scholarly journals Is the Dynamical History at Odds with the Chemical History?

1996 ◽  
Vol 169 ◽  
pp. 367-379
Author(s):  
F. Matteucci
Keyword(s):  
High Velocity ◽  
Galactic Plane ◽  
Ultraviolet Excess ◽  
Orbital Eccentricity ◽  
Chemical Abundance ◽  
Chemical Enrichment ◽  
Halo Stars ◽  
The Galaxy ◽  
Chemical History ◽  
Formation And Evolution

The paper of Eggen, Lynden-Bell and Sandage (1962)(hereafter ELS) titled “Evidence from the motion of old stars that the Galaxy collapsed” was the first attempt to understand the formation and evolution of our Galaxy. From a study of a kinematically selected sample of high velocity stars, ELS had found a remarkable correlation between chemical abundance and orbital eccentricity, in the sense that stars with the largest ultraviolet excess (a measure of stellar metallicity, in particular Fe), i.e. the lowest metallicity, are invariably moving in highly elliptical orbits. As the average < [Fe/H] > (in the usual notation [Fe/H] = log(Fe/H)∗ – log(Fe/H)⊙) is expected to increase with time, as a consequence of the progressive chemical enrichment of the gas, stars with the lowest [Fe/H] are, on average, the oldest. ELS also found a correlation between abundance and motion of stars perpendicular to the Galactic plane. This correlation suggests a continuous decrease of the perpendicular velocity with decreasing [Fe/H]. To explain these relations ELS proposed that the Galaxy collapsed from a protocloud to a thin disk on a timescale of a few times 108 years, with progressive chemical enrichment as the collapse proceeded. This model was subsequently criticized mainly because of selection effects in their data, i.e. given the data available to ELS one would not expect the sample to contain low abundance, low orbital eccentricity objects even if they existed. They would be absent from the high velocity catalog they used. In addition, the ELS simple model did not account for the fact that almost half of the halo stars have retrograde orbits. This fact led Larson (1969) to consider models of clumpy and turbulent protogalaxies with collapse times that sometimes exceeded 1 Gyr.

scholarly journals Dynamics of Globular Cluster Systems

1983 ◽  
Vol 100 ◽  
pp. 359-364
Author(s):  
K. C. Freeman
Keyword(s):  
Globular Cluster ◽  
Globular Clusters ◽  
Milky Way ◽  
Cluster Formation ◽  
Active Phase ◽  
Chemical Abundance ◽  
Cluster Systems ◽  
Chemical Enrichment ◽  
Wide Range ◽  
The Galaxy

In the Milky Way, the globular clusters are all very old, and we are accustomed to think of them as the oldest objects in the Galaxy. The clusters cover a wide range of chemical abundance, from near solar down to about [Fe/H] ⋍ −2.3. However there are field stars with abundances significantly lower than −2.3 (eg Bond, 1980); this implies that the clusters formed during the active phase of chemical enrichment, with cluster formation beginning at a time when the enrichment processes were already well under way.

scholarly journals Spatial distribution of exoplanet candidates based on Kepler and Gaia data

2020 ◽  
Vol 635 ◽  
pp. A191
Author(s):  
A. Maliuk ◽  
J. Budaj
Keyword(s):  
Spatial Distribution ◽  
Galactic Plane ◽  
Open Clusters ◽  
Field Of View ◽  
Solar Neighbourhood ◽  
Spatial Gradients ◽  
The Galaxy ◽  
Formation And Evolution ◽  
F Stars ◽  
Galactic Longitude

Context. Surveying the spatial distribution of exoplanets in the Galaxy is important for improving our understanding of planet formation and evolution. Aims. We aim to determine the spatial gradients of exoplanet occurrence in the Solar neighbourhood and in the vicinity of open clusters. Methods. We combined Kepler and Gaia DR2 data for this purpose, splitting the volume sampled by the Kepler mission into certain spatial bins. We determined an uncorrected and bias-corrected exoplanet frequency and metallicity for each bin. Results. There is a clear drop in the uncorrected exoplanet frequency with distance for F-type stars (mainly for smaller planets), a decline with increasing distance along the Galactic longitude l = 90°, and a drop with height above the Galactic plane. We find that the metallicity behaviour cannot be the reason for the drop of the exoplanet frequency around F stars with increasing distance. This might have only contributed to the drop in uncorrected exoplanet frequency with the height above the Galactic plane. We argue that the above-mentioned gradients of uncorrected exoplanet frequency are a manifestation of a single bias of undetected smaller planets around fainter stars. When we correct for observational biases, most of these gradients in exoplanet frequency become statistically insignificant. Only a slight decline of the planet occurrence with distance for F stars remains significant at the 3σ level. Apart from that, the spatial distribution of exoplanets in the Kepler field of view is compatible with a homogeneous one. At the same time, we do not find a significant change in the exoplanet frequency with increasing distance from open clusters. In terms of byproducts, we identified six exoplanet host star candidates that are members of open clusters. Four of them are in the NGC 6811 (KIC 9655005, KIC 9533489, Kepler-66, Kepler-67) and two belong to NGC 6866 (KIC 8396288, KIC 8331612). Two out of the six had already been known to be cluster members.

