RR Lyrae stars are pulsating variables crucial for distance determination and galactic structure studies. Metallicities of fundamental-mode (RRab) RR Lyrae stars are commonly derived from photometry using empirical relations involving the Fourier parameter ϕ31 and the pulsation period. We present a new, calibrated G-band relationship between pulsation period P, Fourier parameter ϕ31, and metallicity [Fe/H] for galactic RR Lyrae stars from the Gaia survey. A set of 72 fundamental mode RR Lyrae stars were identified for deriving the relation in the G-band after visual examination of their light curves. Unlike recent large-scale calibrations, our relation prioritizes calibration purity by anchoring exclusively to a homogeneously analyzed sample of high-resolution spectroscopic metallicities from the literature. Our best fit relation is [Fe/H]=(−6.93±0.58)−(6.04±0.37)P+(1.65±0.11)ϕ31. We compare the [Fe/H] predicted by our relation for the stars in our calibration sample with that obtained from previously established relations in the G-band using different approaches. Our calibrated G-band P-ϕ31-[Fe/H] relationship demonstrates high reliability when validated against spectroscopic data, achieving a negligible bias of 0.00 dex and an empirical RMS scatter of 0.26 dex. Furthermore, by applying an Orthogonal Distance Regression (ODR) routine that fully propagates parameter covariance, we establish a mathematically strict empirical baseline whose theoretical uncertainties perfectly align with this observed dispersion. We find that the inclusion of the R21 Fourier parameter offers no significant improvement in metallicity estimation. Comparisons with literature confirm that our linear relation aligns closely with other Gaia DR3-based studies, while offering improved precision over older DR2-based relations.

We present a new, calibrated G-band relationship between pulsation period P, Fourier parameter ϕ_31, and metallicity [Fe/H] for galactic RR Lyrae stars from the Gaia survey. A set of 72 fundamental mode RR Lyrae stars were identified for deriving the relation in the G-band, after visual examination of their light curves. Unlike recent large-scale calibrations, our relation prioritizes calibration purity by anchoring exclusively to a homogeneously analyzed sample of high-resolution spectroscopic metallicities from the literature. Our best fit relation is [Fe/H] = (-6.93 ± 0.58) - (6.04 ± 0.37)P + (1.65 ± 0.11)ϕ_31. We compare the [Fe/H] predicted by our relation for the stars in our calibration sample with that obtained from previously established relations in the G-band using different approaches. Our calibrated G-band P-ϕ_31-[Fe/H] relationship demonstrates high reliability when validated against spectroscopic data, achieving a negligible bias of 0.00 dex and an empirical RMS scatter of 0.26 dex. Furthermore, by applying an Orthogonal Distance Regression (ODR) routine that fully propagates parameter covariance, we establish a mathematically strict empirical baseline whose theoretical uncertainties perfectly align with this observed dispersion. We find that the inclusion of the R_21 Fourier parameter offers no significant improvement in metallicity estimation. Comparisons with literature confirm that our linear relation aligns closely with other Gaia DR3-based studies, while offering improved precision over older DR2-based relations.

Stellar spectral interpolation is critical technique employed by fitting software to derive the physical parameters of stars. This approach is necessary because on-the-go generation of synthetic stellar spectra is not possible due to the complex and high cost of computation. The goal of this study is to develop a spectral interpolator for a synthetic spectral library using artificial neural networks (ANNs). The study aims to test the accuracy of the trained interpolator through self-inversion and, subsequently, to utilize the interpolator to derive the physical parameters of stars in the globular cluster NGC 6397 using spectra obtained from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT). In this study, ANNs were trained to function as spectral interpolators. The ULySS full-spectrum fitting package, integrated with the trained interpolators, was then used to extract the physical parameters of 1587 spectra of 1063 stars in NGC 6397. The trained ANN interpolator achieved precise determination of stellar parameters with a mean difference of 31 K for T_ eff and 0.01 dex for [Fe/H] compared to previous studies. This study demonstrates the efficacy of ANN-based spectral interpolation in stellar parameter determination, offering faster and more accurate analysis.

