J. Sebastian Pineda

Colors and Kinematics of L Dwarfs From the Sloan Digital Sky Survey

We present a sample of 484 L dwarfs, 210 of which are newly discovered from the Sloan Digital Sky Survey (SDSS) Data Release 7 spectroscopic database. We combine this sample with known L dwarfs to investigate their izJHKS colors. Our spectroscopically selected sample has ~0.1 mag bluer median J – KS color at a given spectral type (for L0-L4) than previously known L dwarfs, which reflects a bias toward redder L dwarfs in past selection criteria. We present photometric distance relations based on i – z and i – J colors and derive distances to our L dwarf sample. We combine the distances with SDSS/2MASS proper motions in order to examine the tangential velocities. For the majority of our spectroscopic sample, we measured radial velocities and present three-dimensional kinematics. We also provide Hα detections for the fraction of our sample with sufficient quality spectra. Comparison of the velocities of our L dwarf sample to a kinematic model shows evidence for both cold and hot dynamical populations, consistent with young and old disk components. The dispersions of these components are similar to those found for M dwarfs. We also show that J – KS color is correlated with velocity dispersion, confirming a relationship between J – KS color and age.

AN ACTIVITY–ROTATION RELATIONSHIP AND KINEMATIC ANALYSIS OF NEARBY MID-TO-LATE-TYPE M DWARFS

Using spectroscopic observations and photometric light curves of 238 nearby M dwarfs from the MEarth exoplanet transit survey, we examine the relationships between magnetic activity (quantified by Hα emission), rotation period, and stellar age. Previous attempts to investigate the relationship between magnetic activity and rotation in these stars were hampered by the limited number of M dwarfs with measured rotation periods (and the fact that v sin i measurements probe only rapid rotation). However, the photometric data from MEarth allows us to probe a wide range of rotation periods for hundreds of M dwarf stars (from shorter than one to longer than 100 days). Over all M spectral types that we probe, we find that the presence of magnetic activity is tied to rotation, including for late-type, fully convective M dwarfs. We also find evidence that the fraction of late-type M dwarfs that are active may be higher at longer rotation periods compared to their early-type counterparts, with several active, late-type, slowly rotating stars present in our sample. Additionally, we find that all M dwarfs with rotation periods shorter than 26 days (early-type; M1–M4) and 86 days (late-type; M5–M8) are magnetically active. This potential mismatch suggests that the physical mechanisms that connect stellar rotation to chromospheric heating may be different in fully convective stars. A kinematic analysis suggests that the magnetically active, rapidly rotating stars are consistent with a kinematically young population, while slow-rotators are less active or inactive and appear to belong to an older, dynamically heated stellar population.

CHARACTERIZING THE COOL KOIs. V. KOI-256: A MUTUALLY ECLIPSING POST-COMMON ENVELOPE BINARY

We report that Kepler Object of Interest 256 (KOI-256) is a mutually eclipsing post-common envelope binary (ePCEB), consisting of a cool white dwarf (M_★ = 0.592 ± 0.089 M_☉, R_★ = 0.01345 ± 0.00091 R_☉, T_(eff) = 7100 ± 700 K) and an active M3 dwarf (M_★ = 0.51 ± 0.16 M_☉, R_★ = 0.540 ± 0.014 R_☉, T_(eff) = 3450 ± 50 K) with an orbital period of 1.37865 ± 0.00001 days. KOI-256 is listed as hosting a transiting planet-candidate by Borucki et al. and Batalha et al.; here we report that the planet-candidate transit signal is in fact the occultation of a white dwarf as it passes behind the M dwarf. We combine publicly-available long- and short-cadence Kepler light curves with ground-based measurements to robustly determine the system parameters. The occultation events are readily apparent in the Kepler light curve, as is spin-orbit synchronization of the M dwarf, and we detect the transit of the white dwarf in front of the M dwarf halfway between the occultation events. The size of the white dwarf with respect to the Einstein ring during transit (R_(Ein) = 0.00473 ± 0.00055 R ☉) causes the transit depth to be shallower than expected from pure geometry due to gravitational lensing. KOI-256 is an old, long-period ePCEB and serves as a benchmark object for studying the evolution of binary star systems as well as white dwarfs themselves, thanks largely to the availability of near-continuous, ultra-precise Kepler photometry.

