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\begin{document}


\title{ASTROPHYSICAL JETS: A PHENOMENOLOGICAL EXAMINATION OF 
ACCELERATION AND COLLIMATION}

\author{Mario Livio\\
{\it Space Telescope Science Institute}\\
{\it 3700 San Martin Drive, Baltimore, MD  21218}\\}
\date{}
\maketitle

\begin{abstract}
It is assumed that the acceleration and collimation mechanisms of
jets are the same in all the classes of astrophysical objects
which are observed to produce jets.  These classes now include such objects as:
active galactic nuclei, young stellar objects, massive x-ray
binaries, low mass \mbox{x-ray} binaries, black hole x-ray transients,
symbiotic systems, planetary nebulae, and supersoft x-ray
sources.

On the basis of this assumption, an attempt is made, to identify
the necessary ingredients for the acceleration and collimation
mechanism.  It is argued that: (i)~jets are produced at the
center of accretion disks, and are accelerated and collimated
hydromagnetically, (ii)~the production of {\it powerful\/} jets
requires a hot corona or access to an additional energy source
associated with the central object.
Tentative explanations for the presence of jets
in some classes of objects and absence in others are given.  Some
critical observation that can test the ideas presented in this
paper are suggested.
\end{abstract}

\section{Introduction}

Highly collimated jets are observed in many classes of astrophysical
objects, ranging from active galactic nuclei (AGN) to young stellar 
objects (YSOs).  In the present paper, like in a previous review (Livio 
1997a), I will make the
assumption that the jet formation mechanism, namely, the mechanism
for acceleration and collimation,  is the same in all of the
different classes of objects which exhibit jets.  Adopting a mostly
phenomenological approach, I will then attempt to determine to which
constraints such an assumption can lead.  Previous attempts of a similar
nature were made, for example, by K\"onigl (1986), Pringle (1993),
Blandford (1993; 1998), Spruit (1996), and Livio (1997a). However, with the 
discovery of new classes of objects
which produce jets (see \S~2 below) and with recent developments in
theoretical work, the constraints become more meaningful.  It should be
noted right away that the emission mechanisms which render jets
observable in the different classes of objects, are very different in
objects like, for example, YSOs and AGN.  Here, I therefore concentrate only on
acceleration and collimation.

I should also note that bipolar, only weakly collimated outflows,
are observed in many objects, such as: luminous blue variables,
planetary nebulae, novae in outburst and post asymptotic giant
branch stars.  Models for the formation of these bipolar outflows
exist (\eg Balick 1987; Nota \etal 1995; Paresce \etal 1995; and
see Livio 1997b for a review), but they will not be discussed in
the present paper.

\section{The Disk-Jet Connection}

In this section, I present all the classes of objects which
exhibit jets, and discuss some aspects of the observational
evidence for a connection between accretion disks and jets.

\subsection{Systems Producing Collimated Jets}

In Table 1, I give a list of all the types of objects in which
collimated jets have been observed, and the nature of the
physical system involved.  A few of these objects (symbiotic
stars; low mass x-ray binaries with a neutron star accretor)
require a little explanation, one class (planetary
nebulae), has not yet routinely made it into the jet
literature, another class (supersoft x-ray sources) is relatively
new, and one class (cataclysmic variables) is conspicuously absent.
\begin{table}[t]
\begin{center}
{\bf TABLE 1: Systems Which Exhibit Collimated Jets}
\begin{tabular}{ll}\hline\hline
\multicolumn{2}{c}{\underline{Stellar}}\\\\
\multicolumn{1}{c}{Object} & \multicolumn{1}{c}{Physical System}\\\hline
Young Stellar Objects		&	{\it Accreting} young star\\
\\
Massive X-Ray Binaries		&	{\it Accreting} neutron star or black hole\\
\\
Black Hole X-Ray Transients	&	{\it Accreting} black hole\\
\\
Low Mass X-Ray Binaries		&	{\it Accreting} neutron star\\
\\
Symbiotic Stars			&	{\it Accreting} white dwarf\\
\\
Planetary Nebulae Nuclei	&	{\it Accreting} nucleus\\
& (or ``interacting winds'')\\
\\
Supersoft X-Ray Sources		&	{\it Accreting} white dwarf\\
\\
\\
\multicolumn{2}{c}{\underline{Extragalactic}}\\\\
Active Galactic Nuclei		&	{\it Accreting} supermassive black hole\\\hline
\end{tabular}
\end{center}
\end{table}

Systems which have been traditionally associated with jets are:
many AGN (e.g.\ Marscher 1993; Laing 1993; Wilson 1993) and YSOs 
(e.g.\ Ray \& Mundt 1993; Reipurth \& Heathcote 1993; Bally 1997) 
and some massive x-ray binaries (HMXBs), such
as SS~433 (\eg Margon 1984), Cyg X-3 (\eg Strom, van Paradijs, \&
van der Klis 1989) and the Galactic center source 1E140.7-2942
(Mirabel \etal 1992).  More recently, black hole x-ray transients
have been added as a class (\eg GRS $1915 + 105$, Mirabel \& Rodriguez 1994;
GRO $1655 - 40$, Hjellming \& Rupen 1995).

So far, the only low mass x-ray binary (LMXB) with a neutron star
accretor in which a jet has been observed is Cir X-1 (Stewart
\etal 1993), and even in that case it is not clear how collimated
the flow really is.  

The only symbiotic system in which a jet has been unambiguously observed, 
both in the optical and in the radio, is R~Aqr (Burgarella \& Paresce 1992; 
Dougherty \etal 1995). Spectroscopic evidence suggests the possible presence 
of a jet also in MWC~560 (Hillwig, Livio, \& Honeycutt 1998).

I now turn to the new classes of objects which should, in my
opinion, be from now on routinely included in any discussion of
jets.  In planetary nebulae (PNe), jets have now been directly observed (in 
the optical) in K1-2 (\eg Bond \& Livio 1990; Pollacco \& Bell 1996), in M1-92
(Trammell \& Goodrich 1996), in NGC~6543 (Harrington \& Borkowski 1994), and 
in NGC~7009 (\eg Schwarz, Corradi, \& Melnick 1992; Balick 1997). A spectacular 
two-sided jet (although not particularly well collimated) emanating from the 
center of a disk has been revealed by recent HST NICMOS observations of the 
Egg Nebula (Thompson et~al.\ 1997).  In addition, 
several ``point-symmetric'' PNe have been interpreted as
resulting from precessing jets (Livio \& Pringle 1996, and
references therein and see \S~6).

