NOVA: Planets Must Be Formed Early

[bystander]

Planets Must Be Formed Early
Netherlands Research School for Astronomy (NOVA) | 2020 Jun 18

Artist's conceptualization of the dusty TYC 8241 2652 system as it might have appeared several years ago when it was emitting large amounts of excess infrared radiation. Credit: Gemini Observatory/AURA; Artwork by Lynette Cook

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Scientists have found evidence that planets form in a blink of an eye on a cosmic scale. New results, obtained using the combined power of Atacama Large Millimeter/submillimeter Array (ALMA) and Very Large Array (VLA), show that very young disks, with ages of between 0.1 - 0.5 million years, have more than enough pieces to assemble planetary systems.

A conundrum that’s been puzzling scientists for years has just found a plausible solution. In recent years astronomers weighed disks across the Milky Way with ALMA. The studies of 1-3 million year old disks kept delivering the news that made scientists scratch their heads - there is a shortage of dust in those mature disks to make even a single gas giant planet like Jupiter, let alone larger planets or groups of gas giants as seen in our solar system.

The answer, says Łukasz Tychoniec, a graduate student at Leiden Observatory and lead author of the new paper, is that “we need to look earlier instead of looking for missing mass.” ...

Dust Masses of Young Disks: Constraining the Initial Solid Reservoir for Planet Formation ~ Lukasz Tychoniec et al

• arXiv.org > astro-ph > arXiv:2006.02812 > 04 Jun 2020 (v1), 05 Jun 2020 ( v2)

[BDanielMayfield]

Another line of evidence for the rapid formation of planets is radioactive aluminum-26.

Meteorite research has also shown that 26Al was relatively abundant at the time of formation of our planetary system. Most meteoriticists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.

26Al decays to stable 26Mg with a half-life of only 717,000 years. The Supernova(s) that produced the original 26Al in our proto-Sun's planetary disk could not have happened very long before the Sol system's formation due to 26Al's astronomically brief lifetime. Neither could the formation of planetoids have taken very long, or there would not have have been enough 26Al left to have melted the interiors of asteroids.


Bruce

[bystander]

Another line of evidence for the rapid formation of planets is radioactive aluminum-26.

Studying Radioactive Aluminum in Solar Systems Unlocks Formation Secrets
McDonald Observatory | University of Texas, Austin | 2020 Jul 28

This artist's concept available from NASA illustrates a solar system that is a much younger version of our own. Dusty disks, like the one shown here circling the star, are thought to be the breeding grounds of planets, including rocky ones like Earth. Credit: NASA/JPL-Caltech

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An international team of astronomers ... has proposed a new method for the formation of aluminum-26 in star systems that are forming planets. Because its radioactive decay is thought to provide a heat source for the building blocks of planets, called planetesimals, it’s important for astronomers to know where aluminum-26 comes from. ...

Since its discovery in the Allende meteorite in 1976, astronomers have debated the origin of the considerable amount of aluminum-26 in our early solar system. Some have suggested that it was blown here by supernova explosions and winds from massive stars. However, these scenarios require a good deal of chance: Our Sun and planets would have to form at exactly the right distance from massive stars, which are quite rare.

Offner’s team has proposed an explanation that does not require an outside source. They propose that aluminum-26 formed close to the young Sun, in the inner part of its surrounding planet-forming disk. As material fell from the disk’s inner edge onto the Sun, it created shockwaves that produced high-energy protons known as cosmic rays.

Leaving the Sun at nearly the speed of light, the cosmic rays slammed into the surrounding disk, colliding with the isotopes aluminum-27 and silicon-28, changing them into aluminum-26. ...

Aluminum-26 Enrichment in the Surface of Protostellar Disks Due to Protostellar Cosmic Rays ~ Brandt A. L. Gaches et al

• Astrophysical Journal 898(1):79 (2020 Jul 20) DOI: 10.3847/1538-4357/ab9a38
• arXiv.org > astro-ph > arXiv:2007.12707 > 24 Jul 2020

[BDanielMayfield]

Another line of evidence for the rapid formation of planets is radioactive aluminum-26.

Studying Radioactive Aluminum in Solar Systems Unlocks Formation Secrets
McDonald Observatory | University of Texas, Austin | 2020 Jul 28

This artist's concept available from NASA illustrates a solar system that is a much younger version of our own. Dusty disks, like the one shown here circling the star, are thought to be the breeding grounds of planets, including rocky ones like Earth. Credit: NASA/JPL-Caltech

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An international team of astronomers ... has proposed a new method for the formation of aluminum-26 in star systems that are forming planets. Because its radioactive decay is thought to provide a heat source for the building blocks of planets, called planetesimals, it’s important for astronomers to know where aluminum-26 comes from. ...

Since its discovery in the Allende meteorite in 1976, astronomers have debated the origin of the considerable amount of aluminum-26 in our early solar system. Some have suggested that it was blown here by supernova explosions and winds from massive stars. However, these scenarios require a good deal of chance: Our Sun and planets would have to form at exactly the right distance from massive stars, which are quite rare.

