Variable Star Spectroscopy: Tools, Techniques, and Recent Results

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Variable Star Spectroscopy Tools, Techniques,
and Recent Results

• Dr. Matthew Beaky
• Adam Vogt
• Robert Chapman
• Thomas Vonderhaar

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There have been only a few efforts to monitor
changes in variable star spectra over time
Alfred Joy 1930s 1960s Pioneer in
variable star spectroscopy Mt. Wilson
Observatory Mira  131 spectra over 10
years Michael Castelaz, et al. 1996 2000
Pisgah Astronomical Research Institute low
resolution spectra of 20 Mira variables from
one to five different phases per
star Association T60 Ongoing 60 cm
telescope at Pic du Midi Observatory
monitoring Be stars
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A stars spectrum shows a continuum, absorption
features, and (possibly) emission features
Dense, hot object
Hot, excited gas
Cool gas in front of hot, dense object
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Stellar spectral classes are organized according
to the stars surface temperature
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Stellar spectral classes are organized according
to the stars surface temperature
Hydrogen Balmer Series
Molecular Bands
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Stellar spectra may be presented visually as
images or as graphs of intensity vs. wavelength
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Stellar spectra may be presented visually as
images or as graphs of intensity vs. wavelength
RT CYG July 17, 2007
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A spectrum provides a wealth of information
about the physical properties of a star
Source Sloan Digital Sky Survey
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This presentation will describe the process of
stellar spectroscopy and show some results
Tools
Techniques
Recent Results
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A complete spectroscopic data collection system
consists of several elements
Minimum practical aperture 8 inches
Prism or diffraction grating
CCD camera (sensitive in infrared)
Specialized for spectroscopy
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A grating, a prism, or a combination of both
disperses the starlight into a spectrum
Prism
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A grating, a prism, or a combination of both
disperses the starlight into a spectrum
Transmission Grating
Transmission Grating Prism
Grism
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A grating, a prism, or a combination of both
disperses the starlight into a spectrum
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Rainbow Optics Transmission Grating no slit
200 lines per mm  5 to 10 Å/pixel fits
in 1.25-inch filter cell easily mounted on
CCD nose piece price 249
SBIG Self-Guiding Spectrograph 10 micron
slit 150 lines/mm  5 Å/pixel 600
lines/mm 1 Å/pixel requires SBIG ST-series
camera price 5000 (plus CCD camera)
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Slitless spectrometers allow the spectrum to
blend with other stars and spectra
no Slit
with Slit
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Spectrometer dimensions and choice of grating
determine the wavelength range and resolution
RT CYG July 24, 2007
SBIG SGS 150 lines/mm grating
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Spectrometer dimensions and choice of grating
determine the wavelength range recorded
RT CYG July 24, 2007
SBIG SGS 150 lines/mm grating
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Spectrometer dimensions and choice of grating
determine the wavelength range recorded
RT CYG July 24, 2007
SBIG SGS 150 lines/mm grating
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Spectrometer dimensions and choice of grating
determine the wavelength range recorded
RT CYG July 24, 2007
SBIG SGS 600 lines/mm grating
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Software options for spectrum calibration and
analysis are limited
Visual Spec by Valerie Desnoux freeware
performs nearly all desired functions
translated from French
IRAF from NOAO freely available
comprehensive spectral analysis steep
learning curve runs on Unix or Linux
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CCD spectroscopy uses similar techniques to CCD
photometry, but with important differences
Longer exposure times typically 2 to 30
minutes co-add multiple images Different
guiding requirements without slit tracking
errors in RA no problem with slit must keep
star on slit (1 arcsec guiding) Need for
calibration frames dark frames wavelength
calibration flux calibration
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Spectra acquired with a CCD spectrometer must be
calibrated in wavelength
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Spectra must also be flux calibrated using a
standard source of known intensity
Standard star or lamp intensity vs. wave data
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Spectra must also be flux calibrated using a
standard source of known intensity
Jacoby and Hunter, AJSpS (1984) 56, 257
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Miscellaneous issues Normalization Equivalent
width Line identification
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X Cygni (Cepheid variable) 5 Å/pixel
July 2007
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X Cygni (Cepheid variable) 1 Å/pixel
July 2007
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RT Cygni Mira variable 5 Å/pixel July
2007
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RT Cygni Mira variable 1 Å/pixel July
2007
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RT Cygni began to fade in late August
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We have begun to look at semiregular variable
stars AV Ari and RZ Ari
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Spectroscopic monitoring of variable stars is a
wide-open field with limitless potential
Potential targets Mira variables
semi-regular variables bright novae Be
stars cataclysmic variables many
more Low resolution is acceptable maybe
preferable Tools and techniques are within reach
of observers with experience in CCD photometry