We present high-resolution, H-band imaging observations, collected with Subaru/HiCIAO, of the scattered light from the transitional disk around SAO 206462 (HD 135344B). Although previous sub-mm ...imagery suggested the existence of a dust-depleted cavity at r < or =, slant 46 AU, our observations reveal the presence of scattered light components as close as 0".2 (~28 AU) from the star. Moreover, we have discovered two small-scale spiral structures lying within 0".5 (~70 AU). We present models for the spiral structures using the spiral density wave theory, and derive a disk aspect ratio of h ~ 0.1, which is consistent with previous sub-mm observations. This model can potentially give estimates of the temperature and rotation profiles of the disk based on dynamical processes, independently from sub-mm observations. It also predicts the evolution of the spiral structures, which can be observable on timescales of 10-20 years, providing conclusive tests of the model. While we cannot uniquely identify the origin of these spirals, planets embedded in the disk may be capable of exciting the observed morphology. Assuming that this is the case, we can make predictions on the locations and, possibly, the masses of the unseen planets. Such planets may be detected by future multi-wavelength observations.
Rapid rotation is a fundamental characteristic of classical Be stars and a crucial property allowing for the formation of their circumstellar disks. Past evolution in a mass and angular momentum ...transferring binary system offers a plausible solution to how Be stars attained their fast rotation. Although the subdwarf remnants of mass donors in such systems should exist in abundance, only a few have been confirmed due to tight observational constraints. An indirect method of detecting otherwise hidden companions is offered by their effect on the outer parts of Be star disks, which are expected to be disrupted or truncated. In the context of the infrared and radio continuum excess radiation originating in the disk, the disk truncation can be revealed by a turndown in the spectral energy distribution due to reduced radio flux levels. In this work, we search for signs of spectral turndown in a sample of 57 classical Be stars with radio data, which include new data for 23 stars and the longest-wavelength detections so far (λ 10 cm) for two stars. We confidently detect the turndown for all 26 stars with sufficient data coverage (20 of which are not known to have close binary companions). For the remaining 31 stars, the data are inconclusive as to whether the turndown is present or not. The analysis suggests that many if not all Be stars have close companions influencing their outer disks. If confirmed to be subdwarf companions, the mass transfer spin-up scenario might explain the existence of the vast majority of classical Be stars.
Abstract
Hot massive stars present strong stellar winds that are driven by absorption, scattering, and reemission of photons by the ions of the atmosphere (line-driven winds). A better comprehension ...of this phenomenon, and a more accurate calculation of hydrodynamics and radiative acceleration, is Required to reduce the number of free parameters in spectral fitting and to determine accurate wind parameters such as mass-loss rates and velocity profiles. We use the non-LTE model-atmosphere code CMFGEN to numerically solve the radiative transfer equation in the stellar atmosphere and to calculate the radiative acceleration
g
rad
(
r
). Under the assumption that the radiative acceleration depends only on the radial coordinate, we solve analytically the equation of motion by means of the Lambert
W
-function. An iterative procedure between the solution of the radiative transfer and the equation of motion is executed in order to obtain a final self-consistent velocity field that is no longer based on any
β-
law. We apply the Lambert-procedure to three O supergiant stars (
ζ
Puppis, HD 165763, and
α
Cam) and discuss the Lambert solutions for the velocity profiles. It is found that, even without recalculation of the mass-loss rate, the Lambert-procedure allows the calculation of consistent velocity profiles that reduce the number of free parameters when a spectral fitting using CMFGEN is performed. Synthetic spectra calculated from our Lambert solutions show significant differences compared to the initial
β
-law CMFGEN models. The results indicate the importance of consistent velocity profile calculation in the CMFGEN code and its use in a fitting procedure and interpretation of observed spectra.
