4,319 research outputs found
Ionized Gas Kinematics and Morphology in Sgr B2 Main on 1000 AU Scales
We have imaged the Sgr B2 Main region with the Very Large Array in the BnA
configuration ( = 0\farcs13) in both the H52 (45.453
GHz) radio recombination line (RRL) and 7 mm continuum emission. At a distance
of 8500 pc, this spatial resolution corresponds to a physical scale of 0.005 pc
(1100 AU). The current observations detect H52 emission in 12
individual ultracompact (UC) and hypercompact (HC) HII regions. Two of the
sources with detected H52 emission have broad
(V50 \kms) recombination lines, and two of the sources
show lines with peaks at more than one velocity. We use line parameters from
the H52 lines and our previous H66 line observations to
determine the relative contribution of thermal, pressure and kinematic
broadening, and electron density. These new observations suggest that pressure
broadening can account for the broad lines in some of the sources, but that gas
motions (e.g. turbulence, accretion or outflow) contribute significantly to the
broad lines in at least one of the sources (Sgr B2 F3).Comment: 10 pages, 2 figure
WSRT and VLA Observations of the 6 cm and 2 cm lines of H2CO in the direction of W 58 C1(ON3) and W 58 C2
Absorption in the J{K-K+} = 2{11}-2{12} transition of formaldehyde at 2 cm
towards the ultracompact HII regions C1 and C2 of W 58 has been observed with
the VLA with an angular resolution of ~0.2'' and a velocity resolution of ~1
km/s. The high resolution continuum image of C1 (ON 3) shows a partial shell
which opens to the NE. Strong H2CO absorption is observed against W 58 C1. The
highest optical depth (tau > 2) occurs in the SW portion of C1 near the edge of
the shell, close to the continuum peak. The absorption is weaker towards the
nearby, more diffuse compact HII region C2, tau<~0.3. The H2CO velocity (-21.2
km/s) towards C1 is constant and agrees with the velocity of CO emission,
mainline OH masers, and the H76 alpha recombination line, but differs from the
velocity of the 1720 MHz OH maser emission (~-13 km/s). Observations of the
absorption in the J{K-K+} = 1{10}-1{11} transition of formaldehyde at 6 cm
towards W 58 C1 and C2 carried out earlier with the WSRT at lower resolution
(~4''x7'') show comparable optical depths and velocities to those observed at 2
cm. Based on the mean optical depth profiles at 6 cm and 2 cm, the volume
density of molecular hydrogen n(H2) and the formaldehyde column density N(H2CO)
were determined. The n(H2) is ~6E4 /cm**3 towards C1. N(H2CO) for C1 is ~8E14
/cm**2 while that towards C2 is ~8E13 /cm**2.Comment: AJ in press Jan 2001, 14 pages plus 6 figures (but Fig. 1 has 4
separate parts, a through d). Data are available at
http://adil.ncsa.uiuc.edu/document/00.HD.0
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
Flickering of 1.3 cm Sources in Sgr B2: Towards a Solution to the Ultracompact HII Region Lifetime Problem
Accretion flows onto massive stars must transfer mass so quickly that they
are themselves gravitationally unstable, forming dense clumps and filaments.
