4,234 research outputs found
One Dimensional Dynamical Models of the Carina Nebula Bubble
We have tested the two main theoretical models of bubbles around massive star
clusters, Castor et al. and Chevalier & Clegg, against observations of the well
studied Carina Nebula. The Castor et al. theory over-predicts the X-ray
luminosity in the Carina bubble by a factor of 60 and expands too rapidly, by a
factor of 4; if the correct radius and age are used, the predicted X-ray
luminosity is even larger. In contrast, the Chevalier & Clegg model
under-predicts the X-ray luminosity by a factor of 10. We modify the Castor et
al. theory to take into account lower stellar wind mass loss rates, radiation
pressure, gravity, and escape of or energy loss from the hot shocked gas. We
argue that energy is advected rather than radiated from the bubble. We
undertake a parameter study for reduced stellar mass loss rates and for various
leakage rates and are able to find viable models. The X-ray surface brightness
in Carina is highest close to the bubble wall, which is consistent with
conductive evaporation from cold clouds. The picture that emerges is one in
which the hot gas pressure is far below that found by dividing the
time-integrated wind luminosity by the bubble volume; rather, the pressure in
the hot gas is set by pressure equilibrium with the photoionized gas at T=10^4
K. It follows that the shocked stellar winds are not dynamically important in
forming the bubbles.Comment: Accepted to APJ. 47 pages, 13 figure
Multiwavelength Observations of Massive Stellar Cluster Candidates in the Galaxy
The Galaxy appears to be richer in young, massive stellar clusters than
previously known, due to advances in infrared surveys which have uncovered
deeply embedded regions of star formation. Young, massive clusters can
significantly impact the surrounding interstellar medium (ISM) and hence radio
observations can also be an important tracer of their activity. Several hundred
cluster candidates are now known by examining survey data. Here we report on
multiwavelength observations of six of these candidates in the Galaxy. We
carried out 4.9 and 8.5 GHz VLA observations of the radio emission associated
with these clusters to obtain the physical characteristics of the surrounding
gas, including the Lyman continuum photon flux and ionized gas mass. Spitzer
Infrared Array Camera observations were also made of these regions, and provide
details on the stellar population as well as the dust continuum and polycyclic
aromatic hydrocarbon emission. When compared to the known young, massive
clusters in the Galaxy, the six cluster candidates have less powerful Lyman
ionizing fluxes and ionize less of the H II mass in the surrounding ISM.
Therefore, these cluster candidates appear to be more consistent with
intermediate-mass clusters (10^3-10^4 Msun).Comment: 39 pages, 20 figures. Accepted in the Astronomical Journal; to be
published Fall 201
VLA observations of candidate high-mass protostellar objects at 7 mm
We present radio continuum observations at 7 mm made using the Very Large
Array towards three massive star forming regions thought to be in very early
stages of evolution selected from the sample of Sridharan et al. (2002).
Emission was detected towards all three sources (IRAS 18470-0044, IRAS
19217+1651 and IRAS 23151+5912). We find that in all cases the 7 mm emission
corresponds to thermal emission from ionized gas. The regions of ionized gas
associated with IRAS 19217+1651 and IRAS 23151+5912 are hypercompact with
diameters of 0.009 and 0.0006 pc, and emission measures of 7.0 x 10^8 and 2.3 x
10^9 pc cm^(-6), respectively.Comment: 17 pages, 5 figures, accepted by The Astronomical Journa
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
Prevalence of Vaccine Type Infections in Vaccinated and Non-Vaccinated Young Women: HPV-IMPACT, a Self-Sampling Study.
Background: The human papillomavirus (HPV) vaccination program for young girls aged 11⁻26 years was introduced in Switzerland in 2008. The objective of this study was to evaluate the prevalence of high- and low-risk HPV in a population of undergraduate students using self-sampling for monitoring the HPV vaccination program's effect.
Undergraduate women aged between 18⁻31 years, attending the Medical School and University of Applied Sciences in Geneva, were invited to participate in the study. Included women were asked to perform vaginal self-sampling for HPV testing using a dry cotton swab.
A total of 409 students participated in the study-aged 18⁻31 years-of which 69% of the participants were vaccinated with Gardasil HPV vaccine and 31% did not received the vaccine. About HPV prevalence, 7.2% of unvaccinated women were HPV 16 or 18 positive, while 1.1% of vaccinated women were infected by HPV 16 or 18 (p < 0.01). Prevalence of HPV 6 and 11 was 8.3% in non-vaccinated women versus 2.1% in vaccinated women (p < 0.02). We observed no cross-protection for the other HPV genotypes of a low- and high-risk strain.
