Search

Canis Major OB1 (CMa OB1) is a Galactic stellar association with a very intriguing star-formation scenario. There are more than two dozen known star clusters in its line of sight, but it is not clear which ones are physically associated with CMa OB1. We use a clustering code that employs 5-dimensional data from the Gaia DR2 catalogue to identify physical groups and obtain their astrometric parameters and, in addition, we use two different isochrone-fitting methods to estimate the ages of these groups. We find 15 stellar groups with distances between 570 pc and 1650 pc, including 10 previously known and 5 new open cluster candidates. Four groups, precisely the youngest ones ($<$ 20 Myr), CMa05, CMa06, CMa07 and CMa08, are confirmed to be part of CMa OB1. We find that CMa08, a new cluster candidate, may be the progenitor cluster of runaway stars. CMa06 coincides with the well-studied CMa R1 star-forming region. While CMa06 is still forming stars, due to the remaining material of the molecular cloud associated with the Sh 2-262 nebula, CMa05, CMa07 and CMa08 seem to be in more evolved stages of evolution, with no recent star-forming activity. The properties of these CMa OB1 physical groups fit well in a monolithic scenario of star formation, with a common formation mechanism, and having suffered multiple episodes of star formation. This suggests that the hierarchical model alone, which explains the populations of other parts of the same association, is not sufficient to explain its whole formation history., Comment: 23 pages, 28 figures, 7 tables

An exotic environment surrounds the young stellar groups associated with the Canis Major (CMa) OB1/R1 region, which probably was formed under feedback from at least three supernova events having occurred a few million years ago. We use astrometric data from the Gaia-DR2 to confirm the membership of the stars in CMa R1, based on proper motion and parallax, which revealed 514 new members and candidates. The mean age of 5 Myr estimated from the color-magnitude diagram characterizes the sources as likely pre-main sequence candidates. In total, a sample of 694 stars detected with the T80-South telescope was analyzed according to different color-color diagrams, which were compared with theoretical colors from evolutionary models, aiming to reveal the objects that exhibit color excess due to accretion processes. Accretion and magnetic activity were also explored on the basis of empirical flux-flux relation, such as F660 and F861 that are related to Halpha and Ca II triplet emission, respectively. A low fraction (3 percent) of the sample have Halpha excess and other colors expected for stars exhibiting chromospheric activity. The number of Class I and Class II objects, identified by the infrared (WISE) colors, indicates a disk fraction of 6 percent, which is lower than the expected for stellar clusters with similar age. A such large sample of objects associated with CMa R1 without evidences of circumstellar accretion can be interpreted as a lack of disk-bearing stars, unusual for young star-forming regions. However, this may be explained as the result of supernova events., Comment: 29 pages, 9 tables, 11 figures, accepted for publication in The Astronomical Journal

The study of the acceleration of particles is an essential element of research in the heliospheric science. Here, we discuss the predisposition to the particle acceleration around coronal mass ejections (CMEs)-driven shocks with corrugated wave-like features. We adopt these attributes on shocks formed from disturbances due to the bimodal solar wind, CME deflection, irregular CME expansion, and the ubiquitous fluctuations in the solar corona. In order to understand the role of a wavy shock in particle acceleration, we define three initial smooth shock morphologies each one associated with a fast CME. Using polar Gaussian profiles we model these shocks in the low corona. We establish the corrugated appearance on smooth shock by using combinations of wave-like functions that represent the disturbances from medium and CME piston. For both shock types, smooth and corrugated, we calculate the shock normal angles between the shock normal and the radial upstream coronal magnetic field in order to classify the quasi-parallel and quasi-perpendicular regions. We consider that corrugated shocks are predisposed to different process of particle acceleration due to irregular distributions of shock normal angles around of the shock. We suggest that disturbances due to CME irregular expansion may be a decisive factor in origin of particle acceleration. Finally, we regard that accepting these features on shocks may be the start point for investigating some questions in the sheath and shock, like downstream-jets, instabilities, shock thermalization, shock stability, and injection particle process., Comment: 15 pages, 5 figures, 1 table, Accepted for publication in The Astrophysical Journal

