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Open Access Publications from the University of California

Scholarly Works (9 results)

  • Article
Other Scholarly Work (1990)

The 59,000 km long global mid-ocean ridge system is the site of formation of 20 km3 of oceanic crust yearly. Two-thirds of all heat loss from the interior of our planet is through the ocean floors, 40% of this amount is focused through the ridge. Activity involves complex interactions among a number of processes occurring over wide ranges of depths and lateral distances, including melting of the earth's mantle, delivery of the molten rock to a crustal magma chamber, cooling of the magma intrusion by hydrothermal circulation and volcanic eruption, chemical exchange between hot rock surrounding the magma chamber and the overlying seawater, and even the establishment of exotic biological communities near hydrothermal vents at the ridge axis. These features justify the expanding scientific interest in the study of the ridge.

Transient controlled-source electromagnetics (CSEM) is a geophysical exploration technique capable of determining the electrical conductivity beneath fast-spreading segments of the mid-ocean ridge. Geological structure beneath the mid-ocean ridge that is readily accessible to transient CSEM exploration is located at crustal levels and includes the axial magma chamber and its associated zones of partial melt and hydrothermal activity. Seismic images of the top several kilometers beneath the fast spreading East Pacific Rise (EPR) between 9-13°N have already been obtained. Multi-channel reflection profiles place strong constraints on the geometry of the top of the axial magma chamber but refraction data provide only coarse estimates of the sub-surface temperature, distribution of partial melt and porosity, parameters required to distinguish between proposed petrological models of the ridge. Electrical conductivity is a strong indicator of all these critical parameters and therefore CSEM methods are well-suited to improve the estimates and help characterize the ridge environment.

In this thesis, a pair of forward modeling computer programs have been developed to design ridge-going experiments and assist interpretation of mid-ocean ridge transient CSEM data sets, as they become available. The programs may also be used to evaluate the transient CSEM technique as it might be applied to investigate other tectonically active regions of the seafloor. One program rapidly computes the theoretical response, as a function of time, of an arbitrary, two dimensional earth to a sudden switch-on of electric current in a line source of electromagnetic energy. The other program is more advanced, requires more computer time, and is referred to as a 2.5-D program because it can handle excitation of the earth by a more realistic, finite source.

The programs solve the forward problem as follows. Electromagnetic boundary value problems based on the governing Maxwell's equations are solved by the finite element method in the Laplace frequency s-domain. The calculated electromagnetic field components are then transformed into the time domain by means of the Gaver-Stehfest algorithm. In the 2.5-D program, Maxwell's equations are additionally Fourier transformed in the direction parallel to the strike of the 2-D conductivity structure, and field components are computed in the along-strike wavenumber q-domain. Following the calculation, inverse transforms are performed to obtain the along-strike spatial variations of the field components. The codes have been validated through comparisons with known analytic solutions in which the earth is modeled as a uniformly conducting half-space. Convergence of the finite element approximation is found to be O(h), where h measures the size of the triangles comprising the finite element mesh. An extrapolation formula is described by which numerical solutions on progressively finer meshes are combined. The formula permits great accuracy to be attained in the computed field components, using relatively coarse meshes.

A numerical study of the performance of an idealized transient CSEM system at the East Pacific Rise has been carried out using the 2-D code. The system consists of an infinite source located 5 km west of the ridge axis, and seafloor magnetic field sensors placed at various distances across the ridge crest. The source is oriented with respect to the strike of the ridge so as to produce only the H-polarization mode of electric current flow. The results indicate that this system can detect the axial magma chamber and the associated zones of hydrothermal activity and partial melt by monitoring two electromagnetic response parameters, the diffusion time T and the response amplitude B max , as a function of transmitter/receiver separation. These response parameters are easily extracted from measured data and are diagnostic of the sub-surface electrical conductivity. The presence of a highly conductive magma chamber slows and attenuates signals diffusing beneath the ridge, increasing T and decreasing Bmax. Hydrothermal circulation in the highly fractured, extrusive basalt layer has the same effect on the data for receivers placed within 3 km of the ridge axis, but very little effect elsewhere. Inferences made from the numerical results suggest that a horizontal electric dipole (HED) of moment 10 4 A•m and receivers sampling the seafloor magnetic field at 10-25 Hz with a sensitivity of 1 pT/s over a time window extending to 10 s are sufficient to detect these crustal targets.

Interpretation of transient CSEM data requires forward modeling using a more realistic, finite source. The 2.5-D code is capable of achieving this. Sample field patterns produced in the vicinity of the ridge by a sudden switch-on of electric current in a horizontal electric dipole (HED) are computed. The patterns illustrate diffusion, in three spatial dimensions and time, of various along-strike electromagnetic field components through typical mid-ocean ridge structures. The results demonstrate the utility of the 2.5-D code, i.e. its potential for interpreting data from a transient CSEM ridge-going experiment.

