Ultraviolet Spectroscopic Explorer Research Articles

Unveiling stellar aurorae: simulating auroral emission lines in hot stars induced by high-energy irradiation

ABSTRACT Auroral emission lines result from the interaction between magnetic field and stellar wind, offering valuable insights into physical properties and processes occurring within magnetospheres of celestial bodies. While extensively studied in planetary and exoplanetary atmospheres, in ultracool dwarfs, and as radio emission from early-type stars, the presence of specific auroral emission lines in hot star spectra remains unexplored. In this study, we utilized tlusty code to simulate the auroral lines, while modelling the effect of the interaction between stellar wind and magnetosphere through X-ray irradiation. Utilizing high-resolution synthetic spectra generated from model atmospheres, we identified potential candidate lines indicative of auroral emission, which were absent in non-irradiated spectra. Emission lines in synthetic spectra were present primarily in the infrared domain. The most prominent line generated by irradiation was He ii 69458 Å, which appeared in all our model atmospheres with effective temperatures ranging from 15 kK to 30 kK. We also calculated the minimum irradiation required to detect emission in this most prominent line. The presence of emission lines was interpreted by considering changes in the population of different excited states of given atoms. Besides the appearance of infrared emission lines, high-energy irradiation causes infrared excess. To complement our simulations, we also searched for auroral lines in Far Ultraviolet Spectroscopic Explorer (FUSE) observations, which are deposited in the Multimission Archive at Space Telescope catalogue. The comparison of observed spectra with synthetic spectra did not identify any possible candidate emission lines in FUSE spectra.

Observations of the Ultraviolet-bright Star ZNG 1 in the Globular Cluster M5 (NGC 5904)

We have analyzed archival spectra of the hot UV-bright star ZNG 1 in the globular cluster M5 (NGC 5904) obtained with the Far Ultraviolet Spectroscopic Explorer and the Space Telescope Imaging Spectrograph. From these data, we derive an effective temperature T eff = 43,000 ± 1400 K, a surface gravity = 4.47 ± 0.08, a rotational velocity = 157 ± 12 km s−1, and a mass M = 0.92 ± 0.17 M ☉. The atmosphere is helium-rich (Y = 0.99) and enhanced in CNO (relative to the cluster). The spectrum exhibits wind features with a terminal velocity near 1500 km s−1 and strong discrete absorption components. The high helium abundance, stellar mass, and rotational velocity suggest that the star is a merger remnant, and its parameters are consistent with models of a pair of merging He-core white dwarfs.

Observations of Multiphase, High-velocity, Shocked Gas in the Vela Supernova Remnant* *Based on observations made with the NASA/ESA Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer, obtained from the MAST data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Based in part on

We present an analysis of high-resolution far-UV archival spectra obtained with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope of the star HD 75309, which probes high-velocity shocked gas in the Vela supernova remnant (SNR). We examine high-velocity features from intrinsically strong absorption lines of O i, Si ii, Si ii*, C ii, C ii*, and Si iii. We also detect high-velocity components in the N v doublet and compare these features to observations of high-velocity O vi absorption, available from archival Far Ultraviolet Spectroscopic Explorer data. Kinetic temperatures are derived from the observed fractional abundances of the various ions, while gas densities and thermal pressures are obtained from the relative populations in excited fine-structure levels of C ii and Si ii. Our results indicate that the highly ionized species at high velocity probe gas in a region immediately behind a shock driven into an interstellar cloud, while the lower-ionization species trace material further downstream in the cooling region of the postshock flow. Low-velocity N v and O vi absorption may trace gas in a conductive boundary layer between the unshocked portion of the cloud and the hot X-ray-emitting intercloud medium. Temporal variations in high-velocity Ca ii absorption features observed toward HD 75309 further confirm the highly inhomogeneous nature of the interstellar medium interacting with the Vela SNR.

