293 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
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Expression of target and reference genes in Daphnia magna exposed to ibuprofen
Background: Transcriptomic techniques are now being applied in ecotoxicology and toxicology to measure the impact of stressors and develop understanding of mechanisms of toxicity. Microarray technology in particular offers the potential to measure thousands of gene responses simultaneously. However, it is important that microarrays responses should be validated, at least initially, using real-time quantitative polymerase chain reaction (QPCR). The accurate measurement of target gene expression requires normalisation to an invariant internal control e. g., total RNA or reference genes. Reference genes are preferable, as they control for variation inherent in the cDNA synthesis and PCR. However, reference gene expression can vary between tissues and experimental conditions, which makes it crucial to validate them prior to application. Results: We evaluated 10 candidate reference genes for QPCR in Daphnia magna following a 24 h exposure to the non-steroidal anti-inflammatory drug (NSAID) ibuprofen (IB) at 0, 20, 40 and 80 mg IB l(-1). Six of the 10 candidates appeared suitable for use as reference genes. As a robust approach, we used a combination normalisation factor (NF), calculated using the geNorm application, based on the geometric mean of three selected reference genes: glyceraldehyde-3-phosphate dehydrogenase, ubiquitin conjugating enzyme and actin. The effects of normalisation are illustrated using as target gene leukotriene B4 12-hydroxydehydrogenase (Ltb4dh), which was upregulated following 24 h exposure to 63-81 mg IB l(-1). Conclusions: As anticipated, use of the NF clarified the response of Ltb4dh in daphnids exposed to sublethal levels of ibuprofen. Our findings emphasise the importance in toxicogenomics of finding and applying invariant internal QPCR control(s) relevant to the study conditions
Improved distance determination to M51 from supernovae 2011dh and 2005cs
The appearance of two recent supernovae, SN 2011dh and 2005cs, both in M51,
provides an opportunity to derive an improved distance to their host galaxy by
combining the observations of both SNe. We apply the Expanding Photosphere
Method to get the distance to M51 by fitting the data of these two SNe
simultaneously. In order to correct for the effect of flux dilution, we use
correction factors (zeta) appropriate for standard type II-P SNe atmospheres
for 2005cs, but find zeta ~ 1 for the type IIb SN 2011dh, which may be due to
the reduced H-content of its ejecta. The EPM analysis resulted in D_M51 = 8.4
+/- 0.7 Mpc. Based on this improved distance, we also re-analyze the HST
observations of the proposed progenitor of SN 2011dh. We confirm that the
object detected on the pre-explosion HST-images is unlikely to be a compact
stellar cluster. In addition, its derived radius (~ 277$ R_sun) is too large
for being the real (exploded) progenitor of SN 2011dh. The supernova-based
distance, D = 8.4 Mpc, is in good agreement with other recent distance
estimates to M51.Comment: 6 pages, 5 figures, accepted for publication in A&
SN 1999ga: a low-luminosity linear type II supernova?
Type II-linear supernovae are thought to arise from progenitors that have
lost most of their H envelope by the time of the explosion, and they are poorly
understood because they are only occasionally discovered. It is possible that
they are intrinsically rare, but selection effects due to their rapid
luminosity evolution may also play an important role in limiting the number of
detections. In this context, the discovery of a subluminous type II-linear
event is even more interesting. We investigate the physical properties and
characterise the explosion site of the type II SN 1999ga, which exploded in the
nearby spiral galaxy NGC 2442. Spectroscopic and photometric observations of SN
1999ga allow us to constrain the energetics of the explosion and to estimate
the mass of the ejected material, shedding light on the nature of the
progenitor star in the final stages of its life. The study of the environment
in the vicinity of the explosion site provides information on a possible
relation between these unusual supernovae and the properties of the galaxies
hosting them. Despite the lack of early-time observations, we provide
reasonable evidence that SN 1999ga was probably a type II-linear supernova that
ejected a few solar masses of material, with a very small amount of radioactive
elements of the order of 0.01 solar masses.Comment: 11 pages, 9 figures. Accepted for publication in A&A (March 28, 2009
Supernova 2012ec: Identification of the progenitor and early monitoring with PESSTO
We present the identification of the progenitor of the Type IIP SN 2012ec in
archival pre-explosion HST WFPC2 and ACS/WFC F814W images. The properties of
the progenitor are further constrained by non-detections in pre-explosion WFPC2
F450W and F606W images. We report a series of early photometric and
spectroscopic observations of SN 2012ec. The r'-band light curve shows a
plateau with M(r')=-17.0. The early spectrum is similar to the Type IIP SN
1999em, with the expansion velocity measured at Halpha absorption minimum of
-11,700 km/s (at 1 day post-discovery). The photometric and spectroscopic
evolution of SN 2012ec shows it to be a Type IIP SN, discovered only a few days
post-explosion (<6d). We derive a luminosity for the progenitor, in comparison
with MARCS model SEDs, of log L/Lsun = 5.15+/-0.19, from which we infer an
initial mass range of 14-22Msun. This is the first SN with an identified
progenitor to be followed by the Public ESO Spectroscopic Survey of Transient
Objects (PESSTO).Comment: 6 pages, 3 figures, MNRAS accepte
The Cassiopeia A Supernova was of Type IIB
Cassiopeia A is one of the youngest supernova remnants known in the Milky Way
and a unique laboratory for supernova physics. We present an optical spectrum
of the Cassiopeia A supernova near maximum brightness, obtained from
observations of a scattered light echo - more than three centuries after the
direct light of the explosion swept past Earth. The spectrum shows that
Cassiopeia A was a type IIb supernova and originated from the collapse of the
