737 research outputs found
Calcium-Rich Gap Transients: Tidal Detonations of White Dwarfs?
We hypothesize that at least some of the recently discovered class of
calcium-rich gap transients are tidal detonation events of white dwarfs (WDs)
by black holes (BHs) or possibly neutron stars. We show that the properties of
the calcium-rich gap transients agree well with the predictions of the tidal
detonation model. Under the predictions of this model, we use a follow-up X-ray
observation of one of these transients, SN 2012hn, to place weak upper limits
on the detonator mass of this system that include all intermediate-mass BHs
(IMBHs). As these transients are preferentially in the stellar haloes of
galaxies, we discuss the possibility that these transients are tidal
detonations of WDs caused by random flyby encounters with IMBHs in dwarf
galaxies or globular clusters. This possibility has been already suggested in
the literature but without connection to the calcium-rich gap transients. In
order for the random flyby cross-section to be high enough, these events would
have to be occurring inside these dense stellar associations. However, there is
a lack of evidence for IMBHs in these systems, and recent observations have
ruled out all but the very faintest dwarf galaxies and globular clusters for a
few of these transients. Another possibility is that these are tidal
detonations caused by three-body interactions, where a WD is perturbed toward
the detonator in isolated multiple star systems. We highlight a number of ways
this could occur, even in lower-mass systems with stellar-mass BHs or neutron
stars. Finally, we outline several new observational tests of this scenario,
which are feasible with current instrumentation.Comment: 10 pages, 1 figure, accepted for publication in MNRA
The nebular spectra of SN 2012aw and constraints on stellar nucleosynthesis from oxygen emission lines
We present nebular phase optical and near-infrared spectroscopy of the Type
IIP supernova SN 2012aw combined with NLTE radiative transfer calculations
applied to ejecta from stellar evolution/explosion models. Our spectral
synthesis models generally show good agreement with the ejecta from a MZAMS =
15 Msun progenitor star. The emission lines of oxygen, sodium, and magnesium
are all consistent with the nucleosynthesis in a progenitor in the 14 - 18 Msun
range. We also demonstrate how the evolution of the oxygen cooling lines of [O
I] 5577 A, [O I] 6300 A, and [O I] 6364 A can be used to constrain the mass of
oxygen in the non-molecularly cooled ashes to < 1 Msun, independent of the
mixing in the ejecta. This constraint implies that any progenitor model of
initial mass greater than 20 Msun would be difficult to reconcile with the
observed line strengths. A stellar progenitor of around MZAMS = 15 Msun can
consistently explain the directly measured luminosity of the progenitor star,
the observed nebular spectra, and the inferred pre-supernova mass-loss rate. We
conclude that there is still no convincing example of a Type IIP explosion
showing the nucleosynthesis expected from a MZAMS > 20 Msun progenitor.Comment: Accepted for publication in MNRA
SN 2005 gj: Evidence for LBV supernovae progenitors?
There has been mounting observational evidence in favour of Luminous Blue
Variables (LBVs) being the direct progenitors of supernovae. Here we present
possibly the most convincing evidence yet for such progenitors. We find
multiple absorption component P-Cygni profiles of hydrogen and helium in the
spectrum of SN 2005gj, which we interpret as being an imprint of the
progenitors mass-loss history. Such profiles have previously only been detected
in Luminous Blue Variables. This striking resemblance of the profiles, along
with wind velocities and periods consistent with LBV's leads us to connect SN
2005gj to an LBV progenitor.Comment: Accepted as a letter to A&A, 4 pages,3 figure
Modeling the light curve of the transient SCP06F6
We consider simple models based on core collapse or pair-formation supernovae
to account for the light curve of the transient SCP06F6. A radioactive decay
diffusion model provides estimates of the mass of the required radioactive
nickel and the ejecta as functions of the unknown redshift. An opacity change
such as by dust formation or a recombination front may account for the rapid
decline from maximum. We particularly investigate two specific redshifts:
, for which Gaensicke et al. (2008) have proposed that the
unidentified broad absorption features in the spectrum of SCP06F6 are C
Swan bands, and based on a crude agreement with the Ca H&K and UV
iron-peak absorption features that are characteristic of supernovae of various
types. The ejected masses and kinetic energies are smaller for a more tightly
constrained model invoking envelope recombination. We also discuss the
possibilities of circumstellar matter (CSM) shell diffusion and shock
interaction models. In general, optically-thick CSM diffusion models can fit
the data with the underlying energy coming from an energetic buried supernova.
