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We present an analytical model for the free–free emission at radio wavelengths produced by the interaction of a Coronal Mass Ejection (CME) with the ambient solar wind. Using our previous models, we show that a dense shell bounded by two shock fronts is formed from this interaction, whose dynamical evolution can be calculated based on considerations of the mass and momentum conservation for the shell. This structure undergoes two stages in its dynamical evolution: (1) A first one of constant velocity, when the shell is bounded by two shock fronts, and (2) a second one, in which a one-shock structure (the leading shock) is decelerated. Here, we compute the emission produced by these shocks, and present a comparison with synthetic observations of the 2011 March 7 CME. Our simplified model gives a physical insight into the free–free emission produced by shocks of CMEs.

The 2005 May 13 eruption was an Earth-directed coronal mass ejection (CME) observed using the Multi-Element Radio-Linked Interferometer Network telescope system in the UK as it traversed the Interplanetary Medium on 2005 May 14. Observations of interplanetary scintillation (IPS) provide information on the solar wind conditions, which include velocities and density. In addition, it is also possible to calculate other parameters related to the turbulence and geometry of density irregularities in the solar wind from observations of IPS. Previous analyses have shown that IPS information can be difficult to interpret when a complex structure is crossing the line of sight since the physical properties of the plasma may change quite drastically with distance from the Sun. In order to compare and relate the internal structure of a CME and its physical changes, with the results from observations of IPS obtained previously, we carried out a numerical simulation of the 2005 May 13 eruptive event as it propagates into the IPM, adapting the geometrical properties derived by IPS analysis. In this work, we give a possible explanation of some signatures of CME sub-structure from the point of view of the IPS technique combined with what the modelling reveals.

The nature and origin of the molecular gas component located in the circumstellar vicinity of $\eta$ Carinae are still far from being completely understood. Here, we present Atacama Large Millimeter/Submillimeter Array (ALMA) CO(3$-$2) observations with a high angular resolution ($\sim$0.15$''$), and a great sensitivity that are employed to reveal the origin of this component in $\eta$ Carinae. These observations reveal much higher velocity ($-$300 to $+$270 km s$^{-1}$) blue and redshifted molecular thermal emission than previously reported, which we associate with the lobes of the Homunculus Nebula, and that delineates very well the innermost contours of the red- and blue-shifted lobes likely due by limb brightening. The inner contour of the redshifted emission was proposed to be a {\it disrupted torus}, but here we revealed that it is at least part of the molecular emission originated from the lobes and/or the expanding equatorial skirt. On the other hand, closer to systemic velocities ($\pm$100 km s$^{-1}$), the CO molecular gas traces an inner butterfly-shaped structure that is also revealed at NIR and MIR wavelengths as the region in which the shielded dust resides. The location and kinematics of the molecular component indicate that this material has formed after the different eruptions of $\eta$ Carinae., Comment: Accepted to Astrophysical Journal. 3D animation -> https://drive.google.com/file/d/1MRWaTkij-Lf_uzTfamJfHeGvgzjDe6Oq/view?usp=sharing

The transition from prompt to the afterglow emission is one of the most exciting and least understood phases in gamma-ray bursts (GRBs). Correlations among optical, X-ray and gamma-ray emission in GRBs have been explored, to attempt to answer whether the earliest optical emission comes from internal and/or external shocks. We present optical photometric observations of GRB 180325A collected with the TAROT and RATIR ground-based telescopes. These observations show two strong optical flashes with separate peaks at $\sim50\;$s and $\sim120\;$s, followed by a temporally extended optical emission. We also present X-rays and gamma-ray observations of GRB 180325A, detected by the Burst Alert Telescope (BAT) and X-ray Telescope (XRT), on the Neil Gehrels Swift observatory, which both observed a narrow flash at $\sim80\;$s. We show that the prompt gamma- and X-ray early emission shares similar temporal and spectral features consistent with internal dissipation within the relativistic outflow (e.g. by internal shocks or magnetic reconnection), while the early optical flashes are likely generated by the reverse shock that decelerates the ejecta as it sweeps up the external medium., Accepted for publication in ApJ

