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1 The Astronomical Journal, 125: , 2003 January # The American Astronomical Society. All rights reserved. Printed in U.S.A. HUBBLE SPACE TELESCOPE OBSERVATIONS OF THE OLD NOVA DI LACERTAE 1 Elizabeth Moyer, 2 Edward M. Sion, 3 Paula Szkody, 4 Boris Gänsicke, 5 Steve Howell, 6 and Sumner Starrfield 7 Received 2002 May 5; accepted 2002 October 3 ABSTRACT We have carried out a synthetic spectral analysis of a Hubble Space Telescope Space Telescope Imaging Spectrograph observation of the old nova DI Lacertae (Nova Lacerta 1910). The spectrum, obtained with the E140M disperser, reveals a rising continuum shortward of 1560 Å, a C iv P Cygni profile indicative of wind outflow associated with disk accretion, a deep Ly profile, and strong N v (1238 and 1242 Å) andov (1371 Å) wind/coronal absorption lines. Numerous sharp interstellar resonance lines are also present. A grid of accretion disk models, spanning a wide range of inclinations, accretion rates, and white dwarf masses, was compared with three sets of dereddened data. From the three best fits, we conclude that the most likely parameters characterizing the far-uv spectrum of DI Lac are an inclination below 18, a white dwarf mass between 0.6 and 0.8 M, and an accretion rate between and M yr 1. The scale factors for the three best-fit disk models indicate distances between 2 and 2.5 kpc. The extreme weakness or absence of silicon features in the observed spectrum corresponds to a silicon underabundance of d10 4 times solar. The best-fitting high-gravity photosphere model has log g =8,T eff = 27,000 K, V sin i = 200 km s 1, and a scale factor distance (for a 0.7 M white dwarf) of only 76 pc. Thus, a white dwarf photosphere cannot be contributing appreciably to the far-uv spectrum. Key words: novae, cataclysmic variables stars: individual (DI Lacertae) 1. INTRODUCTION There is a relative scarcity of information about old nova primaries and disks. For example, how high are their accretion rates in the postnova state? How quickly is an accretion disk established following a nova explosion? How hot is the postnova white dwarf and how was it affected by the explosion? These basic questions lack definitive answers and underscore the need for advancing our knowledge of these systems. We have undertaken a study of DI Lac that we will hope will provide some new insights into the characteristics of the white dwarf red dwarf system after a nova explosion. Among the old novae, it is known that both DI Lac and V841 Ophiuchi are the only such systems that have optical and UV spectra resembling the UX UMa type nova-like variables (Warner 1995 and references therein). Emissionline widths are very narrow, indicating a low inclination, as first noted by Kraft (1964) and later confirmed by other studies. A low inclination could explain why these two old novae have the highest optical to X-ray flux ratios. We note, 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS and with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium. 2 Department of Electrical Engineering, Villanova University, Villanova, PA 19085; elizabeth.moyer@villanova.edu. 3 Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085; emsion@ast.vill.edu. 4 Department of Astronomy, University of Washington, Box , Seattle, WA 98195; szkody@astro.washington.edu. 5 Department of Physics and Astronomy, University of Southampton, Hampshire SO17 1BJ, UK; btg@astro.soton.ac.uk. 6 Astrophysics Group, Planetary Science Institute, 620 North Sixth Avenue, Tucson, AZ ; howell@psi.edu. 7 Department of Physics and Astronomy, Arizona State University, P.O. Box , Tempe, AZ 85287; sumner.starrfield@asu.edu. 288 however, that their optical spectra are quite different (Hoard et al. 2000). The optical spectra of DI Lac reveal broad, shallow Balmer and helium absorption lines with superposed weak emission lines, whereas V841 Oph reveals strong single-peaked Balmer, He i, and He ii emission features. DI Lacertae was observed to be a nova in Its UV continuum observed with IUE has not been previously modeled. Comparatively little is known about the UV characteristics of old novae. For example, only a few of the 16 old novae accessible to IUE were studied in detail (Selvelli et al. 1990). Also, little is known about their accreting white dwarfs. There is no known correlation between the time since nova explosion and spectral excitation levels in the remnant shell. Cassatella et al. (1990) have found that with the exception of the old nova T Aur, they could represent the reddening-corrected continuum energy distribution of the old novae by a power-law spectrum of the form F /, where the UV spectral index ranges from 0.9 at high inclinations to 2.8 at low inclination. To identify the optical brightness state and spectral characteristics at the time of the Hubble Space Telescope (HST) observation, we obtained five optical spectra of DI Lac at the Apache Point Observatory (APO) from 10:34 to 11:33 UT on 2000 April 20 within one day of the HST observation. The air mass was quite high, since DI Lac is a southern object, and ranged from 1.85 to 1.55 during the observation. The dual channels of the spectrograph acquire a blue (4172 to 4993 Å) and red spectrum (5792 to 6830 Å) simultaneously at a resolution of 2.5 Å. The five spectra were combined into an average. The blue and red APO spectra are both displayed in Figure 1. Both spectra document that the optical brightness level of the system at the time of the HST observation was normal. This is important since DI Lac has been reported to have optical flare-ups on a 35 day timescale (Honeycutt, Robertson, & Turner 1995; see also Hoard et al. 2000).

