Hubble Space Telescope GHRS Spectroscopy of U Geminorum ...

THE ASTROPHYSICAL JOURNAL, 483 : 907È912, 1997 July 10 ( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.

HUBBL E SPACE T EL ESCOPE GHRS SPECTROSCOPY OF U GEMINORUM DURING TWO OUTBURSTS1 EDWARD M. SION,2 FUHUA CHENG,2 PAULA SZKODY,3 MIN HUANG,2 JUDI PROVENCAL,4 WARREN SPARKS,5 BRIAN ABBOTT,2 IVAN HUBENY,6 JANET MATTEI,7 AND HARRY SHIPMAN4 Received 1996 September 4 ; accepted 1997 January 29

ABSTRACT We obtained Hubble Space T elescope Goddard High-Resolution Spectrograph medium-resolution (G160M grating) phase-resolved spectroscopic observations of the prototype dwarf nova U Geminorum during di†erent stages of two di†erent outbursts. The spectral wavelength ranges were centered on three di†erent line regions : N V (1238 Ó, 1242 Ó), Si III (1300 Ó), and He II (1640 Ó). The spectrum corresponding to the early decline phase of outburst 1 is essentially featureless except for weak N V absorption and narrow interstellar lines, while the spectrum at the peak of outburst 2 reveals broad emission peaks separated by narrow central absorption. The double-peaked emission-line proÐle structure with low-velocity central absorption seen in the second outburst suggests a disk origin, but the emission velocity widths appear narrower than the widths of the optical disk emission features. We interpret the high-excitation emission lines, with central absorption below the continuum, to be due to photoionized material (coronal ?) above the disk plane with the thickened outer disk absorbing the boundary layer or inner disk radiation. The possibility of a wind origin for the proÐles is also discussed, as well as the possibility of an ejected optically thin shell. The N V absorption velocity versus orbital phase traces the motion of the white dwarf, but the He II absorption velocity appears to deviate from the white dwarf motion. We present the results of synthetic accretion disk spectral Ðtting to the data of both outbursts and derive accretion rates for the two outbursts of 6 ] 10~10 M yr~1 and 2 ] 10~9 M yr~1. Implica_ _ tions are discussed. Subject headings : accretion, accretion disks È novae, cataclysmic variables È stars : emission-line, Be È stars : individual (U Geminorum) È ultraviolet : stars 1.

INTRODUCTION

its quiescence, scheduling problems when U Gem unexpectedly went into outburst led to the inadvertent acquisition of data for two sets of outburst spectra, each set sampling a di†erent outburst of U Gem. High-quality far-UV data were obtained, revealing heretofore unknown spectroscopic details of its dwarf nova outburst phase. In this paper we present for the Ðrst time an analysis of these rich spectra and speculate on several possible physical explanations for their structure and variations.

Far-ultraviolet and optical spectroscopic studies of dwarf novae through the outburst have provided an overall characterization of line spectra and continuum energy distribution (e.g., Szkody, Piche, & Feinswog 1990 ; Hessman 1986 ; la Dous 1991 ; Verbunt 1987 ; and references therein), including temporal spectroscopic variations. The farultraviolet spectra of dwarf novae in outburst have been shown to be strongly correlated with orbital inclination, and in most such systems in outburst (with i \ 70¡), there is blueshifted absorption and P Cygni proÐle structure that reveal hot, fast wind outÑow (see la Dous 1991 and references therein). However, moderate- to high-resolution far-UV spectra with orbital phase resolution and resolved line proÐles have rarely been obtained. In the course of a Hubble Space T elescope (HST ) Godard High-Resolution Spectrograph (GHRS) investigation of line formation regions in the prototype dwarf nova U Geminorum during

