985 research outputs found
VLT Spectropolarimetry of the optical transient in NGC300. Evidence for asymmetry in the circumstellar dust
AIMS: The main goal of this work is to study possible signs of asymmetry in
the bright optical transient in NGC300, with the aim of getting independent
information on the explosion mechanism, the progenitor star and its
circumstellar environment.
METHODS: Using VLT-FORS1 we have obtained low-resolution optical linear
spectropolarimetry of NGC300 OT2008-1 on two epochs, 48 and 55 days after the
discovery, covering the spectral range 3600--9330A.
RESULTS: The data show a continuum polarization at a very significant level.
At least two separate components are identified. The first is characterized by
a strong wavelength dependency and a constant position angle (68.6+/-0.3
degrees), which is parallel to the local spiral arm of the host galaxy. The
second shows a completely different position angle (151.3+/-0.4) and displays a
mild but statistically significant evolution between the two epochs. While the
former is identified as arising in the interstellar dust associated with
NGC300, the latter is most likely due to continuum polarization by dust
scattering in the circumstellar environment. No line depolarization is detected
in correspondence of the most intense emission lines, disfavoring electron
scattering as the source of intrinsic polarization. This implies a very small
deviation from symmetry in the continuum-forming region. Given the observed
level of intrinsic polarization, the transient must be surrounded by a
significant amount of dust (>4x10^-5 Msun), asymmetrically distributed within a
few thousand AU. This most likely implies that one or more asymmetric outflow
episodes took place during the past history of the progenitor.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 16
figure
Supernova 2002ic: the collapse of a stripped-envelope, massive star in a dense medium ?
We revisit the case of SN2002ic that recently revived the debate about the
progenitors of SNeIa after the claim of the unprecedented presence of hydrogen
lines over a diluted SNIa spectrum. As an alternative to the previous
interpretation, we suggest that SN2002ic actually was a type Ic SN, the core
collapse of a massive star which lost its hydrogen and helium envelope. In this
scenario the observed interaction with a dense circumstellar material (CSM) is
the predictable consequence of the intense mass-loss of the progenitor and/or
of the presence of a gas rich environment. With this view we establish a link
between energetic SNeIc and highly interacting SNeIIn and add some credits to
the proposed association of some SNeIIn to GRBs.Comment: Accepted for publication on ApJ
Biologically active Phytophthora mating hormone prepared by catalytic asymmetric total synthesis
A Phytophthora mating hormone with an array of 1,5-stereogenic centers has been synthesized by using our recently developed methodology of catalytic enantioselective conjugate addition of Grignard reagents. We applied this methodology in a diastereo- and enantioselective iterative route and obtained two of the 16 possible stereoisomers of Phytophthora hormone α1. These synthetic stereoisomers induced the formation of sexual spores (oospores) in A2 mating type strains of three heterothallic Phytophthora species, P. infestans, P. capsici, and P. nicotianae but not in A1 mating type strains. The response was concentration-dependent, and the oospores were viable. These results demonstrate that the biological activity of the synthetic hormone resembles that of the natural hormone α1. Mating hormones are essential components in the sexual life cycle of a variety of organisms. For plant pathogens like Phytophthora, sexual reproduction is important as a source of genetic variation. Moreover, the thick-walled oospores are the most durable propagules that can survive harsh environmental conditions. Sexual reproduction can thus greatly affect disease epidemics. The availability of synthetic compounds mimicking the activity of Phytophthora mating hormone will be instrumental for further unravelling sexual reproduction in this important group of plant pathogens.
SN 2006gy: was it really extra-ordinary?
