iiinova — where your digital self keeps evolving

8/8 We still need to check that the merging process can trigger the first detonation, and that the first white dwarf's explosion can ignite the second white dwarf. But I'm optimistic that we're on the right track to finally explain how Type Ia supernovae happen!

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7/ Check out our paper on what these two-star explosions would look like, led by an amazing grad student earlier this year. They reproduce observations well, and I expect them to match even better as we input more physics into the calculations. arxiv.org/abs/2401.08011

Two-star explosions from Boos et al. (2024)

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6/ Moreover, when the explosion ejecta hits the donor white dwarf, it can trigger an explosion in that white dwarf as well! We've started exploring these two-star explosions, and I actually think this is what happens for most Type Ia supernovae.

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5/ So as long as one white dwarf gets hit hard enough by the other white dwarf's accretion stream to start a detonation, it will successfully propagate and trigger a complete explosion and subsequent Type Ia supernova.

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4/ But it wasn't clear that white dwarfs actually have enough helium for the first explosion...until now! We calculated realistic white dwarf structures and then tested them to see if the resulting helium layers could support explosions, and indeed they can.

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3/ Theory has shown this is possible as long as there is enough helium on the white dwarf's surface. The explosion starts in the helium layer and then triggers another explosion in the carbon-rich core, which is why these are called "double detonations."

Double detonation from Shen et al. (2024)

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2/ We still don't know for sure what causes Type Ia supernovae. But there's growing evidence they come from white dwarfs roughly the mass of the Sun ("sub-Chandrasekhar-mass white dwarfs") that explode as they start to merge with other white dwarfs.

Explosion during a double white dwarf merger from Guillochon et al. (2010)

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Almost All Carbon/Oxygen White Dwarfs Can Support Double Detonations

1/ New paper thread (well, new last week)! We find that most white dwarfs are born ready to explode. They just need something to hit them hard enough, which should happen when they merge with other white dwarfs. Read on for details! arxiv.org/abs/2405.19417

arxiv.org

Double detonations of sub-Chandrasekhar-mass white dwarfs (WDs) in unstably mass-transferring double WD binaries have become a leading contender to explain most, if not all, Type Ia supernovae....

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Ava Polzin @avapolzin.bsky.social · 08/12/2023

So excited that our SIRIUS B book is now up on arXiv! arxiv.org/abs/2312.04041 It is intended to serve as a primer for students looking to pursue astro professionally. Participants in our 2024 program will get printed copies thanks to the IAU NA-ROAD Women & Girls in Astro mini-grant. 1/ 🧪🔭👩‍🔬

Cover of the SIRIUS B book, "Astronomy as a Field: A Guide for Aspiring Astrophysicists", which features a drawing of a girl holding her hands out toward space.

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Misty Bentz @astronomisty.bsky.social · 10/11/2023

If you are enjoying the new Euclid image of the Horsehead Nebula, you might be interested learn about the person who first discovered this beautiful nebula: Williamina Fleming.

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Alex Ji @alexji.bsky.social · 21/09/2023

On the astro postdoc market this year? @UChicagoAstro has several fellowships, listed here: astrophysics.uchicago.edu/research/pri... If you're interested in near-field cosmology here, please do reach out to me by e-mail! (Link fixed now, thanks to watchful eyes 😅)

astrophysics.uchicago.edu

Postdoctoral Fellowships in Astrophysics | Department of Astronomy and Astrophysics | The University...

The Department of Astronomy & Astrophysics is a world leader in astrophysical research and offers rigorous academic programs at both the graduate and undergraduate level.

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Dr Ashley J Ruiter @astronomash.bsky.social · 14/09/2023

Transients Down Under abstract submission is closing soon! transientsdownunder.github.io #transient #astrophysics @Swinburne

poster of Australia cutout in black with Aboriginal artwork, background is stars and nebulae, below has images of tennis, roos and telescope domes, building in Melbourne, Dark Emu, a koala.

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18/18 I had a great time working with new people and learning new things somewhat outside my comfort zone for this project. It helped remind me why being a scientist is awesome, and that I'm extremely lucky and privileged to be one!

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17/ If instead these binaries merge unstably, they won't be a viable supernova progenitor channel. But I think that's fine because I think a different scenario (mergers of two white dwarfs) causes Type Ia supernovae!

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16/ Aside from (hopefully) solving this interesting puzzle, this scenario also touches on Type Ia supernovae. Mass transfer from a subgiant to a white dwarf has previously been thought to grow the white dwarf stably up to the Chandrasekhar mass, where it then explodes.

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15/ Katie found that there are indeed the right number of subgiant + white dwarf mergers to explain the observed overdensity of crystallizing white dwarfs. In fact, the best match requires a common envelope parameter that agrees with studies of other kinds of binaries!

