PG 1610+062: a runaway B star challenging classical ejection mechanisms

Abstract

Hypervelocity stars are rare objects, mostly main-sequence (MS) B stars, traveling so fast that they will eventually escape from the Milky Way. Recently, it has been shown that the popular Hills mechanism, in which a binary system is disrupted via a close encounter with the supermassive black hole at the Galactic center, may not be their only ejection mechanism. The analyses of Gaia data ruled out a Galactic center origin for some of them, and instead indicated that they are extreme disk runaway stars ejected at velocities exceeding the predicted limits of classical scenarios (dynamical ejection from star clusters or binary supernova ejection). We present the discovery of a new extreme disk runaway star, PG 1610+062, which is a slowly pulsating B star bright enough to be studied in detail. A quantitative analysis of spectra taken with ESI at the Keck Observatory revealed that PG 1610+062 is a late B-type MS star of 4-5 M with low projected rotational velocity. Abundances (C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe) were derived differentially with respect to the normal B star HD 137366 and indicate that PG 1610+062 is somewhat metal rich. A kinematic analysis, based on our spectrophotometric distance (17.3 kpc) and on proper motions from Gaia's second data release, shows that PG 1610+062 was probably ejected from the Carina-Sagittarius spiral arm at a velocity of 550 ± 40 km s −1 , which is beyond the classical limits. Accordingly, the star is in the top five of the most extreme MS disk runaway stars and is only the second among the five for which the chemical composition is known.

Runaway blue main-sequence stars at high Galactic latitudes

“…Of all MS candidates, the possibly unbound runaway, 2207−4329, also has the Galactic plane crossing radius that is the furthest from the Galactic center at R(Z = 0) = 16.5 +5.9 −5.4 . Such a large distance is similar to the case of HD 271791 also originating from the outer Galactic disk (Heber et al 2008a;Przybilla et al 2008), the ejection velocity of which, however, is much lower (390 ± 4 km s −1 ; Irrgang et al 2019). If the MS classification of 2207−4329 is confirmed by future Gaia data releases and higher-resolution spectroscopy, alternative birth places above the Galactic plane (e.g.…”

“…In Fig. 13, our runaway candidates occupy a lower mass range with respect to the Silva & Napiwotzki (2011) and Irrgang et al (2019) samples. The increase of ejection velocities towards lower masses that characterize our sample appears to be in agreement with the predictions for binary supernova ejections in the most extreme scenarios (Tauris 2015;Evans et al 2020).…”

“…However, Renzo et al (2019) suggest that the dominant fraction of runaway stars with large ejection velocities could be mostly formed via dynamical interactions in open clusters, or more exotic scenarios. Interestingly, the ejection velocities of three runaway candidates, 2207−4329, 2259−4931, and 2307−3157, could largely exceed 400-500 km s −1 , which would be challenging the classical runaway scenarios (Irrgang et al 2018a(Irrgang et al , 2019. Of the three stars, 2207−4329 is by far the fastest (802 +129 −115 km s −1 ).…”

Exploring velocity limits in the thermonuclear supernova ejection scenario for hypervelocity stars and the origin of US 708

“…O' Leary & Loeb (2008) pointed out that an encounter with a stellar mass black hole in orbit around Sag A * could also act as a source of HVS with Sesana et al (2009) coming to the same conclusion regarding intermediate mass black holes (IMBH). The latter mechanism has been suggested as a source of certain B-type HVS (Irrgang et al 2019). Very recently, the discovery of an A-type star of 2.35 M , moving at a Galactic rest frame velocity of ∼1700 km s −1 , and which could be traced with high A&A 641, A52 (2020) accuracy back to the Galactic Center (Koposov et al 2020), has greatly bolstered the viability of the SMBH-encounter scenario.…”

World-Universe Model—Alternative to Big Bang Model

“…Weak interaction between DMF3 particles provides integrity of Fermi Bubbles. FBs made up of DMF3 particles resembles a honeycomb filled with DMF1 and DMF2;• FBs radiate X-rays due to the annihilation of DMF3 particles;• Gamma rays up to 1 TeV[52] are the result of annihilation of DMF1 (1.3 TeV) and DMF2 (9.6 GeV) in Dark Matter Objects (DMOs) whose density is sufficient for the annihilation of DMPs to occur. DMOs are much smaller than stars in the World, have a high concentration in FBs and provide nearly uniform gamma ray glow over their colossal surfaces.…”