909 research outputs found
The Eccentric Disc Instability: Dependency on Background Stellar Cluster
In this paper we revisit the "eccentric disc instability", an instability
which occurs in coherently eccentric discs of stars orbiting massive black
holes (MBHs) embedded in stellar clusters, which results in stars achieving
either very high or low eccentricities. The preference for stars to attain
higher or lower eccentricities depends significantly on the density
distribution of the surrounding stellar cluster. Here we discuss its mechanism
and the implications for the Galactic Centre, home to at least one circum-MBH
stellar disc.Comment: Proceedings article to be published in "The Galactic Center: A Window
on the Nuclear Environment of Disk Galaxies", ed. Mark Morris, Daniel Q. Wang
and Feng Yua
Going with the flow: using gas clouds to probe the accretion flow feeding Sgr A*
The massive black hole in our galactic center, Sgr A*, accretes only a small
fraction of the gas available at its Bondi radius. The physical processes
determining this accretion rate remain unknown, partly due to a lack of
observational constraints on the gas at distances between ~10 and ~10
Schwarzschild radii (Rs) from the black hole. Recent infrared observations
identify low-mass gas clouds, G1 and G2, moving on highly eccentric, nearly
co-planar orbits through the accretion flow around Sgr A*. Although it is not
yet clear whether these objects contain embedded stars, their extended gaseous
envelopes evolve independently as gas clouds. In this paper we attempt to use
these gas clouds to constrain the properties of the accretion flow at ~10
Rs. Assuming that G1 and G2 follow the same trajectory, we model the small
differences in their orbital parameters as evolution resulting from interaction
with the background flow. We find evolution consistent with the G-clouds
originating in the clockwise disk. Our analysis enables the first unique
determination of the rotation axis of the accretion flow: we localize the
rotation axis to within 20 degrees, finding an orientation consistent with the
parsec-scale jet identified in x-ray observations and with the circumnuclear
disk, a massive torus of molecular gas ~1.5 pc from Sgr A*. This suggests that
the gas in the accretion flow comes predominantly from the circumnuclear disk,
rather than the winds of stars in the young clockwise disk. This result will be
tested by the Event Horizon Telescope within the next year. Our model also
makes testable predictions for the orbital evolution of G1 and G2, falsifiable
on a 5-10 year timescale.Comment: 14 pages, 10 figures; submitted to MNRA
A new inclination instability reshapes Keplerian disks into cones: application to the outer Solar System
Disks of bodies orbiting a much more massive central object are extremely
common in astrophysics. When the orbits comprising such disks are eccentric, we
show they are susceptible to a new dynamical instability. Gravitational forces
between bodies in the disk drive exponential growth of their orbital
inclinations and clustering in their angles of pericenter, expanding an
initially thin disk into a conical shape by giving each orbit an identical
'tilt' with respect to the disk plane. This new instability dynamically
produces the unusual distribution of orbits observed for minor planets beyond
Neptune, suggesting that the instability has shaped the outer Solar System. It
also implies a large initial disk mass (1-10 Earth masses) of scattered bodies
at hundreds of AU; we predict increasing numbers of detections of minor planets
clustered in their angles of pericenter with high inclinations.Comment: 6 pages, 4 figures, submitted to MNRAS Letter
Giant Planet Influence on the Collective Gravity of a Primordial Scattered Disk
Axisymmetric disks of high eccentricity, low mass bodies on near-Keplerian
orbits are unstable to an out-of-plane buckling. This "inclination instability"
exponentially grows the orbital inclinations, raises perihelia distances and
clusters in argument of perihelion. Here we examine the instability in a
massive primordial scattered disk including the orbit-averaged gravitational
influence of the giant planets. We show that differential apsidal precession
induced by the giant planets will suppress the inclination instability unless
the primordial mass is Earth masses. We also show that the
instability should produce a "perihelion gap" at semi-major axes of hundreds of
AU, as the orbits of the remnant population are more likely to have extremely
large perihelion distances () than intermediate
values
The Effect of General Relativistic Precession on Tidal Disruption Events from Eccentric Nuclear Disks
An eccentric nuclear disk consists of stars moving on apsidally-aligned
orbits around a central black hole. The secular gravitational torques that
dynamically stabilize these disks can also produce tidal disruption events
(TDEs) at very high rates in Newtonian gravity. General relativity, however, is
known to quench secular torques via rapid apsidal precession.
Here we show that for a disk to black hole mass ratio , the
system is in the full loss cone regime. The magnitude of the torque per orbital
period acting on a stellar orbit means that general relativistic precession
does not have a major effect on the dynamics. Thus we find that TDE rates from
eccentric nuclear disks are not affected by general relativistic precession.
Furthermore, we show that orbital elements between successive TDEs from
eccentric nuclear disks are correlated, potentially resulting in unique
observational signatures.Comment: 10 pages, 8 figures, submitted to Ap
Using gas clouds to probe the accretion flow around SgrA*: G2's delayed pericenter passage
We study the dynamical evolution of the putative gas clouds G1 and G2
recently discovered in the Galactic center. Following earlier studies
suggesting that these two clouds are part of a larger gas streamer, we combine
their orbits into a single trajectory. Since the gas clouds experience a drag
force from background gas, this trajectory is not exactly Keplerian. By
assuming the G1 and G2 clouds trace this trajectory, we fit for the drag force
they experience and thus extract information about the accretion flow at a
distance of thousands of Schwarzschild radii from the black hole. This range of
radii is important for theories of black hole accretion, but is currently
unconstrained by observations.
