Intermediate-mass Stars Research Articles

FORCAST Imaging of Two Small Nearby Clusters: The Coronet and B 59

Abstract We present mid-infrared imaging of two young clusters, the Coronet in the CrA cloud core and B 59 in the Pipe Nebula, using the FORCAST camera on the Stratospheric Observatory for Infrared Astronomy. We also analyze Herschel Space Observatory 4 4 Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. PACS and SPIRE images of the associated clouds. The two clusters are at similar, and very close, distances. Star formation is ongoing in the Coronet, which hosts at least one Class 0 source and several pre-stellar cores, which may collapse and form stars. The B 59 cluster is older, although it still has a few Class I sources, and is less compact. The CrA cloud has a diameter of ∼0.16 pc, and we determine a dust temperature of 15.7 K and a star formation efficiency of ∼27%. The B 59 core is approximately twice as large, with a dust temperature of ∼11.4 K and a star formation efficiency of ∼14%. We infer that the gas densities are much higher in the Coronet, which has also formed intermediate-mass stars, while B 59 has only formed low-mass stars.

Characterizing the morphology of the debris disk around the low-mass star GSC 07396-00759

Context. Debris disks have commonly been studied around intermediate-mass stars. Their intense radiation fields are believed to efficiently remove the small dust grains that are constantly replenished by collisions. For lower-mass central objects, in particular M stars, the dust removal mechanism needs to be further investigated given the much weaker radiation field produced by these objects. Aims. We present new observations of the nearly edge-on disk around the pre-main-sequence M-type star GSC 07396-00759, taken with VLT/SPHERE IRDIS in dual-beam polarimetric imaging mode, with the aim to better understand the morphology of the disk, its dust properties, and the star-disk interaction via the stellar mass-loss rate. Methods. We model the polarimetric observations to characterize the location and properties of the dust grains using the Henyey–Greenstein approximation of the polarized phase function. We use the estimated phase function to evaluate the strength of the stellar winds. Results. We find that the polarized light observations are best described by an extended and highly inclined disk (i ≈ 84.3 ° ± 0.3) with a dust distribution centered at a radius r0 ≈ 107 ± 2 au. Our modeling suggests an anisotropic scattering factor g ≈ 0.6 to best reproduce the polarized phase function S12. We also find that the phase function is reasonably well reproduced by small micron-sized dust grains with sizes s > 0.3μm. We discuss some of the caveats of the approach, mainly that our model probably does not fully recover the semimajor axis of the disk and that we cannot readily determine all dust properties due to a degeneracy between the grain size and the porosity. Conclusions. Even though the radius of the disk may be overestimated, our best-fit model not only reproduces the observations well but is also consistent with previous published data obtained in total intensity. Similarly to previous studies of debris disks, we suggest that using a given scattering theory might not be sufficient to fully explain key aspects, such as the shape of the phase function or the dust grain size. Taking into consideration the aforementioned caveats, we find that the average mass-loss rate of GSC 07396-00759 can be up to 500 times stronger than that of the Sun, supporting the idea that stellar winds from low-mass stars can evacuate small dust grains in an efficient way.

Globules and pillars in Cygnus X

IRAS 20319+3958 in Cygnus X South is a rare example of a free-floating globule (mass ~240M⊙, length ~1.5 pc) with an internal H IIregion created by the stellar feedback of embedded intermediate-mass stars, in particular, one Herbig Be star. In Schneider et al. 2012, (A&A, 542, L18) and Djupvik et al. 2017, (A&A, 599, A37), we proposed that the emission of the far-infrared (FIR) lines of [C II] at 158 μm and [O I] at 145 μm in the globule head are mostly due to an internal photodissociation region (PDR). Here, we present aHerschel/HIFI [C II] 158 μm map of the whole globule and a large set of other FIR lines (mid-to high-JCO lines observed withHerschel/PACS and SPIRE, the [O I] 63 μm line and the12CO 16→15 line observed with upGREAT on SOFIA), covering the globule head and partly a position in the tail. The [C II] map revealed that the whole globule is probably rotating. Highly collimated, high-velocity [C II] emission is detected close to the Herbig Be star. We performed a PDR analysis using the KOSMA-τPDR code for one position in the head and one in the tail. The observed FIR lines in the head can be reproduced with a two-component model: an extended, non-clumpy outer PDR shell and a clumpy, dense, and thin inner PDR layer, representing the interface between the H IIregion cavity and the external PDR. The modelled internal UV field of ~2500G°is similar to what we obtained from theHerschelFIR fluxes, but lower than what we estimated from the census of the embedded stars. External illumination from the ~30 pc distant Cyg OB2 cluster, producing an UV field of ~150–600G°as an upper limit, is responsible for most of the [C II] emission. For the tail, we modelled the emission with a non-clumpy component, exposed to a UV-field of around 140G°.

