754 research outputs found
High resolution observations of Cen A: Yellow and red supergiants in a region of jet-induced star formation?
We present the analysis of near infrared (NIR), adaptive optics (AO) Subaru
and archived HST imaging data of a region near the northern middle lobe (NML)
of the Centaurus A (Cen A) jet, at a distance of kpc north-east (NE)
from the center of NGC5128. Low-pass filtering of the NIR images reveals strong
-- above the background mean -- signal at the expected position of
the brightest star in the equivalent HST field. Statistical analysis of the NIR
background noise suggests that the probability to observe signal at
the same position, in three independent measurements due to stochastic
background fluctuations alone is negligible () and, therefore,
that this signal should reflect the detection of the NIR counterparts of the
brightest HST star. An extensive photometric analysis of this star yields
, visual-NIR, and NIR colors expected from a yellow supergiant (YSG) with
an estimated age Myr. Furthermore, the second and third
brighter HST stars are, likely, also supergiants in Cen A, with estimated ages
Myr and Myr, respectively. The ages of
these three supergiants are in good agreement with the ages of the young
massive stars that were previously found in the vicinity and are thought to
have formed during the later phases of the jet-HI cloud interaction that
appears to drive the star formation (SF) in the region for the past
Myr.Comment: 11 pages, 6 figures, 2 tables, accepted for publication in Ap
A study on subarcsecond scales of the ammonia and continuum emission toward the G16.59-0.05 high-mass star-forming region
We wish to investigate the structure, velocity field, and stellar content of
the G16.59-0.05 high-mass star-forming region, where previous studies have
established the presence of two almost perpendicular (NE-SW and SE-NW), massive
outflows, and a rotating disk traced by methanol maser emission. We performed
Very Large Array observations of the radio continuum and ammonia line emission,
complemented by COMICS/Subaru and Hi-GAL/Herschel images in the mid- and
far-infrared (IR). Our centimeter continuum maps reveal a collimated radio jet
that is oriented E-W and centered on the methanol maser disk, placed at the SE
border of a compact molecular core. The spectral index of the jet is negative,
indicating non-thermal emission over most of the jet, except the peak close to
the maser disk, where thermal free-free emission is observed. We find that the
ammonia emission presents a bipolar structure consistent (on a smaller scale)
in direction and velocity with that of the NE-SW bipolar outflow detected in
previous CO observations. After analyzing our previous N2H+(1-0) observations
again, we conclude that two scenarios are possible. In one case both the radio
jet and the ammonia emission would trace the root of the large-scale CO bipolar
outflow. The different orientation of the jet and the ammonia flow could be
explained by precession and/or a non-isotropic density distribution around the
star. In the other case, the N2H+(1-0) and ammonia bipolarity is interpreted as
two overlapping clumps moving with different velocities along the line of
sight. The ammonia gas also seems to undergo rotation consistent with the maser
disk. Our IR images complemented by archival data allow us to derive a
bolometric luminosity of about 10^4 L_sun and to conclude that most of the
luminosity is due to the young stellar object associated with the maser disk.Comment: 11 pages, 12 figures, published in Astronomy and Astrophysic
The physical and chemical structure of Sagittarius B2 VIIIa. Dust and ionized gas contributions to the full molecular line survey of 47 hot cores
Sagittarius B2 (Sgr B2) is a giant molecular cloud complex in the central
molecular zone of our Galaxy hosting several sites of high-mass star formation.
