Discovery of Radio Emission from the Tight M8 Binary: LP 349-25

We present radio observations of 8 ultracool dwarfs with a narrow spectral type range (M8-M9.5) using the Very Large Array at 8.5 GHz. Only the tight M8 binary LP 349-25 was detected. LP 349-25 is the tenth ultracool dwarf system detected in radio and its trigonometric parallax pi = 67.6 mas, recently measured by Gatewood et al., makes it the furthest ultracool system detected by the Very Large Array to date, and the most radio-luminous outside of obvious flaring activity or variability. With a separation of only 1.8 AU, masses of the components of LP 349-25 can be measured precisely without any theoretical assumptions (Forveille et al.), allowing us to clarify their fully-convective status and hence the kind of magnetic dynamo in these components which may play an important role to explain our detection of radio emission from these objects. This also makes LP 349-25 an excellent target for further studies with better constraints on the correlations between X-ray, radio emission and stellar parameters such as mass, age, temperature, and luminosity in ultracool dwarfs.Comment: accepted by ApJ, referee's comments included, typo in equation 1 correcte

Genetic Inhibition of Phosphorylation of the Translation Initiation Factor eIF2alpha Does Not Block Abeta-Dependent Elevation of BACE1 and APP Levels or Reduce Amyloid Pathology in a Mouse Model of Alzheimer's Disease

beta-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) initiates the production of beta-amyloid (Abeta), the major constituent of amyloid plaques in Alzheimer's disease (AD). BACE1 is elevated approximately 2-3 fold in AD brain and is concentrated in dystrophic neurites near plaques, suggesting BACE1 elevation is Abeta-dependent. Previously, we showed that phosphorylation of the translation initiation factor eIF2alpha de-represses translation of BACE1 mRNA following stress such as energy deprivation. We hypothesized that stress induced by Abeta might increase BACE1 levels by the same translational mechanism involving eIF2alpha phosphorylation. To test this hypothesis, we used three different genetic strategies to determine the effects of reducing eIF2alpha phosphorylation on Abeta-dependent BACE1 elevation in vitro and in vivo: 1) a two-vector adeno-associated virus (AAV) system to express constitutively active GADD34, the regulatory subunit of PP1c eIF2alpha phosphatase; 2) a non-phosphorylatable eIF2alpha S51A knockin mutation; 3) a BACE1-YFP transgene lacking the BACE1 mRNA 5' untranslated region (UTR) required for eIF2alpha translational regulation. The first two strategies were used in primary neurons and 5XFAD transgenic mice, while the third strategy was employed only in 5XFAD mice. Despite very effective reduction of eIF2alpha phosphorylation in both primary neurons and 5XFAD brains, or elimination of eIF2alpha-mediated regulation of BACE1-YFP mRNA translation in 5XFAD brains, Abeta-dependent BACE1 elevation was not decreased. Additionally, robust inhibition of eIF2alpha phosphorylation did not block Abeta-dependent APP elevation in primary neurons, nor did it reduce amyloid pathology in 5XFAD mice. We conclude that amyloid-associated BACE1 elevation is not caused by translational de-repression via eIF2alpha phosphorylation, but instead appears to involve a post-translational mechanism. These definitive genetic results exclude a role for eIF2alpha phosphorylation in Abeta-dependent BACE1 and APP elevation. We suggest a vicious pathogenic cycle wherein Abeta42 toxicity induces peri-plaque BACE1 and APP accumulation in dystrophic neurites leading to exacerbated Abeta production and plaque progression

The Search for Signatures Of Transient Mass Loss in Active Stars

The habitability of an exoplanet depends on many factors. One such factor is the impact of stellar eruptive events on nearby exoplanets. Currently this is poorly constrained due to heavy reliance on solar scaling relationships and a lack of experimental evidence. Potential impacts of Coronal Mass Ejections (CMEs), which are a large eruption of magnetic field and plasma from a star, are space weather and atmospheric stripping. A method for observing CMEs as they travel though the stellar atmosphere is the type II radio burst, and the new LOw Frequency ARray (LOFAR) provides a means for detection. We report on 15 hours of observation of YZ Canis Minoris (YZ CMi), a nearby M dwarf flare star, taken in LOFAR's beam-formed observation mode for the purposes of measuring transient frequency-dependent low frequency radio emission. The observations utilized Low-Band Antenna (10-90 MHz) or High-Band Antenna (110-190 MHz) for five three-hour observation periods. In this data set, there were no confirmed type II events in this frequency range. We explore the range of parameter space for type II bursts constrained by our observations Assuming the rate of shocks is a lower limit to the rate at which CMEs occur, no detections in a total of 15 hours of observation places a limit of $\nu_{type II} < 0.0667$ shocks/hr $\leq \nu_{CME}$ for YZ CMi due to the stochastic nature of the events and limits of observational sensitivity. We propose a methodology to interpret jointly observed flares and CMEs which will provide greater constraints to CMEs and test the applicability of solar scaling relations

