Subgiant Research Articles

Stellar chromospheric activity of 1674 FGK stars from the AMBRE-HARPS sample

Aims.The main objective of this project is to characterise chromospheric activity of FGK stars from the HARPS archive. We start, in this first paper, by presenting a catalogue of homogeneously determined chromospheric emission (CE), stellar atmospheric parameters, and ages for 1674 FGK main sequence (MS), subgiant, and giant stars. The analysis of CE level and variability is also performed.Methods.We measured CE in the Ca IIH&K lines using more than 180 000 high-resolution spectra from the HARPS spectrograph, as compiled in the AMBRE project, obtained between 2003 and 2019. We converted the fluxes to bolometric and photospheric corrected chromospheric emission ratio,RHK′. Stellar atmospheric parametersTeff, logg, and [Fe/H] were retrieved from the literature or determined using a homogeneous method.M⋆,R⋆, and ages were determined from isochrone fitting.Results.We show that our sample has a distribution of CE for MS stars that is consistent with an unbiased sample of solar-neighbour MS stars. We analysed the CE distribution for the different luminosity classes and spectral types and confirmed the existence of the very inactive (VI) star and very active star populations at logRHK′ < −5.1 and > − 4.2 dex, respectively. We found indications that the VI population is composed mainly of subgiant and giant stars and that logRHK′ = −5.1 dex marks a transition in stellar evolution. Overall, CE variability decreases with decreasing CE level but its distribution is complex. There appears to be at least three regimes of variability for inactive, active, and very active stars, with the inactive and active regimes separated by a diagonal, extended Vaughan-Preston (VP) gap. We show that stars with low activity levels do not necessarily have low variability. In the case of K dwarfs, which show high CE variability, inactive and active stars have similar levels of activity variability. This means that activity levels alone are not enough to infer the activity variability of a star. We also explain the shape of the VP gap observed in the distribution of CE using the CE variability-level diagram. In the CE variability-level diagram, the Sun is located in the high-variability region of the inactive MS stars zone. A method to extract the probability density function of the CE variability for a given logRHK′level is discussed, and a python code to retrieve it is provided.

Flare Activity and Magnetic Feature Analysis of the Flare Stars. II. Subgiant Branch

Abstract We present an investigation of the magnetic activity and flare characteristics of the subgiant stars mostly from F and G spectral types and compare the results with the main-sequence (MS) stars. The light curve of 352 stars on the subgiant branch (SGB) from the Kepler mission is analyzed in order to infer stability, relative coverage, and contrast of the magnetic structures and also flare properties using three flare indexes. The results show the following. (i) Relative coverage and contrast of the magnetic features along with rate, power, and magnitude of flares increase on the SGB due to the deepening of the convective zone and more vigorous magnetic field production. (ii) Magnetic activity of the F- and G-type stars on the SGB does not show dependency on the rotation rate and does not obey the saturation regime. This is the opposite of what we saw for the MS, in which the G-, K-, and M-type stars show clear dependency on the Rossby number. (iii) The positive relationship between the magnetic features’ stability and their relative coverage and contrast remains true on the SGB; though, it has a lower dependency coefficient in comparison with the MS. (iv) Magnetic proxies and flare indexes of the SGB stars increase with increasing relative mass of the convective zone.

Spectroscopic and photometric analysis of 21 chromospherically active variables: Activity cycles and differential rotation