SPH modelling of wind-companion interactions in eccentric AGB binary systems

Context. The late evolutionary stages of low- and intermediate-mass stars are characterised by mass loss through a dust-driven stellar wind. Recent observations reveal complex structures within these winds, which are believed to be formed primarily via an interaction with a companion. How these complexities arise, and which structures are formed in which type of systems, is still poorly understood. Particularly, there is a lack of studies investigating the structure formation in eccentric systems. Aims. We aim to improve our understanding of the wind morphology of eccentric asymptotic giant branch (AGB) binary systems by investigating the mechanism responsible for the different small-scale structures and global morphologies that arise in a polytropic wind with different velocities. Methods. Using the smoothed particle hydrodynamics (SPH) code PHANTOM, we generated nine different high-resolution, 3D simulations of an AGB star with a solar-mass companion with various wind velocity and eccentricity combinations. The models assume a polytropic gas, with no additional cooling. Results. Compared to the zero-eccentricity situation, we find that for low eccentricities, for the case of a high wind velocity, and hence limited interaction between the wind and the companion, the standard two-edged spiral structure that dominates the shape of the wind in the orbital plane is only minimally affected. When the wind speed is lower, strong compression of the wind material by the companion occurs, causing a high-pressure region around the companion which shapes the wind into an irregular spiral. In extreme cases, with low wind velocity and high eccentricity, these instabilities grow to such proportion that they cause high-speed ejections of matter along the orbital plane, shaping the wind into a highly irregular morphology. In more eccentric orbits, the amplitude of the phase-dependent wind-companion interaction increases significantly, introducing additional complexities that make the outbursts even more energetic, leading in some cases to high-speed polar flows of matter. Further, the orbital motion of the stars tends to flatten the global density distribution of the models with no instabilities. We distinguish global flattening from an equatorial density enhancement, the latter being formed by a strong gravitational interaction of the companion with the wind particles. We classify the resulting morphologies according to these new definitions, and find that (i) all low-velocity models have an equatorial density enhancement and (ii), in general, the flattening increases for decreasing wind velocity, until the low wind velocity results in high-energy outflows that clear away the flattening. Conclusions. We conclude that for models with a high wind velocity, the short interaction with the companion results in a regular spiral morphology, which is flattened. In the case of a lower wind velocity, the stronger interaction results in the formation of a high-energy region and bow-shock structure that can shape the wind into an irregular morphology if instabilities arise. High-eccentricity models show a complex, phase-dependent interaction leading to wind structures that are irregular in three dimensions. However, the significant interaction with the companion compresses matter into an equatorial density enhancement, irrespective of eccentricity.

