131 research outputs found
Constraints on gamma-ray burst and supernova progenitors through circumstellar absorption lines
Long gamma-ray bursts are thought to be caused by a subset of exploding
Wolf-Rayet stars. We argue that the circumstellar absorption lines in early
supernova and in gamma-ray burst afterglow spectra may allow us to determine
the main properties of the Wolf-Rayet star progenitors which can produce those
two events. To demonstrate this, we first simulate the hydrodynamic evolution
of the circumstellar medium around a 40 Msun star up to the time of the
supernova explosion. Knowledge of density, temperature and radial velocity of
the circumstellar matter as function of space and time allows us to compute the
column density in the line of sight to the centre of the nebula, as a function
of radial velocity, angle, and time. Our column density profiles indicate the
possible number, strengths, widths and velocities of absorption line components
in supernova and gamma-ray burst afterglow spectra. Our example calculation
shows four distinct line features during the Wolf-Rayet stage, at about 0, 50,
150-700 and 2200 km/s, with only those of the lowest and highest velocity
present at all times. The 150-700 km/s feature decays rapidly as function of
time after the onset of the Wolf-Rayet stage. It consists of a variable number
of components, and, especially in its evolved stage, is depending strongly on
the particular line of sight. A comparison with absorption lines detected in
the afterglow of GRB 021004 suggests that the high velocity absorption
component in GRB 021004 may be attributed to the free streaming Wolf-Rayet
wind, which is consistent with the steep density drop indicated by the
afterglow light curve. The presence of the intermediate velocity components
implies that the duration of the Wolf-Rayet phase of the progenitor of GRB
021004 was much smaller than the average Wolf-Rayet life time.Comment: 13 pages, 13 figures, accepted by Astronomy & Astrophysics The newest
version contains the changes requested by the A&A style edito
High resolution imaging of NGC 2346 with GSAOI/GeMS: disentangling the planetary nebula molecular structure to understand its origin and evolution
We present high spatial resolution ( 60--90 milliarcseconds) images
of the molecular hydrogen emission in the Planetary Nebula (PN) NGC 2346. The
data were acquired during the System Verification of the Gemini Multi-Conjugate
Adaptive Optics System + Gemini South Adaptive Optics Imager. At the distance
of NGC 2346, 700 pc, the physical resolution corresponds to 56 AU,
which is slightly higher than that an [N II] image of NGC 2346 obtained with
HST/WFPC2. With this unprecedented resolution we were able to study in detail
the structure of the H gas within the nebula for the first time. We found
it to be composed of knots and filaments, which at lower resolution had
appeared to be a uniform torus of material. We explain how the formation of the
clumps and filaments in this PN is consistent with a mechanism in which a
central hot bubble of nebular gas surrounding the central star has been
depressurized, and the thermal pressure of the photoionized region drives the
fragmentation of the swept-up shell.Comment: accepted in ApJ (17 pages, 7 figures, 1 Table
The State of the Circumstellar Medium Surrounding Gamma-Ray Burst Sources and its Effect on the Afterglow Appearance
We present a numerical investigation of the contribution of the presupernova
ejecta of Wolf-Rayet stars to the environment surrounding gamma-ray bursts
(GRBs), and describe how this external matter can affect the observable
afterglow characteristics. An implicit hydrodynamic calculation for massive
stellar evolution is used here to provide the inner boundary conditions for an
explicit hydrodynamical code to model the circumstellar gas dynamics. The
resulting properties of the circumstellar medium are then used to calculate the
deceleration of a relativistic, gas-dynamic jet and the corresponding afterglow
light curve produced as the shock wave propagates through the shocked-wind
medium. We find that variations in the stellar wind drive instabilities that
may produce radial filaments in the shocked-wind region. These comet-like tails
of clumps could give rise to strong temporal variations in the early afterglow
lightcurve. Afterglows may be expected to differ widely among themselves,
depending on the angular anisotropy of the jet and the properties of the
stellar progenitor; a wide diversity of behaviors may be the rule, rather than
the exception.Comment: 17 pages, 7 figures, ApJ in pres
AATR an ionospheric activity indicator specifically based on GNSS measurements
