"Theoretical Study of Pulsar Emission and Winds
NSF 1993 - Declined
Below is the abstract (all anyone seems to read in a proposal these days),
the reviews, and the full proposal should anyone care THAT much.
Abstract
The author has worked on pulsar theory since their discovery and discovered
the critical role of nonneutral plasmas in the magnetospheric structure.
He has an explicit model for how discharges take place in the magnetosphere,
how the discharges produce
coherent radio emission (a central unanswered puzzle), and how the electrical
current system is closed (an equally long-standing problem).
It is safe to say that any useful theory of pulsar function will have to
address exactly these problems.
To the PI's knowledge, NO ONE is working on these problems, with the
exceptions of a few foreign investigators who are still trying unsuccessfully
to make an early model work that was invalidated by the PI's work on nonneutral
plasmas.
Clearly NSF should fund the best pulsar theory available, but to fund
zero theory simply because there is now no consensus model
is a failure of mission: pulsar theory is a linchpin issue.
Radio Pulsars have evolved from being an astrophysical surprise to
being key indicators of distances, stellar evolution, plasma interactions
(pulsar winds with ISM, binary companions, etc.).
Moreover some are also sources of very high energy emission.
Despite this important role in observable astronomical phenomena,
very little theory is supported on how they actually function.
To quantify "little," we might compare to active galactic nuclei,
which have been recognized for a comparable period of time,
which have been comparably resistant to detailed theoretical understanding,
and which have arguably comparable importance in that ignorance of how they
work is a major stumbling block, and on which a lot of theoretical activity
is engaged.
End Abstract
Reviews follow: you don't want to read them. They can be summarized as
(1) praise for my past and present work (although I haven't been funded by
NSF since 1985) (2) acknowledgement that the field is dying, and (3) insistence
that I somehow provide an itemized list of what equations and which approaches
I am going to use by the day. Lacking that they are going to snot all over everything
(except the very first).
This fascination with fine detail might make sense if there were dozens of
people doing similar things; then it might be reasonable to try to show
why Curt Michel's method might be superior to Joe Schmuck's. But nobody is doing
anything! So these reviews only make sense to the clueless, and there is
no excuse to be clueless, all you have to do is check out
NSF "support" of Pulsar Theory. Clicking around here is informative.
I can't find more that 2 man-years of anything vaguely connected with
core theory for how pulsar work out of what looks to be 150 man-years of "funding".
But you can find FAT awards! Looks like NSF has circled the wagons rather
than done any belt-tightening. You might check the facts out before listening
to some guy with cow-eyes whining that they can't fund you because they're
so broke. (See below for specifics.)
Notice that all these referees pass themselves off as being right on top
of pulsar theory. If so, they are neither funded (click "NSF "support"...)
nor do they publish
(Search the data base and see how many fundamental theory papers there are!
Pulsar theory is dying because it was contentious for years and the those
people never agreed with one another so they give contemptuous
(and contemptable) reviews. And NSF makes pulsar theorists compete with
the "cheerleading" sciences like "gravitational theory" for which the dim
bulbs in DC will part with any amount of money, hence the mega-fraud
LIGO . Here we have a theory as internally complete
and observationally successful as electromagnetism, but no one is proposing
to spend a couple of humdred million to test Coulomb's law in space!
Gravitiy Probe B has already established the existence of a "black hole."
And they spend it on the ground too!
In this field, you hardly even have to rewrite the abstract from the
last grant.
Like there's lots of data sitting around here that we don't understand.
Nothing wrong with distinguished researchers seeking support for what they
like to do, but it is a little difficult to spy any coherent NSF policy
when big bucks go to studying problematic objects and essentially no money
is spent on one of them most important, and potentially understandable
(we think - it would be possibly even more important if it could be shown
that known physics was insufficient) objects of this century.
But no, NSF wants it to be a beauty contest where you somehow write a proposal
that will charm seven sullen failed ex-pulsar researchers (or smirking kibitzers)
into delivering all "Excellents" (best I did was 2 "Excellents" amd 2 "Very Goods", which
wasn't enought!).
Four out of seven is about all you get.
Trivia question,"what did the project monitor for `And they spend it on
the ground too!' (directly above) write his thesis on?"
And if you want to see what must be very detailed proposals, check out
First this one,
And then this one
These are great people, but it shows that one person can get funded
on track record while another's track record is dismissed.
I did gag on the first line of the second abstract, you have to know you're
a shoe-in to write that.
The Reviews (at last)
Reviewer A:
Modeling the pulsar mechanism---whatever it may be---is an important
endeavor which impacts many areas of astronomy. The proposer is in the
forefront of such research and brings innovative ideas and skill to the
work. The proposed research is sound, and the idea of testing models
of the pulsar wind against high resolution observations is interesting.
I would not place all bets on electron-positron discharges always being
the radiation mechanism, but that should not detract from the
importance of studying these discharges.
Overall Rating: Excellent
Reviewer B:
This proposal's defensive comparison between the state of
AGN research and the work proposed herein is unnecessary
and counterproductive. The P.I. has worked for many years
in the field of pulsar emission and winds, and has written
extensively on the subject. Not surprisingly, he has
identified many projects to pursue, but the proposal, as
written, does not specify how these questions will be
answered, how likely it is that the research will be fruitful
("...if one or another initiative turns out to be impossible
or impossibly difficult, there are very important alternative
research topics that cry out to be examined."), and what
direct relevance the results of this effort will have on
observational studies. I would much rather have preferred
to see a single topic fleshed out in detail, with a
convincing justification for why the proposed investigation
was i) likely to succeed, ii) how it differs from earlier
or ongoing work, and iii) where the work will lead to.
Overall rating: Good
comment: i) naturaly chose subject that are unlikey to pan out, ii) there
is no "ongoing work": pulsar theory is dead because of reviewers like this,
iii) a better buggy whip, what did you expect?
