121 research outputs found
Impact of distance determinations on Galactic structure. II. Old tracers
Here we review the efforts of a number of recent results that use old tracers
to understand the build up of the Galaxy. Details that lead directly to using
these old tracers to measure distances are discussed. We concentrate on the
following: (1) the structure and evolution of the Galactic bulge and inner
Galaxy constrained from the dynamics of individual stars residing therein; (2)
the spatial structure of the old Galactic bulge through photometric
observations of RR Lyrae-type stars; (3) the three\--dimensional structure,
stellar density, mass, chemical composition, and age of the Milky Way bulge as
traced by its old stellar populations; (4) an overview of RR Lyrae stars known
in the ultra-faint dwarfs and their relation to the Galactic halo; and (5)
different approaches for estimating absolute and relative cluster ages.Comment: Review article, 80 pages (25 figures); Space Science Reviews, in
press (chapter of a special collection resulting from the May 2016 ISSI-BJ
workshop on Astronomical Distance Determination in the Space Age
The Magellanic Edges Survey I. Description and First Results
We present an overview of, and first science results from, the Magellanic
Edges Survey (MagES), an ongoing spectroscopic survey mapping the kinematics of
red clump and red giant branch stars in the highly substructured periphery of
the Magellanic Clouds. In conjunction with Gaia astrometry, MagES yields a
sample of ~7000 stars with individual 3D velocities that probes larger
galactocentric radii than most previous studies. We outline our target
selection, observation strategy, data reduction and analysis procedures, and
present results for two fields in the northern outskirts ( on-sky
from the centre) of the Large Magellanic Cloud (LMC). One field, located in the
vicinity of an arm-like overdensity, displays apparent signatures of
perturbation away from an equilibrium disk model. This includes a large radial
velocity dispersion in the LMC disk plane, and an asymmetric line-of-sight
velocity distribution indicative of motions vertically out of the disk plane
for some stars. The second field reveals 3D kinematics consistent with an
equilibrium disk, and yields km s at a
radial distance of ~10.5kpc from the LMC centre. This leads to an enclosed mass
estimate for the LMC at this radius of
.Comment: 23 pages, 10 figures. Accepted by MNRA
3D kinematics through the X-shaped Milky Way bulge
Context. It has recently been discovered that the Galactic bulge is X-shaped, with the two southern arms of the X both crossing the lines of sight at l = 0 and | b| > 4, hence producing a double red clump in the bulge color magnitude diagram. Dynamical models predict the formation of X-shaped bulges as extreme cases of boxy-peanut bulges. However, since X-shaped bulges were known to be present only in external galaxies, models have never been compared to 3D kinematical data for individual stars.
Aims. We study the orbital motion of Galactic bulge stars in the two arms (overdensities) of the X in the southern hemisphere. The goal is to provide observational constraints to bulge formation models that predict the formation of X-shapes through bar dynamical instabilities.
Methods. Radial velocities have been obtained for a sample of 454 bulge giants, roughly equally distributed between the bright and the faint red clump, in a field at (l,b) = (0, â6). Proper motions were derived for all red clump stars in the same field by combining images from two epochs, which were obtained 11 years apart, with WFI at the 2.2âm at La Silla. The observed field contains the globular cluster NGC 6558, whose member stars were used to assess the accuracy of the proper motion measurement. At the same time, as a by-product, we provide the first proper motion measurement of NGC 6558. The proper motions for the spectroscopic subsample are analyzed for a subsample of 352 stars, taking into account the radial velocities and metallicities measured from near-infrared calcium triplet lines.
