A Tale of Two Origins: In-Situ versus Accreted Nitrogen-Rich Field Stars in the MW

Abstract

Spectroscopic surveys have identified significant numbers of metal-poor nitrogen-rich (N-rich) field stars. These stars are strong candidates for escapees from globular clusters (GCs), as their distinctive nitrogen enhancement mirrors the chemical patterns observed in some of the members of GCs. As part of the effort to characterize their chemodynamical properties, we derived abundances for up to 25 elements in a sample of 33 N-rich field giant stars (18 of them are studied for the first time) using high-resolution optical spectroscopy. We confirm their elevated abundances of N, Na, and Al, strongly supporting a GC origin. Given that Galactic GCs themselves formed within diverse progenitor galaxies, we sought to identify the ancestral systems of these N-rich field stars. By analyzing their dynamical parameters, we separated the sample into high-energy (HE) and low-energy (LE) groups. The HE group exhibits lower [α/Fe] and enhanced r-process abundances compared to the LE group. This indicates that the HE stars likely escaped from GCs accreted from massive dwarf galaxies (e.g., Gaia-Sausage-Enceladus), while the LE stars probably originated from in-situ GCs. We also find that the chemical pattern of these N-rich stars with [Fe/H] {\lessapprox} -1.0 are similar to the high-redshift ''N-emitters''. Furthermore, orbital integrations revealed a close encounter between one N-rich field star and the globular cluster NGC 6235. Our work demonstrates the potential of using chemodynamical analyses to trace Galactic assembly through chemical peculiar stars, while highlighting that larger samples and more precise data in the future are crucial to establish definitive origins.

Figures (10)

• Figure 1: The [N/Fe] versus [C/Fe] plane. Yellow diamonds denote our observed N-rich field stars, while violet diamonds represent 10 GALAH/APOGEE N-rich field stars. The black dots indicate metal-poor RGB halo stars with similar stellar parameters as these N-rich field stars, and they are selected from the APOGEE survey. Typical abundance errors are shown in the corners of abundance panels.
• Figure 2: Panel a: The [Na/Fe] - [O/Fe] plane. Panel b: The [Mg/Fe] - [Al/Fe] plane. Markers of N-rich field stars are same as Fig.\ref{['figcn']}. The Na and O abundances of individual stars in the GCs from 2009AA...505..117C are over-plotted for comparison. Orange and blue symbols correspond to GC first generation (FG) and second generation (SG) stars respectively. Upper limits in O and Al abundances are shown as triangles, while detections are shown as dots. The solid and dashed contours indicate the number densities of FG and SG, respectively. The field stars from ramirez-2012 and NS10 shown as small cyan circles. The magenta dots correspond to MW halo stars halo-fulbright-2000AJ....120.1841Fhalo-cayrel-2004AA...416.1117Chalo-barklem-2005AA...439..129Bhalo-yong-2013ApJ...762...26Yhalo-Roederer-2014AJ....147..136R and black dots are disk stars disk-reddy-2003MNRAS.340..304Rdisk-reddy-2006MNRAS.367.1329Rdisk-bensby-2014AA...562A..71B.
• Figure 3: The [$\alpha$/Fe] – [Fe/H] planes, where $\alpha$ elements include Mg, Si, Ca, and Ti. Violin shaped symbols indicate GC stars from APOGEE Meszaros2020, including the average [Fe/H], maximum and minimum [$\alpha$/Fe] of each GC. In-situ GCs are colored light green, while accreted GCs are light red. Orange circles correspond to Sagittarius sgr_hasselquist_2017sgr-hayes-2020ApJ...889...63H. Blue circles represent stars from Sculptor scl_hill2019. Other symbols are the same as Fig.\ref{['fignao']}.
• Figure 4: The [Cu/Fe] and [Zn/Fe] vs. [Fe/H] planes. Blue circles correspond to stars from dwarf galaxies (Sagittarius, Sculptor, Fornax, Carina and Leo I) sgr_hasselquist_2017scl_hill2019DGs_Shetrone2003. Other symbols are the same as Fig.\ref{['figalpha']}.
• Figure 5: Panel a: The diagram of E vs$L_Z$ of N-rich stars (circles) in two groups and GALAH MW stars (gray dots). HE stars are colored lime and LE stars are dark violet, and the dark orange line shows the boundary between them. See text for more details about their classification. Panel b: The stars' distribution in orbital inclination and eccentricity.