97 research outputs found
Large mass inflow rates in Saturn's rings due to ballistic transport and mass loading
The Cassini mission provided key measurements needed to determine the
absolute age of Saturn's rings, including the extrinsic micrometeoroid flux at
Saturn, the volume fraction of non-icy pollutants in the rings, and the total
ring mass. These three factors constrain the ring age to be no more than a few
100 Myr (Kempf et al., 2023). Observations during the Cassini Grand Finale also
showed that the rings are losing mass to the planet at a prodigious rate. Some
of the mass flux falls as "ring rain" at high latitudes. However, the influx in
ring rain is considerably less than the total measured mass influx of 4800 to
45000 kg/s at lower latitudes (Waite et al., 2018).
In addition to polluting the rings, micrometeoroid impacts lead to ballistic
transport, the mass and angular momentum transport due to net exchanges of
meteoroid impact ejecta. Because the ejecta are predominantly prograde, they
carry net angular momentum outward. As a result, ring material drifts inward
toward the planet. Here, for the first time, we use a simple model to quantify
this radial mass inflow rate for dense rings and find that, for plausible
choices of parameters, ballistic transport and mass loading by meteoroids can
produce a total inward flux of material in the inner B ring and in the C ring
that is on the order of a few x 10^3 to a few x 10^4 kg/s, in agreement with
measurements during the Cassini Grand Finale. From these mass inflow rates, we
estimate that the remaining ring lifetime is ~15 to 400 Myr. Combining this
with a revised pollution age of ~120 Myr, we conclude that Saturn's rings are
not only young but ephemeral and probably started their evolution on a similar
timescale to their pollution age with an initial mass of one to a few Mimas
masses.Comment: 25 pages, 6 figures, Published in Icaru
Constraints on the initial mass, age and lifetime of Saturn's rings from viscous evolutions that include pollution and transport due to micrometeoroid bombardment
The Cassini spacecraft provided key measurements during its more than twelve
year mission that constrain the absolute age of Saturn's rings. These include
the extrinsic micrometeoroid flux at Saturn, the volume fraction of non-icy
pollutants in the rings, and a measurement of the ring mass. These observations
taken together limit the ring exposure age to be < a few 100 Myr if the flux
was persistent over that time (Kempf et al., 2023). In addition, Cassini
observations during the Grand Finale further indicate the rings are losing mass
(Hsu et al., 2018; Waite et al., 2018) suggesting the rings are ephemeral as
well. In a companion paper (Durisen and Estrada, 2023), we show that the
effects of micrometeoroid bombardment and ballistic transport of their impact
ejecta can account for these loss rates for reasonable parameter choices. In
this paper, we conduct numerical simulations of an evolving ring in a
systematic way in order to determine initial conditions that are consistent
with these observations.Comment: 31 pages, 19 figures, 2 tables, Published in Icaru
Planetary ring studies
The following topics are covered: (1) characterization of the fine scale structure in Saturn's A and B rings; (2) ballistic transport modeling and evolution of fine ring structure; (3) faint features in the rings of Saturn; (4) the Encke moonlet; (5) dynamics in ringmoon systems; (6) a nonclassical radiative transfer model; and (7) particle properties from stellar occultation data
Star formation environments and the distribution of binary separations
We have carried out K-band speckle observations of a sample of 114 X-ray
selected weak-line T Tauri stars in the nearby Scorpius-Centaurus OB
association. We find that for binary T Tauri stars closely associated to the
early type stars in Upper Scorpius, the youngest subgroup of the OB
association, the peak in the distribution of binary separations is at 90 A.U.
