We review the wide range of observed properties of Herbig Ae/Be stars and try to combine this rich data set into a consistent picture of their circumstellar environment and evolutionary status. We discuss in some detail the geometry of the circumstellar environment. The presence of disks in Herbig Ae/Be stars is inferred from direct and indirect observational evidence. Envelopes can dominate the spectral energy distributions on large angular scales. In some stars, a fairly massive disk persists until the star has reached the main sequence. The evidence for an evolutionary link between isolated Herbig Ae/Be stars and fl Pictoris is summarized.

HERBIG Ae/Be STARS

UBVR photometric monitoring of Herbig Ae/Be stars and some related objects has been carried out at Maidanak Observatory in Uzbekistan since 1983. More than 71,000 observations of about 230 stars have been obtained and are made available for anonymous ftp. Virtually all Herbig Ae/Be stars observed are irregular variables (called "UXors" after UX Ori), but there is a wide range of amplitudes from barely detectable to more than 4 mag in V. Our data confirm the results of previous studies, which indicate that large-amplitude variability is confined to stars with spectral types later than B8. The distribution of variability ranges is quite similar to what is seen in classical T Tauri stars. A careful search has failed to reveal any evidence for periodic variations up to 30 days, which can be interpreted as rotation periods. This is a clear distinction between the light variations of low-mass and high-mass pre–main-sequence stars. The Herbig Ae/Be stars evidently do not possess either the large, stable cool spots or persistent hot spots associated with strong surface magnetic fields and magnetically funneled accretion in classical T Tauri stars. A wide variety of shapes, timescales, and amplitudes exists, but the most common behavior is well illustrated by the light curve of LkHα 234. There are two principal components: (1) irregular variations on timescales of days around a mean brightness level that changes on a much longer timescale (typically years), sometimes in a quasi-cyclic fashion, and (2) occasional episodes of deep minima, occurring at irregular intervals but more frequently near the low points of the brightness cycles. Our data suggest that many T Tauri stars of K0 and earlier spectral type share the same variability characteristics as Herbig Ae/Be stars and should be regarded as UXors. Two FU Orionis stars ("FUors"), FU Ori and V1515 Cyg, also have recent light curves that are similar, in some respects, to UXors. The most developed model to account for the variations of some large-amplitude UXors involves variable obscuration by circumstellar dust clumps orbiting the star in a disk viewed nearly edge-on. However, there are problems in extending this model to the entire class, which lead us to propose an alternative mechanism, i.e., unsteady accretion. Evidence favoring the accretion model over the obscuration model is presented. It is suggested that the thermal instability mechanism responsible for outbursts in interacting binary system disks, and possibly FUors, may be the cause of the deep minima in UXors.

https://iopscience.iop.org/article/10.1086/300966/pdf

A Photometric Catalog of Herbig Ae/Be Stars and Discussion of the Nature and Cause of the Variations of UX Orionis Stars

Context. This work presents the characterization of 27 young early-type stars, most of them in the age range 1‐10 Myr, and three ‐suspected‐ hot companions of post-T Tauri stars belonging to the Lindroos binary sample. Most of these objects show IR excesses in their spectral energy distributions, which are indicati ve of the presence of disks. The work is relevant in the fields o f stellar physics, physics of disks and formation of planetary systems. Aims. The aim of the work is the determination of the effective temperature, gravity, metallicity, mass, luminosi ty and age of these stars. An accurate modelling of their disks needs, as a previ ous step, the knowledge of most of these parameters, since they will determine the energy input received by the disk and hence, its geometry and global properties. Methods. Spectral energy distributions and mid-resolution spectra were used to estimate Teff, the effective temperature. The comparison of the profiles of the Balmer lines with synthetic pro files provides the value of the stellar gravity, g∗. High-resolution optical observations and synthetic spectra are used to estimate the metallicity, [M/H]. Once Teff, g∗ and [M/H] are known for each star, evolutionary tracks and isochrones provide estimations of the mass, luminosity, age and distances (or upper limits in some cases). The method is original in the sense that it is distance-indep endent, i.e. the estimation of the stellar parameters does n ot require, as it happens in other works, the knowledge of the distance to the object. Results. Stellar parameters (effective temperature, gravity, metallicity, mass, luminosi ty, age and distances ‐or upper limits) are obtained for the sample of stars mentioned above. A detailed discussion on some individual objects, in particular VV Ser, RR Tau, 49 Cet and the three suspected hot companions of post-T Tauris, is presented. Conclusions. These results, apart from their intrinsic interest, would b e extremely valuable to proceed a step further and attempt to model the disks surrounding the stars. The paper also shows the diffi culty posed by the morphology and behaviour of the system star+disk in the computation of the stellar parameters.

