INTRODUCTION

Exoplanets are found to be ubiquitous across most stages of stellar evolution (e.g. Mayor & Queloz 1995 ;Vanderburg et al. 2020 ). To constrain a planet's bulk composition, measurements of its mass and radius are compared to theoretical mass-radius relationships for various interior compositions and structures (e.g. Seager et al. 2007 ;Dorn et al. 2015 ). Ho we v er, de generacies arise because different compositions can produce similar mass-radius curves, thus introducing uncertainties in determination of bulk compositions.

White dwarfs that have been 'polluted' by the accretion of elements heavier than helium, directly sample the bulk elemental composition of exoplanetary material; this is not possible with other observational techniques. Because of the strong surface gravity of white dwarfs their outer layers should contain only hydrogen or helium or both (Fontaine & Michaud 1979 ). Ho we ver, contrary to this, observ ations have re vealed that 25-50 per cent of single white E-mail: laura.rogers@ast.cam.ac.uk dwarfs have atmospheres that are 'polluted' with elements heavier than helium (Zuckerman et al. 2003(Zuckerman et al. , 2010 ; ;Koester, Gaensicke & Farihi 2014 ;Wilson et al. 2019 ). Due to the rapid gravitational settling times ( ∼ days for hot H-dominated DA white dwarfs, and ∼ millions of years for cool He-dominated DBs) in comparison to the white dwarfs' cooling age, there must be ongoing accretion of material (Koester 2009 ). This material is from remnant planetary systems that have survived to the white dwarf phase (Jura 2003 ;Farihi et al. 2010 ). Planetesimals from outer belts can become destabilized and are perturbed on to eccentric star grazing orbits (e.g. Debes & Sigurdsson 2002 ;Bonsor, Mustill & Wyatt 2011 ; Ve r a s et al. 2014 ; Mustill et al. 2018 ). There are several potential pathways that lead to the accretion of the planetary material: tidal disruption into dust, sublimation directly into gas, or direct collision with the white dwarf (Veras et al. 2014 ;Bonsor et al. 2017 ; Brown, Ve r a s & G änsicke 2017 ; McDonald & Ve r a s 2021 ; Steckloff et al. 2021 ;Brouwers, Bonsor & Malamud 2022 ). Spectroscopic observations of white dwarfs combined with atmospheric models reveal the chemical composition of the planetary material that has polluted each white dw arf. So f ar, 23 hea vy elements ha v e been disco v ered across all Note. * Spectroscopic parameters from Melis et al. ( 2020 ).

polluted white dwarfs (see table 1 in Klein et al. 2021 for references).

The polluted white dwarf GD 362 has absorption features from the most elements detected for a given white dw arf (e.g. Zuck erman et al. 2007 ;Xu et al. 2013 ).

In order to obtain absolute abundances of the polluting material, it is crucial to obtain accurate white dwarf parameters. These parameters are most often derived based on spectra, where the H and/or He lines are fitted with white dwarf models to infer the ef fecti ve temperature ( T eff ) and log ( g ) of the white dwarf, or from photometry, where broad-band photometry is fitted to obtain the ef fecti ve temperature, and the parallax is used to constrain log ( g ). Genest-Beaulieu & Bergeron ( 2019 ) find that the spectroscopically deri ved ef fecti ve temperatures of DA stars greater than 14 000 K, are higher than those derived by photometry by 10 per cent. This is thought to be due to the inaccurate treatment of Stark broadening. The selection of the photometric bands used in the fit for the photometric T eff cause the largest disparity in results; for hotter white dwarfs the u -band is crucial to obtain accurate parameters (Bergeron et al. 2019 ). Recent work by Izquierdo et al. ( 2023 ) highlights that for DB white dwarfs, different spectral data can result in a large spread of derived white dwarf parameters: 524 K in T eff , 0.27 dex in log ( g ), and 0.31 dex in log(H/He). Additionally, when deriving the parameters from photometric data, depending on the data used, a spread of 1210 K and 0.13 dex in T eff and log ( g ) respectively were found.

Previous studies have highlighted that there appears to be an optical and ultraviolet discrepancy, where the abundances of the polluting material derived from optical data are significantly discrepant from those derived from ultraviolet data (G änsicke et al. 2012 ;Jura et al. 2012 ;Xu et al. 2019Xu et al. ). G änsicke et al. ( 2012 ) ) consider that this could be due to uncertain atomic data, abundance stratification, or real v ariation. Gi ven that the optical and ultra violet ab undances are most often obtained from multiple studies where different white dwarf parameters are implemented, a more thorough investigation ensuring consistency is key to helping solve this issue.

Dust debris from tidally disrupted planetesimals has been discovered via excess infrared emission around 1.5-4 per cent of white dwarfs (e.g. Becklin et al. 2005 ;Kilic et al. 2006 ;Jura, Farihi & Zuckerman 2007 ;Rebassa-Mansergas et al. 2019 ;Wilson et al. 2019 ;Xu, Lai & Dennihy 2020 ). 21 of the white dwarfs with dust debris also sho w e vidence of circumstellar gas in emission near the same radius as the dust (G änsicke et al. 2006 ;G änsicke, Marsh & Southworth 2007 ;G änsicke et al. 2008 ;Melis et al. 2010 ;Brinkworth et al. 2012 ;Debes et al. 2012 ;Farihi et al. 2012a ;Melis et al. 2012Melis et al. , 2020 ; ;Dennihy et al. 2020 ;Gentile Fusillo et al. 2021 ). These systems are identified by their double peaked emission features, usually strongest at the Ca II infrared triplet. Gaia J0611 -6931 has the most elements detected in emission, with observations of Ca, O, Si, Mg, and Na (Dennihy et al. 2020 ;Melis et al. 2020 ). The gaseous systems show line profiles with Doppler broadened features consistent with the gas rotating as a Keplerian disc. A number of theories have been proposed to explain the production of gas. A proportion of the gas produced at the sublimation radius could viscously spread outwards causing an o v erlap in the location of the dust and gas (Rafikov 2011 ;Metzger, Rafikov & Bochkarev 2012 ). This outwardly spreading gas causes drag on the dust particles and thus accelerates their accretion on to the white dwarf creating a runaway effect; this might explain the highest accretion rates observed in polluted white dwarfs. An alternativ e e xplanation for gas emission is collisional cascades of planetesimals within the Roche radius of the white dwarf (Jura 2008 ;Kenyon & Bromley 2017a , b ), observations of infrared variability in WD 0145 + 234 appear consistent with simple collisional cascade models (Wang et al. 2019 ;Swan et al. 2021 ).

