491 research outputs found
Observations of nitrogen isotope fractionation in deeply embedded protostars
(Abridged) The terrestrial planets, comets, and meteorites are significantly
enriched in 15N compared to the Sun and Jupiter. While the solar and jovian
nitrogen isotope ratio is believed to represent the composition of the
protosolar nebula, a still unidentified process has caused 15N-enrichment in
the solids. Several mechanisms have been proposed to explain the variations,
including chemical fractionation. However, observational results that constrain
the fractionation models are scarce. While there is evidence of 15N-enrichment
in prestellar cores, it is unclear how the signature evolves into the
protostellar phases. Our aim is to measure the 14N/15N ratio around three
nearby, embedded low-to-intermediate-mass protostars. Isotopologues of HCN and
HNC were used to probe the 14N/15N ratio. A selection of H13CN, HC15N, HN13C,
and H15NC transitions was observed with the APEX telescope. The 14N/15N ratios
were derived from the integrated intensities assuming a standard 12C/13C ratio.
The assumption of optically thin emission was verified using radiative transfer
modeling and hyperfine structure fitting. Two sources, IRAS 16293A and R CrA
IRS7B, show 15N-enrichment by a factor of around 1.5-2.5 in both HCN and HNC
with respect to the solar composition. Solar composition cannot be excluded for
the third source, OMC-3 MMS6. Furthermore, there are indications of a trend
toward increasing 14N/15N ratios with increasing outer envelope temperature.
The enhanced 15N abundances in HCN and HNC found in two Class~0 sources
(14N/15N of 160-290) and the tentative trend toward a temperature-dependent
14N/15N ratio are consistent with the chemical fractionation scenario, but
14N/15N ratios from additional tracers are indispensable for testing the
models. Spatially resolved observations are needed to distinguish between
chemical fractionation and isotope-selective photochemistry.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 13
figure
The demographic consequences of adaptation:Evidence from experimental evolution
The process of adaptation toward novel environments is directly connected to the acquisition of higher fitness relative to others. Such increased fitness is obtained by changes in life history traits that may directly impact population dynamics. From a functional perspective, increased fitness can be achieved through higher resource use or more efficient resource use, each potentially having its own impact on population dynamics. In the first case, adaptation is expected to directly translate into higher population growth. In the second case, adaptation requires less energy and hence may lead to higher carrying capacity. Adaptation may thus lead to changes in ecological dynamics and vice versa. Here, by using a combination of evolutionary experiments with spider mites and a population dynamic model, we investigate how an increase in fecundity (a validated proxy for adaptation) affects a population's ecological dynamics. Our results show that adaptation can positively affect population growth rate and either positively or negatively affect carrying capacity, depending on the ecological condition leading to variation in adaptation. These findings show the importance of evolution for population dynamics in changing environments, which may ultimately affect the stability and resilience of populations
An interferometric study of the low-mass protostar IRAS 16293-2422: small scale organic chemistry
Aims: To investigate the chemical relations between complex organics based on
their spatial distributions and excitation conditions in the low-mass young
stellar objects IRAS 16293-2422 A and B. Methods: Interferometric observations
with the Submillimeter Array have been performed at 5''x3'' resolution
revealing emission lines of HNCO, CH3CN, CH2CO, CH3CHO and C2H5OH. Rotational
temperatures are determined from rotational diagrams when a sufficient number
of lines are detected. Results: Compact emission is detected for all species
studied here. For HNCO and CH3CN it mostly arises from source A, CH2CO and
C2H5OH have comparable strength for both sources and CH3CHO arises exclusively
from source B. HNCO, CH3CN and CH3CHO have rotational temperatures >200 K. The
(u,v)-visibility data reveal that HNCO also has extended cold emission.
Conclusions: The abundances of the molecules studied here are very similar
within factors of a few to those found in high-mass YSOs. Thus the chemistry
between high- and low-mass objects appears to be independent of luminosity and
cloud mass. Bigger abundance differences are seen between the A and B source.
The HNCO abundance relative to CH3OH is ~4 times higher toward A, which may be
due to a higher initial OCN- ice abundances in source A compared to B.
Furthermore, not all oxygen-bearing species are co-existent. The different
spatial behavior of CH2CO and C2H5OH compared with CH3CHO suggests that
hydrogenation reactions on grain-surfaces are not sufficient to explain the
observed gas phase abundances. Selective destruction of CH3CHO may result in
the anti-coincidence of these species in source A. These results illustrate the
power of interferometric compared with single dish data in terms of testing
chemical models.Comment: 11 pages, 15 figures, accepeted by A&
Infrared spectroscopy of HCOOH in interstellar ice analogues
Context: HCOOH is one of the more common species in interstellar ices with
abundances of 1-5% with respect to solid H2O. Aims: This study aims at
characterizing the HCOOH spectral features in astrophysically relevant ice
mixtures in order to interpret astronomical data. Methods: The ices are grown
under high vacuum conditions and spectra are recorded in transmission using a
Fourier transform infrared spectrometer. Pure HCOOH ices deposited at 15 K and
145 K are studied, as well as binary and tertiary mixtures containing H2O, CO,
CO2 and CH3OH. The mixture concentrations are varied from 50:50% to ~10:90% for
HCOOH:H2O. Binary mixtures of HCOOH:X and tertiary mixtures of HCOOH:H2O:X with
X = CO, CO2, and CH3OH, are studied for concentrations of ~10:90% and
~7:67:26%, respectively. Results: Pure HCOOH ice spectra show broad bands which
split around 120 K due to the conversion of a dimer to a chain-structure. Broad
single component bands are found for mixtures with H2O. Additional spectral
components are present in mixtures with CO, CO2 and CH3OH. The resulting peak
position, full width at half maximum and band strength depend strongly on ice
structure, temperature, matrix constituents and the HCOOH concentration.
