arXiv subjects

Francisco Najarro

Publications and source records attributed to Francisco Najarro.

At least 19 recordsLinked to original sources

arXiv2026

ALMA measurements of mass loss and wind clumping in the massive stars of the Arches cluster

We present the first Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 (100 GHz) and Band 6 (243 GHz) continuum observations of the Arches cluster, one of the youngest and most massive stellar clusters in the Milky Way. We detect and characterise millimetre emission from 23 massive stars, including WN7-9h Wolf-Rayet stars, O-type supergiants and hypergiants. By combining our ALMA measurements with archival Very Large Array data spanning 5-22.5 GHz, we derive broadband radio-millimetre spectral indices and investigate the radial structure of stellar winds through frequency-dependent clumping diagnostics. The majority of Wolf-Rayet stars exhibit spectral indices clustered around $α\approx 0.7-0.8$, consistent with predominantly thermal free-free emission from dense, partially optically thick winds. In contrast, several O-type stars show flat or negative broadband spectral indices, indicative of non-thermal synchrotron emission likely associated with colliding-wind binaries. Using millimetre flux densities, we derive clumping-scaled mass-loss rates spanning $\log(\dot{M}/\mathrm{M}_{\odot}\,\text{yr}^{-1})\approx-4.1$ to $-4.9$ for the WN stars and $-4.9$ to $-5.4$ for the O super-/hypergiants, consistent with expectations for luminous massive stars in the Galactic Centre environment. We find significant evidence of structured wind clumping at millimetre wavelengths that generally decreases with increasing radius, supporting structured wind models with strong inner-wind inhomogeneities. These results demonstrate the power of combined radio-millimetre observations for constraining mass-loss and wind structure in massive stars, and provide new insight into stellar feedback in extreme cluster environments.

astro-ph.SR↗

arXiv2026

The winds of OBA hypergiants and luminous blue variables: Dynamically-consistent atmosphere models reveal multiple wind regimes

OBA hypergiants (OBAHGs) are evolved massive stars with notable wind features in their optical spectrum. Located at the cool edge of the line-driven wind regime, many are candidate luminous blue variables (LBVs) likely near the Eddington limit. Although brief, this evolutionary stage deeply affects their surroundings and subsequent evolution. We study the mechanisms behind OBAHG winds and spectra, covering the temperature range of non-eruptive LBVs. Using the PoWR atmosphere code, we compute models with an Eddington parameter Gamma_e ~ 0.4 and moderate turbulent pressure, typical for cool hypergiants, varying the effective temperature from ~12.5 to ~38.0 kK at solar metallicity. Our models show a complex temperature-dependent mass-loss pattern, with regions of higher/lower rates linked to two wind solutions: "dense" and "rarefied." Spectra of known OBAHGs and LBVs match models from all solution regions. We find bi-stability jumps -- with sharp mass-loss increases -- at temperatures where Fe IV recombines to Fe III (and Fe III to Fe II). "Drops" in mass loss also occur when the leading Fe ion changes at wind onset, signaling a switch to rarefied solutions under insufficient driving opacity. The resulting velocity fields also reflect these different regimes: rarefied solutions match the empirical terminal velocity vs temperature relation, while dense ones deviate. Turbulent pressure is crucial for wind acceleration at cooler temperatures. We demonstrate that the bi-stability jumps exist in OBAHGs but are part of a broader complex behavior not replicated by current mass-loss recipes. Combining our and other recent results, we suggest that the switch between rarefied and dense solutions only occurs within a certain proximity to the Eddington Limit. Testing this requires future models with broader parameters and advanced treatments of radiatively-driven turbulence.

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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.

astro-ph.IM↗

arXiv2025

The JWST Galactic Center Survey -- A White Paper

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.

astro-ph.GA↗

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.

