Aims. We study the impact of a subsolar metallicity on various properties of non-rotating and rotating stars, such as surface velocities and abundances, lifetimes, evolutionary tracks, and evolutionary scenarios. Methods. We provide a grid of single star models covering a mass range of 0.8 to 120 M with an initial metallicity Z = 0.002 with and without rotation. We discuss the impact of a change in the metallicity by comparing the current tracks with models computed with exactly the same physical ingredients but with a metallicity Z = 0.014 (solar). Results. We show that the width of the main-sequence (MS) band in the upper part of the Hertzsprung-Russell diagram (HRD), for luminosity above log (L/L ) > 5.5, is very sensitive to rotational mixing. Strong mixing significantly reduces the MS width. Here for the first time over the whole mass range, we confirm that surface enrichments are stronger at low metallicity provided that comparisons are made for equivalent initial mass, rotation, and evolutionary stage. We show that the enhancement factor due to a lowering of the metallicity (all other factors kept constant) increases when the initial mass decreases. Present models predict an upper luminosity for the red supergiants (RSG) of log (L/L ) around 5.5 at Z = 0.002 in agreement with the observed upper limit of RSG in the Small Magellanic Cloud. We show that models using shear diffusion coefficient, which is calibrated to reproduce the surface enrichments observed for MS B-type stars at Z = 0.014, can also reproduce the stronger enrichments observed at low metallicity. In the framework of the present models, we discuss the factors governing the timescale of the first crossing of the Hertzsprung gap after the MS phase. We show that any process favouring a deep localisation of the H-burning shell (steep gradient at the border of the H-burning convective core, low CNO content), and/or the low opacity of the H-rich envelope favour a blue position in the HRD for the whole, or at least a significant fraction, of the core He-burning phase.

(Fig. 1). The first BB temperature estimate was obtained via careful extrapolation of the UVOT UV M2 and P60 g + i light curves back to the first P48 (detection) point (see inset of Fig. 1 and text for details). The earlytime BB temperature estimates, within the first half day after explosion, are also in agreement with our temperature estimates from the modeling of the early Keck spectra (Fig. 4), showing the highly ionised emission lines at temperatures > ∼ 50 kK. The shaded region and the solid black line (a running mean of the region) denote bolometric luminosity estimates based on the multiband photometry ( Fig. 1) according to three methods used to calculate the total flux from the SED: interpolation, order-4 polynomial fit, and BB fits. The top end of the shaded region can be regarded as our best lower limit on the real bolometric luminosity, based on the photometric observations. The red triangles denote a (more conservative) lower limit on the bolometric luminosity obtained from our spectra (Fig. 2, Fig. 3), beginning with the early set of 4 Keck spectra at < ∼ 10 hr after explosion and ending with our latest spectrum at 57.2days. The blue triangles show the luminosity as obtained by our best BB temperature and radius estimates (Fig. 4), L = 4πR 2 σT 4 ; the luminosity in the first point, at ∼ 3.8 hr after explosion, exceeds 10 44 erg s −1 .

Stars stripped of their hydrogen-rich envelope through interaction with a binary companion are generally not considered when accounting for ionizing radiation from stellar populations, despite the expectation that stripped stars emit hard ionizing radiation, form frequently and live 10 − 100 times longer than single massive stars. We compute the first grid of evolutionary and spectral models specially made for stars stripped in binaries for a range of progenitor masses (2-20 M ) and metallicities ranging from solar to values representative for pop II stars. For stripped stars with masses in the range 0.3-7 M , we find consistently high effective temperatures (20 000-100 000 K, increasing with mass), small radii (0.2-1 R ) and high bolometric luminosities, comparable to that of their progenitor before stripping. The spectra show a continuous sequence that naturally bridge subdwarf-type stars at the low mass end and Wolf-Rayet like spectra at the high mass end. For intermediate masses we find hybrid spectral classes showing a mixture of absorption and emission lines. These appear for stars with mass loss rates of 10 −8 − 10 −6 M yr −1 , which have semi-transparent atmospheres. At low metallicity, substantial hydrogen-rich layers are left at the surface and we predict spectra that resemble O-type stars instead. We obtain spectra undistinguishable from subdwarfs for stripped stars with masses up to 1.7 M , which questions whether the widely adopted canonical value of 0.47 M is uniformly valid. Only a handful of stripped stars of intermediate mass have currently been identified observationally. Increasing this sample will provide necessary tests for the physics of interaction, internal mixing and stellar winds. We use our model spectra to investigate the feasibility to detect stripped stars next to an optically bright companion and recommend systematic searches for their UV excess and possible emission lines, most notably HeII λ4686 in the optical and HeII λ1640 in the UV. Our models are publicly available for further investigations or inclusion in spectral synthesis simulations.

