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Photometric activity of UX orionis stars and related objects in the near infrared and optical: CO Ori, RR Tau, UX Ori, and VV Ser:
Abstract
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.
... The T Tauri primary star CO Ori A (spectral type G0; Calvet et al. 2004), located in the 6 − 7 Myr old λ Orionis region (Dolan & Mathieu 2001) displays strong levels of irregular photometric variability across optical and infrared (IR) wavelengths (e.g. Shenavrin et al. 2011Shenavrin et al. , 2016 leading to its identification as a UX Ori-type variable star. In addition to their high-amplitude variability (∆V ≈ 2 − 3 mag), UX Ori stars exhibit high linear polarization during their periods of minimum brightness (e.g. ...
The primary star in the young stellar object (YSO) binary CO Ori displays UX Ori-type variability: irregular, high amplitude optical and near-infrared photometric fluctuations where flux minima coincide with polarization maxima. This is attributed to changes in local opacity. In CO Ori A, these variations exhibit a 12.4 yr cycle. Here, we investigate the physical origin of the fluctuating opacity and its periodicity using interferometric observations of CO Ori obtained using VLTI/GRAVITY. Continuum K-band circum-primary and circum-secondary emission are marginally spatially resolved for the first time while Br emission is detected in the spectrum of the secondary. We estimate a spectral type range for CO Ori B of K2-K5 assuming visual extinction, and a distance of 430 pc. From geometric modelling of the continuum visibilities, the circum-primary emission is consistent with a central point source plus a Gaussian component with a full-width-half-maximum of 2.310.04 milliarcseconds (mas), inclined at 30.22.2 and with a major axis position angle of 406. This inclination is lower than that reported for the discs of other UX Ori-type stars, providing a first indication that the UX Ori phenomena may arise through fluctuations in circumstellar material exterior to a disc, e.g. in a dusty outflow. An additional wide, symmetric Gaussian component is required to fit the visibilities of CO Ori B, signifying a contribution from scattered light. Finally, closure phases of CO Ori A were used to investigate whether the 12.4 yr periodicity is associated with an undetected third component, as has been previously suggested. We rule out any additional companions contributing more than 3.6% to the K-band flux within ~7.3-20 mas of CO Ori A.
... In their 1999 paper Herbst & Shevchenko (13) state "It is probably not an exaggeration to say that the HAEBE stars are the only remaining class of variable stars with a substantial membership whose basic variability mechanism is still unclear." Shenavrin et al. (15) reported V-band observations of RR Tau by the All Sky Automated Survey (16) over 13 years beginning in 2002 which show several Algol-like deep minima. The BAAVSS database (17) shows RR Tau varying irregularly over a visual magnitude range of 10.2 to 14.5, equivalent to a factor of 50 in brightness, with occasional non-periodic Algol-like minima. ...
Multicolour broadband photometry and flux calibrated low resolution spectroscopy between 2014 and 2018 are used to investigate the behaviour of the highly variable Herbig Ae star RR Tauri. As the star experienced a deep Algol-like fade by a factor of over 10 in flux, we found the change in spectral continuum to be grey while H{\alpha} emission flux halved. We also observed the (V-Rc) colour index reddening as the star faded suggesting emission detectable in the Rc-band but beyond our spectral response. We confirm that the circumstellar reddening of RR Tau is consistent with an extinction ratio Rv = 5 and that its spectral type is close to A0. According to its luminosity and temperature RR Tau is located in the H-R diagram among other HAEBE stars contracting onto the Zero Age Main Sequence and has a mass between 4 and 5 MSun.
... The T Tauri primary star CO Ori A (spectral type G0; Calvet et al. 2004), located in the 6 − 7 Myr old λ Orionis region (Dolan & Mathieu 2001) displays strong levels of irregular photometric variability across optical and infrared (IR) wavelengths (e.g. Shenavrin et al. 2011Shenavrin et al. , 2016 leading to its identification as a UX Ori-type variable star. In addition to their high-amplitude variability (∆V ≈ 2 − 3 mag), UX Ori stars exhibit high linear polarization during their periods of minimum brightness (e.g. ...
