Convective envelopes in rotating OB stars

We study the effects of rotation on the outer convective zones of massive stars. We examine the effects of rotation on the thermal gradient and on the Solberg--Hoiland term by analytical developments and by numerical models. Writing the criterion for convection in rotating envelopes, we show that the effects of rotation on the thermal gradient are much larger and of opposite sign to the effect of the Solberg-Hoiland criterion. On the whole, rotation favors convection in stellar envelopes at the equator and to a smaller extent at the poles. In a rotating 20 Msun star at 94% of the critical angular velocity, there are two convective envelopes, with the bigger one having a thickness of 13.2% of the equatorial radius. In the non-rotating model, the corresponding convective zone has a thickness of only 4.6% of the radius. The occurrence of outer convection in massive stars has many consequences.Comment: 4 pages, 3 figures, accepted by Astronomy and Astrophysic

Direct Dark Matter Detection with Velocity Distribution in the Eddington approach

Exotic dark matter together with the vacuum energy (associated with the cosmological constant) seem to dominate the Universe. Thus its direct detection is central to particle physics and cosmology. Supersymmetry provides a natural dark matter candidate, the lightest supersymmetric particle (LSP). One essential ingredient in obtaining the direct detection rates is the density and the velocity distribution of the LSP in our vicinity. In the present paper we study simultaneously density profiles and velocity distributions in the context of the Eddington approach. In such an approach, unlike the commonly assumed Maxwell-Boltzmann (M-B) distribution, the upper bound of the velocity arises naturally from the potential.Comment: 21 LaTex pages, 27 figure

Turbulent Mixing in the Surface Layers of Accreting Neutron Stars

During accretion a neutron star (NS) is spun up as angular momentum is transported through its surface layers. We study the resulting differentially rotating profile, focusing on the impact this has for type I X-ray bursts. The predominant viscosity is likely provided by the Tayler-Spruit dynamo. The radial and azimuthal magnetic field components have strengths of ~10^5 G and ~10^10 G, respectively. This leads to nearly uniform rotation at the depths of interest for X-ray bursts. A remaining small shear transmits the accreted angular momentum inward to the NS interior. Though this shear gives little viscous heating, it can trigger turbulent mixing. Detailed simulations will be required to fully understand the consequences of mixing, but our models illustrate some general features. Mixing has the greatest impact when the buoyancy at the compositional discontinuity between accreted matter and ashes is overcome. This occurs at high accretion rates, at low spin frequencies, or may depend on the ashes from the previous burst. We then find two new regimes of burning. The first is ignition in a layer containing a mixture of heavier elements from the ashes. If ignition occurs at the base of the mixed layer, recurrence times as short as ~5-30 minutes are possible. This may explain the short recurrence time of some bursts, but incomplete burning is still needed to explain these bursts' energetics. When mixing is sufficiently strong, a second regime is found where accreted helium mixes deep enough to burn stably, quenching X-ray bursts. We speculate that the observed change in X-ray burst properties near one-tenth the Eddington accretion rate is from this mechanism. The carbon-rich material produced by stable helium burning would be important for triggering and fueling superbursts. (abridged)Comment: Accepted for publication in The Astrophysical Journal, 16 pages, 15 figure

Visualization and spectral synthesis of rotationally distorted stars

Simple spherical, non-rotating stellar models are inadequate when describing real stars in the limit of very fast rotation: Both the observable spectrum and the geometrical shape of the star deviate strongly from simple models. We attempt to approach the problem of modeling geometrically distorted, rapidly rotating stars from a new angle: By constructing distorted geometrical models and integrating standard stellar models with varying temperature, gravity, and abundances, over the entire surface, we attempt a semi-empirical approach to modeling. Here we present our methodology, and present simple examples of applications.Comment: 7 pages, 5 figures. Contribution to GREAT-ESF Workshop on "Stellar Atmospheres in the Gaia Era", Brussels 23-24 June 201

The effects of inclination, gravity darkening and differential rotation on absorption profiles of fast rotators

Mechanisms influencing absorption line profiles of fast rotating stars can be sorted into two groups; (i) intrinsic variations sensitive to temperature and pressure, and (ii) global effects common to all spectral lines. I present a detailed study on the latter effects focusing on gravity darkening and inclination for various rotational velocities and spectral types. It is shown that the line shapes of rapidly and rigidly rotating stars mainly depend on the equatorial velocity $v_{\rm e}$, not on the projected rotational velocity $v \sin{i}$ which determines the lines width. The influence of gravity darkening and spectral type on the line profiles is shown. The results demonstrate the possibility of determining the inclination angle $i$ of single fast rotators, and they show that constraints on gravity darkening can be drawn for stellar samples. While significant line profile deformation occurs in stars rotating as fast as v_{\rm e} \ga 200 km s$^{-1}$, for slower rotators profile distortion are marginal. In these cases spectral signatures induced by, e.g., differential rotation are not affected by gravity darkening and the methods applicable to slow rotators can be applied to these faster rotators, too.Comment: 7 pages, accepted by A&

Asymmetric transit curves as indication of orbital obliquity: clues from the late-type dwarf companion in KOI-13

