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HD 163296: Emerging Insights into Circumstellar Disks

We should be glad to run into the unexpected when doing research, because things we hadn’t foreseen often point to new understanding. That’s certainly the case with infant planetary systems as observed through the circumstellar disks of gas and dust surrounding young stars. ALMA (the Atacama Large Millimeter/submillimeter Array) has been central to the study of such targets. An array of 66 radio telescopes in Chile’s Atacama Desert, the facility works at millimeter and submillimeter wavelengths to provide detailed imaging of emerging systems.

Because it has been revealing a variety of small-scale structures within circumstellar disks, ALMA is giving us insights into planet formation as we observe gaps, rings and spiral arms and their interactions with young planets. This is where the unexpected comes in. For researchers looking at a 5 million year old star called HD 163296 are seeing an unusual amount of dust, more than 300 times the mass of the Earth, despite the detection of at least three proposed planets whose masses range between that of Jupiter and twice that of Uranus.

HD 163296 is a Herbig Ae/Be star, a pre-main-sequence object still embedded in a gas-dust envelope. The star has about twice the mass of the Sun, with a massive disk accounting for approximately one tenth of the Sun’s mass. The disk is about 500 AU wide, making it perhaps twice the outer boundary of our own Kuiper Belt. It’s noteworthy that the disk remains as abundant as it is given the age of the system and its production of at least three planets.

Image: Artist’s impression of protoplanets forming around a young star. Credit: NRAO / AUI / NSF / S. Dagnello.

The conventional model of dust distribution in this environment is migration from the outer regions of the disk inward because of coupling and friction with abundant gas, and the assumption has been that this migration would be stopped by the presence of massive planets. The hypothesized result: Dust piling up outside the orbits of the massive planets, while clearing out inside the orbit of the innermost planet as the dust migrates onto the star.

But the new ALMA observations, in tandem with the researchers’ dynamical simulations of this system and their application of a collisional model to test various outcomes, show a different result. The orbital regions inside the first planet, as well as between the first and second planet, have some of the highest concentrations of dust anywhere in the disk. The new work looks at this dust distribution in terms not only of interactions with gas and the new planets but also with what must be a large population of planetesimals, small objects that provide the building blocks of planet formation.

Diego Turrini is lead author of the study and a researcher at the Institute for Space Astrophysics and Planetary Science (IAPS) of the Italian National Institute of Astrophysics (INAF):

“From the study of the Solar System we know that mature circumstellar disks like HD 163296 are not composed only by gas and dust, but also contain an invisible population of small planetary objects similar to our asteroids and comets. We also know that the appearance of giant planets affects these planetesimals by causing in their orbital evolution a brief but intense spike of dynamical excitation that, while short from the point of view of the long life of a planetary system, can have a duration comparable to the life of circumstellar disks.”

Image: The circumstellar disk surrounding HD 163296 and the system of gaps and rings created by its young giant planets as recently imaged by ALMA (DSHARP Project). Credit: ALMA (ESO/NAOJ/NRAO), S. Dagnello.

Thus the dynamical simulations calculated to show these interactions and test their limits. What emerges is a regime of planetesimal collisions that remains relatively calm until the giant planets reach their ultimate masses. As the planets grow, more and more planetesimals are driven into eccentric, inclined orbits. At this point, planetesimal collisions become intense. We now have an explanation for the renewed dust in the system, and an orbital distribution of that dust markedly different from before, one where dust accumulates in the inner orbital region of the first planet and also forms a ring between the first and second planets.

These findings jibe with the ALMA observations, linking simulations to a living system. Thus Danai Polychroni, co-author of the study and at the time professor at the Universidad de Atacama and adjunct researcher at INAF-IAPS:

“The fast rate at which ALMA is providing new and more detailed data on HD 163296 allowed us to expand our study beyond its original scope. We noticed that many planetesimals are excited to supersonic velocities with respect to the surrounding gas of the disk and can create shocks waves that can heat both planetesimals and gas. While we could not yet model this process in detail, recent observations reported the unexpected presence of CO gas in regions characterized by temperatures where it should be frozen solid and of possible anomalies in the thermal structure of the disk. Both findings can in principle be explained thanks to the presence of these supersonic planetesimals and the shock waves they create.”

Image: The disk of icy planetesimals hidden in HD 163296’s circumstellar disk seen from above and the side. The young giant planets rapidly create a large population of exocomets acting as high-speed projectiles for the other bodies. Credit: D. Turrini (INAF-IAPS).

