2,444 research outputs found
The Telltale Heartbeat: Detection and Characterization of Eccentric Orbiting Planets via Tides on their Host Star
We present an analytic description of tides raised on a star by a small
orbiting body. In particular, we highlight the disproportionate effect of
eccentricity and thus the scope for using these tides to detect and
characterise the orbits of exoplanets and brown dwarfs. The tidal distortions
of the star produced by an eccentric orbit are, in comparison to a circular
orbit, much richer in detail, and potentially visible from any viewing angle.
The magnitude of these variations is much larger than that in a circular orbit
of the same semi-major axis. These variations are visible in both photometric
and spectroscopic data, and dominate other regular sources of phase variability
(e.g reflection and Doppler beaming) over a particularly interesting portion of
parameter space. These tidal signatures will be a useful tool for planet
detection on their own, and used in concert with other methods provide powerful
constraints on planetary and stellar properties.Comment: Accepted by ApJ, 23 pages, 13 figure
A Bright Year for Tidal Disruptions?
When a star is tidally disrupted by a supermassive black hole (BH), roughly
half of its mass falls back to the BH at super-Eddington rates. Being tenuously
gravitationally bound and unable to cool radiatively, only a small fraction
f_in << 1 of the returning debris will likely be incorporated into the disk and
accrete, with the vast majority instead becoming unbound in an outflow of
velocity ~1e4 km/s. This slow outflow spreads laterally, encasing the BH. For
months or longer, the outflow remains sufficiently neutral to block hard EUV
and X-ray radiation from the hot inner disk, which instead becomes trapped in a
radiation-dominated nebula. Ionizing nebular radiation heats the inner edge of
the ejecta to temperatures of T > few 1e4 K, converting the emission to
optical/near-UV wavelengths where photons more readily escape due to the lower
opacity. This can explain the unexpectedly low and temporally constant
effective temperatures of optically-discovered TDE flares. For BHs with
relatively high masses M_BH > 1e7 M_sun the ejecta can become ionized at an
earlier stage, or for a wider range of viewing angles, producing a TDE flare
which is instead dominated by thermal X-ray emission. We predict total radiated
energies consistent with those of observed TDE flares, and ejecta velocities
that agree with the measured emission line widths. The peak optical luminosity
for M_BH < 1e6 M_sun is suppressed due to adiabatic losses in the inner disk
wind, possibly contributing to the unexpected dearth of optical TDEs in
galaxies with low mass BHs. In the classical picture, where f_in ~ 1, TDEs
de-spin supermassive BHs and cap their maximum spins well below theoretical
accretion physics limits. This cap is greatly relaxed in our model, and
existing Fe K-alpha spin measurements provide suggestive preliminary evidence
that f_in < 1.Comment: 13 pages, 7 figures, submitted to MNRA
Periodic accretion-powered flares from colliding EMRIs as TDE Imposters
When a main sequence star undergoes Roche lobe overflow onto a supermassive
black hole (SMBH) in a circular extreme mass ratio inspiral (EMRI), a phase of
steady mass transfer ensues. Over millions of years, the binary evolves to a
period minimum before reversing course and migrating outwards. Because the time
interval between consecutive EMRIs is comparable to the mass-transfer
timescale, the semi-major axes of two consecutive mass-transferring EMRIs will
cross on a radial scale < few AU. We show that such EMRI crossing events are
inevitably accompanied by a series of mildly relativistic, grazing physical
collisions between the stars. Each collision strips a small quantity of mass,
primarily from the more massive star, which generally increases their radial
separation to set up the next collision after a delay of decades to centuries
(or longer) set by further gravitational radiation. Depending on the mass of
the SMBH, this interaction can result in N ~ 1-1e4 gas production events of
mass Msun/N, thus powering a quasi-periodic sequence of SMBH accretion-powered
flares over a total duration of thousands of years or longer. Although the EMRI
rate is 2-3 orders of magnitude lower than the rate of tidal disruption events
(TDE), the ability of a single interacting EMRI pair to produce a large number
of luminous flares - and to make more judicious use of the available stellar
fuel - could make their observed rate competitive with the TDE rate, enabling
them to masquerade as "TDE Imposters." We predict flares with luminosities that
decay both as power laws shallower than t^(-5/3) or as decaying exponentials.