These density perturbations interact with young massive stars, emitting
ionizing radiation, alternately exposing and confining their HII regions. As a
result, the HII regions are predicted to flicker in flux density over periods
of decades to centuries rather than increasing monotonically in size as
predicted by simple Spitzer solutions. We have recently observed the Sgr B2
region at 1.3 cm with the VLA in its three hybrid configurations (DnC, CnB and
BnA) at a resolution of 0.25''. These observations were made to compare in
detail with matched continuum observations from 1989. At 0.25'' resolution, Sgr
B2 contains 41 UC HII regions, 6 of which are hypercompact. The new
observations of Sgr B2 allow comparison of relative peak flux densites for the
HII regions in Sgr B2 over a 23 year time baseline (1989-2012) in one of the
most source-rich massive star forming regions in the Milky Way. The new 1.3 cm
continuum images indicate that four of the 41 UC HII regions exhibit
significant changes in their peak flux density, with one source (K3) dropping
in peak flux density, and the other 3 sources (F10.303, F1 and F3) increasing
in peak flux density. The results are consistent with statistical predictions
from simulations of high mass star formation, suggesting that they offer a
solution to the lifetime problem for ultracompact HII regions.Comment: 12 pages, 3 figures, Accepted for publication in the Astrophysical
Journal Letter
Broad Recombination Line Objects in W49N on 600 AU Scales
High resolution 7 mm observations of the W49N massive star forming region
have detected recombination line emission from the individual ultracompact (UC)
HII regions on 50 milliarcsecond (600 AU) scales. These line observations,
combined with multifrequency, high-resolution continuum imaging of the region
at 7 mm (VLA) and at 3 mm and 1 mm (BIMA), indicate that five to seven of the
eighteen ultracompact sources in W49N are broad recombination line objects
(BRLOs) as described by Jaffe & Martin-Pintado (1999). BRLOs have both broad
radio recombination lines (V60 \kms) and rising spectra
(S), with values greater than 0.4. The broad
line widths of the H52 line are probably related to motions in the
ionized gas rather than pressure broadening. A number of models have been
proposed to explain the long lifetime of UC HII regions, including the
photoevaporated disk model proposed by Hollenbach et al. (1994). This model can
also explain the broad lines, rising spectra and bipolar morphologies of some
sources. We suggestbased on line and continuum observations as well as
source morphologythat in a subset of the W49N ultracompact sources we may be
observing ionized winds that arise from circumstellar disks.Comment: 15 pages, 2 figures, to appear in The Astrophysical Journal (v. 600,
no. 1), 1 January 200
A search for hypercompact HII regions in the Galactic Plane
22 pages, 8 figures, 6 tables, accepted by MNRASWe have carried out the largest and most unbiased search for hypercompact (HC) H II regions. Our method combines four interferometric radio continuum surveys (THOR, CORNISH, MAGPIS, and White2005) with far-infrared and sub-mm Galactic Plane surveys to identify embedded H II regions with positive spectral indices; 120 positive spectrum H II regions have been identified from a total sample of 534 positive spectral index radio sources. None of these H II regions, including the known HC H II regions recovered in our search, fulfills the canonical definition of an HC H II region at 5 GHz. We suggest that the current canonical definition of HC H II regions is not accurate and should be revised to include a hierarchical structure of ionized gas that results in an extended morphology at 5 GHz. Correlating our search with known ultracompact (UC) H II region surveys, we find that roughly half of detected UC H II regions have positive spectral indices, instead of more commonly assumed flat and optically thin spectra. This implies a mix of optically thin and thick emission and has important implications for previous analyses which have so far assumed optically thin emission for these objects. Positive spectrum H II regions are statistically more luminous and possess higher Lyman continuum fluxes than H II regions with flat or negative indices. Positive spectrum H II regions are thus more likely to be associated with more luminous and massive stars. No differences are found in clump mass, linear diameter, or luminosity-to-mass ratio between positive spectrum and non-positive spectrum H II regions.Peer reviewedFinal Accepted Versio
Production of Single Heavy Charged Leptons at a Linear Collider
A sequential fourth generation of quarks and leptons is allowed by precision
electroweak constraints if the mass splitting between the heavy quarks is
between 50 and 80 GeV. Although heavy quarks can be easily detected at the LHC,
it is very difficult to detect a sequential heavy charged lepton, L, due to
large backgrounds. Should the L mass be above 250 GeV, it can not be
pair-produced at a 500 GeV ILC. We calculate the cross section for the one-loop
process e+e- -> L tau. Although the cross section is small, it may be
detectable. We also consider contributions from the two Higgs doublet model and
the Randall-Sundrum model, in which case the cross section can be substantially
higher.Comment: 14 pages, 7 figure
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
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