Revealing the structure of the outer disks of Be stars

Klement, Róbert; Carciofi, A. C.; Rivinius, Thomas; Matthews, L. D.; Vieira, Rodrigo G.; Ignace, Richard; Bjorkman, J. E.; Mota, Bruna Salani; Faes, D. M.; Bratcher, A. D.; Curé, M.; Štefl, S.

doi:10.1051/0004-6361/201629932

Cited by 52 publications

(82 citation statements)

References 68 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…For the less massive disks witḣ M ≤ 10 −8 M yr −1 the value of the same slope parameter within the same region is approximately 3.125. This agrees with observational results of Klement et al (2017), who determine the values of the disk midplane density slope parameter between approximately 3.0 and 3.5 for the Be stars' disks that are in a steady state. They also predict that disks with this parameter higher than 3.5 should correspond to disks that are in the process of formation, and that disks with this parameter lower than 3 are in the process of dissipation; the variations of the values of the slope parameter in the case of very massive disks indicate that these processes occur up to a certain distance (see Sect.…”

Section: Discussionsupporting

confidence: 91%

Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks

Kurfürst

Feldmeier

Krtička

2018

A&A

View full text Add to dashboard Cite

Context. Evolution of massive stars is affected by a significant loss of mass either via (nearly) spherically symmetric stellar winds or by aspherical mass-loss mechanisms, namely the outflowing equatorial disks. However, the scenario that leads to the formation of a disk or rings of gas and dust around massive stars is still under debate. It is also unclear how various forming physical mechanisms of the circumstellar environment affect its shape and density, as well as its kinematic and thermal structure. Aims. We study the hydrodynamic and thermal structure of optically thick, dense parts of outflowing circumstellar disks that may be formed around various types of critically rotating massive stars, for example, Be stars, B[e] supergiant (sgB[e]) stars or Pop III stars. We calculate self-consistent time-dependent models of temperature and density structure in the disk's inner dense region that is strongly affected by irradiation from a rotationally oblate central star and by viscous heating. Methods. Using the method of short characteristics, we specify the optical depth of the disk along the line-of-sight from stellar poles. Within the optically thick dense region with an optical depth of τ > 2/3 we calculate the vertical disk thermal structure using the diffusion approximation while for the optically thin outer layers we assume a local thermodynamic equilibrium with the impinging stellar irradiation. For time-dependent hydrodynamic modeling, we use two of our own types of hydrodynamic codes: two-dimensional operator-split numerical code based on an explicit Eulerian finite volume scheme on a staggered grid, and unsplit code based on the Roe's method, both including full second-order Navier-Stokes shear viscosity. Results. Our models show the geometric distribution and contribution of viscous heating that begins to dominate in the central part of the disk for mass-loss rates higher thanṀ 10 −10 M yr −1 . In the models of dense viscous disks withṀ > 10 −8 M yr −1 , the viscosity increases the central temperature up to several tens of thousands of Kelvins, however the temperature rapidly drops with radius and with distance from the disk midplane. The high mass-loss rates and high viscosity lead to instabilities with significant waves or bumps in density and temperature in the very inner disk region. Conclusions. The two-dimensional radial-vertical models of dense outflowing disks including the full Navier-Stokes viscosity terms show very high temperatures that are however limited to only the central disk cores inside the optically thick area, while near the edge of the optically thick region the temperature may be low enough for the existence of neutral hydrogen, for example.

show abstract

Section: Discussionsupporting

confidence: 91%

Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks

Kurfürst

Feldmeier

Krtička

2018

A&A

View full text Add to dashboard Cite

show abstract

“…Finally, we comment on the disk structure near the central star that is suggested by some results of the present paper. Detailed accounts of successful representations of the observed emission in the Hα line of Be stars and of the continuum energy distribution from the far-UV to the IR, as well as of the polarization in the visible spectral range, are given in a series of rather recent papers and references therein (Araya et al 2017;Arcos et al 2017;Klement et al 2017;Marr et al 2018). In all these studies, the physical structure of the CE of Be stars is studied in the frame of an axisymmetric quasi-Keplerian disk in hydrostatic equilibrium in the vertical axis, where the scale height H is proportional to R 3/2 T 1/2 and R represents the equatorial radius of the disk and T is its local temperature.…”