Circumstellar dust and gas around white dwarfs tell us about the current, potentially violent accretion of planetary material. With > 1000 polluted white dwarfs known, but only 21 systems with both detectable circumstellar dust and gas, this represents an intriguing subsample of polluted white dwarfs with different circumstellar environments. These systems are e xtreme e xamples of polluted white dwarfs, and as such they are perfect targets for studying pollution in their atmospheres and understanding how the planetary material ultimately ends up there. This work focuses on seven such systems, Paper I (this paper) focuses on the methods to obtain the abundances of the metals in the white dwarfs and the limitations involved, and Paper II (Rogers et al., in preparation) provides an in depth analysis of the composition of the planetary material accreted. This paper is structured as follows. Section 2 describes the optical and ultraviolet spectra of these seven systems taken with VLT X-shooter, Keck HIRES, Magellan MIKE, and HST COS. Section 3 explains the methods to determine the white dwarf parameters and the abundances of the metals in the white dwarfs. The effect on the abundances of differing white dwarf parameters and spectral ranges is reported in the results section in Section 4 . Section 5 discusses the results and limitations of the methods with the conclusions presented in Section 6 .

OBSERVATIONS AND DATA REDUCTION

Targets

The targets were selected as those with clear infrared excesses from a dust disc using data from WISE (Xu, Lai & Dennihy 2020 ), which were confirmed with Spitzer photometry (Lai et al. 2021 ). Dennihy et al. ( 2020 ) and Melis et al. ( 2020 ) report that these seven white dwarfs all host circumstellar gaseous discs. These seven systems are listed in Table 1 .

Downloaded from https://academic.oup.com/mnras/article/527/3/6038/7438903 by guest on 28 February 2024 ). For all observations, stare mode was used, with a 1.0 and 0.9 arcsec slit width for the UVB and VIS arms, respecti vely, this gi ves a resolving power ( λ/ λ) of 5400 and 8900. Two exposures were taken lasting 1700 and 1729 s each for the UVB and the VIS arms, respectively. The data reduction was performed using ESOREFLEX (v 2.11.3) with the X-shooter pipeline version 2.9.1 (Freudling et al. 2013 ). The standard reduction procedures were followed including minor alterations that impro v ed the signalto-noise ratio (SNR) of the output spectrum, and reduced the number of cosmic ray contaminants. The details of the X-shooter observations are listed in Table 2 . The SNR at the continuum around the Ca II K (3993 Å) line was 30-134 for the UVB arm and the SNR at the continuum around 6600 Åw a s 35-127 for the VIS arms depending on the flux of the white dwarf and observing conditions.

HIRES

Six of the white dwarfs were observed with the High Resolution Echelle Spectrometer (HIRES) on the Keck I Telescope, Hawaii (Vogt et al. 1994 ). This has 2 modes, HIRESb and HIRESr, with a wav elength co v erage of approximately 3200-5750 Åa n d 4700-9000 Å, respectively. The C5 decker was used, which has a slit width of 1.148 arcsec and a spectral resolution of 37 000. The observations were taken between 2019 July and 2020 October. Data reduction including bias subtraction, flat fielding, wavelength calibration, and spectral extraction were performed using MAKEE following Xu et al. ( 2016 ). The final spectra were continuum normalized using low order polynomials and combined using IRAF functions (Klein et al. 2010 ). The details of the HIRES observations are listed in Table 2 . The SNR for one observation was 32-89 around the continuum at the Ca K line for HIRESb and 31-73 around the continuum at 6600 Åf o r HIRESr.

MIKE

Gaia J0611 -6931 was observed with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph (Bernstein et al. 2003 ) on the 6.5 m Magellan Clay Telescope at Las Campanas Observatory on 2021 August 27 with one exposure of 1800 s followed by a second of 1300 s. Observations were taken at airmass 1.5 with the atmospheric dispersion corrector installed, but possibly not correcting the spectrum optimally, which would result in a lower SNR than expected in the blue. The SNR of the continuum was about 15 near the Ca infrared triplet. The seeing was 0.8 arcsec, and the employed 1 arcsec slit produces spectral resolution of R ≈ 28 000 on the blue side (3500-5060 Å) and 22 000 on the red side (5000-9400 Å). ThAr lamps taken before and after the exposures were used for wavelength calibration. Data reduction with the standard Carnegie PYTHON MIKE PIPELINE included extraction, flat-fielding, and wavelength calibration using methods described in Kelson et al. ( 2000 ) and Kelson ( 2003 ).

HST COS far ultraviolet spectra

Far ultraviolet (FUV) spectroscopic observations of four of the white dwarfs were conducted with the FUV channel of comic origins spectrograph (COS) on the Hubble Space Telescope ( HST ) (Programme ID: 16752), the observations are reported in Table 3 . The G130M grating was used with a central wavelength of 1291 Å, resulting in a wavelength coverage of 1150-1430 Å( 2 0 Åg a p in between the two segments). The data were reduced with the CALCOS reduction pipeline. The data were obtained using the TIME-TAG mode allowing the data taken when the Sun is below the geometric horizon from the point of view of HST ('night' data) to be separated from those taken when it is abo v e the horizon ('day' data). Data taken during the day can contain geocoronal contributions of Lyman alpha and O I emission lines, and all four of the white dwarf spectra show this. There are photospheric lines of O I and Si II that are blended with these emission features, therefore, for the three white dwarfs with night data, the STScI COS notebooksfoot_1 were used to separate out the night and day data. The COS2025 strategy means only FP-POS 3 and 4 can be used, the exposures were split between these positions and the final data resulted in a median stack between these observations. There are small discrepancies in the resolutions of these lifetime positions, ho we ver, tests re veal this af fects the final abundances by

HST COS near ultraviolet spectra

Near ultraviolet (NUV) spectroscopic observations of six of the white dwarfs were conducted with the NUV channel of COS (Programme ID: 16204). The data were reduced with the CALCOS reduction pipeline. The G230L grating was used with a central wavelength of 2950 Å. Unlike the well-calibrated FUV wavelength scale, this grating has a zero point accuracy to within 175 km s -1 , therefore the radial velocities of the lines are offset in comparison to those from the optical and FUV, as seen in Supplementary Tables B1-B13, thus the radial velocities of the NUV data are not meaningful. The SNR reported in Table 3 was calculated from the continuum around the Al III lines at 1854.716 and 1862.790 Å.

