Aspen 2023 LSExo - Schedule

Program Schedule

Registration and the Opening Reception will be held at the Aspen Center for Physics from 5:00 - 7:00 PM on Sunday evening.
Note that while locations will be suggested for the no-host dinners, the cost of these meals is not covered by registration fees.
While we do not have a designated poster session, we encourage participants to bring a scientific poster to this topical conference. Posters will be hung in the same location as the morning/evening breaks and will serve to stimulate discussion.
Our Public Lecture Speaker will be Dr. Susan Mullally (Space Telescope Science Institute).

Monday Talks:

Siyu Xu introduces the topic of white dwarf accretion
Tim Cunningham
- Title: A White Dwarf Accreting Planetary Material Determined from X-ray Observations
  - Abstract: We have recently made the first direct detection of planetary material accreting onto a white dwarf using X-ray observations. This discovery, recently published in Nature, confirms G29—38 - the prototype of all metal-polluted white dwarfs with detected debris disks – as a significant source of soft X-rays. Our detection relied upon a 106 ks exposure with the Chandra X-ray Observatory and provides the first direct evidence of ongoing accretion of planetary material onto a white dwarf. From the measured low-energy X-ray emission and modelled X-ray luminosity, we provide the first independent constraint on the accretion rate at such a system, finding an instantaneous accretion rate consistent with modelling of observed photospheric abundances. We measure a relatively low plasma temperature of kT≈0.5 keV, corroborating the predicted bombardment solution for white dwarfs accreting at low accretion rates. I will present this recent discovery, together with additional joint modelling of X-ray and UV observations, and its implications for the study of evolved planetary systems, including the accretion rates and bulk elemental compositions.
Isabella Trierweiler
- Title: A chondritic solar neighborhood
  - Abstract: A persistent question in exoplanet demographics is whether exoplanetary systems form from similar compositional building blocks to our own. Polluted white dwarf stars offer a unique way to address this question as they provide measurements of the bulk compositions of exoplanetary material. These stars show evidence of recent accretion of rocky bodies in the form of metal lines in their spectra, which tell us about the relative elemental abundances of the accreted material. We present a statistical analysis of the rocks polluting oxygen-bearing white dwarfs and compare their compositions to Solar System rocks, such as chondrites, bulk Earth, and crust. We find that the majority of the extrasolar rocks are consistent with the composition of CI chondrites, a result that is supported by the compositions of stars in the solar neighborhood.
Paula Izquierdo
- Title: First results on planetary debris abundances from the next-generation spectroscopic surveys
  - Abstract: White dwarfs provide the unique opportunity to measure the composition of exoplanets through the analysis of the photospheric metal lines present in their spectra. These metals are the result of the accretion of planetesimals, which pollute the otherwise pristine hydrogen or helium white dwarf photospheres. Even though between 25 to 50% of all white dwarfs display some signature of planetary debris accretion, the vast majority only exhibit absorption lines of one or two elements. Since less than one in a hundred white dwarfs show the presence of multiple refractory and volatile elements, detailed abundance analyses have just been carried out for a few dozen systems, preventing robust statistical studies of the make-up of exoplanetary systems. This situation will only be overcome by a massive increase of spectroscopically confirmed white dwarfs. We will report here the first results of the follow-up of ~300,000 white dwarf candidates, obtained as part of the next-generation spectroscopic surveys DESI, SDSS-V, WEAVE. Within these projects, we have observed a sizable number of new, strongly metal-polluted white dwarfs. Our analysis already enlarges the sample of detailed abundance studies, and by the end of these surveys, we expect to analyse 500-1000 white dwarfs with numerous element detections.

Tuesday Talks:

Claudia Aguilera-Gomez introduces the topic of post main-sequence planetary engulfment
Ricardo Yarza
- Title: The hydrodynamics of post-main-sequence planetary engulfment
  - Abstract: In many planetary systems, the host star will expand during post-main-sequence evolution and engulf its planets and/or brown dwarfs (hereafter substellar bodies, SBs). I will present hydrodynamical simulations of the stellar envelope in the vicinity of an engulfed SB. These simulations help us understand the flow morphology near the SB, and quantitatively determine the drag forces acting on it. We use these drag forces to integrate the trajectory of the SB inside the envelope numerically. This simplified numerical framework enables exploration of a wide parameter space.We found that engulfment can increase the luminosity of the star by up to several orders of magnitude for up to few thousand years, depending on the mass of the SB and the evolutionary stage of the star. SBs as small as twenty Jupiter masses might eject the envelope of a Sun-like star at the tip of the red giant branch.These findings complement missions such as Kepler and TESS, which when coupled with models of planetary system evolution show that SB engulfment is common. These systems are a new frontier in exoplanet science with the potential to offer critical constraints on the late-stage evolution of planetary systems.
Aida Behmard
- Title: Planet Engulfment Detections are Rare According to Observations and Stellar Modeling
  - Planetary system dynamical evolution can cause planets to be engulfed by their host stars. Following engulfment, the stellar photosphere abundance pattern will reflect accretion of rocky material from planets. Multi-star systems are excellent environments to search for such abundance trends because stellar companions form from the same natal gas cloud and are thus expected to share primordial chemical compositions. Abundance measurements have occasionally yielded rocky enhancements, but few observations have targeted known planetary systems, which can shed light on how engulfment shapes planetary architectures. To address this gap, we carried out a Keck-HIRES survey of 36 multi-star systems that host known planets. We found that only HAT-P-4 exhibits an abundance pattern suggestive of engulfment, but is more likely primordial based on its large projected separation (~30,000 AU) that exceeds typical turbulence scales in molecular clouds. To understand the lack of engulfment detections among our systems, we quantified the strength and duration of refractory enrichments in stellar photospheres using MESA stellar models. We found that observable engulfment signatures from 10 MEarth engulfment events last for only 50 Myr to 1.5 Gyr in solar-like stars, indicating that engulfment will rarely be detected in systems that are several Gyr old.
Matteo Cantiello
- Title: Planetary Engulfments
  - Abstract: I will present 1D and 3D simulations of planetary engulfments in post main sequence low-mass stars. The talk will focus on the possible observational signatures of such events, including photometric transients, chemical abundance anomalies, and enhanced rotational velocity of the host star.

