For reasons not entirely obvious, my blog "title" and description had started to overlay the top of the actual content. Not sure when that started, but I've got rid of it now. Too many options there, and it's less than clear what means what. [This seems to apply across multiple "themes" ; has blogger applied some "downdate" and broken existing things?] Nope, it's still doing it. Now the title is nailed to the window, and the posts, sidebars, etc scroll over its top.Ah - maybe if I move the "title" and "description" into the "NavBar"? Nope, that didn't work. You can't move those bits out of their containers. Switching to a "classic" theme has solved the overscrolling error. The sidebars overprint the main body, so can I fix that? OK, so now, because blogger want to put bullshit thingsd on their menu, I havev to learn even more HTML to get away from their shit. Slow. Hand. Clap. Blogger. Sod this, that's enough for today.
- Two more chapters from the incoming Elsevier book on ... planet formation? Star Formation and Stellar Atmospheres. (Are there two books in progress? Quite possibly. Regardless, these look like a couple of edited chapters. Not final versions - one has 3 times the size of glossary as the other, which I would expect the book editors/ assemblers to address down the line.) The first volume goes from GMC (Giant Molecular Cloud - one of which will form many stars, reflecting why questions of stellar multiplicity and stellar interactions remain important - to being a "star". As gravitational and pressure forces interact, the star goes through several pseudo-equilibria. Then as the scale decreases (under an AU for the "core"), magnetic forces also become significant, complicating both accretion of matter onto the core, and ejection of matter from the core. In theory, these counteracting influences should lead to a prediction of the IMF (Initial Mass Function - the probability of stars forming with mass M1, M2, M3 … . "Should" is a big word there - we haven't got anywhere near that good a theoretical model. Unsurprisingly, metallicity has several big effects (on gas transparency ; on magnetic coupling ...). It all remains very complex.
The multiplicity of star formation (whether a core forms on it's own, or with one, 2, 3 ... companions) becomes a major question, with theory and observation not well aligned. Which is approximately the point at which the story is passed on to the next volume. (Which I don't have a link to, yet.)
Stellar Atmospheres are very important through all of the accretion, lifetime, and death of stars, as they buffer the interation between the "surface" of the star (itself, a complex question) and it's environment. Radiation, magnetic flux and material are some of the interacting forces. The presencve of mass flows ("stellar wind") helps make it even more complex. How complex - the table of 23 different computer codes for modelling atmosphere variations in altitude/ pressure/ temperature/ absorbance/ emission may suggest how complex. - Another pair of linked papers. "Irregular moons possibly injected from the outer solar system by a stellar flyby" and "Trajectory of the stellar flyby that shaped the outer solar system" are linked by assuming that there was a star (or other large body) that passed through the early Solar system. Which is a very plausible scenario - when we look at star-forming regions today, they're packed with many bodies in a small volume. OTOH, since it is so long ago (multiple revolutions of the Solar system arounf the galasctic centre ; many multiple revolution of even the most distant Solar system ovjects), it's really only a question that we can address statistically. We can get to high probabilities of such an event, but not certainty. The "trajectory" of such an imposter is even more statistical.
- 10 September 2024 - nothing interesting.
- "Earth’s mesosphere during possible encounters with massive interstellar clouds 2 and 7 million years ago" - The "Protostars to planets" conference proceedings I was looking at a couple of days ago mentioned - in analysis of the solar environment - that the Sun probably entered the expanding "local bubble" about 6 million years ago, and is currently approaching it's mid-point. (Corollary : the "Local Bubble" is local to us and extends in roughly equal directions in all directions purely by happenstance. Worth remembering, that ; I hadn't really appreciated it before.) Wiki says the "Local Bubble" is around 1000 ly (310-odd parsecs) in diameter, which is a large proportion of the thickness of the galactic disc. Wiki displays a map of the Solar environment to 100 pc, which purports to display the Local Bubble, but that is inconsistent with these other measurements. Regardless of that (take Wiki with a pinch of salt! - as has long been known), thisd paper is more about the atmospheric cosequences of passing through the higher interstellar medium (ISM) densities around the margins of the Bubble (and other structures). The results seem likely to be complex. Increased flux of HOX molecules to the upper atmosphere are suggested to produce long-lasting noctilucent clouds (NLCs ; OK, I can accept that) which would reduce solar radiation at the surface (OK ...) by (this paper's estimate) 7%. Which yas, would be a significant climatic forcing. But those same NLCs would also reduce outgoing longwave radiation (i.e. infrared) by Order(½) … leaving what as the climatic consequences? "More data and further work are needed". (The effects on ozone concentrations are similarly mixed, varying in distribution against height more than in total column density.)
