Disentangling the Arcturus stream

Abstract

Context. The Arcturus stream is an over-density of stars in velocity space and its origin has been much debated recently without any clear conclusion. The (classical) dissolved open cluster origin is essentially refuted, instead the discussions try to distinguish between an accretion, a resonant, or an external-perturbation origin for the stream. As kinematic structures are observational footprints of ongoing and past dynamical processes in disk galaxies, resolving the nature of the Arcturus stream can provide clues to the formation history of the Milky Way and its stellar populations. Aims. We aim to characterise the kinematical and chemical properties of the Arcturus stream in order to resolve its origin. Methods. The space velocities, angular momenta and actions for a sample of more than 5.8 million stars, composed from Gaia DR2, are analysed with a wavelet transform method to characterise kinematic over-densities in the Galactic disk. The kinematic characteristics of each identified group is used to select possible members of the groups from the GALAH and APOGEE spectroscopic surveys to further study and constrain their chemical properties. Results. In the velocity and angular momentum spaces the already known Sirius, Pleiades, Hyades, Hercules, AF06, Arcturus and KFR08 streams are clearly identified. The Hercules stream appears to be a mixture of thin and thick disk stars. The Arcturus stream, as well as the AF06 and KFR08 streams, are low-velocity and low-angular momentum structures with chemical compositions similar to the thick disk. These three groups extend further from the Galactic plane compared to the Hercules stream. The detections of all the groups were spaced by approximately 20 − 30 km s −1 in azimuthal velocity. Conclusions. A wide spread of chemical abundances within the Arcturus stream indicates that the group is not a dissolved open cluster. Instead the Arcturus stream, together with the AF06 and KFR08 streams, are more likely to be part of a phase-space wave, that could have been caused by an ancient merger event. This conclusion is based on that the different structures are detected in steps of 20 − 30 km s −1 in azimuthal velocity, that the kinematic and chemical features are different from what is expected for bar-originated structures, and that the lower-velocity streams extend further from the disk than bar-originated structures.

Bar resonances and low angular momentum moving groups in the Galaxy revealed by their stellar ages

“…Although Arcturus has been argued in the past to be a substructure associated with an extra-galactic orign (N04), this interpretation was challenged in Minchev et al (2009), who proposed an alternative interpretation as the response of the disc to a perturber. This was given further credence with Gaia DR2 in Laporte et al (2019), and also corroborated by Kushniruk & Bensby (2019). Here, we provide the final argument in favour of an internal origin through a re-analysis of the samples used in N04 complemented with APOGEE data (Abolfathi et al 2018).…”

Chemo-kinematics of the Milky Way spiral arms and bar resonances: connection to ridges and moving groups in the Solar vicinity

“…Metallicity variations of stars associated with the known v Rv φ moving groups have been reported in a number of previous studies (see, e.g., Bensby et al 2007;Williams et al 2009;Bensby et al 2014;Zhao et al 2014;Kushniruk & Bensby 2019). Broadly speaking most of the v R -v φ -substructures do not show chemical homogeneity with the abundance patterns being almost indistinguishable from that of the background field stars of the Galactic disk (thick or thin, see, e.g., Nordström et al 2004;Bensby & Feltzing 2006;Ramya et al 2012;Kushniruk & Bensby 2019;Kushniruk et al 2020).…”

“…For instance, previously known vertical wave-like motions (Widrow et al 2012) were detected in z − v z space in the form of a snaillike structure (Antoja et al 2018). The precise measurements from Gaia also provided a more comprehensive view on the inplane phase-space structure of the MW (Gaia Collaboration et al 2018b;Kawata et al 2018;Antoja et al 2018), allowing a number of radially extended features and short kinematic arches to be identified in v R -v φ velocity space (Ramos et al 2018;Khanna et al 2019;Kushniruk & Bensby 2019). These complex stellar motions are not easily interpreted considering a single mechanism.…”

Tracing the Origin of Moving Groups. III. Detecting Moving Groups in The LAMOST DR7

“…It was early believed that they came from dissolving open clusters (e.g., HR1614; De Silva et al 2007). However, this interpretation is incompatible with most of known MGs for the inhomogeneity of age and chemistry within them (e.g., Famaey et al 2005;Ramya et al 2012;Kushniruk & Bensby 2019). Later, dynamical mechanism of internal resonances caused by the Galactic bar or spiral arms was proposed (e.g., Dehnen 2000;Quillen & Minchev 2005;Monari et al 2019).…”

“…Furthermore, MGs can be explained as relics of disrupted satellite galaxies or products of perturbations by external accretion events (Minchev et al 2009). For example, Navarro et al (2004) attributed the Arcturus group to remnants of a dwarf galaxy merged with the Milky Way since a tight sequence in the [Fe/H]−[α/Fe] plane was found, although this argument has been disproved using more unbiased data (Kushniruk & Bensby 2019).…”