The Astrophysical Journal, 507:L75±L78, 1998 November 1 ᭧ 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.

NEAR-SIMULTANEOUS OBSERVATIONS OF DIRECT AND RAMAN-SCATTERED LINES IN THE SYMBIOTIC Z ANDROMEDAE Jennifer J. Birriel Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260; [email protected] Brian R. Espey Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218; [email protected] and Regina E. Schulte-Ladbeck Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260; [email protected] Received 1998 July 14; accepted 1998 September 2; published 1998 September 17

ABSTRACT typically consist of a hot compact star orbiting a cool giant. Overlaid on the continuum from these stars is a nebular spectrum produced by the photoionization of the cool star wind by the hot star. Most of the observed emission lines are readily identi®able with common ions; however, a pair of lines at ll6825 and 7082 remained unidenti®ed until relatively recently. A case has been made that these long-unidenti®ed emission lines result from the Raman scattering of O vi ll1032, 1038 resonance photons. We present near- simultaneous far-UV and optical observations of the symbiotic star Z Andromedae, obtained with the space- borne Hopkins Ultraviolet Telescope and the Mount Hopkins 1.5 m telescope. Our data show the presence of both the O vi ll1032, 1038 resonance doublet and the l6825 and l7082 emission lines and provide strong evidence for the Raman effect in Z And. We show that the unusual line ratio of 7:1 observed for the O vi lines

is due to the effects of interstellar H2 absorption. Correction for this effect results in a line ratio of close to 2:1, consistent with optically thin emission. We derive the ef®ciency for the Raman-scattered O vi lines and compare them with previously reported measurements for RR Telescopii. Subject headings: atomic processes Ð binaries: symbiotic Ð stars: individual (Z Andromedae) Ð ultraviolet: stars

