568 research outputs found
Line-dependent veiling in very active T Tauri Stars
The T Tauri stars with active accretion disks show veiled photospheric
spectra. This is supposedly due to non-photospheric continuum radiated by hot
spots beneath the accretion shocks at stellar surface and/or chromospheric
emission lines radiated by the post-shocked gas. The amount of veiling is often
considered as a measure of the mass-accretion rate. We analysed high-resolution
photospheric spectra of accreting T Tauri stars LkHa 321, V1331 Cyg, and AS 353
A with the aim of clarifying the nature of the line-dependent veiling. Each of
these objects shows a highly veiled, strong emission line spectrum and powerful
wind features indicating high rates of accretion and mass loss. Equivalent
widths of hundreds of weak photospheric lines were measured in the observed
spectra and compared with those in synthetic spectra with the same spectral
type. We found that the veiling is strongly line-dependent: larger in stronger
photospheric lines and weak or absent in the weakest ones. No dependence of
veiling on excitation potential within 0 to 5 eV was found. Different physical
processes responsible for these unusual veiling effects are discussed in the
framework of the magnetospheric accretion model. The observed veiling has two
origins: 1) an abnormal structure of stellar atmosphere heated up by the
accreting matter, and 2) a non-photospheric continuum radiated by a hot spot
with temperature lower than 10000 K. The true level of the veiling continuum
can be derived by measuring the weakest photospheric lines with equivalent
widths down to 10 m\AA. A limited spectral resolution and/or low
signal-to-noise ratio results in overestimation of the veiling continuum. In
the three very active stars, the veiling continuum is a minor contributor to
the observed veiling, while the major contribution comes from the
line-dependent veiling.Comment: 10 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
Facing the wind of the pre-FUor V1331 Cyg
The mass outflows in T Tauri stars (TTS) are thought to be an effective
mechanism to remove angular momentum during the pre-main-sequence contraction
of a low-mass star. The most powerful winds are observed at the FUor stage of
stellar evolution. V1331 Cyg has been considered as a TTS at the pre-FUor
stage. We analyse high-resolution spectra of V1331 Cyg collected in 1998-2007
and 20-d series of spectra taken in 2012. For the first time the photospheric
spectrum of the star is detected and stellar parameters are derived: spectral
type G7-K0 IV, mass 2.8 Msun, radius 5 Rsun, vsini < 6 km/s. The photospheric
spectrum is highly veiled, but the amount of veiling is not the same in
different spectral lines, being lower in weak transitions and much higher in
strong transitions. The Fe II 5018, Mg I 5183, K I 7699 and some other lines of
metals are accompanied by a `shell' absorption at radial velocity of about -240
km/s. We show that these absorptions form in the post-shock gas in the jet,
i.e. the star is seen though its jet. The P Cyg profiles of H-alpha and H-beta
indicate the terminal wind velocity of about 500 km/s, which vary on
time-scales from several days to years. A model of the stellar wind is
developed to interpret the observations. The model is based on calculation of
hydrogen spectral lines using the radiative transfer code TORUS. The observed
H-alpha and H-beta line profiles and their variability can be well reproduced
with a stellar wind model, where the mass-loss rate and collimation (opening
angle) of the wind are variable. The changes of the opening angle may be
induced by small variability in magetization of the inner disc wind. The
mass-loss rate is found to vary within (6-11)x10^{-8} Msun/yr, with the
accretion rate of 2.0x10^{-6} Msun/yr.Comment: 11 pages, 12 figures; accepted for publication in MNRAS.
