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A chemodynamical study of $r$-process-enhanced stars

Pallavi Saraf, Thirupathi Sivarani, Carlos Allende Prieto, Shashikiran Ganesh, Drisya Karinkuzhi

TL;DR

This work addresses the origin and environmental dependence of $r$-process enrichment by performing a chemodynamical analysis of 472 metal-poor, $r$-process-enhanced stars. By backward integrating orbits in a Milky Way potential and classifying stars into Galactic components via $r_a$ and $z_{max}$, the authors reveal a nearly balanced distribution of RPE stars between the disk and halo, with a subset clearly accreted (ex-situ) and a large mixed-origin population. They also show that traditional Toomre-diagram separation is inadequate for eccentric/retrograde orbits, and that inner disk, inner halo, and outer halo share similar $n$-capture element trends, while a subset of outer-disk stars traces a coherent, retrograde, highly eccentric event. These results constrain the environments and assembly history relevant to $r$-process production and highlight the need for kinematic context in chemical evolution studies of the Galaxy.

Abstract

The $r$-process enrichment in the Galaxy still remains elusive with regard to its nucleosynthesis conditions and the astrophysical sites where it occurs. As part of ongoing efforts to pinpoint the origin of chemically peculiar $r$-process-enhanced (RPE) stars, we concentrate in this study on the kinematics of RPE stars to investigate possible variations in the $r$-process enrichment among the Galactic components. We calculate the orbital parameters of a sample of 472 metal-poor RPE stars and associate them to the Galactic bulge, disk and halo populations using a physically motivated classification based on apocenter distance and maximum absolute vertical height of the orbit. We show that the Toomre diagram does not properly separate stars in the disk and halo components when they are on highly eccentric and/or retrograde orbits. The Galactic disk and halo share a similar fraction of RPE stars, in contrast to the earlier perception that the majority of RPE stars belong to the halo. We find that the stars most likely to be accreted belong to the halo. However, 3/4 of the stars lie in a mixed-zone. The inner disk, inner halo and outer halo stars exhibit similar abundance trends for the n-capture elements.

A chemodynamical study of $r$-process-enhanced stars

TL;DR

This work addresses the origin and environmental dependence of -process enrichment by performing a chemodynamical analysis of 472 metal-poor, -process-enhanced stars. By backward integrating orbits in a Milky Way potential and classifying stars into Galactic components via and , the authors reveal a nearly balanced distribution of RPE stars between the disk and halo, with a subset clearly accreted (ex-situ) and a large mixed-origin population. They also show that traditional Toomre-diagram separation is inadequate for eccentric/retrograde orbits, and that inner disk, inner halo, and outer halo share similar -capture element trends, while a subset of outer-disk stars traces a coherent, retrograde, highly eccentric event. These results constrain the environments and assembly history relevant to -process production and highlight the need for kinematic context in chemical evolution studies of the Galaxy.

Abstract

The -process enrichment in the Galaxy still remains elusive with regard to its nucleosynthesis conditions and the astrophysical sites where it occurs. As part of ongoing efforts to pinpoint the origin of chemically peculiar -process-enhanced (RPE) stars, we concentrate in this study on the kinematics of RPE stars to investigate possible variations in the -process enrichment among the Galactic components. We calculate the orbital parameters of a sample of 472 metal-poor RPE stars and associate them to the Galactic bulge, disk and halo populations using a physically motivated classification based on apocenter distance and maximum absolute vertical height of the orbit. We show that the Toomre diagram does not properly separate stars in the disk and halo components when they are on highly eccentric and/or retrograde orbits. The Galactic disk and halo share a similar fraction of RPE stars, in contrast to the earlier perception that the majority of RPE stars belong to the halo. We find that the stars most likely to be accreted belong to the halo. However, 3/4 of the stars lie in a mixed-zone. The inner disk, inner halo and outer halo stars exhibit similar abundance trends for the n-capture elements.

Paper Structure

This paper contains 12 sections, 1 equation, 8 figures.

Table of Contents

  1. Introduction
  2. Sample, Orbits, and the Galactic components
  3. Sample selection
  4. Orbit calculation
  5. Galactic bulge, disk, and halo
  6. Results
  7. Orbit based association of RPE stars to Galactic components
  8. Galactic components in the Toomre diagram
  9. In-situ and ex-situ origin of RPE stars
  10. R-process enrichment of Galactic components
  11. Conclusions
  12. Example orbits of disk and halo stars

Figures (8)

  • Figure 1: Bulge, disk, and halo classification: The apocenter distance (r$_{a}$) as a function of maximum vertical height (z$_{max}$) of the stellar orbits. For better visibility, we have set the axes scale to log-scale. The vertical black line at z$_{max}=3$ kpc and horizontal black lines at r$_{a}=3$ kpc and r$_{a}=15$ kpc separate RPE stars into five different Galactic components namely, bulge, inner disk, inner halo, outer disk, and outer halo. Open cyan circles show $r$-I stars, $r$-II stars are represented by magenta crosses, and limited-$r$ stars are displayed with yellow down triangles.
  • Figure 2: Top panel: Toomre diagram for RPE stars, where the large circle with radius 180 km s$^{-1}$ around LSR is used to separate the disk stars from halo stars and the black vertical line separates the stars on retrograde and prograde motions. Open cyan circles show $r$-I stars, $r$-II stars are represented by magenta crosses, and limited-$r$ stars are displayed with yellow down triangles. Bottom panel: Toomre diagram for $r$-process-enhanced stars color-coded with different galactic components as defined in Section \ref{['sec:bulge_disk_halo']}. Colors of different Galactic components are shown in the legend.
  • Figure 3: Orbital eccentricity of $r$-process-enhanced stars as a function of azimuthal velocity. Stars are color-coded with different galactic components as defined in Section \ref{['sec:bulge_disk_halo']}. Colors of different Galactic components are shown in the legend at the top of the figure.
  • Figure 4: This is the specific angular momentum diagram for RPE stars, where the vertical black dotted line separates the stars with retrograde and prograde motions. The vertical red line at $J_{p}=1000$ kpc km s$^{-1}$, and horizontal red line at $J_{\perp}=1300$ kpc km s$^{-1}$ separate in-situ, ex-situ and mixed stars. Open cyan circles show $r$-I stars, $r$-II stars are represented by magenta crosses, and limited-$r$ stars are displayed with yellow down triangles.
  • Figure 5: Metallicity ([Fe/H]) distribution of RPE stars in different Galactic components.
  • ...and 3 more figures