Results. The radial velocity distribution of stars in the bright red clump, which traces the closer overdensity of bulge stars, shows an excess of stars moving towards the Sun. Similarly, an excess of stars receding from the Sun is seen in the far overdensity, which is traced by faint red clump stars. This is explained by the presence of stars on elongated orbits, which are most likely streaming along the arms of the X-shaped bulge. Proper motions for these stars are consistent with qualitative predictions of dynamical models of peanut-shaped bulges. Surprisingly, stars on elongated orbits have preferentially metal-poor (subsolar) metallicities, while the metal rich ones, in both overdensities, are preferentially found in more axisymmetric orbits. The observed proper motion of NGC 6558 has been measured as (ÎŒlcos â (b),ÎŒb) = (0.30 â ± â 0.14, â0.43 ± 0.13), with a velocity dispersion of (Ïlcos(b),Ïb) = (1.8,1.7) mas/yr. This is the first proper motion measurement for this cluster
A RAVE investigation on Galactic open clusters: II. Open cluster pairs, groups and complexes
© ESO, 2017. Context. It is generally agreed upon that stars form in open clusters (OCs) and stellar associations, but little is known about structures in the Galactic OC population; whether OCs and stellar associations are born isolated or if they prefer to form in groups, for example. Answering this question provides new insight into star and cluster formation, along with a better understanding of Galactic structures. Aims. In the past decade, studies of OC groupings have either been based solely on spatial criteria or have also included tangential velocities for identifications. In contrast to previous approaches, we assumed that real OC groupings occupy a well defined area in the sky and show similar velocity vectors. For the first time, we have used 6D phase-space information, including radial velocities from the RAdial Velocity Experiment (RAVE) and other catalogues, for the detection of OC groupings. We also checked the age spread of potential candidates to distinguish between genuine groupings and chance alignments. Methods. We explored the Catalogue of Open Cluster Data (COCD) and determined 6D phase-space information for 432 out of 650 listed OCs and compact associations. The group identification was performed using an adapted version of the Friends-of-Friends algorithm, as used in cosmology, with linking lengths of 100 pc and 10-20 km s-1. For the verification of the identified structures, we applied Monte Carlo simulations with randomised samples. Results. For the linking lengths 100 pc and 10 km s-1, we detected 19 groupings, including 14 pairs, 4 groups with 3-5 members, and 1 complex with 15 members. The Monte Carlo simulations revealed that, in particular, the complex is most likely genuine, whereas pairs are more likely chance alignments. A closer look at the age spread of the complex and the comparison between spatial distributions of young and old cluster populations suggested that OC groupings likely originated from a common molecular cloud
Diverse Hematological Malignancies Including Hodgkin-Like Lymphomas Develop in Chimeric MHC Class II Transgenic Mice
A chimeric HLA-DR4-H2-E (DR4) homozygous transgenic mouse line spontaneously develops diverse hematological malignancies with high frequency (70%). The majority of malignancies were distributed equally between T and B cell neoplasms and included lymphoblastic T cell lymphoma (LTCL), lymphoblastic B cell lymphoma (LBCL), diffuse large B cell lymphoma (DLBCL), the histiocyte/T cell rich variant of DLBCL (DLBCL-HA/T cell rich DLBCL), splenic marginal zone lymphoma (SMZL), follicular B cell lymphoma (FBL) and plasmacytoma (PCT). Most of these neoplasms were highly similar to human diseases. Also, some non-lymphoid malignancies such as acute myeloid leukemia (AML) and histiocytic sarcoma were found. Interestingly, composite lymphomas, including Hodgkin-like lymphomas, were also detected that had CD30+ Hodgkin/Reed-Sternberg (H/RS)-like cells, representing a tumor type not previously described in mice. Analysis of microdissected H/RS-like cells revealed their origin as germinal center B cells bearing somatic hypermutations and, in some instances, crippled mutations, as described for human Hodgkin lymphoma (HL). Transgene integration in an oncogene was excluded as an exclusive driving force of tumorigenesis and age-related lymphoma development suggests a multi-step process. Thus, this DR4 line is a useful model to investigate common molecular mechanisms that may contribute to important neoplastic diseases in man
Two chemically similar stellar overdensities on opposite sides of the plane of the Galaxy
Our Galaxy is thought to have undergone an active evolutionary history
dominated by star formation, the accretion of cold gas, and, in particular,
mergers up to 10 gigayear ago. The stellar halo reveals rich fossil evidence of
these interactions in the form of stellar streams, substructures, and
chemically distinct stellar components. The impact of dwarf galaxy mergers on
the content and morphology of the Galactic disk is still being explored. Recent
studies have identified kinematically distinct stellar substructures and moving
groups, which may have extragalactic origin. However, there is mounting
evidence that stellar overdensities at the outer disk/halo interface could have
been caused by the interaction of a dwarf galaxy with the disk. Here we report
detailed spectroscopic analysis of 14 stars drawn from two stellar
overdensities, each lying about 5 kiloparsecs above and below the Galactic
plane - locations suggestive of association with the stellar halo. However, we
find that the chemical compositions of these stars are almost identical, both
within and between these groups, and closely match the abundance patterns of
the Milky Way disk stars. This study hence provides compelling evidence that
these stars originate from the disk and the overdensities they are part of were
created by tidal interactions of the disk with passing or merging dwarf
galaxies.Comment: accepted for publication in Natur
The Milky Way Bulge: Observed properties and a comparison to external galaxies
The Milky Way bulge offers a unique opportunity to investigate in detail the
role that different processes such as dynamical instabilities, hierarchical
merging, and dissipational collapse may have played in the history of the
Galaxy formation and evolution based on its resolved stellar population
properties. Large observation programmes and surveys of the bulge are providing
