We present extensive datasets for a class of intermediate-luminosity optical transients known as luminous red novae. They show double-peaked light curves, with an initial rapid luminosity rise to a ...blue peak (at −13 to −15 mag), which is followed by a longer-duration red peak that sometimes is attenuated, resembling a plateau. The progenitors of three of them (NGC 4490−2011OT1, M 101−2015OT1, and SNhunt248), likely relatively massive blue to yellow stars, were also observed in a pre-eruptive stage when their luminosity was slowly increasing. Early spectra obtained during the first peak show a blue continuum with superposed prominent narrow Balmer lines, with P Cygni profiles. Lines of Fe II are also clearly observed, mostly in emission. During the second peak, the spectral continuum becomes much redder, Hα is barely detected, and a forest of narrow metal lines is observed in absorption. Very late-time spectra (∼6 months after blue peak) show an extremely red spectral continuum, peaking in the infrared (IR) domain. Hα is detected in pure emission at such late phases, along with broad absorption bands due to molecular overtones (such as TiO, VO). We discuss a few alternative scenarios for luminous red novae. Although major instabilities of single massive stars cannot be definitely ruled out, we favour a common envelope ejection in a close binary system, with possibly a final coalescence of the two stars. The similarity between luminous red novae and the outburst observed a few months before the explosion of the Type IIn SN 2011ht is also discussed.
Extended optical and near-IR observations reveal that SN 2009dc shares a number of similarities with normal Type Ia supernovae (SNe Ia), but is clearly overluminous, with a (pseudo-bolometric) peak ...luminosity of log (L) = 43.47 (erg s−1). Its light curves decline slowly over half a year after maximum light Δm
15(B)true= 0.71, and the early-time near-IR light curves show secondary maxima, although the minima between the first and the second peaks are not very pronounced. The bluer bands exhibit an enhanced fading after ∼200 d, which might be caused by dust formation or an unexpectedly early IR catastrophe. The spectra of SN 2009dc are dominated by intermediate-mass elements and unburned material at early times, and by iron-group elements at late phases. Strong C ii lines are present until ∼2 weeks past maximum, which is unprecedented in thermonuclear SNe. The ejecta velocities are significantly lower than in normal and even subluminous SNe Ia. No signatures of interaction with a circumstellar medium (CSM) are found in the spectra. Assuming that the light curves are powered by radioactive decay, analytic modelling suggests that SN 2009dc produced ∼1.8 M⊙ of 56Ni assuming the smallest possible rise time of 22 d. Together with a derived total ejecta mass of ∼2.8 M⊙, this confirms that SN 2009dc is a member of the class of possible super-Chandrasekhar-mass SNe Ia similar to SNe 2003fg, 2006gz and 2007if. A study of the hosts of SN 2009dc and other superluminous SNe Ia reveals a tendency of these SNe to explode in low-mass galaxies. A low metallicity of the progenitor may therefore be an important prerequisite for producing superluminous SNe Ia. We discuss a number of possible explosion scenarios, ranging from super-Chandrasekhar-mass white-dwarf progenitors over dynamical white-dwarf mergers and Type I
SNe to a core-collapse origin of the explosion. None of the models seems capable of explaining all properties of SN 2009dc, so that the true nature of this SN and its peers remains nebulous.
We assemble a sample of 24 hydrogen-poor superluminous supernovae (SLSNe). Parameterizing the light-curve shape through rise and decline time-scales shows that the two are highly correlated. ...Magnetar-powered models can reproduce the correlation, with the diversity in rise and decline rates driven by the diffusion time-scale. Circumstellar interaction models can exhibit a similar rise–decline relation, but only for a narrow range of densities, which may be problematic for these models. We find that SLSNe are approximately 3.5 mag brighter and have light curves three times broader than SNe Ibc, but that the intrinsic shapes are similar. There are a number of SLSNe with particularly broad light curves, possibly indicating two progenitor channels, but statistical tests do not cleanly separate two populations. The general spectral evolution is also presented. Velocities measured from Fe ii are similar for SLSNe and SNe Ibc, suggesting that diffusion time differences are dominated by mass or opacity. Flat velocity evolution in most SLSNe suggests a dense shell of ejecta. If opacities in SLSNe are similar to other SNe Ibc, the average ejected mass is higher by a factor 2–3. Assuming κ = 0.1 cm2 g−1, we estimate a mean (median) SLSN ejecta mass of 10 M⊙ (6 M⊙), with a range of 3–30 M⊙. Doubling the assumed opacity brings the masses closer to normal SNe Ibc, but with a high-mass tail. The most probable mechanism for generating SLSNe seems to be the core collapse of a very massive hydrogen-poor star, forming a millisecond magnetar.
