392 research outputs found
SiO excitation from dense shocks in the earliest stages of massive star formation
Molecular outflows are a direct consequence of accretion, and therefore they
represent one of the best tracers of accretion processes in the still poorly
understood early phases of high-mass star formation. Previous studies suggested
that the SiO abundance decreases with the evolution of a massive young stellar
object probably because of a decay of jet activity, as witnessed in low-mass
star-forming regions. We investigate the SiO excitation conditions and its
abundance in outflows from a sample of massive young stellar objects through
observations of the SiO(8-7) and CO(4-3) lines with the APEX telescope. Through
a non-LTE analysis, we find that the excitation conditions of SiO increase with
the velocity of the emitting gas. We also compute the SiO abundance through the
SiO and CO integrated intensities at high velocities. For the sources in our
sample we find no significant variation of the SiO abundance with evolution for
a bolometric luminosity-to-mass ratio of between 4 and 50 . We
also find a weak increase of the SiO(8-7) luminosity with the bolometric
luminosity-to-mass ratio. We speculate that this might be explained with an
increase of density in the gas traced by SiO. We find that the densities
constrained by the SiO observations require the use of shock models that
include grain-grain processing. For the first time, such models are compared
and found to be compatible with SiO observations. A pre-shock density of
cm is globally inferred from these comparisons. Shocks with a
velocity higher than 25 km s are invoked for the objects in our sample
where the SiO is observed with a corresponding velocity dispersion. Our
comparison of shock models with observations suggests that sputtering of
silicon-bearing material (corresponding to less than 10% of the total silicon
abundance) from the grain mantles is occurring.Comment: Accepted for publication by A&
No es lo que dices, sino cómo lo dices
La experiencia docente que aquí presento la llevé a cabo durante las clases deespañol como Lengua Extranjera que impartí en Manchester en fecha reciente. Trasestudiar en profundidad el Método Verbo-Tonal y ver diferentes propuestas deactividades, elaboré una clase de pronunciación del español cuyos resultados fueronmuy positivos. En ella intenté abordar todos los niveles de la pronunciación:entonación, ritmo, acento y sonidos
Evolution and excitation conditions of outflows in high-mass star-forming regions
Theoretical models suggest that massive stars form via disk-mediated
accretion, with bipolar outflows playing a fundamental role. A recent study
toward massive molecular outflows has revealed a decrease of the SiO line
intensity as the object evolves. The present study aims at characterizing the
variation of the molecular outflow properties with time, and at studying the
SiO excitation conditions in outflows associated with massive YSOs. We used the
IRAM30m telescope to map 14 massive star-forming regions in the SiO(2-1),
SiO(5-4) and HCO+(1-0) outflow lines, and in several dense gas and hot core
tracers. Hi-GAL data was used to improve the spectral energy distributions and
the L/M ratio, which is believed to be a good indicator of the evolutionary
stage of the YSO. We detect SiO and HCO+ outflow emission in all the sources,
and bipolar structures in six of them. The outflow parameters are similar to
those found toward other massive YSOs. We find an increase of the HCO+ outflow
energetics as the object evolve, and a decrease of the SiO abundance with time,
from 10^(-8) to 10^(-9). The SiO(5-4) to (2-1) line ratio is found to be low at
the ambient gas velocity, and increases as we move to high velocities,
indicating that the excitation conditions of the SiO change with the velocity
of the gas (with larger densities and/or temperatures for the high-velocity gas
component). The properties of the SiO and HCO+ outflow emission suggest a
scenario in which SiO is largely enhanced in the first evolutionary stages,
probably due to strong shocks produced by the protostellar jet. As the object
evolves, the power of the jet would decrease and so does the SiO abundance.
During this process, however, the material surrounding the protostar would have
been been swept up by the jet, and the outflow activity, traced by entrained
molecular material (HCO+), would increase with time.Comment: 31 pages, 10 figures and 5 tables (plus 2 figures and 3 tables in the
appendix). Accepted for publication in A&A. [Abstract modified to fit the
arXiv requirements.
