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


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


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


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

The VLT-FLAMES Tarantula Survey. XXIX. Massive star formation in the local 30 Doradus starburst

Schneider, F. R. N.; Ramírez-Agudelo, O. H.; Tramper, F.; Bestenlehner, J. M.; Castro, N.; Sana, H.; Evans, C. J.; Sabín-Sanjulián, C.; Simón-Díaz, S.; Langer, N.; Fossati, L.; Gräfener, G.; Crowther, P. A.; de Mink, S. E.; de Koter, A.; Gieles, M.; Herrero, A.; Izzard, R. G.; Kalari, V.; Klessen, R. S. Lennon, D. J.; Mahy, L.; Maíz Apellániz, J.; Markova, N.; van Loon, J. Th.; Vink, J. S.; Walborn, N. R.

Astronomy & Astrophysics, Volume 618, A73 (2018)

ADS – Journal – arXiv


The 30 Doradus (30 Dor) nebula in the Large Magellanic Cloud (LMC) is the brightest HII region in the Local Group and a prototype starburst similar to those found in high redshift galaxies. It is thus a stepping stone to understand the complex formation processes of stars in starburst regions across the Universe. Here, we have studied the formation history of massive stars in 30 Dor using masses and ages derived for 452 mainly OB stars from the spectroscopic VLT-FLAMES Tarantula Survey (VFTS). We find that stars of all ages and masses are scattered throughout 30 Dor. This is remarkable because it implies that massive stars either moved large distances or formed independently over the whole field of view in relative isolation. We find that both channels contribute to the 30 Dor massive star population. Massive star formation rapidly accelerated about 8 Myr ago, first forming stars in the field before giving birth to the stellar populations in NGC 2060 and NGC 2070. The R136 star cluster in NGC 2070 formed last and, since then, about 1 Myr ago, star formation seems to be diminished with some continuing in the surroundings of R136. Massive stars within a projected distance of 8 pc of R136 are not coeval but show an age range of up to 6 Myr. Our mass distributions are well populated up to 200 M. The inferred IMF is shallower than a Salpeter-like IMF and appears to be the same across 30 Dor. By comparing our sample of stars to stellar models in the Hertzsprung-Russell diagram, we find evidence for missing physics in the models above log L/L = 6 that is likely connected to enhanced wind mass loss for stars approaching the Eddington limit. Our work highlights the key information about the formation, evolution and final fates of massive stars encapsulated in the stellar content of 30 Dor, and sets a new benchmark for theories of massive star formation in giant molecular clouds.

Keywords: stars: formation; stars: massive; stars: luminosity function; mass function; Magellanic Clouds; galaxies: star clusters: individual: 30 Doradus; Astrophysics – Solar and Stellar Astrophysics

A tale of two periods: determination of the orbital ephemeris of the super-Eddington pulsar NGC 7793 P13

Fürst, F.; Walton, D. J.; Heida, M.; Harrison, F. A.; Barret, D.; Brightman, M.; Fabian, A. C.; Middleton, M. J.; Pinto, C.; Rana, V.; Tramper, F.; Webb, N.; Kretschmar, P.

Astronomy & Astrophysics, Volume 616, A186 (2018)

ADS – Journal – arXiv


We present a timing analysis of multiple XMM-Newton and NuSTAR observations of the ultra-luminous pulsar NGC 7793 P13 spread over its 65 d variability period. We use the measured pulse periods to determine the orbital ephemeris, confirm a long orbital period with Porb = 63.9+0.5-0.6 d, and find an eccentricity of e ≤ 0.15. The orbital signature is imprinted on top of a secular spin-up, which seems to get faster as the source becomes brighter. We also analyze data from dense monitoring of the source with Swift and find an optical photometric period of 63.9 ± 0.5 d and an X-ray flux period of 66.8 ± 0.4 d. The optical period is consistent with the orbital period, while the X-ray flux period is significantly longer. We discuss possible reasons for this discrepancy, which could be due to a super-orbital period caused by a precessing accretion disk or an orbital resonance. We put the orbital period of P13 into context with the orbital periods implied for two other ultra-luminous pulsars, M82 X-2 and NGC 5907 ULX, and discuss possible implications for the system parameters.

Keywords: stars: neutron; X-rays: binaries; techniques: radial velocities; accretion; accretion disks; pulsars: individual: NGC 7793 P13; Astrophysics – High Energy Astrophysical Phenomena