1. INTRODUCTION. Received 2003 July 11; accepted 2003 August 6

Transcription

1 The Astronomical Journal, 6:5 58, 3 November # 3. The American Astronomical Society. All rights reserved. Printed in U.S.A. SDSS WHITE DWARFS WITH SPECTRA SHOWING ATOMIC OXYGEN AND/OR CARBON LINES James Liebert, H. C. Harris, C. C. Dahn, Gary D. Schmidt, 3 S. J. Kleinman, 4 Atsuko Nitta, 4 Jurek Krzesiński, 4,5 Daniel Eisenstein, J. Allyn Smith, 6 Paula Szkody, 7 Suzanne Hawley, 7 Scott F. Anderson, 7 J. Brinkmann, 4 Matthew J. Collinge, 8 Xiaohui Fan, Patrick B. Hall, 8,9 Gillian R. Knapp, 8 Don Q. Lamb, B. Margon, Donald P. Schneider, and Nicole Silvestri 7 Received 3 July ; accepted 3 August 6 ABSTRACT We discuss 8 white dwarfs, one of which (G7-5) was previously known, whose SDSS spectra show lines of neutral and/or singly ionized carbon. At least two and perhaps four show lines of neutral or singly ionized oxygen. Apart from the extremely hot PG 59 stars, these are the first white dwarfs with photospheric oxygen detected in their optical spectra. The photometry strongly suggests that these stars lie in the, 3, K temperature range of the helium-atmosphere DB white dwarfs, though only one of them shows weak neutral helium lines in the spectrum. Trigonometric parallaxes are known for G7-5 and another, previously known white dwarf (G35-6) showing atomic carbon lines, and they indicate that both are massive stars. Theoretical arguments suggest that all members of this class of rare white dwarfs are massive ( M ), and this finding could explain the paucity of massive DB white dwarfs. Key words: stars: abundances stars: atmospheres white dwarfs. INTRODUCTION It is well known that hot white dwarfs divide primarily into two spectroscopic types those showing primarily hydrogen lines and those showing primarily helium lines, respectively, the DA and DB-DO spectral types. The mass distributions of the two types seem to differ: while both show a strong peak near.6 M, the DA distribution includes distinct subgroups with masses considerably lower and considerably higher (e.g., Bergeron, Saffer, & Liebert 99), but the low- and high-mass outliers appear to be rare in the DB distribution (Beauchamp 996; Beauchamp et al. 996). The absence of DB stars in known samples between about 3, and 45, K (Liebert et al. 986) the socalled DB gap is explainable only in terms of spectral evolution from one type to the other as some of the stars cool (Fontaine & Wesemael 987). A third, rare type of hot white dwarf shows a spectrum exhibiting primarily lines of atomic carbon. Only a few of Department of Astronomy and Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 857. US Naval Observatory, Flagstaff Station, P.O. Box 49, Flagstaff, AZ MMT Observatory, University of Arizona, Tucson, AZ Apache Point Observatory, P.O. Box 59, Sunspot, NM Mount Suhora Observatory, Cracow Pedagogical University, ul. Podchorazych, 3-84 Kraków, Poland. 6 Los Alamos National Laboratory, P.O. Box 663, Los Alamos, NM Department of Astronomy, University of Washington, Box 3558, Seattle, WA Princeton University Observatory, Peyton Hall, Princeton, NJ Departamento de Astronomía y Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 36, Santiago, Chile. Department of Astronomy and Astrophysics, University of Chicago, 564 South Ellis Avenue, Chicago, IL Space Telescope Science Institute, 37 San Martin Drive, Baltimore, MD 8. Department of Astronomy and Astrophysics, 55 Davey Laboratory, Pennsylvania State University, University Park, PA these have been known up to now, although they presumably cool into the well-known DQ class showing molecular C Swan bands. Atmospheric analyses of the DQ stars showed long ago that they have helium-dominated atmospheres (Bues 973; Grenfell 974; Koester, Weidemann, & Zeidler 98; Wegner & Yackovich 984), though atomic He i lines are not visible. It is generally believed that the carbon is dredged up from the core when the convective envelope reaches its diffusive-equilibrium tail (Koester et al. 98; Fontaine et al. 984; Wegner & Yackovich 984; Pelletier et al. 986). The absence of He i