MODELLING METHODS

White dwarf atmospheric models were used to derive stellar parameters, and measure the abundance of the polluting material (Dufour et al. 2012 ).

White dwarf parameters

Two methods were used to derive the stellar parameters for each white dwarf. The first fitted white dwarf models to broad-band photometry (the photometric method), and the second fitted white dwarf models to the pressure broadened hydrogen and helium spectral lines (the spectroscopic method). The deri ved v alues for the spectroscopic and photometric methods are reported in Table 1 . These methods are discussed in more detail below.

Photometric method

For the photometric method, white dwarf models were fitted to SDSS, P an-STARRS, Sk yMapper, and GALEX broad-band photometry with parallaxes from Gaia to extract the best-fitting ef fecti ve temperature and log ( g ). For the two DBs in the sample, fixed values of H/He were used that matched the values measured from the spectra, as described in the following section. SDSS to AB corrections were included as outlined in Eisenstein et al. ( 2006 ). Reddening becomes important for objects > 100 pc, and five of the white dw arfs f all into this distance range. For these white dwarfs the observational data were de-reddened using the method as described in Genest-Beaulieu & Bergeron ( 2019 ) before fitting. When available, a combination of SDSS ugriz and Pan-STARRS grizy were used to constrain the white dwarf parameters. 2 As discussed in Genest-Beaulieu & Bergeron ( 2019 ), this provides the most accurate and consistent results. Otherwise, either the SDSS ugriz , Pan-STARRS grizy , or SkyMapper ugrizy were used.

Spectroscopic method

The spectroscopic method fits synthetic white dwarf model spectra to the hydrogen and helium optical absorption lines to extract the best-fitting ef fecti ve temperature and log ( g ). Updated white dwarfs parameters were found for the four white dwarfs with X-shooter data presented here, and for WD 1622 + 587 using the KAST data from Melis et al. ( 2020 ). For the two remaining objects, the parameters derived in Melis et al. ( 2020 ) were used. The model fits to the Balmer lines for the three DA white dwarfs (Gaia J0006 + 2858, Gaia J0611 -6931, and Gaia J2100 + 2122) are shown in Fig. 1 . Doubled peaked emission lines from the circumstellar gas discs are in the hydrogen lines, these features represent a small fraction of the frequency points and are not found to affect the derived parameters.

For the DB white dwarfs, GaiaJ0644 -0352 and WD 1622 + 587, models were fitted to the Helium lines as shown in Fig. 2 , trace H is also present, and this abundance was also determined in this fit. Heavy elements and hydrogen in cool DBZ stars may affect the pressure/temperature structure and therefore affect the derived white dwarf parameters (Dufour et al. 2012 ;Coutu et al. 2019 ). It was tested whether including heavy elements in the models affected the derived parameters; as these white dwarfs are hot ( T eff > 20 000 K), the inclusion of heavy elements had a negligible affect on the derived white dwarf parameters.

The uncertainties in the spectroscopically deri ved ef fecti ve temperature and log ( g ) for DA white dwarfs are from Liebert, Bergeron & Holberg ( 2005 ), 1.4 per cent in T eff and 0.042 dex in log ( g ). For the two DBA white dwarfs, the uncertainties derived in Izquierdo et al. ( 2023 ) are used 524 K and in T eff and 0.27 dex in log ( g ). These are uncertainties on the fitting procedure and do not encapsulate uncertainties from the model atmospheres.

Line identification

A multitude of lines from heavy elements were identified in the spectra of each white dwarf. Using the IRAF task SPLOT (Tody 1986 ) the equi v alent width, line centre, and radial velocity of each spectral line was measured. The atomic data bases of Vienna Atomic Line Database (VALD) 3 , National Institute of Standards and Technology MNRAS 527, 6038-6054 (2024) (NIST)foot_5 , and Va n Hoof ( 2018 ), as well as published line lists of polluted white dwarfs observed with Keck and HST (e.g. Klein et al. 2011 ;G änsicke et al. 2012 ;Jura et al. 2012 ) were utilized to identify which element species are associated with the spectral features. For the equi v alent width, a Voigt function was fitted to the profile of the line five times whilst changing the region used for the continuum fitting. From this the average equi v alent width and standard deviation for each line was found, this was compared to a direct flux summation to ensure accuracy. The uncertainty on the equi v alent width was calculated by combining in quadrature the standard deviation of the additional uncertainties when deriving abundances of metals in the photospheres of white dwarfs. Table E1 shows the non-photospheric measurements of lines in the spectra of the seven white dwarfs in this study. These lines are usually offset from the velocity of the photospheric lines so can be distinguished. For the ultraviolet wavelengths, an interstellar medium model was used to fit Voigt profiles to the non-photospheric contributions, enabling abundance determination for the photospheric component.

There are two Si IV lines observed in the photosphere of Gaia J0006 + 2858 at wavelengths of 1393.76 and 1402.77 Å. Both lines have a blueshifted component offset from the silicon rest frame wavelengths by -196 km s -1 , with measured line centres of 1392.84 and 1401.85 Åa s shown in Fig. 3 . From the atomic data bases, absorption from other elemental species is ruled out. The relative shift between the line centres from the blueshifted component compared to the Si IV photospheric component are: 216 and 222 km s -1 , respectively. It is likely that these two absorption lines are blueshifted Si IV lines. Additional absorption components to these Si IV lines have been previously observed and are thought to be circumstellar absorption from close in hot gas, ho we ver, the velocity offset is much less extreme (G änsicke et al. 2012 ;Fortin-Archambault, Dufour & Xu 2020 ). The width of these blueshifted lines seen in Gaia J0006 + 2858 is consistent with the range in v elocities e xpected for a gas disc which occults the white dwarf. No circumstellar silicon gaseous emission features are observed (Melis et al. 2020 ), so the radial extent of the silicon part of the gas disc cannot be compared. More detailed models of the gas disc are required to understand these observations. The other three white dwarfs observed in the FUV show no additional Si IV absorption components.