Reviewer C:
It is proposed to carry out theoretical research on radio pulsar
emission, a subfield where interpretation lags phenomenology to an almost
depressing degree. In view of the widespread use of pulsars as
astronomical tools, progress in understanding the emission mechanism
would have major implications thoughout astronomy. There may be
spinoff for plasma physics as well. Dr. Michel is one of a dwindling
band of pulsar theorists who has stuck with these problems. He has
contributed several novel ideas and a major text
to the field. Some progress was made under
his last NSF award towards developing one particular model - a
valuable exercise even for those who doubt its validity as it has provided a
framework in which several observations have been discussed.
The PI proposes to model the discharge presumably through some
kinetic simulations, though few details are presented. The gamma ray
transport is probably straightforward given the magnetic geometry.
What would seem more difficult is the response of the accelerating
electric field to the movement of charge and changes in the external
electromagnetic conditions and dependence upon the surface cohesion
energy, which is now believed to be small enough to allow ions to
flow. It is then proposed to use this local analysis in a global
study of the pulsar magnetosphere.
The PI also proposes to study pulsar winds. Here the description is
also somewhat confusing. It seems as though the plan is to start
from a vacuum wave and then inject charges, following them
electrodynamically, and allowing them to modify the field
self-consistently. My concern about this approach is that it may not
be a good approximation to the case, favored by many researchers, of a
high density wind. Nevertheless, as with the discharge modeling, both
problems are important.
Both problems also look like they will require supercomputer time and it is
good to see that this possibility is mentioned.
The budget is realistic for the research
Overall Rating: not provided (not that NSF cares - one "good" and your're already dead)
Reviewer D:
There is no doubt about the significance of contributions made by the PI
with regard to the interrelated topics of pulsar relativistic winds and emission
mechanisms. If the current proposal is to be evaluated based purely on the PI's
prior research work, as seems to be indicated by the lack of the details in the
proposal, then it necessarily would have to be rated high. There is also no
doubt about the fact that we are in urgent need of detailed emission theories
which not only have a reasonable mechanism of producing the right order of
magnitude of coherent radio emission but also show promise of coming close enough
to observations by incorporating the angle between the spin and magnetic axis
as an essential ingredient in the theory.
However, it is to be supposed that the proposal is also to be evaluated
based on the proposed research, and here, a lack of a clear statement of the
assumptions involved makes the task a difficult one. I am assuming that the PI
proposes to essentially extend the analysis of the pair production and bunching
mechanism outlined in Michel (199]b, in the proposal) using essentially the same
assumptions except for the modifications suggested in article C (summary of the
proposed research). I am forced to do so as these same assumptions are not
clearly stated in the proposal and the reference which apparently outlines them
is not yet out, as has been pointed out by the PI. The analysis referred to
above was a single test particle, one dimensional approach. Furthermore, by the
very nature of the calculational procedure, this analysis gives only an upper-
limit to the number of coherent electron positron pairs which can be produced by
this mechanism. These simplifications are, of course, necessary given the
complicated nature of the problem. The current proposal gives no promise of in
any reasonable way estimating how far such a picture could be from an actual 3D
situation likely to be present around a pulsar. It seems there might yet be a
long journey in this direction before actually making contact with any observable
phenomena as far as radio emission from pulsars is concerned. There seems to be
much more of a promise of making contact with some other observations concerning
pulsars from the current proposal. The non-detection of thermal X-rays from
pulsars coupled with the upper limits on the number of electrons impinging on
the polar cap, from this analysis, seems to be one promising alley.
The prospect of having a self-consistent model for pulsar winds to compare
with observations of eclipsing binary pulsars and pulsars with Be star companions
is certainly exciting. The PI assures that there exists a "well defined model
involved or, failing that, a proper referencing to direct one to where such
statements can be found! It is not very clear whether the pulsar wind cal-
culations are to be implicitly dependent on the results of the discharge
modelling or whether there are to be free parameters here to be fixed by com-
parison with observations. The latter might .be preferable considering the
oversimplifications in modelling the discharge. However, one needs also to
model winds from companions etc. in specific cases, thereby increasing the
number of free parameters in the model. In conclusion, both the lines of re-
search proposed in this project seem extremely desirable and likely to in-
crease our understanding of both radio emission from pulsars as well as their
interactions with their surroundings. One can only hope for clearer eluci-
dation of "specific methods" in subsequent publications arising from this
research!
Overall Rating: Good
comment: where do these people live? I suppose that if I were to do a numerical
integration instead of an eigenvalue expansion that they would have some informed
opinion to add!
Reviewer E:
It is very difficult to judge proposals such as this. The PI has generated
a number of innovative ideas and has certainly spurred others to produce
important research, as well. It seems likely that this will remain true
independent of the presently proposed grant.
The concerns with the proposed work, to which the PI is obviously
sensitive, lie in the likelihood of substantial progress in the research
described. There appear to be two research foci in this proposal. The
first is the PI's forte', 'first principals' studies of magnetospheric
processes. Since pulsar radio emission is grossly non-thermal, studying
this production or connecting global models to observed pulse profiles is
widely (and I feel correctly) held to be a difficult problem with no
clear avenues for progress. The argument that extensive pulsar research
without good understanding of the emission process is an embarrassment
is philosophically true; however, the practical observer must of necessity
move on to the wide and important results obtainable using pulsar
radiation as a tool. My primary concern here is that there does not
seem to be a well defined program for progress -- the cascade bunching
computations seem to be versions of sums that have been extant for some
time (albeit for high energy emission eg. Daugherty and Harding 1982).
The other area noted in this work is the study of pulsar winds
and their interactions. This is indeed a lively topic with important new
observational data providing guidance. However, here fluid computations
seem most relevant for comparison with the observations, unless the proposed
plasma computations are to be directed at X-ray emission from the plasma wind.
