A precessing Be disk as a possible model for occultation events in GX 304-1

A precessing Be disk as a possible model for occultation events in GX 304-1

Authors:

Kühnel et al

Abstract:

We report on the RXTE detection of a sudden increase in the absorption column density, NH, during the 2011 May outburst of GX 304-1. The NH increased up to ∼16×1022 atoms cm−2, which is a factor of 3-4 larger than what is usually measured during the outbursts of GX 304-1 as covered by RXTE. Additionally, an increase in the variability of the hardness ratio as calculated from the energy resolved RXTE-PCA light curves is measured during this time range. We interpret these facts as an occultation event of the neutron star by material in the line of sight. Using a simple 3D model of an inclined and precessing Be disk around the Be type companion, we are able to qualitatively explain the NH evolution over time. We are able to constrain the Be-disk density to be on the order of 10−11 g cm−3. Our model strengthens the idea of inclined Be disks as origin of double-peaked outbursts as the derived geometry allows accretion twice per orbit under certain conditions.

Be discs in binary systems: II. Misaligned orbits

Be discs in binary systems: II. Misaligned orbits

Authors:

Cyr et al

Abstract:
We use a smoothed particle hydrodynamics (SPH) code to examine the effects of misaligned binary companions on Be star discs. We systematically vary the degree of misalignment between the disc and the binary orbit, as well as the disc viscosity and orbital period to study their effects on the density in the inner and outer parts of the disc. We find that varying the degree of misalignment, the viscosity, and the orbital period affects both the truncation radius and the density structure of the outer disc, while the inner disc remains mostly unaffected. We also investigate the tilting of the disc in the innermost part of the disc and find the tilt increases with radius until reaching a maximum around 5 stellar radii. The direction of the line of nodes, with respect to the equator of the central star, is found to be offset compared to the orbital line of nodes, and to vary periodically in time, with a period of half a orbital phase. We also compare the scale height of our discs with the analytical scale height of an isothermal disc, which increases with radius as r1.5. We find that this formula reproduces the scale height well for both aligned and misaligned systems but underestimates the scale height in regions of the disc where density enhancements develop.

INVESTIGATING THE CIRCUMSTELLAR DISK OF THE Be SHELL STAR 48 LIBRAE

INVESTIGATING THE CIRCUMSTELLAR DISK OF THE Be SHELL STAR 48 LIBRAE

Authors:

Silaj et al

Abstract:

A global disk oscillation implemented in the viscous decretion disk (VDD) model has been used to reproduce most of the observed properties of the well known Be star ζ Tau. 48 Librae shares several similarities with ζ Tau—they are both early-type Be stars, display shell characteristics in their spectra, and exhibit cyclic V/R variations—but has some marked differences as well, such as a much denser and more extended disk, a much longer V/R cycle, and the absence of the so-called triple-peak features. We aim to reproduce the photometric, polarimetric, and spectroscopic observables of 48 Librae with a self-consistent model, and to test the global oscillation scenario for this target. Our calculations are carried out with the three-dimensional NLTE radiative transfer code HDUST. We employ a rotationally deformed, gravity-darkened central star surrounded by a disk whose unperturbed state is given by the VDD model. A two-dimensional global oscillation code is then used to calculate the disk perturbation and superimpose it on the unperturbed disk. A very good, self-consistent fit to the time-averaged properties of the disk is obtained with the VDD. The calculated perturbation has a period P = 12 years, which agrees with the observed period, and the behavior of the V/R cycle is well reproduced by the perturbed model. The perturbed model improves the fit to the photometric data and reproduces some features of the observed spectroscopic data. Some suggestions to improve the synthesized spectroscopy in a future work are given.

The Hot Companion and Circumbinary Disk of the Be Star HR 2142

The Hot Companion and Circumbinary Disk of the Be Star HR 2142

Authors:

Peters et al

Abstract:

We present a spectroscopic investigation of the Be+sdO binary system HR 2142 that is based upon large sets of ultraviolet observations from the International Ultraviolet Explorer and ground-based H-alpha observations. We measured radial velocities for the Be star component from these spectra, and computed a revised orbit. In order to search for the spectral signature of the hot subdwarf, we cross-correlated the short wavelength end of each IUE spectrum with a model hot star spectrum, and then we used the predicted Doppler shifts of the subdwarf to shift-and-add all the cross-correlation functions to the frame of the subdwarf. This merged function shows the weak signal from the spectral lines of the hot star, and a best fit is obtained with a mass ratio M_2/M_1 = 0.07 +/- 0.02, companion temperature T_{eff} > 43 +/- 5 kK, projected rotational velocity V sin i < 30 km/s, and a monochromatic flux ratio near 1170 Angstroms of f_2/f_1 > 0.009 +/- 0.001. This hot subdwarf creates a one-armed spiral, tidal wake in the disk of the Be star, and we present a circumbinary disk model that can explain the occurrence of shell absorption lines by gas enhancements that occur where gas crossing the gap created by the subdwarf strikes the disk boundaries. The faint companion of HR 2142 may be representative of a significant fraction of Be stars with undetected former mass donor companion stars.