Abundance of polluting metals

The abundances of metals in the atmosphere of each white dwarf were measured following Dufour et al. ( 2012 ). The spectra were divided into panels which co v er a re gion of 5-15 Å around each absorption line. The white dwarf ef fecti ve temperature and log ( g ) was inputted, and the best-fitting abundance for that spectral line was found. The abundances of the lines were fitted using the ef fecti ve temperature and log ( g ) from the photometric and spectroscopic methods separately. When more than one line of a particular element are present in the 5-15 Å region, the lines are fitted together.

Absorption features in the presence of emission features

Some absorption lines also have gaseous emission features present at the same wavelength which makes it difficult to disentangle the contribution from the photosphere from the circumstellar emission. In Supplementary Tables B1-B13 those lines which have photospheric absorption at the same wavelength as the gaseous emission features are noted. High order polynomials are fitted to the spectra to normalize out the broader gaseous emission features. In Klein et al. ( 2010 ) it is noted that through tests which varied the order of the normalization polynomial, the effect of continuum normalization on narrow absorption lines in the presence of bumpy features was < 1 per cent. Ho we ver, in this sample, the spectra contain both broad and sharp gaseous emission features which are less trivial to normalize out in order to obtain accurate equi v alent widths of the photospheric features. To test how the sharp gaseous emission features affect the derived equivalent widths, tests were performed on the 3933 ÅC a II line in Gaia J0006 + 2858 and 7771 ÅO I line in Gaia J0510 + 2315. The equi v alent widths were measured using SPLOT , as explained in Section 3.2 , using both the un-normalized and normalized HIRES spectra. The average equi v alent width de viation was found to be 12 per cent. For those spectral lines with gaseous emission features, this additional error of 12 per cent was added in quadrature with the equi v alent width error. Gaia J0006 + 2858 has abundances determined from two calcium lines, 3179 and 3933 Å, where the later has a weak gaseous emission feature at the same wa velength. The calcium ab undance derived from 3933 Åa n d that derived from 3179 Åa r e consistent within the errors of 0.1 dex, providing confidence that the derived abundances in the presence of gaseous emission features are accurate.

Upper limits

Some important elements are not detected, and so an equi v alent width upper limit that would have resulted in a 3 σ spectral line detection was calculated. Either a detection or upper limit was determined for these elements for each white dwarf: C, O, S, P,

from https://academic.oup.com/mnras/article/527/3/6038/7438903 by guest on 28 February 2024 MNRAS 527, 6038-6054 (2024)

Table 4. Number abundances (log n(Z)/n(H( e)) of the material polluting the white dwarfs calculated using the photometric and spectroscopic white dwarf parameters separately. For the deri v ation of the upper limits the spectroscopic solutions are used. A dash denotes that it was not possible to derive an abundance or upper limit for this element due to strong gaseous emission lines present, strong non-photometric features contaminating the spectrum, or no data available in the required wavelength range.

[X/H(e)] Spec/Phot UV/Op Gaia J0006 Gaia J0347 Gaia J0510 Gaia J0611 ‡ Gaia J0644 WD 1622 Gaia J2100 H -------5.16 -3.37 -C Spec UV -6.90 ± 0.10 -< -8.27 -7.15 ± 0.10 --4.75 ± 0.11 -" Phot UV -6.93 ± 0.10 --7.29 ± 0.10 --4.69 ± 0.25 -O Spec Op < -3.92 * --4.23 ± 0.11 -3.75 ± 0.16 -5.17 ± 0.13 < -4.46 * < -4.10 * " Phot Op --4.35 ± 0.11 -3.84 ± 0.16 -5.60 ± 0.13 " Spec UV -4.48 ± 0.10 --4.98 ± 0.12 -4.28 ± 0.10 --5.39 ± 0.10 -" Phot UV -4.54 ± 0.10 --5.07 ± 0.12 -4.33 ± 0.10 --5.80 ± 0.10 -S Spec UV -6.35 ± 0.12 --6.20 ± 0.10 -5.30 ± 0.25 --5.99 ± 0.10 -" Phot UV -6.46 ± 0.11 --6.19 ± 0.10 -5.47 ± 0.14 --6.01 ± 0.24 -P Spec UV -7.39 ± 0.22 -< -7.90 -7.49 ± 0.12 --7.74 ± 0.15 -" Phot UV -7.55 ± 0.22 --7.66 ± 0.12 --8.01 ± 0.15 -Na Spec Op < -5.11 -< -5.44 -< -5.65 < -4.67 < -5.18 Mg Spec Op -4.95 ± 0.10 -5.78 ± 0.16 -5.23 ± 0.10 -4.61 ± 0.11 -5.73 ± 0.10 -4.91 ± 0.10 -5.08 ± 0.10 " Phot Op -5.03 ± 0.10 -6.05 ± 0.16 -5.35 ± 0.10 -4.68 ± 0.11 -6.33 ± 0.10 -5.49 ± 0.17 -5.35 ± 0.10 " Spec UV ------4.76 ± 0.10 -5.23 ± 0.10 " Phot UV ------5.27 ± 0.10 -5.57 ± 0.10 Al Spec Op < -5.60 < -5.20 < -5.30 < -4.50 -6.76 ± 0.11 < -5.90 < -5.80 " Phot Op -7.05 ± 0.11 " Spec UV -6.5 ± 0.18 -7.34 ± 0.20 -7.04 ± 0.10 -6.46 ± 0.10 --6.28 ± 0.10 -6.48 ± 0.10 " Phot UV -6.5 ± 0.18 -7.26 ± 0.20 -6.99 ± 0.15 -6.71 ± 0.18 --6.38 ± 0.10 -6.49 ± 0.10 Si Spec Op -4.93 ± 0.10 < -5.30 -5.12 ± 0.10 -4.70 ± 0.11 -5.97 ± 0.10 -5.20 ± 0.10 -5.12 ± 0.10 " Phot Op -5.03 ± 0.10 -5.13 ± 0.10 -4.72 ± 0.15 -6.26 ± 0.10 -5.68 ± 0.10 -5.36 ± 0.10 " Spec UV -5.48 ± 0.10 --5.85 ± 0.17 -5.22 ± 0.10 --5.20 ± 0.10 -" Phot UV -5.50 ± 0.10 --5.88 ± 0.11 -5.24 ± 0.10 --5.18 ± 0.17 -Ca Spec Op -6.17 ± 0.10 < -5.68 ± 0.11 † -6.31 ± 0.10 -6.08 ± 0.15 -6.70 ± 0.10 -5.85 ± 0.10 -6.21 ± 0.11 " Phot Op -6.32 ± 0.10 < -5.92 ± 0.35 † -6.80 ± 0.10 -6.37 ± 0.14 -7.41 ± 0.10 -7.02 ± 0.10 -6.65 ± 0.10 Ti Spec Op < -6.30 < -6.43 < -5.86 < -5.64 -8.35 ± 0.11 < -6.77 < -6.69 " Phot Op -9.13 ± 0.10 Cr Spec Op < -5.71 < -5.47 < -4.53 < -3.50 -7.80 ± 0.10 < -5.74 < -5.78 " Phot Op -8.56 ± 0.10 Fe Spec Op < -4.66 * < -4.38 * < -4.10 * --6.51 ± 0.10 < -5.23 * -4.96 ± 0.14 " Phot Op --7.15 ± 0.10 -5.49 ± 0.14 " Spec UV < -5.00 -< -5.60 -5.23 ± 0.10 --5.26 ± 0.10 -" Phot UV --5.45 ± 0.10 --5.55 ± 0.13 -Ni Spec Op < -4.99 < -4.67 < -3.77 < -3.00 < -7.21 < -5.10 < -5.11 " Spec UV < -6.50 -< -7.20 -6.83 ± 0.10 --6.29 ± 0.24 -" Phot UV --7.05 ± 0.10 --6.71 ± 0.28 -Notes. * Denotes gaseous emission present when derived upper limit.