In this paper we extend our previous work by accounting for radial forces due
to possible inflow or outflow of the background gas. Such radial forces drive
precession in the orbital plane, allowing a slightly better fit to the G1 and
G2 data. This precession delays the pericenter passage of G2 by 4-5 months
relative to estimates derived from a Keplerian orbital fit; if it proves
possible to identify the pericenter time observationally, this enables an
immediate test of whether G1 and G2 are gas clouds part of a larger gas
streamer. If G2 is indeed a gas cloud, its closest approach likely occurred in
late summer 2014, after many of the observing campaigns monitoring G2's
anticipated pericenter passage ended. We discuss how this affects
interpretation of the G2 observations.Comment: 9 pages, submitted to MNRA
The Coupling of Galactic Dark Matter Halos with Stellar Bars
Resonant torques couple stellar bars to dark matter halos. Here we use
high-resolution numerical simulations to demonstrate long-term angular momentum
transfer between stellar bars and dark matter orbits of varying orientation. We
show that bar-driven reversals of dark matter orbit orientations can play a
surprisingly large role in the evolution of the bar pattern speed.
In predominantly prograde (co-rotating) halos, dark matter orbits become
trapped in the stellar bar forming a parallel dark matter bar. This dark matter
bar reaches more than double the vertical height of the stellar bar. In halos
dominated by retrograde orbits, a dark matter wake forms oriented perpendicular
to the stellar bar. These dark matter over-densities provide a novel space to
look for dark matter annihilation or decay signals. % We predict that the Milky
Way hosts a dark matter bar aligned with the stellar bar as well as a dark
matter wake the near-side of which should extend from Galactic center to a
galactic longitude of .Comment: 15 pages, 15 figure
The Hills Mechanism and the Galactic Center S-stars
Our Galactic center contains young stars, including the few million year old
clockwise disk between 0.05 and 0.5 pc from the Galactic center, and the S-star
cluster of B-type stars at a galactocentric distance of ~0.01 pc. Recent
observations suggest the S-stars are remnants of tidally disrupted binaries
from the clockwise disk. In particular, Koposov et al. 2020 discovered a
hypervelocity star that was ejected from the Galactic center 5 Myr ago, with a
velocity vector consistent with the disk. We perform a detailed study of this
binary disruption scenario. First, we quantify the plausible range of binary
semimajor axes in the disk. Dynamical evaporation of such binaries is dominated
by other disk stars rather than by the isotropic stellar population. For the
expected range of semimajor axes in the disk, binary tidal disruptions can
reproduce the observed S-star semimajor axis distribution. Reproducing the
observed thermal eccentricity distribution of the S-stars requires an
additional relaxation process. The flight time of the Koposov star suggests
that this process must be effective within 10 Myr. We consider three
possibilities: (i) scalar resonant relaxation from the observed isotropic star
cluster, (ii) torques from the clockwise disk, and (iii) an intermediate-mass
black hole. We conclude that the first and third mechanisms are fast enough to
reproduce the observed S-star eccentricity distribution. Finally, we show that
the primary star from an unequal-mass binary would be deposited at larger
semimajor axes than the secondary, possibly explaining the dearth of O stars
among the S-stars.Comment: 21 pages, 16 figures + Appendix. Accepted for publication in ApJ. The
new version is substantially improved with a more detailed treatment of
resonant and nonresonant relaxation from the observed isotropic cluster in
the Galactic center. In contrast to the first version, we now conclude that
these processes alone can reproduce the S-stars' eccentricity distribution in
less than 10 My
Magnetized Gas Clouds can Survive Acceleration by a Hot Wind
We present three-dimensional magnetohydrodynamic simulations of magnetized
gas clouds accelerated by hot winds. We initialize gas clouds with tangled
internal magnetic fields and show that this field suppresses the disruption of
the cloud: rather than mixing into the hot wind as found in hydrodynamic
simulations, cloud fragments end up co-moving and in pressure equilibrium with
their surroundings. We also show that a magnetic field in the hot wind enhances
the drag force on the cloud by a factor ~(1+v_A^2/v_wind^2)$, where v_A is the
Alfven speed in the wind and v_wind measures the relative speed between the
cloud and the wind. We apply this result to gas clouds in several astrophysical
contexts, including galaxy clusters, galactic winds, the Galactic center, and
the outskirts of the Galactic halo. Our results can explain the prevalence of
cool gas in galactic winds and galactic halos and how such cool gas survives in
spite of its interaction with hot wind/halo gas. We also predict that drag
forces can lead to a deviation from Keplerian orbits for the G2 cloud in the
galactic center.Comment: Submitted to MNRAS Letter
Secular Eccentricity Oscillations in Axisymmetric Disks of Eccentric Orbits
Massive bodies undergo orbital eccentricity oscillations when embedded in an
axisymmetric disk of smaller mass orbits. These eccentricity oscillations are
driven by secular torques that seek to equalize the apsidal precession rates of
all the orbits in the disk. We investigate this mechanism within the context of
detached objects in the outer Solar System, but we find that the oscillation
timescale is too long for it to be dynamically important. It could however be
interesting for phenomenon a bit farther from home; namely, feeding
supermassive blackholes and polluting the surfaces of white dwarf stars.Comment: Accepted for publicatio
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