A remarkable recurrent nova in M 31: The 2010 eruption recovered and evidence of a six-month period

The Andromeda Galaxy recurrent nova M31N 2008-12a has been caught in eruption nine times. Six observed eruptions in the seven years from 2008 to 2014 suggested a duty cycle of ~1 year, which makes this the most rapidly recurring system known and the leading single-degenerate Type Ia Supernova progenitor candidate; but no 2010 eruption has been found so far. Here we present evidence supporting the recovery of the 2010 eruption, based on archival images taken at and around the time. We detect the 2010 eruption in a pair of images at 2010 Nov 20.52 UT, with a magnitude of m_R = 17.84 +/- 0.19. The sequence of seven eruptions shows significant indications of a duty cycle slightly shorter than one year, which makes successive eruptions occur progressively earlier in the year. We compared three archival X-ray detections with the well observed multi-wavelength light curve of the 2014 eruption to accurately constrain the time of their optical peaks. The results imply that M31N 2008-12a might have in fact a recurrence period of ~6 months (175 +/- 11 days), making it even more exceptional. If this is the case, then we predict that soon two eruptions per year will be observable. Furthermore, we predict the next eruption will occur around late Sep 2015. We encourage additional observations.Comment: 4 pages, 3 figures, 2 tables; submitted to A&A Letter

Application of the generalized two-center cluster model to 10Be

A generalized two-center cluster model (GTCM), including various partitions of the valence nucleons around two alpha-cores, is proposed for studies on the exotic cluster structures of Be isotopes. This model is applied to the 10Be = alpha + alpha + n + n system and the adiabatic energy surfaces for alpha-alpha distances are calculated. It is found that this model naturally describes the formation of the molecular orbitals as well as that of asymptotic cluster states dependeing on their relative distance. In the negative-parity state, a new type of the alpha + 6He cluster structure is also predicted.Comment: 5 pages, 3 figure

Triaxial deformation in 10Be

The triaxial deformation in $^{10}$Be is investigated using a microscopic $\alpha+\alpha+n+n$ model. The states of two valence neutrons are classified based on the molecular-orbit (MO) model, and the $\pi$-orbit is introduced about the axis connecting the two $\alpha$-clusters for the description of the rotational bands. There appear two rotational bands comprised mainly of $K^\pi = 0^+$ and $K^\pi = 2^+$, respectively, at low excitation energy, where the two valence neutrons occupy $K^\pi = 3/2^-$ or $K^\pi = 1/2^-$ orbits. The triaxiality and the $K$-mixing are discussed in connection to the molecular structure, particularly, to the spin-orbit splitting. The extent of the triaxial deformation is evaluated in terms of the electro-magnetic transition matrix elements (Davydov-Filippov model, Q-invariant model), and density distribution in the intrinsic frame. The obtained values turned out to be $\gamma = 15^o \sim 20^o$.Comment: 15 pages, latex, 3 figure

Systematic analyses of the t t clustering effect in He isotopes

A systematic study on the ground state structure of He isotopes including 10He is presented through a new method developed on the basis of antisymmetrized molecular dynamics AMD , the generator coordinate method GCM , and the stochastic variational method SVM . In this approach, variational calculations are carried out by means of the GCM with the AMD wave functions produced by means of the SVM. A role of the t t cluster component is examined with the present method, allowing the wider configuration space containing simultaneously the t t valence neutrons structure and 4He valence neutrons strucutur

Effect of Al2O3 or MgO on liquidus line in the FeOX corner of FeOX-SiO2-CaO system at 1523 K under various oxygen partial pressures

The liquidus line in the FeO corner of FeO SiO - CaO system and the effect by adding Al O or MgO to this plain system on the liquidus line were experimentally investigated at 1523 K in a wide range of oxygen partial pressure between about 10 and about 10 atm and for given (mass% CaO/mass% SiO / ratios in the system between 0 and about 1. It was found for the plain system that the liquidus line remarkably shifted toward the low FeO -content region when the partial pressure of oxygen was increased from 10 to 10 atm. It was clarified that the addition of Al O made the homogeneous region wide only in a strongly reducing atmosphere while the addition of MgO made the homogeneous liquid region remarkably narrow at all the oxygen partial pressures and all the (mass% CaO/mass% SiO / ratios investigated in the study

Searching for 4α\alpha linear-chain structure in excited states of 16^{16}O with a covariant density functional theory

A study of 4$\alpha$ linear-chain structure in high-lying collective excitation states of $^{16}$O with a covariant density functional theory is presented. The low-spin states are obtained by configuration mixing of particle-number and angular-momentum projected quadrupole deformed mean-field states with generator coordinate method. The high-spin states are determined by cranking calculations. These two calculations are based on the same energy density functional PC-PK1. We have found a rotational band at low-spin with the dominated intrinsic configuration considered to be the one that 4$\alpha$ clusters stay along a common axis. The strongly deformed rod shape also appears in the high-spin region with the angular momentum $13-18\hbar$; however whether the state is pure $4\alpha$ linear chain or not is less obvious than that in the low-spin states.Comment: 10 pages with 11 figures and 3 table

A massive star origin for an unusual helium-rich supernova in an elliptical galaxy

The unusual helium-rich (type Ib) supernova SN 2005E is distinguished from any supernova hitherto observed by its faint and rapidly fading light curve, prominent calcium lines in late-phase spectra and lack of any mark of recent star formation near the supernova location. These properties are claimed to be explained by a helium detonation in a thin surface layer of an accreting white dwarf (Perets et al. 2010). Here we report on observations of SN 2005cz appeared in an elliptical galaxy, whose observed properties resemble those of SN 2005E in that it is helium-rich and unusually faint, fades rapidly, shows much weaker oxygen emission lines than those of calcium in the well-evolved spectrum. We argue that these properties are best explained by a core-collapse supernova at the low-mass end ($8-12 M_{\odot}$) of the range of massive stars that explode (Smartt 2009). Such a low mass progenitor had lost its hydrogen-rich envelope through binary interaction, having very thin oxygen-rich and silicon-rich layers above the collapsing core, thus ejecting a very small amount of radioactive $^{56}$Ni and oxygen. Although the host galaxy NGC 4589 is an elliptical, some studies have revealed evidence of recent star-formation activity (Zhang et al. 2008), consistent with the core-collapse scenario.Comment: Accepted by Nature (24 March 2010), 32 pages including Supplementary Informatio