Detecting the signature of planets at millimeter wavelengths

1. Francisco Ramos Stierle : David Hughes : Edward Chapin : framos@inaoep.mx dhughes@inaoep.mx echapin@inaoep.mx Detecting the signature of planets at millimeter wavelengths F. Ramos-Stierle, D.H. Hughes, E. L. Chapin (INAOE, Mexico), G.A. Blake ???gab@gps.caltech.edu The study of planet formation mechanisms is a central part of our search for an understanding of the origin of the Earth and Solar System. The motivation to study the environments of planet formation has become more intense since the discovery of the first giant planets around nearby solar-type stars using the Doppler planet-detection technique. One of the most intriguing results of searches for exoplanets (and a challenge to the new theories of planetary formation), is the discovery that the formation process gives rise to considerable diversity. Surveys of young stars at infrared and millimeter wavelengths show that most exhibit thermal emission from small heated particles distributed in disks (PROPLYDs) , with properties similar to those of the young Solar System. Models of their spectral energy distributions (SEDs) and imaging indicate disk sizes of tens to hundreds of AU. These dusty and gas-rich disks are believed to provide the material for proto-stellar sources, as well as the reservoirs of mass for the formation of planetary systems. Although there is now abundant evidence for the existence of circumstellar disks around young low-mass stars, our understanding of the detailed properties of disks, in particular at distances (< 30 AU) associated with planet formation, is still in its early stages. With the combination of high angular resolution and sensitivity in new millimeter experiments (e.g. ALMA and LMT) , we will be able to image the detailed structure of nearby disks, and detect the gaps and inner-holes (both spectrally and spatially) created by the clearing of material during the planet formation process. Modelling the thermal emission from PROPLYDs We know about the existence of PROPLYDs and planets, but we are still waiting for clear proof that both are related. We present optically thin thermal models of the multi-wavelength emission from PROPLYDs, to generate realistic simulated images of the gaps and holes in disks associated with planet formation. Disks contain a mixture of gas and dust. Even though the dust mass is 1% (or less) of the mass in PROPLYD environments, virtually all the continuum radiation from the infrared to the millimeter is due to thermal radiation from dust. Dust warmed by the starlight radiates as a blackbody modified by the emissivity of the grains. The dust temperatures depend on the distance of a grain from the star, and since a continuous disk contains particles over a wide range of distances from the star (out to several hundred AU), dust temperatures range from the sublimation temperature (approximately 1500 K) to a few Kelvin. The result is a broad spectrum of thermal emission from ~ 1 μm to 1000 μm. Link1 Massive planets orbiting a star cause a change in the radial velocity. This Doppler method currently provides the most efficient planet detection method. The composite SED (black line) from a disk with thermal emission at different radii, and different temperatures. The contribution from the individual annuli are shown in colour. HST imaging of PROPLYDs (25 to 500 AU) around YSOs in Orion (D=400pc). Gaps in disks Physical gaps in the disk can be created by the presence of a proto-planet which clears dusty and gaseous material. Removing this material will also reduce the thermal contribution from the region. The gap will therefore produce a depression in the SED at wavelengths that correspond to the temperature of the “missing” dust . Composite SEDs showing the impact of gaps of different widths which are created in a continuous disk. A 15 – 35 AU gap in the PROPLYD caused by the clearance of dust due to proto-planets. The maximum intensity was clipped to show better the effect. Inner-holes in disks While only the largest proto-planets may form gaps of sufficient width to be imaged directly, smaller bodies can produce detectable inner-holes once the bulk of disk material interior to their orbits has been accreted. After sufficient evolution, the entire planetary system can clear all material out to the most distant planet. Composite SEDs calculated for different sizes of inner-holes. The stellar contribution is shown in yellow. A 40 AU inner-hole caused by the clearance of dust from the collective influence of all planets in a young solar system. Telescope simulations • Individual detector signals are generated by an instrument and telescope simulator2 that passes an array of pixels across a composite of the "idealised" maps. • The following effects eare included: • telescope primary aperture • beam shape • array geometry and sensitivity • scan pattern and pointing errors • atmospheric noise and attenuation • detector time constant and 1/f drift • Poisson noise • integration time True image of dust emission around ε Eri at a wavelength of 850μm, using SCUBA at the JCMT (Greaves et al. 1998)3 . A simulation of dust emission around ε Eri at a wavelength of 850μm, observed with the SCUBA camera operating on the 15-m JCMT. Background is a segment of Saturn’s rings. In these simulations we tune the configuration of the interferometer to optimize the synthesized beam size and sensitivity to the structure of interest. Left figure: We choose a maximum baseline of 150m (beamsize 1’’) in order to map the extended structure of the dust disk, and measure the total dust mass. Right figure: ALMA observations with longer baselines (3.5km – beamsize 0.06’’) in order to spatially resolve a gap in a disk at a distance of 20pc. A simulated 100AU PROPLYD (D=20 pc) at 45° inclination with a gap between 3.2 and 11AU (corresponding to the clearance by the orbits of Jupiter + Saturn), observed with ALMA for 3hrs using 3.5km baselines. The lower panel shows a slice though the disk, clearly showing the presence of the gaps. The huge intensity peak is caused by the hot dust located close to the star. Simulated 100AU PROPLYD (D=3pc), at an inclination 45°, with an inner-hole of 40AU, observed with the BOLOCAM-II camera operating at 1.1mm on the LMT. This figure shows the same PROPLYD (Mdust = 5.5 Mℂ, 5% the initial dust of the minimum solar-nebula)4 observed at 1.1mmwith ALMA using the smallest baselines (150m). References 1. http://astron.berkeley.edu/~gmarcy/039marcy.html 2. Chapin et al. 2001, astro-ph/109330 3. Greaves et al. 1998, ApJ 506:L133-L137 4. Ramos-Stierle 2003, MSc thesis