6,703 research outputs found
On the Importance of the Interclump Medium for Superionization: O VI Formation in the Wind of Zeta Pup
We have studied superionization and X-ray line formation in the spectra of
Zeta Pup using our new stellar atmosphere code (XCMFGEN) that can be used to
simultaneously analyze optical, UV, and X-ray observations. Here, we present
results on the formation of the O VI ll1032, 1038 doublet. Our simulations,
supported by simple theoretical calculations, show that clumped wind models
that assume void in the interclump space cannot reproduce the observed O VI
profiles. However, enough O VI can be produced if the voids are filled by a low
density gas. The recombination of O VI is very efficient in the dense material
but in the tenuous interclump region an observable amount of O VI can be
maintained. We also find that different UV resonance lines are sensitive to
different density regimes in Zeta Pup : C IV is almost exclusively formed
within the densest regions, while the majority of O VI resides between clumps.
N V is an intermediate case, with contributions from both the tenuous gas and
clumps.Comment: Accepted for publication in ApJL, 4 pages with 3 figure
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Photometric modeling of a cometary nucleus: taking Hapke modeling to the limit
Further Criteria for the Existence of Steady Line-Driven Winds
In Paper I, we showed that steady line-driven disk wind solutions can exist
by using "simple" models that mimic the disk environment. Here I extend the
concepts introduced in Paper I and discuss many details of the analysis of the
steady/unsteady nature of 1D line-driven winds. This work confirms the results
and conclusions of Paper I, and is thus consistent with the steady nature of
the 1D streamline line-driven disk wind models of Murray and collaborators and
the 2.5D line-driven disk wind models of Pereyra and collaborators. When
including gas pressures effects, as is routinely done in time-dependent
numerical models, I find that the spatial dependence of the nozzle function
continues to play a key role in determining the steady/unsteady nature of
supersonic line-driven wind solutions. I show here that the
existence/nonexistence of local wind solutions can be proved through the nozzle
function without integrating the equation of motion. This work sets a detailed
framework with which we will analyze, in a following paper, more realistic
models than the "simple" models of Paper I.Comment: 30 pages, 5 figures, accepted for publication by The Astrophysical
Journa
The UV Scattering Halo of the Central Source Associated with Eta Carinae
We have made an extensive study of the UV spectrum of Eta Carinae, and find
that we do not directly observe the star and its wind in the UV. Because of
dust along our line of sight, the UV light that we observe arises from
bound-bound scattering at large impact parameters. We obtain a reasonable fit
to the UV spectrum by using only the flux that originates outside 0.033". This
explains why we can still observe the primary star in the UV despite the large
optical extinction -- it is due to the presence of an intrinsic coronagraph in
the Eta Carinae system, and to the extension of the UV emitting region. It is
not due to peculiar dust properties alone. We have computed the spectrum of the
purported companion star, and show that it could only be directly detected in
the UV spectrum preferentially in the Far Ultraviolet Spectroscopic Explorer
(FUSE) spectral region (912-1175 Ang.). However, we find no direct evidence for
a companion star, with the properties indicated by X-ray studies and studies of
the Weigelt blobs, in UV spectra. This might be due to reprocessing of the
companion's light by the dense stellar wind of the primary. Broad FeII and
[FeII] emission lines, which form in the stellar wind, are detected in spectra
taken in the SE lobe, 0.2" from the central star. The wind spectrum shows some
similarities to the spectra of the B & D Weigelt blobs, but also shows some
marked differences in that high excitation lines, and lines pumped by Ly-alpha,
are not seen. The detection of the broad lines lends support to our
interpretation of the UV spectrum, and to our model for Eta Carinae.Comment: To appear in ApJ. 57 pages with 18 figure
Radial dependence of line profile variability in seven O9--B0.5 stars
Massive stars show a variety of spectral variability: presence of discrete
absorption components in UV P-Cygni profiles, optical line profile variability,
X-ray variability, radial velocity modulations. Our goal is to study the
spectral variability of single OB stars to better understand the relation
between photospheric and wind variability. For that, we rely on high spectral
resolution, high signal-to-noise ratio optical spectra collected with the
spectrograph NARVAL on the Telescope Bernard Lyot at Pic du Midi. We
investigate the variability of twelve spectral lines by means of the Temporal
Variance Spectrum (TVS). The selected lines probe the radial structure of the
atmosphere, from the photosphere to the outer wind. We also perform a
spectroscopic analysis with atmosphere models to derive the stellar and wind
properties, and to constrain the formation region of the selected lines. We
show that variability is observed in the wind lines of all bright giants and
supergiants, on a daily timescale. Lines formed in the photosphere are
sometimes variable, sometimes not. The dwarf stars do not show any sign of
variability. If variability is observed on a daily timescale, it can also (but
not always) be observed on hourly timescales, albeit with lower amplitude.
There is a very clear correlation between amplitude of the variability and
fraction of the line formed in the wind. Strong anti-correlations between the
different part of the temporal variance spectrum are observed. Our results
indicate that variability is stronger in lines formed in the wind. A link
between photospheric and wind variability is not obvious from our study, since
wind variability is observed whatever the level of photospheric variability.