Context. The structure of the inner parts of Be star disks (≲ 20 stellar radii) is well explained by the viscous decretion disk (VDD) model, which is able to reproduce the observable properties of ...most of the objects studied so far. The outer parts, on the other hand, are not observationally well-explored, as they are observable only at radio wavelengths. A steepening of the spectral slope somewhere between infrared and radio wavelengths was reported for several Be stars that were previously detected in the radio, but a convincing physical explanation for this trend has not yet been provided. Aims. We test the VDD model predictions for the extended parts of a sample of six Be disks that have been observed in the radio to address the question of whether the observed turndown in the spectral energy distribution (SED) can be explained in the framework of the VDD model, including recent theoretical development for truncated Be disks in binary systems. Methods. We combine new multi-wavelength radio observations from the Karl. G. Jansky Very Large Array (JVLA) and Atacama Pathfinder Experiment (APEX) with previously published radio data and archival SED measurements at ultraviolet, visual, and infrared wavelengths. The density structure of the disks, including their outer parts, is constrained by radiative transfer modeling of the observed spectrum using VDD model predictions. In the VDD model we include the presumed effects of possible tidal influence from faint binary companions. Results. For 5 out of 6 studied stars, the observed SED shows strong signs of SED turndown between far-IR and radio wavelengths. A VDD model that extends to large distances closely reproduces the observed SEDs up to far IR wavelengths, but fails to reproduce the radio SED. Using a truncated VDD model improves the fit, leading to a successful explanation of the SED turndown observed for the stars in our sample. The slope of the observed SEDs in the radio is however not well reproduced by disks that are simply cut off at a certain distance. Rather, some matter seems to extend beyond the truncation radius, where it still contributes to the observed SEDs, making the spectral slope in the radio shallower. This finding is in agreement with our current understanding of binary truncation from hydrodynamical simulations, in which the disk does extend past the truncation radius. Therefore, the most probable cause for the SED turndown is the presence of binary companions that remain undetected for most of our sources.
Context.
Mass loss due to radiatively line-driven winds is central to our understanding of the evolution of massive stars in both single and multiple systems. This mass loss plays a key role in ...modulating the stellar evolution at different metallicities, particularly in the case of massive stars with
M
*
≥ 25
M
⊙
.
Aims.
We extend the evolution models introduced in Paper I, where the mass-loss recipe is based on the simultaneous calculation of the wind hydrodynamics and the line acceleration, by incorporating the effects of stellar rotation.
Methods.
As in Paper I, we introduce a grid of self-consistent line-force parameters (
k
,
α
,
δ
) for a set of standard evolutionary tracks using G
ENEC
. Based on this grid, we analysed the effects of stellar rotation, CNO abundances, and He/H ratio on the wind solutions to derive additional terms for the recipe with which we predict the self-consistent mass-loss rate,
Ṁ
sc
. With this, we generated a new set of evolutionary tracks with rotation for
M
ZAMS
= 25, 40, 70, and 120
M
⊙
, and for metallicities
Z
= 0.014 (Galactic) and 0.006 (Large Magellanic Cloud).
Results.
In addition to the expected correction factor due to rotation, the mass-loss rate decreases when the surface becomes more helium rich, especially in the later moments of the main-sequence phase. The self-consistent approach gives lower mass-loss rates than the standard values adopted in previous G
ENEC
evolution models. This decrease strongly affects the tracks of the most massive models. Weaker winds allow the star to retain more mass, but also more angular momentum. As a consequence, weaker wind models rotate faster and show a less efficient mixing in their inner stellar structure at a given age.
Conclusions.
The self-consistent tracks predict an evolution of the rotational velocities through the main sequence that closely agrees with the range of
v
sin
i
values found by recent surveys of Galactic O-type stars. As subsequent implications, the weaker winds from self-consistent models also suggest a reduction of the contribution of the isotope
26
Al to the interstellar medium due to stellar winds of massive stars during the MS phase. Moreover, the higher luminosities found for the self-consistent evolutionary models suggest that some populations of massive stars might be less massive than previously thought, as in the case of Ofpe stars at the Galactic centre. Therefore, this study opens a wide range of consequences for further research based on the evolution of massive stars.
The circumstellar disk density distributions for a sample of 63 Be southern stars from the BeSOS survey were found by modeling their H emission line profiles. These disk densities were used to ...compute disk masses and disk angular momenta for the sample. Average values for the disk mass are 3.4 × 10−9 and 9.5 × 10−10 M for early (B0-B3) and late (B4-B9) spectral types, respectively. We also find that the range of disk angular momentum relative to the star is (150-200)J /M and (100-150)J /M , again for early- and late-type Be stars, respectively. The distributions of the disk mass and disk angular momentum are different between early- and late-type Be stars at a 1% level of significance. Finally, we construct the disk mass distribution for the BeSOS sample as a function of spectral type and compare it to the predictions of stellar evolutionary models with rapid rotation. The observed disk masses are typically larger than the theoretical predictions, although the observed spread in disk masses is typically large.
Abstract
The Be phenomenon is present in about 20 per cent of B-type stars. Be stars show variability on a broad range of time-scales, which in most cases is related to the presence of a ...circumstellar disc of variable size and structure. For this reason, a time-resolved survey is highly desirable in order to understand the mechanisms of disc formation, which are still poorly understood. In addition, a complete observational sample would improve the statistical significance of the study of stellar and disc parameters. The ‘Be Stars Observation Survey’ (BeSOS) is a survey containing reduced spectra obtained using the Pontifica Universidad Católica High Echelle Resolution Optical Spectrograph (PUCHEROS) with a spectral resolution of 17 000 in the range 4260–7300 Å. BeSOS's main objective is to offer consistent spectroscopic and time-resolved data obtained with one instrument. The user can download or plot the data and obtain stellar parameters directly from the website. We also provide a star-by-star analysis based on photometric, spectroscopic and interferometric data, as well as general information about the whole BeSOS sample. Recently, BeSOS led to the discovery of a new Be star HD 42167 and facilitated study of the V/R variation of HD 35165 and HD 120324, the steady disc of HD 110335 and the Be shell status of HD 127972. Optical spectra used in this work, as well as the stellar parameters derived, are available online at http://besos.ifa.uv.cl.