Models in which the CSM is of lower density so that the shock energy is both
rapidly thermalized and radiated tend not to be self-consistent. We suggest
that a model of SCP06F6 worth futher exploration is one in which the redshift
is 0.57, the spectral features are Ca and iron peak elements, and the
light curve is powered by the diffusive release of a substantial amount of
energy from nickel decay or from an energetic supernova buried in the ejecta of
an LBV-like event.Comment: 27 pages, 6 figure
A new look at the pulsating DB white dwarf GD 358:Line-of-sight velocity measurements and constraints on model atmospheres
We report on our findings of the bright, pulsating, helium atmosphere white
dwarf GD 358, based on time-resolved optical spectrophotometry. We identify 5
real pulsation modes and at least 6 combination modes at frequencies consistent
with those found in previous observations. The measured Doppler shifts from our
spectra show variations with amplitudes of up to 5.5 km/s at the frequencies
inferred from the flux variations. We conclude that these are variations in the
line-of-sight velocities associated with the pulsational motion. We use the
observed flux and velocity amplitudes and phases to test theoretical
predictions within the convective driving framework, and compare these with
similar observations of the hydrogen atmosphere white dwarf pulsators (DAVs).
The wavelength dependence of the fractional pulsation amplitudes (chromatic
amplitudes) allows us to conclude that all five real modes share the same
spherical degree, most likely, l=1. This is consistent with previous
identifications based solely on photometry. We find that a high signal-to-noise
mean spectrum on its own is not enough to determine the atmospheric parameters
and that there are small but significant discrepancies between the observations
and model atmospheres. The source of these remains to be identified. While we
infer T_eff=24kK and log g~8.0 from the mean spectrum, the chromatic
amplitudes, which are a measure of the derivative of the flux with respect to
the temperature, unambiguously favour a higher effective temperature, 27kK,
which is more in line with independent determinations from ultra-violet
spectra.Comment: 14 pages, 11 figures; accepted for publication in A&
A puzzling periodicity in the pulsating DA white dwarf G 117-B15A
We present time-resolved optical spectrophotometry of the pulsating hydrogen
atmosphere (DA) white dwarf G 117-B15A. We find three periodicities in the
pulsation spectrum (215s, 272s, and 304s) all of which have been found in
earlier studies. By comparing the fractional wavelength dependence of the
pulsation amplitudes (chromatic amplitudes) with models, we confirm a previous
report that the strongest mode, at 215s, has l=1. The chromatic amplitude for
the 272s mode is very puzzling, showing an increase in fractional amplitude
with wavelength that cannot be reproduced by the models for any l at optical
wavelengths. Based on archival HST data, we show that while the behaviour of
the 215s mode at ultra-violet wavelengths is as expected from models, the weird
behaviour of the 272s periodicity is not restricted to optical wavelengths in
that it fails to show the expected increase in fractional amplitude towards
shorter wavelengths. We discuss possible causes for the discrepancies found for
the 272s variation, but find that all are lacking, and conclude that the nature
of this periodicity remains unclear.Comment: 9 pages, 9 figures; accepted for publication in A&
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
Optical and Infrared Analysis of Type II SN 2006BC
We present nebular phase optical imaging and spectroscopy and near/mid-IR
imaging of the Type II SN 2006bc. Observations reveal the central wavelength of
the symmetric H line profile to be red-shifted with respect to the host
galaxy H emission by day 325. Such an phenomenon has been argued to
result from an asymmetric explosion in the iron-peak elements resulting in a
larger mass of Ni and higher excitation of hydrogen on the far side of
the SN explosion. We also observe a gradual blue-shifting of this H
peak which is indicative of dust formation in the ejecta. Although showing a
normal peak brightness, V -17.2, for a core-collapse SN, 2006bc fades by
6 mag during the first 400 days suggesting either a relatively low
Ni yield, an increase in extinction due to new dust, or both. A short
duration flattening of the light curve is observed from day 416 to day 541
suggesting an optical light echo. Based on the narrow time window of this echo,
we discuss implications on the location and geometry of the reflecting ISM.
With our radiative transfer models, we find an upper limit of 2 x 10
M of dust around SN 2006bc. In the event that all of this dust were
formed during the SN explosion, this quantity of dust is still several orders
of magnitude lower than that needed to explain the large quantities of dust
observed in the early universe.Comment: 6 pages, 10 figures, accepted for publication in Ap
Revisiting the progenitor of the low-luminosity type II-plateau supernova, SN 2008bk
The availability of updated model atmospheres for red supergiants and
improvements in single and binary stellar evolution models, as well as
previously unpublished data prompted us to revisit the progenitor of
low-luminosity type II-Plateau supernova, SN 2008bk. Using mid-IR data in
combination with dust models, we find that high temperature (4250-4500 K), high
extinction (E(B-V)>0.7) solutions are incompatible with the data. We therefore
favour a cool (~3500-3700 K) progenitor with a luminosity of log(L/Lsun)~4.53.
Comparing with evolutionary tracks, we infer progenitor masses in the 8-10 Msun
range in agreement with some previous studies. This mass is consistent with the
observed pattern of low-luminosity Type IIP SNe coming from the explosion of
RSGs at the lower extremum for core-collapse. We also present multi-epoch data
of the progenitor, but do not find clear evidence of variability.Comment: 9 pages, 6 figure
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