arXiv subjects

A. Pietrinferni

Publications and source records attributed to A. Pietrinferni.

At least 19 recordsLinked to original sources

arXiv2024

Double Red Giant Branch and Red Clump features of Galactic disc stellar populations with Gaia GSPspec

To disentangle the different competing physical processes at play in Galactic evolution, a detailed chrono chemicalkinematical, and dynamical characterisation of the disc bimodality is necessary, including high number statistics. Here we make use of an extremely precise subsample of the Gaia DR3 GSP-Spec catalogue of stellar chemophysical parameters. The selected database is composed of 408 800 stars with a median uncertainty of 10 K, 0.03 and 0.01 dex in Teff , log(g) and [M/H], respectively. The stellar parameter precision allows to break the age-metallicity degeneracy of disc stars. For the first time, the disc bimodality in the Kiel diagramme of giant stars is observed, getting rid of interstellar absortion issues. This bimodality produces double Red Giant Branch sequences and Red Clump features for mono-metallicity populations. A comparison with BaSTI isochrones allows to demonstrate that an age gap is needed to explain the evolutionary sequences separation, in agreement with previous age-metallicity relations obtained using sub-giant stars. A bimodal distribution in the stellar mass-[alpha/Fe] plane is observed at constant metallicity. Finally, a selection of stars with [M/H]=0.45 \pm 0.03 dex shows that the most metal-rich population in the Milky Way disc presents an important proportion of stars with ages in the range 5-13 Gyr. This old, extremely metal-rich population is possibly a mix of migrated stars from the internal Galactic regions, and old disc stars formed before the last major merger of the Milky Way. The Gaia GSP-Spec Kiel diagrammes of disc mono-abundance stellar populations reveal a complex, non linear age-metallicity relation crafted by internal and external processes of Galactic evolution. Their detailed analysis opens new opportunities to reconstruct the puzzle of the Milky Way disc bimodality.

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arXiv2024

Cepheids as distance indicators and stellar tracers

We review the phenomenology of classical Cepheids (CCs), Anomalous Cepheids (ACs) and type II Cepheids (TIICs) in the Milky Way (MW) and in the Magellanic Clouds (MCs). We also examine the Hertzsprung progression in different stellar systems by using the shape of I-band light curves (Fourier parameters) and observables based on the difference in magnitude and in phase between the bump and the minimum in luminosity. The distribution of Cepheids in optical and in optical-near infrared (NIR) color--magnitude diagrams is investigated to constrain the topology of the instability strip. The use of Cepheids as tracers of young (CCs), intermediate (ACs) and old (TIICs) stellar populations are brought forward by the comparison between observations (MCs) and cluster isochrones covering a broad range in stellar ages and in chemical compositions. The different diagnostics adopted to estimate individual distances (period--luminosity, period--Wesenheit, period--luminosity--color relations) are reviewed together with pros and cons in the use of fundamental and overtones, optical and NIR photometric bands, and reddening free pseudo magnitudes (Wesenheit). We also discuss the use of CCs as stellar tracers and the radial gradients among the different groups of elements (iron, alpha, neutron-capture) together with their age-dependence. Finally, we briefly outline the role that near-future space and ground-based facilities will play in the astrophysical and cosmological use of Cepheids.

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arXiv2024

Benchmarking the effective temperature scale of red giant branch stellar models: the case of the metal-poor halo giant HD 122563

There is plenty of evidence in the literature of significant discrepancies between the observations and models of metal-poor red giant branch stars, in particular regarding the effective temperature, teff, scale. We revisit the benchmark star HD 122563 using the most recent observations from Gaia Data Release 3, to investigate if these new constraints may help in resolving this discrepancy. We review the most recent spectroscopic determinations of the metallicity of HD 122563 [Fe/H], and provide a new assessment of its fundamental parameters, i.e. bolometric luminosity, teff, surface gravity, plus a photometric determination of its metal content. Using these constraints, we compare the position of the star in the Hertzsprung-Russell (H-R) diagram with various recent sets of stellar evolution tracks. The H-R diagram analysis reveals a significant disagreement between observed and theoretical teff values, when adopting the most recent spectroscopic estimate of [Fe/H]. On the other hand, by using the photometric determination of [Fe/H] some of the selected sets of stellar tracks appear in fair agreement with observations. The sets with discrepant teff can be made to agree with observations either by modifying the prescription adopted to calculate the models' outer boundary conditions, and/or by reducing the adopted value of the mixing length parameter with respect to the solar-calibration. A definitive assessment of whether the teff scale of metal-poor stellar red giant branch models is consistent with observations requires an even more accurate determination of the fundamental parameters of HD 122563 and also a larger sample of calibrators. From the theoretical side, it is crucial to minimise the current uncertainties in the treatment (boundary conditions, temperature gradient) of the outer layers of stellar models with convective envelopes.

