Surface abundances of OC supergiants

Martins, F.; Foschino, Sacha; Bouret, J. C.; Barbá, R. H.; Howarth, Ian D.

doi:10.1051/0004-6361/201628288

Cited by 7 publications

(9 citation statements)

References 29 publications

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“…For early-B stars, the main spectral-type criterion is the relative strength of Si iv 4089 and Si iii 4552. The former line is not always quantitatively reproduced in spectroscopic analyses (Martins et al 2016), although this is not systematic, as stressed in Sect. 3 and Fig.…”

Section: Limitations Of the Present Studymentioning

confidence: 90%

Spectroscopic evolution of massive stars on the main sequence

Martins

2017

A&A

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Context. The evolution of massive stars depends on several parameters, and the relation between different morphological types is not fully constrained. Aims. We aim to provide an observational view of evolutionary models in the Hertzsprung-Russell diagram, on the main sequence. This view should help compare observations and model predictions. Methods. We first computed evolutionary models with the code STAREVOL for initial masses between 15 and 100 M ⊙ . We subsequently calculated atmosphere models at specific points along the evolutionary tracks, using the code CMFGEN. Synthetic spectra obtained in this way were classified as if they were observational data: we assigned them a spectral type and a luminosity class. We tested our spectral classification by comparison to observed spectra of various stars with different spectral types. We also compared our results with empirical data of a large number of OB stars. Results. We obtain spectroscopic sequences along evolutionary tracks. In our computations, the earliest O stars (O2-3.5) appear only above ∼50 M ⊙ . For later spectral types, a similar mass limit exists, but is lower. A luminosity class V does not correspond to the entire main sequence. This only holds for the 15 M ⊙ track. As mass increases, a larger portion of the main sequence is spent in luminosity class III. Above 50 M ⊙ , supergiants appear before the end of core-hydrogen burning. Dwarf stars (luminosity class V) do not occur on the zero-age main sequence above 80 M ⊙ . Consequently, the distribution of luminosity class V in the HR diagram is not a diagnostic of the length of the main sequence (above 15 M ⊙ ) and cannot be used to constrain the size of the convective core. The distribution of dwarfs and giants in the HR diagram that results from our calculations agrees well with the location of stars analyzed by means of quantitative spectroscopy. For supergiants, there is a slight discrepancy in the sense that luminosity class I is observed slightly earlier (i.e., at higher T eff ) than our predictions. This is mainly due to wind densities that affect the luminosity class diagnostic lines. We predict an upper mass limit for dwarf stars (∼60 M ⊙ ) that is found consistent with the rarity of O2V stars in the Galaxy. Stars with WNh spectral type are not predicted by our models. Stronger winds are required to produce the characteristic emission lines of these objects.

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Section: Limitations Of the Present Studymentioning

confidence: 90%

Spectroscopic evolution of massive stars on the main sequence

Martins

2017

A&A

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“…Slow rotation is indeed an obvious possibility to explain why the star does not show signatures of strong enough mixing that would take the N/C to higher levels. Martins et al (2016) similarly concluded that Galactic OC supergiants likely experienced very little processing through the CN or CNO cycles. They further argued that because the OC phenomenon in Galactic stars is related to the chemically unprocessed surface abundances, it is best seen in more evolved objects for which chemical processing is already strong in morphologically normal stars.…”

Section: Cno Ratiosmentioning

confidence: 96%

“…In the past decade, several studies have been published on the surface abundances of massive stars in the Galaxy (e.g. Martins et al 2015Martins et al , 2016Martins et al , 2017Mahy et al 2015;Cazorla et al 2017;Markova et al 2018;Carneiro et al 2019) or in the Magellanic Clouds (e.g. Rivero González et al 2012;Bouret et al 2013;Grin et al 2017).…”

