The Shellless Supernova Remnant B0532–67.5 in the Large Magellanic Cloud

The supernova remnant (SNR) B0532−67.5 in the Large Magellanic Cloud (LMC) was first diagnosed by its nonthermal radio emission, and its SNR nature was confirmed by the observation of diffuse X-ray emission; however, no optical SNR shell is detected. The OB association LH75, or NGC 2011, is projected within the boundary of this SNR. We have analyzed the massive star population in and around SNR B0532−67.5 using optical photometric data to construct color–magnitude diagrams, using stellar evolutionary tracks to estimate stellar masses, and using isochrones to assess the stellar ages. From these analyses, we find a 20–25 Myr population in LH75 and a younger population less than 10 Myr old to the southwest of LH75. The center of SNR B0532−67.5 is located closer to the core of LH75 than to the massive stars to its southwest. We conclude that the supernova progenitor was probably a member of LH75 with an initial mass of ∼15 M ⊙. The supernova exploded in an H i cavity excavated by the energy feedback of LH75. The low density of the ambient medium prohibits the formation of a visible nebular shell. Despite the low density in the ambient medium, physical properties of the hot gas within the SNR interior do not differ from SNRs with a visible shell by more than a factor of 2–3. The large-scale H i map shows that SNR B0532−67.5 is projected in a cavity that appears to be connected with the much larger cavity of the supergiant shell LMC-4.

Chemo-dynamics and asteroseismic ages of seven metal-poor red giants from the Kepler field

In this work we combine information from solar-like oscillations, high-resolution spectroscopy and Gaia astrometry to derive stellar ages, chemical abundances and kinematics for a group of seven metal-poor Red Giants and characterise them in a multidimensional chrono-chemo-dynamical space. Chemical abundance ratios were derived through classical spectroscopic analysis employing 1D LTE atmospheres on Keck/HIRES spectra. Stellar ages, masses and radii were calculated with grid-based modelling, taking advantage of availability of asteroseismic information from Kepler. The dynamical properties were determined with Galpy using Gaia EDR3 astrometric solutions. Our results suggest that underestimated parallax errors make the effect of Gaia parallaxes more important than different choices of model grid or – in the case of stars ascending the RGB – mass-loss prescription. Two of the stars in this study are identified as potentially evolved halo blue stragglers. Four objects are likely members of the accreted Milky Way halo, and their possible relationship with known accretion events is discussed.

Probing core overshooting using subgiant asteroseismology: The case of KIC10273246

Context. The size of convective cores remains uncertain, despite their substantial influence on stellar evolution, and thus on stellar ages. The seismic modeling of young subgiants can be used to obtain indirect constraints on the core structure during main sequence, thanks to the high probing potential of mixed modes. Aims. We selected the young subgiant KIC10273246, observed by Kepler, based on its mixed-mode properties. We thoroughly modeled this star, with the aim of placing constraints on the size of its main-sequence convective core. A corollary goal of this study is to elaborate a modeling technique that is suitable for subgiants and can later be applied to a larger number of targets. Methods. We first extracted the parameters of the oscillation modes of the star using the full Kepler data set. To overcome the challenges posed by the seismic modeling of subgiants, we propose a method that is specifically tailored to subgiants with mixed modes and uses nested optimization. We then applied this method to perform a detailed seismic modeling of KIC10273246. Results. We obtain models that show good statistical agreements with the observations, both seismic and non-seismic. We show that including core overshooting in the models significantly improves the quality of the seismic fit, optimal models being found for αov = 0.15. Higher amounts of core overshooting strongly worsen the agreement with the observations and are thus firmly ruled out. We also find that having access to two g-dominated mixed modes in young subgiants allows us to place stronger constraints on the gradient of molecular weight in the core and on the central density. Conclusions. This study confirms the high potential of young subgiants with mixed modes to investigate the size of main-sequence convective cores. It paves the way for a more general study including the subgiants observed with Kepler, TESS, and eventually PLATO.

