Telluric Contamination
effects, challenges, and solutions
in the context of EPRV
Sharon X. Wang, Jason T. Wright, Cullen Blake, Pedro Figueira, Nuno Santos,
Peter Plavchan, Andreas Seifahrt, Claire Moutou, François Bouchy
summary of current status
● Telluric lines have ~m/s level impact on RV
precision (either optical or NIR).
● We can model most telluric lines to ~ 2%.
(radiative transfer + atmospheric model + line list)
● Telluric lines are sources of contamination
instead of wavelength calibrator for EPRV.
telluric absorption in optical+NIR
Smette et al. (2015)
500nm
1μm
1μm
2.6μm
pervasive micro lines in the optical
4430 Å
4850 Å
5040 Å
5410 Å
5750 Å
line depth 0.2 -- 2%
Wavelength [Å]
Cunha et al. (2014)
telluric: calibrator, contamination
Griffin & Griffin (1973) suggested 10 m/s possible with O2 (start
of quest for PRVs?)
Now 5-20 m/s in both optical and NIR using tellurics lines as
calibrators (e.g. Barnes+2012, Blake+2010, Figueira+2010a).
Studies in the optical show long-term stability at 10 m/s over 10
years with HARPS (Figueira+2010b) and that it is possible to
improve upon this value using simple empirical models (e.g.
Figueira+ 2012)
EPRV definitely requires better calibrators and proper modeling
of telluric lines (e.g. Bean+2010, upcoming instruments in NIR).
optical (Th-Ar):
alias induced by micro tellurics
Cunha et al. (2014)
Artigau et al. , 2014, SPIE, 9149
Assumption : telluric absorption spectrum = sum of a finite number of chemical
species that vary in relative strength = linear combination of individual
absorbance with weight varying with airmass and weather.
Library of individual absorbance (real telluric standards (hot stars) observed at a
large variety of airmass and water column or TAPAS synthetic spectra) + principal
component analysis to identify independently varying absorbers.
Non-zero velocity of one absorber seen as an second component equal to the
spectral derivative
Science spectra = least-square fit of 1) absorption-free spectra + 2) linear
combination of absorbance.
Requirement = large spectral range + high resolution
HARPS spectra restricted
to 634 – 669 nm
Without excluding telluric
= 10.3 m/s RMS
Excluding telluric (18% of the domain) = 3.6 m/s RMS
PCA subtracted spectrum
= 1.6 m/s RMS
Artigau et al. (2014)
optical (iodine):
alias induced by micro tellurics
simulation
2Å chunk
telluric
0.4% absorption
clean
HD 185144
Wang et al. in prep
NIR: modeling “macro” telluric lines
Mauna Kea, PWV=3 mm, Airmass = 1 (from Gemini webpage)
A different challenge:
τ>>0.1 at many wavelengths
Several different species (H2O, CH4, CO2)
NIR: modeling “macro” telluric lines
Seifahrt+2010
Bean+2010
Blake+2011
NIR: modeling “macro” telluric lines
Seifahrt+2010
High optical depth - approximations that
work at low optical depth not OK
Detail line shape matters more, line mixing
matters
Water, water, everywhere!
Multiple, overlapping species
Blake+2011
HITRAN errors mater more
Good News?
CO2, CH4 are well behaved, only
small seasonal variations in concentration!
modeling the telluric lines:
approaches, codes available
Physical:
TAPAS (Bertaux et al. 2014, web)
TelFit (Gullikson et al. 2014)
Molecfit (Smette et al. 2015)
TERRASPEC (Bender et al. 2012, cbender@psu.edu)
ATRAN (IR only, Lord 1992 + Gemini, web)
Write your own using LBLTRM(Clough+2005)+HITRAN
(Rothman+2012)
Empirical:
telluric standards (Vacca+2003)
taking spectra at different airmass (Wallace+2011, solar atlas)
PCA (Artigau et al. 2014)
Empirical models
FTS Template - τ>0.1, multiple species
Forward modelling using FTS telluric template from Livingston & Wallace 1991
Blake et al. (2010)
challenges & pressing questions?
