scholarly journals On the origin of young stars at the Galactic center

2013 ◽  
Vol 9 (S303) ◽  
pp. 238-241
Author(s):  
Ann-Marie Madigan ◽  
Oliver Pfuhl ◽  
Yuri Levin ◽  
Stefan Gillessen ◽  
Reinhard Genzel ◽  
...  
Keyword(s):  
Black Hole ◽  
Galactic Center ◽  
Stellar Populations ◽  
Massive Black Hole ◽  
B Stars ◽  
Orbital Parameters ◽  
Original Distribution ◽  
Orbital Configuration ◽  
Early Type Stars ◽  
Sgr A

AbstractThe center of our Galaxy is home to a massive black hole, Sgr A*, and a nuclear star cluster containing stellar populations of various ages. While the late type stars may be too old to have retained memory of their initial orbital configuration, and hence formation mechanism, the kinematics of the early type stars should reflect their original distribution. In this contribution we present a new statistic which uses directly-observable kinematic stellar data to infer orbital parameters for stellar populations, and is capable of distinguishing between different origin scenarios. We use it on a population of B-stars in the Galactic center that extends out to large radii (∼0.5 pc) from the massive black hole. We find that the high K-magnitude population (≲15 M⊙) form an eccentric distribution, suggestive of a Hills binary-disruption origin.

scholarly journals Observing a black hole event horizon: (sub)millimeter VLBI of Sgr A*

2009 ◽  
Vol 5 (S261) ◽  
pp. 271-276 ◽  
Author(s):  
Vincent L. Fish ◽  
Sheperd S. Doeleman
Keyword(s):  
Black Hole ◽  
Event Horizon ◽  
Strong Evidence ◽  
Galactic Center ◽  
Strong Field ◽  
Variable Component ◽  
Massive Black Hole ◽  
Black Hole Candidate ◽  
Long Baseline ◽  
Sgr A

AbstractVery strong evidence suggests that Sagittarius A*, a compact radio source at the center of the Milky Way, marks the position of a super massive black hole. The proximity of Sgr A* in combination with its mass makes its apparent event horizon the largest of any black hole candidate in the universe and presents us with a unique opportunity to observe strong-field GR effects. Recent millimeter very long baseline interferometric observations of Sgr A* have demonstrated the existence of structures on scales comparable to the Schwarzschild radius. These observations already provide strong evidence in support of the existence of an event horizon. (Sub)Millimeter VLBI observations in the near future will combine the angular resolution necessary to identify the overall morphology of quiescent emission, such as an accretion disk or outflow, with a fine enough time resolution to detect possible periodicity in the variable component of emission. In the next few years, it may be possible to identify the spin of the black hole in Sgr A*, either by detecting the periodic signature of hot spots at the innermost stable circular orbit or parameter estimation in models of the quiescent emission. Longer term, a (sub)millimeter VLBI “Event Horizon Telescope” will be able to produce images of the Galactic center emission to the see the silhouette predicted by general relativistic lensing. These techniques are also applicable to the black hole in M87, where black hole spin may be key to understanding the jet-launching region.

scholarly journals Observations of the gas cloud G2 in the Galactic center

2013 ◽  
Vol 9 (S303) ◽  
pp. 254-263
Author(s):  
S. Gillessen ◽  
R. Genzel ◽  
T. K. Fritz ◽  
F. Eisenhauer ◽  
O. Pfuhl ◽  
...  
Keyword(s):  
Black Hole ◽  
Galactic Center ◽  
Accretion Rate ◽  
Tidal Effects ◽  
Massive Black Hole ◽  
Radial Orbit ◽  
Sgr A ◽  
The Many ◽  
Gas Cloud ◽  
Luminosity Evolution

AbstractIn 2011, we discovered a compact gas cloud (“G2”) with roughly three Earth masses that is falling on a near-radial orbit toward the massive black hole in the Galactic center. The orbit is well constrained and pericenter passage is predicted for early 2014. Our data beautifully show that G2 gets tidally sheared apart due to the massive black hole's force. During the next months, we expect that in addition to the tidal effects, hydrodynamics get important, when G2 collides with the hot ambient gas around Sgr A*. Simulations show that ultimately, the cloud's material might fall into the massive black hole. Predictions for the accretion rate and luminosity evolution, however, are very difficult due to the many unknowns. Nevertheless, this might be a unique opportunity in the next years to observe how gas feeds a massive black hole in a galactic nucleus.

scholarly journals A study on tidal disruption event dynamics around an Sgr A*-like massive black hole

2020 ◽  
Vol 642 ◽  
pp. A111
Author(s):  
A. Clerici ◽  
A. Gomboc
Keyword(s):  
Black Hole ◽  
Relativistic Effects ◽  
Dynamical Behaviour ◽  
Return Rate ◽  
Newtonian Potential ◽  
Tidal Disruption ◽  
Massive Black Hole ◽  
Orbital Parameters ◽  
Sgr A ◽  
Disruption Event

