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Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies. While these accreted stellar populations can be forensically identified as kinematically distinct structures within the Galaxy, it is difficult in general to precisely date the age at which any one merger occurred. Recent results have revealed a population of stars that were accreted via the collision of a dwarf galaxy, called \textit{Gaia}-Enceladus, leading to a substantial pollution of the chemical and dynamical properties of the Milky Way. Here, we identify the very bright, naked-eye star $\nu$\,Indi as a probe of the age of the early in situ population of the Galaxy. We combine asteroseismic, spectroscopic, astrometric, and kinematic observations to show that this metal-poor, alpha-element-rich star was an indigenous member of the halo, and we measure its age to be $11.0 \pm 0.7$ (stat) $\pm 0.8$ (sys)$\,\rm Gyr$. The star bears hallmarks consistent with it having been kinematically heated by the \textit{Gaia}-Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 Gyr ago at 68 and 95% confidence, respectively. Input from computations based on hierarchical cosmological models tightens (i.e. reduces) slightly the above limits., Comment: Accepted for publication as a Letter in Nature Astronomy (26 pages, 7 figures, including main article and methods section)

We combine high-resolution spectroscopic data from APOGEE-2 Survey Data Release 16 (DR16) with broad-band photometric data from several sources, as well as parallaxes from {\it Gaia} Data Release 2 (DR2). Using the Bayesian isochrone-fitting code {\tt StarHorse}, we derive distances, extinctions and astrophysical parameters for around 388,815 APOGEE stars, achieving typical distance uncertainties of $\sim 6\%$ for APOGEE giants, $\sim 2\%$ for APOGEE dwarfs, as well as extinction uncertainties of $\sim 0.07$ mag when all photometric information is available, and $\sim 0.17$ mag if optical photometry is missing. {\tt StarHorse} uncertainties vary with the input spectroscopic catalogue, with the available photometry, and with the parallax uncertainties. To illustrate the impact of our results, we show that, thanks to {\it Gaia} DR2 and the now larger sky coverage of APOGEE-2 (including APOGEE-South), we obtain an extended map of the Galactic plane, providing unprecedented coverage of the disk close to the Galactic mid-plane ($|Z_{Gal}|<1$ kpc) from the Galactic Centre out to $R_{\rm Gal}\sim 20$ kpc. The improvements in statistics as well as distance and extinction uncertainties unveil the presence of the bar in stellar density, as well as the striking chemical duality in the innermost regions of the disk, now clearly extending to the inner bulge. We complement this paper with distances and extinctions for stars in other public released spectroscopic surveys: 324,999 in GALAH DR2, 4,928,715 in LAMOST DR5, 408,894 in RAVE DR6, and 6,095 in GES DR3, Comment: Accepted to A&A. 17 pages, 11 figures. Data products are available at https://data.aip.de/aqueiroz2020, (doi:10.17876/data/2020_2)

Since the onset of the `space revolution' of high-precision high-cadence photometry, asteroseismology has been demonstrated as a powerful tool for informing Galactic archaeology investigations. The launch of the NASA TESS mission has enabled seismic-based inferences to go full sky -- providing a clear advantage for large ensemble studies of the different Milky Way components. Here we demonstrate its potential for investigating the Galaxy by carrying out the first asteroseismic ensemble study of red giant stars observed by TESS. We use a sample of 25 stars for which we measure their global asteroseimic observables and estimate their fundamental stellar properties, such as radius, mass, and age. Significant improvements are seen in the uncertainties of our estimates when combining seismic observables from TESS with astrometric measurements from the Gaia mission compared to when the seismology and astrometry are applied separately. Specifically, when combined we show that stellar radii can be determined to a precision of a few percent, masses to 5-10% and ages to the 20% level. This is comparable to the precision typically obtained using end-of-mission Kepler data, Comment: Accepted for publication in ApJ Letters

(Abridged). We introduce the Aarhus Red Giants Challenge, a series of detailed comparisons between widely used stellar evolution and oscillation codes aiming at establishing the minimum level of uncertainties in properties of red giants arising solely from numerical implementations. Using 9 state-of-the-art stellar evolution codes, we defined a set of input physics and physical constants for our calculations and calibrated the convective efficiency to a specific point on the main sequence. We produced evolutionary tracks and stellar structure models at fixed radius along the red-giant branch for masses of 1.0 M$_\odot$, 1.5 M$_\odot$, 2.0 M$_\odot$, and 2.5 M$_\odot$, and compared the predicted stellar properties. Once models have been calibrated on the main sequence we find a residual spread in the predicted effective temperatures across all codes of ~20 K at solar radius and ~30-40 K in the RGB regardless of the considered stellar mass. The predicted ages show variations of 2-5% (increasing with stellar mass) which we track down to differences in the numerical implementation of energy generation. The luminosity of the RGB-bump shows a spread of about 10% for the considered codes, which translates into magnitude differences of ~0.1 mag in the optical V-band. We also compare the predicted [C/N] abundance ratio and found a spread of 0.1 dex or more for all considered masses. Our comparisons show that differences at the level of a few percent still remain in evolutionary calculations of red giants branch stars despite the use of the same input physics. These are mostly due to differences in the energy generation routines and interpolation across opacities, and call for further investigations on these matters in the context of using properties of red giants as benchmarks for astrophysical studies., Comment: accepted for publication in Astronomy and Astrophysics

