A New Prescription for the Mass-loss Rates of WC and WO Stars

Tramper, F.; Sana, H.; de Koter, A.

The Astrophysical Journal, Volume 833, Issue 2, 133 (2016)

ADS – Journal – arXiv


We present a new empirical prescription for the mass-loss rates of carbon- and oxygen-sequence Wolf-Rayet stars as a function of their luminosity, surface chemical composition, and initial metallicity. The new prescription is based on results of detailed spectral analyses of WC and WO stars and improves the often applied Nugis and Lamers relation. We find that the mass-loss rates of WC and WO stars (with X = 0 and Y ≲ 0.98) can be expressed as {log} \dot{M}=-9.20+0.85{log}(L/L ) + 0.44 log Y + 0.25 log (Z Fe/Z Fe,☉). This relation is based on mass-loss determinations that assume a volume-filling factor of 0.1, but the prescription can easily be scaled to account for other volume-filling factors. The residual of the fit is σ = 0.06 dex. We investigated whether the relation can also describe the mass loss of hydrogen-free WN stars and showed that it can when an adjustment of the metallicity dependence ({log} \dot{M}\propto 1.3{log}({Z}{Fe}/{Z}{Fe,☉ })) is applied. Compared to that of Nugis and Lamers, \dot{M} is less sensitive to the luminosity and the surface abundance, implying a stronger mass loss of massive stars in their late stages of evolution. The modest metallicity dependence implies that if WC or WO stars are formed in metal-deficient environments, their mass-loss rates are higher than currently anticipated. These effects may result in the formation of a larger number of SNe Ic and fewer black holes and may favor the production of superluminous SNe Ic through interaction with C- and O-rich circumstellar material or dense stellar wind.

Keywords: stars: evolution; stars: fundamental parameters; stars: mass-loss; stars: massive; stars: winds; outflows; stars: Wolf─Rayet; Astrophysics – Solar and Stellar Astrophysics

The mass of the very massive binary WR21a

Tramper, F.; Sana, H.; Fitzsimons, N. E.; de Koter, A.; Kaper, L.; Mahy, L.; Moffat, A.

Monthly Notices of the Royal Astronomical Society, Volume 455, Issue 2, 1275 (2016)

ADS – Journal – arXiv


We present multi-epoch spectroscopic observations of the massive binary system WR21a, which include the 2011 January periastron passage. Our spectra reveal multiple SB2 lines and facilitate an accurate determination of the orbit and the spectral types of the components. We obtain minimum masses of 64.4 ± 4.8 M and 36.3 ± 1.7 M for the two components of WR21a. Using disentangled spectra of the individual components, we derive spectral types of O3/WN5ha and O3Vz ((f*)) for the primary and secondary, respectively. Using the spectral type of the secondary as an indication for its mass, we estimate an orbital inclination of I = 58.8 ± 2.5° and absolute masses of 103.6 ± 10.2 M and 58.3 ± 3.7 M, in agreement with the luminosity of the system. The spectral types of the WR21a components indicate that the stars are very young (1-2 Myr), similar to the age of the nearby Westerlund 2 cluster. We use evolutionary tracks to determine the mass-luminosity relation for the total system mass. We find that for a distance of 8 kpc and an age of 1.5 Myr, the derived absolute masses are in good agreement with those from evolutionary predictions.

Keywords: binaries: close; binaries: spectroscopic; stars: early-type; stars: fundamental parameters; stars: individual: WR21a; stars: Wolf-Rayet; Astrophysics – Solar and Stellar Astrophysics

Discovery of the Massive Overcontact Binary VFTS352: Evidence for Enhanced Internal Mixing

Almeida, L. A.; Sana, H.; de Mink, S. E.; Tramper, F.; Soszyński, I.; Langer, N.; Barbá, R. H.; Cantiello, M.; Damineli, A.; de Koter, A.; Garcia, M.; Gräfener, G.; Herrero, A.; Howarth, I.; Maíz Apellániz, J.; Norman, C.; Ramírez-Agudelo, O. H.; Vink, J. S.

The Astrophysical Journal, Volume 812, Issue 2, 102 (2015)

ADS – Journal – arXiv


The contact phase expected to precede the coalescence of two massive stars is poorly characterized due to the paucity of observational constraints. Here we report on the discovery of VFTS 352, an O-type binary in the 30 Doradus region, as the most massive and earliest spectral type overcontact system known to date. We derived the 3D geometry of the system, its orbital period {P}{{orb}}=1.1241452(4) day, components’ effective temperatures—T1 = 42 540 ± 280 K and T2 = 41 120 ± 290 K—and dynamical masses—{M}1=28.63+/- 0.30 {M}☉ and {M}2=28.85+/- 0.30 {M}☉ . Compared to single-star evolutionary models, the VFTS 352 components are too hot for their dynamical masses by about 2700 and 1100 K, respectively. These results can be explained naturally as a result of enhanced mixing, theoretically predicted to occur in very short-period tidally locked systems. The VFTS 352 components are two of the best candidates identified so far to undergo this so-called chemically homogeneous evolution. The future of VFTS 352 is uncertain. If the two stars merge, a very rapidly rotating star will be produced. Instead, if the stars continue to evolve homogeneously and keep shrinking within their Roche Lobes, coalescence can be avoided. In this case, tides may counteract the spin down by winds such that the VFTS 352 components may, at the end of their life, fulfill the requirements for long gamma-ray burst (GRB) progenitors in the collapsar scenario. Independently of whether the VFTS 352 components become GRB progenitors, this scenario makes VFTS 352 interesting as a progenitor of a black hole binary, hence as a potential gravitational wave source through black hole-black hole merger.

