Abstract
We present analysis of the proper-motion (PM) field of the red clump stars in the Large Magellanic Cloud (LMC) disk using the Gaia Early Data Release 3 catalog. Using a kinematic model based ...on old stars with 3D velocity measurements, we construct the residual PM field by subtracting the center-of-mass motion and internal rotation motion components. The residual PM field reveals asymmetric patterns, including larger residual PMs in the southern disk. Comparisons of the observed residual PM field with those of five numerical simulations of an LMC analog that is subject to the tidal fields of the Milky Way and the Small Magellanic Cloud (SMC) show that the present-day LMC is not in dynamical equilibrium. We find that both the observed level of disk heating (PM residual rms of 0.057 ± 0.002 mas yr
−1
) and kinematic asymmetry are not reproduced by Milky Way tides or if the SMC impact parameter is larger than the size of the LMC disk. This measured level of disk heating provides a novel and important method to validate numerical simulations of the LMC–SMC interaction history. Our results alone put constraints on an impact parameter ≲10 kpc and impact timing <250 Myr. When adopting the impact timing constraint of ∼140–160 Myr ago from previous studies, our results suggest that the most recent SMC encounter must have occurred with an impact parameter of ∼5 kpc. We also find consistent radial trends in the kinematically and geometrically derived disk inclination and line-of-node position angles, indicating a common origin.
Abstract I report the discovery of a stellar stream (Sutlej) using Gaia DR3 proper motions and XP metallicities located ∼15○ north of the Small Magellanic Cloud (SMC). The stream is composed of two ...parallel linear components (“branches”) approximately ∼8○ × 0.6○ in size and separated by 2.5○. The stars have a mean proper motion of (μRA, μDEC)=(+0.08 mas yr−1,−1.41 mas yr−1) which is quite similar to the proper motion of stars on the western side of the SMC. The color magnitude diagram of the stream stars has a clear red giant branch, horizontal branch, and main sequence turnoff that is well-matched by a PARSEC isochrone of 10 Gyr, Fe/H=−1.8 at 32 kpc and a total stellar mass of ∼33,000 M⊙. The stream is spread out over an area of 9.6 square degrees and has a surface brightness of 32.5 mag arcsec−2. The metallicity of the stream stars from Gaia XP spectra extend over −2.5 ≤ M/H ≤ −1.0 with a median of M/H=−1.8. The tangential velocity of the stream stars is 214 km s−1 compared to the values of 448 km s−1 for the Large Magellanic Cloud and 428 km s−1 for the SMC. While the radial velocity of the stream is not yet known, a comparison of the space velocities using a range of assumed radial velocities, shows that the stream is unlikely to be associated with the Magellanic Clouds. The tangential velocity vector is misaligned with the stream by nearly 90○, which might indicate an important gravitational influence from the nearby Magellanic Clouds.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted) We present results from spectroscopic observations with the Michigan/Magellan Fiber System (M2FS) of 147 stellar targets along the ...line of sight to the newly discovered "ultrafaint" stellar systems Tucana 2 (Tuc 2) and Grus 1 (Gru 1). Based on simultaneous estimates of line of sight velocity and stellar-atmospheric parameters, we identify 8 and 7 stars as probable members of Tuc 2 and and Gru 1, respectively. Our sample for Tuc 2 is sufficient to resolve an internal velocity dispersion of ... km s super(-1) about a mean of ... km s super(-1)(solar rest frame), and to estimate a mean metallicity of Fe/H = ... These results place Tuc 2 on chemodynamical scaling relations followed by dwarf galaxies, suggesting a dominant dark matter component with dynamical mass ... M sub(middot in circle) enclosed within the central ~160 pc, and dynamical mass-to-light ratio ... M sub(middot in circle)/Lv .middot in circle. For Gru 1 we estimate a mean velocity of ... km s super(-1) and a mean metallicity of Fe/H = ... but our sample does not resolve Gru 1's velocity dispersion. The radial coordinates of Tuc 2 and Gru 1 in Galactic phase space suggest that their orbits are among the most energetic within a distance of <, ~ 300 kpc. Moreover, their proximity to each other in this space arises naturally if both objects are trailing the Large Magellanic Cloud.
