The TRENDS High-Contrast Imaging Survey. VI. Discovery of a Mass, Age, and Metallicity Benchmark Brown Dwarf

The mass and age of substellar objects are degenerate parameters leaving the evolutionary state of brown dwarfs ambiguous without additional information. Theoretical models are normally used to help distinguish between old, massive brown dwarfs and young, low mass brown dwarfs but these models have yet to be properly calibrated. We have carried out an infrared high-contrast imaging program with the goal of detecting substellar objects as companions to nearby stars to help break degeneracies in inferred physical properties such as mass, age, and composition. Rather than using imaging observations alone, our targets are pre-selected based on the existence of dynamical accelerations informed from years of stellar radial velocity (RV) measurements. In this paper, we present the discovery of a rare benchmark brown dwarf orbiting the nearby ($d=18.69\pm0.19$ pc), solar-type (G9V) star HD 4747 ([Fe/H]=$-0.22\pm0.04$) with a projected separation of only $\rho=11.3\pm0.2$ AU ($\theta \approx$ 0.6"). Precise Doppler measurements taken over 18 years reveal the companion's orbit and allow us to place strong constraints on its mass using dynamics ($m \sin(i) = 55.3\pm1.9M_J$). Relative photometry ($\Delta K_s=9.05\pm0.14$, $M_{K_s}=13.00\pm0.14$, $K_s - L' = 1.34\pm0.46$) indicates that HD 4747 B is most-likely a late-type L-dwarf and, if near the L/T transition, an intriguing source for studying cloud physics, variability, and polarization. We estimate a model-dependent mass of $m=72^{+3}_{-13}M_J$ for an age of $3.3^{+2.3}_{-1.9}$ Gyr based on gyrochronology. Combining astrometric measurements with RV data, we calculate the companion dynamical mass ($m=60.2\pm3.3M_J$) and orbit ($e=0.740\pm0.002$) directly. As a new mass, age, and metallicity benchmark, HD 4747 B will serve as a laboratory for precision astrophysics to test theoretical models that describe the emergent radiation of brown dwarfs.Comment: Accepted to Ap

The TRENDS High-Contrast Imaging Survey. VII. Discovery of a Nearby Sirius-like White Dwarf System (HD 169889)

Monitoring the long-term radial velocity (RV) and acceleration of nearby stars has proven an effective method for directly detecting binary and substellar companions. Some fraction of nearby RV trend systems are expected to be comprised of compact objects that likewise induce a systemic Doppler signal. In this paper, we report the discovery of a white dwarf companion found to orbit the nearby ($\pi = 28.297 \pm 0.066$ mas) G9 V star HD 169889. High-contrast imaging observations using NIRC2 at Keck and LMIRCam at the LBT uncover the ($\Delta H = 9.76 \pm 0.16$, $\Delta L' = 9.60 \pm 0.03$) companion at an angular separation of 0.8'' (28 au). Thirteen years of precise Doppler observations reveal a steep linear acceleration in RV time series and place a dynamical constraint on the companion mass of $M \geq 0.369 \pm 0.010 M_{\odot}$. This "Sirius-like" system adds to the census of white dwarf companions suspected to be missing in the solar neighborhood.Comment: Accepted to Ap

60 Validated Planets from K2 Campaigns 5-8

We present a uniform analysis of 155 candidates from the second year of NASA's $K2$ mission (Campaigns 5-8), yielding 60 statistically validated planets spanning a range of properties, with median values of $R_p$ = 2.5 $R_\oplus$, $P$ = 7.1 d, $T_\mathrm{eq}$ = 811 K, and $J$ = 11.3 mag. The sample includes 24 planets in 11 multi-planetary systems, as well as 18 false positives, and 77 remaining planet candidates. Of particular interest are 18 planets smaller than 2 $R_\oplus$, five orbiting stars brighter than $J$ = 10 mag, and a system of four small planets orbiting the solar-type star EPIC 212157262. We compute planetary transit parameters and false positive probabilities using a robust statistical framework and present a complete analysis incorporating the results of an intensive campaign of high resolution imaging and spectroscopic observations. This work brings the $K2$ yield to over 360 planets, and by extrapolation we expect that $K2$ will have discovered $\sim$600 planets before the expected depletion of its on-board fuel in late 2018.Comment: 33 pages, 13 figures, 5 tables, accepted for publication in A

