Cepheid Metallicity in the Leavitt Law (C-MetaLL) survey: I. HARPS-N@TNG spectroscopy of 47 Classical Cepheid and 1 BL Her variables

Abstract

Classical Cepheids (DCEPs) are the most important primary indicators of the extragalactic distance scale. Establishing the dependence on metallicity of their period-luminosity and period-Wesenheit (𝑃𝐿𝑍/𝑃𝑊 𝑍) relations has deep consequences on the calibration of secondary distance indicators that lead to the final estimate of the Hubble constant (H 0 ). We collected high-resolution spectroscopy for 47 DCEPs plus 1 BL Her variables with HARPS-N@TNG and derived accurate atmospheric parameters, radial velocities and metal abundances. We measured spectral lines for 29 species and characterized their chemical abundances, finding very good agreement with previous results. We re-determined the ephemerides for the program stars and measured their intensity-averaged magnitudes in the 𝑉, 𝐼, 𝐽, 𝐻, 𝐾 𝑠 bands. We complemented our sample with literature data and used the Gaia Early Data Release 3 (EDR3) to investigate the 𝑃𝐿𝑍/𝑃𝑊 𝑍 relations for Galactic DCEPs in a variety of filter combinations. We find that the solution without any metallicity term is ruled out at more than the 5 𝜎 level. Our best estimate for the metallicity dependence of the intercept of the 𝑃𝐿𝐾 𝑠 , 𝑃𝑊 𝐽𝐾 𝑠 , 𝑃𝑊𝑉 𝐾 𝑠 and 𝑃𝑊 𝐻𝑉 𝐼 relations with three parameters, is −0.456±0.099, −0.465±0.071, −0.459±0.107 and −0.366±0.089 mag/dex, respectively. These values are significantly larger than the recent literature. The present data are still inconclusive to establish whether or not also the slope of the relevant relationships depends on metallicity. Applying a correction to the standard zero point offset of the Gaia parallaxes has the same effect of reducing by ∼22% the size of the metallicity dependence on the intercept of the PLZ/PWZ relations.

A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team

“…R21 derived −0.20 ± 0.13 from 66 Cepheids with HST photometry Ripepi et al (2021). used a larger ground-based sample of N = 317 fundamental and first-overtone pulsators to derive −0.37 ± 0.09 on the ground system with similar filters, steeper by ∼ 1.5 σ than R21 but less applicable here due to the presence of overtones and objects with low accuracy Groenewegen (2018).…”

Current Challenges in Cepheid Distance Calibrations Using Gaia EDR3

“…However, we should note that based on their sample size, ±15 µas was the maximum possible precision, which would be insufficient to determine an offset on the small scales of the aforementioned studies. Ripepi et al (2021) found a +15 ± 15 µas offset by comparing 7 of the Cepheids with HST scanning parallaxes from Riess et al (2018) to the EDR3 parallaxes, though they emphasize that this is a small sample where two of the stars have large scatter with respect to the rest of the sample. Their overall result indicates the L21b offsets sufficiently correct the parallaxes within the quoted uncertainties.…”

“…Kennicutt et al 1998;Freedman et al 2001;Sakai et al 2004;Storm et al 2004;Macri et al 2006;Scowcroft et al 2009;Riess et al 2016;Breuval et al 2021). However, many of these studies have error bars which are nearly as large as the quoted magnitude of the metallicity effect itself, and even very recent determinations based on Gaia EDR3 data have disagreed by a factor of two (Breuval et al 2021;Ripepi et al 2021). Further, some studies (e.g.…”

“…Only a few EDR3 studies using primarily very bright sources have been released, but several of them have speculated on the existence of a distinct zero-point offset for sources with G 11 mag (e.g., Riess et al 2021;Zinn 2021;Huang et al 2021), though the significance of the offset is disputed (Stassun & Torres 2021). Furthermore, the existence of such offsets has been shown to be degenerate with the derived metallicity effects on the PL relations (Ripepi et al 2021). With recent literature disagreeing on the exact values of these effects across photometric bands, we feel that an accurate calibration of the Cepheid distance scale via Milky Way Cepheids is not yet within reach.…”