88 research outputs found
Three-component Stackel potentials satisfying recent estimates of Milky Way parameters
We present a set of three-component Stackel potentials defined by five
parameters and designed to model the Milky Way. We review the fundamental
constraints that any model of the Milky Way must satisfy, including the most
recent ones derived from Hipparcos data, and we study how the parameters of the
presented potentials can vary in order to match these constraints. Five
different valid potentials are presented and analyzed in detail: they are
designed to be confronted with kinematical surveys in the future, by the
construction of three-integral analytic distribution functions.Comment: 13 pages, 5 figures, accepted for publication in MNRA
Potentials and distribution functions to be used for dynamical modeling with GAIA-like data
We present new tools to establish axisymmetric equilibrium models of the
Milky Way. The models we wish to establish are pairs (V,F) where V is the
gravitational potential generated by the whole mass distribution including the
dark matter, and F is the distribution function in phase space for late-type
tracer stars. We present a set of Stackel potentials that fit some fundamental
parameters of the Milky Way (mass density in the solar neighbourhood and Oort
constants). Then we define new component distribution functions that can be
combined with these potentials in order to reproduce kinematical data like
those that will be provided by GAIA.Comment: 2 pages, 2 figures, Proceeding of Les Houches summer school to appear
in J.PhysIV Franc
Dynamical streams in the solar neighbourhood
The true nature of the Hyades and Sirius superclusters is still an open
question. In this contribution, we confront Eggen's hypothesis that they are
cluster remnants with the results of a kinematic analysis of more than 6000 K
and M giants in the solar neighbourhood. This analysis includes new radial
velocity data from a large survey performed with the Coravel spectrometer,
complemented by Hipparcos parallaxes and Tycho-2 proper motions (Famaey et al.
2004). A maximum-likelihood method, based on a bayesian approach, has been
applied to the data, in order to make full use of all the available data
(including less precise parallaxes) and to derive the properties of the
different kinematic subgroups. Two such subgroups can be identified with the
Hyades and Sirius superclusters. Stars belonging to them span a very wide range
of age, which is difficult to account for in Eggen's scenario. These groups are
thus most probably "dynamical streams" related to the dynamical perturbation by
spiral waves rather than to cluster remnants.
In this scenario, the Hyades and Ursa Major clusters just happen to be in the
Hyades and Sirius streams, which are purely dynamical features that have
nothing to do with the remnants of more massive primordial clusters. This
mechanism could be the key to understanding the presence of an old metal-rich
population, and of many exoplanetary systems in our neighbourhood. Moreover, a
strong spiral pattern seems to be needed in order to yield such prominent
streams. Since spiral structure is usually baryonic, this would leave very
little room for dark matter. This may be an indication that the era of the
dark-matter paradigm explaining the dynamics of the Galaxy may come to an end,
and is being superseded by modified gravity.Comment: 5 pages, 1 figure, to appear in The Three Dimensional Universe with
GAIA, eds M. Perryman & C. Turo
Keplerian discs around post-AGB stars: a common phenomenon?
Aims: We aim at showing that the broad-band SED characteristics of our sample
of post-AGB stars are best interpreted, assuming the circumstellar dust is
stored in Keplerian rotating passive discs.
Methods: We present a homogeneous and systematic study of the Spectral Energy
Distributions (SEDs) of a sample of 51 post-AGB objects. The selection criteria
to define the whole sample were tuned to cover the broad-band characteristics
of known binary post-AGB stars. The whole sample includes 20 dusty RV Tauri
stars from the General Catalogue of Variable Stars (GCVS). We supplemented our
own Geneva optical photometry with literature data to cover a broad range of
fluxes from the UV to the far-IR.
Results: All the SEDs display very similar characteristics: a large IR excess
with a dust excess starting near the sublimation temperature, irrespective of
the effective temperature of the central star. Moreover, when available, the
long wavelength fluxes show a black-body slope indicative of the presence of a
component of large mm sized grains.