lines in the C -banded objects is understandable, since they are too cool for helium to be excited. However, for white dwarfs showing only atomic carbon lines the absence of He i lines is harder to explain. G35-6 (WD 3+7) shows only lines of C i and H (Liebert 983). Yet the analysis by Thejll et al. (99) found T eff in the, 4, K range, a carbon abundance of only n C.3, and a hydrogen abundance of n H d.. The inference is that 96% of the atmospheric atoms are helium, but the higher opacity of the contaminants masks the dominant constituent. Moreover, Thejll et al. found that their atmospheric fit required a very high gravity log g = implying a mass of..33 M. An accurate, new USNO trigonometric parallax determination confirms a small radius and large mass ( M ) for G35-6 (see Dahn et al. 3). A second object showing neutral carbon and weaker H lines is G7-5 (WD 77+56), analyzed by Wegner & Koester (985). They found T eff =,5 5 K and small trace abundances of n C 3 3 and n H 4 4. However, this analysis assumed the typical white dwarf log g = 8.. An improved trigonometric parallax determination for this star (Dahn et al. 3) allows the inferences of its gravity and mass. In this paper we present 7 new white dwarfs showing atomic carbon lines, plus a new spectrum of G7-5 (WD 77+56). These include stars that should be substantially hotter than those described above. In addition, we report

2 5 LIEBERT ET AL. Vol. 6 the first such white dwarfs in the DB temperature range exhibiting atomic lines of oxygen. The objects were culled from thousands of spectra taken in the Sloan Digital Sky Survey (SDSS, York et al. ). Positions, astrometry, and photometry of the stars are discussed in x, and color temperatures allow a crude comparison with colors predicted from pure-helium model atmospheres. In x 3 we present and describe the spectrophotometry of the stars. In the discussion (x 4) the relationship of the hot DQ and DB stars is explored.. ASTROMETRY, PHOTOMETRY, AND COLOR TEMPERATURES OF NEW SDSS HOT DQ WHITE DWARFS In Table the first column gives a shorthand designation for each object using incomplete celestial coordinates. The full SDSSJ designation, incorporating accurate J. positions, is listed in the second column. The positions have been taken from the SDSS Astrometric Pipeline (Pier et al. 3). Proper motions are calculated using the USNO-A. Catalog (Monet et al. 999) 3 for first-epoch position. Additional information in Table will be discussed below. In Table, after the shorthand designation, the five SDSS magnitudes and colors are listed from version 5.3 of the Photometric Pipeline (Lupton et al.,, and 3). The SDSS bands cover the entire optical range from the UV atmospheric cutoff (3 Å) to the detector s red sensitivity cutoff ( Å) (Fukugita et al. 996; Gunn et al. 998). The photometric calibration is based on the SDSS standard-star system (Smith et al. ) tied to the survey data with the Photometric Telescope (Hogg et al. ). These colors have not been corrected for Galactic extinction, which makes the implicit assumption that the white dwarfs are within the dust layer of the Galactic disk (e.g., scale height 5 pc). In Table the extinction at u band (A u )is taken from Schlegel, Finkbeiner, & Davis (998) for that line of sight, although only some fraction of that absorption 3 Available at may apply to the nearby (brighter, cooler) stars. The listed Galactic latitudes exceed b =6 in all cases. Hot stars such as these are frequently targets for followup spectra in the Sloan selection process, in part because the colors are blackbody-like, thus overlapping with lowredshift QSOs. We have made no attempt to be complete within any subset of available SDSS spectra, but Table lists a selection that shows lines of atomic oxygen and/or carbon, as indicated by the assigned spectral classification in the last column. Proper motion values are also given in the table, linked to positions measured on Palomar Observatory Sky Survey plates (USNO-A and USNO-B catalogs). Since, as discussed previously, the atmospheric composition of these white dwarfs is expected to be