arXiv subjects

Sergio Simón-Díaz

Publications and source records attributed to Sergio Simón-Díaz.

18 recordsLinked to original sources

arXiv2026

The future of high-resolution UV spectroscopy: Science with a UV Échelle spectrograph on the Habitable Worlds Observatory, or a dedicated mission

High-resolution UV spectroscopy serves a diversity of science cases, from small bodies to planets, stars, and galaxies, but is currently limited to the Hubble Space Telescope and bright targets. Major advances require increasing sensitivity by at least one order of magnitude. Here we present the UV science cases for PEGASUS (Planets, Earths, Galaxies, And Stars UV Spectrograph), a UV Échelle high-resolution spectrograph concept, with $R = λ/δλ\sim 100\,000$ (full range 10 000-140 000) and covering 90--400 nm, with a foreseen extension to at least 800 nm. PEGASUS is ideally suited for the Habitable Worlds Observatory (HWO), enabling transformative science across the UV/optical wavelength ranges. PEGASUS will be unique in high sensitivity (effective area) and high spectral resolution -- an uncharted territory -- as well as robustness, thanks to the simplicity of its design. Its UV science cases include: I) Formation and evolution of planets and their habitability: properties of exoplanets and atmospheres, protoplanetary disks, Solar System bodies; II) Stellar lives and deaths at their extremes: the first stars and the origin of the elements, compact and massive stars, Supernovae; III) Gas and metals in the baryon cycle of galaxies: the interstellar, circumgalactic, and intergalactic medium and their roles in galaxy growth. These are essential for the Astro Decadal 2020 Survey, Voyage 2050, and HWO. While this paper focuses on high-impact science enabled by UV high-resolution spectroscopy, PEGASUS will extend into the optical regime and lower spectral resolution, making it a multi-purpose, widely used, workhorse spectrograph for HWO.

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arXiv2026

The IACOB project XIX. Revisiting massive-star evolution with empirical TAMS constraints: updated models, overshoot calibration, and the population of blue supergiants

Massive stars play a fundamental role in the evolution of the Universe. Yet, several physical processes governing their evolution remain poorly constrained. Notably, the main-sequence width is sensitive to the convective boundary mixing efficiency; it becomes necessary to account for binary interactions to explain some observed properties of massive-star populations. We constrain single-star models using recent observations of massive Galactic stars from the IACOB database. We use the latest proposed empirical location of the TAMS to calibrate the convective boundary mixing efficiency, and use this calibration to test single-star evolution by comparing various model predictions to the observed populations of the IACOB sample. We compute several GENEC grids with various overshoot calibrations, angular momentum transport (AMT) treatments, initial masses and velocities. Finally, we generate synthetic populations from the tracks with SYCLIST and perform a direct comparison with the observed population. The calibrated models at slow rotation reproduce the empirical TAMS location. We find that a mass-dependent overshoot efficiency is required to fit the observational constraints. The overall rotational properties of the observed populations are well reproduced with single-star models, independently of the AMT assumptions. Models accounting only for hydrodynamical instabilities are successful at reproducing the rotational properties, unlike previous genec grids, which we attribute to the choice of winds prescription. Although the empirical TAMS of slow rotators is well reproduced, we find that models are unsuccessful at explaining the velocity dependence of the TAMS location observed in the IACOB sample. Finally, we find that single-star models fail at explaining the population of blue supergiants to the right of the TAMS location.

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arXiv2026

You're Gonna Need a Bigger Core: Calibrating Massive Star Models against Galactic OB-type Stars

