14,255 research outputs found
He II recombination lines as a test of the nature of SN Ia progenitors in elliptical galaxies
To date, the question of which progenitor channel can reproduce the observed
rate of Type Ia supernovae (SNe Ia) remains unresolved, with the single and
double degenerate scenarios remaining the leading contenders. The former
implies a large population of hot accreting white dwarfs with photospheric
temperatures of T ~ 10^5-10^6 K during some part of their accretion history. We
show that in early-type galaxies, a population of accreting white dwarfs large
enough to reproduce the SN Ia rate would contribute significantly to the
ionizing UV radiation expected from the stellar population. For mean stellar
ages < ~5 Gyr, single degenerate progenitors would dominate the ionizing
background produced by stars, increasing the continuum beyond the He
II-ionizing limit more than ten-fold. This opens a new avenue for constraining
the progenitors of SNe Ia, through consideration of the spatially extended
low-ionization emission-line regions now found in many early-type galaxies.
Modelling the expected emission, we show that one can constrain the
contribution of the single degenerate channel to the SN Ia rate in E/S0
galaxies from upper limits on the luminosity of He II recombination lines in
the optical and FUV. We discuss future directions, as well as possible
implications for the evolution of SNe Ia in old stellar populations.Comment: 12 pages, 13 figures, MNRA
Cultural trauma: Ron Eyerman and the founding of a new research paradigm
The field of cultural trauma has reached the status of a research paradigm. Ron Eyerman has played a central role in this development. Since he first embarked on research into cultural trauma with several colleagues in 1999, Eyerman has maintained an intensive preoccupation with the topic, resulting in the publication of numerous books and essays. In this article, I review the development of Eyerman’s approach to cultural trauma, with the broader aim of shedding light on this new research paradigm. I focus on several key themes in Eyerman’s work, including the relationship between event and representation; the significance of affect and emotion; the role of collective memory; the adoption of a dramaturgical perspective; and a multidimensional research methodology. To conclude, I discuss potential new directions in the study of cultural trauma
Balmer-Dominated Shocks Exclude Hot Progenitors for Many Type Ia Supernovae
The evolutionary mechanism underlying Type Ia supernova explosions remains
unknown. Recent efforts to constrain progenitor models based on the influence
that their high energy emission would have on the interstellar medium (ISM) of
galaxies have proven successful. For individual remnants, Balmer-dominated
shocks reveal the ionization state of hydrogen in the immediately surrounding
gas. Here we report deep upper limits on the temperature and luminosity of the
progenitors of four Type Ia remnants with associated Balmer filaments: SN 1006,
0509-67.5, 0519-69.0, and DEM L71. For SN 1006, existing observations of helium
line emission in the diffuse emission ahead of the shock provide an additional
constraint on the helium ionization state in the vicinity of the remnant. Using
the photoionization code Cloudy, we show that these constraints exclude any
hot, luminous progenitor for SN 1006, including stably hydrogen or helium
nuclear-burning white dwarfs, as well as any Chandrasekhar-mass white dwarf
accreting matter at yr via a disk. For
0509-67.5, the Balmer emission alone rules out any such white dwarf accreting
yr. For 0519-69.0 and DEM L71, the inferred
ambient ionization state of hydrogen is only weakly in tension with a recently
hot, luminous progenitor, and cannot be distinguished from e.g., a relatively
higher local Lyman continuum background, without additional line measurements.
Future deep spectroscopic observations will resolve this ambiguity, and can
either detect the influence of any luminous progenitor or rule out the same for
all resolved SN Ia remnants.Comment: 9 pages, 3 figures, 1 table. Accepted for publication in Ap
No hot and luminous progenitor for Tycho's supernova
Type Ia supernovae have proven vital to our understanding of cosmology, both
as standard candles and for their role in galactic chemical evolution; however,
their origin remains uncertain. The canonical accretion model implies a hot and
luminous progenitor which would ionize the surrounding gas out to a radius of
10--100 parsecs for 100,000 years after the explosion. Here we
report stringent upper limits on the temperature and luminosity of the
progenitor of Tycho's supernova (SN 1572), determined using the remnant itself
as a probe of its environment. Hot, luminous progenitors that would have
produced a greater hydrogen ionization fraction than that measured at the
radius of the present remnant (3 parsecs) can thus be excluded. This
conclusively rules out steadily nuclear-burning white dwarfs (supersoft X-ray
sources), as well as disk emission from a Chandrasekhar-mass white dwarf
accreting yr (recurrent novae). The lack of a
surrounding Str\"omgren sphere is consistent with the merger of a double white
dwarf binary, although other more exotic scenarios may be possible.Comment: 17 pages, 2 figures, including supplementary information. Original
accepted manuscript (before copyediting/formatting by Nature Astronomy
Upper limits on the luminosity of the progenitor of type Ia supernova SN2014J
We analysed archival data of Chandra pre-explosion observations of the
position of SN2014J in M82. No X-ray source at this position was detected in
the data, and we calculated upper limits on the luminosities of the progenitor.
