637 research outputs found
Revising the kinematics of 12GHz CH3OH masers in W3(OH)
We derive accurate proper motions of the CH3OH 12 GHz masers towards the
W3(OH) UCHII region, employing seven epochs of VLBA observations spanning a
time interval of about 10 yr. The achieved velocity accuracy is of the order of
0.1 km/s, adequate to precisely measure the relative velocities of most of the
12 GHz masers in W3(OH), with amplitude varying in the range 0.3 - 3 km/s.
Towards W3(OH), the most intense 12 GHz masers concentrate in a small area
towards the north (the northern clump) of the UCHII region. We have compared
the proper motions of the CH3OH 12 GHz masers with those (derived from
literature data) of the OH 6035 MHz masers, emitting from the same region of
the methanol masers. In the northern clump, the two maser emissions emerge from
nearby (but likely distinct) cloudlets of masing gas with, in general, a rather
smooth variation of line-of-sight and sky-projected velocities, which suggests
some connection of the environments and kinematics traced by both maser types.
The conical outflow model, previously proposed to account for the 12 GHz maser
kinematics in the northern clump, does not reproduce the new, accurate
measurements of 12 GHz maser proper motions and has to be rejected. We focus on
the subset of 12 GHz masers of the northern clump belonging to the "linear
structure at P.A. = 130-140 degree", whose regular variation of LSR velocities
with position presents evidence for some ordered motion. We show that the
3-dimensional velocities of this "linear distribution" of 12GHz masers can be
well fitted considering a flat, rotating disk, seen almost edge-on.Comment: 32 pages, 10 figures; accepted in ApJ (Main Journal
The modelling of operational risk: experience with the analysis of the data collected by the Basel Committee
The revised Basel Capital Accord requires banks to meet a capital requirement for operational risk as part of an overall risk-based capital framework. Three distinct options for calculating operational risk charges are proposed (Basic Approach, Standardised Approach, Advanced Measurement Approaches), reflecting increasing levels of risk sensitivity. Since 2001, the Risk Management Group of the Basel Committee has been performing specific surveys of banksÂ’ operational loss data, with the main purpose of obtaining information on the industryÂ’s operational risk experience, to be used for the refinement of the capital framework and for the calibration of the regulatory coefficients. The second loss data collection was launched in the summer of 2002: the 89 banks participating in the exercise provided the Group with more than 47,000 observations, grouped by eight standardised Business Lines and seven Event Types. A summary of the data collected, which focuses on the description of the range of individualgross loss amounts and of the distribution of the banksÂ’ losses across the business lines/event types, was returned to the industry in March 2003. The objective of this paper is to move forward with respect to that document, by illustrating the methodologies and the outcomes of the inferential analysis carried out on the data collected through 2002. To this end, after pooling the individual banksÂ’ losses according to a Business Line criterion, the operational riskiness of each Business Line data set is explored using empirical and statistical tools. The work aims, first of all, to compare the sensitivity of conventional actuarial distributions and models stemming from the Extreme Value Theory in representing the highest percentiles of the data sets: the exercise shows that the extreme value model, in its Peaks Over Threshold representation, explains the behaviour of the operational risk data in the tail area well. Then, measures of severity and frequency of the large losses are gained and, by a proper combination of these estimates, a bottom-up operational risk capital figure is computed for each Business Line. Finally, for each Business Line and in the eight Business Lines as a whole, the contributions of the expected losses to the capital figures are evaluated and the relationships between the capital charges and the corresponding average level of Gross Incomes are determined and compared with the current coefficients envisaged in the simplified approaches of the regulatory framework.operational risk, heavy tails, conventional inference, Extreme Value Theory, Peaks Over Threshold, median shortfall, Point Process of exceedances, capital charge, Business Line, Gross Income, regulatory coefficients
A multiple system of high-mass YSOs surrounded by disks in NGC7538 IRS1
NGC7538 IRS1 is considered the best high-mass accretion disk candidate around
an O-type young star in the northern hemisphere. We investigated the 3D
kinematics and dynamics of circumstellar gas with very high linear resolution,
from tens to 1500 AU, with the ultimate goal of building a comprehensive
dynamical model for this YSO. We employed four different observing epochs of
EVN data at 6.7 GHz, spanning almost eight years, which enabled us to measure,
besides line-of-sight (l.o.s.) velocities and positions, also l.o.s.