2. HST

GHRS FAR-ULTRAVIOLET OBSERVATIONS

We obtained two sets of GHRS observations of U Gem, the Ðrst for spectra on 1995 April 15 during an outburst and the second set for spectra on 1995 September 16, during the next outburst. The temporal placement of the observations with respect to the outburst cycles is shown in Figure 1, where we present the AAVSO light-curve data for the two outbursts. The Ðrst set of observations took place at the onset of the decline from outburst, when U Gem was about 1 to 1 magnitude fainter than its peak visual magnitude. 2 Data for the second spectrum were obtained when U Gem was right at the peak of its outburst, roughly at an apparent visual magnitude of 9. The observations for both outbursts were carried out in the ACCUM mode with the D2 detector of GHRS and the G160M disperser. Three wavelength regions were covered by the observations : the N V region (1230È1265 Ó), the Si III region (1275È1305 Ó), and the He II region (1630È1660 Ó), with a resolution of 0.07 Ó. We estimate a typical wavelength scale accuracy of 0.2 Ó at 1300 Ó. The observations had a temporal resolution of 17.9 s. The standard pipeline data reduction method was used. The wavelength scale has an accuracy of D0.10 Ó. The count rates for examples in the Si III region are estimated as

1 Based on observations with the NASA/ESA Hubble Space T elescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. 2 Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085 ; emsion=ucis.vill.edu. 3 Department of Astronomy, University of Washington, Seattle, WA 98195 ; szkody=alicar.astro.washington.edu. 4 Department of Physics and Astronomy, University of Delaware, Newark, DE 19716 ; jlp=astro.udel.edu hls=astro.udel.edu. 5 Nuclear and Hydrodynamic Applications Group, Los Alamos National Laboratory, Los Alamos, NM 87545 ; wms=lanl.gov. 6 Code 681, NASA Goddard Space Flight Center, Greenbelt, MD 20771 ; hubeny=stars.gsfc.nasa.gov. 7 American Association of Variable Star Observers, Cambridge, MA 02138 ; aavso=aavso.org.

907

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Vol. 483

FIG. 1.ÈAAVSO light-curve data (visual magnitude vs. time) showing the placement of the HST observations during both outbursts

follows. The typical Ñux is 10~11 ergs cm~2 s~1 Ó~1 for a photon energy of 1.5 ] 10~11 ergs, thus giving a typical count rate of D0.65 cm~2 s~1 Ó~1. A detailed log of the observations is given in Table 1, where we tabulate for each ion wavelength region the start time of the observation, the total exposure time in seconds, the start and stop times in ModiÐed Julian Date (MJD), and the orbital phase at the beginning and end times of each exposure. For the phasing, we adopted the orbital ephemeris of Marsh et al. (1990), where phase 0.0 corresponds to an inferior conjunction of the secondary star. Since the original objectives of our line formation study were to delineate the white dwarf photosphere in quiescence and obtain maximum velocity displacement and mass information for the white dwarf, the inadvertent outburst observations were obtained close to the quadrature points of the orbit. We determined the orbital phase of the two observations by using the ephemeris by Marsh et al. (1990), in which phase 0.0 corresponds to an inferior conjunction of the secondary. In this phase convention, the white dwarf would have maximum positive velocity at phase 0.75 and maximum negative velocity at phase 0.25. Thus, for the midexposure phases of the two GHRS observations of the N V region for the two outbursts (0.33 and 0.84), we expect the N V velocity to go from negative to positive values between the two outbursts, which is indeed the case, as seen in Table 2. The N V velocity at midexposure phase 0.84 in

the second outburst agrees with the Si IV velocity of the white dwarf at the same phase, as observed by Sion et al. (1994) during the quiescence of U Gem. However, the He II velocity, obtained from an exposure that began over 3 hr later than the N V exposure, is higher than we expected at phase 0.59. The GHRS line region encompassing N V (1238 and 1242 Ó) for the Ðrst and second outbursts is displayed in Figure 2a, with the top spectrum corresponding to outburst 2 and the bottom spectrum corresponding to outburst 1. In Figure 2a, the bottom spectrum (outburst 1) reveals a broad Lya red wing, an apparent broad emission hump at N V whose width is the same as in outburst 2, and barely measurable weak absorption features near the wavelengths of the N V 1238.82 and 1242.80 Ó doublet components. In contrast, the top spectrum, corresponding to the second outburst, reveals very strong, deep absorption features with pronounced emission wings and several narrow interstellar features. The Si III region (1275È1305 Ó) during both outbursts is essentially featureless and is displayed in Figure 2b. We note, however, that in outburst 2 spectra there is evidence of an extremely broad underlying absorption with a possible emission reversal, centered in the Si III region. In Figure 2c we display the He II (1630È1660 Ó) regions for the two outbursts, with the bottom spectrum corresponding to the Ðrst outburst and the top spectrum corresponding to the second outburst. The same proÐle structure

