A recent paper on ArXiv reports a novel idea about the central regions of "our" galaxy.
Remember the hoopla a few years ago about radio-astronomical observations producing an "image" of our central black hole - or rather, an image of the accretion disc around the black hole - long designated by astronomers as "Sagittarius A*" (or SGR-A*)? If you remember the image published then,
one thing should be striking - it's not very symmetrical. If you think about viewing a spinning object, then you'd expect to see something with a "mirror" symmetry plane where we would see the rotation axis (if someone had marked it). If anything, that published image has three bright spots on a fainter ring. And the spots are not even approximately the same brightness.
This paper suggests that the image we see is the result of the light (radio waves) from SGR-A* being "lensed" by another black hole, near (but not quite on) the line of sight between SGR-A* and us.
By various modelling approaches, they then refine this idea to a "best-fit" of a black hole with mass around 1000 times the Sun, orbiting between the distance of the closest-observed star to SGR-A* ("S2" - most imaginative name, ever!), and around 10 times that distance. That's far enough to make a strong interaction with "S2" unlikely within the lifetime of S2 before it's accretion onto SGR-A*.)
The region around SGR-A* is crowded. Within 25 parsecs (~80 light years, the distance to Regulus [in the constellation Leo] or Merak [in the Great Bear]) there is around 4 times more mass in several millions of "normal" stars than in the SGR-A* black hole. Finding a large (not "super massive") black hole in such a concentration of matter shouldn't surprise anyone.
This proposed black hole is larger than anything which has been detected by gravitational waves (yet) ; but not immensely larger - only a factor of 15 or so. (The authors also anticipate the "what about these big black holes spiralling together?" question : quote "and the amplitude of gravitational waves generated by the binary black holes is negligible"
Being so close to SGR-A*, the proposed black hole is likely to be moving rapidly across our line of sight. At the distance of "S2" it's orbital period would be around 26 years (but the "new" black hole is probably further out than than that). Which might be an explanation for some of the variability and "flickering" reported for SGR-A* ever since it's discovery.
As always, more observations are needed. Which, for SGR-A* are frequently being taken, so improving (or ruling out) this explanation should happen fairly quickly. But it's a very interesting, and fun, idea.
Il semble bien que l’intensité de la raie H_alpha augmente sensiblement depuis l’été dernier !
Which my Français-de-chien translates as "It seems that the intensity of the H_alpha line [in the emission spectrum] is considerably stronger now than last summer. Which was specifically one of the things Schaeffer was mentioning as needing close monitoring in his alert of 10 months ago. It hasn't "gone" yet, but that's what would be expected in the run-up to an outburst.
Merci beaucoup à Alain Lecacheux at [PSM observatory], Alain Figer, Al Grycan and Jean Lecacheuxfor the "heads-up!"
Overall, given the rapid and drastic increase in intensity and width of the Hα profile, we strongly anticipate that the T CrB outburst may occur soon.
Spectroscopy on Jan 15, 29 and Feb 07 continue to show rapid increases in the H&aplha; line, in both intensity and width (indicating a larger size of the accretion disc).
In other words, thinga are hotting-up.
Similar changes have been observed in other "cataclysmic variables" in thye run-up to recent outbursts, though none have the historical record of reaching naked-eye visibility that T CrB has. You probably won't need to carry sunglasses all night, unless you're wanting to pose.
I don't see other mention of T CrB in other ATels back to mid-January.
As submitted ; if posted to front page, I'll modify this to reflect the editing. .
Chatter on the "Minor Planets Mailing List" (an email list for people interested in the "small bodies" of the Solar system) indicate that the Tesla Roadster polluting interplanetary space since being dumped there in 2018 has been spotted again. Telescopes in four countries reported the object. From the object's brightness, it appears to remain attached to the Falcon upper stage.
Initially, the object was misidentified as a minor planet, but once there were sufficient observations to establish it's orbit, it's identity as a "rediscovery" of "2018-017A" was established. Since the object is not natural, it is of no further interest to the astronomers and the last batch of data have been assigned to "artificial object 2018-017A, Falcon Heavy Upper stage with the Tesla roadster". Some people here might be interested though.
"Is it coming back?"
Yes, as it does every 1.525 years. This will be it's 4th return.
"Will it hit us?"
No. This time around, it'll be about 0.1 years behind Earth where it crosses Earth's orbit (4 × 1.525 − 6), which would be about 94 million km away. The closest it'll come back is about 240 thousand km away — about 2/3 of the Earth-Moon distance.
I'm not sure if Elon left the keys in the ignition. Recovering it to Earth would be an amusing trick for Bezos. There should be no legal issues since the object was clearly abandoned by it's owner. Set your diaries for 2065-ish.
If it goes onto the front page, it'll probably be edited. Which is why I've got CSS for "Insert" and "Strikeout".
It's time to update my data on "MOND" activity. First, the what/ why/ when.
A common plaint on the Internet is that "interesting" theories are being "suppressed" by … someone, rarely specified, for reasons eve more rarely specified. The Illuminati have an interest in suppressing techniques for running your gas-guzzler on water ; Donald Trump doesn't want the kompromat tapes that Vladimir holds to become public, whatever.
A while ago I was sufficiently irritated by this to actually look at one of the genuine scientific controversies, which greatly irks the Wingnut Fraternity - how "Big Physics" ignores alternatives to General Relativity because … no clearly-stated reasons, but it doesn't take long before someone points out that Einstein was a "cultural" Jew, and therefore a prime candidate for High Wizzzard of the Illuminati etc. etc. Which irritated me, so I decided to collect some data.
If theories are being actively suppressed, then you certainly wouldn't see papers on them being published out in, uh, public, where any YT-kook can see them (if they knew how to look, or cared to). Since most physics papers get published on "the Arχiv" before they go into their respective journals-of-publication, that's an ideal place to look. (The habit is spreading too: bioarxiv.org for the biological sciences; eartharxiv.org for the Earth sciences, and probably others in fields I'm not so familiar with.
Last year I collated the last few years of research results for a number of terms related to the ever-contentious problem of gravity &emdash; how does it relate to the structure of the universe (a lot of people don't like the counter-intuitive consequences of modern cosmology &emdash; even those who don't have particular invisible sky-fairies they want to proselytise for). That collection had some problems, which I address below, but showed that the "non-standard" theories do get some attention ; just not a lot of attention. It's almost as if the "suppression of independent thought" is profoundly inefficient, and instead not many physicists find the question (or this particular "solution" to it) to be interesting or productive. The level of interest is not greatly increasing or decreasing compared to the general changes in science publication.
