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EUV radiation is primarily responsible for driving hydrodynamic mass loss and thus determining whether an exoplanet can retain its atmosphere and water. The Lyman- α flux is primarily responsible for photo-dissociating water and methane, and therefore plays a major role in determining the chemistry in the upper atmospheres of exoplanets. Since interstellar hydrogen absorbs much of the EUV and Lyman- α radiation, reconstruction and theoretical techniques are needed to determine the intrinsic flux levels received by exoplanets. We describe the techniques and their limitations for estimating the extreme ultraviolet (EUV) spectral energy distribution (10–91.2 nm) and hydrogen Lyman- α flux (121.6 nm) emitted by host stars and incident on exoplanet atmospheres. We evaluate how each reconstruction technique can match the observed solar EUV spectral energy distribution and Lyman- α flux. Each technique has its limitations, but the techniques that can reconstruct the observed solar emission and are based on stellar activity observables that are not affected by interstellar absorption should best explain the intrinsic EUV and Lyman- α stellar emission. [ABSTRACT FROM AUTHOR]

This paper reviews past research and new studies underway of the local interstellar environment and its changing influence on the heliosphere. The size, shape, and physical properties of the heliosphere outside of the heliopause are determined by the surrounding environment – now the outer region of the Local Interstellar Cloud (LIC). The temperature, turbulence, and velocity vector of neutral atoms and ions in the LIC and other partially ionized interstellar clouds are measured from high-resolution spectra of interstellar absorption lines observed with the STIS instrument on the HST. Analysis of such spectra led to a kinematic model with many interstellar clouds defined by velocity vectors derived from radial velocity measurements. This analysis identified fifteen clouds located within about 10 pc of the Sun and their mean temperatures, turbulence, and velocity vectors. With the increasing number of sight lines now being analyzed, we find that temperatures and turbulent velocities have spatial variations within the LIC and other nearby clouds much larger than measurement uncertainties, and that these spatial variations appear to be randomly distributed and can be fit by Gaussians. The inhomogeneous length scale is less than 4,000 AU, a distance that the heliosphere will traverse in less than 600 years. The temperatures and turbulent velocities do not show significant trends with stellar distance or angle from the LIC center. If/when the Sun enters an inter-cloud medium, the physical properties of the future heliosphere will be very different from the present. For the heliosheath and the very local interstellar medium (VLISM) just outside of the heliopause, the total pressures are approximately equal to the gravitational pressure of overlying material in the Galaxy. The internal pressure in the LIC is far below that in the VLISM, but there is an uncertain ram pressure term produced by the flow of the LIC with respect to its environment. [ABSTRACT FROM AUTHOR]

The WD 1145+017 system shows irregular transit features that are consistent with the tidal disruption of differentiated asteroids with bulk densities and bulk masses. We use the open-source N-body code REBOUND to simulate this disruption with different internal structures: varying the core volume fraction, mantle/core density ratio, and the presence/absence of a thin low-density crust. We allow the rubble pile to partially disrupt and capture lightcurves at a specific point during the disruption at cadences comparable to those from ground-based photometry. As a proof-of-concept we show that varying these structural parameters have observationally distinguishable effects on the transit lightcurve as the asteroid is disrupted and compare the simulation-generated lightcurves to data from Gary et al. With the caveat that our simulations do not model the sublimation in detail or account for its effects on orbital evolution, we find that a low core fraction and low mantle/core density ratio asteroid is most consistent with the stable transit feature present for multiple weeks circa 2016 April (referred to as G6121 in Gary et al. and A1 in Hallakoun et al.). Connecting tidal disruption simulations to photometry suggests characteristics for the interior structure and composition of an exoplanetary body, information that is only possible because we are observing the death of the planetary system in action. All-sky survey missions such as TESS and LSST will be able to detect other systems like WD 1145+017, creating a sample of subjects for a new subfield of planetary science: necroplanetology. [ABSTRACT FROM AUTHOR]