Prevalence of HPV 6/11/16/18 was lower in vaccinated women versus unvaccinated women. Continued assessment of HPV vaccine effectiveness in real population is needed
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
An 8.5 GHz Arecibo survey of Carbon Recombination Lines toward Ultra-compact \HII regions: Physical properties of dense molecular material
We report here on a survey of carbon recombination lines (RLs) near 8.5 GHz
toward 17 ultra-compact \HII regions (\UCHII s). Carbon RLs are detected in 11
directions, indicating the presence of dense photodissociation regions (PDRs)
associated with the \UCHII s. In this paper, we show that the carbon RLs
provide important, complementary information on the kinematics and physical
properties of the ambient medium near \UCHII s. Non-LTE models for the carbon
line forming region are developed, assuming that the PDRs surround the \UCHII
s, and we constrained the model parameters by multi-frequency RL data. Modeling
shows that carbon RL emission near 8.5 GHz is dominated by stimulated emission
and hence we preferentially observe the PDR material that is in front of the
\UCHII continuum. We find that the relative motion between ionized gas and the
associated PDR is about half that estimated earlier, and has an RMS velocity
difference of 3.3 \kms. Our models also give estimates for the PDR density and
pressure. We found that the neutral density of PDRs is typically 5
10 \cmthree and \UCHII s can be embedded in regions with high ambient
pressure. Our results are consistent with a pressure confined \HII region model
where the stars are moving relative to the cloud core. Other models cannot be
ruled out, however. Interestingly, in most cases, the PDR pressure is an order
of magnitude larger than the pressure of the ionized gas. Further investigation
is needed to understand this large pressure difference.Comment: 28 pages, 7 figures, 5 tables (accepted for publication in ApJ
Morphologies of Ultracompact HII Regions in W49A and Sgr B2: Prevalence of Shells and a Modified Classification Scheme
We have used Very Large Array (VLA) observations of the massive star forming
regions W49A and Sgr B2, obtained with resolutions from 2\farcs0 to 0\farcs04,
to classify the morphologies of nearly 100 ultracompact HII regions. These high
resolution, multi-frequency, multi-configuration VLA observations motivate
several modifications of the existing morphological classification scheme for
UC HII regions. In this work, we describe the modified morphology scheme and
the criteria used in source classification. In particular, we drop the
``core-halo'' classification, add a ``bipolar'' classification, and change the
shell classification to ``shell-like''. We tally the percentage of each
morphology found in the Sgr B2 and W49A regions and find broad agreement with
the Galactic plane surveys in the distribution of morphologies for most types.
However, we find that nearly a third of the sources in these regions are
shell-like, which is a higher percentage by nearly a factor of ten than found
in the surveys of Galactic plane star forming regions by Wood & Churchwell
(1989a) and Kurtz et al. (1994). This difference may be due to physical
differences in the environments of these two extreme star forming regions.
Alternatively, differences in observational technique may be responsible.Comment: 10 pages, 1 figure, 1 table, to appear in The Astrophysical Journal
Letter
Radio Continuum and Recombination Line Study of UC HII Regions with Extended Envelopes
We have carried out 21 cm radio continuum observations of 16 UC HII regions
using the VLA (D-array) in search of associated extended emission. We have also
observed H76 recombination line towards all the sources and
He76 line at the positions with strong H76 line emission. The
UC HII regions have simple morphologies and large (>10) ratios of single-dish
to VLA fluxes. Extended emission was detected towards all the sources. The
extended emission consists of one to several compact components and a diffuse
extended envelope. All the UC HII regions but two are located in the compact
components, where the UC HII regions always correspond to their peaks. The
compact components with UC HII regions are usually smaller and denser than
those without UC HII regions. Our recombination line observations indicate that
the ultracompact, compact, and extended components are physically associated.
The UC HII regions and their associated compact components are likely to be
ionized by the same sources on the basis of the morphological relations
mentioned above. This suggests that almost all of the observed UC HII regions
are not `real' UC HII regions and that their actual ages are much greater than
their dynamical age (<10000 yr). We find that most of simple UC HII regions
previously known have large ratios of single-dish to VLA fluxes, similar to our
sources. Therefore, the `age problem' of UC HII regions does not seem to be as
serious as earlier studies argued. We present a simple model that explains
extended emission around UC HII regions. Some individual sources are discussed.Comment: 29 pages, 28 postscript figures, Accepted for publication in Ap
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