40 research outputs found
Explosion and nucleosynthesis of low redshift pair instability supernovae
Both recent observations and stellar evolution models suggest that
pair-instability supernovae (PISNe) could occur in the local Universe, at
metallicities below Z_Sun/3. Previous PISN models were mostly produced at very
low metallicities in the context of the early Universe. We present new PISNe
models at a metallicity of Z=0.001, which are relevant for the local Universe.
We take the self-consistent stellar evolutionary models of pair-instability
progenitors with initial masses of 150 and 250 solar masses at metallicity of
Z=0.001 by Langer et al. (2007) and follow the evolution of these models
through the supernova explosions, using a hydrodynamics stellar evolution code
with an extensive nuclear network including 200 isotopes. Both models explode
as PISNe without leaving a compact stellar remnant. Our models produce a
nucleosynthetic pattern that is generally similar to that of Population III
PISN models, which is mainly characterized by the production of large amounts
of alpha-elements and a strong deficiency of the odd-charged elements. However,
the odd-even effect in our models is significantly weaker than that found in
Population III models. The comparison with the nucleosynthetic yields from
core-collapse supernovae at a similar metallicity (Z=0.002) indicates that
PISNe could have strongly influenced the chemical evolution below Z=0.002,
assuming a standard initial mass function. The odd-even effect is predicted to
be most prominent for the intermediate mass elements between silicon and
calcium. With future observations of chemical abundances in Population II
stars, our result can be used to constrain the number of PISNe that occurred
during the past evolution of our Galaxy.Comment: 10 pages, 13 figures, 3 tables. Accepted by Astronomy & Astrophysic
How much radioactive nickel does ASASSN-15lh require?
The discovery of the most luminous supernova ASASSN-15lh triggered a
shock-wave in the supernova community. The three possible mechanisms proposed
for the majority of other superluminous supernovae do not produce a realistic
physical model for this particular supernova. In the present study we show the
limiting luminosity available from a nickel-powered pair-instability supernova.
We computed a few exotic nickel-powered explosions with a total mass of nickel
up to 1500 solar masses. We used the hydrostatic configurations prepared with
the GENEVA and MESA codes, and the STELLA radiative-transfer code for following
the explosion of these models. We show that 1500 solar masses of radioactive
nickel is needed to power a luminosity of 2x10^45 erg/s. The resulting light
curve is very broad and incompatible with the shorter ASASSN-15lh time-scale.
This rules out a nickel-powered origin of ASASSN-15lh. In addition, we derive a
simple peak luminosity - nickel mass relation from our data, which may serve to
estimate of nickel mass from observed peak luminosities.Comment: accepted for publication in MNRAS Letter
New soft gamma-ray bursts in the BATSE records and spectral properties of X-ray rich bursts
A population of X-ray dominated gamma-ray bursts (GRBs) observed by Ginga,
BeppoSax and Hete-2 should be represented in the BATSE data as presumably soft
bursts. We have performed a search for soft GRBs in the BATSE records in the
25--100 keV energy band. A softness of a burst spectrum can be a reason why it
has been missed by the on-board procedure and previous searches for untriggered
GRBs tuned to 50--300 keV range. We have found a surprisingly small number
(~20/yr down to 0.1 ph cm s) of soft GRBs where the count rate is
dominated by 25--50 keV energy channel. This fact as well as the analysis of
HETE-2 and common BeppoSAX/BATSE GRBs indicates that the majority of GRBs with
a low Epeak has a relatively hard tail with the high-energy power-law photon
index >-3. An exponential cutoff in GRB spectra below 20 keV may be a
distinguishing feature separating non-GRB events.Comment: Submitted to MNRAS, 5 pages, 3 figures, 2 table
Shock breakouts from red supergiants: analytical and numerical predictions
Shock breakout (SBO) signal is the first signature of the supernova explosion
apart from gravitational waves and neutrinos. Observational properties of SBO,
such as bolometric luminosity and colour temperature, connect to the supernova
progenitor and explosion parameters. Detecting SBO or SBO-cooling will
constrain the progenitor and explosion models of collapsing stars. In the light
of recently launched eROSITA telescope, the rate for detection of SBO is a few
events during a year. In the current study, we examine the analytic formulae
derived by Shussman et al. (2016). We use four red supergiant models from their
study, while running explosions with the radiation hydrodynamics code STELLA.
