1,049 research outputs found
Dust emission from young outflows: the case of L1157
We present new high-sensitivity 1.3 mm bolometer observations of the young
outflow L1157. These data show that the continuum emission arises from four
distinct components: a circumstellar disk, a protostellar envelope, an extended
flattened envelope --the dense remnant of the molecular cloud in which the
protostar was formed--, and the outflow itself, which represents ~20% of the
total flux. The outflow emission exhibits two peaks that are coincident with
the two strong shocks in the southern lobe of L1157. We show that the mm
continuum is dominated by thermal dust emission arising in the high velocity
material. The spectral index derived from the new 1.3 mm data and 850 mu
observations from Shirley et al. (2000), is ~5 in the outflow, significantly
higher than in the protostellar envelope (~3.5). This can be explained by an
important line contamination of the 850 mu map, and/or by different dust
characteristics in the two regions, possibly smaller grains in the post-shocks
regions of the outflow. Our observations show that bipolar outflows can present
compact emission peaks which must not be misinterpreted as protostellar
condensations when mapping star forming regions
The clumpy structure of the chemically active L1157 outflow
We present high spatial resolution maps, obtained with the Plateau de Bure
Interferometer, of the blue lobe of the L1157 outflow. We observed four lines
at 3 mm, namely CH3OH (2_K-1_K), HC3N (11-10), HCN (1-0) and OCS (7-6).
Moreover, the bright B1 clump has also been observed at better spatial
resolution in CS (2-1), CH3OH (2_1-1_1)A-, and 34SO (3_2-2_1). These high
spatial resolution observations show a very rich structure in all the tracers,
revealing a clumpy structure of the gas superimposed to an extended emission.
In fact, the three clumps detected by previous IRAM-30m single dish
observations have been resolved into several sub-clumps and new clumps have
been detected in the outflow. The clumps are associated with the two cavities
created by two shock episodes driven by the precessing jet. In particular, the
clumps nearest the protostar are located at the walls of the younger cavity
with a clear arch-shape form while the farthest clumps have slightly different
observational characteristics indicating that they are associated to the older
shock episode. The emission of the observed species peaks in different part of
the lobe: the east clumps are brighter in HC3N (11-10), HCN (1-0) and CS (2-1)
while the west clumps are brighter in CH3OH(2_K-1_K), OCS (7-6) and 34SO
(3_2-2_1). This peak displacement in the line emission suggests a variation of
the physical conditions and/or the chemical composition along the lobe of the
outflow at small scale, likely related to the shock activity and the precession
of the outflow. In particular, we observe the decoupling of the silicon
monoxide and methanol emission, common shock tracers, in the B1 clump located
at the apex of the bow shock produced by the second shock episode.Comment: 11 pages, 8 figures, accepted for publication in the MNRA
Strategic Aspects of IIASA's Food and Agricultural Model
The linkage model of IIASA's Food and Agriculture Program (FAP) can be described as an econometric world model which investigates the interaction of many national economies on a number of agricultural markets and one residual nonagricultural market. The model contains a number of policy parameters which can be determined by the national governments in order to improve the economic results. Given that one can specify a goal function for every country, the IIASA linkage model can be thus viewed as a strategic game with the governments of the various countries as players.
In this paper some of the strategic aspects of the IIASA linkage model are investigated. Reasons of analytic tractability made it advisable to study a radically simplified version with only two commodities, food and nonfood, and with rather simple assumptions on government objectives such as short-run minimization of the costs of agricultural policy.
One of the major results is that under apparently weak restrictions on the parameters of our model it is an optimal policy for all countries to supply as much food as possible, which seems to contradict commonly held intuitions. Of course, one still has to determine whether the same result would hold for more complex models and whether the conditions would really be satisfied by econometrically specified model parameters. The basic reason for this counter-intuitive result in our game model is that demand is specified by linear expenditure systems. We therefore suggest alternative demand specifications, which would avoid the difficulties of linear expenditure systems
Submillimeter CO emission from shock-heated gas in the L1157 outflow
We present the CO J=6-5, 4-3, and 3-2 spectra from the blueshifted gas of the
outflow driven by the low-mass class 0 protostar in the L1157 dark cloud.
