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The Cepheid period-luminosity (PL) relation (or Leavitt law) has served as the first rung of the most widely used extragalactic distance ladder and is central to the determination of the local value of the Hubble constant ($H_0$). We investigate the influence of metallicity on Cepheid brightness, a term that significantly improves the overall fit of the distance ladder, to better define its wavelength dependence. To this aim, we compare the PL relations obtained for three Cepheid samples having distinct chemical composition (in the Milky Way and Magellanic Clouds) and focusing on the use of improved and recent data while covering a metallicity range of about 1 dex. We estimate the metallicity effect (hereafter $\gamma$) in 15 filters from mid-IR to optical wavelengths, including five Wesenheit indices, and we derive a significant metallicity term in all filters, in agreement with recent empirical studies and models, in the sense of metal-rich Cepheids being brighter than metal-poor ones. We describe the contribution of various systematic effects in the determination of the $\gamma$ term. We find no evidence of $\gamma$ changing over the wavelength range $0.5-4.5 \, \rm \mu m$, indicating that the main influence of metallicity on Cepheids is in their luminosity rather than color. Finally, we identify factors that sharpen the empirical constraints on the metallicity term over past studies, including corrections for the depth of the Magellanic Clouds, better-calibrated Cepheid photometry, improved Milky Way extinction estimates, and revised and expanded metallicity measurements in the LMC., Comment: Accepted to ApJ

Cepheids are pulsating variable stars which play a key role as primary distance indicators thanks to the empirical relation between their pulsation period and intrinsic luminosity, the period-luminosity relation. This law is used to calibrate the brightness of type-Ia supernovaein nearby galaxies, which is in turn used to measure the distance to galaxies in the Hubble flow. This provides an estimate of the current expansion rate of the Universe, known as the Hubble constant (H0). In recent years, a significant tension of at least 4\(\sigma\)has arisen between the early universe measurement of H0 from the Planck satellite, assuming a \(\Lambda\)CDM model, and the late universe direct measurements based on Cepheid distances. The persistence of this tension would imply new physics beyond the standard model of cosmology: it is therefore critical to improve the period-luminosity calibration with precise and reliable Cepheid distance measurements. The GaiaCollaboration recently published trigonometric parallaxes for 1.7 billion stars, allowing for a remarkable improvement in the precision of the distance scale. However, Cepheid parallaxes suffer from calibration issues due to their variability and important brightness. In this thesis, I present an alternative method for calibrating the period-luminosity relation using Cepheid companions and host open clusters, which are not subject to these issues. By adopting these close and unbiased companion stars to determine the distance to Cepheids, I calibrate the period-luminosity relation in the Milky Way and re-evaluate the local value of the Hubble constant. Finally, I study the relation between Cepheid magnitudes and their metal abundance by comparing the Cepheids of the Milky Way and those of the Magellanic Clouds. I conclude that metal-rich Cepheids are brighter than metal-poor ones, with a stronger effect in near-infrared than in optical. This effect may impact the measurement of the Hubble constant and will have to be taken into account more precisely in the future, to better constrain the calibration of the extragalactic distance scale.

We present HST photometry of 17 Cepheids in open clusters and their mean parallaxes from Gaia EDR3. These parallaxes are more precise than those from individual Cepheids (G300 stars per cluster. Cluster parallaxes also have smaller systematic uncertainty because their stars lie in the range (G>13 mag) where the Gaia parallax calibration is most comprehensive. Cepheid photometry employed in the period--luminosity relation was measured using the same instrument(WFC3) and filters(F555W,F814W,F160W) as extragalactic Cepheids in SNIa hosts. We find no evidence of residual parallax offset in this magnitude range, zp=-3+/-4 muas, consistent with Lindegren:2021b and most studies. The Cepheid luminosity (P=10d, solar-metallicity) in the HST near-infrared, Wesenheit system derived from the cluster sample is M_{H,1}^W=-5.902+/-0.025 and -5.890+/-0.018 mag with or without simultaneous determination of a parallax offset, respectively. These results are similar to measurements from field Cepheids, confirming the accuracy of the Gaia parallaxes over a broad range of magnitudes. The SH0ES distance ladder calibrated solely from this sample gives H_0=72.8+/-1.3 and H_0=73.2+/-1.1 km/s/Mpc with or without offset marginalization; combined with all anchors we find H_0=73.01+/-0.99 and 73.15+/-0.97, respectively, a 5% or 7% reduction in the uncertainty and a 5.3 sigma Hubble Tension relative to Planck+LambdaCDM. It appears increasingly difficult to reconcile two of the best measured cosmic scales, parallaxes from Gaia and the angular size of the acoustic scale of the CMB, using the simplest form of LambdaCDM to join the two., 12 pages, submitted to ApJ, comments welcome

