100 research outputs found
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
A method for comparing discrete kinematic data and N-body simulations
This paper describes a method for quantitatively comparing an N-body model
with a sample of discrete kinematic data. The comparison has two stages: (i)
finding the optimum scaling and orientation of the model relative to the data;
and (ii) calculating a goodness of fit, and hence assessing the plausibility of
the model in vew of the data. The method derives from considering the data and
model both as samples from some underlying binned distribution function, and
applying probability theory arguments. As an example, I consider a published
N-body model for the Galactic bulge and disc, and fictitious l,b,v
measurements, and recover (with error estimates) the spatial and velocity
scales of the model and the orientation of the bar. The fictitious data are
actually derived from the model by assuming the mass scale and the solar
position, but their size and extent mimics a recent survey of OH/IR stars. The
results indicate that mass of the bulge and our viewing angle of the bar are
usefully estimable from current surveys.Comment: To appear in A
Reversal in time order of interactive events: Collision of inclined rods
In the rod and hole paradox as described by Rindler (1961 Am. J. Phys. 29
365-6), a rigid rod moves at high speed over a table towards a hole of the same
size. Observations from the inertial frames of the rod and slot are widely
different. Rindler explains these differences by the concept of differing
perceptions in rigidity. Gron and Johannesen (1993 Eur. J. Phys. 14 97-100)
confirmed this aspect by computer simulation where the shapes of the rods are
different as observed from the co-moving frames of the rod and slot. Lintel and
Gruber (2005 Eur. J. Phys. 26 19-23) presented an approach based on retardation
due to speed of stress propagation. In this paper we consider the situation
when two parallel rods collide while approaching each other along a line at an
inclination with their axis. The collisions of the top and bottom ends are
reversed in time order as observed from the two co-moving frames. This result
is explained by the concept of extended present derived from the principle of
relativity of simultaneity
An automated microreactor for semi-continuous biosensor measurements.
Living bacteria or yeast cells are frequently used as bioreporters for the detection of specific chemical analytes or conditions of sample toxicity. In particular, bacteria or yeast equipped with synthetic gene circuitry that allows the production of a reliable non-cognate signal (e.g., fluorescent protein or bioluminescence) in response to a defined target make robust and flexible analytical platforms. We report here how bacterial cells expressing a fluorescence reporter ("bactosensors"), which are mostly used for batch sample analysis, can be deployed for automated semi-continuous target analysis in a single concise biochip. Escherichia coli-based bactosensor cells were continuously grown in a 13 or 50 nanoliter-volume reactor on a two-layered polydimethylsiloxane-on-glass microfluidic chip. Physiologically active cells were directed from the nl-reactor to a dedicated sample exposure area, where they were concentrated and reacted in 40 minutes with the target chemical by localized emission of the fluorescent reporter signal. We demonstrate the functioning of the bactosensor-chip by the automated detection of 50 μgarsenite-As l(-1) in water on consecutive days and after a one-week constant operation. Best induction of the bactosensors of 6-9-fold to 50 μg l(-1) was found at an apparent dilution rate of 0.12 h(-1) in the 50 nl microreactor. The bactosensor chip principle could be widely applicable to construct automated monitoring devices for a variety of targets in different environments
A quasi-time-dependent radiative transfer model of OH104.9+2.4
We investigate the pulsation-phase dependent properties of the circumstellar
dust shell (CDS) of the OH/IR star OH104.9+2.4 based on radiative transfer
modeling (RTM) using the code DUSTY. Our previous study concerning simultaneous
modeling of the spectral energy distribution (SED) and near-infrared (NIR)
visibilities (Riechers et al. 2004) has now been extended by means of a more
detailed analysis of the pulsation-phase dependence of the model parameters of
OH104.9+2.4. In order to investigate the temporal variation in the spatial
structure of the CDS, additional NIR speckle interferometric observations in
the K' band were carried out with the 6 m telescope of the Special
Astrophysical Observatory (SAO). At a wavelength of 2.12 micron the
diffraction-limited resolution of 74 mas was attained. Several key parameters
of our previous best-fitting model had to be adjusted in order to be consistent
with the newly extended amount of observational data. It was found that a
simple rescaling of the bolometric flux F_bol is not sufficient to take the
variability of the source into account, as the change in optical depth over a
full pulsation cycle is rather high. On the other hand, the impact of a change
in effective temperature T_eff on SED and visibility is rather small. However,
observations, as well as models for other AGB stars, show the necessity of
