Daniel Perez-Becker

Surface Layer Accretion in Conventional and Transitional Disks driven by Far-Ultraviolet Ionization

Whether protoplanetary disks accrete at observationally significant rates by the magnetorotational instability (MRI) depends on how well ionized they are. Disk surface layers ionized by stellar X-rays are susceptible to charge neutralization by small condensates, ranging from {approx}0.01 {mu}m sized grains to angstrom-sized polycyclic aromatic hydrocarbons (PAHs). Ion densities in X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI. Here we show that ionization by stellar far-ultraviolet (FUV) radiation enables full-blown MRI turbulence in disk surface layers. Far-UV ionization of atomic carbon and sulfur produces a plasma so dense that it is immune to ion recombination on grains and PAHs. The FUV-ionized layer, of thickness 0.01-0.1 g cm{sup -2}, behaves in the ideal magnetohydrodynamic limit and can accrete at observationally significant rates at radii {approx}> 1-10 AU. Surface layer accretion driven by FUV ionization can reproduce the trend of increasing accretion rate with increasing hole size seen in transitional disks. At radii {approx}<1-10 AU, FUV-ionized surface layers cannot sustain the accretion rates generated at larger distance, and unless turbulent mixing of plasma can thicken the MRI-active layer, an additional means of transport is needed. In the case of transitional disks, it could be provided by planets.

ATMOSPHERIC HEAT REDISTRIBUTION ON HOT JUPITERS

Infrared light curves of transiting hot Jupiters present a trend in which the atmospheres of the hottest planets are less efficient at redistributing the stellar energy absorbed on their daysides—and thus have a larger day-night temperature contrast—than colder planets. To this day, no predictive atmospheric model has been published that identifies which dynamical mechanisms determine the atmospheric heat redistribution efficiency on tidally locked exoplanets. Here we present a shallow-water model of the atmospheric dynamics on synchronously rotating planets that explains why heat redistribution efficiency drops as stellar insolation rises. Our model shows that planets with weak friction and weak irradiation exhibit a banded zonal flow with minimal day-night temperature differences, while models with strong irradiation and/or strong friction exhibit a day-night flow pattern with order-unity fractional day-night temperature differences. To interpret the model, we develop a scaling theory which shows that the timescale for gravity waves to propagate horizontally over planetary scales, τwave, plays a dominant role in controlling the transition from small to large temperature contrasts. This implies that heat redistribution is governed by a wave-like process, similar to the one responsible for the weak temperature gradients in the Earths tropics. When atmospheric drag can be neglected, the transition from small to large day-night temperature contrasts occurs when , where τrad is the radiative relaxation time and Ω is the planetary rotation frequency. Alternatively, this transition criterion can be expressed as τrad ~ τvert, where τvert is the timescale for a fluid parcel to move vertically over the difference in day-night thickness. These results subsume the more widely used timescale comparison for estimating heat redistribution efficiency between τrad and the horizontal day-night advection timescale, τadv. Only because τadv ~ τvert for hot Jupiters does the commonly assumed timescale comparison between τrad and τadv yield approximately correct predictions for the heat redistribution efficiency.

Catastrophic evaporation of rocky planets

Short-period exoplanets can have dayside surface temperatures surpassing 2000 K, hot enough to vaporize rock and drive a thermal wind. Small enough planets evaporate completely. We construct a radiative-hydrodynamic model of atmospheric escape from strongly irradiated, low-mass rocky planets, accounting for dust-gas energy exchange in the wind. Rocky planets with masses 2000 K are found to disintegrate entirely in 0.1 M_Earth/Gyr --- our model yields a present-day planet mass of < 0.02 M_Earth or less than about twice the mass of the Moon. Mass loss rates depend so strongly on planet mass that bodies can reside on close-in orbits for Gyrs with initial masses comparable to or less than that of Mercury, before entering a final short-lived phase of catastrophic mass loss (which KIC 12557548b has entered). Because this catastrophic stage lasts only up to a few percent of the planets life, we estimate that for every object like KIC 12557548b, there should be 10--100 close-in quiescent progenitors with sub-day periods whose hard-surface transits may be detectable by Kepler --- if the progenitors are as large as their maximal, Mercury-like sizes (alternatively, the progenitors could be smaller and more numerous). According to our calculations, KIC 12557548b may have lost ~70% of its formation mass; today we may be observing its naked iron core.

