Cepheid Radii and Distances

Abstract

Laney & Stobie (1995) (LS95) have determined Baade-Wesselink radii for 49 galactic Cepheids using BVRI and JHK photometry. Four of the possible magnitude-colour combinations were studied in detail -namely, (V,B-V), (V,VIc), (K,V-K) and (K,J-K). In that paper and in the present note we have used the maximum likelihood method of Balona (1977) as recently modified to allow for the nonlinear effects of the non-negligible radius excursions of Cepheids.One major advantage of infrared photometry is that the light variation at K is dominated by the change in surface area, unlike the light variation at V which is dominated by temperature changes. Another is that (as shown both empirically and by model atmospheres) one may use a J-K or V-K colour index to calculate the surface brightness at K without having to worry very much about the effects of varying surface gravity or Doppler broadening velocity, while the same is definitely not true when optical colour indices are used to calculate the surface brightness at V.From radius displacement curves it is clear that the systematic and random errors are least in the case of the infrared solutions, (K,V-K) and (K,J-K). (V,B-V) and (V,V-Ic) solutions have large systematic and random errors. Furthermore, "optical" radius determinations appear quite sensitive to whether the phase of the ascending branch is excluded from the solution. "Infrared" solutions show no such effect.The most accurate period-radius relation was derived by averaging the (K,V-K) and (K,J-K) radius solutions. (V,B-V) and (V,V-I) solutions gave systematically incorrect radius determinations because of the effects of surface gravity and microturbulence variations throughout the pulsation cycle. (V,VRj) radius solutions (calculated for about half of our sample) showed systematic errors essentially identical to those seen in (V,V-Ic) solutions.Given the availability of radial velocities contemporaneous with our JHK data for LMC and SMC Cepheids, we have calculated "infrared" radii for as many Cepheids as possible, despite the limited quality and limited precision of the data compared to our galactic sample. Contemporaneous radial velocities were available in the literature. Radius solutions were considered successful only if the surface brightness coefficients were within 0.6 of the expected value at a given period. No solutions for Cepheids with periods longer than 100 days were successful. For 5 LMC and 4 SMC Cepheids (HV 12815, 879, 2338, 2827, 5497, 837, 824, 11157, and 829), radii could be determined, and the adopted radii are plotted in Fig. 1, together with our "adopted" galactic Cepheid radii. The LMC and SMC Cepheid radii show no systematic difference from the galactic Cepheid 254 terms of use, available at https://www.cambridge.org/core/terms. https://doi

Pulsating Stars and the Distance Scale - II

“…For the LMC data discussed in section 7, the strip width at V is about 0.74 mag. c) The 3-D structure of the SMC, though seen most clearly in the available BVI(PLC) data, is confirmed by other Cepheid and non-Cepheid work (see, e.g., Laney & Stobie 1986). For discussions of the PLC and PL relations as well as reddening and metallicity determinations see Feast 1988Feast , 1989Feast , 1991 Barnes: I think you have been unfair to the Baade-Wesselink optical results on Cepheids by comparing (V -MV)BW uncorrected for metallicity with (V -MV)PLC corrected for metallicity.…”

“…It was established a long while ago from BVI photometry of LMC Cepheids (Martin et al 1979) that the scatter in the PL relation at V was intrinsic and not due either to observational error or differential reddening as had at one time been suggested. This showed, empirically, the need for a PLC relation, a result confirmed by Laney & Stobie (1986) who demonstrated the existence of a PLC relation in the infrared. A further, useful, confirmation comes from the results of the EROS project (Beaulieu 1995, Beaulieu et al 1995 which show rather clearly that an instability strip of finite width is necessary to interpret overtone pulsators in the LMC.…”

The Distances of the Magellanic Clouds

“…Within the errors, these zeropoints agree with those found via the Cepheid effective temperature scale (Pel 1978) and from the lunar occultation diameter of ζ Gem (Ridgway et al 1982). Gieren et al (1998) note that their period-radius relation is identical to that found by Laney & Stobie (1995b) despite the use of very different methods. di Benedetto (1997) adopts a steeper period-radius relation (slope 0.73 cf 0.68), more recent evidence (Bono, Caputo, & Marconi 1998) favors the shallower value.…”

“…The results from all three recent investigations are in reasonable agreement. Gieren et al (1988) obtain an LMC modulus of 18.46 (they prefer to apply no metallicity correction, but a correction of +0.06 is their suggested value), Laney & Stobie (1995b) find 18.58 ± 0.04 mag for the LMC and 19.00 ± 0.04 mag for the SMC, and di Benedetto (1997) derives 18.64 ± 0.02 for the LMC and 19.06 ± 0.03 for the SMC, where for the latter two studies we have here increased the author's moduli by 0.06 so that the galactic cluster zeropoint corresponds to a Hyades modulus of 3.33 mag. The zeropoint error is not included in those above.…”

Observational calibration of the projection factor of Cepheids

“…J, H and K near-infrared photometric light curves from Persson et al (2004), Laney & Stobie (1986), Welch et al (1984) and Storm et al (2004) were also collected. Finally, to complete the photometric sample, we also retrieved 3.5 and 4.5µm light curves from Spitzer observations (Scowcroft et al 2011).…”