grains (about 1500 K). This means that the photometric activity of AB Aur must be due to both the dust and gas components of the circumstellar disk.     

The photopolarimetric activity and circumstellar environment of the T Tauri star CO Ori

We present the results of simultaneous UBVRI photometry and polarimetry of the classical T Tauri star CO Ori carried out at the Crimean Astrophysical Observatory during the 18 years between 1986 and 2004. We show that the variations of linear polarization accompanying the star’s brightness variations follow the law characteristic of UX Ori stars. This suggests that the brightness variations of the star are mainly due to changes of the circumstellar extinction due to non-uniform structure of the circumstellar environment, and to an “optimal” orientation of the circumstellar gas and dust disk relative to the observer, whose line of sight crosses the gas and dust atmosphere of the disk. We determine the star’s intrinsic polarization due to scattering of light in the circumstellar disk. The polarization position angle indicates the orientation of the disk’s symmetry axis in the plane of the sky. Our analysis of an archival light curve for CO Ori confirms the existence of a many-year cycle of photometric activity, suspected by us earlier. The refined period of this cycle is 12.4 years. The existence of such activity cycles of UX Ori stars testifies to considerable deviations of their circumstellar disks from axial symmetry, a reflection of either stellar binarity or the commencement of the process of planetary formation.     

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.     

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.     

Photometric and polarimetric activity of RZ Psc

We present the results of synchronous photometric and polarimetric U BV RI observations of the irregular variable star RZ Psc, acquired at the Crimean Astrophysical Observatory in 1989–2002. The star’s photometric behavior is characterized by short, sporadic Algol-like dimmings. We observed only one deep minimum, with a V amplitude of about 1.5^m, during the entire observation time. During this minimum, the star’s linear polarization reached 3.5%. Comparisons with polarization observations of RZ Psc during another deep minimum in 1989 show that the two minima can be described by the same polarization-brightness relation, testifying to an eclipsing nature for the minima. This provides evidence that the optical characteristics of the flattened circumstellar dust envelope that gives rise to the star’s intrinsic polarization have remained virtually unchanged over the last 13 years. We argue that the origin of this stability is the presence of a large dust-free cavity in the central region of the circumstellar dust disk of RZ Psc. The cavity could be associated with binarity of the star or the formation of a planetary system, with most of the dust in the central region of the disk being transformed into large bodies—planetesimals and planets.     

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.     

Observations of a unique minimum of RR Tau in the optical and near infrared

We present observations of the UX Ori star RR Tau in the optical (UBVRI)and near infrared (JHKLM)acquired between November 2000 and April 2001. We recorded a uniquely long (about half a year) Algol-like minimum with an amplitude of ΔV≈2.9. The dimming of RR Tau was accompanied by an increase of the linear polarization, typical of UX Ori stars and testifying to the eclipsing nature of the minimum. The J and H infrared fluxes varied synchronously with the optical variations. However, the K and L brightness changes were in the opposite sense: the flux in these two bands increased for the entire duration of the optical minimum. Our analysis suggests that the source of the K and L radiation is the dust cloud itself, moving at a distance of about 1 AU from the star. The flux increase in these bands was not due to an increase in the dust temperature, but instead to an increase in the number of emitting grains in the cloud. This could be associated either with an actual increase in the number of fine grains due to sublimation and the disruption of larger grains or with the distortion and disruption of the cloud due to tidal perturbation, permitting the star’s light to penetrate and heat the densest regions of the cloud. Based on the observed L fluxes, we estimate the mass of the emitting dust in the cloud to be ≈10²³ g. Taking into account the presence of cool dust and a gaseous component in the cloud in addition to the dust heated by the star’s radiation, and adopting a ratio for the masses of the dust and gas components similar to that in the interstellar medium (1:100), we estimate the cloud’s total mass to be ≥10²⁵ g. Judging from this value and the duration of the minimum, we observed an extremely rare episode associated with a giant gas and dust cloud with a total mass on the order of 0.1 lunar mass or higher, which passed very near the young star (and may be falling onto it).     

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.     