† Non-photospheric lines at approximately the same radial velocity as white dwarf photosphere, so this should be treated as an upper limit.

‡ Cu measured for Gaia J0611 -6931 to 2.8 σ , abundances are: -7.87 ± 0.18 and -7.94 ± 0.18 for the spectroscopic and photometric parameters respectively, see Fig. D6 for the model fit.

Na, Mg, Al, Si, Ca, Ti, Cr, Fe, and Ni. Around the strongest line for a particular element, a spectral line was artificially inserted at decreasing values of equivalent width, corresponding to decreasing abundance. From this, the significance of the absorption feature was calculated, and repeated 10 000 times. The equi v alent width upper limit was taken to be the point at which 99.7 per cent of the lines were detected at 3 σ for a certain equi v alent width. The equi v alent width upper limits are reported in Table C1 . White dwarf models MNRAS 527, 6038-6054 ( 2024)

were used to convert from equi v alent width to abundance assuming the spectroscopically derived T eff and log ( g ); the abundance upper limits are reported in Table 4 . As mentioned in Section 4.2 , the hotter the ef fecti ve temperature of the white dwarf used in the models, the larger the abundance is for the same spectral line and equi v alent width. Therefore, the abundance upper limits consider the abundance error associated with ef fecti ve temperature and are applicable to both the spectroscopic and photometric abundances.

RESULTING ABUNDANCES

Abundances of the accreted material

This work presents seven polluted white dwarfs with well characterized abundances of multiple elemental species. For the optical data, the a verage ab undances from X-shooter and HIRES (or X-shooter and MIKE for Gaia J0611 -6931) are reported in the Supplementary Tables B1-B13. For white dwarfs with observations from different instruments, the abundances are consistent within the uncertainties when measuring the abundance from the high resolution ( R ≈ 40 000) and lower resolution ( R ≈ 5000-9000) spectrographs separately. The higher resolution data provides more spectral lines and elemental species for abundances to be measured. For those white dwarfs with interstellar or circumstellar absorption features, lower resolution data is unable to distinguish these features and consequently gi ves a decepti vely higher abundance. If the system is free from interstellar and circumstellar absorption, then low resolution and high resolution data give consistent ab undances, b ut depending on the requirements, higher resolution ( > 40 000) spectra may be preferable.

The abundances of the planetary material polluting these seven white dwarfs are reported in Table 4 . For those white dwarfs observed with both high resolution and lower resolution instruments, the a verage ab undance of these is used, weighted by the number of panels used to derive the abundance for that instrument. The reported uncertainties have tw o k ey contributions: the spread in abundances derived for a particular element, and the error associated with the measured equi v alent widths. The spread error is taken as the standard error ( σ/ √ N ) of the abundances derived from the panels, and the equi v alent width error is calculated by propagating the individual equi v alent width errors for each spectral line used in the abundance calculation. These two contributions are added in quadrature to give the error for each abundance. If only one line of a particular element is present, the average spread error based on the observations (0.08 dex) was added in quadrature with the equi v alent width error. Given this spread as well as additional unknown uncertainties, an uncertainty floor of 0.1 dex is used. Examples of unknown uncertainties are: limits of the atomic data, uncertainties in white dwarf parameters, and uncertainties introduced from the white dwarf models. Supplementary Figs D1-D11 show the abundances fit to the strongest spectral line for each element in each white dwarf.

Spectroscopic versus photometric white dwarf parameters used to deri v e abundances

The spectroscopic and the photometric methods for determining white dwarf parameters result in different sets of absolute abundances derived for the pollutant planetary material. All derived white dwarf ef fecti ve temperatures are hotter for the spectroscopic method than for the photometric method. Derived heavy element abundance correlates with temperature, so the abundances of heavy elements when compared to the abundance of the principal element (H or He) as derived from the spectroscopic white dwarf parameters are larger. Figs 4 (a) and (b) compare the absolute abundances compared to the abundance ratios, the ˜ χ 2 when testing the goodness-of-fit of a horizontal line at 0 is 10.24 for the absolute abundances versus 1.03 for the abundance ratios. Therefore, abundance ratios are less affected by differing white dwarf parameters for the ef fecti ve temperature range of the white dwarfs in this sample. Fig. 4 (b) shows that the Ca/Mg abundance ratio is most discrepant when comparing spectroscopic versus photometrically derived abundances. This is likely due to the ionization levels of calcium being particularly sensitive to T eff .

Optical versus ultraviolet spectra to derive abundances

The abundances of the material polluting the white dwarfs were determined based on both optical and ultraviolet data. There are discrepancies between these abundances for the three DAZ white dwarfs, as shown in Fig. 5 . The difference between the abundances appears constant with a mean offset of 0.62 dex. In this work, the one DBZ with FUV data does not sho w an apparent of fset, ho we ver, previous studies (e.g. Jura et al. 2012 ;Xu et al. 2019 ) do observe a discrepancy.