We present radiation-magnetohydrodynamic simulations aimed at studying evolutionary properties of H\,{\normalsize II} regions in turbulent, magnetised, and collapsing molecular clouds formed by converging flows in the warm neutral medium. We focus on the structure, dynamics and expansion laws of these regions. Once a massive star forms in our highly structured clouds, its ionising radiation eventually stops the accretion (through filaments) toward the massive star-forming regions. The new over-pressured H\,{\normalsize II} regions push away the dense gas, thus disrupting the more massive collapse centres. Also, because of the complex density structure in the cloud, the H\,{\normalsize II} regions expand in a hybrid manner: they virtually do not expand toward the densest regions (cores), while they expand according to the classical analytical result towards the rest of the cloud, and in an accelerated way, as a blister region, towards the diffuse medium. Thus, the ionised regions grow anisotropically, and the ionising stars generally appear off-centre of the regions. Finally, we find that the hypotheses assumed in standard H\,{\normalsize II}-region expansion models (fully embedded region, blister-type, or expansion in a density gradient) apply simultaneously in different parts of our simulated H\,{\normalsize II} regions, producing a net expansion law ($R \propto t^\alpha$, with $\alpha$ in the range of 0.93-1.47 and a mean value of $1.2 \pm 0.17$) that differs from any of those of the standard models., Comment: Accepted for publication in MNRAS

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We present a two-component supernova remnant model for the case of Magellanic remnant DEM L316 obtained from two-dimensional, axisymmetric hydrodynamic simulations. We study different scenarios which consider a possible collision between the shells and also the effects of thermal conduction. Synthetic X-ray maps were obtained from numerical results in order to directly compare with the observed morphology of this object. We find a good agreement is achieved when thermal conduction is included, reproducing both the observed morphology and the total X-ray luminosity very well. Finally, our results suggest that the two components of DEM L316 are not in physical interaction.

We present 3D hydrodynamical simulations of the superbubble M17, also known as the Omega nebula, carried out with the adaptive grid code yguazu'-a, which includes radiative cooling. The superbubble is modelled considering the winds of 11 individual stars from the open cluster inside the nebula (NGC 6618), for which there are estimates of the mass loss rates and terminal velocities based on their spectral types. These stars are located inside a dense interstellar medium, and they are bounded by two dense molecular clouds. We carried out three numerical models of this scenario, considering different line of sight positions of the stars (the position in the plane of the sky is known, thus fixed). Synthetic thermal X-ray emission maps are calculated from the numerical models and compared with ROSAT observations of this astrophysical object. Our models reproduce successfully both the observed X-ray morphology and the total X-ray luminosity, without taking into account thermal conduction effects.

Here we present a simple, semianalytic model for estimating the free-free radiation (at radio wavelengths) from the P Cygni stellar wind. Recent observations from this source at different epochs show time-dependent flux densities and spectral indices, which clearly deviate from the standard models of ionized extended envelopes around massive stars. In this work, we study the contribution by shocks to the total emission from a completely ionized stellar wind, which results from variations in the wind parameters at injection. In particular, we consider sudden changes in the wind velocity and mass loss rate that instantaneously produce two-shock wave structures (called working surfaces) that travel down the flow. The compressed layer between the two shocks emits continuum radiation that can be detected in the radio band. We show that our model gives a possible explanation for the observed variability in the flux densities and spectral indices from the P Cygni wind.

At centimeter wavelengths, the young stellar system T Tauri is known to be composed of two sources, the northern one associated with the optical star T Tau itself, and the southern one related to the infrared companion T Tau S. Here wereexaminetheoriginoftheradioemissionfromthesetwocomponentsusingarchival2cm,3.6cm,and6cmVLA observations. The emission from the northern member is confirmed to be largely dominated by free-free radiation from an ionized wind, while the southern radio source is confirmed to consist of a compact component of magnetic origin, surrounded by an extended halo. Only moderately variable, the extended structure associated with the southern source is most likely the result of free-free radiation related to stellar winds. However, its flat spectral energy distribution,its extent,andthelackofvariationof itssizewiththefrequencyofobservationareincompatiblewiththe classical picture of a fully ionized wind with constant velocity and mass-loss rate leading to an electron density distribution of ne(r) / r � 2 . Instead, we propose a model in which the ionization results from the impact of a supersonic wind driven by T Tau Sb onto dense surrounding material, possibly associated with the circumbinary disk recently identified around the T Tau Sa/T Tau Sb pair. The timescales for cooling and recombination in such a situation are in good agreement with the observed morphological changes undergone by the extended structure as its driving source moves through the environment. Subject headings: binaries: general — ISM: jets and outflows — radiation mechanisms: general — radio continuum: stars — stars: formation