helium dominated, it is of interest to compare the colors with those calculated by P. Bergeron (3, private communication) in the Sloan bandpasses from models in the DB range of T eff, assuming log g = 8 for a pure helium composition. A pure helium effective temperature based on this comparison is given in the column labeled T He in Table, in thousands of degrees Kelvin. The dependence of the color on gravity is expected to be small. While we will discuss evidence below that the T He value may differ substantially from the real T eff, the former numbers should at least be useful in rankordering the stars by temperature and in showing that they span a considerable range. For estimating a color temperature, we prefer not to use the u-band data for several reasons. The pure-he model colors are based on line-blanketed synthetic spectra, and there are strong lines of He i in the u bandpass; in contrast, the strong carbon opacity present in the white dwarfs masks (at least for the most part) the dominant atmospheric constituent helium such that helium lines are generally not seen at all. In addition, carbon seems to have less opacity in the u bandpass, so that u g colors appear to be bluer than for DB stars with He i of similar T eff. Moreover, the u band is the most affected by the somewhat uncertain extinction. We prefer not to use the z band as well, since these measurements show the largest uncertainties in Table, mainly because the stars have the least flux in this band. The g i color then yields the longest available baseline in wave- TABLE List of Hot Carbon White Dwarfs SDSSJ Name SDSSJ+ g i T He A u b II l P.A. Spec DQ (C ii, mag?) DQ (C ii,oii?) DQ (C ii,ci?) DQA (C i,h) DQ (C i,oi) DQ (C i) DQ (C i, mag) DQ (C i) DQ (C i,c ) DQ (C i,cii) DQ (C i) DQ (C i,cii,oii?) > DQ (C i,cii) DQ (C i,oi) DQ (C i,c ) DQ (C i) DQA (C i, H?) DBQA (C i,h,hei)

3 No. 5, 3 SPECTRA OF SDSS WHITE DWARFS 53 TABLE SDSS Photometry of Hot Carbon White Dwarfs SDSSJ g u g g r r i i z u g r i z length, for which a coarse color temperature can be estimated from the models. Had we used u i for this exercise, the values of T He would generally be higher, with several stars well in excess of 3, K; this would imply that some of them may be hotter than the DB range. These judgments can only be tested by rigorous stellar modelling with the correct atmospheric opacities, a task left for subsequent work. The hottest object, at least by the rank-ordering of T He,is SDSS J337 6, with an estimate exceeding 3, K. The stars span a range down to, K (SDSS J48 6 and SDSS J ); these two reddest stars in g i are the only ones to show definitely the wellknown C Swan bands, which are characteristic of the cool DQ class, in addition to atomic C i lines. A diverse collection of the C -banded stars found in Sloan data is presented by Harris et al. (3). However, many in the present paper are suggested by the g i colors to be hotter than the two previously analyzed hot DQ stars, G7-5 and G35-6. G7-5 was actually observed in the survey (SDSS J ) and is included in Table. Its T He estimate is 7,5 K. The fact that this is 5, K hotter than the value determined in the model atmospheres fitted by Wegner & Koester s (985) analysis warns us that the pure He estimates are likely to be substantially in error. Yet the set of hot DQ stars do seem to overlap most of the DB temperature range. 3. A SPECTROPHOTOMETRIC TOUR OF CARBON, OXYGEN-LINED WHITE DWARFS The spectra described in this section were taken with the dual SDSS.5 m telescope fiber spectrographs. Using fibers with a diameter of 3, about 6 objects are observed on each exposure at a resolution of about 8 (3 Å) over the 38 9 Å range. Exposures are typically 45 minutes (three integrations of 5 minutes). The spectra are extracted automatically. With only a modest number of fibers available to measure the sky, night-sky lines are often not subtracted perfectly; in particular, residuals are often seen at 5577, 589, and 63 Å. Most of the spectra of this class of star appear to show lines of neutral carbon (C i), with lines of C ii absent or weak. In