TABLE 1 HST GHRS OBSERVATIONS OF U GEM Ion

Start Time

t exp (s)

Start (MJD)

'

Stop (MJD)

'

0.25 0.63 0.02 0.41

49,822.45428033 49,822.52101933 49,822.58988218 49,822.63969989

0.40 0.78 0.17 0.45

0.76 0.14 0.51 0.90

49,976.80705788 49,976.87393288 49,976.94085562 49,976.98726766

0.91 0.29 0.67 0.93

Outburst 1 : 4/15/95 N V ....... Si III . . . . . . He II . . . . . . He II . . . . . .

10 : 15 : 37 11 : 51 : 43 13 : 30 : 53 15 : 12 : 17

1767 1767 1767 408

49,822.42751673 49,822.49425284 49,822.56312003 49,822.63353525 Outburst 2 : 9/16/95

N V ....... Si III . . . . . . He II . . . . . . He II . . . . . .

18 : 43 : 37 20 : 19 : 55 21 : 56 : 17 23 : 32 : 47

1767 1767 1767 408

49,976.78029428 49,976.84716928 49,976.91409058 49,976.98110447

No. 2, 1997

HST GHRS OBSERVATIONS OF U GEM

909

TABLE 2 LINE MEASUREMENTS Ion and Central Wavelength (Ó)

FWHM (Ó)

*j

V (km s~1)

0.30 0.35

2.50 3.79

[1.21 [0.09

[293 [22

4.4 ] 10~12 [1.6 ] 10~11 2.2 ] 10~12 [1.5 ] 10~11 3.7 ] 10~12

[0.46 1.41 [0.23 1.37 [0.40

2.46 1.99 0.99 1.90 2.05

[2.38 ]0.45 [1.38 ]0.73 ]3.45

[576 ]108 [333 ]176 ]832

7.48 ] 10~12 [5.05 ] 10~12 5.13 ] 10~12

[1.83 0.90 [1.23

4.14 2.19 3.98

[2.864 ]0.696 ]3.70

[523 ]127 ]676

5.86 ] 10~12 [8.40 ] 10~12 5.16 ] 10~12

[1.42 1.37 [1.22

3.71 3.50 2.72

[2.694 ]0.886 ]3.97

[492 ]162 ]726

... ...

158 [18

Flux (ergs cm~2 s~1)

EW (Ó)

Outburst 1 N V: 1237.61 . . . . . . . . . . . . . . . . . . . . . . 1242.71 . . . . . . . . . . . . . . . . . . . . . .

[8.9 ] 10~13 [1.1 ] 10~12 Outburst 2

N V: 1236.44e . . . . . . . . . . . . . . . . . . . . 1239.27a . . . . . . . . . . . . . . . . . . . . 1241.42e . . . . . . . . . . . . . . . . . . . . 1243.53a . . . . . . . . . . . . . . . . . . . . 1246.25e . . . . . . . . . . . . . . . . . . . . He II : 1637.61 . . . . . . . . . . . . . . . . . . . . . . 1641.17 . . . . . . . . . . . . . . . . . . . . . . 1644.17 . . . . . . . . . . . . . . . . . . . . . . He II : 1637.78 . . . . . . . . . . . . . . . . . . . . . . 1641.36 . . . . . . . . . . . . . . . . . . . . . . 1644.44 . . . . . . . . . . . . . . . . . . . . . .

GHRS Observations of Si IV during Quiescence Si IV 1394.49 . . . . . . . . . . . . . . . . . . . . . . 1393.67 . . . . . . . . . . . . . . . . . . . . . .

Midexposure ' \ 0.80 Midexposure ' \ 0.14

seen in N V is exhibited by He II (1640 Ó) in the second outburst, but the spectrum obtained at the onset of decline is, in marked contrast, absolutely featureless. In Table 2 we present measurements of the strongest emission and absorption features in the three wavelength regions. The only line feature even marginally measurable in the spectra of the Ðrst outburst, other than interstellar lines, was the N V doublet. In the spectra of the second outburst, the N V and He II features have a similar FWHM (in angstroms) and, within the uncertainties, share the same velocity displacement, which suggests that these two ions

... ...