accelerations and proper motions of methanol masers. In addition, we imaged
with the JVLA-B array highly-excited ammonia inversion lines, from (6,6) to
(13,13), which enabled us to probe the hottest molecular gas very close to the
exciting source(s). We found five 6.7 GHz maser clusters which are distributed
over a region extended N-S across ~1500 AU and are associated with three peaks
of the radio continuum. We proposed that these maser clusters identify three
individual high-mass YSOs, named IRS1a, IRS1b, and IRS1c. We modeled the maser
clusters in IRS1a and IRS1b in terms of edge-on disks in centrifugal
equilibrium. In the first case, masers may trace a quasi-Keplerian thin disk,
orbiting around a high-mass YSO, IRS1a, of up to 25 solar masses. This YSO
dominates the bolometric luminosity of the region. The second disk is both
massive (<16 Msun within ~500 AU) and thick, and the mass of the central YSO,
IRS1b, is constrained to be at most a few solar masses. In summary, we present
compelling evidence that NGC7538 IRS1 is not forming just one single high-mass
YSO, but consists of a multiple system of high-mass YSOs, which are surrounded
by accretion disks, and are probably driving individual outflows. This new
model naturally explains all the different orientations and disk/outflow
structures proposed for the region in previous models.Comment: 34 pages, 18 figures, accepted for publication in Astronomy &
Astrophysic
Milliarcsecond structure of water maser emission in two young high-mass stellar objects associated with methanol masers
The 22.2 GHz water masers are often associated with the 6.7 GHz methanol
masers but owing to the different excitation conditions they likely probe
independent spatial and kinematic regions around the powering young massive
star. We compared the emission of these two maser species on milliarcsecond
scales to determine in which structures the masers arise and to test a
disc-outflow scenario where the methanol emission arises in a circumstellar
disc while the water emission comes from an outflow. We obtained high-angular
and spectral resolution 22.2 GHz water maser observations of the two sources
G31.581+00.077 and G33.641-00.228 using the EVN. In both objects the water
maser spots form complex and filamentary structures of sizes 18-160 AU. The
emission towards the source G31.581+00.077 comes from two distinct regions of
which one is related to the methanol maser source of ring-like shape. In both
targets the main axis of methanol distribution is orthogonal to the water maser
distribution. Most of water masers appear to trace shocks on a working surface
between an outflow/jet and a dense envelope. Some spots are possibly related to
the disc-wind interface which is as close as 100-150 AU to the regions of
methanol emission.Comment: 10 pages, accepted to Astronomy and Astrophysic
Masers in star forming regions
Maser emission plays an important role as a tool in star formation studies.
It is widely used for deriving kinematics, as well as the physical conditions
of different structures, hidden in the dense environment very close to the
young stars, for example associated with the onset of jets and outflows. We
will summarize the recent observational and theoretical progress on this topic
since the last maser symposium: the IAU Symposium 242 in Alice Springs.Comment: Submitted to Proceedings of IAU Symposium No. 287, Cosmic masers -
from OH to H
Tracing the base of protostellar wind(s) towards the high-mass star forming region AFGL 5142: VLA continuum and VLBA water maser observations
We have conducted phase-reference multi-epoch observations of the 22.2 GHz
water masers using the VLBA and multi-frequency study of the continuum emission
using the VLA towards the high-mass SFR AFGL 5142. The water maser emission
comes from two elongated structures (indicated as Group I and Group II), with
the measured proper motions aligned along the structures' elongation axes. Each
group consists of two (blue- and red-shifted) clusters of features separated by
a few hundreds and thousands of AU respectively for Group I and Group II. The
maser features of Group II have both positions and velocities aligned along a
direction close to the axis of the outflow traced by HCO+ and SiO emission on
angular scales of tens of arcsec. We predict that the maser emission arises
from dense, shocked molecular clumps displaced along the axis of the molecular
outflow. The two maser clusters of Group I are oriented on the sky along a
direction forming a large angle (> 60 degrees) with the axis of the jet/outflow
traced by Group II maser features. We have detected a compact (8.4 and 22 GHz)
continuum source that falls close to the centroid of Group I masers, indicating
that the source ionizing the gas is also responsible for the excitation of the
water masers. The kinematic analysis indicates that the Group I masers trace
outflowing rather than rotating gas, discarding the Keplerian disk scenario
proposed in a previous paper for Group I. Since the axis joining the two maser
clusters of Group II does not cross the position of the continuum source, Group
II masers might be excited by an (undetected) massive YSO, distinct from the
one (pinpointed by the VLA continuum emission) responsible for the excitation
of the Group I masers.Comment: 12 pages, 3 figures, accepted for publication in A&
Observations of extragalactic masers in bright IRAS sources
We report the first results of an ongoing survey at 22 GHz with the 100-m
Effelsberg telescope to search for water maser emission in bright IRAS sources.
