EDP Sciences
Free Access
Issue
A&A
Volume 565, May 2014
Article Number A39
Number of page(s) 20
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361/201220602
Published online 01 May 2014

Online material

Appendix A: Observational data and modelling

We intend to work as closely as possible with the observational data of the FLAMES Survey, aiming to avoid bias from differences in the data reduction. As a test, raw data for all FLAMES Survey targets observed with the Fiber-fed Extended Range Optical Spectrograph (FEROS) at the 2.2 m telescope at ESO (La Silla, Chile) were retrieved from the ESO archive and reduced using the FEROS pipeline and our additional recipes for continuum normalisation (see, e.g., Nieva & Przybilla 2007). These were compared with the published FLAMES Survey spectra, downloaded from the project webpage7. Good agreement of the two data reductions was found, such that the FLAMES Survey spectra were employed, except for the Hα region, which was missing in the published data.

The sky-corrected single exposures of the individual FLAMES orders as well as the fully-reduced spectra were also downloaded from the project webpage. Multiple exposures of an object were cross-correlated to identify radial-velocity (rv) variables. In the case of rv variables, the single exposures per order were rv-corrected to laboratory rest frame, normalised and coadded (with weighing factors according to S/N), and then the orders merged. In the case of no rv-variation the fully-reduced spectra from the project webpage were employed.

Our atmospheric modelling and spectrum synthesis calculations for early B-type stars on the MS have been discussed in detail by Nieva & Przybilla (2008, 2012). In brief, based on prescribed LTE model atmospheres (Atlas9, Kurucz 1993) non-LTE line-formation calculations were performed using the codes Detail and Surface (Giddings 1981; Butler & Giddings 1985, both updated by K. Butler), abbreviated by Ads henceforth. Such plane-parallel and hydrostatic hybrid non-LTE models have been shown to be equivalent to either plane-parallel and hydrostatic full non-LTE line-blanketed model atmospheres (Nieva & Przybilla 2007; Przybilla et al. 2011), such as applied within the FLAMES Survey, or spherical and hydrodynamic full non-LTE line-blanketed model atmospheres (Nieva & Simón-Díaz 2011) on the MS.

Appendix B: Test case examples

We discuss six prototype examples representative of the four object classes identified in our investigations in Sect. 5 in the following. These comprise normal stars (on the MS and beyond), double-lined objects (SB2 candidates, visual or apparent binaries), Be stars, and a chemically peculiar object. The focus of the discussion, based on Figs. B.1 to B.6, is on the quality of the match of the synthetic with the observed spectra, in particular, with regard to the stellar parameter indicators – the H and He lines, and metal ionisation equilibria. The individual panels of the figures are centred on the diverse diagnostic lines from different chemical species, sorted according to increasing atomic weight, and within one element by increasing ionisation degree and wavelength. Note that several – mostly weaker – metal lines are not implemented in the models, with the intention to reproduce only the FLAMES Survey results. For the assessment of the entire sub-sample of 31 stars drawn from the H+09 work see Table 1.

  • NGC 6611-006.

    Figure B.1 shows an example for one of the best matches between theoryand observation found within our re-investigation of thesub-sample of stars drawn from the H+09 work, for a single,normal star. Good agreement is found for the hydrogen lineprofiles, except for the narrow central Hα emission that is of nebular origin. A good match is also obtained for the He i and most metal lines, while a reasonable match is found for the He ii and C iii lines, and for Si ivλ4654 Å (not considered by H+09). Only C iiλ4267 Å and C iiλλ6578/82 Å in the Hα wing do not fit, with the predicted lines being too strong by a factor of ~2 and a factor of several, respectively. The ionisation balance of C ii/iii is thus not established, suggesting that an overall improved fit could be achieved by a revision of the atmospheric parameters (requiring also an adjustment of chemical abundances).

  • NGC 2004-053.

