Metal-poor Cepheids Research Articles

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.

Determination of the iron content of Cepheids from the shape of their light curves

We present the study of the metallicity dependence of the Fourier amplitude ratios R_21 and R_31 for the light curves of short-period Galactic classical Cepheids in B, V, R_C and I_C bands. Based on the available photometric and spectroscopic data we determined the relations between the atmospheric iron abundance, [Fe/H], and the Fourier parameters. Using these relations we calculated the photometric [Fe/H] values of all program Cepheids with an average accuracy of 0.15 dex. No spectroscopic iron abundance was known before for 14 of these stars. These empirical results provide an alternate method to determine the iron abundance of classical Cepheids too faint for spectroscopic observations. We also checked whether the metal-poor Cepheids of both Magellanic Clouds follow the same relationships, and a good agreement was found.

ON THE FORM OF THESPITZERLEAVITT LAW AND ITS DEPENDENCE ON METALLICITY

The form and metallicity-dependence of Spitzer mid-infrared Cepheid relations are a source of debate. Consequently, Spitzer 3.6 and 4.5 um period-magnitude and period-color diagrams were re-examined via robust routines, thus providing the reader an alternative interpretation to consider. The relations (nearly mean-magnitude) appear non-linear over an extensive baseline (0.45< logPo <2.0), particularly the period-color trend, which to first-order follows constant (3.6-4.5) color for shorter-period Cepheids and may transition into a bluer convex trough at longer-periods. The period-magnitude functions can be described by polynomials (e.g., [3.6 um]=Ko-(3.071+-0.059) logPo-(0.120+-0.032)logPo^2), and Cepheid distances computed using 3.6 and 4.5 um relations agree and the latter provides a first-order consistency check (CO sampled at 4.5 um does not seriously compromise those distances). The period-magnitude relations appear relatively insensitive to metallicity variations ([Fe/H]~0 to -0.75), a conclusion inferred partly from comparing galaxy distances established from those relations and NED-D (n>700), yet a solid conclusion awaits comprehensive mid-infrared observations for metal-poor Cepheids in IC 1613 ([Fe/H] -1). The Cepheid-based distances were corrected for dust obscuration using a new ratio (i.e., A(3.6)/E(B-V)=0.18+-0.06) deduced from GLIMPSE (Spitzer) data.

On the metallicity dependence of classical Cepheid light amplitudes

Classical Cepheids remain a cornerstone of the cosmic distance scale, and thus characterizing the dependence of their light amplitude on metallicity is important. Period-amplitude diagrams constructed for longer-period classical Cepheids in IC 1613, NGC 3109, SMC, NGC 6822, LMC, and the Milky Way imply that very metal-poor Cepheids typically exhibit smaller V-band amplitudes than their metal-rich counterparts. The results provide an alternate interpretation relative to arguments for a null and converse metallicity dependence. The empirical results can be employed to check predictions from theoretical models, to approximate mean abundances for target populations hosting numerous long-period Cepheids, and to facilitate the identification of potentially blended or peculiar objects.

Calibrating the Cepheid Period-Luminosity relation from the near-infrared surface brightness technique

We have applied the near-infrared surface-brightness method to 111 Cepheids in the Milky Way and in the Large and the Small Magellanic Clouds determining distances and luminosities for the individual stars. We find that the K-band Period-Luminosity (PL-)relations for Milky Way and Large Magellanic Cloud Cepheids are almost identical, whereas the zero point of the Wesenheit relation depends significantly on metallicity, metal poor Cepheids being fainter. We determine empirically the relation between the projection factor, p, and pulsational period, P, used for converting the observed radial velocities into the pulsational velocities necessary for applying the near-infrared surface-brightness method. We also determine the p-factor relation on a theoretical basis and find a significantly shallower slope than from our empirical determination, suggesting that there is still some physics related to the method which deserves further investigation. Using the empirical p-factor relation we re-determine the Cepheid PL-relation in the K-band using all 111 Cepheids. We argue that this is the currently best PL-relation for distance determination being largely independent of both metallicity and reddening.

