1,526 research outputs found
Young star clusters in interacting galaxies - NGC 1487 and NGC 4038/4039
We estimate the dynamical masses of several young (~10 Myr) massive star
clusters in two interacting galaxies, NGC 4038/4039 ("The Antennae") and NGC
1487, under the assumption of virial equilibrium. These are compared with
photometric mass estimates from K-band photometry and assuming a standard
Kroupa IMF. The clusters were selected to have near-infrared colors dominated
by red supergiants, and hence to be old enough to have survived the earliest
phases of cluster evolution when the interstellar medium is rapidly swept out
from the cluster, supported by the fact that there is no obvious Halpha
emission associated with the clusters. All but one of the Antennae clusters
have dynamical and photometric mass estimates which are within a factor ~2 of
one another, implying both that standard IMFs provide a good approximation to
the IMF of these clusters, and that there is no significant extra-virial
motion, as would be expected if they were rapidly dispersing. These results
suggest that almost all of the Antennae clusters in our sample have survived
the gas removal phase as bound or marginally bound objects. Two of the three
NGC 1487 clusters studied here have M_dyn estimates which are significantly
larger than the photometric mass estimates. At least one of these two clusters,
and one in the Antennae, may be actively in the process of dissolving. The
process of dissolution contributes a component of non-virial motion to the
integrated velocity measurements, resulting in an estimated M_dyn which is too
high relative to the amount of measured stellar light. The dissolution
candidates in both galaxies are amongst the clusters with the lowest
pressures/densities measured in our sample.Comment: 17 pages, 14 Figures, A&A accepte
Surface differential rotation and prominences of the Lupus post T Tauri star RX J1508.6-4423
We present in this paper a spectroscopic monitoring of the Lupus post T Tauri star RX J1508.6-4423 carried out at two closely separated epochs (1998 May 06 and 10) with the UCL Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Applying least-squares convolution and maximum entropy image reconstruction techniques to our sets of spectra, we demonstrate that this star features on its surface a large cool polar cap with several appendages extending to lower latitudes, as well as one spot close to the equator. The images reconstructed at both epochs are in good overall agreement, except for a photospheric shear that we interpret in terms of latitudinal differential rotation. Given the spot distribution at the epoch of our observations, differential rotation could only be investigated between latitudes 15° and 60°. We find in particular that the observed differential rotation is compatible with a solar-like law (i.e., with rotation rate decreasing towards high latitudes proportionally to sin 2l, where l denotes the latitude) in this particular latitude range. Assuming that such a law can be extrapolated to all latitudes, we find that the equator of RX J1508.6-4423 does one more rotational cycle than the pole every 50 ±10 d, implying a photospheric shear 2 to 3 times stronger than that of the Sun. We also discover that the Hα emission profile of RX J1508.6-4423 is most of the time double-peaked and strongly modulated with the rotation period of the star. We interpret this rotationally modulated emission as being caused by a dense and complex prominence system, the circumstellar distribution of which is obtained through maximum entropy Doppler tomography. These maps show in particular that prominences form a complete and inhomogeneous ring around the star, precisely at the corotation radius. We use the total Hα and Hβ emission flux to estimate that the mass of the whole prominence system is about 10 20g. From our observation that the whole cloud system surrounding the star is regenerated in less than 4 d, we conclude that the braking time-scale of RX J1508.6-4423 is shorter than 1 Gyr, and that prominence expulsion is thus likely to contribute significantly to the rotational spindown of young low-mass stars
Spectral properties and geology of bright and dark material on dwarf planet Ceres
Variations and spatial distributions of bright and dark material on dwarf
planet Ceres play a key role in understanding the processes that have led to
its present surface composition. We define limits for bright and dark material
in order to distinguish them consistently, based on the reflectance of the
average surface using Dawn Framing Camera data. A systematic classification of
four types of bright material is presented based on their spectral properties,
composition, spatial distribution, and association with specific
geomorphological features. We found obvious correlations of reflectance with
spectral shape (slopes) and age; however, this is not unique throughout the
bright spots. Although impact features show generally more extreme reflectance
variations, several areas can only be understood in terms of inhomogeneous
distribution of composition as inferred from Dawn Visible and Infrared
Spectrometer data. Additional material with anomalous composition and spectral
properties are rare. The identification of the composition and origin of the
dark, particularly the darkest material, remains to be explored. The spectral
properties and the morphology of the dark sites suggest an endogenic origin,
but it is not clear whether they are more or less primitive surficial exposures
or excavated subsurface but localized material. The reflectance, spectral
properties, inferred composition, and geologic context collectively suggest
that the bright and dark material tends to gradually change toward the average
surface over time. This could be because of multiple processes, i.e., impact
gardening/space weathering, and lateral mixing, including thermal and aqueous
alteration, accompanied by changes in composition and physical properties such
as grain size, surface temperature, and porosity (compaction).Comment: Meteoritics and Planetary Science; Dawn at Ceres special issu
Winds of Planet Hosting Stars
The field of exoplanetary science is one of the most rapidly growing areas of
astrophysical research. As more planets are discovered around other stars, new
techniques have been developed that have allowed astronomers to begin to
characterise them. Two of the most important factors in understanding the
evolution of these planets, and potentially determining whether they are
