Milky Way archaeology using RR Lyrae and type II Cepheids

II. High-velocity RR Lyrae stars and Milky Way mass

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
e-mail: prudilz@ari.uni-heidelberg.de
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
³ TU Darmstadt, IKP, Schlossgartenstr. 2 (S2|11), 64289 Darmstadt, Germany
⁴ Goethe University Frankfurt, IAP, Max-von-Laue-Str. 12, Frankfurt am Main, 60438 Frankfurt, Germany
⁵ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁶ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
⁷ Space Science Data Center – ASI, Via del Politecnico snc, 00133 Roma, Italy
⁸ Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755, USA
⁹ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁰ Gemini Observatory/NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA
¹¹ Cerro Tololo Inter-American Observatory, NSF’s National Optical-Infrared Astronomy Research Laboratory, Casilla 603, La Serena, Chile

Received: 18 September 2021
Accepted: 26 May 2022

Abstract

We report the discovery of high-velocity candidates among RR Lyrae stars found in the Milky Way halo. We identified nine RR Lyrae stars with Galactocentric velocities exceeding the local escape velocity based on the assumed Galaxy potential. Furthermore, based on a close examination of their orbits’, we ruled out their ejection location in the Milky Way disk and bulge. The spatial distribution revealed that seven out of nine pulsators overlap with the position of the Sagittarius stellar stream. Two out of these seven RR Lyrae stars can be tentatively linked to the Sagittarius dwarf spheroidal galaxy on the basis of their orbits. Focusing on the high-velocity tail of the RR Lyrae velocity distribution, we estimated the escape velocity in the Solar neighborhood to be v_esc = 512₋₃₇⁺⁹⁴ km s⁻¹ (4 to 12 kpc); and beyond the Solar neighborhood as v_esc = 436₋₂₂⁺⁴⁴ km s⁻¹ and v_esc = 393₋₂₆⁺⁵³ km s⁻¹ (for distances between 12 to 20 kpc and 20 to 28 kpc), respectively. We utilized three escape velocity estimates together with the local circular velocity to estimate the Milky Way mass. The resulting measurement M₂₀₀ = 0.83_−0.16^+0.29 × 10¹² M_⊙ falls on the lower end of the current Milky Way mass estimates, but once corrected for the likely bias in the escape velocity (an increase of approximately 10% in terms of the escape velocity), our mass estimate yields M₂₀₀ = 1.26_−0.22^+0.40 × 10¹² M_⊙, which is in agreement with estimates based on different diagnostics of the Milky Way (MW) mass. The MW mass of within 20 kpc then corresponds to M_MW(r < 20 kpc) = 1.9_−0.1^+0.2 × 10¹¹ M_⊙ without any correction for bias, and M_MW(r < 20 kpc) = 2.1_−0.1^+0.2 × 10¹¹ M_⊙ corrected for a likely offset in escape velocities.