WRT: "Discovery of Extensive [O III] Emission Near M31" (January 2023)
by Marcel Drechsler, Xavier Strottner, Yann Sainty, Robert A. Fesen, Stefan Kimeswenger, J. Michael Shull, Bray Falls, Christophe Vergnes, Nicolas Martino, and Sean Walker.
[ Discovery of Extensive [O III] Emission Near M31 ]

The key piece of information in the above research report is the non-detection of Hα emission from the nebula.

To an astronomer, 90% of the atoms in the universe are hydrogen, 10% are helium, and the remaining ~1% are everything else -- carbon, oxygen, neon, iron, etc. (Yes, I know this adds up to more than 100%, but I'm using very round numbers.) Wherever oxygen is found, hydrogen is going to be much more abundantly present. If there's enough energy to ionize oxygen twice there's also going to be enough to ionize hydrogen, and Hα emission will naturally follow. But this appears to be absent.

The conclusion that follows then is that the oxygen must be pure, and thus it has to have an artificial origin. The oxygen lost by the Apollo 13 mission on its way to the Moon more than fifty years ago immediately springs to mind. The accident happened at 55:54:53 into the mission, i.e., after launch (at 03:08 UTC on April 14, 1970; 10:08 PM EST, April 13. Maybe NASA or the other space agencies know of other spacecraft that have vented significant amounts of oxygen into space, but for now Apollo 13 would have to be the leading contender. It lost about 620 pounds of liquid O₂, or 5.46 x 10²⁷ oxygen atoms (9,070 moles).

This then locates the gas within the solar system rather than at the much larger distances people have thus far been considering. The gas cloud's distance is likely on the order of a fraction of an AU, depending on where along earth's orbit one is when looking at it. The steady momentum imparted to the gas by the prolonged small pressure from the solar wind would have moved it out of the earth-moon system to beyond the earth's orbit.

The source of the gas's ionization, then, is not photo-ionization from a star with a very hot surface, but kinetic ionization from its interaction with the solar wind, sometimes called collisional ionization. (The usual case here is an old supernova remnant plowing into a stationary cold interstellar medium.) The slow solar wind has proton velocities corresponding to ~1000 eV, which is more than sufficient to break oxygen molecules apart and then doubly ionize the resulting atoms. The first ionization energy for O is virtually identical to that for H, 13.61 eV. It then takes an additional 35.11 eV to knock a second electron off, for a total just shy of 50 eV needed to make O-III out of atomic oxygen. Depending on the efficiency one assumes for this process you could get anywhere from several to a couple of dozen O-III ions per solar wind proton (because those in the slow wind can have energies from ~500 eV up to about 1500 eV). I suppose even higher ionization states (O-IV, O-V, etc) are feasible with this much energy per particle slamming into the gas. To knock a third electron out, to form O-IV, takes an additional 54.6 eV.

Somebody has probably at some time or another shot 500-1500 eV protons into a fog of frozen O₂ crystals in a vacuum, but a literature search to track it down could take something on the order of a Hubble time.

This second ionization energy (35+ eV) corresponds to a photon wavelength of only 353Å (or lower) -- 10x shorter than the UV photons that cause sunburn -- for the case of photo-ionization, which only the hottest known stars and other objects in the universe emit in any quantity.

Of course it's one thing to create the needed O⁺⁺ ions, but they also have to be excited properly to radiate the light seen. In a regular emission nebula, like the Orion Nebula, the hard UV photon energy in excess of what's needed to ionize hydrogen goes into heating the resultant plasma, to about 10,000°K. At this temperature the typical electron has an energy, kT, of ~1 eV. But, because of the Maxwellian distribution of velocities and energies, there are sufficient electrons with the higher ~2½ eV needed to collisionally excite O⁺⁺ ions from the ground state up to the higher energy level from which they can radiate the observed 5007Å emission line. (By contrast, hydrogen in the ground state has no comparable low-lying energy levels, so it is not similarly collisionally excited, and it is mostly ionized anyway; the observed H emission lines result from free-bound recombinations followed by subsequent cascading down towards the ground state.) The relevant excited state of O⁺⁺ from which the 5007Å originates is relatively long-lived -- the technical term is "meta-stable" and the radiation is said to be "forbidden" -- so in dense enough or hot enough gas it can be collisionally de-excited before a photon is emitted as the ion drops back down to the ground state, or it can be raised to an even higher energy level by another suitable collision.

It isn't clear in the case of the cloud under current discussion which of these well-known processes is in operation, or even relevant. The trace heavy elements like oxygen play no part in the ionization equilibrium in a natural nebula, because of their small numbers. They do however play an important role in the thermal equilibrium because they absorb energy from the hot gas and radiate it away from the nebula. So they mainly act as coolants. A pure O⁺⁺ gas cloud would seem to have a difficult time staying hot enough to stay ionized. Perhaps the solar wind protons can fill both roles, ionizing the gas and then keeping it warm enough to radiate.