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arXiv2022

Signature of a chemical spread in the open cluster M37

Recent Gaia photometry of the open cluster M37 have disclosed the existence of an extended main-sequence turn off -- like in Magellanic clusters younger than about 2 Gyr -- and a main sequence that is broadened in colour beyond what is expected from the photometric errors, at magnitudes well below the region of the extended turn off, where neither age differences nor rotation rates (the candidates to explain the extended turn off phenomenon) are expected to play a role. Moreover, not even the contribution of unresolved binaries can fully explain the observed broadening. We investigated the reasons behind this broadening by making use of synthetic stellar populations and differential colour-colour diagrams using a combination of Gaia and Sloan filters. From our analysis we have concluded that the observed colour spread in the Gaia colour-magnitude diagram can be reproduced by a combination of either a metallicity spread Delta[Fe/H] ~ 0.15 plus a differential reddening across the face of the cluster spanning a total range DeltaE (B - V) ~ 0.06, or a spread of the initial helium mass fraction DeltaY ~ 0.10 plus a smaller range of reddening DeltaE (B - V) ~ 0.03. High-resolution differential abundance determinations of a sizeable sample of cluster stars are necessary to confirm or exclude the presence of a metal abundance spread. Our results raise the possibility that also individual open clusters, like globular clusters and massive star clusters, host stars born with different initial chemical compositions.

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arXiv2022

A new and homogeneous metallicity scale for Galactic classical Cepheids II. The abundance of iron and alpha elements

Classical Cepheids are the most popular distance indicators and tracers of young stellar populations. The key advantage is that they are bright and they can be easily identified in Local Group and Local Volume galaxies. Their evolutionary and pulsation properties depend on their chemical abundances. The main aim of this investigation is to perform a new and accurate abundance analysis of two tens of calibrating Galactic Cepheids using high spectral resolution (R$\sim$40,000-115,000) and high S/N spectra ($\sim$400) covering the entire pulsation cycle. We focus our attention on possible systematics affecting the estimate of atmospheric parameters and elemental abundances along the pulsation cycle. We cleaned the line list by using atomic transition parameters based on laboratory measurements and by removing lines that are either blended or display abundance variations along the pulsation cycle. The spectroscopic approach that we developed brings forward small dispersions in the variation of the atmospheric parameters ($σ$($T_{\rm eff}$)$\sim$50 K, $σ$($\log{g}$)$\sim$0.2 dex, and $σ$($ξ$)$\sim$0.2 km/s) and in the abundance of both iron ($\lesssim$ 0.05 dex) and alpha elements ($\lesssim$0.10 dex) over the entire pulsation cycle. We also provide new and accurate effective temperature templates by splitting the calibrating Cepheids into four different period bins, ranging from short to long periods. For each period bin, we performed an analytical fit with Fourier series providing $θ= 5040/{T_{\rm eff}}$ as a function of the pulsation phase. The current findings are a good viaticum to trace the chemical enrichment of the Galactic thin disk by using classical Cepheids and a fundamental stepping stone for further investigations into the more metal-poor regime typical of Magellanic Cepheids.

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arXiv2021

Lithium abundances as a tracer of AGB stars pollution in the globular cluster NGC 6752

This paper presents the chemical abundance analysis of 217 stars in the metal-poor globular cluster NGC 6752, distributed from the turn-off to the lower red giant branch. Aluminium and Lithium abundances were derived through spectral synthesis applied to spectra collected with FLAMES, both in GIRAFFE and UVES modes. The work aims to gain insight into the nature of the polluter(s) responsible for the abundance variations and C-N, Na-O, Al-Mg anti-correlations associated with the multiple population phenomenon. We found a plateau at A(Li)=2.33$\pm$0.06 dex in unevolved stars, with the average Li content decreasing continuously down to $\sim$1.25 dex at the bottom of the red giant branch. As expected in the classic anti-correlation scenario, we found stars low in Al and high Li abundance and stars high in Al and low in Li. However, in addition, we also found evidence of Al-rich, second-generation stars with high Li content. This finding suggests the need for Li production, known to happen in intermediate-mass ($\sim$4-8M$_{sun}$) AGB stars through the Cameron-Fowler mechanism. It is worth noticing that the Li abundance observed in Al-rich stars never exceeds that in Al-poor stars.

