Richard Alexander
@richardalexander.bsky.social
Astrophysicist at the University of Leicester | Dad | Cyclist | Exiled Scot
https://rdalexander.github.io
https://rdalexander.github.io
These are steady-state models, but real discs evolve. Simin therefore extended her analysis to time-dependent calculations, and explored a wide range of models to understand what multi-wavelength observations of these rings tell us. She was able to draw several interesting new conclusions... 🔭
September 30, 2025 at 6:49 AM
These are steady-state models, but real discs evolve. Simin therefore extended her analysis to time-dependent calculations, and explored a wide range of models to understand what multi-wavelength observations of these rings tell us. She was able to draw several interesting new conclusions... 🔭
Simin first used an analytic model to show that the dust and gas properties are correlated. If the dust is fragile (with a low fragmentation threshold), then seeing rings implies the turbulence is very weak; while stronger turbulence requires much more resilient dust aggregates. 🔭
September 30, 2025 at 6:49 AM
Simin first used an analytic model to show that the dust and gas properties are correlated. If the dust is fragile (with a low fragmentation threshold), then seeing rings implies the turbulence is very weak; while stronger turbulence requires much more resilient dust aggregates. 🔭
New paper, led by @physicsuol.bsky.social PhD student @simintong.bsky.social. Simin looked at the rings we see in planet-forming discs, and combined new models with multi-wavelength ALMA observations to understand the underlying physical properties of the dust and gas. 🔭
arxiv.org/abs/2509.24818
arxiv.org/abs/2509.24818
September 30, 2025 at 6:48 AM
New paper, led by @physicsuol.bsky.social PhD student @simintong.bsky.social. Simin looked at the rings we see in planet-forming discs, and combined new models with multi-wavelength ALMA observations to understand the underlying physical properties of the dust and gas. 🔭
arxiv.org/abs/2509.24818
arxiv.org/abs/2509.24818
Bonus holiday activity - an afternoon watching the Tour de France from the roadside. 🤩 🇫🇷
August 3, 2025 at 9:28 PM
Bonus holiday activity - an afternoon watching the Tour de France from the roadside. 🤩 🇫🇷
Today, in "old man learns Python": having written a 1-D accretion disc code from scratch in a Jupyter notebook, I managed to vectorise it so it runs only moderately slowly. 🎉
Didn't even ask ChatGPT! Still lots to figure out, but I guess old dogs can sometimes teach themselves a new trick or two. 😂
Didn't even ask ChatGPT! Still lots to figure out, but I guess old dogs can sometimes teach themselves a new trick or two. 😂
July 1, 2025 at 7:59 PM
Today, in "old man learns Python": having written a 1-D accretion disc code from scratch in a Jupyter notebook, I managed to vectorise it so it runs only moderately slowly. 🎉
Didn't even ask ChatGPT! Still lots to figure out, but I guess old dogs can sometimes teach themselves a new trick or two. 😂
Didn't even ask ChatGPT! Still lots to figure out, but I guess old dogs can sometimes teach themselves a new trick or two. 😂
This is 15+ years old now, but anecdotally it’s even worse now. Many (most?) people time their submissions to appear nearer the top of the daily listings.
[The old submission deadline was 4pm Eastern time; it changed to 2pm a few years ago.]
arxiv.org/abs/0907.4740
[The old submission deadline was 4pm Eastern time; it changed to 2pm a few years ago.]
arxiv.org/abs/0907.4740
June 24, 2025 at 9:50 PM
This is 15+ years old now, but anecdotally it’s even worse now. Many (most?) people time their submissions to appear nearer the top of the daily listings.
[The old submission deadline was 4pm Eastern time; it changed to 2pm a few years ago.]
arxiv.org/abs/0907.4740
[The old submission deadline was 4pm Eastern time; it changed to 2pm a few years ago.]
arxiv.org/abs/0907.4740
This is really encouraging, as the mass ratio is exactly what previous simulations of Taurus predict for ejections from triple systems. 😀
The simulations also make a clear prediction - the binary orbit resulting from this process should be quite eccentric. Future observations should measure it... 🔭
The simulations also make a clear prediction - the binary orbit resulting from this process should be quite eccentric. Future observations should measure it... 🔭
April 11, 2025 at 7:44 AM
This is really encouraging, as the mass ratio is exactly what previous simulations of Taurus predict for ejections from triple systems. 😀
The simulations also make a clear prediction - the binary orbit resulting from this process should be quite eccentric. Future observations should measure it... 🔭
The simulations also make a clear prediction - the binary orbit resulting from this process should be quite eccentric. Future observations should measure it... 🔭
But...many binary stars are born as higher-order multiples, so what if this used to be a triple? The ejection of the third star could have tilted the disc.
