Magellanic Cloud Investigations. III. The LMC Bar

Abstract

Three -color photographic photometry has been carried out for three small regions at the west end of the LMC bar. Photoelectric calibration observations by Bok and Tifft for 26 stars were utilized. Program stars were selected so that the photographic photometry is uniformly representative to V = 16.6 magnitude in one region and to V = 18.0 magnitude in two smaller ones. The results include more than 600 stars with B and V and more than 160 of the brighter stars with U. Contamination by foreground stars is minimal since the regions are small. Color- magnitude diagrams show a narrow vertical blue sequence 0.25 magnitude wide extending from V = 14.5 to V = 18. The location of this sequence indicates about 0.13 magnitude of uniform reddening in the area. Red supergiants are seen between magnitude 14 and 16. A strong component of fainter red giants indicative of an older population is seen below magnitude 16. The red giants extend to B -V = 1.9. After reddening corrections the reddest normal giants have B -V = 1.8. A few very red stars, B -V = 2.2 to 2.8, are seen near V = 16. Two bright Harvard Variable Cepheids fall within the region under study. Light curves and color curves for both of these stars were obtained. The Cepheids lie to the red of the instability strip consistent with other local reddening effects. Fainter bright giants of intermediate color, including possible Cepheids, have been isolated using the color -color diagram. A period and light curve for one star has been derived. Matching the old giant star population a differential true modulus of 0.65 ±0.1 is derived between the SMC and LMC. For an SMC modulus of 19.1, the LMC modulus is 18.45. The three Cepheids in the field yield a modulus of 18.45 ±0.1 when the P -L -C relationship calibration of Sandage and Tammann (1969) is used. Outlying Cepheids as studied by Gascoigne (1969) are approximately 0.2 magnitudes fainter. Real intrinsic variation in Cepheid luminosities outside of the P -L -C relationship is possible but not large. An overall mean LMC true modulus of 18.5 ±0.1 is indicated.