scholarly journals A New Sample of Thick Disk and Halo Stars

1995 ◽  
Vol 164 ◽  
pp. 386-386
Author(s):  
C. Soubiran ◽  
M.N. Perrin ◽  
R. Cayrel ◽  
E. Chereul
Keyword(s):  
Stellar Population ◽  
Thin Disk ◽  
Thick Disk ◽  
The Other ◽  
Chemical Characteristics ◽  
Fundamental Parameters ◽  
Halo Stars ◽  
Synthetic Spectra ◽  
The Galaxy ◽  
Comparison Stars

The aim of our stellar population study is to investigate the kinematical and chemical characteristics of the thin disk, thick disk and halo of the Galaxy. We have selected 51 stars in 2 astrometric and photometric surveys at l = 42°, b = +79° (Soubiran 1992) and l = 167°, b = +47° (Ojha et al. 1994), on the basis of the Reduced Proper Motion Diagram. They were observed with the 193cm telescope at Observatoire de Haute-Provence, with the CARELEC spectrograph (dispersion of 66Å/mm, FWHM of 3.0Å, range λλ4600 – 5500Å), together with 43 comparison stars with known fundamental parameters. The derivation of Teff, logg and [Fe/H] was done differentially using a grid of synthetic spectra and the comparison stars, as described in Cayrel et al. (1991). Twenty of the target stars were found to be more deficient than −0.5. In the (V, [Fe/H]) distribution, the halo stars are clearly separated from the other stars with a mean of (V, [Fe/H]) ≃ (−210km/s, – 1.4dex). Because of the small size of the sample, it was not possible to discriminate the thick disk from the thin disk. We have taken 200 more spectra, and with these new observations, we hope to be able to deconvolve the 3 populations in the (U, V, W, [Fe/H]) space as we did previously with the (U, V) velocity (Soubiran 1993).

scholarly journals Abundances of α-Process Elements in Thin-Disk, Thick-Disk, and Halo Stars of the Galaxy: Non-LTE Analysis

2019 ◽  
Vol 63 (9) ◽  
pp. 726-738 ◽  
Author(s):  
L. I. Mashonkina ◽  
M. D. Neretina ◽  
T. M. Sitnova ◽  
Yu. V. Pakhomov
Keyword(s):  
Thin Disk ◽  
Thick Disk ◽  
Halo Stars ◽  
The Galaxy ◽  
Process Elements

scholarly journals Solar Neighbourhood Age–Metallicity Relation Based on Hipparcos Data

2004 ◽  
Vol 21 (2) ◽  
pp. 121-125 ◽  
Author(s):  
Akihiko Ibukiyama
Keyword(s):  
Star Formation ◽  
Essential Feature ◽  
Formation Rate ◽  
Vertical Gradient ◽  
Thin Disk ◽  
Thick Disk ◽  
Solar Neighbourhood ◽  
Orbital Parameters ◽  
The Galaxy ◽  
The One

AbstractWe derive age–metallicity relations (AMRs) and orbits for the 1658 solar neighbourhood stars for which accurate distances are measured by the Hipparcos satellite. The sample comprises 1382 thin disk stars, 229 thick disk stars, and 47 halo stars according to their orbital parameters. We find a considerable scatter for thin disk AMRs along the one-zone Galactic chemical evolution (GCE) model. Orbits and metallicities of thin disk stars show no clear relation to each other. The scatter along the AMR exists even if stars with the same orbits are selected. We examine simple extensions of one-zone GCE models which account for inhomogeneity in the effective yield and inhomogeneous star formation rate in the Galaxy. Both extensions of the one-zone GCE model cannot account for the scatter in the age–[Fe/H]–[Ca/Fe] relation simultaneously. We conclude, therefore, that the scatter along the thin disk AMR is an essential feature in the formation and evolution of the Galaxy. The AMR for thick disk stars shows that star formation terminated 8 Gyr ago in the thick disk. As previously reported, thick disk stars are more Ca-rich than thin disk stars with the same [Fe/H]. We find that thick disk stars show a vertical abundance gradient. These three facts — AMR, vertical gradient, and [Ca/Fe]–[Fe/H] relation — support monolithic collapse and/or accretion of satellite dwarf galaxies as likely thick disk formation scenarios.

