Revised Stellar Parameters for V471 Tau, A Post-common Envelope Binary in the Hyades

V471 Tau is a post-common-envelope binary consisting of an eclipsing DA white dwarf and a K-type main-sequence star in the Hyades star cluster. We analyzed publicly available photometry and spectroscopy of V471 Tau to revise the stellar and orbital parameters of the system. We used archival K2 photometry, archival Hubble Space Telescope spectroscopy, and published radial-velocity measurements of the K-type star. Employing Gaussian processes to fit for rotational modulation of the system flux by the main-sequence star, we recovered the transits of the white dwarf in front of the main-sequence star for the first time. The transits are shallower than would be expected from purely geometric occultations owing to gravitational microlensing during transit, which places an additional constraint on the white-dwarf mass. Our revised mass and radius for the main-sequence star is consistent with single-star evolutionary models given the age and metallicity of the Hyades. However, as noted previously in the literature, the white dwarf is too massive and too hot to be the result of single-star evolution given the age of the Hyades, and may be the product of a merger scenario. We independently estimate the conditions of the system at the time of common envelope that would result in the measured orbital parameters today.

Nonstandard Modeling of a Possible Blue Straggler Star, KIC 11145123

Nonstandard modeling of KIC 11145123, a possible blue straggler star, has been asteroseismically carried out based on a scheme to compute stellar models with the chemical compositions in their envelopes arbitrarily modified, mimicking the effects of some interactions with other stars through which blue straggler stars are thought to be born. We have constructed a nonstandard model of the star with the following parameters: M = 1.36 M ⊙, Y init = 0.26, Z init = 0.002, and f ovs = 0.027, where f ovs is the extent of overshooting described as an exponentially decaying diffusive process. The modification is down to the depth of r/R ∼ 0.6 and the extent ΔX, which is a difference in surface hydrogen abundance between the envelope-modified and unmodified models, is 0.06. The residuals between the model and the observed frequencies are comparable with those for the previous model computed assuming standard single-star evolution, suggesting that it is possible that the star was born with a relatively ordinary initial helium abundance of ∼0.26 compared with that of the previous models (∼0.30–0.40), then experienced some modification of the chemical compositions and gained helium in the envelope. Detailed analyses of the nonstandard model have implied that the elemental diffusion in the deep radiative region of the star might be much weaker than that assumed in current stellar evolutionary calculations; we need some extra mechanisms inside the star, rendering the star a much more intriguing target to be further investigated.

Properties of strange quark matter and strange quark stars

A Polyakov chiral $$\text {SU(3)}$$ SU(3) quark mean-field (PCQMF) model is applied to study the properties of strange quark matter (SQM) and strange quark star (SQS) in $$\beta $$ β -equilibrium. The effect of increasing the strength of vector interactions on the effective constituent quark mass, particle fractions, and the thermodynamical properties such as pressure, energy density, and the speed of sound is investigated. We investigate the above properties for the SQM relevant for various stages of star evolution, i.e., considering with/without trapped neutrinos and zero/finite entropy. The finite lepton fraction and the entropy of the medium is observed to cause the stiffness in the equation of state (EoS). Finally, we calculate the mass-radius relation and the dimensionless tidal deformability within the present model calculations and compare the results to the recent studies.

Quark star matter at finite temperature in a quasiparticle model

We study the thermodynamic properties of asymmetric quark matter, large mass quark stars (QSs), and proto-quark stars (PQSs) within the quasiparticle model. Considering the effects of temperature within quasiparticle model can significantly influence the EOS and the entropy of strange quark matter (SQM), quark fractions in SQM, as well as the tidal deformability and the maximum mass of PQSs along the star evolution line. Our results indicate that the recent discovered heavy compact stars PSR J0348+0432, MSR J0740+6620, PSR J2215+5135, and especially the GW190814’s secondary component $$m_2$$ m 2 can be well described as QSs within the quasiparticle model. The tidal deformability for the QSs describing the heavy compact stars is extremely large, which can not well describe GW170817 as QSs, and the effects of the temperature in the heating process along the star evolution will further increase the tidal deformability and the maximum mass of PQSs.

Massive Star Modeling and Nucleosynthesis

After a brief introduction to stellar modeling, the main lines of massive star evolution are reviewed, with a focus on the nuclear reactions from which the star gets the needed energy to counterbalance its gravity. The different burning phases are described, as well as the structural impact they have on the star. Some general effects on stellar evolution of uncertainties in the reaction rates are presented, with more precise examples taken from the uncertainties of the 12C(α, γ)16O reaction and the sensitivity of the s-process on many rates. The changes in the evolution of massive stars brought by low or zero metallicity are reviewed. The impact of convection, rotation, mass loss, and binarity on massive star evolution is reviewed, with a focus on the effect they have on the global nucleosynthetic products of the stars.

Unexpected LIGO events and the mirror world

ABSTRACT We consider the possibility that LIGO events GW190521, GW190425, and GW190814 may have emerged from the mirror world binaries. Theories of star evolution predict so-called upper and lower mass gaps and masses of these merger components lie in that gaps. In order to explain these challenging events very specific assumptions are required and we argue that such scenarios are order of magnitude more probable in mirror world, where star formation begins earlier and matter density can exceed five times the ordinary matter density.

Multiplicity among the cool supergiants in the Magellanic Clouds

The characterisation of multiplicity among of high-mass stars is of fundamental importance to understand their evolution, the diversity of observed core-collapse supernovae and the formation of gravitational wave progenitor systems. Despite that, until recently, one of the final phases of massive star evolution – the cool supergiant phase – has received comparatively little attention. In this study we aim to explore the multiplicity among the cool supergiant (CSG) population in the Large and Small Magellanic Clouds (LMC and SMC, respectively). To do this we compile extensive archival radial velocity (RV) measurements for over 1000 CSGs from the LMC and SMC, spanning a baseline of over 40 years. By statistically correcting the RV measurements of each stellar catalogue to the Gaia DR2 reference frame we are able to effectively compare these diverse observations. We identify 45 CSGs where RV variations cannot be explained through intrinsic variability, and are hence considered binary systems. We obtain a minimum binary fraction of $15\pm 4{{\ \rm per\ cent}}$ for the SMC and of $14\pm 5{{\ \rm per\ cent}}$ for the LMC, restricting our sample to objects with at least 6 and 5 observational epochs, respectively. Combining these results, we determine a minimum binary fraction of $15\pm 3{{\ \rm per\ cent}}$ for CSGs. These results are in good agreement with previous results which apply a correction to account for observational biases. These results add strength to the hypothesis that the binary fraction of CSGs is significantly lower than their main-sequence counterparts. Going forward, we stress the need for long-baseline multi-epoch spectroscopic surveys to cover the full parameter space of CSG binary systems.