Unusual Photometric Activity of the Weak-line T Tauti Star V715 Per

The results of many-year photometric observations of the weak-line T Tauri star V715 Per in the optical (V RI) and infrared (JHK) photometric bands are presented. The period of the star’s lowamplitude brightness variations in the V RI bands detected earlier, 5.23d, is confirmed. This period persists during the entire 14-year time interval of the observations, but its amplitude varies with time. The same period was found in the variations of JHK infrared fluxes, with the brightness variations in the two ranges being almost synchronous. The most probable origin of these variations is periodic eclipses of the star by its own disk, perturbed by its interaction with the magnetosphere (AA Tau-type variations). In addition to the periodic brightness variations, the star also displayed sporadic Algol-like minima characteristic of UX Ori stars during several years (2005–2011). The amplitude of these variations increasedmonotonically, reaching approximately 1^m in the V band by the end of 2010, after which the deep fadings ended. During this entire time interval, the amplitude of the periodic variations was observed to decrease. After 2011, the periodic component began to become stronger again. These changes of the photometric activity of V715 Per can be explained by an increasing rate of gas accretion onto the star, decreasing the radius of warped regions of the disk and leading to accompanying Algol-like fadings. This model assumes a low inclination of the circumstellar disk in V715 Per to the line of sight.     

Diagnostics for accretion disks around UX Ori stars based on Balmer, Paschen, and Brackett Lines

We have modeled the H_α, H_β, and H_γ (Balmer series), P₁₄ (Paschen series), and Br_γ (Brackett series) hydrogen lines formed in the inner regions of the accretion disk around the Herbig Ae star UX Ori. Our calculations are based on spectra obtained with the Nordic Optical Telescope (NOT) and the IRTF. We computed a grid of non-LTE models for a radiating area in the accretion disk and determined the basic parameters of the lines using the method of Sobolev. Analyzing the theoretical and observed line profiles, equivalent widths, and luminosities, we have estimated the accretion rate and electron-temperature distribution in the inner parts of the accretion disk. The accretion rate of UX Ori is about $\dot M_a = (3 - 10) \times 10^{ - 9} M_ \odot /yr$ , and the temperature distribution is consistent with the power law T(r)=T(r _*)(r/r _*)^?1/n , where the electron temperature near the stellar surface T(r _*) is 15000–20000 K and the power-law index n≈2–3 is about two to three. The resulting value for $\dot M_a $ eliminates problems connected with the application of magnetospheric accretion models to Herbig Ae/Be stars. Another important conclusion is that, at the estimated accretion rate, the energy release is substantially (about two orders of magnitude) lower than the stellar luminosity. Therefore, the optical radiation of UX Ori accretion disks cannot appreciably contribute to the observed variability of these stars, which must be determined mainly by variability in the circumstellar extinction.     

A “tsunami” in the protoplanetary disk of SV Cep

The results of ~15 years of photometric observations of the UX Ori star SV Cep in the near-infrared (JHKL) are presented. They demonstrate the presence of a cyclic component with a period of ~7 years in the variations of the IR fluxes. This is clearly seen in all four IR bands, but is absent in the optical. The variation amplitude is highest in the K band: ΔK ≈ 0.68 ^m . The shape of the variations differs slightly in the transition from J to L. However, it is reproduced with good accuracy during two cycles, suggesting a periodic process is observed. If the periodic perturbations in the circumstellar disk of SV Cep are due to a companion’s orbitalmotion, the orbital semi-major axis should be ~5AU, foramass of SVCep of 2.6M _⊙. The absence of a seven-year period in the optical light curve of SV Cep means that the observed period cannot be due to variations in the circumstellar extinction. The IR brightness variations could be due to the companion’s motion along an eccentric orbit, resulting in a periodic modulation of the rate of accretion onto the star.     

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.     

Statistics of BY Draconis variables

A statistical survey of 113 spotted red dwarf stars that are known or suspected BY Draconis variables is presented. Typical indicators of stellar photometric activity—the amplitudes of the rotational modulation and seasonal mean brightness variations ΔV and Δ〈V〉—are compared to the global parameters of the stars. First, photometric variability shows a weak dependence on spectral type; second, ΔV and Δ〈V〉 grow with increasing stellar rotational velocity and decreasing Rossby number, with the dependences saturating at the critical values V _crit～15–20 km/s and Ro_crit～0.2–0.3; and third, the Sun as a star fits well into the derived relations. Thus, the spottedness of stars, like other indicators of stellar activity, depends on their global parameters.     