How good are we?
● How well can we model telluric lines?
o What are the current best codes? How well does
each do? Do they agree?
o What do we do about the challenging lines?
o What’s the current limiting factor? What’s next?
Empirical+physical? More physics?
● How well do we need to model them?
o How well are we doing now?
o What’s their impact on RV precision?
o What’s the requirement for achieving 10cm/s?
challenges & pressing questions?
How good are they?
● How stable (in terms of RV) are the telluric lines?
o The current knowledge?
o What to be done? Different species?
● How stable do we need them to be or need to model?
o What’s the RV limit for using them as calibrators?
o What’s the requirement for achieving 10cm/s?
o Include telluric RV as free parameter? Species?
part 1/4
● How well can we model telluric lines?
o
What are the current best codes? How well does
each do? Do they agree?
o
What do we do about the challenging lines?
o
What’s the current limiting factor? What’s next?
Empirical+physical? More physics?
● How well can we model telluric lines?
o What are the current best codes? How well does
each do? Do they agree?
Cunha et al. (2014)
Seifahrt et al. (2010)
Part 1/4
● How well can we model telluric lines?
o
What are the current best codes? How well does
each do? Do they agree?
o
What do we do about the challenging lines?
o
What’s the current limiting factor? What’s next?
Empirical+physical? More physics?
Limitations of current modeling
(roughly in order of impact)
1. First order problems in molecular databases (e.g. HITRAN)
1. Missing lines
2. Uncertainties/errors in line position, line strength, line shift and
broadening parameters.
Note: data quality in HITRAN is not homogeneous, neither for the
same species nor for different species in a narrow wavelength range!
2. Limitations of current modeling codes (e.g. LBLRTM) from treatment of
correct line profiles (i.e. speed dependence/line mixing effects, Dicke
narrowing). Standard is Voigt profile, but would need Rautian or Galatry
profile for certain species with strong line mixing.
(see, e.g., Hartman et al. 2008)
3. Second order problems in molecular databases (e.g. HITRAN)
1. Missing data or approximate treatment of line coupling and mixing.
Important e.g. for O2, CO2, N2, H2O, etc.
Limitations of current modeling
(roughly in order of impact) - cont.
4. Limited knowledge of atmospheric conditions
1. Standard atmospheric profiles probably fine for well mixed species,
but absolute column densities for, e.g., CH4, or CO2 need to be
adjusted.
2. Water vapor is problematic in standard atmospheric profiles due to
high absolute variability and variable vertical distribution (different
temperature, pressure broadening effects etc.). -> Water vapor
spectrum for same PWV column can look different depending on
vertical distribution.
5. Wind-induced line-shifts
1. Can be substantial, especially for observatories under jet stream with
light path into the jet stream (i.e., azimut and airmass dependent). See
Figuera et al. (2010)
Improvements to current modeling
1. Invest manpower and/or money into codes and (more importantly) groups
providing data for HITRAN database -> unlikely to happen.
2. Build a simplified (and unphysical) astro-HITRAN with line data retrieved
from fitting high-SNR, high-resolution stellar spectra from instruments with
stable and/or well known LSFs (e.g. HARPS, or gas-cell aided
spectrographs). Need to be done over large number of spectra taken
under different conditions to disentangle stellar and telluric features.
Successful example: CRIRES program of Bean et al. 2010.
3. Improve knowledge of local conditions of the atmosphere. Particularly
important for H2O and for wind-speed issues. -> Launch weather balloons
every night? -> Potential use of LIDAR techniques for remote retrieval?
What about GPS or microwave radiometers?
Part 2/4
● How well do we need to model them?
o
How well are we doing now?
o
What’s their impact on RV precision?
o
What’s the requirement for achieving 10cm/s?
