1,043 research outputs found
Characterizing the chemical pathways for water formation -- A deep search for hydrogen peroxide
In 2011, hydrogen peroxide (HOOH) was observed for the first time outside the
solar system (Bergman et al., A&A, 2011, 531, L8). This detection appeared a
posteriori quite natural, as HOOH is an intermediate product in the formation
of water on the surface of dust grains. Following up on this detection, we
present a search for HOOH in a diverse sample of sources in different
environments, including low-mass protostars and regions with very high column
densities, such as Infrared Dark Clouds (IRDCs). We do not detect the molecule
in any other source than Oph A, and derive 3 upper limits for the
abundance of HOOH relative to H lower than in Oph A for most sources. This
result sheds a different light on our understanding of the detection of HOOH in
Oph A, and shifts the puzzle to why this source seems to be special. Therefore
we rediscuss the detection of HOOH in Oph A, as well as the implications of the
low abundance of HOOH, and its similarity with the case of O. Our chemical
models show that the production of HOOH is extremely sensitive to the
temperature, and favored only in the range 2030 K. The relatively high
abundance of HOOH observed in Oph A suggests that the bulk of the material lies
at a temperature in the range 2030 K.Comment: 18 pages, 3 figures, invited refereed paper at the Faraday Discussion
16
Millimeter interferometer observations of the magnetar 4U 0142+61
The Anomalous X‐ray Pulsar 4U 0142+61 is the only neutron star where it is believed that one of the long searched‐for ‘fallback’ disks has been detected in the mid‐IR by Wang et al. [1] using Spitzer. Such a disk originates from material falling back to the NS after the supernova. We search for cold circumstellar material in the 90 GHz continuum using the Plateau de Bure Interferometer. No millimeter flux is detected at the position of 4U 0142+61, the upper flux limit is 150 μJy corresponding to the 3σ noise rms level. The re‐processed Spitzer MIPS 24μm data presented previously by Wang et al. [2] show some indication of flux enhancement at the position of the neutron star, albeit below the 3σ statistical significance limit. At far infrared wavelengths the source flux densities are probably below the Herschel confusion limits
The dynamical state of the First Hydrostatic Core Candidate Cha-MMS1
Observations of First Hydrostatic Core candidates, a theoretically predicted
evolutionary link between the prestellar and protostellar phases, are vital for
probing the earliest phases of star formation. We aim to determine the
dynamical state of the First Hydrostatic Core candidate Cha-MMS1. We observed
Cha-MMS1 in various transitions with the APEX and Mopra telescopes. The
molecular emission was modeled with a radiative transfer code to derive
constraints on the envelope kinematics. We derive an internal luminosity of
0.08 - 0.18 Lsol. An average velocity gradient of 3.1(0.1) km/s/pc over 0.08 pc
is found perpendicular to the filament in which Cha-MMS1 is embedded. The
gradient is flatter in the outer parts and at the innermost 2000 - 4000 AU.
These features suggest solid-body rotation beyond 4000 AU and slower,
differential rotation beyond 8000 AU. The origin of the flatter gradient in the
innermost parts is unclear. The classical infall signature is detected in HCO+
3-2 and CS 2-1. The radiative transfer modeling indicates a uniform infall
velocity in the outer parts of the envelope. An infall velocity field scaling
with r^(-0.5) is consistent with the data for r < 9000 AU. The infall
velocities are 0.1 - 0.2 km/s at r > 3300 AU and 0.04 - 0.6 km/s at r < 3300
AU. Both the internal luminosity of Cha-MMS1 and the infall velocity field in
its envelope are consistent with predictions of MHD simulations for the first
core phase. There is no evidence for a fast, large-scale outflow stemming from
Cha-MMS1 but excess emission from the high-density tracers CS 5-4, CO 6-5, and
CO 7-6 suggests the presence of higher-velocity material at the inner core. Its
internal luminosity excludes Cha-MMS1 being a prestellar core. The kinematical
properties of its envelope are consistent with Cha-MMS1 being a first core
candidate or a very young Class 0 protostar.(abridged).Comment: Accepted for publication in A&A. 27 pages, 22 figures, 13 tables. A
version with high-resolution figures is available on request to the first
autho
Observations of extragalactic masers in bright IRAS sources
We report the first results of an ongoing survey at 22 GHz with the 100-m
Effelsberg telescope to search for water maser emission in bright IRAS sources.
