Artist’s impression of VFTS 352

This animated artist’s impression shows VFTS 352 — the hottest and most massive double star system to date where the two components are in contact and sharing material. The two stars in this extreme system lie about 160 000 light-years from Earth in the Large Magellanic Cloud. This intriguing pair could be heading for a dramatic end, either with the creation of a single monster star or the formation of a binary black hole. Credit: ESO/L. Calçada.

Discovery paperESO press release


Discovery of a Red Supergiant Donor Star in SN2010da/NGC 300 ULX-1

Heida, M.; Lau, R. M.; Davies, B.; Brightman, M.; Fürst, F.; Grefenstette, B. W.; Kennea, J. A.; Tramper, F.; Walton, D. J.; Harrison, F. A.

The Astrophysical Journal Letters, Volume 883, L34 (2019)

ADS – Journal – arXiv

Abstract

SN2010da/NGC 300 ULX-1 was first detected as a supernova impostor in 2010 May and was recently discovered to be a pulsating ultraluminous X-ray source. In this Letter, we present Very Large Telescope/X-shooter spectra of this source obtained in 2018 October, covering the wavelength range 350-2300 nm. The J- and H-bands clearly show the presence of a red supergiant (RSG) donor star that is best matched by a MARCS stellar atmosphere with T eff = 3650-3900 K and log(L bol/L ) = 4.25 ± 0.10, which yields a stellar radius R = 310 ± 70R . To fit the full spectrum, two additional components are required: a blue excess that can be fitted either by a hot blackbody (T ≳ 20,000 K) or a power law (spectral index α ≈ 4) and is likely due to X-ray emission reprocessed in the outer accretion disk or the donor star; and a red excess that is well fitted by a blackbody with a temperature of ̃1100 K, and is likely due to warm dust in the vicinity of SN2010da. The presence of an RSG in this system implies an orbital period of at least 0.8-2.1 yr, assuming Roche-lobe overflow. Given the large donor-to-compact object mass ratio, orbital modulations of the radial velocity of the RSG are likely undetectable. However, the radial velocity amplitude of the neutron star is large enough (up to 40-60 km s-1) to potentially be measured in the future, unless the system is viewed at a very unfavorable inclination.

Keywords: High mass X-ray binary stars;Late-type supergiant stars;Neutron stars;Astrophysics – High Energy Astrophysical Phenomena;Astrophysics – Astrophysics of Galaxies;Astrophysics – Solar and Stellar Astrophysics


Evolved massive stars at low-metallicity. I. A source catalog for the Small Magellanic Cloud

Yang, Ming; Bonanos, Alceste Z.; Jiang, Bi-Wei; Gao, Jian; Gavras, Panagiotis; Maravelias, Grigoris; Ren, Yi; Wang, Shu; Xue, Meng-Yao; Tramper, Frank; Spetsieri, Zoi T.; Pouliasis, Ektoras

Astronomy & Astrophysics, Volume 629, A91 (2019)

ADS – Journal – arXiv

Abstract

We present a clean, magnitude-limited (IRAC1 or WISE1 ≤ 15.0 mag) multiwavelength source catalog for the Small Magellanic Cloud (SMC) with 45 466 targets in total, with the purpose of building an anchor for future studies, especially for the massive star populations at low-metallicity. The catalog contains data in 50 different bands including 21 optical and 29 infrared bands, retrieved from SEIP, VMC, IRSF, AKARI, HERITAGE, Gaia, SkyMapper, NSC, Massey (2002, ApJS, 141, 81), and GALEX, ranging from the ultraviolet to the far-infrared. Additionally, radial velocities and spectral classifications were collected from the literature, and infrared and optical variability statistics were retrieved from WISE, SAGE-Var, VMC, IRSF, Gaia, NSC, and OGLE. The catalog was essentially built upon a 1″ crossmatching and a 3″ deblending between the Spitzer Enhanced Imaging Products (SEIP) source list and Gaia Data Release 2 (DR2) photometric data. Further constraints on the proper motions and parallaxes from Gaia DR2 allowed us to remove the foreground contamination. We estimate that about 99.5% of the targets in our catalog are most likely genuine members of the SMC. Using the evolutionary tracks and synthetic photometry from MESA Isochrones & Stellar Tracks and the theoretical J – KS color cuts, we identified 1405 red supergiant (RSG), 217 yellow supergiant, and 1369 blue supergiant candidates in the SMC in five different color-magnitude diagrams (CMDs), where attention should also be paid to the incompleteness of our sample. We ranked the candidates based on the intersection of different CMDs. A comparison between the models and observational data shows that the lower limit of initial mass for the RSG population may be as low as 7 or even 6 M and that the RSG is well separated from the asymptotic giant branch (AGB) population even at faint magnitude, making RSGs a unique population connecting the evolved massive and intermediate stars, since stars with initial mass around 6 to 8 M are thought to go through a second dredge-up to become AGB stars. We encourage the interested reader to further exploit the potential of our catalog.

Keywords: infrared: stars; Magellanic Clouds; stars: late-type;stars: massive; stars: mass-loss; stars: variables: general; Astrophysics – Solar and Stellar Astrophysics; Astrophysics – Astrophysics of Galaxies


Clues on the Origin and Evolution of Massive Contact Binaries: Atmosphere Analysis of VFTS 352

Abdul-Masih, Michael; Sana, Hugues; Sundqvist, Jon; Mahy, Laurent; Menon, Athira; Almeida, Leonardo A.; De Koter, Alex; de Mink, Selma E.; Justham, Stephen; Langer, Norbert; Puls, Joachim; Shenar, Tomer; Tramper, Frank

The Astrophysical Journal, Volume 880, Issue 2, 115 (2019)

ADS – Journal – arXiv

Abstract

The massive O4.5 V + O5.5 V binary VFTS 352 in the Tarantula Nebula is one of the shortest-period and most massive overcontact binaries known. Recent theoretical studies indicate that some of these systems could ultimately lead to the formation of gravitational waves via black hole binary mergers through the chemically homogeneous evolution pathway. By analyzing ultraviolet-optical phase-resolved spectroscopic data, we aim to constrain atmospheric and wind properties that could be later used to confront theoretical predictions from binary evolution. In particular, surface abundances are powerful diagnostics of the evolutionary status, mass transfer, and internal mixing processes. From a set of 32 Very Large Telescope/FLAMES visual and eight Hubble Space Telescope/Cosmic Origins Spectrograph ultraviolet spectra, we used spectral disentangling to separate the primary and secondary components. Using a genetic algorithm wrapped around the NLTE model atmosphere and the spectral synthesis code FASTWIND, we perform an 11-parameter optimization to derive the atmospheric and wind parameters of both components, including the surface abundances of He, C, N, O, and Si. We find that both components are hotter than expected compared to single-star evolutionary models, indicating that additional mixing processes may be at play. However, the derived chemical abundances do not show significant indications of mixing when adopting baseline values typical of the system environment.

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


Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically homogeneously evolving stars

Kubátová, B.; Szécsi, D.; Sander, A. A. C.; Kubát, J.; Tramper, F.; Krtička, J.; Kehrig, C.; Hamann, W. -R.; Hainich, R.; Shenar, T.

Astronomy & Astrophysics, Volume 623, A8 (2019)

ADS – Journal – arXiv

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.

Keywords: stars: massive; stars: winds; outflows; stars: rotation; galaxies: dwarf; radiative transfer; Astrophysics – Solar and Stellar Astrophysics