Transcript WFCAM Instrument Progress and Problems

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
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 photoelectrons Ne), simulations for v must
include the measurable amount of
Doppler Information (Q) in optical and
NIR spectra
PRVS
Y+J+H
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
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 midlate-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?
 Causes of intrinsic jitter
 Rotation + star spots/surface features
 Activity/variability
 Turbulence and pulsation
Keck optical sample, Wright et al. (2005)
F stars
 Results from optical RV surveys
 For non-active M dwarfs,
average intrinsic jitter ~ 4 m/s
 No significant trend with SpT
G & K stars
 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
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)
Realistic PRVS Simulations
M6V
Teff = 2800 K
Log g = 5
v sin i = 0 km/s
Model
Telluric
OH
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
V
R=70,000
Y
J
 Over-plot FT (Gaussian PSF)
for R=20k, 50k, 70k, 100k
H
 RESULT:
optimum R  70,000
K
Radial Velocity Error Budget
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
M3V
M1V
G2V
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.