a Radial Velocity Reference Jeff Valenti (STScI) Jay Anderson - - PowerPoint PPT Presentation

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a Radial Velocity Reference Jeff Valenti (STScI) Jay Anderson - - PowerPoint PPT Presentation

Nov 14, 2022 298 likes •681 views

Using a Gas Absorption Cell as a Radial Velocity Reference Jeff Valenti (STScI) Jay Anderson (STScI) Presented at Astronomy of Exoplanets with Precise Radial Velocities at Penn State University on Aug 18, 2010 Why a gas cell can be

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“Using a Gas Absorption Cell as a Radial Velocity Reference” Jeff Valenti (STScI) Jay Anderson (STScI)

Presented at “Astronomy of Exoplanets with Precise Radial Velocities” at Penn State University on Aug 18, 2010

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Why a gas cell can be useful… A gas cell imprints on each spectrum the behavior of optics and detector for the actual illumination conditions during that observation Compensate for spectrograph instabilities. Data analysis is nontrivial. Planets still lurking in 15 years of existing data from slit spectrographs.

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Outline Modeling observations Intrinsic stellar spectrum Iodine cell temperature Line spread function Residuals Results

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Using a Gas Absorption Cell Model calculation

Determine wavelength scale of observation Shift intrinsic stellar spectrum by stellar radial velocity Multiply by gas cell transmission spectrum Convolve with local line spread function Determine normalization function to match observation

Free parameters for each observation

Wavelength scale Stellar radial velocity Normalization function Line spread function

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Wavelengths from Iodine Cell Absorption Lines

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Velocity Shift of Intrinsic Stellar Sepctrum

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Line Spread Function of Spectrograph

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Constructed Model of Observation

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Outline Modeling observations Intrinsic stellar spectrum Iodine cell temperature Line spread function Residuals Results

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Three Ways to Determine the Intrinsic Spectrum Observe directly with R ~ 300 000 spectrograph Deconvolve using contemporaneous LSF

Observe B stars with iodine to get an LSF Observe target star without iodine Deconvolve to get intrinsic stellar spectrum Assumes LSF is stable between observations

Deconvolve using simultaneous LSF

Observe target star several/many times with iodine “Grand solution” gives LSF and intrinsic stellar spectrum Still working to understand and tune the algorithm

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Deconvolution using Contemporaneous LSF

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Plenty of Constraints for Grand Solution

New Code

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Stellar Spectrum Rings if Nodes Too Far Apart

New Code

Same Set of Reduced Spectra Completely New Analysis Code

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Stellar Spectra Deconvolved Two Different Ways

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Outline Modeling observations Intrinsic stellar spectrum Iodine cell temperature Line spread function Residuals Results

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Transmission Spectrum of Keck Iodine Cell FTS spectra at three iodine cell temperatures

50, 55, and 60 C Interpolate to other temperatures as needed

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Temperature Sensitivity of Iodine Lines

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Iodine Cell Temperature vs. TEMPIOD1

Temperature variation, but velocities are good

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Environment Can Affect Gas Cell Temperature

TIOD1 TIOD2 TIN Radiative Cooling Control Sensor Second Sensor Thermal Control Stabilized Sensor Second Sensor Calibration Mirror In Out Environment

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Outline Modeling observations Intrinsic stellar spectrum Iodine cell temperature Line spread function Residuals Results

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LSF Changes For Each Exposure Consecutive exposures

67 second cadence

Raw LSF shift

0.0039 pixels 5.2 m/s

After modeling I2

0.5 m/s Factor of 10 better

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LSF Variations for Consecutive Exposures Spectrograph is stable on short time scales Slit illumination may vary

Misguiding Seeing changes

Pupil illumination may vary

Misguiding with telescope out of focus Particular concern for mosaic gratings

Reduce effects with spectrograph design

Fiber feed Precise guiding

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Spline Nodes Describe Narrow LSF Core

Free Centroid at Zero Fixed

New Code

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Works Equally Well for Broader LSF Core

New Code

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Broad LSF Wings Seen in Laser Exposures

5939.32 Å 5433.65 Å 6328.16 Å 1.3% of LSF is

  • utside ±7 pixels
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Outline Modeling observations Intrinsic stellar spectrum Iodine cell temperature Line spread function Residuals Results

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Fit Residuals for B Star Spectra

New Code

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Fit Residuals for 992 B Star Spectra

New Code

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Adjusted Fit Residuals for 992 B Stars

New Code Systematics reduced but not yet eliminated

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σ Dra without Residual Correction

New Code Prior to Reducing Fit Residuals

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σ Dra with Residual Correction and Uniform BC

New Code After Reducing Fit Residuals Systematics reduced but not yet eliminated

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Outline Modeling observations Intrinsic stellar spectrum Iodine cell temperature Line spread function Residuals Results

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Radial Velocities for τ Cet

New Code

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Radial Velocities for HD 9407

New Code

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Radial Velocities for HD 156668

New Code

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Radial Velocities for GJ 412a

1 pixel per node in intrinsic spectrum New Code

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Main Points

Gas cell compensates for spectrograph instabilities Need Instrinsic stellar spectrum

Obtain directly with R ~ 300 000 spectrograph Deconvolve using contemporaneous LSF Deconvolve using simultaneous LSF (“grand solution”)

Iodine cell temperature depends on environment Describe LSF by spline curve

Centroid at zero breaks degeneracy with wavelengths Need to accommodate extended wings seen in laser

Diagnostics of systematic errors

Fit residuals of many stars in iodine reference frame Radial velocity versus barycentric correction

Grand solution is starting to yield precise velocities