Since the relative strengths of several He I lines intervene
in most classification criteria for the MK system in the spectral range
of interest, the spectral classification of Be stars has always been
considered particularly complicated. In many spectra, the in-filling
of He I lines affects the main classification criteria. When
Fe II emission is present, several lines which are used as
classification criteria can be veiled (such as the Si II 4128-4130 Å doublet).
The high resolutions obtainable at high signal-to-noise ratio
with modern CCD cameras improve
the situation, since they allow us to disentangle lines that were blended
at the resolutions formerly used for spectral classification. On the
other hand, the improved resolution means that the traditional criteria
are not always applicable. Our spectra have a much higher resolution
than the 63 Å mm-1 plates used by Walborn (1971) to define the
grid for early-type B stars. Given that the
only acceptable methodological procedure in the MK scheme is the
comparison of spectra (Morgan & Keenan 1973), all this results
in a strong dependence on the choice of standard stars. Unfortunately,
the standard stars available for observation are limited by the position
of the observatory and time of the year. Our standard stars, taken from
the lists of Walborn (1973) and Jaschek & Gómez (1998) are listed
in Table 3. We point out
that Jaschek & Gómez (1998) give HD 23338 (19 Tau) as a B6V standard
and HD 196867 ( Del) as B9V, while
Morgan & Keenan (1973) give
them as B6IV and B9IV respectively. At our resolution, neither of the two
spectra can be justifiably classified as main sequence objects and we
endorse the subgiant classification. Indeed HD 23338 looks remarkably
similar to HD 23302 (17 Tau), which is given by Jaschek & Gómez (1998)
and Lesh (1968) as the primary B6III standard.
For classification purposes, we compared the spectra of the Be stars
with those of the standard stars in the interval
3940-4750
Å both at full instrumental resolution and binned to 1.2 Å/pixel
to mimic the resolution of photographic plates. The comparison was
done both "by eye'' and using the measured equivalent widths of
the relevant features.
All the spectra have
been classified in this scheme without previous knowledge of spectral
classifications existent in the literature. The derived spectral
types are listed in Table 4. Representative spectral and luminosity
sequences are shown in Figs. 1 and 2, and some peculiar spectra from
the sample are
shown in Fig. 3.
We find that the
accuracy that can be obtained in the classification depends in part
on the spectral type of the object, as described below.
For stars earlier than B3, the classification can be adequately performed
using as main indicators the Si IV and Si III lines. The
strength of these lines and of the O II spectrum is very sensitive
to temperature and luminosity variations. Moreover, the emission spectrum
does not generally extend shortwards of Å. As a
consequence, most of our determinations in this spectral range are very
secure. Even though the spectral grid is finer than at later types, we
believe that most objects are correctly classified to the sub-subtype, i.e.,
a spectrum classified as B0.7III is certainly within the range
B0.5III-B1III and both B0.5III and B1III look inadequate
classifications. The luminosity classification is also secure in this
range, where we are able to differentiate clearly between giants and
main sequence objects.
For stars later than B5, all the classification criteria available are
strongly affected by the presence of an emission continuum. This has
resulted in our determinations for this spectral range being slightly
less secure than for earlier spectral types. However, with few exceptions,
we have been able to assign a spectral type to the correct subtype.
This means that we feel that a star classified as B6V would be inadequately
classified as B5 or B7. The luminosity classification is slightly less
certain. For this reason, we have resorted to using two extra criteria,
namely, the number of Balmer lines that could be resolved in the spectrum
as it approaches the Balmer discontinuity and the full width
half maximum of H (3797 Å),
which, among the standard stars, correlates strongly with luminosity class
at a given spectral type and is not generally affected by emission
in the Be stars. Overall, the three methods do not show strong
discrepancies and our luminosity classification can be considered
secure, at least to the point of discriminating between giant and
main sequence stars.
Lesh (1968) defines B8V by the condition Mg II 4481
Å
He I
4471 Å. However, as can
be seen in Fig. 4, the spectrum of HD 214923 (
Peg), given by
Jaschek & Gómez (1998) as B8V standard, shows the Mg II line
to be clearly stronger than the He I line.
Therefore, this object must be of a later spectral type. Comparison
with HD 196867 (
Del) shows
that this object is not later than B9V. We have taken the spectral
type of this object to be B9V, though we believe that B8.5V could be
an adequate interpolation. No object in our sample is so late as
HD 214923 and therefore we assign to BD +55
2411, the only
object in the sample with Mg II
4481 Å clearly
stronger than He I
4471 Å a spectral type B8.5V.
In the spectral region B7-B8, where the ratio between Mg II
4481 Å and He I
4471 Å is the only
classification criteria, in-filling can strongly affect the derived
spectra. For that reason, we have also used the strength of the
Si II doublet and of the whole He I spectrum as
additional information. Moreover, at this resolution, C II
4267 Å is visible in main sequence stars up to spectral
type B7V. Fe II
4232 Å starts to be visible in
the B8V spectra, but we have not used it as a classification indicator
since it can also be a weak shell line.
For the purpose of spectral classification, we have only marked as "shell'' stars those showing narrow absorption Fe II lines, either on top of emission lines or blanketing the continuum. Several other stars show double-peaked emission split by an absorption core in some Balmer lines, but this emission is still inside the photospheric absorption feature and does not reach the continuum (e.g., BD +37 675 and BD +42 1376 in Fig. 1). The shell definition is not applicable since no iron lines are visible (see Hanuschik 1995). Exceptions could be BD +47 857 and BD +50 3430 which seem to show absorption cores in some Fe II emission lines and therefore could be shell stars. We will revisit this question in future papers when we discuss the emission line spectra of the objects.
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