Numerically determined transport laws for fingering ("thermohaline") convection in astrophysics

We present the first three-dimensional simulations of fingering convection performed in a parameter regime close to the one relevant for astrophysics, and reveal the existence of simple asymptotic scaling laws for turbulent heat and compositional transport. These laws can straightforwardly be extrapolated to the true astrophysical regime. Our investigation also indicates that thermocompositional "staircases," a key consequence of fingering convection in the ocean, cannot form spontaneously in the fingering regime in stellar interiors. Our proposed empirically-determined transport laws thus provide simple prescriptions for mixing by fingering convection in a variety of astrophysical situations, and should, from here on, be used preferentially over older and less accurate parameterizations. They also establish that fingering convection does not provide sufficient extra mixing to explain observed chemical abundances in RGB stars.Comment: Submitted to ApJ Letters on October 29th. 15 pages, 4 figures. See Garaud 2010 for companion pape

Meridional flow and differential rotation by gravity darkening in fast rotating solar-type stars

An explanation is presented for the rather strong total surface differential rotation of the observed very young solar-type stars like AB Dor and PZ Tel. Due to its rapid rotation a nonuniform energy flux leaves the stellar core so that the outer convection zone is nonuniformly heated from below. Due to this `gravity darkening' of the equator a meridional flow is created flowing equatorwards at the surface and thus accelerating the equatorial rotation. The effect linearly grows with the normalized pole-equator difference, \epsilon, of the heat-flux at the bottom of the convection zone. A rotation rate of about 9 h leads to \epsilon=0.1 for a solar-type star. In this case the resulting equator-pole differences of the angular velocity at the stellar surface, \delta\Omega, varies from unobservable 0.005/day to the (desired) value of 0.03 day$^{-1}$ when the dimensionless diffusivity factors $c_\nu$ and c_\chi vary between 1 and 0.1 (standard value c_\nu \simeq c_\chi \simeq 0.3, see Table 1.) In all cases the related temperature differences between pole and equator at the surface are unobservably small. The (clockwise) meridional circulation which we obtain flows opposite to the (counterclockwise) circulation appearing as a byproduct in the \Lambda-theory of the nonuniform rotation in outer convection zones. The consequences of this situation for those dynamo theories of stellar activity are discussed which work with the meridional circulation as the dominant magnetic-advection effect in latitude to produce the solar-like form of the butterfly diagram. Key words: Hydrodynamics, Star: rotation, Stars: pre-main sequence, Stellar activityComment: 4 pages, 3 figures, Astronomy and Astrophysics (subm.

Thermohaline mixing in low-mass giants

Thermohaline mixing has recently been proposed to occur in low mass red giants, with large consequences for the chemical yields of low mass stars. We investigate the role of thermohaline mixing during the evolution of stars between 1Msun and 3Msun, in comparison to other mixing processes acting in these stars. We confirm that thermohaline mixing has the potential to destroy most of the ^3He which is produced earlier on the main sequence during the red giant stage. In our models we find that this process is working only in stars with initial mass M <~ 1.5Msun. Moreover, we report that thermohaline mixing can be present during core helium burning and beyond in stars which still have a ^3He reservoir. While rotational and magnetic mixing is negligible compared to the thermohaline mixing in the relevant layers, the interaction of thermohaline motions with differential rotation and magnetic fields may be essential to establish the time scale of thermohaline mixing in red giants.Comment: 6 pages, conference proceedings IAU Symposium 252 (Sanya

Analytical Blowup Solutions to the 2-dimensional Isothermal Euler-Poisson Equations of Gaseous Stars

We study the Euler-Poisson equations of describing the evolution of the gaseous star in astrophysics. Firstly, we construct a family of analytical blowup solutions for the isothermal case in R^2. Furthermore the blowup rate of the above solutions is also studied and some remarks about the applicability of such solutions to the Navier-Stokes-Poisson equations and the drift-diffusion model in semiconductors are included. Finally, for the isothermal case, the result of Makino and Perthame for the tame solutions is extended to show that the life span of such solutions must be finite if the initial data is with compact support.Comment: 15 page

Cygnus X-2: the Descendant of an Intermediate-Mass X-Ray Binary

The X-ray binary Cygnus X-2 (Cyg X-2) has recently been shown to contain a secondary that is much more luminous and hotter than is appropriate for a low-mass subgiant. We present detailed binary-evolution calculations which demonstrate that the present evolutionary state of Cyg X-2 can be understood if the secondary had an initial mass of around 3.5 M_sun and started to transfer mass near the end of its main-sequence phase (or, somewhat less likely, just after leaving the main sequence). Most of the mass of the secondary must have been ejected from the system during an earlier rapid mass-transfer phase. In the present phase, the secondary has a mass of around 0.5 M_sun with a non-degenerate helium core. It is burning hydrogen in a shell, and mass transfer is driven by the advancement of the burning shell. Cyg X-2 therefore is related to a previously little studied class of intermediate-mass X-ray binaries (IMXBs). We suggest that perhaps a significant fraction of X-ray binaries presently classified as low-mass X-ray binaries may be descendants of IMXBs and discuss some of the implications

