Quiescent X-ray emission from an evolved brown dwarf ?

I report on the X-ray detection of Gl569Bab. During a 25ksec Chandra observation the binary brown dwarf is for the first time spatially separated in X-rays from the flare star primary Gl569A. Companionship to Gl569A constrains the age of the brown dwarf pair to ~300-800 Myr. The observation presented here is only the second X-ray detection of an evolved brown dwarf. About half of the observing time is dominated by a large flare on Gl569Bab, the remainder is characterized by weak and non-variable emission just above the detection limit. This emission -- if not related to the afterglow of the flare -- represents the first detection of a quiescent corona on a brown dwarf, representing an important piece in the puzzle of dynamos in the sub-stellar regime.Comment: to appear in ApJ

Radio Observations of a Large Sample of Late-M, L, and T Dwarfs: The Distribution of Magentic Field Strengths

We present radio observations of a comprehensive sample of 90 dwarf stars and brown dwarfs ranging from spectral type M5 to T8. We detect three radio active sources in addition to the six objects previously detected in quiescence and outburst, leading to an overall detection rate of about 10% for objects later than M7. From the properties of the radio emission we infer magnetic field strengths of ~100 G in quiescence and nearly 1 kG during flares, while the majority of the non-detected objects have B<50 G. Depending on the configuration and size of the magnetic loops, the surface magnetic fields may approach 1 kG even in quiescence, at most a factor of few smaller than in early-M dwarfs. With the larger sample of sources we find continued evidence for (i) a sharp transition around spectral type M7 from a ratio of radio to X-ray luminosity of log(L_R/L_X) ~ -15.5 to >-12, (ii) increased radio activity with later spectral type, in contrast to H-alpha and X-ray observations, and (iii) an overall drop in the fraction of active sources from about 30% for M dwarfs to about 5% for L dwarfs, fully consistent with H-alpha and X-ray observations. Taken together, these trends suggest that some late-M and L dwarfs are capable of generating 0.1-1 kG magnetic fields, but the overall drop in the fraction of such objects is likely accompanied by a change in the structure of the chromospheres and coronae, possibly due to the increasingly neutral atmospheres and/or a transition to a turbulent dynamo. A more extended radio survey currently holds the best promise for measuring the magnetic field properties of a large number of dwarf stars. [abridged]Comment: Submitted to ApJ; 14 pages, 4 figures, 2 table

Raman cooling and heating of two trapped Ba+ ions

We study cooling of the collective vibrational motion of two 138Ba+ ions confined in an electrodynamic trap and irradiated with laser light close to the resonances S_1/2-P_1/2 (493 nm) and P_1/2-D_3/2 (650 nm). The motional state of the ions is monitored by a spatially resolving photo multiplier. Depending on detuning and intensity of the cooling lasers, macroscopically different motional states corresponding to different ion temperatures are observed. We also derive the ions' temperature from detailed analytical calculations of laser cooling taking into account the Zeeman structure of the energy levels involved. The observed motional states perfectly match the calculated temperatures. Significant heating is observed in the vicinity of the dark resonances of the Zeeman-split S_1/2-D_3/2 Raman transitions. Here two-photon processes dominate the interaction between lasers and ions. Parameter regimes of laser light are identified that imply most efficient laser cooling.Comment: 8 pages, 5 figure

The role of two-stage phase formation for the solid-state runaway reaction in Al/Ni reactive multilayers

While extensively studied for heating rates below 1.7 K/s and above 1000 K/s, the solid-state phase transformations in Al/Ni reactive multilayers have not been examined at intermediate heating rates between 100 K/s and 1000 K/s. Combined nanocalorimetry and time-resolved synchrotron x-ray diffraction studies are utilized to address this range of heating rates for multilayers with an overall composition of 10 at. % Ni and a bilayer thickness of 220 nm. It was found that a two-stage phase formation of Al$_{3}$Ni proceeds up to a heating rate of 1000 K/s. The two growth stages occur in the solid-state and are kinetically separated. The activation energy of the first growth stage is determined to be 137 kJ/mol, which agrees well with the literature data at low heating rates. At 1000 K/s, a transition to a runaway reaction is observed. Unusual for metallic multilayers, the reaction proceeds completely in the solid-state which is also known as “solid flame.” Using nanocalorimetry, a critical input power density for ignition of 5.8 x 10$^{4}$ W/cm$^{3}$ was determined. The rapid succession of the two Al$_{3}$Ni formation stages was identified as the underlying mechanism for the self-sustaining reaction

Quasi-periodic X-ray Flares from the Protostar YLW15

With ASCA, we have detected three X-ray flares from the Class I protostar YLW15. The flares occurred every ~20 hours and showed an exponential decay with time constant 30-60 ks. The X-ray spectra are explained by a thin thermal plasma emission. The plasma temperature shows a fast-rise and slow-decay for each flare with kT_{peak}~4-6 keV. The emission measure of the plasma shows this time profile only for the first flare, and remains almost constant during the second and third flares at the level of the tail of the first flare. The peak flare luminosities L_{X,peak} were ~5-20 * 10^{31} erg s^{-1}, which are among the brightest X-ray luminosities observed to date for Class I protostars. The total energy released in each flare was 3-6*10^{36} ergs. The first flare is well reproduced by the quasi-static cooling model, which is based on solar flares, and it suggests that the plasma cools mainly radiatively, confined by a semi-circular magnetic loop of length ~14 Ro with diameter-to-length ratio \~0.07. The two subsequent flares were consistent with the reheating of the same magnetic structure as of the first flare. The large-scale magnetic structure and the periodicity of the flares imply that the reheating events of the same magnetic loop originate in an interaction between the star and the disk due to the differential rotation.Comment: Accepted by ApJ, 9 pages incl. 4 ps figure