The AMBRE Project: Origin and evolution of sulfur in the Milky Way

Context. Sulfur is a volatile chemical element that plays an important role in tracing the chemical evolution of the Milky Way and external galaxies. However, its nucleosynthesis origin and abundance variations in the Galaxy are still unclear because the number of available stellar sulfur abundance measurements is currently rather small. Aims. The goal of the present article is to accurately and precisely study the sulfur content of large number of stars located in the solar neighbourhood. Methods. We use the parametrisation of thousands of high-resolution stellar spectra provided by the AMBRE Project, and combine it with the automated abundance determination GAUGUIN to derive local thermodynamic equilibrium sulfur abundances for 1855 slow-rotating FGK-type stars. This is the largest and most precise catalogue of sulfur abundances published to date. It covers a metallicity domain as high as ∼2.5 dex starting at [M/H] ∼ −2.0 dex. Results. We find that the sulfur-to-iron abundances ratio is compatible with a plateau-like distribution in the metal-poor regime, and then starts to decrease continuously at [M/H] ∼ −1.0 dex. This decrease continues towards negative values for supersolar metallicity stars as recently reported for magnesium and as predicted by Galactic chemical evolution models. Moreover, sulfur-rich stars having metallicities in the range [ − 1.0, −0.5] have very different kinematical and orbital properties with respect to more metal-rich and sulfur-poor ones. Two disc components, associated with the thin and thick discs, are thus seen independently in kinematics and sulfur abundances. The sulfur radial gradients in the Galactic discs have also been estimated. Finally, the enrichment in sulfur with respect to iron is nicely correlated with stellar ages: older metal-poor stars have higher [S/M] ratios than younger metal-rich ones. Conclusions. This work has confirmed that sulfur is an α-element that could be considered to explore the Galactic populations properties. For the first time, a chemo-dynamical study from the sulfur abundance point of view, as a stand-alone chemical element, is performed.

Evolution of Earth-like extended exospheres orbiting solar-like stars

Recent observations of the Earth’s exosphere revealed the presence of an extended hydrogenic component that could reach distances beyond 40 planetary radii. Detection of similar extended exospheres around Earth-like exoplanets could reveal crucial facts in terms of habitability. The presence of these rarified hydrogen envelopes is extremely dependent of the planetary environment, dominated by the ionizing radiation and plasma winds coming from the host star. Radiation and fast wind particles ionize the uppermost layers of planetary atmospheres, especially for planets orbiting active, young stars. The survival of the produced ions in the exosphere of such these planets is subject to the action of the magnetized stellar winds, particularly for unmagnetized bodies. In order to address these star-planet interactions, we have carried out numerical 2.5D ideal MHD simulations using the PLUTO code to study the dynamical evolution of tenuous, hydrogen-rich, Earth-like extended exospheres for an unmagnetized planet, at different stellar evolutionary stages: from a very young, solar-like star of 0.1 Gyr to a 5.0 Gyr star. For each star-planet configuration, we show that the morphology of extended Earth-like hydrogen exospheres is strongly dependent of the incident stellar winds and the produced ions present in these gaseous envelopes, showing that the ionized component of Earth-like exospheres is quickly swept by the stellar winds of young stars, leading to large bow shock formation for later stellar ages.

On ancient solar-type stars – II

We report on the progress of our survey on ancient solar-type stars down to main-sequence effective temperatures Teff ≥ 5300 K and within 42 pc of the Sun. High signal-to-noise, high-resolution spectroscopy is presented for a second major subset of the Population II (τ ≥ 12 Gyr) and the intermediate-disc stars (τ ≃ 10 Gyr) within that volume. In conjunction with updates and the analyses of the single and composite sample spectra, we discuss evidence for new companions or candidates from their radial velocities, chromospheric activities, lithium and barium enrichments, and we also draw attention to related sources in the Gaia DR2 data. Among the Population II stars we note a substantial fraction of degenerates, mass transfer, and merger systems that possibly amount to about 20 per cent of that population, with inherently important consequences on the involved stellar ages and the baryonic mass budget. At the present stage, the survey has reached a two-thirds level of local volume-completeness. Key to that objective will be the forthcoming Gaia data, in terms of new companions, companion masses, and precision parallaxes from orbital solutions, in particular at the sample periphery, where many of the sources inevitably reside. In an appendix we describe a subset of about fifty a priori survey candidates, whose analyses discard them as Population I stars.

