Description of the Hardersen IRTF NIR Asteroid Reflectance Spectra bundle V1.0
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Bundle Generation Date: 2025-04-18
Peer Review: Neese_Richardson_Mueller_Migration
Discipline node: Small Bodies Node


Content description for the Hardersen IRTF NIR Asteroid Reflectance Spectra bundle
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Note: This bundle was migrated to PDS4 from the PDS3 data set EAR_A_I0046_3_HARDERSENSPEC_V1_0. For PDS3 data sets migrated to PDS4, the following text is taken verbatim from the data set description and confidence level note of the PDS3 data set catalog file. In these cases, some details may not be correct as a description of the PDS4 bundle. The file hardersen_et_al was renamed to hardersen_etal_spectral_plots in the PDS4 document directory. 


This dataset contains average Near-Infrared (NIR) reflectance spectra for 68 main-belt asteroids that were observed at the NASA Infrared Telescope Facility (IRTF) from April 2001 to May 2015. These asteroids were used as a part of three research investigations to better constrain their NIR spectra, assigned taxonomies, surface mineralogies, and potential meteorite analogs. The asteroids in this dataset were all observed in a very similar manner across the entire dataset. The resulting average NIR asteroid spectra in this dataset were also reduced in a very similar manner using two different software packages.                             
   
The research projects that utilized these average NIR spectra include: 1) an investigation of the spectral and mineralogical diversity of the M-/X-type asteroids (Hardersen et al., 2005, 2011), 2) a study to better define the basaltic asteroid population in the main asteroid belt (Hardersen et al., 2014, 2015, 2016), and 3) an investigation of (1459) Magnya and its spectral and mineralogical comparison to (4) Vesta (Hardersen et al., 2004).                                                 
   
Hardersen et al. (2005) reported the first results of the M-/X-type asteroid study, which included six M-type asteroids, while Hardersen et al. (2011) provided the final results of this effort that included NIR reflectance spectra for 45 M-/X-type asteroids. These works reported on the identification of significant NIR spectral and surface mineralogical diversity among this group of asteroids, the widespread detection of weak mafic silicate absorption features for pyroxene and olivine (1-5% band depths), detections of possible phyllosilicate features on a few asteroids, and widely varying NIR spectral slopes across the entire spectral dataset.                                                         
   
Hardersen et al. (2014, 2015) were the first results of an effort to better constrain the basaltic asteroid population throughout the main asteroid belt. This is being accomplished by obtaining NIR spectra of Carvano et al. (2010) classified Vp-type asteroids with Wide-Field Infrared Survey (WISE)-derived albedos consistent with basalt (Masiero et al., 2011) from a dataset compiled by Mainzer et al. (2012).              
   
All of the average asteroid NIR reflectance spectra in this dataset were observed at the NASA Infrared Telescope Facility (IRTF), Mauna Kea, Hawaii. The asteroids reported in Hardersen et al. (2004, 2005, 2011, 2014) used the first-generation of the SpeX 0.7-5.3 micron spectrograph while the results in Hardersen et al. (2015) and the expected publication of Hardersen et al. (2016) used the second-generation of the SpeX spectrograph. All observations used the low-resolution, prism mode of the SpeX spectrograph (R = 94), the 0.8 arc second slit, an open dichroic, and an open order sorter filter. Most observations taken prior to 2008 were not observed at the parallactic angle while all observations after 2008 were observed at the parallactic angle. The vast majority of the observations that resulted in this dataset were made in clear to mostly clear weather conditions at the summit of Mauna Kea.                      
   
All observations were conducted in a uniform manner. For each asteroid, an associated extinction star (late F- to late G-type main sequence) was observed close to the asteroid on the sky (less than 5 degrees separation). Stellar observations were interspersed with asteroid observations to ensure that the extinction star airmass range exceeded that of the associated asteroid. The extinction star observations are necessary for later removal of the NIR telluric absorptions during data reduction. One well-known solar analog (SAO 31899, SAO 93936, or SAO 120107) was observed each night, which was used in the reduction process to correct for the use of non-G2V extinction stars and to implement a slope correction so the final asteroid spectrum would mimic that if the actual reflected light from the Sun had been used.                        

Data reduction for the asteroids used either SpecPR (Clark, 1980; Gaffey, 2003) or Spextool (Cushing et al., 2004). For those asteroids reduced using SpecPR, the sky background signal removal was accomplished using the Image Reduction and Analysis Facility (IRAF), followed by importing the sky-subtracted data into SpecPR. SpecPR routines included derivation of the 1st-order extinction coefficients from the extinction stars as a function of wavelength and airmass for up to five sets of extinction star observations (10 spectra per set), sub-channel pixel offsets for alignment of multiple spectra, and averaging routines.  Spextool routines include a sky signal subtraction routine and a telluric correction that utilizes two sets of extinction star observations while also including the sub-channel pixel shifting and averaging routines. Wavelength calibration for SpecPR data is conducted manually by matching argon calibration emission lines/wavelengths to discrete channels and applying a polynomial function to convert the NIR spectra from channels to wavelengths. Spextool implements an internal wavelength calibration using one set of argon calibration spectra.                                                      

For all data, each final average asteroid NIR spectrum can be qualitatively summarized as follows: (Asteroid/Sun) = (Asteroid/Extinction Star) / (Solar Analog Star/Extinction Star), where (Asteroid/Sun) represent the final average asteroid NIR spectrum, (Asteroid/Extinction Star) represents an intermediate average asteroid NIR spectrum after telluric and channeling shifting corrections, and (Solar Analog Star/Extinction Star) is the average stellar spectrum used to correct for observations of non-G2V extinction stars. The average (Solar Analog Star/Extinction Star) spectrum is smoothed to remove telluric absorptions as broad absorption features are not expected in stellar spectra and observations of solar analog stars are typically in very divergent parts of the sky during most IRTF observing runs.                               

The data products in this dataset include the average NIR reflectance spectrum for each of the 68 asteroids. Each asteroid has its own text file that includes two or three columns of data that include wavelength, normalized reflectance, and error values. Most of the SpecPR-derived NIR spectra lack errors in the associated files while most of the Spextool data include errors. Where errors are reported, they are standard errors in SpecPR, or standard errors Robust Weighted Mean in Spextool.  Spextool data are normalized at 1.5 microns while SpecPR data are normalized near 1.7 microns.                                                              

The reported errors for the asteroid spectra that were reduced using Spextool are only the formal errors produced by the Spextool software. The reported errors result from the average of each individual asteroid spectrum, using the error method chosen in Spextool, where each individual asteroid spectrum is divided by the associated extinction star spectrum. The fully propagated errors are not reported, but will be reported in future updates to this dataset.                                           

The reported errors for the asteroid spectra reduced using SpecPR are fractions based on the variability of the point-to-point data scatter present in each final average asteroid spectrum reported here. Typical errors across a spectrum for most of the asteroids range from 1-3% with some asteroids having larger errors in spectral regions containing poorly-corrected telluric features and at longer wavelengths where SpeX has reduced sensitivity."  


Caveats to the data user
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Most observations at the NASA IRTF were in clear, photometric conditions, but there were a few instances of cirrus present in the sky.

