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

Abstract

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 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 and chemical abundances, and in particular iron abundances, obtained with the two NLTE codes compare quite favorably. This

UV Spectroscopy of Metal-Poor Massive Stars in the Small Magellanic Cloud

“…Many of these stars have been analysed to derive mass-loss rates confirming the expected theoretical trend of mass-loss with metallicity ( [1], [5], [10], [11], [4]) while theoretical expectations for terminal velocities have also been confirmed [3]. However, a small number of stars were found to have mass-loss rates which are an order of magnitude or more less than theoretical predictions [1], see Fig.3.…”

“…However, a small number of stars were found to have mass-loss rates which are an order of magnitude or more less than theoretical predictions [1], see Fig.3. Additional members of this group have been found in the SMC [6], in all cases these stars are low luminosity O-type dwarfs having anomalous weak winds.…”

Bright OB stars in the Galaxy

“…During the last years, the quantitative analyses of spectra in the far-UV/UV and optical domains (e.g., Herrero et al 2002;Bianchi & Garcia 2002;Crowther et al 2002;Bouret et al 2003;Repolust et al 2004;Massey et al 2004;Heap et al 2006) have unambiguously shown that the inclusion of line-blocking and blanketing and wind effects (if present) significantly modifies the temperature scale of O-stars (for a recent calibration at solar metallicity, see Markova et al 2004;Martins et al 2005). Regarding B-SGs, particularly of later subtype, this issue has not been addressed so far, mostly due to lacking T eff estimates.…”

Star formation in the Small Magellanic Cloud: the youngest star clusters

“…NGC 346 is an extremely young (∼3 Myr; Bouret et al 2003;Sabbi et al 2007) and active star cluster that excites the largest and brightest HII region (N66) in the SMC. NGC346 (J2000: α = 00 h 59 m 05.2 s , δ = −72 • 10 28 ) contains a major fraction of the O stars known in the entire SMC (Walborn 1978;Walborn and Blades 1986;Niemela et al 1986;Massey et al 1989).…”

“…The bright (12.5 ≤ m F814W ≤ 22.0 mag) and blue (−0.3 ≤ (m F555W − m F814W ) ≤ 0.4 mag) MS, clearly visible in the upper left of the CMD, is the most remarkable feature of the youngest stellar population. The majority of these stars belong to NGC346, and have an age of ∼3 Myr (Bouret et al 2003;Sabbi et al 2007). Spectral analysis of NGC346 members revealed the presence of stars as young as ∼1 Myr, (Massey et al 2005), but stars aged ∼5 Myr are also present (Heap et al 2006;Mokiem et al 2006).…”