Data - the Kook's enemy.
Annual publication numbers for various cosmological theory terms, in Arχiv abstracts over the years.
Date
Search terms
(year-end)
Mordehai Milgrom
MOND
Non-Newtonian Gravity
MOG
dark matter
Brans-Dicke [gravity]
Date
Mordehai Milgrom
MOND
Non-Newtonian Gravity
MOG
dark matter
Brans-Dicke [gravity]
Total 1991-09-01 to 2001-12-31
22
54
54
0
3137
251
2001-12-31
4
38
46
16
538
18
2002-12-31
2
12
13
2
576
18
2003-12-31
1
22
17
2
691
25
2004-12-31
1
12
20
2
752
30
2005-12-31
2
35
22
2
876
34
2006-12-31
2
35
27
4
895
34
2007-12-31
2
49
24
2
1053
24
2008-12-31
3
61
20
3
1199
34
2009-12-31
4
51
23
6
1443
35
2010-12-31
5
50
38
4
1306
54
2011-12-31
4
60
35
5
1475
51
2012-12-31
5
42
23
6
1543
41
2013-12-31
6
56
33
3
1602
45
2014-12-31
3
58
33
8
1700
32
2015-12-31
3
40
33
5
1864
48
2016-12-31
6
51
32
6
1799
53
2017-12-31
2
55
39
17
1889
39
2018-12-31
3
48
35
16
1993
44
2019-12-31
4
55
34
8
2128
54
2020-12-31
3
51
46
19
2149
52
2021-12-31
2
43
47
9
2180
37
2022-12-31
4
63
44
11
2320
37
2023-12-31
4
85
51
24
2369
31
2024-12-31
1
59
34
11
2582
34
Notes
The term “non-Newtonian gravity” has a problem : it collects a lot of material like “non-Newtonian rheology” where gravity gets a mention ( e.g. . non-Newtonian fluids flowing on slopes). Which is perfectly valid science (Oh, I remember having to do my drilling engineering hydraulic pressure calculations on "non-Newtonian" models, on power-law models and at least one other ; every morning at 04:30 for the 06:00 report.) So, on no better grounds than that I’m going to swap that term for “Brans-Dicke gravity”, which is a term I’ve seen before. It actually pre-dates the "MOND" concept.
The Arχiv search engine has numerous complications, and I didn’t note last year’s search terms closely. Generally I'm searching in "Abstracts" (except for Mordehai Milgrom, an "Author") ; I'm searching in the "Physics(all)" space ; other terms are covered by this search link, and substitute dates and search terms as desired. That should make it repeatable over the years. Search URL : “ https://arxiv.org/search/advanced?advanced=&terms-0-operator=AND&terms-0-term=Brans+Dicke&terms-0-field=abstract&classification-physics=y&classification-physics_archives=all&classification-include_cross_list=include&date-year=&date-filter_by=date_range&date-from_date=2018-01-01&date-to_date=2018-12-31&date-date_type=announced_date_first&abstracts=show&size=50&order=-announced_date_first ” Don't forget to strip the enclosing quotes!
The different (probably) search details this year returned 3429 “dark matter" results last year, but this years searching, on the appropriate date range, returns 2369. That’s not good repeatability. So I have to re-do at least the "dark matter" results. The other terms are numberically insignificant, and I can't be bothered to repeat the search manually. Let's see what it's like next year. Having worked out the components of that search URL, I should be able to write it into a script for … wget or cURL. But how to parse the results?
Brans-Dicke theory has a Wiki page, and has been around longer than MOND. It’s interesting that this was trending slightly upwards until 2010~2014, but has been declining since.
I’ve re-done the “dark matter” queries with this year’s search parameters. The numbers are down &emdash; I was probably getting “dark” and “matter” last time, but now should just be getting “dark matter”. Or something like that. If I was doing a formal literature search, I’d probably investigate further.
Arχiv got started in August 1991, so searches from 1991-09-01 should work.
I need to get those gridlines aligned to year-ends - every 4th year or something like that.
Last year I posted a graph of the results. Same again this year, but with some more details on the axes and header.
Results
Again, "dark matter" is far and away the most popular of these different cosmologies. The figures for "non-Newtonian gravity" remain "flat" (bearing in mind that contains a significant amount of "viscosity" related research too). "MOG" (a variety of "MOdified Gravity" theories) continues to attract a little attention. My fairly-blind choice to look at "Brans-Dicke" gravity (I recognised the name, that's pretty much all!) has turned out to be interesting : until about 2010 to 2014 it was generating more publications, but since then the number has dropped, and the trend line shows that with reasonable accuracy. Those paper numbers are higher than I think could reasonably be explained by one retirement from the field ; maybe several. This is in contrast to the continuing modest rise in publication rates on "MOND".
TLDR; version : "suppression" is ineffective. Or non-existent.
The question asked above sort of answers itself. Of course Supernovae (and other "big bangs") can move anything they impinge upon. How much is maybe a more important question.
Since the ... early 1970s? isotopic data from lunar, then meteorite, samples have shown that the early solar system had been impregnated with nuclei like 26Al, which had a short half-life (7.17*10^5 years), and so a considerable energy release (per unit mass) into the materials they impregnated. Since Al is easily incorporated into silicate minerals, it got all over the place. In the early Solar system, that was a significant source of energy, probably responsible for the (seemingly) easy melting of "small" asteroids, rapid heating of larger bodies. Then it stopped, and normal service (gravitational reorganisation ; colission ; accumulation og heat from the decay of longer-lived isotopes) was resumed. OK ; not "stopped", slowed down and rapidly became insignificant. But this 26Al was a significant source of energy that was present in the early development of the Solar system, and isn't now.
The putative source of this material, and it's associated energy, is a supernova "near" to the early Solar system in it's early days. Consequences include that relatively small bodies (asteroids 4/ Vesta, 16/ Psyche ...) have obviously melted at masses considerably lower than modern Solar system compositions would suggest.
Until the detection of "daughter" isotopes of 26Al in meteorite samples (which daughters were less abundant than in terrestrial camples), this was a pizzle. The discovery of the "daughter" isotopes moved the problem to that of how big, and when, did the supernova erupt, producing the 26Al and injecting it into the pre-Solar nebula? It's very clear that it did, and the short lives of large (supernova-prone) stars makes it un-surprising. So it becomes a standard part of "planetogenesis". An early nearby supernova is accepted as a thing these decades. (If there is a dissenting opinion, I haven't heard it expressed.)