astro-ph.SR↗

arXiv2025

Near-Eddington mass loss of hydrogen-rich Wolf-Rayet stars

Context. Very massive clusters and regions of intense star formation such as the center of our Milky Way contain young, hydrogen-burning stars very close to the Eddington Limit. Formally classified as hydrogen-rich Wolf-Rayet stars, the winds and spectra of these stars are distinctively different to the more evolved, classical Wolf-Rayet (cWR) stars. Results. We find an overall downward trend of the mass-loss rate with increasing temperature and decreasing metallicity. However, at SMC metallicities and above, we find a maximum in the wind efficiency with the mass-loss eventually decreasing again at lower temperatures. For intermediate metallicities, we also find strong discontinuities in the mass-loss trends, which do not appear at high or very low metallicities. For the lowest metallicities, a more homogeneous behavior is obtained without any maximum in the wind efficiency. The terminal velocities are generally higher for hotter temperatures. For cooler temperatures, the combined effect of metallicity and mass-loss change significantly reduces the changes in terminal velocity with metallicity. Conclusions. Contrary to cWR stars, the spectral appearance of late-type WNh stars rules out supersonic winds launched at the hot iron bump. Instead, a more extended quasi-hydrostatic regime is necessary. The proximity to the Eddington limit and the complex interactions cause a lot of substructure in the global wind parameter trends. While the strong discontinuities show resemblances to the bi-stability jump predicted for the B-supergiant regime, our models reveal a more complex origin. At sub-SMC metallicity, iron is no longer a major key for setting the mass-loss rate in this WNh regime. Instead, other elements (e.g. nitrogen) and continuum contributions become important.

astro-ph.SR↗

arXiv2025

X-Shooting ULLYSES: massive stars at low metallicity: XII. The clumped winds of O-type (super)giants in the Large Magellanic Cloud

Mass loss governs the evolution of massive stars and shapes the stellar surroundings. To quantify the impact of the stellar winds we need to know the exact mass-loss rates; however, empirical constraints on the rates are hampered by limited knowledge of their small-scale wind structure or 'wind clumping'. We aim to improve empirical constraints on the mass loss of massive stars by investigating the clumping properties of their winds, in particular the relation between stellar parameters and wind structure. We analyse the optical and ultraviolet spectra of 25 O-type (super)giants in the LMC, using the model atmosphere code Fastwind and a genetic algorithm. We derive stellar and wind parameters including detailed clumping properties, such as the amount of clumping, the density of the interclump medium, velocity-porosity of the medium, and wind turbulence. We obtain stellar and wind parameters for 24 of our sample stars and find that the winds are highly clumped, with an average clumping factor of $f_{\rm cl}=33\pm14$, an interclump density factor of $f_{\rm ic}=0.2\pm0.1$, and moderate to strong velocity-porosity effects. The scatter around the average values of the wind-structure parameters is large. With the exception of a significant, positive correlation between the interclump density factor and mass loss, we find no dependence of clumping parameters on either mass-loss rate or stellar properties. In the luminosity range that we investigate, the empirical and theoretical mass-loss rates both have a scatter of about 0.5~dex, or a factor 3. Within this uncertainty, the empirical rates and the theoretical predictions agree. The origin of the scatter of the empirical mass-loss rates requires further investigation. It is possible that our description of wind clumping is still not sufficient to capture effects of the structured wind; this could contribute to the scatter.

astro-ph.SR↗

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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arXiv2023

Clumping and X-Rays in cooler B supergiant stars

B supergiants (BSGs) are evolved stars with effective temperatures between 10 to 30 kK and are important to understand massive star evolution. Located on the edge of the line-driven wind regime, the study of their atmospheres is helpful to understand phenomena such as the bi-stability jump. Key UV features of their spectra have so far not been reproduced by models for types later than B1. Here, we aim to remedy this situation via spectral analysis that accounts for wind clumping and X-rays. In addition, we investigate the evolutionary status of our sample stars based on the obtained stellar parameters. We determined parameters via quantitative spectroscopy using CMFGEN and PoWR codes. The models were compared to UV and optical data of four BSGs: HD206165, HD198478, HD53138, and HD164353. We also study the evolutionary status of our sample using GENEC and MESA tracks. When including clumping and X-rays, we find good agreements between synthetic and observed spectra for our sample stars. For the first time, we reproduced key lines in the UV. For that, we require a moderately clumped wind (f_infty > ~0.5). We also infer relative X-ray luminosities of ~10^-7.5 to 10^-8 -- lower than the typical ratio of 10^-7. Moreover, we find a possible mismatch between evolutionary and spectroscopic masses, which could be related to the mass-discrepancy problem present in other OB stars. Our results provide evidence that X-rays and clumping are needed to describe the winds of cool BSGs. However, their winds seem less structured than in earlier type stars. This aligns with observational X-rays and clumping constraints as well as recent hydrodynamical simulations. The BSGs' evolutionary status appears diverse: some objects are potentially post-red supergiants or merger products. The wind parameters provide evidence for a moderate mass-loss rate increase around the bi-stability jump. Abstract abridged

astro-ph.SR↗

arXiv2022

A new reference catalogue for the very metal-poor Universe: +150 OB stars in Sextans A

