Stellar flares

Alternatively, 6Li in low-metallicity stars could be the result ol 4Hc(3Hc, p) reactions in stellar flares, analogous to effects found in some energetic solar flares (Tatischeff Thibaud 2007). In this case there is expected to be real scatter in the 6Li plateau, due to variations in (present or past) stellar rotation speed. 

Exceptionally violent phenomena can nevertheless disturb this fine order. Some of them bring tremendous speeds into play, in both translational and rotational motions. Solar and stellar flares come to mind, along with exploding stars and spinning pulsars (neutron stars). If part of this well-ordered energy is conferred upon an atomic nucleus, it then becomes a cosmic ray. It is thus excluded from the community, whose gentle brethren are no longer able to retain it. 

The coolest stars with just enough mass to fuse hydrogen are the M-dwarfs (see chapter 3). Two new classes of brown dwarfs have been added to the cool end of the stellar spectrum. The L-dwarfs (1,300 to 2,000 Kelvin) are slightly cooler and less massive than M-dwarfs. T-dwarfs are cooler and lighter than the L-dwarfs. Both of these new dwarfs cannot sustain hydrogen fusion. Researchers have recently discovered hundreds of T-dwarfs and tens of L-dwarfs. Even the cool T-dwarfs may have magnetic fields that create occasional stellar flares. [For more information, see Linda Rowan, Cooler dwarf stars, Science 289(5480) 697 (August 4, 2000).]... 

Observations of the Orion nebular cluster by the Chandra X-ray observatory reveal the presence of frequent (one every 6 days), large (up to 0.5 AU), and highly energetic stellar flares (with inferred magnetic fields as large as 3000 G) (Favata et al. 2005). The duration of the flares varies from less than an hour to almost three... 

While most of the short-lived isotopes found in primitive meteorites could be products of stellar nucleosynthesis (Goswami and Vanhala, 2000), evidence indicates that °Be may have also been alive in the early solar nebula (McKeegan et al., 2000). Beryllium-10 cannot be synthesized in stellar interiors, but can be formed by spallation reactions with energetic particles such as those that accompany stellar flares. The evidence for °Be thus suggests that some short-lived isotopes were formed by spallation reactions near the surface of the nebula or in the X-wind region (Gounelle et al., 2001). However, the fact that e does not correlate with the presence of... 

Stellar nucleosynthesis No production of 6Li seems possible in stars, other than a very small surface abundance that can be established by nuclear reactions in solar flares. Even with that small production, stars are net destroyers of 6Li, so when their ejecta return to the interstellar material it is 6Li-poor. So stars are not its source. 

The temperature distribution is not only a function of radius, but also depends on the stellar luminosity, the disk geometry, and may depend on the accretion rate (see Table 8.1 and Section 3.3) for example, at a given radius irradiated flared disks will be warmer than flat disks. Naturally, hotter stars will heat their disks to higher temperatures at a given radius thus, mid-infrared spectroscopy probes different radii in different disks. 

X-ray spectroscopy has also been applied to the interpretation of solar spectra, which are emitted by solar flares. Now stellar objects are under investigation by X-ray satellites such as Chandra and XMM. Whereas the present X-ray telescopes are medium resolution devices, the next generation (Constellation-X, XEUS) will provide sufficient spectral resolution for detailed analysis. The spectra from distant object usually suffer from low statistics solar flares have low emission time and the observation time of stellar objects is limited. In addition, the electron distribution is not Maxwellian, in general, and some of the spectral lines may be polarized. Therefore, verified theoretical data are of great importance to interpret solar and stellar spectra, where they provide the only source of information on the plasma state. 

Within the last 25 years of X-ray spectroscopy on fusion devices, the theory of He-like ions has been developed to an impressive precision. The spectra can be modeled with deviations not more than 10% on all lines. For the modeling, only parameters with physical meaning and no additional approximation factors are required. Even the small effects due to recombination of H-like atoms, which contribute only a few percent to the line intensity, can be used to explain consistently the recombination processes and hence the charge state distribution in a hot plasma. The measurements on fusion devices such as tokamaks or stellarators allow the comparison to the standard diagnostics for the same parameters. As these diagnostics are based on different physical processes, they provide sensitive tests for the atomic physics used for the synthetic spectra. They also allow distinguishing between different theoretical approaches to predict the spectra of other elements within the iso-electronic series. The modeling of the X-ray spectra of astronomical objects or solar flares, which are now frequently explored by X-ray satellite missions, is now more reliable. In these experiments, the statistical quality of the spectra is limited due to the finite observation time or the lifetime of... 

Solid icy surfaces are observed both in the interstellar medium as mantles on silicatic or carbonaceous grains and on many objects in the Solar System." In space, these icy targets are continuously bombarded by energetic ions from solar wind and flares, planetary magnetospheres, stellar winds and galactic cosmic rays. When an energetic ion collides with an icy target produces physico-chemical modifications in the latter. The study of those effects is based on laboratory ion irradiation experiments carried out under physical conditions as close as possible to the astrophysical ones. 

There are several sources of X-rays such as a Coolidge tube, vacuum sparks, hot-dense fusion plasmas, synchrotron, pinch devices, muonic atoms, beam-foil interaction, stellar X-ray emitters, solar flares, etc. The X-rays originating from all these sources can be broadly categorized into main types (1) atomic inner shell transitions, (2) emission by free electrons, (3) X-rays from few electron systems. The basic spectroscopic aspects of the various types of X-rays are discussed in this article. 

These include sources such as laser produced plasmas, tokamak plasmas, pinch plasmas, solar flares, stellar X-ray emitters, etc. In such plasmas, the electron temperature (corresponding to a Maxwellian velocity distribution) can be a few hundreds of eV to several keV. On collision with plasma ions these energetic electrons undergo acceleration/deceleration and thereby emit Bremsstrahlung radiation. Electron-electron collisions do not emit any net radiation as the two colliding electrons undergo exactly equal and opposite accelerations. The radiation emitted by the two electrons is therefore equal in magnitude and opposite in phase. Hence, there is no net radiation emitted. 

However, the early sun was quite different from the sun we know. Its luminosity was only about 70% of its present value. It rotated much faster. SteUar rotation is an important parameter to trigger stellar activity. The present day sun shows an activity cycle with a period of about 11 years. The number of sunspots varies but also the number of flare occurrence (these are energetic outbursts caused by a reconnection... 

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