Offner’s team has proposed an explanation that does not require an outside source. They propose that aluminum-26 formed close to the young Sun, in the inner part of its surrounding planet-forming disk. As material fell from the disk’s inner edge onto the Sun, it created shockwaves that produced high-energy protons known as cosmic rays.

Leaving the Sun at nearly the speed of light, the cosmic rays slammed into the surrounding disk, colliding with the isotopes aluminum-27 and silicon-28, changing them into aluminum-26. ...

Aluminum-26 Enrichment in the Surface of Protostellar Disks Due to Protostellar Cosmic Rays ~ Brandt A. L. Gaches et al

• Astrophysical Journal 898(1):79 (2020 Jul 20) DOI: 10.3847/1538-4357/ab9a38
• arXiv.org > astro-ph > arXiv:2007.12707 > 24 Jul 2020

Thanks sincerely for linking this new report to my comment bystander! This news is right up my interest lanes; element nucleosynthesis, planetary formation, and work coming out of my favorite Observatory, McDonald in west Texas, managed by UT.

The formation of 26Al via spallation reactions in addition to stellar explosions is a very interesting possibility. This is a paper I'll be reading ...

[BDanielMayfield]

The paper, entitled

Aluminum-26 Enrichment in the Surface of Protostellar Disks Due to Protostellar Cosmic Rays

by Brandt A. L. Gaches, Stefanie Walch, Stella S. R. Walch, Carsten Münker
abstract:

The radioactive decay of aluminum-26 (26Al) is an important heating source in early planet formation. Since its discovery, there have been several mechanisms proposed to introduce 26Al into protoplanetary disks, primarily through contamination by external sources. We propose a local mechanism to enrich protostellar disks with 26Al through irradiation of the protostellar disk surface by cosmic rays accelerated in the protostellar accretion shock. We calculate the 26Al enrichment, [26Al/27Al], at the surface of the protostellar disk in the inner AU throughout the evolution of low-mass stars, from M-dwarfs to proto-Suns. Assuming constant mass accretion rates, m˙, we find that irradiation by MeV cosmic rays can provide significant enrichment on the disk surface if the cosmic rays are not completely coupled to the gas in the accretion flow. Importantly, we find that low accretion rates, m˙<10−7 M⊙ yr−1, are able to produce canonical amounts of 26Al, [26Al/27Al]≈5×10−5. These accretion rates are experienced at the transition from Class I- to Class II-type protostars, when it is assumed that calcium-aluminum-rich inclusions condense in the inner disk. We conclude that irradiation of the inner disk surface by cosmic ray protons accelerated in accretion shocks at the protostellar surface may be an important mechanism to produce 26Al. Our models show protostellar cosmic rays may be a viable model to explain the enrichment of 26Al found in the Solar System.

This could very well be a common feature of protostellar systems, helping explain how exoplanets can form as quickly as they apparently do. If so, it improves the odds of Earth-like planet formation. Awesome

Bruce

[neufer]

Click to play embedded YouTube video.

[BDanielMayfield]

There are three 26Al producing nuclear reactions that the authors believe are at work in protostellar disks:

We take into account three different processes:
• 27Al + p →26 Al + pn,
• 26Mg + p →26 Al + n,
• 28Si + p →26 Al + 3He

It's rather amazing that another site of nucleosynthesis could be a proto star's accretion disk, even before the star begins its main sequence lifetime.

Bruce

[neufer]

1969 Allende meteorite slice on display at Arizona State University

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<<The Allende meteorite, which fell in 1969, contained abundant calcium–aluminium-rich inclusions (CAIs). These are very refractory materials and were interpreted as being condensates from a hot solar nebula. then discovered that the oxygen in these objects was enhanced in ¹⁶O by ~5% while the ¹⁷O/¹⁸O was the same as terrestrial. This clearly showed a large effect in an abundant element that might be nuclear, possibly from a stellar source. These objects were then found to contain strontium with very low ⁸⁷Sr/⁸⁶Sr indicating that they were a few million years older than previously analyzed meteoritic material and that this type of material would merit a search for ²⁶Al. ²⁶Al is only present today in the Solar System materials as the result of cosmic reactions on unshielded materials at an extremely low level.

To establish the presence of ²⁶Al in very ancient materials requires demonstrating that samples must contain clear excesses of ²⁶Mg /²⁴Mg which correlates with the ratio of ²⁷Al/²⁴Mg. The stable ²⁷Al is then a surrogate for extinct ²⁶Al. The different ²⁷Al/²⁴Mg ratios are coupled to different chemical phases in a sample and are the result of normal chemical separation processes associated with the growth of the crystals in the CAIs. Clear evidence of the presence of ²⁶Al at an abundance ratio of 5×10⁻⁵ was shown by Lee, et al. The value (²⁶Al/²⁷Al ∼ 5×10⁻⁵) has now been generally established as the high value in early Solar System samples and has been generally used as a refined time scale chronometer for the early Solar System. Lower values imply a more recent time of formation.>>