ALBERT

Description: In this work, we analyse late-time ( t  〉 100 d) optical spectra of low-redshift ( z  〈 0.1) Type Ia supernovae (SNe Ia) which come mostly from the Berkeley Supernova Ia Program (BSNIP) data set. We also present spectra of SN 2011 by for the first time. The BSNIP sample studied consists of 34 SNe Ia with 60 nebular spectra, to which we add nebular spectral feature measurements of 20 SNe Ia from previously published work (Maeda et al. and Blondin et al.), representing the largest set of late-time SN Ia spectra ever analysed. The full width at half-maximum intensity and velocities of the [Fe  iii ] 4701, [Fe  ii ] 7155 and [Ni  ii ] 7378 emission features are measured in most observations of spectroscopically normal objects where the data have signal-to-noise ratios 20 pixel –1 and are older than 160 d past maximum brightness. The velocities of all three features are seen to be relatively constant with time, increasing only a few to ~20 km s –1 d –1 . The nebular velocity ( v neb , calculated by taking the average of the [Fe  ii ] 7155 and [Ni  ii ] 7378 velocities) is correlated with the near-maximum-brightness velocity gradient and early-time ejecta velocity. Nearly all high velocity gradient objects have redshifted nebular lines while most low velocity gradient objects have blueshifted nebular lines. No correlation is found between v neb and m 15 ( B ), and for a given light-curve shape there is a large range of observed nebular velocities. The data also indicate a correlation between observed ( B  – V ) max and v neb .

Description: We use a sample of 58 low-redshift ( z  ≤ 0.03) Type Ia supernovae (SNe Ia) having well-sampled light curves and spectra near maximum light to examine the behaviour of high-velocity features (HVFs) in SN Ia spectra. We take advantage of the fact that Si ii 6355 is free of HVFs at maximum light in all SNe Ia, while HVFs are still strong in the Ca ii near-infrared feature in many SNe, allowing us to quantify the strength of HVFs by comparing the structure of these two lines. We find that the average HVF strength increases with decreasing light-curve decline rate, and rapidly declining SNe Ia ( m 15 ( B ) ≥ 1.4 mag) show no HVFs in their maximum-light spectra. Comparison of HVF strength to the light-curve colour of the SNe Ia in our sample shows no evidence of correlation. We find a correlation of HVF strength with the velocity of Si ii 6355 at maximum light ( v Si ), such that SNe Ia with lower v Si have stronger HVFs, while those SNe Ia firmly in the ‘high-velocity’ (i.e. v Si  ≥ 12 000 km s –1 ) subclass exhibit no HVFs in their maximum-light spectra. While v Si and m 15 ( B ) show no correlation in the full sample of SNe Ia, we find a significant correlation between these quantities in the subset of SNe Ia having weak HVFs. In general, we find that slowly declining (low m 15 ( B )) SNe Ia, which are more luminous and more energetic than average SNe Ia, tend to produce either high photospheric ejecta velocities (i.e. high v Si ) or strong HVFs at maximum light, but not both. Finally, we examine the evolution of HVF strength for a sample of SNe Ia having extensive pre-maximum spectroscopic coverage and find significant diversity of the pre-maximum HVF behaviour.

Description: With archival and new Hubble Space Telescope observations, we have refined the space-velocity measurements of the stars in the central region of the remnant of Tycho's supernova (SN) 1572, one of the historical Galactic Type Ia supernova remnants (SNRs). We derived a proper motion for Tycho-G of (μ αcos ; μ ) J2000.0  = (–2.63; –3.98) ± (0.06; 0.04) [formal errors] ± (0.18; 0.10) [expected errors] mas yr –1 . If the star were at the distance of the SNR (taken here to be 2.83 kpc), its velocity would be v b = –51 ± 1.5 km s –1 . We also reconstruct the binary orbit that Tycho-G should have followed if it were the surviving companion of SN 1572. We redetermine the Ni abundance of this star and compare it with new abundance data from stars of the Galactic disc, finding that [Ni/Fe] is about 1.7 above the Galactic trend. From the high velocity of Tycho-G perpendicular to the Galactic plane ( – 90 ± 3 km s –1  ≤  v b ≤ –45 ± 1 km s –1 , for the allowed range of distances to the star), its metallicity and its Ni excess, we find the probability of it being a chance interloper to be P   0.000 37 at most. The projected rotational velocity of the star should be below current observational limits. The projected position of Tycho-G is, within the uncertainties, consistent with the centroid of the X-ray emission of Tycho's SNR; moreover, its brightness is generally consistent with the post-explosion evolution of the luminosity of an SN companion. Among the other 23 stars having V  〈 22 mag and located within 42 arcsec from the X-ray centroid, only 4 are at distances compatible with that of the SNR, and none of them shows any peculiarity. Therefore, if even Tycho-G is not the surviving companion of SN 1572, the absence of other viable candidates does favour the merging of two white dwarfs as the producer of the SN.