The upcoming Rubin-LSST is expected to revolutionize the field of classical pulsators by offering well-sampled multi-epoch photometric data in multiple wavelengths. Type II Cepheids (T2Cs) exhibit weak or negligible metallicity dependence on period-luminosity (PL) relations. Thus, they may potentially be used as an alternative to classical Cepheids for extragalactic distance estimations, when used together with RR Lyraes and the tip of the red giant branch. It is therefore crucial to study an updated theoretical pulsation scenario of BL Herculis stars (BL Her; the shortest period T2Cs) in the corresponding Rubin-LSST photometric system. We present new theoretical light curves in the Rubin-LSST filters for a fine grid of BL Her models computed using mesa-rsp . We have also derived new theoretical PL and period-Wesenheit (PW) relations in the Rubin-LSST filters with the aim to study the effect of convection parameters and metallicity on these relations. The grid of BL Her models was computed using the non-linear radial stellar pulsation tool mesa-rsp with the input stellar parameters: metallicity (-2.0; dex ≤ Fe/H ≤ 0.0; dex ), stellar mass ($0.5M_ ⊙ -0.8M_ ⊙ $), stellar luminosity ($50L_ ⊙ -300L_ ⊙ $), and effective temperature (across the full extent of the instability strip; in steps of 50K) and using four sets of convection parameters. Bolometric correction tables from MIST were used to transform the theoretical bolometric light curves of the BL Her models into the Rubin--LSST ugrizy filters. The PL relations of the BL Her models exhibit steeper slopes but smaller dispersion with increasing wavelengths in the Rubin-LSST filters. The PL and PW slopes for the complete set of BL Her models computed with radiative cooling (sets B and D) are statistically similar across the grizy filters. The BL Her models exhibit weak or negligible effect of metallicity on the PL relations for wavelengths longer than the g filter for the case of the complete set of models as well as for the low-mass models. However, we find a significant effect of the metallicity on the PL relation in the u filter. Strong metallicity effects are observed in the PWZ relations involving the u filter and are found to have significant contribution from the high-metallicity BL Her models. Due to a negligible metallicity effect for relations involving the Wesenheit indices W(i,g-i), W(z,i-z), and W(y,g-y), we recommend these filter combinations for BL Her stars during observations with Rubin--LSST for use as reliable standard candles.

We present new theoretical light curves in the Rubin-LSST filters for a fine grid of BL Her models computed using MESA-RSP. We also derive new theoretical period-luminosity (PL) and period-Wesenheit (PW) relations in the Rubin-LSST filters with the goal to study the effect of convection parameters and metallicity on these relations. The grid of BL Her models was computed with the input stellar parameters: metallicity (-2.0 dex≤[Fe/H]≤ 0.0 dex), stellar mass (0.5M_⊙-0.8M_⊙), stellar luminosity (50L_⊙-300L_⊙), and effective temperature (across the full extent of the instability strip; in steps of 50K) and using four sets of convection parameters. Bolometric correction tables from MIST were used to transform the theoretical bolometric light curves of the BL Her models into the Rubin-LSST ugrizy filters. The PL relations of the BL Her models exhibit steeper slopes but smaller dispersion with increasing wavelengths in the Rubin-LSST filters. The PL and PW slopes for the complete set of BL Her models computed with radiative cooling (sets B and D) are statistically similar across the grizy filters. The BL Her models exhibit weak or negligible effect of metallicity on the PL relations for wavelengths longer than the g filter for both the cases of the complete set of models as well as the low-mass models. However, we find significant effect of metallicity on the PL relation in the u filter. Strong metallicity effects are observed in the PWZ relations involving the u filter and are found to have significant contribution from the high-metallicity BL Her models. Due to negligible metallicity effect for relations involving the Wesenheit indices W(i,g-i), W(z,i-z) and W(y,g-y), we recommend these filter combinations for BL Her stars when observed with the Rubin-LSST to be used as reliable standard candles.

