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The atmospheres of white dwarfs often contain elements heavier than helium, even though these elements would be expected to settle into the stars’ interiors; observations of the white dwarf WD 1145+017 suggest that disintegrating rocky bodies are orbiting the star, perhaps contributing heavy elements to its atmosphere. A disintegrating planet orbiting a faded white dwarf The atmospheres of white dwarfs often contain elements heavier than helium, even though such elements would be expected to settle into the stars' interiors once they have exhausted their nuclear fuel. Heavy-element abundance ratios in such atmospheres are similar to those of rocky bodies in the Solar System, suggesting that the 'extra' elements may derive from planetary material. This paper presents observations of one or more disintegrating planetesimals in transit across the white dwarf WD 1145+017, with periods ranging from 4.5 to 4.9 hours. The strongest transit signals, occurring every 4.5 hours, are indicative of a small object with a cometary tail of dusty effluent material. This system provides further evidence that the pollution of white dwarfs by heavy elements might originate from disrupted rocky bodies such as asteroids and minor planets. Most stars become white dwarfs after they have exhausted their nuclear fuel (the Sun will be one such). Between one-quarter and one-half of white dwarfs have elements heavier than helium in their atmospheres 1 , 2 , even though these elements ought to sink rapidly into the stellar interiors (unless they are occasionally replenished) 3 , 4 , 5 . The abundance ratios of heavy elements in the atmospheres of white dwarfs are similar to the ratios in rocky bodies in the Solar System 6 , 7 . This fact, together with the existence of warm, dusty debris disks 8 , 9 , 10 , 11 , 12 , 13 surrounding about four per cent of white dwarfs 14 , 15 , 16 , suggests that rocky debris from the planetary systems of white-dwarf progenitors occasionally pollutes the atmospheres of the stars 17 . The total accreted mass of this debris is sometimes comparable to the mass of large asteroids in the Solar System 1 . However, rocky, disintegrating bodies around a white dwarf have not yet been observed. Here we report observations of a white dwarf—WD 1145+017—being transited by at least one, and probably several, disintegrating planetesimals, with periods ranging from 4.5 hours to 4.9 hours. The strongest transit signals occur every 4.5 hours and exhibit varying depths (blocking up to 40 per cent of the star’s brightness) and asymmetric profiles, indicative of a small object with a cometary tail of dusty effluent material. The star has a dusty debris disk, and the star’s spectrum shows prominent lines from heavy elements such as magnesium, aluminium, silicon, calcium, iron, and nickel. This system provides further evidence that the pollution of white dwarfs by heavy elements might originate from disrupted rocky bodies such as asteroids and minor planets.

We report the detection of a planet whose orbit surrounds a pair of low-mass stars. Data from the Kepler spacecraft reveal transits of the planet across both stars, in addition to the mutual eclipses of the stars, giving precise constraints on the absolute dimensions of all three bodies. The planet is comparable to Saturn in mass and size and is on a nearly circular 229-day orbit around its two parent stars. The eclipsing stars are 20 and 69% as massive as the Sun and have an eccentric 41-day orbit. The motions of all three bodies are confined to within 0.5° of a single plane, suggesting that the planet formed within a circumbinary disk.

Of more than a thousand known cataclysmic variables (CVs), where a white dwarf is accreting from a hydrogen-rich star, only a dozen have orbital periods below 75 minutes 1 – 9 . One way to achieve these short periods requires the donor star to have undergone substantial nuclear evolution before interacting with the white dwarf 10 – 14 , and it is expected that these objects will transition to helium accretion. These transitional CVs have been proposed as progenitors of helium CVs 13 – 18 . However, no known transitional CV is expected to reach an orbital period short enough to account for most of the helium CV population, leaving the role of this evolutionary pathway unclear. Here we report observations of ZTF J1813+4251, a 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a temperature comparable to that of the Sun but a density 100 times greater owing to its helium-rich composition, accreting onto a white dwarf. Phase-resolved spectra, multi-band light curves and the broadband spectral energy distribution allow us to obtain precise and robust constraints on the masses, radii and temperatures of both components. Evolutionary modelling shows that ZTF J1813+4251 is destined to become a helium CV binary, reaching an orbital period under 20 minutes, rendering ZTF J1813+4251 a previously missing link between helium CV binaries and hydrogen-rich CVs. A 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a comparable temperature to that of the Sun but a 100 times greater density, accreting onto a white dwarf is reported.

The Ohara glass S‐FTM16 is of considerable interest for near‐infrared optical designs because it transmits well through theKband and because negative S‐FTM16 elements can be used to accurately achromatize positive calcium fluoride elements in refractive collimators and cameras. Glass manufacturers have sophisticated equipment to measure the refractive index at room temperature, but cannot typically measure the refractive index at cryogenic temperatures. Near‐infrared optics, however, operate at cryogenic temperatures to reduce thermal background. Thus, we need to know the temperature dependence of S‐FTM16’s refractive index. We report here our measurements of the thermal dependence of S‐FTM16’s refractive index between room temperature and ∼77 K. Within our measurement errors we find no evidence for a wavelength dependence or a nonlinear temperature term, so our series of measurements can be reduced to a single number. We find that \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $\\Delta n_{\\mathrm{abs}\\,}/ \\Delta T=-2.4\\times 10^{-6}$ \\end{document} K−1between 298 and ∼77 K and in the wavelength range 0.6–2.6 μm. We estimate that the systematic error (which dominates the measurement error) in our measurement is 10%, sufficiently low for most purposes. We also find the integrated linear thermal expansion of S‐FTM16 between 298 and 77 K is −0.00167 m m−1.

