Quantitative Spectroscopy of O Stars at Low Metallicity: O Dwarfs in NGC 346

By J. Bouret, T. Lanz, D. John Hillier, S. Heap, I. Hubeny, D. Lennon, Linda J. Smith, Christopher H. Evans. 2003

209 citations recorded by the source corpus.

We present the results of a detailed analysis of the properties of dwarf O-type stars in a metal-poor environment. High-resolution, high-quality ultraviolet and optical spectra of six O-type stars in the H II region NGC 346 have been obtained from a spectroscopic survey of O stars in the SMC. Stellar parameters and chemical abundances have been determined using non-LTE (NLTE) line-blanketed photospheric models calculated with TLUSTY. Additionally, we have modeled the spectra with the NLTE line-blanketed wind code CMFGEN to derive wind parameters. Stellar parameters, chemical abundances, and in particular iron abundances obtained with the two NLTE codes compare quite favorably. This consistency demonstrates that basic photospheric parameters of main-sequence O stars can be reliably determined using NLTE static model atmospheres. With the two NLTE codes, we need to introduce a microturbulent velocity to match the observed spectra. Our results hint at a decrease of the required microturbulent velocity from a value close to the sonic velocity in early O stars to a low value in late O stars. As in several recent studies of Galactic, LMC, and SMC stars, we derive effective temperatures lower than predicted from the widely used relation between spectral type and Teff, resulting in lower stellar luminosities and lower ionizing fluxes. From evolutionary tracks in the H-R diagram, we find the age 3 × 106 yr for NGC 346. A majority of the stars in our sample reveal CNO cycle-processed material at their surface during the main-sequence stage, thus indicating fast stellar rotation and/or very efficient mixing processes. We obtain an overall metallicity Z = 0.2 Z☉, in good agreement with other recent analyses of SMC stars. We study the dependence of the mass-loss rate on the stellar metallicity and find a satisfactory agreement with recent theoretical predictions for the three most luminous stars of the sample. The wind momentum-luminosity relation for our sample stars derived for these stars agrees with previous studies. However, the three other stars of our sample reveal very weak signatures of mass loss. We obtain mass-loss rates that are significantly lower than 10-8 M☉ yr-1, below the predictions of radiative line-driven wind theory by an order of magnitude or more. Furthermore, evidence of clumping in the wind of main-sequence O stars is provided by O V λ1371. As in previous studies of O star winds, we are unable to reproduce this line with homogeneous-wind models, but we have achieved very good fits with clumped models. Clumped-wind models systematically yield lower mass-loss rates than theoretical predictions.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/31cead5dee33831c079b39042c21432156d3762f

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In Astronomy and Astrophysics

