Special Notice #64: FN Sgr AND VZ Sgr

September 7, 2007


Longtime AAVSO member observer John Bortle, Stormville, NY, reported to the AAVSO in mid-August that the symbiotic variable FN Sgr (RA 18:53:54.80 Dec -18:59:41.0 (2000.0)) was undergoing an outburst. Peter Williams, Heathcote, NSW, Australia, independently reported the outburst on the AVSON list and some VSNET lists on August 28 following the dark provided by the lunar eclipse. A thoughtful assessment of the behavior of the system was subsequently posted by Bortle on September 4 on [cvnet-discussion] and other lists. Other discussion on this system has also appeared on various lists.

The AAVSO light curve on FN Sgr goes back to JD 2440058 (July 1968) with an observation of visual magnitude <11.5 by Wayne Lowder (to see the remarkable light curve of FN Sgr, go to http://www.aavso.org/data/lcg and ask for data since 2440058). Through the mid-1970s FN Sgr declined erratically from ~11.4 to 12.9-13.00 by the beginning of 1980. During the 1980s it varied between 12.6 and 13.6. Then in February 1990 it showed a slight dip, and subsequently displayed a series of brightenings with increasing amplitude: to 12.4 in May 1991, then to 12.0 in May 1994, then to 11.2 in October 1995. Since then the star has shown eclipses to as deep as 14.7 approximately every 564 days (P=563.8d, per Brandi et al., 2005, A&A, 440, 239), with maxima first becoming increasingly bright (the brightest was 10.8 in June 1998), then less bright. Most recently, in July 2006 FN Sgr brightened from mid-13 to 11.6, declining to 12.9 by the end of October 2006, when it entered the seasonal gap. After emergence it was at 13.4 on May 30, declined to 13.7 by June 30, then began brightening in mid-July and is currently at magnitude 11.6.

Whether the current brightening is a new outburst or is simply the egress from eclipse (an eclipse was predicted for ~July) cannot be said. The current brightness matches the brightness of the star in August 2006, however, the behavior of FN Sgr has been erratic enough over the past several years to make a reliable prediction impossible.

Charts for FN Sgr may be produced by entering the star name or the coordinates above into VSP at: http://www.aavso.org/observing/charts/vsp/index.html

Coverage of this extremely interesting system is urgently needed. FN Sgr is located within reach of most northern and southern observers, and although the observing season is ending, it would be extremely valuable to follow it as long as possible, and then pick it up as early in the next observing season as you can.


According to initial reports from John Bortle, Stormville, NY, and Peter Williams, Heathcote, NSW, Australia, the R CrB variable VZ Sgr (RA 18:15:08.60 Dec -29:42:30.0 (2000.0)) is fading from its maximum visual magnitude of 10.0-10.5. Observations reported to the AAVSO show it has faded to visual magnitude 12.1 as of September 7.

Fadings of R CrB stars are unpredictable in timing, amplitude, and duration. According to observations in the AAVSO International Database, the most recent fading of VZ Sgr occurred in June 2001, when the star faded to visual magnitude 14.7, then returned to maximum by December 2001-January 2002. However, minima have been reported as faint as visual magnitude 16.3 (July-August 1988).

Charts for VZ Sgr may be produced by entering the star name or the coordinates above into VSP at: http://www.aavso.org/observing/charts/vsp/index.html

Please monitor VZ Sgr through the rest of this observing season, and pick it up again as soon as possible next season so we may have the most complete coverage as possible of this unpredictable event.

Many thanks for your astronomical efforts and valuable contributions!

This AAVSO Special Notice was prepared by: Elizabeth O. Waagen



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