413 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
Connecting the first galaxies with ultra faint dwarfs in the Local Group: chemical signatures of Population~III stars
We investigate the star formation history and chemical evolution of isolated
analogues of Local Group (LG) ultra faint dwarf galaxies (UFDs; stellar mass
range of 10^2 solar mass < M_star <10^5 solar mass) and gas rich, low mass
dwarfs (Leo P analogs; stellar mass range of 10^5 solar mass < M_star <10^6
solar mass). We perform a suite of cosmological hydrodynamic zoom-in
simulations to follow their evolution from the era of the first generation of
stars down to z=0. We confirm that reionization, combined with supernova (SN)
feedback, is primarily responsible for the truncated star formation in UFDs.
Specifically, haloes with a virial mass of M_vir
90\% of stars prior to reionization. Our work further demonstrates the
importance of Population~III (Pop~III) stars, with their intrinsically high
yields, and the associated external metal-enrichment, in producing
low-metallicity stars () and carbon-enhanced metal-poor
(CEMP) stars. We find that UFDs are composite systems, assembled from multiple
progenitor haloes, some of which hosted only Population~II (Pop~II) stars
formed in environments externally enriched by SNe in neighboring haloes,
naturally producing, extremely low-metallicity Pop~II stars. We illustrate how
the simulated chemical enrichment may be used to constrain the star formation
histories (SFHs) of true observed UFDs. We find that Leo P analogs can form in
haloes with M_vir ~ 4 x 10^9 solar mass (z=0). Such systems are less affected
by reionization and continue to form stars until z=0, causing higher
metallicity tails. Finally, we predict the existence of extremely
low-metallicity stars in LG UFD galaxies that preserve the pure chemical
signatures of Pop~III nucleosynthesis.Comment: 22 pages, 13 figures, Accepted for publication in Ap
Dynamics of the Magellanic Clouds in a LCDM Universe
We examine Milky Way-Magellanic Cloud systems selected from the Millennium-II
Simulation in order to place the orbits of the Magellanic Clouds in a
cosmological context. Our analysis shows that satellites massive enough to be
LMC analogs are typically accreted at late times. Moreover, those that are
accreted at early times and survive to the present have orbital properties that
are discrepant with those observed for the LMC. The high velocity of the LMC,
coupled with the dearth of unbound orbits seen in the simulation, argues that
the mass of the MW's halo is unlikely to be less than 2 x 10^12 Msun. This
conclusion is further supported by statistics of halos hosting satellites with
masses, velocities, and separations comparable to those of the LMC. We further
show that: (1) LMC and SMC-mass objects are not particularly uncommon in
MW-mass halos; (2) the apparently high angular momentum of the LMC is not
cosmologically unusual; and (3) it is rare for a MW halo to host a LMC-SMC
binary system at z=0, but high speed binary pairs accreted at late times are
possible. Based on these results, we conclude that the LMC was accreted within
the past four Gyr and is currently making its first pericentric passage about
the MW.Comment: 14 pages, 13 figures; MNRAS, in press. Minor revisions, conclusions
unchange
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
Quasi-Resonant Theory of Tidal Interactions
When a spinning system experiences a transient gravitational encounter with
an external perturber, a quasi-resonance occurs if the spin frequency of the
victim matches the peak orbital frequency of the perturber. Such encounters are
responsible for the formation of long tails and bridges of stars during galaxy
collisions. For high-speed encounters, the resulting velocity perturbations can
be described within the impulse approximation. The traditional impulse
approximation, however, does not distinguish between prograde and retrograde
encounters, and therefore completely misses the resonant response. Here, using
perturbation theory, we compute the effects of quasi-resonant phenomena on
stars orbiting within a disk. Explicit expressions are derived for the velocity
and energy change to the stars induced by tidal forces from an external
gravitational perturber passing either on a straight line or parabolic orbit.
Comparisons with numerical restricted three-body calculations illustrate the
applicability of our analysis.Comment: 22 pages, 13 figures, ApJ submitted, numerical routines for
evaluation of special functions and analytical results are provided upon
reques
Formation of Narrow Dust Rings in Circumstellar Debris Disks
Narrow dust rings observed around some young stars (e.g., HR 4796A) need to
be confined. We present a possible explanation for the formation and
confinement of such rings in optically thin circumstellar disks, without
invoking shepherding planets. If an enhancement of dust grains (e.g., due to a
catastrophic collision) occurs somewhere in the disk, photoelectric emission
from the grains can heat the gas to temperatures well above that of the dust.
The gas orbits with super(sub)-Keplerian speeds inward (outward) of the
associated pressure maximum. This tends to concentrate the grains into a narrow
region. The rise in dust density leads to further heating and a stronger
concentration of grains. A narrow dust ring forms as a result of this
instability. We show that this mechanism not only operates around early-type
stars that have high UV fluxes, but also around stars with spectral types as
late as K. This implies that this process is generic and may have occurred
during the lifetime of each circumstellar disk. We examine the stringent
upper-limit on the H2 column density in the HR 4796A disk and find it to be
compatible with the presence of a significant amount of hydrogen gas in the
disk. We also compute the OI and CII infrared line fluxes expected from various
debris disks and show that these will be easily detectable by the upcoming
Herschel mission. Herschel will be instrumental in detecting and characterizing
gas in these disks.Comment: Accepted for publication in ApJ; 14 pages, 7 figure
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