Context. In the era of precision stellar astrophysics, classical pulsating stars play a crucial role in determinations of the cosmological distance scale thanks to their period-luminosity (PL) relations. Therefore, it is important to constrain their stellar evolution and pulsation models not only through a comparison of empirical and theoretical PL relations and properties at mean light, but also using their light curve structure over the complete pulsation cycle. Aims. We carried out an extensive light curve comparison of BL Her stars using observations from Gaia DR3 and stellar pulsation models computed using MESA-RSP with the goal of obtaining the best-matched observed-model pairs for BL Her stars in the Large Magellanic Cloud (LMC). Methods. We used the Fourier decomposition technique to analyze the light curves in the G band obtained from Gaia DR3 and from MESA-RSP and used a robust light-curve-fitting approach to score the observed-model pairs with respect to their pulsation periods and over their Fourier parameter space. Results. We obtain the best-fit models for 48 BL Her stars in the LMC and thereby provide the stellar parameter estimates of these stars, 30 of which we classify as our “gold sample” due to their superior light curve fits. We find a relatively flat distribution of stellar masses between 0.5 and 0.65 M⊙ for the gold sample of observed-model pairs. An interesting result is that the majority of the best-matched models in the gold sample were computed using the convection parameter sets without radiative cooling. The period-Wesenheit (PW) relation for the best-matched gold sample of 30 BL Her models has a slope of −2.805 ± 0.164 and the corresponding period-radius relation a slope of 0.565 ± 0.035, both in good agreement with the empirical PW and period-radius slopes from BL Her stars in the LMC, respectively. We also used the Wesenheit magnitudes of the 30 best-matched observed-model pairs to estimate a distance modulus of μLMC = 18.582 ± 0.067 to the LMC, which lies within the bounds of previous literature values. We also discuss the degeneracy in the stellar parameters of the BL Her models that result in similar pulsation periods and light curve structure, and highlight that caution must be exercised while using the stellar parameter estimates.

Determining the physical parameters of pulsating variable stars such as RR Lyrae is essential for understanding their internal structure, pulsation mechanisms, and evolutionary state. In this study, we present a machine learning framework that uses feedforward artificial neural networks (ANNs) to infer stellar parameters—mass (M), luminosity (log(L/L⊙)), effective temperature (log(Teff)), and metallicity (Z)—directly from Transiting Exoplanet Survey Satellite (TESS) light curves. The network is trained on a synthetic grid of RRab light curves generated from hydrodynamical pulsation models spanning a broad range of physical parameters. We validate the model using synthetic self-inversion tests and demonstrate that the ANN accurately recovers the input parameters with minimal bias. We then apply the trained model to RRab stars observed by the TESS. The observed light curves are phase-folded, corrected for extinction, and passed through the ANN to derive physical parameters. Based on these results, we construct an empirical period–luminosity–metallicity (PLZ) relation: log(L/L⊙) = (1.458 ± 0.028) log(P/days) + (–0.068 ± 0.007) [Fe/H] + (2.040 ± 0.007). This work shows that ANN-based light-curve inversion offers an alternative method for extracting stellar parameters from single-band photometry. The approach can be extended to other classes of pulsators such as Cepheids and Miras.