We study the formation of giant dense cloud complexes and of stars within them by means of SPH numerical simulations of the mildly supersonic collision of gas streams (``inflows'') in the warm neutral medium (WNM). The resulting compressions cause cooling and turbulence generation in the gas, forming a cloud that then becomes self-gravitating and undergoes global collapse. Simultaneously, the turbulent, nonlinear density fluctuations induce fast, local collapse events. The simulations show that: a) The clouds are not in a state of equilibrium. Instead, they undergo secular evolution. Initially, their mass and gravitational energy |Eg| increase steadily, while the turbulent energy Ek reaches a plateau. b) When |Eg| becomes comparable to Ek, global collapse begins, causing a simultaneous increase in both that maintains a near-equipartition condition |Eg| ~ 2 Ek. c) Longer inflow durations delay the onset of global and local collapse, by maintaining a higher turbulent velocity dispersion in the cloud over longer times. d) The star formation rate is large from the beginning, without any period of slow and accelerating star formation. e) The column densities of the local star-forming clumps are very similar to reported values of the column density required for molecule formation, suggesting that locally molecular gas and star formation occur nearly simultaneously. The MC formation mechanism discussed here naturally explains the apparent ``virialized'' state of MCs and the ubiquitous presence of HI halos around them. Within their assumptions, our simulations support the scenario of rapid star formation after MCs are formed, although long (>~ 15 Myr) accumulation periods do occur during which the clouds build up their gravitational energy, and which are expected to be spent in the atomic phase., matches version to be published. Corrected link to movies: http://www.astrosmo.unam.mx/~e.vazquez/turbulence/movies/VSetal06/VSetal06.html

In this paper, we compute theoretically the flux density and the spectral index of the free-free radiation at radio wavelengths produced by shocks in the inner bipolar emission nebula called the little Homunculus around the star η Carinae. The little Homunculus is believed to have formed as a result of the minor eruption suffered by the star in the 1890s. In our model, we consider a simplified interacting stellar wind scenario where the post-outburst η Carinae wind collides with the eruptive outflow (both assumed to be bipolar with conical symmetry). As a result of the interaction, shock-wave structures are formed and generate the development of two polar caps moving in opposite directions. After ∼100 yr (i.e. at present times), the polar caps are located ±2.3 arcsec on each side of the star, and remain embedded within the larger bipolar Homunculus that extends from -8 to +8 arcsec along its major axis. Using observational estimates of the characteristics of the eruptive event of the 1890s, and of the ambient wind powered by η Carinae in the decades after the eruption ended, we study the evolution of the polar caps formed as a result of a sudden increase in the wind velocity and an instantaneous drop in the mass-loss rate (just after the eruption) at the injection radius. We found that the little Homunculus emits continuum radiation that can be detected at radio frequencies and that indeed represents an important contribution to the total free-free emission detected from the η Carinae nebula.

In this paper we present a simple, analytic model for the dynamical evolution of supersonic velocity fluctuations at the base of the ambient solar wind. These fluctuations result in the formation of dense working surfaces that travel down the wind. It is shown how the initial parameters of the fluctuations (velocity, density and duration) are related to the characteristics of the working surfaces far from the Sun (for instance at the Earth). We apply the model to the evolution of the coronal mass ejections in the IP medium, finding that the model is in good agreement with satellite observations of these phenomena, thus providing physical insight into their dynamical evolution. Our model may contribute to future ‘space weather forecasting’ on the Earth, based on detailed satellite monitoring of the solar corona.

