Results derived from observations

Table 31.1. Quantitative spectroscopy of OBA stars and their winds in the Galaxy and the MCs, by means of spherically extended model atmospheres

Table 31.1. Quantitative spectroscopy of OBA stars and their winds in the Galaxy and the MCs, by means of spherically extended model atmospheres

Diagnostic

Reference

Atmospheric model

Sample

Ha

Lamers & Leitherer ( 1993) Puis etal, (1996) Kudritzki et al, (1999) Markova et al, (2004)

Approximate Approximate NLTE/unblanketed Approximate

Galactic O stars Galactic/LMC/SMC O stars Galactic BA-supergiants Galactic O stars

UV

Bianchi & Garcia (2002) Garcia & Bianchi (2004) Martins et al, (2004)

NLTE/WM-basic NLTE/WM-basic NLTE/CMFGEN

Galactic O stars Galactic O stars SMC O-dwarfs

UV + optical

Crowther etal, (2002) Hillier et al, (2003) Bouret et al, (2003) Martins et al, (2005) Bouret et al, (2005)

NLTE/CMFGEN NLTE/CMFGEN NLTE/CMFGEN NLTE/CMFGEN NLTE/CMFGEN

LMC/SMC O-supergiants SMC O-supergiants SMC O-dwarfs Galactic O-dwarfs Galctic O stars

Optical

Herrero et al, (2002) Repolust et al, (2004) Trundle et al, (2004) Trundle & Lennon (2005) Massey et al, (2004/2005) Mokiem et al, (2005) Crowther et al, (2006) Mokiem et al, (2006a/b)

NLTE/FASTWIND

NLTE/FASTWIND

NLTE/FASTWIND

NLTE/FASTWIND

NLTE/FASTWIND

NLTE/FASTWIND

NLTE/CMFGEN

NLTE/FASTWIND

Cyg-OB2 OB stars Galactic O stars SMC B-supergiants SMC B-supergiants LMC/SMC O stars Galactic O stars Galactic B-supergiants SMC/LMC OB stars

CMFGEN (Hillier & Miller 1998), WM-basic (Pauldrach et al, 2001), FW (Puis et al, 2005).

CMFGEN (Hillier & Miller 1998), WM-basic (Pauldrach et al, 2001), FW (Puis et al, 2005).

volume). Most of them were performed using line-blanketed NLTE atmosphere codes.

Taken together, the most important results of these investigations can be summarised as follows. Compared with results from previous investigations, the Teff scale has become lower, due to line-blanketing effects. The mass-loss rates in the SMC (and in the LMC, see below) are systematically smaller than in the Galaxy, though the scatter is large. The observed WLRs meet the theoretical predictions, except for (i) O-supergiants with rather dense winds (which might be explained by wind-clumping, see Section 4), (ii) low-luminosity O-dwarfs with observed wind momenta much lower than predicted (the reason for this is unknown) and (iii) a large fraction (but not all) of B2/3 supergiants, for which again the observed wind momenta are 'too low" (this remains unexplained as well).

Wind-momentum rates of O stars from the FLAMES survey of massive stars

Using the FLAMES multi-object spectrograph attached to the VLT, a large collaboration conducted a programme to investigate the stellar contents of clusters of various ages in the Galaxy and the Magellanic Clouds. For the introductory publication, see Evans et al. (2005). With respect to massive stars, roughly 60 O/early-B stars from the SMC/LMC have been analysed in a homogeneous and objective way. This has been achieved by means of an 'automatic' analysis method combining a genetic algorithm (PIKAIA) (Charbonneau 1995) used to obtain the optimum fit and FASTWIND (see Table 31.1) to calculate the synthetic spectra. The method itself has been presented (and tested) by Mokiem et al. (2005), and the analysis of the SMC/LMC data is described in Mokiem et al. (2006, 2007a).

By combining these data with data from previous investigations, the 'observed' metallicity dependences of M and Dmom could be derived with unprecedented precision,

M a Z0-62±°'15, v^a Z°'13, where the scaling-law for v^ had been obtained much earlier, by Leitherer et al. (1992). Both results are in very good agreement with the theory (cf. Section 2 and Figure 31.3).

Beyond the Local Group

The availability of 8-m-class telescopes allows us to extend our investigations to objects in more distant galaxies, not only in the Local Group but also beyond, when concentrating on the visually brightest stars in the sky, the A-supergiants. Examples of recent results are given in Figure 31.4, which shows that the slope

o qE 28

ro o

Figure 31.3. Observed WLRs for the Galaxy and theMCs, as derived by Mokiem et al. (2007b); see also de Koter (2007). The grey shaded areas denote the 1a confidence interval, and the dashed lines represent the theoretical predictions from Vink etal. (2000,2001). For comparison, we have overplotted the wind-momentum rates from two Of+ stars in the Arches cluster, both of them with reff« 30 kK, as analysed by Najarro et al. (2004). Original figure from Mokiem et al. (2007b).

_l , , , , !

/

- o Galaxy

NGC 300 ^ 1

• M31

, £ NGC 3621

P'

Oy /

/

1 , , , , 1

Figure 31.4. The WLR for Galactic and extragalactic A-supergiants. Data from Kudritzki et al. (1999) for the Galaxy, McCarthy et al. (1997) for M31, Bresolin et al. (2001) for M31 and NGC3621 and Bresolin et al. (2002) for NGC 300). Regarding the M31 objects, see also Chapter 35 in this volume. Figure from Bresolin et al. (2002).

of the corresponding WLR is consistent with the theoretical prediction in this parameter range (a' ~ 0.4). So far, no object providing clear evidence for supersolar metallicity has been found (see Chapter 6 of this volume). Further work has been presented by Bianchi et al. (1996) (UV analyses of M31/M33 OB-supergiants), Smartt et al. (2001) (UV/optical analyses of M31 B-supergiants), Urbaneja et al. (2003) (optical analysis of NGC 300 B-supergiants), Urbaneja et al. (2005) (optical analyses of M33 B-supergiants) and Lennon and Trundle (Chapter 6 of this volume, M31 B-supergiants).

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