Coplanar Orbits in Be discs in binary systems

Be discs in binary systems I. Coplanar orbits

Authors:

Panoglou et al

Abstract:

Be stars are surrounded by outflowing circumstellar matter structured in the form of decretion discs. They are often members of binary systems, where it is expected that the decretion disc interacts both radiatively and gravitationally with the companion. In this work we study how various orbital (period, mass ratio, eccentricity) and disc (viscosity) parameters affect the disc structure in coplanar systems. We simulate such binaries with the use of a smoothed particle hydrodynamics code. The main effects of the secondary on the disc are its truncation and the accumulation of material inwards of truncation. In circular or nearly circular prograde orbits, the disc maintains a rotating, constant in shape, configuration, which is locked to the orbital phase. The disc is smaller in size, more elongated and more massive for low viscosity parameter, small orbital separation and/or high mass ratio. Highly eccentric orbits are more complex, with the disc structure and total mass strongly dependent on the orbital phase. We also studied the effects of binarity in the disc continuum emission. Since the infrared and radio SED are sensitive to the disc size and density slope, the truncation and matter accumulation result in considerable modifications in the emergent spectrum. The decretion disc in circular binaries remains constant in shape and phase-locked. Eccentric binaries exhibit strong dependence on the distance to the secondary. We conclude that binarity can serve as an explanation for the variability exhibited in observations of Be stars, and that our model can be used to detect invisible companions.

Optically thin decretion disks of classical Oe/Be stars

Line-driven ablation of circumstellar disks: I. Optically thin decretion disks of classical Oe/Be stars

Authors:

Kee et al

Abstract:

The extreme luminosities of hot, massive stars drive strong stellar winds through UV line-scattering. For OB stars with an orbiting circumstellar disk, we explore the effect of such line-scattering in ablating disk material, initially focusing on the marginally optically thin decretion disks of classical Oe and Be stars. For this we apply a multi-dimensional radiation-hydrodynamics code, assuming optically thin ray tracing for the stellar continuum and a multi-ray Sobolev treatment of the line transfer. This accounts for desaturation of line-absorption by Keplerian shear in the disk, and associated driving by non-radial photons. Results show dense, intermediate-speed surface ablation, consistent with the strong, blue-shifted absorption seen in UV wind lines of Be shell stars. The asymptotic ablation rate is typically an order-unity factor times the stellar wind mass loss rate, leading to disk destruction times of order months to years for Be disks, consistent with observations. The much stronger radiative forces of O stars reduce this time to order days, making sustaining a disk difficult, and so providing a natural explanation for the rarity of Galactic Oe stars. Additionally, the weakened line-driving at lower metallicity implies both a reduction in the winds that help spin-down stars from near-critical rotation, and in the ablation of decretion disks, thus providing a natural explanation for the higher fraction of Classical Be stars, and the presence of Oe stars, in the Magellanic Clouds. We conclude with a discussion of future extensions to study line-driven ablation of denser, optically thick, accretion disks around pre-main-sequence massive stars.

The Pseudo-photosphere Model for the Continuum Emission of Gaseous Disks

The pseudo-photosphere model for the continuum emission of gaseous discs

Authors:

Vieira et al

Abstract:

We investigate the continuum emission of viscous decretion discs around Be stars in this paper. The results obtained from non-LTE (local thermodynamic equilibrium) radiative transfer models show two regimes in the disc surface brightness profile: an inner optically thick region, which behaves as a pseudo-photosphere with a wavelength-dependent size, and an optically thin tenuous outer part, which contributes with about a third of the total flux. The isophotal shape of the surface brightness is well described by elliptical contours with an axial ratio b/a=cosi for inclinations i less than 75∘. Based on these properties, a semi-analytical model was developed to describe the continuum emission of gaseous discs. It provides fluxes and spectral slopes at the infrared within an accuracy of 10% and 5%, respectively, when compared to the numerical results. The model indicates that the infrared spectral slope is mainly determined by both the density radial slope and the disc flaring exponent, being practically independent of disc inclination and base density. As a first application, the density structure of 15 Be stars was investigated, based on the infrared flux excess, and the results compared to previous determinations in the literature. Our results indicate that the decretion rates are in the range of 10−12 to 10−9M⊙yr−1, which is at least two orders of magnitude smaller than the previous outflowing disc model predictions.

The Circumstellar Disk of Omicron Aquarii

The Circumstellar Disk of the Be Star o~Aquarii

Authors:

Sigut et al

Abstract:

Omicron Aquarii is late-type, Be shell star with a stable and nearly symmetric Hα emission line. We combine Hα interferometric observations obtained with the Navy Precision Optical Interferometer (NPOI) covering 2007 through 2014 with Hα spectroscopic observations over the same period and a 2008 observation of the system's near-infrared spectral energy distribution to constrain the properties of o~Aqr's circumstellar disk. All observations are consistent with a circumstellar disk seen at an inclination of 75±3∘ with a position angle on the sky of 110±8∘ measured E from N. From the best-fit disk density model, we find that 90\% of the Hα emission arises from within 9.5 stellar radii, and the mass associated with this Hα disk is ∼1.8×10−10 of the stellar mass and the associated angular momentum, assuming Keplerian rotation for the disk, is ∼1.6×10−8 of the total stellar angular momentum. The occurrence of a central quasi-emission (CQE) feature in Mg\,{\sc ii} λ4481 is also predicted by this best-fit disk model and the computed profile compares successfully with observations from 1999. To obtain consistency between the Hα line profile modelling and the other constraints, it was necessary in the profile fitting to weight the line core (emission peaks and central depression) more heavily than the line wings, which were not well reproduced by our models. This may reflect the limitation of assuming a single power-law for the disk's equatorial density variation. The best-fit disk density model for o~Aqr predicts that Hα is near its maximum strength as a function of disk density, and hence the Hα equivalent width and line profile change only weakly in response to large (factor of ∼5) changes in the disk density.