The Radio Spectrum of TVLM513-46546: Constraints on the Coronal Properties of a Late M Dwarf

We explore the radio emission from the M9 dwarf, TVLM513-46546, at multiple radio frequencies, determining the flux spectrum of persistent radio emission, as well as constraining the levels of circular polarization. Detections at both 3.6 and 6 cm provide spectral index measurement $\alpha$ (where S$_{\nu} \propto \nu^{\alpha}$) of $-0.4\pm0.1$. A detection at 20 cm suggests that the spectral peak is between 1.4 and 5 GHz. The most stringent upper limits on circular polarization are at 3.6 and 6 cm, with $V/I <$15%. These characteristics agree well with those of typical parameters for early to mid M dwarfs, confirming that magnetic activity is present at levels comparable with those extrapolated from earlier M dwarfs. We apply analytic models to investigate the coronal properties under simple assumptions of dipole magnetic field geometry and radially varying nonthermal electron density distributions. Requiring the spectrum to be optically thin at frequencies higher than 5 GHz and reproducing the observed 3.6 cm fluxes constrains the magnetic field at the base to be less than about 500 G. There is no statistically significant periodicity in the 3.6 cm light curve, but it is consistent with low-level variability.Comment: 11 pages, 2 figures Accepted for publication in the Astrophysical Journa

Two-state folding of the outer membrane protein X into a lipid bilayer membrane

Folding and insertion of β-barrel membrane proteins into native membranes is efficiently catalyzed by β-barrel assembly machineries. Understanding this catalysis requires a detailed description of the corresponding uncatalyzed folding mechanisms, which however have so far remained largely unclear. Here, we resolve folding and membrane insertion of the E. coli outer membrane protein X (OmpX) into 1,2-didecanoyl-sn-glycero-3-phosphocholine (PC10:0) membranes at the atomic level. By combining four different experimental techniques, we correlate global folding kinetics with global and local hydrogen bond formation kinetics. Under a well-defined reaction condition, these processes follow single-exponential velocity laws, with rate constants identical within experimental error. The data thus establish at atomic resolution that OmpX folds and inserts into the lipid bilayer of PC10:0 liposomes by a two-state mechanism