    Cover page: Active Electromagnetics At The Mid-Ocean Ridge
    • Article
    • Peer Reviewed
    Dermatology Online Journal, Volume 3, Issue 2 (1997)
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2017)
      We present results from a Keck/HIRES radial velocity campaign to study four sub-Saturn-sized planets, K2-27b, K2-32b, K2-39b, and K2-108b, with the goal of understanding their masses, orbits, and heavy-element enrichment. The planets have similar sizes (RP=4.5-5.5 ), but have dissimilar masses (MP=16-60 ), implying a diversity in their core and envelope masses. K2-32b is the least massive (MP = 16.5 ± 2.7 M) and orbits in close proximity to two sub-Neptunes near a 3:2:1 period commensurability. K2-27b and K2-39b are significantly more massive at MP = 30.9 ± 4.6 M and MP = 39.8 ± 4.4 M, respectively, and show no signs of additional planets. K2-108b is the most massive at MP = 59.4 ± 4.4 M, implying a large reservoir of heavy elements of about ≈50 . Sub-Saturns as a population have a large diversity in planet mass at a given size. They exhibit remarkably little correlation between mass and size; sub-Saturns range from ≈6-60 M, regardless of size. We find a strong correlation between planet mass and host star metallicity, suggesting that metal-rich disks form more massive planet cores. The most massive sub-Saturns tend to lack detected companions and have moderately eccentric orbits, perhaps as a result of a previous epoch of dynamical instability. Finally, we observe only a weak correlation between the planet envelope fraction and present-day equilibrium temperature, suggesting that photo-evaporation does not play a dominant role in determining the amount of gas sub-Saturns accrete from their protoplanetary disks.
        Cover page: Four Sub-Saturns with Dissimilar Densities: Windows into Planetary Cores and EnvelopesCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2016)
        Open clusters have been the focus of several exoplanet surveys, but only a few planets have so far been discovered. The Kepler spacecraft revealed an abundance of small planets around small cool stars, therefore, such cluster members are prime targets for exoplanet transit searches. Kepler's new mission, K2, is targeting several open clusters and star-forming regions around the ecliptic to search for transiting planets around their low-mass constituents. Here, we report the discovery of the first transiting planet in the intermediate-age (800 Myr) Beehive cluster (Praesepe). K2-95 is a faint (Kp = 15.5 mag) dwarf from K2's Campaign 5 with an effective temperature of 3471 ±124 K, approximately solar metallicity and a radius of 0.402± 0.050.R⊕ We detected a transiting planet with a radius of3.47+0.78-0.53R⊕ and an orbital period of 10.134 days. We combined photometry, medium/high-resolution spectroscopy, adaptive optics/speckle imaging, and archival survey images to rule out any false-positive detection scenarios, validate the planet, and further characterize the system. The planet's radius is very unusual as M-dwarf field stars rarely have Neptune-sized transiting planets. The comparatively large radius of K2-95b is consistent with the other recently discovered cluster planets K2-25b (Hyades) and K2-33b (Upper Scorpius), indicating systematic differences in their evolutionary states or formation. These discoveries from K2 provide a snapshot of planet formation and evolution in cluster environments and thus make excellent laboratories to test differences between field-star and cluster planet populations.
          Cover page: K2 DISCOVERS A BUSY BEE: AN UNUSUAL TRANSITING NEPTUNE FOUND IN THE BEEHIVE CLUSTERCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2017)
          We find transient transit-like dimming events within the K2 time series photometry of the young star RIK-210 in the Upper Scorpius OB association. These dimming events are variable in depth, duration, and morphology. High spatial resolution imaging revealed that the star is single and radial velocity monitoring indicated that the dimming events cannot be due to an eclipsing stellar or brown dwarf companion. Archival and follow-up photometry suggest the dimming events are transient in nature. The variable morphology of the dimming events suggests they are not due to a single spherical body. The ingress of each dimming event is always shallower than egress, as one would expect for an orbiting body with a leading tail. The dimming events are periodic and synchronous with the stellar rotation. However, we argue it is unlikely the dimming events could be attributed to anything on the stellar surface based on the observed depths and durations. Variable obscuration by a protoplanetary disk is unlikely on the basis that the star is not actively accreting and lacks the infrared excess associated with an inner disk. Rather, we explore the possibilities that the dimming events are due to magnetospheric clouds, a transiting protoplanet surrounded by circumplanetary dust and debris, eccentric orbiting bodies undergoing periodic tidal disruption, or an extended field of dust or debris near the corotation radius.
            Cover page: A Transient Transit Signature Associated with the Young Star RIK-210Creative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2017)