This paper documents the sixteenth data release (DR16) from the Sloan Digital Sky Surveys; the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the southern hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey (TDSS) and new data from the SPectroscopic IDentification of ERosita Survey (SPIDERS) programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17)., Comment: DR16 release: Monday Dec 9th 2019. This is the alphabetical order SDSS-IV collaboration data release paper. 25 pages, 6 figures, accepted by ApJS on 11th May 2020. Minor changes clarify or improve text and figures relative to v1

The one-dimensional configuration coordinate model (1D-CCM) is widely used for the analysis of photoluminescence in molecules and doped solids, and relies on a linear combination of the equilibrium nuclear configurations of ground and excited states. It delivers an estimation of the energy barrier at which ground and excited state curves cross, semi-classically linked to non-radiative transition rate and thermal quenching. To assess its predictive power for the latter properties, we propose a new \textit{optimized configuration path (OCP) method in which} the ground-state and excited-state forces are mixed instead of their configurations. We also define another one-parameter model thanks a double energy parabola hypothesis (DEPH). We compare the OCP method and the DEPH reference with the 1D-CCM for three paradigmatic 4f-5d phosphors Y$_3$Al$_5$O$_{12}$:Ce, Lu$_2$SiO$_5$:Ce, and YAlO$_3$:Ce. We find that the OCP and DEPH methods yield similar results with geometries that have significantly lower ground-state energies than the 1D-CCM for the same 4f-5d energy difference. However, the OCP method suffers from the appearance of multiple local minima, rendering the clear determination of the optimal geometry very difficult in practice. Still the OCP method allows one to quantify the deviations from the 1D-CCM, therefore increasing confidence in the lower bound obtained from the DEPH for the 4f-5d crossing barrier, and its comparison with the energy of the auto-ionization thermal quenching mechanism. We expect the OCP approach to be applicable to other luminescent materials or molecules., Comment: 12 pages, 6 figure (Accepted by Phys. Rev. B)

The internal working of low-mass stars is of great significance to both the study of stellar structure and the history of the Milky Way. Asteroseismology has the power to directly sense the internal structure of stars and allows for the determination of the evolutionary state -- i.e. has helium burning commenced or is the energy generated only by the fusion in the hydrogen-burning shell? We use observational data from red-giant stars in a combination (known as APOKASC) of asteroseismology (from the \textit{Kepler} mission) and spectroscopy (from SDSS/APOGEE). The new feature of the analysis is that the APOKASC evolutionary state determination is based on the comparison of diverse approaches to the investigation of the frequency-power spectrum. The high level of agreement between the methods is a strong validation of the approaches. Stars for which there is not a consensus view are readily identified. The comparison also facilitates the identification of unusual stars including those that show evidence for very strong coupling between p and g cavities. The comparison between the classification based on the spectroscopic data and asteroseismic data have led to a new value for the statistical uncertainty in APOGEE temperatures. These consensus evolutionary states will be used as an input for methods that derive masses and ages for these stars based on comparison of observables with stellar evolutionary models (`grid-based modeling') and as a training set for machine-learning and other data-driven methods of evolutionary state determination, Comment: 19 pages, 10 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

The Transiting Exoplanet Survey Satellite (TESS) is performing a near all-sky survey for planets that transit bright stars. In addition, its excellent photometric precision enables asteroseismology of solar-type and red-giant stars, which exhibit convection-driven, solar-like oscillations. Simulations predict that TESS will detect solar-like oscillations in nearly 100 stars already known to host planets. In this paper, we present an asteroseismic analysis of the known red-giant host stars HD 212771 and HD 203949, both systems having a long-period planet detected through radial velocities. These are the first detections of oscillations in previously known exoplanet-host stars by TESS, further showcasing the mission's potential to conduct asteroseismology of red-giant stars. We estimate the fundamental properties of both stars through a grid-based modeling approach that uses global asteroseismic parameters as input. We discuss the evolutionary state of HD 203949 in depth and note the large discrepancy between its asteroseismic mass ($M_\ast = 1.23 \pm 0.15\,{\rm M}_\odot$ if on the red-giant branch or $M_\ast = 1.00 \pm 0.16\,{\rm M}_\odot$ if in the clump) and the mass quoted in the discovery paper ($M_\ast = 2.1 \pm 0.1\,{\rm M}_\odot$), implying a change $>30\,\%$ in the planet's mass. Assuming HD 203949 to be in the clump, we investigate the planet's past orbital evolution and discuss how it could have avoided engulfment at the tip of the red-giant branch. Finally, HD 212771 was observed by K2 during its Campaign 3, thus allowing for a preliminary comparison of the asteroseismic performances of TESS and K2. We estimate the ratio of the observed oscillation amplitudes for this star to be $A_{\rm max}^{\rm TESS}/A_{\rm max}^{\rm K2} = 0.75 \pm 0.14$, consistent with the expected ratio of $\sim0.85$ due to the redder bandpass of TESS., Comment: Accepted for publication in ApJ; 17 pages, 4 figures, 3 tables