The evolution of massive stars above 8 M$_\odot$ depends critically on the amount of mixing above the convective core during the main sequence. However, current models typically extrapolate results from lower-mass stars, where constraints from asteroseismology and eclipsing binary systems are more readily available. A new opportunity to study the evolution of massive stars and their distribution in the Hertzsprung--Russell diagram arises by combining the IACOB spectroscopic sample of over 900 Galactic OB-type stars with Gaia distances. We use this homogeneously analyzed sample to place population-level constraints on main-sequence evolution. We analyze the data by forward modeling stellar evolution tracks with MESA and applying Bayesian inference. This enables us to 1) determine a physically-motivated, data-driven location of the terminal-age main sequence, 2) constrain convective boundary mixing and resulting core masses, and 3) provide a set of massive star models calibrated against modern data. We explore how boundary mixing depends on mass and find that it is well described by a constant overshooting parameter in the mass range of 12 to 40 M$_\odot$, with $α_{\mathrm{ov}} = 0.33 \pm 0.02$, or $f_{\mathrm{ov}} = 0.028 \pm 0.003$ in the step and exponential overshooting schemes respectively. We find evidence against a continuation of the trend to increase with mass that is found at lower masses. Instead, the data does not exclude a decreasing trend at the high mass end. We find that the resulting helium core masses are 10 to 40% larger than other commonly used overshooting prescriptions. Combining our findings with existing observational constraints for low- and intermediate-mass stars, we propose a new mass-dependent overshooting prescription for a wide range of masses. Our calibration and model set are particularly useful for population and spectral synthesis applications.

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arXiv2026

Massive Stars in the Thirties: awaiting new Hubble discoveries

Massive stars play a fundamental role in shaping the evolution of galaxies through feedback, chemical enrichment, and their end products as neutron stars and black holes. Despite major progress in the last decade, key uncertainties remain in the physics of massive stars, particularly in mass loss, internal mixing, binary interactions, and the upper end of the initial mass function. These uncertainties directly affect our understanding of stellar populations, gravitational wave progenitors, and the young Universe probed by JWST. HST is uniquely capable to address these open questions. UV diagnostics are essential for determining stellar parameters, tracing stellar winds, and identifying interacting binaries and stripped-envelope stars. Long-term spectroscopic monitoring further enables constraints on variability, wind structure, and presupernova mass loss. We outline a set of questions which need to be addressed in 2030s by combining UV - optical spectroscopy, time - domain monitoring, and archival baseline exploitation of massive stars and star forming regions. These observations will target massive stars across a range of metallicities, resolve the most luminous stellar populations, and identify compact-object binaries and post-interaction systems. Together, these efforts will pave the way to HWO and secure the long-term legacy of HST in massive star astrophysics.

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arXiv2025

Science Enabled by a 30-Meter-Class Telescope in the Northern Hemisphere: Massive Stars at Low Metallicity

Massive stars are at the core of our observations of the Universe up to the reionization epoch, both through their intense ionizing fluxes and through the energetic end products that release fresh elements into the interstellar medium. Our interpretation of very high redshift galaxies and transient phenomena depends on knowledge derived from massive star populations in the Milky Way and nearby galaxies, with characteristics that only remotely resemble the conditions in the early Universe. However, the models supporting these interpretations have been tested in a narrow range of environments and carry significant uncertainties when extrapolated. Advancing in our understanding of the Universe beyond the Local Volume therefore requires extending massive star studies to conditions representative of the early Universe. The next generation of telescopes has the potential to accomplish this goal.

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arXiv2025

A magnetic field detection in the massive O-type bright giant 63 Oph

Surface magnetic fields are detected in less than $10\%$ of the massive O-type star population and even less frequently among `old' massive stars approaching the terminal-age main sequence (TAMS). It is unclear to what extent the rarity of magnetic detections in massive stars near the TAMS is due to magnetic field decay or observational biases. We report the detection of a weak surface magnetic field in the O-type giant 63~Oph ($T_{\mathrm{eff}}=35.0\pm0.3\,$kK, $\log g=3.51\pm0.03$) from new ESPaDOnS circularly polarized spectra. The mean longitudinal field strength associated with the magnetic detection is $\langle B_{z}\rangle=84\pm14\,$G, which we use to set a lower limit on the dipolar field strength of $B_{\mathrm{p}}\geq300\pm50\,$G. We report Balmer line equivalent widths (EW) and radial velocity (RV) measurements from the analysis of spectra primarily obtained by the IACOB project with the FEROS, FIES and HERMES spectrographs. We identify a dominant period of $\sim19.8\,$d in the EWs which we attribute to the effects of a rotating magnetosphere under the Oblique Rotator Model. We do not identify any coherent signals in a time-series analysis of archival Hipparcos, ASAS-SN and K2 photometry. Our findings show that 63~Oph may be a rare link between strongly magnetic massive stars detected on or near the zero-age main sequence and weakly-magnetic O-type supergiants. Additional observations are needed to fully constrain 63~Oph's magnetic field geometry and magnetospheric properties.

