scholarly journals Chemical differences and similarities among the kinematically selected thick disk, inner halo and outer halo stars

2013 ◽  
Vol 9 (S298) ◽  
pp. 71-76
Author(s):  
Miho N. Ishigaki ◽  
Wako Aoki ◽  
Masashi Chiba
Keyword(s):  
Thick Disk ◽  
Chemical Diversity ◽  
Elemental Abundance ◽  
High Dispersion ◽  
Chemical Abundance ◽  
Chemical Enrichment ◽  
Elemental Abundances ◽  
Halo Stars ◽  
Stellar Halo ◽  
Homogeneous Chemical

AbstractChemical diversity among metal-poor stars in the old stellar components in the Milky Way (MW), namely the thick disk and stellar halo, provides clues to understanding the early chemodynamical evolution of our Galaxy. We present our results on a homogeneous chemical abundance analysis for nearby metal-poor stars likely belonging to the MW thick disk, inner and outer stellar halos. Abundances of alpha, sodium, iron-peak and neutron-capture elements in the sample stars have been estimated using high-resolution (R 50000) spectra obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope. The derived abundances are used to examine differences and similarities in elemental abundance ratios among the kinematically defined thick disk, inner and outer halo subsamples in the metallicity range of −3.3 < [Fe/H] < −0.5. We show that, in the metallicity range of [Fe/H] < −2, the three subsamples are similar in most of the elemental abundances. On the other hand, in the higher metallicities, particularly in [Fe/H] > −1.5, the thick disk and the inner/outer halo subsamples show systematically different abundance ratios for some elements including alpha, sodium, zinc and europium. A modest difference in the sodium and zinc abundances between the inner- and outer halo subsamples is also identified. The observed distinct abundances of some elements among the three subsamples implies that their constituent stars originally formed in progenitor systems that have experienced different star formation and chemical enrichment histories.

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 Chemical abundances of field halo stars - Implications for the building blocks of the Milky Way

2019 ◽  
Vol 14 (S351) ◽  
pp. 24-33
Author(s):  
Miho N. Ishigaki
Keyword(s):  
Milky Way ◽  
Building Blocks ◽  
Chemical Information ◽  
Wide Field ◽  
Chemical Abundance ◽  
Chemical Abundances ◽  
Halo Stars ◽  
Dimensional Phase Space ◽  
Stellar Halo ◽  
Detailed Chemical

AbstractI would like to review recent efforts of detailed chemical abundance measurements for field Milky Way halo stars. Thanks to the advent of wide-field spectroscopic surveys up to a several kpc from the Sun, large samples of field halo stars with detailed chemical measurements are continuously expanding. Combination of the chemical information and full six dimensional phase-space information is now recognized as a powerful tool to identify cosmological accretion events that have built a sizable fraction of the present-day stellar halo. Future observational prospects with wide-field spectroscopic surveys and theoretical prospects with supernova nucleosynthetic yields are also discussed.

scholarly journals The Galactic Thick Disk: An Observational Perspective

2009 ◽  
Vol 5 (S265) ◽  
pp. 289-299
Author(s):  
Bacham E. Reddy
Keyword(s):  
Galaxy Formation ◽  
Chemical Properties ◽  
Thick Disk ◽  
Elemental Abundance ◽  
Distinct Component ◽  
Chemical Enrichment ◽  
Abundance Patterns ◽  
The Galaxy ◽  
Star Formation Histories ◽  
And Chemical Properties

AbstractIn this review, we present a brief description of observational efforts to understand the Galactic thick disk and its relation to the other Galactic components. This review primarily focused on elemental abundance patterns of the thick disk population to understand the process or processes that were responsible for its existence and evolution. Kinematic and chemical properties of disk stars establish that the thick disk is a distinct component in the Milky Way, and its chemical enrichment and star formation histories hold clues to the bigger picture of understanding the Galaxy formation.

scholarly journals The Milky Way has no in-situ halo other than the heated thick disc

2019 ◽  
Vol 632 ◽  
pp. A4 ◽  
Author(s):  
P. Di Matteo ◽  
M. Haywood ◽  
M. D. Lehnert ◽  
D. Katz ◽  
S. Khoperskov ◽  
...  
Keyword(s):  
Galactic Halo ◽  
The Sun ◽  
Galactic Disc ◽  
Elemental Abundances ◽  
Thick Disc ◽  
Halo Stars ◽  
Stellar Halo ◽  
Halo Population ◽  
Astrometric Data