The Southern Photometric Local Universe Survey (S-PLUS) is imaging ~9300 deg^2 of the celestial sphere in twelve optical bands using a dedicated 0.8 m robotic telescope, the T80-South, at the Cerro Tololo Inter-American Observatory, Chile. The telescope is equipped with a 9.2k by 9.2k e2v detector with 10 um pixels, resulting in a field-of-view of 2 deg^2 with a plate scale of 0.55"/pixel. The survey consists of four main subfields, which include two non-contiguous fields at high Galactic latitudes (8000 deg^2 at |b| > 30 deg) and two areas of the Galactic plane and bulge (for an additional 1300 deg^2). S-PLUS uses the Javalambre 12-band magnitude system, which includes the 5 u, g, r, i, z broad-band filters and 7 narrow-band filters centered on prominent stellar spectral features: the Balmer jump/[OII], Ca H+K, H-delta, G-band, Mg b triplet, H-alpha, and the Ca triplet. S-PLUS delivers accurate photometric redshifts (delta_z/(1+z) = 0.02 or better) for galaxies with r < 20 AB mag and redshift < 0.5, thus producing a 3D map of the local Universe over a volume of more than 1 (Gpc/h)^3. The final S-PLUS catalogue will also enable the study of star formation and stellar populations in and around the Milky Way and nearby galaxies, as well as searches for quasars, variable sources, and low-metallicity stars. In this paper we introduce the main characteristics of the survey, illustrated with science verification data highlighting the unique capabilities of S-PLUS. We also present the first public data release of ~336 deg^2 of the Stripe-82 area, which is available at http://datalab.noao.edu/splus., Comment: Updated to reflect the published version (MNRAS, 489, 241). For a short introductory video of the S-PLUS project, see https://youtu.be/yc5kHrHU9Jk - The S-PLUS Data Release 1 is available at http://datalab.noao.edu/splus

Based on our previous work (Vidotto et al. 2009a), we investigate the effects on the wind and magnetospheric structures of weak-lined T Tauri stars due to a misalignment between the axis of rotation of the star and its magnetic dipole moment vector. In such configuration, the system loses the axisymmetry presented in the aligned case, requiring a fully 3D approach. We perform 3D numerical MHD simulations of stellar winds and study the effects caused by different model parameters. The system reaches a periodic behavior with the same rotational period of the star. We show that the magnetic field lines present an oscillatory pattern and that by increasing the misalignment angle, the wind velocity increases. Our wind models allow us to study the interaction of a magnetized wind with a magnetized extra-solar planet. Such interaction gives rise to reconnection, generating electrons that propagate along the planet's magnetic field lines and produce electron cyclotron radiation at radio wavelengths. We find that a close-in Jupiter-like planet orbiting at 0.05AU presents a radio power that is ~5 orders of magnitude larger than the one observed in Jupiter, which suggests that the stellar wind from a young star has the potential to generate strong planetary radio emission that could be detected in the near future with LOFAR. This radio power varies according to the phase of rotation of the star. We also analyze whether winds from misaligned stellar magnetospheres could cause a significant effect on planetary migration. Compared to the aligned case, we show that the time-scale tau_w for an appreciable radial motion of the planet is shorter for larger misalignment angles. While for the aligned case tau_w~100Myr, for a stellar magnetosphere tilted by 30deg, tau_w ranges from ~40 to 70Myr for a planet located at a radius of 0.05AU. (Abridged), Comment: 21 pages, 16 figures, 2 tables (emulateapj.cls). Accepted for publication in the ApJ

