ASASSN-24fw: An 8-month long, 4.1 mag, optically achromatic and polarized dimming event
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| Format: | Preprint |
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2025
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| author | Forés-Toribio, R. JoHantgen, B. Kochanek, C. S. Jorstad, S. G. Hermes, J. J. Armstrong, J. D. Ashall, C. Burns, C. R. Gaidos, E. Hoogendam, W. B. Hsiao, E. Y. Medler, K. Morrell, N. Pfeffer, C. Shappee, B. J. Stanek, K. Tucker, M. A. Xiao, H. Auchettl, K. Lu, L. Rowan, D. M. Vaccaro, T. Williams, J. P. |
| author_facet | Forés-Toribio, R. JoHantgen, B. Kochanek, C. S. Jorstad, S. G. Hermes, J. J. Armstrong, J. D. Ashall, C. Burns, C. R. Gaidos, E. Hoogendam, W. B. Hsiao, E. Y. Medler, K. Morrell, N. Pfeffer, C. Shappee, B. J. Stanek, K. Tucker, M. A. Xiao, H. Auchettl, K. Lu, L. Rowan, D. M. Vaccaro, T. Williams, J. P. |
| contents | We discuss ASASSN-24fw, a 13th-magnitude star that optically faded by $Δg = 4.12 \pm 0.02$ mag starting in September 2024 after over a decade of quiescence in ASAS-SN. The dimmimg lasted $\sim$8 months before returning to quiescence in late May 2025. The spectral energy distribution (SED) before the event is that of a pre-main sequence or a modestly evolved F star with some warm dust emission. The shape of the optical SED during the dim phase is unchanged and the optical and near-infrared spectra are those of an F star. The SED and the dilution of some of the F star infrared absorption features near minimum suggest the presence of a $\sim$0.25$M_\odot$ M dwarf binary companion. The 43.8 year period proposed by Nair & Denisenko (2024) appears correct and is probably half the precession period of a circumbinary disk. The optical eclipse is nearly achromatic, although slightly deeper in bluer filters, $Δ(g-z)=0.31\pm0.15$ mag, and the $V$ band emission is polarized by up to 4%. The materials most able to produce such small optical color changes and a high polarization are big ($\sim$20 $μ$m) carbonaceous or water ice grains. Particle distributions dominated by big grains are seen in protoplanetary disks, Saturn-like ring systems and evolved debris disks. We also carry out a survey of occultation events, finding 46 additional systems, of which only 7 (4) closely match $\varepsilon$ Aurigae (KH 15D), the two archetypes of stars with long and deep eclipses. The full sample is widely distributed in an optical color-magnitude diagram, but roughly half show a mid-IR excess. It is likely many of the others have cooler dust since it seems essential to produce the events. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2507_03080 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | ASASSN-24fw: An 8-month long, 4.1 mag, optically achromatic and polarized dimming event Forés-Toribio, R. JoHantgen, B. Kochanek, C. S. Jorstad, S. G. Hermes, J. J. Armstrong, J. D. Ashall, C. Burns, C. R. Gaidos, E. Hoogendam, W. B. Hsiao, E. Y. Medler, K. Morrell, N. Pfeffer, C. Shappee, B. J. Stanek, K. Tucker, M. A. Xiao, H. Auchettl, K. Lu, L. Rowan, D. M. Vaccaro, T. Williams, J. P. Solar and Stellar Astrophysics We discuss ASASSN-24fw, a 13th-magnitude star that optically faded by $Δg = 4.12 \pm 0.02$ mag starting in September 2024 after over a decade of quiescence in ASAS-SN. The dimmimg lasted $\sim$8 months before returning to quiescence in late May 2025. The spectral energy distribution (SED) before the event is that of a pre-main sequence or a modestly evolved F star with some warm dust emission. The shape of the optical SED during the dim phase is unchanged and the optical and near-infrared spectra are those of an F star. The SED and the dilution of some of the F star infrared absorption features near minimum suggest the presence of a $\sim$0.25$M_\odot$ M dwarf binary companion. The 43.8 year period proposed by Nair & Denisenko (2024) appears correct and is probably half the precession period of a circumbinary disk. The optical eclipse is nearly achromatic, although slightly deeper in bluer filters, $Δ(g-z)=0.31\pm0.15$ mag, and the $V$ band emission is polarized by up to 4%. The materials most able to produce such small optical color changes and a high polarization are big ($\sim$20 $μ$m) carbonaceous or water ice grains. Particle distributions dominated by big grains are seen in protoplanetary disks, Saturn-like ring systems and evolved debris disks. We also carry out a survey of occultation events, finding 46 additional systems, of which only 7 (4) closely match $\varepsilon$ Aurigae (KH 15D), the two archetypes of stars with long and deep eclipses. The full sample is widely distributed in an optical color-magnitude diagram, but roughly half show a mid-IR excess. It is likely many of the others have cooler dust since it seems essential to produce the events. |
| title | ASASSN-24fw: An 8-month long, 4.1 mag, optically achromatic and polarized dimming event |
| topic | Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2507.03080 |