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INAF – Osservatorio Astronomico di Brera Università 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 11 th 2004. INAF – Osservatorio Astronomico di Brera Università degli Studi di Milano.

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INAF – Osservatorio Astronomico di Brera Università degli Studi di Milano

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  1. Photometry of the galaxy NGC 6822searching for Cepheid variable stars Alberto Chiodi's Graduation thesis Internal Supervisor: Laura Fracassini Pasinetti External Supervisor: Elio Antonello Merate – June 11th 2004 INAF – Osservatorio Astronomico di BreraUniversità degli Studi di Milano

  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 δ 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: 94x8=752 + 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)

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