46 research outputs found
A refurbished convergent point method for finding moving groups in the Hipparcos Catalogue
The Hipparcos data allow a major step forward in the research of `moving
groups' in the Solar neighbourhood, as the common motion of group members
causes converging proper motions. Previous knowledge on these coherent
structures in velocity space has always been limited by the availability,
reliability, and accuracy of ground-based proper motion measurements.
A refurbishment of Jones' convergent point method is presented which takes
full advantage of the quality of the Hipparcos data. The original
implementation of this method determines the maximum likelihood convergent
point on a grid on the sky and simultaneously selects group members from a
given set of stars with positions and proper motions. The refurbished procedure
takes into account the full covariance matrix of the Hipparcos measurements
instead of standard errors only, allows for internal motions of the stars, and
replaces the grid-based approach by a direct minimization. The method is tested
on Monte Carlo simulations of moving groups, and applied to the Hyades. Despite
the limited amount of data used by the convergent point method, the results for
stars in and around the cluster- centre region agree very well with those of
the recent comprehensive study by Perryman et al. (1998).Comment: 14 pages, 7 Postscript figures, LaTeX using mn.sty and psfig.sty;
accepted for publication in MNRA
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
Peculiarities in Velocity Dispersion and Surface Density Profiles of Star Clusters
Based on our recent work on tidal tails of star clusters (Kuepper et al.
2009) we investigate star clusters of a few 10^4 Msun by means of velocity
dispersion profiles and surface density profiles. We use a comprehensive set of
-body computations of star clusters on various orbits within a realistic
tidal field to study the evolution of these profiles with time, and ongoing
cluster dissolution From the velocity dispersion profiles we find that the
population of potential escapers, i.e. energetically unbound stars inside the
Jacobi radius, dominates clusters at radii above about 50% of the Jacobi
radius. Beyond 70% of the Jacobi radius nearly all stars are energetically
unbound. The velocity dispersion therefore significantly deviates from the
predictions of simple equilibrium models in this regime. We furthermore argue
that for this reason this part of a cluster cannot be used to detect a dark
matter halo or deviations from Newtonian gravity. By fitting templates to the
about 10^4 computed surface density profiles we estimate the accuracy which can
be achieved in reconstructing the Jacobi radius of a cluster in this way. We
find that the template of King (1962) works well for extended clusters on
nearly circular orbits, but shows significant flaws in the case of eccentric
cluster orbits. This we fix by extending this template with 3 more free
parameters. Our template can reconstruct the tidal radius over all fitted
ranges with an accuracy of about 10%, and is especially useful in the case of
cluster data with a wide radial coverage and for clusters showing significant
extra-tidal stellar populations. No other template that we have tried can yield
comparable results over this range of cluster conditions. All templates fail to
reconstruct tidal parameters of concentrated clusters, however. (abridged)Comment: 23 pages, 13 figures, accepted for publication in MNRA
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
The influence of residual gas expulsion on the evolution of the Galactic globular cluster system and the origin of the Population II halo
We present new results on the evolution of the mass function of the globular
cluster system of the Milky Way, taking the effect of residual gas expulsion
into account. We assume that gas embedded star clusters start with a power-law
mass function with slope \beta=2. The dissolution of the clusters is then
studied under the combined influence of residual gas expulsion driven by energy
feedback from massive stars, stellar mass-loss, two-body relaxation and an
external tidal field. The influence of residual gas expulsion is studied by
applying results from a large grid of N-body simulations computed by Baumgardt
& Kroupa (2007).
In our model, star clusters with masses less than 10^5 M_sun lose their
residual gas on timescales much shorter than their crossing time and residual
gas expulsion is the main dissolution mechanism for star clusters, destroying
about 95% of all clusters within a few 10s of Myr. We find that in this case
the final mass function of globular clusters is established mainly by the gas
expulsion and therefore nearly independent of the strength of the external
tidal field, and that a power-law mass function for the gas embedded star
clusters is turned into a present-day log-normal one.
Another consequence of residual gas expulsion and the associated strong
infant mortality of star clusters is that the Galactic halo stars come from
dissolved star clusters. Since field halo stars would come mainly from
low-mass, short-lived clusters, our model provides an explanation for the
observed abundance variations of light elements among globular cluster stars
and the absence of such variations among the halo field stars.Comment: 12 pages, 9 figures, 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
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 = 16–75 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 study’s 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 study’s implications, strengths, and shortcomings are also discussed
Substructure and Tidal Streams in the Andromeda Galaxy and its Satellites
Tidal streams from existing and destroyed satellite galaxies populate the
outer regions of the Andromeda galaxy (M31). This inhomogeneous debris can be
studied without many of the obstacles that plague Milky Way research. We review
the history of tidal stream research in M31, and in its main satellite
galaxies. We highlight the numerous tidal streams observed around M31, some of
which reside at projected distances of up to 120 kpc from the center of this
galaxy. Most notable is the Giant Stellar Stream, a signature of the most
recent significant accretion event in the M31 system. This event involved an
early-type progenitor of ~10^9 solar masses that came within a few kpc of M31's
center roughly a gigayear ago; almost all of the inner halo debris (within 50
kpc) in M31 can be tied either directly or indirectly to this event. We draw
attention to the fact that most of M31's outer halo globular clusters lie
preferentially on tidal streams and discuss the potential this offers to use
these systems as probes of the accretion history. Tidal features observed
around M33, M32, NGC 205 and NGC 147 are also reviewed. We conclude by
discussing future prospects for this field.Comment: 29 pages, 9 figures. Book chapter in Tidal Streams in the Local Group
and Beyond, eds. Heidi Newberg and Jeff Carli