1. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Transitional events in the circumstellar environment of the Be/shell star BU Tau Lubomir Iliev Institute of Astronomy and National Astronomical Observatory Bulgarian Academy of Sciences

2. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Introduction – what the spectral phases of Be stars are and what they are not Changes from Be to Be-shell and/or B normal type spectrum in Be stars are called phase transitions. Except in some cases of firmly determined Be stars in binary systems, phase connected changes generally have unpredictable character (Doazan et al., 1993). In the binary systems containing Be star as one of the component variations of the shell spectrum features are mainly associated with the orbital period and are influenced by eventual transfer of the material between components of the system.

3. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Introduction – what the spectral phases of Be stars are and what they are not Positional visibility of the circumstellar material formations alone can not explain existence of phase transitions. Inclination of the rotational axis of central Be-star may determine the type Be, Be/shell or B normal of observed spectrum only in steady cases of circumstellar environment or in cases of stable over long periods of time Be stars. Naturally passage of a stars through all known Be type spectra can not be elucidated with inclination changes. Small changes of the inclination of shell formations are possible due to precession events caused by influence of distant companion or interaction between the central star and circumstellar environment. They however are not enough to encompass the scale of observed variations during Be-stars phase changes.

4. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Phase biography of 59 Cyg: Phase transitions of the star were observed since 1904. Normal B type spectrum was described based on observations from 1916. Before 1916 spectrum was characterized by weak emission in Balmer lines. In 1917 new emission phase started. This phase however passed through several sub- phases as summarized in Doazan (1982). In the years 1946-48 and in 1972 significant increase of the emission was recorded. Last period of strong emission initiated of “spectacular” variations that continued about 5 years. Straight after development of strong emission in 1972 shell phase started with remarkable strength. Sharp absorption cores of Balmer lines were visible up to H30. This accentuated attention to the extreme conditions in the extended atmosphere of the central Be star or in the regions of circumstellar material responsible for the formation of ‘pheudophotosphere’ around 59 Cyg. Shell spectrum also exhibited completely developed set of singly ionized metals (Doazan et al., 1975). Unfortunately this shell phase was poorly covered with observations (only 2 spectra). Only few months after the shell spectrum of 59 Cyg had completely disappeared (December, 1973)and was replaced by strong emission again. Second shell phase began in November 1974 and this time it was observed more regularly. Shell cores were visible up to H25. Second shell phase of 59 Cyg lasted about 13 months – from September 1974 till October 1975 and was relatively well observed with photographical spectral observations.

5. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Phase biography of 59 Cyg: After this shell phase the star entered “quasi-normal B phase” with emission presented only in H . In the present the star is in Be phase with minor spectral and photometrical changes (Harmanec et al., 2006) that lasts already several decades. Radial velocity variations observed at 59 Cyg spectrum were attributed to an orbital movement in a binary system with a compact secondary. Variations of radial velocity measured in different lines of Balmer series however were reported during the shell phases. Higher member of the series show different expansion velocities and the are higher in the beginning of the phase. Complicated phase evolution of 59 Cyg made one of the best Be- star researchers Vera Doazan to conclude: This shows that a single phenomenon – the appearance or disappearance of a shell phase – can show quite a different behavior in different stars. This is the very hallmark of what we have called the individuality of Be stars.

6. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Phase transitions in 88 Her Phase transitions and spectral and photometrical variations of 88 Her resemble very much those of Pleione. There are however also significant differences. For long time 88 Her was classified as Ap star. Balmer emission was first observed in 1955 and shell type spectrum was presented 4 years later in 1959. All lines of singly ionized metals were in absorption. Shell lines disappeared in 1970 and in the same time emission decreased. Photometrical observations at the same time showed increase in UBV filter and shift to the blue of the star’s color indexes. Maximum of this photometrical changes coincided with disappearance of metallic shell lines and minimum of Balmer emission. After 1970 for seven years 88 Her was in Be phase, characterized by weak H  emission and weak shell type absorption cores of higher members of Balmer series.

7. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Phase transitions in 88 Her New period of significant changes started 1977 with abrupt drop in brightness – 0. m 33 in U filter for 7 months time interval. This went before gradual development of metallic shell lines. In the same time Balmer emission increased and the star increased its brightness. All mentioned amazingly resembles behavior of Pleione during its phase changes. There are however at least two important differences. First 88 Her goes through phases Be shell (hydrogen + metals) => Be shell (hydrogen) => Be shell (hydrogen + metals) while Pleione phases follow Be shell => Be => Be shell cycle. It is important to notice that sharp absorption cores of Balmer lines in the spectrum of 88 Her have never disappeared. Second important difference between 88 Her and Pleione is that for 88 Her no Balmer progression was observed. All the shell lines in its spectrum follow all the time one and the same periodic radial velocity variation with a period of 86 d.7. This is not the case of Pleione where higher members of Balmer series performed strong expansion during disappearance of the shell.

8. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Color-color diagram of different Be-star phase transitions

9. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Phase biography of Pleione Pleione is known for decades for its spectacular phase changes. Long time interval covered with both spectral and photometrical observations allows relatively detailed and comprehensive description of star’s phases completed by many astronomers. Some famous astronomers are among them. Early history of Pleione in the end of XIX and beginning of XX centuries includes two shell phases separated by about 35 years (Doazan, 1982). Most of the time at that period however Pleione spent like normal B type star. From 1905 until 1936 Pleione’s spectrum was much alike spectrum of normal B8 V star. Emission lines in the spectrum of the star were seen in observations from 1948 together with well developed spectrum of singly ionized metals. This shell phase continued till 1951 when shell lines gradually decreased. Several years later Pleione was observed in Be phase. Maximum of the emission of hydrogen and Fe II lines was reached around 1963.

10. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Phase biography of Pleione Large scale decrease of emission was observed in 1972. In December 1972 start of new shell phase was observed. Guliver (1977) gave well documented description of the previous phase transitions. He also was able to show for the first time that the development of emission and shell phases is gradual. Shell phase continued until 1987 when several authors reported about disappearance of shell lines in the spectrum of Pleione. Balmer progression changes were for the first time reported (Iliev et al., 1988). By chance Balmer progression variations as studied from Rozhen Observatory spectra allow relatively accurately to determine end of the process of shell dissipation – November 1987 ).

11. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 H alpha emission data cube – double emission peaks were first seen for Pleione from Rozhen Observatory spectras

12. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Estimations of H alpha and H beta emitting regions of Pleione’s Local Circumstellar Envelope

13. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 H beta profiles of Pleione. Double peak emission structure seen together with V/R variations.

14. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 H alpha emission and absorption component changes during the last period of Pleione’s Be phase.

15. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Complex spectral and photometrical study of phase transitions of Pleione, as well as archive data-mining lead to reconstruction of much more detailed scenario of the just finished phase transition cycle. Based on monitoring of Balmer progression change we can accept November 4 th, 1987 as start of new emission circumstellar envelope formation (Be phase). Study of emission intensity in Balmer lines together with implemented for the first time for the star reliable estimations of the dimensions of the emitting regions points to the date 31 st of December 2002, as a moment of maximal strength of Be phase. Recent photometrical measurements of Pleione have shown that minimum of brightness in V filter was reached in November 2009 and after than increased gradually. Results and conclusions

16. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 The time interval for full development of the Be-phase can be estimated as 15.15 years, while the period of decadent development – of 6.8 years. All together it results in nearly 22 years duration of the just finished emission phase of Pleione which is much larger than the period of about 17 years of Be- shell-Be transitions observed in the second half of XX century Results and conclusions

17. VIII SBAC, Leskovac, Serbia, May 8-12, 2012 Analysis of wide range of observational data lead to the conclusion that phase transitions of the Be-stars can not be interpreted only by positional and geometrical variations of different formations of circumstellar environment. The whole variety of observed in the visual region spectral and photometrical changes of different features of the Be phenomenon imply that complex of physical processes must be the cause of observed phase transitions. Interaction of the activity processes in the central Be-star with the ones in the circumstellar environment must be the fundament of observed variety of events that accompany the phase changes. Results and conclusions