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arXiv2021

Star formation history and metallicity in the Galactic inner bulge revealed by the red giant branch bump

The study of the inner region of the Milky Way's bulge is hampered by high interstellar extinction and extreme source crowding. Sensitive high angular resolution near-infrared imaging is needed to study stellar populations in such a complex environment. We use the 0.2$''$ angular resolution $JHK_s$ data from the GALACTICNUCLEUS survey to study the stellar population within two $8.0'\times 3.4'$ fields, about 0.6$^\circ$ and 0.4$^\circ$ to the Galactic north of the Milky Way's centre and to compare it with one in the immediate surroundings of Sagittarius A*. We also characterise the extinction curve of the two fields. The average interstellar extinction to the outer and the inner field is $A_{K_s} \sim 1.20 \pm 0.08$ mag and $\sim 1.48 \pm 0.10$ mag, respectively. We present $K_{s}$ luminosity functions that are complete down to at least 2 mag below the red clump (RC). We detect a feature in the luminosity functions that is fainter than the RC by $0.80\pm0.03$ and $0.79\pm0.02$ mag, respectively, in the $K_s$-band. It runs parallel to the reddening vector. We identify the feature as the red giant branch bump. Fitting $α$-enhanced BaSTI luminosity functions to our data, we find that a single old stellar population of $\sim12.8 \pm 0.6$ Gyr and $Z = 0.040 \pm 0.003$ provides the best fit. We obtain that the stellar population in the innermost bulge is old, similar to the one at larger distances from the Galactic plane, and that its metallicity increases down to distances as short as about 60 pc from the centre of the Milky Way. Comparing it with previous known values at larger latitudes ($|b|>2^\circ$), our results favour a flattening of the gradient at $|b|<2^\circ$. As a secondary result we obtain that the extinction index in the studied regions agrees the value of $α= 2.30\pm0.08$, derived in Nogueras-Lara et al. 2018 for the very Galactic centre.

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arXiv2020

The updated BaSTI stellar evolution models and isochrones: II. alpha-enhanced calculations

This is the second paper of a series devoted to present an updated release of the BaSTI ( a Bag of Stellar Tracks and Isochrones) stellar model and isochrone library. Following the publication of the updated solar scaled library, here we present the library for a $α-$enhanced heavy element distribution. These new alpha-enhanced models account for all improvements and updates in the reference solar metal distribution and physics inputs, as in the new solar scaled library. The models cover a mass range between 0.1 and $15~M_{\odot}$, 18 metallicities between [Fe/H]=-3.20 and +0.06 with [alpha/Fe]=+0.4 , and a helium to metal enrichment ratio Delta{Y}Δ{Z}=1.31. For each metallicity, He-enhanced stellar models are also provided. The isochrones cover (typically) an age range between 20Myr and 14.5Gyr, including consistently the pre-main sequence phase. Asteroseismic properties of the theoretical models have also been calculated. Models and isochrones have been compared with results from independent calculations, with the previous BaSTI release, and also with selected observations, to test the accuracy/reliability of these new calculations. All stellar evolution tracks, asteroseismic properties and isochrones are made publicly available at http://basti-iac.oa-teramo.inaf.it

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arXiv2020

Evolutionary and pulsation properties of Type II Cepheids

We discuss the observed pulsation properties of Type II Cepheids (TIICs) in the Galaxy and Magellanic Clouds. The period (P) distributions, luminosity amplitudes and population ratios of the three different sub-groups (BL Herculis[BLH, P<5 days], W Virginis [WV, 5 20 days]) are quite similar in different stellar systems, suggesting a common evolutionary channel and a mild dependence on both metallicity and environment. We present a homogeneous theoretical framework based on Horizontal Branch (HB) evolutionary models, envisaging that TIICs are mainly old (t<10 Gyr), low-mass stars. The BLHs are predicted to be post early asymptotic giant branch (PEAGB) stars (double shell burning) on the verge of reaching their AGB track (first crossing of the instability strip), while WVs are a mix of PEAGB and post-AGB stars (hydrogen shell burning) moving from cool to hot (second crossing). Thus suggesting that they are a single group of variable stars. RVTs are predicted to be a mix of post-AGB stars along their second crossing (short-period tail) and thermally pulsing AGB stars (long-period tail) evolving towards their white dwarf cooling sequence. We also present several sets of synthetic HB models by assuming a bimodal mass distribution along the HB. Theory suggests, in agreement with observations, that TIIC pulsation properties marginally depend on metallicity. Predicted period distributions and population ratios for BLHs agree quite well with observations, while those for WVs and RVTs are almost a factor of two smaller and larger than observed, respectively. Moreover, the predicted period distributions for WVs peak at periods shorter than observed, while those for RVTs display a long period tail not supported by observations. We investigate several avenues to explain these differences, but more detailed calculations are required to address them.

