15 research outputs found
A non-spherical core in the explosion of supernova SN 2004dj
An important and perhaps critical clue to the mechanism driving the explosion
of massive stars as supernovae is provided by the accumulating evidence for
asymmetry in the explosion. Indirect evidence comes from high pulsar
velocities, associations of supernovae with long-soft gamma-ray bursts, and
asymmetries in late-time emission-line profiles. Spectropolarimetry provides a
direct probe of young supernova geometry, with higher polarization generally
indicating a greater departure from spherical symmetry. Large polarizations
have been measured for 'stripped-envelope' (that is, type Ic) supernovae, which
confirms their non-spherical morphology; but the explosions of massive stars
with intact hydrogen envelopes (type II-P supernovae) have shown only weak
polarizations at the early times observed. Here we report multi-epoch
spectropolarimetry of a classic type II-P supernova that reveals the abrupt
appearance of significant polarization when the inner core is first exposed in
the thinning ejecta (~90 days after explosion). We infer a departure from
spherical symmetry of at least 30 per cent for the inner ejecta. Combined with
earlier results, this suggests that a strongly non-spherical explosion may be a
generic feature of core-collapse supernovae of all types, where the asphericity
in type II-P supernovae is cloaked at early times by the massive, opaque,
hydrogen envelope.Comment: Accepted for publication by Nature (results embargoed until 23 March
2006); 14 pages, 2 figure
The nebular spectra of the transitional Type Ia Supernovae 2007on and 2011iv: Broad, multiple components indicate aspherical explosion cores
The nebular-epoch spectrum of the rapidly declining, 'transitional' Type Ia supernova (SN) 2007on showed double emission peaks, which have been interpreted as indicating that the SN was the result of the direct collision of two white dwarfs. The spectrum can be reproduced using two distinct emission components, one redshifted and one blueshifted. These components are similar in mass but have slightly different degrees of ionization. They recede from one another at a line-of-sight speed larger than the sum of the combined expansion velocities of their emitting cores, thereby acting as two independent nebulae. While this configuration appears to be consistent with the scenario of two white dwarfs colliding, it may also indicate an off-centre delayed detonation explosion of a near-Chandrasekhar-mass white dwarf. In either case, broad emission line widths and a rapidly evolving light curve can be expected for the bolometric luminosity of the SN. This is the case for both SNe 2007on and 2011iv, also a transitional SN Ia that exploded in the same elliptical galaxy, NGC1404. Although SN 2011iv does not show double-peaked emission line profiles, the width of its emission lines is such that a two-component model yields somewhat better results than a single-component model. Most of the mass ejected is in one component, however, which suggests that SN 2011iv was the result of the off-centre ignition of a Chandrasekhar-mass white dwarf. © 2017 The Authors
Type Ia Supernovae and the Hubble Constant
The focus of this review is the work that has been done during the 1990s on
using Type Ia supernovae (SNe Ia) to measure the Hubble constant (). SNe
Ia are well suited for measuring . A straightforward maximum-light color
criterion can weed out the minority of observed events that are either
intrinsically subluminous or substantially extinguished by dust, leaving a
majority subsample that has observational absolute-magnitude dispersions of
less than mag.