... ...

may form in approximately the same region. Both features have full width at zero intensity (FWZI) of D12 Ó or 2900 km s~1 at 1240 Ó. The shift and velocities of the three emission peaks in the N V doublet proÐle and the two emission peaks in the He II proÐle are given in Table 2. The mean line velocities at the center of each absorption feature are ]142 km s~1 and ]127 km s~1. The emission peak velocities for N V and He II range from [600 km s~1 for the approaching gas to ]800 km s~1 for the receding gas. These values are similar to what is seen in IP Peg during its outburst (Piche & Szkody 1989).

FIG. 2.ÈGHRS observations of U Gem in the regions of (a) N V (1238 and 1242 Ó), (b) Si III (1300 Ó), and (c) He II (1640 Ó) during outburst 1 (bottom spectra) and outburst 2 (top spectra). Data are displayed as Ñux F vs. wavelength (see text for details). j

910

SION ET AL.

An examination of two IUE archival high-resolution spectra of U Gem (SWP 40142HLG and SWP 40181HLG), both obtained during the peak (plateau phase) of the same outburst but separated by 5 days, reveals a similar line structure in the same three regions covered by GHRS, despite the orbital velocity spread introduced by the longer (1 h) IUE exposure times. Comparing the GHRS proÐles with the corresponding proÐles in the two echelle IUE images, we are able to establish that (1) the blueward and redward emission wings change relative strength from spectrum to spectrum (and presumably from outburst to outburst) and (2) the overall velocity structure of the double-peaked features remained essentially unchanged during the plateau phase of the outburst. Why is the Ðrst outburst spectrum at the onset of decline essentially featureless, while the second spectrum, obtained at the plateau phase of outburst 2, has strong doublepeaked emission with central absorption ? A similar behavior is seen in optical spectra of dwarf novae in outburst and quiescence. For example, during the decline from outburst of some dwarf novae, there is a transitional time period during which the optical spectrum appears nearly featureless as the pure absorption spectrum at outburst gradually transforms to a pure emission spectrum at quiescence (see Clarke, Capel, & Bowyer 1984). The essentially featureless HST spectra seen during the onset of decline of the Ðrst outburst may be the counterpart of the above-mentioned optical behavior. We note that the velocities associated with the emission lines and the total FWZI of the emission features is much lower than would be expected if these lines had formed in the inner disk region close to the white dwarf. The velocities for He II and N V listed in Table 2 are far below the Keplerian velocity at the white dwarf surface (4000 km s~1). Moreover, the total velocity widths of the He II and N V features appear to be narrower than the optical lines corresponding to the accretion disk of U Gem at outburst (Warner 1996). A direct comparison is impeded by the apparent absence of a published optical spectrum of U Gem in outburst. However, the published optical spectra of SS Cygni in outburst and quiescence, a system at lower inclination, o†er a useful comparison. The full velocity widths of the optical features of SS Cyg in outburst (Hessman et al. 1984) are 8600 km s~1. Thus, the velocity half-width for SS Cyg at Ha is 4300 km s~1, compared with 1815 km s~1 for N V (1240 Ó) in U Gem during outburst. The optical spectra of Hessman et al. (1984) show that the velocity half-widths of the Balmer lines of SS Cyg during quiescence are nearly the same as their half-widths in outburst. If the same relation between optical features in outburst and quiescence holds for U Gem, then the fact that the optical spectra of Stover (1981) for U Gem in quiescence yield a velocity halfwidth of 2500È3000 km s~1 supports our statement that the velocity half-widths of N V (1240 Ó) and He II (1640 Ó) during outburst are indeed much narrower than the optical lines during U GemÏs outburst. The fact that these emission proÐles with central absorption appear only at outburst and rapidly disappear at the onset of decline implies that it may be reasonable to link their formation with the intense radiation associated with the hot boundary layer demonstrated to be present at outburst by Long et al. (1996). This radiation could photoionize material close to the boundary layer (and probably above the disk plane) that produces the emission proÐles,