We’ve learned, then, that the formation of planets in a circumstellar disk can produce dynamical excitation leading to a resurgence in the ratio of dust to gas that halts the decay of the dust by creating second-generation dust grains through high-velocity collisions. This is useful stuff, for we’re observing more and more possible planets emerging in disks in which planetesimals should be widespread. The disk around HD 163296 may be showing us a stage of planet and system evolution that is common, and one we can now look for in a range of young systems.

From the paper:

The collisional production of second–generation dust in circumstellar disks hosting giant planets …likely represents a common evolutionary phase marking the transition from a circumstellar disk dominated by primordial dust to a debris disk dominated by second generation dust. Whether the amount of collisionally produced dust is high enough to produce observable signatures, like our results suggest being the case for HD 163296’s disk…depends on the characteristics of each specific system, first of all the masses of the giant planets and of the planetesimal disk.

The paper is Turrini et al., “Dust-to-gas ratio resurgence in circumstellar disks due to the formation of giant planets: the case of HD 163296,” Astrophysical Journal Vol. 877, No. 1 (23 May 2019). Abstract / Preprint.

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{ 16 comments… add one }
  • Mark June 1, 2019, 9:50

    Hate to be the grammar police but:
    “These findings gibe with the ALMA observations”

    “gibe” is a verb or noun used when describing taunting, scoffing, or sarcastic remarks, or the making of such.

    The word you are looking for in this case is “jibe:” to be in accord with.

    (actually, who am I kidding. I love to be the grammar police.)

    • Paul Gilster June 1, 2019, 11:05

      Good catch! Will fix.

    • ljk June 3, 2019, 9:47

      On the other hand, I would not be at all surprised if the Universe were gibing at our relatively youthful and primitive interpretations of itself.

      We are only a few hundred years from realizing that the Sun does not go around Earth. Less than a century in accepting that the Milky Way galaxy is not the entire Universe. Only started finding and accepting exoplanet evidence in the early 1990s. Still do not have a clue about any cosmic neighbors living or extinct, intelligent or otherwise.

      • Bruce D Mayfield June 3, 2019, 14:11

        And we haven’t even finished counting up the full number of planets in our own tiny system.

  • Patient Observer June 1, 2019, 10:49

    This may be bad internet etiquette as the following link pertains to a relatively old post regarding the origins of water on Earth thus off-topic for the current post.

    https://www.sciencedaily.com/releases/2019/05/190521101505.htm

    The theory is Theia (aka – the giant impactor) originated from the outer solar nebula and thus may have been laden with water – no comets with their deuterium ratio problems.

  • Geoffrey Hillend June 1, 2019, 17:39

    I like the idea of “The Formation of the Moon Brought Water to Earth” article, but from what I recall reading the Moon and the Earth’s mantle have similar chemistry, so Thea had to come from the inner solar system. The abundance of the oxygen isotopes O16, O17, and O18 indicate that the Moon and the Earth came from the same part of the solar system, Betty, Peterson, and Chaikin, The New Solar System, 4th Ed., P 137. The ratios of these three oxygen isotopes are the same for the Earth and the Moon, but are different for meteorites.

    Chondrites also come from the asteroid belt and the deuterium ratio used to support the idea that Earth’s water came from a comet is not a proven hypothesis. The jury is still out on that one since more evidence would be needed.

    • Patient Observer June 2, 2019, 0:09

      Another scenario (can’t find the link) suggested that the earth was fully molten at the time of impact allowing a very deep mixing of the two bodies. I suppose that such mixing averages out the isotope differences. Another lunar formation scenario:

      Scientists call such a cloud a synestia, a doughnut-shaped ring of debris full of molten rock that forms in the aftermath of a protoplanet collision. In this case, it would have been a massive collision early in our solar system’s history. According to the new theory, the moon formed within a few dozen years after the crash, as the synestia shrank and cooled. The Earth subsequently emerged about 1,000 years after the moon.

      https://www.space.com/39841-moon-formed-from-synestia-earth-crash-theory.html

      I guess this does tie into formation of circumstellar discs as collisions are fundamental to planet and moon formation and dispersion of debris after the collision.

      • AlexT June 3, 2019, 7:11

        …mixing averages out the isotope differences…

        The planet Earth is so huge anomality in Solar System, so I am wandering why someone should be bothered by observed differences in every area … Yes the Earth is deviated from “average” , this difference allows life to exist – obvious things.
        The different hypothesizes, that are not in contradiction with science, but in same time ungrounded on any scientific evidence, do not help to understand our anomality.