We describe the interface between the outer heliosphere and the local interstellar medium (LISM) surrounding the Sun. The components of the inner LISM are the four partially ionized clouds (the Local Interstellar Cloud (LIC), G cloud, Blue cloud, and Aql cloud) that are in contact with the outer heliosphere, and ionized gas produced by EUV radiation primarily from ϵ CMa. We construct the three-dimensional shape of the LIC based on interstellar line absorption along 62 sightlines and show that in the directions of ϵ CMa, β CMa, and Sirius B the neutral hydrogen column density from the center of the LIC is a minimum. We call this region the "hydrogen hole." In this direction, the presence of Blue cloud absorption and the absence of LIC absorption can be simply explained by the Blue cloud lying just outside the heliosphere. We propose that the outer edge of the Blue cloud is a Strömgren shell driven toward the heliosphere by high pressures in the H ii region. We find that the vectors of neutral and ionized helium flowing through the heliosphere are inconsistent with the LIC flow vector, and that the nearby intercloud gas is consistent with ionization by ϵ CMa and other stellar sources without requiring additional sources of ionization or million-degree plasma. In the upwind direction, the heliosphere is passing through an environment of several LISM clouds, which may explain the recent influx of interstellar grains containing 60Fe from supernova ejecta measured in Antarctic snow. [ABSTRACT FROM AUTHOR]

We summarize the results of our long-term program to study the kinematics, morphology, and physical properties of warm partially ionized interstellar gas located within 100 pc of the Sun. Using the Space Telescope Imaging Spectrograph (STIS) and other spectrographs on the Hubble Space Telescope (HST), we measure radial velocities of neutral and singly ionized atoms that identify comoving structures (clouds) of warm interstellar gas. We have identified 15 of these clouds located within 15 pc of the Sun. Each of them moves with a different velocity vector, and they have narrow ranges of temperature, turbulence, and metal depletions. We compute a three-dimensional model for the Local Interstellar Cloud (LIC), in which the Sun is likely embedded near its edge, and the locations and shapes of the other nearby clouds. These clouds are likely separated by ionized Strömgren sphere gas produced by ϵ CMa, Sirius B, and other hot white dwarfs. We propose that some of these partially ionized clouds are shells of the Strömgren spheres. [ABSTRACT FROM AUTHOR]

Over the course of its motion through the Galaxy, our solar system has encountered many interstellar environments of varying characteristics. Interstellar medium (ISM) density variations spanning seven orders of magnitude are commonly seen throughout the general Galactic environment, and a sufficiently dense cloud within this range has the potential to dramatically alter the structure of the heliosphere. We present observations of the ISM environments the Sun has most recently encountered based on high-resolution optical spectra toward nearby stars in the direction of the historical solar trajectory. The data were obtained with the highest-resolution spectrographs available, including the Tull Spectrograph on the Harlan J. Smith Telescope at McDonald Observatory and the Ultra-High-Resolution Facility on the Anglo-Australian Telescope at the Anglo-Australian Observatory. Observations were made of interstellar Na i and Ca ii doublet absorption toward 43 bright stars within ~500 pc. No absorption is seen out to a distance of 120 pc (consistent with the outer boundary of the Local Bubble), but a complex collection of absorbers is seen in stars beyond 120 pc. While common absorbers are consistently seen in most sight lines, significant spatial variation is also detected, even between closely spaced sight lines. This pervasive evidence of small-scale structure not only speaks to the complexity of the morphology or physical properties of the gas in the ISM, but also emphasizes that dramatic structural changes to the heliosphere are common and it is important to understand the implications of such changes, such as the modulation in the cosmic ray flux, on planets. [ABSTRACT FROM AUTHOR]

We obtained over 90 hr of spectroscopic observations of four exoplanetary systems with the Hobby-Eberly Telescope. Observations were taken in transit and out of transit, and we analyzed the differenced spectra--i.e., the transmission spectra--to inspect it for absorption at the wavelengths of the neutral sodium (Na I) doublet at λλ5889, 5895 and neutral potassium (K I) at λ7698. We used the transmission spectrum at Ca I λ6122--which shows strong stellar absorption but is not an alkali metal resonance line that we expect to show significant absorption in these atmospheres--as a control line to examine our measurements for systematic errors. We use an empirical Monte Carlo method to quantify these systematic errors. In a reanalysis of the same data set using a reduction and analysis pipeline that was derived independently, we confirm theopreviously seen Na I absorption in HD 189733b at a level of (-5.26 ± 1.69) x 10-4 (the average value over a 12 Å integration band to be consistent with previous authors). Additionally, we tentatively confirm the Na I absorption seen in HD 209458b (independently by multiple authors) at a level of (-2.63 ± 0.81) x 10-4, though the interpretation is less clear. Furthermore, we find Na I absorption of (-3.16 ± 2.06) x 10-4 at <3σ in HD 149026b; features apparent in the transmission spectrum are consistent with real absorption and indicate this may be a good target for future observations to confirm. No other results (Na I in HD 147506b and Ca I and K I in all four targets) are significant to ≥ 3σ, although we observe some features that we argue are primarily artifacts. [ABSTRACT FROM AUTHOR]

