Definition and analysis of the fundamental parameters of a close binary system Konkoly J064029.1+ 385652.2

In this work, we studied spectroscopic and photometric observations of the close binary system Konkoly J064029.1+385652.2, which is a candidate for young pre-cataclysmic variables with an sdO subdwarf. Observations were obtained in 2017-19 on the Large Azimuth Telescope SAO and the Russian-Turkish RTT-150 telescope.

Research of the characteristics of the close binary system SDSS J162256

In this work, the optical radiation of the young pre-cataclysmic variable (PV) SDSS J162256 was investigated. Spectroscopic observations were carried out at BTA SAO RAS, photometric observations were carried out at RTT-150 telescope. Numerical modeling of theoretical light curves and spectra was done. The measured sets of radial velocities were analyzed taking into account the possible influence of reflection effects. The fundamental parameters of SDSS J162256 were determined based on a comprehensive analysis of observational data. It was shown that the previously proposed method for determining the masses of the PV components by modeling of the radial-velocity curves is effective only for systems with significant reflection effects.

Exploring velocity limits in the thermonuclear supernova ejection scenario for hypervelocity stars and the origin of US 708

Context. Hypervelocity stars (HVS) are a class of stars moving at velocities that are high enough to make them gravitationally unbound from the Galaxy. In recent years, ejection from a close binary system in which one of the components undergoes a thermonuclear supernova (SN) has emerged as a promising candidate production mechanism for the least massive specimens of this class. The explosion mechanisms leading to thermonuclear supernovae, which include the important Type Ia and related subtypes, remain unclear. Aims. This study presents a thorough theoretical analysis of candidate progenitor systems of thermonuclear SNe in the single degenerate helium donor scenario in the relevant parameter space leading to the ejection of HVS. The primary goal is to investigate the previously indeterminate characteristics of the velocity spectra for the ejected component, including possible maxima and minima, as well as the constraints arising from stellar evolution and initial masses. Furthermore, this paper addresses the question of whether knowledge of the ejection velocity spectra may aid in the reconstruction of the terminal state of the supernova progenitor, with a focus on the observed object, US 708. Methods. This study presents the results of 390 binary model sequences computed with the Modules for Experiments in Stellar Astrophysics framework, investigating the evolution of supernova progenitors composed of a helium-rich hot subdwarf and an accreting white dwarf, while avoiding assumption of a specific explosion mechanism as much as possible. The detailed evolution of the donor star as well as gravitational wave radiation and mass transfer-driven orbital evolution were fully taken into account. The results were then correlated with an idealized kinematic analysis of the observed object US 708. Results. This work shows that the ejection velocity spectra reach a maximum in the range of 0.19 M⊙ < MHVS < 0.25 M⊙. Depending on the local Galactic potential, all donors below 0.4 M⊙ are expected to become HVSs. The single degenerate helium donor channel is able to account for runaway velocities up to ∼1150 km s−1 with a Chandrasekhar mass accretor, exceeding 1200 km s−1 when super-Chandrasekhar mass detonations are taken into account. Results show that the previously assumed mass of 0.3 M⊙ for US 708, combined with proper motions that have been obtained more recently, favor a sub-Chandrasekhar mass explosion with a terminal WD mass between 1.1 M⊙ and 1.2 M⊙, while a Chandrasekhar mass explosion requires a mass of > 0.34 M⊙ for US 708. This mechanism may be a source of isolated runaway extremely low-mass white dwarfs. Conclusions. The presence of clear ejection velocity maxima that are terminal accretor mass-dependent, but simultaneously initial-condition independent, provides constraints on the terminal state of a supernova progenitor. Depending on the accuracy of astrometry, it is possible to discern certain types of explosion mechanisms from the inferred ejection velocities alone, with current proper motions allowing for a sub- Chandrasekhar mass SN to explain the origins of US 708. However, more robust reconstructions of the most likely SN progenitor state will require a greater number of observed objects than are currently available.

A survey for variable young stars with small telescopes – III. Warm spots on the active star V1598 Cyg

ABSTRACT Magnetic spots on low-mass stars can be traced and characterized using multiband photometric light curves. Here, we analyse an extensive data set for one active star, V1598 Cyg, a known variable K dwarf which is either pre-main sequence and/or in a close binary system. Our light curve contains 2854 photometric data points, mostly in V, Rc, Ic, but also in U, B, and Hα, with a total baseline of about 4 yr, obtained with small telescopes as part of the HOYS project. We find that V1598 Cyg is a very fast rotator with a period of 0.8246 d and varying amplitudes in all filters, best explained as a signature of strong magnetic activity and spots. We fit the photometric amplitudes in V, Rc, Ic and use them to estimate spot properties, using a grid-based method that is also propagating uncertainties. We verify the method on a partial data set with high cadence and all five broad-band filters. The method yields spot temperatures and fractional spot coverage with typical uncertainties of 100 K and 3–4 per cent, respectively. V1598 Cyg consistently exhibits spots that are a few hundred degrees warmer than the photosphere, most likely indicating that the light curve is dominated by chromospheric plage. The spot activity varies over our observing baseline, with a typical time-scale of 0.5–1 yr, which we interpret as the typical spot lifetime. Combining our light curve with archival data, we find a six year cycle in the average brightness, that is probably a sign of a magnetic activity cycle.

