Astronomy & Astrophysics

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A&A 368, 912-931 (2001)
DOI: 10.1051/0004-6361:20000577

Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars

R. V. Yudin^{1, 2}

¹ Central Astronomical Observatory of the Russian Academy of Sciences at Pulkovo, 196140 Saint-Petersburg, Russia
² Isaac Newton Institute of Chile, St.-Petersburg Branch, Chile

(Received 9 December 1999 / Accepted 22 December 2000)

Abstract
We present the results of statistical analyses of a sample of 627 Be stars. The parameters of intrinsic polarization $(p_{\ast})$ , projected rotational velocity $(v \sin{i})$ , and near IR excesses have been investigated. The values of $p_{\ast}$ have been estimated for a much larger and more representative sample of Be stars ( $\approx$ 490 objects) than previously. We have confirmed that most Be stars of early spectral type have statistically larger values of polarization and IR excesses in comparison with the late spectral type stars. It is found that the distributions of $p_{\ast}$ diverge considerably for the different spectral subgroups. In contrast to late spectral types (B5-B9.5), the distribution of $p_{\ast}$ for B0-B2 stars does not peak at the value $p_{\ast}=0$ % . Statistically significant differences in the mean projected rotational velocities ( $\overline{v\sin{i}}$ ) are found for different spectral subgroups of Be stars in the sense that late spectral type stars (V luminosity class) generally rotate faster than early types, in agreement with previously published results. This behaviour is, however, not obvious for the III-IV luminosity class stars. Nevertheless, the calculated values of the ratio $v_{\rm t}/v_{\rm c}$ of the true rotational velocity, $v_{\rm t}$ , to the critical velocity for break-up, $v_{\rm c}$ , is larger for late spectral type stars of all luminosity classes. Thus, late spectral type stars appear to rotate closer to their break-up rotational velocity. The distribution of near IR excesses for early spectral subgroups is bi-modal, the position of the second peak displaying a maximum value $E(V-L)\approx 1 .\!\!\!^{\rm m}3$ for O-B1.5 stars, decreasing to $E(V-L)\approx0.\!\!\!^{\rm m}8$ for intermediate spectral types (B3-B5). It is shown that bi-modality disappears for late spectral types (B6-B9.5). No correlations were found between $p_{\ast}$ and near IR excesses and between E(V-L) and $v\sin{i}$ for the different subgroups of Be stars. In contrast to near IR excesses, a relation between $p_{\ast}$ and far IR excesses at 12 $\mu$ m is clearly seen. A clear relation between $p_{\ast}$ and $v\sin{i}$ (as well as between $p_{\ast}$ and $\overline{v\sin{i}}/v_{\rm c}$ ) is found by the fact that plots of these parameters are bounded by a "triangular" distribution of $p_{\ast}$ : $v\sin{i}$ , with a decrease of $p_{\ast}$ towards very small and very large $v\sin{i}$ (and $\overline{v\sin{i}}/v_{\rm c}$ ) values. The latter behaviour can be understood in the context of a larger oblateness of circumstellar disks for the stars with a rapid rotation. From the analysis of correlations between different observational parameters we conclude that circumstellar envelopes for the majority of Be stars are optically thin disks with the range of the half-opening angle of $10\degr< \Theta< 40\degr$ .

Key words: classical Be stars: polarization -- projected rotational velocities -- near IR excesses -- far IR excesses

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