The two main centers of activity are Sgr B2(M) and Sgr B2(N), which contain 27
and 20 continuum sources, respectively. Our analysis aims to be a comprehensive
modeling of each core spectrum, where we take the complex interaction between
molecular lines, dust attenuation, and free-free emission arising from HII
regions into account. In this work, we determine the dust and, if HII regions
are contained, the parameters of the free-free thermal emission of the ionized
gas for each core, and derive a self-consistent description of the continuum
levels of each core. Using the high sensitivity of ALMA, we characterize the
physical and chemical structure of these continuum sources and gain better
insight into the star formation process within the cores. We used ALMA to
perform an unbiased spectral line survey of all 47 sources in ALMA band 6 with
a frequency coverage from 211 GHz to 275 GHz. In order to model the free-free
continuum contribution of a specific core, we fit the contained recombination
lines (RRLs) to obtain the electron temperatures and the emission measures,
where we use an extended XCLASS program to describe RRLs and free-free
continuum simultaneously. In contrast to previous analyses, we derived the
corresponding parameters here not only for each core, but also for their local
surrounding envelope, and determined their physical properties. The
distribution of RRLs we found in the core spectra closely fits the distribution
of HII regions described in previous analyses. For the cores we determine
average dust temperatures of around 236 K (Sgr B2(M)) and 225 K (Sgr B2(N)),
while the electronic temperatures are located in a range between 3800 K and
23800 K
Building parameters that influence overheating of apartment buildings in a temperate climate in Southern Europe
Overheating in dwellings is a global concern that is increasing due to global warming and more frequent and
extreme heatwaves. This study assesses the relationship between different building parameters (built period,
floor level, orientation, window area and solar shading) and compares indoor overheating hours during summer
in twelve apartments monitored in Pamplona (North of Spain). They were selected as samples from different
Spanish built periods related to different energy regulations, without mechanical cooling and with some kind of
exterior solar shading. Overheating hours were calculated using the UNE-EN 16798 standard, which establishes a
maximum acceptable operative temperature. This limit is adaptive and it is defined as the exponentially
weighted running mean of the daily outdoor temperature. Multilevel mixed-effects linear and logistic regressions
were used to analyse and compare overheating hours. Floor level, window area and solar shading were the
parameters that showed a significant relationship with indoor overheating hours (p < 0.01). Orientation and
built period did not reach a statistically significant value (p > 0.01). It is particularly noteworthy that the
apartments built under the current Spanish Energy Regulations (after 2006) do not show a significant reduction
in indoor overheating hours compared to those built without any energy regulations. This assessment reveals that
current building energy regulations may not be enough to avoid overheating or ensure adaptation to warmer
conditions. Therefore, this study contributes to establishing the main building parameters to improve in order to
adapt Spanish apartment buildings to warming conditions in temperate climates
Herschel observations of extraordinary sources: Full Herschel/HIFI molecular line survey of Sagittarius B2(M)
We present a full analysis of a broadband spectral line survey of Sagittarius
B2 (Main), one of the most chemically rich regions in the Galaxy located within
the giant molecular cloud complex Sgr B2 in the Central Molecular Zone. Our
goal is to derive the molecular abundances and temperatures of the high-mass
star-forming region Sgr B2(M) and thus its physical and astrochemical
conditions. Sgr B2(M) was observed using the Heterodyne Instrument for the
Far-Infrared (HIFI) on board the Herschel Space Observatory in a spectral line
survey from 480 to 1907 GHz at a spectral resolution of 1.1 MHz, which provides
one of the largest spectral coverages ever obtained toward this high-mass
star-forming region in the submillimeter with high spectral resolution and
includes frequencies > 1 THz unobservable from the ground. We model the
molecular emission from the submillimeter to the far-IR using the XCLASS
program. For each molecule, a quantitative description was determined taking
all emission and absorption features of that species across the entire spectral
range into account. Additionally, we derive velocity resolved ortho / para
ratios for those molecules for which ortho and para resolved molecular
parameters are available. Finally, the temperature and velocity distributions
are analyzed and the derived abundances are compared with those obtained for
Sgr B2(N) from a similar HIFI survey. A total of 92 isotopologues were
identified, arising from 49 different molecules, ranging from free ions to
complex organic compounds and originating from a variety of environments from
the cold envelope to hot and dense gas within the cores. Sulfur dioxide,
methanol, and water are the dominant contributors. For the ortho / para ratios
we find deviations from the high temperature values between 13 and 27 %. In
total 14 % of all lines remain unidentified.Comment: 67 pages, 102 figures, submitted to A&
Kinematics of the ionized-to-neutral interfaces in Monoceros R2
Context. Monoceros R2 (Mon R2), at a distance of 830 pc, is the only
ultra-compact H ii region (UC H ii) where its associated photon-dominated
region (PDR) can be resolved with the Herschel Space Observatory. Aims. Our aim
is to investigate observationally the kinematical patterns in the interface
regions (i.e., the transition from atomic to molecular gas) associated with Mon
R2. Methods. We used the HIFI instrument onboard Herschel to observe the line
profiles of the reactive ions CH+, OH+ and H2O+ toward different positions in
Mon R2. We derive the column density of these molecules and compare them with
gas-phase chemistry models. Results. The reactive ion CH+ is detected both in
emission (at central and red-shifted velocities) and in absorption (at
blue-shifted velocities). OH+ is detected in absorption at both blue- and
red-shifted velocities, with similar column densities. H2O+ is not detected at
any of the positions, down to a rms of 40 mK toward the molecular peak. At this
position, we find that the OH+ absorption originates in a mainly atomic medium,
and therefore is associated with the most exposed layers of the PDR. These
results are consistent with the predictions from photo-chemical models. The
line profiles are consistent with the atomic gas being entrained in the ionized
gas flow along the walls of the cavity of the H ii region. Based on this
evidence, we are able to propose a new geometrical model for this region.
