Miguel A. Urbaneja

Context. Classical Cepheid metallicities trace recent chemical enrichment and help assess systematics in Cepheid-based distance measurements. They are derived mainly from high-resolution optical spectroscopy, which defines the benchmark abundance scale but restricts measurements to nearby systems. Extending such studies to more distant Cepheids with extremely large telescopes requires complementary low-resolution near-infrared approaches and careful treatment of model–data residuals. Aims. We tested whether low-resolution SpeX/IRTF Y + J -band spectra can deliver Cepheid metallicities on a scale compatible with homogenised high-resolution optical abundances, using Galactic calibrators as a first validation step towards more distant applications. Methods. We analysed a homogeneous sample of 14 Galactic classical Cepheids observed with SpeX/IRTF, spanning periods from approximately 13 to 69 d, supplemented by two shorter-period Cepheids from the IRTF Spectral Library. The spectra were analysed with a Bayesian full-spectrum fitting method based on MARCS/TURBOSPECTRUM synthetic spectra and a regularised covariance likelihood, from which we inferred the global metallicity parameter [M/H]. The resulting metallicities were compared with homogenised optical [Fe/H] measurements. Results. For the primary 14-star homogeneous sample, the inferred metallicities reproduce the homogenised optical reference scale with a mean offset of −0.05 dex, a scatter of 0.09 dex, and a robust scatter of 0.12 dex. The median posterior uncertainty is 0.22 dex. Including the two additional stars changes these values only slightly. Relative to simpler diagonal likelihoods, the use of a covariance-aware likelihood reduces the mean offset and improves the stability of the uncertainty calibration. Conclusions. Low-resolution Y + J spectra contain usable metallicity information for classical Cepheids, provided that short-range correlated residuals and localised modelling imperfections are treated explicitly. The resulting SpeX-based near-infrared metallicity scale is tied to homogenised optical [Fe/H] measurements and provides a practical foundation for extending Cepheid metallicity work to larger and more distant samples.

The presence of massive O-type stars with surfaces enriched by CNO-cycle products has been known since the early 1980s. For many years, internal rotational mixing was assumed to be the dominant mechanism responsible for this chemical contamination. However, accumulating evidence suggests that binary interaction may play an equally important, if not dominant, role.

Context . Quantitative spectroscopy of luminous blue stars relies on detailed non-local thermodynamic equilibrium (non-LTE) model atmospheres whose increasing physical realism makes direct iterative analyses computationally demanding. Aims . We introduce Machine-learning assisted Uncertainty inference (MAUI), a statistical framework designed for an efficient Bayesian inference of stellar parameters using emulator-based spectral models. Methods . MAUI employs Gaussian-process-based emulators trained on a limited set of non-LTE simulations, combined with Markov chain Monte Carlo sampling to explore posterior distributions. We validate the approach with recovery experiments and demonstrate it on Galactic late-type O dwarf and early-type B dwarf and subgiant stars. Results . The emulator reproduces the predictions of full atmosphere models within the quoted uncertainties while reducing computational cost by several orders of magnitude. Posterior distributions are well calibrated with a conservative coverage across all stellar parameters. Conclusions . The emulator-driven Bayesian inference retains the accuracy of classical analyses at a fraction of the computational expense, which enables posterior sampling that would be prohibitive with direct model evaluations. This positions emulators as a practical tool for high-fidelity spectroscopy of massive stars as atmosphere models grow more demanding.

Context: Quantitative spectroscopy of luminous blue stars relies on detailed non-LTE model atmospheres whose increasing physical realism makes direct, iterative analyses computationally demanding. Aims: We introduce MAUI (Machine-learning Assisted Uncertainty Inference), a statistical framework designed for efficient Bayesian inference of stellar parameters using emulator-based spectral models. Methods: MAUI employs Gaussian-process-based emulators trained on a limited set of non-LTE simulations, combined with Markov Chain Monte Carlo (MCMC) sampling to explore posterior distributions. We validate the approach with recovery experiments and demonstrate it on Galactic late-type O dwarf and early-type B dwarf/subgiant stars. Results: The emulator reproduces the predictions of full atmosphere models within quoted uncertainties while reducing computational cost by orders of magnitude. Posterior distributions are well calibrated, with conservative coverage across all stellar parameters. Conclusions: Emulator-driven Bayesian inference retains the accuracy of classical analyses at a fraction of the computational expense, enabling posterior sampling that would be prohibitive with direct model evaluations. This positions emulators as a practical tool for high-fidelity spectroscopy of massive stars as atmosphere models grow more demanding.

