Period-luminosity Research Articles

Spectral energy distributions of classical Cepheids in the Magellanic Clouds

In this study, we constructed spectral energy distributions (SEDs) for a sample of 142 Large Magellanic Cloud (LMC) and 77 Small Magellanic Cloud (SMC) fundamental-mode classical Cepheids (CCs) using photometric data from the literature. When possible, the data were taken to be representative of mean light or averaged over the light curve. The sample was built from stars that either have a metallicity determination from high-resolution spectroscopy or have been used in Baade-Wesselink types of analyses, or have a radial velocity curve published in Gaia DR3 or have Walraven photometry, or have their light- and radial-velocity curves modelled by pulsation codes. The SEDs were fitted with stellar photosphere models to derive the best-fitting luminosity and effective temperature. Distance and reddening were taken from the literature. Only one star with a significant infrared (IR) excess was found in the LMC and none in the SMC. IR excess in MW CCs is not uncommon suggesting that IR excess may be more prominent in MW cepheids than in the Magellanic Clouds. The stars were plotted in a Hertzsprung-Russell diagram (HRD) and compared to evolutionary tracks for CCs and to theoretical instability strips. For the large majority of stars, the position in the HRD is consistent with the instability strip. Period-luminosity (PL) and period-radius relations were derived and compared to these relations in the MW. For a fixed slope, the zero point of the bolometric PL relation does not depend on metallicity, contrary to recent findings of a significant metallicity term when considering the PL relation in different photometric bands. The mass-luminosity (ML) relation is derived and it points to an over luminosity of about +0.3 dex with respect to a canonical ML relation. The most intriguing result concerns the flux-weighted gravity (FWG, a quantity derived from gravity and effective temperature) and its relation to period and luminosity. Both relations agree with theory, with the results for the MW and with the independent estimates from the six known LMC eclipsing binaries that contain CCs. However, the FWG (as determined from dedicated high-resolution spectroscopy for the sample) is too low by about 0.8 dex in 90% of the cases. Recent works on time-series data on 20 CCs in the MW were analysed finding a similar (but less extreme) offset in gravity and the FWG. Most importantly, other time-series data on the same 20 CCs are in full agreement with the FWG-period relation. The observed time-series of spectroscopic data and from a two-dimensional hydrodynamical cepheid model was used to investigate the so-called effective gravity, that is, the gravity corrected for a dynamical term related to the time derivative of the radial velocity. There is a reasonable good correspondence between the predicted effective gravity and the observed gravity as a function of pulsation phase, which would potentially allow for an independent estimate of the projection factor, but the dynamical term is too small to explain the overall difference between the observed (flux-weighted) gravity and the (flux-weighted) gravity derived from the SED modelling and stellar mass estimates.

Asteroseismic Modeling of 1153 Kepler Red Giant Branch Stars: Improved Stellar Parameters with Gravity-mode Period Spacings and Luminosity Constraints

This paper reports the estimated stellar parameters of 1153 Kepler red giant branch stars determined with asteroseismic modeling. We use radial-mode oscillation frequencies, gravity-mode period spacings, Gaia luminosities, and spectroscopic data to characterize these stars. Compared with previous studies, we find that the two additional observed constraints, i.e., the gravity-mode period spacing and luminosity, significantly improve the precision of fundamental stellar parameters. The typical uncertainties are 2.9% for the mass, 11% for the age, 1.0% for the radius, 0.0039 dex for the surface gravity, and 0.5% for the helium core mass, making this the best-characterized large sample of red giant stars available to date. With better characterizations for these red giants, we recalibrate the seismic scaling relations and study the surface term on the red giant branch. We confirm that the surface term depends on the surface gravity and effective temperature, but there is no significant correlation with metallicity.