Here we investigate the contribution of surface Alfven wave damping to the heating of the solar wind in minima conditions. These waves are present in regions of strong inhomogeneities in density or magnetic field (e. g., the border between open and closed magnetic field lines). Using a 3-dimensional Magnetohydrodynamics (MHD) model, we calculate the surface Alfven wave damping contribution between 1-4 solar radii, the region of interest for both acceleration and coronal heating. We consider waves with frequencies lower than those that are damped in the chromosphere and on the order of those dominating the heliosphere. In the region between open and closed field lines, within a few solar radii of the surface, no other major source of damping has been suggested for the low frequency waves we consider here. This work is the first to study surface Alfven waves in a 3D environment without assuming a priori a geometry of field lines or magnetic and density profiles. We determine that waves with frequencies >2.8x10^-4 Hz are damped between 1-4 solar radii. In quiet sun regions, surface Alfven waves are damped at further distances compared to active regions, thus carrying additional wave energy into the corona. We compare the surface Alfven wave contribution to the heating by a variable polytropic index and find that it an order of magnitude larger than needed for quiet sun regions. For active regions the contribution to the heating is twenty percent. As it has been argued that a variable gamma acts as turbulence, our results indicate that surface Alfven wave damping is comparable to turbulence in the lower corona. This damping mechanism should be included self consistently as an energy driver for the wind in global MHD models., Comment: Accepted to ApJ (scheduled September '09), 22 pages, 8 figures

By means of numerical simulations, we investigate magnetized stellar winds of pre-main-sequence stars. In particular we analyze under which circumstances these stars will present elongated magnetic features (e.g., helmet streamers, slingshot prominences, etc). We focus on weak-lined T Tauri stars, as the presence of the tenuous accretion disk is not expected to have strong influence on the structure of the stellar wind. We show that the plasma-beta parameter (the ratio of thermal to magnetic energy densities) is a decisive factor in defining the magnetic configuration of the stellar wind. Using initial parameters within the observed range for these stars, we show that the coronal magnetic field configuration can vary between a dipole-like configuration and a configuration with strong collimated polar lines and closed streamers at the equator (multi-component configuration for the magnetic field). We show that elongated magnetic features will only be present if the plasma-beta parameter at the coronal base is beta<<1. Using our self-consistent 3D MHD model, we estimate for these stellar winds the time-scale of planet migration due to drag forces exerted by the stellar wind on a hot-Jupiter. In contrast to the findings of Lovelace et al. (2008), who estimated such time-scales using the Weber & Davis model, our model suggests that the stellar wind of these multi-component coronae are not expected to have significant influence on hot-Jupiters migration. Further simulations are necessary to investigate this result under more intense surface magnetic field strengths (~2-3 kG) and higher coronal base densities, as well as in a tilted stellar magnetosphere., Comment: Accepted for publication in the Astrophysical Journal, 10 pages, 5 figures, emulateapj

By means of self-consistent 3D MHD numerical simulations, we analyze magnetized solar-like stellar winds and their dependence on the plasma-beta parameter. We adopt in our simulations a heating parameter described by gamma, which is responsible for the thermal acceleration of the wind. We analyze winds with polar magnetic field intensities ranging from 1 to 20G. We show that the wind structure presents characteristics that are similar to the solar coronal wind. The steady-state magnetic field topology for all cases is similar, presenting a configuration of helmet streamer-type, with zones of closed field lines and open field lines coexisting. Higher magnetic field intensities lead to faster and hotter winds. The increase of the field intensity generates a larger dead zone in the wind, i. e., the closed loops that inhibit matter to escape from latitudes lower than ~45 degrees extend farther away from the star. The Lorentz force leads naturally to a latitude-dependent wind. We show that by increasing the density and maintaining B0=20G, the system recover back to slower and cooler winds. For a fixed gamma, we show that the key parameter in determining the wind velocity profile is the beta-parameter at the coronal base. Therefore, there is a group of magnetized flows that would present the same terminal velocity despite of its thermal and magnetic energy densities, as long as the plasma-beta parameter is the same. This degeneracy, however, can be removed if we compare other physical parameters of the wind, such as the mass-loss rate. We analyze the influence of gamma in our results and we show that it is also important in determining the wind structure. (Abridged), Comment: Accepted for publication in the Astrophysical Journal, 12 pages, 12 figures, emulateapj. High-resolution version available at http://www.astro.iag.usp.br/~aline/papers/2009apj-solar-like.pdf

