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

Astronomy and Astrophysics (Impact Factor: 4.48). 12/2009; DOI: 10.1051/0004-6361/200913595
Source: arXiv

ABSTRACT 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

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    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.
    Astronomy and Astrophysics 07/2010; 517. · 4.48 Impact Factor
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    ABSTRACT: Context. Jets from solar-type and low-mass stars are typical manifestations of stellar youth. Shocks along these jets produce visible, generally fast-moving Herbig-Haro objects whose proper motions are easily measured in nearby star-forming regions using images taken just a few years apart. Herbig-Haro objects have now been observed in association with objects close to the substellar boundary. Aims: We present second-epoch observations of the central area of the Lupus 3 star-forming region that include two of its most interesting Herbig-Haro systems. One is HH 228, produced by the classical T Tauri star Th 28 (=Sz 102), whereas the other is HH 600, which has its origin in the very low-mass star Par-Lup3-4. Methods: Narrow-band imaging through filters centered on the Halpha and the [SII] lines was obtained with the FORS2 instrument at the Very Large Telescope (VLT) in mid-2010. The images obtained were compared to others obtained in early 2003, leading to the discovery of HH 600. Results: New Herbig-Haro objects are found at large distances from Th 28 and to be associated with it, representing an extension of the HH 228 jet to a projected distance of 0.32 pc from Th 28. The farthest Herbig-Haro object is HH 989, whose possible relationship with Th 28 had been already suggested in a previous study but is now kinematically confirmed. We find other likely Herbig-Haro objects whose proper motions are less indicative of a connection with Th 28, but which may be caused by oblique shocks near the outer walls of its jet. For the HH 600 jet, the knot discovered by ourselves in 2003 to the southeast of Par-Lup3-4 is found to have clearly moved and faded. Using high resolution spectroscopy obtained in 2003 and the proper motion that we can measure now, we determine a spatial velocity of 170 ± 30 km s-1. The northwestern jet is found to have grown in prominence in the intervening years. The possible relationship of other Herbig-Haro objects in the region with Th 28, Par-Lup3-4, and other young stellar objects in the area is discussed. Based on observations collected with the Very Large Telescope (VLT) at the European Southern Observatory, Paranal, Chile, under observing programme 385.C-0535(A); and on VLT archive observations originally obtained under programme 71.C-0429(D).
    Astronomy and Astrophysics 04/2011; 528. · 4.48 Impact Factor
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    ABSTRACT: X-ray emission from about ten protostellar jets has been discovered and it appears as a feature common to the most energetic jets. Although X-ray emission seems to originate from shocks internal to jets, the mechanism forming these shocks remains controversial. One of the best studied X-ray jet is HH 154 that has been observed by Chandra over a time base of about 10 years. We analyze the Chandra observations of HH 154 by investigating the evolution of its X-ray source. We show that the X-ray emission consists of a bright stationary component and a faint elongated component. We interpret the observations by developing a hydrodynamic model describing a protostellar jet originating from a nozzle and compare the X-ray emission synthesized from the model with the X-ray observations. The model takes into account the thermal conduction and radiative losses and shows that the jet/nozzle leads to the formation of a diamond shock at the nozzle exit. The shock is stationary over the period covered by our simulations and generates an X-ray source with luminosity and spectral characteristics in excellent agreement with the observations. We conclude that the X-ray emission from HH 154 is consistent with a diamond shock originating from a nozzle through which the jet is launched into the ambient medium. We suggest that the physical origin of the nozzle could be related to the dense gas in which the HH 154 driving source is embedded and/or to the magnetic field at the jet launching/collimation region.
    The Astrophysical Journal 05/2011; · 6.28 Impact Factor

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