AbstractWe investigate magnetic activity properties of 21 stars via medium resolution optical spectra and long-term photometry. Applying synthetic spectrum fitting method, we find that all targets are cool giant or sub-giant stars possessing overall [M/H] abundances between 0 and$-0.5$. We find that six of these targets exhibit only linear trend in mean brightness, while eight of them clearly shows cyclic mean brightness variation. Remaining seven target appear to exhibit cyclic mean brightness variation, but this cannot be confirmed due to the long timescales of the predicted cycle compared to the current time range of the photometric data. We further determine seasonal photometric periods and compute average photometric period of each target. Analysed sample in this study provides a quantitative representation of a positive linear correlation between the inverse of the rotation period and the cycle period normalised to the rotation period, on the log-log scale. We also observe no correlation between the activity cycle length and the relative surface shear, indicating that the activity cycle must be driven by a parameter rather than the differential rotation. Our analyses show that the relative surface shear is positively correlated with the rotation period and there is a noticeable separation between main sequence stars and our sample. Compared to our sample, the relative surface shear of a main sequence star is larger for a given rotation period. However, dependence of the relative surface shear on the rotation period appears stronger for our sample. Analysis of the current photometric data indicates that the photometric properties of the observed activity cycles in eight targets seem dissimilar to the sunspot cycle.

Element Abundance Analysis of HD 140283 and Astrophysics Sources of Weak r-process Elements

HD 140283, a nearby extremely metal-poor subgiant star, formed shortly after the big bang, is thought to be the oldest star to date, and also a typical weak r-process star. Study of this star is of great significance for understanding the early history of the universe, and enriching the element nucleosynthesis theory. Based on the observed abundances of weak r-process star HD 140283 and theoretical yields of single supernova (SN), we fit the observational elemental abundances (from C to Zn) to the entire library of single SN yield, and derive that HD 140283 may have been born from a nebula polluted by the explosion of supernova whose progenitor mass is 22.5M⊙ (the subscript “⊙” means the sun). Based on the same method, we study the other five typical weak r-process stars, and respectively derive the progenitor masses of supernovae which have polluted the nebulas forming these stars. Furthermore, the possible astrophysics sources of weak r-process are inferred.

Mode angular degree identification in subgiant stars with convolutional neural networks based on power spectrum

ABSTRACT The identification of the angular degrees l of oscillation modes is essential for asteroseismology and it depends on visual tagging before fitting power spectra in a so-called peakbagging analysis. In oscillating subgiants, radial (l = 0) mode frequencies are distributed linearly in frequency, while non-radial (l ≥ 1) modes are p–g mixed modes that have a complex distribution in frequency that increases the difficulty of identifying l. In this study, we trained a one-dimensional convolutional neural network to perform this task using smoothed oscillation spectra. By training simulation data and fine-tuning the pre-trained network, we achieved 95 per cent accuracy for Kepler data.

A photometric-spectroscopic study of the infrared-excess eclipsing binary V2364 Cyg

The eclipsing binary V2364 Cyg is an A7V-type contact binary with a period of 0.5921 d. We present six spectra from LAMOST and BVRc light curves (LCs), which are simultaneously analyzed to obtain the absolute physical parameters. The first light maxima are brighter than the second ones by up to , and for BVRc bands, respectively. These distorted LCs are modeled by a cool spot on the less massive component. Its mass ratio and over-contact degree are q = 0.319(±0.004) and f = 28.4%(±1.0)% respectively. From the mass-luminosity diagram, the primary component is a normal main-sequence star, while the secondary one is a subgiant star. From the (O – C) curve, the orbital period secularly decreases at a rate dP/dt = –1.62(±0.03)×10−7d yr−1, which is mainly attributed to mass loss via stellar wind from the binary system. This may result in the observed infrared excess in the W4 band.

