Nine Type Ia supernovae (SNe Ia) with preexisting Hubble Space Telescope (HST) data on their host galaxies have been close enough (within 25 Mpc) to search for a progenitor (Supplementary Table 1). No progenitor system has been found; only upper limits have been possible 31,8,9,10 . Limits range from M g = −3.9 mag in the case of SN 2004W to M I = −8.3 mag for SN 2003cg 31 . These limits provided only poor constraints; in the case of SN 2006dd and SN 2006mr in NGC 1316, they ruled out normal stars with initial masses greater than 6 M at the tip of the asymptotic giant branch (AGB), young post-AGB stars with initial masses greater than 4 M , and post-red-giant stars with initial masses greater than 9 M 8 .

We present H‐ and K‐band spectroscopy of OB and Wolf–Rayet (WR) members of the Milky Way cluster 1806−20 (G10.0–0.3) to obtain a revised cluster distance, of relevance to the 2004 giant flare from the (soft gamma repeater) SGR 1806−20 magnetar. From GNIRS (Gemini Near‐Infrared Spectrograph) spectroscopy obtained with Gemini South, four candidate OB stars are confirmed as late O/early B supergiants, while we support previous mid‐WN and late WC classifications for two WR stars. Based upon an absolute Ks‐band magnitude calibration for B supergiants and WR stars, and near‐infrared (IR) photometry from NIRI (Near‐Infrared Imager) at Gemini North plus archival VLT/ISAAC (Very Large Telescope/Infrared Spectrometer And Array Camera) data sets, we obtain a cluster distance modulus of 14.7 ± 0.35 mag. The known stellar content of the 1806−20 cluster suggests an age of 3–5 Myr, from which theoretical isochrone fits infer a distance modulus of 14.7 ± 0.7 mag. Together, our results favour a distance modulus of 14.7 ± 0.4 mag (8.7+1.8−1.5 kpc) to the 1806−20 cluster, which is significantly lower than the nominal 15 kpc distance to the magnetar. For our preferred distance, the peak luminosity of the 2004 December giant flare is reduced by a factor of 3 to 7 × 1046 erg s−1, such that the contamination of BATSE (Burst And Transient Source Experiment) short gamma‐ray bursts (GRBs) from giant flares of extragalactic magnetars is reduced to a few per cent. We infer a magnetar progenitor mass of ∼48+20−8 M⊙, in close agreement with that obtained recently for the magnetar in Westerlund 1.

We present ground-based and Hubble Space Telescope optical observations of the optical transients (OTs) of long-duration Gamma Ray Bursts (GRBs) 060729 and 090618, both at a redshift of z = 0.54. For GRB 060729, bumps are seen in the optical light curves (LCs), and the late-time broad-band spectral energy distributions (SEDs) of the OT resemble those of local Type Ic supernovae (SNe). For GRB 090618, the dense sampling of our optical observations has allowed us to detect well-defined bumps in the optical LCs, as well as a change in colour, that are indicative of light coming from a core-collapse SN. The accompanying SNe for both events are individually compared with SN1998bw, a known GRB supernova, and SN1994I, a typical Type Ic supernova without a known GRB counterpart, and in both cases the brightness and temporal evolution more closely resemble SN1998bw. We also exploit our extensive optical and radio data for GRB 090618, as well as the publicly available Swift-XRT data, and discuss the properties of the afterglow at early times. In the context of a simple jet-like model, the afterglow of GRB 090618 is best explained by the presence of a jet-break at t − t o > 0.5 d. We then compare the rest-frame, peak V-band absolute magnitudes of all of the GRB and X-Ray Flash (XRF)-associated SNe with a large sample of local Type Ibc SNe, concluding that, when host extinction is considered, the peak magnitudes of the GRB/XRF-SNe cannot be distinguished from the peak magnitudes of non-GRB/XRF SNe.

We present a Very Large Telescope/Focal Reducer and Low Dispersion Spectrograph #1 (VLT/FORS1) imaging and spectroscopic survey of the Wolf-Rayet (WR) population in the Sculptor group spiral galaxy NGC 7793. We identify 74 emission-line candidates from archival narrow-band imaging, from which 39 were observed with the Multi Object Spectroscopy mode of FORS1. 85 per cent of these sources displayed WR features. Additional slits were used to observe H II regions, enabling an estimate of the metallicity gradient of NGC 7793 using strong line calibrations, from which a central oxygen content of log(O/H) + 12 = 8.6 was obtained, falling to 8.25 at R 25 . We have estimated WR populations using a calibration of line luminosities of Large Magellanic Cloud stars, revealing ∼27 WN and ∼25 WC stars from 29 sources spectroscopically observed. Photometric properties of the remaining candidates suggest an additional ∼27 WN and ∼8 WC stars. A comparison with the WR census of the LMC suggests that our imaging survey has identified ∼80 per cent of WN stars and ∼90 per cent for the WC subclass. Allowing for incompleteness, NGC 7793 hosts ∼105 WR stars for which N(WC)/N(WN) ∼ 0.5. From our spectroscopy of H II regions in NGC 7793, we revise the global Hα star formation rate of Kennicutt et al. upward by 50 per cent to 0.45 M yr −1 . This allows us to obtain N(WR)/N(O) ∼ 0.018, which is somewhat lower than that resulting from the WR census by Schild et al. of another Sculptor group spiral NGC 300, whose global physical properties are similar to NGC 7793. Finally, we also report the fortuitous detection of a bright (m V = 20.8 mag) background quasar Q2358-32 at z ∼ 2.02 resulting from C IV λ1548-51 redshifted to the λ4684 passband.

M101 is an ideal target in which to test predictions of massive star birth and evolution. The large abundance gradient across M101 (a factor of 20) suggests that many more WR stars must be found in the inner parts of this galaxy than in the outer regions. Many H ii regions and massive star-forming complexes have been identified in M101; they should be rich in WR stars, and surrounded by RSG stars. Finally, the Wolf-Rayet stars in M101 may be abundant enough for one to explode as a Type Ib or Ic supernova and/or GRB within a generation. The clear identification of the progenitor of a Type Ib or Ic supernova as a WR star would be a major confirmation of current stellar evolution theory.Motivated by these considerations, we have used the Hubble Space Telescope to carry out a deep, He ii optical narrowband imaging survey of the massive star populations in the ScI spiral galaxy M101. Combined with archival broadband images, we were able to image almost the entire galaxy with the unprecedented depth and resolution that only HST affords.We describe the extent of the survey and our images, as well as our data reduction procedures. A detailed study of a field east of the center of M101, containing the giant star-forming region NGC 5462, demonstrates how we find -2candidates, their properties and spatial distribution, and how we rule out most contaminants. The spatial distributions of the WR and RSG stars near a giant star-forming complex are strikingly different. WR stars dominate the complex core, while RSG dominate the complex halo. Future papers in this series will describe and catalog more than a thousand WR and RSG candidates that are detectable in our images, as well as spectra of many of those candidates.