ABSTRACT
The low-metallicity, kinematically interesting dwarf stars studied by Stephens & Boesgaard (2002, SB02) are re-examined using Gaia DR2 astrometry, and updated model atmospheres and atomic ...line data. New stellar parameters are determined based on the Gaia DR2 parallactic distances and Dartmouth Stellar Evolution Database isochrones. These are in excellent agreement with spectroscopically determined stellar parameters for stars with Fe/H > −2; however, large disagreements are found for stars with Fe/H ≤ −2, with offsets as large as ΔTeff ∼ +500 K and Δlog g ∼ +1.0. A subset of six stars (test cases) are analysed ab initio using high-resolution spectra with Keck HIRES and Gemini GRACES. This sub-sample is found to include two α-challenged dwarf stars, suggestive of origins in a low-mass, accreted dwarf galaxy. The orbital parameters for the entire SB02 sample are re-determined using Gaia DR2 data. We find 11 stars that are dynamically coincident with the Gaia-Sausage accretion event and another 17 with the Gaia-Sequoia event in action space. Both associations include low-mass, metal-poor stars with isochrone ages older than 10 Gyr. Two dynamical subsets are identified within Gaia-Sequoia. When these subsets are examined separately, a common knee in α/Fe is found for the Gaia-Sausage and low orbital energy Gaia-Sequoia stars. A lower metallicity knee is tentatively identified in the Gaia-Sequoia high orbital energy stars. If the metal-poor dwarf stars in these samples are true members of the Gaia-Sausage and Gaia-Sequoia events, then they present a unique opportunity to probe the earlier star formation histories of these systems.
Abstract
We present a photometric analysis of globular cluster 47 Tuc (NGC 104) using near-IR imaging data from the GeMS/GSAOI Galactic Globular Cluster Survey (G4CS), which is in operation at ...Gemini-South telescope. Our survey is designed to obtain AO-assisted deep imaging with near diffraction-limited spatial resolution of the central fields of Milky Way globular clusters. The G4CS near-IR photometry was combined with an optical photometry catalog that was obtained from Hubble Space Telescope survey data to produce a high-quality color–magnitude diagram that reaches down to
K
s
≈ 21 Vega mag. We used the software suite BASE-9, which uses an adaptive Metropolis sampling algorithm to perform a Markov chain Monte Carlo Bayesian analysis, and obtained probability distributions and precise estimates for the age, distance, and extinction cluster parameters. Our best estimate for the age of 47 Tuc is
12.42
−
0.05
+
0.05
± 0.08 Gyr and our true distance modulus estimate is (
m
−
M
)
0
= 13.250
−
0.003
+
0.003
± 0.028 mag, which are in tight agreement with previous studies using Gaia DR2 parallax and detached eclipsing binaries.
ABSTRACT
Using N-body simulations, we demonstrate that satellite dwarf galaxy pairs which undergo significant mixing (∼6 Gyr) can have their respective most bound particles separated great distances ...upon subsequently merging with a more massive host. This may provide an explanation as to the origin of the complex globular cluster NGC 2419, which is found within the tail of the Sagittarius dwarf spheroidal galaxy, yet separated from its central remnant by over 100 kpc. Dynamical investigations could support the chemical evidence which already points to the NGC 2419 being a nuclear star cluster. Motivated by the distinct nature of NGC 2419, we run a suite of simulations whereby an initial pre-infall merger of two satellites is followed by a post-infall merger of the remnant into a Milky Way-like host potential. We present a striking example from our suite in this work, in which this separation is reproduced by the most bound particles of the two pre-infall satellites. Additionally, this double merger scenario can induce unusual on-sky features in the tidal debris of the post-infall merger, such as clouds, overdensities, and potentially new arms that could resemble the bifurcation observed in Sagittarius.
ABSTRACT
In this work, we look for evidence of a non-unity mass ratio binary dwarf galaxy merger in the Sagittarius stream. Simulations of such a merger show that, upon merging with a host, particles ...from the less massive galaxy will often mostly be found in the extended stream and less so in the central remnant. Motivated by these simulations, we use the Apache Point Observatory Galactic Evolution Experiment Data Release 17 chemical data from approximately 1100 stars in both the Sagittarius remnant and stream to look for evidence of contamination from a second dwarf galaxy. We separate the Sagittarius data into its remnant and stream and compare the Mg/Fe content of the two populations. In particular, we select Mg/Fe to search for hints of unique star formation histories among our sample stars. Comparing the stream and remnant populations, we find regions that have distinct Mg/Fe distributions for fixed Fe/H, in addition to distinct chemical tracks in Mg/Fe–Fe/H abundance space. We show that there are large regions of the tracks for which the probability of the two samples being drawn from the same distribution is very low (p < 0.05). Furthermore, we show that the two tracks can be fit with unique star formation histories using simple, one-zone galactic chemical evolution models. While more work must be done to discern whether the hypothesis presented here is true, our work hints at the possibility that Sagittarius may consist of two dwarf galaxy progenitors.
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.