The primary star in the young stellar object (YSO) binary CO Ori displays UX Ori-type variability: irregular, high amplitude optical and near-infrared photometric fluctuations where flux minima coincide with polarization maxima. This is attributed to changes in local opacity. In CO Ori A, these variations exhibit a 12.4 yr cycle. Here, we investigate the physical origin of the fluctuating opacity and its periodicity using interferometric observations of CO Ori obtained using VLTI/GRAVITY. Continuum K-band circum-primary and circum-secondary emission are marginally spatially resolved for the first time while Br emission is detected in the spectrum of the secondary. We estimate a spectral type range for CO Ori B of K2-K5 assuming visual extinction, and a distance of 430 pc. From geometric modelling of the continuum visibilities, the circum-primary emission is consistent with a central point source plus a Gaussian component with a full-width-half-maximum of 2.310.04 milliarcseconds (mas), inclined at 30.22.2 and with a major axis position angle of 406. This inclination is lower than that reported for the discs of other UX Ori-type stars, providing a first indication that the UX Ori phenomena may arise through fluctuations in circumstellar material exterior to a disc, e.g. in a dusty outflow. An additional wide, symmetric Gaussian component is required to fit the visibilities of CO Ori B, signifying a contribution from scattered light. Finally, closure phases of CO Ori A were used to investigate whether the 12.4 yr periodicity is associated with an undetected third component, as has been previously suggested. We rule out any additional companions contributing more than 3.6% to the K-band flux within ~7.3-20 mas of CO Ori A.
The results of photometric, polarimetric, and spectroscopic observations of the young star ZZ Tau IRS in the visible and near-infrared bands are presented. Against the continuum of an M spectral type star about 50 emission lines of allowed (H I, He I, Na I, S II) and forbidden (O I, O II, O III, N I, N II, S II, Ca II, Fe II, Ni II) transitions were identified. It was found that from the autumn of 2020 to the beginning of 2023, the brightness of the star in the visible region decreased (\Delta I\approx 1\overset{\textrm{m}}{.}5) and then began to return to the initial level. As the visible brightness of the star declined, its color indices decreased in the visible region, but increased in the near-IR bands. At light minimum, the degree of polarization in the I band reached approx 13\%, and the equivalent widths of, e.g., the H\alpha and [S II] \lambda 6731 lines increased to 376 and 79 {{\text{\AA}}^{\circ}}, respectively. Arguments are given in favor of ZZ Tau IRS being a UX Ori type star, and its variability being due to eclipses by dust clouds, which are inhomogeneities in the dusty disk wind. Forbidden lines are formed both in the disk wind and in the jet, the axis of what is oriented along PA=61^{\circ}\pm 3^{\circ}. The jet mass-loss rate exceeds 5\times 10^{-10}M_{\odot} yr{}^{-1}, what is abnormally large for a star with a mass less than 0.3M_{\odot}. Apparently, the disk wind of ZZ Tau IRS is not axially symmetric, probably due to the azimuthal asymmetry of the protoplanetary disk found earlier from ALMA observations.
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.
We present initial results of the first panoramic search for high-amplitude near-infrared variability in the Galactic plane.
We analyse the widely separated two-epoch K-band photometry in the fifth and seventh data releases of the UKIDSS Galactic plane survey. We find 45 stars with ΔK > 1 mag, including two previously known OH/IR stars and a Nova. Even though the mid-plane is not yet included in the data
set, we find the majority (66 per cent) of our sample to be within known star-forming regions (SFRs), with two large concentrations
in the Serpens OB2 association (11 stars) and the Cygnus-X complex (12 stars). Sources in SFRs show spectral energy distributions
that support classification as young stellar objects (YSOs). This indicates that YSOs dominate the Galactic population of
high-amplitude infrared variable stars at low luminosities and therefore likely dominate the total high-amplitude population.