Alfven waves have been invoked as an important mechanism of particle acceleration in stellar winds of cool stars. After their identification in the solar wind they started to be studied in winds of stars located in different regions of the HR diagram. We discuss here some characteristics of these waves and we present a direct application in the acceleration of late-type stellar winds., Comment: Accepted for publication in Advances in Space Research. Presented at the World Space Environment Forum 2007, Egypt. 9 pages, 2 figures

We investigate magnetized solar-like stellar winds by means of self-consistent three-dimensional (3D) magnetohydrodynamics (MHD) numerical simulations. We analyze winds with different magnetic field intensities and densities as to explore the dependence on the plasma-beta parameter. By solving the fully ideal 3D MHD equations, we show that the plasma-beta parameter is the crucial parameter in the configuration of the steady-state wind. Therefore, there is a group of magnetized flows that would present the same terminal velocity despite of its thermal and magnetic energy densities, as long as the plasma-beta parameter is the same., Comment: 2 pages, 4 figures, Proceedings of the IAU Symposium 259, "Cosmic Magnetic Fields: From Planets, to Stars and Galaxies", November 2008

The study of Wolf-Rayet stars plays an important role in evolutionary theories of massive stars. Among these objects, ~ 20% are known to be in binary systems and can therefore be used for the mass determination of these stars. Most of these systems are not spatially resolved and spectral lines can be used to constrain the orbital parameters. However, part of the emission may originate in the interaction zone between the stellar winds, modifying the line profiles and thus challenging us to use different models to interpret them. In this work, we analyzed the HeII4686\AA + CIV4658\AA blended lines of WR30a (WO4+O5) assuming that part of the emission originate in the wind-wind interaction zone. In fact, this line presents a quiescent base profile, attributed to the WO wind, and a superposed excess, which varies with the orbital phase along the 4.6 day period. Under these assumptions, we were able to fit the excess spectral line profile and central velocity for all phases, except for the longest wavelengths, where a spectral line with constant velocity seems to be present. The fit parameters provide the eccentricity and inclination of the binary orbit, from which it is possible to constrain the stellar masses., Comment: accepted for publication in the MNRAS

One of the most intriguing spectral features of WR binary stars is the presence of time-dependent line profiles. Long term observations of several systems revealed the periodicity of this variability, synchronized with the orbital movement. Several partially successful models have been proposed to reproduce the observed data. The most promising assume that the origin of the emission is the wind-wind interaction zone. In this scenario, two high velocity and dense winds produce a strong shock layer, responsible for most of the X-rays observed from these systems. As the secondary star moves along its orbital path, the shock region of conical shape, changes its position with relation to the line of sight. As a consequence, the stream measured Doppler shift presents time variations resulting in position changes of the spectral line. In our work, we present an alternative model, introducing turbulence in the shock layer to account for the line broadening and opacity effects for the asymmetry in the line profiles. We showed that the gas turbulence avoids the need of an unnaturally large contact layer thickness to reproduce line broadening. Also, we demonstrated that if the emission from the opposing cone surface is absorbed, the result is a single peaked profile. This result fully satisfies the recent data obtained from massive binary systems, and can help on the determination of both winds and orbital parameters. We successfully applied this model to the Br22 system and determined its orbital parameters., Comment: To appear in the MNRAS

