Previous Red Supergiant Phase Research Articles

Blue supergiants as tests for stellar physics

Context. Massive star evolution is still poorly understood, and observational tests are required to discriminate between different implementations of physical phenomena in stellar evolution codes. Aims. By confronting stellar evolution models with observed properties of blue supergiants, such as pulsations, the chemical composition, and position in the Hertzsprung-Russell diagram, we aim to determine which of the criteria used for convection (Schwarzschild or Ledoux) is best able to explain the observations. Methods. We computed state-of-the-art stellar evolution models with either the Schwarzschild or the Ledoux criterion for convection. Models are for 14 to 35 M⊙ at solar or Large Magellanic Cloud metallicity. For each model, we computed the pulsation properties to know when radial modes are excited. We then compared our results with the position of blue supergiants in the Hertzsprung-Russell diagram, with their surface chemical composition and with their variability. Results. Our results at Large Magellanic Cloud metallicity shows only a slight preference for the Ledoux criterion over the Schwarzschild one in reproducing, at the same time, the observed properties of blue supergiants, even if the Schwarzschild criterion cannot be excluded at this metallicity. We checked that changing the overshoot parameter at solar metallicity does not improve the situation. We also checked that our models are able to reproduce the position of Galactic blue supergiants in the flux-weighted-gravity–luminosity relation. Conclusions. We confirm that overall, models computed with the Ledoux criterion are slightly better in matching observations. Our results also support the idea that most Galactic α Cyg variables are blue supergiants from group 2, that is stars that have been through a previous red supergiant phase where they have lost a large amount of mass.

Pion: simulating bow shocks and circumstellar nebulae

ABSTRACT Expanding nebulae are produced by mass-loss from stars, especially during late stages of evolution. Multidimensional simulation of these nebulae requires high resolution near the star and permits resolution that decreases with distance from the star, ideally with adaptive time-steps. We report the implementation and testing of static mesh-refinement in the radiation-magnetohydrodynamics (R-MHD) code pion, and document its performance for 2D and 3D calculations. The bow shock produced by a hot, magnetized, slowly rotating star as it moves through the magnetized ISM is simulated in 3D, highlighting differences compared with 2D calculations. Latitude-dependent, time-varying magnetized winds are modelled and compared with simulations of ring nebulae around blue supergiants from the literature. A 3D simulation of the expansion of a fast wind from a Wolf–Rayet star into the slow wind from a previous red supergiant phase of evolution is presented, with results compared with results in the literature and analytic theory. Finally, the wind–wind collision from a binary star system is modelled with 3D MHD, and the results compared with previous 2D hydrodynamic calculations. A python library is provided for reading and plotting simulation snapshots, and the generation of synthetic infrared emission maps using torus is also demonstrated. It is shown that state-of-the-art 3D MHD simulations of wind-driven nebulae can be performed using pion with reasonable computational resources. The source code and user documentation is made available for the community under a BSD3 licence.

The Canonical Luminous Blue Variable AG Car and Its Neighbor Hen 3-519 Are Much Closer than Previously Assumed

Abstract The strong mass loss of Luminous Blue Variables (LBVs) is thought to play a critical role in massive-star evolution, but their place in the evolutionary sequence remains debated. A key to understanding their peculiar instability is their high observed luminosities, which often depends on uncertain distances. Here we report direct distances and space motions of four canonical Milky Way LBVs—AG Car, HR Car, HD 168607, and (candidate) Hen 3-519—from the Gaia first data release. Whereas the distances of HR Car and HD 168607 are consistent with previous literature estimates within the considerable uncertainties, Hen 3-519 and AG Car, both at ∼2 kpc, are much closer than the 6–8 kpc distances previously assumed. As a result, Hen 3-519 moves far from the locus of LBVs on the Hertzsprung–Russell diagram, making it a much less luminous object. For AG Car, considered a defining example of a classical LBV, its lower luminosity would also move it off the S Dor instability strip. Lower luminosities allow both AG Car and Hen 3-519 to have passed through a previous red supergiant phase, lower the mass estimates for their shell nebulae, and imply that binary evolution is needed to account for their peculiarities. These results may also impact our understanding of LBVs as potential supernova progenitors and their isolated environments. Improved distances will be provided in the Gaia second data release, which will include additional LBVs. AG Car and Hen 3-519 hint that this new information may alter our traditional view of LBVs.

