A. C. Raga

Compact protoplanetary disks around the stars of a young binary system

Planet formation is believed to occur in the disks of gas and dust that surround young solar-type stars. Most stars, however, form in multiple systems, where the presence of a close companion could affect the structure of the disk and perhaps interfere with planet formation. It has been difficult to investigate this because of the resolution needed. Here we report interferometric observations (at a wavelength of 7 mm) of the core of the star-forming region L1551. We have achieved a linear resolution of seven astronomical units (less than the diameter of Jupiters orbit). The core of L1551 contains two distinct disks, with a separation of 45 AU; these appear to be associated with a binary system. Both disks are spatially resolved, with semi-major axes of about 10 AU, which is about a factor of ten smaller than disks around isolated stars. The disk masses are of order 0.05 solar masses, which could be enough to form planetary systems like our own.

The Hot, Diffuse Gas in a Dense Cluster of Massive Stars

We present an analytic model describing the cluster wind flow that results from the multiple interaction of the stellar winds produced by the stars of a dense cluster of massive stars. The analytic solution (obtained by matching an inner and an outer solution at the radius of the stellar cluster) can have asymptotically subsonic or supersonic behavior, the latter possibility being appropriate for the case of a cluster surrounded by a low-pressure environment. We also present a three-dimensional numerical simulation of such a cluster wind. We find that the behavior of the mean flow computed from the numerical simulation quite closely follows the flow properties deduced from the analytic model. Finally, we discuss the observational properties of the cluster wind produced by dense clusters such as the Arches cluster close to the center of our Galaxy. In particular, we predict that the X-ray emission from the intracluster gas in this stellar cluster could be detectable.

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.

Shadows behind Neutral Clumps in Photoionized Regions

Shadows behind neutral clumps in photoionized regions can have a neutral core surrounded by gas that is photoionized by the diffuse flux produced by the nebula. We present a simple analytic model describing the configuration of such shadows. We also present numerical gasdynamic simulations of the relaxation to the final, steady state. These models have clear applications (e.g., to the cometary knots in the Helix Nebula) but can also be applied in other astrophysical contexts.

Structure and kinematics of the HH 111 jet

Narrow-band CCD images of the HH 111 optical jet complex obtained at the ESO 3.5 m New Technology Telescope under conditions of excellent seeing are presented. These new images reveal the existence of a faint counterjet in the redshifted lobe of this bipolar outflow. The knot at the tip of the main jet is also identified as a further bow shock, in addition to the three already known in this system. By performing a partial deconvolution of the seeing profile, we have been able to resolve the knots in the trunk of the jet, which are found to have widths of about 0.8″-1.1″, and separation-to-width rations of 2-4. The first bright knot at the base of the jet is split in two, perpendicular to the jet. It is suggested that this structure is a ring of enhanced emission, viewed edge-on, such as might result from the passage of a puff of enhanced density through an oblique shock in the jet

A wide-angle outflow with the simultaneous presence of a high-velocity jet in the high-mass Cepheus A HW2 system

We present five epochs of VLBI water maser observations around the massive protostar Cepheus A HW2 with 0.4 mas (0.3 au) resolution. The main goal of these observations was to follow the evolution of the remarkable water maser linear/arcuate structures found in earlier VLBI observations. Comparing the data of our new epochs of observation with those observed 5 yr before, we find that at ‘large’ scales of 1 arcsec (700 au) the main regions of maser emission persist, implying that both the surrounding medium and the exciting sources of the masers have been relatively stable during that time-span. However, at smaller scales of 0.1 arcsec (70 au) we see large changes in the maser structures, particularly in the expanding arcuate structures R4 and R5. R4 traces a nearly elliptical patchy ring of ∼70 mas size (50 au) with expanding motions of ∼5 mas yr −1 (15 km s −1 ), consistent with previous results of Gallimore and collaborators. This structure is probably driven by the wind of a still unidentified YSO located at the centre of the ring (∼0.18 arcsec south of HW2). On the other hand, the R5 expanding bubble structure (driven by the wind of a previously identified YSO located ∼0.6 arcsec south of HW2) is currently dissipating in the circumstellar medium and losing its previous degree of symmetry, indicating a very short lived event. In addition, our results reveal, at scales of ∼1 arcsec (700 au), the simultaneous presence of a relatively slow (∼10– 70 km s −1 ) wide-angle outflow (opening angle of ∼102 ◦ ), traced by the masers, and the fast (∼500 km s −1 ) highly collimated radio jet associated with HW2 (opening angle of ∼18 ◦ ), previously observed with the VLA. This simultaneous presence of a wide-angle outflow and a highly collimated jet associated with a massive protostar is similar to what is found in some low-mass YSOs. There are indications that the primary wind(s) from HW2 could be rotating. The implications of these results in the study of the formation of high-mass stars are discussed.

