ABSTRACT We present new identifications of five red giant stars in the Galactic halo with chemical abundance patterns that indicate they originally formed in globular clusters. Using data from the ...Apache Point Observatory Galactic Evolution Experiment (APOGEE) Survey available through Sloan Digital Sky Survey (SDSS) Data Release 12, we first identify likely halo giants, and then search those for the well-known chemical tags associated with globular clusters, specifically enrichment in nitrogen and aluminum. We find that 2% of the halo giants in our sample have this chemical signature, in agreement with previous results. Following the interpretation in our previous work on this topic, this would imply that at least 13% of halo stars originally formed in globular clusters. Recent developments in the theoretical understanding of globular cluster formation raise questions about that interpretation, and we concede the possibility that these migrants represent a small fraction of the halo field. There are roughly as many stars with the chemical tags of globular clusters in the halo field as there are in globular clusters, whether or not they are accompanied by a much larger chemically untaggable population of former globular cluster stars.
Abstract
The vast majority of Milky Way stellar halo stars were likely accreted from a small number (≲3) of relatively large dwarf galaxy accretion events. However, the timing of these events is ...poorly constrained and predominantly relies on indirect dynamical mixing arguments or imprecise age measurements of stars associated with debris structures. Here, we aim to infer robust stellar ages for stars associated with galactic substructures to more directly constrain the merger history of the Galaxy. By combining kinematic, asteroseismic, and spectroscopic data where available, we infer stellar ages for a sample of 10 red giant stars that were kinematically selected to be within the stellar halo, a subset of which are associated with the Gaia–Enceladus–Sausage halo substructure, and compare their ages to 3 red giant stars in the Galactic disk. Despite systematic differences in both absolute and relative ages determined here, age rankings of stars in this sample are robust. Passing the same observable inputs to multiple stellar age determination packages, we measure a weighted average age for the Gaia–Enceladus–Sausage stars in our sample of 8 ± 3 (stat.) ± 1 (sys.) Gyr. We also determine hierarchical ages using
isochrones
for the populations of Gaia–Enceladus–Sausage, in situ halo and disk stars, finding a Gaia–Enceladus–Sausage population age of 8.0
−
2.3
+
3.2
Gyr. Although we cannot distinguish hierarchical population ages of halo or disk structures with our limited data and sample of stars, this framework should allow a distinct characterization of Galactic substructures using larger stellar samples and additional data available in the near future.
Abstract Stellar models predict that lithium (Li) inside a star is destroyed during the first dredge-up phase, yet 1.2% of red giant stars are Li-rich. We aim to uncover possible origins of this ...population, by analyzing 1099 Li-rich giants ( A (Li) ≥ 1.5) in GALAH DR3. To expose peculiar traits of Li-rich stars, we construct a reference sample of Li-normal (doppelgänger) stars with matched evolutionary state and fiducial iron-peak and alpha-process abundances (Fe/H and Mg/Fe). Comparing Li-rich and doppelgänger spectra reveals systematic differences in the H α and Ca-triplet line profiles associated with the velocity broadening measurement. We also find twice as many Li-rich stars appear to be fast rotators (2% with v broad ≳ 20 km s −1 ) compared to doppelgängers. On average, Li-rich stars have higher abundances than their doppelgängers, for a subset of elements, and Li-rich stars at the base of RGB have higher mean s -process abundances (≥0.05 dex for Ba, Y, Zr), relative to their doppelgängers. External mass-transfer from intermediate -mass AGB companions could explain this signature. Additional companion analysis excludes binaries with mass ratios ≳0.5 at ≳7 au. Finally, we confirm a prevalence of Li-rich stars on the red clump that increases with lithium, which supports an evolutionary state mechanism for Li-enhancement. Multiple culprits, including binary spin-up and mass-transfer, are therefore likely mechanisms of Li-enrichment.