2 DI LACERTAE 289 Fig. 1. Flux f (ergs cm 2 s 1 Å 1 ) vs. wavelength (in angstrom units) for the averaged blue and red optical spectra of DI Lac obtained at the Apache Point Observatory from 10:34 to 11:33 UT on 2000 April 20 within one day of the HST observation. The spectrum documents that the optical brightness level of the system was normal. As noted by Hoard et al. (2000), the narrow emission cores present in DI Lac are different from most nova-like variables, which show strong broad emission lines. While the narrowness of emission cores may be due to the low inclination and thus be of disk origin, they may indicate irradiation of the secondary star as argued in Hoard et al. (2000). The broad absorption surrounding the cores indicate optically thick material. The optical spectra have spectral slopes consistent with an optically thick disk. The orbital period of DI Lac is hr (Kraft 1964; R. Webbink 1982, private communication) with an estimated but highly uncertain inclination of 0 to 30. It was at V = +4.6 at maximum and V = at minimum, with t 2 = 20 days and t 3 = 43 days. The rate of decline makes it a fast nova. Its visual absorption was previously estimated to be A v = 0.48 (Kraft 1964). In the sections below, we describe the observations, fit the dereddened HST Space Telescope Imaging Spectrograph (STIS) data with various accretion disk models and high-gravity photosphere models, and summarize our conclusions. 2. HUBBLE SPACE TELESCOPE OBSERVATIONS Our observations were obtained with the Hubble Space Telescope STIS in TIMETAG mode on 2000 April 19 at 02:38:08 pm EST. The instrumental setup involved the STIS FUV-MAMA detector and utilized the E140M disperser and 0>2 0>2 aperture. This yielded a nominal resolution of 90,000, a center wavelength of 1425 Å, and a wavelength range of 1140 to 1735 Å. The total exposure time was 2127 s. The spectrum is dominated by numerous absorption lines. Among these are a deep Ly absorption, an absorption trough on the longward wing of Ly (N v, Å), sharp features at Si ii (1260 Å) and C ii (1335 Å), a broad feature at 1370 Å (probably O v), a broad and somewhat shallow feature at 1500 Å, a sharp Si ii ( Å) feature, a sharp feature at 1608 Å (likely Fe ii), and a prominent C iv ( Å) P Cygni profile indicative of wind outflow, presumably from the accretion disk (see below). The presence of weak Si iv resonance absorption ( Å) is unlikely because the 1402 Å doublet component is absent. There is a relatively weak emission line at He ii (1640 Å). Table 1 gives the line identifications, observed wavelengths, rest wavelengths, and likely formation region, either the interstellar medium (ISM), nova shell (CSM shell), accretion disk (disk), or wind. We compared the details of this STIS spectrum with two earlier IUE spectra, one obtained in 1986 September 28 (SWP29325), and the second in 1993 August 2 (SWP48286). The flux levels of the two IUE spectra are nearly identical, but the relatively poor signal-to-noise ratio of both spectra preclude the positive identification of all but the strongest absorption features because of Ly, Si iv (1393, 1402), and C iv (1548, 1550). The continuum flux levels of the IUE spectra are roughly the same as in our HST spectrum. The STIS spectrum contains, as seen in Table 1, a rich line spectrum at much higher signal-to-noise ratio and higher resolution than the IUE spectra. There are 21 spectral lines clearly identified, including interstellar, disk, and wind features. The C iv P Cygni resonance absorption seen in the STIS spectrum is appreciably more pronounced than in the IUE spectra. P Cygni profiles such as the C iv feature in DI Lac are seen in numerous dwarf novae during outburst and in nova-like variables during their high states when their accretion disks are very luminous (Heap et al. 1978; Cordova & Mason 1982; Greenstein & Oke 1982; Sion 1985). The blueshifted P Cygni absorption is formed by outflowing gas viewed against the disk and absorbing disk continuum photons while the emission part of the profile is due to photons scattered into the line of sight but not viewed against the background of the disk. The fact that the C iv P Cygni profile in the STIS spectrum is considerably stronger than the same feature in DI Lac observed with the IUE is likely indicating that the wind outflow from DI Lac may be variable. 