Grids of stellar models with rotation

Context. Grids of stellar models, computed with the same physical ingredients, allow one to study the impact of a given physics on a broad range of initial conditions and they are a key ingredient for modeling the evolution of galaxies. Aims. We present here a grid of single star models for masses between 0.8 and 120 M⊙, with and without rotation for a mass fraction of heavy element Z = 0.006, representative of the Large Magellanic Cloud (LMC). Methods. We used the GENeva stellar Evolution Code. The evolution was computed until the end of the central carbon-burning phase, the early asymptotic giant branch phase, or the core helium-flash for massive, intermediate, and low mass stars, respectively. Results. The outputs of the present stellar models are well framed by the outputs of the two grids obtained by our group for metallicities above and below the one considered here. The models of the present work provide a good fit to the nitrogen surface enrichments observed during the main sequence for stars in the LMC with initial masses around 15 M⊙. They also reproduce the slope of the luminosity function of red supergiants of the LMC well, which is a feature that is sensitive to the time-averaged mass loss rate over the red supergiant phase. The most massive black hole that can be formed from the present models at Z = 0.006 is around 55 M⊙. No model in the range of mass considered will enter into the pair-instability supernova regime, while the minimal mass to enter the region of pair pulsation instability is around 60 M⊙ for the rotating models and 85 M⊙ for the nonrotating ones. Conclusions. The present models are of particular interest for comparisons with observations in the LMC and also in the outer regions of the Milky Way. We provide public access to numerical tables that can be used for computing interpolated tracks and for population synthesis studies.

Photoevaporation of Grain-depleted Protoplanetary Disks around Intermediate-mass Stars: Investigating the Possibility of Gas-rich Debris Disks as Protoplanetary Remnants

Abstract Debris disks are classically considered to be gas-less systems, but recent (sub)millimeter observations have detected tens of those with rich gas content. The origin of the gas component remains unclear, but it could be protoplanetary remnants and/or secondary products from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for ≳ 10 Myr . However, previous models predict ≲ 10 Myr lifetimes because of efficient photoevaporation at the late stage of disk evolution. We investigate photoevaporation of gas-rich, optically-thin disks around intermediate-mass stars at a late stage of the disk evolution. The evolved system is modeled like those devoid of small grains ( ≲ 4 μ m ). We find that grain depletion reduces photoelectric heating so that far-ultraviolet photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of the order of 1 × 10 − 11 ( Φ EUV / 10 38 s − 1 ) 1 / 2 M ⊙ yr − 1 , where Φ EUV is the EUV emission rate of the host star. The estimated gas–disk lifetimes are ∼ 100 ( M disk / 10 − 3 M ⊙ ) ( Φ EUV / 10 38 s − 1 ) 1 / 2 Myr and depend on the “initial” disk mass at the point small grains have been depleted in the system. We show that the gas component can survive for a much longer time around A-type stars than lower-mass (F-, G-, K-type) stars owing to their atypical low EUV (and X-ray) luminosities. This trend is consistent with the higher frequency of gas-rich debris disks around A-type stars, implying the possibility of the gas component being protoplanetary remnants.

ATOMIUM: The astounding complexity of the near circumstellar environment of the M-type AGB star R Hydrae

Evolved low- to intermediate-mass stars are known to shed their gaseous envelope into a large, dusty, molecule-rich circumstellar nebula which typically develops a high degree of structural complexity. Most of the large-scale, spatially correlated structures in the nebula are thought to originate from the interaction of the stellar wind with a companion. As part of the ATOMIUMlarge programme, we observed the M-type asymptotic giant branch (AGB) star R Hydrae with the Atacama Large Millimeter/submillimeter Array. The morphology of the inner wind of R Hya, which has a known companion at ∼3500 au, was determined from maps of CO and SiO obtained at high angular resolution. A map of the CO emission reveals a multi-layered structure consisting of a large elliptical feature at an angular scale of ∼10″ that is oriented along the north–south axis. The wind morphology within the elliptical feature is dominated by two hollow bubbles. The bubbles are on opposite sides of the AGB star and lie along an axis with a position angle of ∼115°. Both bubbles are offset from the central star, and their appearance in the SiO channel maps indicates that they might be shock waves travelling through the AGB wind. An estimate of the dynamical age of the bubbles yields an age of the order of 100 yr, which is in agreement with the previously proposed elapsed time since the star last underwent a thermal pulse. When the CO and SiO emission is examined on subarcsecond angular scales, there is evidence for an inclined, differentially rotating equatorial density enhancement, strongly suggesting the presence of a second nearby companion. The position angle of the major axis of this disc is ∼70° in the plane of the sky. We tentatively estimate that a lower limit on the mass of the nearby companion is ∼0.65 M⊙on the basis of the highest measured speeds in the disc and the location of its inner rim at ∼6 au from the AGB star.

