768 research outputs found
Destabilising conventions using temporary interventions
Conventions are an important concept in multi-agent systems as they allow increased coordination amongst agents and hence a more efficient system. Encouraging and directing convention emergence has been the focus of much research, particularly through the use of fixed strategy agents. In this paper we apply temporary interventions using fixed strategy agents to destabilise an established convention by (i) replacing it with another convention of our choosing, and (ii) allowing it to destabilise in such a way that no other convention explicitly replaces it. We show that these interventions are effective and investigate the minimum level of intervention needed
AKARI observations of ice absorption bands towards edge-on YSOs
To investigate the composition and evolution of circumstellar ice around
low-mass YSOs, we observed ice absorption bands in the near infrared (NIR)
towards eight YSOs ranging from class 0 to class II, among which seven are
associated with edge-on disks. We performed slit-less spectroscopic
observations using the grism mode of the Infrared Camera (IRC) on board AKARI,
which enables us to obtain full NIR spectra from 2.5 m to 5 m. The
spectra were fitted with polynomial baselines to derive the absorption spectra.
The molecular absorption bands were then fitted with the laboratory database of
ice absorption bands, considering the instrumental line profile and the
spectral resolution of the grism dispersion element. Towards the class 0-I
sources (L1527, IRC-L1041-2, and IRAS04302), absorption bands of HO,
CO, CO, and XCN are clearly detected. Column density ratios of CO ice
and CO ice relative to HO ice are 21-28% and 13-46%, respectively. If XCN
is OCN, its column density is as high as 2-6% relative to HO ice. The
HDO ice feature at 4.1 m is tentatively detected towards the class 0-I
sources and HV Tau. Non-detections of the CH-stretching mode features around
3.5 m provide upper limits to the CHOH abundance of 26% (L1527) and
42% (IRAS04302) relative to HO. We tentatively detect OCS ice absorption
towards IRC-L1041-2. Towards class 0-I sources, the detected features should
mostly originate in the cold envelope, while CO gas and OCN could originate
in the region close to the protostar, where there are warm temperatures and UV
radiation. We detect HO ice band towards ASR41 and 2MASSJ1628137-243139,
which are edge-on class II disks. We also detect HO ice and CO ice
towards HV Tau, HK Tau, and UY Aur, and tentatively detect CO gas features
towards HK Tau and UY Aur.Comment: Accepted to A&
Discovery of the Coldest Imaged Companion of a Sun-Like Star
We present the discovery of a brown dwarf or possible planet at a projected
separation of 1.9" = 29 AU around the star GJ 758, placing it between the
separations at which substellar companions are expected to form by core
accretion (~5 AU) or direct gravitational collapse (typically >100 AU). The
object was detected by direct imaging of its thermal glow with Subaru/HiCIAO.
At 10-40 times the mass of Jupiter and a temperature of 550-640 K, GJ 758 B
constitutes one of the few known T-type companions, and the coldest ever to be
imaged in thermal light around a Sun-like star. Its orbit is likely eccentric
and of a size comparable to Pluto's orbit, possibly as a result of
gravitational scattering or outward migration. A candidate second companion is
detected at 1.2" at one epoch.Comment: 5 pages, 3 figures, 2 tables. Accepted for publication in ApJ Letter
Five-Year Optical and Near Infrared Observations of the Extremely Slow Nova V1280 Scorpii
We present optical (, , , and ) and near
infrared (, and ) photometric and spectroscopic observations
of a classical nova V1280 Scorpii for five years from 2007 to 2011. Our
photometric observations show a declining event in optical bands shortly after
the maximum light which continues 250 days. The event is most probably
caused by a dust formation. The event is accompanied by a short ( 30
days) re-brightening episode ( 2.5 mag in ), which suggests a
re-ignition of the surface nuclear burning. After 2008, the band
observations show a very long plateau at around = 10.5 for more than 1000
days until April 2011 ( 1500 days after the maximum light). The nova had
taken a very long time ( 50 months) before entering the nebular phase
(clear detection of both [\ion{O}{iii}] 4959 and 5007) and is still continuing
to generate the wind caused by H-burning. The finding suggests that V1280 Sco
is going through the historically slowest evolution. The interval from the
maximum light (2007 February 16) to the beginning of the nebular phase is
longer than any previously known slow novae: V723 Cas (18 months), RR Pic (10
months), or HR Del (8 months). It suggests that the mass of a white dwarf in
the V1280 Sco system might be 0.6 M_\mathrm{\sun} or smaller. The distance,
based on our measurements of the expansion velocity combined with the directly
measured size of the dust shell, is estimated to be 1.1 0.5 kpc.Comment: 17 pages, 14 figures, accepted for publication in A&
A large geometric distortion in the first photointermediate of rhodopsin, determined by double-quantum solid-state NMR