We present an optical photometric and spectroscopic study of the very
luminous type IIn SN 2006gy for a time period spanning more than one year. In
photometry, a broad, bright (M_R~-21.7) peak characterizes all BVRI light
curves. Afterwards, a rapid luminosity fading is followed by a phase of slow
luminosity decline between day ~170 and ~237. At late phases (>237 days),
because of the large luminosity drop (>3 mag), only upper visibility limits are
obtained in the B, R and I bands. In the near-infrared, two K-band detections
on days 411 and 510 open new issues about dust formation or IR echoes
scenarios. At all epochs the spectra are characterized by the absence of broad
P-Cygni profiles and a multicomponent Halpha profile, which are the typical
signatures of type IIn SNe. After maximum, spectroscopic and photometric
similarities are found between SN 2006gy and bright, interaction-dominated SNe
(e.g. SN 1997cy, SN 1999E and SN 2002ic). This suggests that ejecta-CSM
interaction plays a key role in SN 2006gy about 6 to 8 months after maximum,
sustaining the late-time-light curve. Alternatively, the late luminosity may be
related to the radioactive decay of ~3M_sun of 56Ni. Models of the light curve
in the first 170 days suggest that the progenitor was a compact star (R~6-8
10^(12)cm, M_ej~5-14M_sun), and that the SN ejecta collided with massive
(6-10M_sun), opaque clumps of previously ejected material. These clumps do not
completely obscure the SN photosphere, so that at its peak the luminosity is
due both to the decay of 56Ni and to interaction with CSM. A supermassive star
is not required to explain the observational data, nor is an extra-ordinarily
large explosion energy.Comment: 33 pages, 8 figures. Accepted by ApJ. Paper with high-resolution
figures available at
http://web.oapd.inaf.it/supern/sn2006gy_astroph/agnoletto_2006gy.pd
Moderately Luminous type II Supernovae
Core-collapse Supernovae (CC-SNe) descend from progenitors more massive than
about 8 Msun. Because of the young age of the progenitors, the ejecta may
eventually interact with the circumstellar medium (CSM) via highly energetic
processes detectable in the radio, X-ray, ultraviolet (UV) and, sometimes, in
the optical domains. In this paper we present ultraviolet, optical and near
infrared observations of five type II SNe, namely SNe 2009dd, 2007pk, 2010aj,
1995ad, and 1996W. Together with few other SNe they form a group of moderately
luminous type II events. We collected photometry and spectroscopy with several
telescopes in order to construct well-sampled light curves and spectral
evolutions from the photospheric to the nebular phases. Both photometry and
spectroscopy indicate a degree of heterogeneity in this sample. The light
curves have luminous peak magnitudes (). The ejected
masses of ^56\ni for three SNe span a wide range of values
(MsunM(\ni)Msun), while for a fourth
(SN2010aj) we could determine a stringent upper limit (Msun).
Clues of interaction, such as the presence of high velocity (HV) features of
the Balmer lines, are visible in the photospheric spectra of SNe 2009dd and
1996W. For SN2007pk we observe a spectral transition from a type IIn to a
standard type II SN. Modelling the observations of SNe 2009dd, 2010aj and
1995ad with radiation hydrodynamics codes, we infer kinetic plus thermal
energies of about 0.2-0.5 foe, initial radii of 2-5 cm and
ejected masses of 5.0-9.5 Msun. These values suggest moderate-mass,
super-asymptotic giant branch (SAGB) or red super-giants (RSG) stars as SN
precursors, in analogy with other luminous type IIP SNe 2007od and 2009bw.Comment: 28 pages, 27 fig, accepted by A&A, 3 pages of online material,
abstract abridged. revised significantly with respect to the previous versio
Behaviors of susceptible-infected epidemics on scale-free networks with identical infectivity
In this article, we proposed a susceptible-infected model with identical
infectivity, in which, at every time step, each node can only contact a
constant number of neighbors. We implemented this model on scale-free networks,
and found that the infected population grows in an exponential form with the
time scale proportional to the spreading rate. Further more, by numerical
simulation, we demonstrated that the targeted immunization of the present model
is much less efficient than that of the standard susceptible-infected model.
Finally, we investigated a fast spreading strategy when only local information
is available. Different from the extensively studied path finding strategy, the
strategy preferring small-degree nodes is more efficient than that preferring
large-degree nodes. Our results indicate the existence of an essential
relationship between network traffic and network epidemic on scale-free
networks.Comment: 5 figures and 7 page
Cascaded four-wave mixing in tapered plasmonic nanoantenna
We study theoretically the cascaded four-wave mixing (FWM) in broadband
tapered plasmonic nanoantennas and demonstrate a 300-fold increase in nonlinear
frequency conversion detected in the main lobe of the nanoantenna far-field
pattern. This is achieved by tuning the elements of the nanoantenna to resonate
frequencies involved into the FWM interaction. Our findings have a potentially
broad application in ultrafast nonlinear spectroscopy, sensing, on-chip optical
frequency conversion, nonlinear optical metamaterials and photon sources
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