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arXiv2025

Probing Thin Winds in Massive OB Stars of the SMC with JWST/NIRSPEC Br-alpha Spectroscopy

Mass loss is a key physical process in the evolution of massive stars, the impact of which propagates into galactic evolution, population synthesis models, the interpretation of high-redshift galaxies, and explosive events such as supernovae. However, there are currently substantial uncertainties in the low-metallicity, low-luminosity thin wind regime where classical diagnostics (H-alpha and ultraviolet, UV, P Cygni profiles) yield wind momenta that are 1 to 2 orders of magnitude below prescriptions implemented by default in most evolutionary models. Here, we present spectra of the mass-loss diagnostic line Br-alpha in 15 OB-type stars in the Small Magellanic Cloud obtained using the Near Infrared Spectrograph on the James Webb Space Telescope. The line profile morphology, recovered by virtue of the outstanding signal-to-noise ratio of the data and the avoidance of regions with nebular emission, is consistent with predictions based on previous mass-loss rate estimates from optical and UV spectroscopy. Moreover, an initial spectroscopic analysis of sources covering the thin wind regime confirms the low mass-loss rates, indicates a change of slope in the wind-momentum-luminosity relation in this regime with respect to high-luminosity objects, and strengthens the abovementioned discrepancies with commonly used wind-momentum prescriptions.

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arXiv2025

NGC 663 as a laboratory for massive star evolution

Massive young clusters with rich populations of high-mass stars are ideal laboratories to explore their evolutionary paths. Despite being the most prominent cluster in the Perseus-arm Cas OB8 association, NGC 663 remains comparatively little studied. We present a comprehensive investigation of its properties, integrating astrometric, photometric and spectroscopic data for the cluster and its surroundings, including accurate spectral classification for over 150 members. Gaia astrometry indicates over 300 B-type members, possibly rendering NGC 663 the most massive cluster in the Perseus arm, with initial mass likely exceeding 10000 M_\sun . This large population makes NGC 663 an excellent laboratory for studying massive star evolution. Spectral analysis of the earliest members reveals approximately solar metallicity and a turn-off mass of approximate 8.5 M_\sun, consistent with the photometric age of 23 Ma. We identify five spectroscopic blue stragglers, including the Be/X-ray binary RX J0146.9$+$6121. We outline its evolutionary history and compare its properties with other Be stars. Although the cluster contains many Be stars, their relative fraction is not particularly high. Intriguingly, four of the six blue supergiant members appear to have significantly higher masses than the brightest giants near the Hertzsprung gap. These observations suggest that most mid-B supergiants may form via mergers, unless stars of 10-12 M_\sun born as primaries in binaries rarely undergo supernova explosions. Similarly, if Be stars form through the binary channel, then either most are produced through case A evolution or supernovae are uncommon among primaries in this mass range.

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arXiv2025

A reference framework for extremely metal-poor OB star studies: calibrations for stellar parameters and intrinsic colours