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arXiv2020

On the Metamorphosis of the Bailey diagram for RR Lyrae stars

We collected over 6000 high-resolution spectra of four dozen field RR Lyrae (RRL) variables pulsating either in the fundamental (39 RRab) or in the first overtone (9 RRc) mode. We measured radial velocities (RVs) of four strong metallic and four Balmer lines along the entire pulsational cycle and derived RV amplitudes with accuracies better than 1$-$2~\kmsec. The new amplitudes were combined with literature data for 23~RRab and 3~RRc stars (total sample 74 RRLs) which allowed us to investigate the variation of the Bailey diagram (photometric amplitude versus period) when moving from optical to mid-infrared bands and to re-cast the Bailey diagram in terms of RV amplitudes. We found that RV amplitudes for RRab are minimally affected by nonlinear phenomena (shocks) and multi-periodicity (Blazhko effect). The RV slope ($\log P$--A(V$_r$)) when compared with the visual slope ($\log P$--A($V$)) is shallower and the dispersion, at fixed period, decreases by a factor of two. We constructed homogeneous sets of Horizontal Branch evolutionary models and nonlinear, convective pulsation models of RRLs to constrain the impact of evolutionary effects on their pulsation properties. Evolution causes, on the Bailey diagram based on RV amplitudes, a modest variation in pulsation period and a large dispersion in amplitude. The broad dispersion in period of the Bailey diagram is mainly caused by variation in RRL intrinsic parameters (stellar mass, chemical composition). Empirical evidence indicates that RV amplitudes are an optimal diagnostic for tracing the mean effective temperature across the RRab instability strip.

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arXiv2020

The Aarhus red giants challenge II. Stellar oscillations in the red giant branch phase

Context. The large quantity of high-quality asteroseismic data that obtained from space-based photometric missions and the accuracy of the resulting frequencies motivate a careful consideration of the accuracy of computed oscillation frequencies of stellar models, when applied as diagnostics of the model properties. Aims. Based on models of red-giant stars that have been independently calculated using different stellar evolution codes, we investigate the extent to which the differences in the model calculation affect the model oscillation frequencies. Methods. For each of the models, which cover four different masses and different evolution stages on the red-giant branch, we computed full sets of low-degree oscillation frequencies using a single pulsation code and, from these frequencies, typical asteroseismic diagnostics. In addition, we carried out preliminary analyses to relate differences in the oscillation properties to the corresponding model differences. Results. In general, the differences in asteroseismic properties between the different models greatly exceed the observational precision of these properties, in particular for the nonradial modes whose mixed acoustic and gravity-wave character makes them sensitive to the structure of the deep stellar interior. In some cases, identifying these differences led to improvements in the final models presented here and in Paper I; here we illustrate particular examples of this. Conclusions. Further improvements in stellar modelling are required in order fully to utilise the observational accuracy to probe intrinsic limitations in the modelling. However, our analysis of the frequency differences and their relation to stellar internal properties provides a striking illustration of the potential of the mixed modes of red-giant stars for the diagnostics of stellar interiors.

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arXiv2020

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XX. Ages of single and multiple stellar populations in seven bulge globular clusters

In the present work we analyzed seven globular clusters selected from their location in the Galactic bulge and with metallicity values in the range $-1.30\lesssim\rm{[Fe/H]}\lesssim-0.50$. The aim of this work is first to derive cluster ages assuming single stellar populations, and secondly, to identify the stars from first (1G) and second generations (2G) from the main sequence, subgiant and red giant branches, and to derive their age differences. Based on a combination of UV and optical filters used in this project, we apply the Gaussian mixture models to distinguish the multiple stellar populations. Applying statistical isochrone fitting, we derive self-consistent ages, distances, metallicities, and reddening values for the sample clusters. An average of $12.3\pm0.4$ Gyr was obtained both using Dartmouth and BaSTI (accounting atomic diffusion effects) isochrones, without a clear distinction between the moderately metal-poor and the more metal-rich bulge clusters, except for NGC 6717 and the inner halo NGC 6362 with $\sim 13.5$ Gyr. We derived a weighted mean age difference between the multiple populations hosted by each globular cluster of $41\pm170$ Myr adopting canonical He abundances; whereas for higher He in 2G stars, this difference reduces to $17\pm170$ Myr, but with individual uncertainties of $500$ Myr.