for the first time a look into the global view of the Milky Way bulge that can
be compared with the bulges of other galaxies, and be used as a template for
detailed comparison with models. The Milky Way has been shown to have a
box/peanut (B/P) bulge and recent evidence seems to suggest the presence of an
additional spheroidal component. In this review we summarise the global
chemical abundances, kinematics and structural properties that allow us to
disentangle these multiple components and provide constraints to understand
their origin. The investigation of both detailed and global properties of the
bulge now provide us with the opportunity to characterise the bulge as observed
in models, and to place the mixed component bulge scenario in the general
context of external galaxies. When writing this review, we considered the
perspectives of researchers working with the Milky Way and researchers working
with external galaxies. It is an attempt to approach both communities for a
fruitful exchange of ideas.Comment: Review article to appear in "Galactic Bulges", Editors: Laurikainen
E., Peletier R., Gadotti D., Springer Publishing. 36 pages, 10 figure
The Gaia-ESO Survey::Exploring the complex nature and origins of the Galactic bulge populations
Context. As observational evidence steadily accumulates, the nature of the Galactic bulge has proven to be rather complex: the structural, kinematic, and chemical analyses often lead to contradictory conclusions. The nature of the metal-rich bulge - and especially of the metal-poor bulge - and their relation with other Galactic components, still need to be firmly defined on the basis of statistically significant high-quality data samples. Aims. We used the fourth internal data release of the Gaia-ESO survey to characterize the bulge metallicity distribution function (MDF), magnesium abundance, spatial distribution, and correlation of these properties with kinematics. Moreover, the homogeneous sampling of the different Galactic populations provided by the Gaia-ESO survey allowed us to perform a comparison between the bulge, thin disk, and thick disk sequences in the [Mg/Fe] vs. [Fe/H] plane in order to constrain the extent of their eventual chemical similarities. Methods. We obtained spectroscopic data for âŒ2500 red clump stars in 11 bulge fields, sampling the area -10° â„ l â„ +8° and -10° â„ b â„ -4° from the fourth internal data release of the Gaia-ESO survey. A sample of âŒ6300 disk stars was also selected for comparison. Spectrophotometric distances computed via isochrone fitting allowed us to define a sample of stars likely located in the bulge region. Results. From a Gaussian mixture models (GMM) analysis, the bulge MDF is confirmed to be bimodal across the whole sampled area. The relative ratio between the two modes of the MDF changes as a function of b, with metal-poor stars dominating at high latitudes. The metal-rich stars exhibit bar-like kinematics and display a bimodality in their magnitude distribution, a feature which is tightly associated with the X-shape bulge. They overlap with the metal-rich end of the thin disk sequence in the [Mg/Fe] vs. [Fe/H] plane. On the other hand, metal-poor bulge stars have a more isotropic hot kinematics and do not participate in the X-shape bulge. Their Mg enhancement level and general shape in the [Mg/Fe] vs. [Fe/H] plane is comparable to that of the thick disk sequence. The position at which [Mg/Fe] starts to decrease with [Fe/H], called the "knee", is observed in the metal-poor bulge at [Fe/H]knee = -0:37 ± 0:09, being 0.06 dex higher than that of the thick disk. Although this difference is inside the error bars, it suggest a higher star formation rate (SFR) for the bulge than for the thick disk. We estimate an upper limit for this difference of Î[Fe/H]knee = 0:24 dex. Finally, we present a chemical evolution model that suitably fits the whole bulge sequence by assuming a fast (<1 Gyr) intense burst of stellar formation that takes place at early epochs. Conclusions.We associate metal-rich stars with the bar boxy/peanut bulge formed as the product of secular evolution of the early thin disk. On the other hand, the metal-poor subpopulation might be the product of an early prompt dissipative collapse dominated by massive stars. Nevertheless, our results do not allow us to firmly rule out the possibility that these stars come from the secular evolution of the early thick disk. This is the first time that an analysis of the bulge MDF and α-abundances has been performed in a large area on the basis of a homogeneous, fully spectroscopic analysis of high-resolution, high S/N data
The Catalina Surveys Periodic Variable Star Catalog
We present ~47,000 periodic variables found during the analysis of 5.4
million variable star candidates within a 20,000 square degree region covered
by the Catalina Surveys Data Release-1 (CSDR1). Combining these variables with
type-ab RR Lyrae from our previous work, we produce an on-line catalog
containing periods, amplitudes, and classifications for ~61,000 periodic
variables. By cross-matching these variables with those from prior surveys, we
find that > 90% of the ~8,000 known periodic variables in the survey region are
recovered. For these sources we find excellent agreement between our catalog
and prior values of luminosity, period and amplitude, as well as
classification.
We investigate the rate of confusion between objects classified as contact
binaries and type-c RR Lyrae (RRc's) based on periods, colours, amplitudes,
metalicities, radial velocities and surface gravities. We find that no more
than few percent of these variables in these classes are misidentified. By
deriving distances for this clean sample of ~5,500 RRc's, we trace the path of
the Sagittarius tidal streams within the Galactic halo. Selecting 146
outer-halo RRc's with SDSS radial velocities, we confirm the presence of a
coherent halo structure that is inconsistent with current N-body simulations of
the Sagittarius tidal stream. We also find numerous long-period variables that
are very likely associated within the Sagittarius tidal streams system.
Based on the examination of 31,000 contact binary light curves we find
evidence for two subgroups exhibiting irregular lightcurves. One subgroup
presents significant variations in mean brightness that are likely due to
chromospheric activity. The other subgroup shows stable modulations over more
than a thousand days and thereby provides evidence that the O'Connell effect is
not due to stellar spots.Comment: Accepted ApJS, 43 pages, 9 tables, 44 figures (some at reduced
resolution
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