Gaia DR2 White Dwarfs in the Hercules Stream Santiago Torres1,2, Carles Cantero1, María E

A&A 629, L6 (2019) Astronomy https://doi.org/10.1051/0004-6361/201936244 & c ESO 2019 Astrophysics LETTER TO THE EDITOR Gaia DR2 white dwarfs in the Hercules stream Santiago Torres1,2, Carles Cantero1, María E. Camisassa3,4, Teresa Antoja5, Alberto Rebassa-Mansergas1,2, Leandro G. Althaus3,4, Thomas Thelemaque6, and Héctor Cánovas7 1 Departament de Física, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain e-mail: [email protected] 2 Institute for Space Studies of Catalonia, c/Gran Capità 2-4, Edif. Nexus 104, 08034 Barcelona, Spain 3 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina 4 Instituto de Astrofísica de La Plata, UNLP-CONICET, Paseo del Bosque s/n, 1900 La Plata, Argentina 5 Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain 6 Industrial and Informatic Systems Deparment, EPF - École d’Ingénieurs, 21 boulevard Berthelot, 34000 Montpellier, France 7 European Space Astronomy Centre (ESA/ESAC), Operations Deparment, Villanueva de la Cañada, 28692 Madrid, Spain Received 5 July 2019 / Accepted 7 August 2019 ABSTRACT Aims. We analyzed the velocity space of the thin- and thick-disk Gaia white dwarf population within 100 pc by searching for signatures of the Hercules stellar stream. We aimed to identify objects belonging to the Hercules stream, and by taking advantage of white dwarf stars as reliable cosmochronometers, to derive a first age distribution. Methods. We applied a kernel density estimation to the UV velocity space of white dwarfs. For the region where a clear overdensity of stars was found, we created a 5D space of dynamic variables. We applied a hierarchichal clustering method, HDBSCAN, to this 5D space, and identified those white dwarfs that share similar kinematic characteristics. Finally, under general assumptions and from their photometric properties, we derived an age estimate for each object. Results. The Hercules stream was first revealed as an overdensity in the UV velocity space of the thick-disk white dwarf population. Three substreams were then found: Hercules a and Hercules b, formed by thick-disk stars with an age distribution that peaked 4 Gyr in the past and extends to very old ages; and Hercules c, with a ratio of 65:35 of thin to thick stars and a more uniform age distribution that is younger than 10 Gyr. Key words. white dwarfs – Galaxy: kinematics and dynamics – solar neighborhood – methods: data analysis 1. Introduction their spread in ages and metallicities (e.g., Famaey et al. 2005; Antoja et al. 2008; Bovy & Hogg 2010). The stream was more Stars from the solar neighborhood (the volume of the Galaxy likely formed through non-axisymmetries in the Galactic poten- up to a few hundred parsec from the Sun) are far from present- tial. In particular, the Hercules stream has for a long time been ing a uniform and homogeneous velocity distribution. In addi- hypothesized to be caused by the effects of the outer Lindblad tion to the Galactic components of the thin and thick disk and resonance (OLR) of the Galactic bar (Dehnen 2000; Fux 2001). the stellar halo, several kinematic structures left their imprint More recently, alternative origins for Hercules have been pro- on the velocity space. The origin of these structures is intensely posed based on independent evidence for an extended slowly debated and involves a wide variety of hypotheses ranging from rotating bar in the Milky Way (Wegg et al. 2015; Portail et al. non-axisymmetric structural components (such as the bar and 2015) that would place the OLR too far beyond the solar neigh- the spiral arm of the Galaxy) to cluster disruption or past accre- borhood. This means that the Hercules stream could be related to tion events (for a thorough review, see Antoja et al. 2010). In any the corotation resonance of the bar (Pérez-Villegas et al. 2017), case, the origins represent relevant signatures of the structure and dynamical evolution of the Galaxy. the 4:1 OLR of a slowly rotating bar (Hunt & Bovy 2018), One of the most prominent kinematic features among them and a combination of the effects of a slowly rotating bar and is the Hercules stream, also known as the U-anomaly. The Her- spiral arms (Hattori et al. 2019). This issue is far from being cules stream, first seen in Eggen(1958) and later detected in settled, and recent work still supports the original explanation multiple surveys such as Hipparcos, RAVE, LAMOST, and (Hunt et al. 2018; Ramos et al. 2018; Fragkoudi et al. 2019). Gaia (e.g., Dehnen 1998; Antoja et al. 2012; Liang et al. 2017; While Hercules is the group with largest extension in veloc- Gaia Collaboration 2018), is revealed as an elongated region in ity space, many studies have revealed substructures within it. For the UV-plane with a characteristic velocity moving away from instance, two overdensities at approximately the same rotation the Galactic center, U ≈ −30−50 km s−1 and lagging behind the velocity but at different Galactocentric radial velocity have been local standard of rest (LSR), V ≈ −60 km s−1. The hypothesis of observed in Dehnen(1998) and Antoja et al.