UX Ori Stars Research Articles

THE NEAR INFRARED AND OPTICAL PHOTOMETRIC ACTIVITY OF V517 Cyg

A near infrared and optical photometric study of Herbig star V517 Cyg has been carried out. Infrared data were obtained in 2003–2017 at the Campo Imperatory Observatory (Italy) with the Pulkovo telescope AZT-24 in Johnson’s \(JHK\) bands. Optical light curves in Johnson \(UBVR\) bands were obtained at the Maidanak observatory. Additional optical photometry from different sources (ASSAS, WISE, and AAVSO) was also used. It was shown that the behavior of V517 Cyg in the near infrared is typical for UX Ori stars. A considerable contribution to the near infrared variability is due to variable extinction along the line of sight, but the disk radiation becomes strong in the \(H\) and \(K\) bands: there is a significant correlation of \(V\) and \(J\) magnitudes (\(r \sim 0.84\)), while \(H\) and \(K\) bands correlates poorly with \(V\) band. The amplitude of variability in \(J\) is quite large (\( \sim {\kern 1pt} {{1.8}^{m}}\)). The deepest minimum (\(\Delta V \sim {{3.6}^{m}}\)) in the \(V\) band demonstrates quasi-periodic variations with an amplitude of \( \sim {\kern 1pt} {{0.8}^{m}}\) and a period of \( \sim {\kern 1pt} 19\) days, the origin of which is still unclear. It is possible that these quasi-periodic variations are related to rotation period of a second companion, a cold T Tauri star. The spectrum V517 Cyg reveals typical for UX Ori stars the double-peaked emission line Hα. The NaI D doublet has inverse P Cyg profiles, indicating intense accretion of gas onto the star. The accretion rate from an equivalent width of the Hα line is \({{\dot {M}}_{{acc}}} = 3.6 \times {{10}^{{ - 8}}}\,{{M}_{ \odot }}\) per year.

On the Nature of the Rapid Color Changes of the UX Ori Star RY Lupi at Deep Minima

We propose an explanation for the rapid color changes on the color–magnitude diagrams observed by Gahm et al. in the T Tauri star RY Lup during its deep minima. Our calculations have shown that the hot accretion spot on the stellar surface in combination with the inhomogeneous structure of the gas–dust clouds obscuring the star can be responsible for these changes. The observed rate of of the color changes allows one to estimate the velocity of the screen across the stellar disk =100 km s-1. This velocity is close to the typical gas velocities near T Tauri stars.

On the Nature of the Rapid Color Changes of the UX Ori Star RY Lupi at Deep Minima

On the Nature of the Rapid Color Changes of the UX Ori Star RY Lupi at Deep Minima

Modelling UX Ori star eclipses based on spectral observations with the Nordic Optical Telescope − I. RR Tau

ABSTRACT Based on observations obtained with the Nordic Optical Telescope (NOT) we investigate the spectral variability of the Herbig Ae star RR Tau. This star belongs to the UX Ori family, characterized by very deep fadings caused by the screening of the star with opaque fragments (clouds) of the protoplanetary discs. At the moments of such minima one observes strong spectral variability due to the fact that the dust cloud occults, for an observer, not only the star but also a part of the region where the emission spectrum originates. We calculated a series of obscuration models to interpret the observed variability of the H α line parameters. We consider two main obscuration scenarios: (1) the dust screen rises vertically above the circumstellar disc, and (2) the screen intersects the line-of-sight moving azimuthally with the disc. In both cases, the model of the emission region consists of a compact magnetosphere and a magnetocentrifugal disc wind. Comparison with observations shows that the first scenario explains well the variability of the radiation flux, the equivalent width, as well as the asymmetry of the H α line during eclipses, while the second scenario explains them only partly. This permits us to suggest that in the case of RR Tau, the main causes of the eclipses are either a structured disc wind, or the charged dust lifted along the field lines of the poloidal component of the magnetic field of the circumstellar disc.