Accretion rates

The white dwarfs in this sample represent seven of just 21 polluted white dwarf systems with detectable circumstellar gas and dust discs. Theory suggests that white dwarfs with circumstellar gas discs may have enhanced accretion from gas drag (Rafikov 2011 ) and so polluted white dwarfs with gas are hypothesized to accrete at higher rates than the general population of white dwarfs. A systematic approach is taken and investigates whether these systems are distinct in terms of their accretion rate properties. The mass accretion rates were calculated from the magnesium abundance, assuming magnesium makes up 15.8 per cent of the total mass, as in Bulk Earth (All ègre, Manh ès & Lewin 2001 ). Xu et al. ( 2019 ) demonstrated this as a more reliable and consistent way to measure and compare accretion rates for a sample of polluted white dwarfs, therefore only objects with magnesium abundance measurements are included. The derived mass accretion rates for white dwarfs with circumstellar gaseous emission discs with measured photospheric abundances are shown in Table 5 , and those without circumstellar gaseous emission discs are shown in Table 6 . For the seven white dwarfs in this work, the accretion rates were found using the spectroscopic white dwarf parameters and the photometric parameters. The coolest white dwarf disco v ered with a circumstellar gaseous disc is WD 0145 + 234, with a T eff of 12 720 K (Melis et al. 2020 ), thus, when comparing to the population of white dwarfs without circumstellar gas discs, the lo wer ef fecti ve temperature cut of f was set at 12 720 K. There is no correlation between temperature (white dwarf cooling age) and accretion rate abo v e 10 000 K (Wyatt et al. 2014 ;Xu et al. 2019 ).

The accretion rates as a function of white dwarf ef fecti ve temperature are plotted in Fig. 6 , which compares those white dwarfs with detectable circumstellar gas discs to the population of white dwarfs without circumstellar gas discs. The mass accretion rates differ by factors of 1.6-4.2 between the rates derived from the spectroscopic versus photometric white dwarf parameters; this demonstrates that accurate white dwarf parameters are important for determining accurate accretion rates. The white dwarfs with circumstellar gas have accretion rates that span the full range of accretion rates in the plot.

A Kolmogoro v-Smirno v (KS) test was used to test the Downloaded from https://academic.oup.com/mnras/article/527/3/6038/7438903 by guest on 28 February 2024 MNRAS 527, 6038-6054 (2024) The ˜ χ 2 when testing the goodness of fit of a horizontal line at 0 is 10.24 for the absolute abundances versus 1.03 for the abundance ratios. The abundance ratios are less affected by the difference between the spectroscopic and photometric white dwarf parameters.

Figure 5. For the white dwarfs with both FUV and optically derived abundances, the difference between the optical and ultraviolet abundances are shown. For the DAZ white dwarfs, the offset appears constant (mean offset of 0.62), and for the one DBZ there is no apparently offset. null hypothesis that the two samples, the mass accretion rates of those white dwarfs with detectable gas discs versus those without, come from the same distribution. For both the accretion rates derived from the spectroscopic white dwarf parameters and the photometric parameters, the p -values are found to be large, and therefore, the null hypothesis cannot be rejected, and it remains plausible that the two samples came from the same distribution. Therefore, there is no evidence for enhanced accretion rates for white dwarfs with circumstellar gas discs compared to those without detectable circumstellar gas discs.

DISCUSSION

This paper presents the abundances of the planetary material accreted by seven white dwarfs with circumstellar gas and dust. They represent seven of just 21 known polluted white dwarfs with circumstellar gas emission discs; it is crucial to understand the interplay between accretion, observed gas, and atmospheric pollution. The abundances are derived from optical and ultraviolet spectra for two different sets of white dwarf parameters. The absolute abundances in the white dwarf photosphere are most affected by uncertainties in the derived stellar parameters, as well as the quality of the spectroscopic data and the white dwarf models used to analyse and obtain the abundances. Crucially, ho we ver, interpretation of the observed compositions, which is based on elemental ratios, are less affected by uncertainties in the stellar parameters.

In order to determine elemental abundances from the data, a number of systematics must be considered. Sometimes only one (or few) absorption line(s) of a particular element are present in the spectrum (see Supplementary Tables B1-B13 for details). This is especially important for the five DAZ white dwarfs where there are few absorption lines in the optical and they can be weak. Uncertain atomic data can also cause additional uncertainties to arise (Vennes, Kawka & N émeth 2011 ). The abundances are also limited by the white dwarf models, for example, 3D effects such as convection may affect the derived abundances (Cunningham et al. 2019 ).

This work corroborates previous studies which identified the differences in deriving white dwarf parameters from spectral analysis compared to those derived from broad-band photometry. Above 14 000 K spectroscopic T eff , and therefore also log ( g ), can exceed photometric T eff by 5-10 per cent (Genest-Beaulieu & Bergeron 2019 ). All white dwarfs in this study fall into this range, and indeed this is reflected in the derived stellar parameters. The most accurate white dwarf parameters derived from broad-band photometry are those that include the SDSS u -band with additional optical photometry, e.g. Pan-STARRS (Bergeron et al. 2019 ). For those white Table 5. Total accretion rates based on Mg abundances for white dwarfs with an observable gaseous disc with emission features. This is calculated using Ṁ = (100 / 15 . 8) × M WD × 10 q × 10 [ Mg / H(e) ] × A Mg/H(e) /τ Mg , where q = log 10 ( M CVZ / M WD ), A Mg/H(e) is the atomic mass of Mg divided by the atomic mass of H or He, depending on the dominant atmospheric (atm) constituent, and τ Mg is the sinking time of Mg. For the seven white dwarfs in this paper, the accretion rates are calculated for both the spectroscopic and photometric white dwarf parameters and abundances.

