756 research outputs found
An Intermediate Luminosity Transient in NGC300: The Eruption of a Dust-Enshrouded Massive Star
[abridged] We present multi-epoch high-resolution optical spectroscopy,
UV/radio/X-ray imaging, and archival Hubble and Spitzer observations of an
intermediate luminosity optical transient recently discovered in the nearby
galaxy NGC300. We find that the transient (NGC300 OT2008-1) has a peak absolute
magnitude of M_bol~-11.8 mag, intermediate between novae and supernovae, and
similar to the recent events M85 OT2006-1 and SN2008S. Our high-resolution
spectra, the first for this event, are dominated by intermediate velocity
(~200-1000 km/s) hydrogen Balmer lines and CaII emission and absorption lines
that point to a complex circumstellar environment, reminiscent of the yellow
hypergiant IRC+10420. In particular, we detect broad CaII H&K absorption with
an asymmetric red wing extending to ~1000 km/s, indicative of gas infall onto a
massive and relatively compact star (blue supergiant or Wolf-Rayet star); an
extended red supergiant progenitor is unlikely. The origin of the inflowing gas
may be a previous ejection from the progenitor or the wind of a massive binary
companion. The low luminosity, intermediate velocities, and overall similarity
to a known eruptive star indicate that the event did not result in a complete
disruption of the progenitor. We identify the progenitor in archival Spitzer
observations, with deep upper limits from Hubble data. The spectral energy
distribution points to a dust-enshrouded star with a luminosity of about 6x10^4
L_sun, indicative of a ~10-20 M_sun progenitor (or binary system). This
conclusion is in good agreement with our interpretation of the outburst and
circumstellar properties. The lack of significant extinction in the transient
spectrum indicates that the dust surrounding the progenitor was cleared by the
outburst.Comment: Submitted to ApJ; emulateapj style; 39 pages; 26 figure
Stem cell differentiation increases membrane-actin adhesion regulating cell blebability, migration and mechanics
This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder in order to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/K. S. is funded by an EPSRC PhD studentship. S.T. is funded by an EU Marie Curie Intra European Fellowship (GENOMICDIFF)
Relationship between Expression of the Family of M Proteins and Lipoteichoic Acid to Hydrophobicity and Biofilm Formation in Streptococcus pyogenes
Background: Hydrophobicity is an important attribute of bacteria that contributes to adhesion and biofilm formation. Hydrophobicity of Streptococcus pyogenes is primarily due to lipoteichoic acid (LTA) on the streptococcal surface but the mechanism(s) whereby LTA is retained on the surface is poorly understood. In this study, we sought to determine whether members of the M protein family consisting of Emm (M protein), Mrp (M-related protein), Enn (an M-like protein), and the streptococcal protective antigen (Spa) are involved in anchoring LTA in a manner that contributes to hydrophobicity of the streptococci and its ability to form biofilms. Methodology/Principal Findings: Isogenic mutants defective in expression of emm, mrp, enn, and/or spa genes of eight different serotypes and their parental strains were tested for differences in LTA bound to surface proteins, LTA released into the culture media, and membrane-bound LTA. The effect of these mutations on the ability of streptococci to form a hydrophobic surface and to generate biofilms was also investigated. A recombinant strain overexpressing Emm1 was also engineered and similarly tested. The serotypes tested ranged from those that express only a single M protein gene to those that express two or three members of the M protein family. Overexpression of Emm1 led to enhanced hydrophobicity an
Default Risk and Equity Returns: A Comparison of the Bank-Based German and the U.S. Financial System