Description: We present an investigation of the optical spectra of 264 low-redshift ( z 〈 0.2) Type Ia supernovae (SNe Ia) discovered by the Palomar Transient Factory, an untargeted transient survey. We focus on velocity and pseudo-equivalent width measurements of the Si ii 4130, 5972, and 6355 Å lines, as well those of the Ca ii near-infrared (NIR) triplet, up to +5 days relative to the SN B -band maximum light. We find that a high-velocity component of the Ca ii NIR triplet is needed to explain the spectrum in ~95 per cent of SNe Ia observed before –5 days, decreasing to ~80 per cent at maximum. The average velocity of the Ca ii high-velocity component is ~8500 km s –1 higher than the photospheric component. We confirm previous results that SNe Ia around maximum light with a larger contribution from the high-velocity component relative to the photospheric component in their Ca ii NIR feature have, on average, broader light curves and lower Ca ii NIR photospheric velocities. We find that these relations are driven by both a stronger high-velocity component and a weaker contribution from the photospheric Ca ii NIR component in broader light curve SNe Ia. We identify the presence of C ii in very-early-time SN Ia spectra (before –10 days), finding that 〉40 per cent of SNe Ia observed at these phases show signs of unburnt material in their spectra, and that C ii features are more likely to be found in SNe Ia having narrower light curves.

Description: What are Type II-Linear supernovae (SNe II-L)? This class, which has been ill defined for decades, now receives significant attention – both theoretically, in order to understand what happens to stars in the ~15–25 M range, and observationally, with two independent studies suggesting that they cannot be cleanly separated photometrically from the regular hydrogen-rich SNe II-P characterized by a marked plateau in their light curve. Here, we analyse the multiband light curves and extensive spectroscopic coverage of a sample of 35 SNe II and find that 11 of them could be SNe II-L. The spectra of these SNe are hydrogen deficient, typically have shallow Hα absorption, may show indirect signs of helium via strong O  i 7774 absorption, and have faster line velocities consistent with a thin hydrogen shell. The light curves can be mostly differentiated from those of the regular, hydrogen-rich SNe II-P by their steeper decline rates and higher luminosity, and we propose to define them based on their decline in the V band: SNe II-L decline by more than 0.5 mag from peak brightness by day 50 after explosion. Using our sample we provide template light curves for SNe II-L and II-P in four photometric bands.

Description: While performing a survey to detect recoiling supermassive black holes, we have identified an unusual source having a projected offset of 800 pc from a nearby dwarf galaxy. The object, SDSS J113323.97+550415.8, exhibits broad emission lines and strong variability. While originally classified as a supernova (SN) because of its non-detection in 2005, we detect it in recent and past observations over 63 yr and find over a magnitude of rebrightening in the last 2 yr. Using high-resolution adaptive optics observations, we constrain the source emission region to be 12 pc and find a disturbed host-galaxy morphology indicative of recent merger activity. Observations taken over more than a decade show narrow [O iii ] lines, constant ultraviolet emission, broad Balmer lines, a constant putative black hole mass over a decade of observations despite changes in the continuum, and optical emission-line diagnostics consistent with an active galactic nucleus (AGN). However, the optical spectra exhibit blueshifted absorption, and eventually narrow Fe ii and [Ca ii ] emission, each of which is rarely found in AGN spectra. While this peculiar source displays many of the observational properties expected of a potential black hole recoil candidate, some of the properties could also be explained by a luminous blue variable star (LBV) erupting for decades since 1950, followed by a Type IIn SN in 2001. Interpreted as an LBV followed by an SN analogous to SN 2009ip, the multidecade LBV eruptions would be the longest ever observed, and the broad Hα emission would be the most luminous ever observed at late times (〉10 yr), larger than that of unusually luminous SNe such as SN 1988Z, suggesting one of the most extreme episodes of pre-SN mass-loss ever discovered.