Previous studies based on the analysis of Gaia DR2 data have revealed that accreted stars, possibly originating from a single progenitor satellite, are a significant component of the halo of our Galaxy, potentially constituting most of the halo stars at [Fe/H] <  −1 within a few kpc from the Sun and beyond. In this paper, we couple astrometric data from Gaia DR2 with elemental abundances from APOGEE DR14 to characterise the kinematics and chemistry of in-situ and accreted populations up to [Fe/H] ∼ −2. Accreted stars appear to significantly impact the galactic chemo–kinematic relations, not only at [Fe/H] <  −1, but also at metallicities typical of the thick and metal-poor thin discs. They constitute about 60% of all stars at [Fe/H] <  −1, the remaining 40% being made of (metal-weak) thick-disc stars. We find that the stellar kinematic fossil record shows the imprint left by this accretion event, which heated the old galactic disc. We are able to age-date this kinematic imprint, showing that the accretion occurred between nine and 11 Gyr ago, and that it led to the last significant heating of the galactic disc. An important fraction of stars with abundances typical of the (metal-rich) thick disc, and heated by this interaction, is now found in the galactic halo. Indeed, about half of the kinematically defined halo at few kpc from the Sun is composed of metal-rich thick-disc stars. Moreover, we suggest that this metal-rich thick-disc component dominates the stellar halo of the inner Galaxy. The new picture that emerges from this study is one where the standard, non-rotating in-situ halo population, the collapsed halo, seems to be more elusive than ever.

scholarly journals Elemental Abundances of Normal Sharp-Lined B and A Stars from Optical Region Analyses

1985 ◽  
Vol 111 ◽  
pp. 423-426
Author(s):  
Saul J. Adelman
Keyword(s):  
Main Sequence ◽  
Elemental Abundance ◽  
High Dispersion ◽  
The Sun ◽  
Elemental Abundances ◽  
Solar Composition ◽  
Optical Region ◽  
Optical Spectrophotometry ◽  
Good Agreement

Optical-region elemental-abundance analyses were performed for ten sharp-lined main sequence B and A stars. The derived abundances are generally in good agreement with those of the Sun. Multiple high dispersion spectrograms, fully line-blanketed solar composition model atmospheres, optical spectrophotometry, and the most accurate gf values were employed. This study provides initial parameters for studies of these stars in the ultraviolet and a consistent set of values for comparison with abundances of more exotic stars.

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 Chemical similarities between the Galactic bulge and local thick disk red giant stars: analysis from optical data

2009 ◽  
Vol 5 (S265) ◽  
pp. 342-343
Author(s):  
Alan Alves-Brito ◽  
Jorge Meléndez ◽  
Martin Asplund
Keyword(s):  
High Resolution ◽  
Thick Disk ◽  
Elemental Abundance ◽  
Optical Data ◽  
Galactic Bulge ◽  
Open Problems ◽  
Chemical Enrichment ◽  
Giant Stars ◽  
Red Giant Stars ◽  
Red Giant

AbstractThe Galactic structure and composition remain as one of the greatest open problems in modern astrophysics. We show here that there are chemical similarities between the Galactic bulge and local thick disk red giant stars. This finding puts strong constraints on the IMF, SFR and chemical enrichment timescale of the bulge and thick disk. Our results are based upon a detailed elemental abundance analysis of 80 high S/N and high resolution optical spectra of giant stars, in the range −1.5 < [Fe/H] < +0.5.

scholarly journals Searching for extremely alpha-poor stars in the Galactic Halo

2013 ◽  
Vol 9 (S298) ◽  
pp. 449-449
Author(s):  
Qianfan Xing ◽  
Gang Zhao
Keyword(s):  
Galactic Halo ◽  
Element Abundance ◽  
Chemical Abundance ◽  
Stellar Spectra ◽  
Large Sample ◽  
Pilot Survey ◽  
Chemical Enrichment ◽  
Halo Stars ◽  
Abundance Patterns ◽  
Abundance Trends

AbstractA few alpha-poor stars that show severe departures (over 0.4 dex deficiency in alpha-element abundance) from the general enhanced alpha-element chemical abundance trends of the halo have been discovered in recent years, such as BD +80°245, G4-36 and CS 22966-043. These ratios suggest a different chemical enrichment history for these stars than for the majority of the halo. Similarly low-alpha abundance patterns are also seen in the Sagittarius dSph galaxy. We present a method for searching of extremely alpha-poor stars from low-resolution stellar spectra of LAMOST pilot survey and attempt to create a large sample of these particular Galactic halo stars.

scholarly journals Chemical abundance analysis of extremely metal-poor stars in the Sextans dwarf spheroidal galaxy

