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Why observe M dwarfs? Due to current technical limits

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Presentation on theme: "Why observe M dwarfs? Due to current technical limits"— Presentation transcript:

1 Why observe M dwarfs? Due to current technical limits
(~ 1m/s ---), the reflex velocities of earth-mass planets in the HZ are only observable around mid- to late-M dwarf stars M9V M6V M3V M1V

2 Why observe in the near-IR?
GL 406 M6V (IRTF/SpeX R~2000) Radial velocity precision, v = c Q-1 Ne-0.5 Bouchy et al. (2001) Although M dwarfs are much brighter in the NIR than the optical (more photo- electrons Ne), simulations for v must include the measurable amount of Doppler Information (Q) in optical and NIR spectra PRVS Y+J+H

3 Simulations: Q,v vs v sini (8 m)
M3V R=70,000 S/N=300 M6V R=70,000 S/N=300 M9V R=70,000 S/N=300

4 Theory/Obs Comparison
From high R data, M dwarf theoretical models (Peter Hauschildt) underestimate the Doppler Information (Q) in the NIR by factors > 2 Considering models + data there is a clear advantage to observing mid- late-M dwarfs in NIR (Y+J+H bands, photon-limited) over the optical GL 406 (Wolf 359) M6V J-band, R=20,000 Keck/NIRSPEC (McLean et al. 2007) Qmodel ~ 800 Qdata ~ 1600

5 What is the intrinsic RV jitter of M dwarfs?
Keck optical sample, Wright et al. (2005) Causes of intrinsic jitter Rotation + star spots/surface features Activity/variability Turbulence and pulsation Results from optical RV surveys For non-active M dwarfs, average intrinsic jitter ~ 4 m/s No significant trend with SpT Expectations for NIR RV surveys Higher v sin i for late-M dwarfs But 2 x better star spot contrast in NIR means intrinsic jitter likely < 4 m/s for non-active M dwarfs F stars G & K stars M stars

6 Technical challenges of RV in the NIR
Simultaneous wavelength fiducial covering NIR is required for high precision RV spectroscopy No suitable gas/gases for a NIR absorption cell found Use simultaneously exposed arcs (Th-Ar, Kr, Ne, Xe) and ultra-stable spectrograph ~ 300 bright lines to monitor drift during observing (using super exposures and sub-array reads of arc lines) ~ 1000 lines for PSF and wavelength calibration (daytime) Use of a laser comb possible following R&D Significant telluric contamination in the NIR Mask out  30 km/s around telluric features deeper than 2% At R=70,000 (14,000 ft, 2 mm PWV, 1.2 air-mass) this leaves 87% of Y, 34% of J, and 58% of H Simulations indicate resulting ‘telluric jitter’ ~ 0.5 m/s PRVS ‘Pathfinder’ instrument being used at Penn State supports this modeling (see Pathfinder poster below)

7 Realistic PRVS Simulations
M6V Teff = 2800 K Log g = 5 v sin i = 0 km/s Model Telluric OH

8 Fourier Analysis F() FT (f/) Doppler info of spectrum
F() related to f/. FT (f/) = k f(k) where spatial freq k = 2/ Plot k f(k) vs k for M6V and v sin i = 0 km/s Over-plot FT (Gaussian PSF) for R=20k, 50k, 70k, 100k RESULT: optimum R  70,000 V R=70,000 Y J H K

9 Radial Velocity Error Budget

S/N break-even point between optical and NIR surveys is early- to mid-M SpT OPTICAL RV (8 m) PRVS NIR RV Mean intrinsic RV jitter ~ 4 m/s measured in optical Improved intrinsic RV jitter in NIR? M9V M6V M3V M1V G2V

11 including review by Tarter et al.
Habitable zone is more accessible around M dwarfs when observed in the NIR 1.0 m/s 0.1 m/s Required RV precision to detect 1 ME Kasting et al. (1995) M Star Planet Habitability: Special issue of Astrobiology (February 2007), including review by Tarter et al.

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