Astron. Astrophys. 345, 244-248 (1999)

5. Projected rotational velocity distribution

In this section we show the projected rotational velocity distribution defined as the number of stars per interval (vsini bin) divided by the total sample size and by the width of the interval. Similarly to the past studies, we adopted intercomparison with normal B-type stars. The distribution of normal stars was adopted from Wolff et al. (1982) who measured 306 early B-type stars based on intermediate reciprocal resolution, mm^-1. Fig. 5 displays the comparison of He-rich stars and normal B-type stars distributions. A previous study of Walborn (1983) based on low dispersion data, mm^-1, is also displayed. We can basically confirm Walborn's distribution, keeping in mind the small number of stars involved (we have 17 stars in common with Walborn), as well as the fact that most values of Walborn are larger than ours, as Table 1 shows. The spectral-type dependence of vsini values show no significant trend. Fig. 5 shows that most He-rich stars have  km s^-1, with just a few around 130 km s^-1. Normal stars are more uniformly distributed, up to values as large as 350 km s^-1. The difference is probably significant, though the number of He-rich stars is small. This would indicate that slow rotation is characteristic of all main-sequence magnetic stars, including the He-rich ones. The K-S test has been applied to the cumulative distributions of normal and He-rich stars and has shown that the distributions differ from each other at a significance level of 98.5 percent, while the chi-square test applied to the binned distributions gave the same result, at a significance level of 98 percent.

Fig. 5. Projected rotational velocity distribution (probability density) defined as the number of stars per interval divided by the total sample size and by the width of the interval and multiplied by 103. Solid line: He-rich stars, dotted line: normal B-type stars, Wolff et al. (1982), dashed line: He-rich stars after Walborn (1983).

Fig. 6. The cumulative distributions of normal and He-rich stars.

© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999
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