Context. The Orion complex is arguably the most studied star-forming region in the Galaxy. While stars are still being born in the Orion nebula, the oldest part was believed to be no more than 13 Myr ...old. Aims. In order to study the full hierarchy of star formation across the Orion complex, we perform a clustering analysis of the Ori OB1a region using new stellar surveys and derive robust ages for each identified stellar aggregate. Methods. We use Gaia DR2 parameters supplemented with radial velocities from the GALAH and APOGEE surveys to perform clustering of the Ori OB1a association. Five overdensities are resolved in a six-dimensional parameter space (positions, distance, proper motions, and radial velocity). Most correspond to previously known structures (ASCC 16, 25 Orionis, ASCC 20, ASCC 21). We use Gaia DR2, Pan-STARRS1 and 2MASS photometry to fit isochrones to the colour-magnitude diagrams of the identified clusters. The ages of the clusters can thus be measured with ∼10% precision. Results. While four of the clusters have ages between 11 and 13 Myr, the ASCC 20 cluster stands out at an age of 21 ± 3 Myr. This is significantly greater than the age of any previously known component of the Orion complex. To some degree, all clusters overlap in at least one of the six phase-space dimensions. Conclusions. We argue that the formation history of the Orion complex, and its relation to the Gould belt, must be reconsidered. A significant challenge in reconstructing the history of the Ori OB1a association is to understand the impact of the newly discovered 21 Myr old population on the younger parts of the complex, including their formation.
Full text
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Abstract
In this paper, we report the potential detection of a nonmonotonic radial rotation profile in a low-mass lower-luminosity giant star. For most low- and intermediate-mass stars, the rotation ...on the main sequence seems to be close to rigid. As these stars evolve into giants, the core contracts and the envelope expands, which should suggest a radial rotation profile with a fast core and a slower envelope and surface. KIC 9267654, however, seems to show a surface rotation rate that is faster than its bulk envelope rotation rate, in conflict with this simple angular momentum conservation argument. We improve the spectroscopic surface constraint, show that the pulsation frequencies are consistent with the previously published core and envelope rotation rates, and demonstrate that the star does not show strong chemical peculiarities. We discuss the evidence against any tidally interacting stellar companion. Finally, we discuss the possible origin of this unusual rotation profile, including the potential ingestion of a giant planet or unusual angular momentum transport by tidal inertial waves triggered by a close substellar companion, and encourage further observational and theoretical efforts.
ABSTRACT
Since the advent of Gaia astrometry, it is possible to identify massive accreted systems within the Galaxy through their unique dynamical signatures. One such system, Gaia-Sausage-Enceladus ...(GSE), appears to be an early ‘building block’ given its virial mass $\gt 10^{10}\, \mathrm{M_\odot }$ at infall (z ∼ 1−3). In order to separate the progenitor population from the background stars, we investigate its chemical properties with up to 30 element abundances from the GALAH+ Survey Data Release 3 (DR3). To inform our choice of elements for purely chemically selecting accreted stars, we analyse 4164 stars with low-α abundances and halo kinematics. These are most different to the Milky Way stars for abundances of Mg, Si, Na, Al, Mn, Fe, Ni, and Cu. Based on the significance of abundance differences and detection rates, we apply Gaussian mixture models to various element abundance combinations. We find the most populated and least contaminated component, which we confirm to represent GSE, contains 1049 stars selected via Na/Fe versus Mg/Mn in GALAH+ DR3. We provide tables of our selections and report the chrono-chemodynamical properties (age, chemistry, and dynamics). Through a previously reported clean dynamical selection of GSE stars, including $30 \lt \sqrt{J_R / \, \mathrm{kpc\, km\, s^{-1}}} \lt 55$, we can characterize an unprecedented 24 abundances of this structure with GALAH+ DR3. With our chemical selection we characterize the dynamical properties of the GSE, for example mean $\sqrt{J_R / \, \mathrm{kpc\, km\, s^{-1}}} =$$26_{-14}^{+9}$. We find only $(29\pm 1){{\ \rm per\ cent}}$ of the GSE stars within the clean dynamical selection region. Our methodology will improve future studies of accreted structures and their importance for the formation of the Milky Way.
We explore the kinematics and orbital properties of a sample of 323 very metal-poor stars in the halo system of the Milky Way, selected from the high-resolution spectroscopic follow-up studies of ...Aoki et al. and Yong et al. The combined sample contains a significant fraction of carbon-enhanced metal-poor (CEMP) stars (22% or 29%, depending on whether a strict or relaxed criterion is applied for this definition). Barium abundances (or upper limits) are available for the great majority of the CEMP stars, allowing for their separation into the CEMP-s and CEMP-no subclasses. A new method to assign membership to the inner- and outer-halo populations of the Milky Way is developed, making use of the integrals of motion, and applied to determine the relative fractions of CEMP stars in these two subclasses for each halo component. Although limited by small-number statistics, the data suggest that the inner halo of the Milky Way exhibits a somewhat higher relative number of CEMP-s stars than CEMP-no stars (57% versus 43%), while the outer halo possesses a clearly higher fraction of CEMP-no stars than CEMP-s stars (70% versus 30%). Although larger samples of CEMP stars with known Ba abundances are required, this result suggests that the dominant progenitors of CEMP stars in the two halo components were different; massive stars for the outer halo, and intermediate-mass stars in the case of the inner halo.