This might be a good topic for a RoSAT or ASCA study. Some preliminary results
from this sort of work should in any case be shown in the proposal to help
give a clearer idea of the research goals.
In sum, while the PI will probably create innovative ideas and is
to be lauded for the willingness to persevere at difficult problems, there
is insufficient indications of new definitive research routes to strongly
recommend funding such a risky project. The present fiscal constraints
unfortunately make this an implicit criticism.
Overall Rating: Fair
comment: this person doesn't even know what the NSF ratings mean!
End of Reviews
Proposal follows (unfortunately in ptroff format).AM
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Pulsar Theory
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TABLE OF CONTENTS
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Cover Sheet i
Project Summary ii
I. Table of Contents 1
II. Results from Prior NSF Support 2
1. Funding 2
2. Titles 2
3. Results 2
4. Publications Acknowledging NSF 3
III. Project Description 4
1. State of the Field 4
2. Prospects 5
3. Earlier Ideas 6
4. Longer Background 8
5. Proposed Research 8
A. Modeling the Discharge 8
B. Pulsar Winds 9
6. Observational Opportunities 11
IV. Supporting Data 12
1. Bibliography 12
2. Biographical Sketch 12
V. Budget 19
VI. Current and Pending Support 20
VII. Facilities 21
VIII. List of past collaborators 22
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II. RESULTS FROM PRIOR NSF SUPPORT
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1. This investigator was funded by AST-8511709 for period of two years,
with a one year no-cost extension, after a hiatus of 6 years prior to this
1985 award.
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2. That proposal was entitled "Theoretical Studies of Pulsar Magnetospheres."
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3. The original proposal called for a study of the oblique rotator,
the formation of pulsar winds, the role of pair production in providing
current closure
in pulsar models, and the possible role of extraneous matter
to pulsar activation (i.e., accretion of the ISM).
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The role of nonneutral plasma in shutting down
the "standard model" (aligned rotator)
was established using numerical simulation of how plasma
forms about an aligned rotator
(Krause-Polstorff and Michel, Astron. and Astrophys., 144, 78 [1985];
M.N.R.A.S., 213, 43 [1985])
thanks to a grant from NASA.
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We also examined the possibility that spiral shocks were responsible for
the high disk viscosity in accreting binary x-ray sources
(Ap. J., 279, 807 [1984]).
This model for disk viscosity seems now to have some independent supporters
(Sawada et al., MNRAS, 224, 307 [1981];
Spruit, Astron. Astrophys., 184, 173 [1987]).
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We also examined fall-back from a Type II supernova
(Nature, 333, 644 [1988]) with results similar to a more detailed analysis
by Chevalier (1989).
These suggestions were inspired by a discussion of when the pulsar
from SN1987A might be detected
(with C. F. Kennel and W. A. Fowler, Science, 238, 938 [1987]).
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The issue of pickup from the ISM was addressed and it seems
inconsistent with
the expected intense radiation pressure from pulsars
(Ap. J., 312, 271 [1987]; Arons and Barnard, Astron. Ap., 4, 191 [1983]).
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We noted that the observed circular polarization properties
of pulsars is consistent with curvature radiation
(Ap. J., 322, 822 [1987]).
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Efforts to understand what the pulsar wind interactions are with
the companion of the newly-discovered eclipsing binary pulsar PSR 1957+20
were published in Nature, 337, 237 [1989].
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4. Publications Acknowledging NSF Support
Relativistic Wind Termination: Jets and Synchrotron Nebulae, F. Curtis Michel
The Crab Nebula and Related Supernova Remnants, Univ. Cambridge Press 1985,
M. C. Kafatos and R. B. C. Henry, eds., pp. 55-61
Workshop at George Mason Univ., Fairfax, VA, Oct. 11-12, 1984
Magnetospheres of Pulsars, F. Curtis Michel
Proceedings of the La Londe Les Maures Conference (1986)
Quasiperiodic Pulsars, F. Curtis Michel
Physics Today, 9, October 1986.
Pulsars, F. Curtis Michel
The Encyclopedia of Physical Science and Technology, Vol. II, 403-409 (1987).
Academic Press, Inc., Marvin Yelles Executive Editor
A Pulsar Emission Model: Observational Tests, F. Curtis Michel
Astrophysical Journal, 322, 822 (1987)
The Origin of Millisecond Pulsars, F. Curtis Michel
Nature, 329, 310-313 (24 September 1987)
When Will a Pulsar in SN 1987a Be Seen?, F. Curtis Michel, C. F. Kennel, and William A.
Fowler, Science, 238, 938 (1987)
Pulsar Activation by the Interstellar Medium?, F. Curtis Michel
Astrophysical Journal, 312, 271 (1987)
Electromagnetic Jets from Compact Objects, F. Curtis Michel
Astrophysical Journal, 321, 714 (1987)
"Tertiary" Nuclear Burning: Neutron Star Explosions?, F. Curtis Michel
Astrophysical Journal Letters, 327, L81-L84 (1988).
Neutron Star Disk Formation from Supernova Fall-Back and Possible Observational
Consequences, F. Curtis Michel, Nature, 333, 644 (1988)
Gamma-Ray Bursts from Neutron Star Detonation, F. Curtis Michel, in Nuclear Spectroscopy
of Astrophysical Sources, p. 307 (AIP Conference Proc. 170; AIP New York 1988)
Quark Matter or New Particles, F. Curtis Michel
Physical Review Letters, 80, 677-679 (1988)
On the Formation of Black Holes, F. Curtis Michel
Comments on Astrophysics, 12(4), 191-199 (1988)
Nonneutral Plasmas in the Laboratory and Astrophysics, F. Curtis Michel
Comments on Astrophysics, 13(3), 145 (1989) [based on earlier work]
Nature, 337, 237 (1989).
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III. Project Description
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1. State of the Field
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Pulsar theory is in bad shape.
