69 research outputs found
Supernova Asymmetries
All core collapse supernovae are strongly aspherical. The "Bochum event,"
with velocity components displaced symmetrically about the principal H
line, strongly suggests that SN 1987A was a bi-polar rather than a uni-polar
explosion. While there is a general tendency to display a single prominant axis
in images and spectropolarimetry, there is also growing evidence for frequent
departures from axisymmetry. There are various mechanisms that might contribute
to large scale departures from spherical symmetry: jet-induced processes, the
spherical shock accretion instability (SASI) and associated phenomena, and
non-axisymmetric instabilities (NAXI). The MRI gives inevitable production of
large toroidal magnetic fields. In sum: no without B. The role of
magnetic fields, non-axisymmetric instabilities, and of the de-leptonization
phase are discussed.Comment: 8 pages, 3 figures, to be published in the proceedings of the
conference "Supernova 1987A: 20 Years After" Aspen, 200
Pan-chromatic photometric classification of supernovae from multiple surveys and transfer learning for future surveys
Time-domain astronomy is entering a new era as wide-field surveys with higher
cadences allow for more discoveries than ever before. The field has seen an
increased use of machine learning and deep learning for automated
classification of transients into established taxonomies. Training such
classifiers requires a large enough and representative training set, which is
not guaranteed for new future surveys such as the Vera Rubin Observatory,
especially at the beginning of operations. We present the use of Gaussian
processes to create a uniform representation of supernova light curves from
multiple surveys, obtained through the Open Supernova Catalog for supervised
classification with convolutional neural networks. We also investigate the use
of transfer learning to classify light curves from the Photometric LSST
Astronomical Time Series Classification Challenge (PLAsTiCC) dataset. Using
convolutional neural networks to classify the Gaussian process generated
representation of supernova light curves from multiple surveys, we achieve an
AUC score of 0.859 for classification into Type Ia, Ibc, and II. We find that
transfer learning improves the classification accuracy for the most
under-represented classes by up to 18% when classifying PLAsTiCC light curves,
and is able to achieve an AUC score of 0.945 when including photometric
redshifts for classification into six classes (Ia, Iax, Ia-91bg, Ibc, II,
SLSN-I). We also investigate the usefulness of transfer learning when there is
a limited labelled training set to see how this approach can be used for
training classifiers in future surveys at the beginning of operations.Comment: 15 pages, 14 figure
The death of massive stars - II. Observational constraints on the progenitors of type Ibc supernovae
The progenitors of many type II core-collapse supernovae have now been
identified directly on pre-discovery imaging. Here we present an extensive
search for the progenitors of type Ibc supernovae in all available
pre-discovery imaging since 1998. There are 12 type Ibc supernovae with no
detections of progenitors in either deep ground-based or Hubble Space Telescope
archival imaging. The deepest absolute BVR magnitude limits are between -4 and
-5. We compare these limits with the observed Wolf-Rayet population in the
Large Magellanic Cloud and estimate a 16 per cent probability we have failed to
detect such a progenitor by chance. Alternatively the progenitors evolve
significantly before core-collapse or we have underestimated the extinction
towards the progenitors. Reviewing the relative rates and ejecta mass estimates
from lightcurve modelling of Ibc SNe, we find both incompatible with Wolf-Rayet
stars with initial masses >25Msun being the only progenitors. We present binary
evolution models that fit these observational constraints. Stars in binaries
with initial masses <20Msun lose their hydrogen envelopes in binary
interactions to become low mass helium stars. They retain a low mass hydrogen
envelope until approximately 10,000 years before core-collapse; hence it is not
surprising that galactic analogues have been difficult to identify.Comment: Accepted by MNRAS. 31 pages, 12 figures, 8 table
A UV census of the environments of stripped-envelope supernovae
This paper reports an environmental analysis of 41 uniformly-selected
stripped-envelope supernovae (SESNe) based on deep ultraviolet-optical images
acquired by the Hubble Space Telescope. Young stellar populations are detected
in most SN environments and their ages are derived with a hierarchical Bayesian
approach. The age distributions are indistinguishable between Type IIb and Type
Ib while that for Type Ic is systematically younger. This suggests that the
Type Ic SN progenitors are more massive while the Type IIb and Type Ib SNe have
very similar progenitor masses. Our result supports a hybrid envelope-stripping
mechanism, in which the hydrogen envelopes of the SESN progenitors are stripped
via a mass-insensitive process (e.g. binary interaction) while the helium
envelopes are stripped via a mass-sensitive process (e.g. stellar wind of the
post-binary interaction progenitor). We also provide progenitor constraints for
three Type Ibn SNe and two broad-lined Type Ic SNe. All these results
demonstrate the importance of the very diverse mass-loss processes in the
origins of SESNe.Comment: 15 pages, 9 figures, accepted for publication in MNRA
An environmental analysis of the fast transient AT2018cow and implications for its progenitor and late-time brightness
The nature of the newly discovered fast blue optical transients (FBOTs) is
still puzzling astronomers. In this paper we carry out a comprehensive analysis
of the molecular gas, ionized gas and stellar populations in the environment of
the nearby FBOT AT2018cow based on ALMA, VLT/MUSE and HST/WFC3 observations. A
prominent molecular concentration of 6 ( 1) 10 is
found in the vicinity of AT2018cow, which has given rise to two active
star-forming complexes with ages of 4 1 Myr and 2.5 Myr,
respectively. Each star-forming complex has a stellar mass of 3 10
and has photoionized a giant H II region with H luminosity
even comparable to that of the 30 Dor mini-starburst region. AT2018cow is
spatially coincident with one of the star-forming complexes; however, it is
most likely to reside in its foreground since it has a much smaller extinction
than the complex. Its progenitor could have been formed at an earlier epoch in
this area; if it were from a major star-forming event, the non-detection of the
associated stellar population constrains the progenitor's age to be 10
Myr and initial mass to be 20 . We further find the
late-time brightness of AT2018cow is unlikely to be a stellar object. Its
brightness has slightly declined from 2 yr to 4 yr after explosion and is most
likely to originate from AT2018cow itself due to some powering mechanism still
working at such late times.Comment: 13 pages, 10 figures, submitted to MNRA
The Shape of Cas A
Based on optical, IR and X-ray studies of Cas A, we propose a geometry for
the remnant based on a "jet-induced" scenario with significant systematic
departures from axial symmetry. In this model, the main jet axis is oriented in
the direction of strong blue-shifted motion at an angle of 110 - 120 degrees
East of North and about 40 - 50 degrees to the East of the line of sight.