Magnetospheric accretion at the late phases of the pre-main-sequence evolution: the case of RZ Psc

ABSTRACT It has been shown that during the burst of accretion activity observed in UX Ori-type star RZ Psc in 2013, the accretion rate increased approximately by an order of magnitude. This means that the accretion process at the late stages of the pre-main sequence evolution is very unstable. Using the spectra obtained during this episode, we have studied the magnetospheric emission in the H α line. Models of magnetospheric accretion are calculated to obtain the parameters of the magnetosphere from this observation. In this work, we have taken into account the influence of the recombination delay effect during gas motion in the stellar magnetosphere. The accounting for this effect and the presence of the magnetospheric absorption in the IR Ca ii triplet lines and its absence in D Na i resonance lines allowed us to place a lower limit on the temperature in the magnetosphere at $\approx 10\, 000$ K, which significantly improved precision of our estimate of accretion rate. According to the best-fitting model, the logarithm of accretion rate is $\log \dot{M} = -10.1\pm 0.3$ ($\dot{M} \approx 7\times 10^{-11}\ \mathrm{M_\odot \, yr^{-1}}$) and the inclination angle of RZ Psc is 43° ± 3°. It is less than the inclination, typical for the UX Ori stars (about 70°), that explains the weak photometric variability of this star. Using the obtained accretion rate and magnetosphere radius, we estimate the strength of the dipole component of the magnetic field of RZ Psc ≈ 0.1 kGs.

UX Ori stars eclipses by large-scale disc perturbations

ABSTRACT We simulate the polarized radiative transfer in the vicinity of the UX Ori type stars during their minima. Our model of an eclipse by an extended disc perturbation generalizes the compact gas-dust cloud eclipse model. We apply the radiative transfer method based on enumeration using the directions grid to model the influence of the perturbation extensions along azimuth and radius on the eclipse depth and parameters of the linear polarization. We investigate eclipses both for the flared disc and for the disc with a puffing-up in the dust sublimation zone. The puffing-up is obtained by adding a dusty disc wind to the model. Comparison with a compact cloud eclipse model reveals that the eclipse by a large-scale azimuthally extended perturbation may be significantly deeper and show a greater linear polarization degree. We also demonstrate that the perturbation extension together with the disc puffing-up can strongly affect the degree of polarization and colour index of the star during the eclipse. The position angle of the linear polarization may also change markedly during and after an eclipse by a large-scale perturbation for the model with a puffed-up inner rim. Also, in this model, the maximum degree of the linear polarization can be achieved not at the brightness minimum but closer to the end of the eclipse. We discuss the modelling results in the context of the photopolarimetric observations of UX Ori stars.

Modeling of large-scale disk perturbation eclipses of UX Ori stars with the puffed-up inner disks

Abstract Eclipses of UX Ori stars by compact gas–dust clouds and large-scale circumstellar disk perturbations are modeled. A flared disk and a disk with a puffing-up in the dust sublimation zone are considered. The disk puffing-up explains several observed features of eclipses. The linear polarization degree can remain unchanged during the eclipse. There might be no star reddening in the blue and ultraviolet spectral regions. Strong changes (up to 90 ° 90\text{°} ) in the positional angle of the linear polarization may happen when passing from one spectral band to another. An eclipse by a large-scale disk perturbation can be noticeably deeper than an eclipse by a compact gas–dust cloud. The polarization degree in such an eclipse can also be significantly higher. In addition, an eclipse by a large-scale perturbation of a star with a puffed-up disk explains the significant scatter of eclipse parameters at the same fading level. A significant change in the positional angle of the linear polarization may occur during a large-scale disk perturbation eclipse minimum and after it. We attempted parametric identification of long-lasted deep minima of UX Ori and WW Vul. The considered simple model can only partially explain the observed phenomena and need further development.

Color Indices of T Tauri Stars in Variable Circumstellar Extinction Models

Variable circumstellar extinction models as applied to T Tauri stars are considered. In contrast to hotter UX Ori stars, for which simplified eclipse models are applicable, the star is deemed a point source of light, while the circumstellar dust clouds obscuring the star are assumed to be homogeneous. In the case considered here, both these simplifications may not work. Hot accretion spots are formed through accretion on T Tauri stars, while the dust structures obscuring the star from the observer can be inhomogeneous within the stellar disk. The families of color–magnitude diagrams have been computed by taking into account these two factors. It is shown that in such models the color tracks on the diagrams can differ greatly in shape from those computed in eclipse models for hot UX Ori stars. The results obtained may be applicable to AA Tau stars demonstrating eclipses by the inner circumstellar disk regions.

