Ammonia-methane ratios from H-band near-infrared spectra of late-T and Y dwarfs

Martín, E. L.; Zhang, J. -Y.; Esparza, P.; Gracia, F.; Rasilla, J. L.; Masseron, T.; Burgasser, A. J.
Bibliographical reference

Astronomy and Astrophysics

Advertised on:
Number of authors
IAC number of authors
Refereed citations

Aims: Our goals are to investigate the relative absorption strengths of ammonia and methane using low-resolution H-band (1.5−1.7 microns) spectra obtained in the laboratory and compared with observational spectra of late-T and Y dwarfs, and to estimate what can be expected from the wide-angle low-resolution near-infrared spectroscopic survey that will be provided by the upcoming Euclid space mission.
Methods: Gas cells containing ammonia and methane at atmospheric pressure were custom-made in our chemical laboratory. Low-resolution near-infrared spectroscopy of these gas cells was collected in our optical laboratory. It is compared with simulated spectra using the high-resolution transmission molecular absorption database (HITRAN) for temperatures of 300 K and 500 K, and with near-infrared spectra of late-T dwarfs, Y dwarfs, Jupiter, and Saturn. We selected for this investigation the spectral region between 1.5 and 1.7 microns (H band) because it is covered by the Euclid red grism, it is particularly sensitive to the relative proportions of ammonia and methane opacity, and it is free from strong contributions of other abundant molecules, such as water vapor.
Results: The laboratory spectra showed that the ammonia and methane features present in the simulations that used the HITRAN database are incomplete. Using our laboratory spectra, we propose a modified version of the NH3-H spectral ratio with expanded integration limits that increases the amplitude of variation in the index with respect to spectral type. Combinations of our laboratory spectra were used to find the best fits to the observed spectra with the relative absorption ratio of ammonia to methane as a free parameter. A relationship was found between the Teff and the ratio of ammonia to methane from spectral classes T5 to Y2 (1100 K-350 K), in fairly good qualitative agreement with theoretical predictions for high-gravity objects and temperatures from 1100 K to 500 K. The ammonia-to-methane ratios in late-T and Y dwarfs are similar to that of Jupiter, suggesting a similar chemical composition. Simulations of the spectroscopic performance of Euclid suggest that it will yield Teff values and ratios of ammonia to methane for over 103 late-T dwarfs in the entire wide survey.
Related projects
Project Image
Nucleosynthesis and molecular processes in the late stages of Stellar Evolution
Low- to intermediate-mass (M < 8 solar masses, Ms) stars represent the majority of stars in the Cosmos. They finish their lives on the Asymptotic Giant Branch (AGB) - just before they form planetary nebulae (PNe) - where they experience complex nucleosynthetic and molecular processes. AGB stars are important contributors to the enrichment of the
Domingo Aníbal
García Hernández
Projects' name image
Exoplanets and Astrobiology
The search for life in the universe has been driven by recent discoveries of planets around other stars (known as exoplanets), becoming one of the most active fields in modern astrophysics. The growing number of new exoplanets discovered in recent years and the recent advance on the study of their atmospheres are not only providing new valuable
Pallé Bago