Estimation of the Spectral Type of the Progenitor of the ASASSN-17oz Transient

ATel #11011; B. McCollum (American Univ.), Lee Rottler (American Univ.)
on 27 Nov 2017; 04:35 UT
Credential Certification: Bruce McCollum (mccollub@cua.edu)

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We report the result of SED fitting of the precursor object of the ASASSN-17oz transient (ATel #10991) using pre-outburst archival photometry. We used the following magnitudes, from the sources noted: GALEX All-sky FUV (1516A) = ~21 (AB magnitude upper limit) GALEX All-sky NUV (2267A) = ~22 (AB magnitude upper limit) Johnson B = 17.17 +/- 0.55 (USNO-B1.0 catalog, two values) Johnson V = 16.3 (Jayasinghe et al. 2017, ATel #10991) Johnson R = 16.02 (NOMAD catalog, Zacharias 2005) Pan-STARRS g = 17.108 +/- 0.049 Pan-STARRs r = 16.424 +/- 0.017 Pan-STARRS i = 15.885 +/- 0.053 Pan-STARRS z = 15.77 +/- 0.019 2MASS J = 14.544 +/- 0.032 2MASS H = 14.025 +/- 0.04 2MASS Ks = 13.755 +/- 0.044 WISE W1 = 13.78 +/- 0.028 WISE W2 = 14.712 +/- 0.041 WISE W3 = 12.581 (upper limit) These measurements were obtained at epochs spread over a few decades. However, Jayasinghe et al. 2017 report observing no significant variability of this object in V throughout >1400 epochs across almost four years of observations. The B magnitudes in the USNO-B1.0 catalog (Monet et al. 2003, AJ, 125, 984) are different by 0.78 magnitude over the two catalog epochs, which could be significant given the catalog's nominal accuracy of +/- 0.3 magnitude. The USNO-B1.0 B magnitudes are measured from POSS plates obtained two epochs, one between 1949 and 1965, and the other between 1985 and 2000 (Monet et al. 2003). We averaged the two values and treated the difference as an uncertainty in the value. We compared the SED to three sets of stellar model grids: NextGen stellar models (Hauschildt et al. 1999a, ApJ, 512,77, and 1999b, ApJ, 525, 871), the grids of Baraffe et al. (2015, A&A, 577, 42), and BT-NextGen (Allard et al. 2009, RSPTA, 370,2765). We used the SED fitting tool of Bayo et al. (2008, A&A, 492, 277). The fitting tool was allowed to treat Av as a parameter within the range of 0 to 1.0, centered on the Av estimate obtained using the Schlafly & Finkelbeiner (2011, ApJ, 737, 103) extinction map. A good fit, and the best fit, is obtained using the NextGen model grid, which returns the following parameters: Teff = 4900 +/- 50 K, log g = 3.5 +/- 0.25, Z = 0.5 +/- 0.25. All of the models agree that the object's temperature is about 4700-4900 K. NextGen models tend to overestimate the Teff by ~4-8% for both giant and dwarf stars of spectral type K and later (Bertone et al. 2004, AJ, 128, 829), suggesting that a more accurate temperature is ~ 4500 K to 4700 K, which corresponds to K0 to K2 for giants and K3 to K4 for dwarfs. Thus our best estimate for the spectral type is K1-K2 III/IV or K3-K4 IV/V. We think that the fitting has a larger uncertainty in the luminosity class than the formal uncertainty stated above, because we find that the quality of the fit is mostly insensitive to the log g value when different values of log g corresponding to giants, subgiants, and dwarfs are input as fixed values instead of free parameters. We find no infrared excess. The upper limit on the GALEX NUV flux is ~0.08 of the flux in the B band, which puts a strong constraint against a significant amount of UV flux from any emission component. This publication uses 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 NASA and the National Science Foundation; the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and JPL/Caltech, funded by the NASA; the Spanish Virtual Observatory project (VOSA) supported from the Spanish MICINN through grant AyA2011-24052; the Pan-STARRS1 Surveys; the NASA Galaxy Evolution Explorer, operated for NASA by the California Institute of Technology under NASA contract NAS5-98034; and the VizieR catalog tool, CDS, Strasbourg, France.