Simulations of Prominence Formation in the Magnetized Solar Corona by Chromospheric Heating

Starting from a realistically sheared magnetic arcade connecting chromospheric, transition region to coronal plasma, we simulate the in-situ formation and sustained growth of a quiescent prominence in the solar corona. Contrary to previous works, our model captures all phases of the prominence formation, including the loss of thermal equilibrium, its successive growth in height and width to macroscopic dimensions, and the gradual bending of the arched loops into dipped loops, as a result of the mass accumulation. Our 2.5-dimensional, fully thermodynamically and magnetohydrodynamically consistent model mimics the magnetic topology of normal-polarity prominences above a photospheric neutral line, and results in a curtain-like prominence above the neutral line through which the ultimately dipped magnetic field lines protrude at a finite angle. The formation results from concentrated heating in the chromosphere, followed by plasma evaporation and later rapid condensation in the corona due to thermal instability, as verified by linear instability criteria. Concentrated heating in the lower atmosphere evaporates plasma from below to accumulate at the top of coronal loops and supply mass to the later prominence constantly. This is the first evaporation-condensation model study where we can demonstrate how the formed prominence stays in a force balanced state, which can be compared to the Kippenhahn-Schluter type magnetohydrostatic model, all in a finite low-beta corona

The Structure of Close Binaries in Two Dimensions

The structure and evolution of close binary stars has been studied using the two-dimensional (2D) stellar structure algorithm developed by Deupree (1995). We have calculated a series of solar composition stellar evolution sequences of binary models, where the mass of the 2D model is 8Msun with a point-mass 5Msun companion. We have also studied the structure of the companion in 2D, by considering the zero-age main-sequence (ZAMS) structure of a 5Msun model with an 8Msun point-mass companion. In all cases the binary orbit was assumed to be circular and co-rotating with the rotation rate of the stars. We considered binary models with three different initial separations, a = 10, 14 and 20Rsun. These models were evolved through central hydrogen burning or until the more massive star expanded to fill its critical potential surface or Roche lobe. The calculations show that evolution of the deep interior quantities is only slightly modified from those of single star evolution. Describing the model surface as a Roche equipotential is also satisfactory until very close to the time of Roche lobe overflow, when the self gravity of the model about to lose mass develops a noticeable aspherical component and the surface time scale becomes sufficiently short that it is conceivable that the actual surface is not an equipotential.Comment: 22 pages, 10 figures, accepted by Ap

Mass loss out of close binaries

In a liberal evolutionary scenario, mass can escape from a binary during eras of fast mass transfer. We calculate the mass lost by binaries with a B-type primary at birth where mass transfer starts during hydrogen core burning of the donor. We simulate the distribution of mass-ratios and orbital periods for those interacting binaries. The amount of time the binary shows Algol characteristics within different values of mass-ratio and orbital period has been fixed from conservative and liberal evolutionary calculations. We use these data to simulate the distribution of mass-ratios and orbital periods of Algols with the conservative as well as the liberal model. We compare mass-ratios and orbital periods of Algols obtained by conservative evolution with those obtained by our liberal model. Since binaries with a late B-type primary evolve almost conservatively, the overall distribution of mass-ratios will only yield a few Algols more with high mass-ratios than conservative calculations do. Whereas the simulated distribution of orbital periods of Algols fits the observations well, the simulated distribution of mass-ratios produces always too few systems with large values.Comment: 6 pages, 6 figures, accepted for publication in A&A; accepted versio

Radiatively-Driven Outflows and Avoidance of Common-Envelope Evolution in Close Binaries

Recent work on Cygnus X-2 suggests that neutron-star or black-hole binaries survive highly super-Eddington mass transfer rates without undergoing common-envelope evolution. We suggest here that the accretion flows in such cases are radiation pressure-dominated versions of the "ADIOS" picture proposed by Blandford and Begelman (1999), in which almost all the mass is expelled from large radii in the accretion disk. We estimate the maximum radius from which mass loss is likely to occur, and show that common-envelope evolution is probably avoided in any binary in which a main-sequence donor transfers mass on a thermal timescale to a neutron star or black hole, even though the mass transfer rate may reach values of 0.001 solar masses per year. This conclusion probably applies also to donors expanding across the Hertzsprung gap, provided that their envelopes are radiative. SS433 may be an example of a system in this state.Comment: 4 pages, submitted to Astrophysical Journal Letters, 26 March 199