Spectroscopic follow up of the DR5 sample shows at least four stars with clear characteristics of eruptive pre-main-sequence
variables, two of which are deeply embedded. Our results support the recent concept of eruptive variability comprising a continuum
of outburst events with different time-scales and luminosities, but triggered by a similar physical mechanism involving unsteady
accretion. Also, we find what appears to be one of the most variable classical Be stars.
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.
Moreover, the position angle of polarization in deep minima may be used for determination of the symmetry axis projection of circumstellar disc on the sky relative to the direction of local interstellar magnetic field. The latest is important in order to understand the role of the magnetic field at the initial phase of gravitational collapse of protostellar clouds.
Finally, the component of linear polarization which is due to the alignment of nonspherical circumstellar grains may be separate from the observed polarization under certain conditions.
We present an analysis of near-infrared time-series photometry in J, H, and K
bands for about 100 epochs of a 1 square degree region of the Lynds 1003/1004
dark cloud in the Cygnus OB7 region. Augmented by data from the Wide-field
Infrared Survey Explorer (WISE), we identify 96 candidate disk bearing young
stellar objects (YSOs) in the region. Of these, 30 are clearly Class I or
earlier. Using the Wide-Field imaging CAMera (WFCAM) on the United Kingdom
InfraRed Telescope (UKIRT), we were able to obtain photometry over three
observing seasons, with photometric uncertainty better than 0.05 mag down to J
~17. We study detailed light curves and color trajectories of ~50 of the YSOs
in the monitored field. We investigate the variability and periodicity of the
YSOs and find the data are consistent with all YSOs being variable in these
wavelengths on time scales of a few years. We divide the variability into four
observational classes: 1) stars with periodic variability stable over long
timescales, 2) variables which exhibit short-lived cyclic behavior, 3) long
duration variables, and 4) stochastic variables. Some YSO variability defies
simple classification. We can explain much of the observed variability as being
due to dynamic and rotational changes in the disk, including an asymmetric or
changing blocking fraction, changes to the inner disk hole size, as well as
changes to the accretion rate. Overall, we find that the Class I:Class II ratio
of the cluster is consistent with an age of < 1Myr, with at least one
individual, wildly varying, source ~ 100,000 yr old. We have also discovered a
Class II eclipsing binary system with a period of 17.87 days.
Based on long-term series of photoelectric observations of the UX Ori
stars BF Ori, RR Tau, and CO Ori, we obtained evidence for the cyclicity
of their photometric activity on a time scale from several years to ten
years or more. Since the photometric variability of this type of star is
attributable to variable circumstellar extinction and is determined by
the instantaneous dust column density, the presence of activity cycles
points to the existence in circumstellar dust disks of large-scale
nonuniformities in the dust distribution which periodically appear on
the line of sight. The components of a binary (or multiple) system or
protoplanets can be the source of such nonuniformities.
Context. V2492 Cyg is a young eruptive star that went into outburst in 2010.
The near-infrared color changes observed since the outburst peak suggest that
the source belongs to a newly defined sub-class of young eruptive stars, where
time-dependent accretion and variable line-of-sight extinction play a combined
role in the flux changes.
Aims. In order to learn about the origin of the light variations and to
explore the circumstellar and interstellar environment of V2492 Cyg, we
monitored the source at ten different wavelengths, between 0.55 \mu m and 2.2
\mu m from the ground and between 3.6 \mu m and 160 \mu m from space.
Methods. We analyze the light curves and study the color-color diagrams via
comparison with the standard reddening path. We examine the structure of the
molecular cloud hosting V2492 Cyg by computing temperature and optical depth
maps from the far-infrared data.
Results. We find that the shapes of the light curves at different wavelengths
are strictly self-similar and that the observed variability is related to a
single physical process, most likely variable extinction. We suggest that the
central source is episodically occulted by a dense dust cloud in the inner
disk, and, based on the invariability of the far-infrared fluxes, we propose
that it is a long-lived rather than a transient structure. In some respects,
V2492 Cyg can be regarded as a young, embedded analog of UX Orionis-type stars.
