Artist conception of a myriad of terrestrial planets that may be forming around sun-like stars. Based on data collected from the Formation and Evolution of Planetary Systems Spitzer Legacy Science Project .

Michael R. Meyer

Research Projects:

We continue efforts aimed at 1) understanding the origin of the initial mass function (IMF) of stars and sub-stellar objects; 2) using studies of circumstellar disk evolution to constrain theories of planet formation; and 3) combining results from both to examine astronomical constraints on the origin and evolution of life.

The origin of the initial mass function is one of the major outstanding problems in all of astronomy. In its solution are hidden answers to questions such as: What makes up the baryonic mass budget of the Universe? How do galaxies form and evolve? What is the distribution of potentially habitable environments in the Milky Way galaxy? From a star formation perspective, understanding the shape of the initial mass function of stars is a key to any predictive theory. We are engaged in a two-pronged approach to understand the origins of the IMF. First, weare critically investigating the ratio of stars to sub-stellar objects observed toward a number of young star-forming regions (e.g. Greissl et al. 2007; Andersen et al. 2006). By comparing the distribution of observed ratios to those expected from analytic forms of the intial mass function (e.g. Chabrier, 2003) one can explore what ranges of parameter space are not allowed by the data. Comparing the ensemble of observations available to power--law forms of the very low mass IMF, it appears that IMFs that rise in linear units (dN/dm) below the hydrogen--burning limit are ruled out. While formally consistent with recent estimates of the field star IMF (e.g. Allen et al. 2005), this is the first quantitative demonstration that the IMF must turn over at sub-stellar masses (Andersen et al., in preparation). Future work will involve deep, wide-field, infrared surveys for very faint cluster members and measurement of their kinematic properties as a function of mass (e.g. Lloyd-Hart et al. 2006).

In addition to exploring the ratio of stellar to sub-stellar objects, we are applying techniques used to study nearby clusters to study the IMF in rare (and thus more distant) star-forming events. With graduate student Julia Greissl, we are initiating new projects aimed at constraining the ratio of high to low mass stars in regions of extreme star formation in the local group and nearby starburst galaxies. Models of the integrated light of massive "super star clusters" have been constructed varying the initial mass function and age of the population. While previous attempts (e.g. Starburst99) have focused on the color evolution of the integrated light from post-main sequence and main sequence stars, we have included models of pre-main sequence evolution for low mass stars for the first time. Because low mass stars are much more luminous when young, previous models have underestimated the contribution of these stars for very young super--star clusters. Meyer and Greissl (2005) show that it is possible to directly measure the ratio of high to low mass stars in these unique star-forming events by exploiting spectral differences between high mass (hot) and low mass (cool) stars. Additional work is focused on determining optimum wavelength coverage and spectral resolution needed to constrain the IMF, as well as sensitivity of the model results to surface gravity effects. Analysis of spectra obtained for very young super-star clusters in the Antennae galaxy is on-going (Greissl et al.). Future work will utilize AO-fed spectrographs such as ARIES spectrograph in on the 6.5 meter MMT telescope, as well as new programs on the VLT, to survey other nearby active galaxies. Together with research associate Morten Andersen, we are investigating the ratio of high to low mass stars in the most massive HII region in the Milky Way galaxy, W51. Using the ARIES camera with the f/15 adaptive secondary mirror, we obtained images of the W51 cluster in the H- and K-bands. Work presented by Andersen et al. (2005) shows that the ratio of high to low mass stars in W51 appears to be anomalous. The next steps are to explore narrow-band imaging in selected stellar absorption features to obtain rough estimates of spectral type (and thus stellar temperatures to infer stellar masses) and surface gravity (to insure cluster membership in the PMS). Preliminary observations of the massive cluster Westerlund 1 were obtained in 2006 with the VLT NACO instrument to explore further this technique. In the future, integral field near-IR spectroscopy on the VLT, at the combined focus of the Large Binocular Telescope in southern Arizona, and eventually the E-ELT, should enable us to resolve individual stars and study the IMF down below one solar mass in nearby galaxies such as M33.

