Frequencies, chaos, and resonances: A study of orbital parameters of nearby thick-disc and halo stars

ABSTRACT Although the stellar halo accounts for just ∼1 per cent of the total stellar mass of the Milky Way, the kinematics of halo stars encode valuable information about the origins and evolution of our Galaxy. It has been shown that the high transverse velocity stars in Gaia DR2 reveal a double sequence in the Hertzsprung–Russell (HR) diagram, indicating a bifurcation in the local stellar halo within 1 kpc. We fit these stars by updating the popular Besançon/Galaxia model, incorporating the latest observational results for the stellar halo and an improved kinematic description for the thick disc from Schönrich & Binney (2012). We are able to obtain a good match to the Gaia data and provide new constraints on the properties of the Galactic disc and stellar halo. In particular, we show that the kinematically defined thick-disc contribution to this high velocity tail is $\approx 13{{\ \rm per\ cent}}$. We look in greater detail using chemistry from LAMOST DR5, identifying a population of retrograde stars with thick-disc chemistry. Our thick-disc kinematic model cannot account for this population and so we conclude there is likely to be a contribution from heated or accreted stars in the Solar Neighbourhood. We also investigate proposed dynamical substructures in this sample, concluding that they are probably due to resonant orbits rather than accreted populations. Finally, we provide new insights on the nature of the two sequences and their relation with past accretion events and the primordial Galactic disc.

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CN Anomalies in the Halo System

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314005778 ◽

2012 ◽

Vol 10 (H16) ◽

pp. 282-283

Author(s):

Daniela Carollo

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Strong Field ◽

Significant Source ◽

Red Giants ◽

Orbital Parameters ◽

Halo Stars ◽

Halo Population ◽

Absorption Features ◽

Kinematic Properties

AbstractI present an evaluation of the kinematic properties of halo red giants thought to have formed in globular clusters based on the strength of their UV/blue CN and CH absorption features. The sample has been selected from the catalog of Martell et al. (2011). The orbital parameters of CN-strong halo stars are compared to those of the inner and outer halo populations, and to the orbital parameters of globular clusters with well-studied Galactic orbits. It has been found that both the clusters and the CN-strong field stars exhibit kinematic and orbital properties similar to the inner halo population, indicating that globular clusters could be a significant source of inner halo field stars, and suggesting that both globular clusters and CN-strong stars could belong primarily to the inner halo population of the Milky Way.

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

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834929 ◽

2019 ◽

Vol 632 ◽

pp. A4 ◽

Cited By ~ 32

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.

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Ages and kinematics of chemically selected, accreted Milky Way halo stars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3537 ◽

2020 ◽

Vol 493 (4) ◽

pp. 5195-5207 ◽

Cited By ~ 4

Author(s):

Payel Das ◽

Keith Hawkins ◽

Paula Jofré

Keyword(s):

Total Mass ◽

Milky Way ◽

Gaussian Mixture ◽

Contamination Rate ◽

Chemical Abundance ◽

Orbital Parameters ◽

Halo Stars ◽

Systematic Uncertainties ◽

Data Release ◽

Cross Matching

ABSTRACT We exploit the [Mg/Mn]-[Al/Fe] chemical abundance plane to help identify nearby halo stars in the 14th data release from the APOGEE survey that have been accreted on to the Milky Way. Applying a Gaussian Mixture Model, we find a ‘blob’ of 856 likely accreted stars, with a low disc contamination rate of ∼7 per cent. Cross-matching the sample with the second data release from Gaia gives us access to parallaxes and apparent magnitudes, which place constraints on distances and intrinsic luminosities. Using a Bayesian isochrone pipeline, this enables us to estimate new ages for the accreted stars, with typical uncertainties of ∼20 per cent. This does not account for systematic uncertainties. Our new catalogue is further supplemented with estimates of orbital parameters. The blob stars span [Fe/H] between −2.5 to −0.5, and [Mg/Fe] between −0.1 to 0.5. They constitute ∼30 per cent of the metal-poor ([Fe/H] < −0.8) halo at [Fe/H] ∼ −1.4. Our new ages mainly range between 8 to 13 Gyr, with the oldest stars the metal-poorest, and with the highest [Mg/Fe] abundance. If the blob stars are assumed to belong to a single progenitor, the ages imply that star formation lasted 5 Gyr after which the system merged with our Milky Way around 8 Gyr ago. Dynamical arguments suggest that such a single progenitor would have had a total mass of $\sim 10^{11}\, \mathrm{M}_{\odot }$, similar to that found by other authors using chemical evolution models and simulations.

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Dynamical models and Galaxy surveys

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313006297 ◽

2013 ◽

Vol 9 (S298) ◽

pp. 117-129 ◽

Cited By ~ 3

Author(s):

James Binney ◽

Jason L. Sanders

Keyword(s):

Functional Form ◽

Distribution Functions ◽

Selection Function ◽

Dynamical Models ◽

Galaxy Surveys ◽

Thick Disc ◽

Halo Stars ◽

Solid Dynamics

AbstractEquilibrium dynamical models are essential tools for extracting science from surveys of our Galaxy. We show how models can be tested with data from a survey before the survey's selection function has been determined. We illustrate the application of this method by presenting some results for the RAVE survey. We extend our published analytic distribution functions to include chemistry and fit the chosen functional form to a combination of the Geneva–Copenhagen survey (GCS) and a sample of G-dwarfs observed atz~ 1.75 kpc by the SEGUE survey. By including solid dynamics we are able to predict the contribution that the thick disc/halo stars surveyed by SEGUE should make to the GCS survey. We show that the measured [Fe/H] distribution from the GCS includes many fewer stars at [Fe/H] < −0.6 than are predicted. The problem is more likely to lie in discordant abundance scales than with incorrect dynamics.

