US 708 – an unbound hyper-velocity subluminous O star

Hirsch, H.; Heber, U.; O'Toole, S. J.; Bresolin, Fabio

doi:10.1051/0004-6361:200500212

Cited by 188 publications

(241 citation statements)

References 27 publications

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“…If we select only stars within the overdensity region defined by Abadi et al (2009) the spread reduces to ∼250 km s −1 resulting in a minimum progenitor mass still higher than 10 10 M . Concerning the known satellite galaxies the respective authors of the HVS discovery papers have already excluded a kinematical connection to them (Brown et al 2005(Brown et al , 2006a(Brown et al , 2009aEdelmann et al 2005;Hirsch et al 2005). Since we do not expect such a system to have escaped observations to date, we conclude that a satellite origin for the subsample to be unlikely.…”

Section: Discussionmentioning

confidence: 99%

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Kinematics in galactic tidal tails

Piffl¹,

Williams²,

Steinmetz³

2011

A&A

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Context. Recent observations of stars with unusually high radial velocities in the Galactic stellar halo have raised new interest in so-called hypervelocity stars. Traditionally, it is assumed that the velocities of these stars could only come from an interaction with the supermassive black hole in the Galactic center. It was suggested that stars stripped off a disrupted satellite galaxy could reach similar velocities and leave the Galaxy. Aims. In this work we study the kinematics of tidal debris stars in detail to investigate the implications of the new scenario and the probability that the observed sample of hypervelocity stars could partly develop out of such a galaxy collision. Methods. We used a suite of N-body simulations following the encounter of a satellite galaxy with its Milky Way-type host galaxy to gather statistics on the properties of stripped-off stars. We study especially the orbital energy distribution of this population. Results. We quantify the typical pattern in angular and phase space formed by the debris stars. We further develop a simple stripping model to predict the kinematics of stripped-off stars. We show that the distribution of orbital energies in the tidal debris takes a typical form that can be described quite accurately by a simple function. Based on this we develop a method predicting the energy distribution that allows us to evaluate the significance and the implications of highvelocity stars in satellite tidal debris. Conclusions. Generally the tidal collisions of satellite galaxy produce stars that escape into intragalactic space even if the satellite itself is on a bound orbit. The main parameters determining the maximum energy kick a tidal debris star can get is the initial mass of the satellite and only to a lower extent its orbit. Main contributors to an unbound stellar population created in this way are massive satellites (M sat > 10 9 M ). We thus expect intragalactic stars to have a metallicity higher than the surviving satellite population of the Milky Way. However, the probability that the observed HVS population is significantly contaminated by tidal debris stars appears low in light of our results.

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Section: Discussionmentioning

confidence: 99%

“…Recent observations of 17 stars in the Galactic halo with unusually high velocities (Brown et al 2005;Hirsch et al 2005;Edelmann et al 2005;Brown et al 2006aBrown et al ,b, 2007aBrown et al ,b, 2009aTillich et al 2009) have raised new interest in the topic. By the design of the search strategy, these stars typically have blue colors.…”

Section: Introductionmentioning

confidence: 99%

Kinematics in galactic tidal tails

Piffl¹,

Williams²,

Steinmetz³

2011

A&A

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“…The Galactic gravitational potential of Allen & Santillán (1991) perfectly fulfills these criteria and has thus been widely used to calculate trajectories for, e.g., globular clusters (Odenkirchen et al 1997;Allen et al 2008;Lane et al 2012), dwarf spheroidals (Lépine et al 2011), planetary nebulae (Wu et al 2011), white dwarfs (Pauli et al 2003(Pauli et al , 2006, horizontalbranch stars (Altmann & de Boer 2000;Kaempf et al 2005), subluminous B stars (Altmann et al 2004;Tillich et al 2011), halo stars (Schuster et al 2012;Pereira et al 2012), runaway stars (Irrgang et al 2010;Silva & Napiwotzki 2011), and hypervelocity stars (Hirsch et al 2005;Edelmann et al 2005). However, the model parameters can be recalibrated using new and improved observational constraints.…”

