Variability survey of brightest stars in selected OB associations

The stellar evolution theory of massive stars remains uncalibrated with high-precision photometric observational data mainly due to a small number of luminous stars that are monitored from space. Automated all-sky surveys have revealed numerous variable stars but most of the luminous stars are often overexposed. Targeted campaigns can improve the time base of photometric data for those objects. The aim of this investigation is to study the variability of luminous stars at different timescales in young open clusters and OB associations. We monitored 22 open clusters and associations from 2011 to 2013 using a 0.25-m telescope. Variable stars were detected by comparing the overall light-curve scatter with measurement uncertainties. Variability was analysed by the light curve feature extraction tool FATS. Periods of pulsating stars were determined using the discrete Fourier transform code SigSpec. We then classified the variable stars based on their pulsation periods and available spectral information. We obtained light curves for more than 20000 sources of which 354 were found to be variable. Amongst them we find 80 eclipsing binaries, 31 $\alpha$ Cyg, 13 $\beta$ Cep, 62 Be, 16 slowly pulsating B, 7 Cepheid, 1 $\gamma$ Doradus, 3 Wolf-Rayet and 63 late-type variable stars. Up to 55% of these stars are potential new discoveries as they are not present in the Variable Star Index (VSX) database. We find the cluster membership fraction for variable stars to be 13% with an upper limit of 35%.Comment: 36 pages, 11 figures, catalogue in appendix

Evidence for bipolar jets from the optical spectra of the prototypical symbiotic star Z Andromedae

We have studied optical spectra of the symbiotic star Z And, obtained during its latest outburst started in April 2006, with the aim of finding changes in the spectrum yielding clues to the nature of the hot component and its outbursts. The spectroscopic observations of Z And have been made using the 1.5-meter telescope at the Tartu Observatory, Estonia, and processed in a standard way. We have found high velocity satellites to the hydrogen Balmer emission lines. Starting from July 30, 2006, weak additional emission components at velocities of about +/-1150 km/s were detected. Their appearance near the outburst maximum and similarity to the emission features in another symbiotic star Hen 3-1341 imply fast collimated outflows from the hot component of Z And. This finding is consistent with the earlier results by several authors that symbiotic stars can emit bipolar jets at certain stages of their outbursts. A significant decrease in the temperature of the hot component in initial stages of the outburst was detected by the disappearance of the high excitation emission line from the spectrum.Comment: 4 pages, 6 figures, accepted for publication by A&A Letter

Recent outburst activity of the symbiotic binary AG Draconis

The symbiotic binary AG Dra regularly undergoes quiescent and active stages which consist of several outbursts repeating with about 360d interval. The recent outburst activity of AG Dra started by the minor outburst in the late spring of 2015 and was definitely confirmed by the outbursts in April 2016 and May 2017. In the presented work, the photometric and spectroscopic behaviour of the recent outburst activity of AG Dra is presented in detail. Moreover, the temperature of the white dwarf in AG Dra is studied based on the behaviour of the prominent emission lines. We show that a disentanglement of particular effects in the observed changes of the emission lines is crucial to investigate the intrinsic white dwarf temperature variations related to outburst activity of this strongly interacting binary. We also report the effects of the low excitation lines orbital variations and of the H$_{\beta}$ absorption component on their equivalent widths as well as consequences of the approximations used in our previous works.Comment: 16 pages, 4 figures, Proceedings of The Golden Age of Cataclysmic Variables and Related Objects IV, 11-16 September 2017. Palermo, Ital

Spectroscopy of the symbiotic star CH Cygni from 1996 to 2007

We monitored a set of emission lines in the optical spectra of CH Cyg using the 1.5-m telescope at the Tartu Observatory, Estonia. Most of the spectra were registered in the H alpha region, but other Balmer lines and lines of He, N, O, and Fe were also investigated in terms of their equivalent widths, radial velocities, and absolute fluxes. The spectra indicate different stages that CH Cyg has been through in the course of our observations. During quiescence, the strength of the line profiles changes in opposite phase with the star's luminosity. The H alpha profile becomes double-peaked at times, which may point to the temporary presence of some disk-like structure, but also to absorption in neutral gas surrounding the area of the formation of the emission component. In 1999, when a periastron passage is assumed according to the 5700-day model, the Balmer lines strengthen considerably, as might be expected due to an increase in the accretion rate. The eclipse in 2001, however, is not noticed. At certain episodes, the controversial 756-day period of CH Cyg is seen in our data.Comment: 10 pages, 12 figure

Recent photometry of symbiotic stars - XII

We present new photometric observations of 15 symbiotic stars (EG And, Z And, AE Ara, BF Cyg, CH Cyg, CI Cyg, V1329 Cyg, TX CVn, AG Dra, Draco C-1, RW Hya, SY Mus, AR Pav, AG Peg, AX Per) covering their last orbital cycle(s) from 2003.9 to 2007.2. We obtained our data by both classical photoelectric and CCD photometry. Main results are: EG And brightened by about 0.3 mag in U from 2003. A 0.5 mag deep primary minimum developed in the U light curve (LC) at the end of 2006. Z And continues its recent activity that began during the 2000 autumn. A new small outburst started in summer of 2004 with the peak U-magnitude of about 9.2. During the spring of 2006 the star entered a massive outburst. It reached its historical maximum at U = 8.0 in 2006 July. AE Ara erupted in 2006 February with Dm(vis) approx. 1.2 mag. BF Cyg entered a new active stage in 2006 August. A brightness maximum (U approx. 9.4) was measured during 2006 September. CH Cyg persists in a quiescent phase. During 2006 June - December about 2 mag decline in all colours was measured. CI Cyg started a new active phase during 2006 May - June. After 31 years it erupted by about 2 mag in U. TX CVn maintains a bright stage with U approx. 10.5 from 2003. AG Dra entered a new major outburst in 2006 June. It reached its maximum at U = 8.0 in 2006 September. AR Pav persists at a low level of the activity. AG Peg's LC profile varies markedly during different orbital cycles. AX Per continues its quiescent phase.Comment: 4 pages text, 12 figures, 18 tables, accepted for publication in Astron. Nachrichte