1. N. Smith
2. N. Langer
3. Selma de Mink
4. S. Smartt
5. J. Puls
6. J. Vink
7. P. Crowther
8. Michela Mapelli

In Instrumentation

1. N. Smith
2. Selma de Mink
3. N. Langer
4. J. Vink
5. J. Puls
6. S. Smartt
7. F. Najarro
8. P. Crowther

Citations in the corpus

• Evolution of rapidly rotating metal-poor massive stars towards gamma-ray bursts (2005, background): “C1 and C2 withZ = 0.001 indicate that the blue stars as found by Bouret et al. (2003) and Walborn et al. (2004) in the Magellanic Clouds which have the signature of homogeneous evolution might be promising progenitors of GRBs.”
• Lower mass loss rates in O-type stars: Spectral signatures of dense clumps in the wind of two Galactic O4 stars (2004, methodology): “Independently, Bouret et al. (2003) successfully matched for the first time the Oλ1371 line profile using clumped models.”
• A Grid of NLTE Line-blanketed Model Atmospheres of Early B-Type Stars (2006, methodology): “These advances have been well documented with applications of the Ostar2002 grid to the analysis of O stars in the Galaxy and the Small Magellanic Cloud (SMC; e. g., Bouret et al. 2003, Heap et al. 2006, Lanz et al. 2007).”
• The VLT-FLAMES Survey of Massive Stars: Observations in the Galactic Clusters NGC 3293, NGC 4755 and NGC 6611 ⋆ (2005, methodology): “Results from model atmosphere analyses of O- and B-type supergiants in the Magellanic Clouds (Crowther et al. 2002; Bouret et al. 2003; Hillier et al. 2003; Evans et al. 2004a; Trundle et al. 2004; Walborn et al. 2004; Trundle & Lennon 2005) find evidence of significant nitrogen enrichment, that…”
• Classical Be stars (2013, background): “In particular for the Large and Small Magellanic Clouds (LMC and SMC, respectively) this allowed to build extensive databases of the Be stars in these galaxies, and their photometric and spectroscopic behavior.”
• The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region (2006, methodology): “…by the fact that some of the O-type stars observed by Walborn et al. (2000) 636 C. J. Evans et al.: FLAMES survey of massive stars: LMC and SMC clusters were explicitly avoided in the FLAMES survey as state-of-theart analyses have already been presented in the literature (e.g. Bouret et al. 2003).”
• The VLT-FLAMES survey of massive stars: surface chemical compositions of B-type stars in the Magellanic Clouds (2006, methodology): “Their proximity, combined with their relatively low extinction has contributed to the intensity with which they have been studied in recent years (see, for example, Bouret et al. 2003; Korn et al. 2002; Garnett 1999; Westerlund 1997).”
• Single star progenitors of long gamma-ray bursts - I. Model grids and redshift dependent GRB rate (2006, background): “Our models at Z = 0.004 indicate that the WR/O ratio could reach a few percent at this metallicity for constant star formation, which is compatible with the observed WR/O ratio in SMC.”
• Mass loss from late-type WN stars and its Z-dependence: very massive stars approaching the Eddington limit (2008, background): “Note that significant clumping factors in the range of D=10–100 have recently been proposed for OB star winds (Crowther et al. 2002; Hillier et al. 2003; Bouret et al. 2003, 2005; Fullerton et al. 2006).”
• Rotating massive main-sequence stars: II. Simulating a population of LMC early B-type stars as a test of rotational mixing (2011, background): “…wind properties of massive main-sequence stars are yet to be understood, such as the winds of massive stars with luminosities below ∼105.2 L (e.g. Bouret et al. 2003; Martins et al. 2004, 2005; Mokiem et al. 2007; Puls et al. 2008), intermittent discrete absorption components in UV absorption…”
• The Physical Properties and Effective Temperature Scale of O-Type Stars as a Function of Metallicity. II. Analysis of 20 More Magellanic Cloud Stars and Results from the Complete Sample (2005, methodology): “While this doesn’t prove our scale is right, and the Bouret et al. (2003) values wrong, it does emphasize an often overlooked point, namely that changes in the effective 7The version of the data shown by Walborn et al. (2000) lacked nebular subtraction, and made no correction for the moonlight…”
• The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars (2007, methodology): “…was provided by Eversberg et al. (1998) from time series spectroscopy of He ii λ4686 in ζ Pup, and by Crowther et al. (2002); Hillier et al. (2003); Bouret et al. (2003); Massa et al. (2003); Martins et al. (2004, 2005); Bouret et al. (2005), and Fullerton et al. (2006) from analysis of UV…”
• The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars (2009, background): “…anomalies (particularly nitrogen) have long been observed, by, for example, Walborn (1970), Dufton (1972) and, more recently, Gies & Lambert (1992), Kilian (1991), Bouret et al. (2003), Dufton et al. (2005), Lennon et al. (2003), Trundle & Lennon (2005), Korn et al. (2002) and Venn (1999).”
• A new calibration of stellar parameters of Galactic O stars (2005, methodology): “However, other recent analysis show that consistent results with good fits can be obtained for photospheric UV and optical lines (Bouret et al. 2003, Hillier et al. 2003, Martins et al. 2005).”
• O stars with weak winds: the Galactic case (2005, methodology): “Several tests (Martins et al. 2002a, Bouret et al. 2003) indicate that a reasonable change of this parameter has little effect on the emergent spectrum, except for some specific lines (see Sect.”
• ON THE SOURCE OF THE DUST EXTINCTION IN TYPE Ia SUPERNOVAE AND THE DISCOVERY OF ANOMALOUSLY STRONG Na i ABSORPTION (2013, background): “…nearby galaxies which have significantly lower metallicities, by factors of two and five, respectively, compared to the Milky Way (Dufour 1984; Rolleston et al. 2002; Peimbert The Astrophysical Journal, 779:38 (21pp), 2013 December 10 Phillips et al. & Torres-Peimbert 1976; Bouret et al. 2003).”