Blagovest Petrov

Luminous Blue Variables have been suggested to be the direct progenitors of supernova types IIb and IIn, with enhanced mass loss prior to explosion. However, the mechanism of this mass loss is not yet known. Here, we investigate the qualitative behaviour of theoretical stellar wind mass-loss as a function of Teff across two bi-stability jumps in blue supergiant regime and also in proximity to the Eddington limit, relevant for LBVs. To investigate the physical ingredients that play a role in the radiative acceleration we calculate blue supergiant wind models with the CMFGEN non-LTE model atmosphere code over an effective temperature range between 30000 and 8800 K. Although our aim is not to provide new mass-loss rates for BA supergiants, we study and confirm the existence of two bi-stability jumps in mass-loss rates predicted by Vink, de Koter, & Lamers (1999). However, they are found to occur at somewhat lower Teff (20000 and 9000 K, respectively) than found previously, which would ...

The evolutionary state of blue supergiants is still unknown. Stellar wind mass loss is one of the dominant processes determining the evolution of massive stars, and it may provide clues on the evolutionary properties of blue supergiants. As the Hα line is the most oft-used mass-loss tracer in the OB-star regime, we provide a detailed analysis of the Hα line for OB supergiant models over an T_ eff range between 30000 and 12500K. We find a maximum in the Hα equivalent width at 22500 K - at the location of the bi-stability jump. The Hα line-profile behaviour is characterised by two branches of T_ eff: (i) a "hot" branch between 30000 and 22500 K, where Hα emission becomes stronger with decreasing T_ eff, and (ii) a "cool" branch between 22500 and 12500 K, where the line becomes weaker. Our models show that this non-monotonic Hα behaviour is related to the optical depth of Lyα, finding that at the "cool" branch the population of the 2nd level of hydrogen is...

We present results of optical observations of the extremely violently variable blazar S4 0954+65 on intra-night time scales. The object showed flux changes of up to 100% within a few hours. Time delays between optical bands, color changes and "rms-flux" relations are investigated and the results are discussed in terms of existing models of blazar variability.

We present results from a global view on the colliding-wind binary WR 147. We analysed new optical spectra of WR 147 obtained with Gran Telescopio CANARIAS and archive spectra from the Hubble Space Telescope by making use of modern atmosphere models accounting for optically thin clumping. We adopted a grid-modelling approach to derive some basic physical characteristics of both stellar components in WR 147. For the currently accepted distance of 630 pc to WR 147, the values of mass-loss rate derived from modelling its optical spectra are in acceptable correspondence with that from modelling its X-ray emission. However, they give a lower radio flux than observed. A plausible solution for this problem could be if the volume filling factor at large distances from the star (radio-formation region) is smaller than close to the star (optical-formation region). Adopting this, the model can match well both optical and thermal radio emission from WR 147. The global view on the colliding-wind...

Abstract. We report simultaneous observations in 5 bands UBV RcIc of the flickering variability of the cataclysmic variable V794 Aql. The observations are obtained with the Rozhen NAO and Belogradchik AO telescopes on 2009 July 23 and on 2010 August 13. V794 Aql has ...

We present CCD observations of the short time brightness variations (flickering) of the cataclysmic variable MV Lyr in low and high state. We show that the amplitude of the flickering correlates with the average flux of the accreting component asFF k with k = 0.80 in U, B, V , R and I bands. The relation is ascertained in a large range of brightness variations, as the mean flux changes about 1000 times from low to high photometric state.

We present results from a global view on the colliding-wind binary WR 147. We analysed new optical spectra of WR 147 obtained with Gran Telescopio CANARIAS and archive spectra from the Hubble Space Telescope by making use of modern atmosphere models accounting for optically thin clumping. We adopted a grid-modelling approach to derive some basic physical characteristics of both stellar components in WR 147. For the currently accepted distance of 630 pc to WR 147, the values of mass-loss rate derived from modelling its optical spectra are in acceptable correspondence with that from modelling its X-ray emission. However, they give a lower radio flux than observed. A plausible solution for this problem could be if the volume filling factor at large distances from the star (radio-formation region) is smaller than close to the star (optical-formation region). Adopting this, the model can match well both optical and thermal radio emission from WR 147. The global view on the colliding-wind...

Abstract. We report simultaneous observations in 5 bands UBV RcIc of the flickering variability of the cataclysmic variable V794 Aql. The observations are obtained with the Rozhen NAO and Belogradchik AO telescopes on 2009 July 23 and on 2010 August 13. V794 Aql has ...

We report simultaneous observations in 5 bands UBVR_{c}I_{c} of the flickering variability of the cataclysmic variable V794 Aql. The observations are obtained with the Rozhen NAO and Belogradchik AO telescopes on 2009 July 23 and on 2010 August 13. V794 Aql has flickering source with average (U-B)_0 =-0.75 ± 0.20, (B-V)_0 =+0.02 ± 0.10, (V-R)_0 = 0.15 ± 0.05. We find for the flickering source parameters: T_{fl} = 10900 ± 600 K, bar {R}_{fl} =0.10 ± 0.05 R_sun, and bar {L}_{fl} =0.11 ± 0.02 L_sun (using a distance of d=690 ± 105 pc).