Rotational mixing in Be stars: nitrogen abundances for a sample of Be stars from the MiMeS survey

Ahmed, A. A.; Sigut, T. A. A.

doi:10.1093/mnras/stx1737

Cited by 16 publications

(14 citation statements)

References 78 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Hence a nitrogen enhancement in these two objects cannot be discounted. However near baseline nitrogen abundances have previously been found for Be-type stars in both the Galaxy (Ahmed & Sigut 2017) and the Magellanic Clouds (Lennon et al 2005;Dunstall et al 2011), with Ahmed & Sigut (2017) having recently discussed possible explanations.…”

Section: Nitrogen Abundancesmentioning

confidence: 98%

“…Two targets, VFTS 237 and 835, have been identified as Be-type stars and might be expected to show the effects of rotational mixing due to rotational velocities near to critical velocities having been inferred (Townsend et al 2004;Cranmer 2005;Ahmed & Sigut 2017) for such objects. However both have near baseline nitrogen abundance estimates (6.92 and 6.90 dex respectively) implying that little rotational mixing has occurred.…”

Section: Nitrogen Abundancesmentioning

confidence: 99%

See 1 more Smart Citation

The VLT-FLAMES Tarantula Survey

Dufton

Thompson

Crowther

et al. 2018

A&A

View full text Add to dashboard Cite

Previous analyses of the spectra of OB-type stars in the Magellanic Clouds have identified targets with low projected rotational velocities and relatively high nitrogen abundances; the evolutionary status of these objects remains unclear. The VLT-FLAMES Tarantula Survey obtained spectroscopy for over 800 early-type stars in 30 Doradus of which 434 stars were classified as B-type. We have estimated atmospheric parameters and nitrogen abundances using TLUSTY model atmospheres for 54 B-type targets that appear to be single, have projected rotational velocities, ve sin i ≤ 80 km s−1 and were not classified as supergiants. In addition, nitrogen abundances for 34 similar stars observed in a previous FLAMES survey of the Large Magellanic Cloud have been re-evaluated. For both samples, approximately 75–80% of the targets have nitrogen enhancements of less than 0.3 dex, consistent with them having experienced only small amounts of mixing. However, stars with low projected rotational velocities, ve sini ≤ 40 km s−1 and significant nitrogen enrichments are found in both our samples and simulations imply that these cannot all be rapidly rotating objects observed near pole-on. For example, adopting an enhancement threshold of 0.6 dex, we observed five and four stars in our VFTS and previous FLAMES survey samples, yet stellar evolution models with rotation predict only 1.25 ± 1.11 and 0.26 ± 0.51 based on our sample sizes and random stellar viewing inclinations. The excess of such objects is estimated to be 20–30% of all stars with current rotational velocities of less than 40 km s−1. This would correspond to ~2–4% of the total non-supergiant single B-type sample. Given the relatively large nitrogen enhancement adopted, these estimates constitute lower limits for stars that appear inconsistent with current grids of stellar evolutionary models. Including targets with smaller nitrogen enhancements of greater than 0.2 dex implies larger percentages of targets that are inconsistent with current evolutionary models, viz. ~70% of the stars with rotational velocities less than 40 km s−1 and ~6–8% of the total single stellar population. We consider possible explanations of which the most promising would appear to be breaking due to magnetic fields or stellar mergers with subsequent magnetic braking.

show abstract

Section: Nitrogen Abundancesmentioning

confidence: 98%

Section: Nitrogen Abundancesmentioning

confidence: 99%

The VLT-FLAMES Tarantula Survey

Dufton

Thompson

Crowther

et al. 2018

A&A

View full text Add to dashboard Cite

show abstract

“…Two targets, #0023 and #0089 had been classified as Be-type by Evans et al (2006) and subsequently analysed by Dunstall et al (2011). These targets have been included in our analysis but the results should be treated with caution given the complex nature of their spectra (see, e.g., Ahmed & Sigut 2017).…”

