377 research outputs found
Collisional Shaping of Nuclear Star Cluster Density Profiles
A supermassive black hole (SMBH) surrounded by a dense, nuclear star cluster
resides at the center of many galaxies. In this dense environment,
high-velocity collisions frequently occur between stars. About of the
stars within the Milky Way's nuclear star cluster collide with other stars
before evolving off the main-sequence. Collisions preferentially affect
tightly-bound stars, which orbit most quickly and pass through regions of the
highest stellar density. Over time, collisions therefore shape the bulk
properties of the nuclear star cluster. We examine the effect of collisions on
the cluster's stellar density profile. We show that collisions produce a
turning point in the density profile which can be determined analytically.
Varying the initial density profile and collision model, we characterize the
evolution of the stellar density profile over Gyr. We find that old,
initially cuspy populations exhibit a break around pc in their density
profile, while shallow density profiles retain their initial shape outside of
pc. The initial density profile is always preserved outside of a few
tenths of parsec irrespective of initial conditions. Lastly, we comment on the
implications of collisions for the luminosity and color of stars in the
collisionly-shaped inner cluster.Comment: Submitted to ApJ Letters. Comments welcom
Runaway Coalescence at the Onset of Common Envelope Episodes
Luminous red nova transients, presumably from stellar coalescence, exhibit
long-term precursor emission over hundreds of binary orbits, leading to
impulsive outbursts with durations similar to a single orbital period. In an
effort to understand these signatures, we present and analyze a hydrodynamic
model of unstable mass transfer from a giant-star donor onto a more compact
accretor in a binary system. Our simulation begins with mass transfer at the
Roche limit separation and traces a phase of runaway decay leading up to the
plunge of the accretor within the envelope of the donor. We characterize the
fluxes of mass and angular momentum through the system and show that the
orbital evolution can be reconstructed from measurements of these quantities.
The morphology of outflow from the binary changes significantly as the binary
orbit tightens. At wide separations, a thin stream of relatively high-entropy
gas trails from the outer Lagrange points. As the orbit tightens, the orbital
motion desynchronizes from the donor's rotation, and low-entropy ejecta trace a
broad fan of largely ballistic trajectories. An order-of-magnitude increase in
mass ejection rate accompanies the plunge of the accretor with the envelope of
the donor. We argue that this transition marks the precursor-to-outburst
transition observed in stellar coalescence transients.Comment: Revised following peer-review. ApJ accepted. Animated version of
Figure 5 will be available via the Journal's online publicatio
Bound Outflows, Unbound Ejecta, and the Shaping of Bipolar Remnants during Stellar Coalescence
Recent observations have revealed that the remnants of stellar-coalescence
transients are bipolar. This raises the questions of how these bipolar
morphologies arise and what they teach us about the mechanisms of mass ejection
during stellar mergers and common-envelope phases. In this paper, we analyze
hydrodynamic simulations of the lead-in to binary coalescence, a phase of
unstable Roche lobe overflow that takes the binary from the Roche limit
separation to the engulfment of the more compact accretor within the envelope
of the extended donor. As mass transfer runs away at increasing rates, gas
trails away from the binary. Contrary to previous expectations, early mass loss
from the system remains bound to the binary and forms a circumbinary torus.
