4 Discussion

4.1 Spectral classification

The stellar spectra were normalised and smoothed using a 3 pixel window, and classified following the criteria of [Walborn & Fitzpatrick (1990)]. We obtained thus reliable spectral types for 175 stars. This is fewer than the 231 slit spectra mentioned in Sect. 3 because many stars were observed more than once, while a few were discarded because the signal-to-noise was too low to classify them properly. The method we have used for background subtraction allows us to estimate if the features present in the stellar spectra are undoubtedly of stellar origin.

Figures 4 to 24 show the classified spectra grouped by similar spectral type. The neutral hydrogen lines, together with the nebular lines that showed problems throughout the background subtraction, have been left out of scale in order to enhance the weaker features relevant to the classification process.

  

Table 1: Spectral types of the observed stars

$\parbox{16.5cm}{{\it Notes to Table~\ref{... ...if any. The references are:~\cite{me}, (2)~\cite{p1}, (3) WB97, (4)~\cite{mh}.}$

The resulting spectral types are presented in Table 1. The stellar identifications in Col. 1 are those by [Parker (1993)] (with the exception of a few stars which lie beyond the area covered by his photometry, namely stars 10001 to 10008). Column 2 shows the visual magnitude from Paper I when available and [Parker (1993)]. The ones from the latter are indicated with a "$\star$" symbol. The discrepancies between both photometries has already been discussed in Paper I. Column 3 lists our spectral classification, and Col. 4 indicates previous classification taken mostly from the compilation of [Walborn & Blades (1997)] (hereafter WB). Table 2 lists the spectral types available in the literature, for stars not observed by us but which have been included in our analysis.

  

Table 2: Spectral types of 30 Dor stars available in the literature

$\parbox{12.5cm}{{\it Notes to Table \ref{... ...classification. \\ The references are: (1) \cite{me}, (2) \cite{p1}, (3) WB97.}$

A finding chart for our objects is included in Figs. 1 and 2, where slits have been drawn to scale, showing the corresponding identification number.

  

Figure 5: Same as Fig. 4, stars classified as O3-6 are early O stars, with strong nebular contamination on HeI 4471 Å that prevents from the determination of a more accurate spectral type Figure 6: Same as Fig. 4. for the mid O-type dwarfs

 

Figure 7: Same as Fig. 4 for mid-O stars. The O6-8 type indicates weak HeII absorption lines, but with nebular contamination on HeI 4471 Å

 

Figure 8: Same as Fig. 7

  

Figure 9: Same as Fig. 4 for late O-type dwarf stars. As the intensity of the HeII absorption lines diminishes, the feature identified at 4026 Å  is only due to HeI Figure 10: Same as Fig. 9

  

Figure 11: Same as Fig. 9 Figure 12: Same as Fig. 9

  

Figure 13: Same as Fig. 4 for the early B-type dwarfs. Only traces of HeII are detected. SiIII and SiIV, used for classification, are identified here Figure 14: Same as Fig. 13

  

Figure 15: Same as Fig. 13 Figure 16: Same as Fig. 13

  

Figure 17: Same as Fig. 13 Figure 18: Same as Fig. 4 for OB-type subdwarf stars

  

Figure 19: Spectral classification for early O-type giant stars. HeI and HeII features are identified. The luminosity criterium used is the partial filling of the HeII 4686 Å  feature Figure 20: Same as Fig. 19 for late O - early B type giant stars. Together with HeI and HeII, SiIII and SiIV features are identified

  

Figure 21: Same as Fig. 20 for early B-type giant stars Figure 22: Same as Fig. 20

   

Figure 23: Spectral classification for O giant and Wolf-Rayet stars Figure 24: Spectral classification of B supergiant stars

4.2 Comparison with other spectral classifications

4.2.1 Ground based data

There are 70 stars in our sample with published spectral types, mostly from WB. A comparison plot of both classifications is presented in Fig. 25, where we plot the spectral types from the literature against our own spectral types. The bars indicate the uncertainty in the spectral classification using the following convention: an O3-6 star appears in the Fig. 25 as an O4.5 type with a $\pm 1.5$ error bar in spectral class. From the plot it can be seen that:

1.

There is an excellent overall agreement between our spectral types and the ones already published.

2.

The "flat" distribution of points in the early-mid O range is mostly due to a number of stars classified as "O3-6V" in WB due to strong nebular contamination, and which we have been able to classify more accurately.

3.

We seem to classify late O-type and B-type stars with slightly later types, although the agreement is still quite remarkable.

4.

There are 9 stars that fall well away from the diagonal line. These stars are labelled in the diagram and are discussed below.

