67,811 research outputs found
Multi-Phenomena Modeling of the New Bullet Cluster, ZwCl008.8+52, using N-body/hydrodynamical Simulations
We use hydrodynamical/N-body simulations to interpret the newly discovered
Bullet-cluster-like merging cluster, ZwCl 0008.8+5215 (ZwCl 0008 hereafter),
where a dramatic collision is apparent from multi-wavelength observations. We
have been able to find a self-consistent solution for the radio, X-ray, and
lensing phenomena by projecting an off-axis, binary cluster encounter viewed
just after first core passage. A pair radio relics traces well the leading and
trailing shock fronts that our simulation predict, providing constraints on the
collision parameters. We can also account for the observed distinctive
comet-like X-ray morphology and the positions of the X-ray peaks relative to
the two lensing mass centroids and the two shock front locations. Relative to
the Bullet cluster, the total mass is about 70% lower, ( Msun, with a correspondingly lower infall velocity, km/s,
and an impact parameter of kpc. As a result, the gas component of
the infalling cluster is not trailing significantly behind the associated dark
matter as in the case of the Bullet cluster. The degree of agreement we find
between all the observables provides strong evidence that dark matter is
effectively collisionless on large scales calling into question other claims
and theories that advocate modified gravity.Comment: 9 pages, 3 figures, and 1 table, submitted to the Astrophysical
Journal for publicationon on December 18. Coments are welcom
A Multi-Epoch Study of the Radio Continuum Emission of Orion Source I: Constraints on the Disk Evolution of a Massive YSO and the Dynamical History of Orion BN/KL
We present new 7mm continuum observations of Orion BN/KL with the VLA. We
resolve the emission from the protostar radio Source I and BN at several
epochs. Source I is highly elongated NW-SE, and remarkably stable in flux
density, position angle, and overall morphology over nearly a decade. This
favors the extended emission component arising from an ionized disk rather than
a jet. We have measured the proper motions of Source I and BN for the first
time at 43 GHz. We confirm that both sources are moving at high speed (12 and
26 km/s, respectively) approximately in opposite directions, as previously
inferred from measurements at lower frequencies. We discuss dynamical scenarios
that can explain the large motions of both BN and Source I and the presence of
disks around both. Our new measurements support the hypothesis that a close
(~50 AU) dynamical interaction occurred around 500 years ago between Source I
and BN as proposed by Gomez et al. From the dynamics of encounter we argue that
Source I today is likely to be a binary with a total mass on the order of 20
Msun, and that it probably existed as a softer binary before the close
encounter. This enables preservation of the original accretion disk, though
truncated to its present radius of ~50 AU. N-body numerical simulations show
that the dynamical interaction between a binary of 20 Msun total mass (I) and a
single star of 10 Msun mass (BN) may lead to the ejection of both and binary
hardening. The gravitational energy released in the process would be large
enough to power the wide-angle flow traced by H2 and CO emission in the BN/KL
nebula. Assuming the proposed dynamical history is correct, the smaller mass
for Source I recently estimated from SiO maser dynamics (>7 Msun) by Matthews
et al., suggests that non-gravitational forces (e.g. magnetic) must play an
important role in the circumstellar gas dynamics.Comment: 17 pages, 7 figures, 4 tables, accepted by Ap
First Observational Signature of Rotational Deceleration in a Massive, Intermediate-age Star Cluster in the Magellanic Clouds
While the extended main-sequence turn-offs (eMSTOs) found in almost all 1--2
Gyr-old star clusters in the Magellanic Clouds are often explained by
postulating extended star-formation histories, the tight subgiant branches
(SGBs) seen in some clusters challenge this popular scenario. Puzzlingly, the
SGB of the eMSTO cluster NGC 419 is significantly broader at bluer than at
redder colors. We carefully assess and confirm the reality of this
observational trend. If we would assume that the widths of the features in
color--magnitude space were entirely owing to a range in stellar ages, the
star-formation histories of the eMSTO stars and the blue SGB region would be
significantly more prolonged than that of the red part of the SGB. This cannot
be explained by assuming an internal age spread. We show that rotational
deceleration of a population of rapidly rotating stars, a currently hotly
debated alternative scenario, naturally explains the observed trend along the
SGB. Our analysis shows that a `converging' SGB could be produced if the
cluster is mostly composed of rapidly rotating stars that slow down over time
owing to the conservation of angular momentum during their evolutionary
expansion from main-sequence turn-off stars to red giants.Comment: 11 pages, preprint format (uses aastex6.cls); ApJ Letters, in pres
Multiple stellar populations in Magellanic Cloud clusters. III. The first evidence of an extended main sequence turn-off in a young cluster: NGC1856
Recent studies have shown that the extended main-sequence turn off (eMSTO) is
a common feature of intermediate-age star clusters in the Magellanic Clouds
(MCs). The most simple explanation is that these stellar systems harbor
multiple generations of stars with an age difference of a few hundred Myrs.
However, while an eMSTO has been detected in a large number of clusters with
ages between ~1-2 Gyrs, several studies of young clusters in both MCs and in
nearby galaxies do not find any evidence for a prolonged star-formation
history, i.e. for multiple stellar generations. These results have suggested
alternative interpretation of the eMSTOs observed in intermediate-age star
clusters. The eMSTO could be due to stellar rotation mimicking an age spread or
to interacting binaries. In these scenarios, intermediate-age MC clusters would
be simple stellar populations, in close analogy with younger clusters.
Here we provide the first evidence for an eMSTO in a young stellar cluster.
We exploit multi-band Hubble Space Telescope photometry to study the ~300-Myr
old star cluster NGC1856 in the Large Magellanic Cloud and detected a broadened
MSTO that is consistent with a prolonged star-formation which had a duration of
about 150 Myrs. Below the turn-off, the MS of NGC1856 is split into a red and
blue component, hosting 33+/-5% and 67+/-5% of the total number of MS stars,
respectively. We discuss these findings in the context of
multiple-stellar-generation, stellar-rotation, and interacting-binary
hypotheses.Comment: 14 pages, 10 figures, accepted for publication in MNRA
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