A rapidly evolving high-amplitude δ Scuti star crossing the Hertzsprung Gap

People cannot witness the stellar evolution process of a single star obviously in most cases because of its extremely secular time-scale, except for some special time nodes in it (such as the supernova explosion [1]). But in some specific evolutionary phases, we have the chances to witness such process gradually on human times-scales. When a star evolved leaving from the main sequence, the hydrogen nuclei fusion in its core is gradually transferring into the shell. In the Hertzsprung–Russell diagram, its evolutionary phase falls into the Hertzsprung gap, which is one of the most rapidly evolving phases in the life of a star [2]. Here we report a discovery of a rapidly evolving high-amplitude δ Scuti star KIC6382916 (J19480292+4146558) which is crossing the Hertzsprung gap. According to the analysis of the archival data, we find three independent pulsation modes of it, whose amplitudes and frequencies are variating distinctly in 4 years. The period variation rates of the three pulsation modes are one or two orders larger than the best seismic model constructed by the standard evolution theory, which indicates the current theory cannot precisely describe the evolution process in this rapidly evolving phase and needs further upgrades. Moreover, the frequency and amplitude interactions between the three independent pulsation modes and their harmonics/combinations open a new window to the future asteroseismology.

Binary population synthesis with probabilistic remnant mass and kick prescriptions

ABSTRACT We report on the impact of a probabilistic prescription for compact remnant masses and kicks on massive binary population synthesis. We find that this prescription populates the putative mass gap between neutron stars and black holes with low-mass black holes. However, evolutionary effects reduce the number of X-ray binary candidates with low-mass black holes, consistent with the dearth of such systems in the observed sample. We further find that this prescription is consistent with the formation of heavier binary neutron stars such as GW190425, but overpredicts the masses of Galactic double neutron stars. The revised natal kicks, particularly increased ultra-stripped supernova kicks, do not directly explain the observed Galactic double neutron star orbital period–eccentricity distribution. Finally, this prescription allows for the formation of systems similar to the recently discovered extreme mass ratio binary GW190814, but only if we allow for the survival of binaries in which the common envelope is initiated by a donor crossing the Hertzsprung gap, contrary to our standard model.

Fitting formulae for evolution tracks of massive stars under extreme metal-poor environments for population synthesis calculations and star cluster simulations

ABSTRACT We have devised fitting formulae for evolution tracks of massive stars with 8 ≲ M/M⊙ ≲ 160 under extreme metal-poor (EMP) environments for log (Z/Z⊙) = −2, −4, −5, −6, and −8, where M⊙ and Z⊙ are the solar mass and metallicity, respectively. Our fitting formulae are based on reference stellar models which we have newly obtained by simulating the time evolutions of EMP stars. Our fitting formulae take into account stars ending with blue supergiant (BSG) stars, and stars skipping Hertzsprung gap phases and blue loops, which are characteristics of massive EMP stars. In our fitting formulae, stars may remain BSG stars when they finish their core Helium burning phases. Our fitting formulae are in good agreement with our stellar evolution models. We can use these fitting formulae on the sse, bse, nbody4, and nbody6 codes, which are widely used for population synthesis calculations and star cluster simulations. These fitting formulae should be useful to make theoretical templates of binary black holes formed under EMP environments.

Galactic population of black holes in detached binaries with low-mass stripped helium stars: the case of LB-1 (LS V+22 25)

ABSTRACT We model the Galactic population of detached binaries that harbour black holes with 0.5–1.7 M⊙ companions – remnants of case B mass exchange that rapidly cross Hertzsprung gap after the termination of the Roche lobe overflow or as He-shell burning stars. Several such binaries can be currently present in the Galaxy. The range of MBH in them is about 4–10 M⊙, and the orbital periods are tens to hundreds of days. The unique black hole binary LB-1 fits well into this extremely rare class of double stars.

Magnetic fields in single late-type giants in the Solar vicinity: How common is magnetic activity on the giant branches?

We present our first results on a new sample containing all single G, K and M giants down to V = 4 mag in the Solar vicinity, suitable for spectropolarimetric (Stokes V) observations with Narval at TBL, France. For detection and measurement of the magnetic field (MF), the Least Squares Deconvolution (LSD) method was applied (Donati et al. 1997) that in the present case enables detection of large-scale MFs even weaker than the solar one (the typical precision of our longitudinal MF measurements is 0.1-0.2 G). The evolutionary status of the stars is determined on the basis of the evolutionary models with rotation (Lagarde et al. 2012; Charbonnel et al., in prep.) and fundamental parameters given by Massarotti et al. (1998). The stars appear to be in the mass range 1-4 M⊙, situated at different evolutionary stages after the Main Sequence (MS), up to the Asymptotic Giant Branch (AGB).The sample contains 45 stars. Up to now, 29 stars are observed (that is about 64% of the sample), each observed at least twice. For 2 stars in the Hertzsprung gap, one is definitely Zeeman detected. Only 5 G and K giants, situated mainly at the base of the Red Giant Branch (RGB) and in the He-burning phase are detected. Surprisingly, a lot of stars ascending towards the RGB tip and in early AGB phase are detected (8 of 13 observed stars). For all Zeeman detected stars v sin i is redetermined and appears in the interval 2-3 km/s, but few giants with MF possess larger v sin i.

The Hertzsprung-gap giant 31 Comae in 2013: Magnetic field and activity indicators

We have observed the giant star 31 Comae in April and May 2013 with the spectropolarimeter Narval at Pic du Midi Observatory, France. 31 Comae is a single, rapidly rotating giant with rotational period ~6.8 d and vsini ~ 67 km/s. We present measurements and discuss variability of the longitudinal magnetic field (Bl), spectral activity indicators Hα, CaII H&K, Ca II IR triplet and evolutionary status. Our future aim is to perform a Zeeman-Doppler imaging study for the star.