Calibrating the Cepheid period-luminosity relation from the infrared surface brightness technique

Abstract

Aims. We determine period-luminosity relations for Milky Way Cepheids in the optical and near-IR bands. These relations can be used directly as reference for extra-galactic distance determination to Cepheid populations with solar metallicity, and they form the basis for a direct comparison with relations obtained in exactly the same manner for stars in the Magellanic Clouds, presented in an accompanying paper. In that paper we show that the metallicity effect is very small and consistent with a null effect, particularly in the near-IR bands, and we combine here all 111 Cepheids from the Milky Way, the LMC and SMC to form a best relation. Methods. We employ the near-IR surface brightness (IRSB) method to determine direct distances to the individual Cepheids after we have recalibrated the projection factor using the recent parallax measurements to ten Galactic Cepheids and the constraint that Cepheid distances to the LMC should be independent of pulsation period. Results. We confirm our earlier finding that the projection factor for converting radial velocity to pulsational velocity depends quite steeply on pulsation period, p = 1.550− 0.186 log(P) in disagrement with recent theoretical predictions. We find PL relations based on 70 Milky Way fundamental mode Cepheids of M K = −3.33(±0.09)(log(P) − 1.0) − 5.66(±0.03), W VI = −3.26(±0.11)(log(P) − 1.0) − 5.96(±0.04). Combining the 70 Cepheids presented here with the results for 41 Magellanic Cloud Cepheids which are presented in an accompanying paper, we find M K = −3.30(±0.06)(log(P) − 1.0) − 5.65(±0.02), W VI = −3.32(±0.08)(log(P) − 1.0) − 5.92(±0.03). Conclusions. We delineate the Cepheid PL relation using 111 Cepheids with direct distances from the IRSB analysis. The relations are by construction in agreement with the recent HST parallax distances to Cepheids and slopes are in excellent agreement with the slopes of apparent magnitudes versus period observed in the LMC.

The VMC Survey – XIII. Type II Cepheids in the Large Magellanic Cloud★

“…However, several researchers suggested that the p-factor actually does depend on the period of the pulsator (see e.g. Barnes 2009;Laney & Joner 2009;Storm et al 2011a;Nardetto et al 2014, and references therein), hence, for example, different p-factor values should be used for BL Her and W Wir stars. Given the uncertainties on the projection factor discussed above, in the following we will adopt the HST -based distance for κ Pav, and the zero point of the different P L, P W and P LC relations will be estimated including or not the BW-based distances for SW Tau and V533 Cen.…”

Anchors for the cosmic distance scale: the Cepheid QZ Normae in the open cluster NGC 6067

“…A well-sampled calibration featuring long-period Cepheids is desirable, as remote extragalactic Cepheids observed using HST/LBT typically exhibit periods greater than 10 days (Shappee & Stanek 2011;Gerke et al 2011), as their shorter-period counterparts are fainter. A well-sampled long-period cluster Cepheid calibration would likewise foster stronger constraints on the p-factor, which is used to establish IRSB distances (Gieren et al 2005b;Storm et al 2011;Ngeow et al 2012;Joner & Laney 2012;Neilson et al 2012b). A multi-object fiber-fed spectrograph can survey numerous stars surrounding long-period Cepheids, and automated methods can classify the resulting sample and establish spectroscopic parallaxes (Manteiga et al 2009;Mugnes & Robert 2013).…”

Cepheid limb darkening, angular diameter corrections, and projection factor from static spherical model stellar atmospheres

“…The fourth case is the LMC+HST period-projection factor relation from Storm et al (2011a,b), though we could also include results from Mérand et al (2005), Groenewegen (2007) and Ngeow et al (2012). The LMC+HST Pp relation from Storm et al (2011a) has a slope measured from LMC Cepheids while the zero-point is measured from Galactic Cepheids. Our results appear to agree with the Nardetto et al (2009) relation, but not the observed relation, verifying that limb-darkening does not explain the difference between the observed and theoretical relations, hence we must consider different phenomena to explain the difference between the relations from Nardetto et al (2009) and Storm et al (2011a).…”

“…Storm et al (2011b) found similar results for LMC Cepheids, also using the infrared surface brightness technique, for different observations. Furthermore, Storm et al (2011a) used observations of Galactic Cepheids with known HST parallaxes to verify the steeper period dependence of the projection factor; we refer to this type of analysis as the LMC-HST Pp relation and p-factor. Mérand et al (2005) and Groenewegen (2007) measured the p-factor for Galactic Cepheids with known distances using interferometric and spectroscopic observations, both authors measured p = 1.27.…”