[Paper Review] Palomar Optical Spectrum of Hyperbolic Near-Earth Object A/2017 U1

J. Masiero|arXiv (Cornell University)|Oct 27, 2017

Astro and Planetary Science18 citations

TL;DR

This study presents the first optical spectrum of the interstellar object A/2017 U1 (Alphacorpus), obtained at Palomar Observatory using the Double Spectrograph (DBSP). Despite low signal-to-noise, the data reveal a very red surface with a spectral slope of $3.0 \pm 1.5$ normalized reflectance units per micron, consistent with trans-Neptunian objects, and no significant absorption features in the 520–950 nm range, indicating a likely primitive, icy composition typical of interstellar bodies from the outer solar system or beyond.

ABSTRACT

We present optical spectroscopy of the recently discovered hyperbolic near-Earth object A/2017 U1, taken on 25 Oct 2017 at Palomar Observatory. Although our data are at a very low signal-to-noise, they indicate a very red surface at optical wavelengths without significant absorption features.

Motivation & Objective

• To obtain the first optical spectrum of the hyperbolic near-Earth object A/2017 U1, discovered as a likely interstellar interloper.
• To characterize the surface composition of A/2017 U1 through reflectance spectroscopy despite its faint magnitude and poor observing conditions.
• To determine whether the object exhibits spectral features indicative of common asteroidal or cometary materials, or instead shows signs of interstellar origin.
• To assess the reliability of low signal-to-noise data in deriving meaningful spectral properties of faint, fast-moving solar system objects.

Proposed method

• Optical spectroscopy was obtained using the Double Spectrograph (DBSP) on the 5-meter Hale Telescope at Palomar Observatory on October 25, 2017.
• Ten 300-second exposures were combined, with a 1.5 arcsecond slit aligned along the parallactic angle to minimize atmospheric differential refraction.
• Bias, flat-field, and wavelength calibrations were performed using He-Hg and He-Ne-Ar arc lamps for the blue and red arms, respectively.
• Atmospheric extinction was corrected using a local standard star (BD+03 27), and the spectrum was normalized using a solar analog (HD 1368) to correct for spectral type effects.
• Sky background was measured and subtracted from each exposure using median-combined regions 10–30 pixels above and below the spectral trace.
• Spectral reflectance was derived by dividing the object spectrum by the local standard, then correcting for the solar analog trend via linear fitting, with analysis restricted to 520–950 nm to avoid systematics and telluric contamination.

Experimental results

Research questions

• RQ1 What is the surface reflectance spectrum of A/2017 U1 in the optical range, and does it show any significant absorption features?
• RQ2 How red is the surface of A/2017 U1, and does its spectral slope fall within known taxonomic classes of solar system bodies?
• RQ3 Can reliable spectral information be extracted from a faint, fast-moving object with low signal-to-noise data under poor atmospheric conditions?
• RQ4 How do the derived colors of A/2017 U1 compare to those of known trans-Neptunian objects and main-belt asteroids?

Key findings

• The reflectance spectrum of A/2017 U1 shows no significant absorption features in the 520–950 nm range, despite low signal-to-noise conditions.
• The spectral slope is measured at $3.0 \pm 1.5$ normalized reflectance units per micron, indicating a very red surface, consistent with the RR class seen in trans-Neptunian objects.
• The object's colors—$g-r=0.2\pm0.4$, $r-i=0.3\pm0.3$, and $r-z=0.4\pm0.4$—are redder than the bulk of main-belt asteroids and match those of Kuiper Belt objects from the Col-OSSOS survey.
• The spectral slope is only $2\sigma$ from zero, but the redness is significantly greater than typical S- or C-type asteroids, suggesting a primitive, icy surface composition.
• The data are consistent with A/2017 U1 being a dark, red, and spectrally featureless body, typical of trans-Neptunian objects, supporting its likely interstellar origin.
• Despite the low signal-to-noise, the detection of flux is robust, as evidenced by a positive signal-to-noise trend in the histogram of flux over error across the spectrum.