A massive star origin for an unusual helium-rich supernova in an elliptical galaxy

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The unusual helium-rich (type Ib) supernova SN 2005E is distinguished from all supernovae hitherto observed by its faint and rapidly fading light curve, prominent calcium lines in late-phase spectra and lack of any mark of recent star formation near the supernova location. These properties are claimed1 to be explained by a helium detonation in a thin surface layer of an accreting white dwarf. Here we report that the observed properties of SN 2005cz, which appeared in an elliptical galaxy, resemble those of SN 2005E. We argue that these properties are best explained by a core-collapse supernova at the low-mass end (8–12 solar masses) of the range of massive stars that explode2. Such a low-mass progenitor lost its hydrogen-rich envelope through binary interaction, had very thin oxygen-rich and silicon-rich layers above the collapsing core, and accordingly ejected a very small amount of radioactive 56Ni and oxygen. Although the host galaxy NGC 4589 is an elliptical, some studies have revealed evidence of recent star-formation activity3, consistent with the core-collapse model.

At a glance


  1. Early-time spectra of stripped-envelope core-collapse supernovae.
    Figure 1: Early-time spectra of stripped-envelope core-collapse supernovae.

    Red, spectrum of type Ib SN 2005cz taken on 2005 July 28 with the Keck Telescope. Also shown are the spectra of type Ib SN 2000H at t = +8days (black) and t = +29days (grey)18, type IIb SN 1993J at t = +8days (blue) and t = +24days (cyan)19, and type Ic SN 1994I at t = +7days (magenta) and t = +26days (green)20, 21. The type Ib is characterized by strong helium lines and weak silicon lines, while in the type Ic both helium and silicon lines are weak. The type IIb shows a type II-like spectrum characterized by the strong hydrogen features at early times, and becomes type Ib/c-like at late times. All these supernovae are thought to have partly or fully stripped off their outer layers of hydrogen and helium before the explosions. The epochs are also shown in the figure. The overall appearance of spectral features in SN 2005cz is quite similar to those of the type Ib SN 2000H at t = +29 days, the type IIb SN 1993J at t = +24days (despite its stronger H lines), and also the typical type Ic SN 1994I at t = +26days (despite its lack of the strong He lines). The spectra are corrected for the host redshift and the reddening. We adopted a total (Milky Way + host) reddening of E(B-V) = 0.13 (0.03+0.1) mag in SN 2005cz, 0.23 (0.23+0.0) mag in SN 2000H, 0.45mag in SN 1994I, and 0.3mag in SN 1993J. The flux is on an absolute scale for SN 2005cz, calibrated with the Calar Alto photometry obtained four nights later. For the comparison supernovae, the fluxes are on an arbitrary scale and constants are added for presentation. The positions of the prominent Hei lines are shown by the dashed lines. The spectrum of SN 2005cz is consistent with the post-maximum spectra of type Ib supernovae.

  2. Late-time spectra of stripped-envelope core-collapse supernovae and faint supernovae.
    Figure 2: Late-time spectra of stripped-envelope core-collapse supernovae and faint supernovae.

    Red, calcium-rich late-time spectrum of type Ib SN 2005cz taken on 2006 December 27 (t = +179days). Also shown are type Ib SN 2004dk at t392days (black)22, type IIb SN 1993J at t = +203 days (blue)19, type Ic SN 1994I at t = +147 days (magenta)21, peculiar (pec.) type Ia SN 2005hk at t = +232 days (green)23, and peculiar (possibly type I) SN 2008ha at t = +65 days (yellow)24. As time goes by, the ejecta become transparent to optical light, following the expansion and density decrease. Late-time spectra of type Ib/c supernovae are thus characterized by various emission lines, mostly of forbidden transitions. The spectrum of SN 2004dk is typical for type Ib/c supernovae at late times (for example, see figure 2 of ref. 22). The spectra are corrected for the host redshift, but not for reddening. The flux is on an approximate absolute scale for SN 2005cz, calibrated with the spectroscopic standard star (but not with photometry), whereas it is on an arbitrary scale for the comparison supernovae. The asterisk on SN 2004dk indicates days since its discovery (not maximum light). It is unique that SN 2005cz shows only weak [Oi] lines at 6,300Å and 6,364Å, and much stronger [Caii] lines at 7,291Å and 7,323Å than [Oi]. The relatively weak Caii infrared triplet compared with other supernovae might suggest lower density ejecta for SN 2005cz. It is interesting that the [Caii] line is considerably narrow (half-width at half-maximum, 0.005c, which is probably the typical expansion velocity of the inner core emitting the [Caii] line) compared with the blueshift of the absorption in the Caii infrared triplet in the early-time spectrum (~0.04c), which is attributed to the expansion velocity of the outer layer.