Study of rotational splittings in δ Scuti stars using pattern finding techniques

ABSTRACT Detecting and understanding rotation in stellar interiors is nowadays one of the unsolved problems in stellar physics. Asteroseismology has been able to provide insights on rotation for the Sun, solar-like stars, and compact objects like white dwarfs. However, this is still very difficult for intermediate-mass stars. These stars are moderate-to-rapid rotators. Rotation splits and shifts the oscillation modes, which makes the oscillation spectrum more complex and harder to interpret. Here we study the oscillation patterns of a sample of benchmark δ Sct stars belonging to eclipsing binary systems with the objective of finding the frequency spacing related to the rotational splitting (δr). For this task, we combine three techniques: the Fourier transform, the autocorrelation function, and the histogram of frequency differences. The last two showed a similar behaviour. For most of the stars, it was necessary to determine the large separation (Δν) prior to spot δr. This is the first time we may clearly state that one of the periodicities present in the p modes oscillation spectra of δ Sct stars corresponds to the rotational splitting. This is true independent of the stellar rotation rate. These promising results pave the way to finding a robust methodology to determine rotational splittings from the oscillation spectra of δ Sct stars and, thus, understanding the rotational profile of intermediate-mass pulsating stars.

Spitzer Mid-infrared Study of Sh 2-208: Evolution of Protoplanetary Disks in Low-metallicity Environments

Abstract This study presents sensitive mid-infrared (MIR) photometry obtained with the Spitzer/IRAC for a young cluster in Sh 2-208 (S208) located in one of the lowest-metallicity H ii regions in the Galaxy, [ O / H ] = − 0.8 dex. Previous studies suggested that the cluster is ∼0.5 Myr old and has a distance of D = 4 kpc, which is consistent with the astrometric distance from Gaia EDR3. In a ∼ 3 .′ 5 × 4 ′ field, 96 sources were detected in at least one MIR band at ≥10σ, covering intermediate-mass stars with a ∼1.0 M ⊙ mass detection limit. A total of 41 probable cluster members were identified based on the spatial distributions of spectral energy distribution slopes derived from the near-infrared (NIR) K S and IRAC bands and extinctions of the sources. The cumulative distribution of the spectral energy distribution slopes for the S208 cluster was not significantly different from those of other clusters in solar-metallicity environments with approximately the same age for intermediate-mass stars, if one also considers nondetected MIR sources identified as S208 cluster members from NIR observations. This suggests that the degree of dust growth/settling does not significantly change with metallicities as different as ∼1 dex. The fraction of stars with MIR disk emissions for the cluster members with ≥1 M ⊙ was 64%–93%, which is comparable to the results in solar-metallicity environments. Although this may suggest that dominant disk dispersal mechanisms for intermediate-mass stars have either no or very weak dependence on metallicity, it can be argued alternatively that this may suggest that the disk dispersal process does not work effectively at this young stage.

Influence of Irradiation-driven Winds on the Evolution of Intermediate-mass Black Hole X-ray Binaries

Abstract In young dense clusters, an intermediate-mass black hole (IMBH) may get a companion star via exchange encounters or tidal capture and then evolve toward the IMBH X-ray binary by the Roche lobe overflow. It is generally thought that IMBH X-ray binaries are potential ultraluminous X-ray sources (ULXs); hence, their evolution is very significant. However, the irradiation-driven winds by the strong X-ray flux from the accretion disks around the IMBHs play an important role in determining the evolution of IMBH X-ray binaries and should be considered in the detailed binary evolution simulation. Employing the models with the MESA code, we focus on the influence of irradiation-driven winds on the evolution of IMBH X-ray binaries. Our simulations indicate that a high wind-driving efficiency (f = 0.01 for Z = 0.02, and f = 0.002 for Z = 0.001) substantially shortens the duration in the ULX stage of IMBH X-ray binaries with an intermediate-mass (5 M ⊙) donor star. However, this effect can be ignored for high-mass (10 M ⊙) donor stars. The irradiation effect (f = 0.01 or 0.002) markedly shrinks the initial parameter space of IMBH binaries evolving toward ULXs with high luminosity (L X > 1040 erg s−1) and hyperluminous X-ray sources in the donor-star mass versus orbital period diagram. Furthermore, the irradiation effect results in an efficient angular momentum loss, yielding to IMBH X-ray binaries with relatively close orbits. In our simulated parameter space, about 1% of IMBH binaries would evolve toward compact X-ray sources owing to short initial orbital periods, some of which might be detected as low-frequency gravitational-wave sources.

