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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

M6V

M9V

M3V

M1V

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 vmust

include the measurable amount of

Doppler Information (Q) in optical and

NIR spectra

PRVS

Y+J+H

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

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

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)

Fourier Analysis

FT (f/)

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

Y

R=70,000

J

H

K

PRVS SENSITIVITY NICHE

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

G2V

M3V

M1V

Habitable zone is more accessible around M dwarfswhen 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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