We provide the first reference framework for extremely metal-poor (XMP) OB-type stars. We parsed a grid of 0.10 $Z_{\odot}$ FASTWIND models, covering the parameter space of O stars and early-B supergiants, through contemporary spectral classification criteria to deliver a calibration of key stellar properties as a function of spectral type, and tabulated colours for the most common photometric systems. By using an extensive grid of models, we account for the different combinations of stellar parameters that result in the same spectral morphology and provide a range of parameters and colours compatible with each spectral subtype and luminosity class. We supply updated photometric criteria to optimize candidate selection of OB stars in XMP environments. We find 0.10 $Z_{\odot}$ OB stars are 1-6 kK hotter and produce higher ionizing fluxes than their Galactic analogues. In addition, we find a bimodal distribution of the HeII-ionizing flux with spectral type; because of its known dependence on effective temperature and the wind, $\log~q_{HeII}$ for individual XMP late-O type stars could be underestimated by up to 4 orders of magnitude by other calibrations, some of them used by population synthesis codes. Finally, we used our calibrated colours to map the extinction of the 0.10 $Z_{\odot}$ galaxy Sextans A finding that reddening is non-negligible and uneven.

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arXiv2024

The IACOB project XII. A new grid of northern standards for the spectral classification of B-type stars

With the advent of large spectroscopic surveys, automated stellar parameter determination has become commonplace. Nevertheless, spectral classification still offers a quick and useful alternative for obtaining parameter estimates for large samples of spectra of varying quality. We present a new atlas of stellar spectra covering the B-type range, with the intention of providing detailed classification criteria valid for modern spectra and improving the grid of reliable standards. This new grid will be used in future works to provide classification criteria beyond the classical classification range and addressing, in particular, the use of Gaia/RVS spectra. We analysed historical standards by means of multiple high-resolution spectra, marking out problematic cases and complementing the grid with a new set of reliable comparators. We then elaborated on a new set of classification criteria based on high-quality R=4000 spectra. Our new classification grid is much thicker than any previous set of standards, presenting a high degree of self-consistency. Although it is based entirely on morphological criteria, the grid demonstrates a much better correlation with different physical parameters. The new grid can be used to study classification criteria in other spectral ranges, providing a valuable tool for the study of B-type stars that covers a very wide range of temperatures, luminosities, and stellar masses. The very process of classification also offers valuable insights into stellar evolution.

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arXiv2024

The IACOB project XI. No increase of mass-loss rates over the bistability region

The properties of blue supergiants are key for constraining the end of the main sequence (MS) of massive stars. Whether the observed drop in the relative number of fast-rotating stars below $\sim$21$\,$kK is due to enhanced mass-loss rates at the location of the bistability jump, or the result of the end of the MS is still debated. Here, we combine newly derived estimates of photospheric and wind parameters with Gaia distances and wind terminal velocities from the literature to obtain upper limits on the mass-loss rates for a sample of 116 Galactic luminous blue supergiants. The parameter space covered by the sample ranges between 35-15$\,$kK in $T_{\rm eff}$ and 4.8-5.8$\,$dex in log(L/L$_{\odot}$). Our results show no increase in the mass-loss rates over the bistability jump. Therefore, we argue that the drop in rotational velocities cannot be explained by enhanced mass loss. Since a large jump in the mass-loss rates is commonly included in evolutionary models, we suggest an urgent revision of the currently used default prescriptions.

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arXiv2024

The IACOB project X. Large-scale quantitative spectroscopic analysis of Galactic luminous blue stars

Blue supergiants (BSGs) are key objects for understanding the evolution of massive stars. However, discrepancies between theoretical predictions and empirical observations have opened up important questions yet to be answered. Studying statistically significant and unbiased samples of BSGs can help to improve the situation. We aim to perform a homogeneous and comprehensive quantitative spectroscopic analysis of a large sample of Galactic luminous blue stars (being the majority BSGs) from the IACOB spectroscopic database. We derive the projected rotational velocity ($v\sin i$) and macroturbulent broadening ($v_{\rm mac}$) using IACOB-BROAD. We used FASTWIND computations to derive effective temperatures ($T_{\rm eff}$), surface gravities, microturbulences ($ξ$), Si and He surface abundances, and the wind-strength parameter. We provide estimates of these quantities for the largest sample of Galactic luminous O9-B5 stars spectroscopically analyzed to date, with 527 targets. We find a drop in the relative number of stars at ~21 kK, coinciding with a scarcity of fast rotating stars below that temperature. We speculate that this feature might be roughly delineating the location of the Terminal-Age-Main-Sequence in the 15-85M$_\odot$ range. By investigating the main characteristics of the $v\sin i$ distribution as a function of $T_{\rm eff}$, we propose that an efficient mechanism transporting angular momentum from the stellar core to the surface might be operating along the main sequence. We find correlations between $ξ$, $v_{\rm mac}$ and the spectroscopic luminosity. We also find that no more than 20% of the stars in our sample have atmospheres clearly enriched in He, and suggest that the origin of this specific sub-sample might be in binary evolution. We do not find clear empirical evidence of an increase in the wind-strength over the wind bi-stability region towards lower temperatures.

