June 20, 2013: Dr. Andrea Dupree (Harvard-Smithsonian Center for Astrophysics) has requested AAVSO monitoring of the symbiotic variable AG Draconis from now through at least September 2013 for correlation with upcoming Chandra x-ray and HST ultraviolet observations.
Dr. Dupree writes that as a symbiotic variable, AG Dra "exhibits both red and blue spectral features in a single system. This system consists of a cool (red) giant (spectral type K2 or K3) with a very hot (blue) companion (a white dwarf). The binary has a period of 549 days and does not eclipse. Both stars are losing material to a wind, creating a large photoionized nebula that could be bounded by a region where the winds collide with one another. In addition, there may be an accretion disk around the white dwarf formed by material captured from the wind of the red giant. But we have found some puzzling high temperature emission lines in the ultraviolet spectrum, and we don't know where they originate. We also do not know what produces the soft X-rays from the system. To determine the dynamics of the gas and the temperatures and densities in various parts of this complex system, we are taking high resolution spectra in the X-ray region with CHANDRA (during the week of July 22, 2013) and in the ultraviolet region with HST (September 18-28, 2013).
"Complicating the picture, AG Dra periodically enters an active phase where the UV/optical light increases by 1 to 3 magnitudes. Major outbursts seem to occur every 14.5 years, and the next one might be in 2017. We think AG Dra is in quiescence now. However, the system can go into an active phase sporadically!
"We need to know what state the star is in when the CHANDRA and HST observations are made in order to interpret the spectra. Thus it is very important to have photometry and good visual estimates of the brightness of the star. We would be most appreciative if the AAVSO observers could keep an eye on the brightness and variability of AG Dra starting in June and continuing through September."
AG Dra is ~9.9V during quiescence. According to observations in the AAVSO International Database, its last significant optical brightening occurred in July 2006, when AG Dra brightened to visual magnitude 8.1 and returned to magnitude 9.9 in April 2008. Prior to 2006, AG Dra had showed irregular brightenings since its previous outburst in 1994.
Please observe AG Dra nightly from now through the end of the 2013 observing season. Filtered photometry and visual observations are welcome. Time-series observations are not necessary.
Coordinates: R.A. 16 01 41.01 Dec. +66 48 10.2 (2000.0)
Charts for AG Dra may be created using the AAVSO Variable Star Plotter (http://www.aavso.org/vsp).
Please report observations to the AAVSO International Database using the name AG DRA.
For more information on symbiotic variables (Z And stars), please visit the Z And section of the AAVSO's Variable Star of the Season webpages (http://www.aavso.org/vsots_archive#zand). This campaign is being followed on the AAVSO Observing Campaigns page (http://www.aavso.org/observing-campaigns).
This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.
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