Realistic mock observations of the sizes and stellar mass surface densities of massive galaxies in FIRE-2 zoom-in simulations

ABSTRACT The galaxy size–stellar mass and central surface density–stellar mass relationships are fundamental observational constraints on galaxy formation models. However, inferring the physical size of a galaxy from observed stellar emission is non-trivial due to various observational effects, such as the mass-to-light ratio variations that can be caused by non-uniform stellar ages, metallicities, and dust attenuation. Consequently, forward-modelling light-based sizes from simulations is desirable. In this work, we use the skirt dust radiative transfer code to generate synthetic observations of massive galaxies ($M_{*}\sim 10^{11}\, \rm {M_{\odot }}$ at z = 2, hosted by haloes of mass $M_{\rm {halo}}\sim 10^{12.5}\, \rm {M_{\odot }}$) from high-resolution cosmological zoom-in simulations that form part of the Feedback In Realistic Environments project. The simulations used in this paper include explicit stellar feedback but no active galactic nucleus (AGN) feedback. From each mock observation, we infer the effective radius (Re), as well as the stellar mass surface density within this radius and within $1\, \rm {kpc}$ (Σe and Σ1, respectively). We first investigate how well the intrinsic half-mass radius and stellar mass surface density can be inferred from observables. The majority of predicted sizes and surface densities are within a factor of 2 of the intrinsic values. We then compare our predictions to the observed size–mass relationship and the Σ1−M⋆ and Σe−M⋆ relationships. At z ≳ 2, the simulated massive galaxies are in general agreement with observational scaling relations. At z ≲ 2, they evolve to become too compact but still star forming, in the stellar mass and redshift regime where many of them should be quenched. Our results suggest that some additional source of feedback, such as AGN-driven outflows, is necessary in order to decrease the central densities of the simulated massive galaxies to bring them into agreement with observations at z ≲ 2.

Inside-out formation of nuclear discs and the absence of old central spheroids in barred galaxies of the TIMER survey

The centres of disc galaxies host a variety of structures built via both internal and external processes. In this study, we constrain the formation and evolution of these central structures, in particular, nuclear rings and nuclear discs, by deriving maps of mean stellar ages, metallicities, and [α/Fe] abundances. We use observations obtained with the MUSE integral-field spectrograph for the TIMER sample of 21 massive barred galaxies. Our results indicate that nuclear discs and nuclear rings are part of the same physical component, with nuclear rings constituting the outer edge of nuclear discs. All nuclear discs in the sample are clearly distinguished based on their stellar population properties. As expected in the picture of bar-driven secular evolution, nuclear discs are younger, more metal-rich, and exhibit lower [α/Fe] enhancements, as compared to their immediate surroundings. Moreover, nuclear discs exhibit well-defined radial gradients, with ages and metallicities decreasing, and [α/Fe] abundances increasing with radius out to the nuclear ring. Often, these gradients show no breaks from the edge of the nuclear disc up through the centre, suggesting that these structures extend to the very centres of galaxies. We argue that continuous (stellar) nuclear discs may form from a series of bar-built (initially gas-rich) nuclear rings that expand in their radius as the bar evolves. In this picture, nuclear rings are simply the (often) star-forming outer edge of nuclear discs. Finally, by combining our results with those taken from a accompanying kinematic study, we do not find evidence for the presence of large, dispersion-dominated components in the centres of these galaxies. This could be a result of quiet merger histories, despite the large galaxy masses, or, perhaps, due to high angular momentum and strong feedback processes preventing the formation of these kinematically hot components.