Testing ultralow amplitude Cepheid candidates in the Galactic disk by TESS and Gaia

Ultralow amplitude (ULA) and strange mode Cepheids are thought to be pulsating variable stars that are near to or are at the edges of the classical instability strip. Until now, a few dozen such variable star candidates have been found both in the Large Magellanic Cloud and the Milky Way. For the present work, we studied six ULA Cepheid candidates in the Milky Way, identified by using CoRoT and 2MASS data. In order to identify their positions in the period–luminosity and color–magnitude diagrams, we used the Gaia DR3 parallax and brightness data of each star to calculate their reddening-free absolute magnitudes and distances. Furthermore, we calculated the Fourier parameters (e.g., period and amplitude) of the light variations based on CoRoT and TESS measurements, and established the long-term phase shifts for four out of six stars. Based on the results, we conclude that none of the six ULA Cepheid candidates are pulsating variable stars, but rather rotation-induced variable stars (rotational spotted and α2 Canum Venaticorum variables) that are either bluer or fainter than Cepheids would be.

Period–Luminosity Relations for Galactic Classical Cepheids in the Sloan Bands* * Based on data from the Las Cumbres Observatory.

We present the first period–luminosity (PL) and period–Wesenheit (PW) relations in the Sloan–Pan-STARRS g P1 r P1 i P1 bands for classical fundamental-mode Cepheids in the Milky Way. We used a relatively modest number of 76 stars for the PL and 84−85 stars for the PW relations calibration. The data for the project were collected with the network of 40 cm telescopes of the Las Cumbres Observatory, and Gaia Data Release 3 parallaxes were used for the calculations. These gri-band PL and PW relations calibrations will be a useful tool for distance determinations in the era of large sky surveys using the Sloan photometric system, especially with the near-future start of the Large Synoptic Survey of Space and Time.

Miras as a distance indicator in the CSST, JWST, and Gaia era

Mira variables are important distance indicators owing to the clear period–luminosity relation at infrared wavelengths. We have compiled a sample of 343 Galactic Miras selected from the American Association of Variable Star Observers database and Gaia Data Release 3. We used V-band observations from the American Association of Variable Star Observers to generate template light curves and derived mean magnitudes in the near-infrared JHK band from the Two Micron All-Sky Survey. Based on the Gaia-Two Micron All-Sky Survey diagram and the Gaia-LPV2 catalog, we classified 299 O-rich and 44 C-rich Miras. We fitted the Galactic period–luminosity relations for O-rich Miras in the near-infrared JHK band and estimated slopes (–2.74 ± 0.32, –3.14 ± 0.30, and –3.50 ± 0.25) and zero points (–5.58 ± 0.06 mag, –6.40 ± 0.06 mag, and –6.84 ± 0.05 mag) defined at logP (days) = 2.30, separately. Although the uncertainties of our derived Galactic PLRs for Miras are still larger than those of other galaxies, the multi-band sky survey conducted using the China Space Survey Telescope could help build more reliable and accurate PLRs for Miras, particularly in a near-infrared y band (927–1,080 nm).

New Near-infrared Period–Luminosity–Metallicity Relations for Galactic RR Lyrae Stars Based on Gaia EDR3 Parallaxes

We present new period–luminosity and period–luminosity–metallicity relations for Galactic RR Lyrae stars based on a sample of 28 pulsators located at distances of up to 1.5 kpc from the Sun. Near-infrared photometry was obtained at the Cerro Armazones Observatory, and parallaxes were taken from the Gaia Early Data Release 3. Relations were determined for the 2MASS JHK s bands and the W JK Wesenheit index. We compare our results with other calibrations available in the literature and obtain very good agreement with the photometry of RR Lyraes from the Large Magellanic Cloud (LMC) anchored using the distance to the Cloud, which is based on detached eclipsing binaries. We find that the dependence of absolute magnitudes on metallicity of 0.070 ± 0.042 (J band) to 0.087 ± 0.031 (W JK index) mag dex−1 for the population of fundamental pulsators (RRab) is in agreement with previously published phenomenological works. We perform a refined determination of distance to the LMC based on our new calibration and photometry from Szewczyk et al. We study the dependence of the fitted parameters of the fiducial relations and the LMC distance on the systematic parallax offset.

