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 50: a subgiant that dimmed for half a year

about 30%

Why this estimate. The dimming is certain: ZTF recorded it on two of its detectors and flagged it 81 times, and ASAS-SN, a separate network of small telescopes, saw it in each of its three cameras that observed the star. But green light dimmed only somewhat more than red, between starspots and ordinary dust, so the colour alone cannot tell them apart, and the star was slightly brighter in 2018 for reasons that are unclear.

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 a subgiant of about 5,400 K, some 6 billion years old, about 1,750 parsecs away in Cepheus. ZTF saw it steady from 2020 to 2024, though 0.02 to 0.04 magnitudes brighter in 2018. From early July 2019 it dimmed, by 0.07 to 0.10 magnitudes in green light and 0.05 to 0.08 in red from mid-August to early September, about 9 per cent less light, and it was normal again by mid-December. ZTF’s alert system flagged it 81 times, every few nights from June to December 2019. A second, shorter dip followed from 28 April to 8 May 2024, seen by one ZTF field only.

Why it might matter. Blue light dimmed about 1.3 to 1.65 times as much as red light, between starspots (1.1 to 1.3) and ordinary interstellar dust (1.4). WISE observed on 27 August 2019, at the deepest point, and saw only about 0.01 magnitudes of change at 3.4 microns while red light was 0.07 magnitudes faint: spots would have given about 0.03 and a solid body 0.07, so this favours dust, though only mildly. TESS shows no repeating period in any sector.

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 2285208410072626688, a subgiant
Location
RA 331.90027°, Dec +79.68360° (J2000), in Cepheus
Distance
about 1,750 parsecs (5,700 light-years)
Brightness
Gaia G = 15.2, 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
The colour alone cannot tell spots from dust, and the infrared favours dust only mildly. The star was slightly brighter in 2018 for reasons that are unclear, and the 2024 dip was seen on one ZTF field only. 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 white one between the yellow ticks; the faint red star 11 arcseconds to the east-north-east is a neighbour (G = 19.6), and the blue smudges to the west are artefacts of the image. 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 nine sectors. The first two panels, from November and December 2019, were taken at the end of the dimming. No panel shows a repeating period. 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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