696 research outputs found
Reaffirming the connection between the Galactic stellar warp and the Canis Major overdensity
We perform a critical re-analysis and discussion of recent results presented
in the literature which interpret the CMa overdensity as the signature of an
accreting dwarf galaxy or a new substructure within the Galaxy. Several issues
are addressed. We show that arguments against the ``warp'' interpretation are
based on an erroneous perception of the Milky Way. There is nothing anomalous
with colour--magnitude diagrams on opposite sides of the average warp mid-plane
being different. We witnessed the rise and fall of the blue plume population,
first attributed to young stars in a disrupting dwarf galaxy and now discarded
as a normal disc population. Similarly, there is nothing anomalous in the outer
thin+thick disc metallicities being low (-1<[Fe/H]<-0.5), and spiral arms (as
part of the thin disc) should, and do, warp. Most importantly, we show
unambiguously that, contrary to previous claims, the warp produces a stellar
overdensity that is distance-compatible with that observed in CMa.The CMa
over-density remains fully accounted for in a first order approach by Galactic
models without new substructures. Given the intrinsic uncertainties (concerning
the properties of the warp, flare and disc cutoff, the role of extinction and
degeneracy), minor deviations with respect to these models are not enough to
support the hypothesis of an accreted dwarf galaxy or new substructure within
the Milky Way disc.Comment: A&A Letter, accepted, 4 pages, 3 figure
NTT infrared imaging of star cluster candidates towards the central parts of the Galaxy
We address the issue whether the central parts of the Galaxy harbour young
clusters other than Arches, Quintuplet and the Nuclear Young Cluster. A large
sample of centrally projected cluster candidates has been recently identified
from the 2MASS J, H and Ks Atlas. We provide a catalogue of higher angular
resolution and deeper images for 57 2MASS cluster candidates, obtained with the
near-IR camera SOFI at the ESO NTT telescope. We classify 10 objects as star
clusters, some of them deeply embedded in gas and/or dust clouds. Three other
objects are probably star clusters, although the presence of dust in the field
does not exclude the possibility of their being field stars seen through
low-absorption regions. Eleven objects are concentrations of stars in areas of
little or no gas, and are classified as dissolving cluster candidates. Finally,
31 objects turned out to be the blend of a few bright stars, not resolved as
such in the low resolution 2MASS images. By combining the above results with
other known objects we provide an updated sample of 42 embedded clusters and
candidates projected within 7 degrees. As a first step we study Object 11 of
Dutra & Bica (2000) projected at approximately 1 degree from the nucleus. We
present H and Ks photometry and study the colour-magnitude diagram and
luminosity function. Object 11 appears to be a less massive cluster than Arches
or Quintuplet, and it is located at a distance from the Sun d=8 kpc, with a
visual absorption Av=15.Comment: accepted to A&A, 9 pages, 10 figure
Septins focus cellular growth for host infection by pathogenic fungi
This is the final version of the article. Available from Frontiers Media via the DOI in this record.One of the key challenges faced by microbial pathogens is invasion of host tissue. Fungal pathogens adopt a number of distinct strategies to overcome host cell defenses, including the development of specialized infection structures, the secretion of proteins that manipulate host responses or cellular organization, and the ability to facilitate their own uptake by phagocytic mechanisms. Key to many of these adaptations is the considerable morphogenetic plasticity displayed by pathogenic species. Fungal pathogens can, for example, shift their growth habit between non-polarized spores, or yeast-like cells, and highly polarized hyphal filaments. These polarized filaments can then elaborate differentiated cells, specialized to breach host barriers. Septins play fundamental roles in the ability of diverse fungi to undergo shape changes and organize the F-actin cytoskeleton to facilitate invasive growth. As a consequence, septins are increasingly implicated in fungal pathogenesis, with many septin mutants displaying impairment in their ability to cause diseases of both plants and animals. In this mini-review, we show that a common feature of septin mutants is the emergence of extra polar outgrowths during morphological transitions, such as emergence of germ tubes from conidia or branches from hyphae. We propose that because septins detect and stabilize membrane curvature, they prevent extra polar outgrowths and thereby focus fungal invasive force, allowing substrate invasion.NT is funded by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 294702 GENBLAST. MM was funded by NSF Grant IOS1051730 from the Developmental Systems Cluster
A hot horizontal branch star with a close K-type main-sequence companion
Dynamical interactions in binary systems are thought to play a major role in
the formation of extreme horizontal branch stars (EHBs) in the Galactic field.
