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Calibration of an astronomical spectrograph using a laser frequency comb yields a short-term Doppler shift repeatability of 2.5 cm s −1 , which is sufficient to calculate the orbit of an extrasolar planet. High-precision spectroscopy The precise calibration of astronomical spectrographs is central to progress in extrasolar-planet observation and cosmology. A development reported here could pave the way for exciting discoveries in the future, potentially including Earth-sized exoplanets. Precise calibration is key to the effectiveness of a spectrograph, and laser-frequency combs, which emit a 'comb' of equally spaced spectral lines against which targets can be measured, are potentially ideal for this. Here Wilken et al . describe a purpose-built 'astro-comb' that achieves the repeatability and accuracy needed for astronomical observations. They use it to calibrate the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph fitted to the 3.6-metre telescope at La Silla Observatory in Chile. The authors demonstrate the power of the combination by monitoring the star HD 75289 and calculating the orbit of its companion planet. The best spectrographs are limited in stability by their calibration light source 1 . Laser frequency combs are the ideal calibrators for astronomical spectrographs 2 . They emit a spectrum of lines that are equally spaced in frequency 3 and that are as accurate and stable as the atomic clock relative to which the comb is stabilized. Absolute calibration 4 provides the radial velocity of an astronomical object relative to the observer (on Earth). For the detection of Earth-mass exoplanets 5 , 6 in Earth-like orbits around solar-type stars, or of cosmic acceleration 7 , 8 , 9 , the observable is a tiny velocity change of less than 10 cm s −1 , where the repeatability of the calibration—the variation in stability across observations—is important. Hitherto, only laboratory systems 10 , 11 , 12 or spectrograph calibrations of limited performance 4 , 13 , 14 have been demonstrated. Here we report the calibration of an astronomical spectrograph with a short-term Doppler shift repeatability of 2.5 cm s −1 , and use it to monitor the star HD 75289 and recompute the orbit of its planet. This repeatability should make it possible to detect Earth-like planets in the habitable zone of star or even to measure the cosmic acceleration directly.

Stellar ejecta gradually enrich the gas out of which subsequent stars form, making the least chemically enriched stellar systems direct fossils of structures formed in the early Universe 1 . Although a few hundred stars with metal content below 1,000th of the solar iron content are known in the Galaxy 2 – 4 , none of them inhabit globular clusters, some of the oldest known stellar structures. These show metal content of at least approximately 0.2% of the solar metallicity ( [ Fe / H ] ≳ − 2.7 ) . This metallicity floor appears universal 5 , 6 , and it has been proposed that protogalaxies that merged into the galaxies we observe today were simply not massive enough to form clusters that survived to the present day 7 . Here we report observations of a stellar stream, C-19, whose metallicity is less than 0.05% of the solar metallicity ( [ F e / H ] = − 3.38 ± 0.06 ( s t a t i s t i c a l ) ± 0.20 ( s y s t e m a t i c ) ) . The low metallicity dispersion and the chemical abundances of the C-19 stars show that this stream is the tidal remnant of the most metal-poor globular cluster ever discovered, and is significantly below the purported metallicity floor: clusters with significantly lower metallicities than observed today existed in the past and contributed their stars to the Milky Way halo. Observations of a stellar stream below the metallicity floor for a disrupted globular cluster are described.

