that the polarization of the system was variable. Others soon extended this work, including Appenzeller &

Transcription

1 THE ASTRONOMICAL JOURNAL, 115:1576È1591, 1998 April ( The American Astronomical Society. All rights reserved. Printed in U.S.A. SPECTROPOLARIMETRIC EVIDENCE FOR A BIPOLAR FLOW IN b LYRAE JENNIFER L. HOFFMAN Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 5376; jho man=uwast.astro.wisc.edu AND KENNETH H. NORDSIECK AND GEOFFREY K. FOX Space Astronomy Laboratory, University of Wisconsin, 115 University Avenue, Madison, WI 5376; khn=sal.wisc.edu Received 1997 July 8; revised 1997 December 22 ABSTRACT We present ultraviolet and visual spectropolarimetry of the interacting binary star b Lyrae, obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment and the HPOL spectropolarimeter at Pine Blu Observatory. Our observations span 3 years and cover the wavelength range from 14 to 15 Ó, with a resolution of 7.5È16 Ó. Detailed broad- and narrowband spectropolarimetric analysis allows us to begin to decompose the complex spectrum of b Lyr: by examining the polarization behavior of a line or continuum, we can determine which component scatters the light and, ultimately, from which component that light originates. After removing interstellar polarization from our data and rotating the results to the apparent intrinsic position angle of the system, we Ðnd that the polarization of the hydrogen Balmer and vacuum ultraviolet UV bump ÏÏ emission lines, as well as that of the near-uv continuum, remains mostly constant with phase and is oriented at 9 to the visible polarization, indicating that the scattering plane of the light in these three spectral components is perpendicular to the scattering plane of the visible light. We propose that the UV bump, Balmer emission, and near-uv continuum polarization is produced by electron scattering within a bipolar outñow in the b Lyr system. The intrinsic visible polarization of b Lyr shows eclipses that associate it with material near the accretion disk. We Ðnd conñicting evidence regarding both the origin of this visible light and the scattering surface that polarizes it: continuum evidence points toward the secondary object as the illuminator and the accretion disk edge as the scatterer, while line analysis suggests that light from the loser scatters o material between it and the disk. The presence of material away from the orbital plane may help resolve this contradiction. Key words: binaries: close È binaries: eclipsing È stars: activity È stars: individual (b Lyrae) È techniques: polarimetric 1. INTRODUCTION Though it has been among the most studied celestial objects over the last two centuries, b Lyrae A (also HD , ADS 11745A, and HR 716; hereafter b Lyr ÏÏ) is far from being understood completely. Instead, its unusual structure continues to puzzle astronomers, providing us with a complex set of characteristics against which to test new theories and observational methods. This semidetached eclipsing binary star has a well-established period of 12.9 days, which increases at a rate of 19 s yr~1 (Harmanec & Scholz 1993). Beyond this, however, we know very little for certain about b Lyr. Wilson (1974) places the mass ratio (q \ M /M ) between 4.2 and 6 and Ðnds a corresponding inclination gainer loser i of 85 ; Hubeny & Plavec (1991, hereafter HP) take q \ 5.6 and Ðnd i \ 83. Skulskii (1992) prefers q \ 4.28 with i near 8. According to HP, the primary mass-losing star, or loser,ïï is a B6ÈB8 II star of about 2 M, while its companion, the gainer,ïï is probably a main-sequence _ B V star of 12 M (HP); SkulskiiÏs (1992) analysis yields M \ 12.5 M _ and M \ 3 M. It is now generally agreed loser that a thick _ accretion gainer disk obscures _ the mass-gaining star (Huang 1963; Wilson 1974; HP); we will refer to the gainer and disk together as the secondary object.ïï Recently, Harmanec et al. (1996) also found interferometric and spectroscopic evidence for the existence of an extended jet of gas oriented perpendicular to the orbital plane of the binary. Given these geometric complexities, the b Lyr system is an ideal target for polarimetric observations. Because light 1576 polarized by electron scattering preserves a record of the orientation of its scattering surface, polarimetric analysis of b Lyr can provide important geometric information complementary to that obtained by other methods, such as radial velocity and occultation studies. In the visible wavelength range, the system displays a moderately high degree of polarization, which varies with its orbital period. In addition, it has a complex spectrum consisting of several distinct components; determining in which part of the system speciðc spectral features arise has always been one of the most problematic aspects of studying this object. Spectropolarimetry can distinguish between direct and scattered light, thus making it possible to disentangle such compound spectra. Detailed polarimetry in both broad and narrow bands, then, can help us construct a consistent model of the structure of b Lyr, a model that may be applicable to other disk systems such as cataclysmic variables, dwarf novae, and perhaps even quasars. Polarimetry was used to study b Lyr as early as 1934 (O hman 1934), and, in 1962, Shakhovskoi (1962) Ðrst determined that the polarization of the system was variable. Others soon extended this work, including Appenzeller & Hiltner (1967, hereafter AH), who published UBV polarization curves corrected for interstellar polarization e ects. However, most of these early polarimetric observations measured the total percentage polarization of the light received from the star as a function of orbital phase, without investigating individual line polarization or the variation of intrinsic polarization with wavelength. A few studies fea-