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14/ And because the neon and magnesium have extra neutrons and are heavier, this rearrangement releases some gravitational energy, which turns out to be enough to explain the extra cooling delay!

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13/ These crystals are lighter than the liquid, and they float upwards. This displaces the neon- & magnesium-rich liquid downwards in a process that's been termed "distillation" (see ui.adsabs.harvard.edu/abs/2021ApJ..…).

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12/ And second, it has a lot of neon-22 and magnesium-26. Both of these isotopes have 2 extra neutrons. It turns out that when the white dwarf cools and starts to freeze, the crystals that are formed are deficient in neon & magnesium.

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11/ Finally, after helium-burning ends, the star cools back into a white dwarf, but with two interesting features. First, it's massive but still made of carbon & oxygen, which is exactly what we want!

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10/ Later, when the helium starts burning, the nitrogen is converted into oxygen-18, then neon-22, and finally magnesium-26. Meanwhile, the bulk of the helium burns into carbon and oxygen.

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9/ This helium-rich surface layer will contain hydrogen from the subgiant and carbon mixed in from the white dwarf core. The combination of hydrogen and carbon burns to nitrogen through the first steps of our good friend, the CNO cycle.

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8/ And I think we found one: mergers of white dwarfs with slightly evolved ("subgiant") stars! During the merger, the envelope of the subgiant gets ejected and the subgiant's helium-rich core gets added to the white dwarf's surface.

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7/ There have been multiple efforts to come up with an answer. All have required atypical compositions, which may be the answer, but I was hoping for a solution that would work for normal abundances.

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6/ There needs to be some mechanism that 1) produces massive (~1.2 solar masses) carbon/oxygen white dwarfs and 2) causes some delay in their cooling that lasts for billions of years longer than the normal delay from crystallization.

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5/ And for another, there shouldn't even be an overdensity of stars at the blue end (the left side) because this crystallization region is for white dwarfs made of carbon & oxygen, but stars that blue are too massive to form carbon/oxygen white dwarfs on their own.

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4/ But it turns out there are some weird things about this crystallization region, first pointed out by Sihao Cheng (see his thread at twitter.com/SihaoCheng/status/11712…). For one, a lot of the stars in it are older than surrounding stars.

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3/ This energy release halts the cooling for a little bit. We can see this pause as a buildup of white dwarfs along their cooling sequence, helpfully outlined in orange in this paper by Pier-Emmanuel Tremblay.

The white dwarf cooling sequence, with the crystallization region outlined in orange.

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2/ As white dwarfs evolve and cool, they eventually reach a point where they start to crystallize (a process akin to freezing). This releases some energy, just like the opposite process (melting) requires some energy input.

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CosmicRami 🏳️‍🌈🏳️‍⚧️ @rami.spaceaustralia.com · 29/06/2023

1/ Paper thread! I had a fun time exploring white dwarf physics with Simon Blouin and Katie Breivik. Read on to find out more! arxiv.org/abs/2308.04559

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BIG SCIENCE NEWS! 🧪 Global astronomers announce strongest evidence yet of stochastic gravitational wave background! I'm one of the co-authors on these papers 😭😭😭 www.spaceaustralia.com/feature/aust… 📸 @astronerdika.bsky.social

Graphic showing the Earth at the centre of a warped space-time grid. On the grid, surrounding the Earth are several pulsars whose beams are being distorted with space-time. In the distance, and on the edges of the image, are swirling binary black holes.

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10/10 And one last thing: Kareem decided to submit to a free, open-access journal, leading to the attached arXiv submission comments. Perhaps a sign of interesting things to come....?

Comment that reads: "Submitted to the Open Journal of Astrophysics. 25 pages, 17 figures, zero dollars"

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9/ Kareem also identified 2 new LP-40 stars, bringing that total to 6. These are possibly the surviving remnants of stars that partially exploded (as opposed to the D6 stars, which are the companions of stars that completely exploded). Also extremely interesting!

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7/ Kareem is amazing at many, many things, including using Gaia data. He immediately came up with better search criteria and conducted follow-up observations, and within a few weeks had several good candidates.

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6/ Last fall, at our KITP program, I got Kareem El-Badry interested in these stars. This was perhaps the most scientifically useful thing I did at the program!

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5/ But in 2020, when Gaia's third data release came out, I wasn't able to find any new candidates even though the data was better. There was too much noise...or so I thought.

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3/ They'll also be covered in the ash from the explosion that occurred right next to them. These characteristics (their hypervelocities and unusual compositions) should be unique signatures of the D6 scenario.

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2/ Without another star keeping it in orbit, the surviving white dwarf flies away like a stone from a sling at velocities of 1000-2500 km/s (2-6 million mph), arguably faster than any known stars in the Milky Way.

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1/ My colleagues and I have been working on a Type Ia supernova model (the "D6" scenario) involving two white dwarfs that may lead to the explosion and destruction of only the more massive one.

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