Coronagraphy of Earth Analogs: Observables and Goals

Transcription

1 Coronagraphy of Earth Analogs: Observables and Goals Karl Stapelfeldt Exoplanets and Stellar Astrophysics Laboratory, NASA/Goddard With acknowledgements to the TPF-C STDT report Lorentz Center Workshop and the current ExoPAG SAG 5 report

2 Coronagraph Science Goals 1. System architectures: direct imaging census of planetary systems around nearby stars: Jupiters, Neptunes, Super-Earths, Earths in the HZ, and dust structures. Measure brightness and constrain orbital parameters 2. Atmospheric composition: Measure planet colors. Detect major molecular bands (H20, O3, O2, possibly CO2 and CH4). Measure Rayleigh scattering. 3. Infer planet radii and masses (no direct Center Workshop measurement)lorentz

3 Earth reflectance spectrum at R= 70 Des Marais et al Wavelength range µm would encompass O2 and O features, ozone,lorentz and Center onsetworkshop of Rayleigh scattering 23 H October

4 Giant planets well-characterized by µm spectroscopy Depth of cloud deck determines strength of CH4 features Jupiter vs. Neptune easily distinguished R= 70 Lorentz Center Workshop TPF-C STDT report

5 Contrast requirement The Earth intercepts ¼(6,378/1.49e+8)2 = 4.6e-10 of solar output. For 30% albedo, the full-disk reflected contrast is 1.3e-10. At the most-probable quadrature geometry, the halfilluminated phase is fainter by a factor of π. Measurements down to contrast level of 4.4e-11 (δmag= 25.9) are thus needed if we are to have good photometric completeness Lorentz Center Workshop

6 Internal coronagraph performance is approaching TPF requirement 1-4 λ/d Target Target performance performance Lawson et al Trauger et al Lorentz Center Workshop

7 Planet brightness for spectroscopy: Implications for Aperture Size Median R mag= 30.4, distance 15 pc for HZ Earth in TPF top 100 Aperture size needed is a function of exozodi. Allowing 20 days of integration (2x Hubble Deep Field), and requiring S/N= 10 at λ/δλ = 70, No exozodi: 2.5 m telescope would suffice 1 exozodi requires 3.5 m telescope 5 exozodis requires 4.7 m telescope 10 exozodis requires 5.5 m telescope 20 exozodis requires 6.5 m telescope 23 October exozodis Lorentz Workshop requires 7.2Center m telescope Assumes: shot noise only 50% throughput 20% degraded spatial resolution from apodization

8 Inner working angle (IWA) Define the Habitable Zone as AU, scaled by sqrt(stellar luminosity), with 1.0 AU as fiducial For the top 100 TPF stars, the median angular separation corresponding to 1 AU in the HZ is 65 mas To provide for good obscurational completeness, must further reduce the IWA to ~70% of the above value. 100 star mission must provide IWA~45 mas In zodi-driven case (10), telescope diameter >= 5.5 m is needed the coronagraph must enable 0.5e-10 contrast imaging at ring radius (0.045/202625)/(0.5e06 m / 5.5 m)>= 2.4 λ/d. Coronagraphy less challenging If exozodi is at solar system level, a 3.5 m telescope 23might October suffice 2012 Lorentz Center Workshop but needed ring radius is now 1.5 λ/d

9 Orbit determination is critical for assessment of habitability First revisit must remove confusion: show candidate planet does not move like a background star At V= 30, galactic background star density varies from to 1 per sq arcsec. BG star will be unlikely in many cases Relative position measurements needed at 3 epochs. Assume S/N= 10 measurements in broad bandpass on R= 30.4 planet (1 day integration with 5.5 m telescope and 10 exozodi), implies astrometric measurement precision of λ/10d= 2 mas. For HZ target planet at 15 pc median distance, mean orbital motion rate is 400 mas/yr and mean projected rate around quadrature is 130 mas/yr Revisit time interval must be long enough to resolve this motion: 2/130 yr= 6 days, and scales inversely with telescope size. Longer2012 intervals, morelorentz visits preferable for more accurate orbit solutions. 23 October Center Workshop

10 Planet photometric variability In principle, the planet rotation period and constraints on surface albedo features can be derived from extended timeseries photometry (Cowan et al. 2009) In practice the above will be very difficult, as few targets will be bright enough for S/N= 10 photometry on timescales of a few hours. Assuming 1 zodi, only 7 systems would be accessible to a 4.5 m telescope. A 7m aperture would enable for such measurements in 50 stars Seasonal variations might be measured, after accounting for changes in illumination phase around the orbit Lorentz Center Workshop

11 Coronagraphy in binary systems New impetus for this from alpha Cen Bb will want to probe HZ of this star Companion star diffraction can be controlled using specialized occulting masks Planet detection must be accomplished against companion star's halo produced by uncontrolled telescope wavefront errors Feasibility will be a function of the specific Δmag and separation of the stars, and telescope smoothness at spatial frequencies ~100 cycles/aperture For alpha Cen B, 15 separation and very bright exo23 October Lorentzallow Center Workshop Earth2012 (V= 26) would detection against the halo

12 Distinguishing planets from exozodi clumps: Multi-epoch imaging reveals modulation of the illumination phase Isotropic dust scattering Somewhat forward dust scattering Considered 2 zodi clumps leading/trailing the planet by 60 degrees Assumed 1 zodi dust distributed in AU region, planet at 1 AU, 8m telescope, target at 7 pc, inclination 30 deg from edge-on. Simulation by Karl Stapelfeldt Lorentz Center Workshop

13 Super Earths are puffy Valencia et al GJ 1214b density is 1.8 g cm-3 (2.7 Rearth) Neptune core mass is ~1.2 Mearth Lorentz Center Workshop of volatiles At 2.0 Rearth, already indications

14 Need ηrocky ; will Kepler provide it? TPF goal is to find Earthlike planets. Super-Earths may not qualify. Massive atmospheres are unlikely to have large O2 mixing ratio: Timescale may be too long for biological activity to effect conversion from CO2 O2 unstable in excess of H2 TESS+RV followup would allow determination of largest radius 23consistent October 2012with a rocky Lorentz Center Workshop planet Batalha et al. 2012

15 Needed inputs/actions/decisions from the community Frequency of good exo- Focus technical efforts to best utilize limited Earth targets funding available Characterization of exozodi levels in HZ Mission scale: what is an adequate yield of exo-earths? Better advocacy to astronomical community Do not neglect systemlevel architecture issues Rebuild agency-level coordination we had in the TPF/Darwin era Lorentz Center Workshop