R Coronae Borealis stars : characteristics of their decline phase
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
R Coronae Borealis (RCB) stars belong to a rare class of variable stars characterized by sudden and unpredictable declines, which are believed to be caused by dust cloud obscuration. In spite of the fact that these stars are so rare (only about 40 are known in our Galaxy), there are many reasons for investigating them. The unusual variability and peculiar chemical composition make them unique among all known types of variable stars. Their evolution and the nature of their unpredictable minima are still not entirely understood. Very few observations of RCB stars during the decline phase exist. Only three RCB stars (R CrB, RY Sgr and V854 Cen) have been studied in detail and only a few declines have been completely covered by observations. This thesis investigates the spectroscopic and photometric characteristics of RCB variables during their decline phases. A programme of photometric and spectroscopic observations of nine RCB and three HdC stars has been undertaken at Mt John University Observatory (MJUO) over a period of two and a half years. The programme includes some typical examples of RCB stars (Teff ~ 7000 K), as well as some cool ones (Teff ~ 5000 K). One of the most unusual of all RCB stars, V854 Cen, is also included. The photometric observations, as part of the long-term monitoring of RCB stars at MJUO, have provided the UBVRIphotometry and have served as a decline indicator. Complex colour changes during the declines were monitored and compared with the spectroscopy. The photometry during the recovery phases of the nine RCB stars in the last 12 years was used for studying the extinction properties of the gas obscuring the photosphere. An analysis of 26 different declines shows that the material causing the declines has extinction properties similar to those of the interstellar medium. The medium and high-resolution spectroscopy has been obtained for six declines of different programme stars using the 1-m telescope at MJUO. Although the duration and depth of the declines are very different, they all show similar photometric and spectroscopic characteristics. The results have been compared with other observations and used to examine a simple line-region model (E1/E2/BL), which attempts to describe the evolution and origin of emission lines during a decline. In general, the evolution of various emission lines observed in this work is consistent with their classification into these three groups. However, some characteristics of the emission lines indicate a different origin from that suggested by the model. A very rich emission line spectrum was monitored during the 1998 decline of V854 Cen, while only the most prominent lines were observed in the other stars. Short-lived high-excitation lines from the initial decline phase, such as CI and 01, were classified as Ei. They show a characteristic, shock-induced red shift indicating the photospheric origin. Lines classified as E2 are mainly from the low-excitation ions and neutral atoms. All lines from this group appear at the very beginning of the decline and are visible through to the late recovery phase, slightly blue-shifted relative to the stellar velocity. The lines of the low-excitation ions exhibit a complex structure with a strong central and two weaker components, one on each side. Their absolute flux evolution has been compared with the changes in the stellar continuum flux. The behaviour of these lines indicates that they are not affected by the dust cloud in the same way as the photospheric continuum. Assuming that the dust cloud is formed at about 2R* and taking into account the acceleration obtained from the analysis of the high-velocity Nal D absorption lines, the position of the E2 line emitting region was estimated to be about 3R* - 5R*. The third group (BL) consists of broad emission lines, which are a typical feature of all observed declines. The most prominent broad lines present in all RCB stars belong to the Nal D doublet. The observations demonstrate that these lines are the strongest in V854 Cen, due to the significant amount of material produced by its frequent declines. In contrast to the E2 lines, whose fluxes have been found to decrease during the decline, the absolute flux of the broad lines stays constant throughout the whole decline phase. This is consistent with the idea that the broad emission is a permanent feature, whose visibility depends only on the photospheric brightness. Various Nal D components (sharp and broad emission and high-velocity absorption) have been analysed in a number of RCB declines and presented in this thesis. The high-velocity blue-shifted Nal D absorption demonstrates similar velocities (between -230 kms-1 and -400 kms-1), structure and behaviour in the different declines. The observations from the 1998 decline of V854 Cen clearly show that the high-velocity absorption lines can also appear during the initial decline phase. This suggests that they can be associated with the clouds formed in some previous declines, as well as with the current one. The spectroscopic observations of the 1998 decline of V854 Cen obtained in this thesis represent the first almost complete coverage of a decline of this star.