The Pre-Dimming Spectral Type of the Unusual Variable ASASSN-25cd

ATel #17220; Bruce McCollum (American Univ.), Seppo Laine (IPAC/Caltech)
on 4 Jun 2025; 23:05 UT
Credential Certification: Bruce McCollum (bruce.m.mccollum@gmail.com)

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We have used archival photometry to fit the SED of ASASSN-25cd to a set of stellar atmospheric models in order to determine its spectral type before its unusual dimming. JoHantgena et al. (2025, ATel #17211) report that this object is exhibiting a deep, prolonged dimming episode with no prior evidence of variability. For the fitting, we used photometry from the bandpasses: SkyMapper u, v, g, Pan-STARRS1 r and i, ESO Vista Survey I, Z, Y, DENIS survey J and Ks, 2MASS H, and WISE W1, W2, W3, with an upper limit in W4. Values of 2MASS J and Ks were excluded from the fitting because of quality warnings for those measurements. Photometric uncertainties stated in the catalogs are less than one percent.

We fitted against two sets of model atmospheres: 1) BT-Settl-CIFIST2011 (Baraffe et al. 2015, A&A, 577A, 42B), which assumes a fixed metallicity of 0, and 2) BT-Settl-AGS2009 (Allard et al. 2013, MSAIS, 24, 128), in which the metallicity is allowed to vary from -4 to +0.5. For both fits, the object temperature and Av were allowed to vary as independent variables, with the Teff in increments of 100 degrees.

A good fit is obtained with both sets of model atmospheres, with no excess emission indicated in either the blue or the infrared. The BT-Settl-CIFIST2011 best fit is for Teff = 4400 K +/- 50 and Av = 0.1 +/- 0.03. The BT-Settl-AGS2009 best fit is for Teff = 4500 K +/- 60 and Av = 0.1 +/- 0.05. Using the Gaia distance of 2072 pc, based on the archival photometric measurements the fitting tool concludes that the SED corresponds to a bolometric luminosity of 40x solar. This luminosity is consistent with that of a mid-K giant.

A plot of the model fit is available at

https://web.ipac.caltech.edu/staff/seppo/plots/ASASSN-25cd-SED.jpg

This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This publication makes use of VOSA, developed under the Spanish Virtual Observatory project supported from the Spanish MINECO through grant AyA2017-84089. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. We acknowledge use of data from DENIS, which is the result of a joint effort involving human and financial contributions of several Institutes mostly located in Europe. We acknowledge use of data from the Sloan Digital Sky Survey. We acknowledge use of data from the SkyMapper Southern Survey (Wolf et al., 2018, PASA, 10). Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 179.B 2002. This research has made use of the SVO Filter Profile Service "Carlos Rodrigo", funded by MCIN/AEI/10.13039/501100011033/ through grant PID2023-146210NB-I00. This work makes use of data from the Pan-STARRS1 Survey (Chambers, K.C., et al., 2016).