589 research outputs found
On the velocity of the Vela pulsar
It is shown that if the shell of the Vela supernova remnant is responsible
for nearly all the scattering of the Vela pulsar, then the scintillation and
proper motion velocities of the pulsar can only be reconciled with each other
in the case of nonzero transverse velocity of the scattering material. A
possible origin of large-scale transverse motions in the shell of the Vela
supernova remnant is discussed.Comment: 4 pages, 1 figure, revised version accepted for publication in A&
W49A: A starburst triggered by expanding shells
W49A is a giant molecular cloud which harbors some of the most luminous
embedded clusters in the Galaxy. However, the explanation for this
starburst-like phenomenon is still under debate. Methods. We investigated
large-scale Spitzer mid-infrared images together with a Galatic Ring Survey
13CO J = 1-0 image, complemented with higher resolution (~ 11 arcsec) 13CO J =
2-1 and C18O J = 2-1 images over a ~ 15 x 13 pc^2 field obtained with the IRAM
30m telescope. Two expanding shells have been identified in the mid-infrared
images, and confirmed in the position-velocity diagrams made from the 13CO J =
2-1 and C18O J = 2-1 data. The mass of the averaged expanding shell, which has
an inner radius of ~ 3.3 pc and a thickness of ~ 0.41 pc, is about 1.9 x 10^4
M*. The total kinetic energy of the expanding shells is estimated to be ~ 10^49
erg which is probably provided by a few massive stars, whose radiation pressure
and/or strong stellar winds drive the shells. The expanding shells are likely
to have a common origin close to the two ultracompact Hii regions (source O and
source N), and their expansion speed is estimated to be ~ 5 km/s, resulting in
an age of ~ 3-7 x 10^5 years. In addition, on larger (~ 35 x 50 pc^2) scales,
remnants of two gas ejections have been identified in the 13CO J = 1 - 0 data.
Both ejections seem to have the same center as the expanding shells with a
total energy of a few times 10^50 erg. The main driving mechanism for the gas
ejections is unclear, but likely related to the mechanism which triggers the
starburst in W49A
Massive star formation and feedback in W49A: The source of our Galaxy's most luminous water maser outflow
We present high spatial resolution mid-IR images of the ring of UCHII regions
in W49A obtained at Gemini North, allowing us to identify the driving source of
its powerful H2O maser outflow. These data also confirm our previous report
that several radio sources in the ring are undetected in the mid-IR because
they are embedded deep inside the cloud core. We locate the source of the water
maser outflow at the position of the compact mid-IR peak of source G (source
G:IRS1). This IR source is not coincident with any identified compact radio
continuum source, but is coincident with a hot molecular core, so we propose
that G:IRS1 is a hot core driving an outflow analogous to the wide-angle
bipolar outflow in OMC-1. G:IRS1 is at the origin of a larger bipolar cavity
and CO outflow. The water maser outflow is orthogonal to the bipolar CO cavity,
so the masers probably reside near its waist in the cavity walls. Models of the
IR emission require a massive protostar of 45Msun, 3e5Lsun, and an effective
envelope accretion rate of 1e-3Msun/yr. Feedback from the central star could
potentially drive the H2O maser outflow, but it has insufficient radiative
momentum to have driven the large-scale CO outflow, requiring that this massive
star had an active accretion disk over the past 10^4 yr. Combined with the
spatialy resolved morphology in IR images, G:IRS1 in W49 provides compelling
evidence for a massive protostar that formed by accreting from a disk,
accompanied by a bipolar outflow.Comment: 14 pages, MNRAS accepte
Densities and filling factors of the DIG in the Solar neighbourhood
For the first time we have combined dispersion measures and emission measures
towards 38 pulsars at KNOWN distances from which we derived the mean electron
density in clouds, N_c, and their volume filling factor, F_v, averaged along
the line of sight. The emission measures were corrected for absorption by dust
and contributions from beyond the pulsar distance. Results: The scale height of
the electron layer for our sample is 0.93+/-0.13 kpc and the midplane electron
density is 0.023+/-0.004 cm^-3, in agreement with earlier results. The average
density along the line of sight is = 0.018+/-0.002 cm^-3 and nearly
constant. Since = F_v N_c, an inverse relationship between F_v and N_c is
expected. We find F_v(N_c) = (0.011+/-0.003) N_c^{-1.20+/-0.13}, which holds
for the ranges N_c = 0.05-1 cm^-3 and F_v = 0.4-0.01. Near the Galactic plane
the dependence of F_v on N_c is significantly stronger than away from the
plane. F_v does not systematically change along or perpendicular to the
Galactic plane, but the spread about the mean value of 0.08+/-0.02 is
considerable. Conclusions: The inverse F_v-N_c relation is consistent with a
hierarchical, fractal density distribution in the diffuse ionized gas (DIG)
caused by turbulence. The observed near constancy of then is a signature
of fractal structure in the ionized medium, which is most pronounced outside
the thin disk.Comment: 9 pages, 9 figures. Accepted for publication in A&
VLBI study of maser kinematics in high-mass SFRs. I. G16.59-0.05
The present paper focuses on the high-mass star-forming region G16.59-0.05.