    This star is apparently the slowest rotator and yet one of the most nitrogen-rich stars in the LMC sample, making it a role-model for one of the star classes that cannot be explained on the basis of rotational mixing alone (H+09). The broad central and symmetric Hα emission in Fig. B.2 identifies the object as a Be star seen pole-on (feature apparently overlooked by H+09, but correctly identified by Evans et al. 2006). Despite the sharp-lined character of the spectrum, this is, in fact, a fast-rotating star. The spectrum is dominated by light from the hot stellar pole, and smaller contributions from the gravity-darkened stellar equator regions (further diminished because of limb darkening) and the disc. The question is to which extent the non-uniform surface temperature and density, and the emission from the disc affect the continuum and line spectra. Despite the overall rather good match of model spectrum and observations (again except for C iiλ4267 Å and C iiλλ6578/82 Å, and a consistent failure to reproduce the depths of most metal lines) it is at present not possible to judge how realistic the derived parameters and abundances really are. Because of this potential systematic bias they should be treated with caution in any further interpretation. For the moment, the star should be excluded from further interpretation, like other Be-stars (see Sect. 5). However, on observational grounds alone a true nature as a nitrogen-rich fast rotator is indicated.

  • NGC 3293-034.

    Despite being a good match for Balmer lines, He iiλ4686 Å and many metal lines (Fig. B.3) the stellar parameters and abundances derived by H+09 are not adequate. The reason for this is the mismatch of model and observation for the He i lines. These indicate that NGC 3293-034 is a He-strong star (as identified by Evans et al. 2005). Note also that none of the available ionisation equilibria (He i/ii, C ii/iii, Si iii/iv) is matched in a satisfactory way. The higher molecular weight and lower opacity of He (with respect to H) in combination with a pronounced overabundance of the element change the atmospheric temperature and density structure, which requires dedicated model calculations for proper consideration beyond the pre-computed grids employed within the FLAMES Survey. In consequence, this star has to be excluded from further interpretation to avoid biased conclusions.

  • NGC 6611-001.

    The spectrum synthesis provides an overall good match to the observed spectrum, see Fig. B.4, except for some He i/ii lines and the unsatisfactory fit of the C iii-dominated complex around 4650 Å. However, close inspection of isolated strong metal features like C iiλ4267 Å, O iiλ4076 Å, or Si iiiλλ4552, 4574 Å reveals line-profile asymmetries. Such asymmetries become apparent in particular in visual inspection when using line-profile fitting techniques, but are easily overlooked when employing an equivalent-width approach (like by H+09) for the quantitative analysis. Line-profile asymmetries are indicators for a possible SB2 nature of the star, but require confirmation by time-series spectroscopy. Indeed, Sana et al. (2009) can clearly resolve double lines for NGC 6611-001 for some epochs in their spectra (note that the star is also known to be an eclipsing binary, Lefèvre et al. 2009). Consequently, the spectrum, while dominated by the light of the primary, is compromised by second light, and the analysis is therefore biased. To avoid misleading conclusions, the star has to be excluded from further interpretation.

  • NGC 3293-007.

    This is an example of an object evolved off the MS, identified as nitrogen-poor by H+09. The comparison of observed and model spectra in Fig. B.5 shows an overall poor match, with the model largely under-predicting the depths of nearly all lines. In particular, the synthetic N ii lines are too weak by a factor 2–3 in equivalent width, indicating that the finding of nitrogen-poorness is spurious. Moreover, the ionisation equilibria do not match, indicating poorly constrained stellar parameters and therefore, erroneous abundances in general. An independent comparison with non-LTE line-blanketed hydrodynamic models computed with Fastwind and Cmfgen (Hillier & Miller 1998) by M. Urbaneja (priv. comm.) confirms a poor match of observation and models for the published parameters and abundances of H+09. This is in line with the findings of Martins & Palacios (2013) that the abundances for Galactic OB stars determined by H+09, on the one hand, and those determined by other codes differ (Martins et al. 2008; Przybilla et al. 2010; Nieva & Simón-Díaz 2011; Firnstein & Przybilla 2012; Nieva & Przybilla 2012; Bouret et al. 2012, despite some minor differences that also exist between the Ads, Fastwind, and Cmfgen models). A Teff higher by 3000 to 4000 K would seem more appropriate. The object should be excluded from further interpretation.

