421 research outputs found
On interpretation of recent proper motion data for the Large Magellanic Cloud
Recent observational studies using the Hubble Space Telescope (HST) have
derived the center-of-mass proper motion (CMPM) of the Large Magellanic Cloud
(LMC). Although these studies carefully treated both rotation and perspective
effects in deriving the proper motion for each of the sampled fields, they did
not consider the effects of local random motion in the derivation. This means
that the average PM of the fields (i.e., the observed CMPM) could significantly
deviate from the true CMPM, because the effect of local random motion can not
be close to zero in making the average PM for the small number of the fields
(~10). We discuss how significantly the observationally derived CMPM can
deviate from the true CMPM by applying the same method as used in the
observations for a dynamical model of the LMC with a known true CMPM. We find
that the deviation can be as large as ~ 50 km/s (~0.21 mas/yr), if the LMC has
a thick disk and a maximum circular velocity of ~120$ km/s. We also find that
the deviation depends both on the total number of the sampled fields and on
structure and kinematics of the LMC. We therefore suggest that there is a
possibility that the observed CMPM of the LMC deviates from the true one to
some extent. We also show that a simple mean of PM for a large number of the
LMC fields (~1000) can be much closer to the true CMPM.Comment: 12 pages, 4 figures accepted in ApJ
Revisiting the Role of M31 in the Dynamical History of the Magellanic Clouds
We study the dynamics of the Magellanic Clouds in a model for the Local Group
whose mass is constrained using the timing argument/two-body limit of the
action principle. The goal is to evaluate the role of M31 in generating the
high angular momentum orbit of the Clouds, a puzzle that has only been
exacerbated by the latest proper motion measurements. We study the
effects of varying the total Local Group mass, the relative mass of the Milky
Way and M31, the proper motion of M31, and the proper motion of the LMC on this
problem. Over a large part of this parameter-space we find that tides from M31
are insignificant. For a range of LMC proper motions approximately
higher than the mean and total Local Group mass ,
M31 can provide a significant torque to the LMC orbit. However, if the LMC is
bound to the MW, then M31 is found to have negligible effect on its motion and
the origin of the high angular momentum of the system remains a puzzle.
Finally, we use the timing argument to calculate the total mass of the MW-LMC
system based on the assumption that they are encountering each other for the
first time, their previous perigalacticon being a Hubble time ago, obtaining
.Comment: 10 pages, 5 figures, accepted to Ap
A possible common halo of the Magellanic Clouds
Recent observational and theoretical studies on the three-dimensional (3D)
space motions of the Large and the Small Magellanic Clouds (LMC and SMC,
respectively) have strongly suggested that the latest proper motion
measurements of the Magellanic Clouds (MCs) are consistent with their orbital
evolution models in which the MCs have arrived in the Galaxy quite recently for
the first time. The suggested orbital models appear to be seriously
inconsistent with the tidal interaction models in which the Magellanic Stream
(MS) can be formed as a result of the mutual tidal interaction between the MCs
and the Galaxy for the last ~2 Gyr. Based on orbital models of the MCs, we
propose that if the MCs have a common diffuse dark halo with the mass larger
than ~ 2 * 10^{10} M_sun, the MCs can not only have the present 3D velocities
consistent with the latest proper motion measurements but also interact
strongly with each other and with the Galaxy for the last 2 Gyr.These results
imply that if the observed proper motions of the MCs are true ones of the
centers of mass for the MCs, the common halo of the MCs would need to be
considered in constructing self-consistent MS formation models. We discuss
whether the origin of the possible common halo can be closely associated either
with the past binary formation or with the MCs having been in a small group.Comment: 13 pages, 4 figures, accepted in ApJ
Identification of the Microlens in Event MACHO-LMC-20
We report on the identification of the lens responsible for microlensing
event MACHO-LMC-20. As part of a \textit{Spitzer}/IRAC program conducting
mid-infrared follow-up of the MACHO Large Magellanic Cloud microlensing fields,
we discovered a significant flux excess at the position of the source star for
this event. These data, in combination with high resolution near-infrared
\textit{Magellan}/PANIC data has allowed us to classify the lens as an early M
dwarf in the thick disk of the Milky Way, at a distance of kpc. This
is only the second microlens to have been identified, the first also being a M
dwarf star in the disk. Together, these two events are still consistent with
the expected frequency of nearby stars in the Milky Way thin and thick disks
acting as lenses.Comment: 6 pages, 4 figures, submitted to ApJ Letter
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