First SPLAT Publication: An In-depth Analysis of GJ 660.1AB

Screen Shot 2016-01-20 at 10.14.17 PMMorehouse College undergraduate Christian Aganze has led the first result to be published from the SpeX Prism Library Analysis Toolkit (SPLAT) project: an in-depth analysis of the M-dwarf binary system GJ 660.1AB.

Originally discovered in 2011, GJ 660.1B is separated by only 6″ from its M1 primary, and is nearly 100x fainter in the near-infrared (it is just barely seen in the logarithmically-stretched 2MASS image shown above), making it challenging to obtain a resolved spectrum of it. Nevertheless, we were able to split the two sources with the SpeX spectrograph on the NASA Infrared Telescope Facility and obtain low-resolution near-infrared spectra of both components to analyze the system.

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Comparison of the spectrum of GJ 660.1B (black line) to M7-L0 spectral standards.

The spectrum of GJ 660.1B turned out to be surprisingly peculiar, appearing to be very different from standard M and L dwarf spectra. It has a much bluer shape (more light at shorter wavelengths) and a distinct triangular-shaped 1.6 µm peak (see figure to left). The latter peculiarity initially suggested that GJ 660.1B was a 10-100 million-year “young” brown dwarf, as verified by standard surface gravity indices. However, this wasn’t in agreement with the high proper motion of the system, which indicates an old (> 2 billion years) source.

Time to apply some SPLAT tools!

Using routines he and other Cool Star Lab researchers have been developing over the past 2 years, Christian first compared the source to all other spectra in the SpeX Prism Library, and found that the best matches tended to be slightly metal-poor, high proper motion M dwarfs (metals in astronomy are all elements other than hydrogen and helium). He then specifically compared GJ 660.1B to spectra of known young sources and known metal-poor sources, and the latter proved to be better matches. This was the first hint that unusual chemical abundances may explain the peculiar spectrum of GJ 660.1B, and motivated our classification of the source as d/sdM7, the “d/sd” signifying a “mild subdwarf”, or modestly metal-poor star.

Moderate-resolution spectrum of GJ 660.1A, with several of the atomic lines used to measure the metallicity of this source labeled

Moderate-resolution spectrum of GJ 660.1A, with several of the atomic lines used to measure the metallicity of this source labeled

Fortunately, researchers have been developing techniques to measure the metallicities of early M dwarfs as part of the search and characterization of exoplanet host stars.  We obtained a second, higher-resolution, near-infrared spectrum of GJ 660.1A with SpeX and applied published metallicity relations based on potassium, sodium and calcium line strengths and water absorption. From these, we determined that GJ 660.1A was indeed metal-poor, measured astronomically as [M/H] = -0.47±0.07; i.e., the abundances of heavy elements are about 1/3 of those in the Sun.  Assuming that GJ 660.1A and B formed from the same initial material, this result implies that GJ 660.1B must be metal-poor as well.

Comparison of the spectrum of GJ 660.1B to the best-fitting BTSettl2008 model, which is both low surface gravity (log g) and very low metallicity (z = [M/H]).

Comparison of the spectrum of GJ 660.1B to the best-fitting BTSettl2008 model, which is both low surface gravity (log g) and very low metallicity (z = [M/H]).

As a check, Christian used SPLAT tools to compare the near-infrared spectrum of GJ 660.1B to two sets of theoretical atmosphere models. He used a Monte Carlo Markov Chain code written collaboratively by undergraduate researchers in the Cool Star Lab to converge on a best fit model and distribution in the relevant parameters (temperature, surface gravity, metallicity and radius). This analysis proved to be challenging, as the models converged on low surface gravities (and hence young ages) and very low metal abundances, as low as 1/10th those of the Sun. It was clear that something was amiss in the model fits.

At the atmospheric temperatures of late M dwarfs (about 2500˚K), several complex chemical transitions are taking place, the most important of which is the formation of clouds of mineral condensates from VO, TiO and atomic metal gases. Indeed, it is this transition that motivates the definition of a distinct spectral class of stars, the L dwarfs. But gas and condensate chemistry is sensitive to temperature, pressure and the chemical abundances present, and varying the latter can skew the predictions of atmosphere models. In addition, molecular hydrogen may play a confounding role, as absorption from this gas species is broad and pressure-dependent, and is still under investigation.  The mismatch between the model and observed spectra for GJ 660.1B therefore point to needed improvements in the models, particularly with respect to condensate formation and molecular hydrogen absorption.

Despite the inconsistent results between empirical and theoretical analyses of the spectral data, the metal-poor GJ 660.1AB system should prove to be an important benchmark in cool dwarf atmosphere studies, precisely because models don’t quite match the data yet. This system can serve as a test case as improvements are made to the chemistry and physics used in the atmosphere models.

This result will be published as Aganze et al. (2016) in the Astronomical Journal; a preprint can be found on the arXiv.


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