In this paper, we address the question whether the impact of default risk on equity returns depends on the financial system firms operate in. Using an implementation of Merton's option-pricing model for the value of equity to estimate firms' default risk, we construct a factor that measures the excess return of firms with low default risk over firms with high default risk. We then compare results from asset pricing tests for the German and the U.S. stock markets. Since Germany is the prime example of a bank-based financial system, where debt is supposedly a major instrument of corporate governance, we expect that a systematic default risk effect on equity returns should be more pronounced for German rather than U.S. firms. Our evidence suggests that a higher firm default risk systematically leads to lower returns in both capital markets. This contradicts some previous results for the U.S. by Vassalou/Xing (2004), but we show that their default risk factor looses its explanatory power if one includes a default risk factor measured as a factor mimicking portfolio. It further turns out that the composition of corporate debt affects equity returns in Germany. Firms' default risk sensitivities are attenuated the more a firm depends on bank debt financing
Interacting Supernovae: Types IIn and Ibn
Supernovae (SNe) that show evidence of strong shock interaction between their
ejecta and pre-existing, slower circumstellar material (CSM) constitute an
interesting, diverse, and still poorly understood category of explosive
transients. The chief reason that they are extremely interesting is because
they tell us that in a subset of stellar deaths, the progenitor star may become
wildly unstable in the years, decades, or centuries before explosion. This is
something that has not been included in standard stellar evolution models, but
may significantly change the end product and yield of that evolution, and
complicates our attempts to map SNe to their progenitors. Another reason they
are interesting is because CSM interaction is an efficient engine for making
bright transients, allowing super-luminous transients to arise from normal SN
explosion energies, and allowing transients of normal SN luminosities to arise
from sub-energetic explosions or low radioactivity yield. CSM interaction
shrouds the fast ejecta in bright shock emission, obscuring our normal view of
the underlying explosion, and the radiation hydrodynamics of the interaction is
challenging to model. The CSM interaction may also be highly non-spherical,
perhaps linked to binary interaction in the progenitor system. In some cases,
these complications make it difficult to definitively tell the difference
between a core-collapse or thermonuclear explosion, or to discern between a
non-terminal eruption, failed SN, or weak SN. Efforts to uncover the physical
parameters of individual events and connections to possible progenitor stars
make this a rapidly evolving topic that continues to challenge paradigms of
stellar evolution.Comment: Final draft of a chapter in the "SN Handbook". Accepted. 25 pages, 3
fig
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
Probing the atmosphere of a solar-like star by galactic microlensing at high magnification
We report a measurement of limb darkening of a solar-like star in the very
high magnification microlensing event MOA 2002-BLG-33. A 15 hour deviation from
the light curve profile expected for a single lens was monitored intensively in
V and I passbands by five telescopes spanning the globe. Our modelling of the
light curve showed the lens to be a close binary system whose centre-of-mass
passed almost directly in front of the source star. The source star was
identified as an F8-G2 main sequence turn-off star. The measured stellar
profiles agree with current stellar atmosphere theory to within ~4% in two
passbands. The effective angular resolution of the measurements is <1
micro-arcsec. These are the first limb darkening measurements obtained by
microlensing for a Solar-like star.Comment: Accepted for publication in A&A Letters. 5 pages, 2 embedded colour
ps figures plus 1 jpg figure. Version with all figures embedded available
from: http://www.roe.ac.uk/~iab/moa33paper
Supernova 2007bi as a pair-instability explosion
Stars with initial masses 10 M_{solar} < M_{initial} < 100 M_{solar} fuse
progressively heavier elements in their centres, up to inert iron. The core
then gravitationally collapses to a neutron star or a black hole, leading to an
explosion -- an iron-core-collapse supernova (SN). In contrast, extremely
massive stars (M_{initial} > 140 M_{solar}), if such exist, have oxygen cores
which exceed M_{core} = 50 M_{solar}. There, high temperatures are reached at
relatively low densities. Conversion of energetic, pressure-supporting photons
into electron-positron pairs occurs prior to oxygen ignition, and leads to a
violent contraction that triggers a catastrophic nuclear explosion. Tremendous
energies (>~ 10^{52} erg) are released, completely unbinding the star in a