We have detected water vapor emission in IC 342. The maser, associated with a
star forming region ~10-15 arcsec west of the nucleus, consists of a single 0.5
km/s wide feature and reaches an isotropic luminosity of 0.01 L(sun) (D = 1.8
Mpc). Our detection raises the detection rate among northern galaxies with IRAS
point source fluxes S(100micron) > 50 Jy to 16%.Comment: 6 pages, 3 figures. Proceeding of the conference "SRT: the impact of
large antennas on Radio Astronomy and Space Science", Cagliari, Italy, 7-10
Nov. 2001, Eds. Porceddu et al. Needs srt_style.st
Star formation in Chamaeleon I and III: a molecular line study of the starless core population
The Chamaeleon clouds are excellent targets for low-mass star formation
studies. Cha I and II are actively forming stars while Cha III shows no sign of
ongoing star formation. We aim to determine the driving factors that have led
to the very different levels of star formation activity in Cha I and III and
examine the dynamical state and possible evolution of the starless cores within
them. Observations were performed in various molecular transitions with APEX
and Mopra. Five cores are gravitationally bound in Cha I and one in Cha III.
The infall signature is seen toward 8-17 cores in Cha I and 2-5 cores in Cha
III, which leads to a range of 13-28% of the cores in Cha I and 10-25% of the
cores in Cha III that are contracting and may become prestellar. Future
dynamical interactions between the cores will not be dynamically significant in
either Cha I or III, but the subregion Cha I North may experience collisions
between cores within ~0.7 Myr. Turbulence dissipation in the cores of both
clouds is seen in the high-density tracers N2H+ 1-0 and HC3N 10-9. Evidence of
depletion in the Cha I core interiors is seen in the abundance distributions of
C17O, C18O, and C34S. Both contraction and static chemical models indicate that
the HC3N to N2H+ abundance ratio is a good evolutionary indicator in the
prestellar phase for both gravitationally bound and unbound cores. In the
framework of these models, we find that the cores in Cha III and the southern
part of Cha I are in a similar evolutionary stage and are less chemically
evolved than the central region of Cha I. The measured HC3N/N2H+ abundance
ratio and the evidence for contraction motions seen towards the Cha III
starless cores suggest that Cha III is younger than Cha I Centre and that some
of its cores may form stars in the future. The cores in Cha I South may on the
other hand be transient structures. (abridged)Comment: Accepted for publication in A&A. The resolution of Figure 2 has been
degraded and the abstract in the metadata has been shortened to fit within
the limits set by arXi
Negative magnetoresistance and phase slip process in superconducting nanowires
We argue that the negative magnetoresistance of superconducting nanowires,
which was observed in recent experiments, can be explained by the influence of
the external magnetic field on the critical current of the phase slip process.
We show that the suppression of the order parameter in the bulk superconductors
made by an external magnetic field can lead to an enhancement of both the first
and the second critical currents of the phase slip process in
nanowires. Another mechanism of an enhancement of can come from
decreasing the decay length of the charge imbalance at weak
magnetic fields because is inversely proportional to . The
enhancement of the first critical current leads to a larger intrinsic
dissipation of the phase slip process. It suppresses the rate of both the
thermo-activated and/or quantum fluctuated phase slips and results in
decreasing the fluctuated resistance.Comment: 7 pages, 4 figure
Diversity of chemistry and excitation conditions in the high-mass star forming complex W33
The object W33 is a giant molecular cloud that contains star forming regions
at various evolutionary stages from quiescent clumps to developed H II regions.
Since its star forming regions are located at the same distance and the primary
material of the birth clouds is probably similar, we conducted a comparative
chemical study to trace the chemical footprint of the different phases of
evolution. We observed six clumps in W33 with the Atacama Pathfinder Experiment
(APEX) telescope at 280 GHz and the Submillimeter Array (SMA) at 230 GHz. We
detected 27 transitions of 10 different molecules in the APEX data and 52
transitions of 16 different molecules in the SMA data. The chemistry on scales
larger than 0.2 pc, which are traced by the APEX data, becomes more
complex and diverse the more evolved the star forming region is. On smaller
scales traced by the SMA data, the chemical complexity and diversity increase
up to the hot core stage. In the H II region phase, the SMA spectra resemble
the spectra of the protostellar phase. Either these more complex molecules are
destroyed or their emission is not compact enough to be detected with the SMA.
Synthetic spectra modelling of the HCO transitions, as detected with the
APEX telescope, shows that both a warm and a cold component are needed to
obtain a good fit to the emission for all sources except for W33 Main1. The
temperatures and column densities of the two components increase during the
evolution of the star forming regions. The integrated intensity ratios
NH(32)/CS(65) and
NH(32)/HCO(43) show clear trends as a
function of evolutionary stage, luminosity, luminosity-to-mass ratio, and
H peak column density of the clumps and might be usable as chemical
clocks.Comment: 66 pages, 28 figures, 8 tables, accepted for publication at A&
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