Section: Introductionmentioning

confidence: 99%

Milky Way mass models for orbit calculations

Irrgang

Wilcox

Tucker

et al. 2013

A&A

227

339

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Context. Studying the trajectories of objects like stars, globular clusters, or satellite galaxies in the Milky Way allows the dark matter halo to be traced but requires reliable models of its gravitational potential. Aims. Realistic, yet simple and fully analytical, models have already been presented in the past. However, improved, as well as new, observational constraints have become available in the meantime, calling for a recalibration of the respective model parameters. Methods. Three widely used model potentials are revisited. By a simultaneous least-squares fit to the observed rotation curve, in-plane proper motion of Sgr A*, local mass/surface density, and the velocity dispersion in Baade's window, parameters of the potentials are brought up-to-date. The mass at large radii -in particular, that of the dark matter halo -is hereby constrained by requiring that the most extreme known halo, blue horizontal-branch star has to be bound to the Milky Way. Results. The Galactic mass models are tuned to yield a very good match to recent observations. The mass of the dark matter halo is -within the limitations of the applied models -estimated in a fully consistent way. As a first application, the trajectory of the hypervelocity star HE 0437-5439 is investigated again to check its suggested origin in the Large Magellanic Cloud (LMC). Conclusions. Despite their simplicity, the presented Milky Way mass models are very able to reproduce all observational constraints. Their analytical and simple form makes them ideally suited for fast and accurate orbit calculations. The LMC cannot be ruled out as HE 0437-5439's birthplace.

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“…While most of the 17 HVS known today (Brown et al 2009;Tillich et al 2009) are early-type main-sequence stars, there is just one evolved low-mass star, US 708, a hot subdwarf star of spectral type sdO (Hirsch et al 2005). …”

Section: Introductionmentioning

confidence: 99%

The Hyper-MUCHFUSS project: probing the Galactic halo with sdB stars

Tillich

Heber

Geier

et al. 2011

A&A

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Context. High-velocity stars in the Galactic halo, e.g. the so-called hyper-velocity stars (HVS), are important tracers of the properties of the dark matter halo, in particular its mass. Aims. A search for the fastest stars among hot subdwarfs (sdB) in the halo is carried out to identify HVS, unbound to the Galaxy, and bound population II stars, in order to derive a lower limit to the halo mass. Methods. Based on the SDSS DR6 spectral database we selected stars with high rest-frame velocities. These radial velocity (RV) measurements were verified at several telescopes to exclude RV variable stars. Out of 88 stars observed in the follow-up campaign, 39 stars were found to have constant RVs. For twelve of them we measured a proper motion significantly different from zero and obtained spectroscopic distances from quantitative spectral analysis to construct the full 6D phase space information for a kinematical study.Results. All but one programme sdB show halo characteristics, but can be distinguished as two kinematical groups, one (G1) with the low Galactic rotation typical of halo stars and a second one (G2) with rapid retrograde motion. We also investigate the possibility that the programme stars are not genuine halo stars but ejected from the Galactic disc or bulge. The G1 objects crossed the Galactic plane in the central bulge, whereas the G2 stars did so in the outer Galactic disc. J1211+1437 (G2) is an HVS candidate, as it is unbound to the Galaxy if the standard Galactic potential is adopted. Conclusions. We conclude that in the ejection scenario, G1 stars might have been formed via the slingshot mechanism that invokes acceleration by tidal interaction of a binary with the central supermassive black hole. The G2 stars, however, would originate in the outskirts of the Galactic disc and not in the central bulge. J1211+1437 is the first unbound subdwarf B star, for which we can rule out the slingshot mechanism. Alternatively, we may assume that the stars are old population II stars and so they have to be bound. Then the kinematics of J1211+1437 set a lower limit of 2 × 10 12 M to the mass of the Galactic dark matter halo.

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US 708 – an unbound hyper-velocity subluminous O star

Cited by 188 publications

References 27 publications

Kinematics in galactic tidal tails

Kinematics in galactic tidal tails

Milky Way mass models for orbit calculations

The Hyper-MUCHFUSS project: probing the Galactic halo with sdB stars

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