WD Name

Atm

yr) log(Mg/H(e)) Ṁ (g s -1 ) Reference Gaia J0006 + 2858 H 23 920 8.04 0.66 -15.5 -1.43 -4.95 5.86 × 10 8 Spec, this work Gaia J0006 + 2858 H 22 840 7.86 0.56 -15.7 -1.49 -5.03 2.75 × 10 8 Phot, this work Gaia J0347 + 1624 H 21 820 8.10 0.69 -16.2 -1.93 -5.78 6.04 × 10 7 Spec, this work Gaia J0347 + 1624 H 18 850 7.84 0.54 -16.2 -1.78 -6.05 1.72 × 10 7 Phot, this work Gaia J0510 + 2315 H 21 700 8.22 0.76 -16.6 -2.26 -5.23 2.12 × 10 8 Spec, this work Gaia J0510 + 2315 H 20 130 8.13 0.70 -16.5 -2.19 -5.35 1.33 × 10 8 Phot, this work Gaia J0611 -6931 H 17 750 8.14 0.70 -16.7 -2.32 -4.61 6.57 × 10 8 Spec, this work Gaia J0611 -6931 H 16 530 7.81 0.51 -16.3 -1.84 -4.68 3.56 × 10 8 Phot, this work Gaia J0644 -0352 He 18 350 8.18 0.70 -6.4 5.29 -5.73 5.89 × 10 9 Spec, this work Gaia J0644 -0352 He 17 000 7.98 0.58 -5.8 5.85 -6.33 1.41 × 10 9 Phot, this work WD 1622 + 587 He 23 430 7.80 0.50 -7.3 4.92 -4.91 8.42 × 10 9 Spec, this work WD 1622 + 587 He 21 530 7.98 0.59 -6.9 5.08 -5.49 4.61 × 10 9 Phot, this work Gaia J2100 + 2122 H 25 570 8.10 0.69 -15.5 -1.47 -5.23 3.58 × 10 8 Spec, this work Gaia J2100 + 2122 H 22 000 7.92 0.59 -15.8 -1.59 -5.35 1.38 × 10 8 Phot, this work WD 0145 + 234 H 12 720 8.10 0.67 -15.6 -1.55 -5.90 6.35 × 10 7 Melis et al. ( 2020 ) SDSS J0738 + 1835 He 13 950 8.40 0.84 -6.0 5.45 -4.68 1.41 × 10 11 Dufour et al. ( 2012 ) WD 0842 + 572 H 16 225 8.00 0.62 -16.6 -2.16 -3.90 2.78 × 10 9 Melis et al. ( 2020 ) SDSS J0845 + 2257 He 19 780 8.18 0.71 -6.7 5.08 -4.70 5.13 × 10 10 Wilson et al. ( 2015 ) SDSS J0959 -0200 H 13 280 8.06 0.64 -15.8 -1.59 -5.20 2.66 × 10 8 Farihi et al. ( 2012a ) SDSS J1043 + 0855 H 17 880 8.12 0.69 -16.7 -2.29 -5.15 1.86 × 10 8 Manser et al. ( 2016 ) SDSS J1228 + 1040 H 20 900 8.15 0.71 -16.5 -2.15 -5.10 2.62 × 10 8 G änsicke et al. ( 2012 ) HE 1349 -2305 He 18 170 8.13 0.67 -6.3 5.40 -6.50 9.62 × 10 8 Melis et al. ( 2012 ) SDSS J1617 + 1620 H 13 520 8.11 0.68 -15.9 -1.70 -5.02 * 4.16 × 10 8 Wilson et al. ( 2014 ) Note. * Using the Convective at τ R = 3.2 case.

dwarfs with GALEX photometry, the GALEX FUV and/or NUV band magnitudes help us to constrain the white dwarf parameters in a similar way to the u band providing increased accuracy of the parameters. There are significant systematic uncertainties associated with the stellar parameters and a hybrid approach similar to Izquierdo et al. ( 2021 ) may result in impro v ed parameters.

The heavy element abundance uncertainties quoted in Table 4 are measurement errors and do not include systematic errors arising from the different methods of obtaining white dwarf parameters. Instead the abundances based on the two sets of stellar parameters are considered. The difference in T eff and log ( g ) between the two sets of stellar parameters are between 1000-3500 K and 0.1-0.25 de x, respectiv ely. In order to obtain reliable and accurate absolute abundances of the accreted planetary material it is crucial to obtain accurate stellar parameters. Ho we ver, as sho wn in Fig. 4 (b) the relative elemental ratios, which are used to interpret the observed compositions, when comparing abundances derived from the spectroscopic stellar parameters compared with the photometric stellar parameters. The ratios are less sensitive to the derived stellar parameters. Therefore, until the uncertainties on the absolute abundances can be reduced, the differences between the photometric and spectroscopic T eff and log ( g ) will have little affect on the abundance analysis based on the relative ratios of elements. If absolute abundances are important, as when deriving accretion rates, the variations induced by differing white dwarf parameters must be considered. Table 5 and Fig. 6 show the difference in derived accretion rates for the seven white dwarfs when using the spectroscopic parameters versus the photometric parameters. The accretion rates can change by up to a factor of 4.2 which would consequently affect inferences about the accretion and mass of the planetesimal currently present in the white dwarf photosphere. Therefore, when considering accretion rates, careful error propagation considering these systematic errors are crucial.

It has been addressed in the literature that there exists an optical and ultraviolet discrepancy, where the abundances determined from optical data are offset from those derived from the ultraviolet data (G änsicke et al. 2012 ;Jura et al. 2012 ;Xu et al. 2019 ). This work uses a systematic approach, ensuring that the same white dwarf parameters are used when deriving abundances from the optical and ultraviolet. For the three DAZ white dwarfs in the sample, this discrepancy is observed, with an approximately constant offset of 0.62 observed between O and Si derived from the optical and ultraviolet. As is highlighted in previous works, atomic data uncertainties, accretion rate variation, or imperfect white dwarf atmosphere calculations may contribute to this ef fect. Fig. 7 (a) sho ws the depth of formation of the lines in the ultraviolet and the optical. Depth of formation is the Rosseland optical depth, τ r , at which τ ν = 2/3 for each line wavelength. The lines that form in the optical come from the same depth, whereas those from the ultraviolet form from a range of depths. Fig. 7 (b) shows that there is not an abundance dependence on the depth of formation, and lines that form at the same depth have different derived abundances. There may be issues with the white dwarf structure calculations, ho we ver , further in vestigations are outside the scope of this paper. Therefore, until the origin of this discrepancy is discovered, when analysing polluted white dwarf abundances, care must be given when combining results from the optical and UV.