The newest exciting addition to the classes of objects which
produce jets are the supersoft x-ray sources (SSS).  These are
luminous ($L_{bol} \sim 10^{37} - 10^{38}$~erg~s$^{-1}$) objects,
with a characteristic radiation temperature of $(1-10) \times 10^5 K$ (\eg 
Hasinger 1994; Kahabka \& Tr\"umper 1996; Kahabka \& van den Heuvel 1997), 
in which probably a white dwarf accretes mass from a subgiant companion at
such a high rate that it burns hydrogen steadily (\eg van den
Heuvel \etal 1992).  Recent spectroscopic observations of the LMC
source RX J0513.9-6951 reveal what is probably a bipolar collimated
outflow with a velocity of $\sim 3800$~km~s$^{-1}$ (Pakull 1994; Crampton \etal 
1996; Southwell \etal 1996), through the presence of blue- and red-shifted satellite 
emission features to the optical Hydrogen and Helium recombination lines.  
Similar features corresponding to a projected velocity of $\sim 850$~km~s$^{-1}$ 
have now been observed also in the SSS RX~J0019.8+2156 (Becker, Remillard \& 
Rappaport 1998; Tomov et~al.\ 1998). In the latter case satellite lines to 
P$\gamma$, P$\beta$, and Br~$\gamma$ have also been observed (Quaintrell \& 
Fender 1998).  Such ``jet lines'' may also be present in CAL~83 (Crampton et~al.\ 
1987).  The similarity of the spectral features corresponding to the outflow to 
those observed in SS~433 (\eg Vermeulen \etal 1993) is striking.  

As I noted above, conspicuous in their absence from Table~1 are cataclysmic 
variables (CVs), which in many respects are similar to the SSS and symbiotic 
systems (all involve accreting white dwarfs).  While a suggestion for a 
tentative detection of jet satellite lines (to H$\alpha$) in the CV~T~Pyx has 
been made (Shahbaz et~al.\ 1997), more recent observations have revealed that 
the satellite lines are in fact due to [N~II]~$\lambda\lambda$6548, 6584 formed 
in the expanding nova shell (O'Brien \& Cohen 1998; Margon \& Deutsch 1998). 
I will return to possible reasons for the apparent absence of jets in CVs in 
\S~3.4 and \S~7.

An examination of Table~1 reveals that all the objects which
exhibit jets contain {\it accreting} central objects (some models
for jets in PNe and YSOs do not involve accretion, see \S~2.2, but others
do); this leads us naturally to the question in the next section.

\subsection{Does the Formation of Jets Require an Accretion Disk?}

Clearly a complete answer to this question is difficult, since it
requires both a demonstration that disks can produce jets in all
the different classes of objects and that other mechanisms cannot
produce them.  Since I have adopted a phenomenological approach,
I will rather attempt to answer the simpler question: has an accretion disk
been observed in all of the classes of objects which produce jets?

In the case of YSOs, the answer is clearly: {\it yes}, (\eg Beckwith \& Sargent 
1993; Beckwith 1994; O'Dell \& Wen 1994; Najita \etal 1995) with the most 
dramatic manifestation being the disk and jet recently observed in the 
Herlig-Haro object HH~30 (Burrows 
\etal 1996).  Similarly, disks have unambiguously been observed in all
the classes of x-ray binaries (HMXBs, LMXBs, SSS, and black hole
x-ray transients; \eg van Paradijs \& McClintock 1995; Southwell \etal 1996; 
Schandl, Meyer-Hofmeister \& Meyer 1996; G\"ansicke, Beuermann \& de Martino 
1996). Furthermore, in the case of the black hole x-ray transients it has been 
shown that most likely, the IR and radio emitting plasma is ejected from the 
inner disk (Fender et~al.\ 1997; Mirabel et~al.\ 1998; Eikenberg et~al.\ 1998; 
and see also \S~3.1).  The situation with AGN is somewhat more
frustrating.  Although almost all of the researchers in this
field agree that there are accretion disks in AGN, the evidence
is somewhat circumstantial (\eg Netzer 1992; Kinney 1994), and
every now and then there are even attempts to cast doubt on their
existence (\eg Barvainis 1992).  Here I would merely
like to mention a few recent pieces of evidence for the presence
of disks in AGN, which are fairly convincing.  (i)~The iron
$K\alpha$ line in MCG-6-30-15, which is consistent with emission
from a disk, (Tanaka \etal 1995; Fabian \etal 1995; Iwasawa \etal 1996; but see 
also Reynolds \& Begelman 1997; Young, Ross \& Fabian 1998) and a similar line 
from NCG-5-23-16 (Weaver 
\etal 1997) and other AGNs.  (ii)~The fact that the fit to the double peaked 
Balmer lines in 3C~390.3 with an accretion disk, and the superluminal
motion observed in the same source, give an inclination angle for
the disk and the jet which shows that the jet is exactly
perpendicular to the disk (Eracleous, Halpern, \& Livio 1996).
(iii)~The dust torus observed in NGC~4261 (Ferrarese, Ford, \&
Jaffe 1996), which is remarkably consistent with AGN unification
schemes containing an accretion disk (\eg Urry \& Padovani
1995).   (iv)~The warped subparsec-scale molecular disk observed in the maser 
emitting LINER NGC~4258 (Miyoshi et~al.\ 1995; Neufeld \& Maloney 1995; 
Herrnstein, Greenhill \& Moran 1996). (v)~The fact that velocity-delay
maps of optical and ultraviolet emission lines in objects like NGC~5458
(Wanders \etal 1995; Done \& Krolik 1996) and NGC~4151 (Ulrich \& Horne
1996) appear much more consistent with disk kinematics than with
spherical freefall.

Incidentally, for some time there has been a question whether the
double-peaked Balmer emission lines observed in some (mostly radio-loud)
AGN (Eracleous \& Halpern 1994; Eracleous et~al.\ 1997) originate in an accretion 
disk, or in two line emitting cones (formed by two-sided jets; Veilleux
\& Zheng 1991). However, Livio \& Xu (1997) have shown that at least in the case 
of 3C~390.3 the double-peaked lines cannot be produced in a two-sided jet, 
because the emitting region on the receding jet is expected to be obscured from 
view by the accretion disk, which is optically thick up to radii of $R \sim 
10^{18}$~cm $(M_{\rm BH}/10^8 M_{\odot})$  (e.g.\ Collin-Souffrin \& Dumont 1990; 
see also Livio \& Pringle 1996).  The fact that the red wing of a line produced 
in a two-sided jet may be obscured from view by the accretion disk is well known 
from YSOs (see \eg [$OI$] $\lambda$~6300 profiles for T~Tauri stars; Edwards
\etal 1987). 

In the case of PNe, until recently, only theoretical arguments
for the presence of disks in these systems existed (Soker \&
Livio 1994; Livio \& Pringle 1996).  These relied on the fact
that following a common envelope phase (which is required, to
form the observed close binary nuclei; see \eg Livio 1996 for a
review), the somewhat bloated secondary companion is likely to fill its Roche 
lobe.  The recent spectacular images of the Egg Nebula in the optical and infrared
(CRL~2688; Sahai \& Trauger 1996; Thompson et~al.\ 1997), however, revealed a clear 
dust disk, with jets perpendicular to it. A dust disk has also been observed in 
the ``Red Rectangle'' nebula (Bond et~al.\  1998).

