139 research outputs found
A unified model for the spectrophotometric development of classical and recurrent novae: the role of asphericity of the ejecta
There is increasing evidence that the geometry, and not only the filling
factors, of nova ejecta is important in the interpretation of their spectral
and photometric developments. Ensembles of spectra and light curves have
provided general typographies. This Letter suggests how these can be
unified.The observed spread in the maximum magnitude - rate of decline (MMRD)
relation is argued to result from the range of opening angles and inclination
of the ejecta, and not only to their masses and velocities. The spectroscopic
classes can be similarly explained and linked to the behavior of the light
curves. The secondary maximum observed in some dust forming novae is a natural
consequence of the asphericity. Neither secondary ejections nor winds are
needed to explain the phenomenology. The spectrophotometric development of
classical novae can be understood within a single phenomenological model with
bipolar, although not jet-like, mass ejecta. High resolution spectropolarimetry
will be an essential analytical tool.Comment: 4 pages, no figures; accepted for A&A Letters (in press
The Galactic Center Isolated Nonthermal Filaments as Analogs of Cometary Plasma Tails
We propose a model for the origin of the isolated nonthermal filaments
observed at the Galactic center based on an analogy to cometary plasma tails.
We invoke the interaction between a large scale magnetized galactic wind and
embedded molecular clouds. As the advected wind magnetic field encounters a
dense molecular cloud, it is impeded and drapes around the cloud, ultimately
forming a current sheet in the wake. This draped field is further stretched by
the wind flow into a long, thin filament whose aspect ratio is determined by
the balance between the dynamical wind and amplified magnetic field pressures.
The key feature of this cometary model is that the filaments are dynamic
configurations, and not static structures. As such, they are local
amplifications of an otherwise weak field and not directly connected to any
static global field. The derived field strengths for the wind and wake are
consistent with observational estimates. Finally, the observed synchrotron
emission is naturally explained by the acceleration of electrons to high energy
by plasma and MHD turbulence generated in the cloud wake.Comment: Uses AAS aasms4.sty macros. ApJ (in press, vol. 521, 20 Aug
The slow decline of the Galactic recurrent novae T Pyxidis, IM Normae, and CI Aquilae
A distinguishing trait of the three known Galactic recurrent novae with the
shortest orbital periods, T Pyx, IM Nor, and CI Aql, is that their optical
decline time-scales are significantly longer than those of the other recurrent
systems. On the other hand, some estimates of the mass of the ejecta, the
velocity of the ejecta, and the duration of the soft X-rays emission of these
systems are of the order of those of the other recurrent systems and the fast
classical novae. We put forth a tentative explanation of this phenomenon. We
propose that in these systems part of the material transferred from the
companion during the first few days of the eruption remains within the Roche
lobe of the white dwarf, preventing the radiation from ionizing the ejecta of
the system and increasing the optical decline time-scale. We explain why this
phenomenon is more likely in systems with a high mass transfer rate and a short
orbital period. Finally, we present a schematic model that shows that the
material transferred from the companion is sufficient to absorb the radiation
from the white dwarf in these systems, ultimately supporting this scenario as
quantitatively realistic
Colliding stellar winds in the eclipsing Wolf-Rayet binary V444 Cygni
High resolution spectra of V444 Cygni have been obtained using the International Ultraviolet Explorer Satellite. These spectra span both eclipses and include one observation at third quadrature. Together with seven archival spectra, they provide reasonably complete phase coverage for the system. The variations in the P Cygni profiles of the He(II) and N(IV) lines, imply the existence of a low density region in the WR wind. This region occupies a relatively narrow range of orbital phase coinciding with the highest terminal velocities observed in C IV. These data are interpreted to be evidence of an interaction region separating the winds of the O-star and Wolf-Rayet star
Spectroscopic diagnostics of dust formation and evolution in classical nova ejecta
A fraction of classical novae form dust during the early stages of their
outbursts. The classical CO nova V5668 Sgr (Nova Sgr. 2015b) underwent a deep
photometric minimum about 100 days after outburst that was covered across the
spectrum. A similar event was observed for an earlier CO nova, V705 Cas (Nova
Cas 1993) and a less optically significant event for the more recent CO nova
V339 Del (Nova Del 2013). This study provides a "compare and contrast" of these
events to better understand the very dynamical event of dust formation. We show
the effect of dust formation on multiwavelength high resolution line profiles
in the interval 1200\AA\ - 9200\AA\ using a biconical ballistic structure that
has been applied in our previous studies of the ejecta. We find that both V5668
Sgr and V339 Del can be modeled using a grey opacity for the dust, indicating
fairly large grains (at least 0.1 micron) and that the persistent asymmetries
of the line profiles in late time spectra, up to 650 days after the event for
V5668 Sgr and 866 days for V339 Del, point to the survival of the dust well
into the transparent, nebular stage of the ejecta evolution. This is a general
method for assessing the properties of dust forming novae well after the
infrared is completely transparent in the ejecta.Comment: 15 pages 14 figures, accepted for publication in A&A, 2018 June 2
Three-dimensional simulations of turbulent convective mixing in ONe and CO classical nova explosions
Classical novae are thermonuclear explosions that take place in the envelopes
of accreting white dwarfs in binary systems. The material piles up under
degenerate conditions, driving a thermonuclear runaway. The energy released by
the suite of nuclear processes operating at the envelope heats the material up
to peak temperatures about 100 - 400 MK. During these events, about 10-3 - 10-7
Msun, enriched in CNO and, sometimes, other intermediate-mass elements (e.g.,
Ne, Na, Mg, Al) are ejected into the interstellar medium. To account for the
gross observational properties of classical novae (in particular, the large
concentrations of metals spectroscopically inferred in the ejecta), models
require mixing between the (solar-like) material transferred from the secondary
and the outermost layers (CO- or ONe-rich) of the underlying white dwarf.