2 b LYRAE 1577 TABLE 1 DATA AND PHASE INFORMATION FOR THE MIDPOINTS OF OUR OBSERVATIONS OF b LYRAE Datea JD [ 2,4, Phaseb PBO Reticon: 1992 Sep , Oct , Oct , Oct , Dec , Jul , Jun , Jun , Jul , Jul , Jul , Sep , Sep , Nov , PBO CCD: 1994 Mar , May , May , May 27c... 49, May , Jun , Jul , Jul , Jul , Jul , Jul , Aug , Aug 14c... 49, Aug , Sep , WUPPE: 1995 Mar , a PBO \ Pine Blu Observatory.9 m telescope with Reticon or CCD detector; WUPPE \ Wisconsin Ultraviolet Photo-Polarimeter Experiment, Astro-2 mission. b Phases were determined using the ephemeris given in Harmanec & Scholz c These observations used the red grating only; see 2. tured narrowband spectropolarimetry (e.g., O hman 1934; McLean 1977), but their analyses were limited by low resolution and insufficient correction for interstellar polarization. This paper aims to complete the picture by presenting the Ðrst high-resolution intrinsic spectropolarimetry of b Lyr. In 2, we present details of our spectropolarimetric observations; 3 describes the reduction and preliminary analysis of our data, including single-phase WUPPE results, phase-dependent continuum and line results, and broadband visible spectrum results. In 4, we attempt to interpret these Ðndings within the context of a consistent geometric model for the structure of b Lyr. Section 5 summarizes our principal conclusions and suggests directions for further research. 2. OBSERVATIONS In this study we have combined polarimetric and spectral measurements of b Lyr spanning 3 years. We obtained our visible wavelength data with the University of WisconsinÏs HPOL spectropolarimeter and the.9 m telescope at Pine Blu Observatory (PBO), near Madison, Wisconsin. Between 1992 September and 1994 November, we made 14 observations using a dual Reticon array detector; the resulting spectrophotometric and spectropolarimetric measurements cover a wavelength range of 32È76 Ó with a resolution of 15 Ó (see Wol, Nordsieck, & Nook 1996 for further instrumental details). For our 15 observations between 1995 March and September, we used the newer CCD-based system, which has an extended wavelength range of 32È15 Ó, with a resolution of 7.5 Ó below 6 Óand 1 Ó above (Nordsieck & Harris 1996). We also observed b Lyr with the Wisconsin Ultraviolet Photo- Polarimeter Experiment (WUPPE) during the Astro-2 mission in 1995 March. This single observation covered the vacuum ultraviolet (VUV) wavelengths from 14 to 33 Ó with a spectral resolution of about 16 Ó. Bjorkman et al. (1993) and Nordsieck et al. (1994) discuss the WUPPE instrumentation in detail. Table 1 shows the civil and Julian dates (minus 2,4,) for the midpoint of each of these observations, along with the corresponding orbital phase of b Lyr; in calculating phases, we have used the quadratic ephemeris given by Harmanec & Scholz (1993): T \ JD 2,48, ] E] ]1~6E2 pri (where E represents the number of orbits since the primary eclipse at JD 2,48, ). Each observation covers the entire spectral range given above, with two indicated exceptions, which used only the red grating (6È15 Ó) of the CCD system. In order to determine the interstellar polarization spectrum in the direction of b Lyr A, we also observed the nearby (45A away) associated star b Lyr B (see Abt et al. 1962) at PBO with the CCD system on 1995 May 21 and 1996 July RESULTS The data obtained at PBO were reduced and analyzed using the REDUCE software package, described by Wol et al. (1996 and references therein); Nordsieck et al. (1994) discuss the reduction and analysis of the WUPPE data. We corrected for interstellar e ects by Ðtting a Serkowski law to the Ðrst (1995 May) observation of b Lyr B using the calibration described by Wilking, Lebofsky, & Rieke (1982), then removing the resulting interstellar polarization (P \.419% ^ 13%; P.A. \ 151.^.9; j \ max max 465 ^ 26 Ó) from the raw data. The later observation of b Lyr B (1996 July) produced values that agreed, within errors, with those above. By comparison, AH took the interstellar polarization to be a weighted mean of the observed polarization measurements of the associated stars b Lyr B, E, and F and found P \.42% ^ 4% and P.A. \ 153.2^3, while McLean (1977), referring to several previous authors, concluded that the best estimate of interstellar polarization was P \.42% ^ 2% with P.A. \ 153.4^1.4 and j \ 51 max Ó. Finally, for b LyrÏs extinction of E(B[V ) \ 4 max mag (Dobias & Plavec 1985), the relation P ¹ 9.E(B[V ) found by Serkowski, Mathewson, & Ford max (1975) predicts a maximum interstellar polarization of.36%. Our adopted values are consistent with all the above estimates and have signiðcantly smaller errors than those of either AH or McLean (1977). This welldetermined interstellar polarization for b Lyr greatly simpliðes our analysis. We note, however, that the behavior of interstellar polarization below 36 Ó is somewhat less well known than that in the visible. To quantify the UV interstellar polarization spectrum along this sight line, we assumed the above Ser-

3 1578 HOFFMAN, NORDSIECK, & FOX Vol. 115 Flux Position Angle % Polarization HeI 5876 Hα 5 1 Wavelength ( ) FIG. 1.ÈUncorrected Ñux (1~1 ergs cm~2 s~1 Ó~1), percentage polarization, and position angle (degrees) spectra for the WUPPE observation (phase.743, 14È3 Ó) and one PBO observation (phase.749, 3È15 Ó). The Stokes parameters producing the polarization and position angle are binned to constant internal errors of 7% (WUPPE), 3% (blue grating, 3È6 Ó), and 2% (red grating, 6È15 Ó). The Ha and He I j5876 emission lines have been identiðed. kowski law also held (by extrapolation) at shorter wavelengths; however, recent analysis of WUPPE observations by Anderson et al. (1996) shows that this assumption is not always a good one, especially for small values of j such max as ours. Since the results of Anderson et al. show that the Serkowski extrapolation seems to yield a lower bound to the UV interstellar polarization, we know that we have, if anything, undercorrected our data at these wavelengths. We estimated the qualitative results of such an underremoval by constructing an artiðcial super-serkowski ÏÏ polarization spectrum (based on Anderson et al.ïs observations of HD 27198) with our adopted values of P and j, subtracting this spectrum from our uncorrected max observations, and comparing the results with the Serkowski law max results. We will address the minor di erences at appropriate points throughout this paper. 