Presupernova evolution of accreting white dwarfs with rotation

We discuss the effects of rotation on the evolution of accreting carbon-oxygen white dwarfs, with the emphasis on possible consequences in Type Ia supernova (SN Ia) progenitors. Starting with a slowly rotating white dwarf, we simulate the accretion of matter and angular momentum from a quasi-Keplerian accretion disk. The role of the various rotationally induced hydrodynamic instabilities for the transport of angular momentum inside the white dwarf is investigated. We find that the dynamical shear instability is the most important one in the highly degenerate core. Our results imply that accreting white dwarfs rotate differentially throughout,with a shear rate close to the threshold value for the onset of the dynamical shear instability. As the latter depends on the temperature of the white dwarf, the thermal evolution of the white dwarf core is found to be relevant for the angular momentum redistribution. As found previously, significant rotation is shown to lead to carbon ignition masses well above 1.4 Msun. Our models suggest a wide range of white dwarf explosion masses, which could be responsible for some aspects of the diversity observed in SNe Ia. We analyze the potential role of the bar-mode and the r-mode instability in rapidly rotating white dwarfs, which may impose angular momentum loss by gravitational wave radiation. We discuss the consequences of the resulting spin-down for the fate of the white dwarf, and the possibility to detect the emitted gravitational waves at frequencies of 0.1 >... 1.0 Hz in nearby galaxies with LISA. Possible implications of fast and differentially rotating white dwarf cores for the flame propagation in exploding white dwarfs are also briefly discussed.Comment: 22 pages, 16 figures, Accepted to A&

The Evolution of Helium Star Plus Carbon-Oxygen White Dwarf Binary Systems and Implications for Diverse Stellar Transients and Hypervelocity Stars

Helium accretion induced explosions in CO white dwarfs (WDs) are considered promising candidates for a number of observed types of stellar transients, including supernovae (SNe) of Type Ia and Type Iax. However, a clear favorite outcome has not yet emerged. We explore the conditions of helium ignition in the white dwarf and the final fates of helium star-WD binaries as function of their initial orbital periods and component masses. We compute 274 model binary systems with the Binary Evolution Code (BEC), where both components are fully resolved. Stellar and orbital evolution is computed simultaneously, including mass and angular momentum transfer, tides, and gravitational wave emission, as well as differential rotation and internal hydrodynamic and magnetic angular momentum transport. We find that helium detonations are expected only in systems with the shortest initial orbital periods, and for initially massive white dwarfs (MWD > 1.0 MSun ) and lower mass donors (Mdonor < 0.8 MSun), with accumulated helium layers mostly exceeding 0.1 MSun. Upon detonation, these systems would release the donor as a hypervelocity pre-WD runaway star, for which we predict the expected range of kinematic and stellar properties. Systems with more massive donors or initial periods exceeding 1.5 h will likely undergo helium deflagrations after accumulating 0.1 - 0.001 MSun of helium. Helium ignition in the white dwarf is avoided in systems with helium donor stars below - 0.6 MSun, and lead to three distinctly different groups of double white dwarf systems. The size of the parameter space open to helium detonation corresponds to only about 3 % of the galactic SN Ia rate, and to 10 % of the SN Iax rate, while the predicted large amounts of helium (>0.1 MSun) in progenitors cannot easily be reconciled with observations of archetypical SN Ia. ...Comment: Accepted for publication in A&A, 28 pages, 16 figures, 6 table

The metallicity dependence of envelope inflation in massive stars

Recently it has been found that models of massive stars reach the Eddington limit in their interior, which leads to dilute extended envelopes. We perform a comparative study of the envelope properties of massive stars at different metallicities, with the aim to establish the impact of the stellar metallicity on the effect of envelope inflation. We analyse published grids of core-hydrogen burning massive star models computed with metallicities appropriate for massive stars in the Milky Way, the LMC and the SMC, the very metal poor dwarf galaxy I Zwicky 18, and for metal-free chemical composition. Stellar models of all the investigated metallicities reach and exceed the Eddington limit in their interior, aided by the opacity peaks of iron, helium and hydrogen, and consequently develop inflated envelopes. Envelope inflation leads to a redward bending of the zero-age main sequence and a broadening of the main sequence band in the upper part of the Hertzsprung-Russell diagram. We derive the limiting L/M-values as function of the stellar surface temperature above which inflation occurs, and find them to be larger for lower metallicity. While Galactic models show inflation above ~29 Msun, the corresponding mass limit for Population III stars is ~150 Msun. While the masses of the inflated envelopes are generally small, we find that they can reach 1-100 Msun in models with effective temperatures below ~8000 K, with higher masses reached by models of lower metallicity. Envelope inflation is expected to occur in sufficiently massive stars at all metallicities, and is expected to lead to rapidly growing pulsations, high macroturbulent velocities, and might well be related to the unexplained variability observed in Luminous Blue Variables like S Doradus and Eta Carina.Comment: 16 pages (with Appendix), accepted in A&

Off-diagonal magnetoimpedance in field-annealed Co-based amorphous ribbons

The off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons was measured in the low-frequency range using a pick-up coil wound around the sample. The asymmetric two-peak behavior of the field dependence of the off-diagonal impedance was observed. The asymmetry is attributed to the formation of a hard magnetic crystalline phase at the ribbon surface. The experimental results are interpreted in terms of the surface impedance tensor. It is assumed that the ribbon consists of an inner amorphous region and surface crystalline layers. The coupling between the crystalline and amorphous phases is described through an effective bias field. A qualitative agreement between the calculated dependences and experimental data is demonstrated. The results obtained may be useful for development of weak magnetic-field sensors.Comment: 19 pages, 6 figure

On the origin of microturbulence in hot stars

We present results from the first extensive study of convection zones in the envelopes of hot massive stars, which are caused by opacity peaks associated with iron and helium ionization. These convective regions can be located very close to the stellar surface. Recent observations of microturbulence in massive stars from the VLT-Flames survey are in good agreement with our predictions concerning the occurrence and the strength of sub-surface convection in hot stars. We argue further that convection close to the surface may trigger clumping at the base of the stellar wind of massive stars.Comment: to appear in Comm. in Astroseismology - Proceedings of the 38th LIAC/HELAS-ESTA/BAG, 200

Ultra-stripped Type Ic supernovae from close binary evolution

Recent discoveries of weak and fast optical transients raise the question of their origin. We investigate the minimum ejecta mass associated with core-collapse supernovae (SNe) of Type Ic. We show that mass transfer from a helium star to a compact companion can produce an ultra-stripped core which undergoes iron core collapse and leads to an extremely fast and faint SN Ic. In this Letter, a detailed example is presented in which the pre-SN stellar mass is barely above the Chandrasekhar limit, resulting in the ejection of only ~0.05-0.20 M_sun of material and the formation of a low-mass neutron star. We compute synthetic light curves of this case and demonstrate that SN 2005ek could be explained by our model. We estimate that the fraction of such ultra-stripped to all SNe could be as high as 0.001-0.01. Finally, we argue that the second explosion in some double neutron star systems (for example, the double pulsar PSR J0737-3039B) was likely associated with an ultra-stripped SN Ic.Comment: ApJ Letters, in press, 6 pages, 5 figures (emulateapj style). Very minor changes to match printed version. Follow DOI link below for online published versio