- "The early Solar System and its meteoritical witnesses" discusses the problem that (current telescopes can't see well through the debris discs around present-day forming stars (with inferred planetary systems), so for understanding what is happening in them is strongly influenced by the meteorite components we see in the Solar system. Unfortunately, motions within the solar system obscure the question of where a particular meteorite (let alone, it's components) originated 4.5 billion years ago, and radiometric dating to the timescale of the mixing time of a protoplanetary disc is also difficult. The paper's own cited data spans some 30 million years, when the time scale of interest is O(4 Myr).
This seems to be one chapter of a "workshop proceedings" book. Internal placeholders ("Lodders chapter", "Schönbächler chapter", "Krot, Lee chapters") point to other parts of the proceedings. Skipping various details, a basic lesson that"At any rate, obviously, the solar nebula was not homogeneous. It may have inherited heterogeneity from the parental molecular cloud but it also developed some in situ"
is reiterated. Another summmary to remember is that"since the present-day main belt [of asteroids], not exceeding a twentieth of lunar mass, is but a very partial sampling of the original population of planetesimals"
. Seeing (weak) evidence for 95-148 distinguishable parent bodies for meteorite samples suggests the problem of raw taxonomies. The classification they refer to (someone else's work) has 30-odd categories, some overlapping. Verily, "stamp collecting" science. While isotopic ratios and thermal histories do indicate some coherent trends in meteorite composition, not having meaningful indication of the source region of most meteorites' origin leaves the field rather hobbled. That many meteorites have internal evidence of a multi-stage formation history doesn't help.
The bulk of the paper is a review of current planetary disk evolution modelling, from distribution of matter in the nebular disk through to planetesimal assembly, not forgetting the problem of Jupiter. - Oh, this sounds like fun. "Minimum Safe Distances for DE-STAR Space Lasers". "DESTAR" is an acronym where they missed (for incomprehensible reasons) the necessary "ATH" part of the acronym. The authors assert that they mean "Directed Energy Systems for Targeting of Asteroids and exploRation". Essentially, it means "build as large a laser as you can, in space, with a solar panel power supply ; then build lots more of them. They classify them on a basis of the log of the array side size in metres, so a DEATH-STAR (see what I did there?) "4" would be order of 10km on a side. (Just from the abstract, there's a challenge of cooling the laser modules - the innermost ones are going to be radiating heat through 5km thickness of laser machinery, itself radiating in similar frequencies to the particular module. Getting power in is similarly problematic at larger sizes.)
"clearly there is the potential for such an asset to be deployed as a weapon by targeting locations on Earth"
Oh hell yeah! Cynical moi? suspects that no such "Earth Protection System" would ever get off the ground without this "accidental" feature being incorporated. But yeah - put them where they're too far away to actually cause damage to Earth. (What's that Lassie? Of course you don't need to shoot asteroids out of the sky. You do your shooting years (orbits) before the final approach, while the debris has plenty of time to disperse - particulalry across the original Earth-intersecting orbital trajectory. The larger arrays they say would still be able to target Earth from the far side of the Sun, which would rather be the object of the exercise. So you'd need to have a crew onboard steering the DEATH-STAR so it couldn't target Earth.
It may be symptomatic of the times, but that sounds like the description of a really potent Earth-targetting space weapon, manned by the "right stuff" to make sure that the wrong people don't get "accidentally" missed.
"Accidentally Terminating Heathens" would seem to be a suitable filler for the acronym gap. "We cannot," as Dr Strangelove would put it, "allow an acronym gap to develop." - Fucking laptop just crashed the whole post. This is getting unsupportable. "Radial Velocity and Astrometric Evidence for a Close Companion to Betelgeuse" reports more work on understanding Betelgeuse, which suggests that it has a companion of around 2 M☉ (from the brightness variation data), or 0.6 M☉ when adjusted with the radial velocity data. Since the high mass of Betelgeuse limits the time available, such a low-mass core would probably still be condensing, and may not even have achieved thermonuclear fusion yet. That would contribute to - even worsen - the luminosity difference between Betelgeuse and the companion to over a million-fold difference, amply explaining why it hasn't (yet) been detected.
