50 research outputs found
Nitrogen Fractionation in External Galaxies
In star forming regions in our own Galaxy, the 14N/15N ratio is found to vary
from 100 in meteorites, comets and protoplanetary disks up to
1000 in pre-stellar and star forming cores, while in external galaxies the very
few single-dish large scale measurements of this ratio lead to values of
100-450. The extent of the contribution of isotopic fractionation to these
variations is, to date, unknown. In this paper we present a theoretical
chemical study of nitrogen fractionation in external galaxies in order to
determine the physical conditions that may lead to a spread of the 14N/15N
ratio from the solar value of 440 and hence evaluate the contribution of
chemical reactions in the ISM to nitrogen fractionation. We find that the main
cause of ISM enrichment of nitrogen fractionation is high gas densities, aided
by high fluxes of cosmic rays.Comment: Accepted by MNRA
Detectability of Glycine in Solar-type System Precursors
Glycine (NH2CH2COOH) is the simplest amino acid relevant for life. Its
detection in the interstellar medium is key to understand the formation
mechanisms of pre-biotic molecules and their subsequent delivery onto planetary
systems. Glycine has extensively been searched for toward hot molecular cores,
although these studies did not yield any firm detection. In contrast to hot
cores, low-mass star forming regions, and in particular their earliest stages
represented by cold pre-stellar cores, may be better suited for the detection
of glycine as well as more relevant for the study of pre-biotic chemistry in
young Solar System analogs. We present 1D spherically symmetric radiative
transfer calculations of the glycine emission expected to arise from the
low-mass pre-stellar core L1544. Water vapour has recently been reported toward
this core, indicating that a small fraction of the grain mantles in L1544
(~0.5%) has been injected into the gas phase. Assuming that glycine is
photo-desorbed together with water in L1544, and considering a solid abundance
of glycine on ices of ~1E-4 with respect to water, our calculations reveal that
several glycine lines between 67 GHz and 80 GHz have peak intensities larger
than 10 mK. These results show for the first time that glycine could reach
detectable levels in cold objects such as L1544. This opens up the possibility
to detect glycine, and other pre-biotic species, at the coldest and earliest
stages in the formation of Solar-type systems with near-future instrumentation
such as the Band 2 receivers of ALMA.Comment: 5 pages, 2 figures, 1 tables. Accepted for publication in ApJ Letter
The dynamical properties of dense filaments in the infrared dark cloud G035.39-00.33
Infrared Dark Clouds (IRDCs) are unique laboratories to study the initial
conditions of high-mass star and star cluster formation. We present
high-sensitivity and high-angular resolution IRAM PdBI observations of N2H+
(1-0) towards IRDC G035.39-00.33. It is found that G035.39-00.33 is a highly
complex environment, consisting of several mildly supersonic filaments
(sigma_NT/c_s ~1.5), separated in velocity by <1 km s^-1 . Where multiple
spectral components are evident, moment analysis overestimates the non-thermal
contribution to the line-width by a factor ~2. Large-scale velocity gradients
evident in previous single-dish maps may be explained by the presence of
substructure now evident in the interferometric maps. Whilst global velocity
gradients are small (<0.7 km s^-1 pc^-1), there is evidence for dynamic
processes on local scales (~1.5-2.5 km s^-1 pc^-1 ). Systematic trends in
velocity gradient are observed towards several continuum peaks. This suggests
that the kinematics are influenced by dense (and in some cases, starless)
cores. These trends are interpreted as either infalling material, with
accretion rates ~(7 \pm 4)x10^-5 M_sun yr^-1 , or expanding shells with
momentum ~24 \pm 12 M_sun km s^-1 . These observations highlight the importance
of high-sensitivity and high-spectral resolution data in disentangling the
complex kinematic and physical structure of massive star forming regions.Comment: 25 pages, 23 figures, accepted for publication in MNRA
A Virialized Filamentary Infrared Dark Cloud
The initial conditions of massive star and star cluster formation are
expected to be cold, dense and high column density regions of the interstellar
medium, which can reveal themselves via near, mid and even far-infrared
absorption as Infrared Dark Clouds (IRDCs). Elucidating the dynamical state of
IRDCs thus constrains theoretical models of these complex processes. In
particular, it is important to assess whether IRDCs have reached virial
equilibrium, where the internal pressure balances that due to the
self-gravitating weight of the cloud plus the pressure of the external
environmental. We study this question for the filamentary IRDC G035.39-00.33 by
deriving mass from combined NIR & MIR extinction maps and velocity dispersion
from C18O (1-0) & (2-1) line emission. In contrast to our previous moderately
super-virial results based on 13CO emission and MIR-only extinction mapping,
with improved mass measurements we now find that the filament is consistent
with being in virial equilibrium, at least in its central parsec-wide region
where ~1000 M_Sun snakes along several parsecs. This equilibrium state does not
require large-scale net support or confinement by magnetic fields.Comment: 4 pages, 2 figures, Accepted to ApJ
The Submillimeter Polarization Spectrum of M17
We present 450 {\mu}m polarimetric observations of the M17 molecular cloud
obtained with the SHARP polarimeter at the Caltech Submillimeter Observatory.