On the Detectability of Large-Scale Flows by Asteroseismology

Large-scale convective motions are an integral part of stellar interior dynamics and might play a relevant role in stellar dynamo processes. However, they are difficult to detect or characterize. Stellar oscillations are affected by convective flows due to advection. For the Sun, forward calculations of the advective effect of flows on oscillation modes have already been conducted, but the effect has not yet been examined for other types of stars. Suitable candidates are subgiant or red giant stars, since they possess extensive outer convection zones, which likely feature large-scale flow cells with strong flow velocities. We investigate the effects of large-scale flows on oscillation modes of subgiant stars by means of forward calculations based on an exemplary subgiant stellar model. We focus in particular on non-axisymmetric cell formations, also referred to as giant cells. The effects are described in the non-rotating and the rotating case. By solving the forward problem, we evaluate, if large-scale flow cells lead to signatures in asteroseismic data that are suitable for the detection of such flows. The influence of flows is calculated by employing perturbation theory as proposed by Lavely & Ritzwoller (1992), where the flow is treated as a perturbation of a 1D equilibrium stellar model. The presence of a flow leads to a coupling of the modes, which results in frequency shifts and a mixing of the mode eigenfunctions. For a non-rotating star, non-axisymmetric flows lead to degeneracies between coupling modes, which cause an asymmetry in the frequency shifts of modes of opposite azimuthal order. If rotation is included, the degeneracy is lifted in first order, but residual degenerate coupling and third order effects can still lead to asymmetries, depending on whether the modes are of p- or of g-type. For rotating stars, the mode mixing induced by non-axisymmetric flows causes the observational signal of a perturbed mode to be multiperiodic, which becomes visible in the power spectrum. An expression for the amplitudes of the signal’s different components is derived.

On attempting to automate the identification of mixed dipole modes for subgiant stars

Context. The existence of mixed modes in stars is a marker of stellar evolution. Their detection serves for a better determination of stellar age. Aims. The goal of this paper is to identify the dipole modes in an automatic manner without human intervention. Methods. I used the power spectra obtained by the Kepler mission for the application of the method. I computed asymptotic dipole mode frequencies as a function of the coupling factor and dipole period spacing, as well as other parameters. For each star, I collapsed the power in an echelle diagramme aligned onto the monopole and dipole mixed modes. The power at the null frequency was used as a figure of merit. Using a genetic algorithm, I then optimised the figure of merit by adjusting the location of the dipole frequencies in the power spectrum. Using published frequencies, I compared the asymptotic dipole mode frequencies with published frequencies. I also used published frequencies to derive the coupling factor and dipole period spacing using a non-linear least squares fit. I used Monte-Carlo simulations of the non-linear least square fit to derive error bars for each parameter. Results. From the 44 subgiants studied, the automatic identification allows one to retrieve within 3 μHz, at least 80% of the modes for 32 stars, and within 6 μHz, at least 90% of the modes for 37 stars. The optimised and fitted gravity-mode period spacing and coupling factor are in agreement with previous measurements. Random errors for the mixed-mode parameters deduced from the Monte-Carlo simulation are about 30−50 times smaller than previously determined errors, which are in fact systematic errors. Conclusions. The period spacing and coupling factors of mixed modes in subgiants are confirmed. The current automated procedure will need to be improved upon using a more accurate asymptotic model and/or proper statistical tests.

The close binary fraction as a function of stellar parameters in APOGEE: a strong anticorrelation with α abundances

ABSTRACT We use observations from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to explore the relationship between stellar parameters and multiplicity. We combine high-resolution repeat spectroscopy for 41 363 dwarf and subgiant stars with abundance measurements from the APOGEE pipeline and distances and stellar parameters derived using Gaia DR2 parallaxes from Sanders & Das to identify and characterize stellar multiples with periods below 30 yr, corresponding to ΔRVmax ≳ 3 km s−1, where ΔRVmax is the maximum APOGEE-detected shift in the radial velocities. Chemical composition is responsible for most of the variation in the close binary fraction in our sample, with stellar parameters like mass and age playing a secondary role. In addition to the previously identified strong anticorrelation between the close binary fraction and [Fe/H], we find that high abundances of α elements also suppress multiplicity at most values of [Fe/H] sampled by APOGEE. The anticorrelation between α abundances and multiplicity is substantially steeper than that observed for Fe, suggesting C, O, and Si in the form of dust and ices dominate the opacity of primordial protostellar discs and their propensity for fragmentation via gravitational stability. Near [Fe/H] = 0 dex, the bias-corrected close binary fraction (a < 10 au) decreases from ≈100 per cent at [α/H] = −0.2 dex to ≈15 per cent near [α/H] = 0.08 dex, with a suggestive turn-up to ≈20 per cent near [α/H] = 0.2. We conclude that the relationship between stellar multiplicity and chemical composition for sun-like dwarf stars in the field of the Milky Way is complex, and that this complexity should be accounted for in future studies of interacting binaries.

