WINDS OF LOW-METALLICITY OB-TYPE STARS:<i>HST</i>-COS SPECTROSCOPY IN IC 1613

García, M.; Herrero, A.; Najarro, F.; Lennon, D. J.; Urbaneja, M. A.

doi:10.1088/0004-637x/788/1/64

Cited by 66 publications

(84 citation statements)

References 95 publications

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“…In units of the Newtonian gravity, this correction factor is (1 − Γ e ), where Γ e is the Eddington factor for Thomson scattering. This treatment of terminal velocity ignores the large scatter that exists around the ∞ versus esc relation (see discussions in Kudritzki & Puls 2000;Garcia et al 2014 4541,4686,Hδ,Hγ,Hβ,and Hα. For a subset of stars (those with the earliest spectral subtypes, and mid-and late-O supergiants), our set of H and He diagnostic lines was not sufficient to accurately constrain their parameters.…”

Section: Atmosphere Fittingmentioning

confidence: 99%

The VLT-FLAMES Tarantula Survey

Grin

Ramírez-Agudelo

Koter

et al. 2017

A&A

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Context. The Tarantula region in the Large Magellanic Cloud contains the richest population of spatially resolved massive O-type stars known so far. This unmatched sample offers an opportunity to test models describing their main-sequence evolution and massloss properties. Aims. Using ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to determine stellar, photospheric and wind properties of 72 presumably single O-type giants, bright giants and supergiants and to confront them with predictions of stellar evolution and of line-driven mass-loss theories. Methods. We apply an automated method for quantitative spectroscopic analysis of O stars combining the non-LTE stellar atmosphere model fastwind with the genetic fitting algorithm pikaia to determine the following stellar properties: effective temperature, surface gravity, mass-loss rate, helium abundance, and projected rotational velocity. The latter has been constrained without taking into account the contribution from macro-turbulent motions to the line broadening.Results. We present empirical effective temperature versus spectral subtype calibrations at LMC-metallicity for giants and supergiants. The calibration for giants shows a +1kK offset compared to similar Galactic calibrations; a shift of the same magnitude has been reported for dwarfs. The supergiant calibrations, though only based on a handful of stars, do not seem to indicate such an offset. The presence of a strong upturn at spectral type O3 and earlier can also not be confirmed by our data. In the spectroscopic and classical Hertzsprung-Russell diagrams, our sample O stars are found to occupy the region predicted to be the core hydrogen-burning phase by Brott et al. (2011) andKöhler et al. (2015). For stars initially more massive than approximately 60 M ⊙ , the giant phase already appears relatively early on in the evolution; the supergiant phase develops later. Bright giants, however, are not systematically positioned between giants and supergiants at M init ∼ > 25 M ⊙ . At masses below 60 M ⊙ , the dwarf phase clearly precedes the giant and supergiant phases; however this behavior seems to break down at M init ∼ < 18 M ⊙ . Here, stars classified as late O III and II stars occupy the region where O9.5-9.7 V stars are expected, but where few such late O V stars are actually seen. Though we can not exclude that these stars represent a physically distinct group, this behaviour may reflect an intricacy in the luminosity classification at late O spectral subtype. Indeed, on the basis of a secondary classification criterion, the relative strength of Si iv to He i absorption lines, these stars would have been assigned a luminosity class IV or V. Except for five stars, the helium abundance of our sample stars is in agreement with the initial LMC composition. This outcome is independent of their projected spin rates. The aforementioned five stars present moderate projected rotational velocities (i.e., e sin i < 200 km s −1 ) and hence do no...

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Section: Atmosphere Fittingmentioning

confidence: 99%

The VLT-FLAMES Tarantula Survey

Grin

Ramírez-Agudelo

Koter

et al. 2017

A&A

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“…Moreover, the independent works of Garcia et al (2014) and Hosek et al (2014) have found evidence that the Fe abundance in IC 1613 and NGC 3109 may be higher than the 1/7Fe value inferred from oxygen. Since Fe is the main driver of the massloss, this mismatch could explain the strong-wind problem.…”

Section: Introductionmentioning

confidence: 97%

“…Studies like those performed by Kudritzki (2002), Evans & Howarth (2003), Mokiem et al (2004Mokiem et al ( , 2006, Massey et al (2005), Trundle et al (2007), Tramper et al (2011Tramper et al ( , 2014, Garcia & Herrero (2013;hereafter GH13), and Garcia et al (2014) analyzed the dependence of effective temperature, wind properties, or ionizing flux on metallicity.…”

Section: Introductionmentioning

confidence: 99%

“…Tramper et al (2011Tramper et al ( , 2014 analyzed a more extended sample of ten stars in IC 1613, WLM, and NGC 3109 and again find wind momenta that are greater than predicted by theory. These works are based solely on optical spectroscopy and suffer from uncertain terminal velocities estimated from the escape velocity, affecting the WLR (Herrero et al 2012;Garcia et al 2014).…”