Cool giant and supergiant stars generally present low velocity winds with high mass loss rates. Several models have been proposed to explain the acceleration process of these winds. Although dust is known to be present in these objects, the radiation pressure on these particles is uneffective in reproducing the observed physical parameters of the wind. The most promising acceleration mechanism cited in the literature is the transference of momentum and energy from Alfven waves to the gas. Usually, these models consider the wind to be isothermal. We present a stellar wind model in which the Alfven waves are used as the main acceleration mechanism, and determine the temperature profile by solving the energy equation taking into account both the radiative losses and the wave heating. We also determine self-consistently the magnetic field geometry as the result of the competition between the magnetic field and the thermal pressures gradient. As main result, we show that the magnetic geometry present a super-radial index in the region where the gas pressure is increasing. However, this super-radial index is greater than that observed for the solar corona., Comment: Accepted for publication in Space Science Reviews. Presented at the World Space Environment Forum 2005, Austria. 8 pages, 2 figures

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 luminosities, 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, probably, 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 fluxtubes expand with a super-radial factor S>30 near the stellar surface, larger than that used in previous semi-empirical models. The rapid expansion results in a strong spatial dilution of the wave flux. We obtained the wind parameter profiles for a typical supergiant star of 16 M_sun. 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. 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 1.5 r_0 reaching 6000 K. Propagating outwards, the wind cools down mainly due to adiabatic expansion., Comment: to appear in the MNRAS

There are in the literature several theories to explain the mass loss in stellar winds. In particular, for late-type stars, some authors have proposed a wind model driven by an outward-directed flux of damped Alfven waves. The winds of these stars present great amounts of dust particles that, if charged, can give rise to new wave modes or modify the pre-existing ones. In this work, we study how the dust can affect the propagation of Alfven waves in these winds taking into account a specific damping mechanism, dust-cyclotron damping. This damping affects the Alfven wave propagation near the dust-cyclotron frequency. Hence, if we assume a dust size distribution, the damping occurs over a broad band of wave frequencies. In this work, we present a model of Alfven wave-driven winds using the dust-cyclotron damping mechanism. On the basis of coronal holes in the Sun, which present a superradial expansion, our model also assumes a diverging geometry for the magnetic field. Thus, the mass, momentum, and energy equations are obtained and then solved in a self-consistent approach. Our results of wind velocity and temperature profiles for a typical K5 supergiant star shows compatibility with observations. We also show that, considering the presence of charged dust particles, the wave flux is less damped due to the dust-cyclotron damping than it would be if we consider some other damping mechanisms studied in the literature, such as nonlinear damping, resonant surface damping, and turbulent damping., Comment: 10 pages, 5 figures. ApJ, vol. 639, pp 416-422. Minor changes in the text

In this paper we use the 7 mm and 1.3 mm light curves obtained during the 2003.5 low excitation phase of the eta Carinae system to constrain the possible parameters of the binary orbit. To do that we assumed that the mm wave emission is produced in a dense disk surrounding the binary system; during the low excitation phase, which occurs close to periastron, the number of ionizing photons decreases, producing the dip in the radio emission. On the other hand, due to the large eccentricity, the density of the shock region at periastron is very high and the plasma is optically thick for free-free radiation at 7 mm, explaining the sharp peak that was observed at this frequency and lasted for about 10 days. From the shape and duration of the peak we were able to determine the orbital parameters of the binary system, independently of the stellar parameters, such as mass loss rates, wind velocities or temperature at the post-shock region., Comment: accepted for publication by MNRAS

In this paper we present observations of eta Carinae in the 1.3 mm and 7 mm radio continuum, during the 2003.5 low excitation phase. The expected minimum in the light curves was confirmed at both wavelengths and was probably due to a decrease in the number of UV photons available to ionize the gas surrounding the binary system. At 7 mm a very well defined peak was superimposed on the declining flux density. It presented maximum amplitude in 29 June 2003 and lasted for about 10 days. We show that its origin can be free-free emission from the gas at the shock formed by wind-wind collision, which is also responsible for the observed X-ray emission. Even though the shock strength is strongly enhanced as the two stars in the binary system approach each other, during periastron passage the X-ray emission is strongly absorbed and the 7 mm observations represent the only direct evidence of this event.