Figures a and b, several example blue and red spectra are shown, including the bright G7-5 (SDSS J ) at the top. Line identifications, generally taken from the tables of Moore (959), are indicated by tick marks near the top of the figure. The same identification code is used for this and subsequent figures; in particular, strong individual C i lines or closely spaced multiplets (i.e., near 55 Å) are indicated by solid ticks; C ii line positions are indicated by dashed ticks. Long-dashed short-dashed tick marks, displaced to a lower position in the figure, indicate the positions of He i 46, 447 and H (486 Å)on Figure a and H (6563 Å) on Figure b; these are detected in G7-5, but not convincingly in the lower signal-to-noise ratio (S/N) objects. As alluded to earlier, detailed model atmospheres fits to earlier spectra of G7-5 show that the strong carbon opacity is responsible for masking the helium lines, though helium is the dominant atmospheric constituent (Wegner & Koester 985). The middle three spectra in Figure appear similar to that of G7-5, although hydrogen and helium are not clearly detected. Their T He values in the range, 6, K suggest that these stars are cooler than G7-5. More structure is seen in the line profiles of the hotter SDSS J36+65, plotted at the bottom. We believe that this star is a magnetic white dwarf, and it is analyzed in Schmidt et al. s (3) presentation of numerous magnetic objects found in SDSS. The best examples of likely Zeeman triplet splitting are the closely space multiplets near Å, 54 5 Å, and perhaps 538 Å. It is possible that the structure near 4 Å is primarily due to C ii multiplets. In Figure c we extend the plot of the spectrum of G7-5 to the near-infrared range (88 Å). (The remaining stars are not shown because of the low S/N of these spectra.) A few more C i detections are probable, but also an unidentified feature just shortward of 75 Å. There is another possible absorption near 77 Å. In Figures a and b some additional stars also exhibiting primarily C i lines and blends are shown, but these stars span a wider range of T He. The top two appear to be

4 54 LIEBERT ET AL. Vol Fig. a Fig. b Fig. c Fig.. (a) Blue spectra of five white dwarfs showing primarily lines of C i. The top spectrum is the previously known G7-5 (=SDSS J ). All spectra are normalized to have the same relative flux (vertical distance above zero flux) at 45 Å. Spectra are displaced by.4 flux units from each other. Solid tick marks note the positions of strong lines and multiplets of C i. Dashed ticks note the same for C ii, although there is little evidence of these features in the spectra. The short-dashed long-dashed ticks note the positions of He i 46, 447 and H, seen convincingly only in the spectrum of G7-5. (b) Red spectra of the same five white dwarfs, normalized to the same relative flux at 67 Å and displaced vertically by.4 relative flux unit. Tick marks designate features due to C i and C ii as before; the long-dashed short-dashed tick designates H.(c) Near-infrared spectrum of G7-5 (=SDSS J ) only. Other stars are not shown due to poor S/N spectra. Tick marks have the same meaning. considerably hotter than the stars in Figure, with photometric T He -values above, K. The spectra are noisy, but some C ii blends (e.g., 467 Å) appear to be present. Dips due to H and H may be present in the noisy spectrum of SDSS J , but the H line is partially blended with strong carbon lines. Nonetheless, we tentatively classify this star DQA. In contrast, the bottom object (SDSS J48 6) is among the coolest, with T He at, K, and the spectrum shows C bands as well as C i. It is very similar to the longknown white dwarf G47-8 (=WD ) (see Fig. 3a in the spectrophotometric atlas of Wesemael et al. 993). Two model atmospheres analyses using spectrophotometry obtained a T eff estimate of, K for this star (Wegner & Yackovich 984; Grenfell 974), while Koester et al. (98) estimated 96 K. Two additional stars with much nosier spectra, SDSS J (T He of, K) and SDSS J (T He = 3,5 K), show clearly the strongest C i multiplets. We do not show their spectra here.

