scholarly journals The transition between the thick and thin Galactic disks

2007 ◽  
Vol 3 (S248) ◽  
pp. 458-461
Author(s):  
M. Haywood
Keyword(s):  
Stellar Populations ◽  
Thin Disk ◽  
Solar Neighbourhood ◽  
Galactic Disks ◽  
Local Data ◽  
Galactocentric Distance ◽  
The Status ◽  
Solar Galactocentric Distance ◽  
Thick Disks ◽  
Thin Disks

AbstractWe study the transition between the thick and thick disks using solar neighbourhood data, focusing in particular on the status of local metal-poor thin disk stars ([Fe/H]<−0.3 dex, [α/Fe]<0.1 dex). The orbital properties of these stars, which are responsible for the hiatus in metallicity between the two disks, suggest that they most likely originate from the outer Galactic thin disk. It implies that the transition between the two stellar populations at a solar galactocentric distance must have occurred at a metallicity of about −0.3 dex. Transition stars at this metallicity are in fact present in local samples and fill the gap in α-element between the thick and thin disks. These results imply that, at least from the local data, there is a clear evolutionary link between the thick and thin disks.

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.

Estimation of Distributed Unbalance of Rotors

2001 ◽  
Author(s):  
Tachung Yang ◽  
Chunyi Lin
Keyword(s):  
Finite Element ◽  
Singular Value Decomposition ◽  
Element Model ◽  
Singular Value ◽  
Lumped Mass ◽  
Piecewise Polynomials ◽  
Mass Unbalance ◽  
Thick Disks ◽  
Value Decomposition ◽  
Thin Disks

Mass unbalance commonly causes vibration of rotor-bearing systems. Lumped mass modeling of unbalance was adapted in most previous research. The lumped unbalance assumption is adequate for thin disks or impellers, but not for thick disks or shafts. Lee et al. (1993) proposed that the unbalance of shafts should be continuously distributed. Balancing methods based on discrete unbalance models may not be very appropriate for rotors with distributed unbalance. A better alternative is to identify the distributed unbalance of shafts before balancing. In this study, the eccentricity distribution of the shaft is assumed in piecewise polynomials. A finite element model for the distributed unbalance is provided. Singular value decomposition is used to identify the eccentricity curves of the rotor. Numerical validation of this method is presented and examples are given to show the effectiveness of the identification method.

scholarly journals The AMBRE project: The thick thin disk and thin thick disk of the Milky Way

2017 ◽  
Vol 608 ◽  
pp. L1 ◽  
Author(s):  
M. R. Hayden ◽  
A. Recio-Blanco ◽  
P. de Laverny ◽  
S. Mikolaitis ◽  
C. C. Worley
Keyword(s):  
Vertical Velocity ◽  
Velocity Dispersion ◽  
Stellar Populations ◽  
Thin Disk ◽  
Thick Disk ◽  
Solar Neighborhood ◽  
Chemical Enrichment ◽  
History Of ◽  
External Galaxies ◽  
Subgiant Stars

We analyze 494 main sequence turnoff and subgiant stars from the AMBRE:HARPS survey. These stars have accurate astrometric information from Gaia DR1, providing reliable age estimates with relative uncertainties of ±1 or 2 Gyr and allowing precise orbital determinations. The sample is split based on chemistry into a low-[Mg/Fe] sequence, which are often identified as thin disk stellar populations, and high-[Mg/Fe] sequence, which are often associated with thick disk stellar populations. We find that the high-[Mg/Fe] chemical sequence has extended star formation for several Gyr and is coeval with the oldest stars of the low-[Mg/Fe] chemical sequence: both the low- and high-[Mg/Fe] sequences were forming stars at the same time. We find that the high-[Mg/Fe] stellar populations are only vertically extended for the oldest, most-metal poor and highest [Mg/Fe] stars. When comparing vertical velocity dispersion for the low- and high-[Mg/Fe] sequences, the high-[Mg/Fe] sequence has lower vertical velocity dispersion than the low-[Mg/Fe] sequence for stars of similar age. This means that identifying either group as thin or thick disk based on chemistry is misleading. The stars belonging to the high-[Mg/Fe] sequence have perigalacticons that originate in the inner disk, while the perigalacticons of stars on the low-[Mg/Fe] sequence are generally around the solar neighborhood. From the orbital properties of the stars, the high-[Mg/Fe] and low-[Mg/Fe] sequences are most likely a reflection of the chemical enrichment history of the inner and outer disk populations, respectively; radial mixing causes both populations to be observed in situ at the solar position. Based on these results, we emphasize that it is important to be clear in defining what populations are being referenced when using the terms thin and thick disk, and that ideally the term thick disk should be reserved for purely geometric definitions to avoid confusion and be consistent with definitions in external galaxies.

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 The vertical metallicity gradients of mono-age stellar populations in the Milky Way thin disk

2017 ◽  
Vol 13 (S334) ◽  
pp. 281-282
Author(s):  
Ioana Ciucă ◽  
Daisuke Kawata ◽  
Jane Lin ◽  
Luca Casagrande ◽  
George Seabroke ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Milky Way ◽  
Stellar Populations ◽  
Thin Disk ◽  
Dwarf Stars ◽  
Star Forming ◽  
Data Release ◽  
The Cross

AbstractWe investigate the vertical metallicity gradients of five mono-age stellar populations between 0 and 11 Gyr for a sample of 18 435 dwarf stars selected from the cross-matched Tycho-Gaia Astrometric Solution (TGAS) and RAdial Velocity Experiment (RAVE) Data Release 5. We find a correlation between the vertical metallicity gradients and age, with no vertical metallicity gradient in the youngest population and an increasingly steeper negative vertical metallicity gradient for the older stellar populations. We also find that the intrinsic dispersion in metallicity increases steadily with age. Our results are consistent with a scenario that thin disk stars formed from a flaring thin star-forming disk.