Fe ii fluorescence in main-sequence K-dwarfs

ABSTRACT Main-sequence K-dwarfs possess strong emission in the form of the H i Ly α line. There is a close coincidence between the energy corresponding to the transitions H i 1s-2p and Fe ii (5D)5s 4D5/2–(5D)5p 4D5/2. Singly ionized iron has been confirmed being pumped by photo-excitation by accidental resonance (PAR) in planetary nebulae, symbiotic stars, K-giants, and active galactic nebulae. I investigate in this work whether PAR can occur in the atmospheres of main-sequence K-dwarfs, which do not possess the large extended atmospheres of the late-type K-giants. Specifically a search for possible Fe ii fluorescence lines is conducted. For the case when I can confirm PAR, I estimate the total flux leaving the stars in the form of Fe ii fluorescence. I search for emission lines from the Fe ii (5D)5p 4D5/2 level. Since those of these lines with the largest branching fractions correspond to lines at wavelengths covered by the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, a search for archival FUSE spectra from K-dwarfs within 20 ly from the sun is conducted. I retrieve and analyse FUSE spectra for four of these K-dwarfs. In each case I can confirm PAR, I fit the H i Ly α line in Hubble Space Telescope spectra recorded with the Space Telescope Imaging Spectrograph, in order to estimate the efficiency of the PAR mechanism. I can now confirm Fe ii fluorescence in the two closest K-dwarfs, Alpha Centauri B, and Epsilon Eridani. The total power leaving as Fe ii fluorescence are 4.9 × 1017 and 1.30 × 1018 W respectively.

Cold diffuse interstellar medium of Magellanic Clouds: I. HD molecule and cosmic-ray ionization rate

ABSTRACT HD molecule is one of the most abundant molecule in the Universe and due to its sensibility to the conditions in the medium, it can be used to constrain physical parameters in the medium where HD resides. Lately, we have shown that HD abundance can be enhanced in the low-metallicity medium. Large and Small Magellanic Clouds give us an opportunity to study low-metallicity galaxies in detail towards different sightlines due to their proximity to our Galaxy. We revisited Far Ultraviolet Spectroscopic Explorer (FUSE) space telescope archival spectra towards bright stars in Magellanic Clouds to search for HD molecules, associated with the medium of these galaxies. We reanalysed H2 absorption lines and constrained HD column density at the positions of H2 components. We detected HD towards 24 sightlines (including 19 new detections). We try to measure cosmic ray ionization rate for several systems using measured N(HD)/N(H2), and in most cases get loose constraints due to insufficient quality of the FUSE spectra.

FUSE Spectroscopy of Two Luminosity Class II Giants

Analyses have been made of O vi λ1032 and C iii λ977 line profiles in Far Ultraviolet Spectroscopic Explorer spectra of the luminosity class II giants HD 57623 and HD 160635. No conclusive evidence for an outflowing wind could be discerned.

A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium* * Based on observations made with the NASA/ESA Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer mission, obtained from the MAST data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

We present a comprehensive examination of interstellar P and Cl abundances based on an analysis of archival spectra acquired with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. Column densities of P ii, Cl i, and Cl ii are determined for a combined sample of 107 sight lines probing diffuse atomic and molecular gas in the local Galactic interstellar medium. We reevaluate the nearly linear relationship between the column densities of Cl i and H2, which arises from the rapid conversion of Cl+ to Cl0 in regions where H2 is abundant. Using the observed total gas-phase P and Cl abundances, we derive depletion parameters for these elements, adopting the methodology of Jenkins. We find that both P and Cl are essentially undepleted along sight lines showing the lowest overall depletions. Increasingly severe depletions of P are seen along molecule-rich sight lines. In contrast, gas-phase Cl abundances show no systematic variation with molecular hydrogen fraction. However, enhanced Cl (and P) depletion rates are found for a subset of sight lines showing elevated levels of Cl ionization. An analysis of neutral chlorine fractions yields estimates for the amount of atomic hydrogen associated with the H2-bearing gas in each direction. These results indicate that the molecular fraction in the H2-bearing gas is at least 10% for all sight lines with and that the gas is essentially fully molecular at .

A High-precision Survey of the D/H Ratio in the Nearby Interstellar Medium

We present high signal-to-noise ratio measurements of the H i Lyα absorption line toward 16 Galactic targets that are at distances between approximately 190 and 2200 pc, all beyond the wall of the Local Bubble. We describe the models used to remove stellar emission and absorption features and the methods used to account for all known sources of error in order to compute high-precision values of the H i column density with robust determinations of the uncertainties. When combined with H2 column densities from other sources, we find total H column densities ranging from 1020.01 to 1021.25 cm−2. Using deuterium column densities from Far Ultraviolet Spectroscopic Explorer observations we determine the D/H ratio along the sight lines. We confirm and strengthen the conclusion that D/H is spatially variable over these H i column density and target distance regimes, which predominantly probe the interstellar medium outside the Local Bubble. We discuss how these results affect models of Galactic chemical evolution. We also present an analysis of metal lines along the five sight lines for which we have high-resolution spectra and, along with results reported in the literature, discuss the corresponding column densities in the context of a generalized depletion analysis. We find that D/H is only weakly correlated with metal depletion and conclude that the spatial D/H variability is not solely due to dust depletion. A bifurcation of D/Htot as a function of depletion at high depletion levels provides modest support that deuterium-rich gas is infalling onto the Galactic plane.