8 Ϫ3 1. INTRODUCTION planetary nebulae,nee10 cm , and hot, withT 20,000 K (MK). ∼ ∼ The spectra of symbiotic stars often exhibit two strong, broad ( 20 AÊ ) emission lines at 6825 and 7082 AÊ (Allen 1980). These emission∼ features are only observed in symbiotic systems that 2. THE HUT FAR-UV SPECTRUM exhibit high-excitation features such as [Ne v] and [Fe vii] HUT is a 0.9 m telescope equipped with a Rowland circle (Allen 1980). For the identity of these lines was uncertain, spectrograph. It was ¯own aboard the Space Shuttle Endeavour and they became known as the ªunidenti®ed lines.º Schmid during the Astro-2 mission between 1995 March 2 and 18. (1989) proposed that the ll6825, 7082 emission lines result HUT obtained ultraviolet spectra in the wavelength region from from the Raman scattering of the O vi resonance doublet 820 to 1840 AÊ at a spectral resolution that varies from 2 to ll1032, 1038. In his model, O vi photons produced near the 4AÊ. The HUT calibration is described in Kruk et al. (1995). hot component are scattered by neutral atoms near The spectrum of Z And in quiescence was obtained on 1995 the cool component of the binary system. The scattering also March 6 (JD 2,449,782.6) or a phase off ϭ 0.39 (MK, eq. results in a broadening of the width of the doublet lines by [1]). A total of 1367 s of observing time was obtained on target 6.7 times, producing two broad, polarized lines in the red ∼ through a 20Љ aperture. An additional exposure of 100 s of region of the spectrum. blank sky data was obtained during the target acquisition pro- Recently, Espey et al. (1995) used far-UV data from the cedure. These data permit us to determine the location and Hopkins Ultraviolet Telescope (HUT) and ground-based optical amount of airglow emission-line contamination during the data to provide the ®rst simultaneous observations of both the Z And observation itself (see § 5). O vi ll1032, 1038 lines and the ll6825, 7082 lines in The spectrum was extracted from the raw data using spe- RR Tel. In this Letter, we present similar near-simultaneous cialized software. Poisson statistical errors were calculated from ultraviolet and optical observations of Z Andromedae. the raw count spectra and propagated through the data reduction Z And is the prototypical symbiotic star with a well-estab- process. Calibration of the raw data was performed in the same lished binary of 759 days (Mikoøajewska & ∼ manner as for the RR Tel spectrum discussed previously (Espey Kenyon 1996; hereafter, MK). It is composed of a M3-M4 III et al. 1995). The signal-to-noise ratio in the continuum of the 5 giant and a hot component of Th 1 10 K (MK). Until recently, calibrated data ranges from 4 per pixel at the ends of the it was unclear whether the hot component is a or spectrum to 10 near Lya where∼ the sensitivity of the instru- an accreting main-sequence star, but the hot component mass ment is highest.∼ derived from a study of the polarimetric of Z And (Schmid & Schild 1997) favors a white dwarf. Based on this identi®- 3. THE CONTINUUM cation, the repeatedly observed 2±3 mag eruptions are most likely the result of thermonuclear runaway on the dwarf's sur- Analysis of both the continuum and emission-line properties face. The circumbinary is fairly dense compared to of Z And was performed using the IRAF/SPECFIT x2 mini- L75 L76 BIRRIEL, ESPEY, & SHULTE-LADBECK Vol. 507 mization routine developed by Kriss (1994). Below 1450 AÊ , TABLE 1 the ultraviolet continuum of Z And was modeled using a black- Strongest Measured Lines Ê body; above 1450 A, a recombination component with Te ϭ Flux 4 2 # 10 K was added to represent the nebular continuum. We (10Ϫ13 ergs cmϪ2 sϪ1) adopt the extinction curve of Cardelli, Clayton, & Mathis Line Observed Corrected a (1989), with an assumed RV equal to 3.1. For continuum ®tting, S vi l937 ...... 2.0 150 we chose regions free of line features. tot He ii l958 ...... 1.0 60 K (4000 ע Our best-®t model was a blackbody of (111,000 N iiitot l991 ...... 2.6 300 mag. These results are in good Ne vi] l1000 ...... 7.9 300 (0.01 ע and E(B Ϫ V) ϭ (0.21 agreement with temperature estimates by MuÈrset et al. (1991) O vitot l1035 ...... 300 10,000 and extinction estimates by MK, who present an extensive He ii l1085 ...... 13 260 Ne v] l1134 ...... 15 220 discussion of extinction values derived from different methods tot Ê C iv l1168 ...... 1.6 21 and ®nally adopt 0.3 mag. The spectrum below 1130 A is C iii l1176 ...... 5.0 63 affected by the presence of interstellar H i and H2. We ®t the S iii/[Mg vi] l1190 ...... 7.5 66 absorption lines by scaling a template consisting of optical [S v] l1199 ...... 3.7 43 N v l1240 ...... 129 1300 depth versus wavelength. Our template is derived from recent tot O v l1371 ...... 8.2 60 theoretical data of H2 rotational transitions of Abgrall et al. Si iv l1397 ...... 40 280 Ϫ1 tot (1993a, 1993b) and broadened to a FWHM of 5 km s , typical O ivtot l1402 ...... 55 380 of cool interstellar gas (Morton 1975; van Dishoeck & Black N iv] l1486 ...... 31 200 l 1986, and references therein). The relative scaling of the tran- C ivtot 1549 ...... 390 2300 sitions comes from the mean relative column densities obtained [Ne v] l1575 ...... 3.4 20 [Ne iv] l1602 ...... 1.3 7.4 from ORFEUS II observations of Galactic stars (Dixon et al. He ii l1640 ...... 270 1500

1998). O iii]tot l1664 ...... 49 270 The theoretical intensities are convolved with the instru- N iii]tot l1749 ...... 15 81 mental resolution (Kruk et al. 1998) before ®tting to spectral [Mg vi]/Si ii l1807 ...... 6.8 38 a regions affected only by H2; once the absorption was obtained, We use E(B Ϫ V) ϭ 0.24 and the CCM extinction Ê its contribution was ®xed and absorption due to H i was de- law. Lines below 1130 A were corrected for H2 ab- termined, again by ®tting regions affected by its absorption sorption (see § 5). Fluxes for strong lines are accurate to 15% or better, and weak lines are accurate to The subscript ªtotº indicates that the ¯ux .15%±30% ע features only. Our best-®t values are log N(H i) ϭ (21.30 Ϫ2 .cm . is the total of all lines within the multiplet (0.05 ע log N(H 2 ) ϭ (20.06 ,(0.09