Description: The nearby Type Ia supernovae (SNe Ia) 2011fe and 2011by had nearly identical photospheric phase optical spectra, light-curve widths, and photometric colours, but at peak brightness SN 2011by reached a fainter absolute magnitude in all optical bands and exhibited lower flux in the near-ultraviolet (NUV). Based on those data, Foley & Kirshner argue that the progenitors of SNe 2011by and 2011fe had supersolar and subsolar metallicity, respectively, and that SN 2011fe generated 1.7 times the amount of 56 Ni as SN 2011by. With this work, we extend the comparison of these SNe Ia to 10 d before and 300 d after maximum brightness with new spectra and photometry. We show that the nebular phase spectra of SNe 2011fe and 2011by are almost identical, and do not support a factor of 1.7 difference in 56 Ni mass. Instead, we find it plausible that the Tully–Fisher distance for SN 2011by is an underestimate, in which case these SNe Ia may have reached similar peak luminosity, formed similar amounts of 56 Ni, and had lower metallicity progenitors than previously estimated. Regardless of the true distance to SN 2011by, we find that the relative progenitor metallicity difference remains well supported by their disparity in NUV flux, which we show to be even stronger at pre-maximum epochs – although contributions from differences in total ejecta mass, viewing angle, or progenitor density cannot be ruled out. We also demonstrate that, independent of distance modulus, SN 2011by exhibits a late-time luminosity excess that cannot be explained by a light echo, but is more likely to be the result of greater energy trapping by the nucleosynthetic products of SN 2011by.

Description: A growing subset of Type Ia supernovae (SNe Ia) shows evidence via narrow emission lines for unexpected interaction with a dense circumstellar medium (SNe IIn/Ia–CSM). The precise nature of the progenitor, however, remains debated owing to spectral ambiguities arising from a strong contribution from the CSM interaction. Late-time spectra offer potential insight if the post-shock cold, dense shell becomes sufficiently thin and/or the ejecta begin to cross the reverse shock. To date, only a few high-quality spectra of this kind exist. Here we report on the late-time optical and infrared spectra of the SNe Ia–CSM 2012ca and 2013dn. These SNe Ia–CSM spectra exhibit low [Fe iii ]/[Fe ii ] ratios and strong [Ca ii ] at late epochs. Such characteristics are reminiscent of the super-Chandrasekhar-mass candidate SN 2009dc, for which these features suggested a low-ionization state due to high densities, although the broad Fe features admittedly show similarities to the blue ‘quasi-continuum’ observed in some core collapse SNe Ibn and IIn. Neither SN 2012ca nor any of the other SNe Ia–CSM in this paper show evidence for broad oxygen, carbon, or magnesium in their spectra. Similar to the interacting Type IIn SN 2005ip, a number of high-ionization lines are identified in SN 2012ca, including [S iii ], [Ar iii ], [Ar x ], [Fe viii ], [Fe x ], and possibly [Fe xi ]. The total bolometric energy output does not exceed 10 51  erg, but does require a large kinetic-to-radiative conversion efficiency. All of these observations taken together suggest that SNe Ia–CSM are more consistent with a thermonuclear explosion than a core collapse event, although detailed radiative transfer models are certainly necessary to confirm these results.

Description: A growing subset of Type Ia supernovae (SNe Ia) shows evidence via narrow emission lines for unexpected interaction with a dense circumstellar medium (SNe IIn/Ia–CSM). The precise nature of the progenitor, however, remains debated owing to spectral ambiguities arising from a strong contribution from the CSM interaction. Late-time spectra offer potential insight if the post-shock cold, dense shell becomes sufficiently thin and/or the ejecta begin to cross the reverse shock. To date, only a few high-quality spectra of this kind exist. Here we report on the late-time optical and infrared spectra of the SNe Ia–CSM 2012ca and 2013dn. These SNe Ia–CSM spectra exhibit low [Fe iii ]/[Fe ii ] ratios and strong [Ca ii ] at late epochs. Such characteristics are reminiscent of the super-Chandrasekhar-mass candidate SN 2009dc, for which these features suggested a low-ionization state due to high densities, although the broad Fe features admittedly show similarities to the blue ‘quasi-continuum’ observed in some core collapse SNe Ibn and IIn. Neither SN 2012ca nor any of the other SNe Ia–CSM in this paper show evidence for broad oxygen, carbon, or magnesium in their spectra. Similar to the interacting Type IIn SN 2005ip, a number of high-ionization lines are identified in SN 2012ca, including [S iii ], [Ar iii ], [Ar x ], [Fe viii ], [Fe x ], and possibly [Fe xi ]. The total bolometric energy output does not exceed 10 51  erg, but does require a large kinetic-to-radiative conversion efficiency. All of these observations taken together suggest that SNe Ia–CSM are more consistent with a thermonuclear explosion than a core collapse event, although detailed radiative transfer models are certainly necessary to confirm these results.