14 research outputs found
The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets
There is no universally acknowledged criterion to distinguish brown dwarfs
from planets. Numerous studies have used or suggested a definition based on an
object's mass, taking the ~13-Jupiter mass (M_J) limit for the ignition of
deuterium. Here, we investigate various deuterium-burning masses for a range of
models. We find that, while 13 M_J is generally a reasonable rule of thumb, the
deuterium fusion mass depends on the helium abundance, the initial deuterium
abundance, the metallicity of the model, and on what fraction of an object's
initial deuterium abundance must combust in order for the object to qualify as
having burned deuterium. Even though, for most proto-brown dwarf conditions,
50% of the initial deuterium will burn if the object's mass is ~(13.0 +/-
0.8)M_J, the full range of possibilities is significantly broader. For models
ranging from zero-metallicity to more than three times solar metallicity, the
deuterium burning mass ranges from ~11.0 M_J (for 3-times solar metallicity,
10% of initial deuterium burned) to ~16.3 M_J (for zero metallicity, 90% of
initial deuterium burned).Comment: "Models" section expanded, references added, accepted by Ap
Possible Resonances in the 12C + 12C Fusion Rate and Superburst Ignition
Observationally inferred superburst ignition depths are shallower than models
predict. We address this discrepancy by reexamining the superburst trigger
mechanism. We first explore the hypothesis of Kuulkers et al. that exothermic
electron captures trigger superbursts. We find that all electron capture
reactions are thermally stable in accreting neutron star oceans and thus are
not a viable trigger mechanism. Fusion reactions other than 12C + 12C are
infeasible as well since the possible reactants either deplete at much
shallower depths or have prohibitively large Coulomb barriers. Thus we confirm
the proposal of Cumming & Bildsten and Strohmayer & Brown that 12C + 12C
triggers superbursts. We then examine the 12C + 12C fusion rate. The reaction
cross-section is experimentally unknown at astrophysically relevant energies,
but resonances exist in the 12C + 12C system throughout the entire measured
energy range. Thus it is likely, and in fact has been predicted, that a
resonance exists near the Gamow peak energy ~ 1.5 MeV. For such a hypothetical
1.5 MeV resonance, we derive both a fiducial value and upper limit to the
resonance strength and find that such a resonance could decrease the
theoretically predicted superburst ignition depth by up to a factor of 4; in
this case, observationally inferred superburst ignition depths would accord
with model predictions for a range of plausible neutron star parameters. Said
differently, such a resonance would decrease the temperature required for
unstable 12C ignition at a column depth 10^12 g/cm^2 from 6 x 10^8 K to 5 x
10^8 K. Determining the existence of a strong resonance in the Gamow window
requires measurements of the 12C + 12C cross-section down to a center-of-mass
energy near 1.5 MeV, which is within reach of the proposed DUSEL facility.Comment: 13 pages, 4 figures; minor improvements, results and conclusions
unchanged, accepted to Ap
Bör skolan haka pÄ IT-vÄgen
Syftet med detta arbete Àr att undersöka huruvida elever önskar ett ökat anvÀndande av IT i skolan knutet till hemmet för att i större utstrÀckning göra lÀxan i SamhÀllskunskap. Dessutom Àr syftet att undersöka hur en eventuell förÀndring skulle kunna se ut. För att undersöka detta har enkÀter lÀmnats ut till totalt 111 elever pÄ tre olika skolor.Min undersökning visar att de flesta stÀller sig positiva till ett ökande av IT-anvÀndning i skolan i förbindelse med hemmet men det som förvÄnade mig Àr att mÄnga samtidigt Àr tveksamma. Om en förÀndring skulle infinna sig och det blir möjligt att göra lÀxan via Internet önskar de flesta att det sker i form av textlÀsning pÄ datorn eller i form av spel. I dagslÀget Àr en sÄdan förÀndring inte möjlig dÄ det skulle krÀvas att alla elever har tillgÄng till Internet vilket fallet inte Àr idag.För vidare forskning föreslÄs en djupare undersökning av samma slag som denna och Àven en attitydundersökning frÄn pedagogernas synvinkel.Should school hook on the IT-wave?- A study of whether pupils wish an increased application of IT in teaching social studie