Conclusions. The example of V2492 Cyg demonstrates that the light variations
of young eruptive stars are not exclusively related to changing accretion. The
variability provided information on an azimuthally asymmetric structural
element in the inner disk. Such an asymmetric density distribution in the
terrestrial zone may also have consequences for the initial conditions of
planet formation.
We investigate the transition from primordial Population III (Pop III) star
formation to normal Pop II star formation in the first galaxies using new
cosmological hydrodynamic simulations. We find that while the first stars seed
their host galaxies with metals, they cannot sustain significant outflows to
enrich the intergalactic medium, even assuming a top-heavy initial mass
function. This means that Pop III star formation could potentially continue
until z~6 in different unenriched regions of the universe, before being
ultimately shut off by cosmic reionization. Within an individual galaxy, the
metal production and stellar feedback from Pop II stars overtake Pop III stars
in 20-200 Myr, depending on galaxy mass.
We present moderate-resolution optical spectra of the highly variable Herbig Ae star RR Tauri over 12 epochs spanning ~2.5 mag in V. Combining normalized spectra with contemporaneous photometry from two databases, we analyze both equivalent width and flux behavior as a function of system brightness for lines from the Ca II K line in the blue to the Paschen lines in the far red. The wings (Δv > 400 km s-1) of the Balmer lines and the equivalent widths of several weak metal lines are essentially constant, indicating very little change in the underlying photosphere over a factor of 10 change in brightness. We detect no measurable change in spectral type. Variability is apparent in the cores of Hα and Hβ, but the total flux in these lines is not correlated with photometric variability. Forbidden oxygen ([O I] λ6300) has essentially constant flux, indicating a stable low-density wind component. The low-ionization permitted lines of Fe II, Ca II, O I, and Na I are seen strongly in absorption for V ≤ 12.2 in these normalized spectra but change dramatically from absorption to emission during deep minima (V 12.6). Analysis of the Fe II (42) triplet indicates that these lines originate in circumstellar gas that is partially affected by the photometric minima, in that the absorbing gas changes with the stellar continuum (conserving equivalent width), while a weak emitting region is unaffected (roughly constant flux). Our results are consistent with a model in which the stellar minima are caused by an occulting screen of size such that it obscures the stellar surface and the innermost region of circumstellar gas producing permitted metal absorption lines but not the outer parts or the wind. The circumstellar hydrogen, while variable, is not strongly affected by the occultations.
Infrared photometry and spectroscopy covering a time span of a quarter-century are presented for HD 31648 (MWC 480) and HD 163296 (MWC 275). Both are isolated Herbig Ae stars that exhibit signs of active accretion, including driving bipolar flows with embedded Herbig-Haro (HH) objects. HD 163296 was found to be relatively quiescent photometrically in its inner disk region, with the exception of a major increase in emitted flux in a broad wavelength region centered near 3 μm in 2002. In contrast, HD 31648 has exhibited sporadic changes in the entire 3-13 μm region throughout this span of time. In both stars, the changes in the 1-5 μm flux indicate structural changes in the region of the disk near the dust sublimation zone, possibly causing its distance from the star to vary with time. Repeated thermal cycling through this region will result in the preferential survival of large grains, and an increase in the degree of crystallinity. The variability observed in these objects has important consequences for the interpretation of other types of observations. For example, source variability will compromise models based on interferometry measurements unless the interferometry observations are accompanied by nearly simultaneous photometric data.
We report extensive new photometry and spectroscopy of the highly variable
young stellar object PTF 10nvg including optical and near-infrared time series
data as well as mid-infrared and millimeter data. Following the previously
reported 2010 rise, during 2011 and 2012 the source underwent additional
episodes of brightening and dimming events including prolonged faint states.