Warm ISM in the Sgr A complex

Context. The Arches Cluster – Arched Filaments (AF) system is our Galaxy’s prime example of the complexity involved in the interaction between the strong radiation field of numerous OB stars and their surrounding ISM in extremely harsh environments such as the Galactic center (GC) of the Milky Way. It offers a unique opportunity to study the close relationship between photon-dominated regions (PDRs) and H II regions and their relative contributions to the observed [CII] emission. Aims. We aim to investigate the I([CII]) versus I([NII]) integrated intensity behavior in the AF region in order to assess the [CII] emission contribution from the H II region, which is traced by [NII] line observations, and PDR components in the high-metallicity environment of the GC. Methods. We used [CII] 158 μm and [NII] 205 μm fine structure line observations of the AF in the literature to compare their observational integrated intensity distribution to semi-theoretical predictions for the contribution of H II regions and adjacent PDRs to the observed [CII] emission. We explored variations in the [C/N] elemental abundance ratio to explain the overall behavior of the observed relationship. Based on our models, the H II region and PDR contributions to the observed [CII] emission is calculated for a few positions within and near to the AF. Estimates for the [C/N] abundance ratio and [N/H] nitrogen elemental abundance in the AF can then be derived. Results. The behavior of the I([CII]) versus I([NII]) relationship in the AF can be explained by model results satisfying 0.84 < [C/N]AF < 1.41, with model metallicities ranging from 1 Z⊙ to 2 Z⊙, hydrogen volume density log n(H) = 3.5, and ionization parameters log U from −1 to −2. A least-squares fit to the model data points yields log I([CII]) = 1.068 × log I([NII]) + 0.645 to predict the [CII] emission arising from the H II regions in the AF. The fraction of the total observed [CII] emission arising from within PDRs varies between ~0.20 and ~0.75. Our results yield average values for the carbon-to-nitrogen ratio and nitrogen elemental abundances of [C/N]AF = 1.13 ± 0.09 and [N/H]AF = 6.21 × 10−4 for the AF, respectively. They are a factor of ~0.4 smaller and ~7.5 larger than their corresponding Galactic disk values. Conclusions. The large spatial variation of the fraction of [CII] emission arising either from H II regions or PDRs suggests that both contributions must be disentangled before any modeling attempt is made to explain the observed [CII] emission in the AF. We suggest thatsecondary production of nitrogen from low- to intermediate-mass stars in the Galactic bulge is a plausible mechanism to explain the large abundance differences between the GC and the Galactic disk. The mass loss of such stars would enrich the GC ISM with nitrogen as the gas falls into the inner GC orbits where the AF are located. Overall, our results show that tight constraints are needed on the [C/N] abundance ratio for the GC, significantly tighter than previous abundance measurements have discerned.

Constraining stellar evolution theory with asteroseismology of γ Doradus stars using deep learning

