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

Dynamical PN Evolution with Magnetic Fields

Hydrodynamical simulations played an important role in understanding the dynamics and shaping of planetary nebulae in the past century. However, hydrodynamical simulations were just a first order approach. The new millennium arrived with the generalized understanding that the effects of magnetic fields were necessary to study the dynamics of planetary nebulae. Thus, B-fields introduced a whole new number of physical possibilities for the modeling. In this paper, we review observational works done in the last 5 years and several works on magnetohydrodynamics about proto-planetary nebulae, since all the effort has been focused on that stage, and discuss different scenarios for the origin of magnetized winds, and the relation binary-bipolararity.Comment: 7 pages, Review Talk, IAU 243, Planetary Nebulae in our Galaxy and Beyon

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

Turbulent Dynamo in Asymptotic Giant Branch Stars

Using recent results on the operation of turbulent dynamos, we show that a turbulent dynamo can amplify a large scale magnetic field in the envelopes of asymptotic giant branch (AGB) stars. We propose that a slow rotation of the AGB envelope can fix the symmetry axis, leading to the formation of an axisymmetric magnetic field structure. Unlike solar-type alpha-omega dynamos, the rotation has only a small role in amplifying the toroidal component of the magnetic field. The large-scale magnetic field is strong enough for the formation of magnetic cool spots on the AGB stellar surface. The spots can regulate dust formation, hence mass loss rate, leading to axisymmetric mass loss and the formation of elliptical planetary nebulae (PNe). Despite its role in forming cool spots, the large scale magnetic field is too weak to play a dynamic role and directly influence the wind from the AGB star. We find other problems in models where the magnetic field plays a dynamic role in shaping the AGB winds, and argue that they cannot explain the formation of nonspherical PNe.Comment: 12 pages (1 ps file of a table); Submitted to MNRA

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 ($\approx$ 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 $\approx$ 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$_2$ 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

Population genetics of species on the genera Tursiops and Delphinus within the Gulf of California and along the western coast of Baja California.

This present study investigated the evolution of population genetic structure of two closely related cetacean species, bottlenose (Tursiops truncatus) and common dolphins (Delphinus spp.) within the Gulf of California (GC) and along the West Coast of Baja California. In this study, we found evidence of strong genetic differentiation in both bottlenose and common dolphin populations in the absence of physical barriers. The comparison of the patterns of population genetic differentiation found here for bottlenose and common dolphins supports the hypothesis of local habitat dependence and resource specialization at both the population and putative species level. Fine-geographic scale structure was detected in coastal bottlenose dolphins, which seemed to be strongly associated to the biogeographic subdivision of the Gulf of California and western coast of Baja California. This result suggests that gene flow among bottlenose dolphin coastal populations might be restricted by local dependence on diverse ecological conditions. In contrast, the long-beaked common dolphin genetic structure did not reflect the habitat heterogeneity of the region to the same extent. The difference in foraging specialization between coastal and offshore populations of both bottlenose and common dolphins is reflected in the pattern of genetic structure observed at a broader geographic scale. Overall, the results support the hypothesis that local habitat dependence promotes population differentiation in the absence of physical boundaries to dispersal in these highly mobile species. This study provides an unusual insight into the conditions that lead to incipient speciation in these groups. Divergence among common dolphin populations appears to be associated with changes in the paleoceanographic conditions of the region to the extent that reciprocal monophyly between the sympatric D. delphis and D. capensis forms has evolved within the Holocene timeframe