Some of the established workers are still around
(Arons, Ruderman, Ostriker, Goldreich, Cheng, Benford, etc.)
but working on either
tangential issues or on other topics entirely.
NSF is the major source of funding for pulsar research.
NASA indirectly supports only a small amount,
mainly observational work to assist GRO.
In contrast, NSF supports huge radio-telescope facilities
and large observational programs to gather pulsar data,
much of which remains basically enigmatic even today.
Yet support for even one or two theorists to try to
understand how pulsars work is sporadic and problematical.
The lack of a consensus model is a hindrance, but not a logical one
(e.g., gamma-ray bursters, where there has been an even deeper lack over
about the same time scale).
But lacking such a model opens any proposal to sterile second
guessing about what one should do, when the only reasonable course
is to follow one's own nose.
Sterile because it is clear from the literature that, if someone knows
a better approach, they aren't doing anything with it.
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2. Prospects
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In some circles it seems sage to regard pulsar theory as "impossible,"
citing the many who no longer work in the area.
But erratic funding also serves to drive people elsewhere
(plus embitter the likely reviewers).
In fact, pulsars are probably understood better than AGNs, although
there seems to be comparative hordes of people theorizing about AGNs.
If AGNs are massive black holes powered by accretion from a disk and
surrounded by relativistic plasma that is collimated into jets by
swelling of the inner disk edge, then pulsars are neutron stars with
electron-positron cascades that are produced in the attempt of the system
to neutralize itself, but the system cannot owing to loss of plasma at the
light cylinder and above the polar caps.
The coherent radio emission arises owing to bunching in the cascades
(analogous to air-showers, where a single incident cosmic ray ends up
producing a dense pancake of downward moving particles).
Regardless of whether this model for pulsar action is completely correct,
much less
the one for AGNs, the point is that significant progress has been made
at a time when essentially no funds are being allocated for capitalizing
on this progress.
Such a generic word-sketch for how pulsars work is of little help to
the observers, who need to know what controls the spectra, polarization,
intensity, etc. of the radio emission as well as the high-energy emissions
seen from a small subset of pulsars.
Although high-energy emissions seem sexier at present,
there is no reason to believe that they can be understood in isolation
from understanding the cascade and radio production.
Moreover, the evolution of pulsars, an important key to many questions,
is clouded by the absence of any clear theoretical scaling of how
radio luminosity (which is essentially a "dirt effect" involving only
about 0.001% of the available energy output) evolves as the pulsar slows
down.
The research involved is obvious: one must try to model is some tractable
manner the cascade discharges in an inclined rotator and keep track
of the currents and accumulated charges.
Also we need to determine the energy budget to see how much energy is lost
as gamma-rays, how hot the polar caps are heated, and what is injected into
the relativistic wind (which has observable effects in some binary systems
and in some supernova remnant systems [e.g., Crab nebula]).
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3. Earlier ideas
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Since people stopped paying attention to pulsar theory at different
times, let me explain what is new and different in the above program.
A key paper was that by Goldreich and Julian (1969) which purported
to show that an \fIaligned\fR rotator (magnetized neutron star spinning
about the axis of the magnetic moment) would act as a pulsar.
(In this discussion, the distinction between \fIaligned\fR and
\fIinclined\fR is essential.)
The GJ model, and clones, dominated pulsar theory for the next decade until
the proposer discovered that it would not work.
It took another 5 years
to move from theoretical argument (Michel 1980) to numerical demonstration
(Krause-Polstorff and Michel, 1985a,b).
Basically, a neutron star should shield itself with nonneutral plasma
as a result of cascading to the point that there were no accelerating
potentials capable of driving further cascading.
To salvage the \fIaligned\fR rotator, the proposer suggested that maybe
a circumpulsar disk could reactivate the system,
an idea that apparently generated a certain amount of sub rosa criticism
to which it was, by definition, impossible to reply to.
In all such \fIaligned\fR models,
the issue of \fIinclined\fR rotators was peripheral:
inclination would simply break the axisymmetry and lead to the observed
pulsations instead of a fixed beam.
Some workers (Mestel, Shabata, Beskin et al.) continue to try to make
the \fIaligned\fR rotator "work," but I am unaware of any progress
beyond the cartoon approximation, where currents are drawn as somehow
closed in the vicinity of the light cylinder.
Although electron-positron discharges had been discussed from the very
beginning of pulsar theory (Sturrock 1971), it was thought that
something additional (e.g. the two-stream instability) was needed for bunching
to produce coherent radio emission.
Instead, the cascade itself can produce the bunching (Michel 1991)!
And quantitatively, the right order of magnitude for
coherent radio energy output is obtained.
The state of pulsar theory is so sad that no one has even cited this paper.
If getting the right order of coherent radio emission isn't at least
interesting, what is?
At this point a number of things fell into place.
First, the shielding nonneutral plasma about an \fIaligned\fR rotator,
which derailed the consensus model (GJ) of the time, has an interesting
property: if the system is indeed neutralized, then a dome of trapped
nonneutral plasma above each
polar cap has to extend to "infinity."
But in an \fIinclined\fR rotator, the ponder-motive forces will drive
any such plasma away, certainly beyond the light cylinder distance
(aka wave zone).
Suddenly, \fIinclined\fR became more than a mechanism for pulsations,
it becomes the mechanism for closing the current system (a well-known defect
of the Goldreich-Julian model long before the explanation was understood).
Particles of the opposite sign are simply sent along curved field lines
to the equatorial light cylinder, where they are also driven away by centrifugal
as well as ponder-motive forces.
Thus we have current closure and the mechanism for current production
(the cascades) also provides the coherent radio emission.
All of this is hardly to say that the final definitive theory is in hand.
The trouble with cascades is that they run both directions
(positrons running one direction in the accelerating field,
electrons the other), and
potentially drive lots of particles to the pulsar itself, a heat
source.
Whether this is a true problem is one goal of the research.