Context. The efficiency of the transport of angular momentum and chemical elements inside intermediate-mass stars lacks proper calibration, thereby introducing uncertainties on a star’s evolutionary pathway. Improvements require better estimation of stellar masses, evolutionary stages, and internal mixing properties. Aims. Our aim was to develop a neural network approach for asteroseismic modelling, and test its capacity to provide stellar masses, ages, and overshooting parameter for a sample of 37 γ Doradus stars; these parameters were previously determined from their effective temperature, surface gravity, near-core rotation frequency, and buoyancy travel time Π0. Here our goal is to perform the parameter estimation from modelling of individual periods measured for dipole modes with consecutive radial order rather than from Π0. We assess whether fitting these individual mode periods increases the capacity of the parameter estimation. Methods. We trained neural networks to predict theoretical pulsation periods of high-order gravity modes (n ∈ [15, 91]), and to predict the luminosity, effective temperature, and surface gravity for a given mass, age, overshooting parameter, diffusive envelope mixing, metallicity, and near-core rotation frequency. We applied our neural networks for Computing Pulsation Periods and Photospheric Observables (C-3PO) to our sample and compute grids of stellar pulsation models for the estimated parameters. Results. We present the near-core rotation rates (from the literature) as a function of the inferred stellar age and critical rotation rate. We assessed the rotation rates of the sample near the start of the main sequence assuming rigid rotation. Furthermore, we measured the extent of the core overshoot region and find no correlation with mass, age, or rotation. Finally, for one star in our sample, KIC 12066947, we find indications of mode coupling in the period spacing pattern which we cannot reproduce with mode trapping. Conclusions. The neural network approach developed in this study allows the derivation of stellar properties dominant for stellar evolution, such as mass, age, and extent of core-boundary mixing. It also opens a path for future estimation of mixing profiles throughout the radiative envelope, with the aim of inferring these profiles for large samples of γ Doradus stars.

Evolution of neutron capture elements in dwarf galaxies

ABSTRACT We study the evolution of europium (Eu) and barium (Ba) abundances in Local Group dwarf spheroidal and ultrafaint dwarf galaxies by means of detailed chemical evolution models and compare our results with new sets of homogeneous abundances. The adopted models include gas infall and outflow and have been previously tested. We investigate several production scenarios for r-process elements: merging neutron stars and magnetorotational-driven supernovae. Production of Ba through the main s-process acting in low- and intermediate-mass stars is considered as well. We also test different sets of nucleosynthesis yields. For merging neutron stars we adopt either a constant and short delay time for merging or a delay time distribution function. Our simulations show that (i) if r-process elements are produced only by a quick source, it is possible to reproduce the [Eu/Fe] versus [Fe/H], but those models fail in reproducing the [Ba/Fe] versus [Fe/H]. (ii) If r-process elements are produced only with longer delays the opposite happens. (iii) If both a quick source and a delayed one are adopted, such as magnetorotational-driven supernovae and merging neutron stars with a delay time distribution, the [Eu/Fe] abundance pattern is successfully reproduced, but models still fail in reproducing the [Ba/Fe]. (iv) On the other hand, the characteristic abundances of Reticulum II can be reproduced only if both the Eu and the r-process fraction of Ba are produced on short and constant time delays during a single merging event. We discuss also other possible interpretations, including an inhomogeneous mixing of gas that might characterize this galaxy.

An Asymmetric Dust Ring around a Very Low Mass Star ZZ Tau IRS

Abstract We present Atacama Large Millimeter/submillimeter Array (ALMA) gas and dust observations at band 7 (339 GHz: 0.89 mm) of the protoplanetary disk around a very low mass star ZZ Tau IRS with a spatial resolution of 025. The 12CO J = 3 → 2 position–velocity diagram suggests a dynamical mass of ZZ Tau IRS of ∼0.1–0.3 M ☉. The disk has a total flux density of 273.9 mJy, corresponding to an estimated mass of 24–50 M ⊕ in dust. The dust emission map shows a ring at r = 58 au and an azimuthal asymmetry at r = 45 au with a position angle of 135°. The properties of the asymmetry, including radial width, aspect ratio, contrast, and contribution to the total flux, were found to be similar to the asymmetries around intermediate mass stars (∼2 M ☉) such as MWC 758 and IRS 48. This implies that the asymmetry in the ZZ Tau IRS disk shares a similar origin with others, despite the star being ∼10 times less massive. Our observations also suggest that the inner and outer parts of the disk may be misaligned. Overall, the ZZ Tau IRS disk shows evidence of giant planet formation on a ∼10 au scale at a few megayears. If confirmed, it will challenge existing core accretion models in which such planets have been predicted to be extremely hard to form around very low mass stars.

