R. Bonito

Laboratory formation of a scaled protostellar jet by coaligned poloidal magnetic field

Although bipolar jets are seen emerging from a wide variety of astrophysical systems, the issue of their formation and morphology beyond their launching is still under study. Our scaled laboratory experiments, representative of young stellar object outflows, reveal that stable and narrow collimation of the entire flow can result from the presence of a poloidal magnetic field whose strength is consistent with observations. The laboratory plasma becomes focused with an interior cavity. This gives rise to a standing conical shock from which the jet emerges. Following simulations of the process at the full astrophysical scale, we conclude that it can also explain recently discovered x-ray emission features observed in low-density regions at the base of protostellar jets, such as the well-studied jet HH 154. A scaled-down plasma experiment shows that axial magnetic fields in young stars can shape their bipolar jet outflows. Stellar outflows replicated in miniature Astronomers observe tight bright jets beaming from the poles of many celestial objects. But what focuses them so well? Albertazzi et al. recreated a scaled-down plasma jet in a laboratory setting to match the behavior of those in young stellar objects. The experiments show that the jets are collimated by a poloidal magnetic field aligned with the same axis. A conelike shock also emerges, as the expanding plasma is abruptly confined by the magnetic field. Science, this issue p. 325

The discovery of an expanding X-ray source in the HH 154 protostellar jet

Context. Protostellar jets are a new class of X-ray sources which has been discovered with both XMM-Newton and Chandra. The mechanism responsible for the X-ray emission is still not clear. Self-shocking in jets, shocks where the jet hits the surrounding medium, reflected or scattered stellar X-ray emission have all been invoked as possible explanations. Aims. One key diagnostic discriminating among physical emission mechanisms is the motion of the X-ray source: hydrodynamical numerical models of continuous protostellar jets plowing through a uniform medium show an X-ray emitting shock front moving at several hundreds km s -1 . In the nearest X-ray emitting protostellar jet, HH 154, this is detectable, with the spatial resolution of the Chandra X-ray observatory, over a few years baseline, allowing a robust discrimination among different mechanisms. Methods. We have performed, in October 2005, a deep Chandra X-ray observation of HH 154. Comparison with the previous (2001) Chandra observation allows to detect proper motion down to the level predicted by models of X-ray emitting shocks in the jet. Results. The 2005 Chandra observation of HH 154 shows unexpected morphological changes of the X-ray emission in comparison with the 2001 data. Two components are present: a stronger, point-like component with no detectable motion and a weaker component which has expanded in size by approximately 300 AU over the 4 years time base of the two observations. This expansion corresponds to approximately 500 km s -1 , very close to the velocity of the X-ray emitting shock in the simple theoretical models. Conclusions. The 2005 data show a more complex system than initially thought (and modeled), with multiple components with different properties. The observed morphology is possibly indicating a pulsed jet propagating through a non-homogeneous medium, likely with medium density decreasing with distance from the driving source. Detailed theoretical modeling and deeper X-ray observations will be needed to understand the physics of this fascinating class of sources.

The Gaia-ESO Survey: Discovery of a spatially extended low-mass population in the Vela OB2 association ⋆,⋆⋆

The nearby (distance~350-400 pc), rich Vela OB2 association, includes

The Gaia-ESO Survey: Chromospheric emission, accretion properties, and rotation in γ Velorum and Chamaeleon I

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YSO accretion shocks: magnetic, chromospheric or stochastic flow effects can suppress fluctuations of X-ray emission

Velorum, one of the most massive binaries in the solar neighbourhood and an excellent laboratory for investigating the formation and early evolution of young clusters. Recent Gaia-ESO survey observations have led to the discovery of two kinematically distinct populations in the young (10-15 Myr) cluster immediately surrounding

Gaia-ESO Survey: Analysis of pre-main sequence stellar spectra

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

Velorum. Here we analyse the results of Gaia-ESO survey observations of NGC 2547, a 35 Myr cluster located two degrees south of

The X-ray emission mechanism in the protostellar jet HH 154

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The Gaia-ESO Survey: Extracting diffuse interstellar bands from cool star spectra DIB-based interstellar medium line-of-sight structures at the kpc scale

Velorum. The radial velocity distribution of lithium-rich pre-main sequence stars shows a secondary population that is kinematically distinct from and younger than NGC 2547. The radial velocities, lithium absorption lines, and the positions in a colour-magnitude diagram of this secondary population are consistent with those of one of the components discovered around

Generation of radiative knots in a randomly pulsed protostellar jet - II. X-ray emission

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