Metal-poor stars towards the Galactic bulge: A population potpourri

Abstract

We present a comprehensive chemical abundance analysis of five red giants and two horizontal branch (HB) stars towards the southern edge of the Galactic bulge, at (l, b) ∼ (0 • ,−11 • ). Based on high-resolution spectroscopy obtained with the Magellan/MIKE spectrograph, we derived up to 23 chemical element abundances and identify a mixed bag of stars, representing various populations in the central regions of the Galaxy. Although cosmological simulations predict that the inner Galaxy was host to the first stars in the Universe, we see no chemical evidence of the ensuing massive supernova explosions: all of our targets exhibit halo-like, solar [Sc/Fe] ratios, which is in contrast to the low values predicted from Population III nucleosynthesis. One of the targets is a CEMP-s star at [Fe/H] = −2.52 dex, and another target is a moderately metal-poor ([Fe/H] = −1.53 dex) CH star with strong enrichment in s-process elements (e.g., [Ba/Fe] = 1.35). These individuals provide the first contenders of these classes of stars towards the bulge. Four of the carbon-normal stars exhibit abundance patterns reminiscent of halo star across a metallicity range spanning −2.0 to −2.6 dex, i.e., enhanced α-elements and solar Fe-peak and neutron-capture elements, and the remaining one is a regular metal-rich bulge giant. The position, distance, and radial velocity of one of the metal-poor HB stars coincides with simulations of the old trailing arm of the disrupted Sagittarius dwarf galaxy. While their highly uncertain proper motions prohibit a clear kinematic separation, the stars' chemical abundances and distances suggest that these metal-poor candidates, albeit located towards the bulge, are not of the bulge, but rather inner halo stars on orbits that make them pass through the central regions. Thus, we caution similar claims of detections of metal-poor stars as true habitants of the bulge.

Chemical abundances in a high-velocity RR Lyrae star near the bulge

“…This distinction is significant because the inside-out formation of the Galaxy dictates that the oldest and most (extremely) metal-poor stars should be found in the very center (Tumlinson 2010), rendering them inner halo stars (by formation and chemistry) that happen to be located within the bulge. While still low in numbers, the very metal-poor bulge stars known to date show chemical abundances that vastly overlap with those of the metal-poor halo distribution (e.g., Casey & Schlaufman 2015;Koch et al 2016;Howes et al 2015). Their kinematics then indicates that their orbits are mainly confined to the inner few kpc of the Galaxy (although the star we consider here is not confined to the innermost parts).…”

Four Metal-poor Stars in the Sagittarius Dwarf Spheroidal Galaxy∗

“…However, CEMP stars are not found in all environments. For instance, only a few s-process rich CEMP stars have been detected in the direction of the Milky Way bulge (Howes et al 2015;Koch et al 2016) and studies generally tend to find a lower fraction of CEMP stars in dSphs (Kirby et al 2015;Jablonka et al 2015;Chiti et al 2018), but a small number have been detected in the ultrafaint dwarf galaxies (Frebel & Norris 2015).…”

Apogee Kinematics. I. Overview of the Kinematics of the Galactic Bulge as Mapped by Apogee

“…These stars are likely part of the bar in the plane beyond the boxy extent of the bulge l > 10° ; these [ ]< −1.0 show a slower rotation and flatter dispersion that aligns them with a halo population in the innermost region or else a unique metalpoor bulge population formed at early times (see Howes et al 2015;Koch et al 2015).…”