Strong submillimeter CO emission lines with T_mb > 30 K have been detected at
63" (~0.13 pc) south from the protostar. It is remarkable that the blue wings
in the submillimeter lines are stronger by a factor of 3-4 than that of the CO
J=1-0 emission line. The CO line ratios suggest that the blueshifted lobe of
this outflow consists of moderately dense gas of n(H_2) = (1-3)x10^4 cm^-3
heated to T_kin = 50-170 K.It is also suggested that the kinetic temperature of
the outflowing gas increases from ~80 K near the protostar to ~170 K at the
shocked region in the lobe center, toward which the largest velocity dispersion
of the CO emission is observed. A remarkable correlation between the kinetic
temperature and velocity dispersion of the CO emission along the lobe provides
us with direct evidence that the molecular gas at the head of the jet-driven
bow shock is indeed heated kinematically. The lower temperature of ~80 K
measured at the other shocked region near the end of the lobe is explained if
this shock is in a later evolutionary stage, in which the gas has been cooled
mainly through radiation of the CO rotational lines.Comment: 10 pages, 4 PDF figures, APJL in pres
The ortho-to-para ratio of ammonia in the L1157 outflow
We have measured the ortho-to-para ratio of ammonia in the blueshifted gas of
the L1157 outflow by observing the six metastable inversion lines from (J, K) =
(1, 1) to (6, 6). The highly excited (5, 5) and (6, 6) lines were first
detected in the low-mass star forming regions. The rotational temperature
derived from the ratio of four transition lines from (3, 3) to (6, 6) is
130-140 K, suggesting that the blueshifted gas is heated by a factor of ~10 as
compared to the quiescent gas. The ortho-to-para ratio of the NH3 molecules in
the blueshifted gas is estimated to be 1.3--1.7, which is higher than the
statistical equilibrium value. This ratio provides us with evidence that the
NH3 molecules have been evaporated from dust grains with the formation
temperature between 18 and 25 K. It is most likely that the NH3 molecules on
dust grains have been released into the gas phase through the passage of strong
shock waves produced by the outflow. Such a scenario is supported by the fact
that the ammonia abundance in the blueshifted gas is enhanced by a factor of ~5
with respect to the dense quiescent gas.Comment: 16 pages, including 3 PS figures. To appear in the ApJ (Letters).
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Infall and Outflow around the HH 212 protostellar system
HH 212 is a highly collimated jet discovered in H2 powered by a young Class 0
source, IRAS 05413-0104, in the L1630 cloud of Orion. We have mapped around it
in 1.33 mm continuum, 12CO (), 13CO (), C18O (), and SO
() emission at \arcs{2.5} resolution with the
Submillimeter Array. A dust core is seen in the continuum around the source. A
flattened envelope is seen in C18O around the source in the equator
perpendicular to the jet axis, with its inner part seen in 13CO. The structure
and kinematics of the envelope can be roughly reproduced by a simple edge-on
disk model with both infall and rotation. In this model, the density of the
disk is assumed to have a power-law index of or -2, as found in other
low-mass envelopes. The envelope seems dynamically infalling toward the source
with slow rotation because the kinematics is found to be roughly consistent
with a free fall toward the source plus a rotation of a constant specific
angular momentum. A 12CO outflow is seen surrounding the H2 jet, with a narrow
waist around the source. Jetlike structures are also seen in 12CO near the
source aligned with the H2 jet at high velocities. The morphological
relationship between the H2 jet and the 12CO outflow, and the kinematics of the
12CO outflow along the jet axis are both consistent with those seen in a
jet-driven bow shock model. SO emission is seen around the source and the H2
knotty shocks in the south, tracing shocked emission around them.Comment: 17 pages, 11 figures, Accepted by the Ap
Heavy water around the L1448-mm protostar
Context: L1448-mm is the prototype of a low-mass Class 0 protostar driving a
high-velocity jet. Given its bright H2O spectra observed with ISO, L1448-mm is
an ideal laboratory to observe heavy water (HDO) emission. Aims: Our aim is to
image the HDO emission in the protostar surroundings, the possible occurrence
of HDO emission also investigating off L1448-mm, towards the molecular outflow.
Methods: We carried out observations of L1448-mm in the HDO(1_10-1_11) line at
80.6 GHz, an excellent tracer of HDO column density, with the IRAM Plateau de
Bure Interferometer. Results: We image for the first time HDO emission around
L1448-mm. The HDO structure reveals a main clump at velocities close to the
ambient one towards the the continuum peak that is caused by the dust heated by
the protostar. In addition, the HDO map shows tentative weaker emission at
about 2000 AU from the protostar towards the south, which is possibly
associated with the walls of the outflow cavity opened by the protostellar
wind. Conclusions: Using an LVG code, modelling the density and temperature
profile of the hot-corino, and adopting a gas temperature of 100 K and a
density of 1.5 10^8 cm^-3, we derive a beam diluted HDO column density of about
7 10^13 cm^-2, corresponding to a HDO abundance of about 4 10^-7. In addition,
the present map supports the scenario where HDO can be efficiently produced in
shocked regions and not uniquely in hot corinos heated by the newly born star.Comment: Accepted by A&A as Letter; 5 pages, 3 figure
A Flattened Protostellar Envelope in Absorption around L1157
Deep Spitzer IRAC images of L1157 reveal many of the details of the outflow
and the circumstellar environment of this Class 0 protostar. In IRAC band 4, 8
microns, there is a flattened structure seen in absorption against the
background emission. The structure is perpendicular to the outflow and is
extended to a diameter of 2 arcminutes. This structure is the first clear
detection of a flattened circumstellar envelope or pseudo-disk around a Class 0
protostar. Such a flattened morphology is an expected outcome for many collapse
theories that include magnetic fields or rotation. We construct an extinction
model for a power-law density profile, but we do not constrain the density
power-law index.Comment: ApJL accepte
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