scholarly journals The Scatter of Metallicities of Stars in the Solar Neighbourhood

1996 ◽  
Vol 169 ◽  
pp. 431-432 ◽  
Author(s):  
B. Fuchs ◽  
C. Dettbarn ◽  
R. Wielen
Keyword(s):  
Main Sequence ◽  
Thin Disk ◽  
Thick Disk ◽  
Solar Neighbourhood ◽  
Data Sets ◽  
Main Sequence Stars ◽  
The Third ◽  
Nearby Stars ◽  
Velocity Dispersions

It is well known that the velocity dispersions of the stars in the solar neighbourhood increase with their ages (Wielen 1977). In Fig.1 we show |W| weighted velocity dispersions (cf. Wielen 1977) of the stars in the Third Catalog of Nearby Stars (Gliese and Jahreiß 1994). Open symbols indicate main sequence stars and crosses indicate McCormick stars, a kinematically unbiased subset of the CNS3, respectively, whereas the filled symbols are the Edvardsson et al. (1993) data. Stars older than 14 Gyr are not shown because they are probably thick disk stars (Freeman 1991). We have assumed a maximum age of the old thin disk stars of 12 Gyr as suggested by the Edvardsson et al. data. As can be seen from Fig.1 both data sets fit ideally together. The solid line indicates a σ ∝ τ1/2 law.

scholarly journals Theoretical Limits on the Properties of Low-Velocity M-Dwarfs

1975 ◽  
Vol 69 ◽  
pp. 321-324
Author(s):  
P. Biermann
Keyword(s):  
Mass Fraction ◽  
Velocity Dispersion ◽  
Stellar Populations ◽  
Large Mass ◽  
Solar Neighbourhood ◽  
M Dwarfs ◽  
Low Velocity ◽  
Missing Mass ◽  
Stability Properties

Using a version of the Toomre-criterion (Toomre, 1964) for a mixture of stellar populations, limits are derived for the kinematical properties of the newly discovered population of low-velocity M-dwarfs (Weistrop, 1972; Murray and Sanduleak, 1972); these stars are claimed to be a large mass-fraction (≳ 0.5) of all stars in the solar neighbourhood and to have only 10 km s−1 velocity dispersion. They could make up for the locally missing mass (Oort, 1965; Schmidt, 1974). It is found that a configuration of stellar populations with the M-dwarfs included (having such properties) is not stable in the sense of the Toomre-criterion. The limiting stability properties are given.

Estimation of Distributed Unbalance of Rotors

2002 ◽  
Vol 124 (4) ◽  
pp. 976-983 ◽  
Author(s):  
T. Yang ◽  
C. Lin
Keyword(s):  
Finite Element ◽  
Element Model ◽  
Singular Value ◽  
Lumped Mass ◽  
Piecewise Polynomials ◽  
Mass Unbalance ◽  
Rotor Bearing ◽  
Thick Disks ◽  
Value Decomposition ◽  
Thin Disks

Mass unbalance commonly causes vibration of rotor-bearing systems. Lumped mass modeling of unbalance was adapted in most previous research. The lumped unbalance assumption is adequate for thin disks or impellers, but not for thick disks or shafts. Lee et al. (Lee, A. C., et al., 1993, “The Analysis of Linear Rotor-Bearing Systems: A General Transfer Matrix Method,” ASME J. Vib. Acoust., 115, pp. 490–497) proposed that the unbalance of shafts should be continuously distributed. Balancing methods based on discrete unbalance models may not be very appropriate for rotors with distributed unbalance. A better alternative is to identify the distributed unbalance of shafts before balancing. In this study, the eccentricity distribution of the shaft is assumed in piecewise polynomials. A finite element model for the distributed unbalance is provided. Singular value decomposition is used to identify the eccentricity curves of the rotor. Numerical validation of this method is presented and examples are given to show the effectiveness of the identification method.

scholarly journals Powerful Jets from Radiatively Efficient Disks, a Decades-Old Unresolved Problem in High Energy Astrophysics

Galaxies ◽  
2021 ◽  
Vol 9 (1) ◽  
pp. 10
Author(s):  
Chandra B. Singh ◽  
David Garofalo ◽  
Benjamin Lang
Keyword(s):  
Active Galactic Nuclei ◽  
Scale Invariance ◽  
Galactic Nuclei ◽  
High Energy ◽  
Kerr Solution ◽  
High Energy Astrophysics ◽  
Road Map ◽  
Thick Disks ◽  
Thin Disks ◽  
Partial Success

The discovery of 3C 273 in 1963, and the emergence of the Kerr solution shortly thereafter, precipitated the current era in astrophysics focused on using black holes to explain active galactic nuclei (AGN). But while partial success was achieved in separately explaining the bright nuclei of some AGN via thin disks, as well as powerful jets with thick disks, the combination of both powerful jets in an AGN with a bright nucleus, such as in 3C 273, remained elusive. Although numerical simulations have taken center stage in the last 25 years, they have struggled to produce the conditions that explain them. This is because radiatively efficient disks have proved a challenge to simulate. Radio quasars have thus been the least understood objects in high energy astrophysics. But recent simulations have begun to change this. We explore this milestone in light of scale-invariance and show that transitory jets, possibly related to the jets seen in these recent simulations, as some have proposed, cannot explain radio quasars. We then provide a road map for a resolution.

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