James P. Lloyd

Failure diagnosis using decision trees

We present a decision tree learning approach to diagnosing failures in large Internet sites. We record runtime properties of each request and apply automated machine learning and data mining techniques to identify the causes of failures. We train decision trees on the request traces from time periods in which user-visible failures are present. Paths through the tree are ranked according to their degree of correlation with failure, and nodes are merged according to the observed partial order of system components. We evaluate this approach using actual failures from eBay, and find that, among hundreds of potential causes, the algorithm successfully identifies 13 out of 14 true causes of failure, along with 2 false positives. We discuss some results in applying simplified decision trees on eBays production site for several months. In addition, we give a cost-benefit analysis of manual vs. automated diagnosis systems. Our contributions include the statistical learning approach, the adaptation of decision trees to the context of failure diagnosis, and the deployment and evaluation of our tools on a high-volume production service.

Metallicity and Temperature Indicators in M Dwarf K-band Spectra: Testing New and Updated Calibrations with Observations of 133 Solar Neighborhood M Dwarfs

We present K-band spectra for 133 nearby (d < 33 ps) M dwarfs, including 18 M dwarfs with reliable metallicity estimates (as inferred from an FGK type companion), 11 M dwarf planet hosts, more than 2/3 of the M dwarfs in the northern 8 pc sample, and several M dwarfs from the LSPM catalog. From these spectra, we measure equivalent widths of the Ca and Na lines, and a spectral index quantifying the absorption due to H_2O opacity (the H_2O-K_2 index). Using empirical spectral type standards and synthetic models, we calibrate the H_2O-K_2 index as an indicator of an M dwarfs spectral type and effective temperature. We also present a revised relationship that estimates the [Fe/H] and [M/H] metallicities of M dwarfs from their Na I, Ca I, and H_2O-K_2 measurements. Comparisons to model atmosphere provide a qualitative validation of our approach, but also reveal an overall offset between the atomic line strengths predicted by models as compared to actual observations. Our metallicity estimates also reproduce expected correlations with Galactic space motions and Hα emission line strengths, and return statistically identical metallicities for M dwarfs within a common multiple system. Finally, we find systematic residuals between our H_2O-based spectral types and those derived from optical spectral features with previously known sensitivity to stellar metallicity, such as TiO, and identify the CaH1 index as a promising optical index for diagnosing the metallicities of near-solar M dwarfs.

Characterizing the Cool KOIs. III. KOI-961: A Small Star with Large Proper Motion and Three Small Planets

We present the characterization of the star KOI 961, an M dwarf with transit signals indicative of three short-period exoplanets, originally discovered by the Kepler Mission. We proceed by comparing KOI 961 to Barnards Star, a nearby, well-characterized mid-M dwarf. By comparing colors, optical and near-infrared spectra, we find remarkable agreement between the two, implying similar effective temperatures and metallicities. Both are metal-poor compared to the Solar neighborhood, have low projected rotational velocity, high absolute radial velocity, large proper motion and no quiescent H-alpha emission--all of which is consistent with being old M dwarfs. We combine empirical measurements of Barnards Star and expectations from evolutionary isochrones to estimate KOI 961s mass (0.13 ± 0.05 M_⊙), radius (0.17 ± 0.04 R_⊙) and luminosity (2.40 x 10^(-3.0 ± 0.3) L_⊙). We calculate KOI 961s distance (38.7 ± 6.3 pc) and space motions, which, like Barnards Star, are consistent with a high scale-height population in the Milky Way. We perform an independent multi-transit fit to the public Kepler light curve and significantly revise the transit parameters for the three planets. We calculate the false-positive probability for each planet-candidate, and find a less than 1% chance that any one of the transiting signals is due to a background or hierarchical eclipsing binary, validating the planetary nature of the transits. The best-fitting radii for all three planets are less than 1 Re_⊕, with KOI 961.03 being Mars-sized (Rp = 0.57 ± 0.18 R_⊕), and they represent some of the smallest exoplanets detected to date.