This work reviews an ionospheric activity indicator useful for identifying disturbed periods affecting the performance of Global Navigation Satellite System (GNSS). This index is based in the Along Arc TEC Rate (AATR) and can be easily computed from dual-frequency GNSS measurements. The AATR indicator has been assessed over more than one Solar Cycle (2002â2017) involving about 140 receivers distributed world-wide. Results show that it is well correlated with the ionospheric activity and, unlike other global indicators linked to the geomagnetic activity (i.e. DST or Ap), it is sensitive to the regional behaviour of the ionosphere and identifies specific effects on GNSS users. Moreover, from a devoted analysis of different Satellite Based Augmentation System (SBAS) performances in different ionospheric conditions, it follows that the AATR indicator is a very suitable mean to reveal whether SBAS service availability anomalies are linked to the ionosphere. On this account, the AATR indicator has been selected as the metric to characterise the ionosphere operational conditions in the frame of the European Space Agency activities on the European Geostationary Navigation Overlay System (EGNOS). The AATR index has been adopted as a standard tool by the International Civil Aviation Organization (ICAO) for joint ionospheric studies in SBAS. In this work we explain how the AATR is computed, paying special attention to the cycle-slip detection, which is one of the key issues in the AATR computation, not fully addressed in other indicators such as the Rate Of change of the TEC Index (ROTI). After this explanation we present some of the main conclusions about the ionospheric activity that can extracted from the AATR values during the above mentioned long-term study. These conclusions are: (a) the different spatial correlation related with the MOdified DIP (MODIP) which allows to clearly separate high, mid and low latitude regions, (b) the large spatial correlation in mid latitude regions which allows to define a planetary index, similar to the geomagnetic ones, (c) the seasonal dependency which is related with the longitude and (d) the variation of the AATR value at different time scales (hourly, daily, seasonal, among others) which confirms most of the well-known time dependences of the ionospheric events, and finally, (e) the relationship with the space weather events.Postprint (published version
The circumstellar medium around a rapidly rotating, chemically homogeneously evolving, possible gamma-ray burst progenitor
Rapidly rotating, chemically homogeneously evolving massive stars are
considered to be progenitors of long gamma-ray bursts. We present numerical
simulations of the evolution of the circumstellar medium around a rapidly
rotating 20 Msol star at a metallicity of Z=0.001. Its rotation is fast enough
to produce quasi-chemically homogeneous evolution. While conventionally, a star
of 20 Msol would not evolve into a Wolf-Rayet stage, the considered model
evolves from the main sequence directly to the helium main sequence. We use the
time-dependent wind parameters, such as mass loss rate, wind velocity and
rotation-induced wind anisotropy from the evolution model as input for a 2D
hydrodynamical simulation. While the outer edge of the pressure-driven
circumstellar bubble is spherical, the circumstellar medium close to the star
shows strong non-spherical features during and after the periods of
near-critical rotation. We conclude that the circumstellar medium around
rapidly rotating massive stars differs considerably from the surrounding
material of non-rotating stars of similar mass. Multiple blue-shifted high
velocity absorption components in gamma-ray burst afterglow spectra are
predicted. As a consequence of near critical rotation and short stellar
evolution time scales during the last few thousand years of the star's life, we
find a strong deviation of the circumstellar density profile in the polar
direction from the 1/R^2 density profile normally associated with stellar winds
close to the starComment: accepted by Astronomy & Astrophysic
On the feedback from super stellar clusters. I. The structure of giant HII regions and HII galaxies
We review the structural properties of giant extragalactic HII regions and
HII galaxies based on 2D hydrodynamic calculations, and propose an evolutionary
sequence that accounts for their observed detailed structure. The model assumes
a massive and young stellar cluster surrounded by a large collection of clouds.
These are thus exposed to the most important star-formation feedback
mechanisms: photoionization and the cluster wind. The models show how the two
feedback mechanisms compete in the disruption of clouds and lead to two
different hydrodynamic solutions: The storage of clouds into a long lasting
ragged shell that inhibits the expansion of the thermalized wind, and the
steady filtering of the shocked wind gas through channels carved within the
cloud stratum. Both solutions are claimed to be concurrently at work in giant
HII regions and HII galaxies, causing their detailed inner structure. This
includes multiple large-scale shells, filled with an X-ray emitting gas, that
evolve to finally merge with each other, giving the appearance of shells within
shells. The models also show how the inner filamentary structure of the giant
superbubbles is largely enhanced with matter ablated from clouds and how cloud
ablation proceeds within the original cloud stratum. The calculations point at
the initial contrast density between the cloud and the intercloud media as the
factor that defines which of the two feedback mechanisms becomes dominant
throughout the evolution. Animated version of the models can be found at
http://www.iaa.csic.es/\~{}eperez/ssc/ssc.html.Comment: 28 pages, 10 figures, accepted for publication in the ApJ. Animated
version of the models can be found at
http://www.iaa.csic.es/\~{}eperez/ssc/ssc.htm
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