Other work is aimed at understanding the structure and evolution of circumstellar disks around solar-type stars in order to answer the fundamental question: are habitable planetary systems like our own common or rare among ensembles of sun--like stars in the disk of the Milky Way galaxy? Observations at different wavelengths trace different temperatures of circumstellar gas and dust, thus probing different radii in the disk. Tremendous progress in the understanding of circumstellar disks has been made possible with new space-based capabilities such as NASA's Spitzer Space Telescope. We were awarded a Spitzer Legacy Science Program in 2001 to study the formation and evolution of planetary systems (Meyer et al. 2006: http://feps.as.arizona.edu) . The data reduction team, including post-doctoral research associates J. Serena Kim, Murray Silverstone, Ilaria Pascucci, and led by Dean Hines (SSI), is based in Tucson. FEPS has now received all but a minor fraction of the data for the program and delivered most of it the Spitzer Space Telescope Legacy Science Archive in December, 2006. Ten papers based on our early results have been published, with three papers recently submitted, and several more in preparation. Early highlights concerning disk evolution around sun--like stars include: 1) constraints on inner disk lifetimes based on a 3.6--8.0 mocron survey of 3--30 Myr old stars (Silverstone et al. 2005); 2) discovery of a probable inner debris belt (4--6 AU) surrounding the 30 Myr old star HD 12039 (Hines et al. 2005); 3) discovery of several cold debris disks likely extending beyond 30 AU surrounding sun--like stars over a range of ages (Kim et al. 2005); 4) upper limits to the amount of molecular hydrogen gas surrounding stars aged 3--100 Myr placing constraints on the timescale available to form planets (Hollenbach et al. 2005; Pascucci et al. 2006); 5) a survey for warm debris around stars in the $\sim$ 100 Myr old Pleiades cluster (Stauffer et al. 2005); and 6) a study of the possible connection between the presence of radial velocity planets and debris disks (Moro--Martin et al., 2007). Papers in press include: a) detailed study of the grain mineralogy in optically--thick disks surrounding a sample of T Tauri stars (Bouwmann et al.); b) the unexpected detection of [Ne II] gas in the high resolution IRS spectra of young stars (Pascucci et al.); and c) detailed study of one star harboring two radial velocity planets and a debis disk from 20--50 AU (Moro--Martin et al.). One new paper focuses on the frequency of mid--infrared emission from debris disks found around sun--like stars with ages from 10--100 Myr. Results suggest that the processes that led to the oligarchic growth of terrestrial planets in our solar system may be quite common (Meyer et al. 2008). Several follow-up programs are underway including: 1) HST Cycles 14/15 time to search for scattered light from debris disks identified with Spitzer; 2) Spitzer Cycles 2/3 programs to obtain additional 70 and 160 micron photometry for disks identified through FEPS; 3) an approved project for NASA's FUSE telescope to constrain the FUV emission needed to heat molecular hydrogen gas in disks; 4) ground-based programs with the MMT and VLT utilizing adaptive optics to search for planets surrounding stars with debris disks (Apai et al. 2008); 5) complementary ground-based mid--infrared observations that utilize adaptive optics to spatially resolve disks inferred from Spitzer; and 6) extensive mm--wave observations using the SMT, SMA, the Australia Compact Telescope Array, the IRAM 30m, and soon the CARMA array. In a related follow--up program we are part of a team led by John Carpenter (Caltech) to investigate the disk frequency across the stellar mass spectrum in the well--studied Sco Cen OB association. Initial results from this program suggest that primordial disks surrounding lower mass stars survive longer than disks around higher mass stars (Carpenter et al. 2006). This result could have important implications for planet formation around stars as a function of stellar mass. Finally, Mamajek & Meyer (2007) have developed a theory of protoplanet collision to explain the underluminosity of the unique brown dwarf binary 2MASS1207b. Applying concepts originally used to explain the collisions that could have played a role in forming solar system objects, the study has implications regarding the detection of terrestrial planets in formation around other stars.

Last but not least, through the Life and Planets Astrobiology Center "LAPLACE", a node of NASA's Astrobiology Institute, we are working on a number of programs related to detecting and characterizing planets around nearby stars, as well as preparatory work for the Terrestrial Planet Finder mission. We are collaborating on several planet-search programs utilizing the CLIO instrument developed by P. Hinz. . Because CLIO can search for planets in the thermal IR, it can surveyed older (colder) planets. Because older stars are more common that young stars, targets can be found that are closer to the Sun. As a result the physical resolution of the diffraction-limit at 3-5 microns corresponds to physical dimensions comparable to the orbits of planets in our solar system (5-30 AU). Former-graduate student A. Heinze completed a search around G stars for his thesis. E. Mamajek (CfA) is surveying nearby A stars. Post-doctoral research associate Daniel Apai is leading the search around a volume-limited sample of nearby M dwarfs. Results have been presented at a range of conferences and the survey results are nearly ready for publication (Apai et al. in preparation). We are also participating in a number of NASA/ESA mission study proposals including SPACE (galactic astronomy team for ESA dark energy mission), TOPS (NASA Origins Probe to search for terrestrial planets), and EXCEDE (NASA SMEX mission to study disks and planets).

In new efforts at the Institute of Astronomy (ETH) we will utilize the vast observational capabilities of ESO facilities, such as the four 8.4 meter telescopes of the VLT, and the APEX sub-millimeter telescope. We will also focus our efforts on platforms available through ESA including the soon-to-be launched Herschel Space Telescope. Interdisciplinary research into the formation and evolution of planetary systems will be conducted through the framework of the PLANET-Z initiative in collaboration with colleagues throughout the Institute of Astronomy and the Department of Earth Science.