3. SYNTHETIC SPECTRAL FITTING Interpreting the far-uv spectrum of DI Lac in terms of a steady state, optically thick accretion disk is a reasonable first step, since there is some evidence that an accretion disk is reestablished very quickly after the outburst of a classical nova (Warner 1995 and references therein). The presence of a disk in DI Lac is indicated by the slope of the far-uv continuum and by the presence of a strong P Cygni structure at C iv (1548 and 1550 Å), indicative of pronounced wind outflow from a disk (see above). The accretion disk fits of the HST spectrum were completed using the extensive model disk grid of Wade & Hubeny (1998). Selection of trial disk parameters was governed by what is known about the white dwarf mass, M wd, and the orbital inclination, i. Ideally, with these two parameters reliably known, the accretion rate in quiescence should follow directly from the spectral slope of the best-fit disk model. This assumes that the disk is optically thick, as required by the disk models and as evidenced by the absorption troughs visible in the optical spectrum. Unfortunately, it is relatively rare that both M wd and i are known with sufficient accuracy. Furthermore, the far-uv luminosity alone does not necessarily measure the true disk luminosity. Before comparison with the Wade & Hubeny disk models, the HST STIS data were dereddened using Seaton s galactic average extinction (1979). The past reddening

3 290 MOYER ET AL. Vol. 125 TABLE 1 Flux-calibrated Data for DI Lac Identification Observed (Å) Rest (Å) Formation Region Comments C iii CSM Very sharp and strong Si ii ISM N i ISM N I ISM Ly Disk, wind, CSM Deep N v Wind Very broad (12 Å wide) N v S ii ISM S ii ISM S ii ISM Si ii ISM O i ISM Si ii ISM Ni ii ISM C ii ISM C ii ISM Ni ii ISM O v Disk, wind O v... Small and broad (8 Å wide) O v Si iv Disk, wind Si iv Disk, wind Si iv a Si ii ISM C iv P Cygni wind Fe ii ISM He ii e Disk estimates for DI Lac range over E(B V ) = We adopted E(B V ) values of 0.1, 0.3, and 0.5 to cover this range. Accretion disk fits were completed for the entire range of five accretion rates for each of the five different M wd values and six values of inclination in the disk model grid, a total of 150 accretion disk model fits. Each disk model was compared with the dereddened data with E(B V ) = 0.5. In each case, an 18 inclination always generated the best fit, as verified by 2 values. This trend continued with preliminary fits using E(B V ) = 0.1 and 0.3. Thus, further trials were restricted to 18. Of these extensive trials, we selected the three accretion disk models that provided the best overall agreement with the continuum slope and the Ly absorption profile. All models with M wd of 0.35 or less were discarded, as a mass this low is not normally associated with the energetics of a classical nova outburst. The best fitting of the three models is displayed in Figure 2. The STIS observation is fitted with an accretion disk model corresponding to M wd = 0.55 M, _M = M yr 1, i =18, and E(B V ) = 0.3. The scale factor for this fit was S = , which yields a distance of 2.5 kpc. This model accounts for the shape of the Ly profile and the continuum both longward and shortward of Ly. The low inclination is supported by the X-ray/optical ratio. The accretion rate is similar to the values associated with old novae (see discussion). The next-best fit has M wd = 0.80 M, _M = M yr 1, i =18, and E(B V ) = 0.3. This model accounts for the continuum both longward and shortward of the Ly profile but does not fit the Ly profile itself quite as well (see Fig. 3). The distance implied from the scale factor (S = ) for this fit is 2 kpc. Finally, the third good fit has the values M wd = 0.55 M, _M = M yr 1, i =18, and E(B V ) = 0.1. This model accounts for the continuum longward of Ly but predicts an upturn shortward of Ly that is not observed (see Fig. 4). The distance from the scale factor (S = ) for this fit is 2.9 kpc. All three models predict strong Si ii and Si iii absorption features from the disk, which are not observed. The model Fig. 2. Flux f (ergs cm 2 s 1 Å 1 ) vs. wavelength (in angstrom units) disk model with _M = M yr 1, i =18, M wd = 0.55 M, and E(B V ) = 0.3 is shown for comparison.