The evolution of Lithium: implications of a universal Spite plateau

ABSTRACT The cosmological 7Li problem consists in explaining why the primordial Li abundance, as predicted by the standard Big Bang nucleosynthesis theory with constraints from WMAP and Planck, is a factor of 3 larger than the Li abundance measured in the stars of the Spite plateau defined by old, warm dwarf stars of the Milky Way halo. Several explanations have been proposed to explain this difference, including various Li depletion processes as well as non-standard Big Bang nucleosynthesis, but the main question remains unanswered. In this paper, we present detailed chemical evolution models for dwarf spheroidal and ultra faint galaxies, compute the galactic evolution of 7Li abundance in these objects, and compare it with observations of similar objects. In our models, Li is mainly produced by novae and cosmic rays, and to a minor extent, by low and intermediate mass stars. We adopt the yield combination that best fits the Li abundances in the Milky Way stars. It is evident that the observations of dwarf objects define a Spite plateau, identical to that observed in the Milky Way, thus suggesting that the Spite plateau could be a universal feature and its meaning should be discussed. The predictions of our models for dwarf galaxies are obtained by assuming as Li primordial abundance either the one detected in the atmospheres of the oldest halo stars (Spite plateau; A(Li) ∼ 2.2 dex), or the one from cosmological observations (WMAP; A(Li) ∼ 2.66 dex). Finally, we discuss the implications of the universality of the Spite plateau results.

Isotope Systematics of Presolar Silicate Grains: New Insights from Magnesium and Silicon

Abstract We report on Mg and Si isotope data of 86 presolar silicate grains identified through NanoSIMS oxygen ion imaging in thin sections of carbonaceous and ordinary chondrites. The O, Mg, and Si isotope data of 106 presolar silicates (including grains studied previously by our group) suggest division of O isotope Group 1 grains into four subpopulations: (i) “normal,” (ii) 25Mg-rich, (iii) 26Mg-rich, and (iv) 25Mg-poor. Normal Group 1 grains (∼60% of Group 1 grains) formed in the winds of low-mass asymptotic giant branch (AGB) stars, with Mg and Si defining linear arrays with slopes of ∼0.9 and 1.37, respectively, in three-isotope representations, most likely representing Galactic chemical evolution (GCE). The 25Mg-rich grains (∼25%) show enrichments in 25Mg of up to a factor 2.4 relative to solar composition and most likely formed in supernova (SN) ejecta or the winds of intermediate-mass AGB stars. The 26Mg-rich and 25Mg-poor Group 1 grains lie below the Mg GCE line and their isotopic compositions favor origins from supergiants or SNe. The O isotope Group 2 grains show a wide range of Mg-isotopic compositions, similar to Group 1 grains, with likely origins from massive AGB stars, super-AGB stars, supergiants, and SNe. The Mg- and Si-isotopic compositions of Group 4 grains are compatible with previously proposed SN origins. Our results suggest that >30% of presolar silicates formed in the winds of supergiants and in SN ejecta, and that low-mass AGB stars appear to have contributed only some 50% to presolar silicates, less than previously thought.

Star Formation Timescales of the Halo Populations from Asteroseismology and Chemical Abundances*

Abstract We combine asteroseismology, optical high-resolution spectroscopy, and kinematic analysis for 26 halo red giant branch stars in the Kepler field in the range of −2.5 < [Fe/H] < −0.6. After applying theoretically motivated corrections to the seismic scaling relations, we obtain an average mass of 0.97 ± 0.03 M ⊙ for our sample of halo stars. Although this maps into an age of ∼7 Gyr, significantly younger than independent age estimates of the Milky Way stellar halo, we considered this apparently young age to be due to the overestimation of stellar mass in the scaling relations. There is no significant mass dispersion among lower red giant branch stars (log g > 2), which constrains the relative age dispersion to <18%, corresponding to <2 Gyr. The precise chemical abundances allow us to separate the stars with [Fe/H] > −1.7 into two [Mg/Fe] groups. While the [α/Fe] and [Eu/Mg] ratios are different between the two subsamples, [s/Eu], where s stands for Ba, La, Ce, and Nd, does not show a significant difference. These abundance ratios suggest that the chemical evolution of the low-Mg population is contributed by Type Ia supernovae, but not by low- to intermediate-mass asymptotic giant branch stars, providing a constraint on its star formation timescale as 100 Myr < τ < 300 Myr. We also do not detect any significant mass difference between the two [Mg/Fe] groups, thus suggesting that their formation epochs are not separated by more than 1.5 Gyr.