  3. Absolute R-band light curves of relevant supernovae.
    Figure 3: Absolute R-band light curves of relevant supernovae.

    Red circles, light curve of the rapidly fading type Ib SN 2005cz. It is compared with those of type IIb SN 1993J (cyan triangles), type Ic SN 1994I (blue stars), type Ib SN 2007Y (green squares), type IIn SN 2008S (black open circles), and the possibly type I SN 2008ha (orange open squares). Also shown is the light curve of SN 1994I, but dimmed by 1.5mag (magenta open stars), which is magnified in the inset for convenience of comparison with that of SN 2005cz. For SN 2005cz, the first three points denote unfiltered magnitudes, which are approximately R-band magnitudes. The two points with downward arrows are 3σ upper limits. The distance moduli, μ, and total reddening values, E(B-V), both in units of magnitude, are taken as follows: [µ, E(B-V)] = (32.23, 0.13) for SN 2005cz (Supplementary Information section 1), (27.8, 0.3) for SN 1993J, (29.6, 0.45) for SN 1994I, (31.43, 0.112) for SN 2007Y, (31.55, 0.076) for SN 2008ha, and (28.78, 0.687) for SN 2008S. We assume RV = 3.1 to convert the colour excess to the R-band extinction. The data points, as well as the distance and the reddening, are from the literature19, 24, 25, 26, 27. The putative explosion date for SN 2005cz is assumed to be 2005 June 17, 30days before the discovery and 15days before maximum brightness (Supplementary Information section 1). The tail of the light curve of SN 2005cz is similar to those of type IIn SN 2008S and type Ic SN 1994I (dimmed by 1.5mag). From this, we estimate the mass of 56Ni as , assuming a nickel mass of M(56Ni) = 0.07M produced in the typical type Ic SN 1994I28.

  4. Bolometric light curve.
    Figure 4: Bolometric light curve.

    Filled red circles, pseudobolometric light curve of type Ib SN 2005cz. It is compared with a simple γ-ray and positron deposition model with M(56Ni) = 0.02M and (red line), where E51 is the kinetic energy EK expressed in units of 1051erg. We also plot the bolometric light curve of type Ic SN 1994I (open black squares)25 and a simple deposition model with M(56Ni) = 0.07M (black line) for comparison. Except for the last point (upper limit), we simply assume the bolometric correction BCMbolMR = 0.5, derived from SN 1998bw, SN 2002ap and SN 2008D at similar epochs14, 29, 30. As this is a very crude estimate, we adopt an error bar of ±0.5mag for the bolometric luminosity. The deposition models adopt the γ-ray opacity for the Compton scattering ( ) and assume the full deposition of positrons. The decline rate from the intermediate to the late phase is consistent with . Combining this expression with (Mej,/E51)1 as indicated by the similarity in the absorption velocity seen in SN 2005cz and those in SN 1993J and SN 1994I (Fig. 1, Supplementary Fig. 1), we estimate Mej,1 and E511. The luminosity requires that M(56Ni)0.02M. Note that the estimate for M(56Ni) is sensitively affected by the explosion date. The upper limit to M(56Ni) is only M(56Ni)0.005M, if the explosion date is as late as 2005 July 15 (just a few days before the discovery).


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Author information


  1. Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan

    • K. S. Kawabata
  2. Institute for the Physics and Mathematics of the Universe (IPMU), University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan

    • K. Maeda,
    • K. Nomoto &
    • M. Tanaka
  3. Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany

    • S. Taubenberger
  4. Department of Astronomy, School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

    • M. Tanaka
  5. National Astronomical Observatories, CAS, 20A Datun Road, Chaoyang District, Beijing 100012, China

    • J. Deng
  6. INAF Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-3413 Trieste, Italy

    • E. Pian
  7. Subaru Telescope, National Astronomical Observatory of Japan, Hilo, Hawaii 96720, USA

    • T. Hattori
  8. Itagaki Astronomical Observatory, Teppo-cho, Yamagata 990-2492, Japan

    • K. Itagaki


K.S.K., K.M., K.N., J.D. and E.P. organized the observations and discussions; K.M., K.N and K.S.K. wrote the manuscript; K.S.K., S.T. and K.I. were responsible for data acquisition and reduction; J.D. and E.P. were the Principal Investigators of the relevant Subaru programmes, S05B-132 and S05B-054, respectively; M.T. and S.T. contributed to discussions; and T.H. provided expertise on data acquisition at the Subaru Telescope.

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