The blue supergiant Sher 25 and its intriguing hourglass nebula |
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| Authors: | M A Hendry S J Smartt E D Skillman C J Evans C Trundle D J Lennon P A Crowther I Hunter |
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| Institution: | Astrophysics Research Centre, School of Maths and Physics, Queen's University Belfast, Belfast BT7 1NN;Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA;Astronomy Department, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455, USA;UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ;Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA;Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain;Department of Physics &Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH |
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| Abstract: | The blue supergiant Sher 25 is surrounded by an asymmetric, hourglass-shaped circumstellar nebula. Its structure and dynamics have been studied previously through high-resolution imaging and spectroscopy, and it appears dynamically similar to the ring structure around SN 1987A. Here, we present long-slit spectroscopy of the circumstellar nebula around Sher 25, and of the background nebula of the host cluster NGC 3603. We perform a detailed nebular abundance analysis to measure the gas-phase abundances of oxygen, nitrogen, sulphur, neon and argon. The oxygen abundance in the circumstellar nebula (12 + log O/H = 8.61 ± 0.13 dex) is similar to that in the background nebula (8.56 ± 0.07), suggesting that the composition of the host cluster is around solar. However, we confirm that the circumstellar nebula is very rich in nitrogen, with an abundance of 8.91 ± 0.15, compared to the background value of 7.47 ± 0.18. A new analysis of the stellar spectrum with the fastwind model atmosphere code suggests that the photospheric nitrogen and oxygen abundances in Sher 25 are consistent with the nebular results. While the nitrogen abundances are high, when compared to stellar evolutionary models, they do not unambiguously confirm that the star has undergone convective dredge-up during a previous red supergiant phase. We suggest that the more likely scenario is that the nebula was ejected from the star while it was in the blue supergiant phase. The star's initial mass was around 50 M⊙ , which is rather too high for it to have had a convective envelope stage as a red supergiant. Rotating stellar models that lead to mixing of core-processed material to the stellar surface during core H-burning can quantitatively match the stellar results with the nebula abundances. |
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| Keywords: | stars: abundances stars: evolution stars: individual: Sher 25 supergiants |
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