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

García, M.; Herrero, A.; Najarro, F.; Camacho, I.; Lorenzo, Marta

doi:10.1093/mnras/sty3503

Cited by 27 publications

(40 citation statements)

References 52 publications

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“…There is so far only one O star detected in a Z = 1/30 Z galaxy (Leo P, Evans et al 2019). There are a few hot massive stars detected in Local Group galaxies with metallicities between that of the SMC and 1/10 Z (Bresolin et al 2007;Evans et al 2007;Garcia & Herrero 2013;Hosek et al 2014;Tramper et al 2014;Camacho et al 2016;Garcia 2018;Garcia et al 2019). An emblematic galaxy in the low-metallicity range is I Zw 18 (Z ∼ 1/30−1/50 Z , Izotov et al 1999) in which Izotov et al (1997) reported the detection of Wolf-Rayet stars (see also Brown et al 2002).…”

Section: Spectroscopic Sequences At Z = 1/30 Zmentioning

confidence: 99%

“…the wavelength range 4500−8000 Å which will be probed by HARMONI and MOSAIC. We selected this range for the following reasons: it contains a fair number of lines from different elements; at these wavelengths, OB stars emit more flux than in the near-infrared; Local Group dwarf galaxies have relatively low extinction (Tramper et al 2014;Garcia et al 2019). We therefore anticipate that it will be more efficient to detect and characterize new OB stars in this wavelength range.…”

Section: Optical Wavelength Range Of the Eltmentioning

confidence: 99%

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Spectroscopic evolution of massive stars near the main sequence at low metallicity

Martins

2021

A&A

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Context. The evolution of massive stars is not fully understood. Several physical processes affect their life and death, with major consequences on the progenitors of core-collapse supernovae, long-soft gamma-ray bursts, and compact-object mergers leading to gravitational wave emission. Aims. In this context, our aim is to make the prediction of stellar evolution easily comparable to observations. To this end, we developed an approach called “spectroscopic evolution” in which we predict the spectral appearance of massive stars through their evolution. The final goal is to constrain the physical processes governing the evolution of the most massive stars. In particular, we want to test the effects of metallicity. Methods. Following our initial study, which focused on solar metallicity, we investigated the low Z regime. We chose two representative metallicities: 1/5 and 1/30 Z⊙. We computed single-star evolutionary tracks with the code STAREVOL for stars with initial masses between 15 and 150 M⊙. We did not include rotation, and focused on the main sequence (MS) and the earliest post-MS evolution. We subsequently computed atmosphere models and synthetic spectra along those tracks. We assigned a spectral type and luminosity class to each synthetic spectrum as if it were an observed spectrum. Results. We predict that the most massive stars all start their evolution as O2 dwarfs at sub-solar metallicities contrary to solar metallicity calculations and observations. The fraction of lifetime spent in the O2V phase increases at lower metallicity. The distribution of dwarfs and giants we predict in the SMC accurately reproduces the observations. Supergiants appear at slightly higher effective temperatures than we predict. More massive stars enter the giant and supergiant phases closer to the zero-age main sequence, but not as close as for solar metallicity. This is due to the reduced stellar winds at lower metallicity. Our models with masses higher than ∼60 M⊙ should appear as O and B stars, whereas these objects are not observed, confirming a trend reported in the recent literature. At Z = 1/30 Z⊙, dwarfs cover a wider fraction of the MS and giants and supergiants appear at lower effective temperatures than at Z = 1/5 Z⊙. The UV spectra of these low-metallicity stars have only weak P Cygni profiles. He II 1640 sometimes shows a net emission in the most massive models, with an equivalent width reaching ∼1.2 Å. For both sets of metallicities, we provide synthetic spectroscopy in the wavelength range 4500−8000 Å. This range will be covered by the instruments HARMONI and MOSAICS on the Extremely Large Telescope and will be relevant to identify hot massive stars in Local Group galaxies with low extinction. We suggest the use of the ratio of He I 7065 to He II 5412 as a diagnostic for spectral type. Using archival spectroscopic data and our synthetic spectroscopy, we show that this ratio does not depend on metallicity. Finally, we discuss the ionizing fluxes of our models. The relation between the hydrogen ionizing flux per unit area versus effective temperature depends only weakly on metallicity. The ratios of He I and He II to H ionizing fluxes both depend on metallicity, although in a slightly different way. Conclusions. We make our synthetic spectra and spectral energy distributions available to the community.