  • NGC 4755-003.

    This is an example of an evolved nitrogen-rich star of H+09. As in the case of the previous example, the match between observation and model is overall poor, see Fig. B.6. This time, the model over-predicts the depths of almost all lines, including the N ii features. Again, poorly constrained stellar parameters seem to be a major factor for the mismatch. The object should be excluded from further interpretation. A second reason for exclusion is found from closer inspection of the profiles of the stronger lines. They show the presence of a second absorption component in the red wings – this object is double-lined. It appears that the second object is of similar spectral type but less luminous, probably a main-sequence star.

thumbnail Fig. B.1

Comparison of a synthetic spectrum computed by us with Ads for the atmospheric parameters and elemental abundances derived by Hunter et al. (2009, red line) and the Flames/Giraffe observation (black line) for the star NGC 6611-006. Displayed are spectral regions with important features for the analysis, as indicated. The sharp Hα emission feature is of nebular origin. This is an example of one of the best matches between theory and observation found within our re-investigation of the sub-sample of stars drawn from the Hunter et al. (2009) work.

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thumbnail Fig. B.2

Same as Fig. B.1, but for the star NGC 2004-053. This is an example for a model fit to a Be star seen pole-on. The grey-shaded box guides the eye to recognise the signature of the presence of a disc, i.e., the Hα emission. Similar to the models employed by Hunter et al. (2009), the present computations with Ads do not account for the presence of a disc and gravity darkening. The neglect of the true nature of this star yields potentially biased atmospheric parameters and abundances, despite the fact that a good fit to many spectral lines is achieved.

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thumbnail Fig. B.3

Same as Fig. B.1, but for the He-strong star NGC 3293-034. The grey-shaded boxes guide the eye to recognise the signatures of the chemical peculiarity of the star, i.e., the unusually strong He i lines. Similar to the models employed by Hunter et al. (2009), the present computations with Ads do not account for the pronounced helium enrichment and therefore the altered atmospheric structure of the star. As a consequence, the analysis yields incorrect atmospheric parameters and abundances, despite the fact that a good fit to the hydrogen and many metal lines is achieved.

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thumbnail Fig. B.4

Same as Fig. B.1, but for a FEROS observation of the star NGC 6611-001. This is an example of a model fit to a double-lined binary (SB2) system, thus treating the object as a single star. The grey-shaded boxes guide the eye to recognise the binarity signatures in the most pronounced cases, i.e., the contributions of the two stars to the asymmetric line profiles. The neglect of the binary nature (see also Sana et al. 2009; Lefèvre et al. 2009) results in the analysis yielding incorrect atmospheric parameters and elemental abundances. Some deficits in the modelling, such as for the O ii features around 4070 Å, stem from missing lines in the spectrum synthesis (see Fig. B.1).

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thumbnail Fig. B.5

Same as Fig. B.1, but for a FEROS observation of the star NGC 3293-007. This is an example of a model fit to a nitrogen-poor evolved star (Hunter et al. 2009). Except for the hydrogen Balmer lines (Hα is filled by wind emission) the fit to all lines, and in particular to the N ii lines, is poor. This indicates a poor choice for the stellar parameters, implying that the derived abundances are subject to systematic bias.

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thumbnail Fig. B.6

Same as Fig. B.1, but for a FEROS observation of the star NGC 4755-003. This is an example of a model fit to a nitrogen-rich evolved star (Hunter et al. 2009). The fit to the He i and most metal lines (including N ii) is poor, indicating a poor choice for the stellar parameters. The derived elemental abundances are thus systematically biased. Moreover, close inspection finds the star to be double-lined. The grey-shaded boxes guide the eye to recognise the double-lined signatures in the most pronounced cases, i.e., the contributions of the two stars to the asymmetric line profiles.

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