MAPPING THE SHORES OF THE BROWN DWARF DESERT. I. UPPER SCORPIUS

We present the results of a survey for stellar and substellar companions to 82 young stars in the nearby OB association Upper Scorpius. This survey used nonredundant aperture mask interferometry to achieve typical contrast limits of ΔK ~5-6 at the diffraction limit, revealing 12 new binary companions that lay below the detection limits of traditional high-resolution imaging; we also summarize a complementary snapshot imaging survey that discovered seven directly resolved companions. The overall frequency of binary companions (~35 +5 -4% at separations of 6-435 AU) appears to be equivalent to field stars of similar mass, but companions could be more common among lower mass stars than for the field. The companion mass function has statistically significant differences compared to several suggested mass functions for the field, and we suggest an alternate lognormal parameterization of the mass function. Our survey limits encompass the entire brown dwarf mass range, but we only detected a single companion that might be a brown dwarf; this deficit resembles the so-called brown dwarf desert that has been observed by radial velocity planet searches. Finally, our survey’s deep detection limits extend into the top of the planetary mass function, reaching 8-12 MJup for half of our sample. We have not identified any planetary companions at high confidence (≳99.5%), but we have identified four candidate companions at lower confidence (≳97.5%) that merit additional follow-up to confirm or disprove their existence.

CHARACTERIZING THE COOL KEPLER OBJECTS OF INTERESTS. NEW EFFECTIVE TEMPERATURES, METALLICITIES, MASSES, AND RADII OF LOW-MASS KEPLER PLANET-CANDIDATE HOST STARS

We report stellar parameters for late-K and M-type planet-candidate host stars announced by the Kepler Mission. We obtained medium-resolution, K-band spectra of 84 cool (T_eff ≲ 4400 K) Kepler Objects of Interest (KOIs) from Borucki et al. We identified one object as a giant (KOI 977); for the remaining dwarfs, we measured effective temperatures (T_eff) and metallicities [M/H] using the K-band spectral indices of Rojas-Ayala et al. We determine the masses (M_*) and radii (R_*) of the cool KOIs by interpolation onto the Dartmouth evolutionary isochrones. The resultant stellar radii are significantly less than the values reported in the Kepler Input Catalog and, by construction, correlate better with T_eff. Applying the published KOI transit parameters to our stellar radius measurements, we report new physical radii for the planet candidates. Recalculating the equilibrium temperatures of the planet-candidates assuming Earths albedo and re-radiation fraction, we find that three of the planet-candidates are terrestrial sized with orbital semimajor axes that lie within the habitable zones of their host stars (KOI 463.01, KOI 812.03, and KOI 854.01). The stellar parameters presented in this Letter serve as a resource for prioritization of future follow-up efforts to validate and characterize the cool KOI planet candidates.

CHARACTERIZING THE COOL KOIs. II. THE M DWARF KOI-254 AND ITS HOT JUPITER*

We report the confirmation and characterization of a transiting gas giant planet orbiting the M dwarf KOI-254 every 2.455239 days, which was originally discovered by the Kepler mission. We use radial velocity measurements, adaptive optics imaging, and near-infrared spectroscopy to confirm the planetary nature of the transit events. KOI-254 b is the first hot Jupiter discovered around an M-type dwarf star. We also present a new model-independent method of using broadband photometry to estimate the mass and metallicity of an M dwarf without relying on a direct distance measurement. Included in this methodology is a new photometric metallicity calibration based on J – K colors. We use this technique to measure the physical properties of KOI-254 and its planet. We measure a planet mass of M_P = 0.505 M_(Jup), radius R_P = 0.96 R_(Jup), and semimajor axis a = 0.030 AU, based on our measured stellar mass M_* = 0.59 M_☉ and radius R_* = 0.55 R_☉. We also find that the host star is metal-rich, which is consistent with the sample of M-type stars known to harbor giant planets.

Light emission during fracture of a Zr–Ti–Ni–Cu–Be bulk metallic glass

A light emission phenomenon observed during dynamic fracture of a bulk metallic glass, Zr41.2Ti13.8Cu12.5Ni10Be22.5 (at. %), has been investigated using Charpy V-notch impact specimens. Unlike more conventional crystalline metals, these Zr-based amorphous alloys emit intense flashes of visible light when ruptured. The mechanisms for this surprising behavior are unknown and the phenomenon remains uncharacterized. Here we report spectroscopic measurements of the light emitted from specimens fractured in both room air and nitrogen gas. Spectra acquired from specimens ruptured in air exhibited a single broad peak, which could be fit to a blackbody temperature of ∼3175 K. Emission from specimens fractured in nitrogen, however, was at least four orders of magnitude less intense. The spectrum was shifted to the red with an effective blackbody temperature of ∼1400 K. Fracture surfaces of specimens ruptured in both air and nitrogen exhibited local melting, providing further evidence of intense heating during fract...