Then come the next questions : what would be the effect of such a supernova on the materials (and their arrangement) in the early Solar system? And also, what would be their effects in later stages of the Solar system? This paper concerns itself primarily with the effects of a strong interstellar wind on particles in the Solar system (which would be a necessity for implanting the above 26Al, though this paper deals with later stages up to and including today.
Essentially, the strong wind applies a "kick" impulse to the orbits of particles. The effect of the kick is strongly related to the size of the particle being considered - a particle of 10cm or larger would be unaffected, while a particle of 1mm diameter would be given sufficient impulse to be destabilised in it's orbit, if not completely ejected from the system. Orbit destabilisation would likely result in the particle accreting onto a larger body, or again, being ejected by a close encounter. The geological record contains enough medium-lifetime nuclei (specifically 60Fe
, half-life 2.6 × 106 years) to estimate a nearby supernova rate of around 2 per 10 million years, meaning that the Solar system is frequently swept clear of it's dusty components. Including Saturn's finer rings - though their regneration from colissions between the larger bodies should regenerate the dusty component on a rapid enough timescale to explain the rings we see.
Ejection from the Oort cloud of a system ndergoing a supernova is, in passing, suggested as a potential origin for bodies like 1I/ `Oumuamua (not that that body is short of origin proposals, from the sensible to the fantastical).
I checked recent results from AAVSO at about 17:30 local time.
Star
JD
Calendar Date
Magnitude
Error
Filter
Observer
T Crb
2460647.06597
2024 Dec. 02.56597
<6.0
0.1
V
HQA
T Crb
2460647.03
2024 Dec. 02.53000
9.8
-
Vis
BRG
T Crb
2460646.96597
2024 Dec. 02.46597
<5.6
-
Vis
TRIB
T Crb
2460646.45344
2024 Dec. 01.95344
11.341
0.0048
B
DEY
Inevitably, we're under 8/8ths cloud cover here.
18:10 (JD 2,460,647.26182) Not managing to get an updated measurement listing. But that's a fifth of a day ago. (0.19 JD), 4 and a bit hours. Is there a problem in the hardware?
My "Astro-COLIBRI" is reporting an "unclassified optical transient", but that's at a Dec of 31.57° N, which is … actually, that is in the right range (25~35) for the constellation. But T Corb is at dec 25.9°, so … I'm going to have to convert between RA systems.
OK,converter written, the optical transient ("AT2024addv") was only 5 degrees off in declination - which is a plausible error, but 36° off in right ascension, which is not a plusiible error. But the AAVSO website hasn't posted any more results as of 19:15. Then Firefox crashed. [SIGH]
Anything on other astronomy news sites? S&T? Nope. Astronomy.Com ? nope. CBAT ? Nope.
Has T CorB "gone"?
It looks like that was just a glitch - some blockage in AAVSO's pipeline just while the brightest magnitude readings had been posted for ages were at the top of the list. Overnight postings eventually went back to the norm of around 10 mag (Vis). Which is what you expect, but in this case we're poised for a rapid rise cataclysmic eruption.
Oh well, I got one bit of necessary stuff built into my worksheets. Now need the reverse function.
Many years ago, I went walking in the highlands, all over. One place I circled around - literally - was the eclogite field on Beinn Sgritheall to the south of Glenelg, on the coast opposite Barrisdale in Knoydart. Wonderful area. And I've always been interested in eclogites, granulites, and ultra-deep metamorphics. Comes of getting started on the Lewisian foreland, I suppose.
(Oh, you've got to love the OS speelung-chokers. I'm sure they have a good reason for having "Barisdale" farm overlooking "Barrisdale Bay". Hang on! Sandaig - the place "Ring of Bright Water" was set - is in the paper's field area too. And I now have a GPX first-draft of a route for getting to the localities, "Eclogites-v1.gpx" ; that'll need some more work.)
Anyway, I spotted this article going by on EarthArxiv (which I don't pay enough attention to, I know). Even if it doesn't contain much in the way of field guides to this eclogite field, it still interests me. I'm sadly out of practice at this stuff - too long looking at (per Mike Lappin) "crustal ephemera which haven't been down to 100km for 100 Myr, and are clearly nowhere near equilibrium, so can be safely ignored. Otherwise known as the oil industry.
So, what is going on here? They seem to have evidence (structural, geochemical) that these eclogites were obducted onto the Lewisian (Laurentian, even) foreland in the Grenvillian orogeny, about 1200 Myr ago - before the Caledonian orogeny that formed most of Scotland ; before the preceding deposition of the Moinian and Torridonian (very approximate correlates) and their orogeny under the Caledonian ; back into the late assembly of the Laurentian foreland itself, these eclogites were obducted onto the foreland as an ophiolite.
Ah, approaching Real Geology : Pressure-temperature estimations obtained from various lithologies, including the eclogites, indicate peak metamorphic conditions of c. 20 kbar and 730-750°C, consistent with burial to depths of c. 70 km.. but do they give locations? "The eclogites are typically composed of garnet + omphacite + rutile + quartz (Sanders, 1989)" sounds like some fun rocks for the collection. "Omphacite grains occur with symplectites of diopside and plagioclase and are replaced around their rims by hornblende. Rutile has been replaced round the rims by ilmenite" sounds like some good hand-specimen textures are possible.
Oh goody - most of their locations are coastal. That turns an area search into a linear search. Where's my maps - sheet 32 or 33, IIRC.
T-Ż objects are (arguably) theoretical objects where a compact body - a white dwarf or a black hole - becomes entrained in an otherwise normal star, with lots of interesting consequences for both the behaviour of the object and it's evolution. The really interesting thing is, such a peculiar internal state may not be that obvious from the outside.
I wrote a post about these a while ago (2023-05), when some authors discussed whether or not the Sun could actually be hosting such a stellar viper in it's thermonuclear bosom. Their conclusions were that it would be hard to tell, even if the Sun had acquired it's internal parasite early in it's evolution. The energy produced by the accretion of matter onto an asteroid-mass primordial BH would to large degree replace the energy yield from thermonuclear fusion.
Obviously, other people find these objects interesting, in a train-wreck sort of way. This paper is an early version of a chapter on the bodies for an astrophysics textbook/ review forthcoming from Elsevier.
