Proposal #584

Amendment to AAVSOnet Proposal P444

This proposal was prepared by Edward Wiley (WEY) and team leader Richard Gray (GRIG). Wiley will be responsible for image processing.

HD 5501 is a rapidly evolving interacting and eclipsing binary star in Cassiopeia with a highly variable light curve that changes from orbit to orbit in the sense that the primary and secondary eclipses show variable widths and depths and even timings. The star exhibits complex and changing H-alpha emission with absorption components that show blue shifts up to 500 km/s with respect to the systemic velocity. The primary, a B9-A0 II-III star, is evolving rapidly through the Hertzsprung gap and has recently begun mass transfer through the L1 point onto the secondary, which is unseen, but which we believe is a late-B main sequence star. The orbit is quite eccentric (e ~ 0.24), and so mass transfer only occurs near periastron passage. That "phase-dependent Roche lobe overflow" is partially responsible for the rapid evolution of the orbit; CCD photometry dating from 2003 from various sources as well as photometry from the Harvard plate archives shows that dP/dt = -1.2 X 10-7, leading to a timescale for the orbital evolution of about 170,000 years, remarkably rapid. Analysis of TESS photometry indicates that the system is undergoing dynamical chaos with a rapidly changing accretion disk/torus around the secondary star as well as mass loss through the L2 and possibly L3 point.

It is clear that we have caught this interesting binary in a very special evolutionary state. HD 5501 has been followed nightly, when possible, by members of the “Spectroscopy Discussion Interest Group” in spectroscopy and photometry (B and V). Further continuous photometric data on this star will help us to unravel the complex dynamics this system is undergoing, especially the dynamical chaos that manifests itself in the highly variable light curve. This star would benefit from BVRI photometry on a nightly basis for as long as possible. We propose that P444 be extended indefinitely so that the team can follow it over several seasons.

HD 5501 varies between V = 8.6 to 9.05 with a period of 7.53 days. We propose that HD 5501 is observed each night five times (when possible) through the BVRI filters, each of the five “image sets” being comprised of three exposures through each filter, for a total of 60 exposures per night. We use the same integration times as currently used for images in the original P444 proposal, detailed below.

The nightly images take in the original P444 by Arne Henden comprised one set of BBBVVVRRRIII and took about five (5) minutes to complete. We request the same filter order. A characteristic timescale for the micro variability seen in the light curve is ≥ 1 hour, so the greatest science return would come from 5 separate pointings of the telescope to HD5501 separated in time by about an hour with all image sets taken above 30-degrees of altitude. This is the maximum; the number of image sets taken on any given night will, of course, depend on the observing season.
Integration times and maximum SNR of current P444 images are as follows based on data from 24-12-07 and a BBBVVVRRRIII filter order.

B-filter, 20 seconds, SNR ca 590. (2 of 3 images, one was 170, clouds?)
V-Filter, 10 seconds, SNR 440 (3 of 3)
R-filter, 10 seconds, SNR 420 (3 of 3)
I-filter, 15 seconds, SNR 270 (3 of 3)

Value added will be photometry of five additional variables in the field that have SNR-values sufficient for quality photometry: NSV 360, V0551 Cas, and V1010 Cas (all above SNR = 300 in V), V0644 (SNR = 98-100 in V, 120 in B), BD+59 155/TIC 255802766, a recently discovered eclipsing variable. V0644 Cas, a CST A8V non-variable star of similar color to HD 5501, is added as a quality check.

Our photometry to date has used HD236580 (000-BPQ-434, “86,” and BD+59 159 (000-BPQ-436, “99) as the comparison and check stars. These are somewhat mismatched with the program star in color, but the vicinity of HD5501 is a minefield of variables and these have the virtue of stability. Data will be analyzed and transformed by aggregated image sets in VPhot.