An Extremely Top-Heavy Initial Mass Function in the Galactic Center Stellar Disks

Bartko, H.; Martins, F.; Trippe, Sascha; Fritz, Tobias; Genzel, R.; Ott, Thomas; Eisenhauer, F.; Gillessen, S.; Paumard, T.; Alexander, Tal; Dodds-Eden, K.; Gerhard, Ortwin; Levin, Y.; Mascetti, L.; Nayakshin, Sergei; Perets, Hagai B.; Perrin, Guillaume; Pfuhl, O.; Reid, M. J.; Rouan, D.; Zilka, Miri; Sternberg, A.

doi:10.1088/0004-637x/708/1/834

Cited by 344 publications

(642 citation statements)

References 67 publications

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“…Hopman & Alexander (2006b) used Fokker-planck calculations to show that γ should be 1.4 for solar mass MS stars and up to 2 for stellar black holes (SBH). Similar results from MonteCarlo calculations were found by Freitag et al (2006) However, observations of red-giants in the Galactic center suggest a core-like structure in the inner regions of the GC (γ in the range 0 − 0.5; Do et al 2009;Bartko et al 2010;Genzel et al 2010. Nevertheless, it is not yet clear whether the distribution of red-giants reflect the overall distribution of stars in the GC, and various models had been suggested to explain both a "real" core distribution (Merritt 2010) or an apparent one (i.e., only reflecting the distribution of red-giants; Dale et al 2009;Amaro-Seoane & Chen 2014;Aharon & Perets 2015).…”

Section: Cusp and Core Models For The Nuclear Clustersupporting

confidence: 71%

“…Analysis of observations (Salpeter 1955, Chabrier 2001, Kroupa 2002 shows an almost universal power law behavior, with dN (m) ∝ m −α(m) dm, with a typical exponent of α ∼ 2.3, for stars above one Solar mass and up to ∼ 120 M⊙. The analysis done by Bartko et al (2010) based on spectroscopic survey made with SIN-FONI conclude that the present day mass function and the IMF for the disk of young stars in the Galactic center obey a power law, but the slope is flatter than the "regular" IMF. In particular, they conclude that α = 0.45 ± 0.3.…”

Section: Observed Disk Structure and Mass Functionmentioning

confidence: 95%

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Short- and long-term evolution of a stellar disc around a massive black hole: the role of the cusp, stellar evolution and binaries

Mikhaloff¹,

Perets²

2016

Mon. Not. R. Astron. Soc.

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We study the dynamical evolution of a stellar disk orbiting a massive black hole. We explore the role of two-body relaxation, mass segregation, stellar evolution and binary heating in affecting the disk evolution, and consider the impact of the nuclear cluster structure and the stellar-disk mass-function. We use analytic arguments and numerical calculations, and apply them to study the evolution of a stellar disk (similar to that observed in the Galactic center; GC), both on the short (few Myr) and longer (100 Myr) evolutionary timescales. We find the dominant processes affecting the disk evolution are two-body relaxation and mass segregation where as binary heating have only a little contribution. Massive stars play a dominant role in kinematically heating low mass stars, and driving them to high eccentricities/inclinations. Multi-mass models with realistic mass-functions for the disk stars show the disk structure to be mass stratified, with the most massive stars residing in thinner structures. Stellar evolution plays an important role in decreasing the number of massive stars with time, thereby leading to slower relaxation, where the remnant compact objects of these stars are excited to higher eccentricities/inclinations. At these later evolutionary stages dynamical heating by the nuclear cluster plays a progressively more important role. We conclude that the high eccentricities and high inclination observed for the majority on the young O-stars in the Galactic Center suggest that the disk stars had been formed with initially high eccentricities, or that collective or secular processes (not explored here) dominate the disk evolution. The latter processes are less likely to produce mass stratification in the disk; detailed study of the mass-dependent kinematic properties of the disk stars could therefore provide a handle on the processes that dominate its evolution. Finally, we find that the disk structure is expected to keep its coherency, and be observed as a relatively thin disk even after 100 Myrs; two-body relaxation is too inefficient for the disk to assimilate into the nuclear cluster on such timescales. It therefore suggests earlier disks now containing only older, lower mass stars might still be observed in the Galactic center, unless destroyed/smeared by other non-two-body relaxation processes.

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Section: Cusp and Core Models For The Nuclear Clustersupporting

confidence: 71%

Section: Observed Disk Structure and Mass Functionmentioning

confidence: 95%

Short- and long-term evolution of a stellar disc around a massive black hole: the role of the cusp, stellar evolution and binaries

Mikhaloff¹,

Perets²

2016

Mon. Not. R. Astron. Soc.

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“…The absence of luminous red giants has been noted in the inner parts of the Galactic bulge (e.g. Bartko et al 2010;Kieffer & Bogdanović 2016, and references therein). It has been explained as a result of interactions of stars in red giant phase with the accretion disc surrounding the central black hole (Kieffer & Bogdanović 2016).…”

Section: Discussionmentioning

confidence: 93%

Anomalous double-mode RR Lyrae stars in the Magellanic Clouds

Soszyński

Smolec

Dziembowski

et al. 2016

Mon. Not. R. Astron. Soc.