including a variation of T_eff with pulsation phase in the radiative transfer
models. Therefore, our new best-fitting model accounts for these changes.Comment: 7 pages, including 5 postscript figures and 3 tables. Published in
Astronomy and Astrophysics. (v1: accepted version; v2: published version,
minor grammatical changes
OH-selected AGB and post-AGB objects I.Infrared and maser properties
Using 766 compact objects from a survey of the galactic Plane in the 1612-MHz
OH line, new light is cast on the infrared properties of evolved stars on the
TP-AGB and beyond. The usual mid-infrared selection criteria, based on IRAS
colours, largely fail to distinguish early post-AGB stages. A two-colour
diagram from narrower-band MSX flux densities, with bimodal distributions,
provides a better tool to do the latter. Four mutually consistent selection
criteria for OH-masing red PPNe are given, as well as two for early post-AGB
masers and one for all post--AGB masers, including the earliest ones. All these
criteria miss a group of blue, high-outflow post-AGB sources with 60-mum
excess; these will be discussed in detail in Paper II. The majority of post-AGB
sources show regular double-peaked spectra in the OH 1612-MHz line, with fairly
low outflow velocities, although the fractions of single peaks and irregular
spectra may vary with age and mass. The OH flux density shows a fairly regular
relation with the stellar flux and the envelope optical depth, with the maser
efficiency increasing with IRAS colour R21. The OH flux density is linearly
correlated with the 60-mum flux density.Comment: 16 pages, LaTex, 22 figures, AJ (accepted
Evidence of triggered star formation in G327.3-0.6. Dust-continuum mapping of an infrared dark cloud with P-ArT\'eMiS
Aims. Expanding HII regions and propagating shocks are common in the
environment of young high-mass star-forming complexes. They can compress a
pre-existing molecular cloud and trigger the formation of dense cores. We
investigate whether these phenomena can explain the formation of high-mass
protostars within an infrared dark cloud located at the position of G327.3-0.6
in the Galactic plane, in between two large infrared bubbles and two HII
regions. Methods: The region of G327.3-0.6 was imaged at 450 ? m with the CEA
P-ArT\'eMiS bolometer array on the Atacama Pathfinder EXperiment telescope in
Chile. APEX/LABOCA and APEX-2A, and Spitzer/IRAC and MIPS archives data were
used in this study. Results: Ten massive cores were detected in the P-ArT\'eMiS
image, embedded within the infrared dark cloud seen in absorption at both 8 and
24 ?m. Their luminosities and masses indicate that they form high-mass stars.
The kinematical study of the region suggests that the infrared bubbles expand
toward the infrared dark cloud. Conclusions: Under the influence of expanding
bubbles, star formation occurs in the infrared dark areas at the border of HII
regions and infrared bubbles.Comment: 4 page
H2O Maser Observations of Candidate Post-AGB Stars and Discovery of Three High-velocity Water Sources
We present the results of 22 GHz H_2O maser observations of a sample of 85
post-Asymptotic Giant Branch (post-AGB) candidate stars, selected on the basis
of their OH 1612 MHz maser and far-infrared properties. All sources were
observed with the Tidbinbilla 70-m radio telescope and 21 detections were made.
86 GHz SiO Mopra observations of a subset of the sample are also presented. Of
the 21 H_2O detections, 15 are from sources that are likely to be massive AGB
stars and most of these show typical, regular H_2O maser profiles. In contrast,
nearly all the detections of more evolved stars exhibited high-velocity H_2O
maser emission. Of the five sources seen, v223 (W43A, IRAS 18450-0148) is a
well known `water-fountain' source which belongs to a small group of post-AGB
stars with highly collimated, high-velocity H_2O maser emission. A second
source in our sample, v270 (IRAS 18596+0315), is also known to have
high-velocity emission. We report the discovery of similar emission from a
further three sources, d46 (IRAS 15445-5449), d62 (IRAS 15544-5332) and b292
(IRAS 18043-2116). The source d46 is an evolved post-AGB star with highly
unusual maser properties. The H_2O maser emission from d62 is probably
associated with a massive star. The source b292 is a young post-AGB star that
is highly likely to be a water-fountain source, with masers detected over a
velocity range of 210 km s^{-1}.Comment: 47 pages, 9 figures, 4 tables, accepted by Ap
Observations of the 6 Centimeter Lines of OH in Evolved (OH/IR) Stars
Recent observational and theoretical advances have called into question
traditional OH maser pumping models in evolved (OH/IR) stars. The detection of
excited-state OH lines would provide additional constraints to discriminate
amongst these theoretical models. In this Letter, we report on VLA observations
of the 4750 MHz and 4765 MHz lines of OH toward 45 sources, mostly evolved
stars. We detect 4765 MHz emission in the star forming regions Mon R2 and LDN
1084, but we do not detect excited-state emission in any evolved stars. The
flux density and velocity of the 4765 MHz detection in Mon R2 suggests that a
new flaring event has begun.Comment: 4 pages, to appear in ApJ
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