scholarly journals Deep Multi-Colour Starcounts

1994 ◽  
Vol 161 ◽  
pp. 423-424
Author(s):  
I.N. Reid ◽  
S.R. Majewski
Keyword(s):  
Standard Model ◽  
Stellar Population ◽  
Stellar Populations ◽  
The North ◽  
Halo Stars ◽  
The Standard Model ◽  
The Galaxy ◽  
Best Fitting ◽  
Galactic Stellar Populations ◽  
North Galactic

Starcounts remain one of the most effective methods of probing the structure of the Galactic stellar populations. However, studies of the distribution at large distances above the Plane demand accurate photometry extending to faint magnitudes (V > 20), and such datasets are still rare. We (Reid & Majewski 1993) have analyzed data from one field — Majewski's (1992) UJF observations of SA57, the North Galactic Pole field. Our results revealed significant discrepancies with the standard model of the Galaxy (see refs. in Reid & Majewski), notably a paucity in the number of halo stars by a factor of two and the presence of a factor of two more disk stars than predicted — sufficient stars that the disk is the majority stellar population, outnumbering halo stars 2:1 even at V = 21. Majewski et al. (1993) has obtained UJFN photographic data for several other fields, and Fig. 1 shows a preliminary comparison of these observations with the predictions of the best-fitting SA57 model. Given that none of the parameters have been modified, the agreement is surprisingly good.

scholarly journals Impacts of Radial Mixing on the Galactic Thick and Thin Disks

2017 ◽  
Vol 13 (S334) ◽  
pp. 132-135
Author(s):  
Daisuke Kawata
Keyword(s):  
Thin Disk ◽  
Scale Height ◽  
Thick Disk ◽  
The Other ◽  
Galactic Disks ◽  
Spiral Arms ◽  
Other Hand ◽  
Initial Scale ◽  
Thin Disks ◽  
Metallicity Distribution

AbstractUsing N-body simulations of the Galactic disks, we qualitatively study how the metallicity distributions of the thick and thin disk stars are modified by radial mixing induced by the bar and spiral arms. We show that radial mixing drives a positive vertical metallicity gradient in the mono-age disk population whose initial scale-height is constant and initial radial metallicity gradient is tight and negative. On the other hand, if the initial disk is flaring, with scale-height increasing with galactocentric radius, radial mixing leads to a negative vertical metallicity gradient, which is consistent with the current observed trend. We also discuss impacts of radial mixing on the metallicity distribution of the thick disk stars. By matching the metallicity distribution of N-body models to the SDSS/APOGEE data, we argue that the progenitor of the Milky Way’s thick disk should not have a steep negative metallicity gradient.

scholarly journals The Vertical Structure and Kinematics of the Galaxy

1994 ◽  
Vol 161 ◽  
pp. 435-439
Author(s):  
C. Soubiran
Keyword(s):  
Velocity Distribution ◽  
Vertical Structure ◽  
Vertical Gradient ◽  
Thin Disk ◽  
Thick Disk ◽  
Exponential Density ◽  
The Galaxy ◽  
Local Densities ◽  
Discrete Populations

A sample including 2370 stars with (U, V) velocities has been analyzed up to z = 2.5 kpc. It is shown that the observed vertical gradient in the velocity distribution can be explained by the sum of 3 discrete populations with constant kinematics. The observations are well fitted by exponential density laws for the thin disk and the thick disk with scale lengths of 280 pc and 700 pc respectively, and with local densities of 6% and 0.15% for the thick disk and halo respectively.

scholarly journals The Galactic Disk-Halo Transition – Evidence from Stellar Abundances

2008 ◽  
Vol 4 (S254) ◽  
pp. 103-108 ◽  
Author(s):  
Poul Erik Nissen ◽  
William J. Schuster
Keyword(s):  
Milky Way ◽  
Galactic Disk ◽  
Thin Disk ◽  
Thick Disk ◽  
Chemical Abundance ◽  
Elemental Abundances ◽  
Halo Stars ◽  
New Information ◽  
First Results ◽  
Satellite Galaxies

