INAF

1
INAF Osservatorio Astronomico di
BreraUniversità degli Studi di Milano

• Photometry of the galaxy NGC 6822 searching for
  Cepheid variable stars
• Alberto Chiodi's Graduation thesis
• Internal Supervisor Laura Fracassini Pasinetti
• External Supervisor Elio Antonello
• Merate June 11th 2004

2
Topics

• Cepheids
• NGC 6822
• Photometry with IRAF
• Future

3
Variable stars Cepheids

• Instability strip
• P-L relation

4
Cepheids

• Light curve of classical Cepheids
• Fourier analysis
• Comparison with teoretical models predictions

Light curve of d Cephei (period
5.366d) Abscissa phase, arbitrary
units Ordinate apparent magnitude (arbitrary
zero point) (Cox, 1974)
5

• NGC 6822 (Wh band)

6

• White-light band
• Images 94x8752 bias and flat
• Observing runs

7

• Technical data
• Importance
• Proximity
• Sufficiently well resolved
• Comparison with other galaxies of the Local
  Group

• 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

8

• 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

9

• Defringing
• Fringes as an additive phenomenon
• Fringe images has to be normalized to the
  background sky
• Fringes are subtracted from the original image

10

• 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

11
Future

• Merging with previous observations
• (Images of small parts of NGC6822 with the
  Dutch telescope (91 cm))

12
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

13
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)