32 research outputs found
Stellar Orbital Studies in Normal Spiral Galaxies II: Restrictions to Structural and Dynamical parameters on Spiral Arms
Making use of a set of detailed potential models for normal spiral galaxies,
we analyze the disk stellar orbital dynamics as the structural and dynamical
parameters of the spiral arms (mass, pattern speed and pitch angle) are
gradually modified. With this comprehensive study of ordered and chaotic
behavior, we constructed an assemblage of orbitally supported galactic models
and plausible parameters for orbitally self-consistent spiral arms models. We
find that, to maintain orbital support for the spiral arms, the spiral arm
mass, M, must decrease with the increase of the pitch angle, ; if
is smaller than , M can be as large as , ,
of the disk mass, for Sa, Sb, and Sc galaxies, respectively. If
increases up to , the maximum M is of the disk
mass independently in this case of morphological type. For values larger than
these limits, spiral arms would likely act as transient features. Regarding the
limits posed by extreme chaotic behavior, we find a strong restriction on the
maximum plausible values of spiral arms parameters on disk galaxies beyond
which, chaotic behavior becomes pervasive. We find that for smaller than
, , , for Sa, Sb, and Sc galaxies,
respectively, M can go up to , of the mass of the disk. If the
corresponding is around , , , M
is , , of the mass of the disk. Beyond these
values, chaos dominates phase space, destroying the main periodic and the
neighboring quasi-periodic orbits.Comment: 51 pages in preprint format, 30 figures, Accepted for publication in
Ap
The Galactic Branches as a Possible Evidence for Transient Spiral Arms
With the use of a background Milky-Way-like potential model, we performed
stellar orbital and magnetohydrodynamic (MHD) simulations. As a first
experiment, we studied the gaseous response to a bisymmetric spiral arm
potential: the widely employed cosine potential model and a self-gravitating
tridimensional density distribution based model called PERLAS. Important
differences are noticeable in these simulations, while the simplified cosine
potential produces two spiral arms for all cases, the more realistic density
based model produces a response of four spiral arms on the gaseous disk, except
for weak arms -i.e. close to the linear regime- where a two-armed structure is
formed. In order to compare the stellar and gas response to the spiral arms, we
have also included a detailed periodic orbit study and explored different
structural parameters within observational uncertainties. The four armed
response has been explained as the result of ultra harmonic resonances, or as
shocks with the massive bisymmetric spiral structure, among other. From the
results of this work, and comparing the stellar and gaseous responses, we
tracked down an alternative explanation to the formation of branches, based
only on the orbital response to a self-gravitating spiral arms model. The
presence of features such as branches, might be an indication of transiency of
the arms.Comment: 17 pages, 9 figures. Accepted for publication in MNRA
The Sun was not born in M 67
Using the most recent proper-motion determination of the old,
Solar-metallicity, Galactic open cluster M 67, in orbital computations in a
non-axisymmetric model of the Milky Way, including a bar and 3D spiral arms, we
explore the possibility that the Sun once belonged to this cluster. We have
performed Monte Carlo numerical simulations to generate the present-day orbital
conditions of the Sun and M 67, and all the parameters in the Galactic model.
We compute 3.5 \times 10^5 pairs of orbits Sun-M 67 looking for close
encounters in the past with a minimum distance approach within the tidal radius
of M 67. In these encounters we find that the relative velocity between the Sun
and M 67 is larger than 20 km/s. If the Sun had been ejected from M 67 with
this high velocity by means of a three-body encounter, this interaction would
destroy an initial circumstellar disk around the Sun, or disperse its already
formed planets. We also find a very low probability, much less than 10^-7, that
the Sun was ejected from M 67 by an encounter of this cluster with a giant
molecular cloud. This study also excludes the possibility that the Sun and M 67
were born in the same molecular cloud. Our dynamical results convincingly
demonstrate that M67 could not have been the birth cluster of our Solar System.Comment: Astronomical Journal accepted (35 pages, 9 figures
A novel method to bracket the corotation radius in galaxy discs:vertex deviation maps
We map the kinematics of stars in simulated galaxy disks with spiral arms
using the velocity ellipsoid vertex deviation (l). We use test particle
simulations, and for the first time, fully self-consistent high resolution
N-body models. We compare our maps with the Tight Winding Approximation model
analytical predictions. We see that for all barred models spiral arms rotate
closely to a rigid body manner and the vertex deviation values correlate with
the density peaks position bounded by overdense and underdense regions. In such
cases, vertex deviation sign changes from negative to positive when crossing
the spiral arms in the direction of disk rotation, in regions where the spiral
arms are in between corotation (CR) and the Outer Lindblad Resonance (OLR). By
contrast, when the arm sections are inside the CR and outside the OLR, l
changes from negative to positive.We propose that measurements of the vertex
deviations pattern can be used to trace the position of the main resonances of
the spiral arms. We propose that this technique might exploit future data from
Gaia and APOGEE surveys. For unbarred N-body simulations with spiral arms
corotating with disk material at all radii, our analysis suggests that no clear
correlation exists between l and density structures
On the gravitational content of molecular clouds and their cores
(Abridged) The gravitational term for clouds and cores entering in the virial
theorem is usually assumed to be equal to the gravitational energy, since the
contribution to the gravitational force from the mass distribution outside the
volume of integration is assumed to be negligible. Such approximation may not
be valid in the presence of an important external net potential. In the present
work we analyze the effect of an external gravitational field on the
gravitational budget of a density structure. Our cases under analysis are (a) a
giant molecular cloud (GMC) with different aspect ratios embedded within a
galactic net potential, and (b) a molecular cloud core embedded within the
gravitational potential of its parent molecular cloud. We find that for
roundish GMCs, the tidal tearing due to the shear in the plane of the galaxy is
compensated by the tidal compression in the z direction. The influence of the
external effective potential on the total gravitational budget of these clouds
is relatively small, although not necessarily negligible. However, for more
filamentary GMCs, the external effective potential can be dominant and can even
overwhelm self-gravity, regardless of whether its main effect on the cloud is
to disrupt it or compress it. This may explain the presence of some GMCs with
few or no signs of massive star formation, such as the Taurus or the
Maddalena's clouds. In the case of dense cores embedded in their parent
molecular cloud, we found that the gravitational content due to the external
field may be more important than the gravitational energy of the cores
themselves. This effect works in the same direction as the gravitational
energy, i.e., favoring the collapse of cores. We speculate on the implications
of these results for star formation models.Comment: Accepted for publication in MNRA
Tidal foces as a regulator of star formation in Taurus
Only a few molecular clouds in the Solar Neighborhood exhibit the formation
of only low-mass stars. Traditionally, these clouds have been assumed to be
supported against more vigorous collapse by magnetic fields. The existence of
strong magnetic fields in molecular clouds, however, poses serious problems for
the formation of stars and of the clouds themselves. In this {\em Letter}, we
review the three-dimensional structure and kinematics of Taurus --the archetype
of a region forming only low-mass stars-- as well as its orientation within the
Milky way. We conclude that the particularly low star-formation efficiency in
Taurus may naturally be explained by tidal forces from the Galaxy, with no need
for magnetic regulation or stellar feedback.Comment: Minor changes. 5 pages. Accepted by MNRA