Methods: Using the VLBA and the EVN arrays, we conducted phase-referenced
observations of the three most powerful maser species in G16.59-0.05: H2O at
22.2 GHz (4 epochs), CH3OH at 6.7 GHz (3 epochs), and OH at 1.665 GHz (1
epoch). In addition, we performed high-resolution (> 0".1), high-sensitivity (<
0.1 mJy) VLA observations of the radio continuum emission from the star-forming
region at 1.3 and 3.6 cm. Results: This is the first work to report accurate
measurements of the "relative" proper motions of the 6.7 GHz CH3OH masers. The
different spatial and 3-D velocity distribution clearly indicate that the 22
GHz water and 6.7 GHz methanol masers are tracing different kinematic
environments. The bipolar distribution of 6.7 GHz maser l.o.s. velocities and
the regular pattern of observed proper motions suggest that these masers are
tracing rotation around a central mass of about 35 solar masses. The flattened
spatial distribution of the 6.7 GHz masers, oriented NW-SE, suggests that they
can originate in a disk/toroid rotating around the massive YSO which drives the
12CO(2-1) outflow, oriented NE-SW, observed on arcsec scale. The extended,
radio continuum source observed close to the 6.7 GHz masers could be excited by
a wide-angle wind emitted from the YSO associated with the methanol masers, and
such a wind is proven to be sufficiently energetic to drive the NE-SW 12CO(2-1)
outflow. The H2O masers distribute across a region offset about 0".5 to the NW
of the CH3OH masers, in the same area where emission of high-density molecular
tracers, typical of HMCs, was detected. We postulate that a distinct YSO,
possibly in an earlier evolutionary phase than that exciting the methanol
masers, is responsible for the excitation of the water masers and the HMC
molecular lines. (Abridged)Comment: 20 pages, 8 figures, 3 tables, accepted by Astronomy and Astrophysic
Extended warm and dense gas towards W49A: starburst conditions in our Galaxy?
The star formation rates in starburst galaxies are orders of magnitude higher
than in local star-forming regions, and the origin of this difference is not
well understood. We use sub-mm spectral line maps to characterize the physical
conditions of the molecular gas in the luminous Galactic star-forming region
W49A and compare them with the conditions in starburst galaxies. We probe the
temperature and density structure of W49A using H_2CO and HCN line ratios over
a 2'x2' (6.6x6.6 pc) field with an angular resolution of 15" (~0.8 pc) provided
by the JCMT Spectral Legacy Survey. We analyze the rotation diagrams of lines
with multiple transitions with corrections for optical depth and beam dilution,
and estimate excitation temperatures and column densities. Comparing the
observed line intensity ratios with non-LTE radiative transfer models, our
results reveal an extended region (about 1'x1', equivalent to ~3x3 pc at the
distance of W49A) of warm (> 100 K) and dense (>10^5 cm^-3) molecular gas, with
a mass of 2x10^4 - 2x10^5 M_Sun (by applying abundances derived for other
regions of massive star-formation). These temperatures and densities in W49A
are comparable to those found in clouds near the center of the Milky Way and in
starburst galaxies. The highly excited gas is likely to be heated via shocks
from the stellar winds of embedded, O-type stars or alternatively due to UV
irradiation, or possibly a combination of these two processes. Cosmic rays,
X-ray irradiation and gas-grain collisional heating are less likely to be the
source of the heating in the case of W49A.Comment: Accepted for publication in A&A; 11 pages, 9 figure
Coexisting conical bipolar and equatorial outflows from a high-mass protostar
The BN/KL region in the Orion molecular cloud is an archetype in the study of
the formation of stars much more massive than the Sun. This region contains
luminous young stars and protostars, but it is difficult to study because of
overlying dust and gas. Our basic expectations are shaped to some extent by the
present theoretical picture of star formation, the cornerstone of which is that
protostars acrete gas from rotating equatorial disks, and shed angular momentum
by ejecting gas in bipolar outflows. The main source of the outflow in the
BN/KL region may be an object known as radio source I, which is commonly
believed to be surrounded by a rotating disk of molecular material. Here we
report high-resolution observations of silicon monoxide (SiO) and water maser
emission from the gas surrounding source I; we show that within 60 AU (about
the size of the Solar System), the region is dominated by a conical bipolar
outflow, rather than the expected disk. A slower outflow, close to the
equatorial plane of the protostellar system, extends to radii of 1,000 AU.Comment: 10 pages, 2 figures. Accepted by Nature. To appear December 199
Exploring morphological correlations among H2CO, 12CO, MSX and continuum mappings
There are relatively few H2CO mappings of large-area giant molecular cloud