Conclusions. The kinematical patterns of the OH+ and CH+ absorption indicate
the existence of a layer of mainly atomic gas for which we have derived, for
the first time, some physical parameters and its dynamics.Comment: 6 pages, 5 figures. Accepted for publication in A&
Evaluation of thermal comfort and building form attributes in different semi-outdoor environments in a high-density tropical setting
In highly dense tropical cities, a semi-outdoor space (SOS) is frequently used as a social space within tall building forms where people can interact and connect. Thermal comfort in SOSs within tall buildings, however, may vary depending on the type and form attributes that define it. This study classifies 63 SOSs in four tall buildings of Singapore into five types based on literature review: perimeter buffers, sky terraces, horizontal breezeways, breezeway atria and vertical breezeways. Findings suggest that the five SOS types perform differently in terms of thermal comfort (based on PMV*), environmental parameters (air temperature, mean radiant temperature, relative humidity, and air velocity), and building form attributes (height-to-depth ratio, open space ratio, and green plot ratio). Of these five, vertical breezeways and horizontal breezeways are the most thermally comfortable for all activities during a typically warm hour. It is postulated that higher thermal comfort levels in these SOS types are linked to form attributes that enhance air velocity. This study examines the pros and cons of each SOS type in terms of thermal comfort in their role as communal spaces in tall buildings situated within a highly dense tropical city
Microstructural characterization of alumina-zirconia layered ceramics using positron annihilation spectroscopy
Positron annihilation spectroscopy (PAS), indentation, nanoindentation experiments and scanning electron microscopy (SEM) observations were performed on Al₂O₃-ZrO₂ laminates samples to assess the effect of residual stresses on their mechanical and microstructural properties. Layered samples were implemented by slip-casting, constituted by two thin Al₂O₃ external layers and an intermediate thick one, consisting of a mixture of Al₂O₃ and monoclinic ZrO₂ in the range 0-30 vol.%. In these systems residual tensile stresses fields were generated inside the external layers during cooling from the sintering temperature, by the expansion of the adjacent ZrO₂-containing layer. SEM observations showed the microstructural effects due to the level of tension related to the zirconia content. A correlation between the PAS parameters and the microstructural changes caused by the presence of residual stresses was found. Nanoindentation measurements were used to trace the sign and magnitude of the residual stress gradient across the interface between the layers.This work was supported by Spanish Government under Contract MAT2006-01038. The authors gratefully acknowledge the financial support from the Comunidad de Madrid and the Ministry of Education and Science of Spain, through the ESTRUMAT-CM (MAT/77) programs.Publicad
Deuteration as an evolutionary tracer in massive-star formation
Theory predicts, and observations confirm, that the column density ratio of a
molecule containing D to its counterpart containing H can be used as an
evolutionary tracer in the low-mass star formation process. Since it remains
unclear if the high-mass star formation process is a scaled-up version of the
low-mass one, we investigated whether the relation between deuteration and
evolution can be applied to the high-mass regime. With the IRAM-30m telescope,
we observed rotational transitions of N2D+ and N2H+ and derived the deuterated
fraction in 27 cores within massive star-forming regions understood to
represent different evolutionary stages of the massive-star formation process.
Results. Our results clearly indicate that the abundance of N2D+ is higher at
the pre-stellar/cluster stage, then drops during the formation of the
protostellar object(s) as in the low-mass regime, remaining relatively constant
during the ultra-compact HII region phase. The objects with the highest
fractional abundance of N2D+ are starless cores with properties very similar to
typical pre-stellar cores of lower mass. The abundance of N2D+ is lower in
objects with higher gas temperatures as in the low-mass case but does not seem
to depend on gas turbulence. Our results indicate that the N2D+-to-N2H+ column
density ratio can be used as an evolutionary indicator in both low- and
high-mass star formation, and that the physical conditions influencing the
abundance of deuterated species likely evolve similarly during the processes
that lead to the formation of both low- and high-mass stars.Comment: Accepted by A&AL, 4 pages, 2 figures, 2 appendices (one for Tables,
one for additional figures
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