Near-IR photometry of possible progenitor of SN2008bk

ATel #1464; Dan Maoz and Filippo Mannucci (INAF)
on 7 Apr 2008; 10:24 UT
Credential Certification: Filippo Mannucci (filippo@arcetri.astro.it)

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We report J and Ks photometry of the possible progenitor star, identified by W. Li et al. (Atel #1448), of SN 2008bk in NGC 7793, and give constraints on its mass and nature. Using archival ESO data, we measure in the Vega magnitude system:

J=19.29+/-0.05, Ks=18.27+/-0.10

The J and Ks measurements are from VLT-ISAAC images obtained on Oct 10, 2005. The photometry was calibrated relative to three 2MASS stars in the field having J and Ks errors of less than 0.1 mag. An image showing the possible progenitor star in the Ks image and the 2MASS calibration stars can be found at http://wise-obs.tau.ac.il/~dani/sn2008bk/sn2008bkKs.jpg The colors of the star are therefore: J-Ks=1.02+/-0.12 .

The distance modulus to NGC 7793, based on the tip of the RGB, is 27.96+/-0.24 (Karachentsev et al. 2003, A&A, 404, 93), resulting in absolute magnitudes: M_J=-8.67+/-0.29, M_Ks=-9.69+/-0.34 .

To constrain the nature of the possible progenitor, we have transformed the temperatures, luminosities, and surface gravities of stellar evolution models to magnitudes and colors in the observed bands. For the ZAMS to AGB stages we have used the tracks of Girardi et al. (2000,A&AS, 141, 371) for 3-7 M_sun, and from Salasnich et al. (2000, A&A, 361, 1023) for 8-12 M_sun. For the thermal-pulse AGB phase of 3-5 M_sun stars we have used the models by Marigo \& Girardi (2007, A&A, 469, 239). With these tracks, we have used the Basel 3.1 stellar atmosphere models by Westera et al. (2002, A&A, 381, 524), and the observed bandpasses. The tracks were calculated for a range of metallicities. Pilyugin et al. (2004, A&A, 425, 849) find a gas-phase oxygen abundance in HII regions of NGC 773 of 12+log(O/H)=8.34, corresponding 0.45 solar.

A K vs. J-K color-magnitude diagram can be seen at http://wise-obs.tau.ac.il/~dani/sn2008bk/sn2008bkcolormag.jpg . Curves show tracks for ZAMS masses of 3,4,5,6,7,8,9,10,12 Msun, and metallicities of 0.2 and 1.0 solar. The measurements suggest acceptable progenitor masses of 9-12 M_sun within this range of metallicities. However, "super-AGB" stars with ZAMS masses of 6-8 M_sun, whose final stages of evolution are subject to large uncertainties (see, e.g. http://www.astro.keele.ac.uk/e-stars/ras2008/ras2008.html ), could also be possible progenitors of this event.