Oct 05, 2007 UT: Photometry of HS 2331+3905 (= V455 And)

Michael Richmond
Oct 6, 2007

On the night of Oct 04/05, 2007 EDT, Joe Panzik used the RIT Observatory's 12-inch Meade telescope and SBIG ST8 CCD camera to monitor the cataclysmic variable star HS 2331+3905 (also known as V455 And), which was in the early stages of an outburst. This star is a cataclysmic variable similar to WZ Sge. For more information about it, read

• AAVSO Alert Notice 357
• HS 2331+3905: The cataclysmic variable that has it all by Araujo-Betancor et al., 2005, A&A, 430, 629A.
• The vsnet-alert Yahoo group
• The variable-star-forum Yahoo group
• Arne Henden has provided a deep, well-calibrated set of comparison stars in the field , although these are mostly too faint to use when the star is in outburst (as it is now!)

The plan tonight:

• the Meade 12-inch telescope with SBIG ST-8E CCD (no focal reducer); the plate scale is about 1.85 arcsec per pixel
• no filter (instead of V-band)
• exposure time increased to 30 seconds (previously used 15 seconds) because the star has faded so much

Notes from the night

• The conditions were good: no clouds, sky reasonably dark
• The FWHM was steady at 2.3 pix
• No guiding

This is a chart of the field based on images taken on earlier nights. Click on the chart for a larger version. Note the addition of star "K", which is now about the same brightness as HS2331.

The chart has several of the brighter stars in the field labelled with letters, just to keep me straight as I perform the reductions. Some of these stars have good photometry, as mentioned in AAVSO Alert Notice 357 .

my
label     RA               Dec                 V               B       visual
----------------------------------------------------------------------------
      23:37:10.49     +39:27:09.7      8.366 (0.014)   9.430 (0.025)   84
      23:31:57.51     +39:19:43.4      9.248 (0.024)   9.969 (0.031)   93
      23:33:05.63     +39:23:11.8      9.572 (0.029)  10.693 (0.059)   96
      23:33:53.74     +38:57:22.6     10.007 (0.042)  11.115 (0.087)  100
      23:34:46.69     +39:16:44.0     10.310 (0.043)  10.961 (0.054)  103
 B    23:33:23.41     +39:17:58.8     10.481 (0.055)  11.167 (0.070)  105
 A    23:34:23.38     +39:15:34.9     10.900 (0.078)  11.393 (0.076)  109

I'll use the star marked "A" to set the zeropoint of my differential magnitudes back onto the standard system, at least roughly. However, with the longer 30-second exposure times we have started to use, star "A" is now saturated. Going back to previous nights, I find that the star marked as "H" on the chart above is roughly 1.70 magnitudes fainter than star "A" in unfiltered images. Thus, my procedure for determining a rough zero-point for the magnitude scale will be

1. measure differential mag of star "H" on unfiltered image
2. subtract 1.70 mag, yielding approx value of star "A"
3. add a constant to shift star "A" to 10.900, its V-band magnitude

I measured the instrumental magnitude of each star with aperture photometry, using a radius of 5 pixels = 9.3 arcseconds and sky defined by an annulus around each star. Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame.

Below is a graph of the scatter in differential magnitude versus magnitude in the ensemble solution. The brightest four stars were definitely or possibly saturated, so I didn't include them in the ensemble solution.

HS 2331 is the star with large scatter at differential magnitude 3.7. The brightest unsaturated stars, "D", "G" and "C", have scatter of about 0.006 magnitudes from their ensemble mean values.

Light curves for selected stars (HS 2331 and stars A - J) in the field are shown below. HS 2331, shown by light green crosses near the bottom, is clearly variable. I discarded from the solution the bright stars which were saturated in these long exposures: stars A (red), B (blue), E (red circles), F (orange triangles).

Here's a closeup of the variation in HS 2331 itself. I've also plotted the measurments of nearby star "K", which is about the same brightness as the target now, shifting them by 0.4 magnitudes so that they would fit nicely on the graph. Note the two deep, sharp eclipses at times 378.51436 and 378.57059; the time interval between them, 0.05623 days = 80.97 minutes, is very close to the orbital period of the binary system.

You can see that the star has faded considerably since the outburst was discovered on Sep 4.

I've made a table of the measurements themselves, with three different flavors of time. The differential magnitudes from the ensemble solution have been shifted so that star "A" in my chart, TYC 3231-533-1, has value 10.900. Remember: this data is unfiltered, but I am shifting the zeropoint to make star "A" match its V-band value.

Here's the start of the table.

# Measurements of HS 2331+3905 made at RIT Obs, Oct 5, 2007 UT, 
#   made by Joe Panzik. 
# All data taken with 12-inch LX-200 + no filter + SBIG ST-8 CCD 
#    no focal reducer, so at native f/10 
# Each exposure 30 seconds long; tabulated times are midexposure 
#    and accurate only to +/- 1 second. 
# 'mag' is a differential magnitude based on ensemble photometry 
#    which has been shifted so TYC 3231-533-1 mag=10.900 
#    to match value from AAVSO Alert Notice 357. 
# 
# UT day      JD-2,450,000  HJD-2,450,000   mag    uncert
Oct04.99855      4378.49855   4378.50310  14.385  0.035 
Oct04.99903      4378.49903   4378.50358  14.413  0.043 
Oct04.99950      4378.49950   4378.50405  14.464  0.033 

• download UT 2007 Oct 05 HS 2331 photometry (plain ASCII)

Last modified 9/26/2007 by MWR.