The period 2035-50 considered in the ESA Voyage long-term plan will coincide with a series of foreseeable advances in the characterization of the stellar content of the Milky Way. The Gaia mission, combined with large-scale spectroscopic surveys, is helping to build an unprecedented census in terms of the astrometric, kinematic and chemical properties of Galactic stellar populations. Within a decade, precise measurements of such properties will be available for hundreds of millions of stars. Meanwhile, time-domain surveys initiated with CoRoT and Kepler/K2 and carried on by space missions such as TESS and PLATO or ground-based projects like the LSST, will have brought asteroseismology to a high level of maturity. The combination of precise ages from asteroseismology with astrometric and spectroscopic data, on large stellar samples, is allowing Galactic archaeologists to gain new insight into the assembly history of the Milky Way. Recent breakthroughs --- based on the detection of solar-like oscillations in tens of thousands of red-giant stars --- demonstrate the potential of such approach. Therefore, we are convinced that an all-sky, high-cadence, long-duration stellar variability survey will become a scientific priority in the 2035-50 period. The Chronos concept presented here consists in a time-domain extension to Gaia. It will allow for mass and age estimates for half a million red giants within 1.7 kpc from the Sun and hence shed a new light on our understanding of the Galactic dynamics and archaeology. In terms of the targeted pulsators, Chronos will bridge the gap between PLATO and the LSST by surveying stars all the way from the subgiant branch to the early AGB. Finally, it will surpass all previous surveys capable of conducting asteroseismology in terms of the combined sky coverage and duration of the observations (2 x 3.75 months over the whole sky and >5 years in the CVZ)., Comment: White paper submitted in response to the Voyage 2050 long-term plan in the ESA Science Programme

We explore the sensitivity of the frequencies of low-degree solar p-modes to near-surface magnetic flux on different spatial scales and strengths, specifically to active regions with strong magnetic fields and ephemeral regions with weak magnetic fields. We also use model reconstructions from the literature to calculate average frequency offsets back to the end of the Maunder minimum. We find that the p-mode frequencies are at least three times less sensitive (at 95% confidence) to the ephemeral-region field than they are to the active-region field. Frequency shifts between activity cycle minima and maxima are controlled predominantly by the change of active region flux. Frequency shifts at cycle minima (with respect to a magnetically quiet Sun) are determined largely by the ephemeral flux, and are estimated to have been $0.1\,\rm \mu Hz$ or less over the last few minima. We conclude that at epochs of cycle minimum, frequency shifts due to near-surface magnetic activity are negligible compared to the offsets between observed and model frequencies that arise from inaccurate modelling of the near-surface layers (the so-called surface term). The implication is that this will be the case for other Sun-like stars with similar activity, which has implications for asteroseismic modelling of stars., Comment: 5 pages, 3 figures, accepted for publication in Letters of Monthly Notices of the Royal Astronomical Society

In the last decade, the Kepler and CoRoT space-photometry missions have demonstrated the potential of asteroseismology as a novel, versatile and powerful tool to perform exquisite tests of stellar physics, and to enable precise and accurate characterisations of stellar properties, with impact on both exoplanetary and Galactic astrophysics. Based on our improved understanding of the strengths and limitations of such a tool, we argue for a new small/medium space mission dedicated to gathering high-precision, high-cadence, long photometric series in dense stellar fields. Such a mission will lead to breakthroughs in stellar astrophysics, especially in the metal poor regime, will elucidate the evolution and formation of open and globular clusters, and aid our understanding of the assembly history and chemodynamics of the Milky Way's bulge and few nearby dwarf galaxies., Comment: White paper submitted in response to Voyage 2050 long-term plan in the ESA Science Programme, now published in Experimental Astronomy, see https://link.springer.com/article/10.1007/s10686-021-09711-1

Since its discovery, the environmental instability of exfoliated black phosphorus (2D bP) has emerged as a challenge that hampers its wide application in chemistry, physics, and materials science. Many studies have been carried out to overcome this drawback. Here we show a relevant enhancement of ambient stability in few-layer bP decorated with nickel nanoparticles as compared to pristine bP. In detail, the behavior of the Ni-functionalized material exposed to ambient conditions in the dark is accurately studied by TEM (Transmission Electron Microscopy), Raman Spectroscopy, and high resolution X-ray Photoemission and Absorption Spectroscopy. These techniques provide a morphological and quantitative insight of the oxidation process taking place at the surface of the bP flakes. In the presence of Ni NPs, the decay time of 2D bP to phosphorus oxides is more than three time slower compared to pristine bP, demonstrating an improved structural stability within twenty months of observation.

Context. The CoRoT and Kepler missions have paved the way for synergies between exoplanetology and asteroseismology. The use of seismic data helps providing stringent constraints on the stellar properties which directly impact the results of planetary studies. Amongst the most interesting planetary systems discovered by Kepler, Kepler-444 is unique by the quality of its seismic and classical stellar constraints. Its magnitude, age and the presence of 5 small-sized planets orbiting this target makes it an exceptional testbed for exoplanetology. Aims. We aim at providing a detailed characterization of Kepler-444, focusing on the dependency of the results on variations of key ingredients of the theoretical stellar models. This thorough study will serve as a basis for future investigations of the planetary evolution of the system orbiting Kepler-444. Methods. We use local and global minimization techniques to study the internal structure of the exoplanet-host star Kepler-444. We combine seismic observations from the Kepler mission, Gaia DR2 data and revised spectroscopic parameters to precisely constrain its internal structure and evolution. Results. We provide updated robust and precise determinations of the fundamental parameters of Kepler-444 and demonstrate that this low-mass star bore a convective core during a significant portion of its life on the main-sequence. Using seismic data, we are able to estimate the lifetime of the convective core to approximately 8 Gyr out of the 11 Gyr of the evolution of Kepler-444. The revised stellar parameters found by our thorough study are M = 0.754 +- 0.03M_Sun , R = 0.753 +- 0.01R_Sun , Age = 11 +- 1 Gy., Comment: Accepted for publication in Astronomy and Astrophysics