A word of caution is needed in relation to YSOs (and PNe).  While the
presence of accretion disks in the former systems is unquestionable,
some models for the collimation of jets in these systems (and
indeed in PNe), suggest that refraction through oblique shocks is
sufficient to produce highly collimated jets,  without an active
role for the accretion process (\eg Canto \etal 1988; Frank,
Balick, \& Livio 1996; Frank \& Mellema 1996; Mellema \& Frank 1997).  In 
these models, a fast and dilute wind interacts with a
slowly moving or stationary torus in the equatorial plane, and
collimation is achieved via refraction through the oblique shocks
in the interaction region.  Further work on these models will be
required, to establish whether they can indeed produce long-lived,
highly collimated jets.  Here, however, I will not discuss such models further,
since, as explained in the introduction, I am interested in a
universal model for all the classes of objects, while this mechanism (``shock 
focused inertial confinement'') requires the presence of a torus which is not 
expected to exist at least in some of the systems.

To conclude this section therefore, my answer to the question: do
jets {\it require\/} an accretion disk? is: probably yes, although inertial 
collimation deserves more attention.

\subsection{Do Accretion Disks Require Jets or Outflows?}

What I mean by this question is: are outflows/jets the {\it main\/}
mechanism for transport/removal of angular momentum?  The
suggestion that this may be the case has been made by many
authors (\eg Blandford \& Payne 1982; Pudritz \& Norman 1986;
K\"onigl 1989).  The idea here is very simple, the angular
momentum carried away by a disk wind is
\begin{equation}
\dot J_W = \dot M_W~\Omega~r_A^2~~,
\end{equation}
where $\dot M_W$ is the mass loss rate in the wind, $\Omega$ is the
local angular velocity and $r_A$ is the local Alfven radius
(see \S~3).  At the same time, the rate at which angular momentum
needs to be removed from the disk for accretion to occur is
\begin{equation}
\dot J_{acc} = \frac{1}{2}~\Omega~r^2~\dot M_{acc}~~,
\end{equation}
where $\dot M_{acc}$ is the accretion rate through the disk.  If we
require that all the angular momentum is removed by the wind, we
obtain
\begin{equation}
\frac{\dot M_W}{\dot M_{acc}} = \frac{1}{2} \left(
\frac{r}{r_A} \right)^2~~,
\end{equation}
from which it is clear that if $r_A \sim 10r$, then only less than 1\% of
the accreted mass needs to be lost in the wind.  Such mass
loss rates are indeed observed in CVs and YSOs (\eg Drew 1995;
Knigge, Woods, \& Drew 1995; Lizano \etal 1988), so from this
point of view, winds could in principle provide the main mechanism for removal 
of angular momentum.  In particular, external torques (associated with the jet) 
could extract more angular momentum than internal torques (due e.g.\ to internal 
magnetic fields) in systems like SS~433, in which the jets are inferred to be 
more powerful than the underlying accretion disks.

One signature of potential removal of angular momentum by the outflow
would be the detection of rotation in the wind or jet.  Most
models of hydromagnetic acceleration (\S~3.3) predict that the
ratio $V_\varphi/\Omega r_o$ (where $V_\varphi$ is the angular velocity in
the wind and ($\Omega r_o$) is the rotational velocity at the
base of the outflow) should increase approximately linearly with
distance till the Alfven radius (because the magnetic field
enforces rigid corotation), and then decline, while more or less
conserving specific angular momentum.  Indications for rotation
in the disk winds in CVs have been observed in OY Car (Naylor
\etal 1988), and more recently in V347~Pup (Shlosman, Vitello, \&
Mauche 1996).  In V347~Pup in particular, it was found that
during eclipse, the fastest rotating wind is eclipsed.  Evidence
for rotation in jets has also been found in AGN (\eg in NGC~4258;
Cecil, Wilson, \& Tully 1992; or M87, Biretta 1993).  However, it is not easy 
to distinguish observationally between rotation in the jet material
and precession of the jet, which may occur, for example, as a result of a
radiation-induced warping of the accretion disk (Pringle 1996;
Maloney, Begelman, \& Pringle 1996; and see \S~6).

It should be noted, however, that Shlosman \etal (1996) and
Knigge (1995), were able to fit successfully the observed wind
lines in CVs by simply assuming that the wind rotates with the
rotational velocity at the base of the flow ($\Omega~r_o$).
Thus, at present, there is no clear observational evidence for
the type of extraction of angular momentum that is predicted for
hydromagnetically driven winds.

Furthermore, at least in the case of CVs, there exists clear
observational evidence which suggests that winds {\it are not\/} the
main mechanism of removal of angular momentum.  This is related
to the behavior of the disk radius, during dwarf nova outbursts (which are 
caused by a disk instability which results in the sudden accretion of a 
significant fraction of the disk material).  The point is the following, if 
angular momentum is
transported outwards in the disk through viscous processes
(rather than being removed by the wind), then at outburst, since
matter diffuses inwards, the angular momentum of that matter has
to be transferred to the outer parts of the disk, and the radius
is expected to expand (Livio \& Verbunt 1988; Ichikawa \& Osaki 1992), and then 
decrease slowly to its initial size. Observations of the dwarf novae
$U$~Gem and $Z$~Cha in outburst (Smak 1984; O'Donoghue 1986), show that the 
radius behaves exactly as model calculations predict, on the basis of this 
scenario. Furthermore, Harrop-Allin \& Warner (1996) have shown that the
disks in OY~Car, HT~Car, and Z~Cha are all larger in
outburst than in quiescence, which is again consistent with
viscous transport of angular momentum (rather than removal by the
wind).  Coming back, therefore, to the question posed at the
beginning of this section: Do accretion disks require jets
or outflows, as their {\it main\/} mechanism for angular momentum
transport/removal?  In the case of CVs at least the
answer appears to be: no.  More observations of rotation in jets
and bipolar outflows (including velocity gradients across the outflow) are 
needed, in order to settle this question definitively for other classes of 
objects.

\section{Clues on the Jet Formation Mechanism}

Since we have determined that the formation of jets most probably
requires the presence of an accretion disk, we can now
examine some of the properties of jets, in an attempt to
determine which basic ingredients must be associated with the
accretion disk, for the acceleration and collimation mechanisms
to operate.

\subsection{The Jet Origin}

An important conclusion can be drawn from the observed jet
velocities.  In Table~2, I give examples for the ratio
$V_{jet}/V_{escape}$ (where $V_{escape}$ is the escape velocity
from the central object) for the different classes of objects.
It is immediately clear that in {\it all\/} cases the jet velocity
is of the order of the escape velocity from the central object.
This immediately indicates that the jets originate from the {\it center of the
accretion disk\/}, from the vicinity of the central object (see also
Pringle 1993).  This general conclusion has received impressive observational 
confirmation by the HST images of HH~30 (Burrows \etal 1996) and the Egg Nebula 
(Thompson et~al.\ 1997), which show clearly the jet emanating from the center 
of the accretion disk.

