Jiří Krtička

Mass and angular momentum loss via decretion disks

We examine the nature and role of mass loss via an equatorial decretion disk in massive stars with near-critical rotation induced by evolution of the stellar interior. In contrast to the usual stellar wind mass loss set by exterior driving from the stellar luminosity, such decretion-disk mass loss stems from the angular momentum loss needed to keep the star near and below critical rotation, given the interior evolution and decline in the star’s moment of inertia. Because the specific angular momentum in a Keplerian disk increases with the square root of the radius, the decretion mass loss associated with a required level of angular momentum loss depends crucially on the outer radius for viscous coupling of the disk, and can be significantly less than the spherical, wind-like mass loss commonly assumed in evolutionary calculations. We discuss the physical processes that affect the outer disk radius, including thermal disk outflow, and ablation of the disk material via a line-driven wind induced by the star’s radiation. We present parameterized scaling laws for taking account of decretion-disk mass loss in stellar evolution codes, including how these are affected by metallicity, or by presence within a close binary and/or a dense cluster. Effects similar to those discussed here should also be present in accretion disks during star formation, and may play an important role in shaping the distribution of rotation speeds on the ZAMS.

The light variability of the helium strong star HD 37776 as a result of its inhomogeneous elemental surface distribution

We simulate light curves of the helium strong chemically peculiar star HD 37776 assuming that the observed periodic light variations originate as a result of inhomogeneous horizontal distribution of chemical elements on the surface of a rotating star. We show that chemical peculiarity influences the monochromatic radiative flux, mainly due to bound-free processes. Using the model of the distribution of silicon and helium on HD 37776 surface, derived from spectroscopy, we calculate a photometric map of the surface and consequently the uvby light curves of this star. Basically, the predicted light curves agree in shape and amplitude with the observed ones. We conclude that the basic properties of variability of this helium strong chemically peculiar star can be understood in terms of the model of spots with peculiar chemical composition.

Predictions of the effect of clumping on the wind properties of O-type stars

Aims. Both empirical evidence and theoretical findings indicate that the stellar winds of massive early-type stars are inhomogeneous, i.e., porous and clumpy. For relatively dense winds, empirically derived mass-loss rates might be reconciled with predictions if these empirical rates are corrected for clumping. The predictions, however, do not account for structure in the wind. To allow for a consistent comparison, we investigate and quantify the effect of clumpiness and porosity of the outflow on the predicted wind energy and the maximal effect on the mass-loss rate of O-type stars. Methods. Combining non-LTE model atmospheres and a Monte Carlo method to compute the transfer of momentum from the photons to the gas, the effect of clumping and porosity on the energy transferred from the radiation field to the wind is computed in outflows in which the clumping and porosity stratification is parameterized by heuristic prescriptions. Results. The impact of structure in the outflow on the wind energy is complex and is a function of stellar temperature, the density of gas in the clumps, and the physical scale of the clumps. If the medium is already clumped in the photosphere, the emergent radiation field will be softer, slightly increasing the wind energy of relatively cool O stars (30 000 K) but slightly decreasing it for relatively hot O stars (40 000 K). More important is that as a result of recombination of the gas in a clumped wind the line force increases. However, because of porosity the line force decreases, simply because photons may travel in-between the clumps, avoiding interactions with the gas. If the changes in the wind energy only affect the mass-loss rate and not the terminal velocity of the flow, we find that the combined effect of clumpiness and porosity is a small reduction in the mass-loss rate if the clumps are smaller than 1/100th the local density scale height Hρ. In this case, empirical mass-loss determinations based on Hα fitting and theory match for stars with dense winds ( ˙

The extremely rapid rotational braking of the magnetic helium-strong star HD 37776

