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Published March 20, 2017 | Published + Submitted
Journal Article Open

Project 1640 Observations of Brown Dwarf GJ 758 B: Near-infrared Spectrum and Atmospheric Modeling


The nearby Sun-like star GJ 758 hosts a cold substellar companion, GJ 758 B, at a projected separation of ≾30 au, previously detected in high-contrast multi-band photometric observations. In order to better constrain the companion's physical characteristics, we acquired the first low-resolution (R ~ 50) near-infrared spectrum of it using the high-contrast hyperspectral imaging instrument Project 1640 on Palomar Observatory's 5 m Hale telescope. We obtained simultaneous images in 32 wavelength channels covering the Y, J, and H bands (~952–1770 nm), and used data processing techniques based on principal component analysis to efficiently subtract chromatic background speckle-noise. GJ 758 B was detected in four epochs during 2013 and 2014. Basic astrometric measurements confirm its apparent northwest trajectory relative to the primary star, with no clear signs of orbital curvature. Spectra of SpeX/IRTF observed T dwarfs were compared to the combined spectrum of GJ 758 B, with χ 2 minimization suggesting a best fit for spectral type T7.0 ± 1.0, but with a shallow minimum over T5–T8. Fitting of synthetic spectra from the BT-Settl13 model atmospheres gives an effective temperature T_(eff) = 741 ± 25 K and surface gravity log g = 4.3 ± 0.5 dex (cgs). Our derived best-fit spectral type and effective temperature from modeling of the low-resolution spectrum suggest a slightly earlier and hotter companion than previous findings from photometric data, but do not rule out current results, and confirm GJ 758 B as one of the coolest sub-stellar companions to a Sun-like star to date.

Additional Information

© 2017 The American Astronomical Society. Received 2015 July 26; revised 2017 February 18; accepted 2017 February 28; published 2017 March 24. We are grateful for the financial, scientific, and technical support that made this research possible: R.N. was funded by the Swedish Research Council's International Postdoctoral Grant No. 637-2013-474. A portion of this work was supported by NASA Origins of the Solar System grant No. NMO7100830/102190, and NASA APRA grant No. 08-APRA08-0117. E.R. acknowledges support from the National Science Foundation under Grant No. 1211568 and NASA Astrophysics Data Analysis Program (ADAP) award 11-ADAP11-0169. J.A.'s work was facilitated in part by a National Physical Science Consortium Fellowship and by stipend support from the Laboratory for Physical Sciences in College Park, Maryland. Part of the research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We also thank the Palomar mountain crew, especially Bruce Baker, Mike Doyle, Carolyn Heffner, John Henning, Greg van Idsinga, Steve Kunsman, Dan McKenna, Jean Mueller, Kajsa Peffer, Paul Nied, Joel Pearman, Kevin Rykoski, Carolyn Heffner, Jamey Eriksen, and Pam Thompson.

Attached Files

Published - Nilsson_2017_ApJ_838_64.pdf

Submitted - 1703.01023.pdf


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