We report observations from the Hubble Space Telescope (HST) of Cepheid variables in the host galaxies of 42 Type Ia supernovae (SNe Ia) used to calibrate the Hubble constant (H 0). These include the complete sample of all suitable SNe Ia discovered in the last four decades at redshift z ≤ 0.01, collected and calibrated from ≥1000 HST orbits, more than doubling the sample whose size limits the precision of the direct determination of H 0. The Cepheids are calibrated geometrically from Gaia EDR3 parallaxes, masers in NGC 4258 (here tripling that sample of Cepheids), and detached eclipsing binaries in the Large Magellanic Cloud. All Cepheids in these anchors and SN Ia hosts were measured with the same instrument (WFC3) and filters (F555W, F814W, F160W) to negate zero-point errors. We present multiple verifications of Cepheid photometry and six tests of background determinations that show Cepheid measurements are accurate in the presence of crowded backgrounds. The SNe Ia in these hosts calibrate the magnitude–redshift relation from the revised Pantheon+ compilation, accounting here for covariance between all SN data and with host properties and SN surveys matched throughout to negate systematics. We decrease the uncertainty in the local determination of H 0 to 1 km s−1 Mpc−1 including systematics. We present results for a comprehensive set of nearly 70 analysis variants to explore the sensitivity of H 0 to selections of anchors, SN surveys, redshift ranges, the treatment of Cepheid dust, metallicity, form of the period–luminosity relation, SN color, peculiar-velocity corrections, sample bifurcations, and simultaneous measurement of the expansion history. Our baseline result from the Cepheid–SN Ia sample is H 0 = 73.04 ± 1.04 km s−1 Mpc−1, which includes systematic uncertainties and lies near the median of all analysis variants. We demonstrate consistency with measures from HST of the TRGB between SN Ia hosts and NGC 4258, and include them simultaneously to yield 72.53 ± 0.99 km s−1 Mpc−1. The inclusion of high-redshift SNe Ia yields H 0 = 73.30 ± 1.04 km s−1 Mpc−1 and q 0 = −0.51 ± 0.024. We find a 5σ difference with the prediction of H 0 from Planck cosmic microwave background observations under ΛCDM, with no indication that the discrepancy arises from measurement uncertainties or analysis variations considered to date. The source of this now long-standing discrepancy between direct and cosmological routes to determining H 0 remains unknown.

The nature of circumstellar envelopes (CSE) around Cepheids is still a matter of debate. The physical origin of their infrared (IR) excess could be either a shell of ionized gas, or a dust envelope, or both. This study aims at constraining the geometry and the IR excess of the environment of the long-period Cepheid $\ell$ Car (P=35.5 days) at mid-IR wavelengths to understand its physical nature. We first use photometric observations in various bands and Spitzer Space Telescope spectroscopy to constrain the IR excess of $\ell$ Car. Then, we analyze the VLTI/MATISSE measurements at a specific phase of observation, in order to determine the flux contribution, the size and shape of the environment of the star in the L band. We finally test the hypothesis of a shell of ionized gas in order to model the IR excess. We report the first detection in the L band of a centro-symmetric extended emission around l Car, of about 1.7$R_\star$ in FWHM, producing an excess of about 7.0\% in this band. In the N band, there is no clear evidence for dust emission from VLTI/MATISSE correlated flux and Spitzer data. On the other side, the modeled shell of ionized gas implies a more compact CSE ($1.13\pm0.02\,R_\star$) and fainter (IR excess of 1\% in the L band). We provide new evidences for a compact CSE of $\ell$ Car and we demonstrate the capabilities of VLTI/MATISSE for determining common properties of CSEs. While the compact CSE of $\ell$ Car is probably of gaseous nature, the tested model of a shell of ionized gas is not able to simultaneously reproduce the IR excess and the interferometric observations. Further Galactic Cepheids observations with VLTI/MATISSE are necessary for determining the properties of CSEs, which may also depend on both the pulsation period and the evolutionary state of the stars., Comment: 13 pages, 8 figures, accepted in Astronomy and Astrophysics