Sections cover :
Formation,
Internal Structure and Evolution,
and their final fates,
Bearing in mind that none of these bodies have been observed (though proposals have been made - and disputed), the constraints of reality upon theory are relatively slight. More ink will be spilt!
Formation
Thorne and Żytkow originally considered the collapse of a large star's core without the normal disruption of it's envelope in nova/ supernova. However doing this without getting a large amount of "thermonuclear ash" ("metals" to an astrophysicist - any nuclei heavier than helium) on the surface of the resulting body seems challenging. And we have a wealth of spectroscopic data from many such events which do reveal various (super-)nova remnants - but no Thorne-Żytkow Objects.
Thorne and Żytkow also considered merger scenarios where a closely orbiting pair of stars, the heavier of which (most-rapidly evolving) becomes a neutron star (or black hole), and which could then inspiral into it's companion (with various requirements for ejecting material from the pair to conserve energy and angular momentum. That's a complex process, inherently variable ; hard to predict. Examples have been proposed. And disputed.
Direct collision is thought (by some) to be the most plausible formation path, particularly in the dense cores of globular clusters or molecular clouds (which the most massive stars don't have time to migrate away from before evolving into compact-body-hood. Again, the details can be complex - closing energy and angular momentum have to be accounted for.
Internal Structure and Evolution,
The main model is that the compact body has a zone near it's surface where the infall energy of the rest of the system releases large amounts of energy, producing a zone where outwards radiation is dominant, and supports the rest of the star's mass against inflow (exactly as Eddington discussed in the 1920s for formation of regular stars, leading to ideas of the Eddington limit. Beyond this "radiative zone" the star is convective as for normal stars. Potentially, with black-hole cored Thorne and Żytkow objects, the accretionary radiative zone can be surrounded by a conventional nuclear-fusing core, then it's radiative-limited zone, then the convective zone. Distinguishing these from conventional giant to super-giant stars could be very "challenging". If, however, this core material gets mixed into the upper parts of the star, that potentially is observable.
Understanding the nuclear reactions in such systems remains both controversial and challenging. Signals from both stable and unstable nuclear species have been considered.
Understanding the evolution of the objects is obviously complex. Some solutions suggest a Thorne-Żytkow object might have a shorter lifetime than the same mass regular star ; some calculations suggest the Thorne- Żytkow object could have a longer lifetime than the regular star.
And their final fates,
Like many large stars, there are multiple routes to mass loss for Thorne- Żytkow object through their evolution. The envelope mass might decrease enough that the accretionary structures can radiate through to the surface, which would rapidly radiate down to being a regular (-ish) neutron star. Or the NS could collapse to a black hole, triggering an (abnormal, ?) supernova. Many of the models produce periods of pulsation in the Thorne- Żytkow object (another potential observable?).
Fun objects, Thorne- Żytkow objects. The universe should contain such strange objects. Whether it does or not remains to be seen.
W-R stars are getting a deal of attention with the focus on recurrent novae and (potential) supernovæ. That's not particularly because being a WR star is associated with the SN process(-es), but because they're by definition evolved massive stars with a strong stellar wind, which means they've already run a lot of their short life. On the other hand, a powerful WR stage can lead to so much mass loss (into a large planetary nebula) that the star falls out of the window in which a SN can occur. The high mass (Mini >e; ~20 M☉ (before late-stage mass loss) and high luminosity makes for very short lifetimes (for a 20 M☉ star, 3.7~5.5 million years ; for a 40 M☉ star, 2.6 to less than 1.0 million years), which in turn means the stars die (as planetary nebulae, or supernovae) still in their natal molecular clouds. Often they are part of the dismantling process of the collapsing of the cloud. But why am I trying to summarise a review paper? The death of WR stars - there is some evidence of SN being sourced from WR stars, but other arguments that they are too compact to form SN and instead collapse directly. This latter scenario is argued for from the geometry of SNR-BH couples such as Cygnus X-1.
No, I'm not going to get into the "Is Pluto (♇) a planet?" question. If I'd had my 'druthers, I'd have gone for an intrinsic property of planets vs dwarf planets vs other "small bodies", probably based on the "potato radius", self-rounding or something geological. but I can live with the IAU's extrinsic orbit-clearing definition. Hal Levison's "hand-waving" argument about formation mechanisms holds water too. Argument, as far as I'm concerned, over. Yes, I grew up with 9 planets in my Solar system too. I also watched the discovery of Charon, the increasing puzzlement over Pluto's minuscule size, the initial mapping by mutual occultations, and the discovery of the outer Solar System (3rd or 4th most massive element, to date, is Pluto ♇ ) ; maybe the geometrically largest. Science is a process of improving approximations to the truth, and if the solar system has a 9th planet, we've not seen it yet. That said, @PlutoKiller@Twitter.com (the social media handle of the discoverer of Eris, the most massive (known ; to-date) outer Solar system body) has been quiet lately - maybe he's found something? Rant over. Debate not engaged with.
This is a proto-chapter for another Elsevier book. Probably not the same astrophysics book as the previous entry, but there's no law against them having multiple in production at one time.
This "key point" is one of the less "stamp collecting" parts of the field : "The sizes and shapes of Kuiper Belt objects tell us about the details of planet formation, while Kuiper Belt orbital distribution puts constraints exactly how and when the giant planets migrated."
That there are now over 3000 known TNOs brings statistics to the subject of the outer Solar system, in the same way that the Kuiper telescope brought statistics to the subject of planetary systems in general. The classical a [semi-major axis ; ∝ orbital energy] vs. eccentricity (e) plot, reveals the sculpting of the Kuiper belt by interaction with Neptune (incidentally, clarifying why Neptune is a planet and Pluto isn't), while the avs. inclination (i) plot shows that something has been sculpting the Kuiper Belt (Outer Solar system) by dragging everything through the nit-comb of small-number-integer resonances with Neptune.
Detecting, recognising, and calculating the orbits of TNOs is a noisy, bias-prone topic. What the biases are (per instrument/ methodology), how severe they are, and how to de-bias observations towards estimating the underlying population parameters, are important topics. Once orbits have been calculated, they can be classified. But classifications can change over time, as interactions with Neptune (and to a lesser degree, Uranus, Jupiter, Saturn, potentially Planet9 [Brown, Batygin 2016] ...) lead to the orbit evolving over periods of more than a few million years ; few thousand orbits. Classification is a moving goal in many cases, and needs to be tested in all cases. Not all trans-Neptunian Objects are Kuiper Belt Objects ; there are various other classes, some of which enter the inner Solar system (e.g. Centaurs).