We study the propagation of coronal mass ejections (CMES) from near the Sun to 1 AU by comparing results from two different models: a 1-D, hydrodynamic, single-fluid, numerical model (Gonzalez-Esparza et al., 2003a) and an analytical model to study the dynamical evolution of supersonic velocity's fluctuations at the base of the solar wind applied to the propagation of CMES (Canto et al., 2002). Both models predict that a fast CME moves initially in the inner heliosphere with a quasi-constant velocity (which has an intermediate value between the initial CME velocity and the ambient solar wind velocity ahead) until a ‘critical distance’ at which the CME velocity begins to decelerate approaching to the ambient solar wind velocity. This critical distance depends on the characteristics of the CME (initial velocity, density and temperature) as well as of the ambient solar wind. Given typical parameters based on observations, this critical distance can vary from 0.3 to beyond 1 AU from the Sun. These results explain the radial evolution of the velocity of fast CMEs in the inner heliosphere inferred from interplanetary scintillation (IPS) observations (Manoharan et al., 2001, 2003, Tokumaru et al., 2003). On the other hand, the numerical results show that a fast CME and its associated interplanetary (IP) shock follow different heliocentric evolutions: the IP shock always propagates faster than its CME driver and the latter begins to decelerate well before the shock.

In this paper, new two-dimensional hydrodynamical simulations of η Car’s nebulae are performed. In the 1840s, the massive star η Car suffered a major eruption that resulted in the formation of a bipolar structure, which is commonly known as the large Homunculus. During this event, η Car expelled into the circumstellar material a total mass of ~10 M⊙ and released a total energy of Ek ~ 1050 erg over a very short time (≤5 yr). These kinds of explosive events are frequently called supernova impostors due to their resemblance to a type II supernova, but the stars survive the explosion. In the case of η Car, a brief explosion scenario provides a potential explanation for the behavior of the historical light curve of η Car a few years (~10 yr) after the nineteenth century outburst. Here, such an alternative scenario of an explosion is assumed (instead of a super-Eddington wind) in order to investigate whether an explosive event is also able to explain the shape and kinematics of the large Homunculus. I show that the numerical simulations presented here indeed resemble some of the observed features of the nebula, such as the present-day double-shell structure of the Homunculus, with a thin outer dense shell and a thicker inner layer, as well as thermal instabilities (Rayleigh-Taylor and Kelvin-Helmholtz) along the dense shell that may lead to the current mottled appearance of the large Homunculus. Nonetheless, the explosion model for the 1840s major eruption of η Car is not able to account for the estimated age of the large Homunculus.

Young stars of low and intermediate masses frequently show continuum emission in radio frequencies. The observed flux densities and spectral indices indicate that in most cases this emission is of thermal (free-free) origin and is produced in the powerful stellar winds emanating from the stars. However, this interpretation faces the problem of the ionization of the wind. Photoionization can be ruled out because of the insufficient number of UV photons emitted by this kind of star. In this paper, we present a model in which the ionization (and emission) is produced by internal shocks in the wind. These shocks are the result of periodic variations of the velocity of the wind at injection. It is shown that the free-free radio emission predicted by the model is in good agreement with those observed in young stars of low and intermediate masses.

We present a formalism of the dynamics of internal shocks in relativistic jets where the source has a time-dependent injection velocity and mass-loss rate. The variation of the injection velocity produces a two-shock wave structure, the working surface, that moves along the jet. This new formalism takes into account the fact that momentum conservation is not valid for relativistic flows where the relativistic mass lost by radiation must be taken into account, in contrast to the classic regime. We find analytic solutions for the working surface velocity and radiated energy for the particular case of a step function variability of the injection parameters. We model two cases: a pulse of fast material and a pulse of slow material (with respect to the mean flow). Applying these models to gamma ray burst light curves, one can determine the ratio of the Lorentz factors gamma_2 / gamma_1 and the ratio of the mass-loss rates dot{m_2} / dot{m_1} of the upstream and downstream flows. As an example, we apply this model to the sources GRB 080413B and GRB 070318 and find the values of these ratios. Assuming a Lorentz factor gamma_1=100, we further estimate jet mass-loss rates between dot{m_1} ~ 10^{-5}-1 Msun.yr^{-1}. We also calculate the fraction of the injected mass lost by radiation. For GRB 070318 this fraction is ~7%. In contrast, for GRB 080413B this fraction is larger than 50%; in this case radiation losses clearly affect the dynamics of the internal shocks., Comment: 13 pages, 9 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal

The classic HII region M17 is one of the best studied across the electromagnetic spectrum. We present sensitive, high angular resolution observations made with the Jansky Very Large Array (JVLA) at 4.96 and 8.46 GHz that reveal the presence of 38 compact radio sources, in addition to the well known hypercompact cometary HII region M17 UC1. For this last source we find that its spectral index of value $\sim$1 is due to a gradient in opacity across its face. Of the 38 compact radio sources detected, 19 have stellar counterparts detected in the infrared, optical, or X-rays. Finally, we discuss the nature of the radio emission from the massive binary system CEN 1a and 1b, concluding that both are most probably non-thermal emitters, although the first is strongly time variable and the second is steady., 24 pages, 7 figures, To appear in The Astrophysical Journal

