Daniel Pauli

Recent James Webb Space Telescope (JWST) observations have revealed fine-structure lines of ions with ionization potentials ≳50 eV, such as [Ne v ] and [O iv ], in nebular spectra of some low-metallicity galaxies, raising questions about the nature of the ionizing sources. Ultraluminous X-ray sources (ULXs) have been considered as potential contributors, but direct evidence linking individual ULXs to highly ionized nebulae is limited. Here, we present JWST Integral Field Unit (IFU) spectroscopy of the nebula surrounding the template ULX Ho II X–1 in the low-metallicity galaxy Holmberg II, already known to emit He ii λ 4686 and [O iv ] 25.89 μ m lines. We resolve the infrared nebula on spatial scales of ∼15−25 pc and report the first detections of [Ne v ] and [Ne vi ] in this source. We find anisotropic ionizing conditions: emission from the highest-ionization species arises from compact regions offset from the ULX toward the previously detected radio knots, whereas lower-ionization species trace the wider nebula. The preferential alignment of the highest-ionization emission with the radio axis suggests anisotropic escape of the ionizing continuum and a physical connection with jet-ejection activity. Spectral lines across the nebula are narrow and consistent with the instrumental resolution, indicating kinematically quiescent gas and disfavoring shocks as the dominant ionization mechanism. Together, these results provide direct evidence linking an individual ULX to a highly ionized nebula in a low-metallicity galaxy. These JWST observations of Ho II X–1 show that an accreting black hole in a low-metallicity galaxy is responsible for the extreme ionization of the surrounding nebula on ∼100 pc scales.

Context . Massive hot stars are among the major ionizing sources in the Universe. The ionizing flux of a star strongly depends on its temperature, which is best measured spectroscopically using lines of different ionization stages of the same element. For the quantitative spectral analysis of O-type stars, helium lines are commonly employed, and for B-type stars, lines of silicon and magnesium are used. However, at low metallicity, many of the diagnostic metal lines disappear. Aims . Here, we conduct a systematic theoretical analysis of stellar atmosphere models of OB stars to study the effect of metallicity on the behavior of key temperature diagnostic lines and how this impacts their spectral classification. Methods . We computed large grids of state-of-the-art non-local thermal equilibrium (non-LTE) stellar atmosphere models at four different metallicities, ranging from Z _⊙ (i.e., solar) to 1/31 Z _⊙ . We presented contour plots for the equivalent widths of selected diagnostic lines in the temperature-gravity plane and investigated their dependence on metallicity. Using Galactic stellar templates of O- and B-type stars, we further established equivalent width ratios for spectral classification and applied them to our models. Results . For the hottest stars in our model grids, He I lines become detectable at lower metallicity due to the reduced back-warming in the atmosphere, opening new possibilities for their classification. Metal lines used for the analysis of B-type stars are mostly absent in the spectra of stars with metallicity below that of the Small Magellanic Cloud, except Si III λ 4553 and Si IV λ 4089, challenging current classification schemes. Conclusions . Our stellar atmosphere grids reveal how metallicity affects key temperature diagnostics. Due to reduced back-warming and non-LTE effects in denser stellar atmospheres, these diagnostic lines appear at higher stellar temperatures in lower-metallicity environments. However, the decrease in metallicity also causes many metal lines to disappear, leaving different sensitivities of He I lines the only viable temperature indicators for B-type stars in the optical. The ionizing flux of OB-type stars can be partially trapped within the stellar wind. As a result, at lower metallicity, a larger fraction of stars is able to emit hard ionizing radiation.

Context. Massive stars with their strong ionizing radiation and strong stellar winds are the key feedback agents of the Universe. Stellar winds of massive stars are often measured by fitting resonance lines in the UV using non-local thermal equilibrium (non-LTE) stellar atmosphere models. Aims. We aim to conduct the first measurement of the resonance line formation regions in an early-type eclipsing binary in the Small Magellanic Cloud, namely AzV 75. Methods. We employed photometric data from TESS and ASAS-SN in combination with radial velocity measurements from multi-epoch Hubble Space Telescope UV spectra to derive the ephemeris. We examined the intensity changes in the C IV and N V resonance lines in the UV and combined them with a light-curve analysis to estimate the region in the wind where these lines are formed. Results. We derived that AzV 75 has an orbital period of P _orb = 165.925 ± 0.115 d, eccentricity of e = 0.42 ± 0.02, mass ratio of q = 0.72 ± 0.03, and inclination of i = 85.77 ^◦ . With this orbital configuration, no secondary eclipse is expected. We report that the optically thick UV resonance lines exhibit flattening and shortening of the absorption trough, and weakening of their emission features, as they approach the phase of the expected secondary eclipse, while the continuum UV flux appears to remain unaffected. We illustrate that this behaviour can be well explained by the primary’s optically thick wind eclipsing the secondary star. The C IV and N V resonance line formation regions in the primary star extend up to 316 R _⊙ . Conclusions. We have measured the line formation regions of resonance lines in a stellar wind of an early O-type star. These are important benchmarks for 1D as well as 3D non-LTE stellar atmosphere models. Even though we have numerous multi-epoch UV spectra at hand, the eclipse is not fully sampled, leaving some freedom in the derived sizes of the line formation regions. A first comparison to 1D stellar atmosphere models indicates that a classical β law with an exponent of β = 0.5 instead of β = 0.8 might be favoured for the primary star’s velocity field.