The growth of Sn-rich group-IV semiconductors at the nanoscale provides new paths for understanding the fundamental properties of metastable GeSn alloys. Here, we demonstrate the effect of the growth conditions on the morphology and composition of Ge/GeSn core/shell nanowires by correlating the experimental observations with a theoretical interpretation based on a multi-scale approach. We show that the cross-sectional morphology of Ge/GeSn core/shell nanowires changes from hexagonal to dodecagonal upon increasing the supply of the Sn precursor. This transformation strongly influences the Sn distribution as a higher Sn content is measured under the {112} growth front. Ab-initio DFT calculations provide an atomic-scale explanation by showing that Sn incorporation is favored at the {112} surfaces, where the Ge bonds are tensile-strained. A phase-field continuum model was developed to reproduce the morphological transformation and the Sn distribution within the wire, shedding light on the complex growth mechanism and unveiling the relation between segregation and faceting. The tunability of the photoluminescence emission with the change in composition and morphology of the GeSn shell highlights the potential of the core/shell nanowire system for opto-electronic devices operating at mid-infrared wavelengths.

Analyses of data from spectroscopic and astrometric surveys have led to conflicting results concerning the vertical characteristics of the Milky Way. Ages are often used to provide clarity, but typical uncertainties of $>$ 40\,\% restrict the validity of the inferences made. Using the \textit{Kepler} APOKASC sample for context, we explore the global population trends of two K2 campaign fields (3 and 6), which extend further vertically out of the Galactic plane than APOKASC. We analyse the properties of red giant stars utilising three asteroseismic data analysis methods to cross-check and validate detections. The Bayesian inference tool PARAM is used to determine the stellar masses, radii and ages. Evidence of a pronounced red giant branch bump and an [$\alpha$/Fe] dependence on the position of the red clump is observed from the radii distribution of the K2 fields. Two peaks in the age distribution centred at $\sim$5 and and $\sim$12 Gyr are found using a sample with $\sigma_{\rm{age}}$ $<$ 35\,\%. In a comparison with \textit{Kepler}, we find the older peak to be more prominent for K2. This age bimodality is also observed based on a chemical selection of low- ($\leq$ 0.1) and high- ($>$ 0.1) [$\alpha$/Fe] stars. As a function of vertical distance from the Galactic mid-plane ($|Z|$), the age distribution shows a transition from a young to old stellar population with increasing $|Z|$ for the K2 fields. Further coverage of campaign targets with high resolution spectroscopy is required to increase the yield of precise ages achievable with asteroseismology., Comment: 17 pages, 16 figures. Accepted to MNRAS 01/08/2019. Updated manuscript

Strain engineering in Sn-rich group IV semiconductors is a key enabling factor to exploit the direct band gap at mid-infrared wavelengths. Here, we investigate the effect of strain on the growth of GeSn alloys in a Ge/GeSn core/shell nanowire geometry. Incorporation of Sn content in the 10-20 at.% range is achieved with Ge core diameters ranging from 50nm to 100nm. While the smaller cores lead to the formation of a regular and homogeneous GeSn shell, larger cores lead to the formation of multi-faceted sidewalls and broadened segregation domains, inducing the nucleation of defects. This behavior is rationalized in terms of the different residual strain, as obtained by realistic finite element method simulations. The extended analysis of the strain relaxation as a function of core and shell sizes, in comparison with the conventional planar geometry, provides a deeper understanding of the role of strain in the epitaxy of metastable GeSn semiconductors.

We investigate how the stellar and gas-phase He abundances evolve as functions of time within simulated star-forming disc galaxies with different star formation histories. We make use of a cosmological chemodynamical simulation for galaxy formation and evolution, which includes star formation, as well as energy and chemical enrichment feedback from asymptotic giant branch stars, core-collapse supernovae, and Type Ia supernovae. The predicted relations between the He mass fraction, $Y$, and the metallicity, $Z$, in the interstellar medium of our simulated disc galaxies depend on the past galaxy star formation history. In particular, $dY/dZ$ is not constant and evolves as a function of time, depending on the specific chemical element that we choose to trace $Z$; in particular, $dY/dX_{\text{O}}$ and $dY/dX_{\text{C}}$ increase as functions of time, whereas $dY/dX_{\text{N}}$ decreases. In the gas-phase, we find negative radial gradients of $Y$, due to the inside-out growth of our simulated galaxy discs as a function of time; this gives rise to longer chemical enrichment time scales in the outer galaxy regions, where we find lower average values for $Y$ and $Z$. Finally, by means of chemical evolution models, in the galactic bulge and inner disc, we predict steeper $Y$ versus age relations at high $Z$ than in the outer galaxy regions. We conclude that, for calibrating the assumed $Y$-$Z$ relation in stellar models, C, N, and C+N are better proxies for the metallicity than O, because they show steeper and less scattered relations., Comment: Accepted for publication in A&A