helium core of a red supergiant that had lost most of its hydrogen envelope
prior to exploding. Our finding concludes a longstanding debate on the
Cassiopeia A progenitor and provides new insight into supernova physics by
linking the properties of the explosion to the wealth of knowledge about its
remnant.Comment: 17 pages, 4 figures, including online supporting material; to be
published in Science on 30 May 200
Interacting supernovae and supernova impostors. SN 2007sv: the major eruption of a massive star in UGC 5979
We report the results of the photometric and spectroscopic monitoring
campaign of the transient SN 2007sv. The observables are similar to those of
type IIn supernovae, a well-known class of objects whose ejecta interact with
pre-existing circum-stellar material. The spectra show a blue continuum at
early phases and prominent Balmer lines in emission, however, the absolute
magnitude at the discovery of SN 2007sv (M_R = - 14.25 +/- 0.38) indicate it to
be most likely a supernova impostor. This classification is also supported by
the lack of evidence in the spectra of very high velocity material as expected
in supernova ejecta. In addition we find no unequivocal evidence of broad lines
of alpha - and/or Fe-peak elements. The comparison with the absolute light
curves of other interacting objects (including type IIn supernovae) highlights
the overall similarity with the prototypical impostor SN 1997bs. This supports
our claim that SN 2007sv was not a genuine supernova, and was instead a
supernova impostor, most likely similar to the major eruption of a luminous
blue variable.Comment: Accepted for publication in MNRAS. 15 pages, 11 figures, 5 table
Probing the final-stage progenitor evolution for Type IIP Supernova 2017eaw in NGC 6946
We presented a detailed analysis of progenitor properties of type IIP supernova 2017eaw in NGC 6946, based on the pre-explosion images and early-time observations obtained immediately after the explosion. An unusually red star, with MF814W = −6.9 mag and mF606W − mF814W = 2.9 ± 0.2 mag, can be identified at the SN position in the pre-discovery Hubble Space Telescope (HST) images taken in 2016. The observed spectral energy distribution of this star, covering the wavelength of 0.6–2.0 μm, matches that of an M4-type red supergiant (RSG) with a temperature of about 3550 K. These results suggest that SN 2017eaw has a RSG progenitor with an initial mass of 12 ± 2 M⊙. The absolute F814W-band magnitude of this progenitor star is found to evolve from −7.2 mag in 2004 to −6.9 mag in 2016. Such a dimming effect is, however, unpredicted for an RSG in its neon/oxygen burning phase when its luminosity should modestly increase. The spectrum of SN 2017eaw taken a few hours after discovery clearly shows a narrow Hα emission feature blueshifted by ∼160 km s−1. This narrow component disappeared in the spectrum taken two days later, suggesting the presence of a circumstellar material (CSM) shell (i.e. at a distance of <2.1–4.3 × 1014 cm). Combining the inferred distance with the expansion velocity of the CSM, we suggest that the progenitor of SN 2017eaw should have experienced violent mass-loss at about 1–2 yr prior to explosion, perhaps invoked by pulsational envelop ejection. This mechanism may help explain its luminosity decline in 2016 as well as the lack of detections of RSGs with relatively higher initial mass as progenitors of SNe IIP
Polarized kilonovae from black hole-neutron star mergers
We predict linear polarization for a radioactively powered kilonova following the merger of a black hole and a neutron star. Specifically, we perform 3D Monte Carlo radiative transfer simulations for two different models, both featuring a lanthanide-rich dynamical ejecta component from numerical-relativity simulations while only one including an additional lanthanide-free disc-wind component. We calculate polarization spectra for nine different orientations at 1.5, 2.5, and 3.5 d after the merger and in the 0.1-2 μ wavelength range. We find that both models are polarized at a detectable level 1.5 d after the merger while show negligible levels thereafter. The polarization spectra of the two models are significantly different. The model lacking a disc wind shows no polarization in the optical, while a signal increasing at longer wavelengths and reaching ∼ 1-6 per cent at 2 μ depending on the orientation. The model with a disc-wind component, instead, features a characteristic 'double-peak' polarization spectrum with one peak in the optical and the other in the infrared. Polarimetric observations of future events will shed light on the debated neutron richness of the disc-wind component. The detection of optical polarization would unambiguously reveal the presence of a lanthanide-free disc-wind component, while polarization increasing from zero in the optical to a peak in the infrared would suggest a lanthanide-rich composition for the whole ejecta. Future polarimetric campaigns should prioritize observations in the first ∼48 h and in the 0.5-2 μ range, where polarization is strongest, but also explore shorter wavelengths/later times where no signal is expected from the kilonova and the interstellar polarization can be safely estimated
On the Progenitors of Core-Collapse Supernovae
Theory holds that a star born with an initial mass between about 8 and 140
times the mass of the Sun will end its life through the catastrophic
gravitational collapse of its iron core to a neutron star or black hole. This
core collapse process is thought to usually be accompanied by the ejection of
the star's envelope as a supernova. This established theory is now being tested
observationally, with over three dozen core-collapse supernovae having had the
properties of their progenitor stars directly measured through the examination
of high-resolution images taken prior to the explosion. Here I review what has
been learned from these studies and briefly examine the potential impact on
stellar evolution theory, the existence of "failed supernovae", and our
understanding of the core-collapse explosion mechanism.Comment: 7 Pages, invited review accepted for publication by Astrophysics and
Space Science (special HEDLA 2010 issue
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