Kaloyan Penev

We report the discovery by the HATSouth network of HATS-7b, a transiting Super-Neptune with a mass of 0.120+/-0.012 M_Jup, a radius of 0.563+0.046-0.034 R_Jup, and an orbital period of 3.1853 days. The host star is a moderately bright (V = 13.340+/-0.010 mag, K_S = 10.976+/-0.026 mag) K dwarf star with a mass of 0.849+/-0.027 M_Sun, a radius of 0.815+0.049-0.035 R_Sun, and a metallicity of [Fe/H]= +0.250+/-0.080. The star is photometrically quiet to within the precision of the HATSouth measurements, has low RV jitter, and shows no evidence for chromospheric activity in its spectrum. HATS-7b is the second smallest radius planet discovered by a wide-field ground-based transit survey, and one of only a handful of Neptune-size planets with mass and radius determined to 10% precision. Theoretical modeling of HATS-7b yields a hydrogen-helium fraction of 18+/-4% (rock-iron core and H2-He envelope), or 9+/-4% (ice core and H2-He envelope), i.e.it has a composition broadly similar to that of...

The turbulent flow in stellar convective zones is believed to be an important dissipation mechanism for phenomena like tides and stellar pulsations. The current understanding of this dissipation is based on the assumption that the turbulence follows Kolmogorov scaling. This assumption is reasonable for external shear with high time frequency (e.g. solar p-modes). However, for many cases of astrophysical interest (e.g. binary orbits, Cepheid pulsations etc.) the relevant timescales fall outside of the inertial subrange. We present direct calculations of the turbulent dissipation derived from simulations of stratified anelastic convection with external shear built directly into the equations of motion. We show that the observed dissipation is well parametrized as an effective viscosity coefficient and we derive the values of this coefficient as a function of the forcing period and the direction and amplitude of the shear. In addition we justify a perturbative method for finding the ef...

By using the dense coverage of the extrasolar planet survey project HATNet, we Fourier analyze 381 high-probability members of the nearby open cluster Praesepe (Beehive/M44/NGC 2632). In addition to the detection of 10 variables (of \delta Scuti and other types), we identify 180 rotational variables (including the two known planet hosts). This sample increases the number of known rotational variables in this cluster for spectral classes earlier than M by more than a factor of three. These stars closely follow a color/magnitude -- period relation from early F to late K stars. We approximate this relation by polynomials for an easier reference to the rotational characteristics in different colors. The total (peak-to-peak) amplitudes of the large majority (94%) of these variables span the range of 0.005 to 0.04 mag. The periods cover a range from 2.5 to 15 days. These data strongly confirm that Praesepe and the Hyades have the same gyrochronological ages. Regarding the two planet hosts...

We report the discovery of the transiting extrasolar planet HAT-P-49b. The planet transits the bright (V = 10.3) slightly evolved F-star HD 340099 with a mass of 1.54 M_S and a radius of 1.83 R_S. HAT-P-49b is orbiting one of the 25 brightest stars to host a transiting planet which makes this a favorable candidate for detailed follow-up. This system is an especially strong target for Rossiter-McLaughlin follow-up due to the fast rotation of the host star, 16 km/s. The planetary companion has a period of 2.6915 d, mass of 1.73 M_J and radius of 1.41 R_J. The planetary characteristics are consistent with that of a classical hot Jupiter but we note that this is the fourth most massive star to host a transiting planet with both M_p and R_p well determined.

... TIDAL DISSIPATION IN STARS FROM THE DESTRUCTION RATES OF EXOPLANETS. Kaloyan Penev1, Brian Jackson2, Federico Spada3 and ... Zahn 1966, 1970, 1975, 1977, 1989; Goldreich & Nicholson 1977; Scharlemann 1981, 1982; Goodman & Oh 1997; Papaloizou & ...

The current understanding of the turbulent dissipation in stellar convective zones is based on the assumption that the turbulence follows Kolmogorov scaling. This assumption is valid for some cases in which the time frequency of the external shear is high (e.g. solar p-modes). However, for many cases of astrophysical interest (e.g. binary orbits, stellar pulsations e.t.c.) the timescales of interest lie outside the regime of applicability of Kolmogorov scaling. We present direct calculations of the dissipation efficiency of the turbulent convective flow in this regime using simulations of anelastic convection with external forcing. We show that the effects of the turbulent flow are well represented by an effective viscosity coefficient and we provide the values of the effective viscosity as a function of the perturbation frequency. In addition we justify a perturbative method for finding the effective viscosity (proposed by Goodman and Oh 1997) and apply it to actual simulations of ...