However, it is still unclear if the same mechanisms are at work in globular
clusters, where EHBs are predominantly single stars. Here we report on the
discovery of a unique close binary system (period ~1.61 days) in the globular
cluster NGC6752, comprising an EHB and a main-sequence companion of 0.63+-0.05
Msun. Such a system has no counterpart among nearly two hundred known EHB
binaries in the Galactic field. Its discovery suggests that either field
studies are incomplete, missing this type of systems possibly because of
selection effects, or that a particular EHB formation mechanism is active in
clusters but not in the field
On the HI-Hole and AGB Stellar Population of the Sagittarius Dwarf Irregular Galaxy
Using two HST/ACS data-sets that are separated by ~2 years has allowed us to
derive the relative proper-motion for the Sagittarius dwarf irregular (SagDIG)
and reduce the heavy foreground Galactic contamination. The proper-motion
decontaminated SagDIG catalog provides a much clearer view of the young
red-supergiant and intermediate-age asymptotic giant branch populations. We
report the identification of 3 Milky Way carbon-rich dwarf stars, probably
belonging to the thin disk, and pointing to the high incidence of this class at
low Galactic latitudes. A sub-group of 4 oxygen-rich candidate stars depicts a
faint, red extension of the well-defined SagDIG carbon-rich sequence. The
origin of these oxygen-rich candidate stars remains unclear, reflecting the
uncertainty in the ratio of carbon/oxygen rich stars. SagDIG is also a gas-rich
galaxy characterized by a single large cavity in the gas disk (HI-hole), which
is offset by ~360 pc from the optical centre of the galaxy. We nonetheless
investigate the stellar feedback hypothesis by comparing the proper-motion
cleaned stellar populations within the HI-hole with appropriately selected
comparison regions, having higher HI densities external to the hole. The
comparison shows no significant differences. In particular, the centre of the
HI-hole (and the comparison regions) lack stellar populations younger than ~400
Myr, which are otherwise abundant in the inner body of the galaxy. We conclude
that there is no convincing evidence that the SagDIG HI-hole is the result of
stellar feedback, and that gravitational and thermal instabilities in the gas
are the most likely mechanism for its formation.Comment: Accepted for publication in A&A, 11 pages, 6 jpeg figure
An extreme paucity of second population AGB stars in the normal globular cluster M4
Galactic Globular clusters (GCs) are now known to harbour multiple stellar
populations, which are chemically distinct in many light element abundances. It
is becoming increasingly clear that asymptotic giant branch (AGB) stars in GCs
show different abundance distributions in light elements compared to those in
the red giant branch (RGB) and other phases, skewing toward more primordial,
field-star-like abundances, which we refer to as subpopulation one (SP1). As
part of a larger program targeting giants in GCs, we obtained high-resolution
spectra for a sample of 106 RGB and 15 AGB stars in Messier 4 (NGC 6121) using
the 2dF+HERMES facility on the Anglo-Australian Telescope. In this Letter we
report an extreme paucity of AGB stars with [Na/O] > -0.17 in M4, which
contrasts with the RGB that has abundances up to [Na/O] =0.55. The AGB
abundance distribution is consistent with all AGB stars being from SP1. This
result appears to imply that all subpopulation two stars (SP2; Na-rich, O-poor)
avoid the AGB phase. This is an unexpected result given M4's horizontal branch
morphology -- it does not have an extended blue horizontal branch. This is the
first abundance study to be performed utilising the HERMES spectrograph.Comment: 5 pages, 2 figures, 4 tables (full Table 1 online). Accepted for
publication in MNRAS Letter
Modeling Star counts in the Monoceros stream and the Galactic anti-centre
There is a continued debate as to the form of the outer disc of the Milky Way
galaxy, which has important implications for its formation. Stars are known to
exist at a galacto-centric distance of at least 20 kpc. However, there is much
debate as to whether these stars can be explained as being part of the disc or
whether another extra galactic structure, the so called Monoceros ring/stream,
is required. To examine the outer disc of the Galaxy toward the anti-centre to
determine whether the star counts can be explained by the thin and thick discs