Blue supergiant distances of nearby galaxies obtained with the flux-weighted gravity–luminosity relationship are used for a measurement of the zero-points of Tully–Fisher relationships at different photometric passbands. The Cousins I band and the infrared WISE bands W1 and W2 are investigated. The results are compared with previous work using Cepheid and tip of the red giant branch distances. No significant differences were encountered. This supports the large values of the Hubble constant greater than 73 km s ⁻¹ Mpc ⁻¹ found with the Tully–Fisher distance ladder work over the last decade. Applying blue supergiant distances on the I -band Tully–Fisher relation observations yields a Hubble constant H ₀ = 76.2 ± 6.2 km s ⁻¹ Mpc ⁻¹ . The large uncertainty is caused by the still relatively small blue supergiant galaxy sample size but will be reduced in future work.

Blue supergiant distances of nearby galaxies obtained with the flux-weighted gravity-luminosity relationship are used for a measurement of the zero points of Tully-Fisher relationships at different photometric passbands. The Cousins I-band and the infrared WISE bands W1 and W2 are investigated. The results are compared with previous work using Cepheid and Tip-of-the-Red-Giant-Branch distances. No significant differences were encountered. This supports the large values of the Hubble constant greater than 73km/s/Mpc found with the Tully-Fisher distance ladder work over the last decade. Applying blue supergiant distances on the I-band Tully-Fisher relation observations yields a Hubble constant H0 = 76.2+/-6.2 km/s/Mpc. The large uncertainty is caused by the still relatively small blue supergiant galaxies sample size but will be reduced in future work.

We present a quantitative spectroscopic study of 13 blue supergiant stars in the Pinwheel Galaxy M101, based on data obtained with the Low Resolution Imaging Spectrometer available at the Keck I telescope. The average stellar metallicity decreases from ∼1.9 Z _⊙ near the center of the galaxy to ∼0.3 Z _⊙ at the optical outskirts. The galactocentric radial metallicity gradient is statistically consistent with previous studies of the gas-phase oxygen abundance from H ii regions using the direct method. The H ii region-based Cepheid metallicities used by A. G. Riess et al. in their determination of the Hubble constant H ₀ are in substantial agreement with our measurements. The direct method gas-phase metallicities of the 18 star-forming galaxies we have analyzed so far, when adjusted upward for a mean ∼0.15 dex oxygen dust depletion factor, are in good agreement with those we infer from the supergiants, over a factor of 50 in metallicity. From the same data, we derive an expression for the metal-dependent depletion of oxygen in photoionized nebulae. Utilizing the flux-weighted gravity–luminosity relationship (FGLR) of blue supergiants, we measure a distance to M101, D = 6.5 ± 0.2 Mpc (μ = 29.06 ± 0.08), which is within 1σ from determinations based on the tip of the red giant branch and Cepheids. With M101 as a nearby Type Ia supernova host and using the observed standardized B-band magnitude of the supernova, our FGLR distance yields an independent value of H ₀ = 72.5 ± 4.6 km s⁻¹ Mpc⁻¹.

