Globular Cluster NGC 5986

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Äàòà èçìåíåíèÿ: Fri Dec 7 04:22:58 2007
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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, B, 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.)

Click here for our paper (Alves, Bond, & Onken; Astronomical Journal, 121, 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, at M_v = -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, B, and V 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