R. F. González

Molecular Cloud Evolution. II. From Cloud Formation to the Early Stages of Star Formation in Decaying Conditions

We study the formation of giant dense cloud complexes and of stars within them using SPH numerical simulations of the collision of gas streams (‘‘inflows’’) in the WNM at moderately supersonic velocities. The collisions cause compression,cooling,andturbulencegenerationinthegas,formingacloudthatthenbecomesself-gravitatingandbeginsto collapse globally. Simultaneously, the turbulent, nonlinear density fluctuations induce fast, local collapse events. The simulationsshowthat(1)Thecloudsarenotinastateofequilibrium.Instead,theyundergosecularevolution.Duringits early stages, the cloud’s mass and gravitational energy jEgj increase steadily, while the turbulent energy Ek reaches a plateau.(2)When jEgjbecomescomparabletoEk,globalcollapsebegins,causingasimultaneousincreasein jEgjandEk that maintains a near-equipartition condition jEg j� 2Ek. (3) Longer inflow durations delay the onset of global and local collapsebymaintainingahigherturbulentvelocitydispersioninthecloudoverlongertimes.(4)Thestarformationrate islargefrom the beginning,without any periodofslow and acceleratingstar formation.(5) The column densities of the local star-forming clumps closely resemble 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 discussedherenaturallyexplainstheapparent‘‘virialized’’stateofMCsandtheubiquityofHihalosaroundthem.Also, within their assumptions, our simulations support the scenario of rapid star formation after MCs are formed, although long (k15 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.

Molecular Cloud Evolution. I. Molecular Cloud and Thin Cold Neutral Medium Sheet Formation

We discuss molecular cloud formation by large-scale supersonic compressions in the diffuse warm neutral medium (WNM). Initially, a shocked layer forms, and within it, a thin cold layer. An analytical model and high-resolution one-dimensional simulations predict the thermodynamic conditions in the cold layer. After ~1 Myr of evolution, the layer has column density ~2.5 × 1019 cm-2, thickness ~0.03 pc, temperature ~25 K, and pressure ~6650 K cm-3. These conditions are strongly reminiscent of those recently reported by Heiles and coworkers for cold neutral medium sheets. In the one-dimensional simulations, the inflows into the sheets produce line profiles with a central line of width ~0.5 km s-1 and broad wings of width ~1 km s-1. Three-dimensional numerical simulations show that the cold layer develops turbulent motions and increases its thickness until it becomes a fully three-dimensional turbulent cloud. Fully developed turbulence arises on times ranging from ~7.5 Myr for inflow Mach number M1,r = 2.4 to >80 Myr for M1,r = 1.03. These numbers should be considered upper limits. The highest density turbulent gas (HDG, n > 100 cm-3) is always overpressured with respect to the mean WNM pressure by factors of 1.5-4, even though we do not include self-gravity. The intermediate-density gas (IDG, 10 < n/cm-3 < 100) has a significant pressure scatter that increases with M1,r, so that at M1,r = 2.4 a significant fraction of the IDG is at a higher pressure than the HDG. Our results suggest that the turbulence and at least part of the excess pressure in molecular clouds can be generated by the compressive process that forms the clouds themselves and that thin CNM sheets may be formed transiently by this mechanism, when the compressions are only weakly supersonic.