Context. Light and spectrum variations of the magnetic chemically peculiar (mCP) stars are explained by the oblique rigid rotator model with a rotation period usually assumed to be stable on a long time scale. A few exceptions, such as CU Vir or 56 Ari, have been reported as displaying an increase in their rotation period. A possible increase in the period of light and spectrum variations has also been suggested from observations of the helium-strong mCP star HD 37776 (V901 Ori). Aims. In this paper we attempt to confirm the possible period change of HD 37776 and discuss a possible origin of this change as a consequence of i) duplicity; ii) precession; iii) evolutionary changes; and iv) continuous/discrete/transient angular momentum loss. Methods. We analyse all available observations of the star obtained since 1976. These consist of 1707 photometric measurements obtained in uvby(β), (U)BV, V, BTVT ,a ndHp, including 550 of our own recent observations obtained in 2006 and 2007, 53 spectrophotometric measurements of the Hei λ 4026 A line, 66 equivalent width measurements of Hei spectral lines from 23 CFHT spectrograms acquired in 1986, and 69 Hei equivalent measurements from spectral lines present in 35 SAO Zeeman spectrograms taken between 1994 and 2002. All of these 1895 individual observations obtained by various techniques were processed simultaneously by means of specially developed robust codes. Results. We confirm the previously suspected gradual increase in the 1. 5387 period of HD 37776 and find that it has lengthened by a remarkable 17.7 ± 0.7 s over the past 31 years. We also note that a decrease in the rate of the period change is not excluded by the data. The shapes of light curves in all colours were found to be invariable. Conclusions. After ruling out light-time effects in a binary star, precession of the rotational axis, and evolutionary changes as possible causes for the period change, we interpret this ongoing period increase as a braking of the star’s rotation, at least in its surface layers, due to the momentum loss through events or processes in the extended stellar magnetosphere.

Populations of rotating stars - II. Rapid rotators and their link to Be-type stars

Context. Even though it is broadly accepted that single Be stars are rapidly rotating stars surrounded by a flat rotating circumstellar disk, there is still a debate about how fast these stars rotate and also about the mechanisms involved in the angular-momentum and mass input in the disk. Aims. We study the properties of stars that rotate near their critical-rotation rate and investigate the properties of the disks formed by equatorial mass ejections. Methods. We used the most recent Geneva stellar evolutionary tracks for rapidly rotating stars that reach the critical limit and used a simple model for the disk structure. Results. We obtain that for a 9 M-circle dot star at solar metallicity, the minimum average velocity during the main-sequence (MS) phase to reach the critical velocity is around 330 km s(-1), whereas it would be 390 km s(-1) at the metallicity of the Small Magellanic Cloud (SMC). Red giants or supergiants originating from very rapid rotators rotate six times faster and show N/C ratios three times higher than those originating from slowly rotating stars. This difference becomes stronger at lower metallicity. It might therefore be very interesting to study the red giants in clusters that show a large number of Be stars on the MS band. On the basis of our single-star models, we show that the observed Be-star fraction with cluster age is compatible with the existence of a temperature-dependent lower limit in the velocity rate required for a star to become a Be star. The mass, extension, and diffusion time of the disks produced when the star is losing mass at the critical velocity, obtained from simple parametrized expressions, are estimated to be between 9.4 x 10(-12) and 1.4 x 10(-7) M-circle dot (3 x 10(-6) to 4.7 x 10(-2) times the mass of the Earth), 2000 and 6500 R-circle dot, and 10 and 30 yr. These values are not too far from those estimated for disks around Be-type stars. At a given metallicity, the mass and the extension of the disk increase with the initial mass and with age on the MS phase. Denser disks are expected in low-metallicity regions.

NLTE models of line‐driven stellar winds – II. O stars in the Small Magellanic Cloud

We calculate non-local thermodynamic equilibrium (NLTE) line-driven wind models of selected O stars in the spectral range of O4 to O9 in the Small Magellanic Cloud (SMC). We compare predicted basic wind properties, i.e. the terminal velocity and the mass-loss rate with values derived from observation. We found relatively good agreement between theoretical and observed terminal velocities. On the other hand, predicted mass-loss rates and mass-loss rates derived from observation are in a good agreement only for higher mass-loss rates. Theoretical mass-loss rates lower than approximately 10^-7 M yr^-1 are significantly higher than those derived from observation. These results confirm the previously reported problem of weak winds, since our calculated mass-loss rates are in fair agreement with predictions of Vink et al. We study multicomponent models for these winds. For this purpose we develop a more detailed description of wind decoupling. We show that the instability connected with the decoupling of individual wind elements may occur for low-density winds. In the case of winds with very low observed mass-loss rates the multicomponent effects are important for the wind structure, however this is not able to explain consistently the difference between the predicted mass-loss rate and the mass-loss rate derived from observation for these stars. Similar to previous studies, we found the level of dependence of the wind parameters on the metallicity. We conclude that the wind mass-loss rate significantly increases with metallicity as , whereas the terminal velocity of wind on average depends on metallicity only slightly, namely v~Z^0.06 (for studied stars).