Context. In the era of precision stellar astrophysics, classical pulsating stars play a crucial role in determinations of the cosmological distance scale thanks to their period-luminosity (PL) relations. Therefore, it is important to constrain their stellar evolution and pulsation models not only through a comparison of empirical and theoretical PL relations and properties at mean light, but also using their light curve structure over the complete pulsation cycle. Aims. We carried out an extensive light curve comparison of BL Her stars using observations from Gaia DR3 and stellar pulsation models computed using MESA-RSP with the goal of obtaining the best-matched observed-model pairs for BL Her stars in the Large Magellanic Cloud (LMC). Methods. We used the Fourier decomposition technique to analyze the light curves in the G band obtained from Gaia DR3 and from MESA-RSP and used a robust light-curve-fitting approach to score the observed-model pairs with respect to their pulsation periods and over their Fourier parameter space. Results. We obtain the best-fit models for 48 BL Her stars in the LMC and thereby provide the stellar parameter estimates of these stars, 30 of which we classify as our "gold sample" due to their superior light curve fits. We find a relatively flat distribution of stellar masses between 0.5 and 0.65 M circle dot for the gold sample of observed-model pairs. An interesting result is that the majority of the best-matched models in the gold sample were computed using the convection parameter sets without radiative cooling. The period-Wesenheit (PW) relation for the best-matched gold sample of 30 BL Her models has a slope of -2.805 +/- 0.164 and the corresponding period-radius relation a slope of 0.565 +/- 0.035, both in good agreement with the empirical PW and period-radius slopes from BL Her stars in the LMC, respectively. We also used the Wesenheit magnitudes of the 30 best-matched observed-model pairs to estimate a distance modulus of mu LMC = 18.582 +/- 0.067 to the LMC, which lies within the bounds of previous literature values. We also discuss the degeneracy in the stellar parameters of the BL Her models that result in similar pulsation periods and light curve structure, and highlight that caution must be exercised while using the stellar parameter estimates.

In this study, we present analysis of TESS photometry, spectral energy distribution (SED), high-resolution spectroscopy, and spot modeling of the α2 CVn-type star AL Col (HD 46462). The primary objective is to determine its fundamental physical parameters and investigate its surface activity characteristics. Using TESS short-cadence (120 s) SAP flux, we identified a rotational frequency of 0.09655 d−1 (Prot=10.35733 d). Wavelet analysis reveals that while the amplitudes of the harmonic components vary over time, the strength of the primary rotational frequency remains stable. A SED analysis of multi-band photometric data yields an effective temperature (Teff) of 11,750 K. High-resolution spectroscopic observations covering wavelengthrange 4500–7000 Å provide refined estimates of Teff = 13,814 ± 400 K, logg = 4.09 ± 0.08 dex, and υsini = 16 ± 1 km s−1. Abundance analysis shows solar-like composition of O ii, Mg ii, S ii, and Ca ii, while helium is under-abundant by 0.62 dex. Rare earth elements (REEs) exhibit over-abundances of up to 5.2 dex, classifying the star as an Ap/Bp-type star. AL Col has a radius of R=3.74±0.48R⊙, with its H–R diagram position estimating a mass of M=4.2±0.2M⊙ and an age of 0.12±0.01 Gyr, indicating that the star has slightly evolved from the main sequence. The TESS light curves were modeled using a three-evolving-spot configuration, suggesting the presence of differential rotation. This star is a promising candidate for future investigations of magnetic field diagnostics and the vertical stratification of chemical elements in its atmosphere.

Context. In the era of the Hubble tension, it is crucial to obtain a precise calibration of the period-luminosity (PL) relations of classical pulsators. Type II Cepheids (T2Cs; often exhibiting negligible or weak metallicity dependence on PL relations) used in combination with RR Lyraes and the tip of the red giant branch may prove useful as an alternative to classical Cepheids for the determination of extragalactic distances. Aims. We present new theoretical PL and period-Wesenheit (PW) relations for a fine grid of convective BL Her (the shortest period T2Cs) models computed using MESA-RSP in the Gaia passbands and we compare our results with the empirical relations from Gaia DR3. Our goal is to study the effect of metallicity and convection parameters on the theoretical PL and PW relations. Methods. We used the state-of-the-art 1D non-linear radial stellar pulsation tool MESA-RSP to compute models of BL Her stars over a wide range of input parameters: metallicity (−2.0 dex ≤ [Fe/H] ≤ 0.0 dex), stellar mass (0.5 M⊙ − 0.8 M⊙), stellar luminosity (50 L⊙ − 300 L⊙), and effective temperature (across the full extent of the instability strip; in steps of 50 K). We used the Fourier decomposition technique to analyse the light curves obtained from MESA-RSP and Gaia DR3 and then compared the theoretical and empirical PL and PW relations in the Gaia passbands. Results. The BL Her stars in the All Sky region exhibit statistically different PL slopes compared to the theoretical PL slopes computed using the four sets of convection parameters. We find the empirical PL and PW slopes from BL Her stars in the Magellanic Clouds to be statistically consistent with theoretical relations computed using the different convection parameter sets in the Gaia passbands. There is a negligible effect coming from the metallicity on the PL relations in the individual Gaia passbands. However, there is a small but significant negative coefficient of metallicity in the PWZ relations for the BL Her models using the four sets of convection parameters. This could be attributed to the increased sensitivity of bolometric corrections to metallicities at wavelengths shorter than the V band. Our BL Her models also suggest a dependence of the mass-luminosity relation on metallicity. We found the observed Fourier parameter space to be covered well by our models. Higher mass models (> 0.6 M⊙) may be needed to reliably model the observed light curves of BL Her stars in the All-Sky region. We also found the theoretical light curve structures (especially the Fourier amplitude parameters) to be affected by the choice of convection parameters.