Comparison of the solid HCOOH 5.9, 7.2, and 8.1 micron features with
astronomical data toward the low mass source HH 46 and high mass source W 33A
shows that spectra of binary mixtures do not reproduce the observed ice
features. However, our tertiary mixtures especially with CH3OH match the
astronomical data very well. Thus interstellar HCOOH is most likely present in
tertiary or more complex mixtures with H2O, CH3OH and potentially also CO or
CO2, providing constraints on its formation.Comment: 11 pages, 10 figures, accepted by A&
Cold gas as an ice diagnostic toward low mass protostars
Up to 90% of the chemical reactions during star formation occurs on ice
surfaces, probably including the formation of complex organics. Only the most
abundant ice species are however observed directly by infrared spectroscopy.
This study aims to develop an indirect observational method of ices based on
non-thermal ice desorption in the colder part of protostellar envelopes. For
that purpose the IRAM 30m telescope was employed to observe two molecules that
can be detected both in the gas and the ice, CH3 OH and HNCO, toward 4 low mass
embedded protostars. Their respective gas-phase column densities are determined
using rotational diagrams. The relationship between ice and gas phase
abundances is subsequently determined. The observed gas and ice abundances span
several orders of magnitude. Most of the CH3OH and HNCO gas along the lines of
sight is inferred to be quiescent from the measured line widths and the derived
excitation temperatures, and hence not affected by thermal desorption close to
the protostar or in outflow shocks. The measured gas to ice ratio of ~10-4
agrees well with model predictions for non-thermal desorption under cold
envelope conditions and there is a tentative correlation between ice and gas
phase abundances. This indicates that non-thermal desorption products can serve
as a signature of the ice composition. A larger sample is however necessary to
provide a conclusive proof of concept.Comment: accepted by A&A letters, 10 pages including 5 figure
A Comparison of Haptic and Auditory Feedback as a Warning Signal for Slip in Tele-Operation Scenarios
Slip feedback is an important cue in everyday object manipulation, but it is generally missing in tele-operation systems. To test the usefulness of simple, abstract types of feedback that warn the user about slip events, we tested the effect of auditory and haptic vibration feedback in a tele-operation task. Participants were asked to hold an object in a remote robot hand, and the force profiles that they exerted in response to slip events were measured. Haptic feedback did not significantly change the response characteristics, but auditory feedback did significantly improve response latency. A small but significant difference between haptic and auditory reaction times (60 ms) found in our control experiment might explain the difference between the feedback types
Hydrogenation reactions in interstellar CO ice analogues
Hydrogenation reactions of CO in inter- and circumstellar ices are regarded
as an important starting point in the formation of more complex species.
Previous laboratory measurements by two groups on the hydrogenation of CO ices
resulted in controversial results on the formation rate of methanol. Our aim is
to resolve this controversy by an independent investigation of the reaction
scheme for a range of H-atom fluxes and different ice temperatures and
thicknesses. Reaction rates are determined by using a state-of-the-art ultra
high vacuum experimental setup to bombard an interstellar CO ice analog with
room temperature H atoms. The reaction of CO + H into H2CO and subsequently
CH3OH is monitored by a Fourier transform infrared spectrometer in a reflection
absorption mode. In addition, after each completed measurement a temperature
programmed desorption experiment is performed to identify the produced species.
Different H-atom fluxes, morphologies, and ice thicknesses are tested. The
formation of both formaldehyde and methanol via CO hydrogenation is confirmed
at low temperature (12-20 K). We confirm, as proposed by Hidaka et al., that
the discrepancy between the two Japanese studies is mainly due to a difference
in the applied hydrogen atom flux. The production rate of formaldehyde is found
to decrease and the penetration column to increase with temperature. In order
to fully understand the laboratory data, the experimental results are
interpreted using Monte Carlo simulations. This technique takes into account
the layered structure of CO ice. Temperature-dependent reaction barriers and
diffusion rates are inferred using this model. The model is extended to
interstellar conditions to compare with observational H2CO/CH3OH data.Comment: accepted by A. & A., 21 pages, 15 figure
Dimethyl ether in its ground state, v=0, and lowest two torsionally excited states, v11=1 and v15=1, in the high-mass star-forming region G327.3-0.6
The goal of this paper is to determine the respective importance of solid
state vs. gas phase reactions for the formation of dimethyl ether. This is done
by a detailed analysis of the excitation properties of the ground state and the
torsionally excited states, v11=1 and v15=1, toward the high-mass star-forming
region G327.3-0.6. With the Atacama Pathfinder EXperiment 12 m submillimeter
telescope, we performed a spectral line survey. The observed spectrum is
modeled assuming local thermal equilibrium. CH3OCH3 has been detected in the
ground state, and in the torsionally excited states v11=1 and v15=1, for which
lines have been detected here for the first time. The emission is modeled with
an isothermal source structure as well as with a non-uniform spherical
structure. For non-uniform source models one abundance jump for dimethyl ether
is sufficient to fit the emission, but two components are needed for the
isothermal models. This suggests that dimethyl ether is present in an extended
region of the envelope and traces a non-uniform density and temperature
structure. Both types of models furthermore suggest that most dimethyl ether is
present in gas that is warmer than 100 K, but a smaller fraction of 5%-28% is
present at temperatures between 70 and 100 K. The dimethyl ether present in
this cooler gas is likely formed in the solid state, while gas phase formation
probably is dominant above 100 K. Finally, the v11=1 and v15=1 torsionally
excited states are easily excited under the density and temperature conditions
in G327.3-0.6 and will thus very likely be detectable in other hot cores as
well.Comment: 12 pages (excluding appendices), 8 figures, A&A in pres
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