But in any case, if this isn't quite the right model it
may well be close, and only by analyzing a close model can one
seem what to change.
In the same way, the GJ model was interesting enough to generate the
research that uncovered the flaw in it, and many of the basic concepts
introduced by it remain unchanged.
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4. Longer Background
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In the past I would go over the previous brief outline with a detailed
analysis, but it is clear from referee reports, which now typically
reflect the reviewer's general impression of the PI and the field
(aka gossip), that
either (1) no one actually reads this far or (2) no amount of factual
information changes anyones views.
Suffice it to say that I have a publication in a refereed journal supporting
each and every assertion above, which doesn't make the assertions right
but at least defensible.
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5. Proposed Research
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Given the paradigm, there are a lot of very obvious things that
need to be done.
I will concentrate on just two initiatives: (A) model the bulk discharge
mechanism and (B) model the relativistic wind.
If these initiatives also seem obvious to the reviewer, please skip to item
(C) below for a summary, otherwise continue.
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A. Modeling the Discharge
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Modeling of the electron-positron discharge in an accelerating electric field
has been done successfully numerically simply by keeping track of
each curvature photon emitted by an electron (or positron) as it
departs from the magnetic field line and eventually pair-produces,
and keeping track of these pairs as they are accelerated in opposite
directions.
For an accelerating field of intensity appropriate to pulsars,
these secondaries quickly become new primaries, leading to an exponential
growth of a bunch co-moving with the original primary.
This numerical work is quasi-one dimensional and a
number of simplifying assumptions
were made, and a report appears in the Proceedings of the Taos Isolated
Pulsar meeting, which is unfortunately not yet out.
The point of these simulations will be to parameterized the regions
about the pulsar in terms of cascading,
namely given a potential #PHI# drop over distance #L#, what currents
will flow and (secondarily) what degree of coherence will result
(the cascading need not go to completion, which is where the self-field
of the bunches equals the accelerating field and the pairs are no longer
separated).
I could go into great detail but there is no evidence that such detail
serves any purpose other than warding off the devastating
criticism that "there were no equations in the proposal."
This line of investigation is obviously do-able, but is non-trivial.
With such a scaling, we can then characterize all regions about a pulsar
into either being static nonneutron plasma, being a discharge region,
being a relativistic flow
(in the form of bunches, averaged), and possibly some regions will remain
essentially vacuum.
Since the rotator must be inclined in this model, the 3D distribution
could be complicated, but it is premature to venture how complicated
(observationally, the regions seem to be narrowly confined).
It is possible that this analysis might return one to essentially existing
models such as the Ruderman-Sutherland model with the discharge originating
at the surface, but the above approach does not prejudice the solutions
by trying to guess them in advance.
In any event, an \fIaligned\fR RS model cannot work, which was the original
assumption.
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B. Pulsar Winds
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Recently Coroniti has followed up on an issue of what happens to
alternating magnetic fields in a plasma wind (a "stripped" wind),
arguing that magnetic reconnection would eventually cause the regions to merge and neutralize the magnetic field.
His resolution of a paradox originally noted in my review (Michel 1982) opens
the door to a fairly complete understanding of what it is that
comes out of pulsars when coupled with the above model.
This is important because
an increasing suite of observational data are coming available.
One of the most promising is the discovery of a radio pulsar in a
Be star system (Johnson et al. 1992).
If this system consists of neutron star in eccentric orbit about
a companion having a disk or matter also in orbit, one will
effectively have a pulsar wind source and a "target" the disk and B star,
with a time varying separation that can be worked out.
This geometry would virtually be a laboratory set up for discovering what
exactly it is that comes out of a pulsar.
Additionally, the eclipsing systems provide cases where the pulsar wind
interacts with ordinary matter at close quarters as in the cases of
PSR 1957+20 and PSR 1744-24A.
Interpretation of these systems has not proven straightforward.
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What needs to be done with such models?
Consider the magnetic dipole radiation from
pulsars in the interstellar medium.
These wave frequencies are below the plasma frequency of
the interstellar medium (#approx# 5 kHz) so these waves cannot propagate
through the interstellar medium, yet the waves cannot very well push the
interstellar medium away indefinitely either.
In CTB-80 and in the #H sub alpha# nebula around PSR 1957+20,
we could be witnessing the steady-state termination of a pulsar wind,
and in the cases of PSR 1744\-24A and the Be star pulsar
PSR 1259\-63 we have apparently examples of wind/wind interactions.
Thus we can identify several important research questions,
all of which are interrelated to some extent:
.sp
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1) Propagation of a mixed wind/wave.
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2) Pulsar wind termination.
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3) Interaction of a pulsar wind with a companion.
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4) Plasma pickup by wind/waves.
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We could go on in some detail about each of these issues, but the important
point is that until one has a clear idea of what the wind IS,
one cannot do too much, and the PI's model, for better or for worse, will
be quite specific about what the wind is.
Thus we have a well-defined model and goals, and the specific methods
are irrelevant unless the reviewer is unwilling to credit the PI for
the numerous innovations developed in the past.
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C. Summary of Proposed Research
.sp
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Let me pull together this commentary into a brief list.
A. We propose to use and extend the 1-dimensional simulation of the pair discharge as follows:
1. Determine the time-averaged densities, electric fields, and scaling
laws so the the discharge can be approximated with analytical models.
2. Include the shielding of the accelerating field by the space charge
of the bunches themselves.
3. Incorporate this "mean-field" picture into the electrodynamics of
a slightly inclined rotator
(treating the orthogonal dipole component as a perturbation).
This is a tall order and depends in part on the success of the wind modeling
work that follows in (B).
B. We propose to work out the wind/wave paradigm where the large amplitude
EM waves drive away nonneutral plasma and the plasm drags out the aligned
component.
Specifically,
1. Do a simplified version where plasma is injected artificially at
the wind zone.
