3,604 research outputs found
The diversity of supernovae and impostors shortly after explosion
Observational surveys are now able to detect an increasing number of
transients, such as core-collapse supernovae (SN) and powerful non-terminal
outbursts (SN impostors). Dedicated spectroscopic facilities can follow up
these events shortly after detection. Here we investigate the properties of
these explosions at early times. We use the radiative transfer code CMFGEN to
build an extensive library of spectra simulating the interaction of supernovae
and their progenitor's winds/circumstellar medium (CSM). We consider a range of
progenitor mass-loss rates (Mdot = 5e-4 to 1e-2 Msun/yr), abundances (solar,
CNO-processed, and He-rich), and SN luminosities (L = 1.9e8 to 2.5e10 Lsun).
The models simulate events ~1 day after explosion, and we assume a fixed
location of the shock front as Rin=8.6e13 cm. We show that the large range of
massive star properties at the pre-SN stage causes a diversity of early-time
interacting SN and impostors. We identify three main classes of early-time
spectra consisting of relatively high-ionisation (e.g. Ovi), medium-ionisation
(e.g. Ciii), and low-ionisation lines (e.g. Feii/iii). They are regulated by L
and the CSM density. Given a progenitor wind velocity Vinf, our models also
place a lower limit of Mdot > 5e-4 (Vinf/150 km/s) Msun/yr for detection of CSM
interaction signatures in observed spectra. Early-time SN spectra should
provide clear constraints on progenitors by measuring H, He, and CNO abundances
if the progenitors come from single stars. The connections are less clear
considering the effects of binary evolution. Yet, our models provide a clear
path for linking the final stages of massive stars to their post-explosion
spectra at early times, and guiding future observational follow-up of
transients with facilities such as the Zwicky Transient Facility.Comment: Accepted for publication in A&A. 14 pages, 7 figure
Episodic modulations in supernova radio light curves from luminous blue variable supernova progenitor models
Ideally, one would like to know which type of core-collapse SNe is
produced by different progenitors and the channels of stellar evolution leading
to these progenitors. These links have to be very well known to use the
observed frequency of different types of SN events for probing the star
formation rate and massive star evolution in different types of galaxies.
We investigate the link between LBV as SN progenitors and the appearance
of episodic radio light curve modulations of the SN event. We use the
20Msun and 25Msun models with rotation at solar metallicity, part of an
extended grid of stellar models computed by the Geneva team. At their pre-SN
stage, these two models have recently been shown to have spectra similar to
those of LBV stars and possibly explode as Type IIb SNe. Based on the wind
properties before the explosion, we derive the density structure of their
circumstellar medium. This structure is used as input for computing the SN
radio light curve. We find that the 20Msun model shows radio light
curves with episodic luminosity modulations, similar to those observed in some
Type IIb SNe. This occurs because the evolution of the 20Msun model terminates
in a region of the HR diagram where radiative stellar winds present strong
density variations, caused by the bistability limit. The 25Msun model, ending
its evolution in a zone of the HR diagram where no change of the mass-loss
rates is expected, presents no such modulations in its radio SN light curve.
Our results reinforce the link between SN progenitors and LBV
stars. We also confirm the existence of a physical mechanism for a single star
to have episodic radio light curve modulations. In the case of the 25Msun
progenitors, we do not obtain modulations in the radio light curve, but our
models may miss some outbursting behavior in the late stages of massive stars.Comment: 5 pages, 3 figures, accepted by Astronomy & Astrophysics Letter
Magnetization of ferrofluids with dipolar interactions - a Born--Mayer expansion
For ferrofluids that are described by a system of hard spheres interacting
via dipolar forces we evaluate the magnetization as a function of the internal
magnetic field with a Born--Mayer technique and an expansion in the dipolar
coupling strength. Two different approximations are presented for the
magnetization considering different contributions to a series expansion in
terms of the volume fraction of the particles and the dipolar coupling
strength.Comment: 19 pages, 11 figures submitted to PR
Particle dynamics of a cartoon dune
The spatio-temporal evolution of a downsized model for a desert dune is
observed experimentally in a narrow water flow channel. A particle tracking
method reveals that the migration speed of the model dune is one order of
magnitude smaller than that of individual grains. In particular, the erosion
rate consists of comparable contributions from creeping (low energy) and
saltating (high energy) particles. The saltation flow rate is slightly larger,
whereas the number of saltating particles is one order of magnitude lower than
that of the creeping ones. The velocity field of the saltating particles is
comparable to the velocity field of the driving fluid. It can be observed that
the spatial profile of the shear stress reaches its maximum value upstream of
the crest, while its minimum lies at the downstream foot of the dune. The
particle tracking method reveals that the deposition of entrained particles
occurs primarily in the region between these two extrema of the shear stress.
Moreover, it is demonstrated that the initial triangular heap evolves to a
steady state with constant mass, shape, velocity, and packing fraction after
one turnover time has elapsed. Within that time the mean distance between
particles initially in contact reaches a value of approximately one quarter of
the dune basis length
Spectral atlas of massive stars around He I 10830 A
We present a digital atlas of peculiar, high-luminosity massive stars in the near-infrared region (10470-11000 A) at medium resolution (R~7000). The spectra are centered around He I 10830 A, which is formed in the wind of those stars, and is a crucial line to obtain their physical parameters. The instrumental configuration also sampled a rich variety of emission lines of Fe II, Mg II, C I, N I and Pa gamma. Secure identifications for most spectral lines are given, based on synthetic atmosphere models calculated by our group. We also propose that two unidentified absorption features have interstellar and/or circumstellar origin. For the strongest one (10780 A) an empirical calibration between E(B-V) and equivalent width is provided. The atlas displays the spectra of massive stars organized in four categories, namely Be stars, OBA Iape (or luminous blue variables, LBV candidates and ex/dormant LBVs), OB supergiants and Wolf-Rayet stars. For comparison, the photospheric spectra of non emission-line stars are presented. Selected LBVs were observed in different epochs from 2001 to 2004, and their spectral variability reveals that some stars, such as Eta Car, AG Car and HR Car, suffered dramatic spectroscopic changes during this time interval
Inhomogeneous magnetization in dipolar ferromagnetic liquids
At high densities fluids of strongly dipolar spherical particles exhibit
spontaneous long-ranged orientational order. Typically, due to demagnetization
effects induced by the long range of the dipolar interactions, the
magnetization structure is spatially inhomogeneous and depends on the shape of
the sample. We determine this structure for a cubic sample by the free
minimization of an appropriate microscopic density functional using simulated
annealing. We find a vortex structure resembling four domains separated by four
domain walls whose thickness increases proportional to the system size L. There
are indications that for large L the whole configuration scales with the system
size. Near the axis of the mainly planar vortex structure the direction of the
magnetization escapes into the third dimension or, at higher temperatures, the
absolute value of the magnetization is strongly reduced. Thus the orientational
order is characterized by two point defects at the top and the bottom of the
sample, respectively. The equilibrium structure in an external field and the
transition to a homogeneous magnetization for strong fields are analyzed, too.Comment: 17 postscript figures included, submitted to Phys. Rev.
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