The winds of hot massive first stars

We study dynamical aspects of circumstellar environment around massive zero-metallicity first stars. For this pur- pose we apply our NLTE wind models. We show that the hydrogen-helium stellar wind from stationary massive first generatio n (Population III) stars (driven either by the line (bound-bo und) or continuum (bound-free and free-free) transitions) is unlikely. The possibility of expulsion of chemically homogeneous wind and the role of minor isotopes are also discussed. Finally, we estimate the importance of hydrogen and helium lines for shutting off the initial accretion onto first stars and its influe nce on initial mass function of first stars.

The nature of the light variability of the silicon star HR 7224

Context. Although photometric variations of chemically peculiar (CP) stars are frequently used to determine their rotational periods, the detailed mechanism of their light variability remains poorly understood. Aims. We simulate the light variability of the star HR 7224 using the observed surface distribution of silicon and iron. Methods. We used the TLUSTY model atmospheres calculated for the appropriate silicon and iron abundances to obtain the emergent flux and to predict the rotationally modulated light curve of the star. We also obtained additional photometric measurements and employed our own regression procedure to derive a more precise estimate of the light elements. Results. We show that the light variation of the star can be explained as a result of i) the uneven surface distribution of the elements, ii) the flux redistribution from the ultraviolet to the visible part of the spectrum, and iii) rotation of the star. We show that the silicon bound-free transitions and iron bound-bound transitions provide the main contribution to the flux redistribution, although an additional source of opacity is needed. We confirm that numerous iron lines significantly contribute to the well-known depression at 5200 A and discuss the connection between iron abundance and the value of peculiarity index a. Conclusions. The uneven surface distribution of silicon and iron is able to explain most of the rotationally modulated light variation in the star HR 7224.

Multicomponent radiatively driven stellar winds - I. Nonisothermal three-component wind of hot B stars

We computed models of a three-component nonisothermal radiatively driven stellar wind for different spectral types of hot B stars. We showed that friction heats mainly the outer parts of the wind and the modified temperature stratification leads to a decrease of the outflow velocity. Contrary to the isothermal case, even the possibility of decoupling of radiatively absorbing ions and (major) nonabsorbing component is excluded for a realistic form of the driving force. Regardless of the actual form of the driving force, we derived general conditions under which decoupling of a stellar wind may occur. We demonstrated that the possibility of decoupling is closely related to the functional dependence of the driving force and to the ratio of the densities of individual components. We discuss several consequences of a multifluid phenomenon in hot star winds, particularly metallicity effects and the implications of possible helium decoupling on chemically peculiar stars. We propose an explanation of the low terminal velocity of tau Sco based on frictional heating.

Modelling of the ultraviolet and visual SED variability in the hot magnetic Ap star CU Virginis

Context. The spectral energy distribution (SED) in chemically peculiar stars may be significantly affected by their abundance anomalies. The observed SED variations are usually assumed to be a result of inhomogeneous surface distribution of chemical elements, flux redistribution and stellar rotation. However, the direct evidence for this is still only scarce. Aims. We aim to identify the processes that determine the SED and its variability in the UV and visual spectral domains of the heliumweak star CU Vir. Methods. We used the TLUSTY model atmospheres calculated for the appropriate surface chemical composition to obtain the emergent flux and predict the rotationally modulated flux variability of the star. Results. We show that most of the light variations in the vby filters of the Stromgren photometric system are a result of the uneven surface distribution of silicon, chromium, and iron. Our models are only able to explain a part of the variability in the u filter, however. The observed UV flux distribution is very well reproduced, and the models are able to explain most of the observed features in the UV light curve, except for the region 2000−2500 A, where the amplitude of the observed light variations is higher than predicted. The variability observed in the visible is merely a faint gleam of that in the UV. While the amplitude of the light curves reaches only several hundredths of magnitude in the visual domain, it reaches about 1 mag in the UV. Conclusions. The visual and UV light variability of CU Vir is caused by the flux redistribution from the far UV to near UV and visible regions, inhomogeneous distribution of the elements and stellar rotation. Bound-free transitions of silicon and bound-bound transitions of iron and chromium contribute the most to the flux redistribution. This mechanism can explain most of the rotationally modulated light variations in the filters centred on the Paschen continuum and on the UV continuum of the star CU Vir. However, another mechanism(s) has to be invoked to fully explain the observed light variations in the u filter and in the region 2000−2500 A.