Laurits Leedjärv

The peculiar variable V838 Monocerotis

Spectroscopic observations of the peculiar variable V838 Mon during the period from the second light outburst until the fast dimming are presented. We describe high resolution (R ≈ 60 000) high S /N spectra obtained a day before the second light maximum and low resolution (R ≈ 6000) spectra covering the whole period. The temporal run of intensities and radial velocities of various lines is presented. Using Na  D IS lines we determine the reddening distance of V838 Mon d > 3.1 kpc, and kinematic distance d > 4 kpc. We estimate that V838 Mon is slightly metal deficient but otherwise has a quite solar-like chemical composition except for enhanced abundances of Li, Ba and La.

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

Aims. We have studied optical spectra of the symbiotic star Z And, obtained during its latest outburst started in April 2006, wi th the aim of finding changes in the spectrum yielding clues to the na ture of the hot component and its outbursts. Methods. 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. Results. 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 −1 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 An d. This finding is consistent with the earlier results by severa l authors that symbiotic stars can emit bipolar jets at certa in stages of their outbursts. A significant decrease in the temperature o f the hot component in initial stages of the outburst was detected by the disappearance of the high excitation emission lines from the spectrum.

Outburst activity of the symbiotic system AG Dra

AG Dra is a well known bright symbiotic binary with a white dwarf and a pulsating red giant. The long-term photometry monitoring and a new behaviour of the system are presented. The detailed period analysis of photometry as well as spectroscopy was carried out. In the system of AG Dra, two periods of variability are detected. The longer one around 550 days is related to the orbital motion, and the shorter one around 355 days is interpreted as pulsations of the red giant in our older paper. In addition the active stages change distinctively, but the outbursts are repeated with the periods from 359 to 375 days.

Variability survey of brightest stars in selected OB associations

Context. 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. Aims. The aim of this investigation is to study the variability of luminous stars at different timescales in young open clusters and OB associations. Methods. 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. Results. We obtained light curves for more than 20 000 sources of which 354 were found to be variable. Amongst them we find 80 eclipsing binaries, 31 α Cyg, 13 β Cep, 62 Be, 16 slowly pulsating B, 7 Cepheid, 1 γ 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%.

Symbiotic Stars in the Context of Binary Evolution and Metallicity

A brief description of symbiotic binary stars is given, followed by a discussion on their evolutionary status. It is evident that different subtypes of symbiotic stars belong to the different Galactic populations. Discussion of some properties of the hot and cool components of symbiotic stars is presented.

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

Variability Survey of Massive Stars in OB-Associations: Preliminary Results on the Cygnus Region

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Astronomy in the society and culture of Estonia

absorption component on their equivalent widths as well as consequences of the approximations used in our previous works.

Spectroscopic study of the binary star AX Monocerotis

Abstract We present V - and I-passband photometry of massive stars in the Cyg OB1 and Cyg OB2 associations, based on about 80 observing nights spanning 300 days in the 2011 season. The variability of 22 supergiants and 48 OB-stars with luminosity classes III-V is analyzed. We report two new variable OB stars and 15 variable supergiants of which four are new discoveries. The light variations of Schulte 12 are interpreted as microvariability. We also present light curves of the red supergiants BC Cyg and BI Cyg which exhibit brightness drop of more than 0.4 mag during the season.

Emission lines in the spectrum of the symbiotic star AG Draconis from 1997 to 2003

History and present state of astronomy in a small North-Eastern European country are considered. There is a rather big number (about 35) of professional astronomers in Estonia, including 21 IAU members. Through some outstanding persons, astronomy in Estonia has significant relations with the society. The same can be said about the culture. Well-developed astronomy has contributed into the cooperation of Estonia with the European Space Agency, and thus, has an indirect effect to the countrys economy.