713 research outputs found
New class I methanol masers
We review properties of all known collisionally pumped (class I) methanol
maser series based on observations with the Australia Telescope Compact Array
(ATCA) and the Mopra radio telescope. Masers at 36, 84, 44 and 95 GHz are most
widespread, while 9.9, 25, 23.4 and 104 GHz masers are much rarer, tracing the
most energetic shocks. A survey of many southern masers at 36 and 44 GHz
suggests that these two transitions are highly complementary. The 23.4 GHz
maser is a new type of rare class I methanol maser, detected only in two
high-mass star-forming regions, G357.97-0.16 and G343.12-0.06, and showing a
behaviour similar to 9.9, 25 and 104 GHz masers. Interferometric positions
suggest that shocks responsible for class I masers could arise from a range of
phenomena, not merely an outflow scenario. For example, some masers might be
caused by interaction of an expanding HII region with its surrounding molecular
cloud. This has implications for evolutionary sequences incorporating class I
methanol masers if they appear more than once during the evolution of the
star-forming region. We also make predictions for candidate maser transitions
at the ALMA frequency range.Comment: 8 pages, 2 figures, to appear in proceedings for IAUS 287: Cosmic
Masers - from OH to H
High-velocity feature of the class I methanol maser in G309.38-0.13
The Australia Telescope Compact Array (ATCA) has been used to map class I
methanol masers at 36 and 44 GHz in G309.38-0.13. Maser spots are found at nine
locations in an area of 50''x30'', with both transitions reliably detected at
only two locations. The brightest spot is associated with shocked gas traced by
4.5 micron emission. The data allowed us to make a serendipitous discovery of a
high-velocity 36-GHz spectral feature, which is blue-shifted by about 30 km/s
from the peak velocity at this frequency, but spatially located close to
(within a few arcseconds of) the brightest maser spot. We interpret this as
indicating an outflow parallel to the line of sight. Such a high velocity
spread of maser features, which has not been previously reported in the class I
methanol masers associated with a single molecular cloud, suggests that the
outflow most likely interacts with a moving parcel of gas.Comment: 6 pages, 2 figures, accepted by MNRAS Letter
Discovery of the new class I methanol maser transition at 23.4 GHz
We report the first detection of a methanol maser in the 10(1)-9(2)A-
transition at 23.4 GHz, discovered during the H2O southern Galactic Plane
Survey (HOPS) with the 22-m Mopra radio telescope. In the region covered by
HOPS, the 23.4 GHz maser was found at only one location, G357.97-0.16, which
was also a prominent source of maser emission in the J(2)-J(1)E series near 25
GHz. The Australia Telescope Compact Array (ATCA) was used to follow up these
detections at high angular resolution and prove the maser nature of the
observed emission. The analysis shows that the new methanol maser at 23.4 GHz
is a class I maser, which has properties similar to the 9.9 and 25 GHz masers
(i.e. traces strong shocks with higher than average temperature and density).
