Up: The ionising cluster of 30 Doradus
In Paper I we detected evidence for mass segregation in the 30 Dor cluster.
To further investigate this important point we have
combined our spectral classification, with that available in
the literature, to obtain a total of 235 stars with reliable spectral types
within the 30 Dor cluster. This allows us to examine the spatial
distribution of stars, grouping them in spectral class bands.
The result is shown in Fig. 26 where we have split the stars in four
main groups: O dwarfs, B dwarfs, OB giants and OB supergiants
(including Wolf-Rayet stars).
![\begin{figure}
\includegraphics [angle=-90,width=8.8cm]{H1193fig26.ps}\end{figure}](/articles/aas/full/1999/10/h1193/Timg35.gif) |
Figure 26:
Spatial distribution of stars classified in this paper. The
different distribution of early and late type stars can
be seen in this diagram. Coordinates are in arcsec, measured from R136.
The dotted circles indicate radial distances from R136, at 50 arcsec steps |
It was pointed out by
[Underhill (1983)] that determination of evolutionary stages from spectral
types is unreliable. Still there are some clear points that come out of the
distribution:
- 1.
- Giant and supergiant stars are evenly distributed in the sampled region
(our sample is intrinsically stretched in the N-S direction).
- 2.
- O dwarfs are notoriously concentrated towards R136.
- 3.
- There is a large region, south-east of R136, where the number of late
type stars is much larger than that of early type ones. These stars can be
identified in the center of Fig. 2.
We have also estimated the stellar masses of stars from their spectral
types interpolating from the relationship tabulated by [Schmidt-Kaler (1982)]. With these
values we analysed the radial distribution of stars according to their masses.
As the determination of a star's mass from it's spectral type is not very
accurate, we split the dataset in two large groups: high mass and low
mass stars, with the transition at
.
Figure 27 shows the cumulative distribution of stars from
the centre of the cluster. It can be readily seen that the massive stars show a
concentrated distribution, while the lower
mass stars have a flatter distribution,
with few stars in the inner region. Thus, there is evidence for mass segregation
in the 30Dor cluster, as it is also suggested from the photometric
analysis of Paper I. A Kolmogorov-Smirnov test to both
distributions, yields a significance level of 0.007, which indicates that the
distribution of high mass stars is significantly different from the less
massive ones.
![\begin{figure}
\includegraphics [angle=-90,width=8.8cm]{H1193fig27.ps}\end{figure}](/articles/aas/full/1999/10/h1193/Timg37.gif) |
Figure 27:
Cumulative radial distribution of stars according to masses determined from
their spectral types. Both distributions are plotted with dashed and continuous
line styles, for the high-mass and low-mass stars respectively. The
different distribution for both subsets indicates the presence of
mass segregation |
In previous works, [Malumuth & Heap (1994)] and [Brandl et al. (1996)] had also found
mass segregation in the inner region of 30Dor, close to R136, which agrees
with our findings.
Malumuth & Heap detected a difference in the slope of the IMF of stars within
a 3.3 arcsec radius of R136a and of those outside this area. Brandl et al. detected changes in the IMF slope and a clear trend towards smaller core
radii for brighter -more massive- stars.
Thus, there is evidence for mass segregation at the core and outer
regions
of 30 Dor. We believe that these results are also
compatible with the lack
of change of the IMF slope found
by [Hunter et al. (1996)] given the narrow mass range
and the restricted radial range of their data.
A more detailed analysis of this important point will be discussed
in Paper III where we combine the
masses derived from our photometric and spectroscopic observations.
Up: The ionising cluster of 30 Doradus
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