Correlations between absolute magnitude and one or more distance-independent SN
Ia or parent-galaxy observables can be used to further standardize the absolute
magnitudes to better than 0.2 mag. The absolute magnitudes can be calibrated in
two independent ways --- empirically, using Cepheid-based distances to parent
galaxies of SNe Ia, and physically, by light curve and spectrum fitting. At
present the empirical and physical calibrations are in agreement at or -19.5. Various ways that have been used to match
Cepheid-calibrated SNe Ia or physical models to SNe Ia that have been observed
out in the Hubble flow have given values of distributed throughout the
range 54 to 67 km/s Mpc. Astronomers who want a consensus value of
from SNe Ia with conservative errors could, for now, use km/s
Mpc^{-1}$.Comment: 46 pages. Hard copies of figures, all from the published literature,
can be obtained from the author. With permission, from the Annual Review of
Astronomy and Astrophysics, Volume 36, copyright 1998, by Annual Review
An asymmetric explosion as the origin of spectral evolution diversity in type Ia supernovae
Type Ia Supernovae (SNe Ia) form an observationally uniform class of stellar
explosions, in that more luminous objects have smaller decline-rates. This
one-parameter behavior allows SNe Ia to be calibrated as cosmological `standard
candles', and led to the discovery of an accelerating Universe. Recent
investigations, however, have revealed that the true nature of SNe Ia is more
complicated. Theoretically, it has been suggested that the initial
thermonuclear sparks are ignited at an offset from the centre of the
white-dwarf (WD) progenitor, possibly as a result of convection before the
explosion. Observationally, the diversity seen in the spectral evolution of SNe
Ia beyond the luminosity decline-rate relation is an unresolved issue. Here we
report that the spectral diversity is a consequence of random directions from
which an asymmetric explosion is viewed. Our findings suggest that the spectral
evolution diversity is no longer a concern in using SNe Ia as cosmological
standard candles. Furthermore, this indicates that ignition at an offset from
the centre of is a generic feature of SNe Ia.Comment: To appear in Nature, 1st July 2010 issue. 36 pages including
supplementary materials. 4 figures, 3 supplementary figures, 1 supplementary
tabl
A flash of polarized optical light points to an aspherical ‘cow’
The astronomical transient AT2018cow is the closest example of the new class of luminous, fast blue optical transients (FBOTs). Liverpool Telescope RINGO3 observations of AT 2018cow are reported here, which constitute the earliest polarimetric observations of an FBOT. At 5.7 days post-explosion, the optical emission of AT2018cow exhibited a chromatic polarization spike that reached ∼7% at red wavelengths. This is the highest intrinsic polarization recorded for a non-relativistic explosive transient, and is observed in multiple bands and at multiple epochs over the first night of observations, before rapidly declining. The apparent wavelength dependence of the polarization may arise through depolarization or dilution of the polarized flux, due to conditions in AT 2018cow at early times. A second ‘bump’ in the polarization is observed at blue wavelengths at ∼12 days. Such a high polarization requires an extremely aspherical geometry that is only apparent for a brief period (<1 day), such as shock breakout through an optically thick disk. For a disk-like configuration, the ratio of the thickness to radial extent must be ∼10%
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure
Liverpool telescope 2: a new robotic facility for rapid transient follow-up
The Liverpool Telescope is one of the world's premier facilities for time domain astronomy. The time domain landscape is set to radically change in the coming decade, with surveys such as LSST providing huge numbers of transient detections on a nightly basis; transient detections across the electromagnetic spectrum from other facilities such as SVOM, SKA and CTA; and the era of `multi-messenger astronomy', wherein events are detected via non-electromagnetic means, such as gravitational wave emission. We describe here our plans for Liverpool Telescope 2: a new robotic telescope designed to capitalise on this new era of time domain astronomy. LT2 will be a 4-metre class facility co-located with the LT at the Observatorio del Roque de Los Muchachos on the Canary island of La Palma. The telescope will be designed for extremely rapid response: the aim is that the telescope will take data within 30 seconds of the receipt of a trigger from another facility. The motivation for this is twofold: firstly it will make it a world-leading facility for the study of fast fading transients and explosive phenomena discovered at early times. Secondly, it will enable large-scale programmes of low-to-intermediate resolution spectral classification of transients to be performed with great efficiency. In the target-rich environment of the LSST era, minimising acquisition overheads will be key to maximising the science gains from any follow-up programme. The telescope will have a diverse instrument suite which is simultaneously mounted for automatic changes, but it is envisaged that the primary instrument will be an intermediate resolution, optical/infrared spectrograph for scientific exploitation of transients discovered with the next generation of synoptic survey facilities. In this paper we outline the core science drivers for the telescope, and the requirements for the optical and mechanical design