(GMCs). H2CO absorption lines are good tracers for low-temperature molecular
clouds towards star formation regions. Thus, the aim of the study was to
identify H2CO distributions in ambient molecular clouds. We investigated
morphologic relations among 6-cm continuum brightness temperature (CBT) data
and H2CO (111-110; Nanshan 25-m radio telescope), 12CO (1--0; 1.2-m CfA
telescope) and midcourse space experiment (MSX) data, and considered the impact
of background components on foreground clouds. We report simultaneous 6-cm H2CO
absorption lines and H110\alpha radio recombination line observations and give
several large-area mappings at 4.8 GHz toward W49 (50'\times50'), W3
(70'\times90'), DR21/W75 (60'\times90') and NGC2024/NGC2023 (50'\times100')
GMCs. By superimposing H2CO and 12CO contours onto the MSX color map, we can
compare correlations. The resolution for H2CO, 12CO and MSX data was about 10',
8' and 18.3", respectively. Comparison of H2CO and 12CO contours, 8.28-\mu m
MSX colorscale and CBT data revealed great morphological correlation in the
large area, although there are some discrepancies between 12CO and H2CO peaks
in small areas. The NGC2024/NGC2023 GMC is a large area of HII regions with a
high CBT, but a H2CO cloud to the north is possible against the cosmic
microwave background. A statistical diagram shows that 85.21% of H2CO
absorption lines are distributed in the intensity range from -1.0 to 0 Jy and
the \Delta V range from 1.206 to 5 km/s.Comment: 18 pages, 22 figures, 5 tables. Accepted to be published in
Astrophysics and Space Scienc
Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: a ubiquitous tracer of molecular gas
We discuss the detection of absorption by interstellar hydrogen fluoride (HF)
along the sight line to the submillimeter continuum sources W49N and W51. We
have used Herschel's HIFI instrument in dual beam switch mode to observe the
1232.4762 GHz J = 1 - 0 HF transition in the upper sideband of the band 5a
receiver. We detected foreground absorption by HF toward both sources over a
wide range of velocities. Optically thin absorption components were detected on
both sight lines, allowing us to measure - as opposed to obtain a lower limit
on - the column density of HF for the first time. As in previous observations
of HF toward the source G10.6-0.4, the derived HF column density is typically
comparable to that of water vapor, even though the elemental abundance of
oxygen is greater than that of fluorine by four orders of magnitude. We used
the rather uncertain N(CH)-N(H2) relationship derived previously toward diffuse
molecular clouds to infer the molecular hydrogen column density in the clouds
exhibiting HF absorption. Within the uncertainties, we find that the abundance
of HF with respect to H2 is consistent with the theoretical prediction that HF
is the main reservoir of gas-phase fluorine for these clouds. Thus, hydrogen
fluoride has the potential to become an excellent tracer of molecular hydrogen,
and provides a sensitive probe of clouds of small H2 column density. Indeed,
the observations of hydrogen fluoride reported here reveal the presence of a
low column density diffuse molecular cloud along the W51 sight line, at an LSR
velocity of ~ 24kms-1, that had not been identified in molecular absorption
line studies prior to the launch of Herschel.Comment: 4 pages, 3 figures, A&A Letter special issue, accepted on 07/13/201
Tracing light propagation to the intrinsic accuracy of space-time geometry
Advancement in astronomical observations and technical instrumentation
requires coding light propagation at high level of precision; this could open a
new detection window of many subtle relativistic effects suffered by light
while it is propagating and entangled in the physical measurements. Light
propagation and its subsequent detection should indeed be conceived in a fully
relativistic context, in order to interpret the results of the observations in
accordance with the geometrical environment affecting light propagation itself,
as an unicum surrounding universe. One of the most intriguing aspects is the
boost towards the development of highly accurate models able to recon- struct
the light path consistently with General Relativity and the precepts of
measurements. This paper deals with the complexity of such a topic by showing
how the geometrical framework of models like RAMOD, initially developed for
astrometric observations, constitutes an appropriate physical environment for
back tracing a light ray conforming to the intrinsic accuracy of space-time.
This article discusses the reasons why RAMOD stands out among the existent
approaches applied to the light propagation problem and provides a proof of its
capability in recasting recent literature cases.Comment: 15 pages, 5 figures. Revised version, references and appendixes
added. PRD re-submitte
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