4. THE EMISSION LINES observation. The discrepancy may result from differences in Emission lines were ®tted as Gaussians relative to the ab- the ®t to the continuum and/or some real variation in the line sorbed and reddened continuum adopted in § 3, including air- intensities, as is evident in the case of RR Tel (Schmid et al. glow subtraction as needed. For ¯ux measurements on closely 1998). The absorption template technique that we used for the spaced multiplet lines, such as O vi ll1032, 1038, it was continuum will not work for the emission lines, since the emis- necessary to ®x the wavelength ratios using the atomic data of sion line pro®les are relatively narrow with respect to the H Morton (1991) and the line widths to that of the strongest 2 features and the details of the absorption are hidden through member of the blend. The ¯uxes of the strongest lines are convolution with the instrumental response. presented in Table 1. Line ¯uxes are good to 15% for strong We calculate the effect of H lines by observing the amount lines and to 15%±30% for weaker lines. ≤ 2 of ¯ux absorbed from a Gaussian pro®le of similar character- We redeter∼mine the extinction toward Z And through use of istics to the far-UV lines. The H template is generated at the He ii lines ll1085, 1640, 4686 from the HUT and optical 2 0.01 AÊ resolution with the column density and the FWHM set spectra. Taking the theoretical case B He ii line ratios from to the values used for the continuum ®t (§ 4). The emission Storey & Hummer (1995) and assuming that the emitting gas lines are modeled as Gaussians with FWHM ϭ 30 km sϪ1, as -mag, con (0.03 ע is optically thin yields E(B Ϫ V) ϭ (0.24 observed for the high-ionization He ii l4686 line (Ivison, Bode, sistent with our continuum ®t. We note that MK ®nd the line & Meaburn 1994). We set the relative velocity of the absorption ratio of l1640/l4686 to be phase dependent, which sheds con- and emission components to be zero, consistent with the small siderable doubt on the applicability of this line ratio as an of the emission lines (MK). The amount of extinction diagnostic. However, our observations were taken at absorption of each far-UV emission line is estimated by inte- a phase when the line ratio is observed to be fairly constant, grating the product of the emission- and absorption-line pro®les and we also ®nd that l1640/l1085 and l1085/l4686 produce across each emission line. By this method, we calculate that the same result. As is the case with many symbiotic stars, the O vi l1032 line suffers only about 4% absorption, while extinction values are notoriously dif®cult to derive; we cor- the l1038 line undergoes a 65% decrease. Correction for these rected all line ¯uxes assuming a value of 0.24 mag. ע values results in an estimate of F(1032)/F(1038) ϭ 2.56 0.38, where the error estimate includes the effects of errors in 5. O vi the ¯ux measurements and H2 determinations. Other lines suf- We expect the ratio of O vi l1032 to l1038 lines to be fering considerable absorption include S vi ll937, 944 (5% between the optically thin and thick values of 2:1 and 1:1, and 9% respectively), N iii ll989.8, 991.5, and 991.6 (13%, respectively. The observed ratio relative to the absorbed con- 98%, and 95%), and the Ne vi] multiplet at 1000 AÊ (between tinuum is closer to 7:1 due to selective absorption by H2. 3% and 67%). These factors have been incorporated into the Schmid et al. (1998) report F(1032)/F(1038) 10 using OR- corrected ¯ux values shown in Table 1. Even before allowance FEUS I data, which is signi®cantly different≈from the HUT is made for interstellar absorption, Figure 1 and Table 1 show No. 1, 1998 LINES IN Z ANDROMEDAE L77

