Low-metallicity massive single stars with rotation

Abstract

Context. Metal-poor massive stars are assumed to be progenitors of certain supernovae, gamma-ray bursts, and compact object mergers that might contribute to the early epochs of the Universe with their strong ionizing radiation. However, this assumption remains mainly theoretical because individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the Small Magellanic Cloud. Aims. Here we explore the predictions of the state-of-the-art theories of stellar evolution combined with those of stellar atmospheres about a certain type of metal-poor (0.02 Z⊙) hot massive stars, the chemically homogeneously evolving stars that we call Transparent Wind Ultraviolet INtense (TWUIN) stars. Methods. We computed synthetic spectra corresponding to a broad range in masses (20−130 M⊙) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage. We investigated the influence of mass loss and wind clumping on spectral appearance and classified the spectra according to the Morgan-Keenan (MK) system. Results. We find that TWUIN stars show almost no emission lines during most of their core hydrogen-burning lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission, and even some metal lines (oxygen and carbon, but still almost no nitrogen) are detected. Mass loss and clumping play a significant role in line formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early-O type giant or supergiant, and we find Wolf–Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early-O type star observed in a low-metallicity galaxy could be the result of chemically homogeneous evolution and might therefore be the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe because they are hot without showing prominent emission lines during most of their lifetime.

Spectroscopic evolution of massive stars near the main sequence at low metallicity

“…In our spectroscopic sequences at Z = 1/30 Z most of the O2 stars are dwarfs. Kubátová et al (2019) calculated theoretical spectra of metal-poor stars (Z = 1/50 Z ) following quasi chemically homogeneous evolution. This type of evolution is different from the one followed in our computations, because it requires that rotational mixing be taken into account.…”

“…This type of evolution is different from the one followed in our computations, because it requires that rotational mixing be taken into account. However, the ZAMS models of Kubátová et al (2019) 2019) prefer <O4III. We therefore agree on the spectral type but find a different luminosity class.…”

“…These latter authors found that such objects emit a large number of ionizing photons, equivalent to Wolf-Rayet stars. Kubátová et al (2019) looked at the spectral appearance of stars undergoing quasi-chemically homogeneous evolution (Maeder 1987;Yoon et al 2006), focusing on metal-poor objects (Z = 1/50 Z ), for this type of evolution seems to be more easily achieved at that metallicity (e.g., Brott et al 2011). Kubátová et al (2019) concluded that for most of their evolution, which proceeds directly leftward of the zero age main sequence (ZAMS), stars show only absorption lines in their synthetic spectra, therefore appearing as early-type O stars.…”

“…Kubátová et al (2019) looked at the spectral appearance of stars undergoing quasi-chemically homogeneous evolution (Maeder 1987;Yoon et al 2006), focusing on metal-poor objects (Z = 1/50 Z ), for this type of evolution seems to be more easily achieved at that metallicity (e.g., Brott et al 2011). Kubátová et al (2019) concluded that for most of their evolution, which proceeds directly leftward of the zero age main sequence (ZAMS), stars show only absorption lines in their synthetic spectra, therefore appearing as early-type O stars. In the present work, similarly to Martins & Palacios (2017), we focus on the MS and early post-MS evolution because these phases are the least affected by uncertainties (see Martins & Palacios 2013).…”

High-mass X-ray binaries in nearby metal-poor galaxies: on the contribution to nebular He ii emission

“…Yet despite commensurate advances in stellar modeling, including state-of-the-art treatments of atmospheres, binarity, and rotation (e.g. Szécsi et al 2015;Götberg et al 2017;Eldridge et al 2017;Götberg et al 2018;Stanway & Eldridge 2019;Kubátová et al 2019), there is as-yet no clear solution to the apparent modeling deficit of hard ionizing photons.…”