Different photospheric lines also show different degrees of variability.Comment: 13 pages, 9 figures + appendix. A&A accepted. Figures degraded for
arxiv submissio
Quantitative spectroscopic analysis of and distance to SN1999em
This work presents a detailed quantitative spectroscopic analysis of, and the
determination of the distance to, the type II supernovae (SN) SN1999em with
CMFGEN (Dessart & Hillier 2005a), based on spectrophotometric observations at
eight dates up to 40 days after discovery. We use the same iron-group metal
content for the ejecta, the same power-law density distribution (with exponent
n~10), and a Hubble-velocity law at all times. We adopt a H/He/C/N/O abundance
pattern compatible with CNO-cycle equilibrium values for a RSG/BSG progenitor,
with C/O enhanced and N depleted at later times. Based on our synthetic fits to
spectrophotometric observations of SN1999em, we obtain a distance of 11.5Mpc,
similar to that of Baron et al. (2004) and the Cepheid distance to the galaxy
host of 11.7Mpc (Leonard et al. 2003). Similarly, based on such models, the
Expanding Photosphere Method (EPM) delivers a distance of 11.6Mpc, with
negligible scatter between photometric bandpass sets; there is thus nothing
wrong with the EPM as such. Previous determinations using the tabulated
correction factors of Eastman et al. (1996) all led to 30-50% underestimates:
we find that this is caused by 1) an underestimate of the correction factors
compared to the only other study of the kind by Dessart & Hillier (2005b), 2) a
neglect of the intrinsic >20% scatter of correction factors, and 3) the use of
the EPM at late times when severe line blanketing makes the method inaccurate.
The need of detailed model computations for reliable EPM distance estimates
thus defeats the appeal and simplicity of the method. However, detailed fits to
SN optical spectra, based on tailored models for individual SN observations,
offers a promising approach to obtaining distances with 10-20% accuracy, either
through the EPM or a la Baron et al. (2004).Comment: 20 pages, 13 figures, accepted for publication in A&
The Atomic Physics Underlying the Spectroscopic Analysis of Massive Stars and Supernovae
We have developed a radiative transfer code, CMFGEN, which allows us to model
the spectra of massive stars and supernovae. Using CMFGEN we can derive
fundamental parameters such as effective temperatures and surface gravities,
derive abundances, and place constraints on stellar wind properties. The last
of these is important since all massive stars are losing mass via a stellar
wind that is driven from the star by radiation pressure, and this mass loss can
substantially influence the spectral appearance and evolution of the star.
Recently we have extended CMFGEN to allow us to undertake time-dependent
radiative transfer calculations of supernovae. Such calculations will be used
to place constraints on the supernova progenitor, to place constraints on the
supernova explosion and nucleosynthesis, and to derive distances using a
physical approach called the "Expanding Photosphere Method". We describe the
assumptions underlying the code and the atomic processes involved. A crucial
ingredient in the code is the atomic data. For the modeling we require accurate
transition wavelengths, oscillator strengths, photoionization cross-sections,
collision strengths, autoionization rates, and charge exchange rates for
virtually all species up to, and including, cobalt. Presently, the available
atomic data varies substantially in both quantity and quality.Comment: 8 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Unconventional superconductivity in the cage type compound ScRhSn
We have examined the superconducting ground state properties of the caged
type compound ScRhSn using magnetization, heat capacity, and
muon-spin relaxation or rotation (SR) measurements. Magnetization
measurements indicate type-II superconductivity with an upper critical field
= 7.24 T. The zero-field cooled and field cooled
susceptibility measurements unveil an onset of diamagnetic signal below = 4.4 K. The interpretation of the heat capacity results below
using the BCS model unveils the value of = 2.65, which gives
the dimensionless ratio 2 = 5.3, intimating that
ScRhSn is a strong-coupling BCS superconductor. The zero-field
SR measurements in the longitudinal geometry exhibit a signature of a
spontaneous appearance of the internal magnetic field below the superconducting
transition temperature, indicating that the superconducting state is
characterized by the broken time-reversal symmetry (TRS). We have compared the
results of broken TRS in ScRhSn with that observed in
RRhSn (R = Lu and Y).Comment: 6 pages, 4 figures. arXiv admin note: text overlap with
arXiv:1411.687
Tree-Structured Grid Model of Line and Polarization Variability from Massive Binaries
We have developed a 3-D Monte Carlo radiative transfer model which computes
line and continuum polarization variability for a binary system with an
optically thick non-axisymmetric envelope. This allows us to investigate the
complex (phase-locked) line and continuum polarization variability features
displayed by many massive binaries: W-R+O, O+O, etc. An 8-way tree data
structure constructed via a ``cell-splitting'' method allows for high precision
with efficient use of computer resources. The model is not restricted to binary
systems; it can easily be adapted to a system with an arbitrary density
distribution and large density gradients. As an application to a real system,
the phase dependent Stokes parameters (I, Q, U) and the phase dependent He I
(5876) profiles of the massive binary system V444 Cyg (WN5+O6 III-V) are
computed.Comment: 11 pages, 14 figures, accepted by Astronomy & Astrophysic
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