3-D SUBMILLIMETER SPECTROSCOPY FOR ASTROPHYSICS AND SPECTRAL

ASSIGNMENT

SARA FORTMAN, CHRISTOPHER NEESE, IVAN R. MEDVEDEV, FRANK C. DE LUCIA, Department of Physics, The Ohio State University, Columbus, OH 43210-1106, USA.

Midwest Astrochemistry Meeting

Urbana, IL

November 8th, 2008

3 4 5 6 8 97

Herschel High BandHerschel Low Band0 200 1000800600400 1200 1400 1600 1800 2000

Too Many Weeds

Frequency/GHz

1 mm Survey of Orion with IRAM 30-m Telescope

Class 1 WeedsMethanol – CH3OHMethyl Formate – HCOOCH3

Dimethyl Ether – CH3OCH3

Ethyl Cyanide – CH3CH2CN

Class 2 WeedsVinyl Cyanide – C2H3CNSulfur Dioxide – SO2

Methyl Cyanide – CH3CNCyanoacetylene – HC3NAcetaldehyde – CH3CHO

The consensus is that most of the unknownlines come from these molecules and theirisotopologues.

The challenge becomes solving this problem inthe context of ALMA’s great sensitivity andHerschel’s new spectral regions.

courtesy of J. Cernicharo

Ethyl Cyanide at 300K

Ethyl Cyanide as a Function of Temperature

• If you want an intensity in our temperature range, we know it.• This temperature range is too high for most astronomical spectra.• We can use collisional cooling to reach lower temperatures.

Ratios to Obtain Lower State Energy

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We can plot the log of the ratio in log(1/T) space and expect to see a straight line.

Consider taking the ratio of two lines of which one is assigned and the other is unassigned.

• We could extrapolate to low temperatures, but this will give large errors.

• We want to determine the lower state energies in order to create a catalogue.

Lower State Energy vs. Thermal Behavior

InSb detector

Aluminum cell: length 6 m; diameter 15 cm

Lens

Path of microwaveradiation

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Glass rings used to suppress reflections

Data acquisition system

Computer

Thermal enclosure

AgilentSynthesizer

Submillimeter Spectrometer

VDI

Lens

Temperature Control

• Temperature Range: 228 – 405 K (-45 – 132 °C) at ~.8 degrees/min• Take 350 scans over 4 hours with the solid state system• Take a scan every 38.7 seconds

Recent Results

SO2

• 160 total lines• 60 reference lines• Temperature Range: 234 – 403 K (-39 – 131 °C)• Standard deviation: 21.9 cm^-1

C2H5CN• 1645 total lines • 405 reference lines.• Range: 234 – 389 K (-39 – 116 °C)• Standard deviation of known lines: ~50 cm^-1

• Took sulfur dioxide (SO2) and ethyl cyanide (C2H5CN) spectra from 570 – 650 GHz• Calculated the lower state energy of all lines by taking ratios with a subset of the known lines• Assumed the lower state energy is the average of the energies calculated from the subset of known lines• Checked the results of the known lines

Summary

The Problem• There are too many weed lines for the traditional assignment method of

spectroscopy.

A Solution• Intensity calibrated complete spectra over the ALMA and Herschel Bands by

– Direct measurement at astrophysical temperatures and/or– Lower state energy / Einstein coefficient modeling for catalogues.

• The error in the predicted intensities of the interpolated spectra is comparable to the error in experimental intensity measurements.

Ethyl Cyanide as a Function of Temperature

Sulfur Dioxide as a Function of Temperature

Astronomy

• The smallest errors in intensities will come when the calculated temperature is bounded by experimental temperatures

• The error in the predicted intensity will be of the order the error in the observations (or better because we make many observations).

Propagation of Error and Uncertainties

Spectroscopy

We expect to reduce uncertainties by a factor of 10 by:• Replacing the peak finder with analysis• Fitting a model to the baseline ripple• Using a grand fit of all assigned lines as the reference line

instead of a single line• Getting a proper average over the ends by using the

spectroscopic temperature• Operating over a larger temperature range (using a

collisional cooling cell to 2K)

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Graphing in Two and Three Dimensions

Frequency (MHz)

Intensity

(nm2*MHz)

Lower State Energy (cm-1)

162977 5.1963711 631.1015

163119 17.025509 113.2438

163568 5.0442872 400.8251

163606 37.162086 65.264397

163925 4.3062572 488.5152

• Traditional approach uses a 2D (intensity vs. frequency) plot

• New approach creates a 3D plot from the intensity, frequency and lower state energy data

Two Related Objectives

Spectroscopy Challenge• Bootstrap Assignment in Complex Spectra• FASSST spectra may contain >10^5 lines in many

vibrational states

Traditional Approach• Use 2D (intensity, frequency) spectra to assign and

bootstrap in each vibrational state

New Approach• Observe intensity calibrated variable temperature

spectrum and calculate lower state energies.• Use intensity, frequency and lower state energies in

the bootstrap assignment

Astronomy Challenge• Current telescopes approach confusion limit• Many unassigned lines• New systems (Alma, Herschel) will be more

powerful

Traditional Approach• Quantum Mechanical predictions of astrophysical

spectra give intensity and frequency as a function of temperature

• Spectroscopists calculate and fit what we can, not what astronomers need

New Approach• Predict intensity and frequency as a function of

temperature without assignment

courtesy of J. Cernicharo

Intensity Calibrated Variable Temperature Spectroscopy• Observe 2D spectra at many temperatures• Calculate intensity, frequency and lower state

energies for assigned and unassigned lines• Give astronomers what they want• Give spectroscopists more information