habitable, are the behaviour of the winds of the host star and the way in which
they interact with the planet. The purpose of this project is to reconstruct
the magnetic fields of planet hosting stars from spectropolarimetric
observations, and to use these magnetic field maps to inform simulations of the
stellar winds in those systems using the Block Adaptive Tree Solar-wind Roe
Upwind Scheme (BATS-R-US) code. The BATS-R-US code was originally written to
investigate the behaviour of the Solar wind, and so has been altered to be used
in the context of other stellar systems. These simulations will give
information about the velocity, pressure and density of the wind outward from
the host star. They will also allow us to determine what influence the winds
will have on the space weather environment of the planet. This paper presents
the preliminary results of these simulations for the star Bo\"otis,
using a newly reconstructed magnetic field map based on previously published
observations. These simulations show interesting structures in the wind
velocity around the star, consistent with the complex topology of its magnetic
field.Comment: 8 pages, 2 figures, accepted for publication in the peer-reviewed
proceedings of the 14th Australian Space Research Conference, held at the
University of South Australia, 29th September - 1st October 201
Dynamical Masses of Young Star Clusters in NGC 4038/4039
In order to estimate the masses of the compact, young star clusters in the
merging galaxy pair, NGC 4038/4039 (``the Antennae''), we have obtained medium
and high resolution spectroscopy using ISAAC on VLT-UT1 and UVES on VLT-UT2 of
five such clusters. The velocity dispersions were estimated using the stellar
absorption features of CO at 2.29 microns and metal absorption lines at around
8500 \AA, including lines of the Calcium Triplet. The size scales and light
profiles were measured from HST images. From these data and assuming Virial
equilibrium, we estimated the masses of five clusters. The resulting masses
range from 6.5 x 10^5 to 4.7 x 10^6 M_sun. These masses are large, factor of a
few to more than 10 larger than the typical mass of a globular cluster in the
Milky Way. The mass-to-light ratios for these clusters in the V- and K-bands in
comparison with stellar synthesis models suggest that to first order the IMF
slopes are approximately consistent with Salpeter for a mass range of 0.1 to
100 M_sun. However, the clusters show a significant range of possible IMF
slopes or lower mass cut-offs and that these variations may correlate with the
interstellar environment of the cluster. Comparison with the results of
Fokker-Planck simulations of compact clusters with properties similar to the
clusters studied here, suggest that they are likely to be long-lived and may
lose a substantial fraction of their total mass. This mass loss would make the
star clusters obtain masses which are comparable to the typical mass of a
globular cluster.Comment: 16 pages, 12 figures, A&A accepte
The origin of very wide binary systems
The majority of stars in the Galactic field and halo are part of binary or
multiple systems. A significant fraction of these systems have orbital
separations in excess of thousands of astronomical units, and systems wider
than a parsec have been identified in the Galactic halo. These binary systems
cannot have formed through the 'normal' star-formation process, nor by capture
processes in the Galactic field. We propose that these wide systems were formed
during the dissolution phase of young star clusters. We test this hypothesis
using N-body simulations of evolving star clusters and find wide binary
fractions of 1-30%, depending on initial conditions. Moreover, given that most
stars form as part of a binary system, our theory predicts that a large
fraction of the known wide 'binaries' are, in fact, multiple systems.Comment: 4 pages, 1 figure, to appear in the proceedings of IAU Symposium 266,
eds. R. de Grijs & J.R.D. Lepin
K-band Spectroscopy of Clusters in NGC 4038/4039
Integral field spectroscopy in the K-band (1.9-2.4um) was performed on four
IR-bright star clusters and the two nuclei in NGC 4038/4039 (``The Antennae'').
Two of the clusters are located in the overlap region of the two galaxies, and
together comprise ~25% of the total 15um and ~10% of the total 4.8 GHz emission
from this pair of merging galaxies. The other two clusters, each of them
spatially resolved into two components, are located in the northern galaxy, one
in the western and one in the eastern loop of blue clusters. Comparing our
analysis of Brgamma, CO band-heads, He I (2.058um), Halpha (from archival HST
data), and V-K colors with stellar population synthesis models indicates that
the clusters are extincted (A_V ~ 0.7 - 4.3 mags) and young, displaying a
significant age spread (4-13 Myrs). The starbursts in the nuclei are much older
(65 Myrs), with the nucleus of NGC 4038 displaying a region of recent star
formation northward of its K-band peak. Using our derived age estimates and
assuming the parameters of the IMF (Salpeter slope, upper mass cut-off of 100
M_sun, Miller-Scalo between 1 M_sun and 0.1 M_sun), we find that the clusters
have masses between 0.5 and 5 * 10^6M_sun.Comment: 10 pages, 3 figures, ApJ accepte
The XIIIth Banff Conference on Allograft Pathology: The Banff 2015 Heart Meeting Report: Improving Antibody-Mediated Rejection Diagnostics: Strengths, Unmet Needs, and Future Directions.
The 13th Banff Conference on Allograft Pathology was held in Vancouver, British Columbia, Canada from October 5 to 10, 2015. The cardiac session was devoted to current diagnostic issues in heart transplantation with a focus on antibody-mediated rejection (AMR) and small vessel arteriopathy. Specific topics included the strengths and limitations of the current rejection grading system, the central role of microvascular injury in AMR and approaches to semiquantitative assessment of histopathologic and immunophenotypic indicators, the role of AMR in the development of cardiac allograft vasculopathy, the important role of serologic antibody detection in the management of transplant recipients, and the potential application of new molecular approaches to the elucidation of the pathophysiology of AMR and potential for improving the current diagnostic system. Herein we summarize the key points from the presentations, the comprehensive, open and wide-ranging multidisciplinary discussion that was generated, and considerations for future endeavors
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