D. Falceta-Goncalves

Wind-wind collision in the η Carinae binary system: a shell-like event near periastron

ABSTRACT The exact nature of η Carinae is still an open issue. Strict periodicity in the lightcurves at several wavelengths seem to point out to a binary system, but the observedradial velocities, measured from space with high spatial resolution are in conflict withthe ground based observations used to calculate the binary orbit. Also, the observed2-10 keV X-ray flux is much larger that what is expected from a single star, andfavors the wind-wind collision hypothesis, characteristic of high mass binary systems.However, to explain the duration of the dip in the light curve by wind collisions, it isnecessary to postulate a very large increase in the η Carinae mass loss rate. Finally, theoptical and UV light curves are better explained by periodic shell-ejection events. Inthis paper we conciliate the two hypothesis. We still assume a binary system to explainthe strong X-ray emission, but we also take into account that, near periastron andbecause of the highly eccentric orbit, the wind emerging from η Carinae accumulatesbehind the shock and can mimic a shell-like ejection event. For this process to beeffective, at periastron the secondary star should be located between η Carinae andthe observer, solving also the discrepancy between the orbital parameters derived fromground and space based observations. We show that, as the secondary moves in itsorbit, the shell cools down and the number of available stellar ionizing photons is notenough to maintain the shell temperature at its equilibrium value of about 7500 K.The central part of the shell remains cold and under these conditions grain formationand growth can take place in timescales of hours. We also calculated the neutral gascolumn density intercepting the line of sight at each point of the orbit near periastron,and were able to reproduce the form and duration of the X-ray light curve withoutany change in the η Carinae mass loss rate. This same column density can explain theobserved Hα light curve observed during the 2003 event.Key words: stars: individual (η Car) binaries: general stars: variable X-rays: stars

The Effects of Alfvén Waves and Radiation Pressure in Dust Winds of Late-Type Stars

In the present study, we analyze the effects of a flux of Alfven waves acting together with radiation pressure on grains as an acceleration mechanism of the wind of late-type stars. In the wind model, we simulate the presence of grains through a strong damping of the waves, and we use a nonisothermal profile for the temperature that is coherent with grain formation theories. We examine the changes in the velocity profile of the wind, and we show that if the grains are created in the region 1.1 < r/r0 < 2.0, their presence will affect the mass loss and terminal velocity. The model is applied to a K5 supergiant star and to Betelgeuse (α Ori).

Wind-wind collision in the η Carinae binary system — II. Constraints to the binary orbital parameters from radio emission near periastron passage

ABSTRACT In this paper we use the 7 mm and 1.3 mm light curves obtained during the2003.5 low excitation phase of the η Carinae system to constrain the possibleparameters of the binary orbit. To do that we assumed that the mm waveemission is produced in a dense disk surrounding the binary system; duringthe low excitation phase, which occurs close to periastron, the number ofionizing photons decreases, producing the dip in the radio emission. On theother hand, due to the large eccentricity, the density of the shock region atperiastron is very high and the plasma is optically thick for free-free radiationat 7 mm, explaining the sharp peak that was observed at this frequency andlastedforabout10days.Fromtheshapeanddurationofthepeakwewereableto determine the orbital parameters of the binary system, independently ofthe stellar parameters, such as mass loss rates, wind velocities or temperatureat the post-shock region.Key words: stars: individual (η Car) binaries: general stars: variable radiocontinuum: general

On the magnetic structure and wind parameter profiles of Alfvén wave driven winds in late-type supergiant stars

