INAF – Osservatorio Astronomico di BreraUniversità degli Studi di Milano
Photometry of the galaxy NGC 6822 searching for Cepheid variable stars
Alberto Chiodi's Graduation thesisInternal Supervisor: Laura Fracassini PasinettiExternal Supervisor: Elio Antonello
Merate – June 11th 2004
Topics
● Cepheids● NGC 6822● Photometry with IRAF● Future
Variable stars: Cepheids
● Instability strip
● P-L relation
(Tammann & Sandage 2003)Filled symbols: 25 Cepheids in open clustersopen symbols: 28 Cepheids with BBW distances
(Cox, 1974)
0.5 1.0 1.5 2.0 Log P
Cepheids
● Light curve of classical Cepheids
● Fourier analysis
● Comparison with teoretical models predictions
Light curve of δ Cephei (period: 5.366d)Abscissa: phase, arbitrary unitsOrdinate: apparent magnitude (arbitrary zero point)
(Cox, 1974)
NG
C 6
82
2 (
Wh
ban
d)
● White-light band
● Images: 94x8=752 + bias and flat
● Observing runs: 18-24 Oct 200002-08 Nov 200011-12 J ul 200109-13 Oct 200121-26 Oct 200106 Nov 2001
NGC 6822 or Barnard galaxy
type: irregular dwarf
Irr1 (Sandage 1961)
IB(s)m (De Vaucouleur et al 1964)
first observation: 1886 (Barnard)
first images: 1906 (Wolf)
first classification: 1925 (Hubble)
Distance: ~500 Kpc
Technical data:
Importance:● Proximity
● Sufficiently well resolved
● Comparison with other galaxies of the Local Group
Images reduction with IRAF packages
● Header correction
● Splitting images in chip
(one chip at a time)
● Trimming for overscan
● Bias correction
● Flat-field correction with normalized flat images
● Problem of fringes in Wh-light images
Original image
Bias image
Flat-field image
Defringing
● Fringes as an additive phenomenon
● Fringe images has to be normalized to the background sky
● Fringes are subtracted from the original image
Original image
Fringe image elaborated at ESO
Defringed image
● Photometry analysis using DIGIPHOT-DAOPHOT packages
● Problem of photometric Zero point correction of instrumental magnitudes
● A star catalogue of the whole field of NGC6822
● Search for the variable stars
● Construction
of the light-
curve
● Classification
of the variables
Future
Merging with previous observations
(Images of small parts of NGC6822 with the “Dutch” telescope (91 cm))
Previuos work on NGC 6822Imagines from Dutch(Mauro Stefanon thesis)
Present work withthe WFI
Future
Cepheids in NGC6822:
● Hubble (1925): 11
● Kayser (1967): 13
● Humphreys (1980) and Schmidt & Spear (1989) : 40
● Antonello, E., Fugazza, D., Mantegazza, L., Stefanon, M., Covino, S., (2002): 53
New Cepheids in field A of NGC 6822 (Antonello et al. 2002)
Future
● Luca Fossati's graduation thesis IC1613 on the determination of the P-L relation from B,V,R,I WFI observations of the galaxy
● Giovanni Grassi's graduation thesis on the other Wh-band images of NGC6822 obtained with the Dutch
● Determination of the P-L relation of NGC6822 in B, V, R, I bands
(using the light curves in the Wh-band, and just one or two points in B, V, R, I bands; extension of the Freedman method used for photographic light curves)
