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 52: an orange dwarf that dimmed smoothly for a year and a half

about 25%

Why this estimate. ZTF recorded the dimming in green and red light and flagged it 75 times, and its difference images show a clean dark spot, but only one ZTF detector covers the star, and ASAS-SN, a separate network of small telescopes, sees it too close to its faint limit to give more than a hint. Green light dimmed about twice as much as red, as behind fine dust.

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 an orange dwarf of about 5,000 K, about 530 parsecs away in Taurus, seen through a thick layer of interstellar dust that makes it look red. ZTF measured it about 840 times from 2018 to 2025. It was steady except from late 2020 to early 2022, when it faded gradually, reaching 0.21 magnitudes fainter in green light and 0.10 in red in mid-2021, 18 and 9 per cent of its light, and it was back to normal by mid-2022. Stars of similar colour on the same detector changed by less than 0.02 magnitudes. TESS watched it for 24 days in September and October 2021, near the deepest point, and saw no dips or eclipses: the dimming was smooth.

Why it might matter. Blue light dimmed about 2.2 times as much as red light. A solid body would dim all colours equally, ordinary interstellar dust gives 1.4 and spots or a cooler surface about 1.1 to 1.3, so the steep ratio fits a cloud of very fine dust crossing the line of sight. NEOWISE saw at most 0.02 to 0.03 magnitudes of fading at 3.4 microns, which also rules out a solid body. Averaged over July to December 2021, ASAS-SN found the star 0.11 magnitudes faint, in agreement with ZTF, but with an uncertainty of 0.05.

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. A machine label calls it a possible young star, but no person has classified it.

Star
Gaia DR3 3415149842485194240, a dust-reddened orange (K-type) dwarf
Location
RA 76.78700°, Dec +21.69702° (J2000), in Taurus, 11 degrees below the Milky Way’s mid-plane
Distance
about 530 parsecs (1,700 light-years)
Brightness
Gaia G = 16.2, 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
Only one ZTF detector covers the star, and ASAS-SN’s support is only a hint. The star is not perfectly quiet at other times, so it may be an active, spotted star. A young star dimmed by its own dusty disc would behave similarly, though this one shows no infrared excess. 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 between the yellow ticks; no other star lies within 20 arcseconds. 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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