ACME Stellar Spectra. I. Absolutely Calibrated, Mostly Empirical Flux Densities of 55 Cancri and its Transiting Planet 55 Cancri e

TL;DR

The paper introduces the ACME Spectra project, providing calibrated, mostly empirical spectra of 55 Cancri for exoplanet analysis, and reports a revised brightness temperature for its transiting planet 55 Cnc e.

Contribution

It presents a new method for obtaining calibrated stellar spectra and applies it to determine the brightness temperature of 55 Cnc e, improving previous estimates.

Findings

Calibrated spectrum of 55 Cancri from 0.81-5.05 microns.

Brightness temperature of 55 Cnc e is 1950 K at 4.5 microns.

Planetary flux at 4.5 microns is 0.44 mJy.

Abstract

The ACME Spectra project provides absolutely calibrated, mostly empirical spectra of exoplanet host stars for use in analysis of the stars and their planets. Spectra are obtained from ground-based telescopes and are tied directly to calibrated ground- and space-based photometry. The spectra remain only "mostly" empirical because of telluric absorption, but interpolation of stellar models over the gaps in wavelength coverage provides continuous stellar spectra. Among other uses, the spectra are suitable for precisely converting observed secondary eclipses (occultations) into absolute flux units with minimal recourse to models. In this letter I introduce ACME's methods and present a calibrated spectrum of the nearby, super-Earth hosting star 55 Cancri that spans the range from 0.81-5.05 micron. This spectrum is well-suited for interpreting near- and thermal-infrared eclipse observations. With this spectrum I show that the brightness temperature of the small, low-mass transiting planet 55 Cnc e is 1950 +260/-190 K at 4.5 micron (cooler than previously reported), which corresponds to a planetary flux of 0.44 +0.12/-0.08 mJy. This result suggests the planet has some combination of a nonzero albedo, a moderately efficient redistribution of absorbed stellar irradiation, and/or an optically thick atmosphere, but more precise eclipse measurements are required to distinguish between these scenarii.