The exact nature of eta Carinae is still an open issue. In this paper we assume a binary system to explain the strong X-ray emission, but we also take into account that, near periastron and because of the highly eccentric orbit, the wind emerging from eta Carinae accumulates behind the shock and can mimic a shell-like ejection event. For this process to be effective, at periastron the secondary star should be located between eta Carinae and the observer, solving also the discrepancy between the orbital parameters derived from ground and space based observations. We show that, as the secondary moves in its orbit, the shell cools down and the number of available stellar ionizing photons is not enough 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 formation and growth can take place in timescales of hours. Using recently published HST data for the optical and UV continuum during the 2003 shell event, we estimated that a fraction of 0.01 of the interstellar dust to H ratio is enough to explain the observations. We also calculated the neutral gas column 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 without any change in the eta Carinae mass loss rate. This same column density can explain the observed H$\alpha$ light curve observed during the 2003 event., Comment: accepted to MNRAS

It has been shown that magnetic fields play an important role in the stability of molecular clouds, mainly perpendicularly to the field direction. However, in the parallel direction the stability is a serious problem still to be explained. Interstellar turbulence may allow the generation of Alfv\'en waves that propagate through the clouds in the magnetic field direction. These regions also present great amounts of dust particles which can give rise to new wave modes, or modify the pre-existing ones. The dust-cyclotron damping affects the Alfv\'en wave propagation near the dust- cyclotron frequency. On the other hand, the clouds present different grain sizes, which carry different charges. In this sense, a dust particle distribution has several dust-cyclotron frequencies and it will affect a broad band of wave frequencies. In this case, the energy transfer to the gas is more efficient than in the case where the ion-cyclotron damping is considered alone. This effect becomes more important if a power law spectrum is considered for the wave energy flux, since the major part of the energy is concentrated in low-frequency waves. In this work we calculate the dust- cyclotron damping in a dusty and magnetized dwarf molecular cloud, as well as determine the changes in the Alfv\'en wave flux. Then, we use these results to study the gravitational stability of the cloud. We show that, considering the presence of charged dust particles, the wave flux is rapidly damped due to dust-cyclotron damping. Then the wave pressure acts in a small length scale, and cannot explain the observable cloud sizes, but can explain the existence of small and dense cores., Comment: accepted for publication in ApJ

We present 2.5-dimensional time-dependent simulations of nonrelativistic and nonradiative outflows from sinusoidally perturbed Keplerian accretion disks. A sinusoidal perturbation is introduced in the velocity of the gas ejected from the surface of the disk into a cold corona. In the simulations, the disk is a fixed boundary from which the gas is ejected with a pulsed velocity. The maximum value of this velocity is taken to be a thousandth of the local Keplerian disk velocity. It was found that for large periods, the structures in the jet tend to fragment into smaller substructures. For small values of the period, the structures tend to dissipate, while for medium values of the period, they tend to persist., Comment: 27 pages, 11 figures (by e-mail to braulo@astro.iag.usp.br), accepted for publication in ApJ

Excitation of a Global Alfven wave (GAW) is proposed as a viable mechanism to explain plasma heating in the magnetosphere of young stars. The wave and basic plasma parameters are compatible with the requirement that the dissipation length of GAWs be comparable to the distance between the shocked region at the star's surface and the truncation region in the accretion disk. A two-fluid magnetohydrodynamic plasma model is used in the analysis. A current carrying filament along magnetic field lines acts as a waveguide for the GAW. The current in the filament is driven by plasma waves along the magnetic field lines and/or by plasma crossing magnetic field lines in the truncated region of the disk of the accreting plasma. The conversion of a small fraction of the kinetic energy into GAW energy is sufficient to heat the plasma filament to observed temperatures., Comment: Submitted to ApJ, aheatf.tex, 2 figures