All class I masers were found to originate at the same spatial location (within
the measurement uncertainty of 0.5 arcseconds) in the vicinity of the dominant
infrared source, but at a clearly distinct position from nearby OH, H2O and
class II methanol masers at 6.7 GHz. All maser species are distributed
approximately on a line, but it is not clear at present whether this has any
physical significance. We also detected a weak (1.3 mJy) continuum source at 25
GHz near the OH maser (at the most northern site, associated with a class II
methanol maser and an H2O maser renowned for its extremely wide spread of
velocity components). The continuum source has not been reported at lower
frequencies and is therefore a candidate hypercompact HII region. We also used
the ATCA to find the strongest and only fifth known 9.9 GHz maser towards
G357.97-0.16 and another 23.4 GHz maser towards G343.12-0.06 not seen in HOPS.Comment: 7 pages, 2 figures, 2 tables, accepted by MNRA
Second-layer nucleation in coherent Stranski-Krastanov growth of quantum dots
We have studied the monolayer-bilayer transformation in the case of the
coherent Stranski-Krastanov growth. We have found that the energy of formation
of a second layer nucleus is largest at the center of the first-layer island
and smallest on its corners. Thus nucleation is expected to take place at the
corners (or the edges) rather than at the center of the islands as in the case
of homoepitaxy. The critical nuclei have one atom in addition to a compact
shape, which is either a square of i*i or a rectangle of i*(i-1) atoms, with
i>1 an integer. When the edge of the initial monolayer island is much larger
than the critical nucleus size, the latter is always a rectangle plus an
additional atom, adsorbed at the longer edge, which gives rise to a new atomic
row in order to transform the rectangle into the equilibrium square shape.Comment: 6 pages, 4 figures. Accepted version, minor change
Frontiers, challenges, and solutions in modeling of swift heavy ion effects in materials
Since a few breakthroughs in the fundamental understanding of the effects of
swift heavy ions (SHI) decelerating in the electronic stopping regime in the
matter have been achieved in the last decade, it motivated us to review the
state-of-the-art approaches in the modeling of SHI effects. The SHI track
kinetics occurs via several well-separated stages: from attoseconds in
ion-impact ionization depositing energy in a target, to femtoseconds of
electron transport and hole cascades, to picoseconds of lattice excitation and
response, to nanoseconds of atomic relaxation, and even longer macroscopic
reaction. Each stage requires its own approaches for quantitative description.
We discuss that understanding the links between the stages makes it possible to
describe the entire track kinetics within a multiscale model without fitting
procedures. The review focuses on the underlying physical mechanisms of each
process, the dominant effects they produce, and the limitations of the existing
approaches as well as various numerical techniques implementing these models.
It provides an overview of ab-initio-based modeling of the evolution of the
electronic properties; Monte Carlo simulations of nonequilibrium electronic
transport; molecular dynamics modeling of atomic reaction on the surface and in
the bulk; kinetic Mote Carlo of atomic defect kinetics; finite-difference
methods of tracks interaction with chemical solvents describing etching
kinetics. We outline the modern methods that couple these approaches into
multiscale multidisciplinary models and point to their bottlenecks, strengths,
and weaknesses. The analysis is accompanied by examples of important results
improving the understanding of track formation in various materials.
Summarizing the most recent advances in the field of the track formation
process, the review delivers a comprehensive picture and detailed understanding
of the phenomena.Comment: to be submitte
The Performance and Calibration of the CRAFT Fly's Eye Fast Radio Burst Survey
Since January 2017, the Commensal Real-time ASKAP Fast Transients survey
(CRAFT) has been utilising commissioning antennas of the Australian SKA
Pathfinder (ASKAP) to survey for fast radio bursts (FRBs) in fly's eye mode.
This is the first extensive astronomical survey using phased array feeds
(PAFs), and a total of 20 FRBs have been reported. Here we present a
calculation of the sensitivity and total exposure of this survey, using the
pulsars B1641-45 (J1644-4559) and B0833-45 (J0835-4510, i.e.\ Vela) as
calibrators. The design of the survey allows us to benchmark effects due to PAF
beamshape, antenna-dependent system noise, radio-frequency interference, and
fluctuations during commissioning on timescales from one hour to a year.
