The VLT-FLAMES survey of massive stars: wind properties and evolution of hot massive stars in the Large Magellanic Cloud

Abstract

We have studied the optical spectra of a sample of 28 O-and early B-type stars in the Large Magellanic Cloud, 22 of which are associated with the young star forming region N11. Our observations sample the central associations of LH9 and LH10, and the surrounding regions. Stellar parameters are determined using an automated fitting method (Mokiem et al. 2005), which combines the stellar atmosphere code fastwind ) with the genetic algorithm based optimisation routine pikaia (Charbonneau 1995). We derive an age of 7.0 ± 1.0 and 3.0 ± 1.0 Myr for LH9 and LH10, respectively. The age difference and relative distance of the associations are consistent with a sequential star formation scenario in which stellar activity in LH9 triggered the formation of LH10. Our sample contains four stars of spectral type O2. From helium and hydrogen line fitting we find the hottest three of these stars to be ∼49−54 kK (compared to ∼45−46 kK for O3 stars). Detailed determination of the helium mass fraction reveals that the masses of helium enriched dwarfs and giants derived in our spectroscopic analysis are systematically lower than those implied by non-rotating evolutionary tracks. We interpret this as evidence for efficient rotationally enhanced mixing leading to the surfacing of primary helium and to an increase of the stellar luminosity. This result is consistent with findings for SMC stars by Mokiem et al. (2006). For bright giants and supergiants no such mass discrepancy is found; these stars therefore appear to follow tracks of modestly or non-rotating objects. The set of programme stars was sufficiently large to establish the mass loss rates of OB stars in this Z ∼ 1/2 Z environment sufficiently accurate to allow for a quantitative comparison with similar objects in the Galaxy and the SMC. The mass loss properties are found to be intermediate to massive stars in the Galaxy and SMC. Comparing the derived modified wind momenta D mom as a function of luminosity with predictions for LMC metallicities by Vink et al. (2001) yields good agreement in the entire luminosity range that was investigated, i.e. 5.0 < log L/L < 6.1.

Surface abundances of ON stars

“…The relatively large helium enrichment of most ON stars indicates a peculiar chemical history compared to normal OB stars, which do not systematically show values of He/H higher than 0.1 (e.g. Mokiem et al 2007). Figure 3 shows the surface gravity as a function of effective temperature in our sample.…”

C+emission from the Magellanic Clouds

“…These illuminate a variety of discrete H II and CO clouds (Israel et al 1993;Mac Low et al 1998). A striking feature of N 11 is the super-bubble (Meaburn et al 1989) formed by the action of stellar winds from the OB association LH 9 (NGC 1760; age 7 Myr, Mokiem et al 2007) at its center. The bubble is a prominent source of soft X-rays (Mac Low et al 1998;Nazé et al 2004;Maddox et al 2009) implying significant interaction between the stars and the surrounding gas.…”

“…The bubble is a prominent source of soft X-rays (Mac Low et al 1998;Nazé et al 2004;Maddox et al 2009) implying significant interaction between the stars and the surrounding gas. At the northern edge of the bubble is the much younger OB association LH 10 (NGC 1763; 3 Myr, Mokiem et al 2007) which is still embedded in the bright nebula N 11B (cf. Heydari-Malayeri & Testor 1983).…”

“…An even younger compact object, N 11A (IC 2116, Mac Low et al 1998Nazé et al 2001;Heydari-Malayeri et al 2001), occurs in the same area. The location and apparent age difference of LH 10 and LH 9, as well as the gas kinematics, have led to the suggestion that the formation of LH 10 and other stellar groups was triggered by the expanding shells emanating from LH 9 Rosado et al 1996;Barba et al 2003;Hatano et al 2006;Mokiem et al 2007). …”

A peculiar Of star in the Local Group galaxy IC 1613

“…Very many O-stars have been analysed in the Magellanic Clouds (see, among others, Bouret et al 2003;Hillier et al 2003;Mokiem et al 2006;Mokiem et al 2007a;Massey et al 2004 or Massey et al 2005). However, going to lower metallicities implies a large distance jump.…”