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Discovery of an M-type Companion to the Herbig Ae Star V1787 Ori

The intermediate mass Herbig Ae star V1787 Ori is a member of the L1641 star-forming region in the Orion A molecular cloud. We report the detection of an M-type companion to V1787 Ori at a projected separation of 6.66" (corresponding to 2577 au), from the analysis of VLT/NACO adaptive optics $K_s$-band image. Using astrometric data from Gaia DR2, we show that V1787 Ori A and B share similar distance ($d$ $\sim$ 387 pc) and proper motion, indicating that they are physically associated. We estimate the spectral type of V1787 Ori B to be M5 $\pm$ 2 from color--spectral type calibration tables and template matching using SpeX spectral library. By fitting PARSEC models in the Pan-STARRS color-magnitude diagram, we find that V1787 Ori B has an age of 8.1$^{+1.7}_{-1.5}$ Myr and a mass of 0.39$^{+0.02}_{-0.05}$ $M_\odot$. We show that V1787 Ori is a pre-main sequence wide binary system with a mass ratio of 0.23. Such a low mass ratio system is rarely identified in Herbig Ae/Be binary systems. We conclude this work with a discussion on possible mechanisms for the formation of V1787 Ori wide binary system.