The composition of TNOs/ KBOs are generally only available by spectroscopy (if you can get the time on a light-bucket) or colour in different filters (if you can't get the light-bucket time). This gives a hint of evolution, from the polymerisation of surface organic matter to dark-red "tholin" mixtures. The properties of TNOs eventually tend towards those of the dust of the outer Solar system, which can be compared to the dust- and debris- disks surrounding other stars. A 2024 result from the dust-detector on the New Horizons spacecraft [Doner et al (2024), Feb.] suggests that there is more dust than models of the 2010s would suggest, pointing to the Kuiper Belt being more populous and extending further from the Sun than thought in the 2010s.
Review article on ... well, as the title says. Io excepted, these are N2 - CH4 dominated atmospheres, with the outer bodies (Pluto, Triton) developing seasonal methane frosts. Io is different - it's atmosphere is dominated by SO2 with minor SO, but these components can freeze out rapidly when Io goes into eclipse behind Jupiter. Complicated systems, worth review.
The prospect of (sub)solar mass primordial black holes comes up on an almost monthly basis when people are discussing the problem of Dark Matter. Last year someone, for reasons not at all clear, speculated that the putative "Planet9" [of Brown & Batygin, 2016, as modified] might be such a "primordial" black hole. It's a pretty dead idea - if they were present in significant amounts (mass-wise), then we'd have seen them in gravitational lensing experiments (observation projects) like MACHO and OGLE. They're not(MACHO, <25% of necessary dark mass) there. To mis-quote Feynmann, a beautiful hypothesis slain by an ugly fact.
Anyway, this paper suggests that moderste mass, sub-stellar black holes (so, presumably "primordial"), particulalrly those in highly eccentric orbits, might be marginally detectable by the in-work LISA mission, and more detectable by planned missions such as DECIGO.
Back to top. And that, I think is enough for this one. Plough through more backlog now.
Still catching up with the backlog. for a change, I'm looking at my most-recent listing.
Planet Formation Mechanisms (https://arxiv.org/pdf/2410.14430) - a book chapter for something that hasn't come out yet. (Is it a proceedings volume/ review collection from Protostars and Planets VII, April 10th – 15th of 2023, Kyoto, Japan ? Lots of links on that page to recordings of the conference proceedings, such as "The Solar Neighborhood in the Age of Gaia" - now that looks a valuable seam to mine, once the current problems with downloading from YT are fixed.) Quick read ... disk-instability versus core-accretion models (which are end-members of a continuum). It all happens quickly - order of a million years, and in the middle of a dusty nebula, making it inherently hard to observe. (More data, extending into the thousands of exoplanet, improves the chances of finding examples in that 1-in-5000 time period for the Solar system analogues.) All in, that looks a very valuable resource. Time for me to watch some YT, while making supper.
[Later] Lots of stuff over my head, so far concentrating on aspects of assembling matter into protostars. (Well the conference was titled "Protosatars and Planets", and as presented on YT, the conference seems to have worked systematicay from interstellar space down to assembling planets. So, more watching to do. Worth the investment - and when YT's download blocks are broken again, I'll be back to DL them. Probably worth it's own posting, when I've got recordings that I can pause/ rewind.
"Projections of Earth’s technosphere. I. Scenario modeling worldbuilding, and overview of remotely detectable technosignatures." - a bit difficult to asssess this one. The astronomical point of view is that looking into the future like this helps sharpen the focus on potential bio- and techno-signatures we could find in the atmospheres of exoplanets. Worth doing. Not sure how to assess this though. Interesting, and not terribly optimistic that their Abstract sums up the results as "Our scenarios include three with zero-growth stability, two that have collapsed into a stable state, one that oscillates between growth and collapse, and four that continue to grow. Only one scenario includes rapid growth that could lead to interstellar expansion."
I'm not sure how to assess their methodologies, but it's one for the "futurologists" to consider (if they do anything other than navel-gazing and tea-leaf reading).
"The Accelerating Decline of the Mass Transfer Rate in the Recurrent Nova T Pyxidis" This just caught my eye and reminded me to check on how "T CorBor" is going (Magnitude 10.2 ; Error - JD 2460607.194 ; Calendar Date - 2024 Oct. 23.69400; Magnitude 10.2; Filter Vis.; Observer MQA. It hasn't gone yet.) In comparison to T CorBor, "The recurrent nova T Pyxidis has erupted six times since 1890, with its last outburst in 2011," - 20-odd years, which compares to the 80-year recurrence (one recurrence!, poised on tenterhooks for the second) of T CorBor ; "[...] indicates that T Pyx must have a massive white dwarf accreting at a high rate."
Well, it does if there is any validity to the accreting WD model - which I've heard no serious counter-proposals against. "the magnitude decline of T Pyx from ∼ 13.8 (before 1890) to 15.7 just before the 2011 eruption" Ah, that would explain why it's less well known - you need ... at least a 150mm (six-inch) telescope to see that and do any meaningful measurements on it. In the context of T CorBor being around a month "late" for it's bookings as a TOO (Target Of Opportunity) for just about ever professional telescope in the northern hemisphere, the complexity of this stars varying recurrence rate makes the delay in recurrence all the more understandable.
This star is inferred to have at least one feedback system, where the heat of material transferring onto the WD component inflates one side of the companion star to increase the transfer rate ... leading to more complex behaviour. T Pyx' recurrence intervals of 12, 18, 24, 23, and 44 years suggests there is a lot more complexity to it's behaviour than implied for T CorBor.
T Pyx is a bit odd. If Wiki is right (an important "if"), then the WD has a mass of 0.7 M☉ while the companion has a mass of 0.13M☉. Which puts it down in the brown dwarf margin. And the total mass of the system is 0.87 M☉ - which is well below the Chandrasekhar limit (about 1.4 M☉. So the big concern is ... ? Whatever things T.Pyx has up it's binary sleeve, a type 1A supernova isn't among them. [Checking the references for that mass estimate … Uthas et al (Mon. Not. R. Astron. Soc. 409, 237–246 (2010)) do give that figure, but in a context of a WD:donor mass ratio of 5:1. That still leaves the donor as being very small, for feasible WD masses (≤ Chandrasekhar), and the total mass marginal for producing a SN.
