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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
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Topics
● Cepheids● NGC 6822● Photometry with IRAF● Future
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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
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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)
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NG
C 6
82
2 (
Wh
ban
d)
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● 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
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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
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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
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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
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● 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
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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
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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)
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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)