Vol. 483

while the thickened outer disk at outburst or material farther out produces the absorption at low velocity. The broad, shallow, low-excitation absorption seen at 1300 Ó during outburst 2 may represent still another line formation region distinct from the high-velocity, high-excitation emission and the low-velocity, high-excitation absorption. The above physical picture has been invoked successfully to explain the similar behavior of the optical emission lines with central absorption seen during the outbursts of SW Sex stars (Szkody & Piche 1990) and the outbursts of IP Peg (Piche & Szkody 1989 ; Marsh & Horne 1990). While the disk interpretation of the line proÐle behavior appears promising, three other possibilities remain. We note there are similarities between the emission-line structure of U Gem and the wind emission lines seen in the GHRS G160M spectra of the eclipsing prototype nova-like variable UX UMa (Mason et al. 1995). The emission lines of UX UMa appear blue asymmetric, and the doublet absorption is stationary and not as deep or symmetric as we observe in U Gem, but the FWZI of U GemÏs emission features are essentially the same velocity width as UX UMaÏs lines. For U GemÏs inclination of D70¡, the absorption should be deeper than in UX UMa, if it is indeed due to absorption by expanding gas viewed against the background disk continuum, but the absorption in U GemÏs proÐle is not stationary with respect to the binary as in the case of the C IV absorption of UX UMa. We also note that the He II and N V features appear as though a broad emission feature is centrally self-absorbed by an absorbing shell. The He II region, for example, reveals redshifted and blueshifted emission peaks, while the central absorption has a depth only slightly below the level of the continuum, which suggests that there is little emission Ñux at the line center. This may suggest that the emission is associated with an expanding optically thin shell, with the redshifted component produced by gas moving away from the observer and the blueshifted emission arising from approaching gas. However, the N V absorption features in outburst 2 are much deeper than the He II absorption. This could arise from the fact that since the N V feature remains in absorption into quiescence, that same source of N V absorption in quiescence may also be contributing at outburst. Finally, we mentioned the possibility that the doublepeaked features, with their velocities too low to be associated with the inner disk/boundary layer region, may represent a truncated disk. This could imply that the white dwarf is magnetic, which in turn could point to curtain accretion during outburst, not unlike an intermediate polar, rather than tangential accretion through a disk. Magnetic accretion could account for the slow observed rotation of the white dwarf, the absence of an obvious wind at outburst, the presence of EUV/soft X-rayÈabsorbing material above the disk plane during outburst, and the possible absence of a prominent, rapidly spinning accretion belt. However, a Ðeld strong enough to truncate the disk would probably have been detected by now. Since the disk instability model of dwarf nova outbursts predicts rapid accretion of the inner disk at outburst (Osaki 1996), the disk could be truncated in the sense that its inner part is missing altogether or too depleted of gas for it to be an important emitting contributor during outburst. Several models attempting to explain the type of outbursts and long outburst timescale in U Gem have invoked the truncation of the inner disk at

No. 2, 1997

HST GHRS OBSERVATIONS OF U GEM

quiescence (Zhang & Robinson 1987 ; Ludwig, MeyerHofmeister, & Ritter 1994). However, there is as yet no deÐnitive observational proof that disk truncation exists. 3.

SYNTHETIC ACCRETION DISK SPECTRA

Since, during the high-accretion phase associated with the outburst of U Gem, an accretion disk is expected to dominate the far-UV light, we have constructed synthetic accretion disk spectra to obtain data to compare with the GHRS data. Our computational procedure was as follows : We used J RaymondÏs code to calculate the radiation from the accretion disk (Raymond 1993). In this code the emitting Ñux from an area element on the disk surface is computed with IUE and Kitt Peak stellar Ñuxes provided by S. Kenyon. This simple method has been successfully applied to Cyg X-2, Sco X-1, and Her X-1 (Vrtilek et al. 1990, 1991 ; Cheng, Vrtilek, & Raymond 1995 ; Vrtilek & Cheng 1996). We divided the disk surface into 1000 annuli. For each annulus we calculated the temperature following standard optically thick accretion disk relations. Then we calculated the radiation from the area element using the method mentioned above. Since our observations were made in narrow (35 Ó) ranges surrounding the N V, Si III, and He II line regions, our wavelength coverage is grossly inadequate for precise synthetic spectral Ðttings. However, it is possible to roughly estimate the mass transfer rates by assuming that the main contribution arises from the accretion disk. Model calculations reveal that the Ñux level at binary phase 0.5 is higher by a factor of 1.2 than that at binary phase 0.0. Since the observations of the He II spectrum of outburst 2 were obtained around binary phase 0.0, we increased the He II Ñux by a factor of 1.2. Table 3 lists the parameters used in calculating the accretion disk models and the mass transfer