Influence of the Disk Wind on the Intrinsic Polarization of Young Stars

The behavior of the linear polarization parameters of UX Ori stars during their eclipses by circumstellar dust clouds is studied. A circumstellar disk with a disk wind creating a puffing in the dust sublimation zone is considered. We show that the disk puffing can strongly affect the degree of polarization and color index of the star during its eclipse. A strong wind can change the orientation of the plane of linear polarization. The scattered radiation from a thin disk is polarized perpendicularly to its plane, but the radiation from a disk with a strong wind can be polarized along the disk plane. A situation where the disk-scattered radiation is not polarized in a certain spectral band is possible owing to the disk puffing. There can be different orientations of the linear polarization of the disk radiation in different spectral bands.

Variable Circumstellar Extinction in UX Ori Stars of Late Spectral Types: V695 Per

Eclipses of young stars by dust fragments in protoplanetary disks are the main reason for stochastic variability in the luminosity of UX Ori stars. In analyses and modeling of these kinds of eclipses, the star is often treated as a point light source. This approach is fully justified in the case of hot stars in this family, since the sublimation radius for the dust in their circumstellar disk is much greater than the star’s radius. This approach may not apply to the case of cold stars, since the evaporation zone for the circumstellar dust is close to the star. Here it is shown that accounting for the finite size of the radiation source leads to a change in the shape of a color track on a “color-magnitude” diagram. Calculations for the star V695 Per simultaneously illustrate the variability of the brightness of UX Ori and AA Tau stars for the coldest currently known star of this type.

GaiaDR2 study of Herbig Ae/Be stars

Aims.We useGaiaData Release 2 (DR2) to place 252 Herbig Ae/Be stars in the Hertzsprung–Russell diagram and investigate their characteristics and properties.Methods.For all known Herbig Ae/Be stars with parallaxes inGaiaDR2, we collected their atmospheric parameters and photometric and extinction values from the literature. To these data we added near-infrared and mid-infrared photometry, and collected Hαemission line properties such as equivalent widths and line profiles, and their binarity status. In addition, we developed a photometric variability indicator fromGaia’s DR2 information.Results.We provide masses, ages, luminosities, distances, photometric variabilities and IR excesses homogeneously derived for the most complete sample of Herbig Ae/Be stars to date. We find that high-mass stars have a much smaller IR excess and have much lower optical variabilities compared to lower-mass stars, with the break at around 7M⊙. Hαemission is generally correlated with IR excess, with the correlation being stronger for IR emission at wavelengths tracing the hot dust closest to the star. The variability indicator as developed by us shows that ∼25% of all Herbig Ae/Be stars are strongly variable. We observe that the strongly variable objects display doubly peaked Hαline profiles, indicating an edge-on disk.Conclusions.The fraction of strongly variable Herbig Ae stars is close to that found for A-type UX Ori stars. It had been suggested that this variability is in most cases due to asymmetric dusty disk structures seen edge-on. The observation here is in strong support of this hypothesis. Finally, the difference in dust properties occurs at 7M⊙, while various properties traced at UV/optical wavelengths differ at a lower mass, 3M⊙. The latter has been linked to different accretion mechanisms at work, whereas the differing IR properties and photometric variabilities are related to different or differently acting (dust-)disk-dispersal mechanisms.

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 1m 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.

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 ≈ 0.68m. 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.

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.

Possible Reasons for the Slow Rotation of BF Ori

Possible reasons for the very low projected rotation velocity of BF Ori compared to other UX Ori stars are discussed. The hypothesis of a close companion that slows down the star’s rotation by a tidal interaction is examined. Based on a theory of synchronization and modern models of evolution, the interaction is calculated numerically for different masses of the companion and values of the semi-major axis. It is shown that in order to obtain the projected velocity observed for BF Ori, the companion must have a mass greater than 0.5M⨀ . Such a large companion should have been discovered observationally. It is suggested that the low rotation velocity of BF Ori is more likely to be related to the distribution of the angular momentum of a protostellar cloud between the angular momentum of the star and the orbital angular momentum of a low-mass companion.

