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D. Nowak and H. Cugier
Astronomical Institute of the Wroc
aw University,
Kopernika 11, 51-622 Wroc
aw, Poland,
E-mail:nowak
astro.uni.wroc.pl
line is the most pronounced absorption
feature in spectra of B-type stars.
Two effects play the dominant role in observed behavior of 
:
the Stark broadening of the line absorption coefficient and interstellar
absorption. These two effects may be separated by considering late B and
early B-type stars. We search for the best description of the Stark
effect using observations collected
by Copernicus, IUE and HST satellites.
Having a credible description of the broadening mechanisms of
we determined interstellar component of the observed
line profiles of early B-type
stars. The interstellar column densities, N(H I), are derived and
correlations with various signatures of interstellar extinction
are also examined.
Keywords: B-type stars, H I 
line, Stark effect, interstellar
absorption
Theoretical profiles of the hydrogen 
line are calculated
for Kurucz's (1979) models of atmospheres corresponding to B-type stars.
The line absorption coefficient is a convolution of the Voigt and Stark
profiles. We search for the best description of the broadening mechanisms
of the line absorption coefficient using various data for the Stark
effect. We found that Vidal's et al. (1973) and Clausset's et al. (1994)
results are almost identical for 
.
Feautrier's (1976) exact quantum calculations indicate significantly stronger
line wings, cf. Fig. 1 where two examples are shown (cf. also Tran Minh
et al. 1980 and Hubený 1981).
Figure:
(a) Predicted 
line profiles for model atmospheres of
, 
. The 
profile corresponding to Feautrier's et al. (1976) description of the Stark effect
(solid curve) is markedly stronger than that of calculated using
Vidal's et al. (1973) or Clausset's et al. (1994) (dotted line) data.
(b) The same as Panel a but for stellar model of
and 
.
Figure 2 illustrates how the interstellar H I 
absorption
influences the photospheric spectra of 
1216Å,
Si III 1204Å and C III 1175Å.
Figure:
(a) The photospherical flux for
, 
(solid line) and the
synthetic profile with the interstellar component corresponding to the
hydrogen column density equal to 
(dotted line).
(b) The same as Panel a but for the model atmosphere of
, 
(solid line), and
(dotted line) and 
(dashed line), respectively.
We analyze archival observations of 
line collected by
Copernicus, IUE and HST satellites.
The basic stellar parameters 
and
as well as color excess 
are derived
from 
photometry using Kurucz's (1991) grids of uvby colors
and 
indices calibrated by Smalley 
Dworetsky (1995). Following to
Shobbrook (1978) we adopted the relations 
and
in the de-reddening procedure. The results are shown
in Table 1.
Table: Basic parameters of the program stars and hydrogen
column densities 
, 
. (The number 
means 
).
Figure 3 displays the observations of 
Lyr and 
CMa A
in comparison with the predicted photospheric spectra near 
1216Å, Si III 1204Å and C III 1175Å.
Figure:
(a) Comparison of the predicted 
line profiles
with observations of 
Lyr (Vega). The 
profile corresponding to Feautrier's et al. (1976) description of the Stark effect
is shown as solid curve, whereas calculations based on
Clausset's et al. (1994) data are shown as dotted line.
(b) The same as Panel a but for 
CMa A (Sirius).
As one can see, only line profiles with Feautrier's et al. (1976) data for the hydrogen Stark effect are able to reproduce the observations. The same is true for the remaining late B-type stars investigated. Thus having a good description of the Stark effect we investigated early B-type stars, where interstellar absorption may play an important role. A few examples are show in Fig. 4 (IUE observations) and Fig. 5 (HST observations).
Figure: Examples of the IUE observations in comparison with
theoretical spectra (see text).
The dotted lines in Figs. 4 and 5 mean only the photospherical
fluxes, whereas the synthetic spectra with the interstellar component of
H I 
line are shown as the solid lines.
The stellar rotational broadening does not influence strongly the
profile due to large widths of this line and,
therefore, we do not include this effect in our calculations.
Note that in the case of 
Pic (HD 42933) the photospherical spectrum
is shifted relative to 
line to long
wavelength side (cf. Fig. 5) supporting the interpretation of this line as
mainly produced by interstellar gas.
For 
Gru there are two pieces of the HST/GHRS spectrum which
does not cover the whole profile of 
, cf. Fig. 5.
Figure: Examples of the HST observations in comparison with
theoretical spectra.
The largest collection of hydrogen column densities comes from OAO-2 and Copernicus ( OAO-3) satellites, cf. e.g., Hobbs (1974), Bohlin et al. (1978) and Shull & Van Steenberg (1985). These data are compared with our results for stars in common. The agreement is satisfactory, cf. Fig. 6, but not perfect.
Figure 7 shows published values of 
in comparison with our determination
of 
mentioned in Section 2. The linear relation between these quantities
is the following: 
(straight line in Fig. 7).
Figure: The correlation between hydrogen column densities N (H I) obtained
in this paper and other ones, given by Hobbs (1974) (solid circles),
Bohlin (1978) (open squares) and Shull (1985) (stars).
Figure: The value of 
obtained from 
photometry plotted versus
taken from Bohlin (1978) (open squares) and Shull (1985) (stars).
In this paper we search for the best description of the Stark effect
using observations of late B, A0 and A1 type stars. We found that
Feautrier's et al. (1976) exact quantum calculations match well the
observations. It can be achieved using atmospheric models with
parameters derived from 
photometry as described in Sect. 3.
Having a credible description of the broadening mechanisms of
we determined interstellar component of the observed
line profiles of early B-type stars. Contrary to previous investigations
mentioned in Sect. 3, our approach does not
assume that the stellar 
line is much narrower than the
interstellar absorption and therefore no limitation for spectral types of
about B2 and earlier takes place.
Absorption in the H I 
line provides a fundamental measurement
of interstellar gas.
For instance, in the direction to stars numbered as
6, 7, 11 and 12 in Table 1, the Hydrogen column densities, N(H I),
show almost the same values despite of significant differences in 
.
It is interesting to note that in these cases the molecular hydrogen densities
(cf. Table 1) achieve the largest values in our sample.
The analysis which takes
into account a more complete list of stars is in progress.
We would like to acknowledge ST-DADS and HST/ESO/CFHT Archive Services for the Space Telescope Data. STARCAT interface developed by ST-ECF, CADC and ESO and installed on SPARCstation 2 computer at AI WrU was used. ESA IUE Observatory at VILSPA provided the IUE tapes. CDS and SIMBAD Services provided the Copernicus Catalog III/77 by Internet network. To them all we express our thanks.
This work was supported by the research grant No. 2 P03D 001 08 from the Polish Scientific Research Committee (KBN).
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