Dippers from TESS Full-frame Images. II. Spectroscopic Characterization of Four Young Dippers

Photometric monitoring by the Transiting Exoplanet Survey Satellite (TESS) has discovered not only periodic signals by transiting exoplanets but also episodic or quasiperiodic dimming around young stellar objects. The dimming mechanisms of these objects, the so-called "dippers," are thought to be related to either the accretion property or the structure of protoplanetary disks especially in regions close to the host star. Recently, we have created a catalog of dippers from one year of TESS full-frame image data. In this paper, we report on the spectral features of four newly found dippers in that catalog and show that they potentially shed light on the dimming mechanisms. We found that all of the targets exhibit the H alpha emission line, which is an indicator of ongoing accretion. Based on their line profiles and/or their variability, we characterized the properties of the disks of each source, which can support dimming mechanisms via a dusty disk wind or an accretion-driven inner-disk warp. Also, we found an interesting dipper (TIC 317873721), a "close-in binary dipper," showing a complex variability of the line profile and a large radial velocity variation. Because the dimming intervals are similar to the orbital period of the binary, we suggest that the dips are caused by dust in the accretion warp from a circumbinary disk onto stars. Such a close-in (<0.1 au) binary dipper has been rarely reported thus far; further investigation will reveal new aspects of disk evolution and planetary formation.