Detection of cesium in the atmosphere of the hot He-rich white dwarf HD 149499B

ABSTRACT We report the first detection of cesium (Z = 55) in the atmosphere of a white dwarf. Around a dozen absorption lines of Cs iv, Cs v, and Cs vi have been identified in the Far Ultraviolet Spectroscopic Explorer spectrum of the He-rich white dwarf HD 149499B (Teff = 49 500 K, log g = 7.97). The lines have equivalent widths ranging from 2.3 to 26.9 mÅ. We performed a spectral synthesis analysis to determine the cesium content in the atmosphere. Non-local thermodynamic equilibrium atmosphere models were computed by considering cesium explicitly in the calculations. For this purpose we calculated oscillator strengths for the bound–bound transitions of Cs iv–Cs vi with both autostructure (multiconfiguration Breit–Pauli) and grasp2k (multiconfiguration Dirac–Fock) atomic structure codes as neither measured nor theoretical values are reported in the literature. We determined a cesium abundance of log N(Cs)/N(He) = −5.45 ± 0.35, which can also be expressed in terms of the mass fraction log XCs = −3.95 ± 0.35.

A Detection of H2 in a High-velocity Cloud toward the Large Magellanic Cloud

This work presents a new detection of H2 absorption arising in a high-velocity cloud associated with either the Milky Way or the Large Magellanic Cloud (LMC). The absorber was found in an archival Far Ultraviolet Spectroscopic Explorer spectrum of the LMC star Sk-70°32. This is the fifth well-characterized H2 absorber to be found in the Milky Way’s halo and the second such absorber outside the Magellanic Stream and Bridge. The absorber has a local standard of rest central velocity of +140 km s−1 and a H2 column density of 1017.5 cm−2. It is most likely part of a cool and relatively dense inclusion (T ≈ 75 K, n H ∼ 100 cm−3) in a warmer and more diffuse halo cloud. This halo cloud may be part of a still-rising Milky Way Galactic fountain flow or an outflow from the Large Magellanic Cloud.

The H i Column Density Distribution of the Galactic Disk and Halo

Abstract We present a census of neutral gas in the Milky Way disk and halo down to limiting column densities of N(H i) ∼ 1014 cm−2 using measurements of H i Lyman series absorption from the Far Ultraviolet Spectroscopic Explorer. Our results are drawn from an analysis of 25 AGN sight lines spread evenly across the sky with Galactic latitude ∣b∣ ≳ 20°. By simultaneously fitting multi-component Voigt profiles to 11 Lyman series absorption transitions covered by FUSE (Lyβ–Lyμ) plus HST measurements of Lyα, we derive the kinematics and column densities of a sample of 152 H i absorption components. While saturation prevents accurate measurements of many components with column densities 17 ≲ log N(H i) ≲ 19, we derive robust measurements at log N(H i) ≲ 17 and log N(H i) ≳ 19. We derive the first ultraviolet H i column density distribution function (CDDF) of the Milky Way, both globally and for low-velocity (ISM), intermediate-velocity clouds (IVCs), and high-velocity clouds (HVCs). We find that IVCs and HVCs show statistically indistinguishable CDDF slopes, with β IVC = − 1.01 − 0.14 + 0.15 and β HVC = − 1.05 − 0.06 + 0.07 . Overall, the CDDF of the Galactic disk and halo appears shallower than that found by comparable extragalactic surveys, suggesting a relative abundance of high column density gas in the Galactic halo. We derive the sky-covering fractions as a function of H i column density, finding an enhancement of IVC gas in the northern hemisphere compared to the south. We also find evidence for an excess of inflowing H i over outflowing H i, with −0.88 ± 0.40 M ⊙ yr−1 of HVC inflow versus ≈0.20 ± 0.10 M ⊙ yr−1 of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.