Across the observed region, the magnetic field orientation is consistent with
previous submillimeter and far-infrared polarization measurements. Our
observations are centered on a region of the molecular cloud that has been
compressed by stellar winds from a cluster of OB stars. We have compared these
new data with previous 350 {\mu}m polarimetry and find an anti-correlation
between the 450 to 350 {\mu}m polarization magnitude ratio and the ratio of 21
cm to 450 {\mu}m intensity. The polarization ratio is lower near the east end
of the studied region where the cloud is exposed to stellar winds and
radiation. At the west end of the region, the polarization ratio is higher. We
interpret the varying polarization spectrum as evidence supporting the
radiative alignment torque (RAT) model for grain alignment, implying higher
alignment efficiency in the region that is exposed to a higher anisotropic
radiation field.Comment: 24 pages, 10 figure
Observations of CHOH and CHCHO in a Sample of Protostellar Outflow Sources
Iram 30-m Observations towards eight protostellar outflow sources were taken
in the 96-\SI{176}{\giga\hertz} range. Transitions of CHOH and CHCHO
were detected in seven of them. The integrated emission of the transitions of
each species that fell into the observed frequency range were measured and fit
using RADEX and LTE models. Column densities and gas properties inferred from
this fitting are presented. The ratio of the A and E-type isomers of CHOH
indicate that the methanol observed in these outflows was formed on the grain
surface. Both species demonstrate a reduction of terminal velocity in their
line profiles in faster outflows, indicating destruction in the post-shock gas
phase. This destruction, and a near constant ratio of the CHOH and
CHCHO column densities imply it is most likely that CHCHO also forms on
the grain surface.Comment: Accepted for publication in Ap
Distinct Chemical Regions in the "Prestellar" Infrared Dark Cloud G028.23–00.19
We have observed the Infrared Dark Cloud (IRDC) G028.23–00.19 at 3.3 mm using the Combined Array for Research in Millimeter-wave Astronomy. In its center, the IRDC hosts one of the most massive (~1520 M_☉) quiescent, cold (12 K) clumps known (MM1). The low temperature, high NH2D abundance, narrow molecular line widths, and absence of embedded infrared sources (from 3.6 to 70 μm) indicate that the clump is likely prestellar. Strong SiO emission with broad line widths (6-9 km s^(–1)) and high abundances ((0.8-4) × 10^(–9)) is detected in the northern and southern regions of the IRDC, unassociated with MM1. We suggest that SiO is released to the gas phase from the dust grains through shocks produced by outflows from undetected intermediate-mass stars or clusters of low-mass stars deeply embedded in the IRDC. A weaker SiO component with narrow line widths (~2 km s^(–1)) and low abundances (4.3 × 10^(–11)) is detected in the center-west region, consistent with either a "subcloud-subcloud" collision or an unresolved population of a few low-mass stars. We report widespread CH_3OH emission throughout the whole IRDC and the first detection of extended narrow methanol emission (~2 km s^(–1)) in a cold, massive prestellar clump (MM1). We suggest that the most likely mechanism releasing methanol into the gas phase in such a cold region is the exothermicity of grain-surface reactions. HN^(13)C reveals that the IRDC is actually composed of two distinct substructures ("subclouds") separated in velocity space by ~1.4 km s^(–1). The narrow SiO component arises where the subclouds overlap. The spatial distribution of C2H resembles that of NH_2D, which suggests that C_2H also traces cold gas in this IRDC