The use of double-mode RR Lyrae stars as robust distance and metallicity indicators

AbstractRR Lyrae stars are one of the primary distance indicators for old stellar populations such as globular clusters, dwarf galaxies and galaxies. Typically, fundamental-mode RR Lyr stars are used for distance measurements, and their accuracy is strongly limited by the dependence of absolute magnitudes on metallicity, in both the optical and infrared bands. Here, we report the discovery of a period–(period ratio)–metallicity relation for double-mode RR Lyr stars, which can predict metallicity as accurately as the low-resolution spectra. With theoretical and observational evidence, we propose that the period–luminosity relation of double-mode RR Lyr stars is not affected by the metallicity. Combining the Large Magellanic Cloud distance and Gaia parallaxes, we calibrate the zero point of the period–luminosity relation to an error of 0.022 mag, which means that in the best case double-mode RR Lyr stars can anchor galaxy distances to an accuracy of 1.0%. For four globular clusters and two dwarf galaxies, we obtain distances using double-mode RR Lyr stars with a distance accuracy of 2–3% and 1–2%, respectively. With future telescopes such as the China Space Station Telescope and the Vera C. Rubin Observatory, double-mode RR Lyr stars will be established as an independent distance ladder in the near-field universe.

Gaia Data Release 3

Context. The third Gaia data release provides photometric time series covering 34 months for about 10 million stars. For many of those stars, a characterisation in Fourier space and their variability classification are also provided. This paper focuses on intermediate- to high-mass (IHM) main sequence pulsators (M ≥ 1.3 M⊙) of spectral types O, B, A, or F, known as β Cep, slowly pulsating B (SPB), δ Sct, and γ Dor stars. These stars are often multi-periodic and display low amplitudes, making them challenging targets to analyse with sparse time series. Aims. We investigate the extent to which the sparse Gaia DR3 data can be used to detect OBAF-type pulsators and discriminate them from other types of variables. We aim to probe the empirical instability strips and compare them with theoretical predictions. The most populated variability class is that of the δ Sct variables. For these stars, we aim to confirm their empirical period-luminosity (PL) relation, and verify the relation between their oscillation amplitude and rotation. Methods. All datasets used in this analysis are part of the Gaia DR3 data release. The photometric time series were used to perform a Fourier analysis, while the global astrophysical parameters necessary for the empirical instability strips were taken from the Gaia DR3 gspphot tables, and the v sin i data were taken from the Gaia DR3 esphs tables. The δ Sct PL relation was derived using the same photometric parallax method as the one recently used to establish the PL relation for classical Cepheids using Gaia data. Results. We show that for nearby OBAF-type pulsators, the Gaia DR3 data are precise and accurate enough to pinpoint them in the Hertzsprung-Russell (HR) diagram. We find empirical instability strips covering broader regions than theoretically predicted. In particular, our study reveals the presence of fast rotating gravity-mode pulsators outside the strips, as well as the co-existence of rotationally modulated variables inside the strips as reported before in the literature. We derive an extensive period–luminosity relation for δ Sct stars and provide evidence that the relation features different regimes depending on the oscillation period. We demonstrate how stellar rotation attenuates the amplitude of the dominant oscillation mode of δ Sct stars. Conclusions. The Gaia DR3 time-series photometry already allows for the detection of the dominant (non-)radial oscillation mode in about 100 000 intermediate- and high-mass dwarfs across the entire sky. This detection capability will increase as the time series becomes longer, allowing the additional delivery of frequencies and amplitudes of secondary pulsation modes.

Synthetic Population of Binary Cepheids. II. The Effect of Companion Light on the Extragalactic Distance Scale

Because of their period–luminosity relation (PLR), classical Cepheids play a key role in the calibration of the extragalactic distance scale and the determination of the Hubble–Lemaître constant H 0. Recent findings show that the majority of classical Cepheids should be in binary or multiple systems, which might undermine their accuracy, as the extra—and unaccounted for—light from the companions of Cepheids causes a shift in the PLR. We quantify this shift using synthetic populations of binary Cepheids that we developed for this purpose, as described in Paper I of this series. We find that while all PLRs are shifted toward brighter values due to the excess light from the companions, the bias in the relative distance modulus between two galaxies hosting binary Cepheids can be either positive or negative, depending on the percentage of binary Cepheids in them. If the binarity percentage in the two galaxies is similar, the effect of binarity is canceled. Otherwise, it introduces a shift in the distance modulus of the order of millimagnitudes in the near-infrared passbands and Wesenheit indices, and tens of millimagnitudes in the visual domain; its exact value depends on the variant of the synthetic population (a unique combination of metallicity, star formation history, shape and location of the instability strip, and initial parameter distributions). Such shifts in the distance moduli to Type Ia supernova host galaxies introduce an additional statistical error on H 0, which however does not prevent measuring H 0 with a precision of 1%.