Section: Introductionmentioning

confidence: 99%

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OB stars at the lowest Local Group metallicity

Camacho

García

Herrero

2015

A&A

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Context. Massive stars play an important role in the chemical and dynamical evolution of the Universe. The first metal-poor stars may have started the reionization of the Universe. To understand these early epochs it is necessary to know the behavior and the physical properties of massive stars in very metal-poor environments. We focus on the massive stellar content of the metal-poor irregular galaxy Sextans A. Aims. Our aim is to find and classify OB stars in Sextans A, so as to later determine accurate stellar parameters of these blue massive stars in this low-metallicity region (Z ∼ 0.1 Z ). Methods. Using UBV photometry, the reddening-free index Q and GALEX imaging, we built a list of blue massive star candidates in Sextans A. We obtained low-resolution (R ∼ 1000) GTC-OSIRIS spectra for a fraction of them and carried out spectral classification. For the confirmed O-stars, we derived preliminary stellar parameters. Results. The target selection criteria and observations were successful and have produced the first spectroscopic atlas of OB-type stars in Sextans A. From the whole sample of 18 observed stars, 12 were classified as early OB-types, including 5 O-stars. The radial velocities of all target stars are in agreement with their Sextans A membership, although three of them show significant deviations. We determined the stellar parameters of the O-type stars using the stellar atmosphere code FASTWIND and revisited the sub-SMC temperature scale. Two of the O-stars are consistent with relatively strong winds and enhanced helium abundances, although results are not conclusive. We discuss the position of the OB stars in the HRD. Initial stellar masses run from slightly below 20 up to 40 solar masses. Conclusions. The target selection method worked well for Sextans A. The stellar temperatures are consistent with findings in other galaxies. Some of the targets deserve follow-up spectroscopy because of indications of a runaway nature, an enhanced helium abundance, or a relatively strong wind. We observe a correlation between HI and OB associations similar to the irregular galaxy IC 1613, confirming the previous result that the most recent star formation of Sextans A is currently ongoing near the rim of the H i cavity.

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“…The bi-stability jump (Pauldrach & Puls 1990) is observed as a drop in the ratio between the terminal and escape velocities ( ∞ / esc ) of B-type supergiants (Bsgs) by a factor 2, when the effective temperature (T eff ) falls below ∼21 000 K (Lamers et al 1995;Crowther et al 2006;Markova & Puls 2008;Garcia et al 2014). Vink et al (1999) showed that this was the result of the recombination of the dominant wind driver from Fe iv to Fe iii.…”

Section: Introductionmentioning

confidence: 99%

Two bi-stability jumps in theoretical wind models for massive stars and the implications for luminous blue variable supernovae

Petrov¹,

Gräfener²

2016

Mon. Not. R. Astron. Soc.

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Luminous Blue Variables have been suggested to be the direct progenitors of supernova types IIb and IIn, with enhanced mass loss prior to explosion. However, the mechanism of this mass loss is not yet known. Here, we investigate the qualitative behaviour of theoretical stellar wind mass-loss as a function of T eff across two bi-stability jumps in blue supergiant regime and also in proximity to the Eddington limit, relevant for LBVs. To investigate the physical ingredients that play a role in the radiative acceleration we calculate blue supergiant wind models with the cmfgen non-LTE model atmosphere code over an effective temperature range between 30 000 and 8 800 K. Although our aim is not to provide new mass-loss rates for BA supergiants, we study and confirm the existence of two bi-stability jumps in mass-loss rates predicted by Vink, de Koter, & Lamers (1999). However, they are found to occur at somewhat lower T eff (20 000 and 9 000 K, respectively) than found previously, which would imply that stars may evolve towards lower T eff before strong mass-loss is induced by the bistability jumps. When the combined effects of the second bi-stability jump and the proximity to Eddington limit are accounted for, we find a dramatic increase in the mass-loss rate by up to a factor of 30. Further investigation of both bi-stability jumps is expected to lead to a better understanding of discrepancies between empirical modelling and theoretical mass-loss rates reported in the literature, and to provide key inputs for the evolution of both normal AB supergiants and LBVs, as well as their subsequent supernova type II explosions.

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WINDS OF LOW-METALLICITY OB-TYPE STARS:HST-COS SPECTROSCOPY IN IC 1613

Cited by 66 publications

References 95 publications

The VLT-FLAMES Tarantula Survey

The VLT-FLAMES Tarantula Survey

OB stars at the lowest Local Group metallicity

Two bi-stability jumps in theoretical wind models for massive stars and the implications for luminous blue variable supernovae

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