Spectral Type of the Unusual Variable ASASSN-V J213939.3-702817.4

ATel #12849; B. McCollum (American Univ.), S. Laine (Caltech/IPAC)
on 8 Jun 2019; 00:12 UT
Credential Certification: Bruce McCollum (mccollub@cua.edu)

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We report using archival photometry to fit the SED of ASASSN-V J213939.3-702817.4, which Jayasingh et al. (2019, ATel # 12836) reported to exhibit an unusual dimming episode. We used the BT-Settl-CIFIST set of model atmospheres (Baraffe et al. 2015, A&A, 577A, 42B). A total of nineteen data points were used for the fitting, covering a wavelength range from the GALEX NUV channel to an upper limit from the WISE W4 bandpass.

In the fitting, the object temperature was allowed to vary from 1200 K to 7000 K in 100 K increments. The Av was allowed to vary as an independent variable from 0.01 to 1.5.

The best fit is obtained from the following parameters: T = 6800 +/- 110 K, Av = 0.83 +/- 0.08, where the uncertainties represent two sigma values. A plot of the model fit is available at the link below.

Jayasingh et al. (ATel #12836) report that the object has an absolute magnitude Mv ~ 2.5 outside of the eclipse based on the distance found by Gaia which is 1113 +/- 33 pc (Bailer-Jones et al. 2018, AJ, 156, 58). With no extinction, this value of Mv corresponds to that of an F0V star. A 6800 K dwarf star would have Mv ~ 3.0 (Pecaut & Mamajek 2013, ApJS, 208, 9). In our range of best fits, the best match to the Gaia Mv corresponds to T = 6700 K (spectral type F3V) and Av = 0.75, which would imply Mv ~ 2.35. The Galactic extinction in this direction is only ~0.082 in V (Shlafly & Finkbeiner 2011, ApJ, 737, 103S), which suggests that there is a substantial amount of circumstellar extinction to this object.

Considering that the data fitted were obtained at several different epochs over approximately the past 20 years, the fact that all of the available photometry can be closely fitted to the model SED suggests an absence of any long-duration brightness variations. Jayasingh et al. (2019), also, point out the absence of any brightness variations in their data apart from the one dip. We also note that the close fit of the IR SED, given the small uncertainties (~0.02 mag, except 0.12 in W3) in the 2MASS and WISE data, is consistent with the absence of an IR excess in the WISE bandpasses as noted by Jayasingh et al. (2019). Thus the strong extinction suggested by our fitting must result either from ISM dust or circumstellar material too cold to radiate significantly in the ~1 to 12 um range.

This research has made use of the NASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. 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. This work presents results from the European Space Agency (ESA) space mission Gaia. Gaia data are being processed by the Gaia Data Processing and Analysis Consortium (DPAC). 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.

ASASSN-V J213939.3-702817.4 SED fit