Sparse Logistic Regression for RR Lyrae versus Binaries Classification

RR Lyrae (RRL) stars are old, low-mass, radially pulsating variable stars in their core helium burning phase. They are popular stellar tracers and primary distance indicators since they obey well-defined period–luminosity relations in the near-infrared regime. Their photometric identification is not trivial; indeed, RRL star samples can be contaminated by eclipsing binaries, especially in large data sets produced by fully automatic pipelines. Interpretable machine-learning approaches for separating eclipsing binaries from RRL stars are thus needed. Ideally, they should be able to achieve high precision in identifying RRL stars while generalizing new data from different instruments. In this paper, we train a simple logistic regression classifier on Catalina Sky Survey (CSS) light curves. It achieves a precision of 87% at 78% recall for the RRL star class on unseen CSS light curves. It generalizes on out-of-sample data (ASAS/ASAS-SN light curves) with a precision of 85% at 96% recall. We also considered a L1-regularized version of our classifier, which reaches 90% sparsity in the light-curve features with a limited trade-off in accuracy on our CSS validation set and—remarkably—also on the ASAS/ASAS-SN light-curve test set. Logistic regression is natively interpretable, and regularization allows us to point out the parts of the light curves that matter the most in classification. We thus achieved both good generalization and full interpretability.

The period–luminosity relation for Mira variables in the Milky Way using Gaia DR3: a further distance anchor for H0

ABSTRACT Gaia DR3 parallaxes are used to calibrate preliminary period–luminosity relations of O-rich Mira variables in the 2MASS J, H, and Ks bands using a probabilistic model accounting for variations in the parallax zero-point and underestimation of the parallax uncertainties. The derived relations are compared to those measured for the Large and Small Magellanic Clouds, the Sagittarius dwarf spheroidal galaxy, globular cluster members, and the subset of Milky Way Mira variables with VLBI parallaxes. The Milky Way linear JHKs relations are slightly steeper and thus fainter at short period than the corresponding LMC relations, suggesting population effects in the near-infrared are perhaps larger than previous observational works have claimed. Models of the Gaia astrometry for the Mira variables suggest that, despite the intrinsic photocentre wobble and use of mean photometry in the astrometric solution of the current data reduction, the recovered parallaxes should be on average unbiased but with underestimated uncertainties for the nearest stars. The recommended Gaia EDR3 parallax zero-point corrections evaluated at $\nu _\mathrm{eff}=1.25\, \mu \mathrm{m}^{-1}$ require minimal ($\lesssim 5\, \mu \mathrm{as}$) corrections for redder five-parameter sources, but overcorrect the parallaxes for redder six-parameter sources, and the parallax uncertainties are underestimated at most by a factor ∼1.6 at $G\approx 12.5\, \mathrm{mag}$. The derived period–luminosity relations are used as anchors for the Mira variables in the Type Ia host galaxy NGC 1559 to find $H_0=(73.7\pm 4.4)\, \mathrm{km\, s}^{-1}\, \mathrm{Mpc}^{-1}$.

An Estimate of the Distance to AN Ser using an RR Lyrae Period–Luminosity Relationship

The relationship between the period of a Cepheid variable star and its absolute magnitude—the Period-Luminosity, or Leavitt Law—can be used with a measurement of apparent magnitude to measure the star’s distance. RR Lyrae stars are evolved low-mass stars with shorter periods and lower luminosities than Cepheids, but they are particularly useful for estimating distances to the oldest stellar clusters. We observed the brightness of an RR Lyrae star over time using the robotic telescopes of Las Cumbres Observatory to determine its period and apparent magnitude in four bands. We used that information and theoretical period–luminosity relationships for RR Lyrae stars to determine the distance to AN Ser to be 876 ± 25 pc. Our measured distance is smaller than that modeled by GAIA DR3 via stellar parallax, BP/RP spectra, and G-band photometry (959 pc).

