144 research outputs found
Explaining UXOR variability with self-shadowed disks
In this Letter we propose a new view on UX Orionis type variability. 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, because the standard disk
models have a flaring geometry, it is mostly the outer regions of the disk that
obscure the star. The time scales for such obscuration events would be too long
to match the observed time scales of weeks to months. Recent 2-D
self-consistent models of Herbig Ae/Be protoplanetary disks (Dullemond et al.
2002,2003 henceforth D02/DD03), however, have indicated that for Herbig Ae/Be
star disks there exists, in addition to the usual flared disks, also a new
class of disks: those that are fully self-shadowed. 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 towards the star skims the
upper parts of the puffed-up inner rim, while passing high over the surface of
outer disk regions. Small hydrodynamic fluctuations in the puffed-up inner rim
could then be held responsible for 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 D02/DD03 that flared disks have a strong far-IR excess
and can be classified as `group I' (in the classification of Meeus et al.
2001), 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.Comment: Accepted for publication in ApJ Letters (a few lines added to
original version to accommodate comments of referee
The evolutionary status of the UX Orionis star RZ Piscium
The star RZ Psc is one of the most enigmatic members of the UX Ori star
family. It shows all properties that are typical for these stars (the light
variability, high linear polarization in deep minima, the blueing effect)
except for one: it lacks any signatures of youth. With the Li I line, as a
rough estimate for the stellar age, we show that the "lithium" age of RZ Psc
lies between the age of stars in the Pleiades (approximately 70 Myr) and the
Orion (approximately 10 Myr) clusters. We also roughly estimated the age of RZ
Psc based on the proper motion of the star using the Tycho-2 catalog. We found
that the star has escaped from its assumed birthplace near to the Galactic
plane about 30-40 Myr ago. We conclude that RZ Psc is a post-UXOr star, and its
sporadic eclipses are caused by material from the debris disk
Bimodal Brightness Oscillations in Models of Young Binary Systems
We consider a model for the cyclic activity of young binary stars that
accrete matter from the remnants of a protostellar cloud. If the orbit of such
a binary system is inclined at a small angle to the line of sight, then the
streams of matter and the density waves excited in the circumbinary disk can
screen the primary component of the binary from the observer. To study these
phenomena by the SPH (smoothed particle hydrodynamics) method, we have computed
grids of hydrodynamic models for binary systems based on which we have
constructed the light curves as a function of the orbital phase.The main
emphasis is on investigating the properties of the brightness oscillations.
Therefore, the model parameters were varied within the following ranges: the
component mass ratio q = M2 : M1 = 0.2 - 0.5 and the eccentricity e = 0 - 0.7.
The parameter that defined the binary viscosity was also varied. We adopted
optical grain characteristics typical of circumstellar dust. Our computations
have shown that bimodal oscillations are excited in binaries with eccentric
orbits, provided that the binary components do not differ too much in mass. In
this case, the ratios of the periods and amplitudes of the bimodal oscillations
and their shape depend strongly on the inclination of the binary plane and its
orientation relative to the observer. Our analysis shows that the computed
light curves can be used in interpreting the cyclic activity of UX Ori stars.Comment: 14 pages, 5 figures, 1 tabl
Hydrodynamic Processes in Young Binary Systems as a Source of Cyclic Variations of Circumstellar Extinction
Hydrodynamic models of a young binary system accreting matter from the
remnants of a protostellar cloud have been calculated by the SPH method. It is
shown that periodic variations in column density in projection onto the primary
component take place at low inclinations of the binary plane to the line of
sight. They can result in periodic extinction variations. Three periodic
components can exist in general case. The first component has a period equal to
the orbital one and is attributable to the streams of matter penetrating into
the inner regions of the binary. The second component has a period that is a
factor of 5-8 longer than the orbital one and is related to the density waves
generated in a circumbinary (CB) disk. The third, longest period is
attributable to the precession of the inner asymmetric region of CB disk. The
relationship between the amplitudes of these cycles depends on the model
parameters as well as on the inclination and orientation of the binary in
space. We show that at a dust-to-gas ratio of 1:100 and and a mass extinction
coefficient of 250 cm g, the amplitude of the brightness variations
of the primary component in the V-band can reach at a mass accretion rate
onto the binary components of yr and a
inclination of the binary plane to the line of sight. We discuss possible
applications of the model to pre-main-sequence stars.Comment: 13 pages, 6 figures, published in Astronomy Letters (v.33, 2007
Flares of accretion activity of the 20 Myr old UXOR RZ Psc
We discuss a revision of accretion activity and kinematics of the enigmatic
isolated UX Ori type star RZ Psc. Previously, RZ Psc was known to possess only
spectroscopic signatures of outflow in the low-excitation lines of alkali
metals. The archival high-resolution spectra reveal a short-lived episode of
magnetospheric accretion in the system observed via inverse P Cyg profiles at
the H and CaII 8542 \unicode{x212B} lines. The simultaneous presence
of accretion and outflow signatures at CaII 8542 \unicode{x212B} is
suggestive of an accretion-driven origin of the RZ Psc wind. We argue that RZ
Psc experiences matter ejection via the magnetic propeller mechanism but
variable accretion episodes allow it to sometimes move in the magnetospheric
accretion regime. The presence of the weak accretion in the system is also
supported by the radiation of the hot accretion spot on the stellar surface
observed spectroscopically at the deep photometric minimum of the star. The
Galactic motion of RZ Psc calculated with new GAIA DR2 astrometric data
suggests possible membership in Cas-Tau OB association with an age of
