Advanced

Characteristics of the Two Sequences Seen in the High-velocity Hertzsprung-Russell Diagram in Gaia DR2

Sahlholdt, Christian L. LU ; Casagrande, Luca and Feltzing, Sofia LU (2019) In Astrophysical Journal Letters 881(1).
Abstract

In this study we use a sample of about 9 million SkyMapper stars with metallicities to investigate the properties of the two stellar populations seen in the high-velocity (V T > 200 km s-1) Gaia DR2 Hertzsprung-Russell diagram. Based on 10,000 red giant branch (RGB) stars (out of 75,000 with high velocity), we find that the two sequences have different metallicity distribution functions; one peaks at -1.4 dex (blue sequence) and the other at -0.7 dex (red sequence). Isochrones with ages in the range 11-13.5 Gyr, and metallicities chosen to match the observations for each sequence, fit the turnoffs and broad RGBs well, indicating that the two populations formed at comparable times within the uncertainties. We... (More)

In this study we use a sample of about 9 million SkyMapper stars with metallicities to investigate the properties of the two stellar populations seen in the high-velocity (V T > 200 km s-1) Gaia DR2 Hertzsprung-Russell diagram. Based on 10,000 red giant branch (RGB) stars (out of 75,000 with high velocity), we find that the two sequences have different metallicity distribution functions; one peaks at -1.4 dex (blue sequence) and the other at -0.7 dex (red sequence). Isochrones with ages in the range 11-13.5 Gyr, and metallicities chosen to match the observations for each sequence, fit the turnoffs and broad RGBs well, indicating that the two populations formed at comparable times within the uncertainties. We find that the mean tangential velocity of disk stars increases steadily with decreasing metallicity, and that the red sequence is made up of the high-velocity stars at the lowest metallicities of the thick-disk population. Using relative number densities, we further find that the red-sequence stars are more centrally concentrated in the Galaxy, and we estimate the radial scale length of this population to be on the order of 2-3 kpc. The blue-sequence stars, on the other hand, follow a nearly flat radial density profile. These findings tighten the link between the red-sequence stars and the chemically defined thick disk.

(Less)
Please use this url to cite or link to this publication:
author
; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Astrophysical Journal Letters
volume
881
issue
1
article number
L10
pages
7 pages
publisher
IOP Publishing
external identifiers
  • scopus:85071145790
ISSN
2041-8205
DOI
10.3847/2041-8213/ab321e
project
Stellar ages in the era of Gaia and large spectroscopic surveys
language
English
LU publication?
yes
id
b258f1ac-cfb9-40a5-ba6d-334e5e201e0f
date added to LUP
2019-10-01 16:38:50
date last changed
2020-10-07 06:43:57
@article{b258f1ac-cfb9-40a5-ba6d-334e5e201e0f,
  abstract     = {<p>In this study we use a sample of about 9 million SkyMapper stars with metallicities to investigate the properties of the two stellar populations seen in the high-velocity (V <sub>T</sub> &gt; 200 km s<sup>-1</sup>) Gaia DR2 Hertzsprung-Russell diagram. Based on 10,000 red giant branch (RGB) stars (out of 75,000 with high velocity), we find that the two sequences have different metallicity distribution functions; one peaks at -1.4 dex (blue sequence) and the other at -0.7 dex (red sequence). Isochrones with ages in the range 11-13.5 Gyr, and metallicities chosen to match the observations for each sequence, fit the turnoffs and broad RGBs well, indicating that the two populations formed at comparable times within the uncertainties. We find that the mean tangential velocity of disk stars increases steadily with decreasing metallicity, and that the red sequence is made up of the high-velocity stars at the lowest metallicities of the thick-disk population. Using relative number densities, we further find that the red-sequence stars are more centrally concentrated in the Galaxy, and we estimate the radial scale length of this population to be on the order of 2-3 kpc. The blue-sequence stars, on the other hand, follow a nearly flat radial density profile. These findings tighten the link between the red-sequence stars and the chemically defined thick disk.</p>},
  author       = {Sahlholdt, Christian L. and Casagrande, Luca and Feltzing, Sofia},
  issn         = {2041-8205},
  language     = {eng},
  month        = {08},
  number       = {1},
  publisher    = {IOP Publishing},
  series       = {Astrophysical Journal Letters},
  title        = {Characteristics of the Two Sequences Seen in the High-velocity Hertzsprung-Russell Diagram in Gaia DR2},
  url          = {http://dx.doi.org/10.3847/2041-8213/ab321e},
  doi          = {10.3847/2041-8213/ab321e},
  volume       = {881},
  year         = {2019},
}