2025 PSW Member Lecture
Real-Time Observations of Early Solar System Evolution
The Case of ~1 Myr Old Sun-like RW Aurigae A
Carey Lisse
Senior Scientist
Johns Hopkins University Applied Physics Laboratory
Sponsored by PSW Science Member Andrea Meredith and UMBF, Inc.
About the Lecture
The RW Auriga T-Tauri System (“RW Aur A”) is a special laboratory for studying processes occurring in very young T Tauri stars, like the Sun was at ~1 Myr age. This lecture will discuss recent evidence for destructive accretion of migrating differentiated asteroids in the evolution of the solar system in the RW AurA system.
These observations start with a tremendous drop in RW Aur A’s normal visible and soft XUV emission reported from Chandra in 2017. This, coupled with an observed concomitant large increase in Fe K-shell X-rays, suggested that huge amounts of obscuring gaseous Fe and fine “neutral” refractory rocky dust able to withstand temperatures > 1600 K was quickly created around a central protostar. Subsequent near-infrared spectrophotometric observations exhibited evidence for a highly excited system with a bright, hot, asymmetric accretion disk, numerous hot atomic emission lines from the protostar’s atmosphere, and a new stochastic emission event in its high-speed focused outflow jets moving away at 100 – 200 km/s. The hot (T~20,000K) bifurcated jet spectral signature a few years later was seen to decay back to its normal pre-event level, suggesting that Vesta-sized amounts of excess hot Fe and S, from a disrupted object’s core had been blown out of the system’s jets in ~7 years. Conspicuously absent in the jet spectral signatures were any traces of rock forming elements like Ca, Mg, Na, K, or Al that occur planetesimal lithospheres. These species must have remained in the solid phase, producing the large amounts of reported new “neutral extinctors” in the protostar’s atmosphere and the signatures of refractory inclusions and chondrules found in Spitzer IRS spectra of the system.
Overall, we believe these observations are explained by the catastrophic disruption of a primordial, differentiated, migrating planetesimal, of ~250 km radius, similar in size to the largest asteroids in our solar system, ejecting its iron core material, and the reprocessing and redistribution of its rocky components into materials akin to the first solids found in our own solar system. This processing, which appears to be starting again now, is consistent with theories that all the solar system’s asteroids were first formed big with liquid metal cores, then disrupted and re-accreted into the bodies seen today.
About the Speaker
Carey M. Lisse is Principal Staff Scientist in the Applied Physics Laboratory at Johns Hopkins University. Prior to assuming his current position he held a variety of posts at APL and NASA.
Carey has been active in the fields of astronomy and physics for four decades. He began his career as an instrument scientist at NASA/Goddard and worked on the Nobel prize-winning COBE project. He later turned his attention to planetary studies, writing his dissertation on comets detected in the COBE all sky survey. Since then he has focused primarily on solar system relic fossils, like comets, making important discoveries about their X-ray emissions and working from start to finish on the NASA Deep Impact (DI) mission, including using the Spitzer Space Telescope to observe dust excavated from the comet 9P/Tempel 1 by the DI Impactor. He also studies asteroids and x-ray emission from planets, and searches for the presence of asteroids and comets around other stars. He has also served as a member of the New Horizons and SPHEREx science teams. He last spoke at PSW in 2016.
Among other honors, Carey received awards as part of the COBE team, the JPL Stardust flight team, the EPOXI flight team, and the New Horizons Science Team. He is the recipient of a Space Telescope Science Institute Science Merit Award, a Space Foundation Space Achievement Award, a Johns Hopkins Applied Physics Laboratory Special Achievement Award, and he was awarded the Isaac Newton Institute Visiting Fellowship at Cambridge University. Carey is a AAAS Fellow, and The Asteroid 12226 Caseylisse is named for him. Carey also has appeared in numerous documentaries about planetary science and cosmology.
Carey earned a BA in Chemistry at Princeton, an MS in Chemistry at UC-Berkeley, and an MS and a PhD in Physics at U. Maryland – College Park.
Minutes
On May 9, 2025, Members of the Society and guests joined the speaker for a reception and dinner at 5:45 PM in the Members’ Dining Room at the Cosmos Club. Thereafter they joined other attendees in the Powell Auditorium for the lecture proceedings. In the Powell Auditorium of the Cosmos Club in Washington, D.C., President Larry Millstein called the lecture portion of the 2,515th meeting of the Society to order at 8:04 p.m. ET. He began by welcoming attendees, thanking sponsors for their support, announcing new members, and inviting guests to join the society. Scott Mathews then read the minutes of the previous meeting which included the lecture by James Bock, titled “SPHEREx: An All-sky Infrared Spectral Survey Explorer Satellite”. The minutes were approved, pending a minor correction.
