61 research outputs found
Luminous Blue Variables as the progenitors of supernovae with quasi-periodic radio modulations
The interaction between supernova ejecta and circumstellar matter, arising
from previous episodes of mass loss, provides us with a means with which to
constrain the progenitors of supernovae. Radio observations of a number of
supernovae show quasi-periodic deviations from a strict power-law decline at
late times. Although several possibilities have been put forward to explain
these modulations, no single explanation has proven to be entirely
satisfactory. Here we suggest that Luminous Blue Variables undergoing S-Doradus
type variations give rise to enhanced phases of mass loss which are imprinted
on the immediate environment of the exploding star as a series of density
enhancements. The variations in mass loss arise from changes in the ionization
balance of Fe, the dominant ion that drives the wind. With this idea, we find
that both the recurrence timescale of the variability, as well as the amplitude
of the modulations are in line with the observations. Our scenario thus
provides a natural, single-star explanation for the observed behaviour that is,
in fact, expected on theoretical grounds.Comment: A&A Letters (accepted
SN 1987A at the end of its second decade
After nearly two decades at least five emission mechanisms can be found in SN
1987A. The ejecta continue to glow as a result of the radioactive decay of
long-lived nuclei (mostly 44Ti), but is fading continuously because of the
expansion and the reduced opacity. The nearly stationary rings around SN 1987A
are still fluorescing from the recombination of matter originally excited by
the soft X-ray emission from the shock breakout at explosion. The supernova
shock reached the inner circumstellar ring about ten years ago and the forward
shock is moving through the inner ring and leaves shocked material behind. This
material is excited and accelerated. The reverse shock illuminates the
fast-moving supernova ejecta as it catches up. And, finally light echoes in
nearby interstellar matter can still be observed. We present here high
resolution spectroscopy in the optical and integral-field spectroscopy in the
near infrared of SN 1987A and its rings.Comment: 8 pages, 6 figures, to be published in The Multicoloured Landscape of
Compact Objects and their Explosive Origins, Proceedings of the Cefalu
conferenc
Early-time Spitzer observations of the type II-Plateau supernova, 2004dj
We present mid-infrared observations with the Spitzer Space Telescope of the
nearby type II-P supernova, SN 2004dj, at epochs of 89 to 129 days. We have
obtained the first mid-IR spectra of any supernova apart from SN 1987A. A
prominent [NiII] 6.64 micron line is observed, from which we deduce that the
mass of stable nickel must be at least 2.2e10(-4) Msun. We also observe the red
wing of the CO-fundamental band. We relate our findings to possible progenitors
and favour an evolved star, most likely a red supergiant, with a probable
initial mass between ~10 and 15 Msun.Comment: ApJ Letters (accepted
Spherical symmetry in the kilonova AT2017gfo/GW170817
The mergers of neutron stars expel a heavy-element enriched fireball which
can be observed as a kilonova. The kilonova's geometry is a key diagnostic of
the merger and is dictated by the properties of ultra-dense matter and the
energetics of the collapse to a black hole. Current hydrodynamical merger
models typically show aspherical ejecta. Previously, Sr was identified in
the spectrum of the the only well-studied kilonova AT2017gfo, associated with
the gravitational wave event GW170817. Here we combine the strong Sr P
Cygni absorption-emission spectral feature and the blackbody nature of kilonova
spectrum, to determine that the kilonova is highly spherical at early epochs.
Line shape analysis combined with the known inclination angle of the source
also shows the same sphericity independently. We conclude that energy injection
by radioactive decay is insufficient to make the ejecta spherical. A magnetar
wind or jet from the black hole disk could inject enough energy to induce a
more spherical distribution in the overall ejecta, however an additional
process seems necessary to make the element distribution unifor
Exoplanets or Dynamic Atmospheres? The Radial Velocity and Line Shape Variations of 51 Pegasi and Tau Bootis
Because of our relatively low spectral resolution, we compare our
observations with Gray's line bisector data by fitting observed line profiles
to an expansion in terms of orthogonal (Hermite) functions. To obtain an
accurate comparison, we model the emergent line profiles from rotating and
pulsating stars, taking the instrumental point spread function into account. We
describe this modeling process in detail.
We find no evidence for line profile or strength variations at the radial
velocity period in either 51 Peg or in Tau Boo. For 51 Peg, our upper limit for
line shape variations with 4.23-day periodicity is small enough to exclude with
10 sigma confidence the bisector curvature signal reported by Gray & Hatzes;
the bisector span and relative line depth signals reported by Gray (1997) are
also not seen, but in this case with marginal (2 sigma) confidence. We cannot,
however, exclude pulsations as the source of 51 Peg's radial velocity
variation, because our models imply that line shape variations associated with
pulsations should be much smaller than those computed by Gray & Hatzes; these
smaller signals are below the detection limits both for Gray & Hatzes' data and
for our own.
