Still working the backlog, but it looks as if I've got to cut my own HTML to manage Blogger's stylistic incompetence.
Eclogites
Eclogites and basement terrane tectonics in the northern arm of the Grenville orogen, NW ScotlandMany 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.
I'd better go pack the tent!
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Thorne-Żytkow Objects
https://arxiv.org/pdf/2410.02896T-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.
Back to top.
A veritable slew of book chapters in preprint.
Wolf-Rayet stars
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.
Small Bodies in the Distant Solar System
No, I'm not going to get into the "Is Pluto (
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 ;
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.
Atmospheres of Solar System Moons and Pluto
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.
Detection prospects for the GW background of Galactic (sub)solar mass primordial black holes
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.