ABSTRACT
The view of globular clusters (GCs) as simple systems continues to unravel, revealing complex objects hosting multiple chemical peculiarities. Using differential abundance analysis, we probe ...the chemistry of the Type I GC, NGC 288 and the Type II GC, NGC 362 at the 2 per cent level for the first time. We measure 20 elements and find differential measurement uncertainties of the order of 0.01–0.02 dex in both clusters. The smallest uncertainties are measured for Fe i in both clusters, with an average uncertainty of ∼0.013 dex. Dispersion in the abundances of Na, Al, Ti i, Ni, Fe i, Y, Zr, Ba, and Nd are recovered in NGC 288, none of which can be explained by a spread in He. This is the first time, to our knowledge, a statistically significant spread in s-process elements and a potential spread in metallicity has been detected in NGC 288. In NGC 362, we find significant dispersion in the same elements as NGC 288, with the addition of Co, Cu, Zn, Sr, La, Ce, and Eu. Two distinct groups are recovered in NGC 362, separated by 0.3 dex in average differential s-process abundances. Given strong correlations between Al and several s-process elements, and a significant correlation between Mg and Si, we propose that the s-process rich group is younger. This agrees with asymptotic giant branch star (AGB) enrichment between generations, if there is overlap between low- and intermediate-mass AGBs. In our scenario, the older population is dominated by the r-process with a ΔLa–ΔEu ratio of −0.16 ± 0.06. We propose that the r-process dominance and dispersion found in NGC 362 are primordial.
ABSTRACT
In this study we combine asteroseismic, spectroscopic, and kinematic information to perform a detailed analysis of a sample of 16 stars from the Kepler field. Our selection focuses on stars ...that appear to contradict Galactic chemical evolution models: young and α-rich, old and metal-rich, as well as other targets with unclear classification in past surveys. Kinematics are derived from Gaia DR3 parallaxes and proper motions, and high-resolution spectra from HIRES/Keck are used to calculate chemical abundances for over 20 elements. This information is used to perform careful checks on asteroseismic masses and ages derived via grid-based modelling. Among the seven stars previously classified as young and α-rich, only one seems to be an unambiguously older object masking its true age. We confirm the existence of two very old (≥11 Gyr), super metal-rich (≥0.1 dex) giants. These two stars have regular thin disc chemistry and in-plane solar circle orbits that fit well in the picture of radial migration via the churning mechanism. The alternative explanation that these stars have younger ages would require mass-loss rates that strongly increase with increasing metallicity. Finally, we suggest further investigations to explore the suitability of Zn as a chemical clock in red giants.
ABSTRACT
Detailed chemical studies of F/G/K – or solar-type – stars have long been routine in stellar astrophysics, enabling studies in both Galactic chemodynamics and exoplanet demographics. ...However, similar understanding of the chemistry of M and late-K dwarfs – the most common stars in the Galaxy – has been greatly hampered both observationally and theoretically by the complex molecular chemistry of their atmospheres. Here, we present a new implementation of the data-driven Cannon model, modelling Teff, log g, Fe/H, and Ti/Fe trained on low–medium resolution optical spectra (4000–7000 Å) from 103 cool dwarf benchmarks. Alongside this, we also investigate the sensitivity of optical wavelengths to various atomic and molecular species using both data-driven and theoretical means via a custom grid of MARCS synthetic spectra, and make recommendations for where MARCS struggles to reproduce cool dwarf fluxes. Under leave-one-out cross-validation, our Cannon model is capable of recovering Teff, log g, Fe/H, and Ti/Fe with precisions of 1.4 per cent, $\pm 0.04\,$ dex, $\pm 0.10\,$ dex, and $\pm 0.06\,$ dex respectively, with the recovery of Ti/Fe pointing to the as-yet mostly untapped potential of exploiting the abundant – but complex – chemical information within optical spectra of cool stars.
ABSTRACT We report the discovery, tracking, and detection circumstances for 85 trans-Neptunian objects (TNOs) from the first 42 deg2 of the Outer Solar System Origins Survey. This ongoing r-band ...solar system survey uses the 0.9 deg2 field of view MegaPrime camera on the 3.6 m Canada-France-Hawaii Telescope. Our orbital elements for these TNOs are precise to a fractional semimajor axis uncertainty <0.1%. We achieve this precision in just two oppositions, as compared to the normal three to five oppositions, via a dense observing cadence and innovative astrometric technique. These discoveries are free of ephemeris bias, a first for large trans-Neptunian surveys. We also provide the necessary information to enable models of TNO orbital distributions to be tested against our TNO sample. We confirm the existence of a cold "kernel" of objects within the main cold classical Kuiper Belt and infer the existence of an extension of the "stirred" cold classical Kuiper Belt to at least several au beyond the 2:1 mean motion resonance with Neptune. We find that the population model of Petit et al. remains a plausible representation of the Kuiper Belt. The full survey, to be completed in 2017, will provide an exquisitely characterized sample of important resonant TNO populations, ideal for testing models of giant planet migration during the early history of the solar system.
ABSTRACT
In this work, we combine information from solar-like oscillations, high-resolution spectroscopy, and Gaia astrometry to derive stellar ages, chemical abundances, and kinematics for a group ...of seven metal-poor red giants and characterize them in a multidimensional chrono-chemo-dynamical space. Chemical abundance ratios were derived through classical spectroscopic analysis employing 1D LTE atmospheres on Keck/HIRES spectra. Stellar ages, masses, and radii were calculated with grid-based modelling, taking advantage of availability of asteroseismic information from Kepler. The dynamical properties were determined with galpy using Gaia EDR3 astrometric solutions. Our results suggest that underestimated parallax errors make the effect of Gaia parallaxes more important than different choices of model grid or – in the case of stars ascending the red giant branch – mass-loss prescription. Two of the stars in this study are identified as potentially evolved halo blue stragglers. Four objects are likely members of the accreted Milky Way halo, and their possible relationship with known accretion events is discussed.
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