The observed properties of the local interstellar medium (LISM) have been facilitated by a growing ultraviolet and optical database of high spectral resolution observations of interstellar absorption toward nearby stars. Such observations provide insight into the physical properties (e.g., temperature, turbulent velocity, and depletion onto dust grains) of the population of warm clouds (e.g., 7000 K) that reside within the Local Bubble. In particular, I will focus on the dynamical properties of clouds within ∼15 pc of the Sun. This simple dynamical model addresses a wide range of issues, including, the location of the Sun as it pertains to the relationship between local interstellar clouds and the circumheliospheric interstellar medium (CHISM), the creation of small cold clouds inside the Local Bubble, and the association of interacting warm clouds and small-scale density fluctuations that cause interstellar scintillation. Local interstellar clouds that can be easily distinguished based on their dynamical properties also differentiate themselves by other physical properties. For example, the two nearest local clouds, the Local Interstellar Cloud (LIC) and the Galactic (G) Cloud, show distinct properties in temperature and depletion of iron and magnesium. The availability of large-scale observational surveys allows for studies of the global characteristics of our local interstellar environment, which will ultimately be necessary to address fundamental questions regarding the origins and evolution of the local interstellar medium. [ABSTRACT FROM AUTHOR]

The origin and evolution of CO gas in debris discs has been debated since its initial detection. The gas could have a primordial origin, as a remnant of the protoplanetary disc or a secondary exocometary origin. This paper investigates the origin of gas in two debris discs, HD110058 and HD131488, using Hubble Space Telescope (HST) observations of CI and CO, which play critical roles in the gas evolution. We fitted several electronic transitions of CI and CO rovibronic bands to derive column densities and temperatures for each system, revealing high CO column densities (∼3–4 orders of magnitude higher than β Pictoris), and low CI/CO ratios in both. Using the exogas model, we simulated the radial evolution of the gas in the debris disc assuming a secondary gas origin. We explored a wide range of CO exocometary release rates and α viscosities, which are the key parameters of the model. Additionally, we incorporated photodissociation due to stellar ultraviolet (UV) to the exogas model and found that it is negligible for typical CO-rich discs and host stars, even at a few au due to the high radial optical depths in the Extreme ultraviolet radiation (EUV). We find that the current steady-state secondary release model cannot simultaneously reproduce the CO and CI HST -derived column densities, as it predicts larger CI/CO ratios than observed. Our direct UV measurement of low CI/CO ratios agrees with results derived from recent Atacama Large Millimetre/submillimetre Array findings and may point to vertical layering of CI, additional CI removal, CO shielding processes, or different gas origin scenarios. [ABSTRACT FROM AUTHOR]

High-resolution UV spectra of stellar H i Lyα lines from the Hubble Space Telescope (HST) provide observational constraints on the winds of coronal main-sequence stars, thanks to an astrospheric absorption signature created by the interaction between the stellar winds and the interstellar medium. We report the results of a new HST survey of M dwarf stars, yielding six new detections of astrospheric absorption. We estimate mass-loss rates for these detections and upper limits for nondetections. These new constraints allow us to characterize the nature of M dwarf winds and their dependence on coronal activity for the first time. For a clear majority of the M dwarfs, we find winds that are weaker than or comparable in strength to that of the Sun, i.e.,. However, two of the M dwarfs have much stronger winds: YZ CMi (M4 Ve;) and GJ 15AB (M2 V+M3.5 V;). Even these winds are much weaker than expectations if the solar relation between flare energy and coronal mass ejection (CME) mass extended to M dwarfs. Thus, the solar flare/CME relation does not appear to apply to M dwarfs, with important ramifications for the habitability of exoplanets around M dwarfs. There is evidence for some increase in with coronal activity as quantified by X-ray flux, but with much scatter. One or more other factors must be involved in determining wind strength besides spectral type and coronal activity, with magnetic topology being one clear possibility. [ABSTRACT FROM AUTHOR]