Modeling the effect of storage temperature on the respiration rate and texture of fresh cut pineapple
The effect of temperature on the respiration rate and texture of fresh cut pineapple was studied over the course of 10 days of storage. The thermal exchange between the pineapple trays and the cooling environment was simulated using the finite element method and tested at 6 °C. The temperatures on pineapple wedges differed between the cold point and points near the surface, indicating that the respiration rate may be affected in pineapple subjected to temperature abuse. The experimental respiration rates obtained were used to develop a model relating respiration to O2 and CO2concentrations at different temperatures using the closed system method. The O2 consumption and CO2 production of pineapple wedges was accurately modeled using Michaelis–Menten kinetics. The texture degradation of pineapple wedges follows a zero-order kinetic reaction at different temperatures and the thermal dependence of the model’s parameters for both respiration rate and texture degradation was described by Arrhenius-type equations
Complex organics in IRAS 4A revisited with ALMA and PdBI: Striking contrast between two neighbouring protostellar cores
We used the Atacama Large (sub-)Millimeter Array (ALMA) and the IRAM Plateau
de Bure Interferometer (PdBI) to image, with an angular resolution of 0.5
(120 au) and 1 (235 au), respectively, the emission from 11 different
organic molecules in the protostellar binary NGC1333 IRAS 4A. We clearly
disentangled A1 and A2, the two protostellar cores present. For the first time,
we were able to derive the column densities and fractional abundances
simultaneously for the two objects, allowing us to analyse the chemical
differences between them. Molecular emission from organic molecules is
concentrated exclusively in A2 even though A1 is the strongest continuum
emitter. The protostellar core A2 displays typical hot corino abundances and
its deconvolved size is 70 au. In contrast, the upper limits we placed on
molecular abundances for A1 are extremely low, lying about one order of
magnitude below prestellar values. The difference in the amount of organic
molecules present in A1 and A2 ranges between one and two orders of magnitude.
Our results suggest that the optical depth of dust emission at these
wavelengths is unlikely to be sufficiently high to completely hide a hot corino
in A1 similar in size to that in A2. Thus, the significant contrast in
molecular richness found between the two sources is most probably real. We
estimate that the size of a hypothetical hot corino in A1 should be less than
12 au. Our results favour a scenario in which the protostar in A2 is either
more massive and/or subject to a higher accretion rate than A1, as a result of
inhomogeneous fragmentation of the parental molecular clump. This naturally
explains the smaller current envelope mass in A2 with respect to A1 along with
its molecular richness.Comment: Accepted in Astronomy and Astrophysic
Retinal nerve fiber layer atrophy is associated with physical and cognitive disability in multiple sclerosis
Studying axonal loss in the retina is a promising biomarker for
multiple sclerosis (MS). Our aim was to compare optical coherence tomography
(OCT) and Heidelberg retinal tomography (HRT) techniques to measure the thickness
of the retinal nerve fiber layer (RNFL) in patients with MS, and to explore the
relationship between changes in the RNFL thickness with physical and cognitive
disability. We studied 52 patients with MS and 18 proportionally matched controls
by performing neurological examination, neuropsychological evaluation using the
Brief Repetitive Battery-Neuropsychology and RNFL thickness measurement using OCT
and HRT. RESULTS: We found that both OCT and HRT could define a reduction in the
thickness of the RNFL in patients with MS compared with controls, although both
measurements were weakly correlated, suggesting that they might measure different
aspects of the tissue changes in MS. The degree of RNFL atrophy was correlated
with cognitive disability, mainly with the symbol digit modality test (r=0.754,
P<0.001). Moreover, temporal quadrant RNFL atrophy measured with OCT was
associated with physical disability. CONCLUSION: In summary, both OCT and HRT are
able to detect thinning of the RNFL, but OCT seems to be the most sensitive
technique to identify changes associated with MS evolution
End-to-End V2X Latency Modeling and Analysis in 5G Networks
networks provide higher flexibility and improved performance compared to previous cellular technologies. This has raised expectations on the possibility to support advanced Vehicle to Everything (V2X) services using the cellular network via Vehicle-to-Network (V2N) and Vehicle-to-Network-to-Vehicle (V2N2V) connections. The possibility to support critical V2X services using 5G V2N2V or V2N connections depends on their end-to-end (E2E) latency. The E2E latency of V2N2V or V2N connections depends on the particular 5G network deployment, dimensioning and configuration, in addition to the network load. To date, few studies have analyzed the capabilities of V2N2V or V2N connections to support critical V2X services, and most of them focus on the 5G radio access network or consider dedicated 5G pilot deployments under controlled conditions. This paper progresses the state-of-the-art by introducing a novel E2E latency model to quantify the latency of 5G V2N and V2N2V communications. The model includes the latency introduced at the radio, transport, core, Internet, peering points and application server (AS) when vehicles are supported by a single mobile network operator (MNO) and when they are supported by multiple MNOs. The model can quantify the latency experienced when the V2X AS is deployed from the edge of the network (using MEC platforms) to the cloud. Using this model, this study estimates the E2E latency of 5G V2N2V connections for a large variety of possible 5G network deployments and configurations. The analysis helps identify which 5G network deployments and configurations are more suitable to meet V2X latency requirements. To this aim, we consider as case study the cooperative lane change service. The conducted analysis highlights the challenge for centralized network deployments that locate the V2X AS at the cloud to meet the latency requirements of advanced V2X services. Locating the V2X AS closer to the cell edge reduces the latency. However, it requires a higher number of ASs and also a careful dimensioning of the network and its configuration to ensure sufficient network and AS resources are dedicated to serve the V2X traffic