Section: Discussionmentioning

confidence: 99%

Be and Bn stars: Balmer discontinuity and stellar-class relationship

Cochetti

Zorec

Cidale

et al. 2020

A&A

View full text Add to dashboard Cite

Context. A significant number of Be stars show a second Balmer discontinuity (sBD) attributed to an extended circumstellar envelope (CE). The fast rotational velocity of Be stars undoubtedly plays a significant role in the formation of the CE. However, Bn stars, which are also B-type rapidly rotating stars, do not all present clear evidence of being surrounded by circumstellar material. Aims. We aim to characterize the populations of Be and Bn stars, and discuss the appearance of the sBD as a function of the stellar parameters. We expect to find new indices characterizing the properties of CEs in Be stars and properties relating Be and Bn stars. Methods. We obtained low-and high-resolution spectra of a sample of Be and Bn stars, derived stellar parameters, characterized the sBD, and measured the emission in the Hα line. Results. Correlations of the aspect and intensity of the sBD and the emission in the Hα line with the stellar parameters and the Vsin i are presented. Some Bn stars exhibit the sBD in absorption, which may indicate the presence of rather dense CEs. Six Bn stars show emission in the Hα line, so they are reclassified as Be stars. The sBD in emission appears in Be stars with Vsin i 250 km s −1 , and in absorption in both Be and Bn stars with Vsin i 50 km s −1 . Low-mass Be and Bn stars share the same region in the Hertzsprung-Russell diagram. The distributions of rotational to critical velocity ratios of Be and Bn stars corresponding to the current stellar evolutionary stage are similar, while distributions inferred for the zero-age main sequence have different skewness. Conclusions. We found emission in the Hα line and signs of a CE in some Bn stars, which motivated us to think that Bn and Be stars probably belong to the same population. It should be noted that some of the most massive Bn stars could display the Be phenomenon at any time. The similarities found among Be and Bn stars deserve to be more deeply pursued.

show abstract

“…where Λ = 1/(1 −r − 1 2 out ), withr out = R out /R eq , is a number usually just slightly larger than 1, as R out R eq . The steadystate AM loss rate, therefore, depends very little (through the factor Λ) on the outer radius of the disc, whose value is poorly known for a few Be stars, and completely unknown for most (Klement et al 2017). In the simulations presented in this work, we have setr out = 1000.…”

Section: Model Descriptionmentioning

confidence: 99%

The life cycles of Be viscous decretion discs: The case of ω CMa

Ghoreyshi

Carciofi

Rímulo

et al. 2018

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

We analyzed V -band photometry of the Be star ω CMa, obtained during the last four decades, during which the star went through four complete cycles of disc formation and dissipation. The data were simulated by hydrodynamic models based on a timedependent implementation of the viscous decretion disc (VDD) paradigm, in which a disc around a fast-spinning Be star is formed by material ejected by the star and driven to progressively larger orbits by means of viscous torques. Our simulations offer a good description of the photometric variability during phases of disc formation and dissipation, which suggests that the VDD model adequately describes the structural evolution of the disc. Furthermore, our analysis allowed us to determine the viscosity parameter α, as well as the net mass and angular momentum (AM) loss rates. We find that α is variable, ranging from 0.1 to 1.0, not only from cycle to cycle but also within a given cycle. Additionally, build-up phases usually have larger values of α than the dissipation phases. Furthermore, during dissipation the outward AM flux is not necessarily zero, meaning that ω CMa does not experience a true quiescence but, instead, switches between a high to a low AM loss rate during which the disc quickly assumes an overall lower density but never zero. We confront the average AM loss rate with predictions from stellar evolution models for fast-rotating stars, and find that our measurements are smaller by more than one order of magnitude.

show abstract

Revealing the structure of the outer disks of Be stars

Cited by 52 publications

References 68 publications

Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks

Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks

Be and Bn stars: Balmer discontinuity and stellar-class relationship

The life cycles of Be viscous decretion discs: The case of ω CMa

Contact Info

Product

Resources

About