Description of the Gartrelle et al. IRTF Asteroid Spectra bundle V1.0 ===================================================================== Bundle Generation Date: 2021-02-25 Peer Review: 2020_Asteroid_Review Discipline node: Small Bodies Node Content description for the Gartrelle et al. IRTF Asteroid Spectra bundle ========================================================================= During the past four decades, D-type asteroids have been the subject of a myriad of investigations, yet they remain shrouded in mystery. Their steep linear reddish spectra have revealed no confirmed mineral absorption features. Thus, we have no idea what these dark low-albedo objects are made of, where they originated from, under what conditions they formed, or how they evolved dynamically. The purpose of this research is to understand the VNIR (0.7-2.45 micron) spectral distinctions of D-type asteroids at different heliocentric distance and determine if these differences can be associated with other constraining orbital, observational, or physical characteristics. The twenty-five spectra in this data set were combined with sixty-one D-type VNIR spectra, obtained from IRTF/SpeX in previously published investigations, and used as input in a variety of analyses including correlation, regression, Principal Component Analysis, Monte Carlo Analysis, radiative transfer modeling, and curve matching. Our research determined D-types show increased reddening with decreasing distance, with the segment from 1.5-2.45 µm, driving the trend for the full slope (Gartrelle et al., 2020). The low end of the wave range (0.7-1.35 µm) exhibits an opposite trend of increasing redness with increasing distance. Principal components show strong connection to the 0.7-1.35 µm slope and inclination of D-type Jupiter Trojans. Principal component combinations, magnitudes, and positive/negative direction relate strongly to both observed and derived differences between the D-type L4 and L5 Trojan populations. The L5 population is less evolved spectrally and dynamically than L4 counterparts, perhaps due to lower dynamical instabilities inside the L5 cloud. Surface compositional modeling with Shkuratov radiative transfer theory suggests D-type surfaces are composed of combinations of iron-poor olivine, magnesium-rich saponite, magnetite, siderite, calcite, dolomite, plus varying combinations of opaques and H2O ice (Gartrelle et al., 2021). This is similar bulk chemistry to the ungrouped chondrites and supports D-types as their parent bodies. Observations were conducted at the 3.5 m NASA Infrared Telescope Facility (IRTF) in Mauna Kea, Hawaii using the SpeX spectrograph covering the 0.69-2.53 µm wavelength range. IRTF/SpeX offers state-of-the-art equipment for obtaining asteroid spectra due to the combination of the high-altitude of Mauna Kea; a comparatively high percentage of clear, photometric nights; and a spectrograph optimized for spectral study of small solar system bodies (Rayner and Connelly, 2020; Rayner et al., 2003). SpeX was employed in Low-Resolution Prism Mode using the 0.8 x 15" slit. This mode, specifically designed for occultations and faint-object spectroscopy, delivers spectral resolution of R~200 when matched to the 0.3 x 15" slit (Rayner and Connelly, 2020; Rayner et al., 2003). As R scales proportionally lower with increasing slit width, R~75 is achieved using the 0.8 x 15" slit (Rayner and Connelly, 2020; Rayner et al., 2003). Observing consisted of approximately twenty-nine hours over eight sessions beginning in December, 2016 and ending in January, 2019. Six sessions were conducted under good to excellent seeing conditions and two sessions (10/11/17 and 8/19/18) were conducted in marginal/variable conditions. All sessions were performed remotely with the exception of a twelve-hour full-night session on the summit of Mauna Kea on December 30-31, 2017. Target exposures (AS) were obtained at integration times determined by target brightness. Times ranged between 60-200 seconds. A G-type standard star (SS), in the same field of view as the asteroid, was imaged ten times immediately prior and after imaging each asteroid. A solar analog star (SA), preferably a G2V spectral type, was imaged between twenty and thirty-two times during each observing night except for 8/5/2017 when one was unavailable during the observing window. Observatory arcs and flats were acquired each night using an automated routine provided at the telescope. A standard ABBA telescope nodding pattern was used to facilitate subtraction of the residual sky background. Exposures were taken as close to the meridian as possible to increase seeing quality. Airmass