Section: Dependence On Metallicitymentioning

confidence: 99%

Massive stars in the Small Magellanic Cloud

Bouret

Martins

Hillier

et al. 2021

A&A

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Context. The evolution of massive stars depends on several physical processes and parameters. Metallicity and rotation are among the most important, but their quantitative effects are not well understood. Aims. To complement our earlier study on main-sequence stars, we study the evolutionary and physical properties of evolved O stars in the Small Magellanic Cloud (SMC). We focus in particular on their surface abundances to further investigate the efficiency of rotational mixing as a function of age, rotation, and global metallicity. Methods. We analysed the UV and optical spectra of 13 SMC O-type giants and supergiants using the stellar atmosphere code CMFGEN to derive photospheric and wind properties. We compared the inferred properties to theoretical predictions from evolution models. For a more comprehensive analysis, we interpret the results together with those we previously obtained for O-type dwarfs. Results. Most dwarfs of our sample lie in the early phases of the main sequence. For a given initial mass, giants are farther along the evolutionary tracks, which confirms that they are indeed more evolved than dwarfs. Supergiants have higher initial masses and are located past the terminal-age main-sequence in each diagram. We find no clear trend of a mass discrepancy, regardless of the diagram that was used to estimate the evolutionary mass. Surface CNO abundances are consistent with nucleosynthesis from the CNO cycle. Comparisons to theoretical predictions reveal that the initial mixture is important when the observed trends in the N/C versus N/O diagram are to be reproduced. A trend for stronger chemical evolution for more evolved objects is observed. Above about 30 M⊙, more massive stars are on average more chemically enriched at a given evolutionary phase. Below 30 M⊙, the trend vanishes. This is qualitatively consistent with evolutionary models. A principal component analysis of the abundance ratios for the whole (dwarfs and evolved stars) sample supports the theoretical prediction that massive stars at low metallicity are more chemically processed than their Galactic counterparts. Finally, models including rotation generally reproduce the surface abundances and rotation rates when different initial rotational velocities are considered. Nevertheless, for some objects, a stronger braking and/or more efficient mixing is required.

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“…A direct reading of the CNO abundances suggests that they would have to be be primordial in AV 69. Martins et al (2016) similarly concluded that Galactic OC supergiants likely experienced very little processing through the CN or CNO cycles. They further argued that because the OC phenomenon in Galactic stars is related to the chemically unprocessed surface abundances, it is best seen in more evolved objects for which chemical processing is already strong in morphologically normal stars.…”

Section: Cno Ratiosmentioning

confidence: 96%

“…In the past decade, several studies have been published on the surface abundances of massive stars in the Galaxy (e.g. Martins et al 2015;Mahy et al 2015;Martins et al 2016Martins et al , 2017Cazorla et al 2017;Markova et al 2018;Carneiro et al 2019) or in the Magellanic Clouds (e.g. Rivero González et al 2012;Bouret et al 2013;Grin et al 2017).…”

Section: Dependence On Metallicitymentioning

confidence: 99%

Massive stars in the Small Magellanic Cloud : Evolution, rotation and surface abundances

Bouret,

Martins,

Hillier

et al. 2021

Preprint

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Context. The evolution of massive stars depends on several physical processes and parameters. Metallicity and rotation are among the most important, but their quantitative effects are not well understood. Aims. To complement our earlier study on main-sequence stars, we study the evolutionary and physical properties of evolved O stars in the Small Magellanic Cloud (SMC). We focus in particular on their surface abundances to further investigate the efficiency of rotational mixing as a function of age, rotation, and global metallicity. Methods. We analysed the UV and optical spectra of 13 SMC O-type giants and supergiants using the stellar atmosphere code CMFGEN to derive photospheric and wind properties. We compared the inferred properties to theoretical predictions from evolution models. For a more comprehensive analysis, we interpret the results together with those we previously obtained for O-type dwarfs. Results. Most dwarfs of our sample lie in the early phases of the main sequence. For a given initial mass, giants are farther along the evolutionary tracks, which confirms that they are indeed more evolved than dwarfs. Supergiants have higher initial masses and are located past the terminal-age main-sequence in each diagram. We find no clear trend of a mass discrepancy, regardless of the diagram that was used to estimate the evolutionary mass. Surface CNO abundances are consistent with nucleosynthesis from the CNO cycle. Comparisons to theoretical predictions reveal that the initial mixture is important when the observed trends in the N/C versus N/O diagram are to be reproduced. A trend for stronger chemical evolution for more evolved objects is observed. Above about 30 M , more massive stars are on average more chemically enriched at a given evolutionary phase. Below 30 M , the trend vanishes. This is qualitatively consistent with evolutionary models. A principal component analysis of the abundance ratios for the whole (dwarfs and evolved stars) sample supports the theoretical prediction that massive stars at low metallicity are more chemically processed than their Galactic counterparts. Finally, models including rotation generally reproduce the surface abundances and rotation rates when different initial rotational velocities are considered. Nevertheless, for some objects, a stronger braking and/or more efficient mixing is required.

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Surface abundances of OC supergiants

Cited by 7 publications

References 29 publications

Spectroscopic evolution of massive stars on the main sequence

Spectroscopic evolution of massive stars on the main sequence

Massive stars in the Small Magellanic Cloud

Massive stars in the Small Magellanic Cloud : Evolution, rotation and surface abundances

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