KOI-3890 is a highly eccentric, 153-day period eclipsing, single-lined spectroscopic binary system containing a red-giant star showing solar-like oscillations alongside tidal interactions. The combination of transit photometry, radial velocity observations, and asteroseismology have enabled the detailed characterisation of both the red-giant primary and the M-dwarf companion, along with the tidal interaction and the geometry of the system. The stellar parameters of the red-giant primary are determined through the use of asteroseismology and grid-based modelling to give a mass and radius of $M_{\star}=1.04\pm0.06\;\textrm{M}_{\odot}$ and $R_{\star}=5.8\pm0.2\;\textrm{R}_{\odot}$ respectively. When combined with transit photometry the M-dwarf companion is found to have a mass and radius of $M_{\mathrm{c}}=0.23\pm0.01\;\textrm{M}_{\odot}$ and $R_{\mathrm{c}}=0.256\pm0.007\;\textrm{R}_{\odot}$. Moreover, through asteroseismology we constrain the age of the system through the red-giant primary to be $9.1^{+2.4}_{-1.7}\;\mathrm{Gyr}$. This provides a constraint on the age of the M-dwarf secondary, which is difficult to do for other M-dwarf binary systems. In addition, the asteroseismic analysis yields an estimate of the inclination angle of the rotation axis of the red-giant star of $i=87.6^{+2.4}_{-1.2}$ degrees. The obliquity of the system\textemdash the angle between the stellar rotation axis and the angle normal to the orbital plane\textemdash is also derived to give $\psi=4.2^{+2.1}_{-4.2}$ degrees showing that the system is consistent with alignment. We observe no radius inflation in the M-dwarf companion when compared to current low-mass stellar models., Comment: 11 pages, 5 figures, accepted for publication in MNRAS

Combining the precise parallaxes and optical photometry delivered by Gaia's second data release (Gaia DR2) with the photometric catalogues of PanSTARRS-1, 2MASS, and AllWISE, we derive Bayesian stellar parameters, distances, and extinctions for 265 million stars brighter than G=18. Because of the wide wavelength range used, our results substantially improve the accuracy and precision of previous extinction and effective temperature estimates. After cleaning our results for both unreliable input and output data, we retain 137 million stars, for which we achieve a median precision of 5% in distance, 0.20 mag in V-band extinction, and 245 K in effective temperature for G<14, degrading towards fainter magnitudes (12%, 0.20 mag, and 245 K at G=16; 16%, 0.23 mag, and 260 K at G=17, respectively). We find a very good agreement with the asteroseismic surface gravities and distances of 7000 stars in the Kepler, the K2-C3, and the K2-C6 fields, with stellar parameters from the APOGEE survey, as well as with distances to star clusters. Our results are available through the ADQL query interface of the Gaia mirror at the Leibniz-Institut f\"{u}r Astrophysik Potsdam (gaia.aip.de) and as binary tables at data.aip.de. As a first application, in this paper we provide distance- and extinction-corrected colour-magnitude diagrams, extinction maps as a function of distance, and extensive density maps, demonstrating the potential of our value-added dataset for mapping the three-dimensional structure of our Galaxy. In particular, we see a clear manifestation of the Galactic bar in the stellar density distributions, an observation that can almost be considered a direct imaging of the Galactic bar., Comment: 25 pages, 23 figures + appendix, accepted for publication in A&A. Data (doi:10.17876/gaia/dr.2/51) are available through ADQL queries at gaia.aip.de

Asteroseismology provides fundamental stellar parameters independent of distance, but subject to systematics under calibration. Gaia DR2 has provided parallaxes for a billion stars, which are offset by a parallax zero-point. Red Clump (RC) stars have a narrow spread in luminosity, thus functioning as standard candles to calibrate these systematics. This work measures how the magnitude and spread of the RC in the Kepler field are affected by changes to temperature and scaling relations for seismology, and changes to the parallax zero-point for Gaia. We use a sample of 5576 RC stars classified through asteroseismology. We apply hierarchical Bayesian latent variable models, finding the population level properties of the RC with seismology, and use those as priors on Gaia parallaxes to find the parallax zero-point offset. We then find the position of the RC using published values for the zero-point. We find a seismic temperature insensitive spread of the RC of ~0.03 mag in the 2MASS K band and a larger and slightly temperature-dependent spread of ~0.13 mag in the Gaia G band. This intrinsic dispersion in the K band provides a distance precision of ~1% for RC stars. Using Gaia data alone, we find a mean zero-point of -41 $\pm$ 10 $\mu$as. This offset yields RC absolute magnitudes of -1.634 $\pm$ 0.018 in K and 0.546 $\pm$ 0.016 in G. Obtaining these same values through seismology would require a global temperature shift of ~-70 K, which is compatible with known systematics in spectroscopy., Comment: Accepted for publication in MNRAS

The importance of studying the Gaia DR2 parallax zero-point by external means was underlined by Lindegren et al. (2018), and initiated by several works making use of Cepheids, eclipsing binaries, and asteroseismology. Despite a very efficient elimination of basic-angle variations, a small fluctuation remains and shows up as a small offset in the Gaia DR2 parallaxes. By combining astrometric, asteroseismic, spectroscopic, and photometric constraints, we undertake a new analysis of the Gaia parallax offset for nearly 3000 red-giant branch (RGB) and 2200 red clump (RC) stars observed by Kepler, as well as about 500 and 700 red giants (both RGB and RC) selected by the K2 Galactic Archaeology Program in campaigns 3 and 6. Engaging into a thorough comparison of the astrometric and asteroseismic parallaxes, we are able to highlight the influence of the asteroseismic method, and measure parallax offsets in the Kepler field that are compatible with independent estimates from literature and open clusters. Moreover, adding the K2 fields to our investigation allows us to retrieve a clear illustration of the positional dependence of the zero-point, in general agreement with the information provided by quasars. Lastly, we initiate a two-step methodology to make progress in the simultaneous calibration of the asteroseismic scaling relations and of the Gaia DR2 parallax offset, which will greatly benefit from the gain in precision with the third Data Release of Gaia., Comment: 15 pages, 17 figures, Accepted for publication in A&A