AbstractNew information on the relations between the Galactic disks, the halo, and satellite galaxies is being obtained from elemental abundances of stars having metallicities in the range −1.5 < [Fe/H] < −0.5. The first results for a sample of 26 halo stars and 13 thick-disk stars observed with the ESO VLT/UVES spectrograph are presented. The halo stars fall in two distinct groups: one group (9 stars) has [α/Fe] = 0.30 ± 0.03 like the thick-disk stars. The other group (17 stars) shows a clearly deviating trend ranging from [α/Fe] = 0.20 at [Fe/H] = −1.3 to [α/Fe] = 0.08 at [Fe/H] = −0.8. The kinematics of the stars are discussed and the abundance ratios Na/Fe, Ni/Fe, Cu/Fe and Ba/Y are applied to see if the “low-alpha” stars are connected to the thin disk or to Milky Way satellite galaxies. Furthermore, we compare our data with simulations of chemical abundance distributions in hierarchically formed stellar halos in a ΛCDM Universe.

scholarly journals Vertical Metallicity Gradient of the Galaxy Based on UBV Starcount Data

1994 ◽  
Vol 161 ◽  
pp. 420-422
Author(s):  
T. Yamagata ◽  
Y. Yoshii
Keyword(s):  
Spatial Distribution ◽  
Least Squares ◽  
High Latitude ◽  
Vertical Gradient ◽  
Thin Disk ◽  
Thick Disk ◽  
The North ◽  
The Galaxy ◽  
Metal Abundance ◽  
North Galactic

The spatial distribution of metal abundance in the Galaxy has been analyzed using the UBV starcount data recently obtained in two high-latitude regions of the North Galactic Pole (NGP) and Selected Area 54 (SA54). A least-squares analysis was performed to determine the vertical metallicity gradient for each of the thin and thick disk components that gives a reasonable fit to the observed U-B and B-V colour distributions to V = 18 mag. The most probable value of the vertical gradient is obtained as d[Fe/H]/dz = −0.5 kpc−1 for the thin disk, and −0.1 kpc−1 for the thick disk.

scholarly journals Is Fornax 4 the nuclear star cluster of the Fornax dwarf spheroidal galaxy?

2020 ◽  
Vol 495 (4) ◽  
pp. 4518-4528
Author(s):  
S Martocchia ◽  
E Dalessandro ◽  
M Salaris ◽  
S Larsen ◽  
M Rejkuba
Keyword(s):  
Stellar Population ◽  
Hubble Space Telescope ◽  
Detailed Comparison ◽  
Observational Evidence ◽  
Star Cluster ◽  
The Other ◽  
Horizontal Branch ◽  
The Galaxy ◽  
Red Giant

ABSTRACT Fornax 4 is the most distinctive globular cluster in the Fornax dwarf spheroidal. Located close to the centre of the galaxy, more metal-rich and potentially younger than its four companions (namely, Fornax clusters number 1, 2, 3, and 5), it has been suggested to have experienced a different formation than the other clusters in the galaxy. Here, we use Hubble Space Telescope/WFC3 photometry to characterize the stellar population content of this system and shed new light on its nature. By means of a detailed comparison of synthetic horizontal branch and red giant branch with the observed colour–magnitude diagrams, we find that this system likely hosts stellar sub-populations characterized by a significant iron spread up to Δ[Fe/H] ∼ 0.4 dex and possibly by also some degree of He abundance variations ΔY ∼ 0.03. We argue that this purely observational evidence, combined with the other peculiarities characterizing this system, supports the possibility that Fornax 4 is the nuclear star cluster of the Fornax dwarf spheroidal galaxy. A spectroscopic follow-up for a large number of resolved member stars is needed to confirm this interesting result and to study in detail the formation and early evolution of this system and more in general the process of galaxy nucleation.

scholarly journals Beryllium abundances and the formation of the halo and the thick disk

2009 ◽  
Vol 5 (S268) ◽  
pp. 483-488
Author(s):  
Rodolfo Smiljanic ◽  
L. Pasquini ◽  
P. Bonifacio ◽  
D. Galli ◽  
B. Barbuy ◽  
...  
Keyword(s):  
Star Formation ◽  
Formation Rate ◽  
Cosmic Ray ◽  
Thick Disk ◽  
Star Formation History ◽  
Halo Stars ◽  
Formation History ◽  
The Galaxy ◽  
History Of ◽  
Early Galaxy

AbstractThe single stable isotope of beryllium is a pure product of cosmic-ray spallation in the ISM. Assuming that the cosmic-rays are globally transported across the Galaxy, the beryllium production should be a widespread process and its abundance should be roughly homogeneous in the early-Galaxy at a given time. Thus, it could be useful as a tracer of time. In an investigation of the use of Be as a cosmochronometer and of its evolution in the Galaxy, we found evidence that in a log(Be/H) vs. [α/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. We also found evidence that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation.

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