7,546 research outputs found
The UN in the lab
We consider two alternatives to inaction for governments combating terrorism, which we term Defense and Prevention. Defense consists of investing in resources that reduce the impact of an attack, and generates a negative externality to other governments, making their countries a more attractive objective for terrorists. In contrast, Prevention, which consists of investing in resources that reduce the ability of the terrorist organization to mount an attack, creates a positive externality by reducing the overall threat of terrorism for all. This interaction is captured using a simple 3×3 “Nested Prisoner’s Dilemma” game, with a single Nash equilibrium where both countries choose Defense. Due to the structure of this interaction, countries can benefit from coordination of policy choices, and international institutions (such as the UN) can be utilized to facilitate coordination by implementing agreements to share the burden of Prevention. We introduce an institution that implements a burden-sharing policy for Prevention, and investigate experimentally whether subjects coordinate on a cooperative strategy more frequently under different levels of cost sharing. In all treatments, burden sharing leaves the Prisoner’s Dilemma structure and Nash equilibrium of the game unchanged. We compare three levels of burden sharing to a baseline in a between-subjects design, and find that burden sharing generates a non-linear effect on the choice of the efficient Prevention strategy and overall performance. Only an institution supporting a high level of mandatory burden sharing generates a significant improvement in the use of the Prevention strategy
An Extinction Study of the Taurus Dark Cloud Complex
We present a study of the detailed distribution of extinction in a region of
the Taurus dark cloud complex. Our study uses new BVR images of the region,
spectral classification data for 95 stars, and IRAS Sky Survey Atlas (ISSA) 60
and 100 micron images. We study the extinction of the region in four different
ways, and we present the first inter-comparison of all these methods, which
are: 1) using the color excess of background stars for which spectral types are
known; 2) using the ISSA 60 and 100 micron images; 3) using star counts; and 4)
using an optical (V and R) version of the average color excess method used by
Lada et al. (1994). We find that all four methods give generally similar
results, with important exceptions. To study the structure in the dust
distribution, we compare the ISSA extinction and the extinction measured for
individual stars. From the comparison, we conclude that in the relatively low
extinction regions studied, with 0.9 < A_V < 3.0 mag (away from filamentary
dark clouds and IRAS cores), there are no fluctuations in the dust column
density greater than 45% (at the 99.7% confidence level), on scales smaller
than 0.2 pc. We also report the discovery of a previously unknown stellar
cluster behind the Taurus dark cloud near R.A 4h19m00s, Dec. 27:30:00 (B1950)Comment: 49 pages (which include 6 pages of tables and 6 pages of figures
A New Galactic Extinction Map of the Cygnus Region
We have made a Galactic extinction map of the Cygnus region with 5' spatial
resolution. The selected area is 80^\circ to 90^\circ in the Galactic longitude
and -4^\circ to 8^\circ in the Galactic latitude. The intensity at 140 \mum is
derived from the intensities at 60 and 100 \mum of the IRAS data using the
tight correlation between 60, 100, and 140 \mum found in the Galactic plane.
The dust temperature and optical depth are calculated with 5' resolution from
the 140 and 100 \mum intensity, and Av is calculated from the optical depth. In
the selected area, the mean dust temperature is 17 K, the minimum is 16 K, and
the maximum is 30 K. The mean Av is 6.5 mag, the minimum is 0.5 mag, and the
maximum is 11 mag. The dust temperature distribution shows significant spatial
variation on smaller scales down to 5'. Because the present study can trace the
5'-scale spatial variation of the extinction, it has an advantage over the
previous studies, such as the one by Schlegel, Finkbeiner, & Davis, who used
the COBE/DIRBE data to derive the dust temperature distribution with a spatial
resolution of 1^\circ. The difference of Av between our map and Schlegel et
al.'s is \pm 3 mag. A new extinction map of the entire sky can be produced by
applying the present method.Comment: 27 pages, 14 figures, accepted for publication in Ap
TMC-1C: an accreting starless core
We have mapped the starless core TMC-1C in a variety of molecular lines with
the IRAM 30m telescope. High density tracers show clear signs of
self-absorption and sub-sonic infall asymmetries are present in N2H+ (1-0) and
DCO+ (2-1) lines. The inward velocity profile in N2H+ (1-0) is extended over a
region of about 7,000 AU in radius around the dust continuum peak, which is the
most extended ``infalling'' region observed in a starless core with this
tracer. The kinetic temperature (~12 K) measured from C17O and C18O suggests
that their emission comes from a shell outside the colder interior traced by
the mm continuum dust. The C18O (2-1) excitation temperature drops from 12 K to
~10 K away from the center. This is consistent with a volume density drop of
the gas traced by the C18O lines, from ~4x10^4 cm^-3 towards the dust peak to
~6x10^3 cm^-3 at a projected distance from the dust peak of 80" (or 11,000 AU).
