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Register a new userExploring the production and depletion of lithium in the Milky Way stellar disk
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[ABRIDGED] We determine Li abundances for a well-studied sample of 714 F and G dwarf, turn-off, and subgiant stars in the solar neighbourhood. The analysis is based on line synthesis of the Li line at 6707 {AA} in high-resolution and high signal-to-noise ratio echelle spectra, obtained with the MIKE, FEROS, SOFIN, UVES, and FIES spectrographs. The presented Li abundances are corrected for non-LTE effects. Out of the sample of 714 stars we are able to determine Li abundances for 420 stars and upper limits on the Li abundance for another 121 stars. 18 of the stars with well-determined Li abundances are listed as exoplanet host stars. Our main finding is that there are no signatures of Li production in the thick disk, but the Li abundance for stars of the same effective temperature is independent of metallicity for stars that can be associated with the Galactic thick disk. Significant Li production is however seen in the thin disk, with a steady increase towards super-solar metallicities. At the highest metallicities, however, around [Fe/H]~ +0.3, we tentatively confirm the recent discovery that the Li abundances level out. We hence contradict the recent finding in other studies that found that Li is also produced in the thick disk. This is likely due to the chemically defined selection criteria those studies used to define their thick disk samples. Age criteria that we use here, produce a thick disk stellar sample that is much less contaminated by thin disk stars, and hence more reliable abundance trends. [ABRIDGED] A conclusion that can be drawn is that no significant Li production, relative to the primordial abundance, took place during the first few billion years of the Milky Way, an era coinciding with the formation and evolution of the thick disk. [ABRIDGED]
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We employ numerical simulations and simple analytical estimates to argue that dark matter substructures orbiting in the inner regions of the Galaxy can be efficiently destroyed by disk shocking, a dynamical process known to affect globular star clusters. We carry out a set of fiducial high-resolution collisionless simulations in which we adiabatically grow a disk, allowing us to examine the impact of the disk on the substructure abundance. We also track the orbits of dark matter satellites in the high-resolution Aquarius simulations and analytically estimate the cumulative halo and disk shocking effect. Our calculations indicate that the presence of a disk with only 10% of the total Milky Way mass can significantly alter the mass function of substructures in the inner parts of halos. This has important implications especially for the relatively small number of satellites seen within ~30 kpc of the Milky Way center, where disk shocking is expected to reduce the substructure abundance by a factor of ~2 at 10^9 M$_{odot}$ and ~3 at 10^7 M$_{odot}$. The most massive subhalos with 10^10 M$_{odot}$ survive even in the presence of the disk. This suggests that there is no inner missing satellite problem, and calls into question whether these substructures can produce transient features in disks, like multi-armed spiral patterns. Also, the depletion of dark matter substructures through shocking on the baryonic structures of the disk and central bulge may aggravate the problem to fully account for the observed flux anomalies in gravitational lens systems, and significantly reduces the dark matter annihilation signal expected from nearby substructures in the inner halo.
We present stellar age distributions of the Milky Way (MW) bulge region using ages for $sim$6,000 high-luminosity ($log(g) < 2.0$), metal-rich ($rm [Fe/H] ge -0.5$) bulge stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). Ages are derived using {it The Cannon} label-transfer method, trained on a sample of nearby luminous giants with precise parallaxes for which we obtain ages using a Bayesian isochrone-matching technique. We find that the metal-rich bulge is predominantly composed of old stars ($>$8 Gyr). We find evidence that the planar region of the bulge ($|Z_{rm GC}| le 0.25$ kpc) enriched in metallicity, $Z$, at a faster rate ($dZ/dt sim$ 0.0034 ${rm Gyr^{-1}}$) than regions farther from the plane ($dZ/dt sim$ 0.0013 ${rm Gyr^{-1}}$ at $|Z_{rm GC}| > 1.00$ kpc). We identify a non-negligible fraction of younger stars (age $sim$ 2--5 Gyr) at metallicities of $rm +0.2 < [Fe/H] < +0.4$. These stars are preferentially found in the plane ($|Z_{rm GC}| le 0.25$ kpc) and between $R_{rm cy} approx 2-3$ kpc, with kinematics that are more consistent with rotation than are the kinematics of older stars at the same metallicities. We do not measure a significant age difference between stars found in and outside of the bar. These findings show that the bulge experienced an initial starburst that was more intense close to the plane than far from the plane. Then, star formation continued at super-solar metallicities in a thin disk at 2 kpc $lesssim R_{rm cy} lesssim$ 3 kpc until $sim$2 Gyr ago.