For binary T Tauri stars located in the direction of an older subgroup, but not
closely associated to early type stars, the peak in the distribution is at 215
A.U. A Kolmogorov-Smirnov test indicates that the two binary populations do not
result from the same distibution at a significance level of 98%. Apparently,
the same physical conditions which facilitate the formation of massive stars
also facilitate the formation of closer binaries among low-mass stars, whereas
physical conditions unfavorable for the formation of massive stars lead to the
formation of wider binaries among low-mass stars. The outcome of the binary
formation process might be related to the internal turbulence and the angular
momentum of molecular cloud cores, magnetic field, the initial temperature
within a cloud, or - most likely - a combination of all of these. We conclude
that the distribution of binary separations is not a universal quantity, and
that the broad distribution of binary separations observed among main-sequence
stars can be explained by a superposition of more peaked binary distributions
resulting from various star forming environments. The overall binary frequency
among pre-main-sequence stars in individual star forming regions is not
necessarily higher than among main-sequence stars.Comment: 7 pages, Latex, 4 Postscript figures; also available at
http://spider.ipac.caltech.edu/staff/brandner/pubs/pubs.html ; accepted for
publication in ApJ Letter
Gravitational instabilities in a protosolar-like disc - I. Dynamics and chemistry
MGE gratefully acknowledges a studentship from the European Research Council (ERC; project PALs 320620). JDI gratefully acknowledges funding from the European Union FP7-2011 under grant agreement no. 284405. ACB's contribution was supported, in part, by The University of British Columbia and the Canada Research Chairs program. PC and TWH acknowledge the financial support of the European Research Council (ERC; project PALs 320620).To date, most simulations of the chemistry in protoplanetary discs have used 1 + 1D or 2D axisymmetric α-disc models to determine chemical compositions within young systems. This assumption is inappropriate for non-axisymmetric, gravitationally unstable discs, which may be a significant stage in early protoplanetary disc evolution. Using 3D radiative hydrodynamics, we have modelled the physical and chemical evolution of a 0.17 M⊙ self-gravitating disc over a period of 2000 yr. The 0.8 M⊙ central protostar is likely to evolve into a solar-like star, and hence this Class 0 or early Class I young stellar object may be analogous to our early Solar system. Shocks driven by gravitational instabilities enhance the desorption rates, which dominate the changes in gas-phase fractional abundances for most species. We find that at the end of the simulation, a number of species distinctly trace the spiral structure of our relatively low-mass disc, particularly CN. We compare our simulation to that of a more massive disc, and conclude that mass differences between gravitationally unstable discs may not have a strong impact on the chemical composition. We find that over the duration of our simulation, successive shock heating has a permanent effect on the abundances of HNO, CN and NH3, which may have significant implications for both simulations and observations. We also find that HCO+ may be a useful tracer of disc mass. We conclude that gravitational instabilities induced in lower mass discs can significantly, and permanently, affect the chemical evolution, and that observations with high-resolution instruments such as Atacama Large Millimeter/submillimeter Array (ALMA) offer a promising means of characterizing gravitational instabilities in protosolar discs.Publisher PDFPeer reviewe
The Thermal Regulation of Gravitational Instabilities in Protoplanetary Disks II. Extended Simulations with Varied Cooling Rates
In order to investigate mass transport and planet formation by gravitational
instabilities (GIs), we have extended our 3-D hydrodynamic simulations of
protoplanetary disks from a previous paper. Our goal is to determine the
asymptotic behavior of GIs and how it is affected by different constant cooling
times. Initially, Rdisk = 40 AU, Mdisk = 0.07 Mo, M* = 0.5 Mo, and Qmin = 1.8.
Sustained cooling, with tcool = 2 orps (outer rotation periods, 1 orp ~ 250
yrs), drives the disk to instability in ~ 4 orps. This calculation is followed
for 23.5 orps. After 12 orps, the disk settles into a quasi-steady state with
sustained nonlinear instabilities, an average Q = 1.44 over the outer disk, a
well-defined power-law Sigma(r), and a roughly steady Mdot ~ 5(-7) Mo/yr. The
transport is driven by global low-order spiral modes. We restart the
calculation at 11.2 orps with tcool = 1 and 1/4 orp. The latter case is also
run at high azimuthal resolution. We find that shorter cooling times lead to
increased Mdots, denser and thinner spiral structures, and more violent dynamic
behavior. The asymptotic total internal energy and the azimuthally averaged
Q(r) are insensitive to tcool. Fragmentation occurs only in the high-resolution
tcool = 1/4 orp case; however, none of the fragments survive for even a quarter
of an orbit. Ring-like density enhancements appear and grow near the boundary
between GI active and inactive regions. We discuss the possible implications of
these rings for gas giant planet formation.Comment: Due to document size restrictions, the complete manuscript could not
be posted on astroph. Please go to http://westworld.astro.indiana.edu to
download the full document including figure
Gravitational Radiation from Nonaxisymmetric Instability in a Rotating Star
We present the first calculations of the gravitational radiation produced by
nonaxisymmetric dynamical instability in a rapidly rotating compact star. The
star deforms into a bar shape, shedding of its mass and
of its angular momentum. The gravitational radiation is calculated in the
quadrupole approximation. For a mass M and radius km, the gravitational waves have frequency kHz and amplitude
at the distance of the Virgo Cluster. They carry off
energy and radiate angular momentum .Comment: 16 pages, LaTeX with REVTEX macros, reprints available - send mailing
address to [email protected]. Published: PRL 72, 1314 (1994
Simulated Observations of Young Gravitationally Unstable Protoplanetary Discs
The formation and earliest stages of protoplanetary discs remain poorly
constrained by observations. ALMA will soon revolutionise this field.