Evidence for the fact that jets originate in the inner disk is provided also 
by multiwavelength observations of the black hole x-ray transient (``microquasar'') 
system GRS~1915+105 (Fender et~al.\ 1997; Pooley \& Fender 1997; Eikenberry et~al.\ 
1998; Mirabel et~al.\ 1998; Greiner, Morgan \& Remillard 1996).  These 
observations have demonstrated convincingly that there exists a one-to-one 
correspondence between x-ray and IR (and probably radio) flares, and the 
constant time delay between the x-ray/IR peaks indicates that these are 
triggered by the same event.  This, in turn, implies that initially, the 
emitting regions of the x-ray and IR are in close proximity to each other.  
The fact that subsequently the IR and x-ray emission appear to decouple suggests 
that the emitting regions separate significantly at later times.  A picture in 
which the inner disk ejects a relativistic plasma which produces the IR and 
radio flares by synchrotron emission is consistent with the existing data 
(especially since GRS~1915+105 has actually been observed to eject relativistic 
blobs which produce synchrotron emission; Mirabel \& Rodriguez 1994),
although not all the details have been clarified (e.g.\ Mirabel \&
Rodriguez 1998).

Finally, VLBI observations of M87 (Junor \& Biretta 1995) have shown that the 
jet is quite well collimated (opening angle$\aplt 20^{\rm o}$) already at a 
transverse distance of $l \sim 3 \times 10^{16}$~cm from its origin.  Since the 
mass of the central black hole is $M_{\rm BH} \sim 3 \times 10^9 M_{\odot}$ 
(Macchetto et~al.\ 1997), we find that $l \sim 70 R_g$ (where $R_g = 
GM_{\rm BH}/c^2$ is the gravitational radius).  Thus the jet has to originate 
from the immediate vicinity of the hole.

\small{
\begin{table}[t]
\begin{center}
{\bf TABLE 2: The Ratio of Jet Velocity to the Escape Velocity
from the Central Object}
\begin{tabular}{lcl}\hline\hline
\multicolumn{1}{c}{Object} & $V_{jet}/V_{escape}$ &
\multicolumn{1}{c}{Example}\\\hline
Young Stellar Objects  &  $\sim$ 1  &  HH30, HH34\\
	&	&			$V_{jet} \sim 100 - 350$ km~s$^{-1}$\\
\\
Active Galactic Nuclei	&  $\sim$ 1  &  Radio sources, $\gamma \aplt 10$\\
	&		&		M87, $\gamma \apgt 3$\\
\\
X-Ray Binaries	&	$\sim$ 1	&	SS 433, Cyg X--3\\
		&		&	$V_{jet} \sim 0.26c$\\
\\
Black Hole X-Ray Transients	&  $\sim$ 1   &  GRO 1655-40, GRS 1915+105\\
		&		&  $V_{jet} \apgt 0.9c$\\
\\
Planetary Nebulae  &  $\sim$ 1   &   FLIERS, Ansae, hot winds\\
		&		&   $V \sim 200-1000$ km~s$^{-1}$\\
\\
Supersoft X-Ray Sources   &  $\sim$ 1   &   RX J0513.9-6951, RX J019.8+2156\\
&	&			$V_{jet} ({\rm projected}) \sim 3800$ km~s$^{-1}$,\\
&	&			 $V_{jet} ({\rm projected}) \sim 850$~km~s$^{-1}$\\\hline
\end{tabular}
\end{center}
\end{table}
}
\normalsize

\subsection{Ingredients Which May Not Be Universally Essential for the 
Acceleration and Collimation of Jets}

In Table~3, I list a few properties which at one time or another have been 
suggested as being associated with the formation of jets (I include CVs for
completeness, even though collimated jets have not been observed
from CVs).  It should be remembered that we are considering
mechanisms which can operate in all the classes of objects, and
therefore, any ingredient which has ``NO'' for any of the classes
is considered to be not absolutely essential, unless it can be shown that an
equivalent ingredient is present for that class.  A few words of
explanation are in order.  There is no question that the central
object does not (generally) need to be near break-up rotation, although
models relying on this property have been suggested in the past for YSOs (\eg
Shu \etal 1988; Shu 1991; see however Shu \etal 1994).  Similarly, it is quite 
clear that the central object does not need to be relativistic (the compactness 
of the central object merely determines the escape speed and thereby the jet 
speed).  The question of funnels is somewhat more ambiguous, since one may argue 
that an ion torus or some form of an advection dominated flow in AGN (\eg Rees 
\etal 1982; Narayan, Yi \& Mahadevan 1995), or a torus formed by a slowly moving 
wind in PNe and YSOs (\eg Frank and Mellema 1996; Mellema \& Frank 1997) can 
provide for a form of inertial collimation (see also model for SS~433 by 
Begelman \& Rees 1984 and see discussion \S~2.2).  At present, however, there
is no reason to suspect that a funnel is present, for example, in the SSS.
Furthermore, some of the types of structures proposed in the past are believed to be either 
globally unstable to non-axisymmetric modes (\eg Papaloizou \& Pringle 1984) or 
to generate too much radiation drag to be able to produce the observed (in black 
hole sources) superluminal motions (\eg Blandford 1993). I therefore at present 
do not regard funnels as a necessary ingredient, but more work on this mechanism 
is definitely needed.  It is very clear that the source luminosity does not 
exceed the Eddington luminosity in a number of classes of objects, and therefore 
it is perhaps unlikely that jets are driven by radiation pressure alone, as a 
universal mechanism.  Nevertheless, I should note that it has been shown that 
radiation pressure on resonance lines can accelerate disk winds (e.g.\ Murray, 
Chiang, Grossman \& Voit 1995; Proga \& Drew 1997).
\small{
\begin{table}[t]
\begin{center}
{\bf TABLE 3.  Ingredients Which May Not be
 Absolutely Necessary for the Formation of Jets}
\begin{tabular}{lcccccc}\hline\hline
	& YSOs & AGN & XRBs & SSS & PNe & CVs\\\hline
Central object & NO & NO & NO & ? & NO & NO\\
near break-up rotation\\
\\
Relativistic 	      & NO & YES & YES & NO & NO & NO\\
central object\\
\\
``Funnel''			      & ? & ? & NO(?) & NO & YES(?) & NO\\
\\
$L \apgt L_{edd}$  &  NO  &  NO  &  NO  &  YES  &  YES  &  NO\\
(Radiation pressure\\
driven)\\
\\
Extensive hot &   YES(?) & YES & NO & NO & YES(?) & NO\\
atmosphere\\
(Gas pressure)\\
\\
Boundary Layer & NO(?) & NO & NO/YES & YES(?) & YES(?) & YES(?)\\\hline
\end{tabular}
\end{center}
\end{table}
}
\normalsize

The surrounding gas pressure of the extensive hot atmosphere in
elliptical galaxies has been suggested to be an essential
ingredient in the production of jets in radio loud AGN (Fabian \&
Rees 1995).  However, such an environment certainly does not
exist in some of the classes of objects in Table~3.

