Forbidden hugs in pandemic times: IV. Panchromatic evolution of three luminous red novae

We present photometric and spectroscopic data on three extragalactic luminous red novae (LRNe): AT 2018bwo, AT 2021afy, and AT 2021blu. AT 2018bwo was discovered in NGC 45 (at about 6.8 Mpc) a few weeks after the outburst onset. During the monitoring period, the transient reached a peak luminosity of 1040 erg s−1 . AT 2021afy, hosted by UGC 10043 (∼49.2 Mpc), showed a double-peaked light curve, with the two peaks reaching a similar luminosity of 2.1(±0.6) × 1041 erg s−1 . Finally, for AT 2021blu in UGC 5829 (∼8.6 Mpc), the pre-outburst phase was well-monitored by several photometric surveys, and the object showed a slow luminosity rise before the outburst. The light curve of AT 2021blu was sampled with an unprecedented cadence until the object disappeared behind the Sun, and it was then recovered at late phases. The light curve of LRN AT 2021blu shows a double peak, with a prominent early maximum reaching a luminosity of 6.5 × 1040 erg s−1 , which is half of that of AT 2021afy. The spectra of AT 2021afy and AT 2021blu display the expected evolution for LRNe: a blue continuum dominated by prominent Balmer lines in emission during the first peak, and a redder continuum consistent with that of a K-type star with narrow absorption metal lines during the second, broad maximum. The spectra of AT 2018bwo are markedly different, with a very red continuum dominated by broad molecular features in absorption. As these spectra closely resemble those of LRNe after the second peak, AT 2018bwo was probably discovered at the very late evolutionary stages. This would explain its fast evolution and the spectral properties compatible with that of an M-type star. From the analysis of deep frames of the LRN sites years before the outburst, and considerations of the light curves, the quiescent progenitor systems of the three LRNe were likely massive, with primaries ranging from about 13 M for AT 2018bwo, to 14+4 −1 M for AT 2021blu, and over 40 M for AT 2021afy

SN 2013df, a double-peaked IIb supernova from a compact progenitor and an extended H envelope

Optical observations of the type IIb SN 2013df from a few days to about 250 days after explosion are presented. These observations are complemented with UV photometry taken by \textit{SWIFT} up to 60 days post-explosion. The double-peak optical light curve is similar to those of SNe 1993J and 2011fu although with different decline and rise rates. From the modelling of the bolometric light curve, we have estimated that the total mass of synthesised $^{56}$Ni in the explosion is $\sim0.1$ M$_{\odot}$, while the ejecta mass is $0.8-1.4$ M$_{\odot}$ and the explosion energy $0.4-1.2 \times 10^{51}$erg. In addition, we have estimated a lower limit to the progenitor radius ranging from $64-169$ $R_{\odot}$. The spectral evolution indicates that SN 2013df had a hydrogen envelope similar to SN 1993J ($\sim 0.2$ M$_{\odot}$). The line profiles in nebular spectra suggest that the explosion was asymmetric with the presence of clumps in the ejecta, while the [O\,{\sc i}] $\lambda$$\lambda$$6300$, $6364$ luminosities, may indicate that the progenitor of SN 2013df was a relatively low mass star ( $\sim 12-13$ M$_{\odot}$).Comment: 18 pages, 11 figures, 9 tables, accepted for publication in MNRA

The supernova impostor PSN J09132750+7627410 and its progenitor

We report the results of our follow-up campaign of the supernova impostor PSN J09132750+7627410, based on optical data covering $\sim250\,\rm{d}$. From the beginning, the transient shows prominent narrow Balmer lines with P-Cygni profiles, with a blue-shifted absorption component becoming more prominent with time. Along the $\sim3\,\rm{months}$ of the spectroscopic monitoring, broad components are never detected in the hydrogen lines, suggesting that these features are produced in slowly expanding material. The transient reaches an absolute magnitude $M_r=-13.60\pm0.19\,\rm{mag}$ at maximum, a typical luminosity for supernova impostors. Amateur astronomers provided $\sim4\,\rm{years}$ of archival observations of the host galaxy, NGC 2748. The detection of the quiescent progenitor star in archival images obtained with the Hubble Space Telescope suggests it to be an $18-20$\msun white-yellow supergiant.Comment: 7 pages, 4 figures, supplemental material available in the source file. Accepted for publication on Astrophysical Journal Letter

Massive stars exploding in a He-rich circumstellar medium - VIII. PSN J07285387+3349106, a highly reddened supernova Ibn

We present spectroscopic and photometric observations for the Type Ibn supernova (SN) dubbed PSN J07285387+3349106. Using data provided by amateur astronomers, we monitored the photometric rise of the SN to maximum light, occurred on 2015 February 18.8 UT (JD(max,V) = 2457072.0 +- 0.8). PSN J07285387+3349106 exploded in the inner region of an infrared luminous galaxy, and is the most reddened SN Ibn discovered so far. We apply multiple methods to derive the total reddening to the SN, and determine a total colour excess E(B-V)(tot) = 0.99 +- 0.48 mag. Accounting for the reddening correction, which is affected by a large uncertainty, we estimate a peak absolute magnitude of M(V) = -20.30 +- 1.50. The spectra are dominated by continuum emission at early phases, and He I lines with narrow P-Cygni profiles are detected. We also identify weak Fe III and N II features. All these lines show an absorption component which is blue-shifted by about 900-1000 km/s. The spectra also show relatively broad He I line wings with low contrast, which extend to above 3000 km/s. From about 2 weeks past maximum, broad lines of O I, Mg II and the Ca II near-infrared triplet are identified. The composition and the expansion velocity of the circumstellar material, and the presence of He I and alpha-elements in the SN ejecta indicate that PSN J07285387+3349106 was produced by the core-collapse of a stripped-envelope star. We suggest that the precursor was WNE-type Wolf-Rayet star in its dense, He-rich circumstellar cocoon.Comment: 12 pages, 7 figures, 2 tables. Accepted for publication in MNRA