White dwarf studies carry significant implications across multiple fields of astrophysics, including exoplanets, supernova explosions, and cosmological investigations. Thus, accurate determinations of their fundamental parameters (T eff and log  g) are of utmost importance. While optical surveys have provided measurements for many white dwarfs, there is a lack of studies utilizing ultraviolet (UV) data, particularly focusing on the warmer ones that predominantly emit in the UV range. Here, we present the medium-resolution far-UV spectroscopic survey of 311 DA white dwarfs obtained with Cosmic Origins Spectrograph (COS) onboard Hubble Space Telescope confirming 49 photometric Gaia candidates. We used 3D extinction maps, parallaxes, and hydrogen atmosphere models to fit the spectra of the stars that lie in the range |$12\, 000 \lt \mbox{$T_{\mathrm{eff}}$}\lt 33\, 000$|  K, and |$7 \le \mbox{$\log g$}\lt 9.2$|⁠. To assess the impact of input physics, we employed two mass–radius relations in the fitting and compared the results with previous studies. The comparisons suggest the COS T eff are systematically lower by 3 per cent, on average, than Balmer line fits while they differ by only 1.5 per cent from optical photometric studies. The mass distributions indicate that the COS masses are smaller by ≈0.05  and 0.02 M⊙ than Balmer lines and photometric masses, respectively. Performing several tests, we find that the discrepancies are either arising due to issues with the COS calibration, broadening theories for hydrogen lines, or interstellar reddening which needs further examination. Based on comparative analysis, we identify 30 binary candidates drawing attention for follow-up studies to confirm their nature. [ABSTRACT FROM AUTHOR]

The article reports on a paper in the issue by McComas and colleagues using measurements collected by the Interstellar Boundary Explorer (IBEX) spacecraft to study the interstellar atoms which pass through the heliosphere into the solar system. IBEX measured the velocity and direction of the motion of the interstellar gases and medium around the heliosphere. Topics include possible changes in the structure of the heliosphere as the solar system passes from its interstellar cloud, the shielding of the Earth from galactic charged particles by the heliosphere, and the existence of heliospheres around other stars and planetary systems.

The article reports on a study which discovers a fluorescent emission of methane origin in the upper atmosphere of the Jovian extrasolar planet HD 189733b. It discusses how fluorescence occurs, the ability of methane to do the phenomenon, and the frequency of fluorescent emission in astrophysical environments. It notes the implication of the presence of methane in the upper atmosphere in the study of the planet's surface. It also emphasizes the importance of complementary ground- and space-based studies on planets. Moreover, it cites the significance of the observations in the continued quest for biological life beyond Earth.

We analyze high-resolution spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope toward 34 nearby stars (⩽100 pc) to record Mg II, Fe II, and Mn II absorption due to the local interstellar medium (LISM). Observations span the entire sky, probing previously unobserved regions of the LISM. The heavy ions studied in this survey produce narrow absorption features that facilitate the identification of multiple interstellar components. We detected one to six individual absorption components along any given sight line, and the number of absorbers roughly correlates with the pathlength. This high-resolution near-ultraviolet (NUV) spectroscopic survey was specifically designed for sight lines with existing far-UV (FUV) observations. The FUV spectra include many intrinsically broad absorption lines (i.e., of low atomic mass ions) and are often observed at medium resolution. The LISM NUV narrow-line absorption component structure presented here can be used to more accurately interpret the archival FUV observations. As an example of this synergy, we present a new analysis of the temperature and turbulence along the line of sight toward ε Ind. The new observations of LISM velocity structure are also critical in the interpretation of astrospheric absorption derived from fitting the saturated H I Lyα profile. As an example, we reanalyze the spectrum of λ And and find that this star likely does have an astrosphere. Two stars in the sample that have circumstellar disks (49 Cet and HD141569) show evidence for absorption due to disk gas. Finally, the substantially increased number of sight lines is used to test and refine the three-dimensional kinematic model of the LISM and search for previously unidentified clouds within the Local Bubble. We find that every prediction made by the Redfield & Linsky kinematic model of the LISM is confirmed by an observed component in the new lines of sight. [ABSTRACT FROM AUTHOR]

