66 research outputs found
Modulating terrestrial impacts from Oort cloud comets by the adiabatically changing galactic tides
Time modulation of the flux of new Jupiter-dominated Oort cloud comets is the subject of interest here. The major perturbation of these comets during the present epoch is due to the tidal field of the relatively smooth distribution of matter in the galactic disk. A secondary source of the near-parabolic comet flux are stars penetrating the inner Oort cloud and providing impulses that create brief comet showers. Substantial stellar-induced showers occur approximately every 100 m.y. Less frequent (but stronger) impulses due to giant molecular clouds can also perturb comets from the inner cloud. These occur on timescales of approximately equal to 500 m.y. In contrast to these infrequent stochastic shower phenomena is the continuously varying tidal-induced flux due to the galaxy. As the Sun orbits the galactic center it undergoes quasiharmonic motion about the galactic midplane, which is superimposed on the small eccentricity, near-Keplerian motion in the plane having epicycle period approximately equal to 150 m.y. In the process the galactic tidal field on the Sun/cloud system will vary causing a modulation of the observable Oort cloud flux. We have created a model of the galactic matter distribution as it affects the solar motion over a time interval ranging from 300 m.y. in the past to 100 m.y. into the future. As constraints on the disk's compact dark matter component we require consistency with the following: (1) the observed galactic rotation curve, (2) today's flux distribution of new comets, (3) the studies of K-giant distributions, and (4) the periodicity found in the terrestrial cratering record. The adiabatically varying galactic tidal torque is then determined and used to predict the time dependence of the flux. We find that a model in which approximately half the disk matter is compact is consistent with these constraints. Under such circumstances the peak-to-trough flux variation will be approx. equal to 5:1 with a full width of 9 m.y. This variability will manifest in the terrestrial cratering record and is consistent with the observed cratering periodicity, if over half of the impacts on Earth are caused by comets or asteroids that originate in the outer Oort cloud
Equilibrium Disk-Bulge-Halo Models for the Milky Way and Andromeda Galaxies
We describe a new set of self-consistent, equilibrium disk galaxy models that
incorporate an exponential disk, a Hernquist model bulge, an NFW halo and a
central supermassive black hole. The models are derived from explicit
distribution functions for each component and the large number of parameters
permit detailed modeling of actual galaxies. We present techniques that use
structural and kinematic data such as radial surface brightness profiles,
rotation curves and bulge velocity dispersion profiles to find the best-fit
models for the Milky Way and M31. Through N-body realizations of these models
we explore their stability against the formation of bars. The models permit the
study of a wide range of dynamical phenomenon with a high degree of realism.Comment: 58 pages, 20 figures, submitted to the Astrophysical Journa
A universal angular momentum profile for galactic halos
[Abridged] We study the angular-momentum profiles of a statistical sample of
halos drawn from a high-resolution N-body simulation of the LCDM cosmology. We
find that the cumulative mass distribution of specific angular momentum, j, in
a halo of mass Mv is well fit by a universal function, M(<j) = Mv \mu
j/(j_0+j). This profile is defined by one shape parameter (\mu or j_0) in
addition to the global spin parameter \lambda. It follows a power-law over most
of the mass, and flattens at large j, with the flattening more pronounced for
small values of \mu. Compared to a uniform sphere in solid-body rotation, most
halos have a higher fraction of their mass in the low- and high-j tails of the
distribution. The spatial distribution of angular momentum in halos tends to be
cylindrical and is well-aligned within each halo for ~80% of the halos. We
investigate two ideas for the origin of this profile. The first is based on a
revised version of linear tidal-torque theory combined with extended
Press-Schechter mass accretion, and the second focuses on j transport in minor
mergers. Finally, we briefly explore implications of the M(<j) profile on the
formation of galactic disks assuming that j is conserved during an adiabatic
baryonic infall. The implied gas density profile deviates from an exponential
disk, with a higher density at small radii and a tail extending to large radii.
The steep central density profiles may imply disk scale lengths that are
smaller than observed. This is reminiscent of the "angular-momentum problem"
seen in hydrodynamic simulations, even though we have assumed perfect j
conservation. A possible solution is to associate the central excesses with
bulge components and the outer regions with extended gaseous disks.Comment: 19 pages LaTeX, uses emulateapj5, 22 embedded figures, 1 separate
figure, Submitted to ApJ, version with higher quality figures available at
http://www.astronomy.ohio-state.edu/~james/PAPER/parts.htm
Tidal torques. A critical review of some techniques
We point out that the MacDonald formula for body-tide torques is valid only
in the zeroth order of e/Q, while its time-average is valid in the first order.