Here the trajectories are relatively simple and it may be possible to set up an
iterative scheme starting with the vacuum ("Deutsch") fields, correcting
them for the plasma space charge and currents, and iterating to some
approximate self-consistent result (the idea is more to get trends than
precise solutions).
2. Model nonneutral plasma transport.
We can use the Krause-Polstorff code (which converges to a fixed dome and
torus geometry once all the available surface charge on the neutron star
is exhausted) and introduce at some point in space a mathematical pair
creation which adds equal charge to both dome and torus.
This simulation would determine how the dome evolves when new charge is
added and one can track the solutions to determine what the particle flux
is.
3. Extend this model to interface with item 3
of the discharge research
(the regions of pair creation can be introduced to conform to the actual
electric and magnetic fields, while the ponder-motive force can be approximated
to be spherically or cylindrically symmetric, obviating the need for
a third dimensional degree of freedom.
4. Calculate how this wind might be modified by plasma pickup,
termination in the ISM, or interaction with companion objects.
5. Compare where possible with observation
(see section 6, which follows).
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These are the obvious sorts of approaches one would anticipate simply given
the nature of the paradigm.
Clearly obstacles will appear as well as better ideas inspired by just
getting into the problem and definiting it more precisely.
Theorists can no more predict how they will end up treating a complex
physical problems than observers can predict what they will discover.
The reviewer will well note that there is more here than can likely be
accomplished in 3 years, but the point is that if one or another initiative
turns out to be impossible or impossibly difficult, there are very important
alternative research topics that cry out to be examined.
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6. Observational Opportunities
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Can we glean more from the available or obtainable data?
The answer is presumably "yes."
We have recently completed a joint departmental effort with R. J. Dufour
and a student to analyze polarized CCD images of the Crab Nebula to see
what information about the so-called
"wisps" can be inferred, and what they can tell us about the pulsar wind/wave
(Michel, Scowen, Dufour, and Hester 1991).
This work illustrates the type of fruitful interactions that can take
place between observers and theorists.
Given that the Crab Nebula has been so extensively studied,
one might wonder how anything "new" can be added.
The answer is (1) technology which with CCD photometry and
IRAF data reduction one can quantitatively analyze and intercompare
entire fields in different frequencies, and (2) almost none of the
detailed structure of the nebula has been used to
constrain any of the theoretical models.
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Data from new sources, particularly with
improved optics on HST should importantly increase search
sensitivity.
One can hope that more examples of interactions of pulsar winds
with the interstellar medium will crop up, as was found for
PSR 1957+20, PSR 1744\-24A, the pulsar in CTB-80, and the Be star pulsar
PSR 1259\-63.
We need to understand the non-observations just as much as we need
to understand this one (so far) detection.
In a sense, this section is a sort of "virtual" proposed research
given that we are on the one hand developing a specific theoretical
models while on the other hand the observers are providing a wealth
of observational information that will either fit or fail to fit
the modeling.
Given that the two would proceed hand in hand one can't be very specific
about what opportunities will arise or how connections might be made.
Of course if there is no one doing theory there is no possibility of
synergistic interaction.
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IV. Supporting Data
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1. Bibliography
.sp
Al'bert, Ya. I., Krotova, Z. N., and Ehjdman, V. Ya. 1975, \fIAstrfizika\fR, \fB11\fR, 28 (translated in \fIAstrophysics\fR, \fB11\fR (No. 2), 189).
.sp 0
Arons, J. 1981, \fIAp. J.\fR, \fB248\fR, 1099.
.sp 0
Cheng, K. S., Ho, C., and Ruderman, M. 1986, \fIAp. J.\fR, \fB300\fR, 500.
.sp 0
Cheng, A. F., Ruderman, M., and Sutherland 1976, \fIAp. J.\fR, \fB203\fR, 209.
.sp 0
Chevalier, R. 1989, \fIAp. J.\fR, \fB349\fR, 847.
.sp 0
Coroniti, F. V. 1990, \fIAp. J.\fR, 349, 538.
.sp 0
Deutsch, A. J. 1955, \fIAnn. d'Astrophysique\fR, 18, 1.
.sp 0
Epstein, R. I. 1985, \fIAp. J.\fR, \fB291\fR, 822.
.sp 0
Fawley, W. M. 1978, University of California at Berkeley Ph. D. Thesis.
.sp 0
Goldreich, P., and Julian, W. H. 1969, \fIAp. J.\fR, \fB157\fR, 869.
.sp 0
Johnson, S., Manchester, R. N., Lyne, A. G., Bailes, M., Kaspi, V. M., Guojun, Q. and Lucy, L. B. 1992, \fIAp. J. Letters\fR, 387, L37.
.sp 0
Kennel, C. F., and Coroniti, F. V. 1984a, \fIAp. J.\fR, 283, 694.
.sp 0
Kennel, C. F., and Coroniti, F. V. 1984b, \fIAp. J.\fR, 283, 710.
.sp 0
Krause-Polstorff, J., and Michel, F. C. 1985a, \fIAstr. Ap.\fR, 144, 72.
.sp 0
Krause-Polstorff, J., and Michel, F. C. 1985b, \fIM.N.R.A.S.\fR, 213, 43p.
.sp 0
Lyne, A. G., and Manchester, R. N. 1988, \fIM. N. R. A. S.\fR, 234, 477.
.sp 0
Mestel, L., Robertson, J. A., Wang, Y. -M., and Westfold, K. C. 1985, \fIM.N.R.A.S.\fR, \fB217\fR, 443.
.sp 0
Michel, F. C. 1969, \fIAstrophys. J.\fR, 158, 727.
.sp 0
Michel, F. C. 1977a, \fIAp. J.\fR, \fB213\fR, 836.
.sp 0
Michel, F. C. 1977b, \fIAp. J.\fR, \fB214\fR, 261.
.sp 0
Michel, F. C. 1977c, \fIAp. J.\fR, \fB216\fR, 838.
