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


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

Abstract

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

Abstract

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


An excess of massive stars in the local 30 Doradus starburst

Schneider, F. R. N.; Sana, H.; Evans, C. J.; Bestenlehner, J. M.; Castro, N.; Fossati, L.; Gräfener, G.; Langer, N.; Ramírez-Agudelo, O. H.; Sabín-Sanjulián, C.; Simón-Díaz, S.; Tramper, F.; Crowther, P. A.; de Koter, A.; de Mink, S. E.; Dufton, P. L.; Garcia, M.; Gieles, M.; Hénault-Brunet, V.; Herrero, A. Izzard, R. G.; Kalari, V.; Lennon, D. J.; Maíz Apellániz, J.; Markova, N.; Najarro, F.; Podsiadlowski, Ph.; Puls, J.; Taylor, W. D.; van Loon, J. Th.; Vink, J. S.; Norman, C.

Science, Volume 359, Issue 6371, 69 (2018)

ADS – Journal – arXiv

Abstract

The 30 Doradus star-forming region in the Large Magellanic Cloud is a nearby analog of large star-formation events in the distant universe. We determined the recent formation history and the initial mass function (IMF) of massive stars in 30 Doradus on the basis of spectroscopic observations of 247 stars more massive than 15 solar masses (M☉). The main episode of massive star formation began about 8 million years (My) ago, and the star-formation rate seems to have declined in the last 1 My. The IMF is densely sampled up to 200 M☉ and contains 32 ± 12% more stars above 30 M☉ than predicted by a standard Salpeter IMF. In the mass range of 15 to 200 M☉, the IMF power-law exponent is 1.90-0.26+0.37, shallower than the Salpeter value of 2.35.

Keywords: ASTRONOMY; Astrophysics – Solar and Stellar Astrophysics; Astrophysics – Astrophysics of Galaxies


Massive pre-main-sequence stars in M17

Ramírez-Tannus, M. C.; Kaper, L.; de Koter, A.; Tramper, F.; Bik, A.; Ellerbroek, L. E.; Ochsendorf, B. B.; Ramírez-Agudelo, O. H.; Sana, H.

Astronomy & Astrophysics, Volume 604, A78 (2017)

ADS – Journal – arXiv

Abstract

The formation process of massive stars is still poorly understood. Massive young stellar objects (mYSOs) are deeply embedded in their parental clouds; these objects are rare, and thus typically distant, and their reddened spectra usually preclude the determination of their photospheric parameters. M17 is one of the best-studied H II regions in the sky, is relatively nearby, and hosts a young stellar population. We have obtained optical to near-infrared spectra of previously identified candidate mYSOs and a few OB stars in this region with X-shooter on the ESO Very Large Telescope. The large wavelength coverage enables a detailed spectroscopic analysis of the photospheres and circumstellar disks of these candidate mYSOs. We confirm the pre-main-sequence (PMS) nature of six of the stars and characterise the O stars. The PMS stars have radii that are consistent with being contracting towards the main sequence and are surrounded by a remnant accretion disk. The observed infrared excess and the double-peaked emission lines provide an opportunity to measure structured velocity profiles in the disks. We compare the observed properties of this unique sample of young massive stars with evolutionary tracks of massive protostars and propose that these mYSOs near the western edge of the H II region are on their way to become main-sequence stars ( 6-20 M) after having undergone high mass accretion rates (Ṁacc 10-4-10-3Myr-1). Their spin distribution upon arrival at the zero age main-sequence is consistent with that observed for young B stars, assuming conservation of angular momentum and homologous contraction.

Keywords: stars: pre-main sequence; stars: massive; stars: early-type; HII regions; stars: variables: T Tauri; Herbig Ae/Be; accretion; accretion disks; Astrophysics – Solar and Stellar Astrophysics; Astrophysics – Astrophysics of Galaxies