scholarly journals The Hertzsprung-Russell Diagram of Metal-Poor Disk Stars

1978 ◽  
Vol 80 ◽  
pp. 273-276
Author(s):  
Sidney van den Bergh
Keyword(s):  
Velocity Field ◽  
Globular Cluster ◽  
High Velocity ◽  
Population Model ◽  
Open Cluster ◽  
Ultraviolet Excess ◽  
Red Giants ◽  
The Sun ◽  
The Galaxy ◽  
Red Giant

A quarter of a century ago Keenan and Keller (1953) showed that the majority of high-velocity stars near the Sun outline a Hertzsprung-Russell diagram similar to that of old Population I. This result, which did not appear to fit into Baade's (1944) two-population model of the Galaxy was ignored (except by Roman 1965) for the next two decades. Striking confirmation of the results of Keenan and Keller was, however, obtained by Hartwick and Hesser (1972). Their work appears to show that high-velocity field stars with an ultraviolet excess (which measures Fe/H) of δ(U-B) ≃ +0m.11 lie on a red giant branch that is more than a magnitude fainter than the giant branch of the strong-lined globular cluster 47 Tuc for which δ(U-B) ≃ +0m.10. Furthermore Demarque and McClure (1977) show that the red giants in the old metal poor [δ(U-B) ≃ +0m.11] open cluster NGC 2420 are significantly fainter than are those in 47 Tuc. Calculations by these authors show that the observed differences between the giants in 47 Tuc and in NGC 2420 can be explained if either (1) 47 Tuc is richer in helium than NGC 2420 by ΔY ≃ 0.1 or (2) if 47 Tuc has a ten times lower value of Z(CNO) than does NGC 2420.

Very Metal-Poor Stars and the Early Universe

2017 ◽  
Vol 13 (S334) ◽  
pp. 3-10
Author(s):  
John E. Norris
Keyword(s):  
Near Field ◽  
Field Studies ◽  
Far Field ◽  
Star Forming ◽  
Chemical Enrichment ◽  
Physical Conditions ◽  
The Galaxy ◽  
Formation And Evolution ◽  
The Universe ◽  
Insight Into

AbstractVery metal-poor stars ([Fe/H] &lt; –2.0) inform our understanding of the formation and evolution of the Galaxy, and the physical conditions in the earliest star-forming environments of the Universe. They play an integral part in the paradigms of stellar populations, stellar archaeology, and near-field cosmology. We review the carbon-rich and carbon-normal sub-populations of the most iron-poor stars, providing insight into chemical enrichment at the earliest times in the Universe. We also discuss the role of very metal-poor stars in providing insight into the Galaxy’s halo, thick disk, and bulge, and the promise they hold for the future. A comparison between the abundances obtained for the nine most Fe-poor stars ([Fe/H] &lt; –4.5) (all but one of which is C-rich) with abundances obtained from far-field cosmology suggests that the former are the most chemically primitive objects yet observed and probably older than the DLA- and sub-DLA systems for which data are currently available from far-field studies.

scholarly journals 9. New Spectroscopic Results on Subluminous Stars, V

1971 ◽  
Vol 42 ◽  
pp. 46-60 ◽  
Author(s):  
J. L. Greenstein
Keyword(s):  
High Velocity ◽  
Classification Scheme ◽  
Ultraviolet Excess ◽  
Horizontal Branch ◽  
Line Profiles ◽  
Hydrogen Line ◽  
Dwarf Stars ◽  
White Dwarf Stars ◽  
Halo Stars

Determination of temperature and surface gravity by colors and hydrogen-line profiles have been carried out for hot halo stars. A narrow horizontal branch is found stretching to above 40000 K; the hot O subdwarfs show a nearly vertical sequence, dropping towards the hot white dwarfs.Spectra for 285 white dwarf stars have been obtained, and the classification scheme is reviewed. Theoretical problems of these spectra remain, largely, unsolved.The red subluminous stars found by Eggen were studied spectroscopically; among 68 stars only one new red degenerate star was found. The others are very metal-poor, high-velocity stars with large ultraviolet excess.

scholarly journals The Milky Way: a halo, a corona, or both?

1985 ◽  
Vol 106 ◽  
pp. 415-420
Author(s):  
Klaas S. De Boer
Keyword(s):  
Transition Zone ◽  
Equilibrium Model ◽  
High Velocity ◽  
Milky Way ◽  
Galactic Plane ◽  
Hydrostatic Equilibrium ◽  
Hot Gas ◽  
Halo Stars ◽  
Intergalactic Space ◽  
Gaseous Halo

