High-Velocity Stars In The Cores Of Globular Clusters: The Illustrative Case Of NGC 2808

dc.contributor.utaustinauthorGebhardt, K.en_US
dc.creatorLutzgendorf, N.en_US
dc.creatorGualandris, A.en_US
dc.creatorKissler-Patig, M.en_US
dc.creatorGebhardt, K.en_US
dc.creatorBaumgardt, H.en_US
dc.creatorNoyola, E.en_US
dc.creatorKruijssen, J. M. D.en_US
dc.creatorJalali, B.en_US
dc.creatorde Zeeuw, P. T.en_US
dc.creatorNeumayer, N.en_US
dc.description.abstractContext. We report the detection of five high-velocity stars in the core of the globular cluster NGC 2808. The stars lie on the red giant branch and show total velocities between 40 and 45 kms(-1). For a core velocity dispersion sigma(c) = 13.4 kms(-1), this corresponds to up to 3.4 sigma(c). These velocities are close to the estimated escape velocity (similar to 50 km s(-1)) and suggest an ejection from the core. Two of these stars have been confirmed in our recent integral field spectroscopy data and we will discuss them in more detail here. These two red giants are located at a projected distance of similar to 0.3 pc from the center. According to their positions on the color magnitude diagram, both stars are cluster members. Aims. We investigate several possible origins for the high velocities of the stars and conceivable ejection mechanisms. Since the velocities are close to the escape velocity, it is not obvious whether the stars are bound or unbound to the cluster. We therefore consider both cases in our analysis. Methods. We perform numerical simulations of three-body dynamical encounters between binaries and single stars and compare the resulting velocity distributions of escapers with the velocities of our stars. If the stars are bound, the encounters must have taken place when the stars were still on the main sequence. We compare the predictions for a single dynamical encounter with a compact object with those of a sequence of two-body encounters due to relaxation. If the stars are unbound, the encounter must have taken place recently, when the stars were already in the giant phase. Results. After including binary fractions and black-hole retention fractions, projection effects, and detection probabilities from Monte-Carlo simulations, we estimate the expected numbers of detections for all the different scenarios. Based on these numbers, we conclude that the most likely scenario is that the stars are bound and were accelerated by a single encounter between a binary of main-sequence stars and a similar to 10 M-circle dot black hole. Finally, we discuss the origin of previously discovered fast stars in globular clusters, and conclude that the case of NGC2808 is most likely a representative case for most other detections of fast stars in globular clusters. We show that with the present analysis we are able to explain high-velocity stars in the clusters M3 and 47 Tucanae with simple dynamical encounters.en_US
dc.description.sponsorshipDFG cluster of excellence Origin and Structure of the Universeen_US
dc.description.sponsorshipAustralian Research Council FT0991052en_US
dc.identifier.citationLützgendorf, Nora, Alessia Gualandris, Markus Kissler-Patig, Karl Gebhardt, Holger Baumgardt, Eva Noyola, J. M. D. Kruijssen, Behrang Jalali, P. Tim de Zeeuw, and Nadine Neumayer. >High-velocity stars in the cores of globular clusters: the illustrative case of NGC 2808.> Astronomy & Astrophysics, Vol. 543 (Jul., 2012): A82.en_US
dc.relation.ispartofserialAstronomy & Astrophysicsen_US
dc.rightsAdministrative deposit of works to Texas ScholarWorks: This works author(s) is or was a University faculty member, student or staff member; this article is already available through open access or the publisher allows a PDF version of the article to be freely posted online. The library makes the deposit as a matter of fair use (for scholarly, educational, and research purposes), and to preserve the work and further secure public access to the works of the University.en_US
dc.subjectglobular clusters: individual: ngc 2808en_US
dc.subjectstars: kinematics and dynamicsen_US
dc.subjectmass black-holesen_US
dc.subjectspace-telescope observationsen_US
dc.subjectlong-period variablesen_US
dc.subjectcentral kinematicsen_US
dc.subject3-body encountersen_US
dc.subjectmira variablesen_US
dc.subjectastronomy & astrophysicsen_US
dc.titleHigh-Velocity Stars In The Cores Of Globular Clusters: The Illustrative Case Of NGC 2808en_US
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