A GLOBAL PHYSICAL MODEL FOR CEPHEIDS

We perform a global fit to ~5,000 radial velocity and ~177,000 magnitude measurements in 29 photometric bands covering 0.3 to 8.0 microns distributed among 287 Galactic, LMC, and SMC Cepheids with P > 10 days. We assume that the Cepheid light curves and radial velocities are fully characterized by distance, reddening, and time-dependent radius and temperature variations. We construct phase curves of radius and temperature for periods between 10 and 100 days, which yield light curve templates for all our photometric bands and can be easily generalized to any additional band. With only 4 to 6 parameters per Cepheid, depending on the existence of velocity data and the amount of freedom in the distance, the models have typical rms light and velocity curve residuals of 0.05 mag and 3.5 km/s. The model derives the mean Cepheid spectral energy distribution and its derivative with respect to temperature, which deviate from a black body in agreement with metal-line and molecular opacity effects. We determine a mean reddening law towards the Cepheids in our sample, which is not consistent with standard assumptions in either the optical or near-IR. Based on stellar atmosphere models we predict the biases in distance, reddening, and temperature determinations due to the metallicity and we quantify the metallicity signature expected for our fit residuals. The observed residuals as a function of wavelength show clear differences between the individual galaxies, which are compatible with these predictions. In particular, we find that metal-poor Cepheids appear hotter. Finally, we provide a framework for optimally selecting filters that yield the smallest overall errors in Cepheid parameter determination, or filter combinations for suppressing or enhancing the metallicity effects on distance determinations. We make our templates publicly available.

New period-luminosity and period-color relations of classical Cepheids

The metal-poor, fundamental-mode (P0) and first-overtone (P1) Cepheids in the dwarf galaxies IC 1613, WLM, Pegasus, Sextans A, Sextans B, and Leo A are compared with the about equally metal-poor Cepheids of the Small Magellanic Cloud (SMC). The period-color (P-C) and period-luminosity (P-L) relations of the seven galaxies are indistinguishable, but differ distinctly from those in the Large Magellanic Cloud (LMC) and the solar neighborhood. Adopting (m-M)^{0}_{SMC}=18.93 from independent evidence, one can determine reliable distance moduli for the other dwarf galaxies of (m-M)^{0} = 24.34+/-0.03, 24.95+/-0.03, 24.87+/-0.06, 25.60+/-0.03 (mean for Sextans A & B), and 24.59+/-0.03, respectively.

Bright metal-poor variables: Why “Anomalous” Cepheids?

We investigate the theoretical scenario concerning the large sample of variables recently discovered in the dwarf, metal-poor irregular galaxy Leo A, focusing the attention on the “Anomalous” Cepheid phenomenon and its correlation with RR Lyrae stars, Classical and Population II Cepheids. To this purpose, we make use of suitable stellar and pulsation models to depict the pulsational history of evolutionary structures with metallicity . We find that He-burning pulsators are expected only outside the mass interval ~0.8–1.7 . Stars from ~1.8 to 4 , a mass range including both Anomalous and Classical Cepheids, populate to good approximation a common MV-logPF instability strip, independently of the previous occurrence of a He flash event. Their periods and luminosities increase with the stellar mass and they are at a lower luminosity level mag, as observed in Leo A. The class of less massive pulsators ( , namely RR Lyrae stars and Population II Cepheids) populate a distinct instability strip, where the magnitudes become brighter and the periods longer when the pulsator mass decreases. The dependence on metal content in this scenario has been investigated over the range to 0.008. The edges of the pulsational strip for the more massive class of pulsators appear to be independent of metallicity, but with a minimum mass of these bright pulsators which decreases with decreasing metallicity, thus decreasing the predicted minimum luminosity and period. Comparison with data for Cepheids in Leo A and in the moderately metal-rich extragalactic stellar system Sextans A discloses an encouraging agreement with the predicted pulsational scenario. On this basis, we predict that in a stellar system where both RR Lyrae stars and Cepheids are observed their magnitude difference may help in constraining both the metal content and the distance. The current classification of metal-poor Cepheids is briefly discussed and suggestion is advanced for an updated terminology abreast with the current knowledge of stellar evolution.