Six Large Coronal X-ray Flares Observed With Chandra

A study of the six largest coronal X-ray flares in the Chandra archive is presented. The flares were observed on II Peg, OU And, Algol, HR 1099, TZ CrB and CC Eri, all with the High Energy Transmission Grating spectrometer (HETG) and the ACIS detectors. We reconstruct an Emission Measure Distribution EMD(T), using a spectral line analysis method, for flare and quiescence states separately and compare the two. Subsequently, elemental abundaces are obtained from the EMD. We find similar behaviour of the EMD in all flares, namely a large high-T component appears while the low-T (kT < 2 keV) plasma is mostly unaffected, except for a small rise in the low-T Emission Measure. In five of the six flares we detect a First Ionization Potential (FIP) effect in the flare abundances relative to quiescence. This may contradict previous suggestions that flares are the cause of an inverse FIP effect in highly active coronae.Comment: 18 pages, 6 figures, accepted for publication in A&A. Corrected luminocities on table

Wideband dynamic radio spectra of two ultra-cool dwarfs

A number of radio-loud ultra-cool dwarf (UCD ) stars exhibit both continuous broadband and highly polarized pulsed radio emission. In order to determine the nature of the emission and the physical characteristics in the source region, we have made multi-epoch, wideband spectral observations of TVLM 0513-46 and 2M 0746+20. We combine these observations with archival radio data to fully characterize both the temporal and spectral properties of the radio emission. The continuum spectral energy distribution can be well modeled using gyrosynchrotron emission from mildly relativistic electrons in a dipolar field. The pulsed emission exhibits a variety of time-variable characteristics, including frequency drifts, frequency cutoffs, and multiple pulses per period. For 2M 0746+20 we determine a pulse period consistent with previously determined values. We modeled locations of pulsed emission using an oblique rotating magnetospheric model with beamed electron-cyclotron maser ( ECM) sources. The bestfit models have narrow ECM beaming angles aligned with the local source magnetic field direction, except for one isolated burst from 2M 0746+20. For TVLM 0513-46, the best-fit rotation axis inclination is nearly orthogonal to the line of sight. For 2M0746+20 we found a good fit using a fixed inclination i = 36°, determined from optical observations. For both stars the ECM sources are located near feet of magnetic loops with radial extents 1.2Rs-2.7Rsand surface fields 2.2-2.5 kG. These results support recent suggestions that radio over-luminous UCDs have a global "weak field" non-axisymmetric magnetic topologies

Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf

[Abridged] We present an 8.5-hour simultaneous radio, X-ray, UV, and optical observation of the L dwarf binary 2MASSW J0746+20. We detect strong radio emission, dominated by short-duration periodic pulses at 4.86 GHz with P=124.32+/-0.11 min. The stability of the pulse profiles and arrival times demonstrates that they are due to the rotational modulation of a B~1.7 kG magnetic field. A quiescent non-variable component is also detected, likely due to emission from a uniform large-scale field. The H-alpha emission exhibits identical periodicity, but unlike the radio pulses it varies sinusoidally and is offset by exactly 1/4 of a phase. The sinusoidal variations require chromospheric emission from a large-scale field structure, with the radio pulses likely emanating from the magnetic poles. While both light curves can be explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out a symmetric dipole topology since it would result in a phase lag of 1/2 (poloidal field) or zero (toroidal field). We therefore conclude that either (i) the field is dominated by a quadrupole configuration, which can naturally explain the 1/4 phase lag; or (ii) the H-alpha and/or radio emission regions are not trivially aligned with the field. Regardless of the field topology, we use the measured period along with the known rotation velocity (vsini=27 km/s), and the binary orbital inclination (i=142 deg), to derive a radius for the primary star of 0.078+/-0.010 R_sun. This is the first measurement of the radius of an L dwarf, and along with a mass of 0.085+/-0.010 M_sun it provides a constraint on the mass-radius relation below 0.1 M_sun. We find that the radius is about 30% smaller than expected from theoretical models, even for an age of a few Gyr.Comment: Submitted to Ap