Transient evolution of C-type shocks in dusty regions of varying density
Outflows of young stars drive shocks into dusty, molecular regions. Most
models of such shocks assume that they are steady and propagating perpendicular
to the magnetic field. Real shocks often violate both of these assumptions and
the media through which they propagate are inhomogeneous. We use the code
employed previously to produce the first time-dependent simulations of
fast-mode, oblique C-type shocks interacting with density perturbations. We
include a self-consistent calculation of the thermal and ionisation balances
and a fluid treatment of grains. We identify features that develop when a
multifluid shock encounters a density inhomogeneity to investigate whether any
part of the precursor region ever behaves in a quasi-steady fashion. If it does
the shock may be modelled approximately without solving the time-dependent
hydromagnetic equations. Simulations were made for initially steady oblique
C-type shocks encountering density inhomogeneities. For a semi-finite
inhomogeneity with a density larger than the surrounding medium, a transmitted
shock evolves from being J-type to a steady C-type shock on a timescale
comparable to the ion-flow time through it. A sufficiently upstream part of the
precursor of an evolving J-type shock is quasi-steady. The ion-flow timescale
is also relevant for the evolution of a shock moving into a region of
decreasing density. The models for shocks propagating into regions in which the
density increases and then decreases to its initial value cannot be entirely
described in terms of the results obtained for monotonically increasing and
decreasing densities. For the latter model, the long-term evolution to a C-type
shock cannot be approximated by quasi-steady models.Comment: 11 pages, 9 figure
Infall, outflow, and rotation in the G19.61-0.23 hot molecular core
Aims: The main goal of this study is to perform a sub-arcsecond resolution
analysis of the high-mass star formation region G19.61-0.23, both in the
continuum and molecular line emission. While the centimeter continuum images
will be discussed in detail in a forthcoming paper, here we focus on the
(sub)mm emission, devoting special attention to the hot molecular core.
Results: Our observations resolve the HMC into three cores whose masses are on
the order of 10^1-10^3 Msun. No submm core presents detectable free-free
emission in the centimeter regime, but they appear to be associated with masers
and thermal line emission from complex organic molecules. Towards the most
massive core, SMA1, the CH3CN (18_K-17_K) lines reveal hints of rotation about
the axis of a jet/outflow traced by H2O maser and H13CO+ (1--0) line emission.
Inverse P-Cygni profiles of the 13CO (3--2) and C18O (3--2) lines seen towards
SMA1 indicate that the central high-mass (proto)star(s) is (are) still gaining
mass with an accretion rate Msun/yr. Due to the linear scales
and the large values of the accretion rate, we hypothesize that we are
observing an accretion flow towards a cluster in the making, rather than
towards a single massive star.Comment: A&A accepted; 18 pages; Preprint with full-resolution figures is
available at http://subarutelescope.org/staff/rsf/publication.htm
Search for radio jets from massive young stellar objects. Association of radio jets with H<sub>2</sub>O and CH<sub>3</sub>OH masers
Context. Recent theoretical and observational studies debate the similarities of the formation process of high- (>8 M⊙) and low-mass stars. The formation of low-mass stars is directly associated with the presence of disks and jets. Theoretical models predict that stars with masses up to 140 M⊙ can be formed through disk-mediated accretion in disk-jet systems. According to this scenario, radio jets are expected to be common in high-mass star-forming regions.
Aims. We aim to increase the number of known radio jets in high-mass star-forming regions by searching for radio-jet candidates at radio continuum wavelengths.
Methods. We used the Karl G. Jansky Very Large Array (VLA) to observe 18 high-mass star-forming regions in the C band (6 cm, ≈1′′.0 resolution) and K band (1.3 cm, ≈0′′.3 resolution). We searched for radio-jet candidates by studying the association of radio continuum sources with shock activity signs (e.g., molecular outflows, extended green objects, and maser emission). Our VLA observations also targeted the 22 GHz H2O and 6.7 GHz CH3OH maser lines.
Results. We have identified 146 radio continuum sources, 40 of which are located within the field of view of both images (C and K band maps). We derived the spectral index, which is consistent with thermal emission (between − 0.1 and + 2.0) for 73% of these sources. Based on the association with shock-activity signs, we identified 28 radio-jet candidates. Out of these, we identified 7 as the most probable radio jets. The radio luminosity of the radio-jet candidates is correlated with the bolometric luminosity and the outflow momentum rate. About 7–36% of the radio-jet candidates are associated with nonthermal emission. The radio-jet candidates associated with 6.7 GHz CH3OH maser emission are preferentially thermal winds and jets, while a considerable fraction of radio-jet candidates associated with H2O masers show nonthermal emission that is likely due to strong shocks.
Conclusions. About 60% of the radio continuum sources detected within the field of view of our VLA images are potential radio jets. The remaining sources could be compact H I
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