Conclusions: We argue that in all systems, gravitationally bound dusty discs
are present. The discs must be puffed-up to cover a large opening angle for the
central star and we argue that the discs have some similarity with the passive
discs detected around young stellar objects. We interpret the presence of a
disc to be a signature for binarity of the central object, but this will need
confirmation by long-term monitoring of the radial velocities. We argue that
dusty RV Tauri stars are those binaries which happen to be in the Population II
instability strip.Comment: 29 pages, 5 figures, accepted for publication in A&
Generalisations of the Tully-Fisher relation for early and late-type galaxies
We study the locus of dwarf and giant early and late-type galaxies on the
Tully-Fisher relation (TFR), the stellar mass Tully-Fisher relation (sTFR) and
the so-called baryonic or HI gas+stellar mass Tully-Fisher relation (gsTFR). We
show that early-type and late-type galaxies, from dwarfs to giants, trace
different yet approximately parallel TFRs. Surprisingly, early-type and
late-type galaxies trace a single yet curved sTFR over a range of 3.5 orders of
magnitude in stellar mass. Moreover, all galaxies trace a single, linear gsTFR,
over 3.5 orders of magnitude in HI gas+stellar mass. Dwarf ellipticals,
however, lie slightly below the gsTFR. This may indicate that early-type
dwarfs, contrary to the late-types, have lost their gas, e.g. by galactic winds
or ram-pressure stripping. Overall, environment only plays a secondary role in
shaping these relations, making them a rather ``clean'' cosmological tool. LCDM
simulations predict roughly the correct slopes for these relations.Comment: 4 pages, 2 figures, accepted for publication in Astrophysical Journa
The Compression of Dark Matter Halos by Baryonic Infall
The initial radial density profiles of dark matter halos are laid down by
gravitational collapse in hierarchical structure formation scenarios and are
subject to further compression as baryons cool and settle to the halo centers.
We here describe an explicit implementation of the algorithm, originally
developed by Young, to calculate changes to the density profile as the result
of adiabatic infall in a spherical halo model. Halos with random motion are
more resistant to compression than are those in which random motions are
neglected, which is a key weakness of the simple method widely employed.
Young's algorithm results in density profiles in excellent agreement with those
from N-body simulations. We show how the algorithm may be applied to determine
the original uncompressed halos of real galaxies, a step which must be computed
with care in order to enable a confrontation with theoretical predictions from
theories such as LCDM.Comment: Revised version for ApJ. 8 pages, 8 figures, latex uses emulateap
Shape of the Galactic Orbits in the CNOC1 Clusters
We present an analysis of the orbital properties in 9 intermediate-redshifts
cluster of the CNOC1 survey and we compare them to a control sample of 12
nearby clusters. Similar to the nearby elliptical galaxies, the bulge-dominated
galaxies in clusters at redshifts ~0.1-0.4 present orbits that are more
eccentric than those for disk-dominated galaxies. However, the orbital
segregation is less significant than that found for elliptical and spiral
galaxies in nearby cluster. When galaxies are separated by colors - red
galaxies with colors in the rest frame (U-V)_o > 1.4, and blue galaxies with
(U-V)_o =< 1.4 - the strongest orbital segregation is found. Therefore, the
segregation we found seems to modify more efficiently the star formation
activity than the internal shape of the galaxies. When we compare the orbits of
early-type galaxies at intermediate-redshift with those for z=0, they seem to
develop significant changes getting much more eccentric. A different behavior
is observed in the late-type galaxies, which present no-significant evolution
in their orbit shapes.Comment: Accepted for publication in ApJ, April 2000. Latex with aaspp4.sty,
20 pages, 4 tables, 6 eps figure
Internal Dynamics, Structure and Formation of Dwarf Elliptical Galaxies: I. A Keck/HST Study of Six Virgo Cluster Dwarfs
Keck/ESI spectroscopy is presented for six Virgo Cluster dwarf elliptical
(dE) galaxies. The mean line-of-sight velocity and velocity dispersion are
resolved as a function of radius along the major axis of each galaxy, nearly
doubling the total number of dEs with spatially-resolved stellar kinematics.