Normal to this axis would be an expanding torus as predicted by jet-induced
models. In the proposed geometry, iron-peak elements in the main jet-like flow
could appear "beyond" the portions of the remnant rich in silicon by projection
effects, not the effect of mixing. In the context of the proposed geometry, the
displacement of the compact object from the kinematic center of the remnant at
a position angle of ~169 degrees can be accommodated if the motion of the
compact object is near to, but slightly off from, the direction of the main
"jet" axis by of order 30 degrees. In this model, the classical NE "jet," the
SW "counter-jet" and other protrusions, particularly the "hole" in the North,
are non-asymmetric flows approximately in the equatorial plane, e.g., out
through the perimeter of the expanding torus, rather than being associated with
the main jet. We explore the spoke-like flow in the equatorial plane in terms
of Rayleigh-Taylor, Richtmyer-Meshkov and Kelvin-Helmholz instabilities and
illustrate these instabilities with a jet-induced simulation.Comment: 25 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
The Dwarf Starburst Host Galaxy of a Type Ia SN at z = 1.55 from CANDELS
We present VLT/X-shooter observations of a high redshift, type Ia supernova
host galaxy, discovered with HST/WFC3 as part of the CANDELS Supernova project.
The galaxy exhibits strong emission lines of Ly{\alpha}, [O II], H{\beta}, [O
III], and H{\alpha} at z = 1.54992(+0.00008-0.00004). From the emission-line
fluxes and SED fitting of broad-band photometry we rule out AGN activity and
characterize the host galaxy as a young, low mass, metal poor, starburst galaxy
with low intrinsic extinction and high Ly{\alpha} escape fraction. The host
galaxy stands out in terms of the star formation, stellar mass, and metallicity
compared to its lower redshift counterparts, mainly because of its high
specific star-formation rate. If valid for a larger sample of high-redshift SN
Ia host galaxies, such changes in the host galaxy properties with redshift are
of interest because of the potential impact on the use of SN Ia as standard
candles in cosmology.Comment: 25 pages, 8 figures. Accepted for publication in Ap
The birth place of the type Ic Supernova 2007gr
We report our attempts to locate the progenitor of the peculiar type Ic SN
2007gr in HST pre-explosion images of the host galaxy, NGC 1058. Aligning
adaptive optics Altair/NIRI imaging of SN 2007gr from the Gemini (North)
Telescope with the pre-explosion HST WFPC2 images, we identify the SN position
on the HST frames with an accuracy of 20 mas. Although nothing is detected at
the SN position we show that it lies on the edge of a bright source, 134+/-23
mas (6.9 pc) from its nominal centre. Based on its luminosity we suggest that
this object is possibly an unresolved, compact and coeval cluster and that the
SN progenitor was a cluster member, although we note that model profile fitting
favours a single bright star. We find two solutions for the age of this assumed
cluster; 7-/+0.5 Myrs and 20-30 Myrs, with turn-off masses of 28+/-4 Msun and
12-9 Msun respectively. Pre-explosion ground-based K-band images marginally
favour the younger cluster age/higher turn-off mass. Assuming the SN progenitor
was a cluster member, the turn-off mass provides the best estimate for its
initial mass. More detailed observations, after the SN has faded, should
determine if the progenitor was indeed part of a cluster, and if so allow an
age estimate to within ~2 Myrs thereby favouring either a high mass single star
or lower mass interacting binary progenitor.Comment: 12 pages, 3 figures, resolution of fig 1. has been reduced, some
revision based on referee's comments, Accepted ApJL 27 Nov 200
Rapid formation of large dust grains in the luminous supernova SN 2010jl
The origin of dust in galaxies is still a mystery. The majority of the
refractory elements are produced in supernova explosions but it is unclear how
and where dust grains condense and grow, and how they avoid destruction in the
harsh environments of star-forming galaxies. The recent detection of 0.1-0.5
solar masses of dust in nearby supernova remnants suggests in situ dust
formation, while other observations reveal very little dust in supernovae the
first few years after explosion. Observations of the bright SN 2010jl have been
interpreted as pre-existing dust, dust formation or no dust at all. Here we
report the rapid (40-240 days) formation of dust in its dense circumstellar
medium. The wavelength dependent extinction of this dust reveals the presence
of very large (> 1 micron) grains, which are resistant to destructive
processes. At later times (500-900 days), the near-IR thermal emission shows an
accelerated growth in dust mass, marking the transition of the supernova from a
circumstellar- to an ejecta-dominated source of dust. This provides the link
between the early and late dust mass evolution in supernovae with dense
circumstellar media.Comment: 62 pages, 13 figures, 1 table. Author version of the Letter to
Nature, published online July 9 2014 (Nature, 511, 7509, pp. 326-329 (2014)),
prior to the final editorial changes to conform to Journal style; includes
Methods and Extended Data Figures and the Supplementary Information. See
published version
http://www.nature.com/nature/journal/v511/n7509/full/nature13558.htm
- …