Fabio De Colle

In the interpretation of stellar jet observations, the physical parameters are usually determined from emission line ratios, obtained from spectroscopic observations or using the information contained in narrow band images. The basic hypothesis in the interpretation of the observations is that the emitting region is homogeneous along the line of sight. Actually, stellar jets are in general not homogeneous, but harbour gradients in density, temperature, ionisation from axis to edge, and therefore line of sight convolution effects may lead to the main uncertainty in the determination of the physical parameters. In a series of two papers, we first showed the errors introduced by the homogeneity hypothesis, and second proposed the use of a new approach to analyse the data obtained from high angular resolution imaging of stellar jets.

We use data of the HH30 jet taken by Hartigan & Morse (2007) with the Hubble space telescope using the slitless spectroscopy technique. Using a non-parametric Tikhonov regularization technique, we determine the volumetric emission line intensities of the [SII]&lambda&lambda 6716,6731, [OI] &lambda6300 and [NII]&\lambda 6583 forbidden emission lines. From our tomographic analysis of the corresponding line ratios, we produce ``three-dimensional'' images of the physical parameters. The reconstructed density, temperature and ionization fraction present much steeper profiles than those inferred using the assumption of homogeneity. Our technique reveals that the reconstructed jet is much more collimated than the observed one close to the source. In addition, our results show a much more fragmented and irregular jet structure than the classical analysis, suggesting that the the ejection history of the jet from the star-disk system has a shorter timescale component superimposed on a longer, previously observed timescale (of a few years).

Figure: Electron density for two epochs for the original (left panels) and reconstructed emission line images (right panels).

• "Tomographic reconstruction of the three-dimensional structure of the HH30 jet", De Colle, et al., ApJ, accepted (2010).

• "Diagnostics of inhomogeneous stellar jets", De Colle, et al., A&A, 485, 765-722 (2008).

Stellar jets are normally constituted by chains of knots with some periodicity in their spatial distribution, corresponding to a variability of order of several years in the ejection from the protostar/disk system. Since their discovery several scenarios have been suggested for the origin of steady outflows from young stellar objects. In the stellar wind model, material is accelerated by thermal pressure gradients. Magnetohydrodynamics (MHD) models rely on the magnetocentrifugal launching mechanism. For the X-wind scenario the jet is magnetically driven from the so-called ``X-annulus'' where the young star's magnetosphere interacts with the disk. In the disk wind scenario the jet is launched from an extended region of the disk surface. The analytical models mentioned above have focused mainly on the steady-state aspect of the ejection phenomena.

Observations indicate a significantly longer timescale is associated with the appearance of knots in stellar outflows. Nearly all observed jets present small scale knots up to 0.1 pc from the central source, with a spacing between the knots corresponding to a timescale of 1-20 yr While the long term variation of jets can be explained by variations in the accretion rates (e.g. during FU-Orionis phases), the possible origin of small scale knots is still unclear.

Stellar jets are normally constituted by chains of knots with some periodicity in their spatial distribution, corresponding to a variability of order of several years in the ejection from the protostar/disk system. A widely accepted theory for the presence of knots is related to the generation of internal working surfaces due to variations in the jet ejection velocity.

In this work showed that a stellar magnetic field variation produces a change in the inner radius and therefore in the jet velocity. Therefore, a stellar magnetic field variation, if present, represents a natural candidate to produce stellar knots similar to the observed.

• "The Effect of a Stellar Magnetic Variation on the Jet Velocity", F. De Colle et al., ApJ, vol. 688, pp. 1137-1141 (2008).

We studied the H&alpha emission from jets using two-dimensional axisymmetrical simulations. We compared the emission obtained from hydrodynamic (HD) simulations with that obtained from magnetohydrodynamics (MHD) simulations. The magnetic field is supposed to be present in the jet only, and with a toroidal configuration. The simulations have time-dependent ejection velocities and different intensities for the initial magnetic field. The results showed an increase in the H&alpha emission along the jet for the magnetized cases with respect to the HD case. The increase in the emission is due to a better collimation of the jet in the MHD case, and to a small increase in the shock velocity.

• "The Effect of a Stellar Magnetic Variation on the Jet Velocity", F. De Colle et al., ApJ, vol. 688, pp. 1137-1141 (2008).