Asteroseismic inference of subgiant evolutionary parameters with deep learning

ABSTRACT With the observations of an unprecedented number of oscillating subgiant stars expected from NASA’s TESS mission, the asteroseismic characterization of subgiant stars will be a vital task for stellar population studies and for testing our theories of stellar evolution. To determine the fundamental properties of a large sample of subgiant stars efficiently, we developed a deep learning method that estimates distributions of fundamental parameters like age and mass over a wide range of input physics by learning from a grid of stellar models varied in eight physical parameters. We applied our method to four Kepler subgiant stars and compare our results with previously determined estimates. Our results show good agreement with previous estimates for three of them (KIC 11026764, KIC 10920273, KIC 11395018). With the ability to explore a vast range of stellar parameters, we determine that the remaining star, KIC 10005473, is likely to have an age 1 Gyr younger than its previously determined estimate. Our method also estimates the efficiency of overshooting, undershooting, and microscopic diffusion processes, from which we determined that the parameters governing such processes are generally poorly constrained in subgiant models. We further demonstrate our method’s utility for ensemble asteroseismology by characterizing a sample of 30 Kepler subgiant stars, where we find a majority of our age, mass, and radius estimates agree within uncertainties from more computationally expensive grid-based modelling techniques.

A search for transiting planets around FGKM dwarfs and subgiants in the TESS full frame images of the Southern ecliptic hemisphere

ABSTRACT In this work, we present the analysis of 976 814 FGKM dwarf and subgiant stars in the Transiting Exoplanet Survey Telescope (TESS) full frame images (FFIs) of the Southern ecliptic hemisphere. We present a new pipeline, DIAmante, developed to extract optimized, multisector photometry from TESS FFIs and a classifier, based on the Random Forest technique, trained to discriminate plausible transiting planetary candidates from common false positives. A new statistical model was developed to provide the probability of correct identification of the source of variability. We restricted the planet search to the stars located in the least crowded regions of the sky and identified 396 transiting planetary candidates among which 252 are new detections. The candidates’ radius distribution ranges between 1 R⊕ and 2.6 RJ with median value of 1 RJ and the period distribution ranges between 0.25 and 105 d with median value of 3.8 d. The sample contains four long period candidates (P > 50 d), one of which is new, and 64 candidates with periods between 10 and 50 d (42 new ones). In the small planet radius domain (4R < R⊕), we found 39 candidates among which 15 are new detections. Additionally, we present 15 single transit events (14 new ones), a new candidate multiplanetary system, and a novel candidate around a known TOI. By using Gaia dynamical constraints, we found that 70 objects show evidence of binarity. We release a catalogue of the objects we analysed and the corresponding light curves and diagnostic figures through the MAST and ExoFOP portals.

Fast and Automated Peak Bagging with DIAMONDS (FAMED)