The effect of metallicity on the Cepheid Period-Luminosity relation from a Baade-Wesselink analysis of Cepheids in the Galaxy and in the Small Magellanic Cloud

(ABRIDGED) We have applied the near-IR Barnes-Evans realization of the Baade-Wesselink method as calibrated by Fouque & Gieren (1997) to five metal-poor Cepheids with periods between 13 and 17 days in the Small Magellanic Cloud as well as to a sample of 34 Galactic Cepheids to determine the effect of metallicity on the period-luminosity (P-L) relation. For ten of the Galactic Cepheids we present new accurate and well sampled radial-velocity curves. The Baade-Wesselink analysis provides accurate individual distances and luminosities to the Cepheids in the two samples, allowing us to constrain directly, in a purely differential way, the metallicity effect on the Cepheid P-L relation. For the Galactic Cepheids we provide a new set of P-L relations which have zero-points in excellent agreement with astrometric and interferometric determinations. These relations can be used directly for the determination of distances to solar-metallicity samples of Cepheids in distant galaxies, circumventing any corrections for metallicity effects on the zero-point and slope of the P-L relation. We find evidence for both such metallicity effects in our data. Comparing our two samples of Cepheids at a mean period of about 15 days, we find a weak effect of metallicity on the luminosity similar to that adopted by the HST Key Project on the Extragalactic Distance Scale.

Intermediate‐Mass Star Models with Different Helium and Metal Contents

We present a comprehensive theoretical investigation of the evolutionary properties of intermediate- mass stars. The evolutionary sequences were computed from the zero-age main sequence up to the central He exhaustion and often up to the phases that precede the carbon ignition or to the reignition of the H-shell, which marks the beginning of the thermal pulse phase. The evolutionary tracks were con- structed by adopting a wide range of stellar masses 15) and chemical compositions. In (3 M/M _ order to account for current uncertainties on the He to heavy elements enrichment ratio (*Y /*Z), the stellar models were computed by adopting at Z 0.02 two diUerent He contents (Y 0.27, 0.289) and at Z 0.04 three diUerent He contents (Y 0.29, 0.34, and 0.37). Moreover, to supply a homogeneous evo- lutionary scenario that accounts for young Magellanic stellar systems the calculations were also extended toward lower metallicities (Z 0.004, Z 0.01), by adopting diUerent initial He abundances. We evalu- ated for both solar (Z 0.02) and supermetal-rich (SMR, Z 0.04) models the transition mass Mup between the stellar structures igniting carbon and those that develop a full electron degeneracy inside the carbon-oxygen core. We found that Mup is of the order of 7.7 ^ 0.5 for solar composition, while for M _ SMR structures an increase in the He content causes a decrease in Mup, and indeed it changes from 9.5 ^ 0.5 at Y 0.29, to 8.7 ^ 0.2 at Y 0.34, and to 7.7 ^ 0.2 at Y 0.37. We also show M _ M _ M _ that Mup presents a nonlinear behavior with metallicity, and indeed it decreases when moving from Z 0.04 to Z B 0.001 and increases at lower metal contents. This —nding con—rms the predictions by Cassisi & Castellani and more recently by Umeda et al. and suggests that the rate of SNe type Ia depends on the chemical composition of the parent stellar population. This approach allows us to inves- tigate in detail the evolutionary properties of classical Cepheids. In particular, we —nd that the range of stellar masses that perform the blue loop during the central He-burning phase narrows when moving toward metal-rich and SMR structures. This evidence and the substantial decrease in the evolutionary time spent by these structures inside the instability strip bring out that the probability of detecting long- period Cepheids in SMR stellar systems is substantially smaller than in more metal-poor systems. Moreover, and even more important, we —nd that the time spent by Cepheids along the subsequent crossings of the instability strip also depends on the stellar mass. In fact, our models suggest that low- mass, metal-poor Cepheids spend a substantial portion of their lifetime along the blueward excursion of the blue loop, while at higher masses the time spent along the redward excursion becomes (M/M _ 8) longer. Models at solar chemical composition present an opposite behavior, i.e., the time spent along the redward excursion is longer than the blueward excursion among low-mass Cepheids and vice versa for high-mass Cepheids. Oddly enough, the time spent along the blueward excursion is for models at Z 0.01 longer than the redward excursion over the entire mass range. This suggests a nonlinear depen- dence of crossing times on metallicity. The time spent along the —rst crossing of the instability strip is generally negligible with the exception of high-mass, metal-poor stellar structures for which it becomes of the order of 15%¨20% of the total crossing time. Subject heading: Cepheidsstars: early-typestars: evolutionstars: oscillations ¨ stars: variables: other