Therefore, it is important to provide predictions which will be valuable for
the interpretation of future high sensitivity and high angular resolution
observations. Here we present simulated ALMA observations based on radiative
transfer modelling of a relatively massive (0.39 M_solar) self-gravitating disc
embedded in a 10 M_solar dense core, with structure similar to the pre-stellar
core L1544. We focus on simple species and conclude that C17O 3-2, HCO+ 3-2,
OCS 26-25 and H2CO 404-303 lines can be used to probe the disc structure and
kinematics at all scales.Comment: 12 pages, 15 figures, Accepted by MNRA
Combined Structural and Compositional Evolution of Planetary Rings Due to Micrometeoroid Impacts and Ballistic Transport
We introduce improved numerical techniques for simulating the structural and compositional evolution of planetary rings due to micrometeoroid bombardment and subsequent ballistic transport of impact ejecta. Our current, robust code is capable of modeling structural changes and pollution transport simultaneously over long times on both local and global scales. In this paper, we describe the methodology based on the original structural code of Durisen et al. (1989, Icarus 80, 136-166) and on the pollution transport code of Cuzzi and Estrada (1998, Icarus 132, 1-35). We provide demonstrative simulations to compare with, and extend upon previous work, as well as examples of how ballistic transport can maintain the observed structure in Saturn's rings using available Cassini occultation optical depth data. In particular, we explicitly verify the claim that the inner B (and presumably A) ring edge can be maintained over long periods of time due to an ejecta distribution that is heavily biased in the prograde direction through a balance between the sharpening effects of ballistic transport and the broadening effects of viscosity. We also see that a "ramp"-like feature forms over time just inside that edge. However, it does not remain linear for the duration of the runs presented here unless a less steep ejecta velocity distribution is adopted. We also model the C ring plateaus and find that their outer edges can be maintained at their observed sharpness for long periods due to ballistic transport. We hypothesize that the addition of a significant component of a retrograde-biased ejecta distribution may help explain the linearity of the ramp and is probably essential for maintaining the sharpness of C ring plateau inner edges. This component would arise for the subset of micrometeoroid impacts which are destructive rather than merely cratering. Such a distribution will be introduced in future work
Hydraulic/Shock-Jumps in Protoplanetary Disks
In this paper, we describe the nonlinear outcome of spiral shocks in
protoplanetary disks. Spiral shocks, for most protoplanetary disk conditions,
create a loss of vertical force balance in the post-shock region and result in
rapid expansion of the gas perpendicular to the disk midplane. This expansion
has characteristics similar to hydraulic jumps, which occur in incompressible
fluids. We present a theory to describe the behavior of these hybrids between
shocks and hydraulic jumps (shock bores) and then compare the theory to
three-dimensional hydrodynamics simulations. We discuss the fully
three-dimensional shock structures that shock bores produce and discuss
possible consequences for disk mixing, turbulence, and evolution of solids.Comment: 39 pages, 18 figures, 1 table. Edited to match as closely as possible
the ApJ proofs, which resulted in the correction of several typos. In
addition, section 5.3 was slightly altered because an error in an analysis
tool was discovered; the differences between the entropy gradient method and
the Schwarzschild criterion method are minor. Figure 18 now only includes
what was Figure18
- …