96 research outputs found
CSO and CARMA Observations of L1157. II. Chemical Complexity in the Shocked Outflow
L1157, a molecular dark cloud with an embedded Class 0 protostar possessing a
bipolar outflow, is an excellent source for studying shock chemistry, including
grain-surface chemistry prior to shocks, and post-shock, gas-phase processing.
The L1157-B1 and B2 positions experienced shocks at an estimated ~2000 and 4000
years ago, respectively. Prior to these shock events, temperatures were too low
for most complex organic molecules to undergo thermal desorption. Thus, the
shocks should have liberated these molecules from the ice grain-surfaces en
masse, evidenced by prior observations of SiO and multiple grain mantle species
commonly associated with shocks. Grain species, such as OCS, CH3OH, and HNCO,
all peak at different positions relative to species that are preferably formed
in higher velocity shocks or repeatedly-shocked material, such as SiO and HCN.
Here, we present high spatial resolution (~3") maps of CH3OH, HNCO, HCN, and
HCO+ in the southern portion of the outflow containing B1 and B2, as observed
with CARMA. The HNCO maps are the first interferometric observations of this
species in L1157. The maps show distinct differences in the chemistry within
the various shocked regions in L1157B. This is further supported through
constraints of the molecular abundances using the non-LTE code RADEX (Van der
Tak et al. 2007). We find the east/west chemical differentiation in C2 may be
explained by the contrast of the shock's interaction with either cold, pristine
material or warm, previously-shocked gas, as seen in enhanced HCN abundances.
In addition, the enhancement of the HNCO abundance toward the the older shock,
B2, suggests the importance of high-temperature O-chemistry in shocked regions.Comment: Accepted for publication in the Astrophysical Journa
Informal seed traders: The backbone of seed business and African smallholder seed supply
To work well and be sustainable, seed systems have to o er a range of crops and varieties
of good quality seed and these products have to reach farmers, no matter how remote or poor they
may be. Formal seed sector interventions alone are not delivering the crop portfolio or achieving the
social and geographic breadth needed, and the paper argues for focus on informal seed channels and
particularly on traders who move ‘potential seed’ (informal or local seed) even to high stress areas.
This paper provides the first in-depth analysis on potential seed trader types and actions, drawing on
data collected on 287 traders working in 10 African countries. The research delves into four themes:
the types and hierarchies of traders; the technical ways traders manage seed using 11 core practices;
the price di erential of +50% of potential (local) seed over grain, and the pivotal roles which traders
play in remote and crisis contexts. Traders are the backbone of smallholder seed security and need
to be engaged, not ignored, in development and relief e orts. An action framework for leveraging
seed trader skills is presented, with the paper addressing possible legal and donor constraints for
engaging such market actors more fully
Seed systems smallholder farmers use
Seed can be an important entry point for promoting productivity, nutrition and resilience among smallholder farmers. While investments have primarily focused on strengthening the formal sector, this article documents the degree to which the informal sector remains the core for seed acquisition, especially in Africa. Conclusions drawn from a uniquely comprehensive data set, 9660 observations across six countries and covering 40 crops, show that farmers access 90.2 % of their seed from informal systems with 50.9 % of that deriving from local markets. Further, 55 % of seed is paid for by cash, indicating that smallholders are already making important investments in this arena. Targeted interventions are proposed for rendering formal and informal seed sector more smallholder-responsive and for scaling up positive impacts
A Search for Light Hydrides in the Envelopes of Evolved Stars
We report a search for the diatomic hydrides SiH, PH, and FeH along the line
of sight toward the chemically rich circumstellar envelopes of IRC+10216 and VY
Canis Majoris. These molecules are thought to form in high temperature regions
near the photospheres of these stars, and may then further react via gas-phase
and dust-grain interactions leading to more complex species, but have yet to be
constrained by observation. We used the GREAT spectrometer on SOFIA to search
for rotational emission lines of these molecules in four spectral windows
ranging from 600 GHz to 1500 GHz. Though none of the targeted species were
detected in our search, we report their upper limit abundances in each source
and discuss how they influence the current understanding of hydride chemistry
in dense circumstellar media. We attribute the non-detections of these hydrides
to their compact source sizes, high barriers of formation, and proclivity to
react with other molecules in the winds.Comment: Accepted for publication in ApJ. 14 pages, 4 figures, 3 table
Non-detection of HC_(11)N towards TMC-1: constraining the chemistry of large carbon-chain molecules
Bell et al. reported the first detection of the cyanopolyyne HC_(11)N towards the cold dark cloud TMC-1; no subsequent detections have been reported towards any source. Additional observations of cyanopolyynes and other carbon-chain molecules towards TMC-1 have shown a log-linear trend between molecule size and column density, and in an effort to further explore the underlying chemical processes driving this trend, we have analysed Green Bank Telescope observations of HC_9N and HC_(11)N towards TMC-1. Although we find an HC_9N column density consistent with previous values, HC_(11)N is not detected and we derive an upper limit column density significantly below that reported in Bell et al. Using a state-of-the-art chemical model, we have investigated possible explanations of non-linearity in the column density trend. Despite updating the chemical model to better account for ion–dipole interactions, we are not able to explain the non-detection of HC_(11)N, and we interpret this as evidence of previously unknown carbon-chain chemistry. We propose that cyclization reactions may be responsible for the depleted HC11N abundance, and that products of these cyclization reactions should be investigated as candidate interstellar molecules
Non-detection of HC_(11)N towards TMC-1: constraining the chemistry of large carbon-chain molecules
Bell et al. reported the first detection of the cyanopolyyne HC_(11)N towards the cold dark cloud TMC-1; no subsequent detections have been reported towards any source. Additional observations of cyanopolyynes and other carbon-chain molecules towards TMC-1 have shown a log-linear trend between molecule size and column density, and in an effort to further explore the underlying chemical processes driving this trend, we have analysed Green Bank Telescope observations of HC_9N and HC_(11)N towards TMC-1. Although we find an HC_9N column density consistent with previous values, HC_(11)N is not detected and we derive an upper limit column density significantly below that reported in Bell et al. Using a state-of-the-art chemical model, we have investigated possible explanations of non-linearity in the column density trend. Despite updating the chemical model to better account for ion–dipole interactions, we are not able to explain the non-detection of HC_(11)N, and we interpret this as evidence of previously unknown carbon-chain chemistry. We propose that cyclization reactions may be responsible for the depleted HC11N abundance, and that products of these cyclization reactions should be investigated as candidate interstellar molecules
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