With the help of Gaia DR2, we are able to obtain the full 6D phase space information for stars from the Large Sky Area Multi-Object Fibre Spectroscopic Telescope DR5. With high precision of the ...position, velocity, and metallicity, the rotation of the local stellar halo is presented using the K-giant stars with Fe/H < −1 dex within 4 kpc from the Sun. By fitting the rotational velocity distribution with the three-Gaussian components, stellar halo, disk, and counterrotating hot population, we find that the local halo progradely rotates with km s−1, providing the local standard of the rest velocity of VLSR = 232 km s−1. Meanwhile, we obtain the dispersion of rotational velocity is km s−1. Although the rotational velocity strongly depends on the choice of VLSR, the trend of prograde rotation is substantial even when VLSR is set at as low as 220 km s−1. Moreover, we derive the rotation for subsamples with different metallicities and find that the rotational velocity is essentially not correlated with Fe/H. This may hint at a secular evolution origin of the prograde rotation. It shows that the metallicity of the progradely rotating halo is peaked within −1.9 < Fe/H < −1.6 without considering the selection effect. We also find a small fraction of counterrotating stars with larger dispersion and a lower metallicity. Finally, the disk component rotates with km s−1 and km s−1, which is quite consistent with the metal-weak thick disk population.
Abstract
We construct a new catalog of the blue horizontal-branch (BHB) stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) DR5 data set, which contains 5355 + 81 BHB ...stars at high Galactic latitude (∣Glat∣ > 20°). We combine the spectral line indices with a set of Balmer line profile selection criteria to identify the BHB stars. During the selection process, we use the line index of Ca
ii
K to exclude the metal-rich A-type dwarfs. We obtain their atmospheric parameters by crossmatching our BHB stars with the catalog provided by Xiang et al. The results show that our sample is consistent with the theoretical
T
eff
–log
g
evolutionary tracks of the BHB stars, indicating that our method is robust for identifying BHB stars from the LAMOST spectra. Their spatial distribution indicates that most of our BHB stars are located in the inner halo or the disk of the Milky Way. Combined with other BHB samples from the literature, the BHB stars can cover a large Galactic volume, which makes them a better probe for studying the kinematics, dynamics, and structural characteristics of the Milky Way.
We investigate the extinction, together with the radial velocity (RV) dispersion and distribution of red clump stars in the anticenter direction using spectra obtained with Hectospec on the MMT. We ...find that extinction peaks at Galactocentric radii of about 9.5 and 12.5 kpc, right in front of the locations of the Perseus and Outer arms and in line with the relative position of dust and stars in external spiral galaxies. The RV dispersion peaks around 10 kpc, which coincides with the location of the Perseus arm, yields an estimated arm-interarm density contrast of 1.3-1.5, and is in agreement with previous studies. Finally, we discover that the RV distribution bifurcates around 10-11 kpc into two peaks at +27 km s super(-1) and -4 km s super(-1). This seems to be naturally explained by the presence of the outer Lindblad resonance of the Galactic bar, but further observations will be needed to understand if the corotation resonance of the spirals arms also plays a role.
The Old Moving Groups in the Field of Taurus Liu, Jiaming; Fang, Min; Tian, Hao ...
The Astrophysical journal. Supplement series,
05/2021, Letnik:
254, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
In this work, we present a systematic search for stellar groups in the Taurus field by applying the DBSCAN algorithm to the data from Gaia DR2. We find 22 groups, consisting of 8 young ...groups (Groups 1–8) at ages of 2–4 Myr and distances of ∼130–170 pc, and 14 old groups (Groups 9–22) at ages of 8–49 Myr and distances of ∼110–210 pc. We characterize the disk properties of group members and find 19 new disk-bearing stars, 8 of which are in the young groups with 11 others belonging to the comparatively old groups at the ages of 8–11 Myr. We characterize the accretion properties of the group members with H
α
emission lines in their Large Sky Area Multi-Object Fibre Spectroscopic Telescope spectra, and discover one source in Group 10 at an age of 10 Myr which still shows accretion activity. We investigate the kinematic relations among the old groups, find that Group 9 is kinematically related to the known Taurus members, and exclude any kinematic relations between Groups 10–22 and the known Taurus members.
ABSTRACT
With the calculated guiding centre radius Rguiding and birth radius Rbirth, we investigate the role of radial migration on the description of lithium evolution in the Galactic disc based on ...the upper envelope of the A(Li) versus Fe/H diagram. Using migration distances, we find that stars in the solar neighbourhood are born at different locations in the Galactic disc, and cannot all be explained by models of chemical evolution in the solar neighbourhood. It is found that the upper envelope of the A(Li) versus Fe/H diagram varies significantly with Rbirth, which explains the decrease of Li for super-metal-rich (SMR) stars because they are non-young stars born in the inner disc. The upper envelope of Li-Rbirth plane fits very well with chemical evolution models by Grisoni et al. for Rbirth = 7–12 kpc, outside which young stars generally lack sufficient time to migrate to the solar neighbourhood. For stars born in the solar neighbourhood, the young open clusters and the upper envelope of field stars with age <3 Gyr fit well with theoretical prediction. We find that calculations using stars with ages less than 3 Gyr are necessary to obtain an undepleted Li upper envelope, and that stars with solar age (around 4.5 Gyr) have depleted around 0.3 dex from the original value based on the chemical evolution model of Grisoni et al.