(2015). Recently, the disruption of a cluster as its origin was discarded because of Gaia data showed the splitting of the Hercules stream into two or Article published by EDP Sciences L6, page 1 of5 A&A 629, L6 (2019) three branches of approximately constant Galactocentric radial Thin disk velocity (Gaia Collaboration 2018; Ramos et al. 2018). On the other hand, white dwarfs are long-living objects 6e−04 that represent the most common evolutionary remnants of low- 5e−04 and intermediate-mass stars, that is, stars with M . 8 ∼ 11 M (e.g., Siess 2007). Nuclear fusion reactions have ceased in 4e−04 white dwarf interiors; the pressure due to the degenerate elec- 3e−04 trons is responsible for preventing the gravitational collapse V (km/s) 2e−04 of these compact objects. White dwarfs are then subjected to a long process of gravothermal cooling, and their character- 1e−04 istics are reasonably well understood from a theoretical point 0e+00 of view; see, for instance, the review by Althaus et al.(2010). −150 −100 −50−100 0 50 −50 0 50 100 Hence, white dwarfs have been considered ideal candidates U (km/s) for constraining the age and formation history of the different Galactic components (e.g., Fontaine et al. 2001; Reid 2005; Thick disk García-Berro & Oswalt 2016). However, to date, the use of white dwarfs as Galactic tracers 0.00025 of the dynamic evolution has been limited. In particular, only a few modest studies of the white dwarf population have been ded- 0.00020 icated to the search of stellar streams in the solar neighborhood (e.g., Fuchs & Dettbarn 2011) or to study the possible imprints 0.00015 of a merger episode in the Galactic disk (e.g., Torres et al. 2001). 0.00010 There are several reasons for this: first, we lack radial velocity V (km/s) measurements for white dwarfs unless optimum resolution spec- 0.00005 tra are available (e.g., Pauli et al. 2006; Anguiano et al. 2017). 0.00000 This is a consequence of the broadening of the Balmer spectral −150 −100 −50 0 50 lines that arises from the high surface gravity that characterizes −100 −50 0 50 100 U (km/s) white dwarf atmospheres. Second, also due to this huge gravitational pull, metals are sunk in the deep interiors of white dwarf Fig. 1. UV density diagram for the thin- (top panel) and thick-disk envelopes, which precludes associating a metallicity value with (bottom panel) white dwarf population within 100 pc from the Sun. these objects. Finally, the number of white dwarfs that are iden- A clear overdensity is revealed in the thick-disk population (delim- tified in volume-limited samples is rather small. ited by a red rectangle). For comparative purposes, we show the Fortunately, the advent of large data surveys such as Gaia thin-disk dispersion ellipsoid for 1σ (continuous line) and 3σ (dashed has dramatically increased the number of known white dwarfs, line) levels corresponding to the thin-disk population (Torres et al. thus providing statistically significant large complete samples. 2019), and we illustrate the number density (right scales) of white dwarf stars per 1 × 1 (km s)−2. The location of the Hercules substreams from Together with the potential of white dwarfs as cosmochronome- Antoja et al.(2012) is shown with red crosses. See text for details. ters, this opens the door to using these objects as reliable tracers of the dynamic Galactic evolution. In this Letter we aim to show the Hercules stream signature in the population of white dwarfs of the local neighborhood. 3. Identifying white dwarfs in the Hercules stream Additionally, we aim to identify white dwarfs that belong to The Hercules stream, as many other similar stellar streams, is the Hercules stream and to derive a first approach to their age first revealed as an overdensity of stars in the UV plane (e.g., distribution. Dehnen 2000). Thus, as shown in Fig.1, we started by ana- lyzing the UV space velocity plane by means of a density ker- 2. Gaia DR2 white dwarf 100 pc sample nel estimation (Chen et al. 1997) for the 100 pc Gaia thin- and thick-disk white dwarf populations. No relevant feature depart- The second data release (DR2) of the Gaia mission has pro- ing from a Gaussian distribution appears when the thin-disk vided an unprecedented wealth of accurate astrometric and pho- white dwarf population is depicted (top panel of Fig.1).

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