We report on a search for Hα absorption in four exoplanets. Strong features at Hα are observed in the transmission spectra of both HD 189733b and HD 209458b. We attempt to characterize and remove the effects of stellar variability in HD 189733b, and along with an empirical Monte Carlo test the results imply a statistically significant transit-dependent feature of (– 8.72 ± 1.48) × 10–4 integrated over a 16 Å band relative to the adjacent continuum. We interpret this as the first detection of this line in an exoplanetary atmosphere. A previous detection of Lyα in HD 189733b's atmosphere allows us to calculate an excitation temperature for hydrogen, Texc = 2.6 × 104 K. This calculation depends significantly on certain simplifying assumptions. We explore these assumptions and argue that Texc is very likely much greater than the radiative equilibrium temperature (the temperature the planet is assumed to be at based on stellar radiation and the planetary distance) of HD 189733b. A large Texc implies a very low density that is not in thermodynamic equilibrium with the planet's lower atmosphere. We argue that the n = 2 hydrogen required to cause Hα absorption in the atmosphere is created as a result of the greater UV flux at HD 189733b, which has the smallest orbit and most chromospherically active central star in our sample. Though the overall integration of HD 209458b's transmission spectrum over a wide band is consistent with zero, it contains a dramatic, statistically significant feature in the transmission spectrum with reflectional symmetry. We discuss possible physical processes that could cause this feature. Our remaining two targets (HD 147506b and HD 149026b) do not show any clear features, so we place upper limits on their Hα absorption levels. [ABSTRACT FROM AUTHOR]

Observations of H i Lyman α, the brightest UV emission line of late-type stars, are critical for understanding stellar chromospheres and transition regions, modeling photochemistry in exoplanet atmospheres, and measuring the abundances of neutral hydrogen and deuterium in the interstellar medium. Yet Lyα observations are notoriously challenging owing to severe attenuation from interstellar gas, hindering our understanding of this important emission line's basic morphology. We present high-resolution far- and near-UV spectroscopy of five G, K, and M dwarfs with radial velocities large enough to Doppler-shift the stellar Lyα emission line away from much of the interstellar attenuation, allowing the line core to be directly observed. We detect self-reversal in the Lyα emission-line core for all targets, and we show that the self-reversal depth decreases with increasing surface gravity. Mg ii self-reversed emission-line profiles provide some useful information to constrain the Lyα line core, but the differences are significant enough that Mg ii cannot be used directly as an intrinsic Lyα template during reconstructions. We show that reconstructions that neglect self-reversal could overestimate intrinsic Lyα fluxes by as much as 60%–100% for G and K dwarfs and 40%–170% for M dwarfs. The five stars of our sample have low magnetic activity and subsolar metallicity; a larger sample size is needed to determine how sensitive these results are to these factors. [ABSTRACT FROM AUTHOR]

ABSTRACT We report observations of HD 80606 using the 10.4-m Gran Telescopio Canarias and the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) tunable filter imager. We acquired very high precision, narrow-band photometry in four bandpasses around the K i absorption feature during the 2010 January transit of HD 80606b and during out-of-transit observations conducted in 2010 January and April. We obtained differential photometric precisions of ∼2.08 × 10−4 for the in-transit flux ratio measured at 769.91 nm, which probes the K i line core. We find no significant difference in the in-transit flux ratio between observations at 768.76 and 769.91 nm. Yet, we find a difference of ∼8.09 ± 2.88 × 10−4 between these observations and observations at a longer wavelength that probes the K i wing (777.36 nm). While the presence of red noise in the transit data has a non-negligible effect on the uncertainties in the flux ratio, the 777.36-769.91 nm colour during transit shows no effects from red noise and also indicates a significant colour change, with a mean value of ∼8.99 ± 0.62 × 10−4. This large change in the colour is equivalent to a ∼4.2 per cent change in the apparent planetary radius with wavelength, which is much larger than the atmospheric scaleheight. This implies the observations probed the atmosphere at very low pressures as well as a dramatic change in the pressure at which the slant optical depth reaches unity between ∼770 and 777 nm. We hypothesize that the excess absorption may be due to K i in a high-speed wind being driven from the exoplanet's exosphere. We discuss the viability of this and alternative interpretations, including stellar limb darkening, star-spots and effects from Earth's atmosphere. We strongly encourage follow-up observations of HD 80606b to confirm the signal measured here. Finally, we discuss the future prospects for exoplanet characterization using tunable filter spectrophotometry. [ABSTRACT FROM AUTHOR]

