The contribution of N-rich stars to the Galactic stellar halo using APOGEE red giants

Horta, Danny; Mackereth, J. Ted; Schiavon, Ricardo P.; Hasselquist, Sten; Bovy, Jo; Allende Prieto, Carlos; Beers, Timothy C.; Cunha, Katia; García-Hernández, D. A.; Kisku, Shobhit S.; Lane, Richard R.; Majewski, Steven R.; Mason, Andrew C.; Nataf, David M.; Roman-Lopes, Alexandre; Schultheis, Mathias
Bibliographical reference

Monthly Notices of the Royal Astronomical Society

Advertised on:
1
2021
Number of authors
16
IAC number of authors
2
Citations
32
Refereed citations
29
Description
The contribution of dissolved globular clusters (GCs) to the stellar content of the Galactic halo is a key constraint on models for GC formation and destruction, and the mass assembly history of the Milky Way. Earlier results from APOGEE pointed to a large contribution of destroyed GCs to the stellar content of the inner halo, by as much as 25 ${{\ \rm per\ cent}}$ , which is an order of magnitude larger than previous estimates for more distant regions of the halo. We set out to measure the ratio between nitrogen-rich (N-rich) and normal halo field stars, as a function of distance, by performing density modelling of halo field populations in APOGEE DR16. Our results show that at 1.5 kpc from the Galactic Centre, N-rich stars contribute a much higher 16.8 $^{+10.0}_{-7.0}\, {{\ \rm per\ cent}}$ fraction to the total stellar halo mass budget than the 2.7 $^{+1.0}_{-0.8}\, {{\ \rm per\ cent}}$ ratio contributed at 10 kpc. Under the assumption that N-rich stars are former GC members that now reside in the stellar halo field, and assuming the ratio between first and second population GC stars being 1:2, we estimate a total contribution from disrupted GC stars of the order of 27.5 $^{+15.4}_{-11.5}\, {{\ \rm per\ cent}}$ at r = 1.5 kpc and 4.2 $^{+1.5}_{-1.3}\, {{\ \rm per\ cent}}$ at r = 10 kpc. Furthermore, since our methodology requires fitting a density model to the stellar halo, we integrate such density within a spherical shell from 1.5 to 15 kpc in radius, and find a total stellar mass arising from dissolved and/or evaporated GCs of MGC,total = 9.6 $^{+4.0}_{-2.6}\, \times$ 107 M☉.
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