The situation with the boundary layer is somewhat more problematic.  A boundary 
layer between the accretion disk and the central object may (in principle at 
least) exist in all of the objects, with the
exception of those containing a black hole (AGN and black hole x-ray
transients).  Pringle (1989), suggested in fact that the
origin of the energetic winds in YSOs is the boundary layer (the
driving being due to shear-generated toroidal magnetic fields).
In view of the fact, however, that the black hole sources (and indeed many 
YSOs, in which the flow is channelled onto magnetic field lines) do not
contain a boundary layer, we have to conclude at this point that
a boundary layer is not an essential ingredient in the formation
of jets.

As a consequence of all of the above (basically, if we are looking for a 
{\it universal\/} mechanism then all the mechanisms in Table~3 are excluded), 
we are now led to examine the formation of jets in the context of what is 
regarded as the most promising model for jet acceleration and
collimation: {\it an accretion disk threaded by a reasonably
ordered, poloidal, large scale magnetic field}.

\subsection{Hydromagnetic Jet Acceleration and Collimation}

The suggestion that accretion disks could generate a magnetically driven outflow 
was first made about 20 years ago (\eg Blandford 1976; Lovelace 1976; 
Bisnovatyi-Kogan \& Ruzmaikin 1976; see also Michel 1973). The fact that in an 
expanding wind the magnetic pressure declines more slowly than the gas pressure 
also means that magnetic collimation is likely to dominate eventually.  Much of 
the work done presently (\eg Spruit 1996; K\"onigl 1997; Ostriker 1997; 
Matsumoto \& Shibata 1997; Ogilvie \& Livio 1998) relies on the seminal model 
of Blandford \& Payne (1982).  Significant progress has also been achieved in 
numerical simulations (\eg Kudoh \& Shibata 1997; Matsumoto 1998; Meier et~al.\ 
1997; Ouyed, Pudritz \& Stone 1997).  The basic idea is that at least some
fraction of the magnetic flux is in open field lines, which form some angle 
with the disk surface.  The magnetic energy density is larger (above the disk) 
than the thermal and kinetic energy densities, and hence the outflowing 
(ionized) material is forced to follow field lines.  Since these lines are 
corotating with their foot points in the disk, material is accelerated by
the centrifugal force like a bead on a wire.  It turns out that the acceleration 
in this model can occur only for an inclination (of the field lines to the 
vertical at the disk surface) larger than 30$^{\rm o}$ (Blandford \& Payne
1982).  Blandford and Payne have shown that for angles smaller than 30$^{\rm o}$ 
there is an effective potential barrier, while for angles larger than 30$^{\rm o}$ 
an outflow can be driven spontaneously (although they recognized the fact that 
in such a case their model of a `cold' wind does not describe the flow in the 
neighborhood of the disk adequately).  More recently, Ogilvie \& Livio (1998) 
have demonstrated (by analyzing the dynamics of a transonic flow in the disk's 
corona) that {\it a certain potential difference must be overcome even for angles 
larger than 30$^{\rm o}$} (see \S~5 for further discussion).

In the magneto-centrifugal model, the acceleration stops at the Alfven surface,
where the kinetic energy density becomes comparable to the
magnetic energy density.  The collimation however, occurs in this
picture, outside the Alfven surface.  Since the gas is no longer
attached to the field lines, the field gets wound up by the
rotation, generating loops which are carried by the outflow to
form a spiral field.  The curvature force exerted by
the toroidal magnetic field on the outflowing material acts in
the direction of the rotation axis, and thus collimation of the
flow by these ``hoop stresses'' can in principle be obtained (\eg
Sakurai 1985; Begelman, \& Li 1994; Heyvaerts \& Norman 1996).
However, magnetic fields which are wound up to the point at which
the toroidal component dominates, are known to be unstable to
kink instabilities (\eg Parker 1979), similar in nature to those
of a twisted rubber band.  Once the instability sets in,
collimation by hoop stresses is strongly reduced.  Since the
effects of kink instabilities have not yet been properly
incorporated in numerical simulations, and therefore, it is not clear how
effective collimation by hoop stresses really is, it is
important to examine other possible collimation mechanisms.

A poloidal magnetic field can also act as a collimator (by exerting a magnetic 
pressure gradient), if the radius of the disk is large compared to the radius of 
the central object, and if the magnetic flux is largest at the outer disk 
(Blandford 1993; Spruit 1994; Ostriker 1997).  For example, for a vertical field 
of the form $B_Z \sim (r^2/R^2_{in}~+~1)^{-1/2}$ (where $R_{in}$ is the radius 
at the inner edge of the disk), good collimation is obtained for $R_{Alfven}
\sim R_{disk}$ (\eg Spruit 1996; K\"onigl \& Kartje 1994; Ostriker 1997). In 
this case collimation is achieved simply because the field lines have a naturally 
collimating shape (e.g.\ parabolas).

\subsection{Additional Observational Consequences of the Models for Acceleration 
and Collimation}

Analytic and numerical models of magnetized disks are now capable of producing 
collimated outflows from the disk center (e.g.\ Kudoh \& Shibata 1997; Ostriker 
1997; Ouyed, Pudritz \& Stone 1997; Ouyed \& Pudritz 1997a,b; Meier et~al.\ 1997). 
The terminal jet velocities that are obtained are generally of the order of the 
Keplerian velocity at the footpoint of the jet (Michel 1969; Begelman \& Li 1994; 
Kudoh \& Shibata 1997; Matsumoto 1998), with a relatively weak dependence on the 
strength of the magnetic field (although for Alfven speeds in the disk corona 
larger than some critical value the jet speed could increase considerably, 
e.g.\ Meier et~al.\ 1997).  As I noted in \S~3.1, the observed jet speeds are 
always of the order of the escape speed.  

A second important consequence of poloidal collimation is that
the minimum opening angle of the jet can be estimated.  For a vertical field 
with a radial dependence of the form above, and assuming that all the
collimation occurs before the Alfven surface (due, for example,
to kink instabilities after the Alfven surface), the minimum jet
opening angle is given by (\eg Spruit 1996)
\begin{equation}
\Theta_{min} \simeq (R_{in}/R_{out})^{\frac{1}{2}}~~.
\end{equation}
Here, $R_{out}$ is the outer disk radius.  In Table~4, I give the expected values 
of $\Theta_{min}$ for all the classes of objects which produce jets (again CVs 
are also included although jets have not been observed in CVs).  An
examination of Table~4 reveals something extremely interesting.
For YSOs, HMXBs, the black hole x-ray transients, AGN, and
$R$~Aqr, the opening angle is indeed very small, which is consistent with the 
observation of highly collimated, powerful jets in these systems.  In
CVs, the opening angle is quite large, which is consistent with
the fact that no highly collimated outflows have been observed
from CVs.  However, in the SSS (and possibly in the PNe) the expected opening 
angle is similar to that in CVs, and yet these classes were found to produce 
jets.  This suggests perhaps that while poloidal collimation is important, it
may not (in its simplest form) represent the entire picture.