Local Group (LG) very metal-poor massive stars are the best proxy for the First Stars of the Universe and fundamental to modelling the evolution of early galaxies. These stars may follow new evolutionary pathways restricted to very low metallicities, such as chemically homogeneous evolution (CHE). However, given the great distance leap needed to reach very metal-poor galaxies of the LG and vicinity, no comprehensive spectroscopic studies have been carried out at metallicities lower than the Small Magellanic Cloud (SMC, Z = 1/5 Z$_{\odot}$) until now. After five observing campaigns at the 10.4-m Gran Telescopio Canarias, we have assembled a low-resolution (R $\sim$ 1000) spectroscopic collection of more than 150 OB stars in the 1/10 Z$_{\odot}$ galaxy Sextans A, increasing by an order of magnitude the number of massive stars known in this galaxy. The catalogue includes 38 BA-type supergiants, 4 red supergiants, and the first candidate 1/10 Z$_{\odot}$ binary systems, CHE sources and systems hosting stripped stars. The sample massive stars mainly overlap the higher concentrations of neutral gas of Sextans A. However, we find some sources in low HI column-density regions. The colour-magnitude diagram of the galaxy presents large dispersion, which suggests uneven, internal extinction in Sextans A. This is the largest catalogue of OB-type stars ever produced at sub-SMC metallicities. This sample constitutes a fundamental first step to unveiling the evolutionary pathways and fates of very metal-poor massive stars, analyzing the dependence of radiation-driven winds with metallicity, and studying binary systems in an environment analogue to the early Universe.

astro-ph.GA↗

arXiv2022

New hydrodynamic solutions for line-driven winds of hot massive stars using Lambert $W$-function

Hot massive stars present strong stellar winds that are driven by absorption, scattering and re\-emission of photons by the ions of the atmosphere (\textit{line-driven winds}). A better comprehension of this phenomenon, and a more accurate calculation of hydrodynamics and radiative acceleration is required to reduce the number of free parameters in spectral fitting, to determine accurate wind parameters such as mass-loss rates and velocity profiles. We use the non-LTE model-atmosphere code CMFGEN to numerically solve the radiative transfer equation in the stellar atmosphere and to calculate the radiative acceleration $g_\text{rad}(r)$. Under the assumption that the radiative acceleration depends only on the radial coordinate, we solve analytically the equation of motion by means of the Lambert $W$-function. An iterative procedure between the solution of the radiative transfer and the equation of motion is executed in order to obtain a final self-consistent velocity field that is no longer based on any $β$-law. We apply the Lambert-procedure to three O supergiant stars ($ζ$-Puppis, HD~165763 and $α$-Cam) and discuss the Lambert-solutions for the velocity profiles. It is found that, even without recalculation of the mass-loss rate, the Lambert-procedure allows the calculation of consistent velocity profiles that reduce the number of free parameters when a spectral fitting using CMFGEN is performed. Synthetic spectra calculated from our Lambert-solutions show significant differences compared to the initial $β$-law CMFGEN models. The results indicate the importance of consistent velocity profile calculation in the CMFGEN code and its usage in a fitting procedure and interpretation of observed spectra.

astro-ph.SR↗

arXiv2020

2D kinematics of massive stars near the Galactic Center

The presence of massive stars (MSs) in the region close to the Galactic Center (GC) poses several questions about their origin. The harsh environment of the GC favors specific formation scenarios, each of which should imprint characteristic kinematic features on the MSs. We present a 2D kinematic analysis of MSs in a GC region surrounding Sgr A* based on high-precision proper motions obtained with the Hubble Space Telescope. Thanks to a careful data reduction, well-measured bright stars in our proper-motion catalogs have errors better than 0.5 mas yr$^{-1}$. We discuss the absolute motion of the MSs in the field and their motion relative to Sgr A*, the Arches and the Quintuplet. For the majority of the MSs, we rule out any distance further than 3-4 kpc from Sgr A* using only kinematic arguments. If their membership to the GC is confirmed, most of the isolated MSs are likely not associated with either the Arches or Quintuplet clusters or Sgr A*. Only a few MSs have proper motions suggesting they are likely members of the Arches cluster, in agreement with previous spectroscopic results. Line-of-sight radial velocities and distances are required to shed further light on the origin of most of these massive objects. We also present an analysis of other fast-moving objects in the GC region, finding no clear excess of high-velocity escaping stars. We make our astro-photometric catalogs publicly available.