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arXiv2019

The Aarhus Red Giants Challenge I: Stellar structures in the red giant branch phase

(Abridged). We introduce the Aarhus Red Giants Challenge, a series of detailed comparisons between widely used stellar evolution and oscillation codes aiming at establishing the minimum level of uncertainties in properties of red giants arising solely from numerical implementations. Using 9 state-of-the-art stellar evolution codes, we defined a set of input physics and physical constants for our calculations and calibrated the convective efficiency to a specific point on the main sequence. We produced evolutionary tracks and stellar structure models at fixed radius along the red-giant branch for masses of 1.0 M$_\odot$, 1.5 M$_\odot$, 2.0 M$_\odot$, and 2.5 M$_\odot$, and compared the predicted stellar properties. Once models have been calibrated on the main sequence we find a residual spread in the predicted effective temperatures across all codes of ~20 K at solar radius and ~30-40 K in the RGB regardless of the considered stellar mass. The predicted ages show variations of 2-5% (increasing with stellar mass) which we track down to differences in the numerical implementation of energy generation. The luminosity of the RGB-bump shows a spread of about 10% for the considered codes, which translates into magnitude differences of ~0.1 mag in the optical V-band. We also compare the predicted [C/N] abundance ratio and found a spread of 0.1 dex or more for all considered masses. Our comparisons show that differences at the level of a few percent still remain in evolutionary calculations of red giants branch stars despite the use of the same input physics. These are mostly due to differences in the energy generation routines and interpolation across opacities, and call for further investigations on these matters in the context of using properties of red giants as benchmarks for astrophysical studies.

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arXiv2019

A DECam View of the Diffuse Dwarf Galaxy Crater II: The Colour-Magnitude Diagram

We present a deep Blanco/DECam colour-magnitude diagram (CMD) for the large but very diffuse Milky Way satellite dwarf galaxy Crater II. The CMD shows only old stars with a clearly bifurcated subgiant branch (SGB) that feeds a narrow red giant branch. The horizontal branch (HB) shows many RR Lyrae and red HB stars. Comparing the CMD with [Fe/H] = -2.0 and [$α$/Fe] = +0.3 alpha-enhanced BaSTI isochrones indicates a mean age of 12.5 Gyr for the main event and a mean age of 10.5 Gyr for the brighter SGB. With such multiple star formation events Crater II shows similarity to more massive dwarfs that have intermediate age populations, however for Crater II there was early quenching of the star formation and no intermediate age or younger stars are present. The spatial distribution of Crater II stars overall is elliptical in the plane of the sky, the detailed distribution shows a lack of strong central concentration, and some inhomogeneities. The 10.5 Gyr subgiant and upper main sequence stars show a slightly higher central concentration when compared to the 12.5 Gyr population. Matching to Gaia DR2 we find the proper motion of Crater II: $μ_α\cos δ$=-0.14 $\pm$ 0.07 , $μ_δ$=-0.10 $\pm$ 0.04 mas yr$^{-1}$, approximately perpendicular to the semi-major axis of Crater II. Our results provide constraints on the star formation and chemical enrichment history of Crater II, but cannot definitively determine whether or not substantial mass has been lost over its lifetime.

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arXiv2019

A panchromatic view of the bulge globular cluster NGC 6569

We used high-resolution optical HST/WFC3 and multi-conjugate adaptive optics assisted GEMINI GeMS/GSAOI observations in the near-infrared to investigate the physical properties of the globular cluster NGC 6569 in the Galactic bulge. We have obtained the deepest purely NIR color-magnitude diagram published so far for this cluster using ground-based observations, reaching $K_{s}$ $\approx$ 21.0 mag (two magnitudes below the main-sequence turn-off point). By combining the two datasets secured at two different epochs, we determined relative proper motions for a large sample of individual stars in the center of NGC 6569, allowing a robust selection of cluster member stars. Our proper motion analysis solidly demonstrates that, despite its relatively high metal content, NGC 6569 hosts some blue horizontal branch stars. A differential reddening map has been derived in the direction of the system, revealing a maximum color excess variation of about $δE(B-V)$ $\sim$ 0.12 mag in the available field of view. The absolute age of NGC 6569 has been determined for the first time. In agreement with the other few bulge globular clusters with available age estimates, NGC 6569 turns out to be old, with an age of about 12.8 Gyr, and a typical uncertainty of 0.8-1.0 Gyr.

