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Register a new user$textit{Siriusly}$, a newly identified intermediate-age Milky Way stellar cluster: A spectroscopic study of $textit{Gaia}$ 1
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We confirm the reality of the recently discovered Milky Way stellar cluster $textit{Gaia}$ 1 using spectra acquired with the HERMES and AAOmega spectrographs of the Anglo-Australian Telescope. This cluster had been previously undiscovered due to its close angular proximity to Sirius, the brightest star in the sky at visual wavelengths. Our observations identified 41 cluster members, and yielded an overall metallicity of [Fe/H]$=-0.13pm0.13$ and barycentric radial velocity of $v_r=58.30pm0.22$ km/s. These kinematics provide a dynamical mass estimate of $12.9^{+4.6}_{-3.9}times10^3$ M$_{odot}$. Isochrone fits to $textit{Gaia}$, 2MASS, and Pan-STARRS1 photometry indicate that $textit{Gaia}$ 1 is an intermediate age ($sim3$ Gyr) stellar cluster. Combining the spatial and kinematic data we calculate $textit{Gaia}$ 1 has a circular orbit with a radius of about 12~kpc, but with a large out of plane motion: $z_textrm{max}=1.1^{+0.4}_{-0.3}$ kpc. Clusters with such orbits are unlikely to survive long due to the number of plane passages they would experience.
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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]$le-2$, with offsets as large as $Delta$T$_{rm eff}sim+500$ K and $Delta$log,$gsim+1.0$. 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 $alpha$-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 textit{Gaia} DR2 data. We find 11 stars that are dynamically coincident with the textit{Gaia}-Sausage accretion event and another 17 with the textit{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 textit{Gaia}-Sequoia. When these subsets are examined separately, a common knee in [$alpha$/Fe] is found for the textit{Gaia}-Sausage and low orbital energy textit{Gaia}-Sequoia stars. A lower metallicity knee is tentatively identified in the textit{Gaia}-Sequoia high orbital energy stars. If the metal-poor dwarf stars in these samples are true members of the textit{Gaia}-Sausage and textit{Gaia}-Sequoia events, then they present a unique opportunity to probe the earlier, more pristine, star formation histories of these systems.
We investigate the stellar kinematics of the Galactic disc in 7 $<$ $R$ $<$ 13,kpc using a sample of 118,945 red giant branch (RGB) stars from LAMOST and Gaia. We characterize the median, dispersion and skewness of the distributions of the 3D stellar velocities, actions and orbital parameters across the age-metallicity and the disc $R$ -- $Z$ plane. Our results reveal abundant but clear stellar kinematic patterns and structures in the age -- metallicity and the disc $R$ -- $Z$ plane. The most prominent feature is the strong variations of the velocity, action, and orbital parameter distributions from the young, metal-rich thin disc to the old, metal-poor thick disc, a number of smaller-scale structures -- such as velocity streams, north-south asymmetries, and kinematic features of spiral arms -- are clearly revealed. Particularly, the skewness of $V_{phi}$ and $J_{phi}$ reveals a new substructure at $Rsimeq12$,kpc and $Zsimeq0$,kpc, possibly related to dynamical effects of spiral arms in the outer disc. We further study the stellar migration through analysing the stellar orbital parameters and stellar birth radii. The results suggest that the thick disc stars near the solar radii and beyond are mostly migrated from the inner disc of $Rsim4 - 6$,kpc due to their highly eccentrical orbits. Stellar migration due to dynamical processes with angular momentum transfer (churning) are prominent for both the old, metal-rich stars (outward migrators) and the young metal-poor stars (inward migrators). The spatial distribution in the $R$ -- $Z$ plane for the inward migrators born at a Galactocentric radius of $>$12,kpc show clear age stratifications, possibly an evidence that these inward migrators are consequences of splashes triggered by merger events of satellite galaxies that have been lasted in the past few giga years.