Aims. The interaction of two isotropic stellar winds is studied in order to calculate the free-free emission from the wind collision region. The effects of the binary separation and the wind momentum ratio on the emission from the wind-wind interaction region are investigated. Methods. We developed a semi-analytical model for calculating the thermal emission from colliding stellar winds. Assuming radiative shocks for the compressed layer, which are expected in close binaries, we obtained the emission measure of the thin shell. Then, we computed the total optical depth along each line of sight to obtain the emission from the whole configuration. Results. Here, we present predictions of the free-free emission at radio frequencies from analytic, radiative shock models in colliding wind binaries. It is shown that the emission from the wind collision region mainly arises from the optically thick region of the compressed layer and scales as ~ D^{4/5}, where D is the binary separation. The predicted flux density from the wind collision region becomes more important as the frequency increases, showing higher spectral indices than the expected 0.6 value from the unshocked winds. We also investigate the emission from short-period WR+O systems calculated with our analytic formulation. In particular, we apply the model to the binary systems WR 98 and WR 113 and compare our results with the observations. Our theoretical results are in good agreement with the observed thermal spectra from these sources., Comment: Accepted for publication in Astronomy and Astrophysics, 12 pages, 10 figures

We present here new results of two-dimensional hydrodynamical simulations of the eruptive events of the 1840s (the great) and the 1890s (the minor) eruptions suffered by the massive star $\eta$ Car. The two bipolar nebulae commonly known as the Homunculus and the little Homunculus were formed from the interaction of these eruptive events with the underlying stellar wind. As in previous work (Gonzalez et al. 2004a, 2004b), we assume here an interacting, nonspherical multiple-phase wind scenario to explain the shape and the kinematics of both Homunculi, but adopt a more realistic parametrization of the phases of the wind. During the 1890s eruptive event, the outflow speed {\it decreased} for a short period of time. This fact suggests that the little Homunculus is formed when the eruption ends, from the impact of the post-outburst $\eta$ Car wind (that follows the 1890s event) with the eruptive flow (rather than by the collision of the eruptive flow with the pre-outburst wind, as claimed in previous models; Gonzalez et al. 2004a, 2004b). Our simulations reproduce quite well the shape and the observed expansion speed of the large Homunculus. The little Homunculus (which is embedded within the large Homunculus) becomes Rayleigh-Taylor unstable and develop filamentary structures that resembles the spatial features observed in the polar caps. In addition, we find that the interior cavity between the two Homunculi is partially filled by material that is expelled during the decades following the great eruption. This result may be connected with the observed double-shell structure in the polar lobes of the $\eta$ Car nebula. Finally, as in previous work, we find the formation of tenuous, equatorial, high-speed features that seem to be related to the observed equatorial skirt of $\eta$ Car., Comment: accepted for publication in MNRAS

"We present the analysis of VLA archive observation made in the year 2000 at 8.46 GHz toward M17. In the region of the stellar cluster NGC 6618 we detect seven compact radio sources, of which four have Chandra X-ray counterparts. Two of the radio+X-ray sources coincide positionally with the two O4 stars that form the binary systen CEN 1. These stars are the main ionizing sources of the M17 H II region and the brightest X-ray objects in the zone. The observed 8.46 GHz ux densities exceed by an order of magnitude the values expected from free-free emission from a wind. An additional archive observation made in 1988 at 4.86 GHz suggests variability in the radio sources associated with CEN 1."