2020 ◽  
Vol 636 ◽  
pp. A111 ◽  
Author(s):  
M. Aoki ◽  
W. Aoki ◽  
P. François
Keyword(s):  
Milky Way ◽  
Galactic Halo ◽  
Dwarf Galaxies ◽  
Dwarf Galaxy ◽  
Building Blocks ◽  
High Resolution Spectroscopy ◽  
High Dispersion ◽  
Chemical Abundance ◽  
Halo Structure ◽  
Halo Stars

Context. Metal-poor components of dwarf galaxies around the Milky Way could be remnants of the building blocks of the Galactic halo structure. Low-mass stars that are currently observed as metal-poor stars are expected to have formed in chemically homogeneous clusters in the early phases of galaxy formation. They should have already disintegrated and should exhibit large scatter in abundance ratios of some sets of elements (e.g., Sr/Ba) in the Milky Way field stars. However, chemical abundance ratios are expected to cluster in very metal-poor stars in dwarf galaxies because the number of clusters formed in individual galaxies in the very early phase is expected to be quite limited. Aims. We examine the possible clustering of abundance ratios of Sr and Ba in the Sextans dwarf galaxy to test for the clustering star formation scenario. Methods. We investigate a total of 11 elements (C, Mg, Ca, Sc, Ti, Cr, Mn, Ni, Zn, Sr, Ba) in five stars in the Sextans dwarf galaxy. Previous studies suggest that these have similar abundance ratios. In this study, we focus on the abundance ratio of Sr to Ba. The observations are based on high-resolution spectroscopy (R = 40 000) using the Subaru Telescope High Dispersion Spectrograph. Results. The distribution of α/Fe abundance ratios of the Sextans dwarf galaxy stars is slightly lower than the average of the values of stars in the Galactic halo. The Sr/Ba abundance ratios for the five metal-poor stars are in good agreement, and this clumping is distinctive compared to the [Sr/Ba] spread seen in the metal-poor halo stars. We find that the probability of such clumping is very small if the Sextans stars have distributions of Sr and Ba abundances similar to halo stars. Conclusions. In the Sextans dwarf galaxy, five out of six of the extremely metal-poor stars for which abundance ratios are well studied so far show clear clustering in abundance ratios including Sr/Ba. These observations tend to support the hypothesis that these stars were formed from a cloud of homogeneous chemical composition.

scholarly journals What is the Milky Way outer halo made of?

2017 ◽  
Vol 608 ◽  
pp. A145 ◽  
Author(s):  
G. Battaglia ◽  
P. North ◽  
P. Jablonka ◽  
M. Shetrone ◽  
D. Minniti ◽  
...  
Keyword(s):  
Milky Way ◽  
Formation Rate ◽  
Large Magellanic Cloud ◽  
Solar Neighborhood ◽  
Asymptotic Giant Branch ◽  
Chemical Abundance ◽  
Data Set ◽  
Halo Stars ◽  
Stellar Halo ◽  
Giant Branch Stars

In a framework where galaxies form hierarchically, extended stellar haloes are predicted to be an ubiquitous feature around Milky Way-like galaxies and to consist mainly of the shredded stellar component of smaller galactic systems. The type of accreted stellar systems are expected to vary according to the specific accretion and merging history of a given galaxy, and so is the fraction of stars formed in situ versus accreted. Analysis of the chemical properties of Milky Way halo stars out to large Galactocentric radii can provide important insights into the properties of the environment in which the stars that contributed to the build-up of different regions of the Milky Way stellar halo formed. In this work we focus on the outer regions of the Milky Way stellar halo, by determining chemical abundances of halo stars with large present-day Galactocentric distances, >15 kpc. The data-set we acquired consists of high resolution HET/HRS, Magellan/MIKE and VLT/UVES spectra for 28 red giant branch stars covering a wide metallicity range, −3.1 ≲ [Fe/H] ≲−0.6. We show that the ratio of α-elements over Fe as a function of [Fe/H] for our sample of outer halo stars is not dissimilar from the pattern shown by MW halo stars from solar neighborhood samples. On the other hand, significant differences appear at [Fe/H] ≳−1.5 when considering chemical abundance ratios such as [Ba/Fe], [Na/Fe], [Ni/Fe], [Eu/Fe], [Ba/Y]. Qualitatively, this type of chemical abundance trends are observed in massive dwarf galaxies, such as Sagittarius and the Large Magellanic Cloud. This appears to suggest a larger contribution in the outer halo of stars formed in an environment with high initial star formation rate and already polluted by asymptotic giant branch stars with respect to inner halo samples.

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