Wednesday Talks

Thomas Barclay
- Title: Detecting Planets Orbiting Post-MS with Roman
  - Abstract: The Roman Space Telescope will conduct a survey of the Galactic Bulge. This survey will collect data at approximately a 15-minute cadence of 2 square degrees, which will result in time series data on millions of stellar sources. Data from this survey is anticipated to include as many as 100,000 transiting planets and will enable us to perform hitherto impossible exoplanet population studies. We have developed software to create realistic simulations of the Roman Wide Field Instrument Galactic Bulge Survey data that injects transiting planets, eclipsing binaries, and astrophysical noise into time series of images. To explore what Roman can contribute to the understanding of exoplanets around evolved stars we have injected a realistic population of planets and realistic stellar noise into the red giant and subgiant stars in synthetic time series images, extracted light curves, and performed exoplanet detection. We will report on the expected population of transiting planets that we forecast that Roman will find and discuss synergies with asteroseismic detections of these same sources. This sample of planets will dramatically increase the number of know planets orbiting evolved stars, and provide an ideal sample for population studies.
Nora Eisner
- Finding longer-period planets around post-MS stars with citizen science
  - I will discuss the value of citizen science for identifying planets around post-MS stars that are typically missed by automated search algorithms. While standard automated transit detection methods are biased towards finding short-period planets (< 30 days in TESS data) and RV detection surveys are able to detect the very-long period planets (> 300 days), citizen science can provide the missing link to tie these two populations together by increasing the sample of intermediate-period planets (30 < P < 300 days). In particular, I will present the Planet Hunters TESS project, which harnesses the power of citizen science to find transit events in the TESS data by engaging tens of thousands of volunteers. Our results show that humans can outperform the automated detection pipelines for certain types of transits, in particular intermediate-period planets around both MS and post-MS systems. I will discuss how we have augmented the original Planet Hunters TESS project to specifically search for these planets around evolve stars in the TESS full frame images, and highlight some of the most interesting post-MS planet systems found with citizen science to date.
Sabrina Poulsen
- Title: A Search for the Giant Planets that Drive White Dwarf Accretion
  - Abstract: The presence of metals in the photospheres of white dwarf stars is a long standing puzzle. The strong gravitational field of these stars pulls heavier elements down on very short timescales leaving a chemically pure exterior of hydrogen, meaning the DAZs must be actively accreting metals. It is hypothesized that massive planets which survive the red-giant phase occasionally perturb the orbits of planetesimals, causing them to fall within the Roche limit of the star where they are disintegrated and accreted onto the DAZ. If this model for white dwarf contamination is correct, every DAZ hosts a planetary system containing a giant planet. To test this theory, we will use JWST’s MIRI imager to search for both resolved and unresolved Jupiter-mass companions to 4 nearby DAZs. If we do not find any planet candidates around these stars, we will have almost ruled-out a favored model for creating DAZs. In this presentation, I will discuss the design of our program, the on-sky abilities of MIRI to detect white dwarf companions, and share any preliminary candidates.

Thursday Talks:

Santiago Torres introduces the topic of dynamics in late-stage systems
Sihao Cheng
- Title: Planet candidates around massive white dwarfs
  - Abstract: Despite the large number of exoplanets confirmed today, we still know very little about planets around intermediate-mass stars (> 2 solar mass), due to a number of observational limitations. Fortunately, evolving into the white dwarf phase, those stars become much fainter and more stable, providing a unique window to directly detect light from planets. I will present a search for planets around young, massive white dwarfs using a characteristic infrared signature of giant planets. We found one high-credibility candidate of a 6-jupiter-mass planet and a few other candidates with marginal credibility. They are ideal targets for direct imaging follow-ups and almost guaranteed to be resolved by the James Webb Space Telescope.
Hanno Rein
- Title: Can exo-Oort clouds pollute white dwarfs?
  - Abstract: Strong metal absorption lines are found in the spectra of 25 50 of white dwarf atmospheres These elements are expected to sink quickly, pointing to a scenario of materials accretion delivered from elsewhere in these evolved exoplanetary systems Furthermore, observational evidence revealed that the accretion stays roughly constant over the course of Gigayears Here, we are contributing to prior work on white dwarf pollution via an exo Oort cloud by finding the flux rate of comets experiencing the following perturbations galactic
Andrew Winter
- Title: The dynamical evolution of planets in hot and dense stellar systems
  - Abstract: Short period, massive planets, known as hot Jupiters must have accumulated most of their mass at much greater distances from their host star than their present orbit. One way to induce inward migration is by exciting large eccentricity, such that close approaches with the host star raise tides in the planet that dissipate orbital energy. This ‘high eccentricity migration’ (HEM) can be instigated by dynamical perturbations due to neighbouring stars. I present a new analytic framework to quantify the role of dynamical encounters in instigating migration. In doing so, I demonstrate the diffusive nature of planet eccentricity evolution in kinematically hot stellar systems. This has general consequences for planet system evolution, as well as the rate at which planets and companions collide with their host star. I apply the framework to the globular cluster 47 Tuc to obtain the surprising result that the expected fraction of hot Jupiters is consistent with present constraints, assuming a similar initial planet population to field stars. Finally, I discuss how short period planets can constrain planet formation theory.

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