I would also like to point out that the images of the disk and
jet in HH~30 (Burrows \etal 1996) are also possibly in mild conflict with
the simple poloidal collimation picture.  These images show that
the jet is collimated to probably much better than 20~AU in
diameter, already at 70~AU from the source (while the disk radius
is $\sim 250$~AU), and there may also be evidence for
recollimation taking place.

\section{The Origin of the Large Scale Magnetic Field}

One of the important questions in relation to the model for
acceleration and collimation described in the previous sections
is what is the origin of the large-scale magnetic field that is
assumed to thread the disk (\eg Begelman 1993).  Two main
possibilities exist:  (i)~the field is advected inwards by
the accreting matter in the disk (\eg Blandford \& Payne 1982;
K\"onigl 1989; Lovelace, Romanova \& Newman 1994; Pelletier \&
Pudritz 1992), or (ii)~the field is generated locally by
the same disk dynamo (Tout \& Pringle 1996) which is responsible
for the disk viscosity (\eg Balbus \& Hawley 1991; Hawley \&
Balbus 1991; Stone \etal 1996; Brandenburg \etal 1995).
\begin{table}[t]
\begin{center}
{\bf TABLE 4:  Minimum opening Angle of the Jet in\\
Poloidal Collimation (see text)}\\
\begin{tabular}{lccc}\hline\hline
Object & $R_{in}$ (cm) & $R_{out}$ (cm) & $\Theta_{min}$\\\hline
YSOs  &  10\rlap{$^{11}$}  &  10\rlap{$^{15}$}  & 0.01\\
XRBs, XRTs & 10\rlap{$^6$} & 10\rlap{$^{11}$} & \llap{$<$~}0.01\\
AGN & 10\rlap{$^{14}$} & \llap{$>$~}10\rlap{$^{17}$} & \llap{$\sim$~}0.01\\
R Aqr & 10\rlap{$^9$} & 10\rlap{$^{13}$} & 0.01\\
CVs & 10\rlap{$^9$} & 10\rlap{$^{11}$} & 0.1\enspace\\
PNe & \llap{$\aplt$}10\rlap{$^{10}$} & \llap{$\apgt$}10\rlap{$^{11}$} & \llap{$\aplt$}0.2\enspace\\
SSS & 10\rlap{$^9$} & 10\rlap{$^{11}$} & 0.1\enspace\\\hline
\end{tabular}
\end{center}
\end{table}

In the first case one might expect a field distribution in which
the vertical field is proportional to the surface density, $B_Z
\sim \Sigma$, (\eg Spruit 1994; Begelman 1993).  If the disk is
standard, then $\Sigma \sim \alpha^{-1} R^{-1/2}
(H/R)^{-2}$, where $\alpha$ is the Shakura \& Sunyaev (1973)
viscosity parameter and $H$ is the disk half-thickness.  If in
addition we assume that $\alpha \sim (H/R)^{1.5}$, as may be suggested
by the observed exponential decays in the outbursts of black hole 
transients (Cannizzo, Chen, \& Livio 1995; Cannizzo 1998), then we
obtain (for a standard disk) $B_Z \sim R^{-81/92}$, which
is very close to the $B_Z \sim R^{-1}$ used in several models
(\eg Ostriker 1997).  Under different assumptions
(\eg that motions in the $r-\varphi$ plane generate a magnetic
diffusivity $\eta_{mag}$), one can obtain $B_Z \sim R^{-\gamma}$
with $\gamma = 3/2~\nu/\eta_{mag}$ (where $\nu$ is the kinematic viscosity; \eg 
Spruit 1994).  The situation may be more complicated if the accretion process is
non-stationary. This could be the case, for example, in the black hole
systems (\eg Blandford 1998). If the black hole has a high spin, then it
can be a powerful source of angular momentum (see \S 5) which can stop the
accretion and expel the material away. In this way a cyclic process may
arise, in which the magnetic field builds up close to the hole until the
inner disk is driven away, from where the process repeats itself.

If the field is generated locally, then one still needs to
explain the origin of the {\it large scale} (length scale of
order $R$) field (which is required for the collimation of the
hydromagnetic wind).  This is required, since the dynamo generated
fields have a length scale only of order $H$ (\eg Hawley, Gammie,
\& Balbus 1995; Matsumoto \& Tajima 1995).  It has been suggested
by Tout \& Pringle (1996), that through reconnection of magnetic
loops, an inverse cascade process results, which leads to the
generation of large scale fields.  Here I will adopt a strictly
phenomenological approach, in an attempt to place constraints on
the required model.  First, it is easy to show (\eg Pringle 1993)
that if: viscosity is generated by a dynamo (and hence $\alpha
\sim B^2_D/4\pi \rho_D C_S^2$, where $B_D$ is the magnetic field in
the disk, $\rho_D$ is the density and $C_S$ the speed of sound),
$B_Z \sim B_\varphi$, and the jet velocity is of the order of the
Keplerian velocity then
\begin{equation}
		\frac{B_Z}{B_D} \sim \left[
		\frac{\dot M_j}{\dot M_{acc}} \frac{H}{R} \right]
		^{\frac{1}{2}}~~.
\end{equation}
Here $\dot M_j$ is the mass loss rate in the jet.  If we now
assume that the large scale field is generated by reconnection of
magnetic loops with a length distribution $n(l) \sim l^{-
\delta}$, then (since the length scale of $B_D$ is $H$ and of
$B_Z$ is $R$; Tout \& Pringle 1996)
\begin{equation}
		\frac{B_Z}{B_D} \sim \left( \frac{H}{R} \right)
		^{\delta -1}~~.
\end{equation}
Combining eqs.~(5) and (6) we obtain
\begin{equation}
		\frac{\dot M_j}{\dot M_{acc}} \sim \left( \frac{H}{R}
		\right) ^{2\delta -3}~~.
\end{equation}

Now, from observations we know that $H/R$ is in the range
0.03--0.3, while $\dot M_j/\dot M_{acc}$ is in the range 0.01--0.3
(\eg Pringle 1993 and references therein).  Therefore, if
reconnection of magnetic loops is indeed the process through which the
large scale field is generated, then irrespective of how the
process works, the length distribution of loops must satisfy
\begin{equation}
		1.7 \aplt \delta \aplt 3.4~~.
\end{equation}

In the particular model of Tout \& Pringle (1996), they obtained
$\delta = 2$, which satisfies condition~(8).

It is also interesting to note that in a simulation of the global
development of MHD turbulence in a disk in which vertical stratification
has been omitted, the magnetic field was found to contain power (in
power spectra) on the largest scales (although it was patchy, with
typical scale $\sim H$; Armitage 1998).

It is of course possible that the large scale field is obtained through a 
combination of local generation and advection. (I should note that under some 
conditions the field of the central star could stabilize at least some fraction 
of the disk against the magnetorotational (Balbus-Hawley) instability; Ogilvie \& 
Livio 1998).