Section: Atmospheric Parametersmentioning

confidence: 99%

The NGC 346 massive star census

Dufton

Evans

Lennon

et al. 2020

A&A

View full text Add to dashboard Cite

Previous analyses of two large spectroscopic surveys of early-type stars in the Large Magellanic Cloud (LMC) have found an excess of nitrogen enriched B-type targets with a ve sin i ≤ 40 km s−1 compared with the predictions of single star evolutionary models that incorporate rotational mixing. By contrast, the number of such targets with 40 < ve sin i ≤ 80 km s−1 was consistent with such models. We have undertaken a similar analysis for 61 B-type targets which lie towards the young cluster, NGC 346 in the Small Magellanic Cloud (SMC). These again have projected rotational velocities, ve sin i ≤ 80 km s−1, are not classified as supergiants, and are apparently single. Approximately 65% of these SMC targets could have nitrogen enhancements of less than 0.3 dex, which is consistent with them having experienced only small amounts of mixing due to their low rotational velocities. However, as with the previous LMC surveys, an excess of stars with low projected rotational velocities, ve sin i ≤ 40 km s−1, and significant nitrogen enrichments is found. This is estimated to be approximately 5% of the total population of apparently single B-type stars or 40% of all stars with current rotational velocities of less than 40 km s−1; these percentages are similar to those found previously for the two LMC samples. For all three surveys, the presence of undetected binaries and other uncertainties imply that these percentages might be underestimated and that it is indeed possible for all the single stars with current rotational velocities of less than 40 km s−1 to be nitrogen enriched. Two possible explanations incorporate the effects of the magnetic field, via either a stellar merger followed by magnetic braking or the evolution of a single star with a large magnetic field. Both mechanisms would appear to be compatible with the observed frequency of nitrogen-enriched stars in the Magellanic Clouds. Differences in the properties of the nitrogen-enriched stars compared with the remainder of the sample would be consistent with the former mechanism. For the latter, a qualitative comparison with Galactic evolutionary models that incorporate magnetic fields is encouraging in terms of the amount of nitrogen enrichment and its presence in stars near the zero-age main sequence.

show abstract

“…To warn the reader of the much larger uncertainties than the formal ones listed in Table 1, v sini values for Be stars have been listed in brackets. They were derived neglecting gravitational darkening and disk contamination, which however seem less important from the recent comprehensive investigation of Ahmed & Sigut (2017) than implied previously (e.g. Townsend et al 2004;Fremat et al 2005).…”

Section: Rotationmentioning

confidence: 99%

Different Stellar Rotations in the Two Main Sequences of the Young Globular Cluster NGC 1818: The First Direct Spectroscopic Evidence^*

Marino

Przybilla

Milone

et al. 2018

View full text Add to dashboard Cite

We present a spectroscopic analysis of main sequence (MS) stars in the young globular cluster NGC 1818 (age∼40 Myrs) in the Large Magellanic Cloud. Our photometric survey on Magellanic Clouds clusters has revealed that NGC 1818, similarly to the other young objects with age 600 Myrs, displays not only an extended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age∼1-2 Gyrs), but also a split MS. The most straightforward interpretation of the double MS is the presence of two stellar populations: a sequence of slowly-rotating stars lying on the blue-MS and a sequence of fast rotators, with rotation close to the breaking speed, defining a red-MS. We report the first direct spectroscopic measurements of projected rotational velocities v sini for the double MS, eMSTO and Be stars of a young cluster. The analysis of line profiles includes non-LTE effects, required for correctly deriving v sini values. Our results suggest that: (i) the mean rotation for blue-and red-MS stars is v sini =71±10 km s −1 (σ=37km s −1 ) and v sini =202±23 km s −1 (σ=91km s −1 ), respectively; (ii) eMSTO stars have different v sini , which are generally lower than those inferred for red-MS stars, and (iii) as expected, Be stars display the highest v sini values. This analyis supports the idea that distinct rotational velocities play an important role in the appearence of multiple stellar populations in the color-magnitude diagrams of young clusters, and poses new constraints to the current scenarios.

show abstract

Rotational mixing in Be stars: nitrogen abundances for a sample of Be stars from the MiMeS survey

Cited by 16 publications

References 78 publications

The VLT-FLAMES Tarantula Survey

The VLT-FLAMES Tarantula Survey

The NGC 346 massive star census

Different Stellar Rotations in the Two Main Sequences of the Young Globular Cluster NGC 1818: The First Direct Spectroscopic Evidence^*

Contact Info

Product

Resources

About

Rotational mixing in Be stars: nitrogen abundances for a sample of Be stars from the MiMeS survey

Cited by 16 publications

References 78 publications

The VLT-FLAMES Tarantula Survey

The VLT-FLAMES Tarantula Survey

The NGC 346 massive star census

Different Stellar Rotations in the Two Main Sequences of the Young Globular Cluster NGC 1818: The First Direct Spectroscopic Evidence*

Contact Info

Product

Resources

About

Different Stellar Rotations in the Two Main Sequences of the Young Globular Cluster NGC 1818: The First Direct Spectroscopic Evidence^*