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arXiv2018

New Near-Infrared $JHK_s$ light-curve templates for RR Lyrae variables

We provide homogeneous optical (UBVRI) and near-infrared (JHK) time series photometry for 254 cluster (omega Cen, M4) and field RR Lyrae (RRL) variables. We ended up with more than 551,000 measurements. For 94 fundamental (RRab) and 51 first overtones (RRc) we provide a complete optical/NIR characterization (mean magnitudes, luminosity amplitudes, epoch of the anchor point). The NIR light curves of these variables were adopted to provide new and accurate light-curve templates for both RRc (single period bin) and RRab (three period bins) variables. The templates for the J and the H band are newly introduced, together with the use of the pulsation period to discriminate among the different RRab templates. To overcome subtle uncertainties in the fit of secondary features of the light curves we provide two independent sets of analytical functions (Fourier series, Periodic Gaussian functions). The new templates were validated by using 26 omega Cen and Bulge RRLs covering the four period bins. We found that the difference between the measured mean magnitude along the light curve and the mean magnitude estimated by using the template on a single randomly extracted phase point is better than 0.01 mag (sigma=0.04 mag). We also validated the template on variables for which at least three phase points were available, but without information on the phase of the anchor point. The accuracy of the mean magnitudes is ~0.01 mag (sigma=0.04 mag). The new templates were applied to the Large Magellanic Cloud (LMC) globular Reticulum and by using literature data and predicted PLZ relations we found true distance moduli of 18.47+-0.10+-0.03 mag (J) and 18.49+-0.09+-0.05 mag (K). We also used literature optical and mid-infrared data and we found a mean true distance modulus of 18.47+-0.02+-0.06 mag, suggesting that Reticulum is ~1 kpc closer than the LMC.

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arXiv2018

unVEil the darknesS of The gAlactic buLgE (VESTALE)

The main aim of this experiment is to provide a complete census of old (t > 10 Gyr, RR Lyrae, type II Cepheids, red horizontal branch), intermediate age (red clump, Miras) and young (classical Cepheids) stellar tracers across the Galactic Bulge. To fully exploit the unique photometric quality of LSST images, we plan to perform a Shallow minisurvey (ugrizy, -20 < l < 20 deg, -15 < b < 10 deg) and a Deep minisurvey (izy, -20 < l < 20 deg, -3 < b < 3 deg). The former one is aimed at constraining the 3D structure of the galactic Bulge across the four quadrants, and in particular, the transition between inner and outer Bulge. The u,g,r,i,z,y LSST bands provide fundamental diagnostics to constrain the evolutionary properties of low and intermediate-mass stars when moving from a metal-poor to a metal-rich regime. The deep minisurvey is aimed at tracing RR Lyrae, Red Clump stars, Miras and classical Cepheids in highly reddened regions of the Galactic center. These images will allow us to investigate the role that baryonic mass and dark matter played in the early formation and evolution of the MW.

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arXiv2018

On the Impact of Helium Content on the RR Lyrae Distance Scale

We constructed new sets of He-enhanced (Y = 0.30, Y = 0.40) nonlinear, time-dependent convective hydrodynamical models of RR Lyrae (RRL) stars covering a broad range in metal abundances (Z from 0.0001 to 0.02). The increase in He content from the canonical value (Y = 0.245) to Y = 0.30 and 0.40 causes a simultaneous increase in stellar luminosity and in pulsation period. To investigate the dependence of the RRL distance scale on the He abundance, we computed new optical (RI) and near-infrared (JHK ) period luminosity metallicity helium relations. Interestingly enough, the increase in He content causes a minimal change in the coefficients of both period and metallicity terms, since canonical and He-enhanced models obey similar PLZ relations. On the contrary, the classical B and V band mean magnitude metallicity relations and the R band PLZ relation display a significant dependence on the He content. The He enhanced models are, at fixed metal content, 0.2 to 0.5 mag brighter than canonical ones. This variation is only marginally affected by evolutionary effects. The quoted distance diagnostics once calibrated with trigonometric parallaxes (Gaia) will provide the opportunity to estimate the He content of field and cluster RRLs. Moreover, the use of either spectroscopic or photometric metal abundances will pave the way to new empirical constraints on the universality of the helium to metal enrichment ratio in old stellar tracers.

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