Observations of the Bright Star in the Globular Cluster 47 Tucanae (NGC 104)

Abstract The Bright Star in the globular cluster 47 Tucanae (NGC 104) is a post-asymptotic giant branch (post-AGB) star of spectral type B8 III. The ultraviolet spectra of late-B stars exhibit myriad absorption features, many due to species unobservable from the ground. The Bright Star thus represents a unique window into the chemistry of 47 Tuc. We have analyzed observations obtained with the Far Ultraviolet Spectroscopic Explorer, the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, and the Magellan Inamori Kyocera Echelle Spectrograph on the Magellan Telescope. By fitting these data with synthetic spectra, we determine various stellar parameters (T eff = 10,850 ± 250 K, log g = 2.20 ± 0.13 ) and the photospheric abundances of 26 elements, including Ne, P, Cl, Ga, Pd, In, Sn, Hg, and Pb, which have not previously been published for this cluster. Abundances of intermediate-mass elements (Mg through Ga) generally scale with Fe, while the heaviest elements (Pd through Pb) have roughly solar abundances. Its low C/O ratio indicates that the star did not undergo third dredge-up and suggests that its heavy elements were made by a previous generation of stars. If so, this pattern should be present throughout the cluster, not just in this star. Stellar-evolution models suggest that the Bright Star is powered by a He-burning shell, having left the AGB during or immediately after a thermal pulse. Its mass (0.54 ± 0.16M ⊙) implies that single stars in 47 Tuc lose 0.1–0.2 M ⊙ on the AGB, only slightly less than they lose on the red giant branch.

Revisiting FUSE O vi Emission in Galaxy Halos

Abstract A significant fraction of baryons in galaxies are in the form of diffuse gas of the circumgalactic medium (CGM). One critical component of the multiphases of CGM, the so-called “coronal” warm-hot phase gas (105–106 K) traced by O vi 1031.93, 1037.62 Å resonance lines, has rarely been detected in emission from galaxy halos other than that of the Milky Way. Here we report four additional detections of O vi emission gas in the halos of nearby edge-on galaxies, NGC 4631 and NGC 891, using archival Far Ultraviolet Spectroscopic Explorer data and an updated data pipeline. We find the most intense O vi emission to be from fields forming a vertical line near the center of NGC 4631, despite the close proximity to the disk of two other fields. The detected O vi emission surface brightness is about (1.1 ± 0.3) × 10−18 to (3.9 ± 0.8) × 10−18 erg s−1 cm−2 arcsec−2. The spatial distribution of the five 30″ × 30″ O vi detection fields in NGC 4631 can be interpreted as the existence of filamentary structures of more intense O vi emission superimposed within a diffuse and faint O vi halo in star-forming galaxies. Volume-filled O vi emission mapping is greatly needed to determine the structure and prevalence of warm-hot gas and the role it plays in the cycling of gas between the galaxy disk and the halo. Finally, we present the sensitivity of future funded and proposed UV missions (LUVOIR-A, LUVOIR-B, CETUS, and Aspera) to the detection of diffuse and faint O vi emission in nearby galaxy halos.

Laboratory Data on the EUV/Soft-X-Ray Flux Problem of Fe xviii and Fe xix

Abstract Observations by the spacecraft Extreme Ultraviolet Explorer (EUVE), Far Ultraviolet Spectroscopic Explorer (FUSE), Chandra, and XMM-Newton of Capella (α Aurigae) have encountered problems with the relative intensities of the Fe xviii and Fe xix line emission in the soft-X-ray (XUV) and extreme-ultraviolet spectral ranges versus various model predictions based on theoretical atomic data. The reason may either lie in astrophysical phenomena, e.g., at emission or because of absorption by the interstellar medium, in the theoretical atomic data, in one or more spectrometer calibrations, or in the spectral modeling. By measurements using an electron beam ion trap we provide laboratory data obtained under conditions that are reasonably close to stellar emission regions. The laboratory data on Fe xviii and Fe xix show line ratios that are rather similar to the observations of Capella. The measurements, therefore, rule out astrophysical phenomena and calibration errors and point to issues with the modeling of the observed Capella emission.