We present new theoretical period-luminosity (PL) and period-Wesenheit (PW) relations for a fine grid of convective BL Her, the shortest period T2Cs, models computed using MESA-RSP and compare our results with the empirical relations from Gaia DR3. We use the state-of-the-art 1D non-linear radial stellar pulsation tool MESA-RSP to compute models of BL Her stars over a wide range of input parameters - metallicity (-2.0 dex ≤ [Fe/H] ≤ 0.0 dex), stellar mass (0.5M_⊙-0.8M_⊙), stellar luminosity (50L_⊙-300L_⊙) and effective temperature (full extent of the instability strip; in steps of 50K). The BL Her stars in the All Sky region exhibit statistically different PL slopes compared to the theoretical PL slopes computed using the four sets of convection parameters. We find the empirical PL and PW slopes from BL Her stars in the Magellanic Clouds to be statistically consistent with the theoretical relations computed using the different convection parameter sets in the Gaia passbands. There is negligible effect of metallicity on the PL relations in the individual Gaia passbands. However, there exists a small but significant negative coefficient of metallicity in the PWZ relations for the BL Her models using the four sets of convection parameters. This could be attributed to the increased sensitivity of bolometric corrections to metallicities at wavelengths shorter than the V band. Our BL Her models also suggest a dependence of the mass-luminosity relation on metallicity. We found the observed Fourier parameter space to be covered well by our models. Higher mass models (> 0.6M_⊙) may be needed to reliably model the observed light curves of BL Her stars in the All Sky region. We also found the theoretical light curve structures (especially the Fourier amplitude parameters) to be affected by the choice of convection parameters.

We present a comprehensive photometric study of RR Lyrae stars in the M3 globular cluster, utilizing a vast data set of 3140 optical (UBVRI) CCD images spanning 35yr from astronomical data archives. We have successfully identified previously known 238 RR Lyrae stars from the photometric data, comprising 178 RRab, 49 RRc, and 11 RRd stars. Multiband periodogram was used to significantly improve the long-term periods of 65 percent of RR Lyrae stars in our sample, thanks to the unprecedentedly long temporal coverage of the observations. The light curve templates were used to obtain accurate and precise mean magnitudes and amplitudes of all RR Lyrae variables. We combined optical (UBVRI) and near-infrared (NIR, JHK(s)) photometry of RR Lyrae variables to investigate their location in the colour-magnitude diagrams as well as the pulsation properties such as period distributions, Bailey diagrams and amplitude ratios. The period-luminosity relations in R and I bands and Period-Wesenheit relations were derived after excluding outliers identified in CMDs. The Period-Wesenheit relations calibrated via the theoretically predicted relations were used to determine a distance modulus of mu = 15.04 +/- 0.04 (stats) +/- 0.19 (syst.) mag (using metal-independent W-BV Wesenheit) and mu = 15.03 +/- 0.04 (stats) +/- 0.17 (syst.) mag (using metal-dependent W-VI Wesenheit). Our distance measurements are in excellent agreement with published distances to M3 in the literature. We also employed an artificial neural network based comparison of theoretical and observed light curves to determine physical parameters (mass, luminosity, and effective temperature) for 79 non-Blazhko RRab stars that agree with limited literature measurements.