alone. Using Sloan star counts and extracting the late F and early G dwarfs it
is possible to directly determine the density of stars out to a galacto-centric
distance of about 25 kpc. These are then compared with a simple flared disc
model. A flared disc model is shown to reproduce the counts along the line of
sights examined, if the thick disc does not have a sharp cut off. The flare
starts at a Galacto-centric radius of 16 kpc and has a scale length of
4.5+/-1.5 kpc. Whilst the interpretation of the counts in terms of a
ring/stream cannot be definitely discounted, it does not appear to be
necessary, at least along the lines of sight examined towards the anti centre.Comment: 11 pages, 4 figures, accepted to be published in A&
The Na-O anticorrelation in horizontal branch stars. III. 47 Tuc and M 5
To check the impact of the multiple population scenario for globular clusters
on their HB, we present an analysis of the composition of 110 red HB (RHB)
stars in 47 Tucanae and of 61 blue HB (BHB) and 30 RHB stars in M5. In 47 Tuc
we found tight relations between the colours of the stars and their abundances
of p-capture elements. This strongly supports the idea that the He content -
which is expected to be closely correlated with the abundances of p-capture
elements - is the third parameter (after overall metallicity and age) that
determines the colour of HB stars. However, the range in He abundance must be
small (Delta Y<0.03) in 47 Tuc to reproduce our observations; this agrees with
previous analyses. There is possibly a correlation between the abundances of p-
and n-capture elements in 47 Tuc. If confirmed, this might suggest that AGB
stars of moderate mass contributed to the gas from which second-generation
stars formed. Considering the selection effects in our sample (which does not
include stars warmer than 11000 K and RR Lyrae variables) is important to
understand our results for M5. In this case, we find that, as expected, RHB
stars are Na-poor and O-rich, and likely belong to the primordial population.
There is a clear correlation of the [Na/O] ratio and N abundance with colour
along the BHB. A derivation of the He abundance for these stars yields a low
value of Y=0.22\pm 0.03. This is expected because HB stars of a putative
He-rich population in this cluster should be warmer than 11000 K, and would
accordingly not have been sampled by our analysis. However, we need some
additional source of scatter in the total mass loss of stars climbing up the
RGB to reproduce our results for M5. Finally, we found a C-star on the HB of 47
Tuc and a Ba-rich, fast-rotating, likely binary star on the HB of M5. These
stars are among the brightest and coolest HB stars.Comment: Accepted on Astronomy and Astrophysics. 16 pages, 19 figure
NGC 362: another globular cluster with a split red giant branch
We obtained FLAMES GIRAFFE+UVES spectra for both first and second-generation
red giant branch (RGB) stars in the globular cluster (GC) NGC 362 and used them
to derive abundances of 21 atomic species for a sample of 92 stars. The
surveyed elements include proton-capture (O, Na, Mg, Al, Si), alpha-capture
(Ca, Ti), Fe-peak (Sc, V, Mn, Co, Ni, Cu), and neutron-capture elements (Y, Zr,
Ba, La, Ce, Nd, Eu, Dy). The analysis is fully consistent with that presented
for twenty GCs in previous papers of this series. Stars in NGC 362 seem to be
clustered into two discrete groups along the Na-O anti-correlation, with a gap
at [O/Na] 0 dex. Na-rich, second generation stars show a trend to be more
centrally concentrated, although the level of confidence is not very high. When
compared to the classical second-parameter twin NGC 288, with similar
metallicity, but different horizontal branch type and much lower total mass,
the proton-capture processing in stars of NGC 362 seems to be more extreme,
confirming previous analysis. We discovered the presence of a secondary RGB
sequence, redder than the bulk of the RGB: a preliminary estimate shows that
this sequence comprises about 6% of RGB stars. Our spectroscopic data and
literature photometry indicate that this sequence is populated almost
exclusively by giants rich in Ba, and probably rich in all s-process elements,
as found in other clusters. In this regards, NGC 362 joins previously studied
GCs like NGC 1851, NGC 6656 (M 22), and NGC 7089 (M 2).Comment: 16 pages, 23 figures, 11 tables, accepted for publication on
Astronomy and Astrophysic
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