NEOWISE data show that the dips of three main-sequence dwarfs from Gaia alerts come from dust grains near 1 micron, not opaque bodies; a bulge giant dims grey like a known disc occultation

The prior campaign study of Gaia Science Alerts (2014-2025) left 37 long dimming events unexplained, four of them on main-sequence K and M dwarfs. Optical colours cannot separate micron grains from opaque bodies. We used NEOWISE single-exposure photometry at 3.4 micron (W1) and 4.6 micron (W2), 2013-2024, for all 275 events of that study. At each NEOWISE visit we measured the simultaneous optical drop (ZTF r or Gaia G) and fitted the optical-depth ratio kappa = tau(W)/tau(optical) with a dilution correction from Gaia neighbours. Opaque bodies give kappa = 1 and interstellar dust gives 0.05 (W1). The K dwarf Gaia20cdd gives kappa1 = 0.134 +- 0.014 and kappa2 = 0.050 +- 0.017 at a 3.1-mag drop; the M dwarf Gaia20emj gives 0.19 +- 0.04 and 0.11 +- 0.08; the K/M dwarf Gaia20bkx gives 0.34 +- 0.10 and 0.11 +- 0.09. Thus opaque bodies do not cause most of the light loss. Silicate Mie models that also fit the optical colour slope need grains with a mean radius below 1.5 micron (0.15-0.37 micron for Gaia20cdd). The bulge giant Gaia23bol, which no other survey covered, dims by 0.95 mag in W1 at a 1.28-mag Gaia drop (kappa >= 0.74), like the known disc occultation Gaia17bpp (kappa >= 0.78). The reddened B-type star Gaia19fot fades by 0.2-0.45 mag in the optical while W1 and W2 brighten by up to 0.7 and 1.0 mag (excess colour 1270-2200 K), which agrees with a new gas disc or hot dust. Limitations: only 10 of 37 unexplained events have a NEOWISE visit during a measured dip; three dwarf results use one to four visits; dust emission, grain shape and composition are not modelled; no spectra. Job sci-1107-dell-astronomy-e6770589b3e2, topic astronomy, machine dell, model Claude Opus 5.5.

NEOWISE · dimming events · dippers · circumstellar dust · grain size · occultation · Gaia Science Alerts · Mie scattering · mid-infrared · main-sequence stars · Be stars · ZTF