VERY METAL-POOR OUTER-HALO STARS WITH ROUND ORBITS

Journal Article · Sun Jan 20 04:00:00 EST 2013 · Astrophysical Journal Letters

DOI:https://doi.org/10.1088/2041-8205/763/1/L17· OSTI ID:22078319

Hattori, Kohei; Yoshii, Yuzuru ^[1]; Beers, Timothy C ^[2]; Carollo, Daniela ^[3]

---

1. Institute of Astronomy, School of Science, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015 (Japan)
2. National Optical Astronomy Observatories, Tucson, AZ 85719 (United States)
3. Department of Physics and Astronomy, Macquarie University, Sydney, 2109 NSW (Australia)

The orbital motions of halo stars in the Milky Way reflect the orbital motions of the progenitor systems in which they formed, making it possible to trace the mass-assembly history of the Galaxy. Direct measurement of three-dimensional velocities, based on accurate proper motions and line-of-sight velocities, has revealed that the majority of halo stars in the inner-halo region move in eccentric orbits. However, our understanding of the motions of distant, in situ halo-star samples is still limited, due to the lack of accurate proper motions for these stars. Here we explore a model-independent analysis of the line-of-sight velocities and spatial distribution of a recent sample of 1865 carefully selected halo blue horizontal-branch (BHB) stars within 30 kpc of the Galactic center. We find that the mean rotational velocity of the very metal-poor ([Fe/H] < -2.0) BHB stars significantly lags behind that of the relatively more metal-rich ([Fe/H] > -2.0) BHB stars. We also find that the relatively more metal-rich BHB stars are dominated by stars with eccentric orbits, as previously observed for other stellar samples in the inner-halo region. By contrast, the very metal-poor BHB stars are dominated by stars on rounder, lower-eccentricity orbits. Our results indicate that the motion of the progenitor systems of the Milky Way that contributed to the stellar populations found within 30 kpc correlates directly with their metal abundance, which may be related to their physical properties such as gas fractions. These results are consistent with the existence of an inner/outer halo structure for the halo system, as advocated by Carollo et al.

• OSTI ID: 22078319
• Journal Information: Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 1 Vol. 763; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABUNDANCE
GALACTIC EVOLUTION
METALS
MILKY WAY
ORBITS
PHYSICAL PROPERTIES
PROPER MOTION
SPATIAL DISTRIBUTION
STARS
THREE-DIMENSIONAL CALCULATIONS
VELOCITY