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.     

Disk wind in the radiation of young intermediate-mass stars

We consider the role of the disk wind in the formation of hydrogen emission spectra of young intermediate-mass Herbig Ae stars. The assumed parameters for the kinematic characteristics of the wind region are typical of the theory of magnetocentrifugal disk winds. Our computations of the excitation and ionization states for atoms in the emitting region are based on the Sobolev approximation for a medium with a large velocity gradient. The calculations show that hydrogen-line emission profiles can have a complex structure that depends on the disk-wind parameters, as well as on the system’s orientation relative to the line of sight. The model proposed is able to explain most types of Hα profiles observed for Herbig Ae stars.     

The lower temperature limit of accretors

There should be a universal correlation between the main observational parameters of magnetized accreting stars (neutron stars, white dwarfs, and possibly T Tauri stars): their luminosities, periods, and temperatures. To first approximation, such a dependence is obeyed reasonably well for X-ray pulsars, intermediate polars, and T Tauri stars. In contrast, the parameters of anomalous pulsars (so-called “magnetars”) and soft gamma-ray repeaters differ sharply from this dependence, and even occupy a “forbidden” region in the parameter space. This presents a serious argument against the idea that these are accretingneutron stars.     

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.     

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.     

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.     

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.     

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.     

A new classification scheme for T Tauri light curves

Based on many years of observational data from a photometric database on young stars, we propose a new classification scheme for the light curves of classical T Tauri stars. Our analysis of master light-curve shapes for 28 classical T Tauri stars is used to distinguish up to five light-curve types. The proposed scheme suggests a qualitative interpretation in terms of interaction of the central star with its circumstellar accretion disk.     

Calculation of profiles of the CIV 1550 doublet formed in an accretion shock in a T Tauri star: Axially symmetric radial accretion

We have calculated profiles for the CIV 1550 doublet arising in an accretion shock in a T Tauri star assuming that (i) the accretion zone at the stellar surface is axially symmetric (a circular spot or spherical belt), (ii) the velocity and density of the gas in front of the shock do not vary within the accretion zone, and (iii) the gas falls radially inward toward the star. The calculated CIV 1550 profiles differ qualitatively from those observed in the spectra of T Tauri stars, probably because the velocity of the infalling gas in T Tauri stars has a tangential component of some tens of km/s due to the nonradial magnetic field near the stellar surface.     

Search for and study of hot circumstellar dust envelopes

Long-term (1984–2008) JHKLM photometry for 254 objects is presented. The observations were carried out in the standard JHKLM photometric system using an original method and a modern IR photometer designed and built at the Sternberg Astronomical Institute. Our program of studies included searches for and studies of relatively hot circumstellar dust envelopes. The most important results obtained using these observations include the following. We have detected relatively hot dust envelopes in a number of objects for the first time, including the RCB star UV Cas, RX Cas, several classical symbiotic stars, etc. A model has been calculated for the dust envelope of FG Sge, which formed around the star as a result of several successive cycles of dust condensation beginning in Autumn 1992. Several dust-condensation episodes have been traced in the envelopes of symbiotic systems (CH Cyg, V1016 Cyg, HM Sge, etc.), as well as the role of the hot component in the formation of the dust envelopes. We have established from variations of the IR emission that the cool components in the symbiotic novae V1016 Cyg and HM Sge, and possibly CH Cyg, are Miras. The binarity of V1016 Cyg and HM Sge has also been firmly established. The variability of a whole series of object has been studied, including the stellar components of close binary systems and several dozen Mira and semi-regular variables. The ellipsoidality of the components in the RX Cas system (a prototype W Ser star) and the cool component in the symbiotic systems CI Cyg and BF Cyg has been firmly established. We have obtained the first IR light curve for the eclipsing system V444 Cyg (WN5+O6), and determined the wavelength dependence of the obtained parameters of the WN5 star. Analysis of the IR light curves of several novae indicate the condensation of dust envelopes in the transition periods of Cygnus 1992, Aquila 1993, and Aquila 1995. The IR light curve of R CrB has been obtained over a long period and analyzed. IR observations of the nova-like variable V4334 Sgr have been carried out over four years, over which the star passed through four stages during its motion along its post-AGB evolutionary track; the star’s bolometric flux and optical depth of its dust envelope have been estimated, and the structure and mass of the dust layer determined. We have analyzed the IR variability of the symbiotic star V407 Cyg over 14 years, and found its cool component to be a Mira with a period of 745 days. The observed pulsations and trend are associated with the luminosity and temperature variations of the Mira, as well as the optical depth of the dust envelope. The size of the dust grains and mass-loss rate of the Mira have been determined. We have obtained JHKL light curves for the Seyfert galaxy NGC 4151 over 23 years. The IR brightness of the galaxy grew from 1985 through 1996 (by ∼0.9 ^m at 1.25 μm, ∼1 ^m at 1.65 μm, ∼1.1 ^m at 2.2 μm, and ∼1.3 ^m at 3.5 μm), while the galaxy simultaneously reddened. The “cool” variable source in NGC 4151 was still in the active state in 1998, although its luminosity had decreased by approximately 15%–20%. If the “cool” component of the variable source in this galaxy is a dust envelope heated by the central “hot” source, it should be optically thin to the radiation of this source: its mean optical depth is in the range 0.05–0.15. Emission from dust particles heated to temperatures of 600–800 K was observed in the near IR at a distance of several parsecs from the nucleus during the period of activity in 1995–1998; the inferred mass of emitting dust was 5–20 M _⊙. In 1994–2003, we observed a tendency for NGC 4151 to become bluer at 1.25–1.65 μm while simultaneously reddening at 2.2–3.5 μm. Beginning in Autumn 2000, the galaxy began to emerge from a minimum, which lasted from March 2000 through April 2001 in the IR; a flare of the galactic nucleus was observed and followed in detail in the IR in this same period. We confirm the IR variability of the nucleus of the Seyfert galaxy NGC 1068, which can be located in various stages of activity. The variability of NGC 1068 is associated with a complex source. A periodic component has been detected in the J brightness variations of the oxygen Mira V2108 Oph; we have shown that this star is immersed in a fairly dense dust envelope, and have calculated amodel for this envelope. We have calculated an (axially symmetrical) dust-envelope model for the carbon semi-regular variable RWLMi with a density distribution characteristic for the “superwind” stage. This envelope model is able to reproduce the observed fluxes over the entire observable spectrum, and displays a good agreement with the observational data. The three hot supergiants V482 Cas, QZ Sge, and HD 338926 may be variable in the IR. Long-term photometry of eight planetary nebulae in the near IR (1.25–5 μm) has enabled us to firmly establish the IR brightness and color variability of these nebulae on time scales from several tens of days to six-to-eight years. We have analyzed long-term JK photometry of the X-ray binary Cyg X-1 (V1357 Cyg). Periodicity with a characteristic time scale of ∼11.5 years is visible in the JK brightness variations of Cyg X-1 in 1984–2007, possibly due to periodic variations of the temperature, radius and luminosity of the optical component of the binary with P ∼11.5 yr. Fourteen-year IR light curves of five RV Tau stars (R Sge, RV Tau, AC Her, V Vul, and R Sct) and the yellow supergiant and protoplanetary-nebula candidate V1027 Cyg have been analyzed. A spherically symmetrical dust-envelope model has been calculated for V1027 Cyg.     