We have investigated the degree to which turbulence in the Very Local Interstellar Clouds resembles the highly studied turbulence in the solar corona and the solar wind, particularly the high-speed solar wind. The turbulence diagnostics for the Local Clouds are the absorption line widths measured along 32 lines of sight to nearby stars, yielding measurements for 53 absorption components. We have tested whether the Local Cloud turbulence has the following properties of turbulence in the solar corona or the solar wind: (1) velocity fluctuations mainly perpendicular to the average magnetic field, (2) a temperature anisotropy in the sense that the perpendicular temperature is larger than the parallel temperature (or at least enhanced relative to expectation), and (3) an ion temperature which is dependent on the ion Larmor radius, in the sense that more massive ions have higher temperatures. Our analysis of the data does not show compelling evidence for any of these properties in Local Cloud turbulence, indicating possible differences with the albrementioned heliospheric plasmas. In the case of anisotropy of velocity fluctuations, although the expected observational signature is not seen, we cannot exclude the possibility of relatively high degrees of anisotropy (anisotropy parameter ε ~ 0.50-0.70), if some other process in the Local Clouds is causing variations in the turbulent line width from one line of sight to another. We briefly consider possible reasons for differences between heliospheric turbulence and that in the Local Clouds. The apparent absence of anisotropy of the velocity fluctuations and ion temperature might be due to randomization of the interstellar magnetic field on spatial scales ~ 10 pc, but this would not explain the absence of ion mass dependence in the ion temperature. A likely explanation for the absence of temperature anisotropy and mass-proportional temperature in the Local Clouds, and the presence of these effects in the corona and high-speed solar wind, is the greater collisionality, due to ion-neutral collisions, of the partially ionized Local Cloud plasma. [ABSTRACT FROM AUTHOR]

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 Teff 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. [ABSTRACT FROM AUTHOR]

We present a 5 Å–100 μm spectral energy distribution (SED) of the ultracool dwarf star TRAPPIST-1, obtained as part of the Mega-MUSCLES Treasury Survey. The SED combines ultraviolet and blue-optical spectroscopy obtained with the Hubble Space Telescope, X-ray spectroscopy obtained with XMM-Newton, and models of the stellar photosphere, chromosphere, transition region, and corona. A new differential emission measure model of the unobserved extreme-ultraviolet spectrum is provided, improving on the Lyα–EUV relations often used to estimate the 100–911 Å flux from low-mass stars. We describe the observations and models used, as well as the recipe for combining them into an SED. We also provide a semiempirical, noise-free model of the stellar ultraviolet spectrum based on our observations for use in atmospheric modeling of the TRAPPIST-1 planets. [ABSTRACT FROM AUTHOR]

The photospheric metal pollution of white dwarfs is now well established as the signature of the accretion of planetary debris. However, the origin of the trace hydrogen detected in many white dwarfs with helium atmospheres is still debated. Here, we report the analysis of GD 424: a metal-polluted, helium-atmosphere white dwarf with a large amount of trace hydrogen. We determined the atmospheric parameters using a hybrid analysis that combines the sensitivity of spectroscopy to the atmospheric composition, log(H/He), with that of photometry and astrometry to the effective temperature, T eff, and surface gravity, log  g. The resulting white dwarf mass, radius, and cooling age are |$\mbox{$M_{\mathrm{WD}}$}=0.77\pm 0.01\, \mbox{$\mathrm{M}_{\odot }$}$|⁠ , |$\mbox{$R_{\mathrm{WD}}$}=0.0109\pm 0.0001\, \mbox{$\mathrm{R}_{\odot }$}$|⁠ , and τcool = 215 ± 10 Myr, respectively. We identified and measured the abundances of 11 photospheric metals and argue that the accretion event is most likely either in the increasing or in steady state, and that the disrupted planetesimal resembles either CI chondrites or the bulk Earth in terms of its composition. We suggest that the observed 1.33 × 1022 g of trace hydrogen in GD 424 was at least partly acquired through accretion of water-rich planetary debris in an earlier accretion episode. [ABSTRACT FROM AUTHOR]

