13 research outputs found
Spectral Energy Distributions and Multiwavelength Selection of Type 1 Quasars
We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ~ 3, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGNs) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA, and ROSAT data, where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to errors as large as 50% for individual quasars when inferring a bolometric luminosity from an optical luminosity. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio
Hot Jupiter Magnetospheres
(Abridged) The upper atmospheres of close-in gas giant exoplanets are
subjected to intense heating/tidal forces from their parent stars.
Atomic/ionized hydrogen (H) layers are sufficiently rarefied that magnetic
pressure may dominate gas pressure for expected planetary magnetic field
strength. We examine the magnetospheric structure using a 3D isothermal
magnetohydrodynamic model that includes: a static "dead zone" near the magnetic
equator containing magnetically confined gas; a "wind zone" outside the
magnetic equator in which thermal pressure gradients and the
magneto-centrifugal-tidal effect give rise to transonic outflow; and a region
near the poles where sufficiently strong tidal forces may suppress transonic
outflow. Using dipole field geometry, we estimate the size of the dead zone to
be ~1-10 planetary radii for a range of parameters. To understand appropriate
base conditions for the 3D isothermal model, we compute a 1D thermal model in
which photoelectric heating from the stellar Lyman continuum is balanced by
collisionally-excited Lyman {\alpha} cooling. This 1D model exhibits a H layer
with temperatures T=5000-10000K down to pressures of 10-100 nbar. Using the 3D
isothermal model, we compute H column densities and Lyman {\alpha} transmission
spectra for parameters appropriate to HD 209458b. Line-integrated transit
depths of 5-10% can be achieved for the above base conditions. Strong magnetic
fields increase the transit signal while decreasing the mass loss, due to
higher covering fraction and density of the dead zone. In our model, most of
the transit signal arises from magnetically confined gas, some of which may be
outside the L1 equipotential. Hence the presence of gas outside the L1
equipotential does not directly imply mass loss. Lastly, we discuss the domain
of applicability for the magnetic wind model described in this paper and in the
Roche-lobe overflow model.Comment: 26 pages, 17 figures (5 color), 2 appendices; submitted to ApJ;
higher resolution version available at
http://www.astro.virginia.edu/~gbt8f/HotJupMag_fullres_astroph.pd
Nanoscale Effects on Heterojunction Electron Gases in GaN/AlGaN Core/Shell Nanowires
The electronic properties of heterojunction electron gases formed in
GaN/AlGaN core/shell nanowires with hexagonal and triangular cross-sections are
studied theoretically. We show that at nanoscale dimensions, the non-polar
hexagonal system exhibits degenerate quasi-one-dimensional electron gases at
the hexagon corners, which transition to a core-centered electron gas at lower
doping. In contrast, polar triangular core/shell nanowires show either a
non-degenerate electron gas on the polar face or a single quasi-one-dimensional
electron gas at the corner opposite the polar face, depending on the
termination of the polar face. More generally, our results indicate that
electron gases in closed nanoscale systems are qualitatively different from
their bulk counterparts.Comment: 16 pages, 7 figures. To appear in Nano Letters. Corrected some typo