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2010 Annual Science Report

NASA Goddard Space Flight Center Reporting  |  SEP 2009 – AUG 2010

Evolution of Protoplanetary Disks

Project Summary

Drs. Aki Roberge and Carol Grady are pursuing studies related to Theme 2 of the NASA GSFC Astrobiology Node, “From Molecular Cores to Planets: Our Interstellar Heritage.” Over the last year, they have begun work on two Open Time Key Projects for the Herschel Space Observatory, an ESA mission launched in May 2009. Herschel is expected to spearhead the next big advances in our knowledge of planet formation, protoplanetary disk evolution, and debris disks. One project (GASPS) will illuminate the evolution of gas abundances and chemistry in protoplanetary disks over the planet-forming phase. The other (DUNES) will sensitively probe the Sun’s nearest neighbors for signs of cold debris disks associated with extrasolar Kuiper Belts. Both projects have begun to produce exciting results, including discovery of a possible new class of ultra-cold debris disks that challenge theories of debris disk evolution and planet formation.

4 Institutions
3 Teams
9 Publications
0 Field Sites
Field Sites

Project Progress

The last year has seen a major increase in activities using 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). Over the last year, Drs. Roberge and Grady begun reducing and analyzing the copious data flowing in from Herschel.

Herschel will provide 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 observe roughly 200 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.

The GASPS spectra will focus on detecting one ionized carbon emission line and two neutral oxygen lines, as well as several water vapor lines. The first result will be direct measurement of relative gas abundances as a function of age and stellar mass. Then we will use sophisticated new thermochemical disk models to convert relative abundances into actual gas masses. A detailed description of the team’s disk modeling code, called “ProDiMo”, may be found in Woitke et al. (2009).
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. Ionized carbon emission, on the other hand, is weaker than expected based on pre-launch modeling, providing a challenge to the disk thermochemical models described above. Drs. Roberge and Grady are co-authors on 4 refereed journal articles published by the GASPS team this year.

Dr. Roberge is 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 data on 5 previously known disks. Under the supervision of Dr. Roberge, Jessica is developing a dust disk modeling code, which will be used to characterize the dust around these stars. We expect a journal article on this work to be published in the coming year.

Figure 1B. ​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 will obtain far-IR photometry of nearby stars to sensitively probe them for circumstellar dust coming from planetesimals within extrasolar Kuiper Belts. This survey will push to unprecedentedly low dust levels and provide the most complete survey 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).

Already, the DUNES project has been highly successful. Every target has been detected at the expected level. Many targets show excess emission coming from cold circumstellar dust, at a higher rate than 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 2 refereed journal articles published by the DUNES team this year.

In addition to these activities, an invited chapter written by Dr. Roberge for a University of Arizona Press Space Science Series book entitled “Exoplanets” is in press. The chapter reviews theory and observations of protoplanetary and debris disks (Roberge & Kamp 2010, in press).

Dr. Grady has continued a program of pan-chromatic and high-contrast imaging studies of protoplanetary disks, systems with large gaps in the disk, and debris disks. Such studies allow us to constrain accretion processes onto the PMS stars, the degree of dust settling in the disk as a function of age, and place limits on the massive planet inventory in such disks. Two papers from this effort have been published in 2010, while work is continuing on other systems. In the first, a study of a 7 Myr-old Herbig Ae star, we find that the accretion footprint lies at high temperate latitudes on the star, similar to young Solar analogs, and that the circumstellar disk is highly stratified in this protoplanetary system.

In the second paper, a NIR study of the disk around the 3-5 Myr old young Solar-analog, LkCa 15, we resolve the gap in the disk in NIR scattered light. Our data show an offset of the nebulosity contours along the major axis, likely corresponding to a physical pericenter offset of the disk gap. This reinforces the leading theory that dynamical clearing by at least one orbiting body is the cause of the gap. Based on evolutionary models, our high-contrast imagery imposes an upper limit of 21 MJupiter on companions at separations outside of 0.1” and of 13 MJupiter outside of 0.2”. Thus, we find that a planetary system around LkCa 15 is the most likely explanation for the disk architecture. On-going study of the outer disk, using archival HST imagery suggests that the outer disk has structure which is consistent with the presence of additional bodies which have not yet grown to the point that they clear annuli in the disk.

Refereed Publications:

Mathews, G. S., Dent, W. R. F., Williams, J. P., Howard, C. D., Meeus, G., Riaz, B., Roberge, A., Sandell, G., et al. (2010). “Gas in Protoplanetary Systems: I. First Results.” Astronomy & Astrophysics, 518, L127

Thi, W.-F., Mathews, G. S., Menard, F., Woitke, P., Meeus, G., Riviere-Marichalar, P., Pinte, C., Howard, C. D., Roberge, A., et al. (2010). “Herschel-PACS observation of the 10 Myr old T Tauri disk TW Hya: Constraining the disk gas mass.” Astronomy & Astrophysics, 518, L125

Meeus, G., Pinte, C., Woitke, P., Montesinos, B., Mendigutia, I., Riviere-Marichalar, P., Eiroa, C., Mathews, G., Vandenbussche, B., Howard, C. D., Roberge, A., et al. (2010). “Gas in the protoplanetary disc of HD169142: Herschel’s view.” Astronomy & Astrophysics, 518, L124

Pinte, C., Woitke, P., Menard, F., Duchene, G., Kamp, I., Meeus, G., Mathews, G., Howard, C. D., et al. (2010). “Gas and dust in protoplanetary discs as seen by Herschel/GASPS: Statistical comparison with the DENT grid of models.” Astronomy & Astrophysics, 518, L126

Eiroa, C., Fedele, D., Maldonado, J., Gonzalez-Garcia, B. M., Rodmann, J., Heras, A. M., Pilbratt, G. L., Augereau, J.-C., et al. (2010). “Cold DUst around NEarby Stars (DUNES). First Results: A Resolved Exo-Kuiper Belt around the Solar-like Star ζ2 Ret.” Astronomy & Astrophysics, 518, L131

Liseau, R., Eiroa, C., Fedele, D., Augereau, J.-C., Olofsson, G., Gonzalez, B., Maldonado, J., Montesinos, B., et al. (2010). “Resolving the cold debris disc around a planet-hosting star: PACS photometric imaging observations of q1 Eri (HD10647, HR506).” Astronomy & Astrophysics, 518, L132

Roberge, A. & Kamp, I. (2010). “Protoplanetary & Debris Disks.” In Exoplanets (ed. S. Seager), The Univ. of Arizona Space Science Series, Univ. of Arizona Press, in press

Grady, C.A. et al. (2010). “Locating the Accretion Footprint on a Herbig Ae Star: MWC 480.” Astrophysical Journal, 719, 1565

Thalmann, C., Grady, C.A. et al. (2010). “Imaging of a Transitional Disk Gap in Reflected Light: Indications of Planet Formation Around the Young Solar Analog LkCa 15.” Astrophysical Journal, 718, L87

    Aki Roberge Aki Roberge
    Project Investigator
    Carol Grady

    Kenji Hamaguchi

    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 3.2
    Origins and evolution of functional biomolecules