The Be Star Newsletter, Volume 40 - March 2011
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The end of an active Be state in 66 Oph
A. S. Miroshnichenko1,
S. V. Zharikov2,
J. Fabregat3,
D. E. Reichart4,
The main feature of a star or a stellar system with the Be phenomenon is a circumstellar gaseous disk that is responsible for emission lines in its spectrum. The line emission is variable and can disappear completely for some time. Numerous cases of transition from a Be to a normal B-type state and back have been reported, but only a few of them were well documented (e.g., μ Cen, δ Sco, and π Aqr, Peters 1979, Miroshnichenko et al. 2001, and Bjorkman et al. 2002, respectively). A normal B-type state has been found to be accompanied by no infrared excess, which is always observed during an active Be state. This suggests that the circumstellar material almost completely vanishes when such a transition occurs. Observations of a Be star in a normal B-type state therefore present an opportunity to measure true fundamental parameters, which are masked by the disk during an active Be state. Here we report our detection of a disk loss by 66 Oph, a well-studied object that has spent the last ∼50–60 years in an active Be state. 66 Oph was first mentioned to exhibit Balmer lines in emission by Merrill & Burwell (1933). Sparse observations in the 1930's – 1960's indicated a variable emission-line spectrum with a minimum occurred in mid 1950's which can be considered as the onset of the active Be state. The emission-line strength was steadily increasing until 1993 and decreasing since then (Floquet et al. 2002). A maximum equivalent width (EW) of the Hα line of 60 Å was observed by Hanuschik et al. (1995). Silaj et al. (2010) reported a weak Hα line with an EW of 3.6 Å in 2007. Granada, Arias, & Cidale (2010) observed weak infrared hydrogen emission lines in August 2008 and suggested that the active Be state might be coming to an end.
66 Oph is a recognized binary system with a secondary component
detected by interferometry (Mason et al. 2009) and direct imaging in
the K-band (Oudmaijer & Parr 2010). Radial velocity variations
were detected in its spectrum long ago by Struve (1925), but no
orbital period has been determined from these and later
spectroscopic data. Floquet et al. (2002) concluded on the existence
of non-radial pulsations in the primary component and derived its
fundamental parameters
( We took photometric UBVRI observations of 66 Oph on April 4, 11, and 19, 2010 using two 0.4-m robotic PROMPT telescopes (Reichart et al. 2005) and JHKL observations on April 7, 2010 using a 0.75-m telescope at the SAAO. Optical spectroscopic observations were obtained on October 15, 2010 with an échelle spectrograph at the 2.1-m telescope of the San Pedro Martir Observatory in Mexico with a spectral resolving power of R ∼ 20,000 between ∼4000 and 6800 Å. We have found virtually no signs of the line emission and the IR excess (see Fig. 1).
The strength of the diffuse interstellar bands at λ5780 and
λ5797 Å in our spectrum and the color-index
A weak emission component near the bottom of the Hα line manifests that the circumstellar matter is still present in the system. It may either belong to the debris of the former disk around the primary component or be located near the secondary component, as was found in the binary system of π Aqr when its primary component has lost its disk (Bjorkman et al. 2002). In the latter case, monitoring the motion of this remaining emission may lead to constraining the yet unknown secondary's orbit. The 2011 observing season can be very important for further studies of 66 Oph and the Be phenomenon in whole. Observations of π Aqr showed that the primary's circumstellar disk has shown no signs of recovery for at least five years. If this is the case for 66 Oph, there will be enough time to search for radial velocity variations in order to constrain the system properties. Closely watching the disk-less state will get us more insights into reasons for the disk renewal as well. We propose to monitor 66 Oph starting in the summer of 2011 for at least a few years or until the new active state begins. The campaign should include moderately high-resolution optical spectroscopy (R ≥ 10000) as well as optical/near-IR photometry, both as frequent as a few times a month. A webpage is set up to coordinate efforts of both professional and amateurs (Miroshnichenko 2011).
Figure 1. Left panel: Balmer lines in the spectrum of 66 Oph on October 15, 2010. Hα is shown by the solid line, Hβ is shown by the dashed line. Intensity is normalized to the nearby continuum, radial velocity is heliocentric. Telluric lines were not removed from the Hα region. Right panel: Spectral energy distribution of 66 Oph in April 2010. The circles show our UBVRIJHKL data with the interstellar reddening removed using
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