Astrophotography by Anthony Ayiomamitis

Differential Photometry - WASP-33 in Andromeda

A variable star, as its name suggests, is a star whose magnitude varies intrinsically, in contrast to eclipsing binaries whose magnitude varies as a result of one star in the binary system eclipsing the other. True variables are one of five types, namely Mira stars, semiregular stars, cepheids, eruptive variables and, finally, cataclysmic variables. Minimum to maximum magnitude can range from days to many months with some variables displaying irregular periods.

A popular method for the study of variable stars, particularly short-term variables, is by the use of the technique known as "differential photometry". Rather than measure the (variable) magnitude of a variable star on an absolute scale, measurements are made over time relative to one or more non-variable star(s) and these differences are then plotted so as to study and illustrate the relative or differential change in magnitude. Due to the very large number of variables stars, the field of differential photometry represents one of the key fields in astronomy whereby the amateur astronomer can make a meaningful and long-lasting contribution to both science and astronomy.

More recently, the search for extrasolar planets (925 discovered so far) has identified yet another interesting application for the practice of differential photometry whereby the minute drops in magnitude of a star hosting an exoplanet are studied. Further details for the interested party are available here.

Note: The light curve for exoplanet WASP-33b in Andromeda depicted below is one of the latest (and larger) transiting exoplanets, having being announced in Apr/2010, and represents the thirty-third discovery by the WASP (Wide Angle Search for Planets) team. WASP-33b is characterized with a mass less than 4.1 times that of Jupiter while its radius is equivalent to 1.497 Jupiter radii, thus making this exoplanet one of the larger discoveries to-date. WASP-33b requires 163 minutes to transit its parent star at a depth of 15.1 mmag or 1.52%. With an orbital period of only 1.22 days, WASP-33b is also one of the most irradiated exoplanets found so far with an estimated temperature of 2710° K. WASP-33b is characterized with a retrograde orbit and is one of the few exoplanet finds so far where such an orbit has been discovered. The parent star, HD 15082, is an A5 star estimated to have a mass of 1.495 solar masses, a radius equivalent to 1.444 solar radii, a temperature of 7,430° K and to lie at a distance of 378 light-years away with a visual magnitude of 8.3. Further details regarding WASP-33 and WASP-33b are available in the paper published by the discovery team led by Collier Cameron et al here. For a recent paper describing the variable nature of the host star with pulsations suggestive of a delta-Scuti variable, see the paper by Herrero et al.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were SAO 55556 (mag 9.18) and SAO 55557 (mag 9.36) respectively.

Image Details
Light Curve for Exoplanet WASP-33b
Imaging Details
Parent Star:
WASP-33

GSC/SAO/HD Catalog:
HD 15082

Constellation:
Andromeda

RA / Dec:
02h 26m 51.06s /
+37° 33' 01.7"


Magnitude:
8.3

Distance:
378 light-years
Exoplanet:
WASP-33b

Period:
1.2198669 + 0:0000012 d

Transit Duration:
163 mins

Transit Depth:
15.1 mmag

Minimum Mass:
<4.1 MJup

Radius:
1.497 RJup

Pred Transit Details:
Ingress :  20:41 UT
Mid-trans :  22:02 UT
Egress :  23:24 UT


Date:
Oct 28-29, 2013
21:40:00 - 02:25:00 UT+2


Location:
Athens, Greece

Equipment:
AP 305/f3.8 Riccardi-Honders
AP 1200GTO GEM
SBIG ST-10XME
SBIG CFW-10
SBIG LRGB filters


Integrations:
Red :  229 x 60 sec
Dark :  010 x 60 sec
Flat :  ~22,200 ADU
Binning :  1x1

Temperatures:
Ambient : + 18.0 ° C
CCD Chip : - 17.5 ° C

Software:
CCDSoft V5.00.201
AIP4Win V2.4


Processing:
Reduction
Differential Photometry