So the formula cannot be used for analysis in higher orders of e/Q. This
necessitates corrections in the theory of tidal despinning and libration
damping.
We prove that when the inclination is low and phase lags are linear in
frequency, the Kaula series is equivalent to a corrected version of the
MacDonald method. The correction to MacDonald's approach would be to set the
phase lag of the integral bulge proportional to the instantaneous frequency.
The equivalence of descriptions gets violated by a nonlinear
frequency-dependence of the lag.
We explain that both the MacDonald- and Darwin-torque-based derivations of
the popular formula for the tidal despinning rate are limited to low
inclinations and to the phase lags being linear in frequency. The
Darwin-torque-based derivation, though, is general enough to accommodate both a
finite inclination and the actual rheology.
Although rheologies with Q scaling as the frequency to a positive power make
the torque diverge at a zero frequency, this reveals not the impossible nature
of the rheology, but a flaw in mathematics, i.e., a common misassumption that
damping merely provides lags to the terms of the Fourier series for the tidal
potential. A hydrodynamical treatment (Darwin 1879) had demonstrated that the
magnitudes of the terms, too, get changed. Reinstating of this detail tames the
infinities and rehabilitates the "impossible" scaling law (which happens to be
the actual law the terrestrial planets obey at low frequencies).Comment: arXiv admin note: sections 4 and 9 of this paper contain substantial
text overlap with arXiv:0712.105
LCDM-based models for the Milky Way and M31 I: Dynamical Models
We apply standard disk formation theory with adiabatic contraction within
cuspy halo models predicted by the standard LCDM cosmology. The resulting
models score remarkably well when confronted with the broad range of
observational data available for the Milky Way and M31 galaxies, giving a Milky
Way virial mass of 1-2x10^12Msun and concentration C=12-17. We consider two
types of models, in which: (A) baryons conserve angular momentum and (B) some
of the angular momentum of the baryons is transferred to the dark matter.
Type-A models produce good agreement with observed rotation curves and obey
constraints in the solar neighborhood, but may have too much dark matter in the
center to allow a fast rotating bar. The type-B models with angular momentum
transport have a slightly more massive disk and less dark matter in the central
part, allowing a fast rotating bar to persist. Both classes of models probably
have sufficient baryonic mass in the central 3.5kpc to reproduce recent
observational values of the optical depth to microlensing events towards the
Galactic center. All models require that about 1/2 of all baryons expected
inside the virial radius must not be in the disk or bulge. We investigate
whether the range of virial masses allowed by our dynamical models is
compatible with constraints from the galaxy luminosity function, and find a
range of parameter space that is allowed by this constraint. We conclude that
rotation curves and dynamical properties of ``normal'' high surface brightness
spiral galaxies appear to be consistent with standard LCDM.Comment: 26 pages, 11 figures, submited to Ap
Planetary Dynamics and Habitable Planet Formation In Binary Star Systems
Whether binaries can harbor potentially habitable planets depends on several
factors including the physical properties and the orbital characteristics of
the binary system. While the former determines the location of the habitable
zone (HZ), the latter affects the dynamics of the material from which
terrestrial planets are formed (i.e., planetesimals and planetary embryos), and
drives the final architecture of the planets assembly. In order for a habitable
planet to form in a binary star system, these two factors have to work in
harmony. That is, the orbital dynamics of the two stars and their interactions
with the planet-forming material have to allow terrestrial planet formation in
the habitable zone, and ensure that the orbit of a potentially habitable planet
will be stable for long times. We have organized this chapter with the same
order in mind. We begin by presenting a general discussion on the motion of
planets in binary stars and their stability. We then discuss the stability of
terrestrial planets, and the formation of potentially habitable planets in a
binary-planetary system.Comment: 56 pages, 29 figures, chapter to appear in the book: Planets in
Binary Star Systems (Ed. N. Haghighipour, Springer publishing company
Formation and evolution of compact binaries in globular clusters: II. Binaries with neutron stars
In this paper, the second of a series, we study the stellar dynamical and
evolutionary processes leading to the formation of compact binaries containing
neutron stars (NSs) in dense globular clusters (GCs). For this study, 70 dense
clusters were simulated independently, with a total stellar mass ~2x10^7Msun,
exceeding the total mass of all dense GCs in our Galaxy.