Our O-type classification was done using better quality data than the previous observations. The improvement comes mostly from our method for subtraction of nebular contamination, and also from a better S/N ratio. However, for the B-type stars the classification is more sensible to spectral resolution, which is slightly better in WB, than to S/N. These facts are relevant in the following discussion of the 9 discrepant cases.

Parker 706:  Previously classified as O3V. The spectrum presents HeI absorption lines which indicate a later type (O6V). The residual of the [OIII]5007 Å line is smaller than 4%, which indicates that the chance of HeI being due to over-subtraction is small.

Parker 1013:  This star was classified as O8:V by WB, but our data shows no strong HeI absorption features. The nebular spectrum extracted close to the star indicates low probability of HeI under-subtraction. Thus, our O4V type appears more secure.

Parker 547:  WB give O8-9V for this star, but our data shows no strong HeI features, although with some uncertainty because the nebular contribution had a high residual before the correction. However, the nebular scaling correction did not change our initial O6V spectral type which we retain.

Parker 1247:  We detect a faint HeII 4686 Å  feature together with SiIV 4089 - 4116 and SiIII 4552 - 4567. The presence of HeII is not expected in a B2-3 star, as classified in WB, so our B0.5IV type appears more secure.

Parker 1553:  This is one of the clearest cases in which there is no HeI emission detected in the background, so the undoubtedly stellar absorption line HeI 4471 makes it an O7V type star.

Parker 1797:  The long error bar shows our uncertainty when classifying this star. Notice, however, that according to our estimated contribution of nebular Balmer lines, the hydrogen emission lines seen in the spectrum are intrinsic to this star, so we classify it as Be.

  

Figure 25: Comparison between spectral classification described in this paper, and the one found in the literature. Square markers identify the comparison with Walborn & Blades (1997) (WB97) and [Parker (1993)]. Triangle markers indicate the comparison with spectral types in MH. Labelled points are discussed in the text

4.2.2 HST data

There are six stars in common with the work of [Massey & Hunter (1998)] where they present spectral types for stars within the core of the cluster obtained using the Faint Object Spectrograph (FOS) (numbers 860, 863, 1029, 1036, 1080, 1350 in Table 1; plotted in triangles in Fig. 25.) The FOS observations were obtained through a 0.26'' diameter aperture which reduces the nebular contamination considerably, making it negligible for H and HeI lines. From the ratio of slit areas we estimate that the nebular contamination in the FOS spectra is about 40 times smaller than in our NTT observations. On the other hand, there is a big difference in the light collecting areas so, since the exposure times are slightly larger for the NTT data, the overall S/N ratio is significantly larger in our data. Furthermore, the Digicon detector of the FOS is known to introduce problems due to "dead" and noisy diodes. The observational procedure, described by MH, shifts the spectra along 5 neighbouring diodes at a step of 1/4 diode, which means that a diode turned off reduces the S/N ratio by a factor of $\sim 20\%$, while a noisy diode introduces spurious emission or absorption features that are 20 pixels wide. In particular, at the redshift of 30 Doradus, the HeI 4471 feature lies approximately in diode # 408. According to the information available in the FOS manual, diode # 409 is turned off and diode # 410 is reported as possibly noisy, though still turned on. This reduces considerably the S/N ratio in this region of the spectrum, which unfortunately makes the detection of weak HeI 4471 features rather difficult.

By comparing the FOS intensities of the H$\delta$and H$\gamma$ absorption lines, which are free from nebular contamination in the FOS data, with our NTT data, we can obtain an independent check of the goodness of our nebular subtraction method. Below we present the result of this comparison for the 3 stars for which we disagree with the classification of MH.

Parker 860 (=MH 28) We classify this star as O7:V((f)) although the [OIII] residuals indicate significant nebular contamination of the HeI lines. Comparison of the Balmer lines with the FOS data yield the same estimate, but we still consider that there is real HeI absorption in the spectrum, so the type O3V given by MH is probably not correct.

Parker 863 (=MH 29) We classify this star as O6.5V. Comparison of the Balmer lines in the FOS spectrum indicates that our nebular subtraction is good. Moreover, in this particular case we have detected the stellar HeI 4471 absorption even without background subtraction. So again the O3V type given by MH is probably incorrect.

Parker 1350 (=MH H96-28) We classify this star as O6III(f^*) while MH give O3III(f^*). The presence of NIV 4058 Å  emission in the FOS spectra favours the classification of MH, but this feature is probably of instrumental origin. Although there is no report of problems in that area of the detector, the NIV emission feature is 20 pixels wide, matching the extension of a noisy diode feature. Moreover, the NIV feature is not detected in our higher S/N spectrum of the star.