Recent IR observations indicate that many massive binary systems present dust formation episodes, in regions close to the stars, during the periastron passage. These systems are known to be high-energy sources, and it is believed that wind collisions are the origin of the emission. In this work we show that wind collisions not only increase the X-ray emission but also allow dust formation. As an application we study eta Car, which presents, near periastron, an increase in the X-ray emission followed by a sudden decrease that lasts for about a month. We reproduce this feature calculating the optical depth due to dust formation along the orbital period., Comment: proceedings of "Winds, Bubbles and Explosions - in honour to John Dyson" - Patzcuaro, Mexico (Sep 2002)

Massive binary star systems are not uncommon, and neither the supersonic collision of their winds. In the present work we study these shocks and the further consequences on wind structure. The post-shock gas is a warm and high-density environment, which allows dust to form and grow. We show that this growth is fast, of just a few hours. An application for etaCarinae shows that, probably, the decline of X-rays fluxes observed in its light curve is the consequence of its high absorption in periodic dust formation events, on the periastron passage., Comment: 2 pages, in: A Massive Star Odyssey, from Main Sequence to Supernova (IAU212)

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, we used a non-isothermal profile for temperature, 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/r_0 < 2.0 their presence will affect the mass loss and terminal velocity. The model is applied to a K5 supergiant star and for Betelgeuse (alpha Ori)., Comment: 6 pages, 3 figures, accepted in ApJ (Sep, 2002)

We examine the role of Alfven wave damping in heating the plasma in the magnetic funnels of magnetospheric accretion models of young stars. We study four different damping mechanisms of the Alfven waves: nonlinear, turbulent, viscous-resistive and collisional. Two different possible origins for the Alfven waves are discussed: 1) Alfven waves generated at the surface of the star by the shock produced by the infalling matter; and 2) Alfven waves generated locally in the funnel by the Kelvin-Helmholtz instability. We find that, in general, the damping lengths are smaller than the tube length. Since thermal conduction in the tube is not efficient, Alfven waves generated only at the star's surface cannot heat the tube to the temperatures necessary to fit the observations. Only for very low frequency Alfven waves ~10^{-5} the ion cyclotron frequency, is the viscous-resistive damping length greater than the tube length. In this case, the Alfven waves produced at the surface of the star are able to heat the whole tube. Otherwise, local production of Alfven waves is required to explain the observations. The turbulence level is calculated for different frequencies for optically thin and thick media. We find that turbulent velocities varies greatly for different damping mechanisms, reaching \~100 km s^{-1} for the collisional damping of small frequency waves., Comment: 29 pages, 12 figures, to appear in The Astrophysical Journal

The formation of quasar broad-line region (BLR) clouds via thermal instability in the presence of Alfven heating has been discussed by Goncalves et al. (1993a, 1996). In particular, these studies showed the relevance of Alfven heating in establishing the stability of BLR clouds in the intercloud medium. The present paper shows the results of time-dependent calculations (we use a time-dependent hydrodynamic code) following the evolution of BLR clouds, since their formation from the 10^7 K intercloud medium. We also calculate the UV and optical line emission associated with the clouds in order to compare with observations. Our results are compared with those of UV and optical monitoring of well-studied AGN, which suggest that the BLR is most probably composed of at least two different regions, each one giving rise to a kind of line variability, since low- and high-ionization lines present different patterns of variability. We discuss the alternative scenario in which lines of different ionization could be formed at the same place but heated/excited by distinct mechanisms, considering as a non-radiative mechanism the Alfven heating., Comment: 9 pages, 4 figures, MNRAS in press

We investigate the effects of heating generated by damping of Alfven waves on protostellar accretion disks. Two mechanisms of damping are investigated, nonlinear and turbulent, which were previously studied in stellar winds (Jatenco-Pereira & Opher 1989a, b). For the nominal values studied, f=delta v/v_{A}=0.002 and F=varpi/Omega_{i}=0.1, where delta v, v_{A} and varpi are the amplitude, velocity and average frequency of the Alfven wave, respectively, and Omega_{i} is the ion cyclotron frequency, we find that viscous heating is more important than Alfven heating for small radii. When the radius is greater than 0.5 AU, Alfvenic heating is more important than viscous heating. Thus, even for the relatively small value of f=0.002, Alfvenic heating can be an important source of energy for ionizing protostellar disks, enabling angular momentum transport to occur by the Balbus-Hawley instability., Comment: 21 pages, 9 figures. Accepted for publication in ApJ

