880 research outputs found
Approximated maximum likelihood estimation in multifractal random walks
We present an approximated maximum likelihood method for the multifractal
random walk processes of [E. Bacry et al., Phys. Rev. E 64, 026103 (2001)]. The
likelihood is computed using a Laplace approximation and a truncation in the
dependency structure for the latent volatility. The procedure is implemented as
a package in the R computer language. Its performance is tested on synthetic
data and compared to an inference approach based on the generalized method of
moments. The method is applied to estimate parameters for various financial
stock indices.Comment: 8 pages, 3 figures, 2 table
Doing good by doing nothing? The role of social norms in explaining default effects in altruistic contexts
We explore whether the known preference for default options in choice contextsâdefault effectsâoccur in altruistic contexts and the extent to which this can be explained through appeal to social norms. In four experiments, we found that (i) participants were more likely to donate money to charity when this was the default option in an altruistic choice context; (ii) participants perceived the default option to be the socially normative option; (iii) perceptions of social norms mediated the relationship between default status and charitable donations; and (iv) a transfer effect, whereby participants translated social norms they inferred from the default option in one domain into behavior in a second, related domain. Theoretically, our analysis situates default effects within a comprehensive body of social psychological research concerning social norms and the attitudeâbehavior relationship, providing novel empirical predictions. Practically, these findings highlight that the way donation policies are framed can have an important impact on donation behavior: in our third study, we found that 81% donated half of their earnings for taking part in the experiment to charity when this was the default option, compared with only 19% when keeping the money was the default. Our work suggests that making use of default effects could be an effective tool to increase altruistic behavior without compromising freedo
Concern for others leads to vicarious optimism
An optimistic learning bias leads people to update their beliefs in response to better-than-expected good news but neglect worse-than-expected bad news. Because evidence suggests that this bias arises from self-concern, we hypothesized that a similar bias may affect beliefs about other peopleâs futures, to the extent that people care about others. Here, we demonstrated the phenomenon of vicarious optimism and showed that it arises from concern for others. Participants predicted the likelihood of unpleasant future events that could happen to either themselves or others. In addition to showing an optimistic learning bias for events affecting themselves, people showed vicarious optimism when learning about events affecting friends and strangers. Vicarious optimism for strangers correlated with generosity toward strangers, and experimentally increasing concern for strangers amplified vicarious optimism for them. These findings suggest that concern for others can bias beliefs about their future welfare and that optimism in learning is not restricted to oneself
Modelling CO emission from Mira's wind
We have modelled the circumstellar envelope of {\it o} Ceti (Mira) using new
observational constraints. These are obtained from photospheric light scattered
in near-IR vibrational-rotational lines of circumstellar CO molecules at 4.6
micron: absolute fluxes, the radial dependence of the scattered intensity, and
two line ratios. Further observational constraints are provided by ISO
observations of far-IR emission lines from highly excited rotational states of
the ground vibrational state of CO, and radio observations of lines from
rotational levels of low excitation of CO. A code based on the Monte-Carlo
technique is used to model the circumstellar line emission.
We find that it is possible to model the radio and ISO fluxes, as well as the
highly asymmetric radio-line profiles, reasonably well with a spherically
symmetric and smooth stellar wind model. However, it is not possible to
reproduce the observed NIR line fluxes consistently with a `standard model' of
the stellar wind. This is probably due to incorrectly specified conditions of
the inner regions of the wind model, since the stellar flux needs to be larger
than what is obtained from the standard model at the point of scattering, i.e.,
the intermediate regions at approximately 100-400 stellar radii (2"-7") away
from the star. Thus, the optical depth in the vibrational-rotational lines from
the star to the point of scattering has to be decreased. This can be
accomplished in several ways. For instance, the gas close to the star (within
approximately 2") could be in such a form that light is able to pass through,
either due to the medium being clumpy or by the matter being in radial
structures (which, further out, developes into more smooth or shell-like
structures).Comment: 18 pages, 3 figures, accepted for publication in Ap
Complex organics in IRAS 4A revisited with ALMA and PdBI: Striking contrast between two neighbouring protostellar cores
We used the Atacama Large (sub-)Millimeter Array (ALMA) and the IRAM Plateau
de Bure Interferometer (PdBI) to image, with an angular resolution of 0.5
(120 au) and 1 (235 au), respectively, the emission from 11 different
organic molecules in the protostellar binary NGC1333 IRAS 4A. We clearly
disentangled A1 and A2, the two protostellar cores present. For the first time,
we were able to derive the column densities and fractional abundances
simultaneously for the two objects, allowing us to analyse the chemical
differences between them. Molecular emission from organic molecules is
concentrated exclusively in A2 even though A1 is the strongest continuum
emitter. The protostellar core A2 displays typical hot corino abundances and
its deconvolved size is 70 au. In contrast, the upper limits we placed on
molecular abundances for A1 are extremely low, lying about one order of
magnitude below prestellar values. The difference in the amount of organic
molecules present in A1 and A2 ranges between one and two orders of magnitude.