- There's nothing like thinking big! "Substantial extension of the lifetime of the terrestrial biosphere". Most people don't think about it, but the slow increase in solar luminosity as helium "ash" accumulates in the star's core (the root of the "Faint Young Sun Paradox") means that, regardless of what humans do, life will become extinct on Earth a long time before the Sun turns red giant. A LONG time. One fairly ha limit is when the oceans boil - or to be more precise, when the surface temperature becomes such that the vapour pressure of surface water reaches the point that it increases the greenhouse effect to raise the surface temperature, to raise the water vaour pressure to ... a positive feedback loop ending as the bottoms of the ocean basins sizzle away the last of their water. Probably not long after (a few hundred million years), plate tectonics stop.
That's a pretty hard limit. Our current water-vapour greenhouse warming is about 15 K, and we're worrying about anthropogenic CO2 greenhouse warming of 3~5 K. What temperature the feedback kicks in is unclear. But we probably don't have to worry about that for a couple of Gyr yet. (Whereas the Sun will go red giant in about 5Gyr.) But these authors intorduce another limit, which will probably kick in earlier. While there is still water around, the main constraint on CO2 concentrations in the atmosphere is it's absorbtion by rocks during weathering (turning silicates into silica and metal carbonates - principally calcium and magnesium carbonates). These chemical reactions are significantly temperature sensitive (the old rule of thumb, if you don't have actual kinematic measurements, is that a 10 K temperature increase will double a reaction rate), so as surface temperatures increase, the atmospheric CO2 levels will be decreased. Which will continue until there is too little CO2 for plants to succeed in fixing carbon. At which point, the biosphere collapses, pretty rapidly, as the various processes that mineralise carbon drop carbonates into the ocean trenches, and plants cannot extract CO2 from the emissions of volcanoes before the mineralising chemistry throws that back into the subduction zones.
These authors estimate that that tipping point is about 1 Gyr in the future from today. But they also posit that this is amenable to modification. They look at the temperature sensitivity of land plants, citing various examples of modern plants that continue to photosynthesise at temperatures up to 63 or 65°ree;C and note that aquatic cyanobacteria can survive to 74°ree;C. They also mention the way that "C4" plants use an extra-chlorophyll mechanism to increase the [CO2] around the sites of crabon fixation - which is a relatively novel evolutionary adaptation compared to "C3" plants.
Secondly, they consider the sensitivity of weathering processes to CO2 concentrations and temperatures, which are less amenable to manipulation, and the influence of soils on [CO2] (which is, potentially, amenable to manipulation). These seem to suggest that CO2 fixation by weathering is not as temperature-dependent as earlier (1992) models.
Between the changed sensitivity of wathering to temperature, the expanded (somewhat theoretical) temperature ranges of plants, and the switch from C4 to C3 plants (which is very definitely within human manipulation), they estimate that the lifetime of plants on Earth might be closer to 1.8 Gyr then the previously estimated 0.9 to 1.25 Gyr. Which is great. As long as we, as a species, can survive the coming 0.0001 Gyr, we'll cave some prospective modifications to the biosphere to do. That coming 0.0001 Gyr, though - that's going to be a problem. - "The Symbiotic Recurrent Nova V745 Sco at Radio Wavelengths" Recurrent novæ are a bit of a thing for me at the moment. To occur repeatedly the event that they represent must not be so powerful as to destroy the originating system, but they are quite violent events. They are thought to be related to the formation of supernovae class Ia, where the incremental addition of mass onto a white dwarf eventually reaches the Chandrasekhar Limit. At this point, the mass is 1.44 M☉ and the collapse releases a nearly constant amount of energy (that's a significant question), making them a "standard candle" of cosmological importance. But recurrent novæ are rare things, making it hard to study the process in detail. This paper summarises the process as
A nova is a thermonuclear explosion that ignites at the bottom of a layer of accreted material on a white dwarf (WD) in a binary system. The companion star is usually a main-sequence star, but is occasionally a more evolved sub-giant or giant star. The companion transfers H-rich gas onto the WD, accumulating an envelope of accreted material on its surface. As this material is compressed, the pressure and temperature at the base of the accreted layer increase, and nuclear reactions accelerate, until conditions are reached for thermonuclear runaway.