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Section: Spectroscopic Sequences At Z = 1/30 Zmentioning

confidence: 99%

Section: Optical Wavelength Range Of the Eltmentioning

confidence: 99%

Spectroscopic evolution of massive stars near the main sequence at low metallicity

Martins

2021

A&A

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“…While the quantitative spectroscopy of individual, hot, massive stars is also possible nowadays in galaxies further away (e.g. Garcia et al 2019), such studies are only in their infancy.…”

Section: Introductionmentioning

confidence: 99%

New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars

Björklund

Sundqvist

Puls

et al. 2021

A&A

114

103

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Context. Reliable predictions of mass-loss rates are important for massive-star evolution computations. Aims. We aim to provide predictions for mass-loss rates and wind-momentum rates of O-type stars, while carefully studying the behaviour of these winds as functions of stellar parameters, such as luminosity and metallicity. Methods. We used newly developed steady-state models of radiation-driven winds to compute the global properties of a grid of O-stars. The self-consistent models were calculated by means of an iterative solution to the equation of motion using full non-local thermodynamic equilibrium radiative transfer in the co-moving frame to compute the radiative acceleration. In order to study winds in different galactic environments, the grid covers main-sequence stars, giants, and supergiants in the Galaxy and both Magellanic Clouds. Results. We find a strong dependence of mass-loss on both luminosity and metallicity. Mean values across the grid are Ṁ~L*2.2 and Ṁ~L*0.95; however, we also find a somewhat stronger dependence on metallicity for lower luminosities. Similarly, the mass loss-luminosity relation is somewhat steeper for the Small Magellanic Cloud (SMC) than for the Galaxy. In addition, the computed rates are systematically lower (by a factor 2 and more) than those commonly used in stellar-evolution calculations. Overall, our results are in good agreement with observations in the Galaxy that properly account for wind-clumping, with empirical Ṁ versus Z* scaling relations and with observations of O-dwarfs in the SMC. Conclusions. Our results provide simple fit relations for mass-loss rates and wind momenta of massive O-stars stars as functions of luminosity and metallicity, which are valid in the range Teff = 28 000–45 000 K. Due to the systematically lower values for Ṁ, our new models suggest that new rates might be needed in evolution simulations of massive stars.

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“…Extremely metal-poor H regions give their name to extremely metal-poor galaxies (XMPs), which are arbitrarily defined to have O/H ≤ (O/H) /10 in the ionized gas (Kunth & Östlin 2000), or equivalently, using 12+log(O/H) = 8.69 as the solar reference value (Asplund et al 2009), 12+log(O/H) ≤ 7.56. 1 Unfortunately, the three known nearby XMPs where individual massive stars can be observed (Sextans A, SagDIG, and Leo P) are not strongly starforming (Camacho et al 2016;Garcia 2018;Garcia et al 2019;Evans et al 2019). Thus, their stars do not significantly cover the parameter space of mass, luminosity, effective temperature, age, rotation, dynamical interaction in dense massive clusters, and in the case of stars in binary systems, orbital parameters and mass ratio combinations, which is relevant for either OB-type star astrophysics or as a reference to model distant starburst galaxies.…”

Section: Introductionmentioning

confidence: 99%

Stars and gas in the most metal-poor galaxies – I. COS and MUSE observations of SBS 0335−052E

Wofford

Vidal-García

Feltre

et al. 2020

Monthly Notices of the Royal Astronomical Society

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Among the nearest most metal-poor starburst-dwarf galaxies known, SBS 0335-052E is the most luminous in integrated nebular He ii λ4686 emission. This makes it a unique target to test spectral synthesis models and spectral interpretation tools of the kind that will be used to interpret future rest-frame UV observations of primeval galaxies. Previous attempts to reproduce its He ii λ4686 luminosity found that X-ray sources, shocks, and single Wolf-Rayet stars are not main contributors to the He ii-ionizing budget; and that only metal-free single rotating stars or binary stars with a top-heavy IMF and an unphysically-low metallicity can reproduce it. We present new UV (COS) and optical (MUSE) spectra which integrate the light of four super star clusters in SBS 0335-052E. Nebular He ii, [C iii], C iii], C iv, and O iii] UV emission lines with equivalent widths between 1.7 and 5 Å, and a C iv λλ1548, 1551 P-Cygni like profile are detected. Recent extremely-metal poor shock + precursor models and binary models fail to reproduce the observed optical emission-line ratios. We use different sets of UV and optical observables to test models of constant star formation with single non-rotating stars which account for very massive stars, as blueshifted O v λ1371 absorption is present. Simultaneously fitting the fluxes of all high-ionization UV lines requires an unphysically-low metallicity. Fitting the P-Cygni like + nebular components of C iv λλ1548, 1551 does not constrain the stellar metallicity and time since the beginning of star formation. We obtain 12+log(O/H) = 7.45 ± 0.04 and log(C/O)$\, =-0.45^{+0.03}_{-0.04}$ for the galaxy. Model-testing would benefit from higher spatial resolution UV and optical spectroscopy of the galaxy.

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

Cited by 27 publications

References 52 publications

Spectroscopic evolution of massive stars near the main sequence at low metallicity

Spectroscopic evolution of massive stars near the main sequence at low metallicity

New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars

Stars and gas in the most metal-poor galaxies – I. COS and MUSE observations of SBS 0335−052E

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