ABSTRACT
Classical B emission (Be) stars are fast rotating, near-main-sequence B-type stars. The rotation and the presence of circumstellar discs profoundly modify the observables of active Be stars. ...Our goal is to infer stellar and disc parameters, as well as distance and interstellar extinction, using the currently most favoured physical models for these objects. We present BeAtlas, a grid of $61\, 600$ non-local thermodynamic equilibrium radiative transfer models for Be stars, calculated with the hdust code. The grid was coupled with a Monte Carlo Markov chain (MCMC) code to sample the posterior distribution. We test our method on two well-studied Be stars, α Eri and β CMi, using photometric, polarimetric, and spectroscopic data as input to the code. We recover literature determinations for most of the parameters of the targets, in particular the mass and age of α Eri, the disc parameters of β CMi, and their distances and inclinations. The main discrepancy is that we estimate lower rotational rates than previous works. We confirm previously detected signs of disc truncation in β CMi and note that its inner disc seems to have a flatter density slope than its outer disc. The correlations between the parameters are complex, further indicating that exploring the entire parameter space simultaneously is a more robust approach, statistically. The combination of BeAtlas and Bayesian-MCMC techniques proves successful, and a powerful new tool for the field: The fundamental parameters of any Be star can now be estimated in a matter of hours or days.
Context.
Mass loss through radiatively line-driven winds is central to our understanding of the evolution of massive stars in both single and multiple systems. This mass loss plays a key role in ...modulating massive star evolution at different metallicities, especially in the case of very massive stars (
M
*
≥ 25
M
⊙
).
Aims.
Here we present evolutionary models for a set of massive stars, introducing a new prescription for the mass-loss rate obtained from hydrodynamical calculations in which the wind velocity profile,
v
(
r
), and the line-acceleration,
g
line
, are obtained in a self-consistent way. These new prescriptions cover most of the main sequence phase of O-type stars.
Methods.
We made a grid of self-consistent mass-loss rates
Ṁ
sc
for a set of standard evolutionary tracks (i.e. using the old prescription for mass-loss rate) with different values for initial mass and metallicity. Based on this grid, we elaborate a statistical analysis to create a new simple formula for predicting the values of
Ṁ
sc
from the stellar parameters alone, without assuming any extra condition for the wind description. Therefore, replacing the mass-loss rates at the main sequence stage provided by the standard Vink’s formula with our new recipe, we generate a new set of evolutionary tracks for
M
ZAMS
= 25, 40, 70, and 120
M
⊙
and metallicities
Z
= 0.014 (Galactic),
Z
= 0.006 (LMC), and
Z
= 0.002 (SMC).
Results.
Our new derived formula for mass-loss rate predicts a dependence
Ṁ
∝
Z
a
, where
a
is no longer constant but dependent on the stellar mass: ranging from
a
∼ 0.53 when
M
*
∼ 120
M
⊙
, to
a
∼ 1.02 when
M
*
∼ 25
M
⊙
. We find important differences between the standard tracks and our new self-consistent tracks. Models adopting the new recipe for
Ṁ
(which starts off at around three times weaker than the mass-loss rate from the old formulation) retain more mass during their evolution, which is expressed as larger radii and consequently more luminous tracks over the Hertzsprung-Russell diagram. These differences are more prominent for the cases of
M
ZAMS
= 70 and 120
M
⊙
at solar metallicity, where we find self-consistent tracks are ∼0.1 dex brighter and retain up to 20
M
⊙
more than with the classical models using the previous formulation for mass-loss rate. Later increments in the mass-loss rate for tracks when self-consistency is no longer used, attributed to the LBV stage, produce different final stellar radii and masses before the end of the H-burning stage, which are analysed case by case. Moreover, we observe remarkable differences in the evolution of the radionuclide isotope
26
Al in the core and on the surface of the star. As
Ṁ
sc
is weaker than the commonly adopted values for evolutionary tracks, self-consistent tracks predict a later modification in the abundance of
26
Al in the stellar winds. This new behaviour could provide useful information about the real contribution of this isotope from massive stars to the Galactic interstellar medium.
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