Inconsistency of Chemical Properties of Stellar Populations in the Thick Disk Subsystem of our Galaxy

Abstract

Using modern published data on velocities and spectroscopic definitions of chemical elements in stellar objects of the Galaxy, we investigated the relationship of chemical composition with the kinematics of different populations. The paper shows that the old stellar populations of the Galaxy, belonging (by the kinematic criterion) to the thick disk subsystem-globular clusters, field variables of the type RR Lyrae (lyrids), as well as close F-G dwarfs and field giants, have different chemical composition. In particular, the dwarfs and giants of the field are on average more metallic than the globular clusters and lyrids of the field. Moreover, the relative abundances of α-elements in the range [Fe/H] > −1.0 are the highest for globular clusters, and are the lowest for for field variables of the RR Lyrae type. Based on the analysis of the nature of the dependences of [α/Fe] on [Fe/H] for these objects it was suggested that the thick disk subsystem in the Galaxy is composite and at least three components exist independently within it. The oldest one includes metal-rich globular clusters that formed from a single proto-galactic cloud shortly after the start of type Ia supernovae outbursts. Then the subsystem of field stars of a thick disk was formed as a result of "heating" of stars of already formed thin disk of the Galaxy by a rather massive dwarf satellite galaxy that fell on it. And finally, subsystems of field stars with the kinematics of not only a thick, but even a thin disk that fell on the Galaxy from this captured satellite galaxy.

Physical and Chemical Properties of Galactic Global Clusters with Various Origins Identified from the Gaia DR2 Data

“…It is clearly seen that among genetically related clusters, half of the bulge clusters and a third of the disk clusters have low metallicities, which are characteristic of accreted clusters. This property of genetically related clusters confirms the conclusion of our study [7] that it is incorrect to distinguish globular clusters as belonging to the subsystem of the thick disk or halo by metallicity [Fe/H] = −1.0, as it is usually done (see references in [7]). Note that clusters of the low-energy group have the same metallicity range as the in-situ clusters, although the vast majority of them are metal-poor clusters.…”

“…We have also drawn the lower envelope "by eye" (as a gray broken line) as a reference for genetically related stars; the position of this line, as we can see, nearly coincides with the one proposed by the authors of [22]. As for the position of metal-rich ([Fe/H] > −1.0) globular clusters in the [Fe/H]-[α/Fe] diagrams, we described it in detail in [7].…”

“…Recall that in [5], [6], [7] we identified clusters as genetically related (i. e., born from the same protogalactic cloud) if they were in direct orbits and stayed within 15 kpc from the galactic center. In addition, these clusters should demonstrate the same residual velocities as the field stars of the Galaxy's disk subsystems (i. e., close to circular) and not be considered the remnants of known satellite galaxies disrupted at that time.…”

“…In [5], [6], [7] we calculated the probabilities of the clusters belonging to the subsystems of the thick disk and halo using the method described by Bensby et al [8] with the aid of ground-based measurements of their residual velocities. Only the clusters that belonged to the thick disk were considered to be genetically related to a single protogalactic cloud; as for the halo, we kept only the clusters in direct orbits within 15 kpc from the galactic center.…”

Peculiarities of Abundances of Selected Elements in Metal-Rich Field RR Lyrae Stars

“…Based on the above, it is conceivable that metal-rich field RR Lyraes must have formed from interstellar matter with the chemical evolution history different from that of the parent interstellar matter of most of the field stars in the solar vicinity. We suggested in our previous papers that such helium-rich stars could have come to the solar neighborhood as a result of radial migration from central regions of our Galaxy, where such stars have already been found (Marsakov et al, 2019a), or as a result of the capture of a massive companion dwarf galaxy during early stages of the evolution of our Galaxy (Marsakov et al, 2020).…”