Keywords: binaries: close; binaries: eclipsing; binaries: spectroscopic; stars: early-type; stars: individual: VFTS 352; stars: massive; Astrophysics – Solar and Stellar Astrophysics

Massive stars on the verge of exploding: the properties of oxygen sequence Wolf-Rayet stars

Tramper, F.; Straal, S. M.; Sanyal, D.; Sana, H.; de Koter, A.; Gräfener, G.; Langer, N.; Vink, J. S.; de Mink, S. E.; Kaper, L.

Astronomy & Astrophysics, Volume 581, A110 (2015)

ADS – Journal – arXiv


Context. Oxygen sequence Wolf-Rayet (WO) stars are a very rare stage in the evolution of massive stars. Their spectra show strong emission lines of helium-burning products, in particular highly ionized carbon and oxygen. The properties of WO stars can be used to provide unique constraints on the (post-)helium burning evolution of massive stars, and their remaining lifetimes and the expected properties of their supernovae.
Aims: We aim to homogeneously analyze the currently known presumed-single WO stars to obtain the key stellar and outflow properties and to constrain their evolutionary state.
Methods: We use the line-blanketed non-local thermal equilibrium atmosphere code cmfgen to model the X-Shooter spectra of the WO stars and to deduce the atmospheric parameters. We calculate dedicated evolutionary models to determine the evolutionary state of the stars.
Results: The WO stars have extremely high temperatures that range from 150 kK to 210 kK, and very low surface helium mass fractions that range from 44% down to 14%. Their properties can be reproduced by evolutionary models with helium zero-age main sequence masses of MHe,ini = 15-25 M that exhibit a fairly strong (a few times 10-5M yr-1), homogeneous (fc> 0.3) stellar wind.
Conclusions: WO stars represent the final evolutionary stage of stars with estimated initial masses of Mini = 40-60 M. They are post core-helium burning and predicted to explode as type Ic supernovae within a few thousand years.

Keywords: stars: Wolf-Rayet; stars: massive; stars: winds; outflows; stars: atmospheres; stars: fundamental parameters; stars: early-type; Astrophysics – Solar and Stellar Astrophysics

The VLT-FLAMES Tarantula Survey. XXI. Stellar spin rates of O-type spectroscopic binaries

Ramírez-Agudelo, O. H.; Sana, H.; de Mink, S. E.; Hénault-Brunet, V.; de Koter, A.; Langer, N.; Tramper, F.; Gräfener, G.; Evans, C. J.; Vink, J. S.; Dufton, P. L.; Taylor, W. D.

Astronomy & Astrophysics, Volume 580, A92 (2015)

ADS – Journal – arXiv


Context. The initial distribution of spin rates of massive stars is a fingerprint of their elusive formation process. It also sets a key initial condition for stellar evolution and is thus an important ingredient in stellar population synthesis. So far, most studies have focused on single stars. Most O stars are, however, found in multiple systems.
Aims: By establishing the spin-rate distribution of a sizeable sample of O-type spectroscopic binaries and by comparing the distributions of binary subpopulations with one another and with that of presumed-single stars in the same region, we aim to constrain the initial spin distribution of O stars in binaries, and to identify signatures of the physical mechanisms that affect the evolution of the spin rates of massive stars.
Methods: We use ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial rotational velocities (νesini) for components of 114 spectroscopic binaries in 30 Doradus. The νesini values are derived from the full width at half maximum (FWHM) of a set of spectral lines, using a FWHM vs. νesini calibration that we derive based on previous line analysis methods applied to single O-type stars in the VFTS sample.
Results: The overall νesini distribution of the primary stars resembles that of single O-type stars in the VFTS, featuring a low-velocity peak (at νesini< 200 kms-1) and a shoulder at intermediate velocities (200 < νesini< 300 kms-1). The distributions of binaries and single stars, however, differ in two ways. First, the main peak at νesini ~ 100kms-1 is broader and slightly shifted towards higher spin rates in the binary distribution than that of the presumed-single stars. This shift is mostly due to short-period binaries (Porb~< 10 d). Second, the νesini distribution of primaries lacks a significant population of stars spinning faster than 300 kms-1, while such a population is clearly present in the single-star sample. The νesini distribution of binaries with amplitudes of radial velocity variation in the range of 20 to 200 kms-1 (mostly binaries with Porb ~ 10-1000 d and/or with q< 0.5) is similar to that of single O stars below νesini~< 170kms-1.
Conclusions: Our results are compatible with the assumption that binary components formed with the same spin distribution as single stars, and that this distribution contains few or no fast-spinning stars. The higher average spin rate of stars in short-period binaries may either be explained by spin-up through tides in such tight binary systems, or by spin-down of a fraction of the presumed-single stars and long-period binaries through magnetic braking (or by a combination of both mechanisms). Most primaries and secondaries of SB2 systems with Porb~< 10 d appear to have similar rotational velocities. This is in agreement with tidal locking in close binaries where the components have similar radii. The lack of very rapidly spinning stars among binary systems supports the idea that most stars with νesini~> 300kms-1 in the single-star sample are actually spun-up post-binary interaction products. Finally, the overall similarities (low-velocity peak and intermediate-velocity shoulder) of the spin distribution of binary and single stars argue for a massive star formation process in which the initial spin is set independently of whether stars are formed as single stars or as components of a binary system.

Keywords: stars: rotation; binaries: spectroscopic; Magellanic Clouds; galaxies: star clusters: individual: 30 Doradus; line: profiles; Astrophysics – Solar and Stellar Astrophysics