WR 25 is a massive colliding-wind binary in the Carina nebula comprising a WN6ha primary star with an O5 companion in an eccentric 208-day orbit. Recent spectroscopic analysis estimates the total binary mass to approach 100 M _⊙ , and a primary-to-secondary mass ratio of q = M ₁ / M ₂ ≈ 2, hinting at a very massive primary star. The presence of additional spectroscopic signatures from a third, “intruder” star was also noted, making it a candidate hierarchical triple system. In this study, we present a VLTI/PIONIER interferometric observation of WR 25, spatially resolving all three components for the first time. For the inner WN6ha + O5 binary, we find a projected angular separation of 1.68 ± 0.02 milliarcseconds (mas), corresponding to a projected physical separation of 3.90 ± 0.21 au at a distance of 2.32 ± 0.12 kpc. Leveraging the fortunate timing of the VLTI observation, which was obtained when the two components were passing the line of nodes, we determined the orbital semi-major axis a = 3.11 ± 0.20 au. Subsequently, the newly constrained total dynamical binary mass is 93 ± 18 M _⊙ , with a primary mass M ₁ = 62 ± 13 M _⊙ and secondary mass M ₂ = 31 ± 7 M _⊙ . We detect the third star or tertiary with a projected angular separation of 27.69 ± 0.02 mas from the primary, with a chance alignment probability lower than 10 ⁻⁴ . Using newly obtained brightness ratios between all components, we revisit archival spectroscopic data of WR 25 to disentangle spectra for individual components and derive their stellar parameters. The tertiary, which has a spectral type O7, is broadly coeval with the inner binary and has an evolutionary mass M ₃ = 25.6 _−2.3 ^+2.8 M _⊙ . Based on simulations, we estimate the tertiary period to be in the range 19–82 years. The newly confirmed triple nature of WR 25 thus makes it an important benchmark system to measure accurate dynamical masses of the inner binary and potentially the tertiary, to calibrate stellar evolution and atmosphere models, and to study its formation and stability as a hierarchical triple system.

Massive stars, the progenitors of neutron stars and black holes, play a crucial role in shaping the chemical and radiative properties of entire galaxies through their winds and explosive deaths. Stellar pulsations are a common phenomenon in massive stars and asteroseismology – the study of such pulsations – provides crucial constraints on the physics of massive star interiors. The excitation of heat-driven pulsations in massive stars is expected to depend on a star’s metallicity, but this remains largely uncalibrated in evolution models due to a lack of a sufficient observations. While Transiting Exoplanet Survey Satellite (TESS) has dramatically improved the statistics for Galactic massive stars, obtaining TESS light curves for low-metallicity massive stars beyond the Milky Way is challenging, due to their faintness and heavy crowding. In this paper, we present a novel point-spread-function-based light-curve extraction method called lemons, which overcomes these challenges. We also demonstrate the limitations of the often-used simple aperture photometry method that can provide heavily contaminated light curves. With this new technique, accurate light curves of 91 Small Magellanic Cloud (SMC) massive stars in the Binarity at LOw Metallicity sample are extracted. They reveal a variety of variability types including indications of binarity (e.g. eclipses and ellipsoidal modulation) and stellar pulsations. They also enable us to investigate stochastic low-frequency (SLF) variability for massive stars in the SMC. Furthermore, we demonstrate how the morphology of SLF variability probes a star’s location in the Hertzsprung–Russell diagram, which appears similar to Galactic massive stars thus indicating that the underlying physical mechanism could be insensitive to metallicity.