President Millstein then introduced the speaker for the evening, Carey Lisse, of the Johns Hopkins Applied Physics Laboratory. His lecture was titled “Real-Time Observations of Early Solar System Evolution”.
The speaker began by saying that throughout the evolution of the Universe, there has been a process of accretion and aggregation of bodies, occurring over and over again, on many length scales. He said that his job was to explain how this process led to the formation of our solar system. He introduced the RW Aurigae system, saying that on a galactic scale, it was very close to our solar system, and that it was a good model for our solar system at the age of about 1 million years.
Lisse described the steps in the formation of a solar system: the collapse of matter into a protostar, the formation of a proto-stellar disk and jets, the formation of comets and gas giants, the subsequent formation of terrestrial and rocky planets, and finally a mature system with planets and small relic bodies. The speaker provided “show-and-tell”, passing around a sample from a 4.5-billion-year-old iron meteorite, found in northern Argentina. He showed a sample of a carbonaceous chondrite meteor, as well as micrographs from a similar specimen. The micrograph included CAI’s (Calcium Aluminum Inclusions) and chondrules (melted rock). Lisse discussed the conditions under which such materials form, and the debate amongst the research community as to what processes created those conditions in the early solar system. He said “There are limits to what meteoritical studies can tell us, as the evidence has literally been baked, boiled, blasted, melted and in many cases submerged in water for millions of years.”
The speaker then presented some details of RW Aurigae: a binary protostar, about 165 pc from Earth, in which the B-component orbits the A-component, both having roughly one solar mass, each shining not from nuclear fusion, but from accretion energy alone. He indicated that B appears to be mature, due to its lack of stellar jets, while A has strong jets and is therefore immature. Lisse said that although B is acting like a 3-million-year-old star, A acts as if it were only 1 million years old. He showed the light curve for RW Aurigae A, which indicated a significant decrease in light output, starting in 2014 and lasting approximately 6 years. This variation in brightness appears to be consistent with measured changes in X-ray emissions from the same period of time. He described the “outburst sequence”, in which a large object, like an asteroid, disrupts the typical accretion process, breaks-up, and creates a ring of debris that obscures the protostar and emits radiation characteristic of the debris. Lisse showed how infrared spectroscopy could be used to identify a number of important elements as a function of time, confirming “impulse delivery” to RW Aurigae A. He showed how elements present in the jets could be distinguished from elements in the atmosphere or on the surface by the observation of bifurcated emission lines. Lisse described the compositional difference revealed by infrared spectroscopy, before and after the “upset event”, arguing that the event was likely caused by the destruction of a planetesimal core.
The speaker described the current model of asteroid formation: asteroids grow to large sizes by accretion, they differentiate into a core and mantle, they are fragmented and transformed by collisions, and finally they re-agglomerate into inhomogeneous, mixed bodies. Lisse said that the spectroscopic observations of RW Aurigae, before and after the impulse event, are consistent with this model and account for the formation of CAI’s and chondrules.
The speaker ended his talk by summarizing some of the observations and conclusions mentioned in the lecture:
• RW Aurigae A, appears to act like a 1 Myr old protostar, without a highly excited stellar atmosphere, which allows for better observation.
• A massive, system-wide stochastic event occurred between 2014 and 2020.
• X-ray and IR spectroscopy indicate the conditions and elemental composition for the creation of CAI’s and chondrules.
• Recent measurements show a new drop in brightness and may indicate a more recent stochastic event.
The lecture was followed by a Question and Answer session.
A member asked about the core of the Earth having both a solid and liquid portion, and if this was consistent with the observations of RW Aurigae. Lisse responded that it was consistent, but that the planetesimal which presumably crashed into RW Aurigae A may have had a significantly different mass, as compared to the Earth, and that therefore the process of forming a solid-liquid core might occur at a different rate.
A member asked about the physical process by which the jets are formed. Lisse responded that astro-physical jets are still somewhat of a mystery. He said “We understand that they are extracting energy and angular momentum from the system, but we don’t understand the exact physics.”
A member on the live stream asked what materials make up star dust? Lisse said most of the dust contains oxygen, magnesium, silicon, iron, and sulphur. These are the “rock forming” elements that are found in relatively high abundance in the universe.
After the question and answer period, President Millstein thanked the speaker and presented him with a PSW rosette, a signed copy of the announcement of his talk, and a signed copy of Volume 17 of the PSW Bulletin. He then announced speakers of up-coming lectures and made a number of housekeeping announcements. He adjourned the 2,515th meeting of the society at 9:43 pm ET.
Temperature in Washington, DC: 13.3° Celsius
Weather: Fair
Audience in the Powell auditorium: 54
Viewers on the live stream: 32
For a total of 86 viewers
Views of the video in the first two weeks: 369
Respectfully submitted, Scott Mathews: Recording Secretary