Milky Way globular clusters (MW GCs) are difficult to identify at low Galactic latitudes because of high differential extinction and heavy star crowding. The new deep near-IR images and photometry from the VISTA Variables in the Via Lactea Extended Survey (VVVX) allow us to chart previously unexplored regions. Our long term aim is to complete the census of MW GCs. The immediate goals are to estimate the astrophysical parameters, measuring their reddenings, extinctions, distances, total luminosities, proper motions, sizes, metallicities and ages. We use the near-IR VVVX survey database, in combination with Gaia DR2 optical photometry, and with the Two Micron All Sky Survey (2MASS) photometry. We report the detection of a heretofore unknown Galactic Globular Cluster at $RA =$ 14:09:00.0; $DEC=-$65:37:12 (J2000). We calculate a reddening of $E(J-K_s)=(0.3pm 0.03)$ mag and an extinction of $A_{K_s}=(0.15pm 0.01)$ mag for this new GC. Its distance modulus and corresponding distance were measured as $(m-M)=(15.93pm0.03)$ mag and $D=(15.5pm1.0)$ kpc, respectively. We estimate the metallicity and age by comparison with known GCs and by fitting PARSEC and Dartmouth isochrones, finding $[Fe/H]=(-0.70pm0.2)$ dex and $t=(11.0pm1.0)$ Gyr. The mean GC PMs from Gaia are $mu_{alpha^ast}=(-4.68 pm 0.47 )$ mas $yr^{-1}$ and $mu_{delta}=(-1.34 pm 0.45)$ mas $yr^{-1}$. The total luminosity of our cluster is estimated to be $M_{Ks}=(-7.76pm 0.5)$ mag. We have found a new low-luminosity, old and metal-rich globular cluster, situated in the far side of the Galactic disk, at $R_{G}=11.2$ kpc from the Galactic centre, and at $z=1.0$ kpc below the plane. Interestingly, the location, metallicity and age of this globular cluster are coincident with the Monoceros Ring (MRi) structure.
Milky Way open clusters are very diverse in terms of age, chemical composition, and kinematic properties. Intermediate-age and old open clusters are less common, and it is even harder to find them inside the solar Galactocentric radius, due to the high mortality rate and strong extinction inside this region. NGC 6802 is one of the inner disk open clusters (IOCs) observed by the $Gaia$-ESO survey (GES). This cluster is an important target for calibrating the abundances derived in the survey due to the kinematic and chemical homogeneity of the members in open clusters. Using the measurements from $Gaia$-ESO internal data release 4 (iDR4), we identify 95 main-sequence dwarfs as cluster members from the GIRAFFE target list, and eight giants as cluster members from the UVES target list. The dwarf cluster members have a median radial velocity of $13.6pm1.9$ km s$^{-1}$, while the giant cluster members have a median radial velocity of $12.0pm0.9$ km s$^{-1}$ and a median [Fe/H] of $0.10pm0.02$ dex. The color-magnitude diagram of these cluster members suggests an age of $0.9pm0.1$ Gyr, with $(m-M)_0=11.4$ and $E(B-V)=0.86$. We perform the first detailed chemical abundance analysis of NGC 6802, including 27 elemental species. To gain a more general picture about IOCs, the measurements of NGC 6802 are compared with those of other IOCs previously studied by GES, that is, NGC 4815, Trumpler 20, NGC 6705, and Berkeley 81. NGC 6802 shows similar C, N, Na, and Al abundances as other IOCs. These elements are compared with nucleosynthetic models as a function of cluster turn-off mass. The $alpha$, iron-peak, and neutron-capture elements are also explored in a self-consistent way.
The central kiloparsecs of the Milky Way are known to host an old, spheroidal stellar population, whose spatial and kinematical properties set it apart from the boxy/peanut structure that constitutes most of the central stellar mass. The nature of this spheroidal population, whether a small classical bulge, the innermost stellar halo or a population of disk stars with large initial velocity dispersion, remains unclear. This structure is also a promising candidate to host some of the oldest stars in the Galaxy. Here we address the topic of the inner stellar spheroid age, using spectroscopic and photometric metallicities for a sample of 935 RR Lyrae stars that are constituents of this component. By means of stellar population synthesis, we derive an age-metallicity relation for RR Lyrae populations. We infer, for the RR Lyrae stars in the bulge spheroid, an extremely ancient age of $13.41 pm 0.54$ Gyr and conclude they were among the first stars to form in what is now the Milky Way galaxy. Our age estimate for the central spheroid shows remarkable agreement with the age profile that has been inferred for the Milky Way stellar halo, suggesting a connection between the two structures. However, we find mild evidence for a transition in the halo properties at $r_{rm GC} sim 5$~kpc. We also investigate formation scenarios for metal-rich RR Lyrae stars, such as binarity and helium variations, and whether they can provide alternative explanations for the properties of our sample. We conclude that, within our framework, the only viable alternative is to have younger, slightly helium-rich, RR Lyrae stars, a hypothesis that would open intriguing questions for the formation of the inner stellar spheroid.
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