The Be Star Newsletter, Volume 39 - December 2008

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Observing the next periastron of the δ Scorpii Be binary A. S. Miroshnichenko Department of Physics and Astronomy, University of North Carolina at Greensboro, Greensboro, NC 27402, USA email:  a_mirosh@uncg.edu Received: Oct 27, 2008;  Accepted: Dec 4, 2008 δ Scorpii is a bright, interferometrically detected binary system with a B0-type primary and supposedly a B-type secondary at an eccentric orbit (e = 0.94). The primary component served as a standard of spectral classification in the 20th century. The system is under continuous attention since the summer of 2000 because of a detected brightening, which turned out to be due to development of a circumstellar disk (Fabregat, Reig, & Otero 2000). The process seemed to begin a few months before the periastron passage, which occurred on 2000 September 9±3 and was constrained by spectroscopy of the Hα line (Miroshnichenko et al. 2001). In other words, the primary component of δ Sco became a Be star. The system was continuously brightening for a few years (Otero, Fraser, & Lloyd 2001; Gandet et al. 2002) and became as bright as V ∼ 1.6 mag by the first half of 2003. At the same time, its emission-line spectrum was getting stronger with a decreasing separation between the line peaks, which were initially resolvable with a moderate dispersion (Miroshnichenko et al. 2003). These observations indicated that the disk was growing. In February 2005 the system suddenly started fading in both optical and near-IR regions and almost reached its quiescent brightness level (V ∼ 2.3 mag) in October 2005 (Otero 2008; Carciofi et al. 2006). Nevertheless, the spectrum remained emission-line with decreasing line widths, nearly constant intensities, and growing equivalent widths. The equivalent widths showed a maximum simultaneously with the minimum brightness. This suggests that the disk survived, but underwent a transformation to a ring-like structure (e.g., Rivinius et al. 2001). It might have lost its inner part probably due to a lower mass loss rate, which was always unstable resulting in cyclic variations of the line strengths (Miroshnichenko et al. 2003). After reaching the minimum, the system optical brightness has been gradually increasing and showing ∼60-day variation cycles with an amplitude of ∼0.3 mag (Otero 2008). All the described events occurred as the secondary was on its way to apastron. The orbital period (10.58±0.08 years) was constrained by interferometry that was unable to separate the companions during nearly half of the orbit due to the high eccentricity (see Miroshnichenko et al. 2001). Even now, with improved capabilities, interferometry may not be successful near periastron because of the presence of the disk. Therefore, spectroscopy will remain the most reliable technique to detect the new periastron moment due to a large change of the primary's radial velocity as the companions come very close together. The system is going to come to this phase some time in the first half of 2011. Since the current orbital period may contain a systematic error (the periastron in 2000 occurred a few months after the interferometric predictions, see Miroshnichenko et al. 2001), frequent monitoring of the system has to begin in the fall of 2010. This is not the only task to be performed to know this remarkable object better. While most Be stars with non-degenerate secondary components exhibit circular orbits, the one of δ Sco is very eccentric. At periastron, the components will be separated by a distance of ∼24 primary's radii R1. The primary's Roche lobe size of ∼15 R1 at that time may be smaller than the disk size, which was estimated to be ∼7 R1 already in 2001 (Miroshnichenko et al. 2003; Carciofi et al. 2006) and continued to grow. One can expect a very interesting behavior of the observable parameters, which could give us new important insights into the disk properties and another source of information about the secondary component. We only know that it is optically 1.5–2.0 mag fainter than the primary, a result not yet confirmed (Bedding 1993). Observing the next periastron is also a unique opportunity to study the secondary's effect on the mass loss from the primary. This is important, as the mechanisms triggering mass loss in Be stars are not yet known. Although δ Sco is being closely watched by both professionals (high-resolution spectroscopy) and amateurs (brightness estimates and low-resolution spectroscopy), we definitely need coordinated efforts to document this rare event as fully as possible. I propose the following observations to be done as well as goals to reach in the framework of this campaign. High-resolution (spectral resolving power R = λ/δλ ≥ 20000) optical spectroscopy covering at least the Hα line region to monitor the profile and radial velocity variations. Spectroscopy of higher members of the Balmer series (at least Hβ and Hγ) as well as of the HeI lines at 5876 and 6678 Å is desirable to probe different regions of the disk. Lower-resolution optical/near-IR spectroscopy to monitor the line equivalent widths. Multicolor photometry in the optical/IR region (at least UBVRIJHK) to follow variations of the circumstellar contribution to the overall system brightness. If the object saturates the detector at the shortest possible exposure, one can either defocus the image (see technique used by Halonen et al. 2008) or partially cover the mirror. Interferometry to constrain the orbit and the components' brightness ratio. Optical and near-IR polarimetry to constrain the disk properties in combination with all the above data (see Carciofi et al. 2006, for discussion). Suggested observation frequency is once a week before Fall 2010 and more frequently later. Near the periastron time, every opportunity to observe δ Sco has to be used. Since for most observatories in the northern hemisphere the object becomes invisible in October, the close periastron watch needs to begin from the southern hemisphere. It would be useful to create a data bank of all observations of δ Sco. A webpage with general information about δ Sco and links to electronic copies of publications devoted to observations of its active phase as well as to online observational data has been created and will be maintained by the author (Miroshnichenko 2008). Any other suggestions for observing activities as well as any information sharing are welcome. I hope that the Working Group on Active B Stars will support this proposal and help coordinating the campaign. References: Bedding, T.R. 1993, AJ, 106, 768 Carciofi, A.C., Miroshnichenko, A.S., Kusakin, A.V., et al. 2006, ApJ, 652, 1617 Fabregat, J., Reig, P., & Otero, S. 2000, IAUC 7461 Gandet, T.L., Otero, S., Fraser, B., & West, J.D. 2002, IBVS, 5352 Halonen, R.T., Jones, C.E., Sigut, T.A., et al. 2008, PASP, 120, 498 Miroshnichenko, A.S., Fabregat, J., Bjorkman, K.S., et al. 2001, A&A, 377, 485 Miroshnichenko, A.S., Bjorkman, K.S., Morrison, N.D., et al. 2003, A&A, 408, 305 Miroshnichenko, A.S. 2008, http://www.uncg.edu/∼a_mirosh/Delta_Sco Otero, S. 2008, http://ar.geocities.com/varsao/delta_Sco.htm Otero, S., Fraser, B., & Lloyd, C.. 2001, IBVS 5026 Rivinius, Th., Baade, D., Stefl, S., & Maintz, M. 2001, A&A, 379, 257