AG Dra monitoring requested for multiwavelength campaign

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AG Dra monitoring requested for multiwavelength campaign

文章 PTS » 週五 21 6月, 2013 06:13

AAVSO Alert Notice 485

AG Dra monitoring requested for multiwavelength campaign
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
pages (http://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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