ABSTRACT We present the results of optical, near-infrared, and mid-infrared observations of M101 OT2015-1 (PSN J14021678+5426205), a luminous red transient in the Pinwheel galaxy (M101), spanning a ...total of 16 years. The light curve showed two distinct peaks with absolute magnitudes and , on 2014 November 11 and 2015 February 17, respectively. The spectral energy distributions during the second maximum show a cool outburst temperature of K and low expansion velocities ( km s−1) for the H i, Ca ii, Ba ii, and K i lines. From archival data spanning 15-8 years before the outburst, we find a single source consistent with the optically discovered transient, which we attribute to being the progenitor; it has properties consistent with being an F-type yellow supergiant with L ∼ 8.7 L , K, and an estimated mass of M . This star has likely just finished the H-burning phase in the core, started expanding, and is now crossing the Hertzsprung gap. Based on the combination of observed properties, we argue that the progenitor is a binary system, with the more evolved system overfilling the Roche lobe. Comparison with binary evolution models suggests that the outburst was an extremely rare phenomenon, likely associated with the ejection of the common envelope of a massive star. The initial mass of the primary fills the gap between the merger candidates V838 Mon (5−10 M ) and NGC 4490-OT (30 M ).
We present the results of the one-year long observational campaign of the type II plateau SN 2005cs, which exploded in the nearby spiral galaxy M51 (the Whirlpool galaxy). This extensive data set ...makes SN 2005cs the best observed low-luminosity, 56Ni-poor type II plateau event so far and one of the best core-collapse supernovae ever. The optical and near-infrared spectra show narrow P-Cygni lines characteristic of this SN family, which are indicative of a very low expansion velocity (about 1000 km s−1) of the ejected material. The optical light curves cover both the plateau phase and the late-time radioactive tail, until about 380 d after core-collapse. Numerous unfiltered observations obtained by amateur astronomers give us the rare opportunity to monitor the fast rise to maximum light, lasting about 2 d. In addition to optical observations, we also present near-infrared light curves that (together with already published ultraviolet observations) allow us to construct for the first time a reliable bolometric light curve for an object of this class. Finally, comparing the observed data with those derived from a semi-analytic model, we infer for SN 2005cs a 56Ni mass of about 3 × 10−3 M⊙, a total ejected mass of 8–13 M⊙ and an explosion energy of about 3 × 1050 erg.
In order to assess qualitatively the ejecta geometry of stripped-envelope core-collapse supernovae (SNe), we investigate 98 late-time spectra of 39 objects, many of them previously unpublished. We ...perform a Gauss-fitting of the O iλλ6300, 6364 feature in all spectra, with the position, full width at half maximum and intensity of the λ6300 Gaussian as free parameters, and the λ6364 Gaussian added appropriately to account for the doublet nature of the O i feature. On the basis of the best-fitting parameters, the objects are organized into morphological classes, and we conclude that at least half of all Type Ib/c SNe must be aspherical. Bipolar jet models do not seem to be universally applicable, as we find too few symmetric double-peaked O i profiles. In some objects, the O i line exhibits a variety of shifted secondary peaks or shoulders, interpreted as blobs of matter ejected at high velocity and possibly accompanied by neutron-star kicks to assure momentum conservation. At phases earlier than ∼200 d, a systematic blueshift of the O iλλ6300, 6364 line centroids can be discerned. Residual opacity provides the most convincing explanation of this phenomenon, photons emitted on the rear side of the SN being scattered or absorbed on their way through the ejecta. Once modified to account for the doublet nature of the oxygen feature, the profile of Mg iλ4571 at sufficiently late phases generally resembles that of O iλλ6300, 6364, suggesting negligible contamination from other lines and confirming that O and Mg are similarly distributed within the ejecta.
