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Young stars raining through the Galactic Halo: the nature and orbit of Price-Whelan 1

Abstract : We present radial velocities for five member stars of the recently discovered young (age 100 − 150 Myr) stellar system Price-Whelan 1 (PW 1), that is located far away in the Galactic Halo (D 29 kpc, Z 15 kpc), and that is probably associated to the Leading Arm (LA) of the Magellanic Stream. We measure the systemic radial velocity of PW 1, V r = 275 ± 10 km/s, significantly larger than the velocity of the LA gas in the same direction. We re-discuss the main properties and the origin of this system in the light of these new observations, computing the orbit of the system and comparing its velocity with that of the H i in its surroundings. We show that the bulk of the gas at the velocity of the stars is more than 10 • (5 kpc) away from PW 1 and the velocity difference between the gas and the stars become larger as gas closer to the stars is considered. We discuss the possibilities that (a) the parent gas cloud was dissolved by the interaction with the Galactic gas, and (b) that the parent cloud is the high velocity cloud HVC 287.5+22.5+240, lagging behind the stellar system by 25 km/s and 10 • 5 kpc. This HVC, that is part of the LA, has metallicity similar to PW 1, displays a strong magnetic field that should help to stabilise the cloud agains ram pressure, and shows traces of molecular hydrogen. We also show that the system is constituted of three distinct pieces that do not differ only by position in the sky but also by stellar content.
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Michele Bellazzini, Rodrigo Ibata, Nicolas Martin, Khyati Malhan, Antonino Marasco, et al.. Young stars raining through the Galactic Halo: the nature and orbit of Price-Whelan 1. MNRAS, 2019, 490 (2), pp.2588-2598. ⟨10.1093/mnras/stz2788⟩. ⟨hal-02984965⟩

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