Professor of Mathematics

Paata Ivanisvili

pivanisv@uci.edu · x.com/PI010101

Department of Mathematics, University of California, Irvine

Paata Ivanisvili

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Candidate 30: an old star that dimmed for three winters

about 40%

Why this estimate. The dimming is certain: ZTF recorded it on two of its detectors and ASAS-SN, a separate network of small telescopes, saw it too. NEOWISE observed the star at the deepest point and saw no change in the infrared, which rules out starspots and a solid body and fits dust. The estimate is held down by the possible companion that Gaia’s measurements hint at, and by the lack of a spectrum.

How to read the pictures. The finder chart shows a patch of sky two arcminutes across, about a fifteenth of the Moon’s width, with north up and east to the left. The star sits between the two yellow ticks, and the white bar is 30 arcseconds long. On a light curve the vertical axis is magnitude, a brightness scale on which larger numbers mean fainter stars, and it is drawn so that down means fainter. Purple shading marks the dimming. Each survey uses slightly different filters, so each is compared with its own normal level. The ZTF pictures show ZTF’s reference image of the star, an image taken during the dimming, and the difference between the two, in which only what changed remains: blue means fainter and red brighter. On the TESS light curves the star is black and the comparison stars are coloured.

Five graphs. In the upper four, of brightness against time, the points stay level apart from a dip, shaded purple. The fifth compares the change in green and red light.
Top three rows: brightness from 2010 to 2025 in green, red and infrared light, each against the star’s normal level, from Pan-STARRS1 (a survey telescope in Hawaii), ZTF, and WISE and NEOWISE (NASA’s infrared space telescope); gold diamonds and orange crosses are ASAS-SN, and a note marks the years of Gaia, the European Space Agency’s star-mapping satellite, which publishes only averages. Purple marks the dimming. Middle: a closer view, with the star against its comparison stars, ZTF’s alerts and ASAS-SN’s monthly medians. Bottom: the change in green against that in red, with lines for dimming equal in all colours, ordinary dust and fine dust; the solid line is this star’s. Bottom right: the star and its comparison stars on two separate ZTF fields, recorded by different detectors. Down means fainter. The note under the graphs sums up the measurements.

What changed. The star is an old subgiant, about 5,500 K, poorer in heavy elements than the Sun and about 11.5 billion years old, a member of the Milky Way’s thick disk some 610 parsecs away in Andromeda. ZTF saw it steady from mid-2018 to late 2024 apart from one smooth dip: 0.04 magnitudes fainter in green light in November 2018, 0.15 in green and 0.08 in red in December, and back within 0.015 magnitudes by February 2019. At the deepest, on 20 December, it had lost 17 per cent of its green light. Smaller dimmings followed in the winters of 2020–21 and 2021–22. ZTF’s alert system flagged it 16 times.

Why it might matter. WISE observed the star from 15 to 19 December 2018, near the deepest point, and saw no change at 3.4 microns, while red light was 0.06 to 0.08 magnitudes faint. Starspots would have dimmed the infrared by about 0.035 magnitudes and a solid body by about 0.07, so both are ruled out, while dust would dim it by only about 0.005. That makes dust the likeliest cause. Gaia’s measurements of the star scatter more than those of a single star (its RUWE is 2.26), which hints at a companion.

No record of the dimming could be found. The star is not in SIMBAD, VSX (the variable-star index), Gaia’s variable-star lists or the other catalogues consulted, nor among the dimming stars of Tzanidakis et al. (2025), and there is no Transient Name Server entry, Gaia alert or paper giving its Gaia number. ZTF’s alert system holds only machine-generated classifications for it.

Star
Gaia DR3 2881413844294611968, an old subgiant of the Milky Way’s thick disk
Location
RA 358.82898°, Dec +38.76450° (J2000), in Andromeda
Distance
about 610 parsecs (2,000 light-years)
Brightness
Gaia G = 13.3, far too faint for the eye
Probability
This candidate is unconfirmed. The percentage under the title is an estimate of the chance that it is a real, previously unrecorded case of the kind described. The estimate has not been checked by outside experts. See the introduction to all fifty-eight candidates.
Caveat
Blue light dimmed about 1.9 times as much as red over the whole dip but only 1.1 to 1.6 times on each ZTF field separately, so the colour on its own is not decisive. Gaia hints at an unseen companion, which could complicate the picture. There is no spectrum.
A patch of sky with a star at the centre between two yellow ticks.
Pan-STARRS1 colour image in the g, r and i filters, from 2010 to 2014, two arcminutes by two. The star is the bright one between the yellow ticks; the two faint dots just south of it are much fainter neighbours (G = 20.4 to 20.6). North is up, east is left, and the white bar is 30 arcseconds.
Three square images. The star, between orange ticks, looks much the same in the left and middle ones and leaves a dark blue spot in the right.
Cutouts from one ZTF alert, one arcminute across; orange ticks mark the star. Left: the reference image, made in 2018 before the dimming. Middle: an image taken during the dimming. Right: the difference between them, in which blue means fainter; the star leaves a dark blue spot, while its neighbours subtract away.
Wide panels of dotted light curves, one per TESS sector. In each the star is the black row at the bottom, below the coloured rows of comparison stars.
TESS light curves from three sectors, in 2019, 2022 and 2024, all outside the dimming. No panel shows the steady wave of a spotted star. In each panel the star is in black at the bottom, below comparison stars on the same images, each divided by the others, averaged over two hours and offset for clarity.

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