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Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

Analysis of in B-Type Stars

D. Nowak and H. Cugier
Astronomical Institute of the Wroc aw University, Kopernika 11, 51-622 Wroc aw, Poland, E-mail:nowakastro.uni.wroc.pl

 

Abstract:

The hydrogen 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

Model Calculations

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.

Comparison with Observations

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.

Comparison with Other Measurements of Interstellar Extinction

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).

Conclusions

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.

Acknowledgments:

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).

References:

Bohlin, R.C., Savage, B.D., & Drake, J.F. 1978, ApJ, 224, 132

Clausset, F., Stehlé, C., & Artru, M.-C. 1994, A&A, 287, 666

Feautrier, N., Tran Minh, N., & Van Regemorter, H. 1976, J.Phys. B, 9, 1871

Hobbs, L.M. 1974, ApJ, 191, 381

Hubený, I. 1981, A&A, 98, 96

Kurucz, R. 1979, ApJ, 40, 1

Kurucz, R. 1991, preprint

Shobbrook 1978, MNRAS, 214, 33

Shull, J.M. & Van Steenberg, M.E. 1985, ApJ, 294, 599

Smalley, B. & Dworetsky, M.M. 1995, A&A, 293, 446

Tran Minh, N. & Feautrier, N. 1980, JQSRT, 23, 377

Vidal, C.R., Cooper, J., & Smith, E.W. 1973, , 214, 37



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