Odd system. Big can of worms. (I also see that Schaeffer - the guy who was headlining the "T CorBor is gonna blow!" story - has a long history publishing in the recurrent nova field. As one would hope. The data densities for nova records from the 1920s to [recent] are instructive - dozens of photometric measurements increasing to hundreds per eruption.) Moving on.
A cosmic formation site of silicon and sulphur revealed by a new type of supernova explosion. Everyone knows the "onion" model for nucleosynthesis in (massive) stars. Less well-known is the existence of "stripped" stars where hot massive stars lose their hydrogen-dominated envelopes, revealing a He-dominated core (Wolf-Rayet stars, particularly sub-type WN). Digging deeper (ejecting more of the envelope, one gets the carbon/oxygen shell exposed (Wolf-Rayet WC/WO stars) and their corresponding type 1CN supernovae, with their unusual sets of emission lines. This paper reports a supernova whose spectrum implies stripping all the way to expose the sulphur/silicon layer of the core. SN 2021yf is proposed to show such a star's destruction with lines of multiply ionised silicon, sulphur, and argon (SiIII-IV, SIII-IV, and ArIII with an absence of lighter element lines. Which they interpret as being the detonation of such a core stripped back well into the S-Si shell.
Nice find. I was aware thaat W-R stars were stripped, and hot, bright (so, short-lived) stars. But I hadn't realised they were - at their extremes - taking their own cores apart. "Die young, stay pretty", on a stellar scale.
And that's another couple of days worth skim-read.
Oh, that's interesting. Mars presents a lot of questions because it is the closest Earth-a-like we can study in any detail.
On the other hand, many people forget how different Mars is to Earth (@twitter.com@elonmusk - are you listening? Of course not - you talk, not listen.) Yes (FTFA), "It turns out that a moon of less than a third of the lunar mass was capable of producing a sufficient initial triaxiality." may be true, but it glosses over that Mars is now (and probably always was) one tenth of Earth's mass. Is that comparison with the Moon in absolute mass, or relative mass? In either case it is ridiculously larger than Phobos or Deimos, or their combination.
Where did this Moon go? And why?
I saw an interesting SETI "lunchtime lecture" on the Martian "hemispheric dichotomy" (N. Polar Basin vs Southern Highlands) a number of years ago. Accepting the "giant impact" hypothesis for that structure (itself a natural expectation of "hierarchical growth" [should that be "oligarchic growth"? From Wiki, The next stage is called oligarchic accretion. It is characterized by the dominance of several hundred of the largest bodies - oligarchs - which continue to slowly accrete planetesimals. No body other than the oligarchs can grow. ] - little things accrete to make bigger things - models of planetary growth), then the possibility that after the last "giant impact" the body is significantly non-spherical becomes ... well, plausible, but not guaranteed. Late-stage impacts are going to deliver a lot of energy so that the planet is effectively a droplet of a low-viscosity fluid. And you've got to have a large enough body ("Moon-size", or larger ; the Moon is about 1.25% of the mass of the Earth), close enough to affect the shape of the (slowly) cooling mass.
Time to RTFP!
"Motivation :" Mars’ triaxiality makes itself most evident through the equatorial ellipticity produced by the Tharsis Rise and by a less prominent elevation located almost diametrically opposite to Tharsis and constituted by Syrtis Major Planum and an adjacent part of Terra Sabaea Yeah, well we all know Tharsis - volcanoes, possibly still recently active. Maybe a mark of "single plate tectonics and where the heat gets out. Tharsis, volcanic peaks excluded, is about 7km above the mean elevation of the planet (or is it to a reference elevation, not a "mean" - a bit of Martian cartography I'll have to check up on) while the elevation he gives for Terra Sabaea is only 2.1~2.3 km. The author then goes on to consider the ellipticity of Mars without the Tharsis contribution (which the mappers, Zuber and Smith (1997), had also considered). Even [without Tharsis] Mars retained much of its triaxiality. - Which I'll take as read. They then propose the initiation of a "seed" triaxial component from their putative moon, later amplified by tectonic processes dumping heat and magma onto the Tharsis high point. Unfortunately, this gets rather iffy already. Mars is reported to undergo a lot more "polar wander" than Earth (justifying the horrible SF consequences of losing the Moon, and all sorts of other doom) and that the current near-polar position of the North Polar Basin and the (sub-equatorial) Tharsis bulge are near-coincidence. I don't think you can have both at the same time.
I agree with this next quote - but am not blind to the problems of moons turning up then going away : The seed asymmetry of the equator was considerable if the synchronous moon existed already at the magma-ocean epoch, and was weaker if the moon showed up at the solidification stage.
Whence had it come, whither gone?
The author's title, not mine. But yes, it's a big question.
Had the impact happened during the magma-ocean stage, it would hardly have influenced the subsequent development of Mars’ global structure.
I couldn't put it more succinctly myself. See my above "droplet of a low-viscosity fluid" comment.
On the other hand, had it [a large impact] happened during the formation of crust, it may have, speculatively, left some signature - whence the question arises whether that impact could be the one responsible for the north-south hemispherical dichotomy, a theme beyond the scope of our study.
I don't think the author has seen Marinova's SETI lecture on her work, or the associated papers. Her modelling of a Polar-basin forming impact has the redistribution of 10~20 km thickness of crustal thickness from the (putative) impact site to the rest (other 2/3) of Mars' surface - which would literally outweigh this proposed minor lunar re-shaping. There's the non-trivial point too that the crust and upper mantle would have isostatically adjusted towards following the (gravitational) spheroid or (rotational ellipsoid. Rocks are not solid, even on a cold, dead planet like Mars - they creep under forces.
He doesn't really address the "whence" question - he lists some features of protoplanetary discs, and says they might be factors, while ignoring the blunt fact that most people in the field accept the really large satellites in the Solar system (Luna, Charon) are the products of "giant impacts", and this "Nerio" (some Roman mythological associate of Mars/ Ares) would fall into that category too.
What does he say about "whither"? Well, he blames it on the LHB (Late Heavy Bombardment), with a proviso that it would have to have been early in the LHB, so that later LHB impacts would overprint the expected equator-biased impacts from bits of the moon falling to Mars.
Colour me unconvinced on that front. It's plausible, but far from convincing. The whole "LHB" concept is itself rather dependent on a relatively small number of radiometric dates from a relatively small area of the Moon, all rather close to the Imbrium Basin. There are geological challenges from terrestrial observations too. It's an idea seriously needing better support (e.g. from sample-return missions from the Lunar far-side).