Viscous spreading of the gas disks produced by the accumulation of previous
mass-stripping events places substantial mass on radial scales > 10-100 AU,
providing a reprocessing source required to explain the unexpectedly high
optical luminosities of some flares.Comment: 13 pages, submitte
An enhanced rate of tidal disruptions in the centrally overdense E+A galaxy NGC 3156
Time domain optical surveys have discovered roughly a dozen candidate stellar
tidal disruption flares in the last five years, and future surveys like the
{\it Large Synoptic Survey Telescope} will likely find hundreds to thousands
more. These tidal disruption events (TDEs) present an interesting puzzle: a
majority of the current TDE sample is hosted by rare post-starburst galaxies,
and tens of percent are hosted in even rarer E+A galaxies, which make up of all galaxies in the local universe. E+As are therefore
overrepresented among TDE hosts by 1-2 orders of magnitude, a discrepancy
unlikely to be accounted for by selection effects. We analyze {\it Hubble Space
Telescope} photometry of one of the nearest E+A galaxies, NGC~3156, to estimate
the rate of stellar tidal disruption produced as two-body relaxation diffuses
stars onto orbits in the loss cone of the central supermassive black hole. The
rate of TDEs produced by two-body relaxation in NGC~3156 is large when compared
to other galaxies with similar black hole mass: . This suggests that the preference of TDEs for E+A hosts
may be due to central stellar overdensities produced in recent starbursts.Comment: 8 pages, 4 figures, 2 tables. Minor changes made to match published
version in ApJ
A Dynamical Potential-Density Pair for Star Clusters With Nearly Isothermal Interiors
We present a potential-density pair designed to model nearly isothermal star
clusters (and similar self-gravitating systems) with a central core and an
outer turnover radius, beyond which density falls off as . In the
intermediate zone, the profile is similar to that of an isothermal sphere
(density ), somewhat less steep than the King 62 profile,
and with the advantage that many dynamical quantities can be written in a
simple closed form. We derive new analytic expressions for the cluster binding
energy and velocity dispersion, and apply these to create toy models for
cluster core collapse and evaporation. We fit our projected surface brightness
profiles to observed globular and open clusters, and find that the quality of
the fit is generally at least as good as that for the surface brightness
profiles of King 62. This model can be used for convenient computation of the
dynamics and evolution of globular and nuclear star clusters.Comment: 6 pages, 5 figures. Published in ApJL; changes to match published
versio
Circularization of Tidally Disrupted Stars around Spinning Supermassive Black Holes
We study the circularization of tidally disrupted stars on bound orbits
around spinning supermassive black holes by performing three-dimensional
smoothed particle hydrodynamic simulations with Post-Newtonian corrections. Our
simulations reveal that debris circularization depends sensitively on the
efficiency of radiative cooling. There are two stages in debris circularization
if radiative cooling is inefficient: first, the stellar debris streams
self-intersect due to relativistic apsidal precession; shocks at the
intersection points thermalize orbital energy and the debris forms a
geometrically thick, ring-like structure around the black hole. The ring
rapidly spreads via viscous diffusion, leading to the formation of a
geometrically thick accretion disk. In contrast, if radiative cooling is
efficient, the stellar debris circularizes due to self-intersection shocks and
forms a geometrically thin ring-like structure. In this case, the dissipated
energy can be emitted during debris circularization as a precursor to the
subsequent tidal disruption flare. The possible radiated energy is up to
~2*10^{52} erg for a 1 Msun star orbiting a 10^6 Msun black hole. We also find
that a retrograde (prograde) black hole spin causes the shock-induced
circularization timescale to be shorter (longer) than that of a non-spinning
black hole in both cooling cases. The circularization timescale is remarkably
long in the radiatively efficient cooling case, and is also sensitive to black
hole spin. Specifically, Lense-Thirring torques cause dynamically important
nodal precession, which significantly delays debris circularization. On the
other hand, nodal precession is too slow to produce observable signatures in
the radiatively inefficient case. We also discuss the relationship between our
simulations and the parabolic TDEs that are characteristic of most stellar
tidal disruptions.Comment: 23 pages, 18 figures, 1 appendix, accepted for publication in MNRAS
(with significant improvement
Circumnuclear Media of Quiescent Supermassive Black Holes
We calculate steady-state, one-dimensional hydrodynamic profiles of hot gas
in slowly accreting ("quiescent") galactic nuclei for a range of central black
hole masses , parametrized gas heating rates, and
observationally-motivated stellar density profiles. Mass is supplied to the
circumnuclear medium by stellar winds, while energy is injected primarily by
stellar winds, supernovae, and black hole feedback. Analytic estimates are
derived for the stagnation radius (where the radial velocity of the gas passes
through zero) and the large scale gas inflow rate, , as a function of
and the gas heating efficiency, the latter being related to the
star-formation history. We assess the conditions under which radiative
instabilities develop in the hydrostatic region near the stagnation radius,
both in the case of a single burst of star formation and for the average star