Reconstructing the Extreme Ultraviolet Emission of Cool Dwarfs Using Differential Emission Measure Polynomials

Abstract Characterizing the atmospheres of planets orbiting M dwarfs requires understanding the spectral energy distributions of M dwarfs over planetary lifetimes. Surveys like MUSCLES, HAZMAT, and FUMES have collected multiwavelength spectra across the spectral type’s range of T eff and activity, but the extreme ultraviolet (EUV, 100–912 Å) flux of most of these stars remains unobserved because of obscuration by the interstellar medium compounded with limited detector sensitivity. While targets with observable EUV flux exist, there is no currently operational facility observing between 150 and 912 Å. Inferring the spectra of exoplanet hosts in this regime is critical to studying the evolution of planetary atmospheres because the EUV heats the top of the thermosphere and drives atmospheric escape. This paper presents our implementation of the differential emission measure technique to reconstruct the EUV spectra of cool dwarfs. We characterize our method’s accuracy and precision by applying it to the Sun and AU Mic. We then apply it to three fainter M dwarfs: GJ 832, Barnard’s star, and TRAPPIST-1. We demonstrate that with the strongest far-ultraviolet (FUV, 912–1700 Å) emission lines, observed with the Hubble Space Telescope and/or Far Ultraviolet Spectroscopic Explorer, and a coarse X-ray spectrum from either the Chandra X-ray Observatory or XMM-Newton, we can reconstruct the Sun’s EUV spectrum to within a factor of 1.8, with our model’s formal uncertainties encompassing the data. We report the integrated EUV flux of our M dwarf sample with uncertainties of a factor of 2–7 depending on available data quality.

The search for magnetic fields in two Wolf–Rayet stars and the discovery of a variable magnetic field in WR 55

ABSTRACT Magnetic fields in Wolf–Rayet (WR) stars are not well explored, although there is indirect evidence, e.g. from spectral variability and X-ray emission, that magnetic fields should be present in these stars. Being in an advanced stage of their evolution, WR stars have lost their hydrogen envelope, but their dense winds make the stellar core almost unobservable. To substantiate the expectations on the presence of magnetic fields in the most-evolved massive stars, we selected two WR stars, WR 46 and WR 55, for the search of the presence of magnetic fields using FORS 2 spectropolarimetric observations. We achieve a formally definite detection of a variable mean longitudinal magnetic field of the order of a few hundred gauss in WR 55. The field detection in this star, which is associated with the ring nebula RCW 78 and the molecular environment, is of exceptional importance for our understanding of star formation. No field detection at a significance level of 3σ was achieved for WR 46, but the variability of the measured field strengths can be rather well phased with the rotation period of 15.5 h previously suggested by FUSE(Far Ultraviolet Spectroscopic Explorer) observations.

Project AMIGA: The Circumgalactic Medium of Andromeda*

Abstract Project AMIGA (Absorption Maps In the Gas of Andromeda) is a survey of the circumgalactic medium (CGM) of Andromeda (M31, ≃ 300 kpc) along 43 QSO sightlines at impact parameters 25 ≤ R ≤ 569 kpc (25 at R ≲ ). We use ultraviolet absorption measurements of Si ii, Si iii, Si iv, C ii, and C iv from the Hubble Space Telescope/Cosmic Origins Spectrograph and O vi from the Far Ultraviolet Spectroscopic Explorer to provide an unparalleled look at how the physical conditions and metals are distributed in the CGM of M31. We find that Si iii and O vi have a covering factor near unity for R ≲ 1.2 and ≲1.9 , respectively, demonstrating that M31 has a very extended ∼104–105.5 K ionized CGM. The metal and baryon masses of the 104–105.5 K CGM gas within are ≳108 and ≳4 × 1010 (Z/0.3 Z ⊙)−1 M ⊙, respectively. There is not much azimuthal variation in the column densities or kinematics, but there is with R. The CGM gas at R ≲ 0.5 is more dynamic and has more complicated, multiphase structures than at larger radii, perhaps a result of more direct impact of galactic feedback in the inner regions of the CGM. Several absorbers are projected spatially and kinematically close to M31 dwarf satellites, but we show that those are unlikely to give rise to the observed absorption. Cosmological zoom simulations of ∼L* galaxies have O vi extending well beyond as observed for M31 but do not reproduce well the radial column density profiles of the lower ions. However, some similar trends are also observed, such as the lower ions showing a larger dispersion in column density and stronger dependence on R than higher ions. Based on our findings, it is likely that the Milky Way has a ∼104–105.5 K CGM as extended as for M31 and their CGM (especially the warm–hot gas probed by O vi) are overlapping.