The difficulty with this scenario is that Apollo 13 was on almost the exact "wrong" side of the Sun when the accident occurred. The date was April 14 (Universal Time), and around that date the line from the Earth (and the Earth-Moon system as well) to the Andromeda Galaxy is nearly coincident with the line to the Sun, particularly in longitude around the Earth's orbit, where the difference would have been less than 4°. (Andromeda is then ~33 1/3° N of the plane of the Earth's orbit.) So we'd need to get the expelled gas cloud about 180° around the Earth's orbit before it left the earth-moon system, to the opposite side from where the accident occurred, in order for it to now appear almost in the direction of the Andromeda Galaxy.

There are two different kinds of parallax measurements that could test this hypothesis:

The first involves simultaneous measures made from two widely separated locations. This is what was done to calibrate the astronomical unit in regular distance units using a transit of Venus back in the 18th century, where two widely separated observers saw tracks of the planet across the sun that were offset from one another as a result of their differing perspective. If the measures of the nebula were made at nearly the same longitude but at a latitude difference of 40° or 45°, the baseline might be of the order of 4,000 km. If the nebula was about ½ AU away at the time (because the view may be across a good section of the earth's orbit), or roughly 80 million kilometers, the displacement would be on the order of 10 arc-seconds, or 5 pixels at the scale of the instrument used to discover the nebula (as reported in the research paper). Since the cloud has no real knots or hard edges to serve as a reference, one might use cross correlation techniques (after removing the stars) to put the two images in registration on top of each other; one would then put the stars back in their proper location and then look for a displacement of the stars in the two images.

The other kind of parallax measurement would be sequential in time from a fixed location, using the earth's orbital motion to provide a changing perspective, reflected in the nebula appearing to move relative to the background of stars. This motion could conceivably already be present in the original data obtained, though it would have been smeared out and lost when all the images, taken over a period of several weeks, were "stacked" (added together) to provide the final, ultra-long exposure time picture that's been widely distributed.

One would need to be careful not to make the images to be used near the two times in the year when the earth was heading almost directly towards or away from the cloud. These would roughly correspond to the stationary points in the retrograde motion of an outer planet, where the apparent motion of the planet stops before reversing direction. At about the time when the Andromeda Galaxy was at opposition from the sun (around Oct 4), the earth moves ~2.6 million kilometers per day, so for two images taken a day apart that would be the baseline. At ½ AU to the cloud, the displacement would then be almost 2° per day if the cloud was stationary in space, which would be an obvious amount of motion. In actuality, of course, the cloud would have started with about the same angular momentum as the earth, and in an orbit just a little bit larger than earth's would have nearly the same velocity. For an a=1.02 AU orbit the average speed is 29.49 km/s rather than the 29.786 km/s for the earth; the Δ-v works out to just 25,375 km per day relative to the earth, but at only 1/50th of an AU this still works out to ~¼° per day. That the research paper says the confirming observations had the nebula at the same location suggests the likelihood that if it's in the solar system it would have to be much farther than a fraction of an AU away for any motion to have escaped notice, though it's not specified what time interval the original and confirming observations were made over.

But one can never be sure. If for some reason the nebula is no longer there the better fraction of a year later when people go to look for it again, it could just be in a different place. For that a=1.02 AU orbit, the period is ~11 days longer than our year, so it would be "behind" us in getting around the sun by something like 10-12°.

There's only one instance in the astronomical literature that I know of where nearly pure oxygen is measured to exist: in a few of the bright, fast-moving knots in the bright radio source Cas A, a supernova remnant some 350 years old. The measurements suggest the knots are surviving chunks from deep inside the progenitor star, namely from near the base of the helium-burning shell, where almost all the helium and carbon had been converted into oxygen -- and maybe small amounts of a few other elements beyond, like sulfur and neon.

This page currently under construction...