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arXiv2023

A calibration point for stellar evolution from massive star asteroseismology

Massive stars are progenitors of supernovae, neutron stars and black holes. During the hydrogen-core burning phase their convective cores are the prime drivers of their evolution, but inferences of core masses are subject to unconstrained boundary mixing processes. Moreover, uncalibrated transport mechanisms can lead to strong envelope mixing and differential radial rotation. Ascertaining the efficiency of the transport mechanisms is challenging because of a lack of observational constraints. Here we deduce the convective core mass and robustly demonstrate non-rigid radial rotation in a supernova progenitor, the $12.0^{+1.5}_{-1.5}$ solar-mass hydrogen-burning star HD 192575, using asteroseismology, TESS photometry, high-resolution spectroscopy, and Gaia astrometry. We infer a convective core mass ($M_{\rm cc} = 2.9^{+0.5}_{-0.8}$ solar masses), and find the core to be rotating between 1.4 and 6.3 times faster than the stellar envelope depending on the location of the rotational shear layer. Our results deliver a robust inferred core mass of a massive star using asteroseismology from space-based photometry. HD 192575 is a unique anchor point for studying interior rotation and mixing processes, and thus also angular momentum transport mechanisms inside massive stars.

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arXiv2023

The IACOB project IX. Building a modern empirical database of Galactic O9-B9 supergiants: sample selection, description, and completeness

Blue supergiants (BSGs) are important objects to study the intermediate phases of massive star evolution, helping to constrain evolutionary models. However, the lack of a holistic study of a statistically significant and unbiased sample of these objects makes several long-standing questions about their nature to remain unsolved. The present and other upcoming papers of the IACOB series are focused in studying - from a pure empirical point of view - a sample of 500 Galactic O9 - B9 stars with luminosity classes I and II (plus 250 late O- and early B-type stars with luminosity classes III, IV and V) and covering distances up to 4 kpc from the Sun. We compile an initial set of 11000 high-resolution spectra of 1600 Galactic late O- and B-type stars. We use a new novel spectroscopic strategy based on a simple fitting of the Hbeta line to select stars in a specific region of the spectroscopic HR diagram. We evaluate the completeness of our sample using the Alma Luminous Star catalog (ALS III) and Gaia-DR3 data. We show the benefits of the proposed strategy for identifying BSGs descending from stellar objects born as O-type stars, in the context of single star evolution. The resulting sample reaches a high level of completeness with respect to the ALS III catalog, gathering the 80% for all-sky targets brighter than Bmag < 9 located within 2 kpc. However, we identify the need for new observations in specific regions of the Southern hemisphere. In conclusion, we have explored a very fast and robust method to select BSGs, providing a valuable tool for large spectroscopic surveys like WEAVE-SCIP or 4MIDABLE-LR, and highlighting the risk of using spectral classifications from the literature. Upcoming works will make use of this large and homogeneous spectroscopic sample to study specific properties of these stars in detail. We initially provide first results about their rotational properties.

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arXiv2020

MOBSTER: Identifying Candidate Magnetic O Stars through Rotational Modulation of TESS Photometry

Being relatively rare, the properties of magnetic O stars are not fully understood. To date fewer than a dozen of these stars have been confirmed, making any inference of their global properties uncertain due to small number statistics. To better understand these objects it is necessary to increase the known sample. The MOBSTER collaboration aims to do this by identifying candidate magnetic O, B, and A stars from the identification of rotational modulation in high-precision photometry from the Transiting Exoplanet Survey Satellite (TESS). Here we discuss the collaboration's efforts to detect rotational modulation in TESS targets to identify candidate magnetic O stars for future spectropolarimetric observations.

