Frequency separation variations of the solar low-degree p-modes

Variations of frequency separations of low-degree p-modes are studied over the solar activity cycle. The separations studied are obtained from the frequencies of low-degree p-modes of the Global Oscillation Network Group (GONG). 10.7 cm radio flux is used as an index of solar activity. Small separations of the p-mode frequencies are considered to be mainly dependent on the conditions in stellar interiors. Thus they could be applied to diagnose the changes in the stellar interior. Our calculation results show that the magnitudes of variations of the mean large separations are less than 1 $\sigma$ over the solar activity cycle. Small separations show different behaviors in the ascending and descending phases of activity. In the ascending phase, variations of the small separations are less than 1 $\sigma$. However, the small separations have systematic shifts during 2004 - 2007. The shifts are roughly 1 $\sigma$ or more. The variations of the ratios of the small to large separations with time are similar to the changes of the small separations. The effects of the changes in the large separations on the ratios are negligible. The variations of the separations may be a consequence of the influence from the surface activity or systematic errors in measurements or some processes taking place in the solar interior.Comment: 12 pages, 6 figures. Accepted for publication in PAS

WD+MS Systems as the Progenitors of Type Ia Supernovae with Different Metallicities

The single-degenerate model for the progenitors of type Ia supernovae (SNe Ia) is one of the two most popular models, in which a carbon-oxygen white dwarf (CO WD) accretes hydrogen-rich material from its companion, increases its mass to the Chandrasekhar mass limit, and then explodes as a SN Ia. Incorporating the prescription of Hachisu et al. (1999a) for the accretion efficiency into Eggleton's stellar evolution code and assuming that the prescription is valid for \emph{all} metallicities, we carried out a detailed binary evolution study with different metallicities. We show the initial and final parameter space for SNe Ia in a ($\log P-M_{\rm 2}$) plane. The positions of some famous recurrent novae in the ($\log P-M_{\rm 2}$) plane, as well as a supersoft X-ray source (SSS), RX J0513.9-6951 are well explained by our model, and our model can also explain the space velocity and mass of Tycho G, which is now suggested to be the companion star of Tycho's supernova . Our study indicates that the SSS, V Sge, is a potential progenitor of supernovae like SN 2002ic if the delayed dynamical-instability model in Han & Podsiadlowski (2006) is appropriate.Comment: 10 pages, 12 figures, accepted for publication in PAS

The birth rate of subluminous and overluminous type Ia supernovae

Based on the results of Chen & Li (2009) and Pakmor et al. (2010), we carried out a series of binary population synthesis calculations and considered two treatment of common envelope (CE) evolution, i.e. $\alpha$-formalism and $\gamma$-algorithm. We found that the evolution of birth rate of these peculiar SNe Ia is heavily dependent on how to treat the CE evolution. The over-luminous SNe Ia may only occur for $\alpha$-formalism with low CE ejection efficiency and the delay time of the SNe Ia is between 0.4 and 0.8 Gyr. The upper limit of the contribution rate of the supernovae to all SN Ia is less than 0.3%. The delay time of sub-luminous SNe Ia from equal-mass DD systems is between 0.1 and 0.3 Gyr for $\alpha$-formalism with $\alpha=3.0$, while longer than 9 Gyr for $\alpha=1.0$. The range of the delay time for $\gamma$-algorithm is very wide, i.e. longer than 0.22 Gyr, even as long as 15 Gyr. The sub-luminous SNe Ia from equal-mass DD systems may only account for no more than 1% of all SNe Ia observed. The super-Chandrasekhar mass model of Chen & Li (2009) may account for a part of 2003fg-like supernovae and the equal-mass DD model developed by Pakmor et al. (2010) may explain some 1991bg-like events, too. In addition, based on the comparison between theories and observations, including the birth rate and delay time of the 1991bg-like events, we found that the $\gamma$-algorithm is more likely to be an appropriate prescription of the CE evolution of DD systems than the $\alpha$-formalism if the equal-mass DD systems is the progenitor of 1991bg-like SNe Ia.Comment: 8 pages, 2 figures, accepted for publication in A&