The column density implied by the gas and dust show similar N2H+ and CO
depletion factors (f_D < 6). This can be explained with a simple scenario in
which: (i) the TMC-1C core is embedded in a relatively dense environment (H2
~10^4 cm^-3), where CO is mostly in the gas phase and the N2H+ abundance had
time to reach equilibrium values; (ii) the surrounding material (rich in CO and
N2H+) is accreting onto the dense core nucleus; (iii) TMC-1C is older than
3x10^5 yr, to account for the observed abundance of N2H+ across the core
(~10^-10 w.r.t. H2); and (iv) the core nucleus is either much younger (~10^4
yr) or ``undepleted'' material from the surrounding envelope has fallen towards
it in the past 10,000 yr.Comment: 29 pages, including 5 tables and 15 figure
The NH3-Abating effect of Slurry Injection as Influenced by Soil Moisture in a Semiarid Arable Soil
Accumulation of large volumes of dilute slurries is considered one of the major problems related to intensive farming (Sommer et al., 2004). In the EU-27, more than
half of the total N excretion is applied to croplands due to technical advantages for farmers (e.g. reuse of nutrients). However, the N use efficiency of slurries produced by livestock is low, i.e. only 20-52% of the excreted N is recovered by crops. Much of the remainder can be lost into the atmosphere as ammonia (NH3), nitrous oxide (N2O),
dinitrogen (N2) and nitrogen oxides (NOx)
Warm SiO gas in molecular bullets associated with protostellar outflows
In this paper we present the first SiO multiline analysis (from J=2-1 to
J=11-10) of the molecular bullets along the outflows of the Class 0 sources
L1448-mm and L1157-mm, obtained through observations with IRAM and JCMT. We
have computed the main physical parameters in each bullet and compared them
with other tracers of warm and dense gas and with models for the SiO excitation
in shocks. We find that the bullets close to L1448--mm, associated with high
velocity gas, have higher excitation conditions (n(H2) ~ 10^{6} cm^{-3}, T >
500 K) with respect to the L1157 bullets (n(H2) ~1-5 10^{5} cm^{-3}, T ~
100-300 K). In both the sources, there is a clear evidence of the presence of
velocity components having different excitation conditions, with the denser
and/or warmer gas associated with the gas at the higher speed. In L1448 the
bulk of the emission is due to the high-excitation and high velocity gas, while
in L1157 most of the emission comes from the low excitation gas at ambient
velocity. The observed velocity-averaged line ratios are well reproduced by
shocks with speeds v_s larger than ~ 30 km/s and densities ~ 10^{5} - 10^{6}
cm^{-3}. Plane-parallel shock models, however, fail to predict all the observed
line profiles and in particular the very similar profiles shown by both low and
high excitation lines. The overall observations support the idea that the L1157
clumps are shock interaction events older than the L1448 bullets close to the
driving source. In the latter objects, the velocity structure and the
variations of physical parameters with the velocity resemble very closely those
found in optical/IR jets near the protostar, suggesting that similar launching
and excitation mechanisms are also at the origin of collimated jets seen at
millimetre wavelengths.Comment: 11pages, 9 figures, A&A accepte
Jets and Outflows From Star to Cloud: Observations Confront Theory
In this review we focus on the role jets and outflows play in the star and
planet formation process. Our essential question can be posed as follows: are
jets/outflows merely an epiphenomenon associated with star formation or do they
play an important role in mediating the physics of assembling stars both
individually and globally? We address this question by reviewing the current
state of observations and their key points of contact with theory. Our review
of jet/outflow phenomena is organized into three length-scale domains: Source
and Disk Scales ( au) where the connection with protostellar and disk
evolution theories is paramount; Envelope Scales ( au) where the
chemistry and propagation shed further light on the jet launching process, its
variability and its impact on the infalling envelope; Parent Cloud Scales
( au) where global momentum injection into cluster/cloud
environments become relevant. Issues of feedback are of particular importance
on the smallest scales where planet formation regions in a disk may be impacted
by the presence of disk winds, irradiation by jet shocks or shielding by the
winds. Feedback on envelope scales may determine the final stellar mass
(core-to-star efficiency) and envelope dissipation. Feedback also plays an
important role on the larger scales with outflows contributing to turbulent
support within clusters including alteration of cluster star formation
efficiencies (feedback on larger scales currently appears unlikely). A
particularly novel dimension of our review is that we consider results on jet
dynamics from the emerging field of High Energy Density Laboratory Astrophysics
(HEDLA). HEDLA is now providing direct insights into the 3-D dynamics of fully
magnetized, hypersonic, radiative outflows.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C.
Dullemond, Th. Hennin
Wide-Field Infrared Imaging Polarimetry of the NGC 6334 Region: A Nest of Infrared Reflection Nebulae
We report the detection of eighteen infrared reflection nebulae (IRNe) in the
, , & linear polarimetric observations of the NGC 6334 massive
star-formation complex, of which 16 IRNe are new discoveries. Our images cover
180 square arcminutes, one of the widest near-infrared polarization data
in star-formation regions so far. These IRNe are most likely associated with
embedded young OB stars at different evolutionary phases, showing a variety of
sizes, morphologies, and polarization properties, which can be divided into
four categories. We argue the different nebula characteristics to be a possible
evolutionary sequence of circumstellar structures around young massive stars.Comment: 4 pages, 1 figur
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