Periodic Optical Variability of Radio Detected Ultracool Dwarfs

A fraction of very low mass stars and brown dwarfs are known to be radio active, in some cases producing periodic pulses. Extensive studies of two such objects have also revealed optical periodic variability and the nature of this variability remains unclear. Here we report on multi-epoch optical photometric monitoring of six radio detected dwarfs, spanning the $\sim$M8 - L3.5 spectral range, conducted to investigate the ubiquity of periodic optical variability in radio detected ultracool dwarfs. This survey is the most sensitive ground-based study carried out to date in search of periodic optical variability from late-type dwarfs, where we obtained 250 hours of monitoring, delivering photometric precision as low as $\sim$0.15%. Five of the six targets exhibit clear periodicity, in all cases likely associated with the rotation period of the dwarf, with a marginal detection found for the sixth. Our data points to a likely association between radio and optical periodic variability in late-M/early-L dwarfs, although the underlying physical cause of this correlation remains unclear. In one case, we have multiple epochs of monitoring of the archetype of pulsing radio dwarfs, the M9 TVLM 513-46546, spanning a period of 5 years, which is sufficiently stable in phase to allow us to establish a period of 1.95958 $\pm$ 0.00005 hours. This phase stability may be associated with a large-scale stable magnetic field, further strengthening the correlation between radio activity and periodic optical variability. Finally, we find a tentative spin-orbit alignment of one component of the very low mass binary LP 349-25.Comment: Accepted to The Astrophysical Journal; 22 pages; 12 figure

Gene prediction using the Self-Organizing Map: automatic generation of multiple gene models

Background: Many current gene prediction methods use only one model to represent proteincoding regions in a genome, and so are less likely to predict the location of genes that have an atypical sequence composition. It is likely that future improvements in gene finding will involve the development of methods that can adequately deal with intra-genomic compositional variation. Results: This work explores a new approach to gene-prediction, based on the Self-Organizing Map, which has the ability to automatically identify multiple gene models within a genome. The current implementation, named RescueNet, uses relative synonymous codon usage as the indicator of protein-coding potential. Conclusions: While its raw accuracy rate can be less than other methods, RescueNet consistently identifies some genes that other methods do not, and should therefore be of interest to geneprediction software developers and genome annotation teams alike. RescueNet is recommended for use in conjunction with, or as a complement to, other gene prediction methods

Multi-Epoch VLBI of L Dwarf Binary 2MASS J0746+2000AB: Precise Mass Measurements and Confirmation of Radio Emission from Both Components

Surveys have shown that up to one tenth of all ultracool dwarfs (UCDs) are appreciable radio emitters, with their emission attributed to a combination of gyrosynchrotron radiation and the electron cyclotron maser instability (ECMI). 2M J0746+2000AB is a close stellar binary comprised of an L0 and L1.5 dwarf that was previously identified as a source of 5 GHz radio emission. We used very-long-baseline interferometry (VLBI) to precisely track the radio emission over seven epochs in 2010-2017, and found both components to be radio emitters -- the first such system identified -- with the secondary component as the dominant source of emission in all epochs. The previously identified 2.07 h periodic bursts were confirmed to originate from the secondary component, although an isolated burst was also identified from the primary component. We additionally fitted the VLBI absolute astrometric positions jointly with existing relative orbital astrometry derived from optical/IR observations with Markov-chain Monte Carlo (MCMC) methods to determine the orbital parameters of the two components. We found the masses of the primary and secondary optical components to be 0.0795 +/- 0.0003 Msun and 0.0756 +/- 0.0003 Msun, respectively, representing the most precise mass estimates of any UCDs to date. Finally, we place a 3-sigma upper limit of 0.9 Mjup au on the mass and separation of planets orbiting either of the two components.Comment: 14 pages, 7 figures, 5 tables; ApJ, accepte

Multiepoch VLBI of L Dwarf Binary 2MASS J0746+2000AB: Precise Mass Measurements and Confirmation of Radio Emission from Both Components

Surveys have shown that up to 1/10th of all ultracool dwarfs (UCDs) are appreciable radio emitters, with their emission attributed to a combination of gyrosynchrotron radiation and the electron cyclotron maser instability. 2M J0746+2000AB is a close stellar binary comprised of an L0 and L1.5 dwarf that was previously identified as a source of 5 GHz radio emission. We used Very Long Baseline Interferometry (VLBI) to precisely track the radio emission over seven epochs in 2010–2017, and found both components to be radio emitters—the first such system identified—with the secondary component as the dominant source of emission in all epochs. The previously identified 2.07 hr periodic bursts were confirmed to originate from the secondary component, although an isolated burst was also identified from the primary component. We additionally fitted the VLBI absolute astrometric positions jointly with existing relative orbital astrometry derived from optical/infrared observations with Markov Chain Monte Carlo methods to determine the orbital parameters of the two components. We found the masses of the primary and secondary optical components to be 0.0795 ± 0.0003 M⊙, and 0.0756 ± 0.0003 M⊙, respectively, representing the most precise mass estimates of any UCDs to date. Finally, we place a 3σ upper limit of 0.9 M_(jup) au on the mass and separation of planets orbiting either of the two components