Dr Aaron Rizzuto
Planets in Young Clusters and Association: Evolution and Formation
Planets undergo the most dynamic changes in the first several hundred million years of the life, starting with formation in the first ~10 million years for gas giants, and the first 10-100 million years for rocky planets. The structure, density and size of the birth cluster at the epoch of formation all play a significant role in the types of systems that survive to adulthood. Planet continue to evolve for several hundreds of millions of years following formation via loss of atmosphere from high energy stellar radiation and core heating, gravitational interaction with other planets of passing stars, and accretion of rocky material.
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The vast majority of the exoplanets we know about are either very old (>1 billion years old) or have unknown ages. This is a problem as we then see only the output of a complicated set of processes, making understanding planet evolution difficult. The reason for the lack of young planets is difficulty in detection, with both radial velocity and transit detection of young planets being quite difficult due to stellar variability, and direct imaging being limited to only the widest and most massive planets.
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The TESS Hunt for Young and Maturing Exoplanets (THYME) and the Zodiacal Exoplanets in Time (ZEIT) surveys used space based photometry from the Transiting Exoplanet Survey Sattelite (TESS) and Kepler's K2 mission to search for transiting exoplanets in young groups of stars and open clusters using novel methods of removing stellar noise. So far we've identified 15 planets with K2, including the 11 Myr old K2-33b in Sco-Cen and two 650 Myr old multiple planet systems in Hyades and Praesepe. The THYME survey now continues with TESS and is taking data of young stars across the whole sky. The goal is to do detailed population statistics at different ages spanning 2-1000 Myrs.
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M-dwarf hosted planets in Sco-Cen (11 Myr), Hyades (650 Myr) and Praesepe (650 Myr) appear larger in overall compared to older planets discovered by Kepler prime, even when survey completeness is accounted for (Rizzuto et al., 2018).
The occurrence rate of young exoplanets as a function of cluster or association age from the ZEIT survey, corrected for survey completeness. There is a deficit of transiting exoplanets in at younger ages suggesting a dynamical migration process (Rizzuto et al., in prep).
Finding Planetary Transits in Noisy Young Star Data
The primary difficulty in finding transit signatures in the light curves of very young, active stars is in removing activity and rotational systematics from the data. For young stars (<1 Gyr) stellar rotational signals are orders of magnitude larger than exoplanet transits.
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I have developed a detrending and planet-search pipeline specifically for the case of highly variable young stars in the K2 and TESS samples (Rizzuto et al., 2017) that out- performs all other transit search methods for highly variable stars. To solve this problem I developed the notch-filter algorithm. I use a combination of a small (<1 day) sliding fitting window, and a sliding transit-shaped notch to remove rotation while preserving transit-like features. For the most rapid rotators (Prot < 0.3 days), I model each rotation as a Locally Optimized Combination of Rotation (LOCoR), i.e. a linear combination of past and future rotation profiles.
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These methods outperform other methods of transit detection, and have identified all young (<1 Gyr) transiting planets known to date, and are yielding further detections with TESS.
Notch Filter
(Above) Notch filtering for the 11 Myr Upper Sco planet K2-33b for K2 campaign 2. (Below) A single correction window from the notch filter.
The Structure of Clusters at the Epoch of Planet Formation
Stars of all types, including those like the sun, generally do not form in isolation, but rather as members of clusters or associations. These clusters vary significantly in size, stellar content, and density (e.g., β -Pic vs Sco-Cen). The structure of these clusters at the epoch of planet formation (0-3 Myr) and the following ∼50 Myr of planetesimal formation and orbital evolution sculpts the architecture of planetary systems that survive to maturity: Interaction with other cluster members, the proximity to supernovae, and ionizing radiation and stellar wind pressure from high-mass members all affect disks and forming planets.
I am mapping clusters such as the Sco-Cen association in 8 dimensions, this includes the 6D position-velocity phase space and stellar temperature and luminosity. The goal is to characterize the full populations of these young associations, and perform kinematic traceback to the epoch of star and planet formation to measure the key properties that shape planetary systems, namely densities and stellar mass content of subclustered regions.
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Kinematic membership of the Sco-Cen association from Gaia DR2 and the Bayesian selection method I developed (Rizzuto et al, 2011, 2015).
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Dynamical Masses of Young Stars with Interferometry
Uncertainties in model-derived properties are the dominant systematic error for in-situ measurements of the IMF for young populations, determinations of star cluster ages, comparison of protoplanetary disk properties to those of mature planetary systems, and binary formation studies. Mass/age ambiguities also strongly limit exoplanet demographics at young ages, where the host-star properties directly establish the planet mass.
The gold standard for calibrating stellar evolutionary models is to measure orbits and dynamical masses for visual binary systems. The empirical masses are then directly comparable to a given model’s prediction for that position in the HR diagram. This test has traditionally been difficult since young stellar populations are distant, meaning that only long-period systems can be resolved as visual binaries.
I am using Non-Redundant Aperture Mask (NRM) interferomtery with the NIRC2 imager at Keck to monitor a sample of some 40 young binary systems. NRM allows us to resolve binary companions at or below the diffraction limit (10-20 mas, or <10 AU in Taurus or Sco Cen), so binary orbits with periods of ~10 years are accessible. We are producing dynamical mass measurements to 1-2% and increasing the number of young binary dynamical masses by a factor 2.
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Thus far, we find that the G-type binary systems in Upper Scorpius have model ages of ∼11.5Myr, which is consistent with the latest age estimates for Upper Scorpius, while the M-type binary systems have significantly younger model ages of ∼6.5 Myr. Based on our fits, this age discrepancy in the models corresponds to a luminosity under prediction of 0.5-0.1 L_sun for the M-type stars at a given premain-sequence age. The same discrepancy is seen in our Taurus binary systems, suggesting model incompleteness for premain-sequence stars.
A sample of orbits from our young binaries sample observed with Keck NIRC2 masking.​ (Below) The orbit and dynamical mass of Taurus binary FF Tau.