(I'm not sure all novæ follow this prescription, but that's certainly the model for recurrent novæ.)
So, what is V745 Sco? Working the name, it's a variable star ("V") in the constellation Scorpio ("Sco"), and it was the 745th such variable noted in that constellation. When it underwent it's second nova eruption in 1989, it kept the variable star designation (well, it is still a variable star!) but went onto the (short!) list of recurrent novæ.
The (short) list of recurrent novæ : (this will go out of date, hopefully quite soon!)
From the paper :Full name Discoverer Distance (ly) Apparent magnitude range Days to drop 3 magnitudes from peak Known eruption years Interval (years) Years since latest eruption CI Aquilae K. Reinmuth 8590 ± 830 8.6 - 16.3 40 1917, 1941, 2000 24–59 24 V394 Coronae Australis L. E. Errol 17000 ± 3000 7.2–19.7 6 1949, 1987 38 37 T Coronae Borealis J. Birmingham 2987 ± 75 2.5 - 10.8 6 1217, 1787, 1866, 1946 80 78 IM Normae I. E. Woods 9800 ± 1600 8.5 - 18.5 70 1920, 2002 ≤82 22 RS Ophiuchi W. Fleming 8740 ± 850 4.8 - 11 14 1898, 1907, 1933, 1958, 1967, 1985, 2006, 2021 9 - 26 3 V2487 Ophiuchi K. Takamizawa (1998) 20900 ± 5200 9.5 - 17.5 9 1900, 1998 98 26 T Pyxidis H. Leavitt 9410 ± 780 6.4 - 15.5 62 1890, 1902, 1920, 1944, 1967, 2011 12 - 44 13 V3890 Sagittarii H. Dinerstein 16000 8.1 - 18.4 14 1962, 1990, 2019 28 - 29 5 U Scorpii N. R. Pogson 31300 ± 2000 7.5 - 17.6 2.6 1863, 1906, 1917, 1936, 1979, 1987, 1999, 2010, 2022 8 - 43 2 V745 Scorpii L. Plaut 25400 ± 2600 9.4 - 19.3 7 1937, 1989, 2014 25–52 10 Rising by ten magnitudes is normal. That is a factor of 10000 in brightness on Earth, and presumably at the source. (Small world syndrome : the "five magnitudes =100 × " definition is due to Norman Pogson in 1856, 6 years before the eruption of U Sco noted in the table above.) Four of these ten are ‘symbiotic’ binaries with giant companions (Kenyon 1986), implying that evolved companion stars may be over-represented amongst recurrent novae.
The implication is that less-evolved (smaller!) companion stars are associated with recurrence times that exceed the human (instrumental) timescale. A complicating factor (still under investigation) is if both stars are WDs, then (probably) the transfer rate would be slower, and the chemical characteristics (spectroscopy!) of the SN different. A theory requiring data.
The WDs in most recurrent novae have been observed to be massive, approaching the Chandrasekhar mass. Using the effective temperature of the WD, V745 Sco was found to have a MW > 1.3 M☉. The effective temperatures of the WDs in two other recurrent novae, RS Oph and V3890 Sgr also suggest high masses. V3890 Sgr has a MW = 1.25 − 1.3 M☉ and RS Oph was found to have a MW = 1.2 M☉.
That's a lot more than that inferred for ... oh, it was Betelgeuse's companion that I was considering yesterday - not really in the recurrent nova stakes (yet). The WD in T CrB is thought to have a mass of 1.37 M☉. returning to Betelgeuse's companion briefly, V745 Sco was proposed to have a brightness varition similar to that identified for Betelgeuse (510 days, versus 416 days for Betelgeuse), but both have since been ascribed to internal pulsations in the red giant, rather than orbital signals.
All of which is good background (for me), but what is the new science in this paper? The authors report and analyse radio data collected with several instruments at different times to improve certain propoerties of V745 Sco (notably the distance and the galactic reddening extinction along the line of sight). Between eruptions, the circumstellar medium is below the (radio) detection threashold, which means it would be hard to see in a SN (it would only show in the first few days of observations).
And that's enough for this post.