911 TABLE 3

SYNTHETIC DISK FITTING PARAMETERS

M (M ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . wd _ R (cm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . wd R (cm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d Distance (pc) . . . . . . . . . . . . . . . . . . . . . . . . . . Inclination angle (deg) . . . . . . . . . . . . . . . . Mass accretion rate (M yr~1) . . . . . . _

Outburst 1

Outburst 2

1.0 5.0 ] 108 4.0 ] 1010 78 70 6.3 ] 10~10

1.0 5.0 ] 108 4.0 ] 1010 78 70 2.1 ] 10~9

rates determined from the synthetic spectral Ðttings. The model spectra bearing the closest resemblance to the GHRS Ñux distribution are shown in Figure 3. The accretion rates in Table 3 are similar to previously derived values for U Gem in outburst (e.g., Panek & Holm 1984). 4.

CONCLUDING REMARKS

We have analyzed three sets of HST GHRS data during di†erent portions of two di†erent outbursts. The data were obtained at unprecedented far-ultraviolet spectral and temporal resolution for a dwarf nova in outburst. The three spectra obtained in the early decline of one outburst are essentially line featureless (except for weak N V absorption and broad Lya absorption), while the three spectra obtained at the peak of the second outburst reveal rich line proÐle structures suggestive of a photoionized truncated disk or gaseous shell. These proÐles bear little resemblance to the expected P Cyg line structure of an outÑowing wind, seen during outburst in other dwarf novae and nova-like variables in systems at low inclination (i \ 70¡) e.g., SS Cyg, YZ Cnc, IX Vel, and V3885 Sge ; see la Dous 1991, Drew, Hoare, & Woods 1991). However, the widths of the

FIG. 3.ÈCombined GHRS spectra for outburst 1 (bottom) and for outburst 2 (top) compared with the energy distribution of a synthetic spectrum for an optically thick accretion disk.

912

SION ET AL.

broad emission features of N V and He II are comparable to the emission widths of the eclipsing nova-like variables UX UMa (Mason et al. 1995), which suggests the possibility of a wind origin, although important di†erences are seen as well. Given the shortward and longward emission peaks with little Ñux at line center (as seen best in He II), a possible, albeit unlikely, interpretation of the proÐles could be that of an optically thin expanding shell associated with a discrete ejection event. We note that the velocities associated with the emission lines and the total FWZI of the emission features are much lower than would be expected if these lines had formed in the inner disk region close to the white dwarf. The velocities for He II and N V listed in Table 2 are far below the Keplerian velocity at the white dwarf surface (4000 km s~1). Moreover, the total velocity widths of the He II and N V features are much narrower than the optical lines corresponding to the accretion disk of U Gem at outburst. If the double-peaked features represent a truncated disk, then it is possible that the white dwarf is magnetic. This property would imply that curtain accretion not unlike an intermediate polar may be taking place during outburst (when the ram pressure of the accreting gas exceeds the magnetic pressure) rather than tangential accretion through a disk. This property could account for the slow observed rotation of the white dwarf, the possible absence of a wind at out-

burst, the narrowness of the far-UV double-peaked proÐles, and the apparent absence of a rapidly spinning accretion belt. Alternatively, the disk could be truncated as part of the outburst scenario. Finally, a synthetic disk spectral Ðtting analysis of the combined GHRS data sets for the two outbursts of U Gem yields an accretion rate of 6 ] 10~10 M yr~1 at the time of _ observation of the Ðrst outburst and 2 ] 10~9 M yr~1 at _ the time of observation of the second outburst. These values are not inconsistent with previous determinations of the outburst accretion rate in U Gem, based on optical and IUE spectra (see, e.g., Panek & Holm 1984). We thank an anonymous referee for pointing out the HST GHRS observations of UX Ursa Majoris and for useful comments. We acknowledge with gratitude the support of this work by NASA through grant GO5412.0193A (to Villanova University) and GO-05412.02-934 (to Los Alamos National Laboratory) from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Partial support was also provided by NASA Long-Term Space Astrophysics (LTSA) grant NAGW-3726 and NSF grant AST 90-16283, both to Villanova University, and by NASA LTSA grant NAGW-3158 to the University of Washington.

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