3.1 Single-Phase Results Figure 1 shows the data from the WUPPE observation and from a PBO observation of comparable phase (WUPPE,.743; PBO,.749), both uncorrected for interstellar polarization. The top, middle, and bottom panels show the variation of total observed Ñux, percentage polarization, and position angle, respectively, with wavelength. The Ha and He I j5876 lines have been labeled; these lines will be important in our analysis below. Figure 2a shows percentage polarization, position angle, and polarized Ñux (percentage polarization times Ñux) spectra for the same observations as in Figure 1, with Serkowski law interstellar polarization e ects removed (see 3 above). The abrupt 9 change in position angle at 365 Ó (the Balmer jump) indicates that the ultraviolet light is linearly polarized perpendicular to the visible light. The position angle plot also shows that the Ha and He I j5876 lines agree more closely in angle with the near-uv and VUV continua than with the visual continuum. The polarized Ñux spectrum, shown in the bottom panels of Figures 2a and 2b, is obtained by Position Angle % Polarization Polarized Flux HeI 5876 Hα x4 5 1 Wavelength ( ) FIG. 2a Flux % Polarization Polarized Flux MnIII CIV FeII AlII NiIII { FeIII,NiIII FeIII { CrIII,NiII MnII,FeII MgII Wavelength ( ) FIG. 2.È(a) From top: Percentage polarization, position angle (degrees), and polarized Ñux (1~12 ergs cm~2 s~1 Ó~1) spectra for the same observations as in Fig. 1, corrected for interstellar polarization (see 3). In the visible region, position angle data have been wrapped ÏÏ so that they take on low negative, rather than high positive, values. For clarity, the polarized Ñux plot also shows a magniðed spectrum above 4 Ó. Stokes parameters for the WUPPE data (14È3 Ó) are unbinned in percentage polarization and polarized Ñux, but binned to a constant internal error of 45% in the position angle plot. For the visible data, they are binned to 2% for the top two panels; in the polarized Ñux plot, they are binned to 5 ] 1~14 ergs cm~2 s~1 Ó~1 for the blue data (3È6 Ó) and to 2 ] 1~14 ergs cm~2 s~1 Ó~1 for the red data (6È15 Ó). The same lines have been identiðed as in Fig. 1. (b) Flux (1~1 ergs cm~2 s~1 Ó~1), percentage polarization, and polarized Ñux (1~12 ergs cm~2 s~1 Ó~1) spectra for the WUPPE data only, corrected for interstellar polarization (see 3.3). Binning in the polarization and polarized Ñux spectra is the same as in (a). VUV metal lines have been identiðed. FIG. 2b

4 No. 4, 1998 b LYRAE 1579 multiplying the Ñux spectrum of an observation by its percentage polarization; it gives, in essence, a picture of the spectrum seen ÏÏ by the scattering region. Figure 2b presents an expanded view of the WUPPE data, showing total Ñux, percentage polarization, and polarized Ñux spectra for the VUV region, with Serkowski law interstellar polarization removed. With the help of MazzaliÏs IUE line identiðcations (Mazzali 1987), we have labeled VUV emission lines of ionized manganese, carbon, iron, aluminum, nickel, chromium, synthetic Ðlters to the ISM-corrected PBO data: one (V ) covering the Johnson-Cousins V passband (from Bessell 199), to represent the visible component, and one (BJ[) covering the range below the Balmer jump from 32 to 365 Ó, to represent the near-uv component. Tables 2 and 3 show our numerical results. Our synthetic Ðlter polarization routine produced values and associated internal errors for each observation; however, in comparing data from many di erent observations, we must also take into account and magnesium, which make up the UV bump,ïï or the systematic variation in instrumental polarization 2 Ó bulge ÏÏ: excess ultraviolet Ñux in the spectrum of b Lyr (Mazzali 1987; HP). These lines also appear strongly in the polarized Ñux spectrum. Removal of super-serkowski interstellar polarization from the WUPPE data increases the level of the VUV continuum in both polarization and polarized Ñux but does not change either the emission lines or the position angle spectrum. Figures 1 and 2 show that the continuum below 365 Ó, the lines of the UV bump, and the Ha and He I j5876 lines are all polarized at near 9 from the visible polarization, indicating that these spectral features are physically associated. We suggest that the light in these features is scattered by a component of the system geometrically distinct from, and oriented perpendicular to, the component that scatters the visible light. 3.2 Phase-dependent Continuum Results To investigate the variation of continuum polarization with orbital phase, we obtained intrinsic Stokes parameters between nights. Systematic errors are periodically evaluated by comparing observations of unpolarized standard stars; the errors thus derived are included in Tables 2 and 3. The error bars for each point in Figures 3 and 4 represent the larger of the internal and systematic errors of that observation. Figure 6 below (see 3.3) depicts only internal errors, since the systematic e ects cancel out in our line polarization analysis. The binning in Figures 1, 2, 5 ( 3.3), and 7 ( 3.4) also reñects internal errors only. Figure 3 compares our synthetic Ðlter results with the normalized Fourier Ðt to the V -band light curve of b Lyr calculated by Harmanec et al. (1996). As is evident from Figure 3a, we found that while the BJ[ position angle varies widely over the cycle, the position angle for the V results remains roughly constant with phase at a mean value of 163.5^.18. We interpret this angle to represent the intrinsic physical axis of the binary system. To check this interpretation, we also calculated a Balmer jump Ðlter index for both the combined CCD and combined Reticon for the spectral regions of interest by applying two spectra; this quantity measures the vector di erence TABLE 2 INTRINSIC POLARIZATION VALUES FOR THE HPOL SYNTHETIC V FILTER Internal Systematic Projected Projected Error Error P.A. P.A. Error Phase %Q %U %Q a P %U a P (%) (%) %P (deg) (deg) Reticon: [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ CCD: [ [ [ [ [ [ b... [628 [924 [43 [ [ b [ [ [ [ [ [ [ [ [ a Projected ÏÏ quantities have been rotated to 164 ; see 3.2. b These observations used the red grating only; see 2.