4 No. 1, 2003 DI LACERTAE 291 Fig. 3. Flux f (ergs cm 2 s 1 Å 1 ) vs. wavelength (in angstrom units) disk model with _M = M yr 1, i =18, M wd = 0.80 M, and E(B V ) = 0.3 is shown for comparison. disk features are much broader even at a disk inclination of only 18. Any higher model disk inclination yields even broader absorption profiles. The only Si ii and Si iii features are sharp interstellar lines. This discrepancy is troubling because the composition of the material in the accretion disk should reflect the mix of elements in the mass-transferring secondary star s atmosphere, which should be close to solar. The absence of obvious Si absorption features in the STIS observation (other than the extremely sharp interstellar lines) speaks to a large depletion of Si in the disk gas. To quantify this underabundance, we carried out model calculations characteristic of the same parameters as the best-fit accretion disks models used in Figures 1, 2, and 3. All elements were kept at their solar abundance except Si. The trial abundances of Si were 0.5, 0.2, 0.1, 0.05, 0.01, 0.005, 0.001, Fig. 4. Flux f (ergs cm 2 s 1 Å 1 ) vs. wavelength (in angstrom units) disk model with _M = M yr 1, i =18, M wd = 0.55 M, and E(B V ) = 0.1 is shown for comparison times solar. We found that the Si ii, Siiii, and Si iv absorption lines in the model nearly disappeared when the Si abundance was d times solar. The absence of observed Si ii and Si iii led us to examine IUE archival spectra of other classes of cataclysmic variables with wind outflow associated with a luminous disk as a means of comparing line spectra. Two subclasses characterized by bright disks and wind outflow are dwarf novae in outburst and nonmagnetic nova-like variables in their high states. We found that all but five of 42 dwarf novae in outburst and all but one of 23 nova-like variables in a high state, the C iv P Cygni absorption profiles, were accompanied by Si iv absorption features of commensurate strength while in all these systems, Si ii and Si iii were very weak to moderately strong from system to system. The five exceptions to this rule among the dwarf novae are SS Aur, CZ Ori, HL CMa, SS Cygni, and X Leo. In their outburst spectra, there are very weak or absent Si iv and very weak or absent Si ii and Si iii. The one exception among the novalike variables, QU Car, has no apparent Si iv,si ii,or Si iii. We note, however, that in DI Lac there are only three wind features detected in the far-uv, the ions N v,ov,and C iv, with only weak or absent Si lines. This same mix of ions, namely N v, Ov, and C iv (and no Si iv), are seen in the wind outflow of the central star of Abell 78. Therefore, it is possible that the origin of the outflow absorption features of DI Lac bears a physical similarity to the wind outflow of a H-deficient planetary nebula central star. Current models of such outflows are parameterized semianalytic treatments to fit the profiles. Any such modeling of the DI Lac wind features is beyond the scope of this paper. Next we compared the STIS dereddened data with highgravity photosphere models for white dwarfs to see how well a white dwarf photosphere accounted for the STIS spectra. Theoretical spectra were computed using the model atmosphere code TLUSTY (Hubeny 1988), which calculates the atmospheric structure, and SYNSPEC (Hubeny & Lanz 1995), which generates synthetic line profiles for specified effective temperatures, gravities, and abundances. With fixed solar abundances, we varied the temperature, log g, and E(B V ) values. Each trial fit used our 2 minimization routine IUEFIT to generate a 2 value and the scale factor between the observed flux and the model flux. The goodness of fit was based upon visual inspection and the minimum 2 value achieved. We explored the best-fit photosphere models for E(B V ) = 0.1 and 0.3 with 15 values of T eff in the range 15,000 to 29,000 K in increments of 1000 K for V sin i = 200 km s 1, and log g = 8.0. The best-fitting highgravity photosphere model has log g = 8.0, T eff = 27,000 K, and E(B V ) of 0.3 (see Fig. 5) with a minimum 2 = , scale factor S = , and distance = 76 pc. It is clear that on the assumption that a hot white dwarf dominates the far-uv energy distribution, a very close distance is implied, which is inconsistent with the reddening and the observed optical brightness in outburst. It is not unreasonable therefore to ascribe the UV spectrum to a large bright accretion disk. The slope of the far-uv continuum both shortward and longward of the Ly profile agrees with the dereddened data. Note the model prediction of very strong Si ii (1260 and 1265 Å) but the absence in the STIS spectrum of the Si ii 1265 Å component. This is a clear result of the fact that the Si ii 1260 Å component is almost purely interstellar since