.sp 0
Michel, F. C. 1980, \fIAp. Space Sci.\fR, \fB72\fR, 175.
.sp 0
Michel, F. C. 1982, \fIRev. Mod. Phys.\fR, \fB54\fR, 1.
.sp 0
Michel, F. C. 1985, \fIAp. J.\fR, \fB290\fR, 721.
.sp 0
Michel, F. C. 1991a, \fITheory of Neutron Star Magnetospheres\fR, (University of Chicago Press: Chicago).
.sp 0
Michel, F. C. 1991b, \fIAp. J.\fR, 383, 808.
.sp 0
Michel, F. C., and Dessler, A. J. 1981, \fIAp. J.\fR, 251, 654.
.sp 0
Michel, F. C., Scowen, P. A., Dufour, R. J., and Hester, J. J. 1991c,
\fIAp. J.\fR, 368, 463.
.sp 0
Ruderman, M. A., and Sutherland, P. G. 1975, \fIAp. J.\fR, 196, 51.
.sp 0
Shibata, S. 1991, \fIAp. J.\fR, 378, 239.
.sp 0
Sturrock, P. A. 1971, \fIAp. J.\fR, \fB164\fR, 529.
.sp 0
Sturrock, P. A., Harding, A. K., and Daugherty, J. K. 1989, \fIAp. J.\fR, \fB346\fR, 950.
.DE
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2. Biographical Sketch
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F. C. Michel, Principal Investigator
.sp
.ti 3
The PI has had a long-term and
pervasive interest in the properties of neutrons stars
and their magnetospheres.
Other research interests of the PI,
such as the fate of neutron
stars pushed over their mass limit, the nature of the binary eclipsing pulsar
PSR 1967+20, infall of matter in SN 1987A, etc.,
which will not be discussed here.
The PI's research has generally been targeted on those issues that promised
to be resolvable through fundamental theoretical analysis.
Moreover, the huge data base available on radio pulsars (over 400)
provided the essential service of underscoring what properties are
ubiquitous to pulsars and which are individual peculiarities.
.sp
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The PI, in collaboration with Dick Wolf and Wallace
Tucker, were among the first to propose a neutron star model
for the newly discovered radio pulsars
[Goddard Institute, 1968] (Tommy Gold also proposed a neutron-star model,
Jerry Ostriker proposed "spots" on white dwarfs, etc.).
This launched an effort for the PI to understand more clearly what was happening
in neutron star magnetosphere (the PI's immediately
previous work involved solar wind
interactions and the Earth's magnetosphere).
Goldreich and Julian's 1969 paper seemed to contain the essential gist
of why radio pulsars might be dynamic (centrifugal slinging of plasma
away from the neutron star), and the PI spent several years trying to
mold this model into an internally self-consistent form
(most other theorists chose to concentrate on phenomenological ideas
of what was happening in the magnetic polar caps to explain the coherent
radio-emission).
The discovery of binary X-ray pulsars, gamma-ray bursters, etc. hardly
went unnoticed and the author devoted considerable theoretical effort
to the issue of how plasma can gain access through the magnetosphere of
an accreting X-ray pulsar (e.g., Michel 1977a, b, c).
The PI did no work whatsoever on gamma-ray bursters at that time
because he had no idea what they might be, whereas the other issues
were fairly well defined.
By the early 80's it became clear why it was so difficult to construct a
self-consistent pulsar model: a rotating neutron star need not fill its
magnetosphere, so there is no plasma to spin off and one instead has "quiet"
solutions of seemingly no interest to pulsar function
(Michel 1980, Krause-Polstorff and Michel 1985a, b).
As with most scientific progress, this resolution was implicit in the works
of a number of workers, starting with Holloway and continuing with Rylov,
Jackson, Ruderman and Cheng, etc.
(as reviewed at an early state in Michel 1982).
.sp
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The Goldreich-Julien model had performed the service of concentrating
theoretical effort (hitherto scattered),
but now that a new model was needed, it was not
quite clear what direction go.
The PI and Alex Dessler decided to explore disk models (1981),
while other theorists sought to see if the Goldreich-Julian model might
be resurrected in some form.
The disk model, however, provided an interesting paradigm in which an
old pulsar might become a gamma-ray burster, and for the first time
the PI had something specific to work with (Michel 1985).
Although this paper is probably considered by most as simply a "disk
model for gamma-ray bursters" (Epstein and others published related models
at around the same time), a central conclusion was that such neutron star
had still to be strongly magnetized.
However, many in the radio pulsar community had become convinced that
neutron star magnetic fields decay away.
If so, it would be very difficult for neutron stars to
impulsively accrete enough material because the magnetic fields would
be too weak to "grab" onto the required mass.
This led to a considerable effort at tracking down how the data
was interpreted to give magnetic field decay.
.sp
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Returning to the question of pulsar action,
the PI tried to understand theoretical arguments that
pair-production discharges might work to
refill the magnetosphere and revive the Goldreich-Julien model
(Mestel et al. 1985, Shabata 1991).
Recently, we have focussed on an alternative that seems attractive.
Rather than fill the magnetosphere, we examine the consequences of
it being chronically depleted.
Such a system permits pair-production with an entirely different consequence,
namely of permitting pair-production avalanches that automatically
create an outgoing wind and form intensely charged bunches capable of
radiating coherently.
More importantly to the present proposal, the behavior and geometry
of the gamma-ray emitting electrons is highly constrained.
Thus the pattern and spectrum of gamma-ray emission,
which is largely a mystery if one does not even know where
or how the radio emission is formed,
now appears accessible to theoretical analysis.
.sp
Selected Publications (by F. C. Michel unless otherwise noted)
The Quiet Aligned Rotator, Astrophys. Space Sci., 72, 175 (1980).
.sp
The Power-Law Spectrum of Shock-Accelerated Relativistic Particles,
Astrophys. J., 247, 664 (l98l).
