Astrophotography by Anthony Ayiomamitis

Differential Photometry - WASP-3 in Lyra

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 (275 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-3b in Lyra depicted below is one of the latest transitting exoplanets, having being announced in early 2008, and which represents the third discovery of the WASP (Wide Angle Search for Planets) team. It is characterized with a mass nearly twice that of Jupiter and a very brief orbital period of only 44.3 hours. WASP-3b requires 160 minutes to transit its parent star at a depth of 1.14%. The parent star, GSC 2636:195, is estimated to have a mass of 1.24 solar masses, a radius equivalent to 1.31 solar radii and a temperature of 6,400 K. The high mass observed for WASP-3b in combination with its proximity to its hot parent star make it one of the largest and hottest exoplanets discovered so far. The light curve below is characterized with a slight increase in brightness following egress and which has been observed elsewhere as well, thus suggesting the possible existence of a ring system leading to forward scatter. Further details regarding WASP-3 and WASP-3b are available in the paper published by the discovery team led by Don Pollacco et al (click here).

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 2632:1381 (mag 10.70) and GSC 2636:246 (mag 10.90) respectively.

Image Details
Light Curve for Exoplanet WASP-3b
Imaging Details
Parent Star:
WASP-3

GSC Catalog:
GSC 2636:195

Constellation:
Lyra

RA / Dec:
18h 34m 31.62s /
+35 39' 41.4"


Magnitude:
10.64

Distance:
727 light-years
Exoplanet:
WASP-3b

Period:
1.846834 + 0.000002 d

Transit Duration:
159.84 minutes

Transit Depth:
11.4 mmag

Minimum Mass:
1.76 MJup

Radius:
1.31 RJup

Distance:
730 light-years

Pred Transit Details:
Ingress :  20:35 UT
Mid-trans :  21:56 UT
Egress :  23:18 UT

Moon :  10-days old
Date:
Jun 13-14, 2008
23:00:00 - 03:00:15 UT+3


Location:
Athens, Greece

Equipment:
AP 160 f/7.5 Starfire EDF
AP 1200GTO GEM
SBIG ST-2000XM
SBIG CFW-10
Baader IR Pass


Integrations:
Infrared :  290 x 45 sec
Dark :  015 x 45 sec
Flat :  ~16,500 ADU
Binning :  2x2

Temperatures:
Ambient : + 25.0 C
CCD Chip : - 07.5 C

Software:
CCDSoft V5.00.188
AIP4Win V2.2


Processing:
Reduction
Differential Photometry