astro-ph.SR↗

arXiv2020

A new candidate Luminous Blue Variable

We identify IRAS 16115-5044, which was previously classified as a protoplanetary nebula (PPN), as a candidate luminous blue variable (LBV). The star has high luminosity (>10$^{5.75}$ L_Sun), ensuring supergiant status, has a temperature similar to LBVs, is photometrically and spectroscopically variable, and is surrounded by warm dust. Its near-infrared spectrum shows the presence of several lines of HI, He I, Fe II, Fe [II], MgII, and Na I with shapes ranging from pure absorption and P Cygni profiles to full emission. These characteristics are often observed together in the relatively rare LBV class of stars, of which only $\approx$20 are known in the Galaxy. The key to the new classification is the fact that we compute a new distance and extinction that yields a luminosity significantly in excess of those for post-AGB PPNe, for which the initial masses are <8 M_Sun. Assuming single star evolution, we estimate an initial mass of $\approx$40 M_Sun.

astro-ph.SR↗

arXiv2020

GALACTICNUCLEUS: A high-angular-resolution JHKs imaging survey of the Galactic centre III. Evidence for wavelength dependence of the extinction curve in the near-infrared

The characterisation of the extinction curve in the near infrared (NIR) is fundamental to analyse the structure and stellar population of the Galactic centre (GC), whose analysis is hampered by the extreme interstellar extinction ($A_V\sim 30$ mag) that varies on arc-second scales. Recent studies indicate that the behaviour of the extinction curve might be more complex than previously assumed, pointing towards a variation of the extinction curve as a function of wavelength. We aim at analysing the variations of the extinction index, $α$, with wavelength, line-of-sight, and absolute extinction, extending previous analysis to a larger area of the innermost regions of the Galaxy. We analysed the whole GALACTICNUCLEUS survey, a high-angular resolution ($\sim 0.2''$) $JHK_s$ NIR survey specially designed to observe the GC in unprecedented detail. It covers a region of $\sim 6000$\,pc$^2$, comprising fields in the nuclear stellar disc, the inner bulge, and the transition region between them. We applied two independent methods based on red clump (RC) stars to constrain the extinction curve and analysed its variation superseding previous studies. We used more than 165,000 RC stars and increased significantly the size of the regions analysed to confirm that the extinction curve varies with the wavelength. We estimated a difference $Δα= 0.21\pm0.07$ between the obtained extinction indices, $α_{JH}=2.44\pm0.05$ and $α_{HK_s} = 2.23\pm0.05$. We also concluded that there is no significant variation of the extinction curve with wavelength, with the line-of-sight or the absolute extinction. Finally, we computed the ratios between extinctions, $A_J/A_H = 1.87\pm0.03$ and $A_{H}/A_{K_s} = 1.84\pm0.03$, consistent with all the regions of the GALACTICNUCLEUS catalogue.

astro-ph.GA↗

arXiv2019

A Significant Feature in the General Relativistic Time Evolution of the Redshift of Photons Coming from a Star Orbiting Sgr A*

The star S0-2, orbiting the Galactic central massive black hole candidate Sgr A*, passed its pericenter in May 2018. This event is the first chance to detect the general relativistic (GR) effect of a massive black hole, free from non-gravitational physics. The observable GR evidence in the event is the difference between the GR redshift and the Newtonian redshift of photons coming from S0-2. Within the present observational precision, the 1st post-Newtonian (1PN) GR evidence is detectable. In this paper, we give a theoretical analysis of the time evolution of the 1PN GR evidence, under a presupposition that is different from used in previous papers. Our presupposition is that the GR/Newtonian redshift is always calculated with the parameter values (the mass of Sgr A*, the initial conditions of S0-2, and so on) determined by fitting the GR/Newtonian motion of S0-2 with the observational data. It is then revealed that the difference of the GR redshift and the Newtonian one shows two peaks before and after the pericenter passage. This double-peak-appearance is due to our presupposition, and reduces to a single peak if the same parameter values are used in both GR and Newtonian redshifts as considered in previous papers. In addition to this theoretical discussion, we report our observational data obtained with the Subaru telescope by 2018. The quality and the number of Subaru data in 2018 are not sufficient to confirm the detection of the double-peak-appearance.