5 No. 5, 3 SPECTRA OF SDSS WHITE DWARFS Fig. a Fig. b Fig.. (a) Blue spectra of three white dwarfs showing carbon features, normalized to the same relative flux at 45 Å and offset by.6 from each other. The bottom star (SDSS J48 6) shows the C molecular Swan bands, in addition to atomic C i. The tick marks are the same as for Fig. a, including a long-dashed short-dashed tick for the position of H.(b) Red spectra of the same three white dwarfs, normalized to the same relative flux at 67 Å and offset by.6 from each other. Tick marks have the same meaning, including a long-dashed short-dashed tick for the position of H. Figures 3a and 3b show blue and red spectra of two objects, SDSS J and SDSS J9+575, which exhibit lines of both C i and O i. AtT He estimates of,5 and 5, K, respectively, these may lie near or within the cool DB temperature range. The O i features are marked by short dash-dot tick marks. Especially striking are the O i features in the red spectra, i.e., Å and Å. Figures 4a 4b display three hotter stars with T He e 8, K. Separate spectra of SDSS J53+56 allow the evaluation of the validity of various lines and blends in rather noisy spectra. Stronger multiplets of C ii are now seen in these spectra, while O ii multiplets, plotted as dotted ticks, may also be detected in the top blue spectrum (SDSS J337 6). No H or He features are evident. One Fig. 3a Fig. 3b Fig. 3. (a) Blue spectra of two white dwarfs showing C i and O i; two independent spectra of SDSS J9+575 are shown. Spectra are again normalized to 45 Å, and the offset is.45 flux unit. Solid tick marks are again C i features, and dashed-dotted features designate lines and multiplets of O i. (b) Red spectra of the same two white dwarfs, normalized to 67 Å and offset by.4 flux unit. Tick marks have the same meaning as previously.

6 56 LIEBERT ET AL. Vol Fig. 4a Fig. 4b Fig. 4. (a) Blue spectra of white dwarfs showing C i and C ii features, and possible O ii. Two independent spectra of SDSS J53+56 are displayed. Spectra are again normalized at 45 Å and offset by.4 unit. Dotted ticks designate O ii features, other ticks are the same as before. (b) Red spectra of the same stars, normalized at 67 Å and offset by.6 unit. Tick marks have the same meaning. additional hot star showing possible C ii multiplets is SDSS J5, shown in Figure 5 of Schmidt et al. (3), where it is discussed as a possible magnetic DQ. Finally, in Figures 5a and 5b we plot two stars with T He near, K whose spectra are puzzling, and tick marks for C i, Cii, Oi, andoii (dotted ticks) are displayed at the top. It is not clear to us which combination(s) of ions are required here. The bottom spectrum (SDSS J6+53) is particularly puzzling note the strong, almost banded features in the blue spectrum, along with the much smoother red spectrum. The indicated temperature is too hot for C bands. It is possible that the bottom spectrum requires a different interpretation than some combination of carbon and/or oxygen ions Fig. 5a Fig. 5b Fig. 5. (a) Blue spectra of two final white dwarfs, which might show some combination of the four ions discussed previously. These are again normalized at 45 Å, and the top spectrum is offset by.6 of a vertical flux unit. Tick marks have the same meaning as before. (b) Red spectra of the same two stars, normalized at 67 Å, with the top spectrum offset by.6 unit (and a gap in the spectrum near 77 Å).