(Abridged) The Maunakea Spectroscopic Explorer (MSE) is an end-to-end science platform for the design, execution and scientific exploitation of spectroscopic surveys. It will unveil the composition and dynamics of the faint Universe and impact nearly every field of astrophysics across all spatial scales, from individual stars to the largest scale structures in the Universe. Major pillars in the science program for MSE include (i) the ultimate Gaia follow-up facility for understanding the chemistry and dynamics of the distant Milky Way, including the outer disk and faint stellar halo at high spectral resolution (ii) galaxy formation and evolution at cosmic noon, via the type of revolutionary surveys that have occurred in the nearby Universe, but now conducted at the peak of the star formation history of the Universe (iii) derivation of the mass of the neutrino and insights into inflationary physics through a cosmological redshift survey that probes a large volume of the Universe with a high galaxy density. MSE is positioned to become a critical hub in the emerging international network of front-line astronomical facilities, with scientific capabilities that naturally complement and extend the scientific power of Gaia, the Large Synoptic Survey Telescope, the Square Kilometer Array, Euclid, WFIRST, the 30m telescopes and many more., Comment: 9 chapters, 301 pages, 100 figures. This version of the DSC is a comprehensive update of the original version, released in 2016, which can be downloaded at arXiv:1606.00043. A detailed summary of the design of MSE is available in the MSE Book 2018, available at arXiv:1810.08695

We present a new method for determining the location of active bands of latitude on solar-type stars, which uses stellar-cycle-induced frequency shifts of detectable solar-like oscillations. When near-surface activity is distributed in a non-homogeneous manner, oscillation modes of different angular degree and azimuthal order will have their frequencies shifted by different amounts. We use this simple concept, coupled to a model for the spatial distribution of the near-surface activity, to develop two methods that use the frequency shifts to infer minimum and maximum latitudes for the active bands. Our methods respond to the range in latitude over which there is significant magnetic flux present, over and above weak basal ephemeral flux levels. We verify that we are able to draw accurate inferences in the solar case, using Sun-as-a-star helioseismic data and artificial data. We then apply our methods to Kepler data on the solar analogue HD173701, and find that its active bands straddle a much wider range in latitude than do the bands on the Sun., Comment: 13 pages, 15 figures, MNRAS accepted

We present the first chemical evolution model for Enceladus, alias the Gaia Sausage, to investigate the star formation history of one of the most massive satellites accreted by the Milky Way during a major merger event. Our best chemical evolution model for Enceladus nicely fits the observed stellar [$\alpha$/Fe]-[Fe/H] chemical abundance trends, and reproduces the observed stellar metallicity distribution function, by assuming low star formation efficiency, fast infall time scale, and mild outflow intensity. We predict a median age for Enceladus stars $12.33^{+0.92}_{-1.36}$ Gyr, and - at the time of the merger with our Galaxy ($\approx10$ Gyr ago from Helmi et al.) - we predict for Enceladus a total stellar mass $M_{\star} \approx 5 \times 10^{9}\,\text{M}_{\odot}$. By looking at the predictions of our best model, we discuss that merger events between the Galaxy and systems like Enceladus may have inhibited the gas accretion onto the Galaxy disc at high redshifts, heating up the gas in the halo. This scenario could explain the extended period of quenching in the star formation activity of our Galaxy about 10 Gyr ago, which is predicted by Milky Way chemical evolution models, in order to reproduce the observed bimodality in [$\alpha$/Fe]-[Fe/H] between thick- and thin-disc stars., Comment: Accepted for publication in MNRAS Letters

The Maunakea Spectroscopic Explorer (MSE) is a planned 11.25-m aperture facility with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. A rebirth of the 3.6m Canada-France-Hawaii Telescope on Maunakea, MSE will use 4332 fibers operating at three different resolving powers (R ~ 2500, 6000, 40000) across a wavelength range of 0.36-1.8mum, with dynamical fiber positioning that allows fibers to match the exposure times of individual objects. MSE will enable spectroscopic surveys with unprecedented scale and sensitivity by collecting millions of spectra per year down to limiting magnitudes of g ~ 20-24 mag, with a nominal velocity precision of ~100 m/s in high-resolution mode. This white paper describes science cases for stellar astrophysics and exoplanet science using MSE, including the discovery and atmospheric characterization of exoplanets and substellar objects, stellar physics with star clusters, asteroseismology of solar-like oscillators and opacity-driven pulsators, studies of stellar rotation, activity, and multiplicity, as well as the chemical characterization of AGB and extremely metal-poor stars., Comment: 31 pages, 11 figures; To appear as a chapter for the Detailed Science Case of the Maunakea Spectroscopic Explorer

The relative stability of SiC polytypes, changing with temperature, has been considered a paradox for about thirty years, due to discrepancies between theory and experiments. Based on ab-initio calculations including van der Waals corrections, a temperature-dependent polytypic diagram consistent with the experimental observations is obtained. Results are easily interpreted based on the influence of the hexagonality on both cohesive energy and entropy. Temperature-dependent stability of stacking faults is also analyzed and found to be in agreement with experimental evidences. Our results suggest that lower temperatures during SiC crystal deposition are advantageous in order to reduce ubiquitous stacking faults in SiC-based power devices., Comment: 5 pages, 4 figures