The TRENDS High-contrast Imaging Survey. VIII. Compendium of Benchmark Objects

The physical properties of faint stellar and substellar objects often rely on indirect, model-dependent estimates. For example, the masses of brown dwarfs are usually inferred using evolutionary models, which are age dependent and have yet to be properly calibrated. With the goal of identifying new benchmark objects to test low-mass stellar and substellar models, we have carried out a comprehensive adaptive optics survey as part of the TaRgetting bENchmark-objects with the Doppler Spectroscopy high-contrast imaging program. Using legacy radial velocity measurements from the High Resolution Echelle Spectrometer at Keck, we have identified several dozen stars that show long-term Doppler accelerations. We present follow-up high-contrast observations from the campaign and report the discovery of 31 comoving companions, as well as 11 strong candidate companions, to solar-type stars with well-determined parallax and metallicity values. Benchmark objects of this nature lend themselves to orbit determinations, dynamical mass estimates, and independent compositional assessment. This compendium of benchmark objects will serve as a convenient test group to substantiate theoretical evolutionary and atmospheric models near the hydrogen fusing limit

The TRENDS High-contrast Imaging Survey. VIII. Compendium of Benchmark Objects

The physical properties of faint stellar and substellar objects often rely on indirect, model-dependent estimates. For example, the masses of brown dwarfs are usually inferred using evolutionary models, which are age dependent and have yet to be properly calibrated. With the goal of identifying new benchmark objects to test low-mass stellar and substellar models, we have carried out a comprehensive adaptive optics survey as part of the TaRgetting bENchmark-objects with the Doppler Spectroscopy high-contrast imaging program. Using legacy radial velocity measurements from the High Resolution Echelle Spectrometer at Keck, we have identified several dozen stars that show long-term Doppler accelerations. We present follow-up high-contrast observations from the campaign and report the discovery of 31 comoving companions, as well as 11 strong candidate companions, to solar-type stars with well-determined parallax and metallicity values. Benchmark objects of this nature lend themselves to orbit determinations, dynamical mass estimates, and independent compositional assessment. This compendium of benchmark objects will serve as a convenient test group to substantiate theoretical evolutionary and atmospheric models near the hydrogen fusing limit

Discovery of a Transiting Adolescent Sub-Neptune Exoplanet with K2

The role of stellar age in the measured properties and occurrence rates of exoplanets is not well understood. This is in part due to a paucity of known young planets and the uncertainties in age-dating for most exoplanet host stars. Exoplanets with well-constrained ages, particularly those which are young, are useful as benchmarks for studies aiming to constrain the evolutionary timescales relevant for planets. Such timescales may concern orbital migration, gravitational contraction, or atmospheric photo-evaporation, among other mechanisms. Here we report the discovery of an adolescent transiting sub-Neptune from K2 photometry of the low-mass star K2-284. From multiple age indicators we estimate the age of the star to be 120 Myr, with a 68% confidence interval of 100-760 Myr. The size of K2-284 b ($R_P$ = 2.8 $\pm$ 0.1 $R_\oplus$) combined with its youth make it an intriguing case study for photo-evaporation models, which predict enhanced atmospheric mass loss during early evolutionary stages.Comment: Accepted to AJ, 36 pages, 17 figures, 5 table

Analysis of Neptune's 2017 Bright Equatorial Storm

We report the discovery of a large ($\sim$8500 km diameter) infrared-bright storm at Neptune's equator in June 2017. We tracked the storm over a period of 7 months with high-cadence infrared snapshot imaging, carried out on 14 nights at the 10 meter Keck II telescope and 17 nights at the Shane 120 inch reflector at Lick Observatory. The cloud feature was larger and more persistent than any equatorial clouds seen before on Neptune, remaining intermittently active from at least 10 June to 31 December 2017. Our Keck and Lick observations were augmented by very high-cadence images from the amateur community, which permitted the determination of accurate drift rates for the cloud feature. Its zonal drift speed was variable from 10 June to at least 25 July, but remained a constant $237.4 \pm 0.2$ m s$^{-1}$ from 30 September until at least 15 November. The pressure of the cloud top was determined from radiative transfer calculations to be 0.3-0.6 bar; this value remained constant over the course of the observations. Multiple cloud break-up events, in which a bright cloud band wrapped around Neptune's equator, were observed over the course of our observations. No "dark spot" vortices were seen near the equator in HST imaging on 6 and 7 October. The size and pressure of the storm are consistent with moist convection or a planetary-scale wave as the energy source of convective upwelling, but more modeling is required to determine the driver of this equatorial disturbance as well as the triggers for and dynamics of the observed cloud break-up events.Comment: 42 pages, 14 figures, 6 tables; Accepted to Icaru