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.

Herbig-Haro Jets from Orbiting Sources

The origin of the wiggles detected in the trajectory of Herbig-Haro (HH) jets, microjets, and radio continuum jets from young stellar objects is investigated. We propose that the orbital motion of a binary stellar system is the cause of these outflow morphologies. The analytical trajectories of a ballistic jet ejected by a source moving along a circular orbit have been derived, and their validity has been checked through a comparison with three-dimensional gasdynamic simulations. We propose a simple method to calculate the mass of the outflow source and the orbital parameters using observational measurements (i.e., the opening angle of the wiggling jet pattern, the separation between successive wiggles, the jet velocity, and the orientation of the outflow with respect to the plane of the sky), and we apply it to the DG Tau microjet, HH 47, and the Serpens radio continuum jet. For these three objects we obtain orbital parameters and masses that are reasonable for pre–main-sequence binaries. From this result we conclude that the observed wiggles (of these three outflows) can indeed be interpreted in terms of a model of an outflow ejected from a source with an orbital motion. Subject headings: ISM: Herbig-Haro objects — ISM: jets and outflows — stars: pre–main-sequence

Photoevaporating Flows from the Cometary Knots in the Helix Nebula (NGC 7293)

We explain the Hα emission of the cometary knots in the Helix Nebula (NGC 7293) with an analytical model that describes the emission of the head of the globules as a photoevaporated flow produced by the incident ionizing radiation of the central star. We compare these models with the Hα emission obtained from the Hubble Space Telescope (HST) archival images of the Helix Nebula. From a comparison of the Hα emission with the predictions of the analytical model we obtain a rate of ionizing photons from the central star of about 5 × 1045 s-1, which is consistent with estimates based on the total Hβ flux of the nebula. We also model the tails of the cometary knots as a photoevaporated wind from a neutral shadow region produced by the diffuse ionizing photon field of the nebula. A comparison with the HST images allows us to obtain a direct determination of the value of the diffuse ionizing flux. We compare the ratio of diffuse to direct stellar flux as a function of radius inside an H II region with those obtained from the observational data through the analytical tail and head wind model. The agreement of this model with the values determined from the observations of the knots is excellent.

Structure, Excitation, and Kinematics of the Luminous Herbig-Haro Objects 80/81*

We present a detailed study of the Herbig-Haro objects HH 80/81, twin working surfaces in the highly collimated outflow driven by a luminous young star. High angular resolution emission line images obtained with the Hubble Space Telescope are used together with ground-based low- and high-dispersion spectroscopy, and proper motion measurements to provide a comprehensive picture of the structure and kinematics of these remarkable objects. The two principle knots HH 80A and HH 81A have emission lines with widths of 700 km s-1 and 625 km s-1 (FWZI), respectively—far broader than previously observed in any HH object—and also have large tangential velocities of about 350 km s-1. In addition, they are both of exceptionally high excitation, having the strongest [O III] emission of any known HH object. Although the kinematics of these objects are broadly consistent with expectations from radiative bow shock models, the very high shock velocities implied, in excess of 600 km s-1, mean that the postshock cooling distance at the apex is a few times greater than their size. Consequently, these bow shocks must have adiabatic tips and only become radiative in their wings. At the spatial resolution of our Hubble Space Telescope images the structure of HH 80/81 is seen to be far more complex than was thought on the basis of ground-based images. While HH 80A does bear some resemblance to a bow shock, HH 81A has an intricate filamentary structure. The HH 80/81 outflow is highly collimated from the source out to HH 81A and HH 80A, but then abruptly broadens into a network of faint shock excited streamers, terminating in a giant bow shock. The point at which the flow begins to diverge coincides with the apparent edge of the molecular cloud. We suggest that this morphology results because fast, very hot bullets like HH 80A and HH 81A violently expand as they escape from the cloud into its low pressure surroundings.