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Disentangling the Arcturus Stream Iryna Kushniruk and Thomas Bensby

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Disentangling the Arcturus Stream Iryna Kushniruk and Thomas Bensby A&A 631, A47 (2019) Astronomy https://doi.org/10.1051/0004-6361/201935234 & c ESO 2019 Astrophysics Disentangling the Arcturus stream Iryna Kushniruk and Thomas Bensby Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden e-mail: [email protected], [email protected] Received 8 February 2019 / Accepted 9 September 2019 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 disc galaxies, resolving the nature of the Arcturus stream may 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 were analysed with a wavelet transform method to characterise kinematic over-densities in the Galactic disc. 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 disc stars. The Arcturus stream, as well as the AF06 and KFR08 streams, are high-velocity and low-angular momentum structures with chemical compositions similar to the thick disc. 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 a 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 higher-velocity streams extend further from the disc than bar-originated structures. Key words. stars: kinematics and dynamics – Galaxy: formation – galaxies: kinematics and dynamics – galaxies: evolution 1. Introduction merger history of the Milky Way (e.g. Navarro et al. 2004; Helmi et al. 2006, 2017, 2018; Koppelman et al. 2018). The analysis of The process by which large spiral galaxies form and evolve into theGaiaDR2(GaiaCollaboration2018a,b)revealedthatthekine- the complicated structures that are observed today is an active matic over-densities are a part of a much more complicated struc- area of research, and presents many challenges, both theoreti- ture that is seen as arches and ridges across velocity space and as cally and observationally. As the Milky Way is the only galaxy clumps in action space (Trick et al. 2019). This structure is pos- where stars and structures can be studied in great detail, it serves sibly caused by spiral arms (Quillen et al. 2018) or is a result of as a benchmark galaxy when constraining models of galaxy for- the phase-mixing due to a past merger event (e.g. Katz et al. 2019; mation. It is therefore of utmost importance to obtain a detailed Antoja et al. 2018; Ramos et al. 2018; Laporte et al. 2019), as first map of the Milky Way looks like, and to decipher where the proposed by Minchev et al.(2009). observed stellar populations and structures come from. Cur- This structure is possibly caused by spiral arms or is a result rently, the Milky Way contains a plethora of structures, both of phase-mixing due to a past merger event (e.g. Antoja et al. physical and kinematic, whose nature and origins are unclear. 2018; Ramos et al. 2018; Quillen et al. 2018). As these stud- Many studies have shown that the velocity distribution of stars ies have shown, learning more about the nature of kinematic in the Milky Way disc is clumpy (e.g. Dehnen 1998; Skuljan et al. structures can improve our understanding of the evolution of the 1999; Famaey et al. 2005; Antoja et al. 2008, 2012; Kushniruk Milky Way. In this paper we investigate the properties and origin et al. 2017; Ramos et al. 2018). The kinematic and chemical prop- of the Arcturus stream. erties of such structures can be used to constrain the properties A set of about 50 stars, including the star Arcturus (α Boo- and the formation history of the Milky Way. For example, the Her- tis), was discovered by Eggen(1971) to have a very similar V cules stream has been widely used to probe the pattern speed and space velocity component of V ' −100 km s−1. Eggen(1971) the length of the Galactic bar (e.g. Dehnen 2000; Minchev et al. proposed that this over-density in velocity space is composed 2007; Antoja et al. 2014; Wegg et al. 2015; Pérez-Villegas et al. of stars that escaped from an open cluster and it was therefore 2017). Kinematic structures can be used to study the spiral struc- named the Arcturus moving group. Nowadays the hypothesis ture of the Milky Way (e.g. Quillen & Minchev 2005; Chakrabarty of the Arcturus over-density being a moving group is refuted 2007; Sellwood et al. 2019; Quillen et al. 2018). Studies of kine- as there is no chemical homogeneity within the group (e.g. matic streams especially in the Galactic halo can tell us about the Williams et al. 2009; Ramya et al. 2012; Bensby et al. 2014), Article published by EDP Sciences A47, page 1 of 18 A&A 631, A47 (2019) which there should be if the stars originate from the same open estimate goodness of astrometric fits. Selecting those targets cluster (e.g. De Silva et al. 2007; Bovy 2016). We have there- with RUWE < 1:4 leaves us with 6 692 285 targets. Photometric fore chosen to adopt the “stream” nomenclature when referring filtering that rids the sample of stars with poor astrometric solu- to this Arcturus over-density of stars in velocity space. tions (see Eq. (2) in Arenou et al.(2018)) was also applied. This Two other possible origins of the Arcturus stream are now cut leaves us with 6 683 408 stars. favoured and are widely discussed. The first is an accretion event – Space velocities U; V; W1 together with angular momenta scenario, where a small satellite galaxy merged with the Milky and actions that are used below were computed using the galpy2 Way and caused this dynamical structure (e.g. Navarro et al. package (Bovy 2015). For action estimates we used a MWPoten- 2004; Helmi et al. 2006; Minchev et al. 2009). The second possi- tial2014 axisymmetric gravitational potential model pre-defined bility is that it has originated due to resonances with the Galactic in galpy. Velocity uncertainties σU , σV ; σW were computed fol- bar or spiral arms that cause kinematic over-densities (e.g. lowing equations from Johnson & Soderblom(1987). The veloc- Gardner & Flynn 2010; Monari et al. 2013). The chemical prop- ities are given relative to the Local Standard of Rest: (U , V , −1 erties of the stream do not show any chemical peculiarities, W ) = (11.1, 12.24, 7.25) km s (Schönrich et al. 2010). which would be expected in the case of an extra-Galactic origin Taking into account the results from, for example, Zhao (e.g. Ramya et al. 2012; Bensby et al. 2014). At the same time et al.(2014), the typical size of kinematical structures is around −1 −1 the low angular momentum and the low velocity of the stream 20 km s .ThereforeweneedtocutstarswithσU ; σV > 20 km s , indicate that it could be another substructure of tidal debris in because such large velocity uncertainties will influence the pre- the Galactic halo (e.g. Arifyanto & Fuchs 2006; Klement et al. cision of the results, that is the position in velocity space of the 2008; Zhao et al. 2014). Despite numerous approaches to study structures. This leaves us with 6 002 514 stars. the origin of the Arcturus stream (e.g., numerical simulations, – Next, the sample was constrained to stars that are located kinematic analysis, and studies of elemental abundances), there within a distance of 5 kpc from the Sun. This filters out stars is no consensus on its origin. that are located in the outskirts or in the very inner parts of the The aim of this paper is to characterise the nature of the Galaxy, and thus, cannot be a part of any of the local kinematic Arcturus stream and constrain its origin. We start by detecting structures. The limit of 5 kpc was chosen to avoid regions in and characterising the velocities of the Arcturus stream using a direct contact with for example the Galactic bar, whose half- large stellar sample constructed from the Gaia DR2 catalogue length is about 3 kpc (e.g. Dehnen 2000; Minchev et al. 2010; (see Sect.2). We then search for over-densities in the velocity, Monari et al. 2017). According to Bailer-Jones(2015), distance angular momentum, and action spaces to obtain the kinematic estimates should not be dominated by using pre-Gaia informa- characteristics of the stream (see Sects.3–5).
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