5 158 HOFFMAN, NORDSIECK, & FOX Vol. 115 TABLE 3 INTRINSIC POLARIZATION VALUES FOR THE HPOL SYNTHETIC BJ [ FILTER (32È365 Ó) AND THE SINGLE WUPPE OBSERVATION (21È275 Ó) Internal Systematic Projected Projected Error Error P.A. P.A. Error Phase %Q %U %Q a P %U a P (%) (%) %P (deg) (deg) Reticon:... [.2219 [72 [.151 [ [.1141 [446 [731 [ [742 [ [ [.1757 [3 [.1331 [ [ [ [ [.181 [ [ [.1782 [ [ [ [ [ [ [ [.183 [ [.2512 [392 [.1923 [ CCD: [.3639 [.1266 [.2415 [ [ [ [ [.2188 [ [ [.1978 [ [ [ [ [ [ [ [ [ [841 [137 [641 [ [ [756 [ [ [ [ [ WUPPE: [52.16 [ a Projected ÏÏ quantities have been rotated to 164 ; see 3.2. between polarization above and below the Balmer jump and is independent of any interstellar polarization. We found the position angle of the Balmer jump index to be 163.8^.37 for the Reticon spectra and 166.2^.54 for the CCD spectra. Taking the error-weighted mean of the three values, we found that the system axis lies at a P.A. of 163.8^.15. In order to orient our data with respect to this axis, we rotated all our polarization results to 164 and present in this paper the resulting projected Stokes parameter %Q, which we indicate by %Q. This quantity is convenient because it can be either positive P or negative, while total %P is necessarily always positive. The projection causes data points with a P.A. near 164 to have positive %Q, while negative %Q values indicate a P.A. near 74 (perpendicular P to the positive P data points). Hereafter, unless otherwise speciðed, the phrase projected polarization ÏÏ will refer to this quantity %Q, and the terms positive ÏÏ and negative ÏÏ will indicate the P above position angles. In Figure 3b, we have plotted projected polarization versus phase for our observations. In the V Ðlter (middle), the polarization shows signiðcant phase-dependent variations: an increase at primary eclipse and a decrease at secondary eclipse. However, it never drops below %, since the position angle in the visible remains near 164 throughout the cycle. In the BJ[ Ðlter (bottom), the polarization shows no such clear variation with phase, but mostly scatters between % and [.25%. The projected polarization of the VUV continuum, determined from the WUPPE observation with a Ðlter covering the wavelength range from 21 to 275 Ó, is shown both as a point (triangle) at phase.743 and as a heavy horizontal line at [.1% for comparison with the BJ[ CCD data. A super-serkowski interstellar polarization spectrum causes the projected polarization of the VUV continuum to be somewhat more negative, and to have a position angle closer to 74. Figure 3 also displays a marked asymmetry around secondary eclipse in the polarization curves for both Ðlters. In each case, the mean %Q at phase.75 is signiðcantly more positive than at phase.25. P In the BJ[ Ðlter, the projected polarization becomes positive in this part of the curve, an e ect called a polarization reversal,ïï which indicates a near-9 change in position angle between the secondary and primary eclipses. This is shown more clearly by the BJ[ position angle plot in Figure 3a: between the eclipses, three points lie near the line denoting a P.A. of 164, and two more fall close to (equivalent to 18 ). When we took the 3.3 km~1 (similar to BJ[) data of Coyne (197), recalculated the phases using the Harmanec & Scholz (1993) ephemeris, and applied the 164 rotation, we found that his results also show this polarization reversal after secondary eclipse. We also performed the phase correction and 164 rotation for the results of AH; the resulting projected polarization curve is represented by the envelope shown in the middle panel of Figure 4. Our V data points, plotted in the same panel, vary with phase in good agreement with the AH results. The bottom panel of Figure 4 shows V -band polarized

6 No. 4, 1998 b LYRAE 1581 PA Flux V Flux %Q P V 1 BJ- BJ PA 1 5 %Q P Phase Phase FIG. 3a FIG. 3b FIG. 3.È(a) Position angle in degrees and (b) projected polarization (%Q ) vs. phase for synthetic V and belowèbalmer jump Ðlters applied to PBO observations, for both the Reticon and CCD detectors. The top panel in each case P shows the Fourier Ðtted V -band light curve of Harmanec et al. (1996), in normalized Ñux units. All data have been wrapped in phase so that more than one complete period is shown. Error bars are shown when larger than the symbol; they represent the larger of the internal and systematic errors. In (a), the light solid lines represent a P.A. of 164, while the heavy solid line indicates 74. In (b), the horizontal dashed lines indicate a %Q of zero; the heavy solid line represents the mean %Q of the ultraviolet continuum, as determined from P P our WUPPE observation (see 3.2). Vertical dashed lines mark eclipses. In both (a) and (b), open symbols represent Reticon data, Ðlled symbols represent CCD data, and triangles represent WUPPE data. Observations for which only the red CCD grating was used (see Table 1 and 2) have been excluded. Ñux versus phase for our observations; we obtained this quantity by multiplying the %Q value for each point by the relative Ñux at the appropriate P phase (calculated from the Fourier Ðtted light curve of Harmanec et al. 1996, which is shown in the top panel). We note that although the secondary eclipse appears in the polarized Ñux plot, the primary eclipse does not. An asymmetry between phases.25 and.75 is easily visible in this Ðgure: the polarized Ñux is signiðcantly lower before secondary eclipse than after. Our continuum results indicate that the scattering material that gives rise to the BJ[ continuum polarization is not signiðcantly eclipsed, since the BJ[ projected polarization shows no eclipse e ects. The visual polarization, by contrast, shows a decrease at secondary eclipse, which suggests that the material scattering the visual light is associated with the secondary, fainter component of the binary. We also observe signiðcant asymmetries between phases.25 and.75 in the polarization curves of both Ðlters. The lack of a primary eclipse in the visual polarized Ñux seems to indicate that the scattered V -continuum light originates from the secondary object. This conclusion, however, contradicts an earlier Ðnding by Kruszewski (1974), as well as our own spectropolarimetric evidence ( 3.3); see 4.1 for a full discussion. 3.3 Phase-dependent L ine Results In Figure 5, we present combined Ñux, polarization (both total %P and projected %Q ), and projected polarized Ñux spectra of b Lyr, which we obtained P with an error-weighted vector sum of all our CCD observations. We have identiðed spectral lines of many di erent species, some of which display signiðcant polarization e ects and may warrant further study, particularly, the Ca II H and K lines and those of various other metals. Other lines of interest will be