5 292 MOYER ET AL. Fig. 5. Flux f (ergs cm 2 s 1 Å 1 ) vs. wavelength (in angstrom units) recorded by HST STIS of old nova DI Lac (Nova Lac 1910). The best-fitting high-gravity (log g = 8) model with T eff = 27,000 K is shown for comparison. the Si ii 1265 Å arises from an excited level requiring a high density to populate the lower level. old novae is around 10 L (Krautter et al. 1981). There is no apparent correlation between L uv and P orb, decline rate t3, or time since the last nova. If the disk luminosity L disk is roughly twice the observed far-uv luminosity, then L disk =20L. A representative accretion rate for old novae is therefore is M yr 1. Our model analysis of the STIS spectrum of DI Lac indicates a strong probability that a luminous disk dominates the far-uv luminosity. Adopting the most reasonable combination of disk parameters from our fitting, we believe the accretion rate of DI Lac is between 10 9 and M yr 1 with an inclination less than 18 and a white dwarf mass between 0.6 and 0.8 M. The best model fits were achieved with a reddening value E(B V ) = 0.3. The silicon abundance appears to be subsolar, as the (solar abundance) disk models predict strong Si ii and Si iii absorption, which is not observed. We have determined that the extreme weakness or absence of Si absorption features in the observed spectrum implies a depletion of silicon corresponding to d times solar. This result is not likely due to smearing effects since the inclination is very low. A white dwarf model with a surface temperature of roughly 27,000 K provides a good fit, but the scale factor yields an implausibly low distance of only 76 pc. Further study of DI Lac and other old novae is clearly warranted. 4. DISCUSSION The presence of wind outflow in DI Lac (as indicated by the C iv P Cygni profile) and its energy distribution, strongly similar to UX UMa nova-like systems, make it highly likely that DI Lac is a disk-dominated system. However the relationship between the UX UMa systems and old novae remains unclear. The observed far-uv luminosity of This research was supported by HST GO A, in part by NSF grant (E. M. S.), NASA ADP grant NAG (E. M. S.), and by summer research funding from the NASA Delaware Space Grant Colleges Consortium. S. Starrfield acknowledges partial support from NASA and NSF grants to ASU; B. G. acknowledges support by a PPARC Advanced Fellowship. Cassatella, A., Selvelli, P. L., Gilmozzi, R., Bianchini, A., & Friedjung, M. 1990, in Accretion-powered Compact Binaries, ed. C. Mauche (Cambridge: Cambridge Univ. Press), 373 Cordova, F., & Mason, K. 1982, ApJ, 260, 716 Greenstein, J. L., & Oke, J. B. 1982, ApJ, 258, 209 Heap, S., et al. 1978, Nature, 275, 385 Hoard, D. W., Szkody, P., Honeycutt, R. K., Robertson, J., Desai, V., & Hillwig, T. 2000, PASP, 112, 1595 Honeycutt, K., Robertson, J., & Turner, W. 1995, in Cataclysmic Variables, ed. A Bianchini, M. Della Valle, & M. Orio (Dordrecht: Kluwer) 75 Hubeny, I. 1988, Comput. Phys. Commun., 52, 103 REFERENCES Hubeny, I., & Lanz, T. 1995, ApJ, 439, 875 Kraft, R. 1964, ApJ, 139, 457 Krautter, J., Vogt, N., Klare, G., Wolf, B., Duerbeck, H. W., Rahe, J., & Wargau, W. 1981, A&A, 102, 337 Seaton, M. 1979, MNRAS, 187, P73 Selvelli, P. L., et al. 1990, in IAU Colloq. 122, Physics of Classical Novae, ed. A. Cassatella & R. Viotti (Berlin: Springer), 65 Sion, E. M. 1985, ApJ, 292, 601 Wade, R. A., & Hubeny, I. 1998, ApJ, 509, 350 Warner, B. 1995, Cataclysmic Variable Stars (Cambridge: Cambridge Univ. Press)

THE ASTROPHYSICAL JOURNAL, 557:978È982, 2001 August 20 ( The American Astronomical Society. All rights reserved. Printed in U.S.A.

THE ASTROPHYSICAL JOURNAL, 557:978È982, 2001 August 20 ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A. CHEMICAL ABUNDANCES OF THE ACCRETING MAGNETIC WHITE DWARF IN THE