294 research outputs found
Atmospheric Circulation of Exoplanets
We survey the basic principles of atmospheric dynamics relevant to explaining
existing and future observations of exoplanets, both gas giant and terrestrial.
Given the paucity of data on exoplanet atmospheres, our approach is to
emphasize fundamental principles and insights gained from Solar-System studies
that are likely to be generalizable to exoplanets. We begin by presenting the
hierarchy of basic equations used in atmospheric dynamics, including the
Navier-Stokes, primitive, shallow-water, and two-dimensional nondivergent
models. We then survey key concepts in atmospheric dynamics, including the
importance of planetary rotation, the concept of balance, and scaling arguments
to show how turbulent interactions generally produce large-scale east-west
banding on rotating planets. We next turn to issues specific to giant planets,
including their expected interior and atmospheric thermal structures, the
implications for their wind patterns, and mechanisms to pump their east-west
jets. Hot Jupiter atmospheric dynamics are given particular attention, as these
close-in planets have been the subject of most of the concrete developments in
the study of exoplanetary atmospheres. We then turn to the basic elements of
circulation on terrestrial planets as inferred from Solar-System studies,
including Hadley cells, jet streams, processes that govern the large-scale
horizontal temperature contrasts, and climate, and we discuss how these
insights may apply to terrestrial exoplanets. Although exoplanets surely
possess a greater diversity of circulation regimes than seen on the planets in
our Solar System, our guiding philosophy is that the multi-decade study of
Solar-System planets reviewed here provides a foundation upon which our
understanding of more exotic exoplanetary meteorology must build.Comment: In EXOPLANETS, edited by S. Seager, to be published in the Spring of
2010 in the Space Science Series of the University of Arizona Press (Tucson,
AZ) (refereed; accepted for publication
Pre-Merger Localization of Gravitational-Wave Standard Sirens With LISA I: Harmonic Mode Decomposition
The continuous improvement in localization errors (sky position and distance)
in real time as LISA observes the gradual inspiral of a supermassive black hole
(SMBH) binary can be of great help in identifying any prompt electromagnetic
counterpart associated with the merger. We develop a new method, based on a
Fourier decomposition of the time-dependent, LISA-modulated gravitational-wave
signal, to study this intricate problem. The method is faster than standard
Monte Carlo simulations by orders of magnitude. By surveying the parameter
space of potential LISA sources, we find that counterparts to SMBH binary
mergers with total mass M~10^5-10^7 M_Sun and redshifts z<~3 can be localized
to within the field of view of astronomical instruments (~deg^2) typically
hours to weeks prior to coalescence. This will allow targeted searches for
variable electromagnetic counterparts as the merger proceeds, as well as
monitoring of the most energetic coalescence phase. A rich set of astrophysical
and cosmological applications would emerge from the identification of
electromagnetic counterparts to these gravitational-wave standard sirens.Comment: 29 pages, 12 figures, version accepted by Phys Rev
Ionization, Magneto-rotational, and Gravitational Instabilities in Thin Accretion Disks Around Supermassive Black Holes
We consider the combined role of the thermal ionization, magneto-rotational
and gravitational instabilities in thin accretion disks around supermassive
black holes. We find that in the portions of the disk unstable to the
ionization instability, the gas remains well coupled to the magnetic field even
on the cold, neutral branch of the thermal limit cycle. This suggests that the
ionization instability is not a significant source of large amplitude
time-dependent accretion in AGN. We also argue that, for accretion rates
greater than 10^{-2} solar masses per year, the gravitationally unstable and
magneto-rotationally unstable regions of the accretion disk overlap; for lower
accretion rates they may not. Some low-luminosity AGN, e.g. NGC 4258, may thus
be in a transient phase in which mass is building up in a non-accreting
gravitationally and magneto-rotationally stable ``dead zone.'' We comment on
possible implications of these findings.Comment: Accepted for publication in Ap
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