Auroral radio emission from late L and T dwarfs: A new constraint on dynamo theory in the substellar regime

We have observed 6 late-L and T dwarfs with the Karl G. Jansky Very Large Array (VLA) to investigate the presence of highly circularly polarized radio emission, associated with large-scale auroral currents. Previous surveys encompassing ~60 L6 or later targets in this spectral range have yielded only one detection. Our sample includes the previously detected T6.5 dwarf 2MASS 10475385+2124234 as well as 5 new targets selected for the presence of H-alpha emission or optical/infrared photometric variability, which are possible manifestations of auroral activity. We detect 2MASS 10475385+2124234, as well as 4 of the 5 targets in our biased sample, including the strong IR variable SIMP J01365662+0933473 and bright H-alpha emitter 2MASS 12373919+6526148, reinforcing the possibility that activity at these disparate wavelengths is related. The radio emission frequency corresponds to a precise determination of the lower-bound magnetic field strength near the surface of each dwarf and this new sample provides robust constraints on dynamo theory in the low mass brown dwarf regime. Magnetic fields >2.5 kG are confirmed for 5/6 targets. Our results provide tentative evidence that the dynamo operating in this mass regime may be inconsistent with predicted values from a recently proposed model. Further observations at higher radio frequencies are essential for verifying this assertion.

A SURVEY FOR Hα EMISSION FROM LATE L DWARFS AND T DWARFS

Recently, studies of brown dwarfs have demonstrated that they possess strong magnetic fields and have the potential to produce radio and optical auroral emissions powered by magnetospheric currents. This emission provides the only window on magnetic fields in the coolest brown dwarfs and identifying additional benchmark objects is key to constraining dynamo theory in this regime. To this end, we conducted a new red optical (6300–9700 A) survey with the Keck telescopes looking for Hα emission from a sample of late L dwarfs and T dwarfs. Our survey gathered optical spectra for 29 targets, 18 of which did not have previous optical spectra in the literature, greatly expanding the number of moderate-resolution (R ~ 2000) spectra available at these spectral types. Combining our sample with previous surveys, we confirm an Hα detection rate of 9.2±^(3.5)_(2.1)% for L and T dwarfs in the optical spectral range of L4–T8. This detection rate is consistent with the recently measured detection rate for auroral radio emission from Kao et al., suggesting that geometrical selection effects due to the beaming of the radio emission are small or absent. We also provide the first detection of Hα emission from 2MASS 0036+1821, previously notable as the only electron cyclotron maser radio source without a confirmed detection of Hα emission. Finally, we also establish optical standards for spectral types T3 and T4, filling in the previous gap between T2 and T5.

Two Stars Two Ways: Confirming a Microlensing Binary Lens Solution with a Spectroscopic Measurement of the Orbit

Light curves of microlensing events involving stellar binaries and planetary systems can provide information about the orbital elements of the system due to orbital modulations of the caustic structure. Accurately measuring the orbit in either the stellar or planetary case requires detailed modeling of subtle deviations in the light curve. At the same time, the natural, Cartesian parameterization of a microlensing binary is partially degenerate with the microlens parallax. Hence, it is desirable to perform independent tests of the predictions of microlens orbit models using radial velocity time series of the lens binary system. To this end, we present 3.5 years of RV monitoring of the binary lens system OGLE-2009-BLG-020L, for which Skowron et al. (2011) constrained all internal parameters of the 200--700 day orbit. Our RV measurements reveal an orbit that is consistent with the predictions of the microlens light curve analysis, thereby providing the first confirmation of orbital elements inferred from microlensing events.

A Panchromatic View of Brown Dwarf Aurorae

Stellar coronal activity has been shown to persist into the low-mass star regime, down to late M-dwarf spectral types. However, there is now an accumulation of evidence suggesting that at the end of the main sequence there is a transition in the nature of the magnetic activity from chromospheric and coronal to planet-like and auroral, from local impulsive heating via flares and MHD wave dissipation to energy dissipation from strong large-scale magnetospheric current systems. We examine this transition and the prevalence of auroral activity in brown dwarfs through a compilation of multi-wavelength surveys of magnetic activity, including radio, X-ray, and optical. We compile the results of those surveys and place their conclusions in the context of auroral emission as the consequence of large-scale magnetospheric current systems that accelerate energetic electron beams and drive the particles to impact the cool atmospheric gas. We explore the different manifestation of auroral phenomena in brown dwarf atmospheres, like H

The Sloan Digital Sky Survey Data Release 7 Spectroscopic M Dwarf Catalog. III. The Spatial Dependence of Magnetic Activity in the Galaxy

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A Deep Radio Limit for the TRAPPIST-1 System

, and define their distinguishing characteristics. We conclude that large amplitude photometric variability in the near infrared is most likely a consequence of clouds in brown dwarf atmospheres, but that auroral activity may be responsible for long-lived stable surface features. We report a connection between auroral H

The Sloan Digital Sky Survey Data Release 7 Spectroscopic M Dwarf Catalog. I. Data

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