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Register a new userDiagonal Ridge pattern of different age populations found in Gaia DR2 with LAMOST Main-Sequence-Turn-Off and OB type Stars
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We revisit the diagonal ridge feature (diagonal distributions in the $R, v_{phi}$ plane) found in $Gaia$ and present timing analysis for it between Galactocentric distances of $R=7.5$ and 12 ,kpc, using Main-Sequence-Turn-Off and OB stars selected from the LAMOST Galactic spectroscopic surveys. We recover the ridge pattern in the $R$--$v_{phi}$ plane color coded by mean radial velocity and find this feature is presented from very young (OB stars, few hundred ,Myr) to very old populations ($tau$ = 9$-$14 ,Gyr). Meanwhile, some ridge features are also revealed in the metallicity [Fe/H], [$alpha$/Fe] and $v_{z}$ distributions. In the $L_{Z}, v_{phi}$ plane, one of the ridge patterns, with constant angular momentum per unit mass, shows variations with different age populations compared. However, the remaining two ones are relatively stable, implying there might have two kinds of ridge patterns with different dynamical origins and evolution.
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We construct a sample of nearly 30,000 main-sequence stars with 4500K $<Trm_{eff}<$ 5000K and stellar ages estimated by the chromospheric activity$-$age relation. This sample is used to determine the age distribution in the $R-Z$ plane of the Galaxy, where $R$ is the projected Galactocentric distance in the disk midplane and $Z$ is the height above the disk midplane. As $|Z|$ increases, the percentage of old stars becomes larger. It is known that scale-height of Galactic disk increases as $R$ increases, which is called flare. A mild flare from $R$ $sim$ 8.0 to 9.0 kpc in stellar age distribution is found. We also find that the velocity dispersion increases with age as confirmed by previous studies. Finally we present spiral-shaped structures in $Z-upsilon_{Z}$ phase space in three stellar age bins. The spiral is clearly seen in the age bin of [0, 1] Gyr, which suggests that a vertical perturbation to the disk probably took place within the last $sim$ 1.0 Gyr.
The kinematic properties of young stars that have not yet reached the stage of the main sequence are studied. The selection of these stars was recently carried out by Marton et al. (2019) and Vioque et al. (2020) according to the Gaia DR2 catalog using a number of photometric infrared surveys. We have determined the rotation parameters of the Galaxy and the parameters of the ellipsoids of the residual velocities. The linear velocity of the circular rotation of a solar region around the center of the Galaxy, found using 4431 stars, is equal to V_0=229.1+-4.4 km/s. The following ellipsoid parameters of their residual velocities are found from low-mass stars (of type T Tau): $sigma_{1,2,3}=(9.45,6.99,6.61)pm(0.94,0.43,0.32)$ km/s. For stars of intermediate masses (Herbig Ae/Be stars), their values turned out to be somewhat larger $sigma_{1,2,3}=(13.67,9.25,7.26)pm(2.40,2.44,0.88)$ km/s. Distant stars from both Catalogs trace the local spiral arm well. For 1212 stars, a new estimate of the pitch angle of the Local spiral arm is equal to i=-8.9+-0.1 deg.
Main sequence turn-off (MSTO) stars have advantages as indicators of Galactic evolution since their ages could be robustly estimated from atmospheric parameters. Hundreds of thousands of MSTO stars have been selected from the LAMOST Galactic sur- vey to study the evolution of the Galaxy, and it is vital to derive accurate stellar parameters. In this work, we select 150 MSTO star candidates from the MSTO stars sample of Xiang that have asteroseismic parameters and determine accurate stellar parameters for these stars combing the asteroseismic parameters deduced from the Kepler photometry and atmospheric parameters deduced from the LAMOST spectra.With this sample, we examine the age deter- mination as well as the contamination rate of the MSTO stars sample. A comparison of age between this work and Xiang shows a mean difference of 0.53 Gyr (7%) and a dispersion of 2.71 Gyr (28%). The results show that 79 of the candidates are MSTO stars, while the others are contaminations from either main sequence or sub-giant stars. The contamination rate for the oldest stars is much higher than that for the younger stars. The main cause for the high contamination rate is found to be the relatively large systematic bias in the LAMOST surface gravity estimates.
We present estimates of stellar age and mass for 0.93 million Galactic disk main sequence turn-off and sub-giant stars from the LAMOST Galactic Spectroscopic Surveys. The ages and masses are determined by matching with stellar isochrones using Bayesian algorithm, utilizing effective temperature $T_{rm eff}$, absolute magnitude ${rm M}_V$, metallicity [Fe/H] and $alpha$-element to iron abundance ratio [$alpha$/Fe] deduced from the LAMOST spectra. Extensive examinations suggest the age and mass estimates are robust. The overall sample stars have a median error of 34 per cent for the age estimates, and half of the stars older than 2,Gyr have age uncertainties of only 20--30 per cent. Median error for the mass estimates of the whole sample stars is $sim8$ per cent. The huge dataset demonstrates good correlations among stellar age, [Fe/H] ([$alpha$/H]) and [$alpha$/Fe]. Particularly, double sequence features are revealed in the both the age--[$alpha$/Fe] and age--[Fe/H]([$alpha$/H]) spaces. In the [Fe/H]--[$alpha$/Fe] space, stars of 8--10,Gyr exhibit both the thin and thick disk sequences, while younger (older) stars show only the thin (thick) disk sequence, indicating that the thin disk became prominent 8--10,Gyr ago, while the thick disk formed earlier and almost quenched 8,Gyr ago. Stellar ages exhibit positive vertical and negative radial gradients across the disk, and the outer disk of $Rgtrsim$,9,kpc exhibits a strong flare in stellar age distribution.
Most star clusters at an intermediate age (1-2 Gyr) in the Large and Small Magellanic Clouds show a puzzling feature in their color-magnitude diagrams (CMD) that is not in agreement with a simple stellar population. The main sequence turn-off of these clusters is much broader than would be expected from photometric uncertainties. One interpretation of this feature is that age spreads of the order 200-500 Myr exist within individual clusters, although this interpretation is highly debated. Such large age spreads should affect other parts of the CMD, which are sensitive to age, as well. In this study, we analyze the CMDs of a sample of 12 intermediate-age clusters in the Large Magellanic Cloud that all show an extended turn-off using archival optical data taken with the Hubble Space Telescope. We fit the star formation history of the turn-off region and the red clump region independently with two different theoretical isochrone models. We find that in most of the cases, the age spreads inferred from the red clumps are smaller than the ones resulting from the turn-off region. However, the age ranges resulting from the red clump region are broader than would be expected for a single age. Only two out of 12 clusters in our sample show a red clump which seems to be consistent with a single age. As our results are not unambiguous, we can not ultimately tell if the extended main sequence turn-off feature is due to an age spread, or not, by fitting the star formation histories to the red clump regions. However, we find that the width of the extended main sequence turn-off feature is correlated with the age of the clusters in a way which would be unexplained in the age spread interpretation, but which may be expected if stellar rotation is the cause of the spread at the turn-off.
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