The observed high-amplitude variations are largely consistent with extinction
changes having a 220 day quasi-periodic signal. Spectral evolution includes not
only changes in the spectral slope but correlated variation in the prominence
of TiO/VO/CO bands and atomic line emission, as well as anticorrelated
variation in forbidden line emission which, along with H_2, dominates optical
and infrared spectra at faint epochs. Neutral and singly-ionized atomic species
are likely formed in an accretion flow and/or impact while the origin of
zero-velocity atomic LiI 6707 in emission is unknown. Forbidden lines,
including several rare species, exhibit blueshifted emission profiles and
likely arise from an outflow/jet. Several of these lines are also seen
spatially offset from the continuum source position, presumably in a shocked
region of an extended jet. CARMA maps resolve on larger scales a spatially
extended outflow in mm-wavelength CO. We attribute the observed photometric and
spectroscopic behavior in terms of occultation of the central star as well as
the bright inner disk and the accretion/outflow zones that renders shocked gas
in the inner part of the jet amenable to observation at the faint epochs. We
discuss PTF 10nvg as a source exhibiting both accretion-driven (perhaps
analogous to V1647 Ori) and extinction-driven (perhaps analogous to UX Ori or
GM Cep) high-amplitude variability phenomena.
Context. The study of pulsation in pre-main-sequence intermediate mass stars represents an important tool for deriving information on the stellar parameters and structure, as well as for testing the validity of current theoretical models. The interest in this class of variable stars has significantly increased during the last decade and about 30 members are presently known in the literature. Aims. A new observational study of the Herbig Ae star VV Ser has been performed to detect and accurately measure pulsation frequencies in the δ Scuti range, thus enlarging the sample of known pulsators and contributing to the empirical definition of the pre-main-sequence instability strip. As it belongs to the continuous field of view of the asteroseismological satellite COROT, this study also aims at characterizing the properties of VV Ser as a potential "COROT additional program" candidate. Methods. CCD time series photometry in the Johnson V filter has been obtained for three consecutive years. The resulting light curves have been subject to detailed frequency analysis and the derived frequencies have been compared to model predictions. Results. Seven pulsation frequencies have been measured on the basis of the best data set obtained in 2004, ranging from ∼31 to ∼118 µHz, with an accuracy of the order of 0.5 µHz. The comparison with an extensive set of asteroseismological models shows that all the observed periodicities can be reproduced if the stellar mass is close to 4 M . Conversely, the measured frequencies can be associated with p modes only if the effective temperature is significantly lower than that obtained from the spectral type conversion. Conclusions. The present results seem to suggest that more accurate spectral type determination is necessary to discriminate the best-fit model solution. In any case, the stellar mass of VV Ser is close to the upper mass limit (∼4 M) for this class of pulsators.
We present the results of our infrared JHK photometry for the unusual UX Ori star V1184 Tau. Comparison with previous observations performed before the catastrophic decline in its optical brightness in 2004 (when the star faded approximately by a factor of 100) has shown the following: the star faded approximately by 2m
and 1m
in the J and H bands, respectively, while its K brightness remained almost constant. This pattern of infrared variability seems incompatible with the mechanism of variable circumstellar extinction responsible for the dramatic decline in the star’s optical brightness. However, if this mechanism is considered in the context of an accretion disk model with a puffed-up inner rim in the dust sublimation zone and with a disk wind producing an expanding gas-dust atmosphere above the disk surface, then the paradox can be resolved. In this model, the photometric activity of V1184 Tau in both visible and near-infrared spectral ranges, including the sharp brightness decline in 2004, can be explained by an increase in the geometric thickness of the disk in the dust sublimation zone caused by enhanced accretion of circumstellar matter onto the star. There is reason to believe that such events occur periodically and result from the presence of a companion to V1184 Tau moving in a highly eccentric orbit. The offered interpretation of the photometric activity of V1184 Tau allows this object to be classified as an UX Ori star based on the observed photometric effect and, at the same time, as a FU Ori star based on the pattern of the physical process that produced this effect.