The Draco Dwarf spheroidal (dSph) galaxy is one of the nearest and the most dark-matter-dominated satellites of the Milky Way. We obtained multiepoch near-infrared (NIR, JHK _s ) observations of the central region of Draco dSph covering a sky area of ∼21′ × 21′ using the WIRCam instrument at the 3.6 m Canada–France–Hawaii Telescope. Homogeneous JHK _s time-series photometry for 212 RR Lyrae (173 fundamental-mode, 24 first-overtone, and 15 mixed-mode variables) and five Anomalous Cepheids in Draco dSph are presented and used to derive their period–luminosity relations at NIR wavelengths for the first-time. The small scatter of ∼0.05 mag in these empirical relations for RR Lyrae stars is consistent with those in globular clusters and suggests a very small metallicity spread, up to ∼0.2 dex, among these centrally located variables. Based on empirically calibrated NIR period–luminosity–metallicity relations for RR Lyrae in globular clusters, we determined a distance modulus to Draco dSph of μ _RRL = 19.557 ± 0.026 mag. The calibrated K _s -band period–luminosity relations for Anomalous Cepheids in the Draco dSph and the Large Magellanic Cloud exhibit statistically consistent slopes but systematically different zero points, hinting at possible metallicity dependence of ∼ − 0.3 mag dex ^−1 . Finally, the apparent magnitudes of the tip of the red-giant branch in I and J bands also agree well with their absolute calibrations with the adopted RR Lyrae distance to Draco. Our recommended ∼1.5% precise RR Lyrae distance, D _Draco = 81.55 ± 0.98(statistical) ± 1.17(systematic) kpc, is the most accurate and precise distance to Draco dSph galaxy.

V5579 Sgr was a fast nova discovered in 2008 April 18.784 UT. We present the optical spectroscopic observations of the nova observed from the Castanet Tolosan, SMARTS and CTIO observatories spanning over 2008 April 23 to 2015 May 11. The spectra are dominated by hydrogen Balmer, Fe II and O I lines with P-Cygni profiles in the early phase, typical of an Fe II class nova. The spectra show He I and He II lines along with forbidden lines from N, Ar, S, and O in the nebular phase. The nova showed a pronounced dust formation episode that began about 20 days after the outburst. The dust temperature and mass were estimated using the WISE data from spectral energy distribution (SED) fits. The PAH-like features are also seen in the nova ejecta in the mid-IR Gemini spectra taken 522 d after the discovery. Analysis of the light curve indicates values of t2 and t3 about 9 and 13 days, respectively, placing the nova in the category of fast nova. The best fit cloudy model of the early decline phase JHK spectra obtained on 2008 May 3 and the nebular optical spectrum obtained on 2011 June 2 shows a hot white dwarf source with T-BB similar to 2.6 x 10(5) K having a luminosity of 9.8 x 10(36) ergs s(-1). Our abundance analysis shows that the ejecta is significantly enhanced relative to solar, O/H = 32.2, C/H = 15.5 and N/H = 40.0 in the early decline phase and O/H = 5.8, He/H = 1.5 and N/H = 22.0 in the nebular phase.

Despite having different astronomical characteristics, the studies of mira variables and ultra-cool dwarfs frequently show similar red colors, which could cause leading to photometric misclassification. This study uses photometric data from the WISE, 2MASS, and Pan-STARRS surveys to construct color-based selection criteria for red dwarfs, brown dwarfs, and Mira variables. On analyzing the color indices, we developed empirical rules that separate these objects with an overall classification accuracy of approximately 91 the differentiation between red dwarfs and both Mira variables and brown dwarfs is effective, challenges remain in distinguishing Mira variables from brown dwarfs due to overlapping color indices. The robustness of our classification technique was validated by a bootstrap analysis, highlighting the significance of color indices in large photometric surveys for stellar classification.