Stars of low and intermediate mass that exhibit oscillations may show tens of detectable oscillation modes each. Oscillation modes are a powerful tool to constrain the internal structure and rotational dynamics of the star, hence allowing one to obtain an accurate stellar age. The tens of thousands of solar-like oscillators that have been discovered thus far are representative of the large diversity of fundamental stellar properties and evolutionary stages available. Because of the wide range of oscillation features that can be recognized in such stars, it is particularly challenging to properly characterize the oscillation modes in detail, especially in light of large stellar samples. Overcoming this issue requires an automated approach, which has to be fast, reliable, and flexible at the same time. In addition, this approach should not only be capable of extracting the oscillation mode properties of frequency, linewidth, and amplitude from stars in different evolutionary stages, but also able to assign a correct mode identification for each of the modes extracted. Here we present the new freely available pipeline FAMED (Fast and AutoMated pEak bagging with DIAMONDS), which is capable of performing an automated and detailed asteroseismic analysis in stars ranging from the main sequence up to the core-helium-burning phase of stellar evolution. This, therefore, includes subgiant stars, stars evolving along the red giant branch (RGB), and stars likely evolving toward the early asymptotic giant branch. In this paper, we additionally show how FAMED can detect rotation from dipolar oscillation modes in main sequence, subgiant, low-luminosity RGB, and core-helium-burning stars.

Semianalytic Expressions for the Isolation and Coupling of Mixed Modes

Abstract In the oscillation spectra of giant stars, nonradial modes may be seen to undergo avoided crossings, which produce a characteristic “mode bumping” of the otherwise uniform asymptotic p- and g-mode patterns in their respective echelle diagrams. Avoided crossings evolve very quickly relative to typical observational errors and are therefore extremely useful in determining precise ages of stars, particularly in subgiants. This phenomenon is caused by coupling between modes in the p- and g-mode cavities that are near resonance with each other. Most theoretical analyses of the coupling between these mode cavities rely on the Jeffreys–Wentzel–Kramers–Brillouin approach, which is strictly speaking inapplicable to the low-order g modes observed in subgiants or the low-order p modes seen in very evolved red giants. We present both a nonasymptotic prescription for isolating the two mode cavities, as well as a perturbative (and also nonasymptotic) description of the coupling between them, which we show to hold good for the low-order g and p modes in these physical situations. Finally, we discuss how these results may be applied to modeling subgiant stars and determining their global properties from oscillation frequencies. We also make our code for all of these computations publicly available.

Chemically Peculiar A and F Stars with Enhanced s-process and Iron-peak Elements: Stellar Radiative Acceleration at Work

Abstract We present ≳15,000 metal-rich ([Fe/H] > −0.2 dex) A and F stars whose surface abundances deviate strongly from solar abundance ratios and cannot plausibly reflect their birth material composition. These stars are identified by their high [Ba/Fe] abundance ratios ([Ba/Fe] > 1.0 dex) in the LAMOST DR5 spectra analyzed by Xiang et al. They are almost exclusively main-sequence and subgiant stars with T eff ≳ 6300 K. Their distribution in the Kiel diagram (T eff– ) traces a sharp border at low temperatures along a roughly fixed-mass trajectory (around 1.4 M ⊙) that corresponds to an upper limit in convective envelope mass fraction of around 10−4. Most of these stars exhibit distinctly enhanced abundances of iron-peak elements (Cr, Mn, Fe, Ni) but depleted abundances of Mg and Ca. Rotational velocity measurements from GALAH DR2 show that the majority of these stars rotate slower than typical stars in an equivalent temperature range. These characteristics suggest that they are related to the so-called Am/Fm stars. Their abundance patterns are qualitatively consistent with the predictions of stellar evolution models that incorporate radiative acceleration, suggesting they are a consequence of stellar internal evolution, particularly involving the competition between gravitational settling and radiative acceleration. These peculiar stars constitute 40% of the whole population of stars with mass above 1.5 M ⊙, affirming that “peculiar” photospheric abundances due to stellar evolution effects are a ubiquitous phenomenon for these intermediate-mass stars. This large sample of Ba-enhanced, chemically peculiar A/F stars with individual element abundances provides the statistics to test more stringently the mechanisms that alter the surface abundances in stars with radiative envelopes.