Our results suggest that the optical depth of dust emission at these
wavelengths is unlikely to be sufficiently high to completely hide a hot corino
in A1 similar in size to that in A2. Thus, the significant contrast in
molecular richness found between the two sources is most probably real. We
estimate that the size of a hypothetical hot corino in A1 should be less than
12 au. Our results favour a scenario in which the protostar in A2 is either
more massive and/or subject to a higher accretion rate than A1, as a result of
inhomogeneous fragmentation of the parental molecular clump. This naturally
explains the smaller current envelope mass in A2 with respect to A1 along with
its molecular richness.Comment: Accepted in Astronomy and Astrophysic
The census of interstellar complex organic molecules in the Class I hot corino of SVS13-A
We present the first census of the interstellar Complex Organic Molecules
(iCOMs) in the low-mass Class I protostar SVS13-A, obtained by analysing data
from the IRAM-30m Large Project ASAI (Astrochemical Surveys At IRAM). They
consist of an high-sensitivity unbiased spectral survey at the 1mm, 2mm and 3mm
IRAM bands. We detected five iCOMs: acetaldehyde (CHCHO), methyl formate
(HCOOCH), dimethyl ether (CHOCH), ethanol (CHCHOH) and
formamide (NHCHO). In addition we searched for other iCOMs and ketene
(HCCO), formic acid (HCOOH) and methoxy (CHO), whose only ketene was
detected. The numerous detected lines, from 5 to 37 depending on the species,
cover a large upper level energy range, between 15 and 254 K. This allowed us
to carry out a rotational diagram analysis and derive rotational temperatures
between 35 and 110 K, and column densities between and
cm on the 0."3 size previously determined by
interferometric observations of glycolaldehyde. These new observations clearly
demonstrate the presence of a rich chemistry in the hot corino towards SVS13-A.
The measured iCOMs abundances were compared to other Class 0 and I hot corinos,
as well as comets, previously published in the literature. We find evidence
that (i) SVS13-A is as chemically rich as younger Class 0 protostars, and (ii)
the iCOMs relative abundances do not substantially evolve during the
protostellar phase.Comment: 24 pages, MNRAS in pres
Discovery of SiCSi in IRC+10216: A missing link between gas and dust carriers of SiC bonds
We report the discovery in space of a disilicon species, SiCSi, from
observations between 80 and 350 GHz with the IRAM 30m radio telescope. Owing to
the close coordination between laboratory experiments and astrophysics, 112
lines have now been detected in the carbon-rich star CWLeo. The derived
frequencies yield improved rotational and centrifugal distortion constants up
to sixth order. From the line profiles and interferometric maps with the
Submillimeter Array, the bulk of the SiCSi emis- sion arises from a region of 6
arcseconds in radius. The derived abundance is comparable to that of SiC2. As
expected from chemical equilibrium calculations, SiCSi and SiC2 are the most
abundant species harboring a SiC bond in the dust formation zone and certainly
both play a key role in the formation of SiC dust grains.Comment: To be published in the Astrophysical Journal Letters; Accepted May 6
201
Levy stable distributions via associated integral transform
We present a method of generation of exact and explicit forms of one-sided,
heavy-tailed Levy stable probability distributions g_{\alpha}(x), 0 \leq x <
\infty, 0 < \alpha < 1. We demonstrate that the knowledge of one such a
distribution g_{\alpha}(x) suffices to obtain exactly g_{\alpha^{p}}(x), p=2,
3,... Similarly, from known g_{\alpha}(x) and g_{\beta}(x), 0 < \alpha, \beta <
1, we obtain g_{\alpha \beta}(x). The method is based on the construction of
the integral operator, called Levy transform, which implements the above
operations. For \alpha rational, \alpha = l/k with l < k, we reproduce in this
manner many of the recently obtained exact results for g_{l/k}(x). This
approach can be also recast as an application of the Efros theorem for
generalized Laplace convolutions. It relies solely on efficient definite
integration.Comment: 12 pages, typos removed, references adde
Redshifted formaldehyde from the gravitational lens B0218+357
The gravitational lens toward B0218+357 offers the unique possibility to
study cool moderately dense gas with high sensitivity and angular resolution in
a cloud that existed half a Hubble time ago. Observations of the radio
continuum and six formaldehyde (H2CO) lines were carried out with the VLA, the
Plateau de Bure interferometer, and the Effelsberg 100-m telescope. Three radio
continuum maps indicate a flux density ratio between the two main images, A and
B, of ~ 3.4 +/- 0.2. Within the errors the ratio is the same at 8.6, 14.1, and
43 GHz. The 1_{01}-0_{00} line of para-H2CO is shown to absorb the continuum of
image A. Large Velocity Gradient radiative transfer calculations are performed
to reproduce the optical depths of the observed two cm-wave "K-doublet" and
four mm-wave rotational lines. These calculations also account for a likely
frequency-dependent continuum cloud coverage. Confirming the diffuse nature of
the cloud, an n(H2) density of < 1000 cm^{-3} is derived, with the best fit
suggesting n(H2) ~ 200 cm^{-3}. The H2CO column density of the main velocity
component is ~5 * 10^{13} cm^{-2}, to which about 7.5 * 10^{12} cm^{-2} has to
be added to also account for a weaker feature on the blue side, 13 km/s apart.
N(H2CO)/N(NH3) ~ 0.6, which is four times less than the average ratio obtained
from a small number of local diffuse (galactic) clouds seen in absorption. The
ortho-to-para H2CO abundance ratio is 2.0 - 3.0, which is consistent with the
kinetic temperature of the molecular gas associated with the lens of B0218+357.
With the gas kinetic temperature and density known, it is found that optically
thin transitions of CS, HCN, HNC, HCO+, and N2H+ (but not CO) will provide
excellent probes of the cosmic microwave background at redshift z=0.68.Comment: Accepted for A&A, 8 Pages, 3 Figures, 5 Table
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