7 1582 HOFFMAN, NORDSIECK, & FOX Flux %Q P Projected Polarized Flux Phase FIG. 4.ÈTop: Harmanec et al. (1996) Fourier Ðtted V -band light curve, in normalized Ñux units. Middle: Projected polarization (%Q ) vs. phase for the V points in Fig. 3a, and for the envelope of Appenzeller P & HiltnerÏs (1967) data points, represented by the solid lines. See 3.2 for details of the comparison. Error bars, shown in Fig. 3a, have been left out here for clarity. Bottom: Projected polarized Ñux, i.e., projected polarization multiplied by the normalized Harmanec light curve, vs. phase for the V points in Fig. 3a. Error bars arise from the errors in %Q only. All symbols and lines P have the same meanings as in Fig. 3, and all data have been similarly wrapped in phase. those that appear in the Ñux spectrum but not in the polarized Ñux spectrum, since these lines are unpolarized and thus most likely unscattered. The Ha and He I j5876 lines are prominent in this Ðgure, and we note that each shows two components, oppositely polarized (red wing positive for Ha, negative for He I j5876); this suggests that each line has a complex proðle, formed by multiple contributions from various components of the system. We also note that radial velocity studies (e.g., Sahade et al. 1959; Flora & Hack 1975) have shown the spectral lines identiðed in Figure 5 to arise from the mass-losing star, and that most of these lines also appear in the polarized Ñux spectrum. This factèthat the polarized Ñux has the spectrum of the loserèis an important result, which seems to indicate, in contradiction to the results of 3.2, that the loser is the source of the light that becomes polarized by scattering. We discuss the implications of this discrepancy in 4.1. In order to compare the behavior of the spectral lines of b Lyr with that of the optical and BJ[ continua, we Ðrst studied the polarization of the two most prominent spectral features in the optical regionèthe emission lines Ha and He I j5876èas a function of phase. We determined line polarization by the Ñux equivalent width method: For each line, we chose two wavelengths deðning the boundaries of the line; we also speciðed two continuum samples, one on either side of the line. We then calculated the projected Stokes parameters for the line alone using the equation Q P,line (j) \ / *j Q P,tot (j)f tot (j)dj [ SQ P,cont (j)f cont (j)t*j / *j F cont (j)dj [ SF cont (j)t*j where F (j) and Q (j) are the total Ñux and total %Q observed tot within the P,tot chosen line extent *j, and where the P average continuum Ñux F (j) and average continuum projected polarization Q cont (j) were found by inter- P,cont polation between the continuum samples (a similar equation holds for %U ). Table 4 shows the wavelengths we P used to deðne each spectral line and continuum sample and details the results. We have not included line polarization results for the Reticon observations, because these displayed large errors. Also, the measurements of the helium line are somewhat less reliable than those of Ha, because He I j5876 lies very close to the wavelength limit of our blue grating and because the interstellar sodium doublet falls within the adopted line boundaries. Figure 6 shows position angle (Fig. 6a) and %Q (Fig. 6b) P versus phase for these two lines. The Ha line is negatively polarized at near [.1% and shows little variation with phase; its behavior is similar to that of the BJ[ continuum shown in Figure 3, though it does not show a polarization reversal near phase.75. In position angle, the Ha line also shows little systematic variation, scattering fairly evenly around 74. The helium line, however, shows dramatic variations with phase in both %Q and (to a lesser extent) P position angle, showing a large change in each, especially around phase.45. This does not agree well with either of our two continuum polarization curves. Evidence for asymmetry around secondary eclipse is questionable for either line because of the size of the error bars. We have also included two points (triangles) in each of Figures 6a and 6b, representing the position angles and projected polarization values of the C IV j1548/j1551 and Mg II j2796/j283 lines in the WUPPE ultraviolet spectrum. These values, included in Table 4, have also been obtained by the Ñux equivalent width method detailed above, though the lack of a clear continuum in the VUV spectrum casts doubt on their reliability. Nevertheless, it is evident that while the %Q of these lines is much more negative than that of either Ha P or He I j5876, their position angles are comparable to both. A super-serkowski interstellar polarization spectrum does not change these line results signiðcantly. We next compared Ha and He I j5876 with other hydrogen and helium emission lines by dividing the PBO CCD data into four phase bins: primary eclipse (phases.9è1.1); after primary eclipse (phases.1è.4); secondary eclipse (phases.4è.6); and after secondary eclipse (phases.6è.9). For each bin, the corresponding Ñux and polarization spectra were combined using error-weighted vector addi-,

8 { HI CaII HeI Hδ FeII { Hγ HeI FeII { { HeI Hβ HeI { % Polarization Flux Projected Polarization (%Q P ) Projected Polarized Flux NII HeI NaI Wavelength ( ) FIG. 5a Hα HeI HeI { OI HI { HI % Polarization Flux Projected Polarization (%Q P ) Projected Polarized Flux Wavelength ( ) FIG. 5b FIG. 5.È(a) From top: Flux (1~1 ergs cm~2 s~1 Ó~1), total percentage polarization, projected polarization (%Q ), and projected polarized Ñux (%Q multiplied by Ñux, 1~13 ergs cm~2 s~1 Ó~1) spectra for all our blue (3È6 Ó) CCD observations, combined using P error-weighted vector addition. P Stokes parameters for the bottom three spectra are binned to constant internal errors of 5%, 5%, and 8 ] 1~14 ergs cm~2 s~1 Ó~1, respectively. Several spectral lines have been identiðed. Horizontal lines indicate the zero points of projected polarization and projected polarized Ñux. (b) Same as (a), but for the red end of the spectrum (6È15 Ó). Stokes parameters for the percentage polarization, projected polarization, and projected polarized Ñux are binned to constant internal errors of 4%, 4%, and 2 ] 1~14 ergs cm~2 s~1 Ó~1, respectively. The circled plus sign indicates a geocoronal line.

