An infrared excess that appeared in 2018

A K dwarf 250 parsecs away was a bare photosphere for four years. Then, between two NEOWISE visits in 2018, 1000 K dust appeared around it, and it is still there.

Gaia DR3 5792808347020475136, a K3 main-sequence star, was photospheric from 2010 to 2017. Between two NEOWISE visits in 2018 its W1 and W2 rose by 0.17 and 0.36 mag (33-38 sigma); the excess (16% and 38% of the star) has a ~1000 K colour temperature and persisted into 2024. Twenty-seven field stars stay flat, the WISE position does not move, and the star is not young. Its variability is catalogued in VarWISE (Paz et al. 2026) as a long-period variable; Oddsky's contribution is the interpretation. No optical dimming was seen in 2018. Refereed: about 93% astrophysical.

Claims

C1 [computed] Field test (IRSA Gator neowiserp1bspsd, all exposures within 300" of the target; outside the tools): 27 AllWISE-matched field stars with pre-step W2 9.5-12.5 show median post-minus-pre change dW1 = +0.006 mag (robust sigma 0.005, none brighter by 0.1) and dW2 = +0.014 (robust sigma 0.005) across MJD 58260; per-visit field zero-points stay within +-0.02 mag in every visit. The target's -0.168 (W1) / -0.359 (W2) step is 30 sigma outside the field distribution. The 2018 step is not a local photometric, background, scan-pattern or calibration systematic.
C2 [computed] Centroid and PSF-fit quality across the step (single exposures, Gaia DR3 position propagated with pm -22.1/-13.4 mas/yr; outside the tools): the mean WISE position moves by 0.004 +- 0.006" (RA) and 0.012 +- 0.005" (Dec) between 9 pre-step and 12 post-step visits; w1rchi2 3.65 - 3.54, w2rchi2 1.33 - 1.44; nb = 1, phqual AA, w1sat = w2sat = 0 in every visit. For a ~25% combined flux increase this places any contaminating source within ~0.2" of the K dwarf.
C3 [computed] No catalogued IR neighbour can be responsible: unWISE has a W1-only source 7.4" away at 5% of the target's W1 flux (undetected in W2), CatWISE2020 a W1 14.7 source at 9.4", AllWISE sources at 13.2" (W1 13.2) and 16.2" (W1 12.35), and a W1 8.75 star at 32" that sets the 'h' halo flag. Each of those three AllWISE neighbours is itself flat across 2018 (dW2 +0.026, +0.027, +0.007 mag), and any of them brightening enough to add 39% of the target's W2 flux would drag the centroid by 2".
C4 [computed] Pre-event photosphere: AllWISE 2010 single-exposure epochs (MJD 55257, 55438) give W1 11.18/11.20, W2 11.23/11.24, identical to the 2014-2017 NEOWISE baseline (11.188/11.259); AllWISE W1-W3 = 0.17 +- 0.10 and 2MASS Ks-W1 = 0.06 are photospheric for a K3 dwarf. The excess appeared between MJD 58178.5 and 58341.7 (2018 Feb 26 - Aug 8); the first post-step visit was the brightest in W2 (10.82), then a plateau at 10.86-10.94 through 2024.
C5 [inferred] Energetics (blackbody, inferred): excess colour W1-W2 ~ 0.87 mag gives T ~ 1040-1050 K (K15, K16); for a blackbody this is LIR/L ~ 0.02, an emitting area ~8 times the stellar disk, and an equilibrium distance ~0.04 AU for L ~ 0.3 Lsun. Larger, cooler areas are possible for small superheated grains. A fractional luminosity of ~2% sustained for 6 years is extreme-debris-disk territory (ID8, ASASSN-21qj, BD+20 307 range).
C6 [computed] Not a pre-main-sequence or obviously young star: with GSP-Phot AG = 0.24 and E(BP-RP) = 0.13, the star sits at MG0 ~ 6.31, (BP-RP)0 ~ 1.17, on the K3V main sequence (no PMS overluminosity), with logg 4.58. Heliocentric UVW = (-24.1, -18.4, -2.7) km/s (RV -0.47 +- 1.9 km/s, 17 transits) differs from Chamaeleon I/II, eps Cha and LCC (U ~ -8 to -11, W ~ -6 to -10 km/s) by 12 km/s. It is 6-12 deg from the Chamaeleon clouds on the sky, but at 247 pc it is not kinematically associated. An age of ~100 Myr is likely but unproven; lithium is needed.
C7 [computed] TESS is weaker than the case implies: sectors S100 and S101 contain orbit-length ramps of 0.2-0.4 mag and a flat +0.6 mag offset lasting 5 days (MJD 61107-61111), which are pipeline/scattered-light systematics, so dips there are uninterpretable. Per-sector Lomb-Scargle peaks wander from 6.6 to 20 d at low power, so the '~11 d' rotation period is not established. The other sectors (2019-2025) show no dips beyond ~5% at 30-min noise of 1.6% (K18, K19). There is no optical photometry of any kind across the 2018 onset.
C8 [observed] Optical across the onset (outside the tools; ASAS-SN V and SkyMapper DR4 via the sub-agent): ASAS-SN V is 13.41 with robust scatter 0.036 over 2014-2018, and flat from MJD 58160 to 58378 apart from a gap at 58243-58300. A ~0.1-mag bright excursion at 58300-58306 also appears in an ASAS-SN comparison star and is likely systematic. SkyMapper on 2018-04-24 (MJD 58232, inside the 58178-58341 onset window) gives u 16.52, v 16.19, g 14.12, r 13.55, matching 2014 to 0.03 mag. There is no optical dip or flash beyond ~0.1-0.15 mag in ASAS-SN (diluted by ~1.6x through blending with the 16" neighbour and HD 128641), which argues against a bright engulfment- or merger-type optical outburst unless it fell in the May-June 2018 gap.
C9 [observed] Prior catalogue status (via the sub-agent): VarWISE (Paz et al. 2026) already lists this source as a WISE variable with W1/W2 amplitudes 0.18/0.375, classified 'lpv' (long-period variable). The IR variability is therefore catalogued, but the classification is wrong (a K3V main-sequence dwarf is not an LPV) and the step-plus-reddening, IR-excess interpretation is new. SIMBAD and the literature contain nothing beyond Gaia/VSX [ROT]. Not a member of Chamaeleon I/II, eps Cha or LCC in the membership lists checked. The AllWISE 'hh00' ccflags come from HD 128641 (V 9.66, F0V) at 32", which is flat. The TIC contamination ratio is 3.78: the target supplies only ~21% of the TESS aperture flux, so TESS dip limits on the star are ~5x weaker than they appear.

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