CR Boo update

(vsnet 625, Joe Patterson)

                  CR BOO CAMPAIGN

We have started a 23-day monitoring campaign on the helium cataclysmic variable CR Bootis (PG 1346+082), with the main objective of learning the eruption timescales of this enigmatic star. Starting with Gary Poyner's observation on May 4.97, we have kept a moderately good watch over the first 6 days. The star is cooperating, cycling very rapidly (<1 day) in visual magnitude between about 14.0 and 15.2. Most importantly, it has stayed out of the low state (17.5), which is astrophysically interesting but hard to study because of its faintness. These up-and-down excursions in light are amazing to watch. By comparison CR Boo makes the ER UMa stars - the speed demons of the dwarf novae - look like they're stuck in molasses.

Some observers are doing time-series photometry (continuous monitoring), to resolve the 25 minute variability of this star. But that signal is usually quite weak, and most observers will do fine by acquiring just a few points over the course of a night. Anyone with photoelectric or CCD capability who can monitor the star continuously can bring even more punch to the project.

We are more or less doing this in V light, but we can use other bands including an unfiltered CCD (as long as we know what CCD and what comparison star you used). The natural comparison star is a V=11.40 star 4 arcmin E of CR Boo; it is however quite red (M star), and a better choice might be a V=13.76 star 150" NNW of the bright red guy. We would love to get more observers involved in the project, and I hope to hear from more as the month progresses. I also hope to extend the project through about June 15.

     The 2000 coords are:

CR Boo:                       13h 48m 55.3s  +7d 57m 33s
eastern comp (naughtily red): 13h 49m 11.6s  +7d 57m 50s

These are telescope coords, perhaps good to about 10". You can locate a finder chart in the "PG Catalogue", which is in Ap J Supp 61, 305 (1986). Or, I can FAX you a better one if you ask (home 212-678- 0540, work 212-854-3276, or e-mail). Also, a most excellent paper on this star is Ap J 313, 757.

                                Joe Patterson

  rough chart, north up and west to the right, about 11 arcmin across:



        1                     CR                            ***


 Star 1 is the naughty red star, V=11.40.  Here are some rough mags we've
           V      B-V     U-B     V-R     R-I

Comp 1   11.40   1.60    1.95    0.96    1.04
Comp 2   15.53   0.86    0.32    0.46    0.39
Comp 3   13.76   1.23    1.24    0.78    0.63
Comp 4   14.87   0.54   -0.03    0.33    0.31
Comp 5   14.19   0.67   -0.01    0.43    0.42

***  is I think the Star of Bethlehem.  Whew.  Seems only appropriate
     to have one to point the way to CR Boo.

Most CCD observers are choosing comp 3. This is a K star, usually a fine choice for a comparison star although definitely redder than we like (because of uncertainty in calibrating unfiltered magnitudes). Its effective magnitude for an unfiltered, red-sensitive CCD is 13.01; in other words, close to an R magnitude. CR Boo (and for that matter most cataclysmic variables) has V approximately = R. So if you are using an unfiltered red CCD and you assume this comp to have mag 13.01, you should get a fair approximation to a V mag of a blue program star like a cataclysmic variable. Naturally a direct V-filter observation would be more accurate.

(I'll later refine these numbers.)

Some more information on photometry

(vsnet 626, Taichi Kato)

The Ouda Station has been moderately suffering from clouds these days (as can be seen from reduced number of reports from Japan). I believe Nogami-san will manage to expel every cloudlet tonight, with full of his "power" and ever-lasting enthusiasm.

One more thing to point out: given such a large difference between the variable and comarison stars, we will have to consider the secondary extinction color term. In the V-band, this is something like 0.04*delta(B-V)*F(z) mag? With a difference of 1.2 mag in B-V (assumed), one may expect systematic difference of 0.05 mag due to the atomospheric extinction between at f(z)=0 and f(z)=1. This effect may be even larger in unfiltered CCD photometry. Observers (or analysts) should note spurious 0.5-day or 1-day periodicity may easiliy detected owing to these kinds of systematic errors.

Fortunately the field of CR Boo is close to the Guide Star Photometric Catalog field P558. Taking this field under different air masses may be helpful in calibrating the photometric systems.

Taichi Kato

CCD image of the field

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