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

Abstract

Abstract. We present the results of statistical analyses of a sample of 627 Be stars. The parameters of intrinsic polarization (p * ), projected rotational velocity (v sin i), and near IR excesses have been investigated. The values of p * have been estimated for a much larger and more representative sample of Be stars (≈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 * diverge considerably for the different spectral subgroups. In contrast to late spectral types (B5-B9.5), the distribution of p * for B0-B2 stars does not peak at the value p * = 0%. Statistically significant differences in the mean projected rotational velocities (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 vt/vc of the true rotational velocity, vt, to the critical velocity for break-up, vc, 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) ≈ 1 . m 3 for O-B1.5 stars, decreasing to E(V − L) ≈ 0 . 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 * 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 * and far IR excesses at 12 µm is clearly seen. A clear relation between p * and v sin i (as well as between p * and v sin i/vc) is found by the fact that plots of these parameters are bounded by a "triangular" distribution of p * : v sin i, with a decrease of p * towards very small and very large v sin i (and v sin i/vc) 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• < Θ < 40• .

Critical study of the distribution of rotational velocities of Be stars

“…It is thus clear that a significantly larger number of objects would be necessary to draw more precise results on the evolution of the ratio V/V c . However, these distributions confirm the known fact that V/V c becomes globally larger as stars evolve from early MS evolutionary phases to later evolutionary phases, whether or not they are rapid rotators (Yudin 2001;Zorec et al 2004;Cranmer 2005). Nevertheless, Fig.…”

“…It is however important to inquire which statistical tendencies can be brought out from rotation-velocity distributions of stars in different evolutionary time intervals, in particular on the evolution of the ratio V/V c . For early-type stars, few studies have attempted to tackle this issue by putting stars into luminosityclass groups (Balona 1975;Zorec 1986;Zorec et al 1988Zorec et al , 1990Zorec et al , 2004Yudin 2001;Cranmer 2005). When dealing with rapid rotators, however, the MK luminosity classes are apparent stellar properties whose relation with the actual evolutionary characteristics is not straightforward.…”

Kinematics and geometrical study of the Be stars 48 Persei andψPersei with the VEGA/CHARA interferometer

“…With these inclination angles, we obtain a position angle for the disk major axis of 115±20 • and 108±1 • for ψ Per and 48 Per, which again agree with our independent fit within the whole Hα line with the LITpro code. Yudin (2001) obtain a polarization of 0.17% for 48 Per and a position angle for the major axis of 110 • . This position angle is very close to our measurement of 108 • for the major axis position, thus the polarization seems to be parallel to the disk, which is quite unusual for a Be star.…”

Herbig Ae/Be Stars in nearby OB Associations

“…We compared the dereddened J À H and H À K colors of a samples of HAeBe (Hernández et al 2004) and CBe stars (Yudin 2001) in order to estimate the loci of these objects, showing that the HAeBe and the CBe occupy well-defined different regions on this diagram.…”

“…All these objects were corrected by reddening using the A V calculated by Hernández et al (2004) and the mean value of R V adequate for this sample (R V ¼ 5:0). We also plot in Figure 2 the dereddened colors of a sample of CBe from Yudin (2001), distinguishing stars with spectral type B5 or later and spectral type earlier than B5. To get a homogeneous CBe sample, we selected objects with 2MASS photometry and with Hipparcos spectral types taken from the Michigan catalogs (Houk & Cowley 1975;Houk 1978Houk , 1982Houk & Smith-Moore 1988).…”