Two Substellar Companions Orbiting HD 168443

Precise Doppler measurements during 4.4 yr from the Keck/HIRES spectrometer reveal two superimposed Keplerian velocity variations for HD 168443 (G6 IV). A simultaneous orbital —t to both companions yields companion masses of M sin i \ 7.7 and 17.2 orbital periods of P \ 58 days and 4.8 yr, M JUP , semimajor axes of a \ 0.29 and 2.9 AU, and eccentricities of e \ 0.53 and 0.20. An upper limit to the mass of the outer companion of 42 is derived from the lack of astrometric wobble. The outer M JUP companion was not detected with Keck adaptive optics in the near-IR. Dynamical simulations show that the system is remarkably stable for all possible masses of both companions. The two orbiting companions have masses that are probably near and slightly above the upper end of the observed mass distribution of ii planets ˇˇ at 10 Formation in a protoplanetary disk seems plausible. But these objects M JUP . present a puzzle about their formation and dynamical history, as well as about their possible kinship with planetary systems and triple-star systems.

Near-infrared Metallicities, Radial Velocities and Spectral Types for 447 Nearby M Dwarfs

We present metallicities, radial velocities and near-infr ared spectral types for 447 M dwarfs determined from moderate resolution (R ≈ 2000) near-infrared (NIR) spectra obtained with IRTF/SpeX. These M dwarfs are primarily targets of the MEarth Survey, a transiting planet survey searching for super Earths around mid-to-late M dwarfs within 33pc. We present NIR spectral types for each star and new spectral templates for IRTF in the Y , J , H and K-bands, created using M dwarfs with near-solar metalliciti es. We developed two spectroscopic distance calibrations that use NIR spectral type or an index based on the curvature of the K-band continuum. Our distance calibration has a scatter of 14%. We searched 27 NIR spectral lines and 10 spectral indices for metallicity sensitive features, taking into account co rrelated noise in our estimates of the errors on these parameters. We calibrated our relation using 36 M dwarfs in common proper pairs with an F, G or K-type star of known metallicity. We validated the physical association of these pairs using proper motions, radial velocities and spectroscopic distance estimates. Our resu lting metallicity calibration uses the sodium doublet at 2.2µm as the sole indicator for metallicity. It has an accuracy of 0.12 dex inferred from the scatter between the metallicities of the primaries and the estimated metall icities of the secondaries. Our relation is valid for NIR spectral types from M1V to M5V and for−1.0 < [Fe/H] < +0.35 dex. We present a new color-color metallicity relation using J − H and J − K colors that directly relates two observables: the distance from the M dwarf main sequence and equivalent width of the sodium line at 2.2µm. We measured radial velocities by modeling telluric features to determine the absolute wavelength calibration of our spectra, and used M dwarf binaries, observations at different epochs, and comp arison to precisely measured radial velocities to demonstrate 4 km s −1 accuracy.

THE SOLAR-SYSTEM-SCALE DISK AROUND AB AURIGAE

The young star AB Aurigae is surrounded by a complex combination of gas-rich and dust-dominated structures. The inner disk, which has not been studied previously at sufficient resolution and imaging dynamic range, seems to contain very little gas inside a radius of least 130 AU from the star. Using adaptive optics coronagraphy and polarimetry, we have imaged the dust in an annulus between 43 and 302 AU from the star, a region never seen before. An azimuthal gap in an annulus of dust at a radius of 102 AU, along with a clearing at closer radii inside this annulus, suggests the formation of at least one small body at an orbital distance of ~100 AU. This structure seems consistent with crude models of mean motion resonances or accumulation of material at two of the Lagrange points relative to the putative object and the star. We also report a low significance detection of a point source in this outer annulus of dust. This source may be an overdensity in the disk due to dust accreting onto an unseen companion. An alternate interpretation suggests that the objects mass is between 5 and 37 times the mass of Jupiter. The results have implications for circumstellar disk dynamics and planet formation.