We explore the local volume of the Milky Way via chemical and kinematical measurements from high quality astrometric and spectroscopic data recently released by the Gaia, APOGEE and GALAH programs. We chemically select $1137$ stars up to $2.5$~kpc of the Sun and $rm{[Fe/H]} le -1.0$~dex, and find evidence of statistically significant substructures. Clustering analysis in velocity space classifies $163$ objects into eight kinematical groups, whose origin is further investigated with high resolution N-body numerical simulations of single merging events. The two retrograde groups appear associated with Gaia-Sausage-Enceladus, while the slightly prograde group could be connected to GSE or possibly Wukong. We find evidence of a new 44-member-strong prograde stream we name Icarus; to our knowledge, Icarus is the fast-rotating stream closest to the Galactic disk to date ($langle Z_{rm max} rangle lesssim 0.5$~kpc, $langle V+V_{rm{LSR}}rangle simeq 231~rm{km~s^{-1}}$). Its peculiar chemical ($langle rm{[Fe/H]}rangle simeq -1.45$, $langle rm{[Mg/Fe]}rangle simeq -0.02$) and dynamical (mean eccentricity $simeq 0.11$) properties are consistent with the accretion of debris from a dwarf galaxy progenitor with a stellar mass of $sim 10^9 M_sun$ on an initial prograde low-inclination orbit, $sim 10^circ$. The remaining prograde groups are either streams previously released by the same progenitor of Icarus (or Nyx), or remnants from different satellites accreted on initial orbits at higher inclination.
ABRIDGED: METHODS: We have conducted a high-resolution spectroscopic study of 714 F and G dwarf and subgiant stars in the Solar neighbourhood. The star sample has been kinematically selected to trace the Galactic thin and thick disks to their extremes...... The determination of stellar parameters and elemental abundances is based on a standard 1-D LTE analysis using equivalent width measurements in high-resolution (R=40000-110000) and high signal-to-noise (S/N=150-300) spectra obtained with..... RESULTS: .... Our data show that there is an old and alpha-enhanced disk population, and a younger and less alpha-enhanced disk population. While they overlap greatly in metallicity between -0.7<[Fe/H]<+0.1, they show a bimodal distribution in [alpha/Fe]. This bimodality becomes even clearer if stars where stellar parameters and abundances show larger uncertainties (Teff<5400 K) are discarded, showing that it is important to constrain the data set to a narrow range in the stellar parameters if small differences between stellar populations are to be revealed. We furthermore find that the alpha-enhanced population has orbital parameters placing the stellar birthplaces in the inner Galactic disk while the low-alpha stars mainly come from the outer Galactic disk........... We furthermore have discovered that a standard 1-D, LTE analysis, utilising ionisation and excitation balance of Fe I and Fe II lines produces a flat lower main sequence. As the exact cause for this effect is unclear we chose to apply an empirical correction. Turn-off, and more evolved, stars, appears to be un-affected.
Studying the Milky Way disk structure using stars in narrow bins of [Fe/H] and [alpha/Fe] has recently been proposed as a powerful method to understand the Galactic thick and thin disk formation. It has been assumed so far that these mono-abundance populations (MAPs) are also coeval, or mono-age, populations. Here we study this relationship for a Milky Way chemo-dynamical model and show that equivalence between MAPs and mono-age populations exists only for the high-[alpha/Fe] tail, where the chemical evolution curves of different Galactic radii are far apart. At lower [alpha/Fe]-values a MAP is composed of stars with a range in ages, even for small observational uncertainties and a small MAP bin size. Due to the disk inside-out formation, for these MAPs younger stars are typically located at larger radii, which results in negative radial age gradients that can be as large as 2 Gyr/kpc. Positive radial age gradients can result for MAPs at the lowest [alpha/Fe] and highest [Fe/H] end. Such variations with age prevent the simple interpretation of observations for which accurate ages are not available. Studying the variation with radius of the stellar surface density and scale-height in our model, we find good agreement to recent analyses of the APOGEE red-clump (RC) sample when 1-4 Gyr old stars dominate (as expected for the RC). Our results suggest that the APOGEE data are consistent with a Milky Way model for which mono-age populations flare for all ages. We propose observational tests for the validity of our predictions and argue that using accurate age measurements, such as from asteroseismology, is crucial for putting constraints on the Galactic formation and evolution.
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