Draft version November 6, 2018 Preprint typeset using LATEX style emulateapj v. 5/2/11 THE PROGENITOR OF THE TYPE IIB SN 2008AX REVISITED Gaston´ Folatelli1,2,3, Melina C. Bersten1,2,3, Hanindyo Kuncarayakti4,5, Omar G. Benvenuto1,2, Keiichi Maeda6,3, and Ken'ichi Nomoto3,7, Draft version November 6, 2018 ABSTRACT Hubble Space Telescope observations of the site of the supernova (SN) 2008ax obtained in 2011 and 2013 reveal that the possible progenitor object detected in pre-explosion images was in fact multiple. Four point sources are resolved in the new, higher-resolution images. We identify one of the sources with the fading SN. The other three objects are consistent with single supergiant stars. We conclude that their light contaminated the previously identified progenitor candidate. After subtraction of these stars, the progenitor appears to be significantly fainter and bluer than previously measured. Post-explosion photometry at the SN location indicates that the progenitor object has disappeared. If single, the progenitor is compatible with a supergiant star of B to mid-A spectral type, while a Wolf- Rayet (WR) star would be too luminous in the ultraviolet to account for the observations. Moreover, our hydrodynamical modelling shows the pre-explosion mass was 4{5 M and the radius was 30{50 R , which is incompatible with a WR progenitor. We present a possible interacting binary progenitor computed with our evolutionary models that reproduces all the observational evidence. A companion star as luminous as an O9-B0 main-sequence star may have remained after the explosion. Subject headings: supernovae: general { supernovae: individual (SN 2008ax) 1. INTRODUCTION al. 2011), hotter than the progenitors of Type II-Plateau The question of what stellar system produce the differ- SNe. Moreover, they were suggested to have formed close ent types of core-collapse supernovae (SNe) is still open. binary systems by the possible detection of their compan- While it is accepted that hydrogen-rich (Type II-Plateau) ion stars (Maund et al. 2004; Fox et al. 2014; Folatelli SNe arise from relatively low-mass stars in the red super- et al. 2014). SNe 2008ax and 2013df are the other two giant phase (Smartt et al. 2009), the nature of hydrogen- SNe IIb with possible progenitor identifications in pre- deficient (Types Ib and Ic) SNe progenitors remains un- explosion imaging. The case of SN 2008ax is revised in clear (Eldridge et al. 2013). One crucial aspect is un- this work using newly available data. For SN 2013df, the derstanding what mechanisms are at play for envelope candidate is again a cool, K-type supergiant star (Van removal. In this respect it is interesting to focus the Dyk et al. 2014). These cases appear to indicate a pre- attention on the intermediate group of Type IIb SNe vailing close binary origin for SNe IIb. However, Gal- (SNe IIb) whose spectra evolve from hydrogen rich to hy- Yam et al. (2014) found signatures of a WR-type wind drogen poor and which has been suggested to originate in very ealy spectra of the SN IIb 2013cu. from stars that lost most of their H-rich envelopes. Pro- The SN IIb 2008ax has been an important case of posed mass-loss mechanisms are those of strong stellar study due to its small distance and the availability of pre- winds or interaction in close binary systems. The former explosion Hubble Space Telescope (HST) imaging and ex- mechanism requires a very large mass of the SN progen- tensive multi-wavelength followup. Crockett et al. (2008) itor, possibly with a Wolf-Rayet (WR) or luminous blue studied the pre-SN images and found a source at the SN variable (LBV) nature. location that was possibly the progenitor. With the then Remarkably, the firmest identifications of SN-IIb pro- available data, the authors selected two out of a series genitors in pre-explosion images|i.e., those of SNe 1993J of possible scenarios: a WR star that retained a small and 2011dh|have been associated to moderate-mass su- amount of hydrogen, or an interacting binary system. pergiants of spectral types F to K (Aldering et al. 1994; The light curves and spectra indicated a relatively low arXiv:1509.01588v1 [astro-ph.SR] 4 Sep 2015 Van Dyk et al. 2002; Maund et al. 2011; Van Dyk et ejected mass and normal kinetic energy (Pastorello et al. 2008; Roming et al. 2009; Tsvetkov et al. 2009; Mau- rer et al. 2010; Taubenberger et al. 2011; Jerkstrand et [email protected] al. 2015). The low mass was generally interpreted as an 1 Instituto de Astrof´ısicade La Plata, Argentina 2 Facultad de Ciencias Astron´omicasy Geof´ısicas, Universi- indication against a single massive WR progenitor. How- dad Nacional de La Plata, Paseo del Bosque