Part 3/4
● How stable (in terms of RV) are the telluric lines?
o
The current knowledge?
o
What to be done? Different species?
● How stable (in terms of RV) are the telluric lines?
o The current knowledge?
o What to be done? Different species?
Figueira et al. (2010)
O2 lines in HARPS data
HARPS - Telluric O2 line RVs relative to star: ~2 m/s with simple
model over years, 5 m/s “out of the box”
● How stable (in terms of RV) are the telluric lines?
o The current knowledge?
o What to be done? Different species?
Balthasar+1982 - Telluric O2 lines can be modeled to 3 m/s
Caccin+1985 - Pressure-induced asymmetries+winds < 5
m/s
Smith 1982 - ~5 m/s on Arcturus using O2 lines
What about H2O lines?
Part 4/4
● How stable do we need them to be or need to model?
o
What’s the RV limit for using them as calibrators?
o
What’s the requirement for achieving 10cm/s?
o
Include telluric RV as free parameter? Species?
Bibliography (to be completed)
http://bit.ly/eprv_telluric_references
Stay tuned for summary of the session!
extra slides
feel free to add.
be sure to give citations and add notes!
optical (iodine):
micro telluric lines everywhere!
using TERRASPEC
optical (iodine):
alias induced by micro tellurics
Recipe: How to clean your deconvolved
stellar template
Ingredients: high SNR stellar template
spectra (taken with no iodine), bracketing
telluric standard observations through
nearby B star (optional but desirable),
bracketing nearby B star spectra through
iodine cell (to provide spectral PSF).
Prepare: telluric model generator which
can generate models under different
atmospheric conditions (e.g. varying
water vapor), tuned to your observatory.
Your favorite deconvolution algorithm.
Cook: 1. Fit the B star telluric standards to get atmosphere model. 2. Fit the B star
+ iodine spectra with iodine atlas + telluric model to get the correct PSF. 3.
Deconvolve the stellar template spectra with PSF. 4. Divide out telluric model from
deconvolved stellar template (note: first solve for absolute stellar RV!).
● How well can we model telluric lines?
o What are the current best codes? How well does
each do? Do they agree?
3-5%
correction with TelFit modeling
correction with telluric standards
Gullikson et al. (2014)
● How well can we model telluric lines?
o What are the current best codes? How well does
each do? Do they agree?
2%
Smette et al. (2015)
NIR:
modeling “macro”
telluric lines
Bean et al. (2010)
Theoretical Telluric Spectrum
Terraspec: Chad Bender
Telluric Line RV Shifts
Telluric Lines are
Asymmetric!
Optical Depth
Pressure Shifts: ΔRV< 20 m/s
Winds: ΔRV< 20 m/s
Frequency
Altitude (km)
Atmospheric Pressure
ΔP/P
1σ Variation in Pressure
Pressure with Altitude
Δ~0.01
For H20, CO2, CH4, O2
•1% Pressure Change is ΔRV=3 m/s at 1 micron
•Depends on Zenith Angle φ:
Atmospheric Winds
Alt=20 Km
Alt=5 Km
Jet Stream
East Average
East-West
North-South
Vertical
Average Winds
January
March
Radial Wind Speed
June
September
Monte Carlo Simulations
100 night atmosphere models over 12 months
Models for mountain site in New Mexico
10 random sight lines each night, AM<1.2
•
•
•
Telluric Regions
Barycenter
O2
0.685-0.695μm
H 2O
0.9-1.0 μm
CO2
1.59-1.62μm
CH4
2.28-2.3μm
Wavenumber
•
Yearly average atmosphere model subtracted:
EW wind, NS wind, pressure profile
•
Assumption: H2O well-mixed
Simulation Results
RV residuals after correcting for zenith
angle and yearly average wind profile
Altitude of Line Formation
Histograms of altitudes where
lines reach optical depth τ=1