Astrophotography by Anthony Ayiomamitis

Differential Photometry - HAT-P-23 in Delphinus

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 750 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 HAT-P-23b in Delphinus depicted below is one of the latest transiting exoplanets, having being announced in Aug/2010 along with three other exoplanet finds, and represents the twenty-third discovery by the Hungarian-based HATNet Project team. HAT-P-23b is characterized with a mass 2.090 times that of Jupiter while its radius is equivalent to 1.368 Jupiter radii, thus making this exoplanet a massive "hot Jupiter". HAT-P-23b requires 130.75 minutes to transit its parent star at a depth of 7.6 millimag or 0.76%. It has a nearly circular orbit with a period of 1.212884 days with significant tidal effects at play due to its proximity to the host star. The parent star, GSC 1632:01396, is a class G0 dwarf estimated to have a mass of 1.13 solar masses, a radius equivalent to 1.20 solar radii, a temperature of 5,905° K and to lie at a distance of 1,282 light-years away with a visual magnitude of 12.432.

HAT-P-23b not only has one of the shortest orbital periods known (1.212884 days) but also one of the shortest in-fall times and which has been estimated to be 7.5 million years. Further details regarding HAT-P-23 and HAT-P-23b are available in the paper published by the discovery team led by Gaspar Bakos et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 1632:1319 (mag 11.5) and GSC 1632:1169 (mag 11.8) respectively.

Image Details
Light Curve for Exoplanet HAT-P-23b
Imaging Details
Parent Star:
HAT-P-23

GSC/SAO Catalog:
GSC 1632:1396

Constellation:
Delphinus

RA / Dec:
20h 24m 29.73s /
+16° 45' 44.3"


Magnitude:
12.432

Distance:
1,282 light-years
Exoplanet:
HAT-P-23b

Period:
1.212884 + 0.000002 d

Transit Duration:
130.75 mins

Transit Depth:
7.6 mmag

Minimum Mass:
2.090 MJup

Radius:
1.368 RJup

Pred Transit Details:
Ingress :  21:15 UT
Mid-trans :  22:21 UT
Egress :  23:26 UT


Date:
Jul 20-21, 2012
23:15:00 - 03:30:53 UT+3


Location:
Athens, Greece

Equipment:
AP 305/f3.8 Riccardi-Honders
AP 1200GTO GEM
SBIG ST-10XME
SBIG CFW10
SBIG LRGB filters


Integrations:
Lum :  207 x 60 sec
Dark :  015 x 60 sec
Flat :  ~22,000 ADU
Binning :  1x1

Temperatures:
Ambient : + 22.0 ° C
CCD Chip : - 15.0 ° C

Software:
CCDSoft V5.00.201
AIP4Win V2.4.0


Processing:
Reduction
Differential Photometry