Nine metal-polluted white dwarfs are observed with medium-resolution optical spectroscopy, where photospheric abundances are determined and interpreted by comparison with Solar system objects. An improved method for making such comparisons is presented, which overcomes potential weaknesses of prior analyses, with numerous sources of error considered to highlight the limitations on interpretation. The stars are inferred to be accreting rocky, volatile-poor asteroidal materials with origins in differentiated bodies, in line with the consensus model. The most heavily polluted star in the sample has 14 metals detected, and appears to be accreting material from a rocky planetesimal, whose composition is mantle-like with a small Fe–Ni core component. Some unusual abundances are present. One star is strongly depleted in Ca, while two others show Na abundances elevated above bulk-Earth abundances; it is speculated that either the latter reflect diversity in the formation conditions of the source material, or they are traces of past accretion events. Another star shows clear signs that accretion ceased around 5 Myr ago, causing Mg to dominate the photospheric abundances, as it has the longest diffusion time of the observed elements. Observing such post-accretion systems allows constraints to be placed on models of the accretion process. [ABSTRACT FROM AUTHOR]

We present new Hubble Space Telescope (HST) ultraviolet and ground-based optical observations of the hot, metal-rich white dwarf GD 394. Extreme-ultraviolet (EUV) observations in 1992–1996 revealed a 1.15 d periodicity with a 25 per cent amplitude, hypothesized to be due to metals in a surface accretion spot. We obtained phase resolved HST /Space Telescope Imaging Spectrograph high resolution far-ultraviolet spectra of GD 394 that sample the entire period, along with a large body of supplementary data. We find no evidence for an accretion spot, with the flux, accretion rate, and radial velocity of GD 394 constant over the observed time-scales at ultraviolet and optical wavelengths. We speculate that the spot may have no longer been present when our observations were obtained, or that the EUV variability is being caused by an otherwise undetected evaporating planet. The atmospheric parameters obtained from separate fits to optical and ultraviolet spectra are inconsistent, as is found for multiple hot white dwarfs. We also detect non-photospheric, high ionisation absorption lines of multiple volatile elements, which could be evidence for a hot plasma cocoon surrounding the white dwarf. [ABSTRACT FROM AUTHOR]

We know that planetary systems are just as common around white dwarfs as around mainsequence stars. However, self-consistently linking a planetary system across these two phases of stellar evolution through the violent giant branch poses computational challenges, and previous studies restricted architectures to equal-mass planets. Here, we remove this constraint and perform over 450 numerical integrations over a Hubble time (14 Gyr) of packed planetary systems with unequal-mass planets. We characterize the resulting trends as a function of planet order and mass. We find that intrusive radial incursions in the vicinity of the whitedwarf become less likely as the dispersion amongst planet masses increases. The orbital meandering which may sustain a sufficiently dynamic environment around a white dwarf to explain observations is more dependent on the presence of terrestrial-mass planets than any variation in planetary mass. Triggering unpacking or instability during the white dwarf phase is comparably easy for systems of unequal-mass planets and systems of equal-mass planets; instabilities during the giant branch phase remain rare and require fine-tuning of initial conditions. We list the key dynamical features of each simulation individually as a potential guide for upcoming discoveries. [ABSTRACT FROM AUTHOR]

We present new Hubble Space Telescope observations of H I Lyα absorption toward the F8 V star HD 35296. This line of sight is only a few degrees from the downwind direction of the local interstellar medium flow vector. As a consequence, Lyα absorption from the heliotail is detected in the spectrum, consistent with three previous downwind detections of heliotail absorption. The clustering of the heliotail absorption detections around the downwind direction demonstrates that the heliotail is pointed close to that direction, limiting the extent to which the interstellar magnetic field might be distorting and deflecting the heliotail. We explore this issue further using three-dimensional MHD models of the global heliosphere. The three computed models represent the first three-dimensional MHD models with both a kinetic treatment of neutrals and an extended grid in the tail direction, both of which are necessary to model Lyα absorption downwind. The models indicate only modest heliotail asymmetries and deflections, which are not large enough to be inconsistent with the clustering of heliotail absorption detections around the downwind direction. The models are reasonably successful at reproducing the observed absorption, but they do overpredict the Lyα opacity by a factor of 2-3. We discuss implications of these results in light of observations of the heliotail region from the Interstellar Boundary Explorer mission. [ABSTRACT FROM AUTHOR]