ABSTRACT
Lithium depletion and enrichment in the cosmos is not yet well understood. To help tighten constraints on stellar and Galactic evolution models, we present the largest high-resolution ...analysis of Li abundances A(Li) to date, with results for over $100\, 000$ GALAH (Galactic Archeology with HERMES) field stars spanning effective temperatures $5900\, \mathrm{K} \lesssim T_{\mathrm{eff}}\lesssim 7000\, \mathrm{K}$ and metallicities −3 ≲ Fe/H ≲ +0.5. We separated these stars into two groups, on the warm and cool sides of the so-called Li dip, a localized region of the Kiel diagram wherein lithium is severely depleted. We discovered that stars in these two groups show similar trends in the A(Li)–Fe/H plane, but with a roughly constant offset in A(Li) of $0.4\, \mathrm{dex}$, the warm group having higher Li abundances. At $\rm Fe/H\gtrsim -0.5$, a significant increase in Li abundance with increasing metallicity is evident in both groups, signalling the onset of significant Galactic production. At lower metallicity, stars in the cool group sit on the Spite plateau, showing a reduced lithium of around $0.4\, \mathrm{dex}$ relative to the primordial value predicted from big bang nucleosynthesis (BBN). However, stars in the warm group between Fe/H = −1.0 and −0.5 form an elevated plateau that is largely consistent with the BBN prediction. This may indicate that these stars in fact preserve the primordial Li produced in the early Universe.
ABSTRACT Chemical tagging promises to use detailed abundance measurements to identify spatially separated stars that were, in fact, born together (in the same molecular cloud) long ago. This idea has ...not yielded much practical success, presumably because of the noise and incompleteness in chemical-abundance measurements. We have succeeded in substantially improving spectroscopic measurements with The Cannon, which has now delivered 15 individual abundances for stars observed as part of the APOGEE spectroscopic survey, with precisions around 0.04 dex. We test the chemical-tagging hypothesis by looking at clusters in abundance space and confirming that they are clustered in phase space. We identify (by the k-means algorithm) overdensities of stars in the 15-dimensional chemical-abundance space delivered by The Cannon, and plot the associated stars in phase space. We use only abundance-space information (no positional information) to identify stellar groups. We find that clusters in abundance space are indeed clusters in phase space, and we recover some known phase-space clusters and find other interesting structures. This is the first-ever project to identify phase-space structures at the survey-scale by blind search purely in abundance space; it verifies the precision of the abundance measurements delivered by The Cannon; the prospects for future data sets appear very good.
ABSTRACT We investigate the light-element behavior of red giant stars in northern globular clusters (GCs) observed by the SDSS-III Apache Point Observatory Galactic Evolution Experiment. We derive ...abundances of 9 elements (Fe, C, N, O, Mg, Al, Si, Ca, and Ti) for 428 red giant stars in 10 GCs. The intrinsic abundance range relative to measurement errors is examined, and the well-known C-N and Mg-Al anticorrelations are explored using an extreme-deconvolution code for the first time in a consistent way. We find that Mg and Al drive the population membership in most clusters, except in M107 and M71, the two most metal-rich clusters in our study, where the grouping is most sensitive to N. We also find a diversity in the abundance distributions, with some clusters exhibiting clear abundance bimodalities (for example M3 and M53) while others show extended distributions. The spread of Al abundances increases significantly as cluster average metallicity decreases as previously found by other works, which we take as evidence that low metallicity, intermediate mass AGB polluters were more common in the more metal-poor clusters. The statistically significant correlation of Al/Fe with Si/Fe in M15 suggests that 28Si leakage has occurred in this cluster. We also present C, N, and O abundances for stars cooler than 4500 K and examine the behavior of A(C+N+O) in each cluster as a function of temperature and Al/Fe. The scatter of A(C+N+O) is close to its estimated uncertainty in all clusters and independent of stellar temperature. A(C+N+O) exhibits small correlations and anticorrelations with Al/Fe in M3 and M13, but we cannot be certain about these relations given the size of our abundance uncertainties. Star-to-star variations of -element (Si, Ca, Ti) abundances are comparable to our estimated errors in all clusters.
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