Astrophotography by Anthony Ayiomamitis

Differential Photometry - WASP-1 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 (over 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-1b in Andromeda depicted below is one of the earlier transiting exoplanets, having being announced in Nov/2007, and represents the first discovery by the WASP (Wide Angle Search for Planets) team. WASP-1b is characterized with a mass 0.85 times that of Jupiter while its radius is equivalent to 1.39 Jupiter radii, thus making this exoplanet less dense than Jupiter. WASP-1b requires 226 minutes (3.75 hrs) to transit its parent star at a depth of 1.3%. The parent star, GSC 2265:0107, is an F7v star estimated to have a mass of 1.15 solar masses, a radius equivalent to 1.24 solar radii, a temperature of 6,200° K and to lie at a distance of 1,234 light-years away with a visual magnitude of 11.79. Further details regarding WASP-1 and WASP-1b are available in the paper published by the discovery team led by A. Collier Cameron et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 2265:0082 (mag 11.4) and GSC 2265:0112 (mag 10.30) respectively. The result below was taken during heavy humidity and will be revisited at first available opportunity with much better transparency.

Image Details
Light Curve for Exoplanet WASP-1b
Imaging Details
Parent Star:
WASP-1

GSC/SAO Catalog:
GSC 2265:0107

Constellation:
Pegasus

RA / Dec:
00h 20m 40.1s /
+31° 59' 23.7"


Magnitude:
11.79

Distance:
1,234 light-years
Exoplanet:
WASP-1b

Period:
2.51995 + 0.00001 d

Transit Duration:
224.4 mins

Transit Depth:
13 mmag

Minimum Mass:
0.85 MJup

Radius:
1.39 RJup

Pred Transit Details:
Ingress :  18:10 UT
Mid-trans :  20:03 UT
Egress :  21:57 UT


Date:
Nov 09-10, 2013
19:00:00 - 01:02:28 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:
Lum :  295 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