            We recently used near-infrared spectroscopy to improve the characterization of 76 low-mass stars around which K2 had detected 79 candidate transiting planets. 29 of these worlds were new discoveries that had not previously been published. We calculate the false positive probabilities that the transit-like signals are actually caused by non-planetary astrophysical phenomena and reject five new transit-like events and three previously reported events as false positives. We also statistically validate 17 planets (7 of which were previously unpublished), confirm the earlier validation of 22 planets, and announce 17 newly discovered planet candidates. Revising the properties of the associated planet candidates based on the updated host star characteristics and refitting the transit photometry, we find that our sample contains 21 planets or planet candidates with radii smaller than 1.25 R ⊕, 18 super-Earths (1.25-2 R ⊕), 21 small Neptunes (2-4 R ⊕), three large Neptunes (4-6 R ⊕), and eight giant planets (>6 R ⊕). Most of these planets are highly irradiated, but EPIC 206209135.04 (K2-72e, ), EPIC 211988320.01 (), and EPIC 212690867.01 () orbit within optimistic habitable zone boundaries set by the "recent Venus" inner limit and the "early Mars" outer limit. In total, our planet sample includes eight moderately irradiated 1.5-3 R ⊕ planet candidates (F p ≲ 20 F ⊕) orbiting brighter stars (Ks < 11) that are well-suited for atmospheric investigations with the Hubble, Spitzer, and/or James Webb Space Telescopes. Five validated planets orbit relatively bright stars (Kp < 12.5) and are expected to yield radial velocity semi-amplitudes of at least 2 m s-1. Accordingly, they are possible targets for radial velocity mass measurement with current facilities or the upcoming generation of red optical and near-infrared high-precision RV spectrographs.
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2017)
              A main goal of NASA's Kepler Mission is to establish the frequency of potentially habitable Earth-size planets (). Relatively few such candidates identified by the mission can be confirmed to be rocky via dynamical measurement of their mass. Here we report an effort to validate 18 of them statistically using the BLENDER technique, by showing that the likelihood they are true planets is far greater than that of a false positive. Our analysis incorporates follow-up observations including high-resolution optical and near-infrared spectroscopy, high-resolution imaging, and information from the analysis of the flux centroids of the Kepler observations themselves. Although many of these candidates have been previously validated by others, the confidence levels reported typically ignore the possibility that the planet may transit a star different from the target along the same line of sight. If that were the case, a planet that appears small enough to be rocky may actually be considerably larger and therefore less interesting from the point of view of habitability. We take this into consideration here and are able to validate 15 of our candidates at a 99.73% (3σ) significance level or higher, and the other three at a slightly lower confidence. We characterize the GKM host stars using available ground-based observations and provide updated parameters for the planets, with sizes between 0.8 and 2.9 R ⊕. Seven of them (KOI-0438.02, 0463.01, 2418.01, 2626.01, 3282.01, 4036.01, and 5856.01) have a better than 50% chance of being smaller than 2 R ⊕ and being in the habitable zone of their host stars.
                Cover page: Validation of Small Kepler Transiting Planet Candidates in or near the Habitable ZoneCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2016)
                We present 197 planet candidates discovered using data from the first year of the NASA K2 mission (Campaigns 0-4), along with the results of an intensive program of photometric analyses, stellar spectroscopy, high-resolution imaging, and statistical validation. We distill these candidates into sets of 104 validated planets (57 in multi-planet systems), false positives, and 63 remaining candidates. Our validated systems span a range of properties, with median values of RP= 2.3 R⊕, P = 8.6 days, Teff = 5300 K, and Kp = 12.7mag. Stellar spectroscopy provides precise stellar and planetary parameters for most of these systems. We show that K2 has increased by 30% the number of small planets known to orbit moderately bright stars (1-4 R R⊕, Kp = 9-13 mag). Of particular interest are planets smaller than 2 R⊕, orbiting stars brighter than Kp = 11.5 mag, 5 receiving Earth-like irradiation levels, and several multi-planet systems - including 4 planets orbiting the M dwarf K2-72 near mean-motion resonances. By quantifying the likelihood that each candidate is a planet we demonstrate that our candidate sample has an overall false positive rate of 15%-30%, with rates substantially lower for small candidates (<2R⊕) and larger for candidates with radii >8 R⊕ and/or with P<3 days. Extrapolation of the current planetary yield suggests that K2 will discover between 500 and 1000 planets in its planned four-year mission, assuming sufficient follow-up resources are available. Efficient observing and analysis, together with an organized and coherent follow-up strategy, are essential for maximizing the efficacy of planet-validation efforts for K2, TESS, and future large-scale surveys.
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2019)
                  Planets occur most frequently around cool dwarfs, but only a handful of specific examples are known to orbit the latest-type M stars. Using TESS photometry, we report the discovery of two planets transiting the low-mass star called LP 791-18 (identified by TESS as TOI 736). This star has spectral type M6V, effective temperature 2960 K, and radius 0.17 R o, making it the third-coolest star known to host planets. The two planets straddle the radius gap seen for smaller exoplanets; they include a 1.1R ⊕ planet on a 0.95 day orbit and a 2.3R ⊕ planet on a 5 day orbit. Because the host star is small the decrease in light during these planets' transits is fairly large (0.4% and 1.7%). This has allowed us to detect both planets' transits from ground-based photometry, refining their radii and orbital ephemerides. In the future, radial velocity observations and transmission spectroscopy can both probe these planets' bulk interior and atmospheric compositions, and additional photometric monitoring would be sensitive to even smaller transiting planets.
                    Cover page: A Super-Earth and Sub-Neptune Transiting the Late-type M Dwarf LP 791-18Creative Commons 'BY' version 4.0 license
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