We present quasi-simultaneous, multi-frequency VLA observations at 4.8, 8.4, and 22.5 GHz, of a sample of 13 Wolf Rayet (WR) stars, aimed at disentangling the nature of their radio emission and the possible detection of a non-thermal behavior in close binary systems. We detected 12 stars from our sample, for which we derived spectral information and estimated their mass loss rates. From our data, we identified four thermal sources (WR 89, 113, 138, and 141), and three sources with a composite spectrum (similar contribution of thermal and non-thermal emission; WR 8, 98, and 156). On the other hand, from the comparison with previous observations, we confirm the non-thermal spectrum of one (WR 105), and also found evidence of a composite spectrum for WR 79a, 98a, 104, and 133. Finally, we discuss the possible scenarios to explain the nature of the emission for the observed objects., Comment: 19 pages, 1 figure, Accepted by ApJ

We present three numerical simulations of randomly driven, isothermal, non-magnetic, self-gravitating turbulence with different rms Mach numbers Ms and physical sizes L, but approximately the same value of the virial parameter, alpha approx 1.2. We obtain the following results: a) We test the hypothesis that the collapsing centers originate from locally Jeans-unstable ("super-Jeans"), subsonic fragments; we find no such structures. b) We find that the fraction of small-scale super-Jeans structures is larger in the presence of self-gravity. c) The velocity divergence of subregions of the simulations exhibits a negative correlation with their mean density. d) The density probability density function (PDF) deviates from a lognormal in the presence of self-gravity. e) Turbulence alone in the large-scale simulation does not produce regions with the same size and mean density as those of the small-scale simulation. Items (b)-(e) suggest that self-gravity is not only involved in causing the collapse of Jeans-unstable density fluctuations produced by the turbulence, but also in their {it formation}. We also measure the star formation rate per free-fall time, as a function of Ms for the three runs, and compare with the predictions of recent semi-analytical models. We find marginal agreement to within the uncertainties of the measurements. However, the hypotheses of those models neglect the net negative divergence of dense regions we find in our simulations. We conclude that a) part of the observed velocity dispersion in clumps must arise from clump-scale inwards motions, and b) analytical models of clump and star formation need to take into account this dynamical connection with the external flow and the fact that, in the presence of self-gravity, the density PDF may deviate from a lognormal., Comment: 10 pages. Accepted by MNRAS. Replacement matches accepted version. Six new figure panels, extended discussion, conclusions unchanged. Abstract abridged

We present an analytical model that describes the evolution of two consecutive interplanetary coronal mass ejections (ICMEs), their interaction, and the evolution of the merged region. In this model, the coronal mass ejections (CMEs) are seen as velocity and density (mass loss rate) fluctuations of the solar wind that result in the formation of shocks traveling into the ambient solar wind (ASW). The dynamical evolution of these structures depends on both the CME parameters such as the initial velocity, density, and the duration of the eruption as well as the solar wind conditions (ejection velocity and mass loss rate). The probability of an interaction of two consecutive CMEs launched in the same direction is high, basically depending on their speeds and the interval of time between the CMEs. As a result of the interaction, a merged region is formed. Its dynamical evolution is also addressed by the present model. Given a set of initial CMEs and ASW conditions, our model is able to predict the time and distance of the interaction between the ICMEs and the velocity of the merged region as a function of the heliospheric distance. These are fundamental parameters of space weather predictions. Also, the model is able to predict the arrival time and velocity of the merged region at 1 AU. In this work, we validate the model through its application to well documented interaction events observed on 2007 January 24, 2010 May 23, 2010 August 01, and 2012 November 09.

The Astrophysical Journal 807.1 (2015): 55 reproduced by permission of the AAS, We observed with the Jansky Very Large Array at 3.6 and 1.3 cm a sample of 11 proto-brown dwarf (BD) candidates in Taurus in a search for thermal radio jets driven by the most embedded BDs. We detected for the first time four thermal radio jets in proto-BD candidates. We compiled data from UKIDSS, 2MASS, Spitzer, WISE, and Herschel to build the spectral energy distribution (SED) of the objects in our sample, which are similar to typical Class I SEDs of young stellar objects (YSOs). The four proto-BD candidates driving thermal radio jets also roughly follow the well-known trend of centimeter luminosity against bolometric luminosity determined for YSOs, assuming they belong to Taurus, although they present some excess of radio emission compared to the known relation for YSOs. Nonetheless, we are able to reproduce the flux densities of the radio jets modeling the centimeter emission of the thermal radio jets using the same type of models applied to YSOs, but with corresponding smaller stellar wind velocities and mass-loss rates, and exploring different possible geometries of the wind or outflow from the star. Moreover, we also find that the modeled mass outflow rates for the bolometric luminosities of our objects agree reasonably well with the trends found between the mass outflow rates and bolometric luminosities of YSOs, which indicates that, despite the "excess" centimeter emission, the intrinsic properties of proto-BDs are consistent with a continuation of those of very low-mass stars to a lower mass range. Overall, our study favors the formation of BDs as a scaled-down version of low-mass stars, This work is based in part on data obtained as part of the UKIRT Infrared Deep Sky Survey. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. O. Morata is supported by the MOST (Ministry of Science and Technology, Taiwan) ALMA-T grant MOST 103-2119-M-001-010-MY to the Institute of Astronomy & Astrophysics, Academia Sinica. A. Palau acknowledges financial support from UNAM-DGAPA-PAPIIT IA102815 grant, México. I. de Gregorio is supported by MICINN (Spain) grant AYA2011-30228-C03-01. R.F. González is funded by UNAM/PAPIIT grant IN112014. H. Bouy is funded by the the Ramón y Cajal fellowship program number RYC-2009-04497. H. Bouy, D. Barrado, N. Huélamo, and M. Morales Calderón are supported by MICINN (Spain) grant AYA2012-38897-C02-01. A. Bayo acknowledges financial support from the Proyecto Fondecyt de Iniciación 11140572. L. F. Rodríguez acknowledges the financial support from UNAM, and CONACyT, México