The Sloan Digital Sky Survey III's Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a high-resolution near-infrared spectroscopic survey covering all of the major components of the ...Galaxy, including the dust-obscured regions of the inner Milky Way disk and bulge. Here we present a sample of 10,341 likely red-clump stars (RC) from the first two years of APOGEE operations, selected based on their position in color- metallicity-surface-gravity-effective-temperature space using a new method calibrated using stellar evolution models and high-quality asteroseismology data. The narrowness of the RC locus in color-metallicity-luminosity space allows us to assign distances to the stars with an accuracy of 5%-10%. The sample extends to typical distances of about 3 kpc from the Sun, with some stars out to 8 kpc, and spans a volume of approximately 100 kpc super(3) over 5 kpc lap R lap 14 kpc, |Z| lap 2 kpc, and -15degrees lap Galactocentric azimuth lap 30degrees. The APOGEE red-clump (APOGEE-RC) catalog contains photometry from the Two Micron All Sky Survey, reddening estimates, distances, line-of-sight velocities, stellar parameters and elemental abundances determined from the high-resolution APOGEE spectra, and matches to major proper motion catalogs. We determine the survey selection function for this data set and discuss how the RC selection samples the underlying stellar populations. We use this sample to limit any azimuthal variations in the median metallicity within the approximately 45degrees azimuthal region covered by the current sample to be < or =, slant0.02 dex, which is more than an order of magnitude smaller than the radial metallicity gradient. This result constrains coherent non-axisymmetric flows within a few kiloparsecs from the Sun.
Abstract
The Milky Way (MW) stellar disk has both a thin and a thick component. The thin disk is composed mostly of younger stars (≲8 Gyr) with a lower abundance of
α
-elements, while the thick disk ...contains predominantly older stars (≳8–12 Gyr) with a higher
α
abundance, giving rise to an
α
-bimodality most prominent at intermediate metallicities. A proposed explanation for the bimodality is an episode of clumpy star formation, where high-
α
stars form in massive clumps that appear in the first few billion years of the MW’s evolution, while low-
α
stars form throughout the disk and over a longer time span. To better understand the evolution of clumps, we track them and their constituent stars in two clumpy MW simulations that reproduce the
α
-abundance bimodality, one with 10% and the other with 20% supernova feedback efficiency. We investigate the paths that these clumps take in the chemical space (O/Fe–Fe/H) as well as their mass, star formation rate (SFR), formation location, lifetime, and merger history. The clumps in the simulation with lower feedback last longer on average, with several lasting hundreds of millions of years. Some of the clumps do not reach high-
α
, but the ones that do on average have a higher SFR, longer lifetime, greater mass, and form closer to the Galactic center than the ones that do not. Most clumps that reach high-
α
merge with others and eventually spiral into the Galactic center, but shed stars along the way to form most of the thick-disk component.
The Large and Small Magellanic Clouds are unique local laboratories for studying the formation and evolution of small galaxies in exquisite detail. The Survey of the MAgellanic Stellar History ...(SMASH) is an NOAO community Dark Energy Camera (DECam) survey of the Clouds mapping 480 deg2 (distributed over ∼2400 square degrees at ∼20% filling factor) to ∼24th mag in ugriz. The primary goals of SMASH are to identify low surface brightness stellar populations associated with the stellar halos and tidal debris of the Clouds, and to derive spatially resolved star formation histories. Here, we present a summary of the survey, its data reduction, and a description of the first public Data Release (DR1). The SMASH DECam data have been reduced with a combination of the NOAO Community Pipeline, the PHOTRED automated point-spread-function photometry pipeline, and custom calibration software. The astrometric precision is ∼15 mas and the accuracy is ∼2 mas with respect to the Gaia reference frame. The photometric precision is ∼0.5%-0.7% in griz and ∼1% in u with a calibration accuracy of ∼1.3% in all bands. The median 5 point source depths in ugriz are 23.9, 24.8, 24.5, 24.2, and 23.5 mag. The SMASH data have already been used to discover the Hydra II Milky Way satellite, the SMASH 1 old globular cluster likely associated with the LMC, and extended stellar populations around the LMC out to R ∼ 18.4 kpc. SMASH DR1 contains measurements of ∼100 million objects distributed in 61 fields. A prototype version of the NOAO Data Lab provides data access and exploration tools.