\section{Is the Picture Complete?}

At this point it is appropriate to ask whether there are
additional ingredients (other than the accretion disk, threaded
by a magnetic field) which are necessary, in order to produce
{\it powerful} jets.  This question is, in my opinion,
inevitable, at least because of the following four puzzles (the first two
of which have been known for quite some time): (i)~why are there
radio-loud and radio-quiet AGN? (ii)~What is the difference
between objects like SS~433 (which possibly contains a neutron star) 
and other neutron star LMXBs (of which only 
Cir X-1 has perhaps a jet)? (iii)~Why CVs appear not to produce
jets, while SSS and PN nuclei which are very similar systems do
produce jets?  (iv)~Why do some of the black hole x-ray transients produce
powerful jets while the others appear not to?

In an attempt to answer all of these questions the following conjecture has been 
proposed (Livio 1997a): {\it Powerful jets are produced by systems in which on 
top of an accretion disk threaded by a vertical field, there exists an 
additional source of energy/wind (possibly associated with the central object).}

While the above conjecture has been originally based only on phenomenology, more 
recent theoretical work has given it a more rigorous basis. Ogilvie \& Livio (1998) 
solved for the local vertical structure of an accretion disk threaded by a poloidal 
magnetic field.  They showed that for sufficiently strong fields, stable equilibria (
with respect to the Balbus-Hawley instability) could be found for any value (in 
the range $0 \aplt i < 90^{\rm o}$) of the inclination angle between the magnetic 
field and the vertical to the disk surface.  Most importantly however, by 
analyzing the dynamics of the transonic outflow in the disk corona, Ogilvie and 
Livio showed that a certain potential difference must be overcome even when 
$i > 30^{\rm o}$.  Thus, the launching of an outflow from an accretion disk 
requires a hot corona or access to an additional source of energy, in accordance 
with the above conjecture.  One may attempt to identify the additional 
energy/wind source in the Galactic objects which produce jets; this is done in 
Table~5.

A comment should also be made about x-ray binaries with neutron
star accretors.  In these systems, typically, the inner disk is
radiation pressure dominated.  Since the magnetic pressure cannot
normally exceed the gas pressure, these systems will normally
have $P_{mag} \aplt P_{gas} \ll P_{rad}$.  Consequently, jets will
not be routinely produced by these systems, and a strong
energy/wind source associated with the central object is probably
absolutely necessary.  This may be the reason why only SS~433,
which has supercritical accretion, produces unambiguous powerful jets
among the neutron star x-ray binaries (in fact it is not even entirely 
clear if SS~433 contains a neutron star or a black hole).

I now turn to the extragalactic sources and the long standing
question of what are the differences between radio-loud and
radio-quiet AGN.  This problem has been recently reviewed by
Wilson (1996) and discussed by Blandford \& Levinson (1995) and
Fabian \& Rees (1995).  Generally, there are two classes of
explanations for the fact that powerful jets are found in
radio-loud AGN and not in radio-quiet ones: (i)~the central
engines in radio-louds and radio-quiets are the same, but either
the formation and/or the propagation of the jets is prohibited by
some external circumstances in the radio-quiets.  (ii)~Only the
central engines in the radio-louds are capable of producing
powerful jets.
\begin{table}[t]
\begin{center}
{\bf TABLE 5. Potential Energy/Wind Sources that are Associated with\\
the Central Objects in Jet Producing Galactic Sources}
\begin{tabular}{l|l}\hline\hline
\multicolumn{1}{c}{Object} & \multicolumn{1}{c}{Energy/Wind
Source}\\\hline
Young Stellar Objects & Energy released at magnetosphere/disk interface\\
                      & or at stellar surface\\
SS 433        & Supercritical accretion onto central object\\
Supersoft X-Ray Sources & Hot central star (due to steady nuclear
burning)\\
Planetary Nebula Nuclei & Hot central star (due to nuclear
burning)\\\hline
\end{tabular}
\end{center}
\end{table}

Examples of possibility (i) include the suggestion by Blandford \& Levinson 
(1995), that mass losing stars in spiral galaxies prevent the hydromagnetic 
wind from self collimating (by never becoming super-Alfvenic), and the 
suggestion by Fabian \& Rees (1995) that the gas pressure of the hot atmosphere 
in ellipticals is essential for the production of jets (in some sense this is 
similar to the suggestion of inertial collimation in PNe and YSOs, see \S2.2).  
It is impossible at present to rule out such possibilities.  I should note 
however, that while the central environments of S0 galaxies are generally 
similar to those in ellipticals, few S0 galaxies are powerful radio sources.
At any rate, since my basic assumption has been that the
formation and collimation of jets {\it is the same\/} in all
classes of objects, while these types of scenarios are specific
to AGN only, I will proceed under the assumption that only the
central engines in radio louds are capable of producing powerful
jets (see also Wilson 1996, for a discussion).

Figure 1 (taken from Xu, Livio \& Baum 1998; see also Rawlings 1994, Baum \& 
Heckman 1989) shows the radio luminosity (which is related to $\dot M_j$) as 
a function of the [O~III] $\lambda$5007 luminosity (which is probably related 
to $\dot M_{acc}$; \eg Falcke, Malkan, \& Biermann 1995; Miller et~al.\ 1992).  
The figure shows what appears to be a separation between
radio-louds and radio-quiets, with a correlation between the
radio and [O~III]~$\lambda$5007 luminosities existing in each of
the groups.  It is important to note that the {\it range} in the
[O~III] luminosity (and hence, presumably the range in $\dot M_{acc}$) in the 
two groups is similar for sources at low redshifts ($z \aplt 0.2$; Xu, Livio 
\& Baum 1998). However, radio-loud sources at higher redshifts reach higher 
values of [O~III] luminosity (higher $\dot M_{\rm acc}$).  We may now attempt 
to interpret Fig.~1, in the context of the ideas presented in the
present paper.  First, the existence of a correlation between the
power in the jet and the accretion luminosity is a direct
consequence of hydromagnetic jets driven by accretion (see
eq.~(7)).  Second, the mass of the central black hole determines
the Eddington luminosity, and therefore, the {\it maximum\/} $\dot
M_{acc}$ which an object can have (how far to the right, on the
[O~III] axis, the object can be found).  Hence, only the most massive black 
holes can occupy the upper right portion of the correlation for each
group.  Given the fact that the radio-loud sources at high redshifts extend to 
higher values of $\dot M_{\rm acc}$, this may indicate that the most massive 
black holes in these systems have higher masses than those of the most
massive black holes in the radio-quiets. We now come to the most difficult 
question, namely, which parameter distinguishes the upper group (radio-louds) 
from the lower one (radio-quiets).  In the context of the new conjecture
(Livio 1997a), {\it an additional energy/wind source must be identified}.  A 
natural such source can be provided by the black hole {\it spin}, since the
rotational energy can be extracted from the spinning black hole
by the Blandford \& Znajek (1977) mechanism (essentially
equivalent to a resistor rotating in a magnetic field).  In fact,
the suggestion that the spin of the black hole is what
distinguishes radio-louds from radio-quiets has already been made
in the past (\eg Blandford 1993).  I feel that the new suggestion
made in the present work, on the basis of a universal
mechanism for all the classes of objects, makes the central
black hole's spin an even more attractive possibility (although the magnetic 
field strengths that are required for the Blandford-Znajek process to work 
are somewhat on the high side compared to those expected from disk dynamics; 
Ghosh \& Abramowicz 1997).  I should note, however, that at some level, the 
suggestion of Fabian \& Rees (1995), of an extensive hot atmosphere around 
the nucleus being essential, can perhaps also be regarded as the additional
energy source.

