1,986 research outputs found
Recent progress in understanding the eruptions of classical novae
Dramatic progress has occurred in the last two decades in understanding the physical processes and events leading up to, and transpiring during the eruption of a classical nova. The mechanism whereby a white dwarf accreting hydrogen-rich matter from a low-mass main-sequence companion produces a nova eruption has been understood since 1970. The mass-transferring binary stellar configuration leads inexorably to thermonuclear runaways detected at distances of megaparsecs. Summarized here are the efforts of many researchers in understanding the physical processes which generate nova eruptions; the effects upon nova eruptions of different binary-system parameters (e.g., chemical composition or mass of the white dwarf, different mass accretion rates); the possible metamorphosis from dwarf to classical novae and back again; and observational diagnostics of novae, including x ray and gamma ray emission, and the characteristics and distributions of novae in globular clusters and in extragalactic systems. While the thermonuclear-runaway model remains the successful cornerstone of nova simulation, it is now clear that a wide variety of physical processes, and three-dimensional hydrodynamic simulations, will be needed to explain the rich spectrum of behavior observed in erupting novae
A direct N-body model of core-collapse and core oscillations
We report on the results of a direct N-body simulation of a star cluster that
started with N = 200 000, comprising 195 000 single stars and 5 000 primordial
binaries. The code used for the simulation includes stellar evolution, binary
evolution, an external tidal field and the effects of two-body relaxation. The
model cluster is evolved to 12 Gyr, losing more than 80% of its stars in the
process. It reaches the end of the main core-collapse phase at 10.5 Gyr and
experiences core oscillations from that point onwards -- direct numerical
confirmation of this phenomenon. However, we find that after a further 1 Gyr
the core oscillations are halted by the ejection of a massive binary comprised
of two black holes from the core, producing a core that shows no signature of
the prior core-collapse. We also show that the results of previous studies with
N ranging from 500 to 100 000 scale well to this new model with larger N. In
particular, the timescale to core-collapse (in units of the relaxation
timescale), mass segregation, velocity dispersion, and the energies of the
binary population all show similar behaviour at different N.Comment: 9 pages, 8 figures, accepted for publication in MNRA
Progenitor constraints on the Type-Ia supernova SN2011fe from pre-explosion Hubble Space Telescope HeII narrow-band observations
We present Hubble Space Telescope (HST) imaging observations of the site of
the Type-Ia supernova SN2011fe in the nearby galaxy M101, obtained about one
year prior to the event, in a narrow band centred on the HeII 4686 \AA{}
emission line. In a "single-degenerate" progenitor scenario, the hard photon
flux from an accreting white dwarf (WD), burning hydrogen on its surface over
Myr should, in principle, create a HeIII Str\"{o}mgren sphere or shell
surrounding the WD. Depending on the WD luminosity, the interstellar density,
and the velocity of an outflow from the WD, the HeIII region could appear
unresolved, extended, or as a ring, with a range of possible surface
brightnesses. We find no trace of HeII 4686 \AA{} line emission in the HST
data. Using simulations, we set upper limits on the HeII 4686 \AA{}
luminosity of erg s for a point
source, corresponding to an emission region of radius pc. The upper
limit for an extended source is erg
s, corresponding to an extended region with pc. The largest
detectable shell, given an interstellar-medium density of 1 cm, has a
radius of pc. Our results argue against the presence, within the
yr prior to the explosion, of a supersoft X-ray source of luminosity
erg s, or of a super-Eddington
accreting WD that produces an outflowing wind capable of producing cavities
with radii of 2-6 pc.Comment: Accepted by MNRAS Letters; revised version following referee report
and readers' comment
Growth and Development in Lady Beetles
Living in a group can potentially put stress on an animal. This is particularly true for species like Hippodamia convergens, the convergent lady beetle, the larvae of which will cannibalize each other when food sources are limited. This can raise the question of whether or not living in a group affects the growth and development of an individual, and how the growth rate might differ for an individual who is raised alone. This experiment attempts to answer that question by comparing 12 colonies of lady beetles, some of which were reared in groups, some reared alone, and recording the pupation lengths. The hypothesis was that the individuals raised alone would grow and pupate faster than the groups, because they would not have to compete for space or resources. It was ultimately found that while pupation and hatch dates for individual lady beetles were far more variable, there were no large differences in the average length of pupation between groups and individuals
