The brightening of Cyg OB2-4 B: A phase change in a Be star
ATel #5571; J. Salas (Spain), J. Maiz Apellaniz (IAA-CSIC, Spain), R. C. Gamen (IALP-CONICET, Argentina), R. H. Barba (ULS, Chile), A. Sota (IAA-CSIC, Spain), J. Sanchez-Bermudez (IAA-CSIC, Spain), & E. J. Alfaro (IAA-CSIC, Spain)
on 12 Nov 2013; 05:39 UT
Credential Certification: Rodolfo Barba (rbarba@dfuls.cl)
Subjects: Optical, Star, Variables
As part of a photometric campaign of Cyg OB2 carried out with an amateur 35 cm telescope, we discovered a brightening of Cyg OB2-4 B (RA=20:32:13.117, dec=+41:27:24.25 J2000). The star maintained approximately constant magnitudes of RJohnson = 11.23 and IJohnson = 10.47 during the 2012 summer campaign. When the campaign resumed in 4 June 2013 the star retained the previous magnitudes but in a few days it started a progressive brightening that reached RJohnson = 10.66 and IJohnson = 9.78 in late August. After that, the star dimmed progressively until reaching RJohnson = 10.94 and IJohnson = 10.08 in early November.
We carried out differential photometry of Cyg OB2-4 B with respect to the nearby A and C components using the AstraLux Lucky Imaging instrument at the 2.2 m telescope in Calar Alto. Comparing observations taken in September 2013 with previous ones from November 2007 and September 2011, we also detect a brightening of 0.60 magnitudes in the Sloan i band and of 0.60-0.65 magnitudes in the z band.
As part of the Galactic O-Star Spectroscopic Survey (GOSSS), we also obtained R~2500 blue-violet spectroscopy of Cyg OB2-4 B with the William Herschel Telescope in September 2012 and October 2013. The 2012 spectrum is purely in absorption and can be classified as B0: Vn. The 2013 spectrum remains close to B0 but now a number of lines, most noticeably H beta, show double-peaked emission profiles.
Finally, we also observed Cyg OB2-4 B as part of the Calar Alto Fiber-fed Échelle Binary Evolution Andalusian Northern Survey (CAFÉ-BEANS), obtaining two high-resolution optical spectra of the star in late September. In both spectra H alpha appears in emission with a double-peaked profile.
These phase changes between a Be and a normal B phase are observed in other Be and Oe stars (see Gamen et al. 2012, A&A 546, A92). From the information in different filters it appears that the star is slightly redder during the Be phase. Looking at additional data, we discovered that this could be a relatively common occurrence in Cyg OB2-4 B. A GOSSS spectrum taken with the 3.5 m CAHA telescope on October 2008 also shows a double-peak profile in H beta. An AstraLux z-band image obtained on June of that year also shows a magnitude difference between A and B similar to the ones from September 2013. Therefore, Cyg OB2-4 B appears to have been in its normal B phase in 2007, in its Be phase in 2008, in its B phase again in 2012, and to have recently switched to its Be phase again.
Additional data is available at http://jmaiz.iaa.es/research/CygOB24B/CygOB24B.html.
