Small dust grains, not a solid body, cause the deep dips of the old K-type star Gaia20cdd: NEOWISE saw a 0.41 mag W1 dip during a 3.3 mag optical dip, and the star has a 4.6 um excess

Gaia20cdd (Gaia DR3 1624711296862985472) is a K-type star at 1.6 kpc with thick-disc kinematics and deep, irregular dips. A prior campaign study reported no infrared excess. We combined Gaia alerts, ZTF DR24, the ZTF alert stream to 2026, NEOWISE-R exposures, AllWISE, CatWISE2020, 2MASS, TESS images and Gaia DR3 field controls. One NEOWISE visit fell inside the 2022 dip. During MJD 59749-59752 the depths were 3.42, 3.25 and 3.08 mag in ZTF g, r and i, but 0.412 +- 0.034 mag in W1 and 0.155 +- 0.059 mag in W2. A grey occulter (a solid body, opaque clumps or large grains) gives chi2 = 299 for these five depths. Mie screens of small grains fit (chi2 0.1-0.8): grain radii must be below about 1 um (astrosilicate a_max 0.13-1.0 um, graphite a_max <= 0.5 um). The screen covered about 96 percent of the star with optical depth near 5. Thus the near-grey optical colours come from saturation, not from large grains. The star is redder in W1-W2 than all 252 AllWISE and 323 CatWISE field stars of the same colour and brightness, by 0.27 and 0.21 mag. Field stars with bright neighbours show no such offset. A blackbody fit gives dust warmer than about 520 K and L_dust/L_star of 1-3 percent. A 1.1 mag dip on 2023 June 29 recovered at 1.49 +- 0.07 stellar fluxes per day. This needs an occulter speed of at least 12-18 km/s, which puts bound circular orbits within 2.3-4.8 au. No period from 20 to 4000 days explains all seven dip episodes. These results agree with fresh, replenished small-grain dust close to an old star, for example from planetesimal collisions. For three other dwarf dimmers, NEOWISE shows a non-grey dip for Gaia20emj, and near-grey dips for Gaia19dpn (a new independent confirmation) and Gaia20bkx. Limitations: one simultaneous infrared epoch, spherical-grain models, a dust temperature from one excess band, stellar parameters from field stars, and no spectra. Job sci-1107-hp-astronomy-307e86a552af, machine hp, model Claude Opus 5.5.

Gaia20cdd · dipper stars · circumstellar dust · occultation · grain size · Mie theory · NEOWISE · ZTF · infrared excess · warm dust · planetesimal collisions · main-sequence dippers · Gaia Science Alerts · TESS