In this paper we present the first comprehensive results extracted from the spectroscopic campaigns carried out by the EXPORT (EXoPlanetary Observational Research Team) consortium. During 1998-1999, EXPORT carried out an intensive observational effort in the framework of the origin and evolution of protoplanetary systems in order to obtain clues on the evolutionary path from the early stages of the pre-main sequence to stars with planets already formed. The spectral types of 70 stars, and the projected rotational velocities, , of 45 stars, mainly Vega-type and pre-main sequence, have been determined from intermediate- and high-resolution spectroscopy, respectively. The first part of the work is of fundamental importance in order to accurately place the stars in the HR diagram and determine the evolutionary sequences; the second part provides information on the kinematics and dynamics of the stars and the evolution of their angular momentum. The advantage of using the same observational configuration and methodology for all the stars is the homogeneity of the set of parameters obtained. Results from previous work are revised, leading in some cases to completely new determinations of spectral types and projected rotational velocities; for some stars no previous studies were available.
Aiming at statistically studying the variability in the mid-IR of young
stellar objects (YSOs), we have compared the 3.6, 4.5, and 24 um Spitzer fluxes
of 1478 sources belonging to the C2D (Cores to Disks) legacy program with the
WISE fluxes at 3.4, 4.6, and 22 um. From this comparison we have selected a
robust sample of 34 variable sources. Their variations were classified per
spectral Class (according to the widely accepted scheme of Class I/flat/II/III
protostars), and per star forming region. On average, the number of variable
sources decreases with increasing Class and is definitely higher in Perseus and
Ophiuchus than in Chamaeleon and Lupus. According to the paradigm Class <=>
Evolution, the photometric variability can be considered to be a feature more
pronounced in less evolved protostars, and, as such, related to accretion
processes. Moreover, our statistical findings agree with the current knowledge
of the star formation activity in different regions. The 34 selected variables
were further investigated for similarities with known young eruptive variables,
namely the EXors. In particular we analyzed : (1) the shape of the spectral
energy distribution (SED); (2) the IR excess over the stellar photosphere; (3)
magnitude versus color variations; and (4) output parameters of model fitting.
This first systematic search for EXors ends up with 11 bona fide candidates
that can be considered as suitable targets for monitoring or future
investigations.
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.
Young stars with masses 2-8 Suns, called the Herbig Ae and Be stars, often
show a near-infrared excess too large to explain with a
hydrostatically-supported circumstellar disk of gas and dust. At the same time
the accretion flow carrying the circumstellar gas to the star is thought to be
driven by magneto-rotational turbulence, which according to numerical MHD
modeling yields an extended low-density atmosphere supported by the magnetic
fields. We demonstrate that the base of the atmosphere can be optically-thick
to the starlight and that the parts lying near 1 AU are tall enough to double
the fraction of the stellar luminosity reprocessed into the near-infrared. We
generate synthetic spectral energy distributions (SEDs) using Monte Carlo
radiative transfer calculations with opacities for sub-micron silicate and
carbonaceous grains. The synthetic SEDs closely follow the median Herbig SED
constructed recently by Mulders and Dominik, and in particular match the large
near-infrared flux, provided the grains have a mass fraction close to
interstellar near the disk's inner rim.
The problem of the study of variable stars is examined in a
comprehensive manner. The principal types of variable stars are examined
with emphasis on their evolutionary status, physical mechanisms of
variability, and the relation of variable star studies to the
investigation of the structure of the Galaxy and stellar evolution. The
discussion covers pulsing, eruptive, and eclipsing variables;
discoveries and classifications of variable stars, and a short review of
the methods and organization of observations.