Double-quantum magic-angle-spinning NMR experiments were performed on 11,12-C-13(2)-retinylidene-rhodopsin under illumination at low temperature, in order to characterize torsional angle changes at the C11-C12 photoisomerization site. The sample was illuminated in the NMR rotor at low temperature (similar to 120 K) in order to trap the primary photointermediate, bathorhodopsin. The NMR data are consistent with a strong torsional twist of the HCCH moiety at the isomerization site. Although the HCCH torsional twist was determined to be at least 40A degrees, it was not possible to quantify it more closely. The presence of a strong twist is in agreement with previous Raman observations. The energetic implications of this geometric distortion are discussed
Occasional errors can benefit coordination
The chances solving a problem that involves coordination between people are increased by introducing robotic players that sometimes make mistakes. This finding has implications for real-world coordination problems
Imaging of a Transitional Disk Gap in Reflected Light: Indications of Planet Formation Around the Young Solar Analog LkCa 15
We present H- and Ks-band imaging data resolving the gap in the transitional
disk around LkCa 15, revealing the surrounding nebulosity. We detect sharp
elliptical contours delimiting the nebulosity on the inside as well as the
outside, consistent with the shape, size, ellipticity, and orientation of
starlight reflected from the far-side disk wall, whereas the near-side wall is
shielded from view by the disk's optically thick bulk. We note that
forward-scattering of starlight on the near-side disk surface could provide an
alternate interpretation of the nebulosity. In either case, this discovery
provides confirmation of the disk geometry that has been proposed to explain
the spectral energy distributions (SED) of such systems, comprising an
optically thick outer disk with an inner truncation radius of ~46 AU enclosing
a largely evacuated gap. Our data show an offset of the nebulosity contours
along the major axis, likely corresponding to a physical pericenter offset of
the disk gap. This reinforces the leading theory that dynamical clearing by at
least one orbiting body is the cause of the gap. Based on evolutionary models,
our high-contrast imagery imposes an upper limit of 21 Jupiter masses on
companions at separations outside of 0.1" and of 13 Jupiter masses outside of
0.2". Thus, we find that a planetary system around LkCa 15 is the most likely
explanation for the disk architecture.Comment: 5 pages, 4 figures, accepted for publication in ApJ Letters. Minor
change to Figure
Direct Imaging of a Cold Jovian Exoplanet in Orbit around the Sun-like Star GJ 504
Several exoplanets have recently been imaged at wide separations of >10 AU
from their parent stars. These span a limited range of ages (<50 Myr) and
atmospheric properties, with temperatures of 800--1800 K and very red colors (J
- H > 0.5 mag), implying thick cloud covers. Furthermore, substantial model
uncertainties exist at these young ages due to the unknown initial conditions
at formation, which can lead to an order of magnitude of uncertainty in the
modeled planet mass. Here, we report the direct imaging discovery of a Jovian
exoplanet around the Sun-like star GJ 504, detected as part of the SEEDS
survey. The system is older than all other known directly-imaged planets; as a
result, its estimated mass remains in the planetary regime independent of
uncertainties related to choices of initial conditions in the exoplanet
modeling. Using the most common exoplanet cooling model, and given the system
age of 160 [+350, -60] Myr, GJ 504 b has an estimated mass of 4 [+4.5, -1.0]
Jupiter masses, among the lowest of directly imaged planets. Its projected
separation of 43.5 AU exceeds the typical outer boundary of ~30 AU predicted
for the core accretion mechanism. GJ 504 b is also significantly cooler (510
[+30, -20] K) and has a bluer color (J-H = -0.23 mag) than previously imaged
exoplanets, suggesting a largely cloud-free atmosphere accessible to
spectroscopic characterization. Thus, it has the potential of providing novel
insights into the origins of giant planets, as well as their atmospheric
properties.Comment: 20 pages, 12 figures, Accepted for publication in ApJ. Minor updates
from the version
Spitzer and HHT observations of starless cores: masses and environments
We present Spitzer observations of a sample of 12 starless cores selected to
have prominent 24 micron shadows. The Spitzer images show 8 and 24 micron
shadows and in some cases 70 micron shadows; these spatially resolved
absorption features trace the densest regions of the cores. We have carried out
a 12CO (2-1) and 13CO (2-1) mapping survey of these cores with the Heinrich
Hertz Telescope (HHT). We use the shadow features to derive optical depth maps.
We derive molecular masses for the cores and the surrounding environment; we
find that the 24 micron shadow masses are always greater than or equal to the
molecular masses derived in the same region, a discrepancy likely caused by CO
freeze--out onto dust grains. We combine this sample with two additional cores
that we studied previously to bring the total sample to 14 cores. Using a
simple Jeans mass criterion we find that ~ 2/3 of the cores selected to have
prominent 24 micron shadows are collapsing or near collapse, a result that is
supported by millimeter line observations. Of this subset at least half have
indications of 70 micron shadows. All cores observed to produce absorption
features at 70 micron are close to collapse. We conclude that 24 micron
shadows, and even more so the 70 micron ones, are useful markers of cloud cores
that are approaching collapse.Comment: 41 pages, 28 figures, 5 tables; accepted by Ap
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