183 research outputs found
Interbacterial signaling via Burkholderia contact-dependent growth inhibition system proteins
How bacteria interact with one another has implications for human health and disease because complex bacterial communities like biofilms can impact agriculture, infection transmission, and disease progression. Contact-dependent growth inhibition systems are proteins produced by many bacteria that deliver toxins to neighboring bacteria, allowing the producing cell to inhibit competitors that do not make the correct antidote. Here, we show that these systems not only antagonize competitors but also mediate communication and cooperation between bacteria that produce identical toxin/antidote pairs. Antidote-containing bacteria that are targeted by toxins change their gene expression to promote biofilm formation. Leading to a more complete understanding of how these antibacterial protein systems function, this study will inform future development of therapeutics, vaccines, and decontaminants
Methanol masers : Reliable tracers of the early stages of high-mass star formation
The GLIMPSE and MSX surveys have been used to examine the mid-infrared
properties of a statistically complete sample of 6.7 GHz methanol masers. The
GLIMPSE point sources associated with methanol masers are clearly distinguished
from the majority, typically having extremely red mid-infrared colors, similar
to those expected of low-mass class 0 young stellar objects. The intensity of
the GLIMPSE sources associated with methanol masers is typically 4 magnitudes
brighter at 8.0 micron than at 3.6 micron. Targeted searches towards GLIMPSE
point sources with [3.6]-[4.5] > 1.3 and an 8.0 micron magnitude less than 10
will detect more than 80% of class II methanol masers. Many of the methanol
masers are associated with sources within infrared dark clouds (IRDC) which are
believed to mark regions where high-mass star formation is in its very early
stages. The presence of class II methanol masers in a significant fraction of
IRDC suggests that high-mass star formation is common in these regions.
Different maser species are thought to trace different evolutionary phases of
the high-mass star formation process. Comparison of the properties of the
GLIMPSE sources associated with class II methanol masers and other maser
species shows interesting trends, consistent with class I methanol masers
tracing a generally earlier evolutionary phase and OH masers tracing a later
evolutionary phase.Comment: 45 pages, 19 figures, accepted for publication in Ap
ISOCAM observations of the rho Ophiuchi cloud: Luminosity and mass functions of the pre-main sequence embedded cluster
We present the results of the first extensive mid-infrared (IR) imaging
survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera
on board the ISO satellite. The main molecular cloud L1688, as well as L1689N
and L1689S, have been completely surveyed for point sources at 6.7 and 14.3
micron. A total of 425 sources are detected including 16 Class I, 123 Class II,
and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR
sources coincide with near-IR sources, but a large proportion of them are
recognized for the first time as YSOs. Our dual-wavelength survey allows us to
identify essentially all the YSOs with IR excess in the embedded cluster down
to Fnu ~ 10 - 15 mJy. It more than doubles the known population of Class II
YSOs and represents the most complete census to date of newly formed stars in
the rho Ophiuchi central region. The stellar luminosity function of the
complete sample of Class II YSOs is derived with a good accuracy down to L=
0.03 Lsun. A modeling of this lumino- sity function, using available pre-main
sequence tracks and plausible star for- mation histories, allows us to derive
the mass distribution of the Class II YSOs which arguably reflects the IMF of
the embedded cluster. We estimate that the IMF in rho Ophiuchi is well
described by a two-component power law with a low- mass index of -0.35+/-0.25,
a high-mass index of -1.7 (to be compared with the Salpeter value of -1.35),
and a break occurring at M = 0.55+/-0.25 Msun. This IMF is flat with no
evidence for a low-mass cutoff down to at least 0.06 Msun.Comment: A&A Document Class -- version 5.01, 27 pages, 10 figures v2: typos
added including few changes in source numberin
ISOCAM observations of the L1551 star formation region
The results of a deep mid-IR ISOCAM survey of the L1551 dark molecular cloud
are presented. The aim of this survey is a search for new YSO (Young Stellar
Object) candidates, using two broad-band filters centred at 6.7 and 14.3
micron. Although two regions close to the centre of L1551 had to be avoided due
to saturation problems, 96 sources were detected in total (76 sources at 6.7
micron and 44 sources at 14.3 micron). Using the 24 sources detected in both
filters, 14 were found to have intrinsic mid-IR excess at 14.3 micron and were
therefore classified as YSO candidates. Using additional observations in B, V,
I, J, H and K obtained from the ground, most candidates detected at these
wavelengths were confirmed to have mid-IR excess at 6.7 micron as well, and
three additional YSO candidates were found. Prior to this survey only three
YSOs were known in the observed region (avoiding L1551 IRS5/NE and HL/XZ Tau).
