Evidence of quasi-chemically homogeneous evolution of massive stars up to solar metallicity

Abstract

Context. Long soft gamma-ray bursts (LGRBs) are usually associated with the death of the most massive stars. A large amount of core angular momentum in the phases preceding the explosion is required to form LGRBs. A very high initial rotational velocity can provide this angular momentum. This velocity strongly influences the way the star evolves: it is mixed in a chemically homogeneous way and evolves directly towards the blue part of the Hertzsprung-Russell (HR) diagram from the main sequence. Aims. We have shown that chemically homogeneous evolution takes place in the Small Magellanic Cloud (SMC) at low metallicity. We want to see whether there is a metallicity threshold above which such an evolution is not possible. Methods. We performed a spectroscopic analysis of H-rich early-type WN stars in the Large Magellanic Cloud (LMC) and the Galaxy. We used the code CMFGEN to determine the fundamental properties (T eff , L) and the surface composition of the target stars. We then placed the stars in the HR diagram and determined their evolution. Results. We show that both the LMC and Galactic WNh stars we selected cannot be explained by standard stellar evolution. They are located on the left of the main sequence but show surface abundances typical of CN equilibrium. In addition, they still contain a large amount of hydrogen. They are thus core-H burning objects. Their properties are consistent with chemically homogeneous evolution. We determine the metallicity of the Galactic stars from their position and Galactic metallicity gradients and conclude that they have 0.6 < Z < 1.0. A moderate coupling between the core and the envelope is required to explain that stellar winds do not extract too much angular momentum to prevent a blueward evolution. Conclusions. We have shown that chemically homogeneous evolution takes place in environments with metallicity up to solar. Since some long gamma-ray bursts appear in (super-)solar environments, such an evolution may be a viable way to form them over a wide range of metallicities.

The Evolution and Physical Parameters of WN3/O3s: A New Type of Wolf–Rayet Star*

“…The uncertainty in our modeling is then the value of this change. The values we found are consistent with the uncertainties found by Martins et al (2013) when modeling LMC WN stars using cmfgen. It should be noted that these uncertainties represent the uncertainty within the particular model and not within the modeling code itself.…”

“…In the Smith et al (1996) classification scheme, the spectral subtype "WNha" is used to denote a WR star with hydrogen absorption lines (see also Crowther 2007). This nomenclature has been assigned to the Galactic stars WR3 (HD9974), WR7, WR10, WR18 and WR128, the LMC stars BAT99-18 and BATAT-63, and seven SMC WRs, all of which show WR-like emission lines with some hydrogen absorption lines (see, e.g., Martins et al 2009Martins et al , 2013. cmfgen and PoWR modeling of these stars (by Marchenko et al 2004, Martins et al 2009, 2013, and Hainich et al 2015 has constrained their physical parameters 8 .…”

“…This nomenclature has been assigned to the Galactic stars WR3 (HD9974), WR7, WR10, WR18 and WR128, the LMC stars BAT99-18 and BATAT-63, and seven SMC WRs, all of which show WR-like emission lines with some hydrogen absorption lines (see, e.g., Martins et al 2009Martins et al , 2013. cmfgen and PoWR modeling of these stars (by Marchenko et al 2004, Martins et al 2009, 2013, and Hainich et al 2015 has constrained their physical parameters 8 . While there are many similarities between the physical properties of these WRha stars and the WN3/O3s, there are also a few key differences.…”

The Evolution and Properties of Rotating Massive Star Populations

“…As a result, the effective photosphere-the τ = 2/3 surface-has a radius larger than the hydrostatic radius reported in the stellar evolutionary in low-metallicity environments, they may also occur in metal-rich systems. See Martins et al (2013) for more details. track, and the inferred temperature from observations, T eff, WR , is cooler than the temperature from stellar evolution calculations, T * .…”

Shear mixing in stellar radiative zones

“…Gies & Lambert 1992;Lyubimkov et al 2000; and more recently Hunter et al 2009 with the VLT-FLAMES survey). Martins et al (2009Martins et al ( , 2013 have even found evidence for the existence of stars in which mixing is so efficient that they are quasi-homogeneous.…”