Structure of the circumbinary envelopes of young binary stars with elliptical orbits

The results of two-dimensional gas-dynamical numerical simulations of the structure of matter flows in the envelopes of a number of T Tauri binary systems with elliptical orbits are considered. The main flow elements in inner regions of protoplanetary disks of these stars are described. The influence of shocks on the size of the gap—a rarified region in the inner parts of the protoplanetary disk—is analyzed. A method is proposed for estimating the size of this gap from the numerical simulations, and the gap sizes for the studied stars are determined and compared with observational results. The flow dynamics in the gap is considered, and the periodic variations of the gap size on time scales of several orbital periods are analyzed. Possible observational manifestations of the studied flows are discussed.     

Photometry and polarimetry of the classical Herbig Ae star VV Ser

We present the results of our long-term photometric and polarimetric observations of the classical Herbig Ae star VV Ser, performed at the Crimean Astrophysical Observatory as part of a program of photometric and polarimetric monitoring of UX Ori stars. We recorded an unusually deep minimum of VV Ser (ΔV≈3^m), with a turn of the color tracks in the V-(U-B) and V-(B-V) diagrams (“the blueing effect”) observed for the first time for this star. The increase of the linear polarization during the minimum brightness was consistent with expectations for variable circumstellar extinction models, and the maximum polarization in the B band reached a record value for UX Ori stars in the deepest part of the minimum (12.8±1.4%). Our results cannot be explained by models with an axially symmetrical circumstellar dust disk consisting of silicate grains. They point to the existence of a large-scale nonuniformity in the azimuthal dust distribution near VV Ser attributable to the presence of a second component or protoplanet.     

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.     

Molecular structure of brown-dwarf disks

We describe typical features of the chemical composition of proto-planetary disks around brown dwarfs. We model the chemical evolution in the disks around a low-mass T Tauri star and a cooler brown dwarf over a time span of 1 Myr using a model for the physical structure of an accretion disk with a vertical temperature gradient and an extensive set of gas-phase chemical reactions. We find that the disks of T Tauri stars are, in general, hotter and denser than the disks of lower-luminosity substellar objects. In addition, they have more pronounced vertical temperature gradients. The atmospheres of the disks around low-mass stars are more strongly ionized by UV and X-ray radiation, while less dense brown-dwarf disks have higher fractional ionizations in their midplanes. Nevertheless, in both cases, most molecules are concentrated in the so-called warm molecular layer between the ionized atmosphere and cold midplane, where grains with ice mantles are abundant.     