Recent multidimensional simulations have demonstrated that Kelvin-Helmholtz
instabilities can naturally produce self-enrichment of the accreted envelope
with material from the underlying white dwarf at levels that agree with
observations. However, the feasibility of this mechanism has been explored in
the framework of CO white dwarfs, while mixing with different substrates still
needs to be properly addressed. Three-dimensional simulations of mixing at the
core-envelope interface during nova outbursts have been performed with the
multidimensional code FLASH, for two types of substrates: CO- and ONe-rich. We
show that the presence of an ONe-rich substrate, as in "neon novae", yields
larger metallicity enhancements in the ejecta, compared to CO,rich substrates
(i.e., non-neon novae). A number of requirements and constraints for such 3-D
simulations (e.g., minimum resolution, size of the computational domain) are
also outlined.Comment: Accepted for publication in Astronomy & Astrophysic
Revisiting V1309 Sco 2008 outburst spectra. Observational evidence for theoretical modeling of stellar mergers
CONTEXT: V1309 Sco is the only certain noncompact stellar merger, due to its
indisputable preoutburst light curve matching that of a contact binary of
almost equal mass stars. Therefore, anything that can be deduced from the
existing observations serves as benchmark constraints for models. AIMS: We
present some observational evidences to guide future hydrodynamical simulations
and common envelope studies. METHODS: Using archive spectra taken at high and
mid spectral resolution during the V1309 Sco outburst and late decline,
together with the inferential methods we developed to study nova ejecta through
panchromatic high resolution spectroscopic follow ups, we constrain the
physical state, structure, dynamics and geometry of the transient originated in
the stellar merger. RESULTS: We found that the emitted spectra arise from two
distinct contributions: matter expelled during the 2008 outburst and
circumbinary gas produced during historic mass loss episodes. These two
components likely have orthogonal geometry with the 2008 mass loss displaying a
dust-laden bipolar ejecta produced by a time limited rapidly accelerating wind
and the circumbinary gas having a donut-like shape. A central source powers
them both, having produced a fluorescent light pulse, but we cannot precisely
determine the time it started or its spectral energy distribution. We can,
however, place its upper energy cutoff at about 54 eV and the bulk of its
emission at 20 eV. We also know that the central source turned off within
months from the outburst and before the ejecta turned optically thin.Comment: 11 pages, 9 figures (png format). Accepted for publication in A&
Production of Energy-dependent Time Delays in Impulsive Solar Flare Hard X-Ray Emission by Short-Duration Spectral Index Variations
Cross-correlation techniques have been used recently to study the relative timing of solar flare hard X-ray emission at different energies. These studies find that for the majority of the impulsive flares observed with BATSE there is a systematic time delay of a few tens of milliseconds between low (approximate to 50 keV) and higher energy emission (approximate to 100 keV). These time delays have been interpreted as energy-dependent time-of-flight differences for electron propagation from the corona, where they are accelerated, to the chromosphere, where the bulk of the hard X-rays are emitted. We show in this paper that crosscorrelation methods fail if the spectral index of the flare is not constant. BATSE channel ratios typically display variations of factors of 2 to 5 over time intervals as short as a few seconds. Using simulated and observed data, we demonstrate that cross-correlating energy channels with identical timing characteristics, but with variations in the amplitudes of one or a small number of relatively strong emission spikes, produces asymmetric time delays of either sign. The reported time delays are therefore largely due to spectral index variations and are not signatures of time-of-flight effects
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