The detection in absorption lines of gas clouds outside the galactic plane at high velocities by Münch and Zirin (1961), high velocities then defined as velocities differing by more than 20 km/s from the LSR, showed that the space outside the Milky-Way disk contains not just stars. Of course, from a continuity argument it had been all along clear that some transition zone had to exist between the dense (relatively speaking) gas of the Milky-Way plane and the vast (almost) emptiness of intergalactic space. The presence of these clouds requires a mechanism to prevent their evaporation, and Spitzer (1956) proposed that dilute hot gas had to exist outside the Milky-Way disk reaching, in his hydrostatic-equilibrium model, temperatures of a few million K at several tens of kpc. These high temperatures led him to name these gases the Galactic Corona. Observational confirmation of the abundance of these cool clouds came from the measurements of 21-cm HI emission, but no one-to-one correspondence with clouds detected in the visual did appear (Habing 1969). For the majority of the high-velocity (HV) clouds (Hulsbosch 1978) no distances are known, and all of those are believed to exist as a gaseous halo with the halo stars. Thus our Milky Way appears to have outside the disk: a halo, a gaseous halo, and a corona.

scholarly journals The dual origin of the Galactic thick disc and halo from the gas-rich Gaia–Enceladus Sausage merger

2020 ◽  
Vol 497 (2) ◽  
pp. 1603-1618 ◽  
Author(s):  
Robert J J Grand ◽  
Daisuke Kawata ◽  
Vasily Belokurov ◽  
Alis J Deason ◽  
Azadeh Fattahi ◽  
...  
Keyword(s):  
Rotation Velocity ◽  
Orbital Eccentricity ◽  
Chemical Abundance ◽  
Thick Disc ◽  
Rapid Formation ◽  
The Galaxy ◽  
Stellar Halo ◽  
Disc Region ◽  
Magnetohydrodynamic Simulations ◽  
Dual Origin

ABSTRACT We analyse a set of cosmological magnetohydrodynamic simulations of the formation of Milky Way-mass galaxies identified to have a prominent radially anisotropic stellar halo component similar to the so-called ‘Gaia Sausage’ found in the Gaia data. We examine the effects of the progenitor of the Sausage (the Gaia–Enceladus Sausage, GES) on the formation of major galactic components analogous to the Galactic thick disc and inner stellar halo. We find that the GES merger is likely to have been gas-rich and contribute 10–50 ${{\ \rm per\ cent}}$ of gas to a merger-induced centrally concentrated starburst that results in the rapid formation of a compact, rotationally supported thick disc that occupies the typical chemical thick disc region of chemical abundance space. We find evidence that gas-rich mergers heated the proto-disc of the Galaxy, scattering stars on to less-circular orbits such that their rotation velocity and metallicity positively correlate, thus contributing an additional component that connects the Galactic thick disc to the inner stellar halo. We demonstrate that the level of kinematic heating of the proto-galaxy correlates with the kinematic state of the population before the merger, the progenitor mass, and orbital eccentricity of the merger. Furthermore, we show that the mass and time of the merger can be accurately inferred from local stars on counter-rotating orbits.

Maser effects in the interstellar hydrogen clouds of the galactic halo

1967 ◽  
Vol 31 ◽  
pp. 291-293
Author(s):  
I. S. Šklovskij
Keyword(s):  
High Velocity ◽  
Population Inversion ◽  
Galactic Halo ◽  
Galactic Plane ◽  
Neutral Hydrogen ◽  
Collisional Excitation ◽  
Hydrogen Atoms ◽  
Hydrogen Density ◽  
The Galaxy ◽  
High Velocity Clouds

It is suggested that the neutral hydrogen atoms in clouds with high negative velocities observed at high galactic latitudes may, when moving towards a galactic H 11 region, be excited by radiation in the red wing of the Lyman-α profile. The steepness of this wing may cause a population inversion of the hyperfine-structure levels. Consequently, estimates of the hydrogen density in the high-velocity clouds, and of the flow of matter towards the galactic plane (or into the Galaxy), when based on the assumption of collisional excitation, may be too high by two orders of magnitude.

scholarly journals High Velocity Clouds in the Galaxy

1995 ◽  
Vol 164 ◽  
pp. 129-132
Author(s):  
Felix J. Lockman
Keyword(s):  
High Velocity ◽  
Large Scale ◽  
Galactic Plane ◽  
Spectral Lines ◽  
Galactic Rotation ◽  
The Sun ◽  
The Galaxy ◽  
High Velocity Clouds ◽  
Major Review ◽  
Lower Limits

Early observers measuring 21 cm HI profiles away from the Galactic plane found not only the emission near zero velocity expected from gas in the immediate vicinity of the Sun, but also occasional emission at velocities reaching several hundred km s−1. It seemed unlikely that these spectral lines could come from gas in normal galactic rotation (they are sometimes found at |b| > 45°), and so began the puzzle of “high-velocity clouds” (HVCs). The early result that all HVCs had negative velocity implying that they were infalling was soon shown to be incorrect with the discovery of many positive velocity clouds in the southern hemisphere. Attempts to determine the distance to HVCs by searching for them in absorption against stars yielded only lower limits, typically > 1 kpc. By 1984 several large-scale surveys had established that a significant fraction of the sky was covered with high velocity HI (e.g., Oort, 1966; Giovanelli, 1980). A recent major review is by Wakker (1991a; see also van Woerden, 1993). For this brief presentation to a specialized audience, I will concentrate on issues that may be relevant to the topic of stellar populations.

Sign in / Sign up

Export Citation Format

Share Document