We present time-series photometry of 21 nearby Type II Cepheids in the near-infrared J, H and Ks passbands. We use this photometry, together with the Third Gaia Early Data Release parallaxes, to determine for the first time period-luminosity relations (PLRs) for Type II Cepheids from field representatives of these old pulsating stars in the near-infrared regime. We found PLRs to be very narrow for BL Herculis stars, which makes them candidates for precision distance indicators. We then use archival photometry and the most accurate distance obtained from eclipsing binaries to recalibrate PLRs for Type II Cepheids in the Large Magellanic Cloud (LMC). Slopes of our PLRs in the Milky Way and in the LMC differ by slightly more than 2{\sigma} and are in a good agreement with previous studies of the LMC, Galactic Bulge and Galactic Globular Clusters Type II Cepheids samples. We use PLRs of Milky Way Type II Cepheids to measure the distance to the LMC and we obtain a distance modulus of 18.540$\pm$0.026(stat.)$\pm$0.034(syst.)mag in the WJKs Wesenheit index. We also investigate the metallicity effect within our Milky Way sample and we find rather significant value of about -0.2mag/dex in each band meaning that more metal-rich Type II Cepheids are intrinsically brighter than their more metal-poor counterparts, in agreement with the value obtained from Type II Cepheids in Galactic Globular Clusters. The main source of systematic error on our Milky Way PLRs calibration and the LMC distance is the current uncertainty of the Gaia parallax zero point., Comment: Accepted for ApJ

Using a sample of 81 galactic, detached eclipsing binary stars we investigated the global zero-point shift of their parallaxes with the Gaia Data Release 2 (DR2) parallaxes. The stars in the sample lay in a distance range of 0.04-2 kpc from the Sun. The photometric parallaxes {\pi}_Phot of the eclipsing binaries were determined by applying a number of empirical surface brightness - color (SBC) relations calibrated on optical-infrared colors. For each SBC relation we calculated the individual differences d{\pi}_i = ({\pi}_Gaia - {\pi}_Phot)_i and then we calculated unweighted and weighted means. As the sample covers the whole sky we interpret the weighted means as the global shifts of the Gaia DR2 parallaxes with respect to our eclipsing binary sample. Depending on the choice of the SBC relation the shifts vary from -0.094 mas to -0.025 mas. The weighted mean of the zero-point shift from all colors and calibrations used is d{\pi} = -0.054 +/- 0.024 mas. However, the SBC relations based on (B-K) and (V-K) colors, which are the least reddening dependent and have the lowest intrinsic dispersions, give a zero-point shift of d{\pi} = -0.031 +/- 0.011 mas in full agreement with results obtained by Lindegren et al. and Arenou et al. Our result confirms the global shift of Gaia DR2 parallaxes of d{\pi} = -0.029 mas reported by the Gaia team, but we do not confirm the larger zero-point shift reported by a number of follow-up papers., Comment: Like original version but with updated grant numbers

X-ray observations have been made of a sample of 20 classical Cepheids, including two new observations (Polaris and {\it l} Car) reported here. The occurrence of X-ray flux around the pulsation cycle is discussed. Three Cepheids are detected ($\delta$ Cep, $\beta$ Dor, and Polaris). X-rays have also been detected from the low--mass F, G, and K companions of 4 Cepheids (V473 Lyr, R Cru, V659 Cen, and W Sgr), and one hot companion (S Mus). Upper limits on the X-ray flux of the remaining Cepheids provide an estimate that 28\% have low mass companions. This fraction of low--mass companions in intermediate mass Cepheids is significantly lower than expected from random pairing with the field IMF. Combining the companion fraction from X-rays with that from ultraviolet observations results in a binary/multiple fraction of 57\% $\pm$12\% for Cepheids with the ratios q $>$ 0.1 and separations a $>$ 1 au. This is a lower limit since M stars are not included. X-ray observations detect less massive companions than other existing studies of intermediate mass stars. Our measured occurrence rate of unresolved, low-mass companions to Cepheids suggests that intermediate-period binaries derive from a combination of disk and core fragmentation and accretion. This yields a hybrid mass-ratio distribution that is skewed toward small values compared to a uniform distribution but is still top-heavy compared to random pairings drawn from the IMF., Comment: Accepted by ApJ