DISAPPEARANCE of the PROGENITOR of SUPERNOVA IPTF13BVN Gaston´ Folatelli1,2, Schuyler D
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Draft version June 10, 2016 Preprint typeset using LATEX style emulateapj v. 08/22/09 DISAPPEARANCE OF THE PROGENITOR OF SUPERNOVA IPTF13BVN Gaston´ Folatelli1,2, Schuyler D. Van Dyk3, Hanindyo Kuncarayakti4,5, Keiichi Maeda6,2, Melina C. Bersten1,2, Ken'ichi Nomoto2,7, Giuliano Pignata8,4, Mario Hamuy5,4, Robert M. Quimby9,2, WeiKang Zheng10, Alexei V. Filippenko10, Kelsey I. Clubb10, Nathan Smith11, Nancy Elias-Rosa12, Ryan J. Foley13,14, and Adam A. Miller15,16 Draft version June 10, 2016 ABSTRACT Supernova (SN) iPTF13bvn in NGC 5806 was the first Type Ib SN to have been tentatively associated with a progenitor in pre-explosion images. We performed deep ultraviolet (UV) and optical Hubble Space Telescope observations of the SN site ∼ 740 days after explosion. We detect an object in the optical bands that is fainter than the pre-explosion object. This dimming is likely not produced by dust absorption in the ejecta; thus, our finding confirms the connection of the progenitor candidate with the SN. The object in our data is likely dominated by the fading SN, implying that the pre-SN flux is mostly due to the progenitor. We compare our revised pre-SN photometry with previously proposed models. Although binary progenitors are favored, models need to be refined. In particular, to comply with our deep UV detection limit, any companion star must be less luminous than a late-O star or substantially obscured by newly formed dust. A definitive progenitor characterization will require further observations to disentangle the contribution of a much fainter SN and its environment. Subject headings: supernovae: general { supernovae: individual (iPTF13bvn) { stars: evolution { galaxies: individual (NGC 5806) 1. INTRODUCTION stars (MZAMS > 25 M ) is strong stellar winds leading The stellar origin of hydrogen-free core-collapse super- to Wolf-Rayet (WR) progenitors (Heger et al. 2003), but novae (SNe) remains unknown chiefly owing to the lack such massive progenitors are difficult to reconcile with of detections of progenitor stars in pre-explosion images the large fraction of stripped-envelope explosions (Smith (Eldridge et al. 2013). The only firm progenitor can- et al. 2011) and their low ejecta masses (Drout et al. 2011; didate found thus far is that of iPTF13bvn, a Type Ib Dessart et al. 2011; Hachinger et al. 2012). The more supernova (SN Ib) in the galaxy NGC 5806 (Cao et al. common alternative is mass transfer in close binary sys- 2013). If confirmed, this case can provide important clues tems (Shigeyama et al. 1990; Podsiadlowski et al. 1992), about the mechanisms of envelope removal among mas- allowing less-massive stars to lose their envelopes (e.g., sive stars. One proposed mechanism for very massive Benvenuto et al. 2013). The relative incidence of each type of progenitor is still unknown. 1 Facultad de Ciencias Astron´omicas y Geof´ısicas,Universidad Cao et al. (2013) used pre-explosion Hubble Space Tele- Nacional de La Plata, Paseo del Bosque S/N, B1900FWA La scope (HST) imaging, together with early-time SN obser- Plata; Instituto de Astrof´ısica de La Plata (IALP), CONICET, vations, to suggest that the progenitor of iPTF13bvn was Argentina 2 Kavli Institute for the Physics and Mathematics of the Uni- a compact WR star. Groh et al. (2013b) found that the verse (WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, pre-explosion photometry could be fit with models of sin- Japan; [email protected] gle WR stars in the initial mass range of 31{35 M . How- 3 IPAC/Caltech, Mailcode 100-22, Pasadena, CA 91125, USA ever, from hydrodynamical light-curve modeling, Bersten 4 Millennium Institute of Astrophysics (MAS), Santiago, Chile et al. (2014) inferred a low pre-SN mass (∼ 3:5 M ) and 5 Departamento de Astronom´ıa, Universidad de Chile, Casilla 36-D, Santiago, Chile disfavored a massive WR star; instead, they presented 6 Department of Astronomy, Kyoto University, Kitashirakawa- close binary system models that could explain the pre- Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan explosion photometry and the progenitor mass and ra- 7 arXiv:1604.06821v2 [astro-ph.SR] 9 Jun 2016 Hamamatsu Professor dius derived from the SN observations. By analyzing the 8 Departamento de Ciencias Fisicas, Universidad Andres Bello, Avda. Republica 252, Santiago, Chile light curves, Fremling et al. (2014) and Srivastav et al. 9 Department of Astronomy, San Diego State University, 5500 (2014) also disfavored a massive WR progenitor. Kun- Campanile Drive, San Diego, CA 92182-1221, USA carayakti et al. (2015) argued for a low-mass progeni- 10 Department of Astronomy, University of California, Berkeley, tor based on the strength of oxygen and calcium lines CA 