The nearby Sun-like star GJ 758 hosts a cold substellar companion , GJ 758 B , at a projected separation of \lesssim 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 \sim 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 ( \sim 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 { \chi } ^ { 2 } minimization suggesting a best fit for spectral type T7.0 \pm 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 _ { \text { eff } } = 741 \pm 25 K and surface gravity \log g = 4.3 \pm 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 .