Fundamental Properties of O‐Type Stars

Abstract

We present a comprehensive analysis of high-resolution, far-ultraviolet HST STIS, FUSE, and optical spectra of 18 O stars in the Small Magellanic Cloud. Our analysis is based on the OSTAR2002 grid of NLTE metal-line-blanketed model atmospheres calculated with our code TLUSTY. We systematically explore and present the sensitivity of various UV and optical lines to different stellar parameters. We have obtained consistent fits of the UV and the optical spectrum to derive the effective temperature, surface gravity, surface composition, and microturbulent velocity of each star. Stellar radii, masses, and luminosities follow directly. For stars of the same spectral subtype, we find a general good agreement between effective temperature determinations obtained with TLUSTY, CMFGEN, and FASTWIND models, which are all lower than the standard T eff calibration of O stars. We propose a new calibration between the spectral type and effective temperature based on our results from UV metal lines, as well as optical hydrogen and helium lines. The lower effective temperatures translate into ionizing luminosities that are smaller by a factor of 3 compared to luminosities inferred from previous standard calibrations. The chemical composition analysis reveals that the surface of about 80% of the program stars is moderately to strongly enriched in nitrogen, while showing the original helium, carbon, and oxygen abundances. Our results support the new stellar evolution models that predict that the surface of fast rotating stars becomes nitrogen-rich during the main-sequence phase because of rotationally induced mixing. Enrichment factors are, however, larger than predicted by stellar evolution models. Most stars exhibit the ''mass discrepancy'' problem, which we interpret as a result of fast rotation that lowers the measured effective gravity. Nitrogen enrichment and low spectroscopic masses are therefore two manifestations of fast rotation. Our study thus emphasizes the importance of rotation in our understanding of the properties of massive stars and provides a framework for investigating populations of low-metallicity massive stars at low and high redshifts.

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

“…The bright end of its stellar population (V ≤ 19.5 mag) has been investigated extensively in the past 20 years. Spectral investigations of the brightest members identified several stars of spectral type O6.5 or earlier (Massey et al 1989;Heap et al 2006;Mokiem et al 2006;Evans et al 2006). Massey et al (1989) found the mass function (MF) of the brightest stars in NGC346-down to 5 M -to have a slope that is consistent with what has been found for massive stars near the Sun and in the Large Magellanic Cloud (LMC).…”

“…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). …”

A quantitative study of O stars in NGC 2244 and the Monoceros OB2 association

“…The first analysis of surface CNO abundances of O stars and B supergiants tends to confirm the theoretical predictions qualitatively: more evolved stars have a higher N content as a result of CNO processing and mixing (Hillier et al 2003;Heap et al 2006;Crowther et al 2006); however, the role of rotation in explaining the chemical patterns of B stars in the Magellanic Clouds has been questioned by the results of the first ESO/VLT Large Programme dedicated to massive stars (Evans et al 2005(Evans et al , 2006. Hunter et al (2008) showed that 60% of their sample of B main-sequence stars in the Large Magellanic Cloud displayed the nitrogen surface abundances expected by evolutionary models including rotational mixing, but the remaining 40% escaped such an agreement.…”

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.…”