Astrophotography by Anthony Ayiomamitis

Differential Photometry - WASP-24 in Virgo

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 400 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-24b in Virgo depicted below is the latest exoplanet discovered, having being announced on April 14/2010 along with another four new discoveries, and represents the twenty-fourth discovery by the WASP (Wide Angle Search for Planets) team. WASP-24b is characterized with a mass 1.032 times that of Jupiter while its radius is equivalent to 1.104 Jupiter radii, thus making this exoplanet a very typical "hot Jupiter". WASP-24b requires 155.5 minutes to transit its parent star at a depth of 12 mmag (1.2%). The parent star, GSC 339:329-1, is an F8-9 star estimated to have a mass of 1.129 solar masses, a radius equivalent to 1.147 solar radii, a temperature of 6,075° K and to lie at a distance of 1080 light-years away with a visual magnitude of 11.3. Further details regarding WASP-24 and WASP-24b are available in the paper published by the discovery team led by Rachel Street et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 339:432 (mag 10.47) and GSC 339:520 (mag 10.65) respectively. The light curve below is the first amateur result of this newly discovered exoplanet in the constellation of Virgo.

Image Details
Light Curve for Exoplanet WASP-24b
Imaging Details
Parent Star:
USNO-B1.0 0923-0348089

GSC/SAO Catalog:
GSC 339:329-1

Constellation:
Virgo

RA / Dec:
15h 08m 51.72s /
+02° 20' 36.1"


Magnitude:
11.3

Distance:
1080 light-years
Exoplanet:
WASP-24b

Period:
2.3412083 + 0.0000049 d

Transit Duration:
155.5 mins

Transit Depth:
12 mmag

Minimum Mass:
1.032 MJup

Radius:
1.104 RJup

Pred Transit Details:
Ingress :  22:08 UT
Mid-trans :  23:26 UT
Egress :  00:43 UT
Date:
Apr 22, 2010
00:00:01 - 04:30:58 UT+3


Location:
Athens, Greece

Equipment:
AP 160 f/7.5 Starfire EDF
AP 1200GTO GEM
SBIG ST-10XME
SBIG CFW-10
SBIG Lum filter


Integrations:
Lum :  129 x 120 sec
Dark :  015 x 120 sec
Flat :  ~19,900 ADU
Binning :  2x2

Temperatures:
Ambient : + 15.0 ° C
CCD Chip : - 20.0 ° C

Software:
CCDSoft V5.00.195
AIP4Win V2.2


Processing:
Reduction
Differential Photometry