We use LAMOST DR4 M giants combined with Gaia DR2 proper motions and ALLWISE photometry to obtain an extremely pure sample of Sagittarius (Sgr) stream stars. Using TiO5 and CaH spectral indices as ...indicators, we selected a large sample of M-giant stars from M-dwarf stars in LAMOST DR4 spectra. Considering the position, distance, proper motion, and angular momentum distribution, we obtained 164 pure Sgr stream stars. We find that the trailing arm has higher energy than the leading arm in the same angular momentum. The trailing arm we detected extends to a heliocentric distance of ∼130 kpc at Λ ˜ ∼ 170 ° , which is consistent with the feature found in RR Lyrae in Sesar et al. Both of these detections of Sgr, in M-giants and in RR Lyrae, imply that the Sgr stream may contain multiple stellar populations with a broad metallicity range.
We study the radial metallicity gradient ΔM/H/ΔRg as a function of Mg/Fe and with the help of a guiding radius based on the Apache Point Observatory Galactic Evolution Experiment and Gaia and then ...analyze the radial migration effect on the radial metallicity gradient and metallicity-rotation gradient between the Galactic thin and thick disks. The derived trend of gradient ΔM/H/ΔRg versus Mg/Fe shows a transition at Mg/Fe ∼0.18 dex, below which the gradient is negative and varies a little as Mg/Fe increases; however, it changes sharply in Mg/Fe ranges of 0.16-0.18, above which the gradient increases linearly with increasing Mg/Fe, being a positive value at Mg/Fe 0.22 dex. These positive gradients in the high-Mg/Fe populations are found at kpc, and there are nearly no gradients toward higher . By comparing the metallicity distributions, the radial metallicity gradients ΔM/H/ΔR, and the metallicity-rotation gradients between the total sample and subsample (or subsample), we find that, for the thick disk, blurring flattens the gradient ΔM/H/ΔR and favors metal-poor high-eccentricity stars. These stars are responsible for the measured positive metallicity-rotation gradient of the thick disk.
We present and analyze the positions, distances, and radial velocities for over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn from SDSS DR8. We search for position-velocity ...substructure in these data, a signature of the hierarchical assembly of the stellar halo. Using a cumulative 'close pair distribution' as a statistic in the four-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical significance among the BHB stars: pairs of BHB stars that are close in position on the sky tend to have more similar distances and radial velocities compared to a random sampling of these overall distributions. We make analogous mock observations of 11 numerical halo formation simulations, in which the stellar halo is entirely composed of disrupted satellite debris, and find a level of substructure comparable to that seen in the actually observed BHB star sample. This result quantitatively confirms the hierarchical build-up of the stellar halo through a signature in phase (position-velocity) space. In detail, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population. BHB stars located beyond 20 kpc from the Galactic center exhibit stronger substructure than at r gc < 20 kpc.
ABSTRACT
We study the mass distribution and kinematics of the giant elliptical galaxy M87 (NGC 4486) using discrete chemo-dynamical, axisymmetric Jeans equation modelling. Our catalogue comprises 894 ...globular clusters (GCs) extending to a projected radius of ∼430 kpc with line-of-sight velocities and colours, and Multi Unit Spectroscopic Explorer integral field unit data within the central 2.4 kpc of the main galaxy. The gravitational potential for our models is a combination of a luminous matter potential with a varying mass-to-light ratio for the main galaxy, a supermassive black hole and a dark matter (DM) potential with a cusped or cored DM halo. The best-fitting models with either a cusped or a cored DM halo show no significant differences and both are acceptable. We obtain a total mass of $(2.16 \pm 0.38) \times 10^{13} \, \mathrm{M}_{\odot }$ within ∼400 kpc. By including the stellar mass-to-light ratio gradient, the DM fraction increases from ∼26 per cent (with no gradient) to ∼73 per cent within $1\, R_e^{\rm maj}$ (major axis of half-light isophote, 14.2 kpc), and from ∼84 per cent to ∼94 per cent within $5\, R_e^{\rm maj}$ (71.2 kpc). Red GCs have moderate rotation with Vmax/σ ∼ 0.4, and blue GCs have weak rotation with Vmax/σ ∼ 0.1. Red GCs have tangential velocity dispersion anisotropy, while blue GCs are consistent with being nearly isotropic. Our results suggest that red GCs are more likely to be born in situ, while blue GCs are more likely to be accreted.
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