Previous work has found that detectable circumstellar gas is a rare phenomena in polluted white dwarfs (Manser et al. 2020 ). Xu et al. ( 2019 ) compared pollution levels for those white dwarfs with and without a detectable circumstellar dust disc, compiling their work with data from the literature (Koester et al. 2011 ; Koester, Gaen- 2019) is expanded to compare the pollution rate between white dwarfs with and without detectable circumstellar gaseous discs, where without refers to all polluted white dwarfs without any evidence of circumstellar gas in emission. Previous studies investigating the link between the presence of a detectable gaseous disc and accretion rate based on a handful of systems found no correlation between accretion rate and the presence of circumstellar gas (Manser et al. 2016(Manser et al. , 2020 ) ). Ho we ver, gas drag may cause enhanced accretion rates abo v e what may be expected from Poynting-Robertson drag alone.

Combining white dwarfs with detectable circumstellar gas discs from the literature (which have reported abundances of at least Mg in their photosphere) with the seven in this work, the sample size is almost doubled. The white dwarfs that have circumstellar gaseous discs appear to trace the mass accretion rates of polluted white dwarfs without detectable gaseous discs, therefore, there is no evidence that accretion of material onto the white dwarfs is enhanced by gas drag.

If instead it was found that gas discs systems accrete at higher rates, it would be difficult to confirm whether the presence of gas causes enhanced accretion, or vice-versa, whether the enhanced accretion results in gas. These conclusions are limited by both small number statistics and that the white dwarfs not studied in this paper have accretion rates derived from different methods of obtaining white dwarf parameters, therefore, the accretion rates for these could be inaccurate. Fig. 6 highlights how the accretion rates of He dominated white dwarfs are higher than those in H dominated atmospheres (Farihi et al. 2012b ;Xu et al. 2019 ). This may be due to the orders of magnitude longer settling time-scales for He dominated white dwarfs and therefore the rates represent an average historical accretion rate. Direct impacts on to the white dwarf (Brown, Ve r a s & G änsicke 2017 ;

McDonald & Ve r a s 2021 ), or the disruption of massive asteroids ( > 500 km) which can collisionally evolve to produce enhanced accretion on short time-scales (Wyatt et al. 2014 ;Brouwers, Bonsor & Malamud 2022 ) can contribute to larger measured average accretion rates for DBZ white dwarfs. Ho we ver, for the sample of objects with dust and gas discs, direct impacts are unlikely to explain the enhanced Figure The accretion rates of white dwarfs with and without detectable gaseous discs in emission as a function of their ef fecti ve temperatures. The data for the accretion rates of polluted white dwarfs without observable circumstellar gaseous discs are from Table 6 , and the accretion rates for gaseous disc systems are from Table 5 . The seven systems reported in this work show both the spectroscopic and photometrically derived accretion rates and these are connected by dashed lines, where the higher ef fecti ve temperatures are those derived from the spectroscopic method and have a black outline. accretion rates as direct impacts do not result in circumstellar discs. Thermohaline mixing may account for some differences between DA and DB accretion rates, but it should be stated the effect is debated (Koester 2014 ). DBZ white dw arfs w ould experience less thermohaline mixing than DAZ white dwarfs (Bauer & Bildsten 2019 ). When including thermohaline mixing into the white dwarf models, larger accretion rates are required in order to account for this instability (Bauer & Bildsten 2018 ). As it disproportionally affects DA white dwarfs, the accretion rates would need to be orders of magnitude larger for the DB white dwarfs to make DA and DB consistent using thermohaline mixing models.

CONCLUSIONS

This paper presents VLT X-shooter, Keck HIRES, and Magellan MIKE optical spectroscopy and HST COS ultraviolet spectroscopy of seven white dwarfs that host both detectable circumstellar gas and dust discs. All sev en hav e accreted heavy elements; between three and 10 of these elements, C, O, S, P, Mg, Al, Si, Ca, Ti, Cr, Fe, and Ni, are detected in the atmosphere of each of the white dwarfs. White dwarfs with circumstellar gaseous discs are good targets for studying photospheric abundances as the y rev eal numerous elements allowing in depth compositional analysis of the polluting planetary material.

All the white dwarfs show non-photospheric lines in their spectra. Most notably, Gaia J0006 + 2858 has a non-photospheric component for two Si IV photospheric lines in the FUV data blueshifted with a velocity of -196 km s -1 . This could be absorption from fast moving hot circumstellar gas, ho we ver, further gas disc modelling is required to confirm this.

Abundances of planetary material in the atmospheres of white dwarfs provide crucial constraints on the composition of exoplanetary material. This work shows that the ratio of abundances within the white dwarf atmosphere, for example, Fe/Mg, are less effected by uncertainties in the white dwarf parameters for the ef fecti ve temperature range considered (16 000-25 500 K) than absolute abundances (e.g.

[Mg/H], [Fe/H]). Thus, it is preferable to use abundance ratios when interpreting planetary composition. This highlights the importance of considering the discrepancy between white dwarf parameters derived by the spectroscopic and photometric method when considering the total accretion onto white dwarfs, in this work the accretion rates differed by factors of 1.6-4.2.

A poorly understood discrepancy between abundances derived from optical or ultraviolet spectroscopy has previously been reported in the literature. This work derives the abundances in the optical and ultraviolet using a consistent approach and finds that there is an approximately constant offset between the optical and ultraviolet abundances of silicon and oxygen in three DAZ white dwarfs of 0.62 dex, and no offset is found for the one DBZ white dwarf analysed in this sample. This work speculates as to whether this discrepancy could be explained by vertical gradients in composition in the white dwarf atmosphere. The optical lines form at approximately the same depth, whereas, the ultraviolet lines form o v er a range of depths. Further work is needed to understand the origin of this discrepancy.

Combining the seven objects from this paper with nine white dwarfs from the literature with circumstellar gaseous discs and Mg abundance measurements of the polluting material, the mass accretion rates of systems with detectable circumstellar gaseous discs were compared to those without. This supports that polluted white dwarfs with circumstellar gaseous discs do not show enhanced accretion when compared to the population of polluted white dwarfs without gaseous discs, and so there is no evidence for enhanced accretion rates from gas drag.

The analysis of the abundances of the material that has accreted onto these seven white dwarfs is presented in the subsequent paper, Paper II, including in depth discussions on the composition and geological history of the planetesimals.

MNRAS 527, 6038-6054 (2024) Table 4 . The data points from the optical and UV are marked separately. by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has al w ays had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This paper includes data gathered with the 6.5 metre Magellan Telescopes located at Las Campanas Observatory, Chile. This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under N ASA contract N AS 5-26555. These observations are associated with programmes 16204 and 16752.