227 research outputs found
Stellar molecular jets traced by maser emission
Highly collimated jets found in AGB and post-AGB stars are expected to play
an important role for shaping planetary nebulae. Recent VLBI observations of
H2O maser sources have revealed that some of the spatio-kinematical structures
of H2O maser sources exhibit stellar jets with extremely spatially and
kinematically high collimation. Such stellar H2O maser jets, so-called "water
fountain" sources, have been identified in about 10 sources to date. Here we
review recent VLBI observations and relevant observational results of the water
fountain sources. They have revealed a typical dynamical age and the detailed
kinematical structures of the water fountains, possibility of the existence of
"equatorial flows", and the evolutionary status of the host stars. The location
and kinematics of one of the water fountain sources in the Galaxy is also
revealed and shown here.Comment: 8 pages, 6 figures, in the Proceedings of the IAU Symposium 242:
Astrophysical Masers and their Environment
The A-X infrared bands of Aluminum Oxide in stars: search and new detections
We describe a search for the A-X infrared bands of AlO with a view to better
understand the characteristics of this radical. These bands are infrequently
encountered in astronomical sources but surprisingly were very prominent in the
spectra of two well-known, nova-like variables (V838 Mon and V4332 Sgr) thereby
motivating us to explore the physical conditions necessary for their
excitation. In this study, we present the detection of A-X bands in the spectra
of 13 out of 17 stars, selected on the basis of their J-K colors as potential
candidates for detection of these bands. The majority of the AlO detections are
in AGB stars viz. 9 OH/IR stars, 2 Mira variables and 2 bright infrared
sources. Our study shows that the A-X bands are fairly prevalent in sources
with low temperature and O-rich environments. Interesting variation in strength
of the AlO bands in one of the sources (IRAS 18530+0817) is reported and the
cause for this is examined. Possible applications of the present study are
discussed in terms of the role of AlO in alumina dust formation, the scope for
estimating the radioactive Al content in AGB stars from the A-X bands,
and providing possible targets for further mm/radio studies of AlO which has
recently been discovered at millimeter wavelengths.Comment: To appear in Astrophysical Journal (Letters), 14 pages, 2 figure
Search for radiative pumping lines of OH masers: I. The 34.6um absorption line towards 1612 MHz OH maser sources
The 1612 MHz hydroxyl maser in circumstellar envelopes has long been thought
to be pumped by 34.6um photons. Only recently, the Infrared Space Observatory
has made possible spectroscopic observations which enable the direct
confirmation of this pumping mechanism in a few cases. To look for the presence
of this pumping line, we have searched the Infrared Space Observatory Data
Archive and found 178 spectra with data around 34.6um for 87 galactic 1612MHz
masers. The analysis performed showed that the noise level and the spectral
resolution of the spectra are the most important factors affecting the
detection of the 34.6um absorption line. Only 5 objects from the sample (3 red
supergiants and 2 galactic center sources) are found to show clear 34.6um
absorption (all of them already known) while two additional objects only
tentatively show this line. The 3 supergiants show similar pump rates and their
masers might be purely radiatively pumped. The pump rates of OH masers in late
type stars are found to be about 0.05, only 1/5 of the theoretical value of
0.25 derived by Elitzur (1992). We have also found 16 maser sources which,
according to the analysis assuming Elitzur's pump rate, should show the 34.6
m absorption line but do not. These non-detections can be tentatively
explained by far-infrared photon pumping, clumpy nature of the OH masing region
or a limb-filling emission effect in the OH shell.Comment: 11 pages, 8 figures, 3 table
Variable number of tandem repeat markers in the genome sequence of Mycosphaerella fijiensis, the causal agent of black leaf streak disease of banana (Musa spp)