/pdf/parameters-of-herbig-ae-be-and-vega-type-stars-2xvxmj62ar.pdf

Parameters of Herbig Ae/Be and Vega-type stars ⋆

In this Letter we propose a new view on the phenomenon of Algol-type minima in the light curves of UX Orionis stars (UXORs). The idea is based on the earlier proposal by various authors that UXORs are nearly edge-on disks in which hydrodynamic fluctuations could cause clumps of dust and gas to cross the line of sight. However, early models of protoplanetary disks were based on the notion that these have a flaring geometry. If so, then it is mostly the outer regions of the disk that obscure the star. The timescales for such obscuration events would be too long to match the observed timescales of weeks to months. Recent two-dimensional self-consistent models of Herbig Ae/Be protoplanetary disks, however, have indicated that for Herbig Ae/Be star disks there exists, in addition to the usual flared disks, a new class of disks: disks that are fully self-shadowed. For these disks only their puffed-up inner rim (at the dust evaporation radius) is directly irradiated by the star, while the disk at larger radius resides in the shadow of the rim. For these disks there exist inclinations at which the line of sight toward the star skims the upper parts of the puffed-up inner rim, while passing high over the surface of outer-disk regions. These outer-disk regions therefore do not obscure the star nor the inner-disk regions, and small hydrodynamic fluctuations in the puffed-up inner rim could cause the extinction events seen in UXORs. If this idea is correct, it makes a prediction for the shape of the SEDs of these stars. It was shown by Dullemond and Dullemond, Dominik, & Natta that flared disks have a strong far-IR excess and can be classified as “Group I” (in the classification of Meeus et al.), while self-shadowed disks have a relatively weak far-IR excess and are classified as “Group II.” Our model therefore predicts that UXORs belong to the “Group II” sources. We show that this correlation is indeed found within a sample of 86 Herbig Ae/Be stars. Subject headings: circumstellar matter — planetary systems: protoplanetary disks — stars: pre–main-sequence — stars: variables: other

/pdf/explaining-ux-orionis-star-variability-with-self-shadowed-5gss845167.pdf

Explaining ux orionis star variability with self-shadowed disks

Context. AA Tau has been monitored for more than 20 years since 1987 and exhibited a nearly constant brightness level of V = 12:5 mag. We report here that in 2011 it suddenly faded, becoming 2 mag fainter in the V-band, and has remained in this deep state since then. Aims. We investigate the origin of the sudden dimming of the AA Tau system. Methods. We report on new optical and near-IR photometry and spectroscopy obtained during the fading event. Results. The system appears to be much redder and fainter than it was in the bright state. Moreover, the 8.2 d photometric period continuously observed for more than 20 years is not detected during most of the deep state. The analysis of the system’s brightness and colors suggests that the visual extinction on the line of sight has increased by about 3‐4 mag in the deep state. At optical wavelengths, the system appears to be dominated by scattered light, probably originating from the upper surface layers of a highly inclined circumstellar disk. The profiles of the Balmer lines have significantly changed as well, with the disappearance of a central absorption component regularly observed in the bright state. We ascribe this change to the scattering of the system’s spectrum by circumstellar dust. Remarkably, the mass accretion rate in the inner disk and onto the central star has not changed as the system faded. Conclusions. We conclude that the deepening of the AA Tau system is due to a sudden increase of circumstellar dust extinction on the line of sight without concomitant change in the accretion rate. We suggest that the enhanced obscuration may be produced by a nonaxisymmetric overdense region in the disk, located at a distance of 7.7 AU or more, that was recently brought into the line of sight by its Keplerian motion around the central star.

/pdf/aa-tauri-s-sudden-and-long-lasting-deepening-enhanced-13i9ytp7b5.pdf

AA Tauri’s sudden and long-lasting deepening: enhanced extinction by its circumstellar disk

High linear polarization (up to 5–8%) discovered by our group in the deep minima of isolated AE-Herbig stars is discussed in the framework of the model of ‘zodiacal light’ produced by scattering matter in a circumstellar dust envelope (probably in protoplanetary discs). The numerical simulations of polarizational and colourimetrical properties of the scattered light based on the Mie theory permit to obtain from the observations some important parameters of circumstellar dust: the size distribution of grains, their rough chemical composition (silicate/graphite), the flatness of the dust envelope.

N. Kh. Minikulov

Papers

The investigations of zodiacal light of isolated AE-Herbig stars with non-periodic Algol-type minima