We present a first determination of distances and extinctions for individual stars in the first release of the APOKASC catalogue, built from the joint efforts of the Apache Point Observatory Galactic ...Evolution Experiment (APOGEE) and the Kepler Asteroseismic Science Consortium (KASC). Our method takes into account the spectroscopic constraints derived from the APOGEE Stellar Parameters and Chemical Abundances Pipeline, together with the asteroseismic parameters from KASC. These parameters are then employed to estimate intrinsic stellar properties, including absolute magnitudes, using the Bayesian tool param. We then find the distance and extinction that best fit the observed photometry in Sloan Digital Sky Survey (SDSS), 2MASS, and WISE passbands. The first 1989 giants targetted by APOKASC are found at typical distances between 0.5 and 5 kpc, with individual uncertainties of just ∼1.8 per cent. Our extinction estimates are systematically smaller than provided in the Kepler Input Catalogue and by the Schlegel et al. maps. Distances to individual stars in the NGC 6791 and NGC 6819 star clusters agree to within their credible intervals. Comparison with the APOGEE red clump and SAGA catalogues provide another useful check, exhibiting agreement with our measurements to within a few per cent. Overall, present methods seem to provide excellent distance and extinction determinations for the bulk of the APOKASC sample. Approximately one third of the stars present broad or multiple-peaked probability density functions and hence increased uncertainties. Uncertainties are expected to be reduced in future releases of the catalogue, when a larger fraction of the stars will have seismically determined evolutionary status classifications.
ABSTRACT We present the results of the Sloan Digital Sky Survey APOGEE INfrared Spectroscopy of Young Nebulous Clusters program (IN-SYNC) survey of the Orion A molecular cloud. This survey obtained ...high-resolution near-infrared spectroscopy of about 2700 young pre-main-sequence stars on a field of view. We have measured accurate stellar parameters ( , , ) and extinctions and placed the sources in the Hertzsprung-Russel diagram (HRD). We have also extracted radial velocities for the kinematic characterization of the population. We compare our measurements with literature results to assess the performance and accuracy of the survey. Source extinction shows evidence for dust grains that are larger than those in the diffuse interstellar medium: we estimate an average RV = 5.5 in the region. Importantly, we find a clear correlation between HRD inferred ages and spectroscopic surface-gravity-inferred ages and between extinction and disk presence; this strongly suggests a real spread of ages larger than a few Myr. Focusing on the young population around NGC 1980/ Ori, which has previously been suggested to be a separate, foreground, older cluster, we confirm its older (∼5 Myr) age and low AV, but considering that its radial velocity distribution is indistinguishable from Orion A's population, we suggest that NGC 1980 is part of Orion A's star formation activity. Based on their stellar parameters and kinematic properties, we identify 383 new candidate members of Orion A, most of which are diskless sources in areas of the region poorly studied by previous works.
We measure the Milky Way's rotation curve over the Galactocentric range 4 kpc lap R lap 14 kpc from the first year of data from the Apache Point Observatory Galactic Evolution Experiment. We ...determine the local value of the circular velocity to be V sub(c)(R sub(0)) = 218+ or -6kms super(-1) and find that the rotation curve is approximately flat with a local derivative between -3.0kms super(-1) kpc super(-1) and 0.4kms super(-1) kpc super(-1). We investigate various systematic uncertainties and find that these are limited to offsets at the percent level, ~2kms super(-1) in V sub(c). Marginalizing over all the systematics that we consider, we find that V sub(c)(R sub(0)) < 235kms super(-1) at >99 % confidence. We find an offset between the Sun's rotational velocity and the local circular velocity of 26 + or - 3kms super(-1), which is larger than the locally measured solar motion of 12kms super(-1). Combining our results with other data, we find that the Milky Way's dark-halo mass within the virial radius is ~8 x 10 super(11) M sub(middot in circle).
Abstract
We present new maps of the Milky Way disk showing the distribution of metallicity (Fe/H),
α
-element abundances (Mg/Fe), and stellar age, using a sample of 66,496 red giant stars from the ...final data release (DR17) of the Apache Point Observatory Galactic Evolution Experiment survey. We measure radial and vertical gradients, quantify the distribution functions for age and metallicity, and explore chemical clock relations across the Milky Way for the low-
α
disk, high-
α
disk, and total population independently. The low-
α
disk exhibits a negative radial metallicity gradient of −0.06 ± 0.001 dex kpc
−1
, which flattens with distance from the midplane. The high-
α
disk shows a flat radial gradient in metallicity and age across nearly all locations of the disk. The age and metallicity distribution functions shift from negatively skewed in the inner Galaxy to positively skewed at large radius. Significant bimodality in the Mg/Fe–Fe/H plane and in the Mg/Fe–age relation persist across the entire disk. The age estimates have typical uncertainties of ∼0.15 in log(age) and may be subject to additional systematic errors, which impose limitations on conclusions drawn from this sample. Nevertheless, these results act as critical constraints on galactic evolution models, constraining which physical processes played a dominant role in the formation of the Milky Way disk. We discuss how radial migration predicts many of the observed trends near the solar neighborhood and in the outer disk, but an additional more dramatic evolution history, such as the multi-infall model or a merger event, is needed to explain the chemical and age bimodality elsewhere in the Galaxy.
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