Tau Boo's large radial velocity amplitude and v*sin(i) make it easier to test
for pulsations in this star. Again we find no evidence for periodic line-shape
changes, at a level that rules out pulsations as the source of the radial
velocity variability. We conclude that the planet hypothesis remains the most
likely explanation for the existing data.Comment: 44 pages, 19 figures, plain TeX, accepted to ApJS (companion to
letter astro-ph/9712279
Spitzer measurements of atomic and molecular abundances in the Type IIP SN 2005af
We present results based on Spitzer Space Telescope mid-infrared (3.6-30
micron) observations of the nearby IIP supernova 2005af. We report the first
ever detection of the SiO molecule in a Type IIP supernova. Together with the
detection of the CO fundamental, this is an exciting finding as it may signal
the onset of dust condensation in the ejecta. From a wealth of fine-structure
lines we provide abundance estimates for stable Ni, Ar, and Ne which, via
spectral synthesis, may be used to constrain nucleosynthesis models.Comment: ApJ Letters (accepted
The Outermost Ejecta of Type Ia Supernovae
The properties of the highest velocity ejecta of normal Type Ia supernovae
(SNe Ia) are studied via models of very early optical spectra of 6 SNe. At
epochs earlier than 1 week before maximum, SNe with a rapidly evolving Si II
6355 line velocity (HVG) have a larger photospheric velocity than SNe with a
slowly evolving Si II 6355 line velocity (LVG). Since the two groups have
comparable luminosities, the temperature at the photosphere is higher in LVG
SNe. This explains the different overall spectral appearance of HVG and LVG
SNe. However, the variation of the Ca II and Si II absorptions at the highest
velocities (v >~ 20,000 km/s) suggests that additional factors, such as
asphericity or different abundances in the progenitor white dwarf, affect the
outermost layers. The C II 6578 line is marginally detected in 3 LVG SNe,
suggesting that LVG undergo less intense burning. The carbon mass fraction is
small, only less than 0.01 near the photosphere, so that he mass of unburned C
is only <~ 0.01 Msun. Radioactive 56Ni and stable Fe are detected in both LVG
and HVG SNe. Different Fe-group abundances in the outer layers may be one of
the reasons for spectral diversity among SNe Ia at the earliest times. The
diversity among SNe Ia at the earliest phases could also indicate an intrinsic
dispersion in the width-luminosity relation of the light curve.Comment: 13 pages, 10 figures, Accepted for publication in The Astrophysical
Journa
Signatures of delayed detonation, asymmetry, and electron capture in the mid-infrared spectra of supernovae 2003hv and 2005df
We present mid-infrared (5.2-15.2 ÎŒm) spectra of the Type Ia supernovae (SNe Ia) 2003hv and 2005df observed with the Spitzer Space Telescope. These are the first observed mid-infrared spectra of thermonuclear supernovae, and show strong emission from fine-structure lines of Ni, Co, S, and Ar. The detection of Ni emission in SN 2005df 135 days after the explosion provides direct observational evidence of high-density nuclear burning forming a significant amount of stable Ni in a SN Ia. The SN 2005df Ar lines also exhibit a two-pronged emission profile, implying that the Ar emission deviates significantly from spherical symmetry. The spectrum of SN 2003hv also shows signs of asymmetry, exhibiting blueshifted [Co III], which matches the blueshift of [Fe II ] lines in nearly coeval near-infrared spectra. Finally, local thermodynamic equilibrium abundance estimates for the yield of radioactive ^(56)Ni give M^(56)Ni â 0.5 Mâ, for SN 2003hv, but only M^(56)Ni â 0.13-0.22 Mâ for the apparently subluminous SN 2005df, supporting the notion that the luminosity of SNe Ia is primarily a function of the radioactive ^(56)Ni yield. The observed emission-line profiles in the SN 2005df spectrum indicate a chemically stratified ejecta structure, which matches the predictions of delayed detonation (DD) models, but is entirely incompatible with current three-dimensional deflagration models. Furthermore, the degree that this layering persists to the innermost regions of the supernova is difficult to explain even in a DD scenario, where the innermost ejecta are still the product of deflagration burning. Thus, while these results are roughly consistent with a delayed detonation, it is clear that a key piece of physics is still missing from our understanding of the earliest phases of SN Ia explosions
- âŠ