Three short-period Jupiters from TESS

We report the confirmation and mass determination of three hot Jupiters discovered by the Transiting Exoplanet Survey Satellite (TESS) mission: HIP 65Ab (TOI-129, TIC-201248411) is an ultra-short-period Jupiter orbiting a bright (V = 11.1 mag) K4-dwarf every 0.98 days. It is a massive 3.213 ± 0.078 MJ planet in a grazing transit configuration with an impact parameter of b = 1.17−0.08+0.10. As a result the radius is poorly constrained, 2.03−0.49+0.61RJ. The planet’s distance to its host star is less than twice the separation at which it would be destroyed by Roche lobe overflow. It is expected to spiral into HIP 65A on a timescale ranging from 80 Myr to a few gigayears, assuming a reduced tidal dissipation quality factor of Qs′ = 107 − 109. We performed a full phase-curve analysis of the TESS data and detected both illumination- and ellipsoidal variations as well as Doppler boosting. HIP 65A is part of a binary stellar system, with HIP 65B separated by 269 AU (3.95 arcsec on sky). TOI-157b (TIC 140691463) is a typical hot Jupiter with a mass of 1.18 ± 0.13 MJ and a radius of 1.29 ± 0.02 RJ. It has a period of 2.08 days, which corresponds to a separation of just 0.03 AU. This makes TOI-157 an interesting system, as the host star is an evolved G9 sub-giant star (V = 12.7). TOI-169b (TIC 183120439) is a bloated Jupiter orbiting a V = 12.4 G-type star. It has a mass of 0.79 ±0.06 MJ and a radius of 1.09−0.05+0.08RJ. Despite having the longest orbital period (P = 2.26 days) of the three planets, TOI-169b receives the most irradiation and is situated on the edge of the Neptune desert. All three host stars are metal rich with [Fe / H] ranging from 0.18 to0.24.

LAMOST J202629.80+423652.0 is not a symbiotic star

LAMOST J202629.80+423652.0 has been recently classified as a new symbiotic star containing a long-period Mira, surrounded by dust (D-type) and displaying in the optical spectra high ionization emission lines, including the Raman-scattered OVI at 6825 Ang. We have observed LAMOST J202629.80+423652.0 photometrically in the BVRI bands and spectroscopically over the 3500-8000 Ang range. We have found it to be a normal G8IV sub-giant star, deprived of any emission line in its spectrum, and reddened by E(B-V)=0.35 mag. Combining our photometry with data from all-sky patrol surveys, we find LAMOST J202629.80+423652.0 to be non variable, so not pulsating as a Mira. We have compiled from existing sources its spectral energy distribution, extending well into the mid-Infrared, and found it completely dominated by the G8IV photospheric stellar emission, without any sign of circumstellar dust. We therefore conclude that LAMOST J202629.80+423652.0 is not a symbiotic star, nor it is pulsating or been enshrouded in dust.

Convective excitation and damping of solar-like oscillations

ABSTRACT The last decade has seen a rapid development in asteroseismology thanks to the CoRoT and Kepler missions. With more detailed asteroseismic observations available, it is becoming possible to infer exactly how oscillations are driven and dissipated in solar-type stars. We have carried out three-dimensional (3D) stellar atmosphere simulations together with one-dimensional (1D) stellar structural models of key benchmark turn-off and subgiant stars to study this problem from a theoretical perspective. Mode excitation and damping rates are extracted from 3D and 1D stellar models based on analytical expressions. Mode velocity amplitudes are determined by the balance between stochastic excitation and linear damping, which then allows the estimation of the frequency of maximum oscillation power, νmax, for the first time based on ab initio and parameter-free modelling. We have made detailed comparisons between our numerical results and observational data and achieved very encouraging agreement for all of our target stars. This opens the exciting prospect of using such realistic 3D hydrodynamical stellar models to predict solar-like oscillations across the Hertzsprung–Russell diagram, thereby enabling accurate estimates of stellar properties such as mass, radius, and age.

