P. F. Velázquez

Jet/cloud collision, 3D gasdynamic simulations of HH 110

We present 3D, gasdynamic simulations of jet/cloud collisions, with the purpose of modelling the HH 270/110 system. From the models, we obtain predictions of Hα and H_2 1–0 s(1) emission line maps, which qualitatively reproduce some of the main features of the corresponding observations of HH 110. We find that the model that better reproduces the observed structures corresponds to a jet that was deflected at the surface of the cloud ~1000 yr ago, but is now boring a tunnel directly into the cloud. This model removes the apparent contradiction between the jet/cloud collision model and the lack of detection of molecular emission in the crossing region of the HH 270 and HH 110 axes.

The X-Ray Luminosities of Herbig-Haro Objects

The recent detection of X-ray emission from HH 2 and HH 154 with the Chandra and XMM-Newton satellites (respectively) have opened up an interesting, new observational possibility in the field of Herbig-Haro objects. In order to be able to plan further X-ray observations of other HH objects, it is now of interest to be able to estimate their X-ray luminosities in order to choose which objects to observe. This Letter describes a simple, analytic model for predicting the X-ray luminosity of a bow shock from the parameters of the flow (i.e., the size of the bow shock, its velocity, and the preshock density). The accuracy of the analytic model is analyzed through a comparison with the predictions obtained from axisymmetric, gasdynamic simulations of the leading working surface of an HH jet. We find that our analytic model reproduces the observed X-ray luminosities of HH 2 and HH 154, and we propose that HH 80/81 is a good candidate for future observations with Chandra.

Investigation of the Large-scale Neutral Hydrogen near the Supernova Remnant W28

The distribution and kinematics of neutral hydrogen have been studied in a wide area around the supernova remnant (SNR) W28. A 25 ? 25 field centered at l = 65, b = 0? was surveyed using the Parkes 64 m radio telescope (half-power beamwidth of 147 at ?21 cm). Even though W28 is located in a complex zone of the Galactic plane, we have found different H I features, which are evidence of the interaction between W28 and its surrounding gas. An extended cold cloud with about 70 M? of neutral hydrogen was detected at the location of W28 as a self-absorption feature, near the local standard of rest velocity of +7 km s-1. This H I feature is the atomic counterpart of the molecular cloud shown by previous studies to be associated with W28. From this detection, we can independently confirm a kinematical distance of about 1.9 kpc for W28. In addition, the neutral hydrogen observed in emission around the SNR displays a ringlike morphology in several channel maps over the velocity interval [-25.0, +38.0] km s-1. We propose that these features are part of an interstellar H I shell that has been swept up by the supernova shock front. Emission from this shell is confused with unrelated gas. Hence, we derive an upper limit for the shell mass of 1200?1600 M?, a maximum radius of the order of 20 pc, an expansion velocity of ~30 km s-1, an initial energy of about 1.4?1.8 ? 1050 ergs, and an age of ~3.3 ? 104 yr. The preexisting ambient medium has a volume density on the order of 1.5?2 cm-3. W28 is probably in the radiative evolutionary phase, although it is not possible to identify the recombined thin neutral shell expected to form behind the shock front with the angular resolution of the present survey.

Emission lines from rotating proto-stellar jets with variable velocity profiles - I. Three-dimensional numerical simulation of the non-magnetic case

Using the Yguazu-a three-dimensional hydrodynamic code, we have computed a set of numerical simulations of heavy, supersonic, radiatively cooling jets including var iabilities in both the ejection direction (precession) and the jet velocity (intermittence). In order to investigate the effects of jet rotation on the shape of the line profiles, we also i ntroduce an initial toroidal rotation velocity profile, in agreement with some r ecent observational evidence found in jets from T Tauri stars which seems to support the presence of a rotation velocity pattern inside the jet beam, near the jet production region. Since th e Yguazu- a code includes an atomic/ionic network, we are able to compute the emission coeffi cients for several emission lines, and we generate line profiles for the H �, (O I)�6300, (S II)�6716 and (N II)�6548 lines. Using initial parameters that are suitable for the DG Tau microjet, we show that the computed radial velocity shift for the medium-velocity component of the line profile as a function of distance from the jet axis is strikingly simi lar for rotating and non-rotating jet models. These findings lead us to put forward some caveats on the interpretation of the observed radial velocity distribution from a few outflows from yo ung stellar objects, and we claim that these data should not be di rectly used as a doubtless confirmation of the magnetocentri fugal wind acceleration models.