A computer-based catalogue of UBVRI photoelectric photometry of T Tauri
stars and their earlier type analogs has been compiled. It presently
includes over 10 000 entries on 80 stars and will be updated on a
regular basis; it is available on Internet. The catalogue is used to
analyze the sometimes bizarre light variations of pre-main-sequence
stars on time scales of days to months in an attempt to illuminate the
nature and causes of the phenomenon. It is useful in discussing their
light variations to divide the stars into three groups according to
their spectra. These are: weak T Tauri stars (WTTS; spectral class later
than K0 and WH-alpha less than 10 A, classical T Tauri stars
(CTTS; spectral class later than K0 and WH-alpha greater than
10 A), and early type T Tauri stars (ETTS; spectral class of K0 or
earlier). Three distinct types of variability are displayed by stars in
the catalogue. Type I variations are periodic in VRI and undoubtedly
caused by rotational modulation of a star with an asymmetric
distribution of cool spots on its surface. Irregular flare activity is
sometimes seen on such stars in U and B. Type I variations are easiest
to see on WTTS but are clearly present on CTTS and ETTS as well. Type II
variations are caused by hot 'spots' or zones and, it is argued, result
from changes in the excess or 'veiling' continuum commonly attributed to
an accretion boundary layer or impact zone of a magnetically channeled
accretion flow. This type of variation is seen predominantly or solely
in CTTS. A sub-category, designated Type IIp, consists of stars which
display periodic variations caused by hot spots. Whereas cool spots may
last for hundreds or thousands of rotations, hot spots appear to come
and go on a much shorter time scale. This suggests that both unsteady
accretion and rotation of the star contribute to Type II variations. It
is shown that a third type of variation exists among ETTS, including
stars as early as A type. UX Ori is a typical example and we call these
Type III variables or UXors. Their distinguishing characteristic is that
they can display very large amplitudes (exceeding 2.8 mag in V) while
showing little or no evidence for a veiling continuum or any substantial
change in their photospheric spectra. If Type III variations are caused
by changes in accretion luminosity, then boundary layers or impact zones
in ETTS must be much different from CTTS which, of course, is possible
since mass accretion rates are probably much higher. However, the
leading hypothesis for explaining Type III variations is variable
obscuration by circumstellar dust. <It is argued that the putative
dust clumps causing such variations cannot be confined to a disk;
otherwise UXors would be rare. &Perhaps magnetic effects are involved in
levitating accreting dust out of the plane, as has been suggested for
CTTS, or perhaps we are witnessing continuing infall of clumps from
placental clouds. A third possibility is that dust may be condensing in
an outflow.
We present comprehensive models for the Herbig Ae stars MWC 275 and AB Aur that aim to explain their spectral energy distribution (from UV to millimeter) and long-baseline interferometry (from near-infrared to millimeter) simultaneously. Data from the literature, combined with new mid-infrared (MIR) interferometry from the Keck Segment Tilting Experiment, are modeled using an axisymmetric Monte Carlo radiative transfer code. Models in which most of the near-infrared (NIR) emission arises from a dust rim fail to fit the NIR spectral energy distribution (SED) and sub-milliarcsecond NIR CHARA interferometry. Following recent work, we include an additional gas emission component with similar size scale to the dust rim, inside the sublimation radius, to fit the NIR SED and long-baseline NIR interferometry on MWC 275 and AB Aur. In the absence of shielding of starlight by gas, we show that the gas-dust transition region in these YSOs will have to contain highly refractory dust, sublimating at ~1850 K. Despite having nearly identical structure in the thermal NIR, the outer disks of MWC 275 and AB Aur differ substantially. In contrast to the AB Aur disk, MWC 275 lacks small grains in the disk atmosphere capable of producing significant 10-20 μm emission beyond ~7 AU, forcing the outer regions into the "shadow" of the inner disk.
Photoelectric UBVR observations of the young irregular variable star V586 Ori are presented and discussed.
Es werden UBVR-Beobachtungen des jungen veränderlichen Sternes V586 Ori dargestellt und ihre Ergebnisse diskutiert.
