The Tarantula Massive Binary Monitoring. I. Observational campaign and OB-type spectroscopic binaries

Almeida, L. A.; Sana, H.; Taylor, W.; Barbá, R.; Bonanos, A. Z.; Crowther, P.; Damineli, A.; de Koter, A.; de Mink, S. E.; Evans, C. J.; Gieles, M.; Grin, N. J.; Hénault-Brunet, V.; Langer, N.; Lennon, D.; Lockwood, S.; Maíz Apellániz, J.; Moffat, A. F. J.; Neijssel, C.; Norman, C. Ramírez-Agudelo, O. H.; Richardson, N. D.; Schootemeijer, A.; Shenar, T.; Soszyński, I.; Tramper, F.; Vink, J. S.

Astronomy & Astrophysics, Volume 598, A84 (2017)

ADS – Journal – arXiv

Abstract

Context. Massive binaries play a crucial role in the Universe. Knowing the distributions of their orbital parameters is important for a wide range of topics from stellar feedback to binary evolution channels and from the distribution of supernova types to gravitational wave progenitors, yet no direct measurements exist outside the Milky Way.
Aims: The Tarantula Massive Binary Monitoring project was designed to help fill this gap by obtaining multi-epoch radial velocity (RV) monitoring of 102 massive binaries in the 30 Doradus region.
Methods: In this paper we analyze 32 FLAMES/GIRAFFE observations of 93 O- and 7 B-type binaries. We performed a Fourier analysis and obtained orbital solutions for 82 systems: 51 single-lined (SB1) and 31 double-lined (SB2) spectroscopic binaries.
Results: Overall, the binary fraction and orbital properties across the 30 Doradus region are found to be similar to existing Galactic samples. This indicates that within these domains environmental effects are of second order in shaping the properties of massive binary systems. A small difference is found in the distribution of orbital periods, which is slightly flatter (in log space) in 30 Doradus than in the Galaxy, although this may be compatible within error estimates and differences in the fitting methodology. Also, orbital periods in 30 Doradus can be as short as 1.1 d, somewhat shorter than seen in Galactic samples. Equal mass binaries (q> 0.95) in 30 Doradus are all found outside NGC 2070, the central association that surrounds R136a, the very young and massive cluster at 30 Doradus’s core. Most of the differences, albeit small, are compatible with expectations from binary evolution. One outstanding exception, however, is the fact that earlier spectral types (O2-O7) tend to have shorter orbital periods than later spectral types (O9.2-O9.7).
Conclusions: Our results point to a relative universality of the incidence rate of massive binaries and their orbital properties in the metallicity range from solar (Z) to about half solar. This provides the first direct constraints on massive binary properties in massive star-forming galaxies at the Universe’s peak of star formation at redshifts z 1 to 2 which are estimated to have Z 0.5 Z.

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


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

Abstract

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

Abstract

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

Abstract

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

Abstract

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