A search for a surviving companion star of the progenitor of type Ia supernova 1006 indicates that fewer than 20 per cent of such supernovae result from the accretion onto the progenitor star of material from a companion that survives the process rather than being destroyed. White dwarf binary as SN 1006 progenitor Type Ia supernovae are thought to occur in binaries containing a white dwarf and a companion that is a red giant, subgiant, main-sequence star or another white dwarf. In the last of these options, the 'double-degenerate' case, the two white dwarfs would merge before the explosion and no companion would be left. In a single-degenerate system the former companion star would survive. Previous searches for remnant companions have revealed one contested case for the Tycho Brahe supernova SN 1572. More recently, observations have restricted surviving companions to small, main-sequence stars, ruling out giant companions. Jonay González Hernández et al . report the result of a search for surviving companions to the progenitor of SN 1006 and find no sign of a former companion. Together with previous results, this suggests that fewer than 20% of type Ia supernovae occur through the single-degenerate channel, the slow accumulation of mass from a large star. The more common trigger, it seems, is the rapid break-up of a smaller orbiting white dwarf. Type Ia supernovae are thought to occur when a white dwarf made of carbon and oxygen accretes sufficient mass to trigger a thermonuclear explosion 1 . The accretion could be slow, from an unevolved (main-sequence) or evolved (subgiant or giant) star 2 , 3 (the single-degenerate channel), or rapid, as the primary star breaks up a smaller orbiting white dwarf 3 , 4 (the double-degenerate channel). A companion star will survive the explosion only in the single-degenerate channel 5 . Both channels might contribute to the production of type Ia supernovae 6 , 7 , but the relative proportions of their contributions remain a fundamental puzzle in astronomy. Previous searches for remnant companions have revealed one possible case for SN 1572 (refs 8 , 9 ), although that has been questioned 10 . More recently, observations have restricted surviving companions to be small, main-sequence stars 11 , 12 , 13 , ruling out giant companions but still allowing the single-degenerate channel. Here we report the results of a search for surviving companions of the progenitor of SN 1006 (ref. 14 ). None of the stars within 4 arc minutes of the apparent site of the explosion is associated with the supernova remnant, and we can firmly exclude all giant and subgiant stars from being companions of the progenitor. In combination with previous results, our findings indicate that fewer than 20 per cent of type Ia supernovae occur through the single-degenerate channel.

The discovery of the most metal-poor stream, C-19, provides us with a fossil record of a stellar structure born very soon after the Big Bang. In this work, we search for new C-19 members over the whole sky by combining two complementary stream-searching algorithms, STREAMFINDER and StarGO,, and utilizing low-metallicity star samples from the Pristine survey as well as Gaia BP/RP spectro-photometric catalogues. We confirm twelve new members, spread over more than 100\\(^\\circ\\), using velocity and metallicity information from a set of spectroscopic follow-up programs that targeted a quasi-complete sample of our bright candidates (\\(G \\lesssim 16.0\\)). From the updated set of stream members, we confirm that the stream is wide, with a stream width of \\(\\sim200\\) pc, and dynamically hot, with a derived velocity dispersion of \\(10.9^{+2.1}_{-1.5}\\) km/s. The tension remains between these quantities and a purely baryonic scenario in which the relatively low-mass stream (even updated to a few \\(10^4M_{\\odot}\\)) stems from a globular cluster progenitor, as suggested by its chemical abundances. Some heating mechanism, such as preheating of the cluster in its own dark matter halo or through interactions with halo sub-structures appears necessary to explain the tension. The impact of binaries on the measured dispersion also remains unknown. Detailed elemental abundances of more stream members as well as multi-epoch radial velocities from spectroscopic observations are therefore crucial to fully understand the nature and past history of the most metal-poor stream of the Milky Way.

We present five far- and near-ultraviolet spectra of the Type II plateau supernova, SN 2022acko, obtained 5, 6, 7, 19, and 21 days after explosion, all observed with the Hubble Space Telescope/Space Telescope Imaging Spectrograph. The first three epochs are earlier than any Type II plateau supernova has been observed in the far-ultraviolet revealing unprecedented characteristics. These three spectra are dominated by strong lines, primarily from metals, which contrasts with the relatively featureless early optical spectra. The flux decreases over the initial time series as the ejecta cools and line-blanketing takes effect. We model this unique dataset with the non-local thermodynamic equilibrium radiation transport code CMFGEN, finding a good match to the explosion of a low mass red supergiant with energy Ekin = 6 x 10^50 erg. With these models we identify, for the first time, the ions that dominate the early UV spectra. We also present optical photometry and spectroscopy, showing that SN 2022acko has a peak absolute magnitude of V = -15.4 mag and plateau length of ~115d. The spectra closely resemble those of SN 2005cs and SN 2012A. Using the combined optical and UV spectra, we report the fraction of flux redwards of the uvw2, U, B, and V filters on days 5, 7, and 19. We also create a spectral time-series of Type II supernovae in the ultraviolet, demonstrating the rapid decline of UV flux over the first few weeks of evolution. Future observations of Type II supernovae will continue to explore the diversity seen in the limited set of high-quality UV spectra.

