328 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
Effect of Thermal Gradients on the Electromigration Lifetime in Power Electronics
The combined effects of electromigration and thermomigration are studied. Significantly shorter electromigration lifetimes are observed in the presence of a temperature gradient. This cannot be explained by thermomigration only, but is attributed to the effect of temperature gradient on electromigration-induced failures
Dynamical structure of the inner 100 AU of the deeply embedded protostar IRAS 16293-2422
A fundamental question about the early evolution of low-mass protostars is
when circumstellar disks may form. High angular resolution observations of
molecular transitions in the (sub)millimeter wavelength windows make it
possible to investigate the kinematics of the gas around newly-formed stars,
for example to identify the presence of rotation and infall. IRAS 16293-2422
was observed with the extended Submillimeter Array (eSMA) resulting in
subarcsecond resolution (0.46" x 0.29", i.e. 55 35~AU) images
of compact emission from the CO (3-2) and CS (7-6) transitions at
337~GHz (0.89~mm). To recover the more extended emission we have combined the
eSMA data with SMA observations of the same molecules. The emission of
CO (3-2) and CS (7-6) both show a velocity gradient oriented
along a northeast-southwest direction with respect to the continuum marking the
location of one of the components of the binary, IRAS16293A. Our combined eSMA
and SMA observations show that the velocity field on the 50--400~AU scales is
consistent with a rotating structure. It cannot be explained by simple
Keplerian rotation around a single point mass but rather needs to take into
account the enclosed envelope mass at the radii where the observed lines are
excited. We suggest that IRAS 16293-2422 could be among the best candidates to
observe a pseudo-disk with future high angular resolution observations.Comment: Accepted for publication in ApJ, 18 pages, 10 figure
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&
A recent accretion burst in the low-mass protostar IRAS 15398-3359: ALMA imaging of its related chemistry
Low-mass protostars have been suggested to show highly variable accretion
rates through-out their evolution. Such changes in accretion, and related
heating of their ambient envelopes, may trigger significant chemical variations
on different spatial scales and from source-to-source. We present images of
emission from C17O, H13CO+, CH3OH, C34S and C2H toward the low-mass protostar
IRAS 15398-3359 on 0.5" (75 AU diameter) scales with the Atacama Large
Millimeter/submillimeter Array (ALMA) at 340 GHz. The resolved images show that
the emission from H13CO+ is only present in a ring-like structure with a radius
of about 1-1.5" (150-200 AU) whereas the CO and other high dipole moment
molecules are centrally condensed toward the location of the central protostar.
We propose that HCO+ is destroyed by water vapor present on small scales. The
origin of this water vapor is likely an accretion burst during the last
100-1000 years increasing the luminosity of IRAS 15398-3359 by a factor of 100
above its current luminosity. Such a burst in luminosity can also explain the
centrally condensed CH3OH and extended warm carbon-chain chemistry observed in
this source and furthermore be reflected in the relative faintness of its
compact continuum emission compared to other protostars.Comment: Accepted for publication in ApJ Letters; 14 pages, 5 figure
Structure of evolved cluster-forming regions
Context. An approach towards understanding the formation of massive
stars and star clusters is to study the structure of their hot core phase, an evolutionary
stage where dust has been heated, but molecules have not yet been destroyed by ultraviolet
radiation. These hot molecular cores are very line-rich, but the interpretation of line
surveys is also hampered by poor knowledge of the physical and chemical structure.
Aims. To constrain the radial structure of high-mass star-forming
regions containing hot cores, we attempt to reproduce by radiative transfer modeling both
the intensity and shape of a variety of molecular lines.
Methods. We observed 12 hot cores with the Atacama Pathfinder EXperiment
(APEX) in lines of HCN, HCO+, CO, and their isotopologues, including
high-J lines and vibrationally excited HCN. We investigate how well the
sources can be modeled as centrally heated spheres with a power-law density gradient,
making use of the radiative transfer code RATRAN and the radial profile of the submm
continuum emission, taken from the APEX Telescope Large Area Survey of the GALaxy
(ATLASGAL).
Results. Most of the observed lines have complicated shapes that
incorporate self-absorption, asymmetries, and line wings. Vibrationally excited HCN is
detected in all sources, and vibrationally excited H13CN in half of the
sources. We are able to successfully model most features seen in the APEX data, such as
the ratio of the isotopologue lines (very high optical depths), self-absorption
(temperature gradient), blue asymmetries (moderate infall), vibrationally excited HCN
(high inner temperatures), and H13CN (high HCN abundance under dense and hot
conditions). Other features could not be reproduced, such as an occasional lack of
self-absorption, the emission from high-J lines in the outer pixels of
the CHAMP+ receiver (15′′−20′′ from the center), the outflow wings, and the red
asymmetric profiles.
Conclusions. The amount of molecular gas, in particular of HCN, at very
high temperatures is larger than previously thought. A complex interplay between infall
and outflow motions is present. Our basic model assumptions of pure central heating and a
power-law radial density distribution can serve as approximations for most sources, but
are too simple to explain all observed lines. In particular, taking into account
clumpiness, multiplicity of heating sources and a more complex velocity field seems to be
necessary to more closely match model calculations to observations. This would require
three-dimensional radiative transfer modeling of high-resolution interferometric data
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