MIRROR AND POINT SYMMETRIES IN A BALLISTIC JET FROM A BINARY SYSTEM

Models of accretion disks around a star in a binary system predict that the disk will have a retrograde precession with a period a factor of ~10 times the orbital period. If the star+disk system ejects a bipolar outflow, this outflow will be subject to the effects of both the orbital motion and the precession. We present an analytic, ballistic model and a three-dimensional gasdynamical simulation of a bipolar outflow from a source in a circular orbit, and with a precessing outflow axis. We find that this combination results in a jet/counterjet system with a small spatial scale, reflection-symmetric spiral (resulting from the orbital motion) and a larger-scale, point-symmetric spiral (resulting from the longer period precession). These results provide interesting possibilities for modeling specific Herbig-Haro jets and bipolar planetary nebulae.

Simulated X-Ray Images and Spectra of the Arches Cluster

We present three-dimensional numerical simulations of the wind from the Arches cluster. In order to make the model as realistic as possible, we have placed stellar wind sources at the positions on the plane of the sky of the 60 brightest stars of the cluster and have calculated three different possible distributions of the stars along the line of sight. From the resulting flows, we have computed the X-ray emission, obtaining emission maps and (spatially integrated) spectra. These results are compared with the recent Chandra observations of the Arches cluster of Yusef-Zadeh and coworkers, and a good qualitative agreement is found.

The time-dependent ejection velocity histories of HH 34 and HH 111

The southern lobe of HH 34 and the western lobe of HH 111 show remarkable similarities. Both objects have a chain of well aligned knots, ending in well defined bow shocks (HH 34S and HH 11 IV, respectively). In this paper, we derive the past ejection velocity histories from previously published radial velocity measurements of the HH 34 and HH 111 jets. This is done under the assumption that the flows are ballistic. From these reconstructed ejection velocity variabilities, we compute axisymmetric gasdynamic simulations of the two flows, and find that we do obtain large working surfaces at the positions of the HH 34S and HH 11 IV bow shocks for the time at which these objects are being observed. This appears to be quite definite proof that these bow shocks are indeed the result of an ejection velocity time-variability. Also, there is the observational fact that the HH 34S bow shock is huge, being wider than HH 11 IV by a factor of ∼3. We find that we can reproduce this difference in size by choosing an appropriate value for the density of the ambient medium. Finally, from our small sample of two reconstructed ejection velocity variability histories, we attempt to make some statements about the general nature of these variabilites, and their implications on the possible ejection/collimation mechanisms.

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.

Line ratios from shocked cloudlets in planetary nebulae

Context. Some PNe and PPNe show compact knots, travelling at high velocities away from the central sources. Aims. We compute a number of models from which we obtain predictions of the emission-line spectrum, which can be compared with the spectra of the observed knots. Methods. We completed a series of 11 axisymmetric simulations of an initially spherical cloudlet, travelling away from a photoionizing source, into a uniform medium. The simulations included a multi-frequency transfer of the ionizing radiation, and a 33 species non-equilibrium ionization network. Results. From our simulations, we computed emission maps and spatially-integrated emission-line spectra. The predictions show a transition from spectra similar to those of shock wave models (for simulations with lower photoionization rates) to spectra similar to those of photoionized regions (for simulations with higher photoionization rates). Conclusions. The spectra from our photoionized cloudlet models have a range of line ratios that agree approximately with the observed spectra when shown in two-line ratio diagnostic diagrams. The predicted and observed spatial distributions of the emission (with high ionization lines extending more towards the source than lower ionization lines) agree in a qualitative way.

Analytic and Numerical Models for the Knots along the HH 111 Jet

We consider a model for HH 111 as a jet ejected with a sawtooth, time-dependent ejection velocity. Such a variability is suggested by the position-velocity (PV) diagrams obtained from Hubble Space Telescope observations. We calculate both analytic and numerical models describing the flow resulting from such an ejection velocity time variability. Analytically, we calculate the flow for the limiting cases of massless working surfaces (i.e., those that efficiently eject mass sideways into the cocoon of the jet) and mass-conserving working surfaces, and we find that the numerically computed flow lies between these two limits. From the numerical simulations, we compute PV diagrams that can be directly compared with the corresponding observations. We find a surprisingly good agreement, which can be seen as a partial confirmation of the interpretation of the knots along HH 111 as the result of a time dependence in the ejection. Also, we show that a sawtooth functional form for the ejection velocity variability appears to be particularly appropriate for modeling the detailed radial velocity structure of the knots along HH 111.