Cool stars at giant and supergiant evolutionary phases present low-velocity and high-density winds, responsible for the observed high mass-loss rates. Although presenting high lumi- nosities, radiation pressure on dust particles is not sufficient to explain the wind acceleration process. Among the possible solutions to this still unsolved problem, Alfven waves are, prob- ably, the most interesting for their high efficiency in transfering energy and momentum to the wind. Typically, models of Alfven wave driven winds result in high-velocity winds if they are not highly damped. In this work, we determine self-consistently the magnetic field geometry and solve the momentum, energy and mass conservation equations, to demonstrate that even a low-damped Alfven wave flux is able to reproduce the low-velocity wind. We show that the magnetic flux tubes expand with a super-radial factor of S > 30 near the stellar surface, larger than that used in previous semi-empirical models. The rapid expansion results in a strong spa- tial dilution of the wave flux. We obtained the wind parameter profiles for a typical supergiant star of 16 M� . The wind is accelerated in a narrow region, coincident with the region of high divergence of the magnetic field lines, up to 100 km s −1 . For the temperature, we obtained a slight decrease near the surface for low-damped waves, because the wave heating mechanism is less effective than the radiative losses. The peak temperature occurs at r � 1.5 r 0 reaching 6000 K. Propagating outwards, the wind cools down mainly due to adiabatic expansion.

MHD numerical simulations of colliding winds in massive binary systems – I. Thermal versus non‐thermal radio emission

In the past few decades detailed observations of radio and X-ray emission from massive binary systems revealed a whole new physics present in such systems. Both thermal and non-thermal components of this emission indicate that most of the radiation at these bands originates in shocks. O and B-type stars and Wolf–Rayet (WR) stars present supersonic and massive winds that, when colliding, emit largely due to the free–free radiation. The non-thermal radio and X-ray emissions are due to synchrotron and inverse Compton processes, respectively. In this case, magnetic fields are expected to play an important role in the emission distribution. In the past few years the modelling of the free–free and synchrotron emissions from massive binary systems have been based on purely hydrodynamical simulations, and ad hoc assumptions regarding the distribution of magnetic energy and the field geometry. In this work we provide the first full magnetohydrodynamic numerical simulations of wind–wind collision in massive binary systems. We study the free–free emission characterizing its dependence on the stellar and orbital parameters. We also study self-consistently the evolution of the magnetic field at the shock region, obtaining also the synchrotron energy distribution integrated along different lines of sight. We show that the magnetic field in the shocks is larger than that obtained when the proportionality between B and the plasma density is assumed. Also, we show that the role of the synchrotron emission relative to the total radio emission has been underestimated.

The mass-loss process in dwarf galaxies from 3D hydrodynamical simulations: the role of dark matter and starbursts

Theoretical

DUSTY MOLECULAR CLOUD COLLAPSE IN THE PRESENCE OF ALFVEN WAVES

\Lambda

Statistics of Reconnection-driven Turbulence

CDM cosmological models predict a much larger number of low mass dark matter haloes than has been observed in the Local Group of galaxies. One possible explanation is the increased difficulty of detecting these haloes if most of the visible matter is lost at early evolutionary phases through galactic winds. In this work we study the current models of triggering galactic winds in dwarf spheroidal galaxies (dSph) from supernovae, and study, based on 3D hydrodynamic numerical simulations, the correlation of the mass loss rates and important physical parameters as the dark matter halo mass and its radial profile, and the star formation rate. We find that the existence of winds is ubiquitous, independent on the gravitational potential. Our simulations revealed that the Rayleigh-Taylor Instability (RTI) may play a major role on pushing matter out of these systems, even for very massive haloes. The instability is responsible for 5 - 40% of the mass loss during the early evolution of the galaxy, being less relevant at

Modelling spectral line profiles of wind–wind shock emissions from massive binary systems

t > 200

On the theory of mass-loss in dwarf galaxies – I. Basic equations and the case of wave/thermal driven winds

Myrs. There is no significant difference in the mass loss rates obtained for the different dark matter profiles studied (NFW and logarithmic). We have also found a correlation between the mass loss rate and both the halo mass and the rate of supernovae, as already reported in previous works. Besides, the epoch in which most of the baryon galactic matter is removed from the galaxy varies depending on the SN rate and gravitational potential. The later, combined to the importance of the RTI in each model, may change our understanding about the chemical evolution of dwarf galaxies, as well as in the heavy element contamination of the intergalactic medium at high redshifts.