This survey reveals 15 new YSO candidates, although several of these are
uncertain due to their extended nature either in the mid-IR or in the
optical/near-IR observations. Two of the sources with mid-IR excess are
previously known YSOs, one is a brown dwarf MHO 5 and the other is the well
known T Tauri star HH30, consisting of an outflow and an optically thick disk
seen edge on.Comment: 14 Pages, 8 Figure
Star Formation in the Milky Way and Nearby Galaxies
We review progress over the past decade in observations of large-scale star
formation, with a focus on the interface between extragalactic and Galactic
studies. Methods of measuring gas contents and star formation rates are
discussed, and updated prescriptions for calculating star formation rates are
provided. We review relations between star formation and gas on scales ranging
from entire galaxies to individual molecular clouds.Comment: 55 pages, 15 figures, in press for Annual Reviews of Astronomy and
Astrophysics; Updated with corrected equation 5, improved references, and
other minor change
Determining the Parameters of Massive Protostellar Clouds via Radiative Transfer Modeling
A one-dimensional method for reconstructing the structure of prestellar and
protostellar clouds is presented. The method is based on radiative transfer
computations and a comparison of theoretical and observed intensity
distributions at both millimeter and infrared wavelengths. The radiative
transfer of dust emission is modeled for specified parameters of the density
distribution, central star, and external background, and the theoretical
distribution of the dust temperature inside the cloud is determined. The
intensity distributions at millimeter and IR wavelengths are computed and
quantitatively compared with observational data. The best-fit model parameters
are determined using a genetic minimization algorithm, which makes it possible
to reveal the ranges of parameter degeneracy as well. The method is illustrated
by modeling the structure of the two infrared dark clouds IRDC-320.27+029 (P2)
and IRDC-321.73+005 (P2). The derived density and temperature distributions can
be used to model the chemical structure and spectral maps in molecular lines.Comment: Accepted for publication in Astronomy Report
Chemistry in Infrared Dark Cloud Clumps: a Molecular Line Survey at 3 mm
We have observed 37 Infrared Dark Clouds (IRDCs), containing a total of 159
clumps, in high-density molecular tracers at 3 mm using the 22-meter ATNF Mopra
Telescope located in Australia. After determining kinematic distances, we
eliminated clumps that are not located in IRDCs and clumps with a separation
between them of less than one Mopra beam. Our final sample consists of 92 IRDC
clumps. The most commonly detected molecular lines are (detection rates higher
than 8%): N2H+, HNC, HN13C, HCO+, H13CO+, HCN, C2H, HC3N, HNCO, and SiO. We
investigate the behavior of the different molecular tracers and look for
chemical variations as a function of an evolutionary sequence based on Spitzer
IRAC and MIPS emission. We find that the molecular tracers behave differently
through the evolutionary sequence and some of them can be used to yield useful
relative age information. The presence of HNC and N2H+ lines do not depend on
the star formation activity. On the other hand, HC3N, HNCO, and SiO are
predominantly detected in later stages of evolution. Optical depth calculations
show that in IRDC clumps the N2H+ line is optically thin, the C2H line is
moderately optically thick, and HNC and HCO+ are optically thick. The HCN
hyperfine transitions are blended, and, in addition, show self-absorbed line
profiles and extended wing emission. These factors combined prevent the use of
HCN hyperfine transitions for the calculation of physical parameters. Total
column densities of the different molecules, except C2H, increase with the
evolutionary stage of the clumps. Molecular abundances increase with the
evolutionary stage for N2H+ and HCO+. The N2H+/HCO+ and N2H+/HNC abudance
ratios act as chemical clocks, increasing with the evolution of the clumps.Comment: Accepted to ApJ. 29 page
Stellar populations in a standard ISOGAL field in the Galactic disk
We aim to identify the stellar populations (mostly red giants and young
stars) detected in the ISOGAL survey at 7 and 15micron towards a field (LN45)
in the direction l=-45, b=0.0. The sources detected in the survey of the
Galactic plane by the Infrared Space Observatory are characterized based on
colour-colour and colour-magnitude diagrams. We combine the ISOGAL catalog with
the data from surveys such as 2MASS and GLIMPSE. Interstellar extinction and
distance are estimated using the red clump stars detected by 2MASS in
combination with the isochrones for the AGB/RGB branch. Absolute magnitudes are
thus derived and the stellar populations are identified based on their absolute
magnitudes and their infrared excess. A standard approach to the analysis of
ISOGAL disk observations has been established. We identify several hundred
RGB/AGB stars and 22 candidate young stellar objects in the direction of this
field in an area of 0.16 deg^2. An over-density of stellar sources is found at
distances corresponding to the distance of the Scutum-Crux spiral arm. In
addition, we determine mass-loss rates of AGB-stars using dust radiative
transfer models from the literature.Comment: 48pages, 38 figures, accepted for publication in A &
Mapping the column density and dust temperature structure of IRDCs with Herschel
Infrared dark clouds (IRDCs) are cold and dense reservoirs of gas potentially
available to form stars. Many of these clouds are likely to be pristine
structures representing the initial conditions for star formation. The study
presented here aims to construct and analyze accurate column density and dust
temperature maps of IRDCs by using the first Herschel data from the Hi-GAL
galactic plane survey. These fundamental quantities, are essential for
understanding processes such as fragmentation in the early stages of the
formation of stars in molecular clouds. We have developed a simple
pixel-by-pixel SED fitting method, which accounts for the background emission.
By fitting a grey-body function at each position, we recover the spatial
variations in both the dust column density and temperature within the IRDCs.
This method is applied to a sample of 22 IRDCs exhibiting a range of angular
sizes and peak column densities. Our analysis shows that the dust temperature
decreases significantly within IRDCs, from background temperatures of 20-30 K
to minimum temperatures of 8-15 K within the clouds, showing that dense
molecular clouds are not isothermal. Temperature gradients have most likely an
important impact on the fragmentation of IRDCs. Local temperature minima are
strongly correlated with column density peaks, which in a few cases reach NH2 =
1 x 10^{23} cm^{-2}, identifying these clouds as candidate massive prestellar
cores. Applying this technique to the full Hi-GAL data set will provide
important constraints on the fragmentation and thermal properties of IRDCs, and
help identify hundreds of massive prestellar core candidates.Comment: Accepted for publication in A&A Herschel special issu
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