We present a multiwavelength analysis of the young star cluster Berkeley 59, based on Gaia data and deep IR observations with the 3.58 m Telescopio Nazionale Galileo and Spitzer space telescope. The mean proper motion of the cluster is found to be μ _α cos δ ∼ −0.63 mas yr ^−1 and μ _δ ∼ −1.83 mas yr ^−1 , and the kinematic distance of the cluster, ∼1 kpc, is in agreement with previous photometric studies. The present data are the deepest available near-IR observations for the cluster so far and reach below 0.03 M _⊙ . The mass function of the cluster region is calculated using the statistically cleaned color–magnitude diagram and is similar to the Salpeter value for the member stars above 0.4 M _⊙ . In contrast, the slope becomes shallower (Γ ∼ 0.01 ± 0.18) in the mass range 0.04–0.4 M _⊙ , comparable to other nearby clusters. The spatial distribution of young brown dwarfs (BDs) and stellar candidates shows a nonhomogeneous distribution. This suggests that the radiation feedback from massive stars may be a prominent factor contributing to the BD population in the cluster Berkeley 59. We also estimated the star-to-BD ratio for the cluster, which is found to be ∼3.6. The Kolmogorov–Smirnov test shows that the stellar and BD populations significantly differ, and stellar candidates are nearer the cluster center compared to the BDs, suggesting mass segregation in the cluster toward the substellar mass regime.

We carry out an extensive light curve comparison of BL Her stars using observations from Gaia DR3 and stellar pulsation models computed using MESA-RSP with the goal to obtain the best-matched modeled-observed pairs for BL Her stars in the LMC. We use the Fourier decomposition technique to analyse the light curves in the G band obtained from Gaia DR3 and from MESA-RSP and use a robust light curve fitting approach to score the modeled-observed pairs with respect to their pulsation periods and over their Fourier parameter space. We obtain the best-fit models for 48 BL Her stars in the LMC and thereby provide the stellar parameter estimates of these stars, 30 of which are labelled as the gold sample with superior light curve fits. We find a relatively flat distribution of stellar masses between 0.5-0.65 Msolar for the gold sample of modeled-observed pairs. An interesting result is that the majority of the best-matched models in the gold sample are computed using the convection parameter sets without radiative cooling. The period-Wesenheit relation for the best-matched gold sample of 30 BL Her models exhibits a slope of -2.805 ± 0.164 while the corresponding period-radius relation exhibits a slope of 0.565 ± 0.035, both in good agreement with the empirical PW and PR slopes from BL Her stars in the LMC, respectively. We also used the Wesenheit magnitudes of the 30 best-matched modeled-observed pairs to estimate a distance modulus of μ_ LMC = 18.582 ± 0.067 to the LMC, which lies within the bounds of previous literature values. We also discuss the degeneracy in the stellar parameters of the BL Her models that result in similar pulsation periods and light curve structure, and highlight that caution must be exercised while using the stellar parameter estimates.