Baade–Wesselink Analysis of Magellanic Cloud Cepheids

AbstractWe apply the Barnes–Evans variant of the Baade–Wesselink method to Cepheids in the LMC and SMC in an attempt to determine the distance directly to individual stars in these galaxies and to determine the metallicity effect on the Cepheid period–luminosity relation. We now haveK-band light curves for a sample of SMC stars as well as for many Cepheids in young clusters in the LMC. Using theFV, (V–K) calibration of Fouqué &amp; Gieren (1997) we find preliminary evidence for a metallicity effect which makes metal poor Cepheids brighter. This is at odds with earlier results based on optical photometry and the reason is not entirely understood yet.

Correcting the PL Relation?

Metal-poor Cepheids do not appear to have smaller radii or cooler temperatures than more metal-rich Cepheids. A correction to the LMC modulus of 0.4 mag, due to metal-poor Cepheids being fainter for their periods, is inconsistent with the data at the 6σ level or greater.

Rebuilding the Cepheid Distance Scale. I. A Global Analysis of Cepheid Mean Magnitudes

We develop a statistical method for using multicolor photometry to determine distances using Cepheid variables, including the effects of temperature, extinction, and metallicity, and apply it to UBVRIJHK photometry of 694 Cepheids in 17 galaxies. We derive homogeneous distance, extinction, and uncertainty estimates for four models, starting from the standard extragalactic method and then adding the physical effects of temperature distributions and extinction distributions, requiring positive definite extinctions and metallicity. While we find general agreement with published distances when we make similar systematic assumptions, there is a clear problem in the standard distances because they require Cepheids with negative extinctions, particularly in low-metallicity galaxies, unless the mean LMC extinction exceeds E(B - V) ≳ 0.20. The problem can be explained by the physically expected metallicity dependence of the Cepheid distance scale, where metal-poor Cepheids are hotter and possibly fainter at V and I than metal-rich Cepheids, or by large systematic errors in Cepheid photometry. For V and I we found that the mean magnitude change is -0.14 ± 0.14 mag dex-1 and the mean color change is 0.13 ± 0.04 mag dex-1, with the change in color dominating the change in distance. The effect on Type Ia supernova estimates of the Hubble constant is dramatic because most were found in the metal-poor galaxies with the bluest Cepheids. The Type Ia multicolor light-curve shape method estimate for H0 formally rises from 69 ± 8 to 80 ± 6 km s-1 Mpc-1 with the metallicity correction.

On the Discrepancy between the Cepheid and RR Lyrae Distance Scales

view Abstract Citations (35) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Discrepancy between the Cepheid and RR Lyrae Distance Scales van den Bergh, Sidney Abstract Available evidence suggests that there is a small discrepancy between distances to Local Group dwarf galaxies determined from Cepheids and RR Lyrae stars. This difference might be due to a slight metallicity dependence of the period luminosity relation for classical Cepheids. Such a dependence would have to be in the sense that metal-poor Cepheids of a given period are slightly fainter than their metal-rich counterparts. If this hypothesis is correct then recent Hubble Space Telescope distances to IC 4182 (SN 1937C) and NGC 5253 (SN 1972E) may have been slightly overestimated, resulting in a small increase in the Hubble parameter H0. Alternatively, the adopted relation between the metallicity and luminosity of RR Lyrae variables might be in error. It is noted that the rather faint Mv(RR) values observed in giant galaxies appear to conflict with the brighter values predicted by horizontal branch models. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175763 Bibcode: 1995ApJ...446...39V Keywords: COSMOLOGY: DISTANCE SCALE; GALAXIES: DISTANCES AND REDSHIFTS; GALAXIES: LOCAL GROUP; STARS: ABUNDANCES; STARS: VARIABLES: CEPHEIDS full text sources ADS | data products SIMBAD (18) NED (8)