ABSTRACT. We searched the genome of Mycosphaerella fijiensis for molecular markers that would allow population genetics analysis of this plant pathogen. M. fijiensis, the causal agent of banana leaf streak disease, also known as black Sigatoka, is the most devastating pathogen attacking bananas (Musa spp). Recently, the entire genome sequence of M. fijiensis became available. We screened this database for VNTR markers. Forty-two primer pairs were selected for validation, based on repeat type and length and the number of repeat units. Five VNTR markers showing multiple alleles were validated with a reference set of isolates from different parts of the world and a population from a banana plantation in Costa Rica. Polymorphism information content values varied from 0.6414 to 0.7544 for the reference set and from 0.0400 and 0.7373 for the population set. Eighty percent of the polymorphism information content values were above 0.60, indicating that the markers are highly informative. These markers allowed robust scoring of agarose gels and proved to be useful for variability and population genetics studies. In conclusion, the strategy we developed to identify and validate VNTR markers is an efficient means to incorporate markers that can be used for fungicide resistance management and to develop breeding strategies to control banana black leaf streak disease. This is the first report of VNTR-minisatellites from the M. fijiensis genome sequence. Key words: Molecular markers; VNTRs; Genetic diversity; Population genetics; Black Sigatok
Polarimetric observations of OH masers in proto-planetary nebulae
The 1612 and 1667 MHz OH maser lines have been measured in all four Stokes
parameters in 47 proto-planetary nebula (PPN) candidates. Out of 42 objects
detected, 40 and 34 are 1612 and 1667 MHz emitters, respectively. The spectral
extent of the 1667 MHz line overshoots that of the 1612 MHz line in about 80%
of the targets. 52% and 26% of the 1612 and 1667 MHz sources, respectively,
show linear polarization in at least some features. Circular polarization is
more frequent, occurring in 78% and 32% of sources of the respective OH lines.
The percentage polarization is usually small (<15%) reaching up to 50-80% in a
few sources. Features of linearly polarized emission are usually weak (0.5-4Jy)
and narrow (0.3-0.5kms^{-1}). The strength of magnetic field inferred from
likely Zeeman pairs in two sources of a few mG is consistent with values
reported elsewhere for those classes of objects. An upper limit of the electron
density in the envelope of OH17.7-2.0 derived from the difference in the
position angle of polarization vectors for the two OH lines is about 1cm^{-3}.
Distinct profiles of polarization position angle at 1612 and 1667 MHz are seen
in about one third of the sources and strongly suggest that the envelopes are
permeated with structured magnetic fields. The geometry of the magnetic field
is implicated as an important cause of the depolarization found in some PPN
candidates. For the subset of targets which show axisymmetric shells in the
optical or radio images we found a dominance of magnetic field components which
are orthogonal to the long axis of the nebulae. This finding supports the
hypothesis that such bipolar lobes are shaped by the magnetic field.Comment: Accepted for publication in A&A; 11 pages, 7 figure
A comprehensive set of transcript sequences of the heavy metal hyperaccumulator Noccaea caerulescens
Noccaea caerulescens is an extremophile plant species belonging to the Brassicaceae family. It has adapted to grow on soils containing high, normally toxic, concentrations of metals such as nickel, zinc, and cadmium. Next to being extremely tolerant to these metals, it is one of the few species known to hyperaccumulate these metals to extremely high concentrations in their aboveground biomass. In order to provide additional molecular resources for this model metal hyperaccumulator species to study and understand the mechanism of adaptation to heavy metal exposure, we aimed to provide a comprehensive database of transcript sequences for N. caerulescens. In this study, 23,830 transcript sequences (isotigs) with an average length of 1025 bp were determined for roots, shoots and inflorescences of N. caerulescens accession “Ganges” by Roche GS-FLEX 454 pyrosequencing. These isotigs were grouped into 20,378 isogroups, representing potential genes. This is a large expansion of the existing N. caerulescens transcriptome set consisting of 3705 unigenes. When translated and compared to a Brassicaceae proteome set, 22,232 (93.2%) of the N. caerulescens isotigs (corresponding to 19,191 isogroups) had a significant match and could be annotated accordingly. Of the remaining sequences, 98 isotigs resembled non-plant sequences and 1386 had no significant similarity to any sequence in the GenBank database. Among the annotated set there were many isotigs with similarity to metal homeostasis genes or genes for glucosinolate biosynthesis. Only for transcripts similar to Metallothionein3 (MT3), clear evidence for an additional copy was found. This comprehensive set of transcripts is expected to further contribute to the discovery of mechanisms used by N. caerulescens to adapt to heavy metal exposur
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