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arXiv2019

Low-frequency gravity waves in blue supergiants revealed by high-precision space photometry

Almost all massive stars explode as supernovae and form a black hole or neutron star. The remnant mass and the impact of the chemical yield on subsequent star formation and galactic evolution strongly depend on the internal physics of the progenitor star, which is currently not well understood. The theoretical uncertainties of stellar interiors accumulate with stellar age, which is particularly pertinent for the blue supergiant phase. Stellar oscillations represent a unique method of probing stellar interiors, yet inference for blue supergiants is hampered by a dearth of observed pulsation modes. Here we report the detection of diverse variability in blue supergiants using the K2 and TESS space missions. The discovery of pulsation modes or an entire spectrum of low-frequency gravity waves in these stars allow us to map the evolution of hot massive stars towards the ends of their lives. Future asteroseismic modelling will provide constraints on ages, core masses, interior mixing, rotation and angular momentum transport. The discovery of variability in blue supergiants is a step towards a data-driven empirical calibration of theoretical evolution models for the most massive stars in the Universe.

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arXiv2015

Open cluster Dolidze 25: Stellar parameters and the metallicity in the Galactic Anticentre

The young open cluster Dolidze 25, in the direction of the Galactic Anticentre, has been attributed a very low metallicity, with typical abundances between $-0.5$ and $-0.7$ dex below solar. We intend to derive accurate cluster parameters and accurate stellar abundances for some of its members. We have obtained a large sample of intermediate- and high-resolution spectra for stars in and around Dolidze 25. We used the FASTWIND code to generate stellar atmosphere models to fit the observed spectra. We derive stellar parameters for a large number of OB stars in the area, and abundances of oxygen and silicon for a number of stars with spectral types around B0. We measure low abundances in stars of Dolidze 25. For the three stars with spectral types around B0, we find $0.3$ dex (Si) and $0.5$ dex (O) below the values typical in the solar neighbourhood. These values, even though not as low as those given previously, confirm Dolidze 25 and the surrounding H II region Sh2-284 as the most metal-poor star-forming environment known in the Milky Way. We derive a distance $4.5\pm0.3\:$kpc to the cluster ($r_{\textrm{G}}\approx12.3\:$kpc). The cluster cannot be older than $\sim3\:$Myr, and likely is not much younger. One star in its immediate vicinity, sharing the same distance, has Si and O abundances at most $0.15\:$dex below solar. The low abundances measured in Dolidze 25 are compatible with currently accepted values for the slope of the Galactic metallicity gradient, if we take into account that variations of at least $\pm0.15\:$dex are observed at a given radius. The area traditionally identified as Dolidze 25 is only a small part of a much larger star-forming region that comprises the whole dust shell associated with Sh2-284 and very likely several other smaller H II regions in its vicinity.

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arXiv2014

Sher 25: pulsating but apparently alone

The blue supergiant Sher25 is surrounded by an asymmetric, hourglass-shaped circumstellar nebula, which shows similarities to the triple-ring structure seen around SN1987A. From optical spectroscopy over six consecutive nights, we detect periodic radial velocity variations in the stellar spectrum of Sher25 with a peak-to-peak amplitude of ~12 km/s on a timescale of about 6 days, confirming the tentative detec-tion of similar variations by Hendry et al. From consideration of the amplitude and timescale of the signal, coupled with observed line profile variations, we propose that the physical origin of these variations is related to pulsations in the stellar atmosphere, rejecting the previous hypothesis of a massive, short-period binary companion. The radial velocities of two other blue supergiants with similar bipolar nebulae, SBW1 and HD 168625, were also monitored over the course of six nights, but these did not display any significant radial velocity variations.

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