Context. In the era of the Hubble tension, it is crucial to obtain a precise calibration of the period-luminosity (PL) relations of classical pulsators. Type II Cepheids (T2Cs; often exhibiting negligible or weak metallicity dependence on PL relations) used in combination with RR Lyraes and the tip of the red giant branch may prove useful as an alternative to classical Cepheids for the determination of extragalactic distances. Aims. We present new theoretical PL and period-Wesenheit (PW) relations for a fine grid of convective BL Her (the shortest period T2Cs) models computed using MESA-RSP in the Gaia passbands and we compare our results with the empirical relations from Gaia DR3. Our goal is to study the effect of metallicity and convection parameters on the theoretical PL and PW relations. Methods. We used the state-of-the-art 1D non-linear radial stellar pulsation tool MESA-RSP to compute models of BL Her stars over a wide range of input parameters: metallicity (-2.0 dex <= [Fe/H] <= 0.0 dex), stellar mass (0.5 M-circle dot - 0.8 M-circle dot), stellar luminosity (50 L-circle dot - 300 L-circle dot), and effective temperature (across the full extent of the instability strip; in steps of 50 K). We used the Fourier decomposition technique to analyse the light curves obtained from MESA-RSP and Gaia DR3 and then compared the theoretical and empirical PL and PW relations in the Gaia passbands. Results. The BL Her stars in the All Sky region exhibit statistically different PL slopes compared to the theoretical PL slopes computed using the four sets of convection parameters. We find the empirical PL and PW slopes from BL Her stars in the Magellanic Clouds to be statistically consistent with theoretical relations computed using the different convection parameter sets in the Gaia passbands. There is a negligible effect coming from the metallicity on the PL relations in the individual Gaia passbands. However, there is a small but significant negative coefficient of metallicity in the PWZ relations for the BL Her models using the four sets of convection parameters. This could be attributed to the increased sensitivity of bolometric corrections to metallicities at wavelengths shorter than the V band. Our BL Her models also suggest a dependence of the mass-luminosity relation on metallicity. We found the observed Fourier parameter space to be covered well by our models. Higher mass models (> 0.6 M-circle dot) may be needed to reliably model the observed light curves of BL Her stars in the All-Sky region. We also found the theoretical light curve structures (especially the Fourier amplitude parameters) to be affected by the choice of convection parameters.

Milky Way Cepheid variables with accurate Hubble Space Telescope photometry have been established as standards for primary calibration of the cosmic distance ladder to achieve a percent-level determination of the Hubble constant ( H _0 ). These 75 Cepheid standards are the fundamental sample for investigation of possible residual systematics in the local H _0 determination due to metallicity effects on their period–luminosity relations. We obtained new high-resolution ( R ∼ 81,000), high-signal-to-noise (S/N ∼ 50–150) multiepoch spectra of 42 out of 75 Cepheid standards using the ESPaDOnS instrument at the 3.6 m Canada–France–Hawaii Telescope. Our spectroscopic metallicity measurements are in good agreement with the literature values with systematic differences up to 0.1 dex due to different metallicity scales. We homogenized and updated the spectroscopic metallicities of all 75 Milky Way Cepheid standards and derived their multiwavelength ( GVIJHK _s ) period–luminosity–metallicity and period–Wesenheit–metallicity relations using the latest Gaia parallaxes. The metallicity coefficients of these empirically calibrated relations exhibit large uncertainties due to low statistics and a narrow metallicity range (Δ[Fe/H] = 0.6 dex). These metallicity coefficients are up to 3 times better constrained if we include Cepheids in the Large Magellanic Cloud and range between −0.21 ± 0.07 and −0.43 ± 0.06 mag dex ^−1 . The updated spectroscopic metallicities of these Milky Way Cepheid standards were used in the Cepheid–supernovae distance ladder formalism to determine H _0 = 72.9 ± 1.0 km s ^−1 Mpc ^−1 , suggesting little variation (∼0.1 km s ^−1 Mpc ^−1 ) in the local H _0 measurements due to different Cepheid metallicity scales.

We present a new technique to generate the light curves of RRab stars in different photometric bands ($I$ and $V$ bands) using Artificial Neural Networks (ANN). A pre-computed grid of models was used to train the ANN, and the architecture was tuned using the $I$ band light curves. The best-performing network was adopted to make the final interpolators in the $I$ and $V$ bands. The trained interpolators were used to predict the light curve of RRab stars in the Magellanic Clouds, and the distances to the LMC and SMC were determined based on the reddening independent Wesenheit index. The estimated distances are in good agreement with the literature. The comparison of the predicted and observed amplitudes, and Fourier amplitude ratios showed good agreement, but the Fourier phase parameters displayed a few discrepancies. To showcase the utility of the interpolators, the light curve of the RRab star EZ Cnc was generated and compared with the observed light curve from the Kepler mission. The reported distance to EZ Cnc was found to be in excellent agreement with the updated parallax measurement from Gaia EDR3. Our ANN interpolator provides a fast and efficient technique to generate a smooth grid of model light curves for a wide range of physical parameters, which is computationally expensive and time-consuming using stellar pulsation codes.