It was shown by dos Santos et al. the importance of Alfv\'en waves to explain the winds of Wolf-Rayet stars. We investigate here the possible importance of Alfv\'en waves in the creation of inhomogeneities in the winds of early-type stars. The observed infrared emission (at the base of the wind) of early-type stars is often larger than expected. The clumping explains this characteristic in the wind, increasing the mean density and hence the emission measure, making possible to understand the observed infrared, as well as the observed enhancement in the blue wing of the $H_\alpha$ line. In this study, we investigate the formation of these clumps a via thermal instability. The heat-loss function used, $H(T,n)$, includes physical processes such as: emission of (continuous and line) recombination radiation; resonance line emission excited by electron collisions; thermal bremsstrahlung; Compton heating and cooling; and damping of Alfv\'en waves. As a result of this heat-loss function we show the existence of two stable equilibrium regions. The stable equilibrium region at high temperature is the diffuse medium and at low temperature the clumps. Using this reasonable heat-loss function, we show that the two stable equilibrium regions can coexist over a narrow range of pressures describing the diffuse medium and the clumps., Comment: 21 pages (psfig.sty), 5 figures (included), ApJ accepted. Also available at http://www.iagusp.usp.br/preprints/preprint.html

The Ha peak intensity, velocity shift and velocity dispersion maps of the giant HII region NGC 604 in M 33, obtained by two dimensional high spatial resolution Fabry-Perot observations with TAURUS II at the 4.2 m William Herschel Telescope in Spain (Sabalisck, 1995), are analyzed via two point correlation functions. The whole system seems to rotate as a rigid body on scales from 50 to 80 pc (the largest studied scale), with a period of $\sim$ 85 Myr. We demonstrate that the cloud seems to be comprised of eddies with varying characteristic scale lengths which range from 10 pc to the largest observed scales. The calculated kinetic energy spectrum may be interpreted as either a manifestation of a double cascading spectrum of forced two-dimensional turbulence, or as a Kolmogorov three-dimensional turbulence (although this last possibility seems unlikely). According to the first interpretation, turbulence is being forced at scales of $\sim$ 10 pc, while an enstrophy (mean-square vorticity) cascade has developed down to the smallest scales resolved and an inverse kinetic energy cascade extends up to scales of $\sim$ 70 pc where a low wave number turn over is observed; if true, this would be the first time that such a phenomenon has been observed outside the Solar System. As for the second interpretation, energy should be injected at the largest scales, $\sim$ 70 pc. In both cases the average intrinsic optical depth consistent with the results is $\sim$ 20 pc., Comment: 27 pages, postscript version including 8 embedded figures

One of the major problems concerning cooling flows is the nature of the mechanism powering the emission lines of the optical filaments seen in the inner regions of cooling flows. In this work we investigate the plausibility of Alfv\'en heating (AH) as a heating/excitation mechanism of optical filaments in cooling flows. We use a time-dependent hydrodynamical code to follow the evolution of cooling condensations arising from the $10^7$ K cooling flow. The filaments contain magnetic fields and AH is at work at several degrees of efficiency (including no AH at all). We consider two damping mechanisms of Alfv\'en waves: nonlinear and turbulent. We calculate the optical line emission associated with the filaments and compare our results to the observations. We find that AH can be an important ionizing and heating source for class I filaments in the line-ratio scheme of Heckman et al. In addition, AH can be an important contributor to [OI]$\lambda 6300$ emission even for the more luminous class II systems., Comment: 7 pages, 2 figures, TeX, aa.cmm style