Bec ran a suite of simulations to explore this process in detail. tl;dr: ejecting a ~0.35Msun star gives more or less the right configuration.🔭
Bec ran a suite of simulations to explore this process in detail. tl;dr: ejecting a ~0.35Msun star gives more or less the right configuration.🔭
April 11, 2025 at 7:44 AM
But...many binary stars are born as higher-order multiples, so what if this used to be a triple? The ejection of the third star could have tilted the disc.
Bec ran a suite of simulations to explore this process in detail. tl;dr: ejecting a ~0.35Msun star gives more or less the right configuration.🔭
Bec ran a suite of simulations to explore this process in detail. tl;dr: ejecting a ~0.35Msun star gives more or less the right configuration.🔭
But how the hell do you form something like that? Tilting a disc that far needs a very big perturbation. And doing so without disrupting the binary or planet orbit is...tricky. The obvious answer is a stellar fly-by, but in Taurus the low stellar density makes that incredibly unlikely. 🔭
April 11, 2025 at 7:42 AM
But how the hell do you form something like that? Tilting a disc that far needs a very big perturbation. And doing so without disrupting the binary or planet orbit is...tricky. The obvious answer is a stellar fly-by, but in Taurus the low stellar density makes that incredibly unlikely. 🔭
Alternatively: "how it started / how it's going" 😀 🔭
[my first undergrad research project / new paper out today ]
[my first undergrad research project / new paper out today ]
February 25, 2025 at 4:51 PM
Alternatively: "how it started / how it's going" 😀 🔭
[my first undergrad research project / new paper out today ]
[my first undergrad research project / new paper out today ]
@simintong.bsky.social is giving a talk (remotely) on our new paper (👇🏻) at the "Pebbles in Planet Formation" conference in Tokyo this week.
[Simin's talk is on Thursday morning; for those attending online, it's overnight Wed/Thu in Europe, or Wed evening in the US.] 🔭
indico2.riken.jp/event/5012/
[Simin's talk is on Thursday morning; for those attending online, it's overnight Wed/Thu in Europe, or Wed evening in the US.] 🔭
indico2.riken.jp/event/5012/
February 12, 2025 at 3:59 PM
@simintong.bsky.social is giving a talk (remotely) on our new paper (👇🏻) at the "Pebbles in Planet Formation" conference in Tokyo this week.
[Simin's talk is on Thursday morning; for those attending online, it's overnight Wed/Thu in Europe, or Wed evening in the US.] 🔭
indico2.riken.jp/event/5012/
[Simin's talk is on Thursday morning; for those attending online, it's overnight Wed/Thu in Europe, or Wed evening in the US.] 🔭
indico2.riken.jp/event/5012/
This "shoulder" feature is potentially a unique fingerprint of the dead zone outer edge, and may already have been observed. 😀 🔭
(Figure from Miley+ 2024; the blue line is the Sz66 disc.)
(Figure from Miley+ 2024; the blue line is the Sz66 disc.)
February 10, 2025 at 9:33 AM
This "shoulder" feature is potentially a unique fingerprint of the dead zone outer edge, and may already have been observed. 😀 🔭
(Figure from Miley+ 2024; the blue line is the Sz66 disc.)
(Figure from Miley+ 2024; the blue line is the Sz66 disc.)
The combined effects of this fragmentation and radial drift means that mm dust can't survive beyond the dead zone, so ALMA observations trace the dead zone size. The transition also creates a dynamical effect (which Simin dubs "hitch-hiking") which causes in faint structures at the dead zone edge. 🔭
February 10, 2025 at 9:31 AM
The combined effects of this fragmentation and radial drift means that mm dust can't survive beyond the dead zone, so ALMA observations trace the dead zone size. The transition also creates a dynamical effect (which Simin dubs "hitch-hiking") which causes in faint structures at the dead zone edge. 🔭
Simin's new idea is that compact dust discs trace the underlying turbulence in the gas.
