# Cloud Atlas: Hubble Space Telescope Near-Infrared Spectral Library of Brown Dwarfs, Planetary-mass companions, and hot Jupiters

## TL;DR

This paper presents a high-quality near-infrared spectral library of 76 brown dwarfs and hot Jupiters, enabling improved atmospheric analysis and comparative studies of ultracool objects.

## Contribution

It provides a comprehensive, high signal-to-noise spectral library from Hubble Space Telescope data, including analysis of peculiar objects and atmospheric features.

## Key findings

- Identification of low- and intermediate-surface-gravity objects.
- Detection of candidates with composite atmospheres.
- Spectral similarity between cold hot Jupiters and mid-L dwarfs.

## Abstract

Bayesian atmospheric retrieval tools can place constraints on the properties of brown dwarfs and hot Jupiters atmospheres. To fully exploit these methods, high signal-to-noise spectral libraries with well-understood uncertainties are essential. We present a high signal-to-noise spectral library (1.10-1.69 microns) of the thermal emission of 76 brown dwarfs and hot Jupiters. All our spectra have been acquired with the Hubble Space Telescope's Wide Field Camera 3 instrument and its G141 grism. The near-infrared spectral types of these objects range from L4 to Y1. Eight of our targets have estimated masses below the deuterium-burning limit. We analyze the database to identify peculiar objects and/or multiple systems, concluding that this sample includes two very-low-surface-gravity objects and five intermediate-surface-gravity objects. In addition, spectral indices designed to search for composite atmosphere brown dwarfs, indicate that eight objects in our sample are strong candidates to have such atmospheres. None of these objects are overluminous, thus their composite atmospheres are unlikely a companion-induced artifact. Five of the eight confirmed candidates have been reported as photometrically variable, suggesting that composite atmospheric indices are useful in identifying brown dwarfs with strongly heterogeneous cloud covers. We compare hot Jupiters and brown dwarfs in a near-infrared color-magnitude diagram. We confirm that the coldest hot Jupiters in our sample have spectra similar to mid-L dwarfs, and the hottest hot Jupiters have spectra similar to those of M-dwarfs. Our sample provides a uniform dataset of a broad range of ultracool atmospheres, allowing large-scale, comparative studies, and providing a HST legacy spectral library.