Discovery of a complex spiral-shell structure around the oxygen-rich AGB star GX Monocerotis

The circumstellar envelopes of asymptotic giant branch (AGB) stars exhibit a wide range of morphologies and chemical compositions that can be exploited to unravel their mass-loss history as well as binary status. Here, we present ALMA Band 6 observations centred upon the oxygen-rich, high mass-loss rate AGB star GX Mon. The resulting CO (2–1) map reveals an intricate, complex circumstellar spiral-arc structure consistent with hydrodynamical models for an AGB experiencing mass loss in a highly eccentric, close binary system with an orbital period of around 140 years. Several other transitions (including SiO, SiS, SO2, and CS) are detected in the data, however only the SO (5–4) map shows a similar – although much weaker – distribution as imaged for the CO.

Luminous red novae: Stellar mergers or giant eruptions?

We present extensive datasets for a class of intermediate-luminosity optical transients known as luminous red novae. They show double-peaked light curves, with an initial rapid luminosity rise to a blue peak (at −13 to −15 mag), which is followed by a longer-duration red peak that sometimes is attenuated, resembling a plateau. The progenitors of three of them (NGC 4490−2011OT1, M 101−2015OT1, and SNhunt248), likely relatively massive blue to yellow stars, were also observed in a pre-eruptive stage when their luminosity was slowly increasing. Early spectra obtained during the first peak show a blue continuum with superposed prominent narrow Balmer lines, with P Cygni profiles. Lines of Fe II are also clearly observed, mostly in emission. During the second peak, the spectral continuum becomes much redder, Hα is barely detected, and a forest of narrow metal lines is observed in absorption. Very late-time spectra (∼6 months after blue peak) show an extremely red spectral continuum, peaking in the infrared (IR) domain. Hα is detected in pure emission at such late phases, along with broad absorption bands due to molecular overtones (such as TiO, VO). We discuss a few alternative scenarios for luminous red novae. Although major instabilities of single massive stars cannot be definitely ruled out, we favour a common envelope ejection in a close binary system, with possibly a final coalescence of the two stars. The similarity between luminous red novae and the outburst observed a few months before the explosion of the Type IIn SN 2011ht is also discussed.

ON THE CHROMOSPHERIC ACTIVITY NATURE OF A LOW-MASS CLOSE BINARY: KIC 12004834

We study the nature of the chromospheric activity of an eclipsing binary KIC 12004834, using Kepler data. We analyse the light curve of the system, the sinusoidal variations at out-of-eclipses and detected flare events. The secondary component’s temperature is found to be 4001±11 K, the mass ratio is 0.743±0.001, and the orbital inclination is 75◦.89±0◦.03. The analysis indicates a stellar spot effect on the variation. Moreover, the OPEA model has been derived over 149 flares. The saturation level called Plateau value, is found to be 2.093±0.236 s. The flare number per hour (known as flare frequency N1) is found to be 0.06644 h−1, while the flare-equivalent duration per hour (known as flare frequency N2) is found to be 0.59 second/hour. According to these results, KIC 12004834 is a very low-mass close binary system with high level of flare activity.

MULTICOLOR STUDY OF V1009 PER, A CLOSE BINARY SYSTEM AT THE BEGINNING OF THE OVERCONTACT PHASE, AND OF CRTS J031642.2+332639, A NEW BINARY SYSTEM IN THE SAME FIELD

The first multicolor observations and light curve solutions of the eclipsing binary systems V1009 Per and CRTS J031642.2+332639 are presented. Using the 2005 version of the Wilson-Devinney code, both systems are found to be W UMa contact binaries. V1009 Per has a mass ratio of q = 0.362±0.002 and a shallow fill out parameter of f = 11.8 ± 0.6% while CRTS J031642.2+332639 has a mass ratio of q = 2.507±0.006 and a fill out of f = 13.6±0.4%. High orbital inclinations, i = 85◦.9 for V1009 Per and i = 83◦.2 for CRTS J031642.2+332639, imply that both systems are total eclipsing binaries and that the photometric parameters here obtained are reliable. Based on 16 times of minimum the orbital period variations of V1009 Per are discussed. The absolute dimensions of the systems are estimated and, from the log M − log L diagram, it is found that both components of the systems follow the general pattern of the W subtype W Ursae Majoris systems.