7 No. 5, 3 SPECTRA OF SDSS WHITE DWARFS DISCUSSION These are the first detections of atomic O i and/or probable O ii in optical spectra of white dwarfs, apart from the extremely hot (T eff 5 K) PG 59 (prototype PG 59 35) white dwarfs or pre white dwarfs in which gravitational diffusion has not yet had time to produce an outer pure-helium layer. Provencal, Shipman, & MacDonald () reported oxygen lines in an ultraviolet spectrum of G7-5, though it was not clear whether the features are photospheric or are instead due to a stellar wind or even to chromospheric activity. The first attempts to detect oxygen via CO bands in infrared spectroscopic observations have been made at McDonald Observatory (Kilic et al. 3), although no positive detections have yet been made. Let us consider the possible implications of the presence of oxygen at the surfaces of what is likely a rare group of white dwarfs. The important new parallaxes of G7-5 and G35-6 and their inferred masses e M, reported by Dahn et al. (3), strengthen the case that the rare white dwarfs showing carbon lines with likely T eff in the DB range (, 3, K) are massive. For the cooler DQ stars showing C bands with trigonometric parallax measurements, there is no evidence that they as a group have an unusually high mean mass (Koester et al. 98), although the well-studied, warm G47-8 certainly has a high mean mass (see also Bergeron, Leggett, & Ruiz ). It is known from theoretical calculations of asymptotic giant branch stars that steady helium-shell burning continues to much lower He-envelope masses as the stellar mass increases (see Kawai, Saio, & Nomoto 988). With a smaller He-layer mass as a white dwarf, the dredge up of carbon should accordingly take place at a hotter T eff, as Thejll et al. (99) argued. If this is the general explanation for the carbon-lined objects at DB temperatures, then the absence of a high-mass tail in the mass distribution of stars classified DB discussed in the introduction may be understood. The detection of oxygen in up to several objects here implies that a diffusive tail of oxygen is broached by the helium convection zone in these stars. In many models of C-O cores, little or no oxygen is present near the edge of the core (see Salaris et al. 997). However, more massive cores have more total oxygen, and the oxygen profile reaches closer to the edge. Moreover, several uncertainties in the physics such as the C (,)O 6 cross-section render such calculated abundance profiles very uncertain at this time. Thejll et al. (99) discussed the possibility that G35-6 could have a core composed of oxygen-neon-magnesium. This could well be the interior composition of any white dwarf with a mass above about. M. In a series of models exploring the evolution of 9 M stars, Garcia-Berro, Iben, and collaborators have managed to generate such cores in a second asymptotic giant branch phase (Garcia- Berro & Iben 994; Ritossa, Garcia-Berro, & Iben 999 and references therein). Such an ONeMg core is expected to have an envelope of CO around it, and perhaps external He and H layers as well, although convective mixing occurs frequently in their calculations of the second AGB phase. It is therefore not implausible that oxygen as well as carbon could be dredged up to the atmosphere of an ONeMG white dwarf. In conclusion, it is important for complete model atmosphere analyses of these rare stars to be undertaken in order to determine the temperatures, gravities, atmospheric abundances, and, of course, masses. The arguments presented earlier in this section suggest that the stars with the higher temperatures may be the most massive, and those showing photospheric oxygen lines may deserve priority for trigonometric