We conclude that there is a good agreement between analytic and numerical
approaches for bolometric luminosity and colour temperature during SBO. The
analytic formulae for the SBO signal based on the global supernova parameters
can be used instead of running time-consuming numerical simulations. We define
spectral range where analytic formulae for the SBO spectra are valid. We
provide improved analytical expression for the SBO spectral energy
distribution. We confirm dependence of colour temperature on radius derived by
analytical studies and suggest to use early time observations to confine the
progenitor radius. Additionally we show the prediction for the SBO signal from
red supergiants as seen by eROSITA instrument.Comment: 12 figures, 2 tables. Accepted for publication in MNRA
Formation of star clusters and enrichment by massive stars in simulations of low-metallicity galaxies with a fully sampled initial stellar mass function
We present new GRIFFIN project hydrodynamical simulations that model the
formation of galactic star cluster populations in low-metallicity ()
dwarf galaxies, including radiation, supernova and stellar wind feedback of
individual massive stars. In the simulations, stars are sampled from the
stellar initial mass function (IMF) down to the hydrogen burning limit of
M. Mass conservation is enforced within a radius of pc for
the formation of massive stars. We find that massive stars are preferentially
found in star clusters and follow a correlation set at birth between the
highest initial stellar mass and the star cluster mass that differs from pure
stochastic IMF sampling. With a fully sampled IMF, star clusters lose mass in
the galactic tidal field according to mass-loss rates observed in nearby
galaxies. Of the released stellar feedback, of the supernova material
and up to of the wind material reside either in the hot interstellar
medium (ISM) or in gaseous, metal enriched outflows. While stellar winds
(instantaneously) and supernovae (delayed) start enriching the ISM right after
the first massive stars form, the formation of supernova-enriched stars and
star clusters is significantly delayed (by Myr) compared to the formation
of stars and star clusters enriched by stellar winds. Overall, supernova ejecta
dominate the enrichment by mass, while the number of enriched stars is
determined by continuous stellar winds. These results present a concept for the
formation of chemically distinct populations of stars in bound star clusters,
reminiscent of multiple populations in globular clusters.Comment: 26 pages, 23 figures. Accepted for publication in MNRA
SN 2021zny: an early flux excess combined with late-time oxygen emission suggests a double white dwarf merger event
We present a photometric and spectroscopic analysis of the ultra-luminous and
slowly evolving 03fg-like Type Ia SN 2021zny. Our observational campaign starts
from hours after explosion (making SN 2021zny one of the earliest
observed members of its class), with dense multi-wavelength coverage from a
variety of ground- and space-based telescopes, and is concluded with a nebular
spectrum months after peak brightness. SN 2021zny displayed several
characteristics of its class, such as the peak brightness ( mag),
the slow decline ( mag), the blue early-time colours,
the low ejecta velocities and the presence of significant unburned material
above the photosphere. However, a flux excess for the first days
after explosion is observed in four photometric bands, making SN 2021zny the
third 03fg-like event with this distinct behavior, while its d spectrum
shows prominent [O I] lines, a very unusual characteristic of thermonuclear
SNe. The early flux excess can be explained as the outcome of the interaction
of the ejecta with of H/He-poor circumstellar
material at a distance of cm, while the low ionization state of
the late-time spectrum reveals low abundances of stable iron-peak elements. All
our observations are in accordance with a progenitor system of two
carbon/oxygen white dwarfs that undergo a merger event, with the disrupted
white dwarf ejecting carbon-rich circumstellar material prior to the primary
white dwarf detonation.Comment: 19 pages, 16 figures, accepted for publication in MNRA