The remaining 27 pages of the paper are mathematical arguments which are over my head. The author obviously thinks they show that his sequence of events is mathematically plausible, and I'm willing to accept that (besides, it's plain from the reference list, that this is his field, and he's worked with many others in this area, and presummably they accept this work when they reviewed the paper. "plausible" ≠ "true".
My summary : plausible, but I don't think it's likely. Worth a read ; not worth studying the maths (which I assume is correct).
Between one thing and another (which includes my laptop deciding to power-down half-way through an OS upgrade), a week of inactivity. Fortunately, nowt published, so let's see what is in the pile.
Bottom of the list : 16 August 2024
Oh, before I go any further, T CorB still hasn't "gone". Yet.
Portability of Fortran's `do concurrent' on GPUs How to execute code on $GPU$ in a relatively transparent manner - because those who write the code may not know who is running the code, and on what hardware. - Really, that's very interesting. But is it astronomy?
28 August 2024 - A synchronous moon as a possible cause of Mars’ initial triaxiality Oh, that's interesting. Mars presents a lot of questions because it isd the closest Earth-a-like we can study in any detail.
On the other hand, many people forget how different Mars is to Earth (@twitter.com@elonmusk - are you listening? Of course not - you talk, not listen.) Yes (FTFA), "It turns out that a moon of less than a third of the lunar mass was capable of producing a sufficient initial triaxiality." may be true, but it glosses over that Mars is now (and probbly always was) one tenth of Earth's mass. So, presenting the same data differently suddenly sounds less impressive : "It turns out that a moon of three times the relative lunar mass of Luna to Earth ..." sounds less impressive. That, and requiring a deus ex machina to take the moon off-stage before we get to see it ... unconvincing. It's plausible that the author (one-off : Michael Efroimsky, US Naval Observatory, Washington DC 20392 michael.efroimsky @ gmail.com! ; always a good sign of something that hasn't passed an in-house peer review before seeing the outside world. The author may be right, but as it stands, it's his name that he'll blacken, not his institution) is correct, but I won't hold my breath.
Maybe worth reading about the "initial tri-axility" though. Between rotational forces, and the possibility of small bodies to exhibit "single-plate tectonics" (never (??) seen on Earth), there are some interesting questions there. Wossname did an interesting SETI "lunchtime lecture" on the Martian "hemispheric dichotomy" (N. Polar Basin vs Southern Highlands) a number of years ago. Accepting the "giant impact" hypothesis for that structure (itself a natural expectation of "hierarchical growth" - little things accrete to make bigger things), then the possibility that after the last "giant impact" the body is significantly non-spherical becomes sort-of obvious. not guaranteed though - late-stage impacts are going to deliver enough energy (which cannot leak away fast enough) that the planet is effectively a drop of a low-viscosity fluid. And you've got to have a large enough body (Moon-size, or larger ; the Moon is about 1.25% of the mass of the Earth) close enough to affect the shape of the (slowly) cooling mass. Hmmm. Before reading TFP, that's not looking very liklely. OK, I've done enough thinking on this, it's worth it's own post.
Slashdot's editors often (daily, if not more often) get a slating for errors, and doing, esentially, nothing. Now, as a regular submitter (occasionally accepted - my count is approaching 130 stories ; whingers - get to that count yourself or stop whinging about the site's content) I know that's bullshit, but I'd never actually sat down to record the difference between what I submitted, and what the Ed. (EditorDavid, one of about a dozen) changed before posting the story to the "front page".
I forgot to mention that I fat-fingered the title in my submission : "shy" not "sky".
I should note that this is not my first brush with the editorial "blue pencil" - in the 1990s and 2000s I volunteered on a community (specifically, trade union) newsletter, supervised by a former newsroom (print and TV) editor, Bob Gibb (also a journalist for Lloyd's List, the shipping newspaper). Bob never seriously discouraged my over-wordy, excessive-detail style. It's easy for me to cut down your work, because I rarely need to add anything; just rearrange it, and clarify it. Far easier than writing it myself!
Vale, Bob.
I submitted my story in the wee sma' hoors of 2024-04-29, and it was accepted at 7:34 (time zone unsure ; I'm Zulu) by "EditorDavid", with these revisions : superseded text (deleted by Ed.) ; inserted text.
For clarification, I've no serious disputes with EditorDavid over this. I'm making notes to learn. No, as Bob would appreciate, "shome mishtake shurely"
The naked-eye sky will briefly host a "new" star.
By "star", I do not mean "comet", "meteorite" or "firefly", but genuine [star] photons arriving here after about 3000 years in flight, causing your eyes to see a bright point on the nighttime sky. When it happens, the star will go from needing a telescope ot good binoculars to see, to being the 50th (or even 30th) brightest star in the sky. PARA For a week or so.PARA
Of course, it could just go full-on supernova, and be visible in daylight for a few weeks, and dominate the night sky for months. But that's unlikely.
Named "T Corona Borealis" (meaning : because it is the 20th variable star studied in the constellation "Corona Borealis") is a variable star in the northern sky - circumpolar ( it's now visible all night, all year) for about 60% of the world's population which although normally you need binoculars to see it.
PARA For over 150 years it has been known to vary in brightness, slightly. But in 1866, it suddenly brightened to become about the 35th brightest star in the sky. "Suddenly" meaning it was invisible one hour, and near full brightness an hour later. That made it a dramatic "nova" ("new star"), if not a "supernova", and people watched it like hungry haws as it faded over the next weeks, and months, and years.
And it faded back into it's previous obscurity, just wobbling a little, well below naked-eye visibility.
Until the late 1930s, when it started to change it's ESTABLISHED 280-day cyclic pattern. Then, in 1946 ... someone turned the switch back on, and again in less than an hour it brightened about 240 times, again becoming about the 50th brightest object in the sky. Which made it almost unique - a recurring nova. Today, only 10 of these are known, and they're extremely important for understanding the mechanisms underlying novae.
In 2016, "T CrB" (as it is known) started showing a similar pattern of changes to what were seen in the late 1930s.