Typographical errors have been corrected after the proof stag
An Infrared Coronagraphic Survey for Substellar Companions
We have used the F160W filter (1.4-1.8 um) and the coronagraph on the
Near-InfraRed Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space
Telescope (HST) to survey 45 single stars with a median age of 0.15 Gyr, an
average distance of 30 pc, and an average H-magnitude of 7 mag. For the median
age we were capable of detecting a 30 M_Jup companion at separations between 15
and 200 AU. A 5 M_Jup object could have been detected at 30 AU around 36% of
our primaries. For several of our targets that were less than 30 Myr old, the
lower mass limit was as low as a Jupiter mass, well into the high mass planet
region. Results of the entire survey include the proper motion verification of
five low-mass stellar companions, two brown dwarfs (HR7329B and TWA5B) and one
possible brown dwarf binary (Gl 577B/C).Comment: 11 figures, accepted by A
Density streams in the disc winds of Classical T Tauri stars
Spectral and photometric variability of the Classical T Tauri stars RY Tau
and SU Aur from 2013 to 2022 is analyzed. We find that in SU Aur the H-alpha
line's flux at radial velocity RV = -50 +- 7 km/s varies with a period P = 255
+- 5 days. A similar effect previously discovered in RY Tau is confirmed with
these new data: P = 21.6 days at RV = -95 +- 5 km/s. In both stars, the radial
velocity of these variations, the period, and the mass of the star turn out to
be related by Kepler's law, suggesting structural features on the disc plane
orbiting at radii of 0.2 AU in RY Tau and 0.9 AU in SU Aur, respectively. Both
stars have a large inclination of the accretion disc to the line of sight - so
that the line of sight passes through the region of the disc wind. We propose
there is an azimuthal asymmetry in the disc wind, presumably in the form of
'density streams', caused by substructures of the accretion disc surface. These
streams cannot dissipate until they go beyond the Alfven surface in the disc's
magnetic field. These findings open up the possibility to learn about the
structure of the inner accretion disc of CTTS on scales less than 1 AU and to
reveal the orbital distances related to the planet's formation.Comment: 9 pages, 10 figure
A search for pulsations in the HgMn star HD 45975 with CoRoT photometry and ground-based spectroscopy
The existence of pulsations in HgMn stars is still being debated. To provide
the first unambiguous observational detection of pulsations in this class of
chemically peculiar objects, the bright star HD 45975 was monitored for nearly
two months by the CoRoT satellite. Independent analyses of the light curve
provides evidence of monoperiodic variations with a frequency of 0.7572 c/d and
a peak-to-peak amplitude of ~2800 ppm. Multisite, ground-based spectroscopic
observations overlapping the CoRoT observations show the star to be a
long-period, single-lined binary. Furthermore, with the notable exception of
mercury, they reveal the same periodicity as in photometry in the line moments
of chemical species exhibiting strong overabundances (e.g., Mn and Y). In
contrast, lines of other elements do not show significant variations. As found
in other HgMn stars, the pattern of variability consists in an absorption bump
moving redwards across the line profiles. We argue that the photometric and
spectroscopic changes are more consistent with an interpretation in terms of
rotational modulation of spots at the stellar surface. In this framework, the
existence of pulsations producing photometric variations above the ~50 ppm
level is unlikely in HD 45975. This provides strong constraints on the
excitation/damping of pulsation modes in this HgMn star.Comment: Accepted for publication in A&A, 14 pages, 15 colour figures (revised
version after language editing
Interpretation of the Veiling of the Photospheric Spectrum for T Tauri Stars in Terms of an Accretion Model
The problem on heating the atmospheres of T Tauri stars by radiation from an
accretion shock has been solved. The structure and radiation spectrum of the
emerging so-called hot spot have been calculated in the LTE approximation. The
emission not only in continuum but also in lines has been taken into account
for the first time when calculating the spot spectrum. Comparison with
observations has shown that the strongest of these lines manifest themselves as
narrow components of helium and metal emission lines, while the weaker ones
decrease significantly the depth of photospheric absorption lines, although
until now, this effect has been thought to be due to the emission continuum
alone. The veiling by lines changes the depth of different photospheric lines
to a very different degree even within a narrow spectral range. Therefore, the
nonmonotonic wavelength dependence of the degree of veiling r found for some
CTTS does not suggest a nontrivial spectral energy distribution of the veiling
continuum. In general, it makes sense to specify the degree of veiling r only
by providing the set of photospheric lines from which this quantity was
determined. We show that taking into account the contribution of lines to the
veiling of the photospheric spectrum can cause the existing estimates of the
accretion rate onto T Tauri stars to decrease by several times, with this being
also true for stars with a comparatively weakly veiled spectrum. Neglecting the
contribution of lines to the veiling can also lead to appreciable errors in
determining the effective temperature, interstellar extinction, radial
velocity, and vsin(i)
CSI 2264: Probing the inner disks of AA Tauri-like systems in NGC 2264
Context. The classical T Tauri star (CTTS) AA Tau has presented photometric variability that was attributed to an inner disk warp, caused by the interaction between the inner disk and an inclined magnetosphere. Previous studies of the young cluster NGC 2264 have shown that similar photometric behavior is common among CTTS.