Zwicky Transient Facility and Globular Clusters: The Period–Luminosity and Period–Wesenheit Relations for SX Phoenicis Variables in the gri Band

SX Phoenicis (SXP) variables are short-period pulsating stars that exhibit a period–luminosity (PL) relation. We derived the gri-band PL and extinction-free period–Wesenheit (PW) relations, as well as the period-color and reddening-free period-Q-index relations for 47 SXP stars located in 21 globular clusters, using the optical light curves taken from Zwicky Transient Facility. These empirical relations were derived for the first time in the gri filters except for the g-band PL relation. We used our gi-band PL and PW relations to derive a distance modulus to Crater II dwarf spheroidal which hosts one SXP variable. Assuming that the fundamental and first-overtone pulsation mode for the SXP variable in Crater II, we found distance moduli of 20.03 ± 0.23 mag and 20.37 ± 0.24 mag, respectively, using the PW relation, where the latter is in excellent agreement with independent RR Lyrae based distance to Crater II dwarf galaxy.

A multiphase study of theoretical and observed light curves of classical Cepheids in the Magellanic Clouds

ABSTRACTWe present an analysis of the theoretical and observed light-curve parameters of the fundamental mode (FU) classical Cepheids in the Magellanic Clouds in V- and I- photometric bands. The state-of-the-art 1D non-linear radial stellar pulsation (RSP) code in mesa (mesa-rsp) has been utilized to generate the theoretical light curves using four sets of convection parameters. Theoretical light curves with two chemical compositions: Z = 0.008 and Z = 0.004 appropriate for the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), respectively, covered a wide range of periods ($3\lt P (\rm {d})\lt 32$). The observed light curves are taken from the OGLE-IV data base. We compare theoretical and observed Fourier parameters (FPs), and investigate the period–luminosity (PL), period–colour (PC), and amplitude–colour (AC) relations as a function of pulsation phase for short (log P < 1), long (log P > 1), and all periods. The multiphase relations obtained from theoretical and observed light curves in the PL/PC/AC plane are found to be dynamic in nature, with the effect more pronounced at Φ ∼ 0.75–0.85. Furthermore, a contrasting behaviour of the theoretical/observed multiphase PL and PC relations between the short and long periods has been found for both LMC and SMC. The analysis shows that multiphase PL relations are more stringent to test the models with observations over the FPs. Distances to the LMC/SMC determined using long period Cepheids are found to be in good agreement with the literature values when the term R21 is added to the PL relation.

Light-curve Recovery with the Vera Rubin Observatory’s LSST. I. Pulsating Stars in Local Group Dwarf Galaxies

This investigation is mainly focused on the LSST Survey Strategy Optimization process, a bottom-up approach that turned out to be quite effective in involving the scientific community in the definition of the LSST observing strategy. We are mainly interested in using radial variables (RR Lyrae, classical Cepheids, long-period variables) as stellar tracers and distance indicators, and we developed a new tool called PulsationStarRecovery to quantify the recovery of the light-curve period and amplitude from an LSST-simulated time series. The outputs of this code are pulsation parameters (period, amplitude, mean magnitude) together with quantitative information concerning the difference between the shape of the light curve and template light curves. Furthermore, we apply the newborn metric to simulate LSST observations and recovery of different types of pulsating stars hosted by selected massive stellar systems (19 Local Group dwarf galaxies and the Large Magellanic Cloud) to show how the recovery changes according to distance and variable-star type. We show that this exercise is essential to understand the potential of LSST in this field since excellent recovery is necessary to optimize the use of predicted period–luminosity, period–amplitude, and color–color relations to constrain the cosmic distance scale and the metallicity distribution function of different stellar populations.

