arXiv subjects

Fabio De Colle

Publications and source records attributed to Fabio De Colle.

60 records · Page 4Linked to original sources

arXiv2010

Tomographic reconstruction of the three-dimensional structure of the HH30 jet

The physical parameters of Herbig-Haro jets are usually determined from emission line ratios, obtained from spectroscopy or narrow band imaging, assuming that the emitting region is homogeneous along the line of sight. Under the more general hypothesis of axisymmetry, we apply tomographic reconstruction techniques to the analysis of Herbig-Haro 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]6716,6731, [OI]6300 and [NII]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 (a width ~ 5 AU vs. ~ 20 AU at a distance of 10 AU from the star), while they have similar widths at larger distances. 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 (~ some months) superimposed on a longer, previously observed timescale (of a few years). Finally, we discuss the possible application of the same technique to other stellar jets and planetary nebulae.

astro-ph.SR↗

arXiv2009

Radiation-magnetohydrodynamic simulations of the photoionisation of magnetised globules

We present the first three-dimensional radiation-magnetohydrodynamic simulations of the photoionisation of a dense, magnetised molecular globule by an external source of ultraviolet radiation. We find that, for the case of a strong ionising field, significant deviations from the non-magnetic evolution are seen when the initial magnetic field threading the globule has an associated magnetic pressure that is greater than one hundred times the gas pressure. In such a strong-field case, the photoevaporating globule will adopt a flattened or "curled up" shape, depending on the initial field orientation, and magnetic confinement of the ionised photoevaporation flow can lead to recombination and subsequent fragmentation during advanced stages of the globule evolution. We find suggestive evidence that such magnetic effects may be important in the formation of bright, bar-like emission features in H II regions. We include simple but realistic fits to heating and cooling rates in the neutral and molecular gas in the vicinity of a high-mass star cluster and show that the frequently used isothermal approximation can lead to an overestimate of the importance of gravitational instability in the radiatively imploded globule. For globules within 2 parsecs of a high-mass star cluster, we find that heating by stellar x rays prevents the molecular gas from cooling below 50 K.

astro-ph↗

arXiv2008

The dynamics of internal working surfaces in MHD jets

The dynamical effects of magnetic fields in models of radiative, Herbig-Haro (HH) jets have been studied in a number of papers. For example, magnetized, radiative jets from variable sources have been studied with axisymmetric and 3D numerical simulations. In this paper, we present an analytic model describing the effect of a toroidal magnetic field on the internal working surfaces that result from a variability in the ejection velocity. We find that for parameters appropriate for HH jets the forces associated with the magnetic field dominate over the gas pressure force within the working surfaces. Depending on the ram pressure radial cross section of the jet, the magnetic field can produce a strong axial pinch, or, alternatively, a broadening of the internal working surfaces. We check the validity of the analytic model with axisymmetric numerical simulations of variable, magnetized jets.

astro-ph↗

arXiv2008

The effect of a stellar magnetic variation on the jet velocity

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 paper we study the effect of variations in the inner disk-wind radius on the jet ejection velocity. We show that a small variation in the inner disk-wind radius produce a variation in the jet velocity large enough to generate the observed knots. We also show that the variation in the inner radius may be related to a variation of the stellar magnetic field.

astro-ph↗

arXiv2006

Structure of Passive Circumstellar Disks: Beyond the Two-Temperature Approximation

Structure and spectral energy distributions (SEDs) of externally irradiated circumstellar disks are often computed on the basis of the two-temperature model of Chiang & Goldreich. We refine these calculations by using a more realistic temperature profile which is continuous at all optical depths and thus goes beyond the two-temperature model. It is based on the approximate solution of the radiation transfer in the disk obtained from the frequency-integrated moment equations in the Eddington approximation. We come up with a simple procedure (``constant g_z approximation'') for treating the vertical structure of the disk in regions where its optical depth to stellar radiation is high. This allows us to obtain expressions for the vertical profiles of density and pressure at every point in the disk and to determine the shape of its surface. Armed with these analytical results we calculate the full radial structure of the disk and demonstrate that it favorably agrees with the results of direct numerical calculations. We also describe a simple and efficient way of the SED calculation based on our adopted temperature profile. Resulting spectra provide very good match (especially at short wavelengths) to the results of more detailed (but also more time-consuming) SED calculations solving the full frequency- and angle-dependent radiation transfer within the disk.

astro-ph↗

arXiv2005

MHD simulations of radiative jets from young stellar objects: Halpha emission

We study the H$α$ emission from jets using two-dimensional axisymmetrical simulations. We compare 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 show an increase in the H$α$ 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. These results could have important implications for the interpretation of the observations of jets from young stellar objects.

astro-ph↗