Distances to Galactic OB Stars: Photometry versus Parallax

Abstract For application to surveys of interstellar matter and Galactic structure, we compute new spectrophotometric distances to 139 OB stars frequently used as background targets for UV spectroscopy. Many of these stars have updated spectral types and digital photometry with reddening corrections from the Galactic O-Star (GOS) spectroscopic survey. We compare our new photometric distances to values used in previous International Ultraviolet Explorer and Far Ultraviolet Spectroscopic Explorer surveys and to parallax distances derived from Gaia-DR2, after applying a standard (0.03 mas) offset from the quasar celestial reference frame. We find substantial differences between photometric and parallax distances at d > 1.5 kpc, with increasing dispersion when parallax errors exceed 8%. Differences from previous surveys arise from new GOS stellar classifications, especially luminosity classes, and from reddening corrections. We apply our methods to two OB associations. For Perseus OB1 (nine O stars) we find mean distances of 2.47 ± 0.57 kpc (Gaia parallax) and 2.99 ± 0.14 kpc (photometric) using a standard grid of absolute magnitudes. For 29 O stars in Car OB1 associated with Trumpler 16, Trumpler 14, Trumpler 15, and Collinder 228 star clusters, we find 2.87 ± 0.73 kpc (Gaia) and 2.60 ± 0.28 kpc (photometric). Using an alternative grid of O-star absolute magnitudes shifts these photometric distances ∼7% closer. Improving the distances to OB stars will require attention to spectral types, photometry, reddening, binarity, and the grid of absolute magnitudes. We anticipate that future measurements in Gaia-DR3 will improve the precision of distances to massive star-forming regions in the Milky Way.

FUSE and IUE Spectroscopy of the Prototype Dwarf Nova ER Ursa Majoris During Quiescence

Abstract ER Ursae Majoris is the prototype for a subset of SU UMa-type dwarf novae characterized by short cycle times between outburst, high outburst frequency, and negative superhumps. It suffers superoutbursts every 43 days, lasting 20 days, normal outbursts every 4 days, and has an outburst amplitude of 3 mag. We have carried out a far-ultraviolet (FUV) spectral analysis of ER UMa in quiescence, by fitting Far Ultraviolet Spectroscopic Explorer and International Ultraviolet Explorer spectra with model accretion disks and high-gravity photosphere models. Using the Gaia parallax distance and an orbital inclination of 50°, we find that during the brief quiescence of only four days, the accretion rate is 7.3 × 10−11 M ⊙ yr−1, with the ER UMa white dwarf contributing 55% of the FUV flux and the accretion disk contributing the remaining 45% of the flux. The white dwarf in ER UMa is markedly hotter (32,000 K) than the other white dwarfs in dwarf novae below the cataclymic variable (CV) period gap, which have typical temperatures of ∼15,000 K. For higher inclinations of 60°–75°, the accretion rates that we derive are roughly an order of magnitude higher, (1–3) × 10−10 M ⊙ yr−1.

A far-UV survey of three hot, metal-polluted white dwarf stars: WD0455−282, WD0621−376, and WD2211−495

ABSTRACT Using newly obtained high-resolution data (R ∼ 1 × 105) from the Hubble Space Telescope, and archival UV data from the Far Ultraviolet Spectroscopic Explorer, we have conducted a detailed UV survey of the three hot, metal-polluted white dwarfs WD0455−282, WD0621−376, and WD2211−495. Using bespoke model atmospheres, we measured Teff, log g, and photospheric abundances for these stars. In conjunction with data from Gaia, we measured masses, radii, and gravitational redshift velocities for our sample of objects. We compared the measured photospheric abundances with those predicted by radiative levitation theory, and found that the observed Si abundances in all three white dwarfs, and the observed Fe abundances in WD0621−376 and WD2211−495, were larger than those predicted by an order of magnitude. These findings imply not only an external origin for the metals, but also ongoing accretion, as the metals not supported by radiative levitation would sink on extremely short time-scales. We measured the radial velocities of several absorption features along the line of sight to the three objects in our sample, allowing us to determine the velocities of the photospheric and interstellar components along the line of sight for each star. Interestingly, we made detections of circumstellar absorption along the line of sight to WD0455−282 with three velocity components. To our knowledge, this is the first such detection of multicomponent circumstellar absorption along the line of sight to a white dwarf.