S/N, B1900FWA ever, by including rotation in the evolutionary models of La Plata, Argentina massive stars Georgy et al. (2012) found a suitable pro- 3 Kavli Institute for the Physics and Mathematics of the Uni- verse (WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, genitor born with 20 M and ending with the correct core Japan mass, hydrogen content, luminosity and color to explain 4 Millennium Institute of Astrophysics (MAS), Casilla 36-D, the complete set of observations. Groh et al. (2013a) Santiago, Chile reinterpreted the final stage of the rotating model as a 5 Departamento de Astronom´ıa,Universidad de Chile, Casilla 36-D, Santiago, Chile luminous blue variable (LBV) star and suggested that 6 Department of Astronomy, Kyoto University, Kitashirakawa- LBVs may be the progenitors of some core-collapse SNe. Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan One intriguing aspect of SNe IIb is the possible re- 7 Hamamatsu Professor 2 Folatelli et al. lation between progenitor radius (or H-envelope mass) imaging obtained in 2011 and 2013. The former dataset and circumstellar medium (CSM) density. If the pro- included images in four optical bands obtained at three genitor star can be detected in pre-SN observations, a different epochs with the Wide Field Planetary Camera 2 radius can be determined. Alternatively, the shape of (WFPC2) mounted on HST. The SN site landed on WF the UV-optical light curve during the first few days after detectors for the F 450W , F 606W , and F 814W frames, explosion can also indicate the extent of the exploding yielding a pixel scale of 000: 1 pixel−1. In the F 300W star depending on the strength of the cooling emission frames the SN site was on the PC chip, with a pixel from the shocked envelope (Woosley et al. 1994; Bersten scale of 000: 046 pixel−1. The new imaging was obtained et al. 2012, 2014). The CSM can be probed through in two epochs (in July 2011 and October 2013) using the radio and X-ray observations and this can be linked to Wide Field Camera 3 (WFC3) of HST with a pixel scale the mass-loss history of the progenitor. When a dense of 000: 04 pixel−1. In 2011 the F 336W , F 438W , F 606W , CSM is present shock-heated gas can also produce opti- F 625W , and F 814W bands were employed, while in 2013 cal emission at late times, typically evident in box-like the imaging was done in the F 275W , F 336W , F 438W , emission lines in the spectra and a slow-down of the light F 555W , and F 814W bands. Along with a higher reso- curve as compared to the usual radioactive-power decline lution, the new images are deeper than the pre-explosion rate. These phenomena were observed in the SNe IIb ones. 1993J (Matheson et al. 2000a,b) and 2013df (Maeda et Processed HST data were downloaded from the Hub- al. 2015). ble Legacy Archive8 (HLA). The WFPC2 observations Chevalier & Soderberg (2010) suggested that ra- were performed in pairs of exposures with each filter. dio properties of core-collapse SNe provide information The combined F 606W image had poor quality due to about the size of the progenitor and proposed that a misalignment of both exposures. We thus performed SNe IIb were divided into those with compact (WR- the image combination for that band using the DRIZ- like) and extended (supergiant) progenitors. SN 2008ax ZLEPAC package. The WFC3 images from 2011 and was placed in the compact category while SN 1993J be- 2013 consisted on four and three integrations per band, longed to the extended one. Bersten et al. (2012), how- respectively. ever, showed that such a connection breaks down for Crockett et al. (2008) obtained ground-based, high- SN 2011dh. Nonetheless, by restricting to the three resolution imaging of the SN while it was still bright in or- SNe IIb with direct progenitor detections and estimates der to accurately determine the location of the SN. These of the CSM density (SNe 1993J, 2011dh, and 2013df), observations were carried out in the K band using adap- Maeda et al. (2015) suggest that SNe with more extended tive optics with the Altair/NIRI instrument mounted on progenitors may have denser surrounding media, possibly the Gemini North Telescope. We obtained the raw im- due to a larger mass-loss rate near the time of explosion. ages from the Gemini Science Archive9 and processed The range of radii goes from ≈200 R in SN 2011dh them through sky subtraction and combination using the to ≈600 R in SNe 1993J and 2013df. There is a wide dedicated tools for NIRI in the gemini package of IRAF. variety of SNe-IIb light curves observed during the first The co-added image has excellent quality, with FHWM few days after explosion, which may indicate a range of of 000: 09 (≈ 4:1 pixels).