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The brightest stars in nearby galaxies. I. The color-magnitude diagram and luminosity function for IC 1613

1976; Institute of Physics; Volume: 81; Linguagem: Inglês

10.1086/111945

1538-3881

A. Sandage, B. Katem,

Astronomy and Astrophysical Research

The color-magnitude diagram for brightest stars in the Local Group galaxy IC 1613 is obtained from photometry that reaches B 21.5, in an effort to improve the absolute-magnitude calibration of brightest stars in late-type spiral and dwarf irregular galaxies just beyond the Local Group. The brightest blue star in IC 1613 is 22A with B = 17.00, B-V = -0.15; or MB = -7.55 and Mv = -7.37 if (m-M) =24.55 and (m -M)A V =24.52. The brightest red supergiant is variable V38 with B = 18.60, V = 16.49 at maximum, which gives M B = -5.95, M V = -8.03. The differential [ (M)] and integral [N(M)] luminosity functions are obtained from Baade's unpublished star counts. The shape of `p(M) agrees well with the mean van Rhijn-Luyten function for the solar neighborhood at the bright end. Stars to B =22 contribute only 20% of the total blue light of IC 1613. The slope of M) at the bright end is steep enough to explain the previously observed correlation of M B (first blue star) and the absolute magnitude of the parent galaxy as a statistical effect that results from the required normalization of (M) to the total galaxian luminosity. The shape of (M ) for a young star as- sociation in IC 1613 differs systematically from the mean (M) at the bright end, probably due to the Salpeter process that changes the birth-rate function into the observed (M) functions that exist in regions of older mean age.