Myr
An active asteroid belt causing the UX Ori phenomenon in RZ Psc
We report the discovery of mid-infrared excess emission in the young object
RZ Psc. The excess constitutes ~8% of its Lbol, and is well fit by a single
500K black-body implying a dust free region within 0.7AU for optically thick
dust. The object displays dust obscuration events (UXOR behaviour) with a
time-scale that suggests dusty material on orbits of 0.5AU. We also report a
12.4 year cyclical photometric variability which can be interpreted as due to
perturbations in the dust distribution. The system is characterized by a high
inclination, marginal extinction (during bright photometric states), a single
temperature for the warm dust, and an age estimate which puts the star beyond
the formation stage. We propose that the dust occultation events present a
dynamical view of an active asteroid belt whose collisional products
sporadically obscure the central star.Comment: Accepted for A&A letter
Self-similar solution of a nonsteady problem of nonisothermal vapour condensation on a droplet growing in diffusion regime
This paper presents a mathematically exact self-similar solution to the joint
nonsteady problems of vapour diffusion towards a droplet growing in a
vapour-gas medium and of removal of heat released by a droplet into a
vapour-gas medium during vapour condensation. An equation for the temperature
of the droplet is obtained; and it is only at that temperature that the
self-similar solution exists. This equation requires the constancy of the
droplet temperature and even defines it unambiguously throughout the whole
period of the droplet growth. In the case of strong display of heat effects,
when the droplet growth rate decreases significantly, the equation for the
temperature of the droplet is solved analytically. It is shown that the
obtained temperature fully coincides with the one that settles in the droplet
simultaneously with the settlement of its diffusion regime of growth. At the
obtained temperature of the droplet the interrelated nonsteady vapour
concentration and temperature profiles of the vapour-gas medium around the
droplet are expressed in terms of initial (prior to the nucleation of the
droplet) parameters of the vapour-gas medium. The same parameters are used to
formulate the law in accordance with which the droplet is growing in diffusion
regime, and also to define the time that passes after the nucleation of the
droplet till the settlement of diffusion regime of droplet growth, when the
squared radius of the droplet becomes proportionate to time. For the sake of
completeness the case of weak display of heat effects is been studied.Comment: 12 pages, 4 figure
EXPORT: optical photometry and polarimetry of Vega-type and pre-main sequence stars
This paper presents optical UBVRI broadband photo-polarimetry of the EXPORT
sample obtained at the 2.5m Nordic Optical Telescope. The database consists of
multi-epoch photo-polarimetry of 68 pre-main-sequence and main-sequence stars.
An investigation of the polarization variability indicates that 22 objects are
variable at the 3sigma level in our data. All these objects are pre-main
sequence stars, consisting of both T Tauri and Herbig Ae/Be objects while the
main sequence, Vega type and post-T Tauri type objects are not variable. The
polarization properties of the variable sources are mostly indicative of the
UXOR-type behaviour; the objects show highest polarization when the brightness
is at minimum. We add seven new objects to the class of UXOR variables (BH Cep,
VX Cas, DK Tau, HK Ori, LkHa 234, KK Oph and RY Ori). The main reason for their
discovery is the fact that our data-set is the largest in its kind, indicating
that many more young UXOR-type pre-main sequence stars remain to be discovered.
The set of Vega-like systems has been investigated for the presence of
intrinsic polarization. As they lack variability, this was done using indirect
methods, and apart from the known case of BD +31.643, the following stars were
found to be strong candidates to exhibit polarization due to the presence of
circumstellar disks: 51 Oph, BD +31.643C, HD 58647 and HD 233517.Comment: A&A accepte
A reconsideration of disk properties in Herbig Ae stars
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 μm, with a peak at about 3 μm. 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 μm silicate features and their variation from star to star
Dynamics of the circumstellar gas in the Herbig Ae stars BF Orionis, SV Cephei, WW Vulpeculae and XY Persei
We present high resolution (lambda / Delta_lambda = 49000) echelle spectra of
the intermediate mass, pre-main sequence stars BF Ori, SV Cep, WW Wul and XY
Per. The spectra cover the range 3800-5900 angstroms and monitor the stars on
time scales of months and days. All spectra show a large number of Balmer and
metallic lines with variable blueshifted and redshifted absorption features
superimposed to the photospheric stellar spectra. Synthetic Kurucz models are
used to estimate rotational velocities, effective temperatures and gravities of
the stars. The best photospheric models are subtracted from each observed
spectrum to determine the variable absorption features due to the circumstellar
gas; those features are characterized in terms of their velocity, v, dispersion
velocity, Delta v, and residual absorption, R_max. The absorption components
detected in each spectrum can be grouped by their similar radial velocities and
are interpreted as the signature of the dynamical evolution of gaseous clumps
with, in most cases, solar-like chemical composition. This infalling and
outflowing gas has similar properties to the circumstellar gas observed in UX
Ori, emphasizing the need for detailed theoretical models, probably in the
framework of the magnetospheric accretion scenario, to understand the complex
environment in Herbig Ae (HAe) stars. WW Vul is unusual because, in addition to
infalling and outflowing gas with properties similar to those observed in the
other stars, it shows also transient absorption features in metallic lines with
no obvious counterparts in the hydrogen lines. This could, in principle,
suggest the presence of CS gas clouds with enhanced metallicity around WW Vul.
The existence of such a metal-rich gas component, however, needs to be
confirmed by further observations and a more quantitative analysis.Comment: 21 pages, 13 figures. Accepted for publication by Astronomy &
Astrophysic
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