[Paper Review] Vertical distribution of Galactic disk stars I - Kinematics and metallicity

C. Soubiran, O. Bienaymé+1 more|ArXiv.org|Oct 29, 2002

Stellar, planetary, and galactic studiesPhysics and Astronomy26 references162 citations

TL;DR

This study presents a high-resolution spectroscopic survey of nearly 400 red clump giants in the North Galactic Pole field, using ELODIE at Haute-Provence Observatory to derive precise kinematics, metallicities, and distances. It deconvolves the old thin and thick disk populations, finding the thick disk has a moderate rotational lag of $-51 \pm 5\ \mathrm{km\,s^{-1}}$, high velocity dispersions ($\sigma_U = 63 \pm 6$, $\sigma_V = 39 \pm 4$, $\sigma_W = 39 \pm 4\ \mathrm{km\,s^{-1}}$), a mean metallicity of [Fe/H] = $-0.48 \pm 0.05$, and a local normalization of 15% $\pm$ 7%, indicating a higher thick disk fraction than previously thought.

ABSTRACT

Nearly 400 Tycho-2 stars have been observed in a 720 square degree field in the direction of the North Galactic Pole with the high resolution echelle spectrograph ELODIE. Absolute magnitudes, effective temperatures, gravities and metallicities have been estimated, as well as distances and 3D velocities. Most of these stars are clump giants and span typical distances from 200pc to 800pc to the galactic mid-plane. This new sample, free of any kinematical and metallicity bias, is used to investigate the vertical distribution of disk stars. The old thin disk and thick disk populations are deconvolved from the velocity-metallicity distribution of the sample and their parameters are determined. The thick disk is found to have a moderate rotational lag of -51+-5 km/s with respect to the Sun with velocity ellipsoid (sigma_U, sigma_V, sigma_W)=(63+-6, 39+-4, 39+-4) km/s, mean metallicity of [Fe/H]=-0.48+-0.05 and a high local normalization of 15+-7%. Combining this NGP sample with a local sample of giant stars from the Hipparcos catalogue, the orientation of the velocity ellipsoid is investigated as a function of distance to the plane and metallicity. We find no vertex deviation for old stars, consistent with an axisymmetric Galaxy. Paper II is devoted to the dynamical analysis of the sample, puting new constraints on the vertical force perpendicular to the galactic plane and on the total mass density in the galactic plane.

Motivation & Objective

• To obtain an unbiased, high-precision sample of disk stars extending up to 800 pc from the galactic plane to study their kinematics and metallicity distribution.
• To deconvolve the old thin disk and thick disk populations from the velocity-metallicity distribution using a non-informative mixture model.
• To determine the kinematic parameters, metallicity, and relative normalization of the thick disk with improved accuracy compared to previous studies.
• To investigate the orientation of the velocity ellipsoid as a function of height above the plane and metallicity, testing for axisymmetry in the galactic disk.
• To provide a foundation for Paper II’s dynamical analysis, including vertical force and mass density constraints in the galactic plane.

Proposed method

• High-resolution echelle spectroscopy with the ELODIE instrument on a 720 square degree field toward the North Galactic Pole to obtain radial velocities, effective temperatures, gravities, and metallicities.
• Use of the TGMET software to estimate atmospheric parameters and absolute magnitudes by cross-correlating observed spectra with a reference library of empirical spectra.
• Distance determination via absolute magnitude estimation, achieving a typical accuracy of 18% for stars at 400 pc.
• Application of a non-informative Gaussian mixture model to separate the thin and thick disk populations based on their kinematic and metallicity distributions.
• Combination of the NGP sample with a local Hipparcos sample of giants to study the vertex deviation of the velocity ellipsoid as a function of metallicity and distance from the plane.
• Comparison of observed distributions with simulations from the Besançon galactic model to validate the derived disk parameters.

Experimental results

Research questions

• RQ1 What are the precise kinematic and metallicity parameters of the thick disk population in the solar neighborhood, as derived from a homogeneous, unbiased sample of red clump giants?
• RQ2 How does the velocity ellipsoid orientation vary with distance above the galactic plane and metallicity, and what does this imply for the axisymmetry of the Milky Way disk?
• RQ3 What is the local normalization of the thick disk relative to the thin disk, and how does it compare to previous estimates?
• RQ4 Can the observed kinematic and metallicity distribution be better explained by a moderate thick disk model or a more extreme one, and how does this relate to formation theories?
• RQ5 To what extent do color and magnitude cuts effectively isolate red clump giants, and what contamination from dwarfs or subgiants remains?

Key findings

• The thick disk exhibits a rotational lag of $-51 \pm 5\ \mathrm{km\,s^{-1}}$ with respect to the Sun, indicating it rotates more slowly than the local standard of rest.
• The velocity dispersion tensor for the thick disk is $\sigma_U = 63 \pm 6$, $\sigma_V = 39 \pm 4$, $\sigma_W = 39 \pm 4\ \mathrm{km\,s^{-1}}$, indicating a kinematically hot population.
• The mean metallicity of the thick disk is [Fe/H] = $-0.48 \pm 0.05$, placing it on the metal-rich end of previous estimates.
• The local normalization of the thick disk is found to be 15% $\pm$ 7%, significantly higher than the typical 2–7% assumed in earlier studies.
• No significant vertical gradient is observed in the parameters of the thin disk, indicating its kinematic properties are consistent across the sampled height range.
• The vertex deviation of the velocity ellipsoid is null for the low-metallicity (old) population, supporting the hypothesis of an axisymmetric galactic disk, while the high-metallicity sample shows a small positive deviation.