Laser frequency combs (LFCs) are well on their way to becoming the next-generation calibration sources for precision astronomical spectroscopy 1 – 6 . This development is considered key in the hunt for low-mass rocky exoplanets around solar-type stars whose discovery with the radial-velocity method requires cm s –1 Doppler precision 7 . In order to prove such precise calibration with an LFC, it must be compared to another calibrator of at least the same precision. Being the best available spectrograph calibrator, this means comparing it to a second—fully independent—LFC. Here, we report on a test in which two separate LFCs were used to simultaneously calibrate an astronomical spectrograph. Our installation of two LFCs at the ultra-stable two-channel spectrograph HARPS allowed characterization of their relative stability and consistency in calibration at the highest available level. Although the test was limited in time, the results confirm the 1 cm s –1 stability that has long been anticipated by the astronomical community. Using two independent laser frequency combs to calibrate an astronomical spectrometer, researchers demonstrate a stability of ~1 cm s –1 , which is required for detecting low-mass rocky exoplanets around Sun-like stars.

Black hole X-ray binaries with large mass ratios and short orbital periods are expected to change their orbital period due to magnetic breaking, mass loss, gravitational radiation, or mass evaporation of the black hole in alternative descriptions of gravity, like in braneworld gravity scenarios. The black hole X-ray binary XTE J1118+480, consisting of a late-type secondary star orbiting a ~ 8 M⊙ black hole in a 4.1-hr period, offers a unique opportunity to test these models. New spectroscopic data allow us to determine the time of the inferior conjunction of the secondary star at different epochs. Observations over a 10 year span will provide constraints on the rate of any orbital period change. We present here a preliminary radial velocity curve obtained with the 10.4m GTC telescope equipped with OSIRIS medium-resolution spectrograph, as part of an ongoing long-term program to study the orbital period evolution in this binary.

We determined effective temperatures, surface gravities, and metallicities for a sample of 343 M dwarfs observed with CARMENES, the double-channel, high-resolution spectrograph installed at the 3.5 m telescope at Calar Alto Observatory. We employed SteParSyn, a Bayesian spectral synthesis implementation particularly designed to infer the stellar atmospheric parameters of late-type stars following a Markov chain Monte Carlo approach. We made use of the BT-Settl model atmospheres and the radiative transfer code turbospectrum to compute a grid of synthetic spectra around 75 magnetically insensitive Fe I and Ti I lines plus the TiO \\(\\gamma\\) and \\(\\epsilon\\) bands. To avoid any potential degeneracy in the parameter space, we imposed Bayesian priors on Teff and log g based on the comprehensive, multi-band photometric data available for the sample. We find that this methodology is suitable down to M7.0 V, where refractory metals such as Ti are expected to condense in the stellar photospheres. The derived \\(T_{\\rm eff}\\), \\(\\log{g}\\), and [Fe/H] range from 3000 to 4200 K, 4.5 to 5.3 dex, and -0.7 to 0.2 dex, respectively. Although our \\(T_{\\rm eff}\\) scale is in good agreement with the literature, we report large discrepancies in the [Fe/H] scales, which might arise from the different methodologies and sets of lines considered. However, our [Fe/H] is in agreement with the metallicity distribution of FGK-type stars in the solar neighbourhood and correlates well with the kinematic membership of the targets in the Galactic populations. Lastly, excellent agreement in \\(T_{\\rm eff}\\) is found for M dwarfs with interferometric angular diameter measurements, as well as in the [Fe/H] between the components in the wide physical FGK+M and M+M systems included in our sample.

We use the Pristine survey CaHK narrow-band photometry, combined with the SDSS ugr photometry, to provide a cleaner sample of blue horizontal branch stars in the Galactic halo out to large distances. We demonstrate a completeness of 91% and a purity of 93% with respect to available spectroscopic classifications. We subsequently use our new clean sample of these standard candles to investigate the substructure in the Galactic halo over the Pristine footprint. Among other features, this allows for a careful tracing of multiple parts of the Sagittarius stream, providing a measurement independent from other tracers used and reaching larger distances. Moreover, we demonstrate with this clean and complete sample that the halo follows a density profile with a negative power-law slope of 3.5 - 4.0. As the relatively shallow SDSS u-band is the limiting factor in this technique, we foresee large potential for combining Pristine survey photometry with the much deeper u-band photometry from the Canada-France-Imaging Survey.