But RockDoctor writes that in 2016, "T CrB" (as it is known) has started showing "a similar pattern of changes" to what happened in the late 1930s when it became one of only 10 "recurring nova" known to science:
In 2023, the pattern continued and the match of details got better.PARA
The star is expected to undergo another "eruption" EN-dash EM-dash becoming one of the brightest few stars in the sky, within the next couple of months. Maybe the next couple of weeks. Maybe the next couple of hours. I'll check the databases before submitting the story, and advise the editors to check too. [I expected this to be deleted]
Last week, astrophysicist Dr Becky Smethurst posted on the expected event in her monthly "Night Sky News" video blog. If you prefer your information in text not video, the AAVSO (variable star observers) posted a news alert for it's observers a while ago. They also hosted a seminar on the star, and why it's eruption is expected Real Soon Now, which is also on YouTube. A small selection of recent papers on the subject are posted here, which also includes information on how to get the most up-to-date (unless you're a HST / JWST / Palomar / Hawai`i / Chile telescope operator) brightness readings. Yes, the "big guns" of astronomy have prepared their "TOO - Target Of Opportunity" plans, and will be dropping normal observations really quickly when the news breaks and slewing TOO the target.
You won't need your eclipse glasses for this (Dr Becky's video covers where you can send them for re-use), but you might want to photograph the appropriate part of the sky so you'll notice when the bomb goes off.
Bomb? Did I say that the best model for what is happening is a thermonuclear explosion like a H-bomb the size of the Earth detonating? Well, that's the best analogue. Understandably, taking a "close" (3000 light years - not close enough?) look at one seems like a good idea.
Preview, check for brightening/ detonation (JD 2460428.55208 = 2024 Apr. 28.05208 mag 9.905 ± 0.0052 - not "Gone" yet!), submit.
This CNN article includes a nice animation from NASA illustrating the multi-star interaction that's causing the event:
The stars in the orbiting pair are close enough to each other that they interact violently. The red giant becomes increasingly unstable over time as it heats up, casting off its outer layers that land as matter on the white dwarf star. The exchange of matter causes the atmosphere of the white dwarf to gradually heat until it experiences a "runaway thermonuclear reaction," resulting in a nova [according to NASA]...
The NASAUniverse account on X, formerly known as Twitter, will provide updates about the outburst and its appearance.
The BBC reiterates the key data points — that "The rare cosmic event is expected to take place sometime before September 2024. When it occurs it will likely be visible to the naked eye. No expensive telescope will be needed to witness this cosmic performance, says NASA."
Footnote
And, in the tradition I established while writing this post, I'll check the database : JD 2460429.6875, date/ time 2024 Apr. 29.18750, magnitude 10.0. No eruption yet!
Well, I've got a backlog to try to work my way through. Thick end of 100 days, which would be pushing 10,000 papers, if I looked at everything. But I've already blindly thrown about 90% of that over the side. Probably including some tatties. Oh well. Modify the default template with the "fonty" stuff.
Nothing attractive in the first part of the pile. But I should go back to the previous post and add anything I see in the archives for T.CrB.
Did the T CrB (see - even i'm not consistent in the abbreviation used!) submission to Slashdot - with a Tyop in the title. Quick look through another day's worth of IArχiv, then bedtime. Bit of a collection post.
A 10-billion solar mass black hole in a low dispersion galaxy with a Kroupa IMF (decided against it)
Recent HTML Learnings - 2024-04
I learned a little about using external fonts, specifically from Google, but I should be able to generalise it, if it's worthwhile. (I'm dubious enough about Google's committment to keeping these fonts generally available, or any of their self-interested "philanthropy", but that'll be another thing to work on.)
This block should be in a silly font. "Google Monoton ". Nope, I'm borked again. Forgotten how to make it work. [...]
Fixed it now. Different funny font, "Jacquard 12 Charted" at 30 pix.
Back to RTFM, and improve my notes.
Where did I (initially) go wrong? I've got (1) the link in the HEAD section, then (2) the font family chosen in the (CSS)STYLE section. (I use PRE for demonstration. Meh.)
The example given encloses the URL for the stylesheet link in only one set of quotes - which is problematic when there are spaces in the font name. Let's get rid of that (and put single-quotes on the outside) for starters.
Yep, that did it. So, names with spaces now.
That looks a bit odd. (Note the different quote marks.) rel="stylesheet" href='https://fonts.googleapis.com/css?family=Monoton|Major Mono Display' works, but I'm sure there were warnings about mixing names with spaces in there. Oh well. Lesson learned, into the default header it goes.
I've also done a little paragraph-level formatting with p style="font-size:30px ; font-family:'Jacquard 12 Charted'" above. Note the arrangement of different quotes in there.
Looks moderately interesting. Total system mass ~25 M☉, of considerably differing sizes, thus MS-durations. Not co-planar (now that's surprising - worse then Pluto-Sun-Jupiter, without the tail-wags-doggery). There's a chain of logic implied from the distribution of system masses to the range of bound NS-NS and NS-BH potential future systems, and a difference between expected [NS] and [BH] occurrence rates seen in GW mergers. Which will need more brain cell than I have tonight. This morning, even.
Abstract
Hierarchical massive quadruple systems are ideal laboratories for examining the theories of star formation, dynamical evolution, and stellar evolution. The successive mergers of hierarchical quadruple systems might explain the mass gap between neutron stars and black holes. Looking for light curves of O-type binaries identified by LAMOST, we find a (2+2) quadruple system: TYC 3340-2437-1, located in the stellar bow-shock nebula (SBN). It has a probability of over 99.99\% being a quadruple system derived from the surface density of the vicinity stars. Its inner orbital periods are 3.390602(89) days and 2.4378(16) days, respectively, and the total mass is about (11.47 + 5.79) + (5.2 + 2.02) = 24.48 M☉. The line-of-sight inclinations of the inner binaries, B$_1$ and B$_2$, are 55.94 and 78.2 degrees, respectively, indicating that they are not co-planar. Based on observations spanning 34 months and the significance of the astrometric excess noise ($D>2$) in Gaia DR3 data, we guess that its outer orbital period might be a few years. If it were true, the quadruple system might form through the disk fragmentation mechanism with outer eccentric greater than zero. This eccentricity could be the cause of both the arc-like feature of the SBN and the noncoplanarity of the inner orbit. The outer orbital period and outer eccentric could be determined with the release of future epoch astrometric data of Gaia.
I met Kroupa IMFs last week - oit's the 4-class IMF, with different power laws indices for each successive mass class.
Look at this one too. Might help me modelling the IMF (and other models).
That's enough for tonight.
Got up to the start of March (mostly by throwing lists away un-examined). I need to thin down (or increasingly specialise) IArχiv for the weighted list. No more work here, just separate posts tomorrow on the "interesting" stuff.