pair-instability SN (PISN), with no compact remnant. Transitional objects with
100 M_{solar} < M_{initial} < 140 M_{solar}, which end up as iron-core-collapse
supernovae following violent mass ejections, perhaps due to short instances of
the pair instability, may have been identified. However, genuine PISNe, perhaps
common in the early Universe, have not been observed to date. Here, we present
our discovery of SN 2007bi, a luminous, slowly evolving supernova located
within a dwarf galaxy (~1% the size of the Milky Way). We measure the exploding
core mass to be likely ~100 M_{solar}, in which case theory unambiguously
predicts a PISN outcome. We show that >3 M_{solar} of radioactive 56Ni were
synthesized, and that our observations are well fit by PISN models. A PISN
explosion in the local Universe indicates that nearby dwarf galaxies probably
host extremely massive stars, above the apparent Galactic limit, perhaps
resulting from star formation processes similar to those that created the first
stars in the Universe.Comment: Accepted version of the paper appearing in Nature, 462, 624 (2009),
including all supplementary informatio
Flash Spectroscopy: Emission Lines from the Ionized Circumstellar Material Around <10-Day-Old Type II Supernovae
The American Astronomical Society. All rights reserved.Supernovae (SNe) embedded in dense circumstellar material (CSM) may show prominent emission lines in their early-time spectra (≤10 days after the explosion), owing to recombination of the CSM ionized by the shock-breakout flash. From such spectra ("flash spectroscopy"), we can measure various physical properties of the CSM, as well as the mass-loss rate of the progenitor during the year prior to its explosion. Searching through the Palomar Transient Factory (PTF and iPTF) SN spectroscopy databases from 2009 through 2014, we found 12 SNe II showing flash-ionized (FI) signatures in their first spectra. All are younger than 10 days. These events constitute 14% of all 84 SNe in our sample having a spectrum within 10 days from explosion, and 18% of SNe II observed at ages <5 days, thereby setting lower limits on the fraction of FI events. We classified as "blue/featureless" (BF) those events having a first spectrum that is similar to that of a blackbody, without any emission or absorption signatures. It is possible that some BF events had FI signatures at an earlier phase than observed, or that they lack dense CSM around the progenitor. Within 2 days after explosion, 8 out of 11 SNe in our sample are either BF events or show FI signatures. Interestingly, we found that 19 out of 21 SNe brighter than an absolute magnitude MR = -18.2 belong to the FI or BF groups, and that all FI events peaked above MR = -17.6 mag, significantly brighter than average SNe II
The Collimation and Energetics of the Brightest Swift Gamma-Ray Bursts
Long-duration gamma-ray bursts (GRBs) are widely believed to be
highly-collimated explosions (opening angle theta ~ 1-10 deg). As a result of
this beaming factor, the true energy release from a GRB is usually several
orders of magnitude smaller than the observed isotropic value. Measuring this
opening angle, typically inferred from an achromatic steepening in the
afterglow light curve (a "jet" break), has proven exceedingly difficult in the
Swift era. Here we undertake a study of five of the brightest (in terms of the
isotropic prompt gamma-ray energy release, E(gamma, iso)) GRBs in the Swift era
to search for jet breaks and hence constrain the collimation-corrected energy
release. We present multi-wavelength (radio through X-ray) observations of GRBs
050820A, 060418, and 080319B, and construct afterglow models to extract the
opening angle and beaming-corrected energy release for all three events.
Together with results from previous analyses of GRBs 050904 and 070125, we find
evidence for an achromatic jet break in all five events, strongly supporting
the canonical picture of GRBs as collimated explosions. The most natural
explanation for the lack of observed jet breaks from most Swift GRBs is
therefore selection effects. However, the opening angles for the events in our
sample are larger than would be expected if all GRBs had a canonical energy
release of ~ 10e51 erg. The total energy release we measure for those
"hyper-energetic" (E(total) >~ 10e52 erg) events in our sample is large enough
to start challenging models with a magnetar as the compact central remnant.Comment: Submitted to ApJ, comments welcom
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