Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.8202j&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #82.02; Bulletin of the American Astronomical Society, Vol. 24, p.1255
Astronomy and Astrophysics
Astronomy
Scientific paper
The Sd galaxy IC 4182 was host to the Type Ia supernova, SN 1937C. Being relatively nearby and well observed, this supernova has been used over the years to set the absolute magnitude, M_B, for the Type Ia class. Until recently, the best distance modulus estimate was given by Sandage and Tammann (1982) as 28.2+/-0.2 from photographic B and V photometry of the brightest red stars. This result was challenged by Pierce, Ressler, and Shure (1992) who derived (m-M)=27.0+/-0.2 using a similar technique (photometry of brightest red stars), but with CCD and IR arrays in the I and K bands where the effects of extinction are much reduced. Even more recently, Sandage et al. (1992) determined the apparent distance modulus to IC 4182 with unprecented precision by identifying 27 Cepheid variables with HST. They find (m-M)=28.47+/-0.05. In order to assess the importance of internal extinction in IC 4182, we have obtained UBVRI images at the KPNO 2.1m and 4m, and CFH 3.6m telescopes. By comparing the color-magnitude diagrams for IC 4182 with those for Local Group galaxies (M33, IC 1613, and NGC 6822) where the reddening has been studied in detail, we find that the young stars in IC 4182 are reddened by E(B-V) ~ 0.32 mag. This comparison eliminates the effects of stellar evolution to first order. The effects of crowding, which can also artificially redden the blue stars, are evaluated by comparing the high resolution CFHT data with the moderate resolution KPNO data. While we find that the true distance modulus for IC 4182 must be reduced from 28.47 to ~ 27.4, the net effect on M_B for SN 1937C cannot be addressed directly since its colors are not well determined. The Type Ia absolute calibration is a very strong function of the extinction because it affects both the distance and the luminosity correction (R_B=4 E_B-V). Thus, even a color uncertainty of 0.1 in B-V for either the Cepheids or SN 1937C is highly significant.
Jacoby George H.
Massey Pedro
Pierce Michael J.
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