Observational Properties of 155 O- and B-type Massive Pulsating Stars

O- and B-type (OB-type) pulsating stars are important objects for studying the structure and evolution of massive stars through asteroseismology. A large amount of data from various sky surveys provides an unprecedented opportunity to search for and study this kind of variable star. We identify 155 OB-type pulsating stars or candidates, including 38 Oe/Be stars or candidates, from the data observed by TESS, LAMOST, and Gaia, which are almost new. Among the 155 objects, 87 samples are identified as slowly pulsating B (SPB) stars including 37 objects with pure low-frequency and 50 objects with both low- and high-frequency pulsation, and 14 samples are identified as β Cephei pulsating variable (BCEP) stars with both low- and high-frequency pulsation. An H-R diagram shows that these SPB and BCEP stars are mainly located in their instability regions and in the evolutionary stage of the main sequence with mass ranges of 2.5–20 M ⊙ and 7–20 M ⊙, respectively. Two special objects show Fourier spectra similar to BCEP stars but with different positions in H-R, period–temperature (P-T), and period–luminosity (P-L) diagrams. Meanwhile, 52 other targets are identified as candidates of OB-type pulsating stars. We also derive the preliminary results of the P-L relation for SPB and BCEP stars, respectively. This work also indicates that in addition to the H-R diagram, the P-T and P-L diagrams are also very useful for the classification of SPB and BCEP stars. Further detailed analysis of these objects can dramatically increase our understanding of the theories of evolution and structure for massive OB-type pulsating stars.

Metallicity estimations of MW, SMC, and LMC classical Cepheids from the shape of the V- and I-band light curves

Context. Estimating the metallicity of classical Cepheids is of prime importance for studying metallicity effects on stellar evolution and the chemical evolution of galaxies, as well as on the period–luminosity relation used on the extragalactic distance scale. Aims. Our first aim is to establish new empirical relations for estimating the iron content of classical Cepheids for short and long periods based on Fourier parameters from the V- and I-band light curves. We go on to apply these relations to Cepheids from data on the Milky Way (MW) as well as the Small and Large Magellanic Clouds (SMC and LMC) from the literature. Methods. We retrieved the metallicities of 586 fundamental-mode Cepheids from spectroscopic determinations in the literature and we found well-sampled light curves for 545 of them in different V-band catalogs. We then described the shape of these light curves by applying a Fourier decomposition and we fit the empirical relations between the Fourier parameters and the spectroscopic metallicities individually, for short-period (2.5 < P < 6.3 days) and long-period Cepheids (12 < P < 40 days). We verified the accuracy of these relations by applying them to V-band light curves of Cepheids from the Small and Large Magellanic Clouds and comparing these derived metallicities to literature values. We calibrated new interrelations of Fourier parameters to convert these empirical relations into the I band. We then used these I-band relations to derive the metallicity of fundamental-mode Cepheids from OGLE-IV for MW, SMC, and LMC (486, 695, and 1697 stars, respectively). Finally, we mapped the metallicity distribution in these galaxies for the purpose of investigating potential applications in galactic archeology. Results. For short-period Cepheids, our best fit is given for a relation based on explicit amplitude terms A1 and A2 of the first and second harmonic, respectively. In the V and I bands, these empirical relations are found with an intrinsic scatter (rms) of 0.12 dex. This relation performs well for estimations of [Fe/H] between about −0.5 and 0.1 dex, but it remains uncertain outside this range because of the lack of a spectroscopic metallicity required for the calibration. For long-period Cepheids, we found a metallicity dependence on the Fourier parameters A1, ϕ21, and R41. We found an intrinsic scatter of 0.25 dex when using this relation. The empirical relations in the V and I bands allow us to derive the mean metallicity of a sample of MW, SMC, and LMC Cepheids that is in agreement with literature values within 1σ. We also show that these relations are precise enough to reconstruct the radial metallicity gradients within the MW from OGLE data. Conclusions. The empirical relations in the V and I bands that are calibrated in this work for short- and long-period Cepheids provide a useful new tool for estimating the metallicity of Cepheids that are not accessible via spectroscopy. The calibration can be improved with further high-resolution spectroscopic observations of metal-poor Cepheids and homogeneous photometries in the V and I bands.

RR Lyrae Mid-infrared Period–Luminosity–Metallicity and Period–Wesenheit–Metallicity Relations Based on Gaia DR3 Parallaxes