DETECTION AND IMAGING OF THE CRAB NEBULA WITH THE NUCLEAR COMPTON TELESCOPE

The Nuclear Compton Telescope (NCT) is a balloon-borne Compton telescope designed for the study of astrophysical sources in the soft gamma-ray regime (200 keV–20 MeV). NCT’s 10 high-purity germanium crossedstrip detectors measure the deposited energies and three-dimensional positions of gamma-ray interactions in the sensitive volume, and this information is used to restrict the initial photon to a circle on the sky using the Compton scatter technique. Thus NCT is able to perform spectroscopy, imaging, and polarization analysis on soft gamma-ray sources. NCT is one of the next generation of Compton telescopes—the so-called compact Compton telescopes (CCTs)—which can achieve effective areas comparable to the Imaging Compton Telescope’s with an instrument that is a fraction of the size. The Crab Nebula was the primary target for the second flight of the NCT instrument, which occurred on 2009 May 17 and 18 in Fort Sumner, New Mexico. Analysis of 29.3 ks of data from the flight reveals an image of the Crab at a significance of 4σ . This is the first reported detection of an astrophysical source by a CCT.

Overview of the Nuclear Compton Telescope

The Nuclear Compton Telescope (NCT) is a balloon-borne telescope designed to study astrophysical sources of nuclear line emission and polarization at soft gamma-ray (0.2-10 MeV) energies. NCT uses high-purity germanium strip detectors for 3D tracking of photon interactions. Compton imaging enables effective background rejection, resulting in a compact but highly efficient instrument. The NCT prototype completed a successful flight from Fort Sumner, New Mexico in 2005. We have since integrated additional detectors, updated the readout electronics, and improved other flight systems. Two flights of the full instrument are upcoming: a conventional flight in New Mexico and a long duration flight from Australia. We give an overview of the instrument and its status prior to the planned balloon flights.

The upcoming long duration balloon flight of the Nuclear Compton Telescope

The nuclear Compton telescope (NCT) is a balloon- borne soft gamma-ray (0.2 MeV-10 MeV) telescope designed to study astrophysical sources of nuclear line emission and polarization. A prototype instrument was successfully launched from Fort Sumner, New Mexico on June 1, 2005. The NCT prototype consisted of two 3D position sensitive high-purity germanium strip detectors (GeDs) fabricated with amorphous Ge contacts. We are currently working toward two balloon flights: another conventional balloon flight from Fort Sumner, New Mexico in September 2008, and a long-duration balloon flight (LDBF) from Alice Springs, Australia in December 2009. The NCT instrument is being upgraded to include all twelve planned GeDs. The electronics for all twelve detectors have been redesigned for smaller size, lower power consumption, and lower noise, and are now being fabricated and tested. Here we present our current progress in preparing for the flights.

Efficiency and polarimetric calibration of the Nuclear Compton Telescope

The Nuclear Compton Telescope (NCT) is a balloon-borne gamma-ray telescope which uses cross-strip germanium detectors to study astrophysical sources of nuclear line emission. The compact design allows for wide-field imaging with excellent efficiency from 0.2–10 MeV. Moreover, the Compton imaging principle utilized by NCT provides polarimetric sensitivity above 200 keV. We conducted an extensive calibration campaign using radioactive sources prior to our flight from Ft. Sumner, New Mexico in Spring 2009. We present the results of our calibration of the effective area throughout NCTs field of view and compare them with Monte Carlo simulations using a detailed mass model. Additionally, we assess NCTs polarimetric capabilities with observations of a partially-polarized beam.

The Data Readout System of the Nuclear Compton Telescope (NCT)

The Nuclear Compton Telescope (NCT) is a balloon-borne telescope based on the 3D-positioning germanium detectors. It is designed to study astrophysical sources of gamma-ray emission in the energy range of 0.2 MeV to 10 MeV. The data readout system of NCT is designed to amplify, digitize and collect signals from a germanium detector according to a certain trigger scheme. It also has an interface to the NCT flight computer to receive commands and transfer data. This paper contains the design and the scientific test result of the readout system.

The spring 2009 balloon flight of the Nuclear Compton Telescope

The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma-ray (0.2-10 MeV) telescope designed to study astrophysical sources of nuclear line emission and polarization. NCT consists of twelve high-purity germanium cross-strip detectors (GeDs) that measure both the position and energy of gamma-ray interactions. A 10-GeD version was flown on May 17-18 2009 from the Columbia Scientific Balloon Facility in Fort Sumner, NM, with a total flight duration of 38.5 hours. Here we summarize the instrument, the calibrations, the flight, and our preliminary science results.

Characterizing and Correcting the Cross-Talk Effect on Depth Measurement in the NCT Detectors

The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma ray (0.2-10 MeV) telescope designed to study astrophysical sources of nuclear line emission and polarization. The heart of NCT is an array of 12 cross-strip germanium detectors, designed to provide 3D positions for each photon interaction with full 3D position resolution to 1.6 mm3. The x and y positions are provided by the orthogonal strips, and the interaction depth (z position) in the detector is measured to an accuracy of 0.4 mm FWHM using the relative timing of the anode and cathode charge collection signals. The charge collection signals are affected by cross-talk when interactions occur in adjacent strips, altering the timing measurement in those interactions. We simulated this effect in our NCT detectors, and have developed a method to correct the timing information. Here we present the simulation and the correction results.