.sp
Theory of Pulsar Magnetospheres,
Rev. Mod. Phys., 54, 1 (1982).
.sp
Relativistic Charge-Separated Winds,
The Astrophysical Journal, 284, 384-388 (1984).
.sp
Cosmic Ray Acceleration by Pulsars,
Advances in Space Research, 4, 387-391 (1984).
.sp
Relativistic Wind Termination: Jets and Synchrotron Nebulae,
The Astrophysical Journal, 288, 138-141 (1985)
.sp
Gamma-ray Bursts from Remnant Neutron Star Disks,
The Astrophysical Journal, 290, 721-727 (1985)
.sp
J. Krause-Polstorff and F. C. Michel, Pulsar Space Charging,
Astronomy and Astrophysics, 144, 72-80 (1985).
.sp
J. Krause-Polstorff and F. C. Michel,
Electrosphere of an Aligned Magnetized Neutron Star,
Monthly Notices of the Royal Astronomical Society, 213, 43p-49p (1985)
.sp
Pulsar Activation by the Interstellar Medium?
Astrophysical Journal, 312, 271 (1987)
.sp
A Pulsar Emission Model: Observational Tests,
Astrophysical Journal, 322, 822 (1987)
.sp
The Origin of Millisecond Pulsars,
Nature, 329, 310-313 (24 September 1987).
.sp
Is PSR1957+20 eclipsed by a comet, magnetosphere or particulate cloud?
Nature 337, 237 (1989).
.sp
Eclipse Models [for PSR 1957+20]
Ann. New York Acad. Sci., 571. 424 (1989).
.sp
F. Curtis Michel, P. A. Scowen, R. J. Dufour, and J. J. Hester,
Observations of a Pulsar Wind: CCD Polarimetry of the Crab Nebula,
Astrophysical Journal (in press).
.sp
F. C. Michel, J. Bland Hawthorn, and A. G. Lyne,
Quasi-linear Response in a Glitching Pulsar,
Monthly Notices of the Royal Astronomical Society (to be published).
.sp
Statistical Search for Magnetic Field Decay, Magnetospheric Structure and Emission Mechanisms of Radio Pulsars (IAU Colloquium No. 128) 17-23 June 1990, Lagow, Poland.
.sp
L. Shier and F. C. Michel, Modeling of Pulsar Polarization,
Magnetospheric Structure and Emission Mechanisms of Radio Pulsars
(IAU Colloquium No. 128) 17-23 June 1990, Lagow, Poland.
.sp
\fITheory of Neutron Star Magnetospheres\fR, University of Chicago Press, (January 1991).
.sp
F. C. Michel,
Formation of Dense Charged Bunches in Vacuum Gaps,
Astrophysical Journal, 383, 808, (1991)
.sp
F. C. Michel,
Evolution of Pulsar Magnetic Fields,
Publications of the Astronomical Society of the Pacific,
103, 770 (1991).
.sp
F. C. Michel and C. D. Dermer,
Pulsar Emissions,
Nature, 356, 483 (1992).
.sp
F. C. Michel,
Self-Consistent Pair-Production Discharges,
Proceedings of the Taos conference on Isolated Pulsars.
.sp
F. C. Michel,
Pulsars with Planets?
Proceedings of the Taos conference on Isolated Pulsars.
.sp
S. J. Sturner, C. D. Dermer, and F. C. Michel,
Resonant Compton Scattering in the Magnetospheres of Radio Pulsars
(in preparation).
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V. Budget
Foreign Travel justifcation.
The proposer has spent a year in France as a Guggenheim fellow and a
year in Germany on a senior US scientist fellowship from the Humboldt
foundation.
Consequently I get frequent invitations to conferences in Europe in addition
to the not-uncommon international conferences.
For example, I attended the 1990 Pulsar conference in Poland at my personal
expense and was invited to give an invited talk on gamma-ray bursters
at a Max-Planck society meeting at Ringberg castle in September 1992.
The castle is owned by the society and devoted to an annual round of
special conferences of one week duration; I was unable to attend owing to
teaching responsibilities.
As an unfunded researcher, my rule of thumb is to mainly limit attendence to
conferences at which I am invited to give a talk.
The request for foreign travel is a generic one in which to respond to the
typical one or two invitations per year owing to contacts in Europe,
which are invaluable in maintaining those contacts.
Plus the inevitable international meeting of interest, such as the July 1993
meeting again in Poland (not relevant, of course, to this proposal).
In practice it is difficult to identify conferences 4 years in advance
because they are neither announced nor organized that far in advance.
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VI. Current and pending support
.ce
Current
NGT-50569 NASA
Student training grant (No PI support)
Theoretical Interpretation of Gamma-Ray Burst Spectra
ends 6/30/1993;
.ce
Pending
This proposal
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VII. Facilities
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We have timesharing through SUN 3/50 and 3/280 link through ethernet.
This service is sufficient for most of our purposes and
we can link to the Rice University computing center
(ICSA, Institute for Computing Services and Applications)
as necessary for very large jobs on their IBM ES/9000.
NSF funding would also permit use of the NCAR Cray for production jobs
and/or separate requests to the new Supercomputer initiative.
R. J. Dufour routinely reduces CCD and digitized plates using the IRAF system.
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VIII. List of past collaborators (within last 5 years)
.sp
.DS
J. L. Burch
C. D. Dermer
A. J. Dessler
R. J. Dufour
R. C. Elphic
J. W. Freeman, Jr.
J. B. Hawthorn
J. J. Hester
T. W. Hill
R. R. Hodges
A. G. Lyne
R. H. Manka
D. J. McComas
C. T. Russell
P. A. Scowen
L. Shier
S. J. Sturner
.DE
\"NSF/nsf93
\"27 Jan 1993
\"Edward G. Schmidt, Stellar Astronomy and Astrophysics