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 54: a distant F star that dimmed for four months

about 25%

Why this estimate. ZTF recorded the dimming in three filters and flagged it six times, and ASAS-SN, a separate network of small telescopes, saw the star faint in October 2019, but that support is weak because its measurements of this star scatter. Green and red light dimmed almost equally, which large dust grains, a solid body or spots could all explain, and WISE did not observe during the dip.

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.

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. Down means fainter. The note under the graphs sums up the measurements.

What changed. The star is a yellow-white star of type F, about 6,400 K, some 2,000 to 2,400 parsecs away in Aries. ZTF saw it steady from 2018 to mid-2019. From mid-September 2019 it dimmed, by 0.08 to 0.12 magnitudes in green light and 0.03 to 0.08 in red until mid-October, about 0.05 magnitudes in November and 0.01 to 0.04 in December and January, and it was steady again from February 2020. On 13 October 2019, near the deepest point, it was 11 per cent fainter in green light. ASAS-SN saw it 0.11 magnitudes faint in October 2019 in all three cameras observing it.

Why it might matter. Blue light dimmed only about 1.15 times as much as red light, close to what a solid body or large dust grains would cause (1.0) and below ordinary interstellar dust (1.4); spots on a star this hot usually give about 1.2. So the colour cannot tell the causes apart. A dark object or a clump of coarse dust passing in front of the star would be the most interesting explanation.

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 61027606923915008, an F-type star
Location
RA 50.78965°, Dec +20.61755° (J2000), in Aries
Distance
2,000 to 2,400 parsecs (6,500 to 7,800 light-years)
Brightness
Gaia G = 15.6, 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
Only one ZTF field covers the star, and ASAS-SN’s support is weak: one other month of its record is also faint when ZTF saw the star normal. The colour does not separate large dust grains from spots, and there is no infrared measurement during the dip. 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 very faint smudge about 7 arcseconds to its south-east is below Gaia’s limit. 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.

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