Later ejecta, generated as the accretor grazes the surface of the donor, have
very different morphologies and are unbound. These two components of mass loss
from the binary interact as later, higher-velocity ejecta collide with the
circumbinary torus formed by earlier mass loss. Unbound ejecta are redirected
toward the poles, and escaping material creates a bipolar outflow. Our findings
show that the transition from bound to unbound ejecta from coalescing binaries
can explain the bipolar nature of their remnants, with implications for our
understanding of the origin of bipolar remnants of stellar-coalescence
transients and, perhaps, some preplanetary nebulae.Comment: ApJ accepte
A Consistent Picture Emerges: A Compact X-ray Continuum Emission Region in the Gravitationally Lensed Quasar SDSS J0924+0219
We analyze the optical, UV, and X-ray microlensing variability of the lensed
quasar SDSS J0924+0219 using six epochs of Chandra data in two energy bands
(spanning 0.4-8.0 keV, or 1-20 keV in the quasar rest frame), 10 epochs of
F275W (rest-frame 1089A) Hubble Space Telescope data, and high-cadence R-band
(rest-frame 2770A) monitoring spanning eleven years. Our joint analysis
provides robust constraints on the extent of the X-ray continuum emission
region and the projected area of the accretion disk. The best-fit half-light
radius of the soft X-ray continuum emission region is between 5x10^13 and 10^15
cm, and we find an upper limit of 10^15 cm for the hard X-rays. The best-fit
soft-band size is about 13 times smaller than the optical size, and roughly 7
GM_BH/c^2 for a 2.8x10^8 M_sol black hole, similar to the results for other
systems. We find that the UV emitting region falls in between the optical and
X-ray emitting regions at 10^14 cm < r_1/2,UV < 3x10^15 cm. Finally, the
optical size is significantly larger, by 1.5*sigma, than the theoretical
thin-disk estimate based on the observed, magnification-corrected I-band flux,
suggesting a shallower temperature profile than expected for a standard disk.Comment: Replaced with accepted version to Ap
Merger of white dwarf-neutron star binaries: Prelude to hydrodynamic simulations in general relativity
White dwarf-neutron star binaries generate detectable gravitational
radiation. We construct Newtonian equilibrium models of corotational white
dwarf-neutron star (WDNS) binaries in circular orbit and find that these models
terminate at the Roche limit. At this point the binary will undergo either
stable mass transfer (SMT) and evolve on a secular time scale, or unstable mass
transfer (UMT), which results in the tidal disruption of the WD. The path a
given binary will follow depends primarily on its mass ratio. We analyze the
fate of known WDNS binaries and use population synthesis results to estimate
the number of LISA-resolved galactic binaries that will undergo either SMT or
UMT. We model the quasistationary SMT epoch by solving a set of simple ordinary
differential equations and compute the corresponding gravitational waveforms.
Finally, we discuss in general terms the possible fate of binaries that undergo
UMT and construct approximate Newtonian equilibrium configurations of merged
WDNS remnants. We use these configurations to assess plausible outcomes of our
future, fully relativistic simulations of these systems. If sufficient WD
debris lands on the NS, the remnant may collapse, whereby the gravitational
waves from the inspiral, merger, and collapse phases will sweep from LISA
through LIGO frequency bands. If the debris forms a disk about the NS, it may
fragment and form planets.Comment: 28 pages, 25 figures, 6 table
The Process of Stellar Tidal Disruption by Supermassive Black Holes. The first pericenter passage
Tidal disruption events (TDEs) are among the brightest transients in the
optical, ultraviolet, and X-ray sky. These flares are set into motion when a
star is torn apart by the tidal field of a massive black hole, triggering a
chain of events which is -- so far -- incompletely understood. However, the
disruption process has been studied extensively for almost half a century, and
unlike the later stages of a TDE, our understanding of the disruption itself is
reasonably well converged. In this Chapter, we review both analytical and
numerical models for stellar tidal disruption. Starting with relatively simple,
order-of-magnitude physics, we review models of increasing sophistication, the
semi-analytic ``affine formalism,'' hydrodynamic simulations of the disruption
of polytropic stars, and the most recent hydrodynamic results concerning the
disruption of realistic stellar models. Our review surveys the immediate
aftermath of disruption in both typical and more unusual TDEs, exploring how
the fate of the tidal debris changes if one considers non-main sequence stars,
deeply penetrating tidal encounters, binary star systems, and sub-parabolic
orbits. The stellar tidal disruption process provides the initial conditions
needed to model the formation of accretion flows around quiescent massive black
