Table 4: Mean NLTE abundances (on the scale in which $\log \epsilon$[H] = 12) and total 1$\sigma $-uncertainties (values for $\xi ^1$ CMa from Morel et al. 2006). The number of spectral lines used is given in brackets. A blank indicates that the abundance of a given element could not be determined. For comparison purposes, we provide in the last column the typical values found for OB dwarfs in the solar neighbourhood (Daflon & Cunha 2004; Gummersbach et al. 1998; Kilian-Montenbruck et al. 1994). We define [N/C] and [N/O] as $\log$ [$\epsilon $(N)/$\epsilon $(C)] and $\log$ [$\epsilon $(N)/$\epsilon $(O)], respectively.
  $\xi ^1$ CMa $\tau $ Sco $\theta $ Car OB stars
He/H 0.098 $\pm$ 0.017 (10) 0.085 $\pm$ 0.027 (9) 0.083 $\pm$ 0.028 (5) 0.10a
$\log \epsilon$(C) 8.18 $\pm$ 0.12 (9) 8.19 $\pm$ 0.14 (15) 7.16 $\pm$ 0.46 (1)b $\sim$8.2
$\log \epsilon$(N) 8.00 $\pm$ 0.17 (34) 8.15 $\pm$ 0.20 (35) 8.56 $\pm$ 0.27 (12) $\sim$7.6
$\log \epsilon$(O) 8.59 $\pm$ 0.17 (34) 8.62 $\pm$ 0.20 (42) 8.38 $\pm$ 0.22 (7) $\sim$8.5
$\log \epsilon$(Mg) 7.37 $\pm$ 0.20 (1) 7.45 $\pm$ 0.09 (2)   $\sim$7.4
$\log \epsilon$(Al) 6.16 $\pm$ 0.22 (4) 6.31 $\pm$ 0.29 (3)   $\sim$6.1
$\log \epsilon$(Si) 7.13 $\pm$ 0.21 (4) 7.24 $\pm$ 0.14 (9) 7.43 $\pm$ 0.23 (10) $\sim$7.2
$\log \epsilon$(S) 6.99 $\pm$ 0.16 (2) 7.18 $\pm$ 0.28 (3) 7.32 $\pm$ 0.33 (1) $\sim$7.2
$\log \epsilon$(Fe) 7.30 $\pm$ 0.22 (32) 7.33 $\pm$ 0.31 (13)   $\sim$7.3c
${\rm [N/C]}$ -0.18 $\pm$ 0.21 -0.04 $\pm$ 0.25 +1.40 $\pm$ 0.53 $\sim$-0.6
${\rm [N/O]}$ -0.59 $\pm$ 0.25 -0.47 $\pm$ 0.29 +0.18 $\pm$ 0.35 $\sim$-0.9
a From Lyubimkov et al. (2004); b derived from a differential analysis of the C II $\lambda $4267 doublet with respect to $\tau $ Sco (see text); c mean NLTE abundance from Morel et al. (2006).

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