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Astron. Astrophys. 325, 1077-1082 (1997)

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3. Results and discussion

The hydrogen lines are one of the main source of information about gravity parameter lg g in the atmosphere of B and A stars. For the lg g determination both equivalent width and form of the hydrogen lines profiles were independently employed. The method applied was as follows. The observed [FORMULA], [FORMULA] and [FORMULA] profiles were compared with theoretical profiles calculated by Kurucz (1979) for a set of parameters [FORMULA] and lg g. This technique is especially efficient for A0-A3 stars, is admissible for A5-A7 stars and yields rather uncertain values of lg g for F0 and later spectral types. For obtaining the parameter lg g the correct value of [FORMULA] should be known. As original value of [FORMULA] we took a mean value of [FORMULA] corresponding to the two-dimensional MK classification, and the temperature [FORMULA] found from the colour index (B-V)0 corrected for interstellar reddening. In cases when the interstellar absorption [FORMULA] was not known we set [FORMULA] for luminosity class III.

The error of the [FORMULA] thus adopted cannot distort substantially the final result. The temperatures of most ALIVARS fall over the range where the equivalent widths of hydrogen lines depend very little on [FORMULA]. Thus, in the interval 7600 K [FORMULA] 9200 K the [FORMULA] equivalent width varies within the limits [FORMULA] =13.7 [FORMULA] 1.7 [FORMULA], i.e. less than 13% for a certain fixed value lg g=3. Then, the theoretical (model) profiles were chosen from Kurucz's model (1979) for given value of [FORMULA] and for the observed value of the equivalent width. It was convoluted with the instrumental profile and compared with the observed profile corrected for scattered light. The Balmer lines [FORMULA], [FORMULA] and [FORMULA] were analysed in that way and the values of lg g were found. The values of lg g are listed in Table 2, first line for every programme star.


[TABLE]

Table 2. The results of the lg g determination of the programme stars


The mentioned method of lg g determination using equivalent widths gives overestimated values of the gravity parameter, because the observed equivalent widths of hydrogen lines comprise equivalent widths of many weak metal lines superposed on the hydrogen lines. For this reason we employed the form of wings of the hydrogen lines together with the equivalent widths method to obtain independent evaluations of lg g. Firstly, we have used the dependence of line half-width at the intensity level 0.8 on the temperature [FORMULA]. Some examples of such a dependence are shown in Fig. 1, for wnich the data were taken from Kurucz (1979).

[FIGURE] Fig. 1. Variation of [FORMULA] half-width at intensity level 0.8 as a function of [FORMULA]

Secondly, we have used the dependence of the relative intensity of the line, at the distance [FORMULA] from its center, on the temperature [FORMULA] (the [FORMULA] 's are equal 7, 8 and 9  [FORMULA]). These relationships are shown in Fig. 2.

[FIGURE] Fig. 2. Variation of relative intensity of [FORMULA] at distant [FORMULA] =8 [FORMULA] from the line center as a function of [FORMULA]

The lg g values determined using these two additional methods are also listed in Table 2, at the second and third lines for each programme star correspondingly. The mean square root error of the lg g determination, determined using programme and standard stars, equals 0.08 dex. All lg g values obtained using the three mentioned methods are in good agreement. Mean values of lg g for individual stars are given in the last column of Table 2 (the statistical weight of each method employed equals to 1).

Standard stars of different luminosity classes were employed to test the validity of the approach used. The lg g values obtained for 4 standard stars (see Table 3) are consistent with that for stars of luminosity classes V and III.


[TABLE]

Table 3. The results of the lg g determination of the standard stars


A comparison of the observed profiles of HD 23194 and the theoretical ones for [FORMULA] =8800 K and lg g=4.1 is shown in Fig. 3 to illustrate the suitability of the method used. It seemed to give fairly reliable results.

[FIGURE] Fig. 3. Comparison of observed (solid line) and calculated (dashed line) profiles of hydrogen lines for standard stars HD 23194

Two more stars - CO Ori and EZ Ori - were added to our observing programme, both being classified as HAeBes (Herbig & Bell 1988) located on the main sequence. Their photosphere temperatures are lower than that for typical ALIVARS and, in addition, CO Ori is considered to be an object of the T Tau group (Herbst et al. 1983). Unfortunately, the method of lg g determination, which have been used, failed to give a proper value of the gravity parameter for temperatures less than 7500 K. However we succeeded to obtain certain values of temperatures of these cool stars using equivalent widths of hydrogen lines.

In those cases where the lg g determinations are not possible we have substituted it by the determination of the luminosity classes. A well known criterium based on the ratio of the specific absorption line intensities was used (Seiter 1970). The following pairs of metal lines were chosen: SrII [FORMULA] 4215  [FORMULA] /FeI [FORMULA] 4250  [FORMULA] ; CaI [FORMULA] 4456  [FORMULA] /FeI [FORMULA] 4462  [FORMULA] ; CrI [FORMULA] 4344  [FORMULA] /FeI [FORMULA] 4415  [FORMULA].

The criteria used unambiguously point out that the ALIVARS BH Cep, BO Cep and BN Ori are more luminous stars than CO Ori and EZ Ori. For this reason proper values of lg g have been assigned to BH Cep, BO Cep and BN Ori which in our opinion correspond with ratios of the chosen line pairs. These values are labeled in Table 2 with the letter "c".

In Fig. 4 the results of the comparison of the observed hydrogen line profiles with theoretical ones for the variable IP Per are shown for illustration. Emissions in [FORMULA] and [FORMULA] are well observable here.

[FIGURE] Fig. 4. Same as Fig. 3, for programme star IP Per

Data obtained allow us to state that the ALIVARS studied here are neither main sequence nor pre-main sequence stars. On the " [FORMULA] -lg g" diagram (see Fig. 5) they occupy a region bounded by giants and stars of higher luminosity. Their parameters of gravity lg g range from 3.8 to 1.4, the mean value being near lg g [FORMULA] 3.0 (see Fig. 6).

[FIGURE] Fig. 5. Position of stars studied in the (lg g- [FORMULA]) diagram
[FIGURE] Fig. 6. Histogram of lg g for programme stars

This conclusion should not be regarded as a result of the erroneous method applied, because for the standard stars reasonable values of lg g were obtained. It might well be that ALIVARS are evolved rather than young stars, especially those lying far away from star-formation regions.

Some ALIVARS on the " [FORMULA] -lg g" diagram have reached a region which neighbours that of the well evolved RCB-type variable stars (R CrB, RY Sgr, XX Cam). It is a very serious argument in favour of the hypothesis of the post-main sequence status of ALIVARS. One of the programme stars, V351 Ori, was shown to be an Ae/Be star which has left the main sequence and in the past was never connected with the Orion star-formation region (Pugach & Kovalchuk 1997). The rotation velocity of V351 Ori is near 87 [FORMULA].

We can't help but agree with Filkenzeller's (Finkenzeller 1985) remark that the locus of HAeBes on the HRD should not be regarded as a final reference of the evolutionary status of the stars. Values of the rotation velocities are needed before a definite conclusion can be drawn. Unfortunately reliable [FORMULA] sin [FORMULA] data are absent at present. However, it is notable that no fast rotators have been found among ALIVARS.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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