None of the observed objects shows evidence of strong rotation: upper limits on
v_rot/sigma are well below those expected for rotationally-flattened objects.
Such limits place strong constraints on dE galaxy formation models. Although
these galaxies continue the trend of low rotation velocities observed in Local
Group dEs, they are in contrast to recent observations of large rotation
velocities in slightly brighter cluster dEs. Using WFPC2 surface photometry and
spherically-symmetric dynamical models, we determine global mass-to-light
ratios 3 < M/L_V < 6. These ratios are comparable to those expected for an
intermediate-age stellar population and are broadly consistent with the (V-I)
colors of the galaxies. This implies that these dEs do not have a significant
dark matter component inside an effective radius. Central black holes more
massive than 10^7 M_sun can be ruled out. For the 5 nucleated dEs in our
sample, we determine kinematic and photometric properties for the central
nucleus separately from the underlying host dE galaxy. These nuclei are as
bright or brighter than the most luminous Galactic globular clusters and lie
near the region of Fundamental Plane space occupied by globular clusters. In
this space, the Virgo dE galaxies lie in the same general region as Local Group
and other nearby dEs, although non-rotating dEs appear to have a slightly
higher mean mass and mass-to-light ratio than their rotating counterparts.Comment: 31 pages, 11 figures. Accepted in the Astronomical Journa
Strong dust processing in circumstellar discs around 6 RV Tauri stars. Are dusty RV Tauri stars all binaries?
We present extended Spectral Energy Distributions (SEDs) of seven classical
RV Tauri stars, using newly obtained submillimetre continuum measurements and
Geneva optical photometry supplemented with literature data. The broad-band
SEDs show a large IR excess with a black-body slope at long wavelengths in six
of the seven stars, R Sct being the noticeable exception. This long wavelength
slope is best explained assuming the presence of a dust component of large
grains in the circumstellar material. We show that the most likely distribution
of the circumstellar dust around the six systems is that the dust resides in a
disc. Moreover, very small outflow velocities are needed to explain the
presence of dust near the sublimation temperature and we speculate that the
discs are Keplerian. The structure and evolution of these compact discs are as
yet not understood but a likely prerequisite for their formation is that the
dusty RV Tauri stars are binaries.Comment: 10 pages, will be published in A&
Building up the Stellar Halo of the Galaxy
We study numerical simulations of satellite galaxy disruption in a potential
resembling that of the Milky Way. Our goal is to assess whether a merger origin
for the stellar halo would leave observable fossil structure in the phase-space
distribution of nearby stars. We show how mixing of disrupted satellites can be
quantified using a coarse-grained entropy. Although after 10 Gyr few obvious
asymmetries remain in the distribution of particles in configuration space,
strong correlations are still present in velocity space. We give a simple
analytic description of these effects, based on a linearised treatment in
action-angle variables, which shows how the kinematic and density structure of
the debris stream changes with time. By applying this description we find that
a single satellite of current luminosity 10^8 L_\sun disrupted 10 Gyr ago
from an orbit circulating in the inner halo (mean apocentre kpc)
would contribute about kinematically cold streams with internal
velocity dispersions below 5 km/s to the local stellar halo. If the whole
stellar halo were built by disrupted satellites, it should consist locally of
300 - 500 such streams. Clear detection of all these structures would require a
sample of a few thousand stars with 3-D velocities accurate to better than 5
km/s. Even with velocity errors several times worse than this, the expected
clumpiness should be quite evident. We apply our formalism to a group of stars
detected near the North Galactic Pole, and derive an order of magnitude
estimate for the initial properties of the progenitor system.Comment: 28 pages, 10 figures, minor changes, matches the version to appear in
MNRAS, Vol. 307, p.495-517 (August 1999
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