A more realistic representation of overshoot at the base of the solar convective envelope as seen by helioseismology

The stratification near the base of the Sun's convective envelope is governed by processes of convective overshooting and element diffusion, and the region is widely believed to play a key role in the solar dynamo. The stratification in that region gives rise to a characteristic signal in the frequencies of solar p modes, which has been used to determine the depth of the solar convection zone and to investigate the extent of convective overshoot. Previous helioseismic investigations have shown that the Sun's spherically symmetric stratification in this region is smoother than that in a standard solar model without overshooting, and have ruled out simple models incorporating overshooting, which extend the region of adiabatic stratification and have a more-or-less abrupt transition to subadiabatic stratification at the edge of the overshoot region. In this paper we consider physically motivated models which have a smooth transition in stratification bridging the region from the lower convection zone to the radiative interior beneath. We find that such a model is in better agreement with the helioseismic data than a standard solar model.Comment: 18 pages, 4 tables, 24 figures - to appear in MNRAS (version a: equation 9 corrected

Stellar Oscillations Network Group

Stellar Oscillations Network Group (SONG) is an initiative aimed at designing and building a network of 1m-class telescopes dedicated to asteroseismology and planet hunting. SONG will have 8 identical telescope nodes each equipped with a high-resolution spectrograph and an iodine cell for obtaining precision radial velocities and a CCD camera for guiding and imaging purposes. The main asteroseismology targets for the network are the brightest (V<6) stars. In order to improve performance and reduce maintenance costs the instrumentation will only have very few modes of operation. In this contribution we describe the motivations for establishing a network, the basic outline of SONG and the expected performance.Comment: Proc. Vienna Workshop on the Future of Asteroseismology, 20 - 22 September 2006. Comm. in Asteroseismology, Vol. 150, in the pres

What Fraction of Boron-8 Solar Neutrinos arrive at the Earth as a nu_2 mass eigenstate?

We calculate the fraction of B^8 solar neutrinos that arrive at the Earth as a nu_2 mass eigenstate as a function of the neutrino energy. Weighting this fraction with the B^8 neutrino energy spectrum and the energy dependence of the cross section for the charged current interaction on deuteron with a threshold on the kinetic energy of the recoil electrons of 5.5 MeV, we find that the integrated weighted fraction of nu_2's to be 91 \pm 2 % at the 95% CL. This energy weighting procedure corresponds to the charged current response of the Sudbury Neutrino Observatory (SNO). We have used SNO's current best fit values for the solar mass squared difference and the mixing angle, obtained by combining the data from all solar neutrino experiments and the reactor data from KamLAND. The uncertainty on the nu_2 fraction comes primarily from the uncertainty on the solar delta m^2 rather than from the uncertainty on the solar mixing angle or the Standard Solar Model. Similar results for the Super-Kamiokande experiment are also given. We extend this analysis to three neutrinos and discuss how to extract the modulus of the Maki-Nakagawa-Sakata mixing matrix element U_{e2} as well as place a lower bound on the electron number density in the solar B^8 neutrino production region.Comment: 23 pages, 8 postscript figures, latex. Dedicated to the memory of John Bahcall who championed solar neutrinos for many lonely year

Solar-like oscillations in the G8 V star tau Ceti

We used HARPS to measure oscillations in the low-mass star tau Cet. Although the data were compromised by instrumental noise, we have been able to extract the main features of the oscillations. We found tau Cet to oscillate with an amplitude that is about half that of the Sun, and with a mode lifetime that is slightly shorter than solar. The large frequency separation is 169 muHz, and we have identified modes with degrees 0, 1, 2, and 3. We used the frequencies to estimate the mean density of the star to an accuracy of 0.45% which, combined with the interferometric radius, gives a mass of 0.783 +/- 0.012 M_sun (1.6%).Comment: accepted for publication in A&

High-precision abundances of elements in Kepler LEGACY stars. Verification of trends with stellar age

HARPS-N spectra with S/N > 250 and MARCS model atmospheres were used to derive abundances of C, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, and Y in ten stars from the Kepler LEGACY sample (including the binary pair 16 Cyg A and B) selected to have metallicities in the range -0.15 < [Fe/H] < +0.15 and ages between 1 and 7 Gyr. Stellar gravities were obtained from seismic data and effective temperatures were determined by comparing non-LTE iron abundances derived from FeI and FeII lines. Available non-LTE corrections were also applied when deriving abundances of the other elements. The results support the [X/Fe]-age relations previously found for solar twins. [Mg/Fe], [Al/Fe], and [Zn/Fe] decrease by ~0.1 dex over the lifetime of the Galactic thin disk due to delayed contribution of iron from Type Ia supernovae relative to prompt production of Mg, Al, and Zn in Type II supernovae. [Y/Mg] and [Y/Al], on the other hand, increase by ~0.3 dex, which can be explained by an increasing contribution of s-process elements from low-mass AGB stars as time goes on. The trends of [C/Fe] and [O/Fe] are more complicated due to variations of the ratio between refractory and volatile elements among stars of similar age. Two stars with about the same age as the Sun show very different trends of [X/H] as a function of elemental condensation temperature Tc and for 16 Cyg, the two components have an abundance difference, which increases with Tc. These anomalies may be connected to planet-star interactions.Comment: 13 pages with 7 figures. Accepted for publication in A&

ASTEC -- the Aarhus STellar Evolution Code

The Aarhus code is the result of a long development, starting in 1974, and still ongoing. A novel feature is the integration of the computation of adiabatic oscillations for specified models as part of the code. It offers substantial flexibility in terms of microphysics and has been carefully tested for the computation of solar models. However, considerable development is still required in the treatment of nuclear reactions, diffusion and convective mixing.Comment: Astrophys. Space Sci, in the pres

A theoretical approach for the interpretation of pulsating PMS intermediate-mass stars

The investigation of the pulsation properties of pre-main-sequence intermediate-mass stars is a promising tool to evaluate the intrinsic properties of these stars and to constrain current evolutionary models. Many new candidates of this class have been discovered during the last decade and very accurate data are expected from space observations obtained for example with the CoRoT satellite. In this context we aim at developing a theoretical approach for the interpretation of observed frequencies, both from the already available ground-based observations and from the future more accurate and extensive CoRoT results. To this purpose we have started a project devoted to the computations of fine and extensive grids of asteroseismic models of intermediate mass pre-main-sequence stars. The obtained frequencies are used to derive an analytical relation between the large frequency separation and the stellar luminosity and effective temperature and to develop a tool to compare theory and observations in the echelle diagram. The predictive capabilities of the proposed method are verified through the application to two test stars. As a second step, we apply the procedure to two true observations from multisite campaigns and we are able to constrain their stellar parameters, in particular the mass, in spite of the small number of frequencies. We expect that with a significantly higher number of frequencies both the stellar mass and age could be constrained and, at the same time, the physics of the models could be tested.Comment: Accepted for publication on A&