KOI-13.01, a planet-sized companion in an optical double star was announced as one of the 1235 Kepler planet candidates in February 2011. The transit curves show significant distortion that was stable over the ~130 days time-span of the data. Here we investigate the phenomenon via detailed analyses of the two components of the double star and a re-reduction of the Kepler data with pixel-level photometry. Our results indicate that KOI-13 is a common proper motion binary, with two rapidly rotating components (v sin i ~ 65--70 km/s). We identify the host star of KOI-13.01 and conclude that the transit curve asymmetry is consistent with a companion orbiting a rapidly rotating, possibly elongated star on an oblique orbit. After correcting the Kepler light curve to the second light of the optical companion star, we derive a radius of 2.2 R_J for the transiter, implying an irradiated late-type dwarf, probably a hot brown dwarf rather than a planet. KOI-13 is the first example for detecting orbital obliquity for a substellar companion without measuring the Rossiter-McLaughlin effect from spectroscopy.Comment: Accepted by ApJ Letters. A new part added about KOI-13.01 (Sect. 3.4 pars 2-4

Superconvergent Perturbation Method in Quantum Mechanics

An analogue of Kolmogorov's superconvergent perturbation theory in classical mechanics is constructed for self adjoint operators. It is different from the usual Rayleigh--Schr\"odinger perturbation theory and yields expansions for eigenvalues and eigenvectors in terms of functions of the perturbation parameter.Comment: 11 pages, LaTe

An Extremely Lithium-Rich Bright Red Giant in the Globular Cluster M3

We have serendipitously discovered an extremely lithium-rich star on the red giant branch of the globular cluster M3 (NGC 5272). An echelle spectrum obtained with the Keck I HIRES reveals a Li I 6707 Angstrom resonance doublet of 520 milli-Angstrom equivalent width, and our analysis places the star among the most Li-rich giants known: log[epsilon(Li)] ~= +3.0. We determine the elemental abundances of this star, IV-101, and three other cluster members of similar luminosity and color, and conclude that IV-101 has abundance ratios typical of giants in M3 and M13 that have undergone significant mixing. We discuss mechanisms by which a low-mass star may be so enriched in Li, focusing on the mixing of material processed by the hydrogen-burning shell just below the convective envelope. While such enrichment could conceivably only happen rarely, it may in fact regularly occur during giant-branch evolution but be rarely detected because of rapid subsequent Li depletion.Comment: 7-page LaTeX file, including 2 encapsulated ps figures + 1 table; accepted for publication in the Astrophysical Journal Letter

Ionization structure in the winds of B[e] supergiants II. Influence of rotation on the formation of equatorial hydrogen neutral zones

Context: B[e] supergiants are known to have non-spherical winds, and the existence of disks that are neutral in hydrogen close to their stellar surface has been postulated. A suitable mechanism to produce non-spherical winds seems to be rapid rotation, and at least for three B[e] supergiants in the Magellanic Clouds rotation velocities at a substantial fraction of their critical velocity have been found. Aims: We want to find suitable recombination distances in the equatorial plane of rapidly rotating stars that explain the observed huge amounts of neutral material in the vicinity of B[e] supergiants. Methods: We perform ionization structure calculations in the equatorial plane around rapidly rotating luminous supergiants. The restriction to the equatorial plane allows us to treat the ionization balance equations 1-dimensionally, while the stellar radiation field is calculated 2-dimensionally, taking into account the latitudinal variation of the stellar surface parameters. The stellar parameters used correspond to those known for B[e] supergiants. The assumptions made in the computations all have in common that the total number of available ionizing photons at any location within the equatorial plane is overestimated, resulting in upper limits for the recombination distances. Results: We find that despite the drop in equatorial surface density of rapidly rotating stars (neglecting effects like bi-stability and/or wind compression), hydrogen and helium recombine at or close to the stellar surface, for mass loss rates Mdot > 5d-5 M_sun/yr and rotation speeds in excess of v(rot,eq)/v(crit)=0.8.Comment: 10 pages, 5 figures, accepted for publication in A&A (03/05/2006), Eq. (3) corrected, language improve

On the Selection of Photometric Planetary Transits

We present a new method for differentiating between planetary transits and eclipsing binaries based on the presence of the ellipsoidal light variations. These variations can be used to detect stellar secondaries with masses ~0.2 M_sun orbiting sun-like stars at a photometric accuracy level which has already been achieved in transit surveys. By removing candidates exhibiting this effect it is possible to greatly reduce the number of objects requiring spectroscopic follow up with large telescopes. Unlike the usual candidate selection method, which are primarily based on the estimated radius of the orbiting object, this technique is not biased against bona-fide planets and brown dwarfs with large radii, because the amplitude of the effect depends on the transiting object's mass and orbital distance. In many binary systems, where a candidate planetary transit is actually due to the partial eclipse of two normal stars, the presence of flux variations due to the gravity darkening effect will show the true nature of these systems. We show that many of the recent OGLE-III photometric transit candidates exhibit the presence of significant variations in their light curves and are likely to be due to stellar secondaries. We find that the light curves of white dwarf transits will generally not mimic those of small planets because of significant gravitationally induced flux variations. We discuss the relative merits of methods used to detect transit candidates which are due to stellar blends rather than planets. We outline how photometric observations taken in two bands can be used to detect the presence of stellar blends.Comment: ApJ, 11 pages, 2 figures, 1 table, replaced with accepted versio