An Extended Grid of Nova Models. III. Very Luminous, Red Novae
Extremely luminous, red eruptive variables like RV in M31 are being suggested
as exemplars of a new class of astrophysical object. Our greatly extended
series of nova simulations shows that classical nova models can produce very
red, luminous eruptions. In a poorly studied corner of 3-D nova parameter space
(very cold, low-mass white dwarfs, accreting at very low rates) we find bona
fide classical novae that are very luminous and red because they eject very
slowly moving, massive envelopes. A crucial prediction of these nova models -
in contrast to the predictions of merging star ("mergeburst") models - is that
a hot remnant, the underlying white dwarf, will emerge after the massive
ejected envelope has expanded enough to become optically thin. This blue
remnant must fade on a timescale of decades - much faster than a "mergeburst",
which must fade on timescales of millennia or longer. Furthermore, the cooling
nova white dwarf and its expanding ejecta must become redder in the years after
eruption, while a contracting mergeburst must become hotter and bluer. We
predict that red novae will always brighten to L~1000 Lsun for about 1 year
before rising to maximum luminosity at L~10^6 - 10^7 Lsun. The maximum
luminosity attainable by a nova is likely to be L~10^7 Lsun, corresponding to
M-12. In an accompanying paper we describe a fading, luminous blue candidate
for the remnant of M31-RV; it is observed with HST to be compatible only with
the nova model.Comment: Accepted for publication in ApJ. Comments welcom
REPRESENTATION OF INDONESIA IN THE 2015 SOUTHEAST ASIA’S HAZE IN THE GUARDIAN AND THE NEW YORK TIMES
Pada tahun 2015, Indonesia bertanggung jawab dalam masalah lingkungan yang berasal dari kabut asap akibat kebakaran hutan yang tersebar hingga ke beberapa negara tetangga. Penelitian ini bertujuan untuk mengetahui representasi Indonesia di koran the Guardian dan the New York Times perihal kabut asap di Asia Tenggara tahun 2015, dan untuk mengungkap ideologi di dalamnya. Penelitian ini menggunakan metode kualitatif dan pendekatan Sociocognitive to Critical Discourse Analysis untuk menganalisis semantik makrostruktur serta menafsirkan hasil yang diperoleh. Data yang digunakan ialah enam artikel dari the Guardian dan tiga artikel dari the New York Times. Hasilnya menunjukkan bahwa Indonesia direpresentasikan secara negatif pada kedua koran Barat tersebut dan ideologi yang dominan di balik representasi tersebut ialah kapitalisme.;--- In 2015 Indonesia was held responsible for an environmental problem caused by the haze resulting from forest fires that spread across several neighbouring countries. The present research aims to examine the representation of Indonesia in the Guardian and the New York Times regarding the 2015 Southeast Asia’s haze and to uncover ideology underlying such a representation. This study employs the qualitative method and the sociocognitive approach to Critical Discourse Analysis to conduct the semantic macrostructures analysis and interpret the results. The data used in the research include six articles from the Guardian and three articles from the New York Times. The results reveal that Indonesia is mostly represented negatively in both Western newspapers. The dominant ideology behind the representations in both newspapers is capitalism
Non-Equipartition of Energy, Masses of Nova Ejecta, and Type Ia Supernovae
The total masses ejected during classical nova eruptions are needed to answer
two questions with broad astrophysical implications: Can accreting white dwarfs
be pushed towards the Chandrasekhar mass limit to yield type Ia supernovae? Are
Ultra-luminous red variables a new kind of astrophysical phenomenon, or merely
extreme classical novae? We review the methods used to determine nova ejecta
masses. Except for the unique case of BT Mon (nova 1939), all nova ejecta mass
determinations depend on untested assumptions and multi-parameter modeling. The
remarkably simple assumption of equipartition between kinetic and radiated
energy (E_kin and E_rad, respectively) in nova ejecta has been invoked as a way
around this conundrum for the ultra-luminous red variable in M31. The deduced
mass is far larger than that produced by any classical nova model. Our nova
eruption simulations show that radiation and kinetic energy in nova ejecta are
very far from being in energy equipartition, with variations of four orders of
magnitude in the ratio E_kin/E_rad being commonplace. The assumption of
equipartition must not be used to deduce nova ejecta masses; any such
"determinations" can be overestimates by a factor of up to 10,000. We
data-mined our extensive series of nova simulations to search for correlations
that could yield nova ejecta masses. Remarkably, the mass ejected during a nova
eruption is dependent only on (and is directly proportional to) E_rad. If we
measure the distance to an erupting nova and its bolometric light curve then
E_rad and hence the mass ejected can be directly measured.Comment: 9 pages, 4 figures, awaiting publication in ApJ
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