holes, and in some cases may also lead to directly observable emission, for
example via shock breakout, gravitational waves or runaway nuclear fusion in
deeply plunging TDEs.Comment: Review chapter in book: 'The Tidal Disruption of Stars by Massive
Black Holes', Space Science Reviews, Springer. Comments welcom
A Description of Quasar Variability Measured Using Repeated SDSS and POSS Imaging
We provide a quantitative description and statistical interpretation of the
optical continuum variability of quasars. The Sloan Digital Sky Survey (SDSS)
has obtained repeated imaging in five UV-to-IR photometric bands for 33,881
spectroscopically confirmed quasars. About 10,000 quasars have an average of 60
observations in each band obtained over a decade along Stripe 82 (S82), whereas
the remaining ~25,000 have 2-3 observations due to scan overlaps. The observed
time lags span the range from a day to almost 10 years, and constrain quasar
variability at rest-frame time lags of up to 4 years, and at rest-frame
wavelengths from 1000A to 6000A. We publicly release a user-friendly catalog of
quasars from the SDSS Data Release 7 that have been observed at least twice in
SDSS or once in both SDSS and the Palomar Observatory Sky Survey, and we use it
to analyze the ensemble properties of quasar variability. Based on a damped
random walk (DRW) model defined by a characteristic time scale and an
asymptotic variability amplitude that scale with the luminosity, black hole
mass, and rest wavelength for individual quasars calibrated in S82, we can
fully explain the ensemble variability statistics of the non-S82 quasars such
as the exponential distribution of large magnitude changes. All available data
are consistent with the DRW model as a viable description of the optical
continuum variability of quasars on time scales of ~5-2000 days in the rest
frame. We use these models to predict the incidence of quasar contamination in
transient surveys such as those from PTF and LSST.Comment: 33 pages, 19 figures, replaced with accepted version. Catalog is
available at http://www.astro.washington.edu/users/ivezic/macleod/qso_dr7
COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses XIII: Time delays and 9-yr optical monitoring of the lensed quasar RX J1131-1231
We present the results from nine years of optically monitoring the
gravitationally lensed z=0.658 quasar RX J1131-1231. The R-band light curves of
the four individual images of the quasar were obtained using deconvolution
photometry for a total of 707 epochs. Several sharp quasar variability features
strongly constrain the time delays between the quasar images. Using three
different numerical techniques, we measure these delays for all possible pairs
of quasar images while always processing the four light curves simultaneously.
For all three methods, the delays between the three close images A, B, and C
are compatible with being 0, while we measure the delay of image D to be 91
days, with a fractional uncertainty of 1.5% (1 sigma), including systematic
errors. Our analysis of random and systematic errors accounts in a realistic
way for the observed quasar variability, fluctuating microlensing magnification
over a broad range of temporal scales, noise properties, and seasonal gaps.
Finally, we find that our time-delay measurement methods yield compatible
results when applied to subsets of the data.Comment: 11 pages, 9 figures, minor additions to the text only, techniques and
results remain unchanged, A&A in pres
Time Delay and Accretion Disk Size Measurements in the Lensed Quasar SBS 0909+532 from Multiwavelength Microlensing Analysis
We present three complete seasons and two half-seasons of Sloan Digital Sky Survey (SDSS) r-band photometry of the gravitationally lensed quasar SBS 0909+532 from the U.S. Naval Observatory, as well as two seasons each of SDSS g-band and r-band monitoring from the Liverpool Robotic Telescope. Using Monte Carlo simulations to simultaneously measure the system’s time delay and model the r-band microlensing variability, we confirm and significantly refine the precision of the system’s time delay to ΔtAB = 50+2 −4 days, where the stated uncertainties represent the bounds of the formal 1σ confidence interval. There may be a conflict between the time delay measurement and a lens consisting of a single galaxy. While models based on the Hubble Space Telescope astrometry and a relatively compact stellar distribution can reproduce the observed delay, the models have somewhat less dark matter than we would typically expect. We also carry out a joint analysis of the microlensing variability in the r and g bands to constrain the size of the quasar’s continuum source at these wavelengths, obtaining log{(rs,r/cm)[cos i/0.5]1/2} = 15.3 ± 0.3 and log{(rs,g/cm)[cos i/0.5]1/2} = 14.8 ± 0.9, respectively. Our current results do not formally constrain the temperature profile of the accretion disk but are consistent with the expectations of standard thin disk theory
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