Call for monitoring of DY Per

(vsnet-alert 708)

Appended below is a note extracted from the "chemically-peculiar cool stars" newsletter, which may be of interest to vsnet folks. I don't expect that many readers will be able to do the spectroscopy, but prompt notification of the start up the expected decline later this year would be beneficial to professional observers.

I see from Gary Poyner's observation report from last night that DY Per is about mag. 10.5 visually. What kind of shape are the comparison stars in?

For those who do not know him, Phil Keenan is the "K" of the MK spectral classification system.



                     UNUSUAL VARIABLE STAR, DY PER

Hoffmeister noticed the large amplitude of light variation of the star now known as DY Per as early as 1940, but it was only in 1994 that Alksnis showed that the light curve made it very probable that DY Per belongs to the R CrB variables. Long thought to be an R star, the spectrum has recently been reclassified to C-Hd 4+ C2 6.5 on the Revised MK system.

DY Per is perhaps the coolest R CrB variable known, though our knowledge of it is based on only a few spectra. The mean period was established by Alksnis as 792 days, but the intervals between the deep minima may vary from 630 to 934 days. Dr. Alksnis has kindly informed us that if DY Per continues to behave as it has during the last few cycles, the next sudden drop to minimum might occur near the beginning of October, 1997. The decline, however, could begin as early as July, or as late as November.

It is obviously difficult to catch the star just as it starts to fade, but observations during the first few weeks of the decline would be particularly valuable, for R CrB variables commonly show a spectrum of many sharp emission lines just at that stage. Because the strong absorption by bands of C2 and CN obliterate most of the atomic lines in the yellow and red regions, it would be desirable to observe the spectrum in the blue region. This will not be easy, since the photo- graphic magnitude even at maximum light is near 13.5, but observations at any dispersion would be valuable.

                                         Philip C. Keenan
                                         The Ohio State University

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