The nature of the velocity field in molecular clouds – I. The non-magnetic case

We present numerical simulations designed to test some of the hypotheses and predictions of recent models of star formation. We consider a set of three numerical simulations of randomly driven, isothermal, non-magnetic, self-gravitating turbulence with different rms Mach numbers Ms and physical sizes L, but with approximately the same value of the virial parameter, α ≈ 1.2. We obtain the following results: (i) we test the hypothesis that the collapsing centres originate from locally Jeans unstable (‘super-Jeans’), subsonic fragments; we find no such structures in our simulations, suggesting that collapsing centres can arise also from regions that have supersonic velocity dispersions but are nevertheless gravitationally unstable. (ii) We find that the fraction of small-scale super-Jeans structures is larger in the presence of self-gravity. (iii) There exists a trend towards more negative values of the velocity field’s mean divergence in regions with higher densities, implying the presence of organized inflow motions within the structures analysed. (iv) The density probability density function (PDF) deviates from a lognormal in the presence of self-gravity, developing an approximate power-law high-density tail, in agreement with previous results. (v) Turbulence alone in the large-scale simulation (L = 9 pc) does not produce regions with the same size and mean density as those of the small-scale simulation (L = 1 pc). Items (ii)–(v) 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 formation. We then measure the ‘star formation rate per free-fall time’, SFRff , 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, within the L = 9 pc simulation, subregions with similar density and size to those of the L = 1p c simulation differ qualitatively from the latter in that they exhibit a global convergence of the velocity field ∇·v ∼− 0.6 km s −1 pc −1 on average. This suggests that the assumption that turbulence in clouds and clumps is purely random is incomplete. We conclude that (i) part of the observed velocity dispersion in clumps must arise from clump-scale inwards motions, even in driven-turbulence situations, and (ii) 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.

Gasdynamical Simulations of the Large and Little Homunculus Nebulae of ? Carinae

Here we present two-dimensional, time-dependent radiatively cooling hydrodynamical simulations of the large and little Homunculus nebulae around η Carinae. We employ an alternative scenario to previous interacting stellar wind models that is supported by both theoretical and observational evidence, where a nonspherical outburst wind (with a latitudinal velocity dependence that matches the observations of the large Homunculus), which is expelled for 20 years, interacts with a preeruptive 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 nonspherical outburst and the preoutburst winds in the equatorial plane.

Radio continuum emission from knots in the DG Tauri jet

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. We aim to search for radio knots to compare them with the optical and X-ray knots. We also aim 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 furthermore 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 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.

Radio-Continuum Emission from Shocked Stellar Winds in Low-Mass Stars

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.

Numerical Modeling of η Carinae Bipolar Outflows

In this paper, we present two-dimensional gasdynamic simulations of the formation and evolution of the ? Car bipolar outflows. Adopting the interacting nonspherical winds model, we have carried out high-resolution numerical simulations, which include explicitly computed time-dependent radiative cooling, for different possible scenarios of the colliding winds. In our simulations, we consider different degrees of nonspherical symmetry for the preoutburst wind and the great eruption of the 1840s produced by the ? Car wind. Different models show important differences in the shape and kinematical properties of the Homunculus structure. In particular, we search for the appropriate combination of wind parameters (which control the degree of nonspherical symmetry) to obtain the numerical results that best match both the observed morphology and the expansion velocity of the ? Car bipolar shell. In addition, our numerical simulations show the formation of a bipolar nebula embedded within the Homunculus (the little Homunculus) that developed from a secondary eruptive event suffered by the star in the 1890s, and also the development of tenuous, high-velocity ejections in the equatorial region that resulted from the impact of the eruptive wind of the 1840s with the preoutburst wind; these ejections could explain some of the high-speed features observed in the equatorial ejecta. The models were, however, unable to produce the equatorial ejections associated with the second eruptive event.

COMPACT RADIO SOURCES IN M17

The classic H II 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 at 4.96 and 8.46 GHz that reveal the presence of 38 compact radio sources, in addition to the well-known hypercompact cometary H II region M17 UC1. For this last source, we find that its spectral index of the order of ~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 CEN 1b, concluding that both are most probably non-thermal emitters, although the first is strongly time variable and the second is steady.

ON THE NATURE OF THE EXTENDED RADIO EMISSION SURROUNDING T TAURI SOUTH

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

DISENTANGLING THE NATURE OF THE RADIO EMISSION IN WOLF-RAYET STARS

We present quasi-simultaneous, multi-frequency Very Large Array 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.