        • Bruce D. Mayfield June 3, 2019, 10:33

          What do you mean by “ungrounded on any scientific evidence” AlexT? Isotopic ratios are indeed tangible evidence. How planetary bodies accrete under the influence of gravity and other forces is a robustly developing science.

          Wether one believes the Earth was purposely designed or not, figuring out how it has come to be a planet swarming with life is an area of great interest. Quantifying the degree of Earth as an anomaly (and if it really is that anomalous) in the universe is a highly important goal to knowing how rare or abundant life might be.

          I often agree with your comments AlexT, but this one makes less sense than most of your observations.

          • AlexT June 3, 2019, 14:38

            …Isotopic ratios are indeed tangible evidence…

            Yes , for sure , measured isotopic ratios are obvious scientific facts…
            But “Theia” impact and/or other hypothesizes quoted here by “Patient Observer” – are only hypothesizes…
            But isotopic ratios distribution – does not need Theia…
            By the way it was discussed in one of previous topics: there is space bodies that has different from the Earth isotopes distribution and there is bodies that have similar distribution…
            I do not touch here in any way any “creationists” theory – it is religious area – I somehow out of this play.
            In same time the phrase: “Let there be light, there was light” is not in contradiction with scientific Big Bang theory (only small difference there should be someone who can spell this phrase right before Big bang :-).
            On my opinion the Theia hypothesis is not far from “let there be light” story, although scientifically it is possible, but in reality supported mostly by faith …

      • ljk June 3, 2019, 9:49

        There is evidence that Earth had life going back 4.1 billion years, which is just 500 million years after our planet formed. How long was it thought to be molten?

        https://www.sciencedaily.com/releases/2015/10/151019154153.htm

        Between the molten state and an object presumably smacking into Earth to make the Moon, amazing that life formed so relatively quickly. Or did something else happen altogether?

  • Harry R Ray June 4, 2019, 11:20

    Tharindu Jayasinghe tweeted this 15 hours ago: “We have just discovered a Dyson Sphere being made in front of our eyes! O K, probably not, but a star that has been non-variable for ~1800 days dropped by more than 1 mag in two days: details via @astromerstpl soon, probably tomorrow@j_tharindu.” Cant wait to see the light curve to see if it is more like Boyajian’s Stars’ D792(Q8) 22% dip, or more like EPIC204376071’s 80% dip. ALSO: Is this star more like the older Boyajian’s Star or the younger EPIC204376071, which is young enough to still have at least remainants of a previous circomstellar disk still orbiting it. Or, maybe the light curve will be symetric and of longer duration, making it more similar to that of Mamajek’s Object. We’ll know soon!

    • Harry R Ray June 5, 2019, 9:56

      The star has already returned to normal brightness, thus eliminating the Mamajek’s Objectscenario from consideration. ALSO: The star’s designation is ASASSN-v-J213939.3-702817.4. Its previously determined distance was logged on the ASASSN catalog as being 3,500 light years away. This has yet to be confirmed by GAIA DR2 data, so this may(if it is IN the DR2 catalog)be subject to change. Still no stellar class or stellar type to report, although this should be determined very quickly now that the star is back to normal brightness. FINALLY: New data! ASASSN saw NO infra-red EXCESS in the data, indicating that whatever blocked the star’s light was not close to the star.

      • Harry R Ray June 6, 2019, 9:57

        Most recent update from https://www.astronomerstelegram.org is as follows: Absolute magnitude is M_V 2.25. Based on this, the possibilities are as follows: ONE, A5(non-metallic)V to F1(metallic)V main sequence star; TWO, G2(non-metallic)IV to G5(metallic)IV subgiant; THREE, Blue Straggler; FOUR, RoAP star similar to Przybylski’s Star.

        • Harry R Ray June 9, 2019, 14:27

          ASASSN-v-J213939.3-702817.4 is almost exactly like Boyajian’s Star in one way and absolutely NOT in another. Its spectral type and class are as follows: F0V. However, even the POSSIBILITY of ANY secular LONG-TERM dimming has been TOTALLY ELIMINATED! Being a F3V main sequence star, Boyajian’s Star is very similar to ASASSN-v-J213939.3-702817.4, but still remains unique with respect to BOTH SHORT- TERM AND LONG-TERM DIMMING. I have always argued that Bouyajian’s Stars’ UNIQUENESS is its most strong argument for a non-natural solution to its chaotic variability. That arguement still stands.

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