9 1584 HOFFMAN, NORDSIECK, & FOX Vol. 115 TABLE 4 POLARIZATION VALUES FOR THE CCD Ha AND He I j5876 AND THE WUPPE C IV j1548/j1551 AND Mg II j2796/j283 SPECTRAL LINES Projected Projected Error P.A. P.A. Error Phase %Q %U %Q a P %U a P (%) %P (deg) (deg) Ha (63È61, 6522È6615, 6719È6828 Ó)b [31 74 [ [ [.2139 [ [ [.1661 [ [ [ [31 [1 [258 [ [77 22 [77 [ [ [ [ [.1345 [ [ [ [.35 6 [.294 [ [ [ [ [.1233 [ [ [ [.46 [.127 [.277 [ [ [.1569 [ He I j5876 (545È55, 5854È5891, 6È61 Ó)b [ [ [.7689 [ [ [ [ [ [ [ [ [ [ [73 81 [ [ [ [ [ [ [ C IV j1548/j1551 (155È153, 154È156, 157È16 Ó)b [ [1.884 [ Mg II j2796/j283 (25È27, 279È2815, 29È31 Ó)b [ [ [ a Projected ÏÏ quantities have been rotated to 164 ; see 3.2. b Red continuum, line, and blue continuum boundaries, respectively. These quantities were used in the Ñux equivalent width line analysis; see 3.3. tion; we then performed the Ñux equivalent width calculation on each of these combined spectra for the lines Hb, He I j6679, and He I j765, along with Ha and He I j5876. (Other hydrogen and helium lines are visible, but the signalto-noise ratio in the combined spectrum prevented meaningful measurements for all but these, which are the strongest lines. Note [Fig. 5] that these helium lines appear to be pure emission lines.) The results for the helium lines are tabulated in Table 5 and show that while He I j5876 and j765 have negative projected polarization and behave similarly with phase, He I j6679 has a positive %Q. This suggests that all the helium emission cannot arise from P the same region of the system. In considering the behavior of the hydrogen line polarization, we noted that, for the dispersion of our observations, the higher Balmer lines are in absorption, as shown in Figure 5; this suggests that the Ha and Hb emission lines are superposed on signiðcant absorption lines. Based on the fact that all the Balmer lines appear in absorption in the polarized Ñux spectrum, which we have seen to be associated with the spectrum of the mass-losing star, we conclude that the underlying Balmer absorption lines arise from the loser as well. If these absorption lines are oppositely polarized to their corresponding emission lines, they can cause polarization measurements of the emission lines to be too negative. We therefore corrected for the presence of the underlying Balmer absorption lines by treating them as part of the polarization continuum. With this adjustment, the equation above becomes Q (j) P,line \ / *j Q P,tot (j)f tot (j)dj [ SQ P,cont (j)f cont (j)t(*j ] EW abs ), / F (j)dj [ SF (j)t(*j ] EW ) *j cont cont abs where EW represents the equivalent width of the absorption line. We abs assumed that any helium absorption lines would be so weak as to be negligible, but performed this calculation for a range of absorption equivalent widths for Ha and Hb, looking for combinations that would yield the standard 3:1 emission line strength ratio. We found that an Ha absorption equivalent width of 8 ^ 2 Ó and an Hb equivalent width of 6 ^ 1 Ó produced the correct ratio for

10 No. 4, 1998 b LYRAE 1585 Flux.6 Flux Hα (uncorrected).6.4 Hα (uncorrected) 1.2 PA 75 %Q P HeI HeI PA 5 25 %Q P Phase Phase FIG. 6a FIG. 6b FIG. 6.È(a) Position angle in degrees and (b) projected polarization (%Q ) vs. phase for the Ha and He I j5876 lines (PBO CCD data only), again compared with the Fourier Ðtted V -band light curve of Harmanec et al. (1996), P in normalized Ñux units. Ha data have not been corrected for underlying absorption e ects (see 3.3). Solid and dashed lines have the same meanings as in Fig. 3, and all data have been similarly wrapped in phase. Error bars represent internal errors (see 3.2); a few points with very large error bars have been excluded. Triangles represent the two WUPPE lines we measured: C IV j1548/j1551 and Mg II j2796/j283. the combined spectra, while also causing both lines to match the BJ[ and VUV continua by showing a projected polarization near [.1% over the entire phase cycle. Table 6 details the results for hydrogen, both with and without consideration of underlying absorption lines. The equivalent widths adopted above for the Ha and Hb absorption lines are consistent with the model atmosphere predictions of Kurucz, Peytremann, & Avrett (1974) and Kurucz (1979) for T B 12,È13, K and log g B 2.5È 3.5, representative parameters for a B6 II primary star; this agreement supports the initial assumption that the Balmer absorption lines, and, by extension, the rest of the visual polarized light, arise from the mass-losing primary star. We note a few limitations to this analysis: First, the T and log g values for the primary are not well known. Second, since the primary has been observed to be helium enriched, a result of having lost its outer layers to its companion (Balachandran et al. 1986; Dimitrov 1987), it is likely that the actual Balmer absorption line widths in its spectrum are greater than those calculated by Kurucz, who assumed solar abundances. Finally, the fact that the primary Ðlls its Roche lobe and is thus distorted from spherical may also a ect line widths, especially as a function of phase. However, despite these uncertainties, the general conclusion of the absorption equivalent width analysis agrees with the results of radial velocity studies, which show that the only Balmer absorption in the b Lyr spectrum arises from the primary. Together, the two pieces of evidence suggest that this star, not the secondary object as indicated by our polarized Ñux phase curve ( 3.2), is the source of the V -continuum polarized light. We discuss this contradiction at length in 4.1 below. 3.4 Full-Spectrum Results We also used the four phase bin method to investigate the variation with phase of the entire polarization spectrum.