Dust grains in discs are fairly fragile. In the "dead zone" turbulence is suppressed, so dust collisions are gentle and lead to growth; but beyond the dead zone, stronger turbulence tends to shatter the grains. 🔭
Dust grains in discs are fairly fragile. In the "dead zone" turbulence is suppressed, so dust collisions are gentle and lead to growth; but beyond the dead zone, stronger turbulence tends to shatter the grains. 🔭
February 10, 2025 at 9:30 AM
Simin's new idea is that compact dust discs trace the underlying turbulence in the gas.
Dust grains in discs are fairly fragile. In the "dead zone" turbulence is suppressed, so dust collisions are gentle and lead to growth; but beyond the dead zone, stronger turbulence tends to shatter the grains. 🔭
Dust grains in discs are fairly fragile. In the "dead zone" turbulence is suppressed, so dust collisions are gentle and lead to growth; but beyond the dead zone, stronger turbulence tends to shatter the grains. 🔭
We've all seen the amazing ALMA images of protoplanetary discs, which show all sorts of complex structures: rings, gaps, spirals, & horseshoes. However, it turns out that ~half the discs we observe are compact (smaller than the solar system), and apparently featureless (gallery from Long+ 2019). 🔭
February 10, 2025 at 9:29 AM
We've all seen the amazing ALMA images of protoplanetary discs, which show all sorts of complex structures: rings, gaps, spirals, & horseshoes. However, it turns out that ~half the discs we observe are compact (smaller than the solar system), and apparently featureless (gallery from Long+ 2019). 🔭
Dragging this back up - we may now have a set up for grabs, as after this hand today I will be retiring. 😀
December 26, 2024 at 5:06 PM
Dragging this back up - we may now have a set up for grabs, as after this hand today I will be retiring. 😀
I never would have said I have a favourite painting, but…
First picture - Musee d’Orsay, in March.
Second picture - Courtauld Gallery, in December.
Third picture is my office - I’ve had that print for nearly 15 years.
Apparently not only do I have a favourite painting, I’m actually stalking it. 🤣
First picture - Musee d’Orsay, in March.
Second picture - Courtauld Gallery, in December.
Third picture is my office - I’ve had that print for nearly 15 years.
Apparently not only do I have a favourite painting, I’m actually stalking it. 🤣
December 16, 2024 at 10:25 PM
I never would have said I have a favourite painting, but…
First picture - Musee d’Orsay, in March.
Second picture - Courtauld Gallery, in December.
Third picture is my office - I’ve had that print for nearly 15 years.
Apparently not only do I have a favourite painting, I’m actually stalking it. 🤣
First picture - Musee d’Orsay, in March.
Second picture - Courtauld Gallery, in December.
Third picture is my office - I’ve had that print for nearly 15 years.
Apparently not only do I have a favourite painting, I’m actually stalking it. 🤣
Still not enough facepalm emojis...🤦🏻♂️
November 27, 2024 at 3:50 PM
Still not enough facepalm emojis...🤦🏻♂️
Superb. 🤩 Just slightly nicer than my own image of M104, taken with Isaac Newton Telescope in La Palma on my one observing run (almost 20 years ago!).
[Our actual targets were clouded out, so we spent a few mins before closing taking images of fun things in the clear part of the sky. 😀]
[Our actual targets were clouded out, so we spent a few mins before closing taking images of fun things in the clear part of the sky. 😀]
November 25, 2024 at 4:05 PM
Superb. 🤩 Just slightly nicer than my own image of M104, taken with Isaac Newton Telescope in La Palma on my one observing run (almost 20 years ago!).
[Our actual targets were clouded out, so we spent a few mins before closing taking images of fun things in the clear part of the sky. 😀]
[Our actual targets were clouded out, so we spent a few mins before closing taking images of fun things in the clear part of the sky. 😀]
Long but enjoyable day in the big smoke (visiting Queen Mary). The view from Tom’s office was pretty good too. 🌅
November 20, 2024 at 10:54 PM
Long but enjoyable day in the big smoke (visiting Queen Mary). The view from Tom’s office was pretty good too. 🌅
Episode 5 - overall it’s getting sillier, but the shot-for-shot recreation of the “testing the rifle” scene from the 1973 film is beautiful. 👌🏻
November 12, 2024 at 11:54 PM
Episode 5 - overall it’s getting sillier, but the shot-for-shot recreation of the “testing the rifle” scene from the 1973 film is beautiful. 👌🏻
Came home and the clouds cleared. This one is from the back garden. 😀 🔭
October 10, 2024 at 9:52 PM
Came home and the clouds cleared. This one is from the back garden. 😀 🔭