We investigate the fundamental properties of core-collapse supernova (SN) progenitors from single stars at solar metallicity. For this purpose, we combine Geneva stellar evolutionary models with initial masses of M ini = 20−120 M with atmospheric and wind models using the radiative transfer code CMFGEN. We provide synthetic photometry and high-resolution spectra of hot stars at the pre-SN stage. For models with M ini = 9−20 M , we supplement our analysis using publicly available MARCS model atmospheres of RSGs to estimate their synthetic photometry. We employ well-established observational criteria of spectroscopic classification and find that, depending on their initial mass and rotation, massive stars end their lives as red supergiants (RSG), yellow hypergiants (YHG), luminous blue variables (LBV), and Wolf-Rayet (WR) stars of the WN and WO spectral types. For rotating models, we obtained the following types of SN progenitors:, and RSGs (9 ≤ M ini ≤ 18 M ). For non-rotating models, we found spectral types WO1-, and RSGs (9 ≤ M ini ≤ 20 M ). Our rotating models indicate that SN IIP progenitors are all RSG, SN IIL/b progenitors are 56% LBVs and 44% YHGs, SN Ib progenitors are 96% WN10-11 and 4% WOs, and SN Ic progenitors are all WO stars. We find that the most massive and luminous SN progenitors are not necessarily the brightest ones in a given filter, since this depends on their luminosity, temperature, wind density, and the way the spectral energy distribution compares to a filter bandpass. We find that SN IIP progenitors (RSGs) are bright in the RIJHK S filters and faint in the UB filters. SN IIL/b progenitors (LBVs and YHGs), and SN Ib progenitors (WNs) are relatively bright in optical/infrared filters, while SN Ic progenitors (WOs) are faint in all optical filters. We argue that SN Ib and Ic progenitors from single stars should be undetectable in the available pre-explosion images with the current magnitude limits, in agreement with observational results.

For the first time, the interior and spectroscopic evolution of a massive star is analyzed from the zero-age main sequence (ZAMS) to the pre-supernova (SN) stage. For this purpose, we combined stellar evolution models using the Geneva code and stellar atmospheric/wind models using CMFGEN. With our approach, we were able to produce observables, such as a synthetic high-resolution spectrum and photometry, thereby aiding the comparison between evolution models and observed data. Here we analyze the evolution of a nonrotating 60 M star and its spectrum throughout its lifetime. Interestingly, the star has a supergiant appearance (luminosity class I) even at the ZAMS. We find the following evolutionary sequence of spectral types: O3 I (at the ZAMS), O4 I (middle of the H-core burning phase), B supergiant (BSG), B hypergiant (BHG), hot luminous blue variable (LBV; end of H-core burning), cool LBV (H-shell burning through the beginning of the He-core burning phase), rapid evolution through late WN and early WN, early WC (middle of He-core burning), and WO (end of He-core burning until core collapse). We find the following spectroscopic phase lifetimes: 3.22 × 10 6 yr for the O-type, 0.34 × 10 5 yr (BSG), 0.79 × 10 5 yr (BHG), 2.35 × 10 5 yr (LBV), 1.05 × 10 5 yr (WN), 2.57 × 10 5 yr (WC), and 3.80 × 10 4 yr (WO). Compared to previous studies, we find a much longer (shorter) duration for the early WN (late WN) phase, as well as a long-lived LBV phase. We show that LBVs arise naturally in single-star evolution models at the end of the MS when the mass-loss rate increases as a consequence of crossing the bistability limit. We discuss the evolution of the spectra, magnitudes, colors, and ionizing flux across the star's lifetime, and the way they are related to the evolution of the interior. We find that the absolute magnitude of the star typically changes by ∼6 mag in optical filters across the evolution, with the star becoming significantly fainter in optical filters at the end of the evolution, when it becomes a WO just a few 10 4 years before the SN explosion. We also discuss the origin of the different spectroscopic phases (i.e., O-type, LBV, WR) and how they are related to evolutionary phases (H-core burning, H-shell burning, He-core burning).