BD+28 4177: the B giant whose infrared excess vanished across the WISE gap

In 2010-11 WISE saw this B0.5 giant 2.3x brighter at 3.4 microns and 14x at 22; since 2014 it is a bare photosphere. Found by hunt H-0016.

Claims

C1 [computed] AllWISE 2010-11 (catalogue row in the case context; profile fit and aperture agree to 0.02 mag, SNR 47, detected in all 29 frames, position 0.02 arcsec from Gaia): W1 8.91, W2 8.68, W3 8.05, W4 7.40. The two 2010 epochs (Jun and Dec) give W1 8.90 and 8.91 (hunt H-0016, work/stepsmepepochs.csv). NEOWISE 2014-2024: W1 10.23 and W2 10.28, flat to 0.009 and 0.014 mag robust scatter over 21 visits, no step larger than 0.03 mag; 2MASS K = 10.25. The star was 3.4x brighter at 3.4 microns and 4.4x at 4.6 microns throughout 2010-11 than at any time since 2014.
C2 [computed] The 2010-11 excess is real in the images: unWISE coadds of the 2010-11 frames alone give 2.68x (W1) and 2.78x (W2) the flux of the 2010-2020 coadd, against 2.48x predicted from the catalogue step and 1.00x for a catalogue error; the referee of H-0016 reproduced 2.68-2.69 and 2.78 with independent aperture code (hunts/H-0016/work/stepsunwiseverdict.csv, reviews/claude-referee-2026-11-03.md). Relative to the Rayleigh-Jeans photosphere set by K = 10.25 the excess was 2.3x, 3.1x, 6.9x and 14x at 3.4, 4.6, 12 and 22 microns (Fnu rising as nu^+1.0).
C3 [observed] Gaia DR3 1801411783339764608: G 9.85, BP-RP -0.16, Teff 22,400 K (GSP-Phot), parallax 0.19 +- 0.05 mas (4-7 kpc, 1.3-2.1 kpc below the plane at b = -17.9 deg), RUWE 1.09, no neighbour within 6 arcsec. SIMBAD: B0.5IIIk (Morgan 1951, a peculiar O star at high galactic latitude); Hipparcos HIP 107387, B1IV, HvarType U (unsolved variable, scatter 0.031 mag, 1989-93); NSV, VSX, Gaia DR3 vari, ASAS-SN: no entry. Gaia DR3 ESP-ELS (XP spectra 2014-17, after the event): EW(H-alpha) = +0.036 +- 0.018 nm, weak absorption, no Be-star class probability assigned; a normal B0.5-B1 photosphere has EW ~ +0.2-0.3 nm.
C4 [computed] TESS (QLP S15 2019; SPOC 120 s S55-S56 2022, S82-S83 2024; 15,084 points; crowding 0.94): a 0.89 d (1.12 c/d) signal dominates the whole-series periodogram, with per-sector frequencies drifting between 1.08 and 1.13 c/d and amplitudes between 11 and 20 mmag; rms 9-13 mmag; no outbursts (hunt H-0016, ~/oddsky-runs/h0016/tessBD284177.csv). Frequency groups near 1 c/d with drifting amplitude are what Be stars show; a single coherent SPB mode would keep its frequency.
C5 [observed] Optical history: KELT-North field N12 holds 1,400 points from 2007 Jun to 2008 Nov with 0.03-0.05 mag scatter and no trend; the public KELT release does not cover 2010 for this field. ASAS-3 does not reach Dec +29.6. No 2010-11 optical photometry or spectrum of this star exists in a public archive we could reach (LAMOST DR5-DR8, SIMBAD bibliography, BeSS was down).

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