Protostellar systems, ranging from low-luminosity T Tauri and Herbig Ae stars
to high-luminosity Herbig Be stars, exhibit a near-infrared (NIR) excess in
their spectra that is dominated by a bump in the monochromatic luminosity with
a peak near 3 microns. The bump can be approximated by a thermal emission
component of temperature 1500 K that is of the order of the sublimation
temperature of interstellar dust grains. In the currently popular "puffed up
rim" scenario, the bump represents stellar radiation that propagates through
the optically thin inner region of the surrounding accretion disk and is
absorbed and reemitted by the dust that resides just beyond the dust
sublimation radius, Rsub. However, this model cannot account for the strongest
bumps measured in these sources, and it predicts a large secondary bounce in
the interferometric visibility curve that is not observed. In this paper we
present an alternative interpretation, which attributes the bump to reemission
of stellar radiation by dust that is uplifted from the disk by a centrifugally
driven wind. Winds of this type are a leading candidate for the origin of the
strong outflows associated with protostars, and there is observational evidence
for disk winds originating on scales ~Rsub. Using a newly constructed Monte
Carlo radiative transfer code, we show that this model can account for the NIR
excess emission even in bright Herbig Ae stars such as AB Auriga and MWC 275,
and that it successfully reproduces the basic features of the visibilities
measured in these protostars. We argue that a robust dusty outflow in these
sources could be self-limiting to a relatively narrow launching region between
Rsub and 2Rsub. Finally, we suggest that our model could also naturally account
for the NIR and scattered-light variability exhibited by a source like MWC 275,
which may be triggered by the uplifting of dust clouds from the disk.
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).
The results of the high-resolution long-term spectral monitoring of the Herbig Ae star HD 31648 in the regions of emission
Hα line, Na I D resonance lines and OI 7774 lines are presented.
We confirmed the conclusion, made in previous papers, that the spectral variability of the star in the region of Hα line have
a cyclic character. It is manifested itself as the changing of the equivalent width and intensity of Hα line of the time scale
of about 1200d.
It is shown, that the stellar wind is non-homogeneous and consists of several components, which are differed each other by
their velocities. They are observed as in the H line as in Na I D resonance lines. The component’s parameters are changed
during the cycle of stellar activity (in the maximum of activity the velocity and density of the wind are taken the largest
values and then they are gradually decreased).
The investigation of rapid variability of the He I 5876 line on the time scale of a few hours allows find the correlation
between the variability of the blue and the red wings of the line. It points at the connection between the accretion and the
outflows. Such connection, in particularly, is predicted by the modeling of the wind from young stars made in the frame of
the magneto-centrifugal model, the accordance of which for the HD 31648 was shown in the previous papers. In the present work
we confirmed this conclusion on the basis of the new data.
We found the weak variability of the stellar brightness (about 0.2m), which is agree with the spectral variability (the brightness
of the star becomes lower in the maximum of the activity). These changes are well explained by the process of the dust transfer
from CS disk by the stellar wind. This process is likely to be more effective in the maximum of activity.
An analysis of the variability of other spectral lines shows the agreement between the changing of the Hα line, the Na I D
resonance doublet lines and KI 7698 line. The weak connection between the He I 5876 and the Na I D lines is also found. Since
the formation regions of He I 5876 and Na I D lines are essentially different, we can conclude that the phenomena, responsible
for the observed cyclic variability, take place in a spacious region of the CS envelope.
We believe that the most plausible reason of found cyclic variability is the reconstruction of the inner structure of the
CS gas envelope, caused by the presence around the star a low mass companion or planet.
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≈3m), 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.
This paper presents state-of-the-art spectral energy distributions (SEDs) of four Herbig Ae stars, based in part on new data in the mid and far-infrared and at millimeter wavelengths. The SEDs are discussed in the context of circumstellar disk models. We show that models of irradiated disks provide a good fit to the observations over the whole range of wavelengths. We offer a possible solution to the long-standing puzzle caused by the excess emission of Herbig Ae stars, where a large fraction of the stellar luminosity is re-radiated between ~1.25 and 7 m, with a peak at about 3 m. We suggest that this general behaviour can be caused by dust evaporation in disks where the gas component is optically thin to the stellar radiation, as expected if the accretion rate is very low. The creation of a puffed-up inner wall of optically thick dust at the dust sublimation radius can account for the near-infrared characteristics of the SEDs. It can also naturally explain the H and K band interferometric observations of AB Aur (Millan-Gabet et al. [CITE]), which reveal a ring of emission of radius ~0.3 AU. Finally, irradiated disk models can easily explain the observed intensity of the 10 m silicate features and their variation from star to star.