Context. The number of observed tidal disruption events is increasing rapidly with the advent of new surveys. Thus, it is becoming increasingly important to improve tidal disruption event models using different stellar and orbital parameters. Aims. We study the dynamical behaviour of tidal disruption events produced by an Sgr A*-like massive black hole by changing different initial orbital parameters, taking into account the observed orbits of S stars. Investigating different types of orbits and penetration factors is important since their variations lead to different timescales of the tidal disruption event debris dynamics, making mechanisms such as self-crossing and pancaking act strongly or weakly and thus affecting the circularisation and accretion disc formation. Methods. We performed smoothed particle hydrodynamics simulations. Each simulation consisted of modelling the star with 105 particles, and the density profile is described by a polytrope with γ = 5/3. The massive black hole was modelled with a generalised post-Newtonian potential, which takes into account the relativistic effects of the Schwarzschild space-time. Results. Our analyses find that mass return rate distributions of solar-like stars and S-like stars with the same eccentricities have similar durations, but S-like stars have higher mass return rate distributions, as expected due to their larger masses. Regarding debris circularisation, we identify four types of evolution related to the mechanisms and processes involved during circularisation: in type 1, the debris does not circularise efficiently, hence a disc is not formed or is formed after a relatively long time; in type 2, the debris slowly circularises and eventually forms a disc with no debris falling back; in type 3, the debris circularises relatively quickly and forms a disc while there is still debris falling back; in type 4, the debris quickly and efficiently circularises, mainly through self-crossings and shocks, and forms a disc with no debris falling back. Finally, we find that the standard relation of circularisation radius rcirc = 2rt holds only for β = 1 and eccentricities close to parabolic.

scholarly journals The Dark Mass Concentration at the Galactic Center

1997 ◽  
Vol 163 ◽  
pp. 637-646
Author(s):  
F. Melia
Keyword(s):  
Black Hole ◽  
Large Scale ◽  
Galactic Center ◽  
Line Emission ◽  
Concentrated Mass ◽  
Massive Black Hole ◽  
Central Mass ◽  
Kinematic Studies ◽  
Sgr A ◽  
Large Scale Flow

AbstractStellar kinematic studies indicate the presence of a concentrated central mass at just under 2 × 106M⊙, in close agreement with the mass deduced from gas velocities measured with the [Ne II] line. Although this mass is most likely a black hole, it may be dominated by a tightly concentrated cluster of stellar remnants. If Sgr A*, a point radio source coincident with this central mass, is a massive black hole embedded in a region with strong gaseous outflows, as suggested by the observation of He I, Brα and Brγ line emission, it is accreting from its environment via the Bondi-Hoyle process. We discuss the consequences of this activity, including the expected mass and angular momentum accretion rate onto the black hole, and the resulting observable characteristics. The latest infrared images of this region appear to rule out the possibility that this large scale flow settles down into a standard α-disk at small radii. We discuss some possible scenarios that might account for this, including strong advection in the disk or the presence of a massive, fossilized disk. Not all of the gas affected in this way by Sgr A*’s strong gravitational field becomes bound. Some of it is redirected into a focused flow that in turn interacts with other coherent gas structures near the black hole. We suggest that the mini-cavity (to the south-west of Sgr A*) may be formed as a result of this activity, and argue that the characteristics of the mini-cavity lend some observational support for the presence of a concentrated mass near Sgr A*. We show, however, that as far as the mini-cavity is concerned, this concentrated mass need not be in the form of a point mass, but may instead be a highly concentrated cluster of stellar remnants.

scholarly journals A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty

2019 ◽  
Vol 625 ◽  
pp. L10 ◽  
Author(s):  
◽  
R. Abuter ◽  
A. Amorim ◽  
M. Bauböck ◽  
J. P. Berger ◽  
...  
Keyword(s):  
Black Hole ◽  
Near Infrared ◽  
Galactic Center ◽  
Accurate Determination ◽  
Massive Black Hole ◽  
Geometric Distance ◽  
Significance Level ◽  
Beam Combiner ◽  
Infrared Interferometry ◽  
Sgr A

We present a 0.16% precise and 0.27% accurate determination of R0, the distance to the Galactic center. Our measurement uses the star S2 on its 16-year orbit around the massive black hole Sgr A* that we followed astrometrically and spectroscopically for 27 years. Since 2017, we added near-infrared interferometry with the VLTI beam combiner GRAVITY, yielding a direct measurement of the separation vector between S2 and Sgr A* with an accuracy as good as 20 μas in the best cases. S2 passed the pericenter of its highly eccentric orbit in May 2018, and we followed the passage with dense sampling throughout the year. Together with our spectroscopy, in the best cases with an error of 7 km s−1, this yields a geometric distance estimate of R0 = 8178 ± 13stat. ± 22sys. pc. This work updates our previous publication, in which we reported the first detection of the gravitational redshift in the S2 data. The redshift term is now detected with a significance level of 20σ with fredshift = 1.04 ± 0.05.