Polarimetry of the protoplanetary nebula AFGL 2688

Our spectroscopic (with resolution R=75 000) and spectropolarimetric (R=15 000) observations with the 6-m telescope of the Special Astrophysical Observatory have enabled us to distinguish photospheric and circumstellar features in the optical spectrum of the bipolar protoplanetary nebula AFGL 2688 for the first time. The linear polarization of the radiation was measured at 5000–6600 Å. The emission in the lines of the sodium doublet and the Swan bands is formed in the envelope, and the mechanism exciting the transitions is resonance fluorescence. We conclude that the circumstellar envelope has a low density. Features of the structure of the nebula are discussed based on published high-angular-resolution photometric and polarimetric (HST NICMOS) data.     

Microvariability of line profiles in the spectra of OB stars: δOri A

We have studied variability of the spectral lines of the OB star δOri A—the brightest component of the δOri triple system. Forty spectra with signal-to-noise ratios ≈500–800 and a time resolution of four minutes were obtained. We detected variability in the HeIIλ4686, HeIλ4713, and Hβ absorption and the CIIIλ5696 emission profiles. The amplitude of the variability is ≈(0.5–1)% of the continuum intensity. The dynamical wavelet spectrum of the profile variations reveals large-scale components in the interval 25–50 km/s that move within the-V sin i to V sin i band for the primary star of the system, Aa¹, with a band crossing time of 4^h–5^h. However, some of the variable features go outside the band, presumably due to either imhomogeneities in the stellar wind from δOri Aa¹ or nonradial pulsations of the weaker components of the system, Aa² or Ab. The detected variability may be cyclic with a period of ≈4^h. We suggest that it is associated with nonradial pulsations of the primary in the sector mode (l,m) = (2, ?2).     

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.     

Optical spectra of three AGB stars

The basic parameters and detailed chemical compositions of three asymptotic giant branch stars with similar effective temperatures and surface gravities have been determined using CCD spectra obtained with the échelle spectrometers of the SAO 6-m telescope. The metallicity and chemical composition of the optical counterpart of the OH/IR star IRAS 18123 + 0511 have been derived for the first time. The abundance [X/H]_⊙ of the iron group elements (V, Cr, Fe) is ?0.45 dex. An overabundance of oxygen, [O/Fe]_⊙=1.44 dex, is detected in the atmosphere of this star. The abundances of s-process heavy elements are not enhanced, and are instead underabundant with respect to the metallicity: the average value of [X/Fe]_⊙ for Y, Zr, Ba, La, Ce, Pr, Nd is ?0.25. The derived abundances confirm that IRAS 18123 + 0511 is in the AGB stage of its evolution. The metallicity of the object, together with its radial velocity V _r=78.0 km/s and Galactic latitude |b|=11°, suggest that it belongs to the old disk population. The expansion velocity of the circumstellar envelope, V _exp≈21 km/s, is derived from the positions of circumstellar absorption bands. The set of parameters obtained for the low-metallicity, highlatitude supergiants BD + 18° 2757 and BD + 18° 2890 (with iron abundances [Fe/H]_⊙=2.10 and ?1.48, respectively) confirm that they are evolved halo stars, and probably UU Her-type stars.     

IR photometry and models for the dust envelopes of two carbon stars

The results of JHKLM photometry of two carbon stars are presented: the irregular variable NQ Cas and the Mira star BD Vul. Data on the mean fluxes supplemented with mid-IR observations with the IRAS, AKARI, andWISE satellites are used to compute spherically symmetrical model dust envelopes for the stars, consisting of particles of amorphous carbon and silicon carbide. The optical depth in the visible for the comparatively cool dust envelope of BD Vul, with a dust temperature at its inner boundary T₁ = 610 K, is fairly low: τ_V = 0.13. The dust envelope of NQ Cas is appreciably hotter (T₁ = 1550 K), and has τ_V = 0.32. The estimated mass-loss rates are 1.5 × 10^?7M_⊙/yr for NQ Cas and 5.9 × 10^?7M_⊙/yr for BD Vul.