Description of the Sanchez_Reddy_small_NEOs bundle V1.0
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Bundle Generation Date: 2024-08-19
Peer Review: 2024_Asteroid_Review
Discipline node: Small Bodies Node


Content description for the Sanchez_Reddy_small_NEOs bundle
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The structure of this bundle consists of two main folders, "data" and "document". The "data" folder contains a subdirectory called "Sanchez_etal_2024_Spectra" with the NIR spectra given as tables (.tab) with three columns:  wavelength in micron, reflectance normalized to 1.5 micron, and reflectance error; as well as their corresponding labels, which also include relevant observational and spectral information. The "document" folder contains the bundle description, a pdf document with thumbnail plots of the spectra, the reference list for the bundle, and information on the observing equipment used.

All spectral observations were obtained using the SpeX instrument on the NASA IRTF in low-resolution prism mode with a 0.8" slit width. Observations were made remotely and in classical mode on site. During the observations, the slit was oriented along the parallactic angle in order to minimize the effects of differential atmospheric refraction. SpeX in low resolution mode has resolving powers of R~100 across the wavelength region from ~0.7 to 2.5 microns. An infrared guider is available to guide on calibration stars (sidereal rates) and asteroids (non-sidereal rates). The main spectrograph uses a 1024x1024 Aladdin 3 InSb array and the guider uses a 512x512 Aladdin 2 InSb array [RAYNERETAL2004]. Low-resolution spectrographs like SpeX are ideal for resolving broad absorption features produced by abundant mafic minerals like olivine and pyroxene that make up many asteroid surface assemblages. The low resolution prism mode also helps in obtaining spectra with higher signal-to-noise-ratios (S/N) and asteroids as faint as Vmag~18.5 are routinely observed.

Spectral observations for this data set were made by taking nodded spectral image pairs of the asteroid, local standard star (for telluric correction) and solar analog stars. The placement of these stellar observations, temporally and spatially on the sky, in relation to the asteroid is important for producing good quality spectra. If the atmosphere over Mauna Kea is stable throughout the observing run (photometric), then the log of the flux (apparent magnitude) of the object will decrease linearly with increasing airmass. Hence, all objects are typically observed at airmasses less than 1.5, which corresponds to a zenith angle of less than 50 degrees. However, if the atmosphere is unstable over Mauna Kea, whether due to an orographic cap cloud or rapid variability of water vapor content, it often produces a non-linear magnitude-airmass relationship.

Local (or extinction) standard stars close to the asteroid are observed to correct for the terrestrial atmospheric water vapor features. Generally, the greater the distance between the local standard star and the asteroid, the poorer the monitoring of the sky conditions for the asteroid. During a typical observing run, a local standard star with spectral properties similar to our Sun (i.e., G-type, main sequence stars) is paired with an asteroid and is observed over a wide airmass range that bracket the airmass range of the asteroid observations. NIR spectra of a solar analog [e.g., HARDORP1978; HARDORP1980B; LANDOLT1992] were also obtained to correct for possible spectral slope variations that could be introduced by the use of a nonsolar local extinction star. Detailed information about the observational circumstances for the NEOs are presented in [SANCHEZETAL2024].

SpeX prism data was reduced using the IDL-based Spextool provided by the NASA IRTF [CUSHINGETAL2004]. The steps followed in the reduction process include: (1) sky background removal by subtracting the image pairs, (2) flat-fielding, (3) cosmic ray and spurious hit removals, (4) wavelength calibration, (5) division of asteroid spectra by the spectrum of the solar analog star, and (6) co-adding of individual spectra. The final files have different numbers of rows due to the quality of the data. All spectra were normalized to unity at 1.5 microns.


Caveats to the data user
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Uncertainties in spectral parameters for near-IR data are crucial for detecting and quantifying surface composition. The average wavelength resolution of the Spextool data is ~0.0035 microns. This is just due to spectral resolution based on the wavelength calibration. When spectra were combined the statistic used was the robust weighted mean. For this, Spextool makes use of a sigma clipping algorithm to identify outliers. The value at each pixel is then the weighted average of the good pixels and the uncertainty is given by the propagated variance. Uncertainties in the data arise primarily due to low S/N of the final average spectrum, incomplete correction of telluric absorption features, and variable sky/weather conditions.