The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars

Aims. We have previously analysed the spectra of 135 early B-type stars in the Large Magellanic Cloud (LMC) and found several groups of stars that have chemical compositions that conflict with the theory of rotational mixing. Here we extend this study to Galactic and Small Magellanic Cloud (SMC) metallicities. Methods. We provide chemical compositions for ~50 Galactic and ~100 SMC early B-type stars and compare these to the LMC results. These samples cover a range of projected rotational velocities up to ~300 km s-1 and hence are well suited to testing rotational mixing models. The surface nitrogen abundances are utilised as a probe of the mixing process since nitrogen is synthesized in the core of the stars and mixed to the surface. Results. In the SMC, we find a population of slowly rotating nitrogen-rich stars amongst the early B type core-hydrogen burning stars, which is comparable to that found previously in the LMC. The identification of non-enriched rapid rotators in the SMC is not possible due to the relatively high upper limits on the nitrogen abundance for the fast rotators. In the Galactic sample we find no significant enrichment amongst the core hydrogen-burning stars, which appears to be in contrast with the expectation from both rotating single-star and close binary evolution models. However, only a small number of the rapidly rotating stars have evolved enough to produce a significant nitrogen enrichment, and these may be analogous to the non-enriched rapid rotators previously found in the LMC sample. Finally, in each metallicity regime, a population of highly enriched supergiants is observed, which cannot be the immediate descendants of core-hydrogen burning stars. Their abundances are, however, compatible with them having gone through a previous red supergiant phase. Together, these observations paint a complex picture of the nitrogen enrichment in massive main sequence and supergiant stellar atmospheres, where age and binarity cause crucial effects. Whether rotational mixing is required to understand our results remains an open question at this time, but could be answered by identifying the true binary fraction in those groups of stars that do not agree with single-star evolutionary models.

The blue supergiant Sher 25 and its intriguing hourglass nebula

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.

The evolutionary status of Sher 25 – implications for blue supergiants and the progenitor of SN 1987A

The blue supergiant Sher 25 in the massive Galactic cluster NGC 3603 is surrounded by a striking emission line nebula. The nebula contains an equatorial ring and probable bi-polar outflows, and is similar in morphology, mass and kine- matics to the shell now visible around SN 1987A. It has been suggested that both nebulae were ejected while Sher 25 and the progenitor of SN 1987A were in previous red supergiant phases. In the case of Sher 25 this is based on the qualitative strengths of nebular (Nii) emission which is indicative of nitrogen enriched gas. This gas may have been dredged up to the stellar surface by convective mixing during a previous red supergiant phase. We present optical high-resolution spectra of Sher 25 and a model photosphere and unified stellar wind analysis which determines the atmospheric parameters, mass-loss rate and photospheric abundances for C, N, O, Mg, and Si. We compare these results, in particular CNO, to other Galactic B-type supergiants and find that Sher 25 does not appear extreme or abnormal in terms of its photospheric nitrogen abundance. The C/ Na nd N/O ratios are compared to surface abundances predicted by stellar evolutionary calculations which assume the star has gone through a red supergiant phase and convective dredge-up. In particular we find that the N/O abundance is incompatible with the star having a previous red supergiant phase, and that the nebulae is likely to have been ejected while the star was a blue supergiant. The results are compatible with some degree of rotationally induced mixing having occurred while the star was on or near the main-sequence. This is similar to what has recently been found for nebulae surrounding LBVs. In addition our wind analysis suggests the star currently has a relatively normal mass-loss rate in comparison with other Galactic B-type supergiants and sits comfortably within the wind momentum-luminosity relationship. In light of the evidence regarding massive evolved early-type stars in the Galaxy we suggest there is no object which shows any evidence of having had a previous red supergiant phase and hence of undergoing blue loops in the HR diagram.

Pinwheel Nebula around WR 98a.

We present the first near-infrared images of the dusty Wolf-Rayet star WR 98a. Aperture-masking interferometry has been utilized to recover images at the diffraction limit of the Keck I telescope, less, similar50 mas at 2.2 µm. Multiepoch observations spanning about 1 yr have resolved the dust shell into a "pinwheel" nebula, the second example of a new class of dust shell first discovered around WR 104 by Tuthill, Monnier, & Danchi. Interpreting the collimated dust outflow in terms of an interacting winds model, the binary orbital parameters and apparent wind speed are derived: a period of 565+/-50 days, a viewing angle of 35&j0;+/-6 degrees from the pole, and a wind speed of 99+/-23 mas yr-1. This period is consistent with a possible approximately 588 day periodicity in the infrared light curve, linking the photometric variation to the binary orbit. Important implications for binary stellar evolution are discussed by identifying WR 104 and WR 98a as members of a class of massive, short-period binaries whose orbits were circularized during a previous red supergiant phase. The current component separation in each system is similar to the diameter of a red supergiant, which indicates that the supergiant phase was likely terminated by Roche lobe overflow, leading to the present Wolf-Rayet stage.

Stellar Winds and the Interstellar Medium

AbstractTheoretical models for the interaction of stellar winds and the interstellar medium are described. Stellar wind energy sources are listed, and observations of expanding shells around stars and star clusters are reviewed. Giant expanding shells, or “superbubbles,” with radius > 100 pc, are formed by the combined action of stellar winds and supernovae from clusters of OB stars. Smaller expanding shells around Wolf-Rayet stars are probably formed by the interaction of the WR stellar wind with matter ejected in a previous red supergiant phase of stellar evolution. Stellar wind activity is evident in regions of star formation in dense molecular clouds.