11 1586 HOFFMAN, NORDSIECK, & FOX Vol. 115 TABLE 5 INTRINSIC POLARIZATION VALUES AND EQUIVALENT WIDTHS, BINNED BY PHASE, FOR SELECTED CCD HELIUM EMISSION LINES Projected Projected Error EW Bina %Q b P %U b P (%) %P (Ó) He I j5876 (545È55, 5854È5891, 6È61 Ó)c PE... [ APE... [ SE ASE... [ All... [ He I j6679 (63È64, 6656È6715, 6719È6828 Ó)c PE [ APE SE [ ASE [ All [ He I j765 (6719È6828, 729È719, 7372È751 Ó)c PE... [ APE... [.16 [ SE... [.178 [ ASE... [.267 [ All... [.253 [ a PE (primary eclipse): phases.9è1.1; APE (after primary eclipse): phases.1è.4; SE (secondary eclipse): phases.4è.6; ASE (after secondary eclipse): phases.6è.9. All ÏÏ indicates the error-weighted sum of all observations. b Projected ÏÏ quantities have been rotated to 164 ; see 3.2. c Red continuum, line, and blue continuum boundaries, respectively. These quantities were used in the Ñux equivalent width line analysis; see 3.3. For each phase bin, we took the combined polarization spectrum and rotated it to a position angle of 164 ; the results for each bin are plotted in Figure 7a. We then multiplied each spectrum in Figure 7a by the corresponding combined Ñux spectrum to obtain a projected polarized Ñux spectrum for each phase bin; these are plotted in Figure 7b. The asymmetry around secondary eclipse ( 3.2) is again apparent, in that the integrated polarized Ñux is lowest during secondary eclipse and highest just after it. The Ha and He I j5876 lines are easily identiðable, and we see again in Figure 7b that Ha remains roughly constant while He I j5876 varies substantially between phases. 4. DISCUSSION 4.1 Geometric Analysis We Ðrst attempt to identify the origin of b LyrÏs polarization below the Balmer jump (BJ[), by noting the lack of eclipses in the BJ[ polarization curves (Fig. 3b); this fact tells us that a large fraction of the near-uvèscattering material is visible from Earth at all phases and, therefore, cannot lie in the binaryïs nearly edge-on orbital plane. We interpret this result as signifying the presence of a fourth component in the b Lyr system: a wind or Ñow that sends ionized material away from the accretion disk. This geometric model was proposed by Harmanec (1992) and further supported by the Ðndings of Harmanec et al. (1996); our results corroborate their conclusions. From the close agreement in position angle between the BJ[ and VUV radiation, we infer that both components arise from the hidden mass-gaining star and scatter o the polar Ñow (see Fig. 8). What, then, is the origin of the visible continuum polarization in b Lyr? Figures 2a, 3a, and 3b all indicate that the average position angles of the V and BJ[ continuum polarization are 9 apart. Since the linear polarization produced by electron scattering is perpendicular to the scattering plane, the visible polarization we observe must originate in a plane at right angles to that in which the BJ[ polarization originates. If, as discussed above, the BJ[ light scatters o a polar Ñow extending perpendicular to the orbital plane, the material that scatters the visible light must Projected Polarization (%Q P ) Wavelength ( ) FIG. 7a Primary Eclipse After Primary Secondary Eclipse After Secondary Projected Polarized Flux Wavelength ( ) FIG. 7b Primary Eclipse After Primary Secondary Eclipse After Secondary FIG. 7.È(a) Projected polarization (%Q ) vs. wavelength for all PBO CCD data in each of four phase bins (see 3.3 for phase binning details). Stokes parameters have been binned to constant internal P errors of ( from top) 36%, 32%, 28%, and 25% for the blue data (3È6 Ó), and 36%, 28%, 25%, and 2% for the red data (6È15 Ó). The dashed lines indicate % projected polarization. For line identiðcations, see Fig. 5. (b) Projected polarized Ñux (1~12 ergs cm~2 s~1 Ó~1) vs. wavelength for all PBO CCD data in each of four phase bins (see 3.3 for details). Stokes parameters have been binned to constant internal errors of ( from top) 6, 8, 6, and9]1~14 ergs cm~2 s~1 Ó~1 for the blue data (3È6Ó) and 2, 3, 3, and 3 ] 1~14 ergs cm~2 s~1 Ó~1 for the red data (6È15 Ó). The dashed lines indicate zero polarized Ñux. For line identiðcations, see Fig. 5.