Fig. 1.ÐAirglow-subtracted HUT spectrum of Z And obtained during the Astro-2 mission. Positions of the Lya l1216 and O i l1304 airglow lines are indicated. quite clearly that the O vi doublet is the strongest emission mounted on the 1.5 m telescope on Mount Hopkins, Arizona. feature in the far-UV. The spectrum covers 3800±7500 AÊ at a resolution of 3 AÊ , We estimate the temperature of the Oϩ5-emitting region as- and the ¯ux calibration is believed to be accurate to∼10% suming that the O v l1371 emission results principally from (see MK). the dielectronic recombination of O vi. Using the method of The optical spectrum of Z And displays a strong red con- Nussbaumer & Storey (1984), the ¯uxes of the O v l1371 and tinuum and TiO absorption bands from the cool giant and neb- O vi l1032 lines, and the O vi collisional strengths of Mendoza ular H i, He i, and He ii emissions. Using SPECFIT, the ob- 4 (1983), we determine Te 2.3 # 10 K. served M giant star continuua of Fluks et al. (1994) were ®tted The nebular density in∼the Oϩ5 region can be estimated with to the continuum of the (see Fig. 2). The best-®t Fluks the Ne v] and Ne vi] diganostics of Espey et al. (1996), since model was M4.1 III. The l6825 and l7082 lines were modeled these lines have ionization potentials bracketing that of O vi. assuming Gaussian pro®les with line widths tied together, and Assuming a temperature of T 2 # 10 4 K, the Ne v] di- the wavelength ratio tied to the ratio of the progenitor l1032 e ∼ yields log ne ϭ 9.2± and l1038 lines, scaled by a factor of 6.7 as expected under 0.56 ע agnostic F(1137)/F(1575) ϭ 3.18 9.4 cmϪ3. The Ne vi] diagnostics ratios are less de®nitive: Raman scattering. The extinction-corrected ¯uxes are Ϫ3 Ϫ12 Ϫ12 Ϫ2 ergs cm 10 # (0.50 ע and (2.23 10 # (0.67 ע yields log ne 5.5 cm , (7.32 0.19 ע F(1006.1)/F(999.6) ϭ 0.78 ≈ Ϫ1 ,0.8 ע gives log ne 6 or 10 s , respectively. The ratio F(6825)/F(7082) is 3.3 0.08 ע and F(1010.6)/F(999.6) ϭ 0.20 found by Schmid 0.7 ע cmϪ3. However, the errors on the ¯ux ratios actually≈allow≈the which is consistent with the ratio of 3.6 Ϫ3 entire range from log ne 5 to 10 cm . There are several & Schild (1997). problems with the Ne vi]≈diagnostics: (1) 1006.1 and 1010.6 The scattering ef®ciency for the Raman process is the ratio are blended, (2) the 1010.6 line is extremely weak, and (3) all of the output to the input photon numbers. The above ¯uxes three of the Ne vi] lines undergo signi®cant absorption by H2. for the Raman lines and the corrected O vi ¯uxes, Ϫ3 Ϫ10 ע and F(1038) ϭ (2.80 10 # (0.87 ע We adopt a density of log ne 1 9 cm for the O vi region. The F(1032) ϭ (7.26 derived temperature of the Oϩ5-emitting region is similar to 0.36) # 10Ϫ10 ergs cmϪ2 sϪ1, yield conversion ef®ciencies of that of RR Tel, but the density for Z And is signi®cantly higher. 7% for l1032 and 5% for l1038. These values are similar ∼to those derived for R∼R Tel, 6% and 3%, respectively. Raman scattering is a dipo∼le process∼; hence, we expect the 6. OPTICAL OBSERVATIONS: THE RAMAN EMISSIONS emission lines at 6825 and 7082 AÊ to be polarized. Contem- poraneous polarization measurements are not available, but a Optical observations were provided by MK. The spectrum recent study of Z And by Schmid & Schild (1997) indicates a was obtained 1995 January 31 using the FAST spectrograph for the l6825 line at f ϭ 0.39; the 1% ע polarization of 6% polarization in the l7082 generally follows that of the l6825 line.

7. SUMMARY Combined far-UV and optical data lead us to conclude that Raman scattering of O vi is occurring in the symbiotic star Z And. This is the second system, after RR Tel, for which a strong case can be made that the ªunidenti®edº lines are the result of this process, and as such it lends further support to the occurrence of Raman scattering in some symbiotic stars.

The authors wish to thank Scott Kenyon and Joanna Fig. 2.ÐO vi ll1032, 1038 emissions and the Raman-scattered O vi lines Mikoøajewska for providing the optical data for this study in at 6825 and 7082 AÊ in the visible. The dotted lines indicate the ®ts discussed advance of publication. We also wish to thank Van Dixon for in the text. supplying data in advance of publication and Stephan Mc- L78 BIRRIEL, ESPEY, & SHULTE-LADBECK Vol. 507

Candliss for generating the H2 optical depth templates from NAG8-1049 to the University of Pittsburgh. J. B. acknowledges laboratory data. support through the departmental Research Development Fund. This work was supported by Astro-2 Guest Investigator grant

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