Aims. The goal of this work is to investigate the main causes of the observed photometric variability of CTTS in NGC 2264 that present AA Tau-like light curves, and verify if an inner disk warp could be responsible for their observed variability.
Methods. In order to understand the mechanism causing these stars’ photometric behavior, we investigate veiling variability in their spectra and u − r color variations and estimate parameters of the inner disk warp using an occultation model proposed for AA Tau. We also compare infrared Spitzer IRAC and optical CoRoT light curves to analyze the dust responsible for the occultations.
Results. AA Tau-like variability proved to be transient on a timescale of a few years. We ascribe this variability to stable accretion regimes and aperiodic variability to unstable accretion regimes and show that a transition, and even coexistence, between the two is common. We find evidence of hot spots associated with occultations, indicating that the occulting structures could be located at the base of accretion columns. We find average values of warp maximum height of 0.23 times its radial location, consistent with AA Tau, with variations of on average 11% between rotation cycles. We also show that extinction laws in the inner disk indicate the presence of grains larger than interstellar grains.
Conclusions. The inner disk warp scenario is consistent with observations for all but one star with AA Tau-like variability in our sample. AA Tau-like systems are fairly common, comprising 14% of CTTS observed in NGC 2264, though this number increases to 35% among systems of mass 0.7 M_⊙ ≲ M ≲ 2.0 M_⊙. Assuming random inclinations, we estimate that nearly all systems in this mass range likely possess an inner disk warp. We attribute this to a possible change in magnetic field configurations among stars of lower mass
Coordinated Multiwavelength Observations of V410Tau
In November 2001 we undertook a coordinated observing campaign to study the
connection between X-ray and optical variability in the weak-line T Tauri star
V410Tau. The observing plan included three 15 ksec observations with Chandra
using the Advanced CCD Imaging Spectrometer for Spectroscopy scheduled for
different phases of the known 1.87 d starspot cycle. Photometric and
spectroscopic monitoring of V410Tau involving telescopes on three different
continents was scheduled simultaneously with the Chandra exposures.Comment: 3 pages, Poster Proceedings of "1st Potsdam Thinkshop on Sunspots and
Starspots", Potsdam, Germany, May 200
CSI 2264: Accretion process in classical T Tauri stars in the young cluster NGC 2264
Context. NGC 2264 is a young stellar cluster (~3 Myr) with hundreds of low-mass accreting stars that allow a detailed analysis of the accretion process taking place in the pre-main sequence.
Aims. Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars to the physical processes acting in the stellar and circumstellar environment, within a few stellar radii from the star.
Methods. NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and photometric and spectroscopic observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several accretion diagnostics and disk parameters.
Results. The morphology of the CoRoT light curve reflects the evolution of the accretion process and of the inner disk region. Accretion burst stars present high mass-accretion rates and optically thick inner disks. AA Tau-like systems, whose light curves are dominated by circumstellar dust obscuration, show intermediate mass-accretion rates and are located in the transition of thick to anemic disks. Classical T Tauri stars with spot-like light curves correspond mostly to systems with a low mass-accretion rate and low mid-IR excess. About 30% of the classical T Tauri stars observed in the 2008 and 2011 CoRoT runs changed their light-curve morphology. Transitions from AA Tau-like and spot-like to aperiodic light curves and vice versa were common. The analysis of the Hα emission line variability of 58 accreting stars showed that 8 presented a periodicity that in a few cases was coincident with the photometric period. The blue and red wings of the Hα line profiles often do not correlate with each other, indicating that they are strongly influenced by different physical processes. Classical T Tauri stars have a dynamic stellar and circumstellar environment that can be explained by magnetospheric accretion and outflow models, including variations from stable to unstable accretion regimes on timescales of a few years
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