12 No. 4, 1998 b LYRAE 1587 TABLE 6 INTRINSIC POLARIZATION VALUES AND EQUIVALENT WIDTHS, BINNED BY PHASE AND ALLOWING FOR ABSORPTION-LINE EFFECTS, FOR THE CCD Ha AND Hb EMISSION LINES Projected Projected Error EW Bina %Q b P %U b P (%) %P (Ó) Ha (63È61, 6522È6615, 6719È6828 Ó)c Ó absorption EW: PE... [ APE... [.291 [ SE... [.19 [ ASE... [ All... [ Ó absorption EW: PE... [ APE... [.136 [ SE... [ ASE... [ All... [ Hb (4745È4816, 4848È4875, 4938È4977 Ó)c Ó absorption EW: PE... [.958 [ APE... [ SE... [ ASE... [3.396 [ All... [1.378 [ Ó absorption EW: PE... [95 [ APE... [ SE... [ ASE... [.112 [ All... [.1 [ a PE (primary eclipse): phases.9è1.1; APE (after primary eclipse): phases.1è.4; SE (secondary eclipse): phases.4è.6; ASE (after secondary eclipse): phases.6è.9. All ÏÏ indicates the error-weighted sum of all observations. b Projected ÏÏ quantities have been rotated to 164 ; see 3.2. c Red continuum, line, and blue continuum boundaries, respectively. These quantities were used in the Ñux equivalent width line analysis; see 3.3. extend parallel to the orbital plane of the binary. (It is interesting to note that Coyne [197] did not address the 9 di erence in position angle, though it is present in his data. McLean [1977], comparing CoyneÏs data with his own, did acknowledge the di erence in sign, though he attributed it to polar brightening.) Given this geometric constraint, we attempt to locate both the scatterer of the visible polarized light and the illuminator, or body from which it originates before scattering. We begin by analyzing the projected polarized Ñux phase curve shown in Figure 4. Because the polarized Ñux of an object (in this case, intrinsic projected %Q times Ñux; see 3.1) represents the scattered light from P the object only, with no contribution from unpolarized light, it is often a more useful quantity than the percentage polarization. Figure 4 indicates that the polarized Ñux of b Lyr decreases at secondary eclipse, which tells us that much of the polarization of the V -continuum light must arise either from the secondary object or from material between the primary and the secondary. The absence of a primary eclipse in the polarized Ñux curve implies that the primary eclipse seen in the visible light curve is an eclipse of unpolarized light only. This is most easily explained if the primary starïs light is unpolarized, and if the light being polarized by the secondary object originates mainly from the secondary object itself. Then, if the visible portion of the secondary is an accretion disk, as has been proposed by several authors (Huang 1963; Wilson 1974; HP), the polarized V - continuum light both originates and scatters within the disk, becoming aligned parallel to the disk axis at a P.A. of 164. In this scenario, which we call the disk-disk case,ïï the increase in projected polarization at primary eclipse, shown in Figure 3b, occurs because of the occultation of the unpolarized primary star. However, we also Ðnd evidence supporting the possibility that the primary star, not the accretion disk, is the illuminator. Most convincing is the presence of the higher Balmer lines as strong absorption lines in the polarized Ñux spectrum (Fig. 5). Absorption lines in the polarized Ñux spectrum must arise either from absorption lines in the spectrum of the illuminator or from negatively polarized emission lines elsewhere in the system. If these lines were due to emission, they would be quite weak and thus would not appear very strongly in the polarized Ñux; since they are prominent in the polarized Ñux spectrum, the lines must arise, at least in part, from the absorption lines of the masslosing star. This line of reasoning leads us to investigate the possible e ects of underlying absorption lines from the loser on the polarization of the Ha and Hb emission lines as a function of orbital phase ( 3.3). We Ðnd, for absorption equivalent widths appropriate to the spectral type of the loser, that the Ha and Hb lines behave similarly with phase and display a 3:1 line-strength ratio; these results lend credibility to the scenario in which the loser is the origin of the visible polarized light (but see 3.3 for discussion of the appropriate ÏÏ line widths). Radial velocity studies (Sahade et al. 1959; Flora & Hack 1975) support this interpretation. If the primary is the illuminator, however, it is difficult to explain the lack of a primary eclipse in our polarized Ñux curve (Fig. 4). In order to explain a decrease in polarized Ñux at one eclipse but not at the other, we must assume that the material o which the primaryïs light scatters is distributed nonaxisymmetrically in the system in such a way that the only polarization we see is oriented in the same sense as if it had scattered o the disk edge (i.e., at a P.A. of 164 ). This limits the location of any possible scattering material: it must lie between the primary and secondary objects and be easily visible over the edge of the disk at primary eclipse. The existence of material outside the orbital plane of the system (see above, this section) may provide a way to explain our apparently contradictory results; for example, material near the Roche lobe of the primary star but outside the orbital plane might produce the observed e ects (see Fig. 8b). We designate this scenario the loser-lobe case.ïï Our polarized Ñux curve (Fig. 4) also shows a clear asymmetry around secondary eclipse, which most likely does not appear in KruszewskiÏs (1974) curve because he did not remove interstellar polarization from his data; this indicates the presence of a corresponding asymmetry in the system, which could be caused by a geometrically nonaxisymmetric disk or by the presence of a gas stream or hot spot,ïï the point where the mass stream from the primary impacts the disk. Radiation emitted from this spot would naturally be highly variable as a result of changes in the mass-loss rate, and would disturb ÏÏ the disk just before secondary eclipse (see Fig. 8), causing more scatter in polarization observations at this phase. Observations do in fact show a large scatter here, as seen most easily in Figure 4, where the AH envelope is signiðcantly wider just before secondary eclipse than just after. The green-ðlter polarization curve of Elias et