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2011 Annual Science Report

NASA Goddard Space Flight Center Reporting  |  SEP 2010 – AUG 2011

Evolution of Protoplanetary Disks

Project Summary

Drs. Aki Roberge and Carol Grady have continued work on two Open Time Key Projects for the Herschel Space Observatory, a far-infrared ESA mission launched in May 2009. One project (GASPS) is illuminating the evolution of gas abundances and chemistry in protoplanetary disks over the planet-forming phase; Drs. Roberge and Grady were co-authors on two GASPS journal published articles this year. The other (DUNES) has sensitively probed the Sun’s nearest neighbors for signs of cold debris disks associated with extrasolar Kuiper Belts; Dr. Roberge was a co-author on two DUNES journal articles published this year. Dr. Roberge was also a science team member for a SOFIA instrument concept that was developed in 2011 and proposed to NASA. The instrument, called the High Resolution Mid-Infrared Spectrometer (HIRMES), has a primary science goal of chemical evolution studies in protoplanetary disks.

4 Institutions
3 Teams
2 Publications
0 Field Sites
Field Sites

Project Progress

Over the last year, many unique new observations of protoplanetary and debris disks have been obtained with the Herschel Space Observatory, a far-infrared/sub-mm telescope launched in May 2009 by the European Space Agency. Drs. Roberge and Grady are Co-Is on a Herschel Open Time Key Project called “Gas in Protoplanetary Systems” (GASPS; PI: William Dent). Dr. Roberge is also a Co-I on another Key Project, “Dust Around Nearby Stars” (DUNES; PI: Carlos Eiroa).

Herschel has provided the first opportunity to sensitively measure gas abundances in a large number of disk systems. To this end, the GASPS project is executing a survey of young stars for far-IR gas emission lines using the PACS spectrograph on Herschel. GASPS will ultimately observe roughly 250 targets, which were carefully chosen to span a wide range of spectral types. The stellar ages are between 1 million years and a few tens of million years, covering the critical gas dissipation phase into the young debris disk phase. It’s during this phase that protoplanetary disks transform from disks of interstellar material left over from star formation to young planetary systems in the late stages of terrestrial planet formation.

To date, the GASPS project has been detecting neutral oxygen emission from disks systems spanning a wide range of parameters, including spectral type, disk mass, and accretion rate. The emission tentatively appears correlated with the strength of the dust continuum emission, but no other parameters. On the other hand, ionized carbon emission from gas-rich disks is weaker than expected based on pre-launch modeling, providing a challenge to current disk thermochemical models. Drs. Roberge and Grady are co-authors on two refereed journal articles published by the GASPS team this year. Dr. Roberge is leading analysis of the GASPS observations of the unique debris disk system 49 Ceti. We plan to submit a Letter on this exciting dataset within the next few months.

Dr. Roberge is also leading analysis of the GASPS study of the Tucana-Horologium Association (Tuc-Hor), a nearby 30 Myr-old stellar association. Her U of MD graduate student, Jessica Donaldson, is in charge of reducing and analyzing the photometric observations of 17 Tuc-Hor stars. In the course of this work, we have found one new debris disk in the association (see Figure 1) and obtained important new data on 5 previously known disks. Under the supervision of Dr. Roberge, Jessica has developed a dust disk modeling code, which was used to characterize the dust around these stars. We expect a journal article on this work to be submitted in Dec. 2011.

Figure 1. ​Spectra energy distribution for HD30051, an intermediate-mass star in the Tucana-Horologium Association. The expected flux from the central star is shown with a solid black line. The new photometric point at 70 microns obtained by the Herschel GASPS project is shown with an asterisk. The fact that the point is much brighter than the expected stellar flux at that wavelength points to the presence of a previously unknown cold debris disk around the star. A simple single-temperature blackbody model for the dust excess flux is shown with the black dashed line.

The DUNES project has obtained far-IR photometry of nearby solar-type stars to sensitively probe them for circumstellar dust coming from planetesimals within extrasolar Kuiper Belts. This survey has pushed to unprecedentedly low dust levels and will provide the most complete inventory to date of planetary material around the Sun’s nearest neighbors. These stars will be the primary targets of future efforts to directly detect and characterize potentially habitable terrestrial planets (aka. exoEarths).

The DUNES project has been highly successful. All data have been acquired and every target was detected with the expected sensitivity. Many targets show excess emission coming from cold circumstellar dust, at a rate than appears higher than rates seen in earlier surveys with the Spitzer Space Telescope. Most notably, DUNES appears to have discovered a new class of ultra-cold debris disks that present a severe challenge to both debris disk modeling codes and to theories of debris disk evolution. Dr. Roberge is a co-author on two refereed journal articles published by the DUNES team this year

Dr. Roberge is a member of the science team for an instrument concept developed for SOFIA. The High Resolution Mid-Infrared Spectrometer (HIRMES) would provide sensitive high resolution spectroscopy (R = 105) between 25 and 65 microns, a spectral region that contains a large number of atomic, ionic, and molecular gas emission lines, as well as two water ice features. As such, this instrument is tailored for studies of the chemical evolution of protoplanetary disks, and would be a unique tool to follow-up disks studied at lower resolution with Herschel. It would provide a valuable complement to upcoming high resolution disk studies with ALMA. Dr. Roberge is a co-investigator on a proposal to build HIRMES submitted to NASA in Sept. 2011 (PI: S. Moseley, GSFC).

    Aki Roberge Aki Roberge
    Project Investigator
    Carol Grady

    Kenji Hamaguchi

    Objective 1.1
    Formation and evolution of habitable planets.