Annu. Rev. Astron. Astrophys. 1999. 37: 445-486 Copyright © 1999 by Annual Reviews. All rights reserved

4. CALIBRATION OF THE CEPHEID P-L RELATION

Baade's plan to recalibrate the zero-point of the Cepheid P-L relation using the difference in apparent magnitude between the Cepheids and RR Lyrae stars in M31 (Baade 1956a) was frustrated by the strong dependence of M(RR) on [Fe/H] just discussed. The calibration of the effect is still in contention at the 0.3 mag level, and until the calibration of M(RR) is beyond doubt, Baade's route to the Cepheid P-L calibration will remain uncertain.

Remarkably, an independent method of calibration of the P-L relation of classical Cepheids came into play in 1958. Peter Doig (1925) first pointed out that the Cepheid U Sgr was probably a member of the galactic cluster M25. Irwin (1955) rediscovered this fact, which, together with Eggen's knowledge that NGC 7790 contains three Cepheids, opened the way to use photometric parallaxes of the parent open clusters to calibrate the Cepheids.

A program of photometry and spectroscopy was begun by Arp, Kraft, and the present writer in 1958, both at Mount Wilson and at Palomar, to obtain the necessary data on distances and reddenings to carry a new calibration of the P-L relation to a preliminary conclusion.

By 1961, six calibrators had been completed - CF Cas (Sandage 1958c), EV Sct (Arp 1958), DL Cas (Arp et al 1959), CV Mon (Arp 1960b), U Sgr (Sandage 1960, Johnson 1960, Wampler et al 1961), and S Nor (Irwin 1958, Fernie 1961). Kraft (1961) discussed all of the photometry of the parent open clusters that existed to 1961, redetermining the cluster reddenings and distance moduli on a uniform basis.

Kraft's new reductions, plus four additional long-period Cepheids in the h and chi Per complex adopted as members on the basis of Eggen's (1965) discussion, were used in a calibration of the P-L relation whose shape was redefined by combining relative relations from M31, NGC 6822, LMC, and SMC (Sandage & Tammann 1968). The double Cepheid CE Cas a and b in NGC 7790 was added later (Sandage & Tammann 1969) following the difficult photometry of the individual components that are separated by only 2.3 arcsec.

The resulting 1968 zero-point, based on only nine fundamental calibrators is, remarkably, within 0.02 mag of the recent Hipparcos zero-point as defined by Feast & Catchpole (1998). It is also brighter by between 0.07 and 0.10 mag than the Feast & Walker (1987) calibration, which itself is an average of 0.11 mag fainter than the Hipparcos calibration (Sandage & Tammann 1998, Seggewiss 1998), suggesting a small systematic error in that calibration of order 0.1 mag.

Hence, the 1961 and 1969 Cepheid calibrations fully supported and refined Baade's 1952 discovery of the error in the previous calibrations from Hertzsprung (1913), to Shapley (1918), to Wilson (1939). Hence, by 1970 Baade's 1948 program was complete.

The dream of the astronomers at the Palomar dedication in 1948, expressed with conviction by Vannevar Bush in his address "Two Observatories Operate as One," had in fact come true.