In disguise or out of reach: first clues about in-situ and accreted stars in the stellar halo of the Milky Way from Gaia DR2 (1805.02617v3)

Abstract: We investigate the nature of the double color-magnitude sequence observed in the Gaia DR2 HR diagram of stars with high transverse velocities. The stars in the reddest-color sequence are likely dominated by the dynamically-hot tail of the thick disk population. Information from Nissen & Schuster (2010) and from the APOGEE survey suggests that stars in the blue-color sequence have elemental abundance patterns that can be explained by this population having a relatively low star-formation efficiency during its formation. In dynamical and orbital spaces, such as the `Toomre diagram', the two sequences show a significant overlap, but with a tendency for stars on the blue-color sequence to dominate regions with no or retrograde rotation and high total orbital energy. In the plane defined by the maximal vertical excursion of the orbits versus their apocenters, stars of both sequences redistribute into discrete wedges. We conclude that stars which are typically assigned to the halo in the solar vicinity are actually both accreted stars lying along the blue sequence in the HR diagram, and the low velocity tail of the old Galactic disk, possibly dynamically heated by past accretion events. Our results imply that a halo population formed in situ and responsible for the early chemical enrichment prior to the formation of the thick disk is yet to be robustly identified, and that what has been defined as the stars of the in situ stellar halo of the Galaxy may be in fact fossil records of its last significant merger.

In Situ and Accreted Stars in the Stellar Halo of the Milky Way: Insights from Gaia Data Release 2

The paper "In Disguise or Out of Reach: First Clues About In Situ and Accreted Stars in the Stellar Halo of the Milky Way from Gaia DR2" by M. Haywood et al., provides an astute examination of the Milky Way's halo stellar populations using data from the second release of the Gaia mission. This paper focuses on the double color-magnitude sequences observed in the Hertzsprung-Russell (HR) diagram for stars with significant transverse velocities. The authors parse the duality as suggestive evidence of both in situ and accreted populations within the Milky Way's halo, challenging previously held perceptions about the galaxy's complex structure.

The investigation into these sequences reveals that red-color sequence stars are generally more metal-rich ([Fe/H] > -0.8) and are dominant in prograde orbits, ostensibly linked to the thick disk of the Milky Way. Conversely, stars populating the blue-color sequence are predominantly metal-poor ([Fe/H] < -1.1) and occupy both prograde and retrograde orbits. The presence of distinctly contrived metallicity distributions, where significant transitional gaps are observed around [Fe/H] ∼ -1.0, further delineates these sequences.

A significant insight within this analysis relates to the kinematics and orbital dynamics of these stars, particularly elucidated through the Toomre diagram. The finding of a high proportion of retrograde orbits within the blue sequence suggests an accreted origin, likely the remnants of merger events. This result aligns with earlier suggestions by Belokurov et al. (2018) concerning the potential accretion history contribution to the halo's formation, illuminating a phase during which the Galactic disk may have been dynamically agitated by substantial accretion episodes.

Furthermore, chemical abundance studies using data cross-matched from APOGEE indicate that high [Fe/H] stars belonging to the blue sequence are less alpha-enhanced compared to disk stars at equivalent metallicities, underscoring the accreted nature of these objects. The correlation of low and high-alpha stars merging into singular abundance sequences bolsters this narrative and suggests prolonged star formation under differing conditions than those prevalent in the Milky Way's inception.

The implications of these findings extend deeply into our understanding of Galactic evolution. This paper posits that what has been broadly categorized as in situ halo stars might actually embody the fossilized remnants of the Galaxy's most recent significant merger. Such a perspective challenges the traditional vision of the Milky Way and implies that refined models of Galactic formation and evolution need to accommodate these nuanced populations.

Future developments in this arena will benefit from more elaborate observational campaigns and theoretical models. Enhanced spectroscopic surveys coupled with advanced simulations can potentially validate the proposed scenarios of galaxy-mass growth through hierarchical accretion. Additionally, extended studies into further astrometric releases from Gaia should similarly refine the precision with which we delineate these separate stellar populations and interpret their historical trajectories.

Overall, this paper enriches the discourse on Milky Way dynamics by proposing the dominance of accreted stars in what has traditionally been considered a homogeneously in situ stellar halo, pushing the boundaries of our understanding of one of the most fundamental components of our Galaxy's structure.