Possibilities for identifying FK com candidates using observations with the Kepler Space Telescope

High-accuracy photometric observations obtained with the Kepler Space Telescope are used to identify candidate FK Com stars-a very rare group of single, rapidly rotating, chromospherically active G-K stars. Published data for more than 40 000 stars are used with available Kepler observations from the Q3 time interval to select four stars with temperature ranges, surface gravities, and rotation periods consistent with those of FK Com stars. These stars also display brightness variations with considerable amplitudes, possibly testifying to the presence of appreciably spotting on their surfaces. The rotation periods of these stars are determined, and the parameters of their differential rotation estimated. The locations (longitudes) of the dominant active regions on the stellar surfaces are identified. In all cases, the active longitude does not remain constant, andmoves across the stellar surface with time. In general, the character of this activeregion movement is the same as that found earlier for FK Com and HD 199178. These displacements are characterized by monotonic motions over hundreds of days, as well as changes in the positions by about 180° (“flip-flops”) or phase shifts not exceeding 0.4 in phase. The number of active-longitude position changes during the studied time interval ranges from one for KIC 11862915 to seven for KIC 5785906 (five phase shifts are also detected for the latter star). The time scale for the position changes of the active longitudes is from 1500 days (about 4 years) to 200 days (0.54 years), comparable to the reported time intervals between flip-flops for FK Com (from 0.8 to 4.4 years). The duration of the stellar activity cycles are estimated by analyzing the amplitude spectrum for changes in the brightness-variation amplitudes for datasets covering a single rotation period. The photometric variations of the stars on various time scales (from the rotation period, which reveals the presence of surface temperature inhomogeneities, to activity cycles lasting for several years) are similar to those derived for FK Com and other stars of this type. The need for spectroscopic observations of the selected candidates to establish whether they are single (do not show signs of binarity), look for emission lines of chromospheric origin, estimate the lithium abundances, and determine the stellar rotation velocities from spectral-line profiles is noted.     

Variations in the accretion rate and luminosity in gravitationally unstable protostellar disks

Self-consistent modeling of a protostar and protostellar disk is carried out for early stages of their evolution. The accretion rate at distances of sevral astronomical units from the protostar is appreciably variable, which is reflected in the protostar’s luminosity. The amplitude of the variations in the accretion rate and luminosity grows together with the sampling period, as a consequence of the nature of gravitationally unstable protostellar disks. A comparison of model luminosity variations with those derived from observations of nearby sites of star formation shows that the model variations are appreciably lower than the observed values for sampling periods of less than 10 years, indicating the presence of additional sources of variability on small dynamical distances from the protostar.     

Multicolor photoelectric WBVR observations of the close binary system HZ Her = Her X-1 in 1986–1988. Method and observations

We present results of four-color (WBVR) photoelectric observations of the close binary HZ Her = Her X-1 in 1986–1988. As a rule, the duration of the observations exceeded two 35-day X-ray orbital periods in the 1986–1988 observing seasons. The accuracy and length of the photoelectric observations facilitated multi-faceted studies, which enabled us to define several fine photometric effects in the light curves of the binary more precisely and attempt to interpret them in a model for the matter flow from the optical component to the accretion disk around the neutron star. This model provides a satisfactory explanation for the inhomogeneity of the gas flow and “hot spot,” as well as the existence of distinct “splashes” moving in their own Keplerian orbits around the outer parts of the Keplerian disk. We present series of light curves for all the observing seasons, as well as color-color diagrams that reflect the physics of various photometric effects. The transformation coefficients for each of the instrumental systems for the three observatories at which the observations were carried out are given. Atmospheric extinction was taken into account duringmulti-color observations of the object, with subsequent correction for atmospheric effects with accuracies ranging from 0.003 ^m to 0.005 ^m for air masses up to M(z) = 2.     

The rotation of the Sun as a star from the green-line emission of the entire corona

A new representation for the database created by J. Sykora on the 5303 Å Fe XIV line emission observed from 1939 to 2001 is proposed. Observations of the corona at an altitude of 60″ above the limb reduced to a uniform photometric scale provide estimates of the emission of the entire visible solar surface. It is proposed to use the resulting series of daily measurements as a new index of the solar activity, GLSun (The Green-Line Sun). This index is purely observational and is free of the model-dependent limitations imposed on other indices of coronal activity. GLSun describeswell both the cyclic activity and the rotational modulation of the brightness of the corona of the Sun as a star. The GLSun series was subject to a wavelet analysis similar to that applied to long-term variability in the chromospheric emission of late-type active stars. We obtain that the brightness inhomogeneities in the solar corona rotate more slowly during epochs of high activity than their average rotational rate over the entire time observations. The time interval of slower rotation of the inhomogeneities is close to the epoch when the Sun’s field represents a horizontal magnetic dipole in each activity cycle, but is somewhat longer than the duration of the polarity reversal in both hemispheres. The difference between the periods for the slower and mean rotation exceeds three days, as is typical for some stars with higher but less regular activity than solar one. The importance of these findings for dynamo theory for the origin and evolution of the magnetic fields of the Sun and other late-type stars is briefly discussed.