Context . Our limited understanding of the winds of massive stars hampers our ability to predict their evolution. We currently often rely on mass-loss rate prescriptions that show strong features that lack a firm empirical confirmation, such as the bi-stability jump. This bi-stability jump is a pronounced increase in mass loss in the decreasing temperature regime T _eff ~28–21 kK, i.e., spectral types B1-B2. Different prescriptions also give different metallicity dependences; these sometimes include a dependence on effective temperature, luminosity or both, that may also differ among prescriptions. Although recent papers have compared empirical results to prescriptions, a large observational sample of B stars for which the wind has been scrutinised over different metallicity ranges is still lacking. Aims . Through the modelling of both ultraviolet (from the ULLYSES programme) and optical (from the XShootU programme) high-resolution spectra, we determined the stellar and wind parameters, including detailed clumping parameters, of 24 SMC B stars ranging in T _eff from 13 to 29 kK. By combining this sample with LMC studies, we compared the wind behaviour of B stars in two different metallicity regimes. We compared our results to common mass-loss rate prescriptions to test features present in these models and their metallicity dependence. Methods . We have used the model atmosphere code FASTWIND and the genetic algorithm code Kiwi-GA to fit the UV and optical spectra simultaneously. This allows us to determine wind properties including clumping behaviour. Results . The moderate to strong metallicity trends present in the mass-loss prescriptions ( Ṁ ∝ Z ^0.41–1.4 ) explored here overestimate the empirical metallicity dependence in the B-star regime, which appears very weak. Similarly to a previous study of B supergiants in the LMC, we do not find a sudden increase in mass-loss rate at approximately spectral type B1, but only a weak temperature trend. As in this LMC study, we show that on average around 40% of the wind mass is located in the wind medium between the clumps. Using other studies from this paper series, we compiled a sample of 80+ O and B stars in the SMC and LMC. From a comparison we find a clear difference in O- and B-type metallicity dependence. Conclusions . The apparent lack of a bi-stability jump in the B-star regime and a very weak metallicity dependence for the same stars offers new empirical constraints to theoretical models of line-driven winds. As differences between these models are large (reaching orders of magnitude in parts of parameter space) such constraints are much needed. More studies exploring the mass-loss rates of B stars and cooler objects will be helpful to our understanding of hot star winds.

Stripped stars are a common product of binary stellar systems, spanning a wide mass range from Wolf-Rayet stars to hot subdwarfs and helium white dwarfs. The recent discovery of intermediate-mass stripped stars, with masses between those of WRs and subdwarfs, provides a continuous evolutionary sequence and valuable test beds for binary interaction models. Population synthesis studies explain the formation of stripped products through stable mass transfer or common envelope evolution. Recently, however, a merger scenario (or, a “failed common envelope”) between a stripped star and a post-main sequence companion was suggested to explain the formation of the magnetic intermediate-mass stripped star HD 45166. In this Letter, we investigate this channel using detailed interacting binary simulations at Galactic metallicity and model their stripped merger products. We find that these merger products can contribute to the population of stripped stars across the Hertzsprung-Russell diagram and produce a blue-straggler effect, with merger masses spanning from 0.6 M _⊙ to 14 M _⊙ . These objects would be consistent with long-lived core He-burning stars retaining thin hydrogen envelopes after a partial CE ejection, and would appear as single or members of wide binaries that were originally hierarchical triples.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

The Binarity at LOw Metallicity (BLOeM) survey is an ESO large programme designed to obtain multi-epoch spectroscopy for 929 massive stars in the Small Magellanic Cloud (SMC). It will provide binary fractions and orbital configurations of binary systems and search for dormant black hole binary candidates (OB+BH). We present projected rotational velocities ( v sin i ) of all sources, and, using the multiplicity properties presented in previous papers, we derive the v sin i distributions of apparent single stars, single-lined spectroscopic (SB1) binaries, and SB2 systems. We identify a locus in the Hertzsprung-Russell diagram where rotational velocities decrease significantly; we interpret this feature as broadly corresponding to the terminal-age main sequence. The main-sequence cohort is distinguished by a broad range of v sin i values, but with a strong peak in the distribution in the range 30–60 km s ⁻¹ , which is close to the resolution limit of ∼30 km s ⁻¹ , indicating the presence of many upper limits. Sources in this low v sin i peak are distributed throughout the main sequence and are also present in the SB1 sample, though less prominent than in the single-star distribution. A preliminary analysis of the lowest v sin i cohort, which includes SB1 systems, implies that roughly one-third may be nitrogen rich, and we speculate that this cohort is a mix of pristine single stars, long-period binaries, and merger products. The SB2 systems appear to be mostly short-period binaries in synchronous rotation, and their v sin i estimates are distributed around a mean value of ∼140 km s ⁻¹ . Higher v sin i sources are also present in the single and SB1 systems, all of which have a tail to higher v sin i values. This is consistent with tidal and mass-transfer effects. The supergiants, with a few exceptions, have low v sin i , and the bulk of these systems is essentially unresolved at the current spectral resolution (∼30 km s ⁻¹ ).