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The high transverse velocity stars in Gaia-LAMOST

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319008603 ◽

2019 ◽

Vol 14 (S353) ◽

pp. 59-60

Author(s):

João A. S. Amarante ◽

Martin C. Smith ◽

Corrado Boeche

Keyword(s):

High Velocity ◽

Milky Way ◽

Transverse Velocity ◽

Double Sequence ◽

Good Match ◽

Thick Disc ◽

Halo Stars ◽

Stellar Halo ◽

Gaia Dr2 ◽

Hr Diagram

AbstractAlthough the stellar halo accounts for just ∼1% of the total stellar mass of the Milky Way, the kinematics of halo stars can tell us a lot about the origins and evolution of our Galaxy. It has been shown that the high transverse velocity stars in Gaia DR2 reveal a double sequence in the Hertzsprung-Russell (HR) diagram, indicating a duality in the local halo within 1 kpc. We fit these stars by updating the popular Besançon/Galaxia model, incorporating the latest observational results for the stellar halo. We are able to obtain a good match to the Gaia data and provide new constraints on the properties of the disc and halo. In particular, we show that the thick disc contribution to this high velocity tail is small, but not negligible, and likely has an influence on the red sequence of the HR diagram.

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Chemical Signature of Gas-rich disc-disc Mergers at high Redshift

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311022885 ◽

2010 ◽

Vol 6 (S277) ◽

pp. 250-254

Author(s):

Hugo Martel ◽

Simon Richard ◽

Chris B. Brook ◽

Daisuke Kawata ◽

Brad K. Gibson ◽

...

Keyword(s):

Numerical Simulations ◽

Velocity Dispersion ◽

High Redshift ◽

Late Type ◽

Orbital Parameters ◽

Chemical Signature ◽

Thick Disc ◽

Disc Galaxies

AbstractWe performed numerical simulations of mergers between gas-rich disc galaxies, which result in the formation of late-type galaxies. Stars formed during the merger end up in a thick disc that is partially supported by velocity dispersion and has high [α/Fe] ratios at all metallicities. Stars formed later end up in a thin, rotationally supported disc which has lower [α/Fe] ratios. While the structural and kinematical properties of the merger remnants depend strongly upon the orbital parameters of the mergers, we find a clear chemical signature of gas-rich mergers.

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Lifetime of short-period binaries measured from their Galactic kinematics

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa400 ◽

2020 ◽

Vol 493 (2) ◽

pp. 2271-2286 ◽

Cited By ~ 3

Author(s):

Hsiang-Chih Hwang ◽

Nadia L Zakamska

Keyword(s):

Main Sequence ◽

Tangential Velocity ◽

Galactic Kinematics ◽

Wide Field ◽

Galactic Latitude ◽

Eclipsing Binary ◽

Multiple Systems ◽

Thick Disc ◽

Halo Stars ◽

Short Period

ABSTRACT As a significant fraction of stars are in multiple systems, binaries play a crucial role in stellar evolution. Among short-period (<1 d) binary characteristics, age remains one of the most difficult to measure. In this paper, we constrain the lifetime of short-period binaries through their kinematics. With the kinematic information from Gaia Data Release 2 and light curves from Wide-field Infrared Survey Explorer (WISE), we investigate the eclipsing binary fraction as a function of kinematics for a volume-limited main-sequence sample. We find that the eclipsing binary fraction peaks at a tangential velocity of 101.3−1.6 km s−1, and decreases towards both low- and high-velocity end. This implies that thick disc and halo stars have eclipsing binary fraction ≳10 times smaller than the thin-disc stars. This is further supported by the dependence of eclipsing binary fraction on the Galactic latitude. Using Galactic models, we show that our results are inconsistent with any known dependence of binary fraction on metallicity. Instead, our best-fitting models suggest that the formation of these short-period binaries is delayed by 0.6–3 Gyr, and the disappearing time is less than the age of the thick disc. The delayed formation time of ≳0.6 Gyr implies that these short-period main-sequence binaries cannot be formed by pre-main sequence interaction and the Kozai–Lidov mechanism alone, and suggests that magnetic braking plays a key role in their formation. Because the main-sequence lifetime of our sample is longer than 14 Gyr, if the disappearance of short-period binaries in the old population is due to their finite lifetime, our results imply that most (≳90 per cent) short-period binaries in our sample merge during their main-sequence stage.

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Distribution of R- and S-Process Elements in the Galactic Halo

Symposium - International Astronomical Union ◽

10.1017/s0074180900035555 ◽

1988 ◽

Vol 132 ◽

pp. 501-506

Author(s):

C. Sneden ◽

C. A. Pilachowski ◽

K. K. Gilroy ◽

J. J. Cowan

Keyword(s):

Heavy Element ◽

Galactic Halo ◽

Low Noise ◽

Heavy Elements ◽

Process Synthesis ◽

Element Abundances ◽

Halo Stars ◽

Abundance Patterns ◽

R Process ◽

Process Elements

Current observational results for the abundances of the very heavy elements (Z>30) in Population II halo stars are reviewed. New high resolution, low noise spectra of many of these extremely metal-poor stars reveal general consistency in their overall abundance patterns. Below Galactic metallicities of [Fe/H] Ã −2, all of the very heavy elements were manufactured almost exclusively in r-process synthesis events. However, there is considerable star-to-star scatter in the overall level of very heavy element abundances, indicating the influence of local supernovas on element production in the very early, unmixed Galactic halo. The s-process appears to contribute substantially to stellar abundances only in stars more metal-rich than [Fe/H] Ã −2.

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