These upper limits allow us to firmly rule out an unobscured supersoft X-ray
source progenitor with a photospheric radius comparable to the radius of white
dwarf near the Chandrasekhar mass (~1.38 M_sun) and mass accretion rate in the
interval where stable nuclear burning can occur. However, due to a relatively
large hydrogen column density implied by optical observations of the supernova,
we cannot exclude a supersoft source with lower temperatures, kT < 80 eV. We
find that the supernova is located in the centre of a large structure of soft
diffuse emission, about 200 pc across. The mass, ~3x10^4 M_sun and short
cooling time of the gas, tau_cool ~ 8 Myrs, suggest that it is a
supernova-inflated super-bubble, associated with the region of recent star
formation. If SN2014J is indeed located inside the bubble, it likely belongs to
the prompt population of type Ia supernovae, with a delay time as short as ~ 50
Myrs. Finally, we analysed the one existing post-supernova Chandra observation
and placed upper limit of ~ (1-2) 10^37 erg/s on the X-ray luminosity of the
supernova itself.Comment: 8 pages, 6 figure
The Formation of Low-Mass Double White Dwarfs through an Initial Phase of Stable Non-Conservative Mass Transfer
Although many double white dwarfs (DWDs) have been observed, the evolutionary
channel by which they are formed from low-mass/long-period
red-giant-main-sequence (RG-MS) binaries remains uncertain. The canonical
explanations involve some variant of double common-envelope (CE) evolution,
however it has been found that such a mechanism cannot produce the observed
distribution. We present a model for the initial episode of mass transfer (MT)
in RG-MS binaries, and demonstrate that their evolution into double white
dwarfs need not arise through a double-CE process, as long as the initial
primary's core mass (Md,c) does not exceed 0.46M. Instead, the first
episode of dramatic mass loss may be stable, non-conservative MT. We find a
lower bound on the fraction of transferred mass that must be lost from the
system in order to provide for MT, and demonstrate the feasibility of this
channel in producing observed low-mass (with M < 0.46M) DWD
systems.Comment: 2 pages, 1 figure, Conference Proceedings for the International
Conference on Binaries, Mykonos, Greec
On the Maximum Mass of Accreting Primordial Supermassive Stars
Supermassive primordial stars are suspected to be the progenitors of the most
massive quasars at z~6. Previous studies of such stars were either unable to
resolve hydrodynamical timescales or considered stars in isolation, not in the
extreme accretion flows in which they actually form. Therefore, they could not
self-consistently predict their final masses at collapse, or those of the
resulting supermassive black hole seeds, but rather invoked comparison to
simple polytropic models. Here, we systematically examine the birth, evolution
and collapse of accreting non-rotating supermassive stars under accretion rates
of 0.01-10 solar masses per year, using the stellar evolution code KEPLER. Our
approach includes post-Newtonian corrections to the stellar structure and an
adaptive nuclear network, and can transition to following the hydrodynamic
evolution of supermassive stars after they encounter the general relativistic
instability. We find that this instability triggers the collapse of the star at
masses of 150,000-330,000 solar masses for accretion rates of 0.1-10 solar
masses per year, and that the final mass of the star scales roughly
logarithmically with the rate. The structure of the star, and thus its
stability against collapse, is sensitive to the treatment of convection, and
the heat content of the outer accreted envelope. Comparison with other codes
suggests differences here may lead to small deviations in the evolutionary
state of the star as a function of time, that worsen with accretion rate. Since
the general relativistic instability leads to the immediate death of these
stars, our models place an upper limit on the masses of the first quasars at
birth.Comment: 5 pages, 4 figures. Accepted ApJ letter
Population synthesis of accreting white dwarfs: II. X-ray and UV emission
Accreting white dwarfs (WDs) with non-degenerate companions are expected to
emit in soft X-rays and the UV, if accreted H-rich material burns stably. They
are an important component of the unresolved emission of elliptical galaxies,
and their combined ionizing luminosity may significantly influence the optical
line emission from warm ISM. In an earlier paper we modeled populations of
accreting WDs, first generating WD with main-sequence, Hertzsprung gap and red
giant companions with the population synthesis code \textsc{BSE}, and then
following their evolution with a grid of evolutionary tracks computed with
\textsc{MESA}. Now we use these results to estimate the soft X-ray
(0.3-0.7keV), H- and He II-ionizing luminosities of nuclear burning WDs and the
number of super-soft X-ray sources for galaxies with different star formation
histories. For the starburst case, these quantities peak at Gyr and
decline by orders of magnitude by the age of 10 Gyr. For stellar
ages of ~10 Gyr, predictions of our model are consistent with soft X-ray
luminosities observed by Chandra in nearby elliptical galaxies and He II
4686 line ratio measured in stacked SDSS spectra of retired
galaxies, the latter characterising the strength and hardness of the UV
radiation field. However, the soft X-ray luminosity and
He~II~4686 ratio are significantly overpredicted for stellar
ages of Gyr. We discuss various possibilities to resolve this
discrepancy and tentatively conclude that it may be resolved by a modification
of the typically used criteria of dynamically unstable mass loss for giant
stars.Comment: 13 pages, 12 figures, MNRAS accepte
Interstellar H^+_3: possible detection of the 1_(10)→1_(11) transition of H_2D^+
An interstellar line has been detected in emission at the expected submillimeter wavelength of the 1_(10)→1_(11) transition of H_(2)D^+, the deuterated version of the primary ion (H^(+)_(3)) in the favored ion-molecule reaction scheme for interstellar gas phase chemistry. The strength of the line is in approximate agreement with the theoretically anticipated H_(2)D^+ abundance
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