94720-3411, USA 11 Steward Observatory, University of Arizona, 933 N. Cherry in the late-time spectrum. Subsequently, Eldridge et al. Ave., Tucson, AZ 85721, USA (2015) (E15, hereafter) revised the pre-explosion pho- 12 INAF-Osservatorio Astronomico di Padova, Vicolo tometry and found the progenitor to be brighter than dell'Osservatorio 5, I-35122 Padova, Italy measured by Cao et al. (2013); see also our measure- 13 Astronomy Department, University of Illinois at Urbana- Champaign, 1002 W. Green Street, Urbana, IL 61801, USA ments in Section 2. With the new magnitudes and using 14 Department of Physics, University of Illinois at Urbana- binary evolution calculations, E15 argued in favor of the Champaign, 1110 W. Green Street, Urbana, IL 61801, USA binary scenario. The same conclusion was found by Kim 15 Jet Propulsion Laboratory, 4800 Oak Grove Drive, MS et al. (2015). Assuming the binary configurations dis- 169-506, Pasadena, CA 91109, USA 16 Hubble Fellow cussed by Bersten et al. (2014), Hirai & Yamada (2015) 2 Folatelli et al. simulated the effect of the SN shock on the companion files while we use the flc files. Additionally, E15 mention star. They found that the companion could bloat and that they adopt the recommended DOLPHOT parame- thus evolve to a red-supergiant structure on a timescale ters. Among these, the sky-fitting algorithm parameter, of a few years after explosion. We note, however, that FitSky, is suggested to be set as 1 for general purposes similar shock simulations performed by Liu et al. (2015), in the DOLPHOT v2.0 ACS module manual. However, although for different companion masses, do not predict for a crowded field such as that of iPTF13bvn, the rec- such a post-explosion evolution. ommendation is to perform the sky fit inside a relatively To better determine the progenitor's nature, the SN large photometry aperture by adopting FitSky=3 (Dal- site had to be reobserved after the ejecta faded enough canton et al. 2009), which was our choice. If we instead to probe the disappearance of the pre-SN object. In this use FitSky=1, we obtain F555W = 26:37 ± 0:05 mag, work we present new HST observations that reveal a de- in close agreement with Eldridge & Maund (2016). We crease in flux relative to the pre-SN observations, con- also obtain large negative sharpness values, as mentioned firming that this is the first SN Ib with a progenitor by those authors, which is not seen with FitSky=3. We detection17. In Section 2 we describe the observations conclude that the main source of discrepancy is likely the and photometry methods. The new data are analyzed in choice of the sky-fitting algorithm, and that for the field Section 3 to constrain the progenitor nature. We present of iPTF13bvn, our procedure is more accurate. Never- our conclusions in Section 4. theless, we note that the main conclusions in the present work would not change if we adopted the photometry in 2. OBSERVATIONS AND PHOTOMETRY E15 and Eldridge & Maund (2016). Figure 2 (left panel) shows the HST magnitudes con- We obtained deep imaging of the field of iPTF13bvn verted to BV I. The conversion was calculated using syn- ∼ 740 days after explosion using HST through Cycle 22 thetic photometry from the 306 day spectrum of Kuncar- programs GO-13684 and GO-13822. Program GO-13684 ayakti et al. (2015), which gave B − F438W = 0:06 mag, was executed between 2015 June 26.37 and 26.60 (UT V −F555W = −0:03 mag, and I −F814W = −0:05 mag. dates are used herein) with the Wide Field Camera 3 We used the F225W image to compute a detection (WFC3) UVIS channel; see Table 1. Program GO-13822 limit at the SN location, employing DOLPHOT to add ∼ comprised observations obtained on 2015 June 30.63 with 50; 000 artificial stars near the SN site with a uniform dis- WFC3/UVIS (F225W filter) and on June 30.90 UT with tribution of brightness over 25.4{27.4 mag. DOLPHOT the Advanced Camera for Surveys (ACS; F814W filter). was run on the resulting images to recover the artificial The 2015 images are shown in Figure 1, along with the sources. A detection limit of 26.4 mag was found where pre-explosion images obtained in 2005 through program the recovery fraction dropped to 50%, which is roughly GO-10187 with ACS. equivalent to a 5σ detection limit (Harris 1990). The SN location in the pre- and post-explosion images We also obtained BV RI imaging of iPTF13bvn until was found by aligning them relative to a F555W image ∼ 280 days with the Katzman Automatic Imaging Tele- obtained through program GO-12888 with WFC3/UVIS scope (KAIT; Filippenko et al. 2001) and the 1 m Nickel on 2013 September 2.37 when the SN was still very telescope at Lick Observatory. Template subtraction was bright. The registration was done with the AstroDriz- performed using additional images obtained after the SN zle package employing 20 point sources in common.