gr-qc↗

arXiv2019

Variability of the near-infrared extinction curve towards the Galactic centre

Due to the extreme extinction towards the Galactic centre ($A_{V} \sim 30$ mag), its stellar population is mainly studied in the near-infrared (NIR) regime. Therefore, a proper analysis of the NIR extinction curve is necessary to fully characterise the stellar structure and population of the inner part of the galaxy. We studied the dependence of the extinction index ($α_λ$) in the NIR on the line of sight, wavelength, and extinction. We used the GALACTICNUCLEUS imaging survey, a high angular resolution catalogue ($0.2''$) for the inner part of the Galaxy in $JHK_s$, and studied the spatial variation in the extinction index. We also applied two independent methods based on red clump stars to compute the extinction index between different bands and its variation with wavelength. We did not detect any significant line-of-sight or extinction variation in $α$ within the studied region in the nuclear stellar disc. The extinction index between $JH$ and $HK_s$ differs by $0.19 \pm 0.05$. We obtained mean values for the extinction indices $α_{JH} = 2.43\pm0.03$ and $α_{HK_s} = 2.23\pm0.03$. The dependence of the extinction index on the wavelength could explain the differences obtained for $α_λ$ in the literature since it was assumed constant for the NIR regime.

astro-ph.SR↗

arXiv2019

GALACTICNUCLEUS: A high-angular-resolution JHKs imaging survey of the Galactic centre II. First data release of the catalogue and the most detailed CMDs of the GC

The high extinction and extreme source crowding of the central regions of the Milky Way are serious obstacles to the study of the structure and stellar population of the Galactic centre (GC). Existing surveys that cover the GC region (2MASS, UKIDSS, VVV, SIRIUS) do not have the necessary high angular resolution. Therefore, a high-angular-resolution survey in the near infrared is crucial to improve the state of the art. Here, we present the GALACTICNUCLEUS catalogue, a near infrared $JHK_s$ high-angular-resolution ($0.2''$) survey of the nuclear bulge of the Milky Way. We explain in detail the data reduction, data analysis, calibration, and uncertainty estimation of the GALACTICNUCLEUS survey. We assess the data quality comparing our results with previous surveys. We obtained accurate $JHK_s$ photometry for $\sim 3.3\times10^6$ stars in the GC detecting around 20 \% in $J$, 65 \% in $H,$ and 90 \% in $K_s$. The survey covers a total area of $\sim0.3$ square degrees, which corresponds to $\sim 6,000$ pc$^2$. The GALACTICNUCLEUS survey reaches 5\,$σ$ detections for $J \sim 22$ mag, $H \sim 21$ mag, and $K_s \sim 21$ mag. The uncertainties are below 0.05 mag at $J \sim 21$ mag, $H \sim 19$ mag, and $K_s \sim 18$ mag. The zero point systematic uncertainty is $\lesssim0.04$ mag in all three bands. We present colour-magnitude diagrams for the different regions covered by the survey.

astro-ph.GA↗

arXiv2019

Ongoing star formation at the outskirts of Sextans A: Spectroscopic detection of early-O type stars

With both nebular- and stellar-derived abundances of $\lesssim$ 1/10 Zsun and low foreground extinction, Sextans A is a prime candidate to replace the Small Magellanic Cloud as reservoir of metal-poor massive stars and reference to study the metal-poor Universe. We report the discovery of two early-O type stars in Sextans A, the earliest O-stars with metallicity < 1/7 Zsun known to date, and two additional O9 stars. Colour-excess estimates towards individual targets, enabled by spectral typing, manifest that internal reddening is neither uniform nor negligible. The four targets define a new region of star formation far from the optically-brightest centre of the galaxy and from its conspicuous HII shells, but not devoid of neutral hydrogen. In fact, we detect a spatial correlation between OB-stars and HI in Sextans A and other dIrr's that leads us to propose that the neutral phase may be fundamental to star formation in low-density environments. According to the existing evidence at least two of the targets formed in isolation, thus suggestive of an stochastic sampling of the initial mass function that would enable low-mass galaxies like Sextans A to form very massive stars. The discovery of these four stars provide spatially-resolved, spectroscopic confirmation of recent findings suggesting that dwarf galaxies can sustain star formation despite the low density of the gas phase.

astro-ph.GA↗