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 på IT-vågen

Syftet med detta arbete är att undersöka huruvida elever önskar ett ökat användande av IT i skolan knutet till hemmet för att i större utsträckning göra läxan i Samhällskunskap. Dessutom är syftet att undersöka hur en eventuell förändring skulle kunna se ut. För att undersöka detta har enkäter lämnats ut till totalt 111 elever på tre olika skolor.Min undersökning visar att de flesta ställer sig positiva till ett ökande av IT-användning i skolan i förbindelse med hemmet men det som förvånade mig är att många samtidigt är tveksamma. Om en förändring skulle infinna sig och det blir möjligt att göra läxan via Internet önskar de flesta att det sker i form av textläsning på datorn eller i form av spel. I dagsläget är en sådan förändring inte möjlig då det skulle krävas att alla elever har tillgång till Internet vilket fallet inte är idag.För vidare forskning föreslås en djupare undersökning av samma slag som denna och även en attitydundersökning från pedagogernas synvinkel.Should school hook on the IT-wave?- A study of whether pupils wish an increased application of IT in teaching social studie