Generation of radiative knots in a randomly pulsed protostellar jet I. Dynamics and energetics

Astronomy and Astrophysics (Impact Factor: 4.38). 12/2009; 511(1). DOI: 10.1051/0004-6361/200913595
Source: arXiv


HH objects are characterized by a complex knotty morphology detected mainly along the axis of protostellar jets in a wide range of bands. Evidence of interactions between knots formed in different epochs have been found, suggesting that jets may result from the ejection of plasma blobs from the source. We aim at investigating the physical mechanism leading to the irregular knotty structure observed in jets in different bands and the complex interactions occurring among blobs of plasma ejected from the stellar source. We perform 2D axisymmetric HD simulations of a randomly ejected pulsed jet. The jet consists of a train of blobs which ram with supersonic speed into the ambient medium. The initial random velocity of each blob follows an exponential distribution. We explore the ejection rate parameter to derive constraints on the physical properties of protostellar jets by comparison of model results with observations. Our model takes into account radiative losses and thermal conduction. We find that the mutual interactions of blobs ejected at different epochs and with different speed lead to a variety of plasma components not described by current models. The main features characterizing the random pulsed jet scenario are: single high speed knots, showing a measurable proper motion in nice agreement with observations; irregular chains of knots aligned along the jet axis and possibly interacting with each other; reverse shocks interacting with outgoing knots; oblique shocks produced by the reflection of shocks at the jet cocoon. All these structures concur to determine the morphology of the jet in different bands. We also find that the thermal conduction plays a crucial role in damping out HD instabilities that would develop within the cocoon and that contribute to the jet breaking. Comment: 10 pages, 10 figures, accepted for publication in A&A

Download full-text


Available from: M. Miceli
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Protostellar jets are known to emit in a wide range of bands, from radio to IR to optical bands, and to date also about ten X-ray emitting jets have been detected, with a rate of discovery of about one per year. We aim at investigating the mechanism leading to the X-ray emission detected in protostellar jets and at constraining the physical parameters that describe the jet/ambient interaction by comparing our model predictions with observations. We perform 2D axisymmetric hydrodynamic simulations of the interaction between a supersonic jet and the ambient. The jet is described as a train of plasma blobs randomly ejected by the stellar source along the jet axis. We explore the parameter space by varying the ejection rate, the initial jet Mach number, and the initial density contrast between the ambient and the jet. We synthesized from the model the X-ray emission as it would be observed with the current X-ray telescopes. The mutual interactions among the ejected blobs and of the blobs with the ambient medium lead to complex X-ray emitting structures within the jet: irregular chains of knots; isolated knots with measurable proper motion; apparently stationary knots; reverse shocks. The predicted X-ray luminosity strongly depends on the ejection rate and on the initial density contrast between the ambient and the jet, with a weaker dependence on the jet Mach number. Our model represents the first attempt to describe the X-ray properties of all the X-ray emitting protostellar jets. The comparison between our model predictions and the observations can provide a useful diagnostic tool necessary for a proper interpretation of the observations. In particular, we suggest that the observable quantities derived from the spectral analysis of X-ray observations can be used to constrain the ejection rate, a parameter explored in our model that is not measurable by current observations. Comment: Accepted for publication in Astronomy and Astrophysics
    Full-text · Article · May 2010 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Context. HH 444 is one of the first Herbig-Haro (HH) jets discovered within a photoionized region. Aims: We re-analyze the Halpha and red [S II] HST images of HH 444, and calculate the width of the jet as a function of distance from the source. We compare the Halpha image with predictions from variable ejection velocity jet models. Methods: The determination of the jet's width is done with a non-parametric, wavelet analysis technique. The axisymmetric, photoionized jet simulations are used to predict Halpha maps that can be directly compared with the observations. Results: Starting with a thin jet (unresolved at the resolution of the observations), we are able to produce knots with widths and morphologies that generally agree with the Halpha knots of HH 444. This agreement is only obtained if the jet axis is at a relatively large, ~45° angle with respect to the sky. This agrees with previous spectroscopic observations of the HH 444 bow shock, which imply a relatively large jet axis/plane of the sky angle. Conclusions: We conclude that the general morphology of the chain of knots close to V510 Ori (the HH 444 source) can be explained with a variable ejection velocity jet model. For explaining the present positions of the HH 444 knots, however, it is necessary to invoke a more complex ejection velocity history than a single-mode, periodic variability.
    Preview · Article · Jul 2010 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The physical parameters of Herbig-Haro (HH) jets are usually determined from emission-line ratios, obtained from spectroscopy or narrowband 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 HH jets. We use data of the HH30 jet taken by Hartigan and Morse 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 [S II]λλ6716,6731, [O I]λ6300, and [N II]λ6583 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 versus ~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 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.
    Full-text · Article · Sep 2010 · The Astrophysical Journal
Show more