Confessions of a Laboratory Spectroscopist

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Confessions of a Laboratory Spectroscopist
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The Discussion 1. Meetings for spectroscopy in
support of X are becoming popular 2. The
spectroscopists dirty little secret We
measure, assign, and model what we can - not what
you need - the catalogues are massively
incomplete
Spectroscopic strategies and custom vs
astrophysical advances 3. Growing consensus that
conventional approach will not work a
semi-infinite job 4. An alternative Complete,
intensity calibrated spectra as a function of
temperature - experiment and theory 5. Really
here to discuss flowers and grand fits The
integration of laboratory and astrophysical
spectroscopy to detect the unobservable Large
molecules emit more photons/GHz into a multiplex
telescope receiver than do small molecules
An example of this type of spectroscopy is
called FASSST, an acronym for FAst Scan
Submillimeter-wave Spectroscopic Technique
developed by Dr. F. De Lucia, which has the
potential to obtain needed data rapidly and with
the accuracy required. . . . In this approach
the entire spectrum is collected at multiple
temperatures and compared with known calibration
lines allowing the line strengths and energy
levels to be determined.
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1 mm Survey of Orion with IRAM 30-m Telescope
courtesy of J. Cernicharo
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U-Lines in the IRAM Survey
After 50 years of submillimeter spectroscopy
gt5000 U lines 40 of total Most
attributable to large molecules -Very large
number of low lying vibrational/torsional states
-Many have perturbations we often analyze
the portion of the spectrum that we can or have
time to -In some lab spectra assign and
fit 10000 out of 100000 lines Baseline often
confusion, not noise limited
courtesy of J. Cernicharo
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The Fundamental Problem A Brief History of
Bootstrap Astrophysical Spectroscopy and
Models In the beginning there were only a few
astrophysical lines H2CO, NH3, CO, . .
. Laboratory mm/submm spectroscopy was ahead of
the astronomy Then there were U-lines - exotic
species like HCO Astrophysical reality made it
easy in the lab Small Molecules
Astrophysically abundant and spectroscopically
strong (good partition function) Also, easy to
characterize in lab simple models were
complete -gt generate complete catalogues But
then along came methanol, methyl formate, and
others Spectral complexity is a very steep
function of molecular size The difficulty of
complete spectroscopic modeling is also a very
steep function of molecular size
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Stated Another Way In the beginning it was easier
to model/predict line frequencies than to measure
them Small, easy to model species - measure a
few well selected transitions, predict the rest
As an important by-product, these models gave
astronomers intensities As an additional
by-product, this led naturally to catalogues
based on these models Now it is easier to measure
spectra than to predict them Orders of
magnitude increase in the difficulty of modeling
(large molecules) Improvements in
experimental approaches Our cataloging
strategy does not reflect this change
Catalogues have become very incomplete in
unpredictable ways - unknown bounds But even with
experimentally measured spectral frequencies, we
have to deal with the intensity/temperature
problem
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Methanol
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Non-Bootstrap Approach Measure every line
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FASSST Spectrum of the Classical Weed Methyl
Formate
lt 0.01 second of data
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BUT! 1. We rarely measure intensities 2.
Even if we did, we need to know them over the
range of astronomical temperatures 3.
Traditional bootstrap Quantum Mechanical models
do this very well
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The Effect of Temperature on the Spectrum of
CH3OH
Observed Calculated
We need spectrum that is not just complete in
frequency, but also in intensity at all
temperatures
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The Calculation of Line Frequencies and
Intensities from Experimental Data Overview of
New Approach
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A MOLECULAR LINE SURVEY OF ORION KL IN THE 350
MICRON BAND C. Comito, P. Schilke, T. G.
Phillips, D. C. Lis, F. Motte, and D. Mehringer
Ap. J. S.S. 156, 127 (2005).
Can we fit astrophysical data as opposed to
simply identifying the lines? Analytical
Chemistry (with intensity calibrated reference
spectra) Number of data points number of
spectral resolution elements Number of
molecular variables number of molecules (their
concentrations) Astrophysics Number of
astrophysical variables ? (temperature(s)-veloci
ty component(s), . .)
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A Motivation From Weeds to Flowers?
Rotational Partition Functions
At a given observational frequency the
distance between band heads is the number
of K-levels associated with each band head is
the number of MJ levels associated with each
K - line is
Sum of line strengths/frequency interval - the
number of spectral photons available to a
multi-channel telescope
Because the spectral space occupied by these
lines grows as R2 (the MJ factor above adds
intensity, but not spectral space)
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Is it possible to recover astronomical molecular
concentrations without individually observable
lines? Some Thoughts and Questions 1. Fit for
individually identifiable U lines and QM
assigned lines. 2. Will fits to complete
spectral libraries eliminate the background
clutter? 3. Are there individually hidden,
but collectively observable flowers in the
astronomical garden? 4. Note the lineshape
problems in the astrophysical spectrum - how big
is this impact?
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Questions -gt A modest Program? 1. What is the
state of the art for already having done this
(i.e. subtract the contributions from assigned
lines) Turbulence and lineshape effects? 2. As
a function of telescope/telescope type/
molecule/astronomical source, how bad is the
approximation that there is an effective
temperature? To drive down clutter to 20 of
current level, this linear average approximation
only needs to be good to a factor of 5? How
well can you use the spectra from simple
molecules to establish the effective
temperature? Are there families of molecules
which occupy similar regions inside of a spatial
resolution element?
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3. A simpler molecular problem? A
glycine-like molecule for which there may be
individually observable lines, but lines near the
clutter limit. 4. If/When Complete Spectra
become available, what might the best
astronomical sources be for which to try this
grand fit approach? Not too hard - Not too
easy 5. Alternatives to effective temperature
Let regions within resolution element have
different temperatures/velocities and use the
very redundant spectral information to choose
sets of concentrations/temperature Include
chemistry models to link species in space
(temperature and turbulence)
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The Relationships Among Spectroscopy, Catalogues,
and Astrophysics have Changed Dramatically and We
Need a New Strategy Elimination of Weeds by use
of Experimental Models From experimental
measurements at two temperatures T1 and T2, it is
possible to calculate spectrum (with intensities)
at an arbitrary T3. For low T3, a relatively
low T2 improves the accuracy of the calculated
spectrum. Collisional cooling provides a
general method for achieving this low T2, with 77
K convenient and suitable for all but the lowest
temperatures. FASSST is a means of obtaining
the needed data rapidly and with chemical
concentrations constant over the data collection
period. It is realistic in a finite time to
produce catalogs complete enough to account even
for the quasi-continua that sets the confusion
limit. In the limit of complete
spectroscopic knowledge, the confusion limit
will probably be set by the unknowns associated
with the complexity of the astrophysical
conditions, but the high spatial resolution of
large telescopes and modern arrays may reduce
this complexity. Finding Flowers - Understanding
Chemistry and Astrophysics With good telescope
intensity calibration and high spatial resolution
there is a good prospect to use a global fitting
approach to detect larger molecules than commonly
assumed. Are analyses which include chemistry
and nonlinearity of spectral signature to
temperature to link concentrations/temperatures/tu
rbulence within a spatial resolution element
possible? The path laid out has challenges, but
they are small in comparison to other challenges
that must be met to get maximum return on
investment for 109 instruments