172

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We report the discovery of a new subclass of double-mode RR Lyrae stars in the Large and Small Magellanic Clouds. The sample of 22 pulsating stars have been extracted from the latest edition of the OGLE collection of RR Lyrae variables in the Magellanic System. The stars pulsating simultaneously in the fundamental (F) and first-overtone (1O) modes have distinctly different properties than regular double-mode RR Lyrae variables (RRd stars). The P 1O /P F period ratios of our anomalous RRd stars are within a range 0.725-0.738, while "classical" double-mode RR Lyrae variables have period ratios in the range 0.742-0.748. In contrast to the typical RRd stars, in the majority of the anomalous pulsators the F-mode amplitudes are higher than the 1O-mode amplitudes. The light curves associated with the F-mode in the anomalous RRd stars show different morphology than the light curves of, both, regular RRd stars and single-mode RRab stars. Most of the anomalous double-mode stars show long-term modulations of the amplitudes (Blazhko-like effect). Translating the period ratios into the abundance parameter, Z, we find for our stars Z ∈ (0.002, 0.005) -an order of magnitude higher values than typical for RR Lyrae stars. The mass range of the RRd stars inferred from the W I vs. P F diagram is (0.55 − 0.75) M ⊙ . These parameters cannot be accounted for with single star evolution assuming a Reimers-like mass loss. Much greater mass loss caused by interaction with other stars is postulated. We blame the peculiar pulsation properties of our stars to the parametric resonance instability of the 1O-mode to excitation of the F-and 2O-modes as with the inferred parameters of the stars 2ω 1O ≈ ω F + ω 2O .

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“…One would expect from theoretical models that, if relaxed, the stellar density near the SMBH should be steeply-rising and form a Bahcall & Wolf (1976) cusp. In contrast, observations by Do et al (2009); Buchholz et al (2009); Bartko et al (2010) show that the distribution of old stars near the SMBH appears to have a core. Understanding the nuclear star cluster dynamics may therefore give useful constraints on the mechanisms by which it formed and evolved (Merritt 2010).…”

Section: Introductionmentioning

confidence: 83%

The old nuclear star cluster in the Milky Way: dynamics, mass, statistical parallax, and black hole mass

Chatzopoulos¹,

Fritz²,

Gerhard³

et al. 2014

Monthly Notices of the Royal Astronomical Society

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171

216

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We derive new constraints on the mass, rotation, orbit structure and statistical parallax of the Galactic old nuclear star cluster and the mass of the supermassive black hole. We combine star counts and kinematic data from Fritz et al. (2014), including 2'500 line-of-sight velocities and 10'000 proper motions obtained with VLT instruments. We show that the difference between the proper motion dispersions σ l and σ b cannot be explained by rotation, but is a consequence of the flattening of the nuclear cluster. We fit the surface density distribution of stars in the central 1000 ′′ by a superposition of a spheroidal cluster with scale ∼ 100 ′′ and a much larger nuclear disk component. We compute the self-consistent two-integral distribution function f (E, L z ) for this density model, and add rotation self-consistently. We find that: (i) The orbit structure of the f (E, L z ) gives an excellent match to the observed velocity dispersion profiles as well as the proper motion and line-of-sight velocity histograms, including the double-peak in the v l -histograms. (ii) This requires an axial ratio near q 1 = 0.7 consistent with our determination from star counts, q 1 = 0.73 ± 0.04 for r < 70 ′′ . (iii) The nuclear star cluster is approximately described by an isotropic rotator model. (iv) Using the corresponding Jeans equations to fit the proper motion and line-of-sight velocity dispersions, we obtain best estimates for the nuclear star cluster mass, black hole mass, and distance M * (r < 100 ′′ ) = (8.94±0.31| stat ±0.9| syst )×10 6 M ⊙ , M • = (3.86±0.14| stat ±0.4| syst )×10 6 M ⊙ , and R 0 = 8.27±0.09| stat ±0.1| syst kpc, where the estimated systematic errors account for additional uncertainties in the dynamical modeling. (v) The combination of the cluster dynamics with the S-star orbits around Sgr A * strongly reduces the degeneracy between black hole mass and Galactic centre distance present in previous S-star studies. A joint statistical analysis with the results of Gillessen et al. (2009) gives M • = (4.23±0.14)×10 6 M ⊙ and R 0 = 8.33±0.11 kpc.

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An Extremely Top-Heavy Initial Mass Function in the Galactic Center Stellar Disks

Cited by 344 publications

References 67 publications

Short- and long-term evolution of a stellar disc around a massive black hole: the role of the cusp, stellar evolution and binaries

Short- and long-term evolution of a stellar disc around a massive black hole: the role of the cusp, stellar evolution and binaries

Anomalous double-mode RR Lyrae stars in the Magellanic Clouds

The old nuclear star cluster in the Milky Way: dynamics, mass, statistical parallax, and black hole mass

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