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Polarimetry of Early Emission Line Stars UvA-DARE (Digital Academic Repository) Polarimetry of Early Emission Line Stars. McDavid, D. Publication date 2001 Link to publication Citation for published version (APA): McDavid, D. (2001). Polarimetry of Early Emission Line Stars. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:28 Sep 2021 Chapterr 4 Multicolorr Polarimetry of Selected Be Stars: 1990-93 3 (originallyy published in 1994 PASP, 106, 949) ABSTRACT T Opticall polarization data in the XJBVRI filter system for eight bright northernn Be stars are presented here as the continuation of a long term mon- itoringg project begun in 1984. There are no strong cases of night-to-night variability,, and the only star showing unmistakable changes in polarization fromfrom year to year over the nine years covered by the program is Pi Aquarii. Evenn though the observed sample spans a wide range in spectral type, v sin i, andd degree of intrinsic polarization, the normalized wavelength dependence off the polarization is surprisingly similar for all of the stars. Analysis of the wavelengthh dependence of the variable polarization of ir Aqr in terms of a simplee equatorial disk model suggests that changes in the circumstellar elec- tronn number density alone may be sufficient to account for the observations, butt it is not clear what real physical mechanism is involved. 1.. Introduction Thiss paper is the third installment in a series reporting on the progress of a long term projectt to annually monitor the broad band optical linear polarization of several bright Bee stars observable from northern latitudes. The purpose of the project is to accumulate aa data base which can be analyzed together with multiwavelength spectroscopy and photometryy by other observers to constrain and test hypotheses about the origin of thee "Be phenomenon": sporadic mass loss by rapidly rotating B stars on unpredictable timee scales ranging from days to decades, which gives rise to a circumstellar envelope producingg emission lines, polarization, and ultraviolet and infrared excesses. The first installmentt of the series (McDavid 1986, hereafter Paper I) tied the observational system 46 6 Chapterr 4 too previous measurements from the literature, and the second installment (McDavid 1990,, hereafter Paper II) reported data from the next four years, with a clear detection off year-to-year variability for Zeta Tauri and Pi Aquarii. Forr the unfamiliar reader, an excellent general summary of the field of Be-star research iss the review by Slettebak (1988). Many of the recent developments can be found inn Balona, Henrichs, & Le Contel (1994), and several background references dealing specificallyy with polarimetry of Be stars are given in Papers I and II. Capps,, Coyne, & Dyck (1973) showed that the intrinsic polarization of Be stars could be explainedd by electron scattering in the circumstellar envelopes or extended atmospheres, withh a wavelength dependence due to absorption and emission by partially ionized hy- drogen.. This model and later refinements (Brown & McLean 1977; Fox 1991), together withh spectroscopic evidence for rapid rotation, still provide the strongest arguments thatt the envelopes must be equatorially flattened in order to give the observed intrinsic polarization,, which can be as large as 2%. Inn practice, polarization is a useful, but problematic, indicator of physical conditions in thee envelopes or extended atmospheres of Be stars, because the models have too many freefree