The Be Star Newsletter, Volume 36 - April 2003

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The bright Be-shell star  CrB possibly entering a new shell phase Th. Rivinius1, S. Stefl2, O. Stahl1, & D. Baade3 1 Landessternwarte Königstuhl, D-69117 Heidelberg, Germany email:  T.Rivinius@lsw.uni-heidelberg.de 2 Astronomical Institute, Academy of Sciences, CZ-251 65 Ondrejov, Czech Republic 3 European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany Received: March 28, 2003

ABSTRACT In a manuscript recently submitted to A&A, we report about our observations of bright Be shell stars obtained from 1991 to 2003. Among other results, we have found  CrB to have become active again in the past time. Our observations are currently interrupted by telescope maintenance until June. The rate of change in  CrB has increased drastically in the last few spectra, taken Mar. 26 and 27, compared to the previous ones of Mar. 16. We want to announce this change to the Be community by publishing the relevant parts of the manuscript currently being in the editorial process, supplemented by the findings in the most recent spectra.

CrB was observed from Tautenburg in 1991 and 1992 with FLASH, with the Ondrejov coudé instrument 1993 to 2000, then with HEROS from Wendelstein in 2000, and from Ondrejov since 2001 also with HEROS. While the Tautenburg, Ondrejov coudé, and Wendelstein data show the purely rotational H profile present since 1981, the spectrum had developed a weak shell signature with a rather broad core in February, 2001. This signature remained stable until May, then disappeared in July. In March, 2002, a weak and broad CQE-type (central quasi emission, Rivinius et al., 1999, A&A 348, 831) signature was present in the core of H that finally disappeared in April, 2002 (Fig. 1, left panel). In February, 2003, however, circumstellar contribution is visible again. Only H showed a similar, but weaker shell effect, while no other line reflected this brief and weak shell episode. HEROS is dismounted end of March 2003 to allow for telescope maintenace. In the very last spectra before this the spectral changes towards a Be shell signature became stronger and more rapid (Fig. 1, right panel). In particular, the emission peaks in H became obvious in the full profiles (i.e. local flux maxima are forming), while in the previous seasons they were seen as emission in the residuals only. The variations are now clearly visible also in H. Figure 1. Evolution of the H profile of  CrB from 2000 to 2003 (left), the residuals from the undisturbed profiles (middle) and a close-up of the 2003 observations (right). The first spectrum was taken May 16, 2000, the last one on Mar. 27, 2003. The mean of the 1991 Tautenburg data is shown as dotted line for comparison. In the right panel the first spectrum was taken Feb. 15, the last one Mar. 27. Note the sudden increase of the violet emission peak in the very last spectrum, the previous one just taken the night before. The offsets are proportional to time, one year per flux unit and 90 day per flux unit in the left and right panels, respectively. By analogy to  Cen (Hanuschik et al. 1993, A&A 274, 356; Rivinius et al., 1998, A&A 333, 125), the difference in enhancement of the blue and the red emission component between the last two nights could mean that  CrB is undergoing an outburst, with the ejected matter not yet having been circularized. If so, a dense series of spectra could provide a rare opportunity for a detailed tomography of such an event in a star seen equator-on. Useful diagnostics could also result from observations of standard metallic shell lines and the Paschen series, which were not yet detected in emission or extra absorption. Given the rate of change in the Balmer lines, they may appear soon, however.