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2015 Annual Science Report

NASA Goddard Space Flight Center Reporting  |  JAN 2015 – DEC 2015

Evolution of Protoplanetary Disks and Preparations for Future Observations of Habitable Worlds

Project Summary

The evolution of protoplanetary disks tells the story of the birth of planets and the formation of habitable environments. Microscopic interstellar materials are built up into larger and larger bodies, eventually forming planetesimals that are the building blocks of terrestrial planets and their atmospheres. With the advent of ALMA and continuing use of the Hubble Space Telescope, we are poised to break open the study of young exo-planetesimals, probing their organic content with detailed observations comparable to those obtained for Solar System bodies. Furthermore, studies of planetesimal debris around nearby mature stars are paving the way for future NASA missions to directly observe potentially habitable exoplanets.

4 Institutions
3 Teams
2 Publications
2 Field Sites
Field Sites

Project Progress

Debris disks are relatively gas-poor disks of rocky material centered around young main sequence stars (~ 10 – 100s of million years old). The younger debris disks are likely in the late stages of terrestrial planet formation, while the older ones correspond to the “clearing” phase of planetary system evolution, where most leftover planetesimals are removed from the system by collisions and ejection. Some older debris disks with anomalously high dust abundances may currently be experiencing a dramatic rearrangement of their planetary systems, like the one that occurred during the Solar System’s Late Heavy Bombardment Period and explained by the so-called “Nice Model.”

While it has long been known that these disks are composed of the destruction products of young extrasolar planetesimals, detailed studies of their composition have been difficult to achieve. However, such studies would provide radical new information on the bulk organic content of young planetesimals formed in environments both similar to and different from the solar nebula, illuminating unobserved processes that occurred in the early Solar System and providing context for Solar System planetesimal compositions.

Aki Roberge is leading a Hubble Space Telescope (HST) ultraviolet absorption study of gas in the unusual 49 Ceti debris disk system. In our 2014 annual report, we discussed our analysis of the bulk stable gas in the disk, which appears to have a super-solar carbon-to-oxygen ratio (Roberge et al. 2014). This year, we analyzed a further aspect of the dataset: time-variable absorption features from atomic gas infalling towards and outgoing from the central star at high velocity (Miles, Roberge, & Welsh 2016). Similar features are a long-studied aspect of the famous Beta Pictoris debris disk and have been attributed to the transits of star-grazing exo-comets.

Two sets of 49 Ceti spectra were obtained with HST, taken 5 days apart. Under GCA’s 2014 URAA program, undergraduate research associate Brittany Miles (UCLA) analyzed the time-variable spectral features. The first 49 Ceti visit showed blueshifted absorption detected in triply-ionized carbon (C IV); in this event the gas was outgoing from the central star at velocities of up to -372 km/s. In the second visit, redshifted absorption was detected in both C II and C IV; the gas was infalling towards the central star at velocities of up to 258 km/s. No other absorption lines from circumstellar gas varied between the two visits. Figure 1 shows the variable C IV gas features.

Figure 1: Transit of star-grazing exocomets around the (young) 40 million-year-old star 49 Ceti. This figure shows time-variable, triply-ionized carbon (C IV) absorption in UV spectra of the star taken during two HST visits separated by 5 days. In both panels, the y-axis shows the spectra from one visit divided by the other, highlighting only the position of circumstellar gas that varied between the two visits. The x-axis shows the velocity of the absorption relative to the central star. In the first visit, blue-shifted C IV absorption is detected (left panel), indicating variable gas falling onto the star at high velocity. In the second visit, red-shifted variable absorption is detected (right panel). Similar features are seen in the famous Beta Pictoris debris disk and are attributed to evaporating exo-comets passing through our line of sight to the central star. Image credit: Miles, Roberge, & Welsh, 2016.

We measured the carbon abundances in the infalling gas and set upper limits on the amount of variable gas from other atomic species. We found that the infalling gas was within 0.2 AU of the central star at the time of transit and also appears to have a super-solar carbon-to-oxygen ratio (C/O > 1.5). This work strengthens the connection between the variable gas in the 49 Ceti disk and star-grazing exo-comets. The presence of both red- and blue-shifted events is difficult to explain with other scenarios. Furthermore, a species as highly ionized as C IV cannot be produced by photoionization in the circumstellar environment of a main sequence star like 49 Ceti (of spectral class ‘A’). It can be naturally explained by collisional ionization in hot, dense gas such as that found in the shock front of a comet coma. Most interestingly, the super-solar C-to-O ratio in the gas coming directly from star-grazing exo-comets may point to a different chemical history for planet formation around this 2.7 solar-mass star.

Aki Roberge is Co-I on an ALMA Cycle 1 project (PI: M. Hughes, Wesleyan University) to map 49 Ceti’s dust and CO content. The data are in hand; analysis shows that mm-sized dust grains extend over a large range of radii, with an increase in surface density at the inner edge of the CO gas disk (Lieman-Sifry et al., in preparation). Roberge is also PI of an approved ALMA Cycle 3 project to map the neutral carbon content of the 49 Ceti disk; these data have not yet been taken. Together, these ALMA datasets will illuminate the relationship between the molecular and atomic gas in this important debris disk.

Finally, Roberge is leading a project to create high-fidelity spatial and spectral models of extrasolar planetary systems (“Finding the Needles in the Haystacks”), to be used in development of future exoplanet direct imaging missions. Under GCA’s 2015 URAA program, undergraduate research associate Tiffany Jansen (Univ. Washington) developed a “Haystacks” model of a planetary system with a Jupiter-mass planet at 1 AU from a Sun-twin star; the Jovian is orbited by an Earth-like moon. In addition to the star, planet, and moon, the model includes a plausible structure for interplanetary dust coming from planetesimals. Jansen performed a preliminary evaluation of the detectability of the habitable moon with the Large UV / Optical / Infrared Surveyor (LUVOIR) mission concept. A Decadal Mission Study of this concept was initiated by NASA in January 2016 and will extend over the next 3 years; Roberge is the LUVOIR Study Scientist.

    Aki Roberge Aki Roberge
    Project Investigator
    Carol Grady

    Objective 1.1
    Formation and evolution of habitable planets.

    Objective 1.2
    Indirect and direct astronomical observations of extrasolar habitable planets.

    Objective 3.1
    Sources of prebiotic materials and catalysts

    Objective 7.2
    Biosignatures to be sought in nearby planetary systems