This paper presents a discussion of our “yguazu‐a” code, which integrates systems of differential equations (generally, the gasdynamic or MHD equations together with and atomic/chemical network) on a hierarchical, binary adaptive grid. Examples of recent calculations are shown, and a general discussion of the capabilities of the code and the published results is presented.

We here present two-dimensional, time-dependent radiatively cooling hydrodynamical simulations of the large and little Homunculus nebulae around $\eta$ Carinae. We employ an alternative scenario to previous interacting stellar wind models which is supported by both theoretical and observational evidence, where a non-spherical outburst wind (with a latitudinal velocity dependence that matches the observations of the large Homunculus), which is expelled for 20 years, interacts with a pre-eruptive slow wind also with a toroidal density distribution, but with a much smaller equator-to-polar density contrast than that assumed in previous models. A second eruptive wind with spherical shape is ejected about 50 years after the first outburst, and causes the development of the little internal nebula. We find that, as a result of an appropriate combination of the parameters that control the degree of asymmetry of the interacting winds, we are able to produce not only the structure and kinematics of both Homunculus, but also the high-velocity equatorial ejecta. These arise from the impact between the non-spherical outburst and the pre-outburst winds in the equatorial plane., Comment: ApJ Lett. in press

Luminous blue variables are massive stars that undergo a series of eruptive events through which they shed their outer layers before becoming Wolf-Rayet stars. During the eruptive events, their winds are denser and expand slower than in the non-eruptive state. In this paper we apply a model for the interaction of the post-eruption fast wind with the slower moving shell of dense material that was ejected during the 1994 eruption of HD 5980, a massive system located in the Small Magellanic Cloud. The model predicts a velocity for the working surface that is consistent with the velocity of insterstellar-like absorption features observed in HST/STIS spectra of 1999 and 2000, leading to the conclusion that we have witnessed for the first time the earliest stages of a circumstellar shell being formed around an LBV.

Context: HH 158, the jet from the young star DG Tau, is one of the few sources of its type where jet knots have been detected at optical and X-ray wavelengths. Aims: To search, using Very Large Array observations of this source, radio knots and if detected, compare them with the optical and X-ray knots. To model the emission from the radio knots. Methods: We analyzed archive data and also obtained new Very Large Array observations of this source, as well as an optical image, to measure the present position of the knots. We also modeled the radio emission from the knots in terms of shocks in a jet with intrinsically time-dependent ejection velocities. Results: We detected radio knots in the 1996.98 and 2009.62 VLA data. These radio knots are,within error, coincident with optical knots. We also modeled satisfactorily the observed radio flux densities as shock features from a jet with intrinsic variability. All the observed radio, optical, and X-ray knot positions can be intepreted as four successive knots, ejected with a period of 4.80 years and traveling away from the source with a velocity of 198 km s$^{-1}$ in the plane of the sky. Conclusions: The radio and optical knots are spatially correlated and our model can explain the observed radio flux densities. However, the X-ray knots do not appear to have optical or radio counterparts and their nature remains poorly understood., 8 pages, 8 figures, to appear in Astronomy & Astrophysics