parameters to fit uniquely based on polarimetry alone. For example, Poeckert & Marlboroughh (1978) required the mutual constraints of continuum and Ha polarimetry, high-resolutionn Ha and H/? line profiles, and visible, infrared, and radio continuum flux measurementss to compute a detailed model for the envelope of Gamma Cassiopeiae. Theyy concluded that their solution was lacking in part because the observations were nott simultaneous, pointing out the complication that most of the observable quanitites aree unpredictably variable with time and notoriously difficult to correlate. Whilee most recent work has been based on brief "snapshots" with relatively high spectral andd time resolution, routine polarization monitoring with broad band filters is still useful too maintain the longest possible time base of continuous systematic observations for the studyy of long term variability, which is the most perplexing problem of the Be stars. Tabless containing all of the annual observations reported in this paper, combined with thee data from Papers I and II, will be included in Vol. 3 of the AAS CD-ROM Series. Tablee 1. Program Be Stars Name e HD D HR R V V Spectrall Type vaini vaini (kmm s"1) 77 Cas 5394 4 264 4 2.47 7 B0.55 IVe 230 0 <f>Per <f>Per1051 6 6 496 6 4.07 7 Bl.55 (V:)e-sheU 400 0 488 Per 25940 0 1273 3 4.04 4 B4Ve e 200 0 CTau u 37202 2 1910 0 3.00 0 Bll IVe-shell 220 0 488 Lib 142983 3 5941 1 4.88 8 B3:IV:e-sheU U 400 0 xOph h 148184 4 6118 8 4.42 2 Bl.55 Ve 140 0 7TT Aqr 212571 1 8539 9 4.66 6 Bll IH-IVe 300 0 oo And 217675 5 8762 2 3.62 2 B6III I 260 0 Selectedd Be Stars: 1990-93 47 7 Tablee 2. Filter System Parameters Filterr Effective Wavelength Bandpass (fwhm) (A)) (A) 3650 0 700 0 Bu B u 4400 0 1000 0 V V 5500 0 900 0 R R 6400 0 1500 0 I I 7900 0 1500 0 Tablee 3. 2H Cam (Standard Star) Date(Mo/Yr) ) q/dq q/dq u/du u/du p/dp p/dp 6/dO 6/dO dpi/dOi dpi/dOi Filter(n) ) (%) (%) (%) (%) n n (%)) (°) 01/90 0 U(6) U(6) -1.86/0.10 0 -2.41/0.12 2 3.04/0.08 8 116.2// 1.3 0.07// 0.7 B(6) B(6) -1.99/0.07 7 -2.56/0.05 5 3.24/0.06 6 116.0// 0.5 0.03// 0.3 V(6) V(6) -2.09/0.05 5 -2.74/0.04 4 3.44/0.03 3 116.3// 0.4 0.03// 0.3 R(6) R(6) -1.99/0.08 8 -2.66/0.06 6 3.32/0.07 7 116.6// 0.5 0.03// 0.3 1(6) 1(6) -1.75/0.06 6 -2.31/0.06 6 2.90/0.07 7 116.4// 0.3 0.05// 0.5 12/90 0 c7(4) ) -1.86/0.13 3 -2.39/0.07 7 3.03/0.12 2 116.1// 0.9 0.11// 1.0 B(4) ) -1.92/0.08 8 -2.70/0.09 9 3.32/0.06 6 117.3// 1.0 0.05// 0.4 vUS vUS -2.07/0.11 1 -2.74/0.05 5 3.44/0.10 0 116.5// 0.6 0.04// 0.4 R(4) R(4) -1.92/0.04 4 -2.62/0.05 5 3.25/0.03 3 116.9// 0.5 0.04// 0.3 1(4) 1(4) -1.78/0.08 8 -2.41/0.07 7 3.00/0.08 8 116.8// 0.6 0.06// 0.6 12/91 1 U(5) U(5) -1.75/0.22 2 -2.45/0.07 7 3.02/0.10 0 117.3// 2.0 0.09// 0.9 B(5) ) -1.95/0.06 6 -2.50/0.06 6 3.17/0.07 7 116.0// 0.4 0.04// 0.4 V(5) V(5) -2.14/0.05 5 -2.71/0.12 2 3.46/0.08 8 115.8// 0.8 0.04// 0.4 R(5) R(5) -2.06/0.04 4 -2.64/0.07 7 3.34/0.04 4 116.0// 0.6 0.03// 0.3 7(5) ) -1.66/0.17 7 -2.39/0.11 1 2.91/0.13 3 117.7// 1.6 0.06// 0.6 01/93 3 U(2) U(2) -1.83/0.01 1 -2.40/0.03 3 3.02/0.04 4 116.3// 0.1 0.08// 0.8 B(2) B(2) -2.03/0.06 6 -2.63/0.08 8 3.32/0.03 3 116.1// 0.9 0.04// 0.3 V{2) V{2) -1.99/0.07 7 -2.74/0.05 5 3.39/0.08 8 117.0// 0.2 0.04// 0.4 R(2) R(2) -1.91/0.03 3 -2.74/0.06 6 3.34/0.07 7 117.6// 0.1 0.03// 0.2 1(2) 1(2) -1.81/0.05 5 -2.34/0.02 2 2.96/0.02 2 116.1// 0.5 0.05// 0.5 Note:: Tables containing all of the annual observations reported in this paper, combinedd with the data from Papers I and II, will be included in Vol.
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