Angular momentum transport, layering, and zonal jet formation by the GSF instability: non-linear simulations at a general latitude

ABSTRACT We continue our investigation into the non-linear evolution of the Goldreich–Schubert–Fricke (GSF) instability in differentially rotating radiation zones. This instability may be a key player in transporting angular momentum in stars and giant planets, but its non-linear evolution remains mostly unexplored. In a previous paper we considered the equatorial instability, whereas here we simulate the instability at a general latitude for the first time. We adopt a local Cartesian Boussinesq model in a modified shearing box for most of our simulations, but we also perform some simulations with stress-free, impenetrable, radial boundaries. We first revisit the linear instability and derive some new results, before studying its non-linear evolution. The instability is found to behave very differently compared with its behaviour at the equator. In particular, here we observe the development of strong zonal jets (‘layering’ in the angular momentum), which can considerably enhance angular momentum transport, particularly in axisymmetric simulations. The jets are, in general, tilted with respect to the local gravity by an angle that corresponds initially with that of the linear modes, but which evolves with time and depends on the strength of the flow. The instability transports angular momentum much more efficiently (by several orders of magnitude) than it does at the equator, and we estimate that the GSF instability could contribute to the missing angular momentum transport required in both red giant and subgiant stars. It could also play a role in the long-term evolution of the solar tachocline and the atmospheric dynamics of hot Jupiters.

Derivation of parameters for 3748 FGK stars usingH-band spectra from APOGEE Data Release 14

Context.The Apache Point Observatory Galactic Evolution Experiment (APOGEE) has observed theH-band spectra of over 200 000 stars withR ∼ 22 000.Aims.The main motivation for this work is to test an alternative method to the standard APOGEE pipeline (APOGEE Stellar Parameter and Chemical Abundances Pipeline, ASPCAP) to derive parameters in the near-infrared for FGK dwarfs.Methods.iSpec and Turbospectrum are used to generate synthetic spectra matching APOGEE observations and to determine the parameters throughχ2minimization.Results.We present spectroscopic parameters (Teff, [M/H], logg,vmic) for a sample of 3748 main-sequence and subgiant FGK stars, obtained from their APOGEEH-band spectra.Conclusions.We compare our output parameters with the ones obtained with ASPCAP for the same stellar spectrum, and find that the values agree within the expected uncertainties. A comparison with the optical samples California Planet Survey, High Accuracy Radial Velocity Planet Searcher – Guaranteed Time Observations, and PASTEL is also available, and median differences below 10 K forTeffand 0.2 dex for [M/H] are found. Reasons for these differences are explored. The fullH-band line list, the line selection for the synthesis, and the synthesized spectra are available at the CDS, as are the calculated parameters and their estimated uncertainties.

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

Lithium abundances are presented for 91 dwarf and subgiant stars in the Galactic bulge. The analysis is based on line synthesis of the 7Li line at 6707 Å in high-resolution spectra obtained during gravitational microlensing events, when the brightnesses of the targets were highly magnified. Our main finding is that bulge stars at sub-solar metallicities that are older than about eight billion years do not show any sign of Li production; that is, the Li trend with metallicity is flat or even slightly declining. This indicates that no lithium was produced during the first few billion years in the history of the bulge. This finding is essentially identical to what is seen for the (old) thick disk stars in the solar neighbourhood, and adds another piece of evidence for a tight connection between the metal-poor bulge and the Galactic thick disk. For the bulge stars younger than about eight billion years, the sample contains a group of stars at very high metallicities at [Fe/H] ≈ +0.4 that have lithium abundances in the range A(Li) = 2.6 − 2.8. In the solar neighbourhood the lithium abundances have been found to peak at A(Li) ≈ 3.3 at [Fe/H] ≈ +0.1 and then decrease by 0.4–0.5 dex when reaching [Fe/H] ≈ +0.4. The few bulge stars that we have at these metallicities seem to support this declining A(Li) trend. This could indeed support the recent claim that the low A(Li) abundances at the highest metallicities seen in the solar neighbourhood could be due to stars from the inner disk, or the bulge region, that have migrated to the solar neighbourhood.