The Sun is the most studied of stars and a laboratory of fundamental physics. However, the understanding of our star is stained by the solar modelling problem which can stem from various causes. We combine inversions of sound speed, an entropy proxy and the Ledoux discriminant with the position of the base of the convective zone and the photospheric helium abundance to test combinations of ingredients such as equation of state, abundance and opacity tables. We study the potential of the inversions to constrain ad-hoc opacity modifications and additional mixing in the Sun. We show that they provide constraints on these modifications to the ingredients and that the solar problem likely occurs from various sources and using phase shifts with our approach is the next step to take., Comment: To appear in the proceedings of the PHOST meeting held in Banyuls, France

We present the target list of solar-type stars to be observed in short-cadence (2-min) for asteroseismology by the NASA Transiting Exoplanet Survey Satellite (TESS) during its 2-year nominal survey mission. The solar-like Asteroseismic Target List (ATL) is comprised of bright, cool main-sequence and subgiant stars and forms part of the larger target list of the TESS Asteroseismic Science Consortium (TASC). The ATL uses Gaia DR2 and the Extended Hipparcos Compilation (XHIP) to derive fundamental stellar properties, calculate detection probabilities and produce a rank-ordered target list. We provide a detailed description of how the ATL was produced and calculate expected yields for solar-like oscillators based on the nominal photometric performance by TESS. We also provide publicly available source code which can be used to reproduce the ATL, thereby enabling comparisons of asteroseismic results from TESS with predictions from synthetic stellar populations., Comment: 13 pages, 11 figures; accepted for publication in Astrophysical Journal Supplement Series

The kinematics of the Milky Way disc as a function of age are well measured at the solar radius, but have not been studied over a wider range of Galactocentric radii. Here, we measure the kinematics of mono-age, mono-$\mathrm{[Fe/H]}$ populations in the low and high $\mathrm{[\alpha/Fe]}$ discs between $4 \lesssim R \lesssim 13$ kpc and $|z| \lesssim 2$ kpc using 65,719 stars in common between APOGEE DR14 and $\it{Gaia}$ DR2 for which we estimate ages using a Bayesian neural network model trained on asteroseismic ages. We determine the vertical and radial velocity dispersions, finding that the low and high $\mathrm{[\alpha/Fe]}$ discs display markedly different age--velocity-dispersion relations (AVRs) and shapes $\sigma_z/\sigma_R$. The high $\mathrm{[\alpha/Fe]}$ disc has roughly flat AVRs and constant $\sigma_z/\sigma_R = 0.64\pm 0.04$, whereas the low $\mathrm{[\alpha/Fe]}$ disc has large variations in this ratio which positively correlate with the mean orbital radius of the population at fixed age. The high $\mathrm{[\alpha/Fe]}$ disc component's flat AVRs and constant $\sigma_z/\sigma_R$ clearly indicates an entirely different heating history. Outer disc populations also have flatter radial AVRs than those in the inner disc, likely due to the waning effect of spiral arms. Our detailed measurements of AVRs and $\sigma_z/\sigma_R$ across the disc indicate that low $\mathrm{[\alpha/Fe]}$, inner disc ($R \lesssim 10\,\mathrm{kpc}$) stellar populations are likely dynamically heated by both giant molecular clouds and spiral arms, while the observed trends for outer disc populations require a significant contribution from another heating mechanism such as satellite perturbations. We also find that outer disc populations have slightly positive mean vertical and radial velocities, likely because they are part of the warped disc., Comment: Accepted for publication in MNRAS - The revised version has various updates to the text following suggestions from the referee, but general results remain the same. Code is available https://github.com/jmackereth/monoage-velocity-dispersion, and the APOGEE DR14 age catalogue can be found alongside a paper summary http://www.astro.ljmu.ac.uk/~astjmack/dynamical-heating.html

A key aspect in the determination of stellar properties is the comparison of observational constraints with predictions from stellar models. Asteroseismic Inference on a Massive Scale (AIMS) is an open source code that uses Bayesian statistics and a Markov Chain Monte Carlo approach to find a representative set of models that reproduce a given set of classical and asteroseismic constraints. These models are obtained by interpolation on a pre-calculated grid, thereby increasing computational efficiency. We test the accuracy of the different operational modes within AIMS for grids of stellar models computed with the Li\`ege stellar evolution code (main sequence and red giants) and compare the results to those from another asteroseismic analysis pipeline, PARAM. Moreover, using artificial inputs generated from models within the grid (assuming the models to be correct), we focus on the impact on the precision of the code when considering different combinations of observational constraints (individual mode frequencies, period spacings, parallaxes, photospheric constraints,...). Our tests show the absolute limitations of precision on parameter inferences using synthetic data with AIMS, and the consistency of the code with expected parameter uncertainty distributions. Interpolation testing highlights the significance of the underlying physics to the analysis performance of AIMS and provides caution as to the upper limits in parameter step size. All tests demonstrate the flexibility and capability of AIMS as an analysis tool and its potential to perform accurate ensemble analysis with current and future asteroseismic data yields., Comment: Accepted for publication in MNRAS. 17 pages, 17 figures