Observation time, solid-angle, and search efficiency are calculated as a
function of FRB fluence threshold. Using this metric, effective survey
exposures and sensitivities are calculated as a function of the source counts
distribution. The implied FRB rate is significantly lower than the
\,sky\,day calculated using nominal exposures and
sensitivities for this same sample by \citet{craft_nature}. At the Euclidean
power-law index of , the rate is \,sky\,day above a threshold of \,Jy\,ms, while for the best-fit index for this sample of , it is
\,sky\,day above a threshold of \,Jy\,ms. This strongly suggests that these calculations be performed
for other FRB-hunting experiments, allowing meaningful comparisons to be made
between them.Comment: 21 pages, 15 figures, 2 tables, accepted for publication in PAS
Class I methanol masers in low-mass star formation regions
Four Class I maser sources were detected at 44, 84, and 95 GHz toward
chemically rich outflows in the regions of low-mass star formation NGC 1333I4A,
NGC 1333I2A, HH25, and L1157. One more maser was found at 36 GHz toward a
similar outflow, NGC 2023. Flux densities of the newly detected masers are no
more than 18 Jy, being much lower than those of strong masers in regions of
high-mass star formation. The brightness temperatures of the strongest peaks in
NGC 1333I4A, HH25, and L1157 at 44 GHz are higher than 2000 K, whereas that of
the peak in NGC 1333I2A is only 176 K. However, rotational diagram analysis
showed that the latter source is also a maser. The main properties of the newly
detected masers are similar to those of Class I methanol masers in regions of
massive star formation. The former masers are likely to be an extension of the
latter maser population toward low luminosities of both the masers and the
corresponding YSOs.Comment: 5 pages, 1 figure, Proc. IAU Symp. 287 "Cosmic Masers: from OH to
H0". LSR velocities of the HH25 masers, which are presented in Table 1, are
correcte
High-velocity OH megamasers in IRAS 20100-4156: Evidence for a Supermassive Black Hole
We report the discovery of new, high-velocity narrow-line components of the
OH megamaser in IRAS 20100-4156. Results from the Australian Square Kilometre
Array Pathfinder (ASKAP)'s Boolardy Engineering Test Array (BETA) and the
Australia Telescope Compact Array (ATCA) provide two independent measurements
of the OH megamaser spectrum. We found evidence for OH megamaser clumps at
409 and 562 km/s (blue-shifted) from the systemic velocity of the galaxy,
in addition to the lines previously known. The presence of such high velocities
in the molecular emission from IRAS 201004156 could be explained by a ~50 pc
molecular ring enclosing an approximately 3.8 billion solar mass black hole. We
also discuss two alternatives, i.e. that the narrow-line masers are dynamically
coupled to the wind driven by the active galactic nucleus or they are
associated with two separate galactic nuclei. The comparison between the BETA
and ATCA spectra provides another scientific verification of ASKAP's BETA. Our
data, combined with previous measurements of the source enabled us to study the
variability of the source over a twenty-six year period. The flux density of
the brightest OH maser components has reduced by more than a factor of two
between 1988 and 2015, whereas a secondary narrow-line component has more than
doubled in the same time. Plans for high-resolution VLBI follow-up of this
source are discussed, as are prospects for discovering new OH megamasers during
the ASKAP early science program.Comment: Accepted to MNRAS. Seven pages, three figure
Molecular line radiative transfer in protoplanetary disks: Monte Carlo simulations versus approximate methods
We analyze the line radiative transfer in protoplanetary disks using several
approximate methods and a well-tested Accelerated Monte Carlo code. A low-mass
flaring disk model with uniform as well as stratified molecular abundances is
adopted. Radiative transfer in low and high rotational lines of CO, C18O, HCO+,
DCO+, HCN, CS, and H2CO is simulated. The corresponding excitation
temperatures, synthetic spectra, and channel maps are derived and compared to
the results of the Monte Carlo calculations. A simple scheme that describes the
conditions of the line excitation for a chosen molecular transition is
elaborated. We find that the simple LTE approach can safely be applied for the
low molecular transitions only, while it significantly overestimates the
intensities of the upper lines. In contrast, the Full Escape Probability (FEP)
approximation can safely be used for the upper transitions (J_{\rm up} \ga 3)
but it is not appropriate for the lowest transitions because of the maser
effect. In general, the molecular lines in protoplanetary disks are partly
subthermally excited and require more sophisticated approximate line radiative
transfer methods. We analyze a number of approximate methods, namely, LVG, VEP
(Vertical Escape Probability) and VOR (Vertical One Ray) and discuss their
algorithms in detail. In addition, two modifications to the canonical Monte
Carlo algorithm that allow a significant speed up of the line radiative
transfer modeling in rotating configurations by a factor of 10--50 are
described.Comment: 47 pages, 12 figures, accepted for publication in Ap
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