We have detected water vapor emission in IC 342. The maser, associated with a
star forming region ~10-15 arcsec west of the nucleus, consists of a single 0.5
km/s wide feature and reaches an isotropic luminosity of 0.01 L(sun) (D = 1.8
Mpc). Our detection raises the detection rate among northern galaxies with IRAS
point source fluxes S(100micron) > 50 Jy to 16%.Comment: 6 pages, 3 figures. Proceeding of the conference "SRT: the impact of
large antennas on Radio Astronomy and Space Science", Cagliari, Italy, 7-10
Nov. 2001, Eds. Porceddu et al. Needs srt_style.st
Water masers in the massive protostar IRAS 20126+4104: ejection and deceleration
We report on the first multi-epoch, phase referenced VLBI observations of the
water maser emission in a high-mass protostar associated with a disk-jet
system. The source under study, IRAS 20126+4104, has been extensively
investigated in a large variety of tracers, including water maser VLBA data
acquired by us three years before the present observations. The new findings
fully confirm the interpretation proposed in our previous study, namely that
the maser spots are expanding from a common origin coincident with the
protostar. We also demonstrate that the observed 3-D velocities of the maser
spots can be fitted with a model assuming that the spots are moving along the
surface of a conical jet, with speed increasing for increasing distance from
the cone vertex. We also present the results of single-dish monitoring of the
water maser spectra in IRAS 20126+4104. These reveal that the peak velocity of
some maser lines decreases linearly with time. We speculate that such a
deceleration could be due to braking of the shocks from which the maser
emission originates, due to mass loading at the shock front or dissipation of
the shock energy.Comment: 11 pages, 8 figures. Accepted for publication in A&
Trigonometric Parallaxes of Massive Star Forming Regions: VIII. G12.89+0.49, G15.03-0.68 (M17) and G27.36-0.16
We report trigonometric parallaxes for three massive star forming regions,
corresponding to distances of kpc for G12.89+0.49 (also
known as IRAS 18089-1732), kpc for G15.03-0.68 (in the
M17 region), and kpc for G27.36-0.16. Both G12.89+0.49 and
G15.03-0.68 are located in the Carina-Sagittarius spiral arm
Trigonometric Parallaxes of Massive Star Forming Regions: II. Cep A & NGC 7538
We report trigonometric parallaxes for the sources NGC 7538 and Cep A,
corresponding to distances of 2.65 [+0.12/-0.11] kpc and 0.70 [+0.04/-0.04]
kpc, respectively. The distance to NGC 7538 is considerably smaller than its
kinematic distance and places it in the Perseus spiral arm. The distance to Cep
A is also smaller than its kinematic distance and places it in the Local arm or
spur. Combining the distance and proper motions with observed radial velocities
gives the location and full space motion of the star forming regions. We find
significant deviations from circular Galactic orbits for these sources: both
sources show large peculiar motions (> 10 km/s) counter to Galactic rotation
and NGC 7538 has a comparable peculiar motion toward the Galactic center.Comment: 21 pages, 8 figures; to appear in the Astrophysical Journa
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