The Globular Cluster NGC 5986 and its Luminous Post-AGB Stars
NGC 5986, a little-studied southern-hemisphere globular cluster,
is remarkable in containing two very luminous A-F type stars, discovered
by me in the 1970's. These objects are probably in their final evolution
off the asymptotic giant branch. I have argued that post-AGB A-F stars
are excellent candidates for a new standard extragalactic candle, since
they are luminous (they are the visually brightest stars in old populations),
should have a narrow luminosity function, and are very easily recognized
because of their large Balmer jumps. (Read more by clicking
here.) Determining the distance to NGC 5986 will help calibrate the
absolute brightnesses of post-AGB A-F stars.
Here is a color picture of NGC 5986, made from u,
and V CCD exposures with the Cerro Tololo 0.9-m reflector. The post-AGB
stars are the two bright, whitish stars just to the right and upper right
of the cluster core. (The even brighter star a little further to the upper
right is a foreground non-member.)
here for our paper (Alves, Bond, & Onken; Astronomical Journal,
318, 2001), which presents a new color-magnitude diagram, and shows the
distance to the cluster to be 10.7 kpc. Our photometry shows that the two
post-AGB stars have the same absolute magnitudes to within +/-0.05 mag,
Mv = -3.35.
NGC 5986: last updated 2001 December 20
Update: A Post-AGB Star in M79
Mike Siegel and I have found a post-AGB star in the globular cluster
M79 (NGC 1904). Its absolute magnitude is nearly identical to those of
the post-AGB stars in NGC 5986, further confirming the utility of these
stars as standard candles.
Below is a color picture of M79, made from u,
CCD exposures with the Cerro Tololo 0.9-m reflector. The newly discovered
post-AGB star is the bright, white star near the top of the image, almost
directly north of the cluster core.
I have been monitoring the brightness of the PAGB star since the
spring of 2007 with the SMARTS 1.3m telescope. The observations show the
star to be a low-amplitude pulsating variable with a period of about 30
days. This is consistent with the star's high luminosity (it has about
the same effective temperature as RR Lyrae stars, which are 4 mag fainter
and whose periods are generally around 0.6 day).
M79: last updated 2007 December 6