for the data ranges between 1.02-1.42 with an average of 1.15 for the data set. Data reduction was accomplished using Spextool to apply the nightly arcs and flats, average the asteroid reflectance spectra, correct for atmospheric interference, and remove solar color from the reflectance (Cushing et al., 2004; Vacca et al., 2003). Additional detail on the observing and data reduction procedures used for this dataset are found in: Gartrelle (2019); Gartrelle et al. (2020, 2021), and (Hardersen et al., 2014). The final end product for each target: Asteroid Reflectance = (AS/SS) / (SA/SS) The asteroid reflectance spectra were normalized to unity at 1.5 µm. In this dataset we provide the full 0.69-2.53 µm spectra from our observations. Data for each observed asteroid contains separate columns containing the wavelength range (in µm) with reflectance values and uncertainty for each wavelength. The observational parameters data file contains columns for target number, target name, CSV data file name, observing start date/time (UTC), observing completion date/time (UTC), number of reduced exposures to construct the final spectrum, net integration time (seconds) per target, apparent magnitude, phase angle (degrees), geocentric distance (AU), heliocentric distance (AU), solar analog star, standard star, slit width (arc seconds), and airmass. It should be noted our published works (Gartrelle, 2019; Gartrelle et al., 2020; Gartrelle et al., 2021) cover discussion and analysis of our spectral data over the 0.7-2.45 µm range in order to be consistent with previous IRTF spectra of D-types from the literature which cover only this range. REFERENCES Cushing, M. C., Vacca, W. D., Rayner, J. T., 2004. SpeXtool: A spectral extraction package for SpeX, a 0.8-5.5 micron cross-dispersed spectrograph. Publications of the Astronomical Society of the Pacific. 116, 362-376. Gartrelle, G. M., 2019. Spectral variations of D-Type asteroids at different heliocentric distances. Space Studies, PhD. Dissertation, University of North Dakota, Grand Forks, ND, pp. 518. Gartrelle, G. M., Hardersen, P. S., Izawa, M. R. M., Nowinski, M. C., 2020. Same family, different neighborhoods: Visible near-infrared (0.7-2.45 μm) Spectral distinctions of D-type asteroids at different heliocentric distances. Icarus (In Press). 0, 0. Gartrelle, G. M., Hardersen, P. S., Izawa, M. R. M., Nowinski, M. C., 2021. Illuminating the dark side of the asteroid population: Surface mineral composition modeling of 81 D-Type asteroids using Shkuratov radiative transfer theory. Icarus 354. Hardersen, P. S., Reddy, V., Roberts, R., Mainzer, A., 2014. More chips off of asteroid (4) Vesta: Characterization of eight Vestoids and their HED meteorite analogs. Icarus. 242, 269-282. Marsset, M., et al., 2020. Twenty years of SpeX: Accuracy limits of spectral slope measurements in asteroid spectroscopy. The Astrophysical Journal Supplement Series. 247, 73. Ostrowski, D. R., Gietzen, K., Lacy, C., Sears, D. W. G., 2010. An investigation of the presence and nature of phyllosilicates on the surfaces of C asteroids by an analysis of the continuum slopes in their near-infrared spectra. Meteoritics & Planetary Science. 45, 615-637. Ostrowski, D. R., Lacy, C. H. S., Gietzen, K. M., Sears, D. W. G., 2011. IRTF spectra for 17 asteroids from the C and X complexes: A discussion of continuum slopes and their relationships to C chondrites and phyllosilicates. Icarus. 212, 682-696. Rayner, J. T., Connelly, M., 2020. SpeX observing manual. University of Hawaii, Honolulu, HI, pp. 44. Rayner, J. T., Onaka, P., Cushing, M., Vacca, W., 2004. Four years of good SpeX. Proceedings: SPIE Astronomical Telescopes & Instrumentation, Vol. 5492. SPIE. Rayner, J. T., et al., 2003. SpeX: A medium-resolution 0.8–5.5 micron spectrograph and imager for the NASA Infrared Telescope Facility. Publications of the Astronomical Society of the Pacific. 115, 362-382. Vacca, W. D., Cushing, M. C., Rayner, J. T., 2003. A method of correcting near-infrared spectra for telluric absorption. Publications of the Astronomical Society of the Pacific. 115, 389-409. Caveats to the data user ======================== Uncertainties were derived from the nominal output from SpeXtool as a result of reducing the asteroid spectral data (Vacca et al. 2003; Cushing et al., 2004). This likely does not capture the total uncertainty and should be taken into account when working with this dataset (Marsset et al., 2020; Ostrowski et al., 2010; Ostrowski et al., 2011). Uncertainties resulting from changing atmospheric conditions, signal-to-noise fluctuation, as well as nightly instrumental or operational variances should also be considered.