Superluminous supernovae (SLSNe) of Type Ic have a tendency to occur in faint host galaxies which are likely to have low mass and low metallicity. PTF12dam is one of the closest and best-studied ...superluminous explosions that has a broad and slowly fading light curve similar to SN 2007bi. Here we present new photometry and spectroscopy for PTF12dam from 200–500 d (rest frame) after peak and a detailed analysis of the host galaxy (SDSS J142446.21+461348.6 at z = 0.107). Using deep templates and image subtraction we show that the light curve can be fit with a magnetar model if escape of high-energy gamma rays is taken into account. The full bolometric light curve from −53 to +399 d (with respect to peak) cannot be fit satisfactorily with the pair-instability models. An alternative model of interaction with a dense circumstellar material (CSM) produces a good fit to the data although this requires a very large mass (∼13 M⊙) of hydrogen-free CSM. The host galaxy is a compact dwarf (physical size ∼1.9 kpc) and with M
g
= −19.33 ± 0.10, it is the brightest nearby SLSN Ic host discovered so far. The host is a low-mass system (2.8 × 108 M⊙) with a star formation rate (5.0 M⊙ yr−1), which implies a very high specific star formation rate (17.9 Gyr−1). The remarkably strong nebular emission provide detections of the O iii λ4363 and O ii λλ7320, 7330auroral lines and an accurate oxygen abundance of 12 + log (O/H) = 8.05 ± 0.09. We show here that they are at the extreme end of the metallicity distribution of dwarf galaxies and propose that low metallicity is a requirement to produce these rare and peculiar SNe.
2002cx-like supernovae are a sub-class of sub-luminous Type Ia supernovae (SNe). Their light curves and spectra are characterized by distinct features that indicate strong mixing of the explosion ...ejecta. Pure turbulent deflagrations have been shown to produce such mixed ejecta. Here, we present hydrodynamics, nucleosynthesis and radiative-transfer calculations for a 3D full-star deflagration of a Chandrasekhar-mass white dwarf. Our model is able to reproduce the characteristic observational features of SN 2005hk (a prototypical 2002cx-like supernova), not only in the optical, but also in the near-infrared. For that purpose we present, for the first time, five near-infrared spectra of SN 2005hk from −0.2 to 26.6 d with respect to B-band maximum. Since our model burns only small parts of the initial white dwarf, it fails to completely unbind the white dwarf and leaves behind a bound remnant of ∼1.03 M - consisting mainly of unburned carbon and oxygen, but also enriched by some amount of intermediate-mass and iron-group elements from the explosion products that fall back on the remnant. We discuss possibilities for detecting this bound remnant and how it might influence the late-time observables of 2002cx-like SNe.
The photometric and spectroscopic properties of 26 well-observed Type Ia Supernovae (SNe Ia) were analyzed with the aim of exploring SN Ia diversity. The sample includes (Branch) normal SNe, as well ...as extreme events such as SN 1991T and SN 1991bg, while the truly peculiar SNe Ia, SN 2000cx and SN 2002cx, are not included in our sample. A statistical treatment reveals the existence of three different groups. The first group (FAINT) consists of faint SNe Ia similar to SN 1991bg, with low expansion velocities and rapid evolution of Si II velocity. A second group consists of normal SNe Ia, also with high temporal velocity gradient (HVG), but with brighter mean absolute magnitude < M sub(B) > = -19.3 and higher expansion velocities than the FAINT SNe. The third group includes both normal and SN 1991T-like SNe Ia: these SNe populate a narrow strip in the Si II velocity evolution plot, with a low-velocity gradient (LVG), but have absolute magnitudes similar to HVGs. While the FAINT and HVG SNe Ia together seem to define a relation between R(Si II) and Delta m sub(15)(B), the LVG SNe either do not conform to that relation or define a new, looser one. The R (Si II) premaximum evolution of HVGs is strikingly different from that of LVGs. We discuss the impact of this evidence on the understanding of SN Ia diversity, in terms of explosion mechanisms, degree of ejecta mixing, and ejecta-circumstellar material interaction.
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