The blue supergiant Sher 25 and its intriguing hourglass nebula

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 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_sun, 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.

arXiv commentRevised after initial referee report, to appear in MNRAS. 24 pages, some figures significantly degraded, high-res copy available at http://star.pst.qub.ac.uk/~sjs/papers/sher25/

References in corpus (21)

• Line formation in solar granulation: IV. [O I], OI and OH lines and the photospheric O abundance
• The Effective Temperature Scale of Galactic Red Supergiants: Cool, But Not As Cool As We Thought
• Presupernova Evolution of Rotating Massive Stars II: -- Evolution of the Surface Properties
• Atmospheric NLTE-Models for the Spectroscopic Analysis of Blue Stars with Winds. II. Line-Blanketed Models
• Physical Parameters and Wind Properties of Galactic Early B Supergiants
• Systematic effects and a new determination of the primordial abundance of 4He and dY/dZ from observations of blue compact galaxies
• The VLT-FLAMES survey of massive stars: Surface chemical compositions of B-type stars in the Magellanic Clouds
• The "Solar Model Problem" Solved by the Abundance of Neon in Stars of the Local Cosmos
• The VLT-FLAMES survey of massive stars: atmospheric parameters and rotational velocity distributions for B-type stars in the Magellanic Clouds
• Bright OB stars in the Galaxy IV. Stellar and wind parameters of early to late B supergiants
• Understanding B-type Supergiants in the Low Metallicity Environment of the SMC
• Discovery of a nearby twin of SN1987A's nebula around the luminous blue variable HD168625: Was Sk--69 202 an LBV?
• Galactic Twins of the Ring Nebula Around SN1987A and a Possible LBV-like Phase for Sk-69 202
• Chemical composition and origin of nebulae around Luminous Blue Variables
• The Massive Star Content of NGC 3603
• And in the Darkness Bind Them: Equatorial Rings, B[e] Supergiants, and the Waists of Bipolar Nebulae
• Ring Nebula and Bipolar Outflows Associated with the B1.5 Supergiant Sher #25 in NGC 3603
• The evolutionary status of Sher25 - implications for blue supergiants and the progenitor of SN1987A
• The Hourglass Nebulae of Sher 25 and SN 1987 A: Two of a Kind?
• The Structure of the Homunculus. III. Forming a Disk and Bipolar Lobes in a Rotating Surface Explosion
• The Slow Merger of Massive Stars

Cited by in corpus (22)

• On the nature of the galactic early-B hypergiants
• Progressive star formation in the young galactic super star cluster NGC 3603
• On the Origin of Mass Segregation in NGC 3603
• Abundances and density structure of the inner circumstellar ring around SN 1987A
• The quest for blue supergiants: binary merger models for the evolution of the progenitor of SN 1987A
• The Physical Properties of the Red Supergiant WOH G64: The Largest Star Known?
• Early Dust Formation and a Massive Progenitor for SN 2011ja?
• Discovery of the first B[e] supergiants in M 31
• Dusty Blue Supergiants: News from High-Angular Resolution Observations
• Evidence of the Evolved Nature of the B[e] Star MWC 137
• Luminous blue variables: An imaging perspective on their binarity and near environment
• The blue supergiant MN18 and its bipolar circumstellar nebula
• Sher 25: pulsating but apparently alone
• Optical spectroscopy of the blue supergiant Sk-69 279 and its circumstellar shell with SALT
• TYC 3159-6-1: a runaway blue supergiant
• Identification of two new HMXBs in the LMC: a $\sim$2013 s pulsar and a probable SFXT
• Tracing back the evolution of the candidate LBV HD168625
• The blue supergiant Sher 25 revisited in the Gaia era
• Unveiling the nitrogen-rich massive star in the metal-poor galaxy NGC 4068
• Dense Molecular Environments of B[e] Supergiants and Yellow Hypergiants
• Two circumstellar nebulae discovered with the Wide-field Infrared Survey Explorer and their massive central stars
• HD 93795: a late-B supergiant star with a square circumstellar nebula