Reasons for considering the Gum nebula a fossil H II region are presented. Arguments for different types of fossil regions are given. Possible research projects are enumerated

Cox, D. P.

English

NASA Technical Reports Server

A NOTE ON THE POSSIBLE IMPORTANCE
OF THE GUM NEBULA
D. P. Cox
Department of Physics
University of Wisconsin
Madison, Wisconsin 53706
I would like at this time to develop my first astronomical catalog to which
future generations of Gum Nebulists may refer. It is a catalog of
Theories and Vested Interests
C-1 There are a number of people for whom it is important that the Gum
Nebula be some kind of fossil H II region, directly and almost wholly
associated with the Vela X supernova remnant. Two groups come to mind:
C-IA is the group who have studied the region and proposed the possi-
bility (Brandt, Stecher, Crawford, Maran, Alexander, . . .);
C-IB is the group who have proposed the time-dependent theory for heat-
ing and ionization of the interstellar medium, and who are studying
the possible consequences on growth of instabilities and star for-
mation (Bottcher, McCray, Dalgarno, Jura, . . .).
C-II There are several groups who are more particular about the kind of fossil
H II region they want:
C-IIA formed by the immediate outburst of UV radiation from the SN
explosion (Morrison, Sartori, Kafatos, . .)
C-IIB formed by the gradual emission of UV radiation from the expanding,
shock-heated remnant (Tucker, me*)
C-IIC formed by the immediate release of low-energy cosmic rays from
the SN explosion, cosmic rays which are able to spiral along the
magnetic field lines without sweeping those lines into a wall and
thereby be confined within a shock, and which instead are able to
* Ph.D. thesis, Lc Jolla 1970, p. 105.
166
https://ntrs.nasa.gov/search.jsp?R=19720004120 2020-03-23T13:40:35+00:00Z
escape beyond the mechanical shock front to penetrate and ionize
the medium beyond. These cosmic rays, although released immedi-
ately, require a substantial length of time to ionize the nebula be-
cause of their low velocities (Ramaty, Colgate, Boldt, Silk, . . .).
CIII One final group is not so much concerned with the remnant, but instead with
the possible relationship between the pulsar(s), high-velocity stars, and
the nearby B-associations in this region (Gott and Ostriker).
It seems to me that there are really two fundamental questions which need
to be answered by the students of the Gum Nebula in order to make some of
these catalog entries retire to the realm of pure historical interest:
(1) To what degree and scale is the ionization of the Gum Nebula region
really due to the occurrence of one supernova event ?
(2) What evidence exists which would indicate the mode or modes of energy
transport from the SN to the interstellar medium, and the amount of
energy transported ?
In this regard, I think a two-step cooperative process between observers
and theoreticians is required:
(1) Detailed observations of the 5°-diameter supernova remnant proper,
coupled with a complete and credible model of the remnant which is
capable of explaining the entire presently-observed spectrum, must
be made.
(2) Detailed observations of the much larger H II region outside the
remnant must be made over the complete spectral range. In addition,
the model of the present remnant must be extrapolated backward in
time to get its spectrum at earlier epochs. The necessary conse-
quences of this earlier radiation on the surrounding interstellar medium
must be subtracted from the presently "observed" distribution of
ionization, temperature, cosmic rays and magnetic field to discover
those residual effects attributable to other possible modes of energy
transport from the SN explosion, not included in the model of the
present remnant.
There are several measurements of the Gum Nebula which seem potentially
very useful:
(1) We need a good series of spectra of some of the optical filaments in the
SNR proper. There is some chance that these spectra can be matched
167
by calculations of the cooling behind a shock wave, such as I have per-
formed for the Cygnus Loop. If so, this might help pin down the shock
velocity and ambient density.
(2) I would like to see density and temperature measured by using [OII] and
[OIII] lines in the regions in the Gum Nebula lying outside the SNR, but
which show high emission measures. These can then be compared with
calculations such as those by Kafatos and Morrison to determine whether
it is possible for this material to have been impulse-heated 10,000
years ago. The studies should also measure the relative populations
of various ions in order to determine the current ionization structure.
(3) It would be worth checking to see if the characteristic X-rays expected
from the cosmic ray model of Ramaty, Boldt, Colgate and Silk are found
in emission outside the SNR.
(4) The He radio recombination lines, enhanced by dielectronic recom-
bination as described by Dupree, should be studied, as should
(5) The intensity and spectrum of continuous X-ray emission outside the
SNR.
(6) If the bright emission regions are actually found to be very hot, one
might search for cooler (T ~ 104 °K) H II regions around the B stars,
imbedded in the hotter material as predicted by Stecher in the discus-
sions here.
In closing, I think that the idea advanced by Colgate to argue against the impulsive
emission of UV by a SN explosion should be evaluated after relaxing the restric-
tion (or verifying its validity) that the emission spectrum would resemble a
black-body.
168


A note on the possible importance of the Gum nebula

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server