parallax measurement. Finally, it may be possible that some of the hottest of these stars actually fall in the temperature range of the pulsating DBV stars. It would be interesting to find out if the carbon and/or oxygen suppresses pulsations, or whether potential DQV stars could exhibit a different mode behavior. Funding for creation and distribution of the Sloan Digital Sky Survey Archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS web site is The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, the Johns Hopkins University, Los Alamos National Laboratory, the Max Planck Institute for Astronomy, the Max Planck Institute for Astrophysics, New Mexico State University, the University of Pittsburgh, Princeton University, the United States Naval Observatory, and the University of Washington. We thank P. Bergeron for calculating the fluxes in the five SDSS bands for his pure-helium white dwarf atmospheric models. We thank an anonymous referee for catching some errors in numbering. Beauchamp, A. 996, Ph.D. thesis, Univ. Montréal Beauchamp, A., Wesemael, F., Fontaine, G., Lamongtagne, R., Saffer, R. A., & Liebert, J. 996, in ASP Conf. Proc. 96, Hydrogen-Deficient Stars and Related Objects, ed. C. S. Jeffery & U. Heber (San Francisco: ASP), 95 Bergeron, P., Leggett, S. K., & Ruiz, M. T., ApJS, 33, 43 Bergeron, P., Saffer, R. A., & Liebert, J. 99, ApJ, 394, 8 Bues, I. 973, A&A, 8, 8 Dahn, C. C., et al. 3, in preparation Fontaine, G., Villeneuve, B., Wesemael, F., & Wegner, G. 984, ApJ, 77, L6 Fontaine, G., & Wesemael, F. 987, in IAU Colloq. 95, The Second Conference on Faint Blue Stars, ed. A. G. D. Philip, D. S. Hayes, & J. Liebert (Schenectady: L. Davis), 39 Fukugita, M., Ichikawa, T., Gunn, J. E., Doi, M., Shimasaku, K., & Schneider, D. P. 996, AJ,, 748 Garcia-Berro, E., & Iben, Jr., I. 994, ApJ, 434, 36 Grenfell, T. C. 974, A&A, 3, 33 REFERENCES Gunn, J. E., et al. 998, AJ, 6, 34 Harris, H. C., et al. 3, AJ, 6, 3 Hogg, D. W., Finkbeiner, D. P., Schlegel, D. J., & Gunn, J. E., AJ,, 9 Kawai, Y., Saio, H., & Nomoto, K. 988, ApJ, 38, 7 Kilic, M., Winget, D. E., von Hippel, T., Lester, D., & Saumon, D. 3, in The XIII European Workshop on White Dwarfs, ed. D. de Martino, R. Silvotti, J.-E. Solheim, & R. Kalytis (Dordrecht: Kluwer), 69 Koester, D., Weidemann, V., & Zeidler-K. T., E. M. 98, A&A, 6, 47 Liebert, J. 983, PASP, 95, 878 Liebert, J., Wesemael, F., Hansen, C. J., Fontaine, G., Shipman, H. L., Sion, E. M., Winget, D. E., & Green, R. F. 986, ApJ, 39, 4 Lupton, R. H., Gunn, J. E., Ivezić, Ž., Knapp, G. R., Kent, S. M., & Yasuda, N., in ASP Conf. Ser. 38, ADASS X, ed. F. R. Harnden, Jr., F. A. Primini & H. E. Payne (San Francisco: ASP), 69 Lupton, R. H., Ivezić, Ž., Gunn, J. E., Knapp, G. R., Strauss, M. A., & Yasuda, N., Proc. SPIE, 4836, 35 Lupton, R. H., et al. 3, in preparation

8 58 LIEBERT ET AL. Monet, D. G., et al. 999, The USNO-A. Catalog Moore, C. E. 959, A Multiplet Table of Astrophysical Interest, NBS Tech. Note 36 (Washington: NBS) Pelletier, C., Fontaine, G., Wesemael, F., Michaud, G., & Wegner, G. 986, ApJ, 37, 4 Pier, J. R., et al. 3, AJ, 5, 559 Provencal, J. L., Shipman, H. L., & MacDonald, J., BAAS, 99, No. 7. Ritossa, C., Garcia-Berro, E., & Iben, Jr., I. 999, ApJ, 55, 38 Salaris, M., Domínguez, I., García-Berro, E., Hernanz, M., Isern, J., & Mochkovitch, R. 997, ApJ, 486, 43 Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 998, ApJ, 5, 55 Schmidt, G. D., et al. 3, ApJ, 595, Smith, J. A., et al., AJ, 3, Thejll, P., Shipman, H. L., MacDonald, J., & MacFarlane, W. M. 99, ApJ, 36, 97 Wegner, G., & Koester, D. 985, ApJ, 88, 746 Wegner, G., & Yackovich, F. H. 984, ApJ, 84, 57 Wesemael, F., et al. 993, PASP, 5, 76 York, D. G., et al., AJ,, 579