Magnetic propeller effect in the spectra of young stars

The origin of the blueshifted narrow absorption components in the resonance sodium doublet lines observed in the spectra of some young stars is discussed. Such components are assumed to be formed by the interaction of the circumstellar gas with the stellar magnetosphere in the magnetic propeller regime. The results of observations for the post UX Ori star RZ Psc are discussed in detail. This star shows distinctive signatures of mass outflow in the absence of any clear accretion signatures. Such a picture is quite possible in the magnetic propeller regime. Estimates show that for this regime to be realized, the star must have a surface magnetic field of ~1 kG at an accretion rate that does not exceed 10-10M. yr-1.

UX Ori Variables in the Cluster IC 348

Results are presented from many years of photometric (VR C I C ) observations of three variable T Tauri type stars in the cluster IC 348: V712 Per, V719 Per, and V909 Per. All three stars have photometric activity characteristic of UX Ori stars. The activity of V719 Per has increased significantly over the last 10 years: the amplitude of its Algol-like minima has increased by roughly a factor of 4 and has reached three stellar magnitudes in the I band. Periodograms of the light curves do not confirm the periods found previously by other authors on the basis of shorter series of observations. The slope of the color tracks on “color-magnitude” diagrams is used to determine the reddening law for these stars owing to selective absorption by circumstellar dust. Modelling of these parameters by the Mie theory shows that the maximum size a max of the dust particles in the protoplanetary disks of these stars is 1.5-2 times greater than in the interstellar medium. In V712 Per and V909 Per, the bulk of the mass of the dust particles is concentrated near a max , while in V719 Per the average mass of the dust particles is determined by the minimum size of the particles. It should be emphasized that these conclusions rely on an analysis of the optical variability of these stars.

An in-depth analysis of the RZ Piscium atmosphere

The star RZ Psc is one of the most unusual members of the UX Ori star family. It demonstrates all properties that are typical of these stars (the light variability, high linear polarization in deep minima, the blueing effect), but until recently there has been no obvious evidence of its youth. Recently, we have shown that RZ Psc is in the transitional evolutionary stage between stars with the primordial and debris disks. In this paper we present the results of the quantitative analysis of the star’s spectrum. Using the high-resolution (R = 46 000) spectrum of RZ Psc obtained with Nordic Optical Telescope, we determined T eff = 5350 ± 150 K, log g = 4.2 ± 0.2, [M/H] = −0.3 ± 0.05. These parameters agree well with the same ones as for the stars recently passed the T Tauri star evolutionary stage. This supports our previous suggestion about the evolutionary status of RZ Psc as the post-T Tauri and post-UX Ori star.

Observation of Circumstellar Gas in the Neighborhood of RZ Psc

The first evidence is found of the existence of circumstellar gas in the nearest surroundings of the UX Ori star RZ Psc. Spectra obtained at the Terskol Observatory, Special Astrophysical Observatory (SAO), and the Nordic Optical Telescope (NOT) reveal a strong variability in the sodium doublet lines that is indicative of a sporadic outflow of matter. Weak variability was also observed in the core of the Hα line. One nontrivial feature of this discovery is that RZ Psc is of spectral class K0 IV. This means that the star has no intrinsic energy resources for creating the observed outflow of matter. There are no emission lines in the star’s spectrum which might indicate that matter is falling into the star so that the observed outflow could be related to an accretion process. We suggest, nevertheless, that the ejection of gas is related to residual (slow) accretion and is driven by a propeller mechanism. The latter is possible if the star has a sufficiently high (on the order of 103 G) magnetic field.

On the kinematic age of RZ Psc

RZ Psc belongs to the family of young UX Ori stars whose photometric activity is due to strong extinction variations in the circumstellar disks surrounding them. However, in contrast to all the remaining stars of this type, no evidence of youth has been detected for RZ Psc until recently. A rough estimate of the star’s kinematic age was made for the first time in our previous paper (Grinin et al. 2010). It shows that RZ Psc is intermediate in its evolutionary status between young stars in Orion and stars with debris disks. In this paper, we provide a refined estimate of the kinematic age for the star confirming this conclusion. According to this estimate, the age of RZ Psc is approximately 25 ± 5 Myr at M* = 1M⊙.