Astronomers have discovered thousands of planets outside the Solar System 1 , most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star 2 , but more distant planets can survive this phase and remain in orbit around the white dwarf 3 , 4 . Some white dwarfs show evidence for rocky material floating in their atmospheres 5 , in warm debris disks 6 – 9 or orbiting very closely 10 – 12 , which has been interpreted as the debris of rocky planets that were scattered inwards and tidally disrupted 13 . Recently, the discovery of a gaseous debris disk with a composition similar to that of ice giant planets 14 demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether these planets can survive the journey. So far, no intact planets have been detected in close orbits around white dwarfs. Here we report the observation of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. We observed and modelled the periodic dimming of the white dwarf caused by the planet candidate passing in front of the star in its orbit. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95 per cent confidence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red giant phase and shrinks owing to friction. In this case, however, the long orbital period (compared with other white dwarfs with close brown dwarf or stellar companions) and low mass of the planet candidate make common-envelope evolution less likely. Instead, our findings for the WD 1856+534 system indicate that giant planets can be scattered into tight orbits without being tidally disrupted, motivating the search for smaller transiting planets around white dwarfs. A giant planet candidate roughly the size of Jupiter but more than 14 times as massive is observed by TESS and other instruments to be transiting the white dwarf star WD 1856+534.

Refractive optics in astronomical instruments are potentially sensitive to temperature gradients and temperature transients. This sensitivity arises from thermally dependent refractive indices, lens spacings, and lens dimensions. In addition, thermal gradients in the instrument structure can cause undesirable image shifts at the detector that degrade instrument calibration. We have therefore undertaken a detailed thermal analysis of Binospec, a wide‐field optical spectrograph under development for the converted Multiple Mirror Telescope (MMT). Our goals are to predict the temperature gradients that will be present in the Binospec optics and structure under realistic operating conditions and to determine how design choices affect these gradients. We begin our analysis by deriving thermal time constants for instrument subassemblies to estimate the magnitude of temperature gradients in the instrument and to determine where detailed thermal models are required. We then generate a low‐resolution finite‐difference model of the entire instrument and high‐resolution models of sensitive subassemblies. This approach to thermal analysis is applicable to a variety of other instruments. We use measurements of the ambient temperature in the converted MMT's dome to model Binospec's thermal environment. In moderate conditions, the external temperature changes by up to 8°C over 48 hr, while in extreme conditions the external temperature changes by up to 17°C in 24 hr. During moderate conditions, we find that the Binospec lens groups develop ∼0.14°C axial and radial temperature gradients and that lens groups of different mass develop ∼0.5°C temperature differences; these numbers are doubled for the extreme conditions. Internal heat sources do not significantly affect these results; heat flow from the environment dominates. The instrument must be periodically opened to insert new aperture masks, but we find that the resulting temperature gradients and thermal stresses in the optics are small. Image shifts at the detector caused by thermal deflections of the Binospec optical bench structure are ∼0.1 pixel hr \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $^{-1}$ \\end{document} . We conclude that the proposed Binospec design has acceptable thermal properties, and we briefly discuss design changes to further reduce temperature gradients.

We present follow-up spectroscopy and a detailed model atmosphere analysis of 29 wide double white dwarfs, including eight systems with a crystallized C/O core member. We use state-of-the-art evolutionary models to constrain the physical parameters of each star, including the total age. Assuming that the members of wide binaries are coeval, any age difference between the binary members can be used to test the cooling physics for white dwarf stars, including potential delays due to crystallization and \\(^{22}\\)Ne distillation. We use our control sample of 14 wide binaries with non-crystallized members to show that this method works well; the control sample shows an age difference of only \\(\\Delta\\)Age = \\(-0.03 \\pm\\) 0.15 Gyr between its members. For the eight crystallized C/O core systems we find a cooling anomaly of \\(\\Delta\\)Age= 1.13\\(^{+1.20}_{-1.07}\\) Gyr. Even though our results are consistent with a small additional cooling delay (\\(\\sim1\\) Gyr) from \\(^{22}\\)Ne distillation and other neutron-rich impurities, the large uncertainties make this result not statistically significant. Nevertheless, we rule out cooling delays longer than 3.6 Gyr at the 99.7% (\\(3\\sigma\\)) confidence level for 0.6-0.9 \\(M_{\\odot}\\) white dwarfs. Further progress requires larger samples of wide binaries with crystallized massive white dwarf members. We provide a list of subgiant + white dwarf binaries that could be used for this purpose in the future.

We present the discovery of 17 double white dwarf (WD) binaries from our on-going search for extremely low mass (ELM) <0.3 Msun WDs, objects that form from binary evolution. Gaia parallax provides a new means of target selection that we use to evaluate our original ELM Survey selection criteria. Cross-matching the Gaia and Sloan Digital Sky Survey (SDSS) catalogs, we identify an additional 36 ELM WD candidates with 17

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