We find that, in order to reproduce the empirically derived formation rate of
low-mass X-ray binaries (LMXBs), we must assume that NSs can be formed via
electron-capture supernovae (ECS) with typical natal kicks smaller than in
core-collapse supernovae. Our results explain the observed dependence of the
number of LMXBs on ``collision number'' as well as the large scatter observed
between different GCs. We predict that the number of quiescent LMXBs in
different GCs should not have a strong metallicity dependence. In our cluster
model the following mass-gaining events create populations of MSPs that do not
match the observations: (i) accretion during a common envelope event with a NS
formed through ECS, and (ii) mass transfer (MT) from a WD donor. Some processes
lead only to a mild recycling. In addition, for MSPs, we distinguish
low-magnetic-field (long-lived) and high-magnetic-field (short-lived)
populations. With this distinction and by considering only those mass-gaining
events that appear to lead to NS recycling, we obtain good agreement of our
models with the numbers and characteristics of observed MSPs in 47 Tuc and
Terzan 5, as well as with the cumulative statistics for MSPs detected in GCs of
different dynamical properties. We find that significant production of merging
double NSs potentially detectable as short gamma-ray bursts occurs only in very
dense, most likely core-collapsed GCs. (abridged)Comment: 25 pages, 7 figures, 12 tables, MNRAS accepte
Field Blue Stragglers and Related Mass Transfer Issues
This chapter contains my impressions and perspectives about the current state
of knowledge about field blue stragglers (FBS) stars, drawn from an extensive
literature that I searched. I conclude my review of issues that attend FBS and
mass transfer, by a brief enumeration of a few mildly disquieting observational
facts.Comment: Chapter 4, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Misaligned Protoplanetary Disks in a Young Binary System
Many extrasolar planets follow orbits that differ from the nearly coplanar
and circular orbits found in our solar system; orbits may be eccentric or
inclined with respect to the host star's equator, and the population of giant
planets orbiting close to their host stars suggests significant orbital
migration. There is currently no consensus on what produces such orbits.
Theoretical explanations often invoke interactions with a binary companion star
on an orbit that is inclined relative to the planet's orbital plane. Such
mechanisms require significant mutual inclinations between planetary and binary
star orbital planes. The protoplanetary disks in a few young binaries are
misaligned, but these measurements are sensitive only to a small portion of the
inner disk, and the three-dimensional misalignment of the bulk of the
planet-forming disk mass has hitherto not been determined. Here we report that
the protoplanetary disks in the young binary system HK Tau are misaligned by
60{\deg}-68{\deg}, so one or both disks are significantly inclined to the
binary orbital plane. Our results demonstrate that the necessary conditions
exist for misalignment-driven mechanisms to modify planetary orbits, and that
these conditions are present at the time of planet formation, apparently due to
the binary formation process.Comment: Published in Nature, July 31 2014. 18 pages. This version has slight
differences from the final published version. Final version is available at
http://www.nature.com/nature/journal/v511/n7511/full/nature13521.htm
The relationships between workaholism and symptoms of psychiatric disorders: a large-scale cross-sectional study
Despite the many number of studies examining workaholism, large-scale studies have been lacking. The present study utilized an open web-based cross-sectional survey assessing symptoms of psychiatric disorders and workaholism among 16,426 workers (Mage = 37.3 years, SD = 11.4, range = 16â75 years). Participants were administered the Adult ADHD Self-Report Scale, the Obsession-Compulsive Inventory-Revised, the Hospital Anxiety and Depression Scale, and the Bergen Work Addiction Scale, along with additional questions examining demographic and work-related variables. Correlations between workaholism and all psychiatric disorder symptoms were positive and significant. Workaholism comprised the dependent variable in a three-step linear multiple hierarchical regression analysis. Basic demographics (age, gender, relationship status, and education) explained 1.2% of the variance in workaholism, whereas work demographics (work status, position, sector, and annual income) explained an additional 5.4% of the variance. Age (inversely) and managerial positions (positively) were of most importance. The psychiatric symptoms (ADHD, OCD, anxiety, and depression) explained 17.0% of the variance. ADHD and anxiety contributed considerably. The prevalence rate of workaholism status was 7.8% of the present sample. In an adjusted logistic regression analysis, all psychiatric symptoms were positively associated with being a workaholic. The independent variables explained between 6.1% and 14.4% in total of the variance in workaholism cases. Although most effect sizes were relatively small, the studyâs findings expand our understanding of possible psychiatric predictors of workaholism, and particularly shed new insight into the reality of adult ADHD in work life. The studyâs implications, strengths, and shortcomings are also discussed
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