Context. Massive stars play a very important role in many astrophysical fields. Despite their scarcity with respect to less-massive counterparts, their influence on the chemo-dynamical evolution of the galaxies is substantial. Yet, some fundamental aspects of their evolution remain poorly constrained. In this regard, there is an open debate on the width of the main-sequence (MS) phase, in which stars spend most of their lifetimes. Aims. We aim to create an updated Hertzsprung-Russell (HR) diagram that includes a volume-limited and statistically significant sample of massive stars with luminosities L ≳ 2 × 10⁴ L_⊙ and effective temperatures T_eff ≳ 14 kK. Our goal is to use this sample to investigate the extension and end of the MS while also incorporating information about projected rotational velocities (v sin i) and spectroscopic binarity status. Methods. We combined spectroscopic parameters derived with the FASTWIND stellar atmosphere code and Gaia distances to obtain estimates of stellar parameters for a sample of 876 Galactic luminous O- and B-type stars gathered within the IACOB project. We used the ALS III catalog of Galactic OB stars to select the best-suited volume-limited sample for our study. We chose the iacob-broad tool to derive v sin i estimates and reviewed multi-epoch spectra to identify single- and double-line spectroscopic binaries (SB1, SB2). Results. We present an HR diagram for a sample of 670 stars located within 2500 pc that has the best balance between completeness and number. We evaluated the extension of the MS in terms of the drop in the relative number of stars as a function of the effective temperature for different luminosity ranges. We found a consistent cool boundary at ≈22.5 kK within the full range of luminosities that we used to delineate the terminal-age main sequence (TAMS). We obtained a smooth decrease of the highest observed v sin i with T_eff along the MS band, which is likely limited by the critical velocity. We consider this effect combined with a lower expected fraction of stars beyond the MS as the best explanation for the lack of fast-rotating objects in the post-MS region. Our results favor low to mild initial rotation (v_ini ≲ 150 km s⁻¹) for the full sample along with a binary past for the well-known tail of fast-rotating stars in the v sin i distribution. The prominence of SB1 and SB2 systems within the MS band and the 25% decrease in the relative fraction of SB1 systems when crossing the TAMS can further delineate its location.

We present a complete set of physical parameters for three early-type eclipsing binary systems in the Large Magellanic Cloud (LMC): OGLE LMC-ECL-17660, OGLE LMC-ECL-18794, and HV 2274, together with the orbital solutions. The first and third systems comprise B-type stars, while the second has O-type components and exhibits a total eclipse. We performed a complex analysis that included modeling light and radial velocity curves, O − C analysis, and additional non-LTE spectroscopic analysis for the O-type system. We found that OGLE LMC-ECL-17660 is at least a triple, and tentatively, a quadruple. A significant nonlinear period decrease was determined for HV 2274. Its origin is unclear, possibly due to a faint, low-mass companion on a wide orbit. The analyzed components have masses ranging from 11.7 M _⊙ to 22.1 M _⊙, radii from 7.0 R _⊙ to 14.2 R _⊙, and temperatures between 22,500 and 36,000 K. For HV 2274, the precision of our masses and radii is about six times higher than in previous studies. The position of the components of all six systems analyzed in this series on the mass–luminosity and mass–radius diagrams indicates they are evolutionarily advanced on the main sequence. Our sample contributes significantly to the knowledge of physical parameters of early-type stars in the mass range of 11 M _⊙–23 M _⊙. A new mass–luminosity relation for O- and B-type stars in the LMC is provided. Additionally, we used the measured apsidal motion of the systems to compare the observational and theoretical internal structure constant.

We present a complete set of physical parameters for three early-type eclipsing binary systems in the Large Magellanic Cloud (LMC): OGLE LMC-ECL-17660, OGLE LMC-ECL-18794, and HV 2274, together with the orbital solutions. The first and third systems comprise B-type stars, while the second has O-type components and exhibits a total eclipse. We performed a complex analysis that included modeling light and radial velocity curves, O-C analysis, and additional non-LTE spectroscopic analysis for the O-type system. We found that OGLE LMC-ECL-17660 is at least a triple and, tentatively, a quadruple. A significant non-linear period decrease was determined for HV 2274. Its origin is unclear, possibly due to a faint, low-mass companion on a wide orbit. The analyzed components have masses ranging from 11.7 M$_\odot$ to 22.1 M$_\odot$, radii from 7.0 R$_\odot$ to 14.2 R$_\odot$, and temperatures between 22500 K and 36000 K. For HV 2274, the precision of our masses and radii is about six times higher than in previous studies. The position of the components of all six systems analyzed in this series on the mass-luminosity and mass-radius diagrams indicates they are evolutionarily advanced on the main sequence. Our sample contributes significantly to the knowledge of physical parameters of early-type stars in the mass range of 11 M$_\odot$ to 23 M$_\odot$. A new mass-luminosity relation for O and B-type stars in the LMC is provided. Additionally, we used the measured apsidal motion of the systems to compare the observational and theoretical internal structure constant.