nai

Project Progress

Co-I Hollenbach working with collaborator Gorti is modeling photoevaporation processes in protoplanetary disks around young stars. Photoevaporation is caused by the heating of the protoplanetary disk surface by EUV (Lyman continuum), FUV, and X-rays from the central star. This photoevaporation clears the disk and helps determine the likelihood of habitable planet formation around a star of a given mass.

In the current reporting period we examined the lifetime of the planet-forming disks as a function of the mass of the central star. It had been proposed by other researchers that the lifetime of disks may have a maximum for central star masses of order 1 to 3 solar masses. We tested this hypothesis in Gorti and Hollenbach (2008a) and found that there is no such maximum, but that stars with mass from about 0.3 to 3 solar masses all have disks with about the same lifetime (1-2 Myr) against photoevaporation. We did find, in accordance with earlier predictions, that stars more massive than 3 solar masses have short lifetimes. Our models also included stars of higher mass at 7 and 30 solar masses and we found that these have lifetimes of only ~0.1 Myr. Thus, habitable planets do have time to form around stars less massive than 3 solar masses, but are unlikely to form around more massive stars.

{{ 1 }}

Gorti and Hollenbach (2008b) report our final results on the infrared emission from planet-forming disks. We find that various infrared lines probe the density, temperature, and chemical abundances of gas at various distances from the central star. Some of the lines probing the terrestrial planet region around 1 AU include the pure rotational lines of H₂, H₂O, and OH, the ground state vibrational lines of CO and H₂O, and the fine structure lines of [FeI], [FeII], [SiII], [OI], [NeII], [NeIII], [ArII] and [SI]. Lines probing the 100 AU region of the outer disk include low J rotational transitions of CO, and the fine structure lines of [CII] and [OI]. Comparisons of [NeII] observational results from the Spitzer Space Telescope with these models have been made in Pascucci, Hollenbach et al (2007). One key conclusion is that this neon emission line is currently the most sensitive way to detect gas in the planet-forming region of a young star. More work is planned to exploit this sensitive probe of planet forming regions.

Co-I Lissauer working with Pilcher (NASA Astrobiology Institute) wrote an article entitled “The Quest for Habitable Worlds and Life Beyond the Solar System”. This article describes progress and prospects for detection of extrasolar planets and biosignatures thereon, and is geared towards a very well-educated but diverse audience which includes philosophers and theologians in addition to scientists and engineers.

Co-Is Davis and Richard continue work on modeling large scale transport in protoplanetary disks and have been working on improving numerical schemes to describe accurately turbulent diffusion and meridional advection along with chemical reactions Davis (2007). Richard is also developing models of optical scattering by astrophysical dust grains as reported in Richard & Davis (2008) and presented at the 2008 Astrobiology conference (Richard & Davis 2008 to further our understanding of their influence on the thermodynamical structure of disks as well as on the optical opacity. These models should also provide additional tools to understand astronomical observations of disks, in particular to probe the structure and composition of the dust component. S. Davis, in collaboration with E. Young (UCLA) reported new findings concerning the spatial distribution of oxygen isotopes in the protoplanetary nebula (Davis 2008).

{{ 2 }}

Co-I Laughlin has been working on several projects during the past year, including: (1) the development of the Systemic Console, a flexible GUI-based computational tool for analyzing radial velocity and transit data for extrasolar planetary systems. The software code base has been completed, and it is fully operational and available for download at www.oklo.org. (2) Investigation of the formation and delectability of the potentially habitable terrestrial planets in orbit around Alpha Centauri B. In the following paragraphs, both projects are described in more detail. In the first instance, we implemented dynamical transit fitting into the Systemic console software. Our user base now exceeds 2000 users, drawn from a wide spectrum of professional astronomers, amateur astronomers, students, and the general public. Systemic users have made several important discoveries, including the first characterizations of the low-mass planets Gl 581c, and 55 Cnc e and f. The project continues to be the subject of a considerable amount of favorable media attention, stemming from an AP news service article (in early 2007) that appeared in newspapers worldwide. The after-effects of this attention continue to drive users to our website. (http://www.space.com/news/070116_ap_space_online.html/)

We are finalizing a paper for publication that describes the detailed operation of the console, including the numerical algorithms that we employ for integration and optimization. Co-I Laughlin has recently joined the Lick-Carnegie Planet Search team (along with Co-Is Steve Vogt and Paul Butler) and the systemic console is being used as the primary analysis tool for this new planet survey.

In area (2) we have also made major progress on our study of planet formation and detectability in the Alpha Centauri Binary system. (Guedes, et al 2008). In brief, we simulated the formation of planetary systems around Alpha Centauri B. The N-body accretionary evolution of a 1/r disk populated with 400-900 lunar-mass protoplanets was followed for 200 million years, and all simulations led to the formation of multiple-planet systems with at least one planet in the 1-2 Earth-mass range at 0.5-1.5 AU. We examined the detectability of our simulated planetary systems by generating synthetic radial velocity observations and including noise based on the radial velocity residuals to the recently published three planet fit to the nearby K0V star HD 69830. Using these synthetic observations, we found that we can reliably detect a 1.8 MEarth planet in the habitable zone of Alpha Centauri B after only three years of high cadence observations. We also found that the planet is detectable even if the radial velocity precision is 3 m/s, as long as the noise spectrum is white. Our results show that the greatest uncertainty in our ability to detect rocky planets in the Alpha Centauri system is the unknown magnitude of ultra-low frequency stellar noise. This paper also generated considerable media interest, including a story in National Geographic News: http://news.nationalgeographic.com/news/2008/03/080310-centauri-earth.html