It is beyond the scope of the present work to attempt to explain
why the spin of the central black holes should be different in
powerful radio sources (Fanaroff-Riley class~II, FR~IIs; which are 
found only in ellipticals) than in
spirals (if the black hole spin is indeed the distinguishing
factor), but major galaxy mergers are certainly an attractive
possibility (Wilson 1996). Other possibilities include differences in
the accretion flow (\eg advection dominated flows vs.\ standard disks),
or in the stability properties of the disks against warping (see \S 6).

In discussing relativistic jets from AGNs it is very instructive to make
comparisons with Galactic black hole X-ray transient systems (\eg
Mirabel \& Ridriguez 1997). In an intriguing recent work, Zhang, Cui \&
Chen (1997) used the observed color temperature and flux from a few
black hole transients in an attempt to determine the black hole spin (by
determining the radius of the innermost stable orbit.) While this
determination is far from secure, the result (if taken at face value) is
very interesting.  Zhang \etal found that the spin of the sources
1124+68, 2000+25, and LMC~X--3 (all of which are not known to have jets)
is consistent with zero, while the black holes in 1655--40 and 1915+105
(both of which have powerful jets) are maximally rotating ($a_*=0.93$
and 0.998 respectively, where $a_*=Jc/GM^2$ is the dimensionless
specific angular momentum). Clearly, if confirmed, this finding would be
consistent with the suggestion that the black hole spin provides the
extra energy that is necessary for the production of powerful jets.

\section{Warps and Precession}

High resolution imaging of planetary nebulae has revealed the fact that
about 5\% of them are ``point-symmetric", namely, their morphology
exhibits a point reflection symmetry about their center (\eg Corradi \&
Schwarz 1995; Manchado \etal 1996). Examples of this morphology include
NGC~6543 (Harrington \& Borkowski 1995) and NGC~5307 (Bond \etal 1998).
It has been suggested that the point-symmetic morphology is produced by
episodic ejection of matter into a two-sided wobbling jet (Raga, Canto
\& Biro 1993; Lopez, Meaburn \& Palmer 1993; Soker \& Livio 1994; Livio
\& Pringle 1996).

Similarly, there exists observational evidence that the jets in AGNs are
frequently not perpendicular to the plane of the outer disks, or that
the disk is warped (\eg Ulvestad \& Wilson 1984; Heckman \etal 1994;
Colbert \etal 1996; Schmitt \etal 1997; Herrnstein \etal 1996; and, most
recently, in Cen~A, Schreier \etal 1998).

A mechanism that can cause the jet to wobble (or precess) was suggested
by Pringle (1996, 1997), who showed that an accretion disk which is
irradiated by a central source can become unstable to warping (see also
Petterson 1977; Iping \& Petterson 1990; Maloney, Begelman \& Pringle
1996). Once the disk becomes warped, it starts to wobble and precess,
and in the non-linear regime it can even undergo complete inversion.
Pringle (1997) and Livio \& Pringle (1997) have shown numerically that
radiation-induced warping occurs at all radii satisfying
\begin{equation}
R/R_s\apgt20\eta^2/\epsilon^2~~,
\end{equation}
where $R_s=2GM/c^2$ is the gravitational radius of the central object,
$\eta=\nu_2/\nu_1$ is the ratio of the $(R,z)$ and $(R,\phi)$ shear
viscosities, and $\epsilon=L_*/\dot{M}_{acc} c^2$ ($L_*$ is the
luminosity of the central source). Pringle (1997), Livio \& Pringle
(1996, 1997), Southwell, Livio \& Pringle (1997), and Wijers \&
Pringle (1998) have further shown that jet wobbling and precession can
occur in AGNs, in planetary nebulae, in some SSSs and some X-ray
binaries.

The following point should be noted. In many systems torques other than
radiative may be even more important in producing warps.  For example,
if a fraction $\xi$ of the incident radiative power produces an
instantaneous wind of speed $\upsilon_w$, then the ablative torque will
exceed the radiative torque as long as $\xi\apgt \upsilon_w/c$.
Furthermore, precessing jets may be obtained also as a result of
instabilities in axisymmetric magnetic fields.

\section{Required Observations}

Some critical observations can help to test, and further advance the
ideas presented in this paper.  In particular, it should be noted that
to my knowledge, there is no direct observation which confirms that jets
are hydromagnetically driven. In this respect Zeeman observations of
the maser line in AGNs can prove very useful in placing meaningful
constraints (the field strength can be determined by polarization
measurements).

Similarly, reliable determinations of the collimation length scale and
of the ratio $\dot{M}_{jet}/\dot{M}_{acc}$ can help in constraining
theoretical models.

Multiwavelength observations of the Galactic black hole sources (\eg
Mirabel \etal 1998) also hold the promise of providing information about
the mechanism of ejection of relativistic plasma. Searches for jets in
other black hole systems are also strongly encouraged. Determinations of
more black hole masses and (possibly) spins in AGNs will certainly
provide useful information toward an understanding of the differences
between radio-louds and radio-quiets (Fig.~1).

As was noted in \S 2.1 and \S 3.4 a firm determination concerning the
presence or absence of jets in CVs can have important implications for
the jet production mechanism. In a recent work, Knigge \& Livio (1998)
derived a relation between the accretion rate through the disk and the
expected equivalent width (EW) of the jet lines. The relation is
approximately of the form EW (line)~$\propto \dot{M}_{acc}^{4/3}$. Knigge
and Livio showed that for typical CVs jet lines (if they exist) are
expected to have EWs of a few hundredths to a few tenths of Angstroms.
A recent attempt to find such lines in a few nova-like variables
produced negative results (Hillwig, Livio \& Honeycutt 1998).

\section*{Acknowledgements}
This work has been supported in part by NASA Grant NAG5-6857.

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\newpage

\begin{figure}
\centerline{\epsfxsize=5in\epsfbox{pp1261f1.eps}}
\caption{The total 5~GHz luminosity vs.\ the [O~III] 5007 line
luminosity for a sample of AGNs.  The filled circles represent sources
that are intermediate between radio-loud and radio-quiet. From Xu, Livio
and Baum (1998).}
\end{figure}

\end{document}