Notes

[ ecliptic long. ~27 1/2 for M31 vs. 23 2/3 for Sun, lat. ~33 1/3° ]

TLI 02:35:46
1st Midcourse correction 30:40:50, burn into hybrid trajectory for Fra Mauro
(~4½-5° S); 40,000 mi off earth's surface if nothing else done
O2 tank explosion 55:54:53, 210,000 mi from earth (84½% of max)
2nd course correction, back to a free return trajectory
61:29:43 (LM's Descent Propulsion System)
stars visible inside moon's shadow 76:42:07
"PC+2" burn; pericynthion + 2 hours TEI 79:27:38.95 (shortened return 12 hrs)
3rd DPS burn 105:18:42, projected entry flight path angle back within safe limits
full power-up of LM 133:00:00
4th LM burn 137:40:13, using reaction control thrusters
service module jettisoned ~138:02 "a considerable amount of debris visible"
undock from lunar module 141:30:00
re-entry 142:40:46
down 142:54:41

"Lovell, looking out the window, reported "a gas of some sort" venting into space, making it clear that there was a serious problem." ... "As soon as Lovell pressed his nose to the glass [~10 minutes after the explosion], his eye caught a thin, white, gassy cloud surrounding his craft, crystallizing on contact with space, and forming an irridescent halo that extended tenuously for miles in all directions." ...
"Tank one, however, was still in a slow leak. Its contents were obviously streaming into space, and the force of the leak was no doubt what was responsible for the out-of-control motion of the ship [for more than an hour after the explosion]. It was nice to know that when the needle finally reached zero Odyssey's oscillations would at last disappear." ...
"It was almost as if some huge, gassy halo had surrounded the ship, spreading out slowly for twenty-five or thirty miles [as seen by Saulietes from a 14" telescope on the ground]." ... "The volume surrounding the spacecraft was filled with myriad small bits of debris from the accident, complicating any efforts to use the stars for navigation." ...
"The view from the LEM [Lunar Excursion Module, aka the lunar lander] was much the same as it had been from the Command Module: a swirling cloud of oxygen ice crystals and particles of debris... hundreds, indeed thousands, of brightly glowing false stars." ... "Everywhere he [Lovell] looked out his window, the cloud of rubble that surrounded Aquarius [the LEM] seemed uniformly dense. Firing the jets to move straight forward, he tried to fly out of the the glowing haze, but it seemed to move with him, almost as if the gravitational attraction of the ships themselves -- with the moon's or the Earth's gravity to compete with them -- was drawing the rubbish particles along like iron filings moving with a magnet. ... Everywhere Lovell looked, his view of the distant stars was obliterated. ... Aligning his [navigation] platform by the stars, he was now convinced, would be impossible." This persisted until after the PC+2 burn, except when they went into the shadow of the Moon ~45 minutes before "PC" and could make out Antares and Nunki (σ Sag) -- as well as the Milky Way, though there were two "eerily dark columns blotting out some of the newly visible stars"; this was ~24 hours after the explosion. Notice that the recently discovered cloud has two main columns.

320 lbs of O2; density of LOX: 1,141 kilogram per cubic meter at -183°C / ~boiling point (9.51 lbs/gallon => 33.7 gallons per tank).
860 psi initially, tank #1 loss rate was "better than a pound per minute." 100 psi minimum needed by fuel cells. After O2 surge tank was isolated (cut off), the rate was -1.7 lbs/minute, and increased to ~3. It took ~2 hrs to drop to 205 psi, and a little less than 3 hrs before being empty.
spacecraft speed: ~2,000 mph = ~3,000 fps; back up to near 5,000 mph ~1 hr after astronauts went into LEM -- it was 3,000 mph (4,400 fps) as the de-hybrid, docked DPS burn was being worked out ~3 1/2 after accident, which only required a delta-v of 16 fps.

Wikipedia: The 10.7 cm radio flux (F10.7) is a measurement of RF [radio frequency] emissions from the Sun and is roughly correlated with the solar EUV flux. Since this RF emission is easily obtained from the ground and EUV flux is not, this value has been measured and disseminated continuously since 1947. The world standard measurements are made by the Dominion Radio Astrophysical Observatory at Penticton, BC, Canada and reported once a day at local noon in solar flux units (10−22W·m−2·Hz−1). F10.7 is archived by the National Geophysical Data Center.

Many of the early instruments were research spacecraft that were re-purposed for space weather applications. One of the first of these was the IMP-8 (Interplanetary Monitoring Platform).[57] It orbited the Earth at 35 Earth radii and observed the solar wind for two-thirds of its 12-day orbits from 1973 to 2006. Since the solar wind carries disturbances that affect the magnetosphere and ionosphere, IMP-8 demonstrated the utility of continuous solar wind monitoring. IMP-8 was followed by ISEE-3, which was placed near the L1 Sun-Earth Lagrangian point, 235 Earth radii above the surface (about 1.5 million km, or 924,000 miles) and continuously monitored the solar wind from 1978 to 1982. The next spacecraft to monitor the solar wind at the L1 point was WIND from 1994 to 1998. After April 1998, the WIND spacecraft orbit was changed to circle the Earth and occasionally pass the L1 point. The NASA Advanced Composition Explorer (ACE) has monitored the solar wind at the L1 point from 1997 to present.