We present new empirical infrared period–luminosity–metallicity (PLZ) and period–Wesenheit–metallicity (PWZ) relations for RR Lyae based on the latest Gaia Early Data Release 3 (EDR3) parallaxes. The relations are provided in the Wide-field Infrared Survey Explorer (WISE) W1 and W2 bands, as well as in the W(W1, V − W1) and W(W2, V − W2) Wesenheit magnitudes. The relations are calibrated using a very large sample of Galactic halo field RR Lyrae stars with homogeneous spectroscopic [Fe/H] abundances (over 1000 stars in the W1 band), covering a broad range of metallicities (−2.5 ≲ [Fe/H] ≲ 0.0). We test the performance of our PLZ and PWZ relations by determining the distance moduli of both galactic and extragalactic stellar associations: the Sculptor dwarf spheroidal galaxy in the Local Group (finding ), the Galactic globular clusters M4 (), and the Reticulum globular cluster in the Large Magellanic Cloud (). The distance moduli determined through all our relations are internally self-consistent (within ≲0.05 mag) but are systematically smaller (by ∼2–3σ) than previous literature measurements taken from a variety of methods/anchors. However, a comparison with similar recent RR Lyrae empirical relations anchored with EDR3 likewise shows, to varying extents, a systematically smaller distance modulus for PLZ/PWZ RR Lyrae relations.

Cepheid metallicity in the Leavitt law (C-MetaLL) survey – III. Simultaneous derivation of theGaiaparallax offset and period–luminosity–metallicity coefficients

ABSTRACTClassical Cepheids (DCEPs) are the most important standard candles in the extra-galactic distance scale thanks to the period–luminosity ($\rm PL$), period–luminosity–colour ($\rm PLC$), and period–Wesenheit ($\rm PW$) relations that hold for these objects. The advent of the Gaia mission, and in particular the Early Data Release 3 (EDR3), provided accurate parallaxes to calibrate these relations. In order to fully exploit Gaia measurements, the zero point (ZP) of Gaia parallaxes should be determined with an accuracy of a few $\rm \mu as$. The individual ZP corrections provided by the Gaia team depend on the magnitude and the position on the sky of the target. In this paper, we use an implicit method that relies on the Cepheid $\rm PL$ and $\rm PW$ relations to evaluate the ensemble Gaia parallax zero point. The best inferred estimation of the offset value needed to additionally correct (after the Gaia team correction) the Gaia parallaxes of the present DCEP sample amounts to $\rm -22\pm 4\, \mu as$. This value is in agreement with the most recent literature values and confirms that the correction proposed by the Gaia team overcorrected the parallaxes. As a further application of our results, we derive an estimate of the Large Magellanic Cloud distance ($\rm \mu _0=18.49\pm 0.06\, mag$) in very good agreement with the currently accepted value obtained through geometric methods.

Precise Empirical Determination of Metallicity Dependence of Near-infrared Period–Luminosity Relations for RR Lyrae Variables

RR Lyrae variables are excellent Population II distance indicators thanks to their well-defined period–luminosity relations (PLRs) at infrared wavelengths. We present results of near-infrared (NIR) monitoring of Galactic globular clusters to empirically quantify the metallicity dependence of NIR PLRs for RR Lyrae variables. Our sample includes homogeneous, accurate, and precise photometric data for 964 RR Lyrae variables in 11 globular clusters covering a large metallicity range (Δ[Fe/H] ∼ 2 dex). We derive JHK s -band period–luminosity–metallicity (PLZ) and period–Wesenheit–metallicity (PWZ) relations anchored using 346 Milky Way field RR Lyrae stars with Gaia parallaxes, and simultaneously solved for independent distances to globular clusters. We find a significant metallicity dependence of ∼0.2 mag dex−1 in the JHK s -band PLZ and PWZ relations for RR Lyrae stars independent of the adopted metallicity scale. The metallicity coefficients and the zero-points of the empirical PLZ and PWZ relations are in excellent agreement with the predictions from the horizontal branch evolution and pulsation models. Furthermore, RR Lyrae–based distances to our sample of globular clusters are also statistically consistent with other independent measurements in the literature. Our recommended empirical JHK s -band PLZ relations for RR Lyrae stars with periods of fundamental mode pulsation (P f) are: MJ=−0.44(±0.03)−1.83(±0.02)log(Pf)+0.20(±0.02)[Fe/H](σ=0.05mag)MH=−0.74(±0.02)−2.29(±0.02)log(Pf)+0.19(±0.01)[Fe/H](σ=0.05mag)MKs=−0.80(±0.02)−2.37(±0.02)log(Pf)+0.18(±0.01)[Fe/H](σ=0.05mag).