scholarly journals Forming misaligned stellar disks around a massive black hole: cloud infall in the Galactic center

2013 ◽  
Vol 9 (S303) ◽  
pp. 245-247
Author(s):  
William Lucas ◽  
Ian Bonnell ◽  
Melvyn Davies ◽  
Ken Rice
Keyword(s):  
Black Hole ◽  
Angular Momentum ◽  
Galactic Center ◽  
Massive Black Hole ◽  
Hydrodynamic Simulations ◽  
Large Spread ◽  
Smoothed Particle ◽  
The Creation ◽  
Sgr A ◽  
Smoothed Particle Hydrodynamic

AbstractThe innermost parsec around Sgr A* has been found to play host to two disks or streamers of O and W-R stars. They are misaligned by an angle approaching 90°. That the stars are approximately coeval indicates that they formed in the same event rather than independently. We have performed smoothed particle hydrodynamic simulations of the infall of a single prolate cloud towards a massive black hole. As the cloud is disrupted, the large spread in angular momentum can, if conditions allow, lead to the creation of misaligned gas disks. In turn, stars may form within those disks. We are now investigating the origins of these clouds in the Galactic center (GC) region.

scholarly journals 7.8. Particle cascades in Sgr A∗: the possibility of observing their γ-ray signature

1998 ◽  
Vol 184 ◽  
pp. 307-308
Author(s):  
Sera Markoff ◽  
Fulvio Melia ◽  
Ina Sarcevic
Keyword(s):  
Black Hole ◽  
Particle Physics ◽  
Galactic Center ◽  
Theoretical Models ◽  
High Energy ◽  
Massive Black Hole ◽  
Inverse Compton Scattering ◽  
Black Hole Candidate ◽  
Γ Ray ◽  
Sgr A

The recent detection of a γ-ray flux from the direction of the Galactic center by EGRET on the Compton GRO raises the question of whether this is a point source (possibly coincident with the massive black hole candidate Sgr A∗) or a diffuse emitter. Using the latest experimental particle physics data and theoretical models, we have examined in detail the γ-ray spectrum produced by synchrotron, inverse Compton scattering and mesonic decay resulting from the interaction of relativistic protons with hydrogen accreting onto a point-like object. Such a distribution of high-energy baryons may be expected to form within an accretion shock as the inflowing gas becomes supersonic. This scenario is motivated by hydrodynamic studies of Bondi-Hoyle accretion onto Sgr A∗, which indicate that many of its radiative characteristics may ultimately be associated with energy liberated as this plasma descends down into the deep potential well. Earlier attempts at analyzing this process concluded that the EGRET data are inconsistent with a massive point-like object (Mastichiadis & Ozernoy, 1994). Our results demonstrate that a more careful treatment of the physics of p-p scattering suggests that a ~ 106M⊙ black hole may be contributing to this high-energy emission.

scholarly journals Hot stars in the Galactic Center

1999 ◽  
Vol 193 ◽  
pp. 449-458
Author(s):  
Andreas Eckart ◽  
Thomas Ott ◽  
Reinhard Genzel ◽  
Dieter Lutz
Keyword(s):  
Black Hole ◽  
Near Infrared ◽  
Milky Way ◽  
Infrared Imaging ◽  
Galactic Center ◽  
Hot Stars ◽  
Massive Black Hole ◽  
Central Cluster ◽  
Short Period ◽  
Sgr A

The central parsec of our Galaxy is powered by a cluster of young massive hot stars which formed a few million years ago. Within that cluster the seven most luminous (L >105.75 L⊙) and moderately hot (T < 104.5 K) blue supergiants contribute half of the ionizing luminosity of that region. These stars probably formed when a dense cloud fell into the center < 107 years ago, was highly compressed there, and became gravitationally unstable. Over six years of high spatial resolution, near-infrared imaging and spectroscopy have made it possible to carry out a detailed investigation of the stars in the central cluster and its enclosed mass. As one result of a detailed variability study of the central cluster stars we found that the bright He I star IRS 16SW is a short-period variable with a period of ∼9.72 days. It is most likely an eclipsing binary with a lower mass limit of 100 solar masses. Line of sight velocities and proper motions have been measured for these hot stars (as well as ∼200 other stars) down to separations of less than five light days from the compact radio source Sgr A* at the dynamic center of the Milky Way. These confirmed measurements imply the presence of a central dark mass of 2.6 × 106 solar masses. The dark mass at the center of the Milky Way is currently the most compelling case for a massive black hole. Simple physical considerations show that this dark mass cannot consist of a stable cluster of stars, stellar remnants, substellar condensations or a degenerate gas of elementary particles but that at least 103 to 105 solar masses must be in the form of a massive black hole associated with Sgr A* itself.

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