formation history predicted by cosmological simulations. By combining a sample
of measured nuclear X-ray luminosities, , of nearby quiescent galactic
nuclei with our results for we address whether the
nuclei are consistent with accreting in a steady-state, thermally-stable manner
for radiative efficiencies predicted for radiatively inefficiency accretion
flows. We find thermally-stable accretion cannot explain the short average
growth times of low mass black holes in the local Universe, which must instead
result from gas being fed in from large radii, due either to gas inflows or
thermal instabilities acting on larger, galactic scales. Our results have
implications for attempts to constrain the occupation fraction of SMBHs in low
mass galaxies using the mean correlation, as well as the
predicted diversity of the circumnuclear densities encountered by relativistic
outflows from tidal disruption events.Comment: 24 pages, 11 figures, 2 tables. Published in MNRA
Assisted Inspirals of Stellar Mass Black Holes Embedded in AGN Disks: Solving the "Final AU Problem"
We explore the evolution of stellar mass black hole binaries (BHBs) which are
formed in the self-gravitating disks of active galactic nuclei (AGN). Hardening
due to three-body scattering and gaseous drag are effective mechanisms that
reduce the semi-major axis of a BHB to radii where gravitational waves take
over, on timescales shorter than the typical lifetime of the AGN disk. Taking
observationally-motivated assumptions for the rate of star formation in AGN
disks, we find a rate of disk-induced BHB mergers (, but with large uncertainties) that is comparable with
existing estimates of the field rate of BHB mergers, and the approximate BHB
merger rate implied by the recent Advanced LIGO detection of GW150914. BHBs
formed thorough this channel will frequently be associated with luminous AGN,
which are relatively rare within the sky error regions of future gravitational
wave detector arrays. This channel could also possess a (potentially transient)
electromagnetic counterpart due to super-Eddington accretion onto the stellar
mass black hole following the merger.Comment: 10 pages, 3 figures, changes made to match MNRAS published versio
The Delay Time Distribution of Tidal Disruption Flares
Recent observations suggest that stellar tidal disruption events (TDE) are
strongly overrepresented in rare, post-starburst galaxies. Several dynamical
mechanisms have been proposed to elevate their TDE rates, ranging from central
stellar overdensities to the presence of supermassive black hole (SMBH)
binaries. Another such mechanism, introduced here, is a radial velocity
anisotropy in the nuclear star cluster produced during the starburst. These,
and other, dynamical hypotheses can be disentangled by comparing observations
to theoretical predictions for the TDE delay time distribution (DTD). We show
that SMBH binaries are a less plausible solution for the post-starburst
preference, as they can only reproduce the observed DTD with extensive
fine-tuning. The overdensity hypothesis produces a reasonable match to the
observed DTD (based on the limited data currently available), provided that the
initial stellar density profile created during the starburst, , is
exceptional in both steepness and normalization. In particular, explaining the
post-starburst preference requires with , i.e. much steeper than the classic Bahcall-Wolf equilibrium
profile of . For "ultrasteep" density cusps (),
we show that the TDE rate decays with time measured since the starburst as
. Radial anisotropies
also represent a promising explanation, provided that initial anisotropy
parameters of are sustainable against the radial orbit
instability. TDE rates in initially anisotropic cusps will decay roughly as
. As the sample of TDEs with well-studied host
galaxies grows, the DTD will become a powerful tool for constraining the
exceptional dynamical properties of post-starburst galactic nuclei.Comment: 15 pages, 10 figures, 2 appendices. Submitted to MNRAS, comments
welcom
Formation of Massive Black Holes in Galactic Nuclei: Runaway Tidal Encounters
Nuclear star clusters (NSCs) and supermassive black holes (SMBHs) both
inhabit galactic nuclei, coexisting in a range of bulge masses, but excluding
each other in the largest or smallest galaxies. We propose that the
transformation of NSCs into SMBHs occurs via runaway tidal captures, once NSCs
exceed a certain critical central density and velocity dispersion. The
bottleneck in this process, as with all collisional runaways, is growing the
first e-fold in black hole mass. The growth of a stellar mass black hole past
this bottleneck occurs as tidally captured stars are consumed in repeated
episodes of mass transfer at pericenter. Tidal captures may turn off as a
growth channel once the black hole reaches a mass ~100-1000 solar masses, but
tidal disruption events will continue and appear capable of growing the seed
SMBH to larger sizes. The runaway slows (becomes sub-exponential) once the seed
SMBH consumes the core of its host NSC. While the bulk of the cosmic mass
density in SMBHs is ultimately produced (via the Soltan-Paczynski argument) by
episodic gaseous accretion in very massive galaxies, the smallest SMBHs have
probably grown from strong tidal encounters with NSC stars. SMBH seeds that
grow for a time entirely through this channel will follow simple power law
relations with the velocity dispersion, , of their host galaxy. In the
simplest regime it is ,
but the exponents and prefactor can differ slightly depending on the details of
loss cone refilling. Current tidal disruption event rates predicted from this
mechanism are consistent with observations.Comment: 18 pages, 9 figures, comments welcom
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