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2.5.2. Further checks for M31

A number of distance estimates are available for the nearest giant spiral M31, and they are shown in TABLE 6 (the underlined numbers are the zero point). Stanek & Garnavich (1998) applied the red clump method to M31, and obtained m - M = 24.47 ± 0.06, which agrees with the M31 modulus 24.44 ± 0.10 from the Cepheid based on the (m - M)LMC = 18.5 calibration, whereas the same method gives 18.1 for the LMC distance. Namely, M31 - LMC largely disagrees between the two. This discrepancy might be ascribed to a metallicity effect of either Cepheids or red clumps, or to the systematic error of either indicator. Mochejska et al. (1999) ascribed it to the error of the Cepheid distance from a crowded stellar population. On the other hand, tip of giant branch (TRGB) gives (m - M)M31-LMC in agreement with the value derived from the Cepheid. The difference from PNLF is also consistent. The value from RR Lyr, however, is consistent with that from red clumps (the numbers in the Table is derived using (4) for the zero point). The results are dichotomous again.

Table 6. Relative distance of M31 to LMC

method M31 LMC M31 - LMC refs.

Cepheid 24.44±0.10 18.5 5.94±0.10 Ferrarese et al. 1999b
red clump 24.47±0.06 18.07±0.04 6.40±0.07 Stanek & Garnavich 1998
TRGB 24.41±(0.19) 18.5 5.91±(0.19) Ferrarese et al. 1999b
RR Lyr (B) 24.50±(0.15) 18.30 6.20± Pritchet & vd Bergh 1989
PNLF 24.44 18.56±0.18 5.82±0.18 Jacoby et al. 1990b

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