On the Cepheid variables of nearby galaxies

Astronomy and Astrophysics – Astronomy

Scientific paper

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Astronomical Photometry, Cepheid Variables, Distance, Galaxies, Local Group (Astronomy), Charge Coupled Devices, Interstellar Extinction, Magellanic Clouds, Stellar Luminosity, Stellar Magnitude

Scientific paper

Five Cepheids in Sextans A and three in Sextans B, already discovered and studied by Sandage & Carlson (1982, 1985), have been measured into a new photometric CCD sequence in B. A zero point offset of 0.22 magnitudes has been detected with respect to previous photographic photometry (CCD magnitudes being brighter). Five additional Cepheid candidates in Sextans A and four in Sextans B have been identified and preliminary periods have been proposed. The new Cepheids allow to extend the period-luminosity relation to one magnitude fainter. For all the variables, single epoch V, R, and I magnitudes are derived. Apparent B, V, and I distance moduli relative to the Large Magellanic Cloud (LMC) have been calculated. Combining the data at different wavelength and assuming a true distance modulus of 18.5 mag for the LMC, we obtain a true distance modulus, corrected for the effects of interstellar extinction, of (m-M)0 = 25.71 +/- 0.20 for Sextans A and (m-M)0 = 25.63 +/- 0.21 for Sextans B. The resulting distances are 20% shorter than previously suggested by Sandage and Carlson. Adopting E(B-V) = 0.08 for the LMC, the interstellar reddening for the Cepheids in Sextans A is E(B-V) = 0.02, and in Sextans B E(B-V) = 0.01. Adopting the above distance moduli and reddening, the absolute magnitudes of the brightest stars are: (MB(3)) = -7.44 for the three brightest blue and (MV(3)) = -7.28 for the three brightest red stars in Sextans A; for Sextans B it is (MB(3)) = -6.39 and (MV(3)) = -6.43.

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