AGB Winds with Gas-Dust Drift in Stellar Evolution Codes

A significant fraction of new metals produced in stars enter the interstellar medium in the form of dust grains. Including dust and wind formation in stellar evolution models of late-stage low- and intermediate-mass stars provides a way to quantify their contribution to the cosmic dust component. In doing so, a correct physical description of dust formation is of course required, but also a reliable prescription for the mass-loss rate. Here, we present an improved model of dust-driven winds to be used in stellar evolution codes and insights from recent detailed numerical simulations of carbon-star winds including drift (decoupling of dust and gas). We also discuss future directions for further improvement.

The effect of the centrifugal acceleration on period spacings of gravito-inertial modes in intermediate-mass stars

Context. The Kepler and Transiting Exoplanet Survey Satellite (TESS) space telescopes delivered high-precision, long-duration photometric time series for hundreds of main-sequence stars, revealing their numerous gravito-inertial (g) pulsation modes. This high precision allows us to evaluate increasingly detailed theoretical stellar models. Recent theoretical work extended the traditional approximation of rotation, a framework to evaluate the effect of the Coriolis acceleration on g modes, to include the effects of the centrifugal acceleration in the approximation of slightly deformed stars, which so far have mostly been neglected in asteroseismology. This extension of the traditional approximation was conceived by re-deriving the traditional approximation in a centrifugally-deformed, spheroidal coordinate system. Aims. We explore the effect of the centrifugal acceleration on g modes and assess its detectability in space-based photometric observations. Methods. We implemented the new theoretical framework to calculate the centrifugal deformation of pre-computed 1D spherical stellar structure models and computed the corresponding g-mode frequencies, assuming uniform rotation. The framework was evaluated for a grid of stellar structure models covering a relevant parameter space for observed g-mode pulsators. Results. The centrifugal acceleration modifies the effect of the Coriolis acceleration on g modes, narrowing the equatorial band in which they are trapped. Furthermore, the centrifugal acceleration causes the pulsation periods and period spacings of the most common g modes (prograde dipole modes and r modes) to increase with values similar to the observational uncertainties of the measured period spacing values in Kepler and TESS data. Conclusions. The effect of the centrifugal acceleration on g modes is formally detectable in modern space photometry. The implementation of the used theoretical framework in stellar structure and pulsation codes will allow for more precise asteroseismic modelling of centrifugally deformed stars in order to assess its effect on mode excitation, trapping, and damping.

An IRTF-iSHELL Survey of 4.52–5.25 μm CO Spectra in Protoplanetary Disks of Intermediate-mass Stars: Preliminary Sample and Analysis

Abstract We are conducting a survey of high-resolution near-infrared CO rovibrational spectra in protoplanetary disks around young intermediate-mass stars. We use IRTF-iSHELL with the 0.″375 and 0.″75 slits, providing 4 km s−1 and 6 km s−1 resolution and covering 4.52–5.25 μm in one single exposure. This includes part of the rovibrational R branch and most of the P branch of the CO fundamental band (Δv = 1), one of the best tracers of warm/hot gas in disks. The high quality of the spectra and the large sample covered in this survey will support multiple investigations to study the structure, kinematics, composition, and evolution of the inner 10 au in disks for years to come. The survey currently includes ∼30 stars mostly within 200 pc, complementing extensive imaging campaigns that are observing their disks at optical, infrared, and millimeter wavelengths and revealing structures and planets.