DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

pathline
which could have led to the formation of the
patera.
pathline A line drawn through a region of
fluid that indicates the path followed by a particular
particle within the fluid.
P-Cygni profile An emission/absorption line
profile which is the result of an expanding shell
of gas around a star; the star P Cyg is the prototype.
Its visible spectrum shows lines that have
a wide emission component and an absorption
component, with the absorption blue shifted and
the emission line approximately at the star’s rest
velocity. A simple explanation is that the absorption
arises in material in front of the star
along our line of sight toward the continuum
source. Since this material is expanding directly
at us, we see the absorption line blue shifted with
respect to the star’s rest frame. The part of the
expanding gas that is moving at right angles to
our line of sight, on either side of the star, withoutthestellarcontinuuminthebackground,produces
emission lines that are roughly centered at
the star’s rest velocity. The degree in which the
absorption portion is shifted depends on the velocity
of the gas that is producing the lines, and
thus these lines allow the measurement of wind
velocities in astronomical objects. The winds in
OB stars and in cataclysmic variables were originally
detected through the observation of these
line profiles in their spectrum.
Peclet number A dimensionless number
quantifying the importance of advective heat
transfer. It is the ratio of heat flux by advection
to heat flux by conduction. For example, if the
characteristic flow velocity in the direction of
heat conduction isv over a characteristic length
L, the Peclet number is defined asPe=vL/κ,
where κ is the thermal diffusivity. If Pe≫ 1,
heat transfer is mainly advective. If Pe≪ 1,
heat transfer is mainly conductive.
peculiar motion In astronomy, the physical
or angular velocity of a source with reference to
afixedframeofinterest. Forstars, thetransverse
peculiar motion is typically given in seconds of
arc per century. For extragalactic objects, transverse
motion is not measurable, but the radial
© 2001 by CRC Press LLC
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peculiar motion is measured by means of red
shifts, and reported in terms of km/sec.
peculiar velocity See peculiar motion.
peeling property The Weyl tensor (a form
of the curvature tensor) of an asymptotically
flat space-time, when expanded in power series
of the affine parameter r of outgoing null
geodesics, has the form
Cijkl = Nijkl
r
IIIijkl
+
r2 IIijkl
+
r3 + Iijkl
+O
r4 1
r 5
where the coefficients Nijkl , IIIijkl , etc. are of
the Petrov type indicated by the kernel letter.
In particular, gravitational radiation encoded in
Nijkl has an amplitude falloff ofr −1 . See Petrov
types.
penetrative convection Sinking plumes in
a convectively mixing surface layer (or rising
plumes in a convectively mixing bottom layer)
impinge with their characteristic velocity scale
onto the adjacent stratified fluid. As the plumes
have momentum, they reach a limited penetration
of the depth scale before the plumes stop
due to entrainment and buoyancy. For instance,
substantial, convective motions are present in
the upper layer of the sun. At the base of the
convection zone there is a gradual transition to
the stable, radiative interior, the convective penetration
zone. Below it, energy is transported
outwards entirely by radiative diffusion. At the
penetrative convection depth, the gas becomes
suddenly less opaque, becomes unstable, and
convective transport becomes dominant.
Penman equation (or combination equation)
Evaporation (E) can be estimated by combining
mass-transfer and energy-balance approaches in
a theoretically sound and dimensionally homogeneous
relation:
E = s (Ta) (K + L) + γKEρwλvua [esat (Ta)] (1 − RH)
ρwλv s (Ta) + γ
where s(Ta) is the slope of the saturation vapor
pressure vs. air temperature (Ta), K is net shortwave
radiation, L is net longwave radiation, γ is