We present the discovery of TOI-197.01, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. TOI-197 (HIP116158) is a bright (V=8.2 mag), spectroscopically classified subgiant which oscillates with an average frequency of about 430 muHz and displays a clear signature of mixed modes. The oscillation amplitude confirms that the redder TESS bandpass compared to Kepler has a small effect on the oscillations, supporting the expected yield of thousands of solar-like oscillators with TESS 2-minute cadence observations. Asteroseismic modeling yields a robust determination of the host star radius (2.943+/-0.064 Rsun), mass (1.212 +/- 0.074 Msun) and age (4.9+/-1.1 Gyr), and demonstrates that it has just started ascending the red-giant branch. Combining asteroseismology with transit modeling and radial-velocity observations, we show that the planet is a "hot Saturn" (9.17+/-0.33 Rearth) with an orbital period of ~14.3 days, irradiance of 343+/-24 Fearth, moderate mass (60.5 +/- 5.7 Mearth) and density (0.431+/-0.062 gcc). The properties of TOI-197.01 show that the host-star metallicity - planet mass correlation found in sub-Saturns (4-8 Rearth) does not extend to larger radii, indicating that planets in the transition between sub-Saturns and Jupiters follow a relatively narrow range of densities. With a density measured to ~15%, TOI-197.01 is one of the best characterized Saturn-sized planets to date, augmenting the small number of known transiting planets around evolved stars and demonstrating the power of TESS to characterize exoplanets and their host stars using asteroseismology., Comment: 12 pages (excluding author list and references), 9 figures, 4 tables, accepted for publication in AJ. An electronic version of Table 3 is available as an ancillary file (sidebar on the right)

Asteroseismology is a unique tool that can be used to study the interior of stars and hence deliver unique information for the studiy of stellar physics, stellar evolution, and Galactic archaeology. We aim to develop a simple model of the information content of asteroseismology and to characterize the ability and precision with which fundamental properties of stars can be estimated for different space missions. We defined and calibrated metrics of the seismic performance. The metrics, expressed by a seismic index ${\mathcal{E}$ defined by simple scaling relations, are calculated for an ensemble of stars. We studied the relations between the properties of mission observations, fundamental stellar properties, and the performance index. We also defined thresholds for asteroseismic detection and measurement of different stellar properties We find two regimes of asteroseismic performance: the first where the signal strength is dominated by stellar properties and not by observational noise; and the second where observational properties dominate. Typically, for evolved stars, stellar properties provide the dominant terms in estimating the information content, while main sequence stars fall in the regime where the observational properties, especially stellar magnitude, dominate. We estimate scaling relations to predict ${\mathcal{E}$ with an intrinsic scatter of around 21%. Incidentally, the metrics allow us to distinguish stars burning either hydrogen or helium. Our predictions will help identify the nature of the cohort of existing and future asteroseismic observations. In addition, the predicted performance for PLATO will help define optimal observing strategies for defined scientific goals., Comment: Accepted in A&A

Context: The observations of solar-like oscillations in evolved stars have brought important constraints on their internal rotation rates. To correctly reproduce these data, an efficient transport mechanism is needed in addition to meridional circulation and shear instability. Aims: We study the efficiency of the transport of angular momentum during the subgiant phase. Results: The precise asteroseismic measurements of both core and surface rotation rates available for the six Kepler targets enable a precise determination of the efficiency of the transport of angular momentum needed for each of these subgiants. These results are found to be insensitive to all the uncertainties related to the modelling of rotational effects before the post-main sequence phase. An interesting exception in this context is the case of young subgiants (typical values of log(g) close to 4), because their rotational properties are sensitive to the degree of radial differential rotation on the main sequence. These young subgiants constitute therefore perfect targets to constrain the transport of angular momentum on the main sequence from asteroseismic observations of evolved stars. As for red giants, we find that the efficiency of the additional transport process increases with the mass of the star during the subgiant phase. However, the efficiency of this undetermined mechanism decreases with evolution during the subgiant phase, contrary to what is found for red giants. Consequently, a transport process with an efficiency that increases with the degree of radial differential rotation cannot account for the core rotation rates of subgiants, while it correctly reproduces the rotation rates of red giant stars. This suggests that the physical nature of the additional mechanism needed for the internal transport of angular momentum may be different in subgiant and red giant stars., Comment: 10 pages, 11 figures, accepted for publication in A&A

We study from first principles the luminescence of Lu$_2$SiO$_5$:Ce$^{3+}$ (LSO:Ce), a scintillator widely used in medical imaging applications, and establish the crucial role of oxygen vacancies (V$_O$) in the generated spectrum. The excitation energy, emission energy and Stokes shift of its luminescent centers are simulated through a constrained density-functional theory method coupled with a ${\Delta}$SCF analysis of total energies, and compared with experimental spectra. We show that the high-energy emission band comes from a single Ce-based luminescent center, while the large experimental spread of the low-energy emission band originates from a whole set of different Ce-V$_O$ complexes together with the other Ce-based luminescent center. Further, the luminescence thermal quenching behavior is analyzed. The $4f-5d$ crossover mechanism is found to be very unlikely, with a large crossing energy barrier (E$_{fd}$) in the one-dimensional model. The alternative mechanism usually considered, namely the electron auto-ionization, is also shown to be unlikely. In this respect, we introduce a new methodology in which the time-consuming accurate computation of the band gap for such models is bypassed. We emphasize the usually overlooked role of the differing geometry relaxation in the excited neutral electronic state Ce$^{3+,*}$ and in the ionized electronic state Ce$^{4+}$. The results indicate that such electron auto-ionization cannot explain the thermal stability difference between the high- and low-energy emission bands. Finally, a hole auto-ionization process is proposed as a plausible alternative. With the already well-established excited state characterization methodology, the approach to color center identification and thermal quenching analysis proposed here can be applied to other luminescent materials in the presence of intrinsic defects., Comment: 13 pages, 8 figures, accepted by Phys. Rev. Materials