The multi-faceted Type II-L supernova 2014G from pre-maximum to nebular phase

We present multi-band ultraviolet, optical, and near-infrared photometry, along with visual-wavelength spectroscopy, of supernova (SN) 2014G in the nearby galaxy NGC 3448 (25 Mpc). The early-phase spectra show strong emission lines of the high ionisation species He II/N IV/C IV during the first 2-3 d after explosion, traces of a metal-rich CSM probably due to pre-explosion mass loss events. These disappear by day 9 and the spectral evolution then continues matching that of normal Type II SNe. The post-maximum light curve declines at a rate typical of Type II-L class. The extensive photometric coverage tracks the drop from the photospheric stage and constrains the radioactive tail, with a steeper decline rate than that expected from the $^{56}$Co decay if $\gamma$-rays are fully trapped by the ejecta. We report the appearance of an unusual feature on the blue-side of H$\alpha$ after 100 d, which evolves to appear as a flat spectral feature linking H$\alpha$ and the O I doublet. This may be due to interaction of the ejecta with a strongly asymmetric, and possibly bipolar CSM. Finally, we report two deep spectra at ~190 and 340 d after explosion, the latter being arguably one of the latest spectra for a Type II-L SN. By modelling the spectral region around the Ca II, we find a supersolar Ni/Fe production. The strength of the O I $\lambda\lambda$6300,6363 doublet, compared with synthetic nebular spectra, suggests a progenitor with a zero-age main-sequence mass between 15 and 19 M$_\odot$.Comment: 24 pages, 14 figure

Optical and near infrared observations of SN 2014ck: an outlier among the Type Iax supernovae

We present a comprehensive set of optical and near-infrared photometric and spectroscopic observations for SN 2014ck, extending from pre-maximum to six months later. These data indicate that SN 2014ck is photometrically nearly identical to SN 2002cx, which is the prototype of the class of peculiar transients named SNe Iax. Similar to SN 2002cx, SN 2014ck reached a peak brightness $M_B=-17.37 \pm 0.15$ mag, with a post-maximum decline-rate $\Delta m_{15} (B) = 1.76 \pm 0.15$ mag. However, the spectroscopic sequence shows similarities with SN 2008ha, which was three magnitudes fainter and faster declining. In particular, SN 2014ck exhibits extremely low ejecta velocities, $\sim 3000$ km s$^{-1}$ at maximum, which are close to the value measured for SN 2008ha and half the value inferred for SN 2002cx. The bolometric light curve of SN 2014ck is consistent with the production of $0.10^{+0.04}_{-0.03} M_{\odot}$ of $^{56}$Ni. The spectral identification of several iron-peak features, in particular Co II lines in the NIR, provides a clear link to SNe Ia. Also, the detection of narrow Si, S and C features in the pre-maximum spectra suggests a thermonuclear explosion mechanism. The late-phase spectra show a complex overlap of both permitted and forbidden Fe, Ca and Co lines. The appearance of strong [Ca~II] $\lambda\lambda$7292, 7324 again mirrors the late-time spectra of SN 2008ha and SN 2002cx. The photometric resemblance to SN 2002cx and the spectral similarities to SN 2008ha highlight the peculiarity of SN 2014ck, and the complexity and heterogeneity of the SNe Iax class.Comment: MNRAS Accepted 2016 March 22. Received 2016 March

SNhunt151: An explosive event inside a dense cocoon

Indexación: Scopus.We thank S. Spiro, R. Rekola, A. Harutyunyan, and M. L. Graham for their help with the observations. We are grateful to the collaboration of Massimo Conti, Giacomo Guerrini, Paolo Rosi, and Luz Marina Tinjaca Ramirez from the Osservatorio Astronomico Provinciale di Montarrenti. The staffs at the different observatories provided excellent assistance with the observations.The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 267251, ‘Astronomy Fellowships in Italy’ (AstroFIt)’. NE-R acknowledges financial support from MIUR PRIN 2010-2011, ‘The Dark Universe and the Cosmic Evolution of Baryons: From Current Surveys to Euclid’. NE-R, AP, SB, LT, MT, and GP are partially supported by the PRIN-INAF 2014 (project ‘Transient Universe: Unveiling New Types of Stellar Explosions with PESSTO’). GP acknowledges support provided by the Millennium Institute of Astrophysics (MAS) through grant IC120009 of the Programa Iniciativa Cientíifica Milenio del Ministerio de Economía, Fomento y Turismo de Chile. TK acknowledges financial support from the Emil Aaltonen Foundation. CRTS was supported by the NSF grants AST-0909182, AST-1313422, and AST-1413600. AVF is grateful for generous financial assistance from the Christopher R. Redlich Fund, the TABASGO Foundation, the Miller Institute for Basic Research in Science (UC Berkeley), and NASA/HST grant GO-14668 from the Space Telescope Science Institute, which is operated by AURA, Inc. under NASA contract NAS5-26555. The work of AVF was conducted in part at the Aspen Center for Physics, which is supported by NSF grantPHY-1607611; he thanks the Center for its hospitality during the neutron stars workshop in June and July 2017. NE-R acknowledges the hospitality of the ‘Institut de Ciències de l'Espai (CSIC), where this work was completed.This research is based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofísica de Canarias; the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma; the Italian Telescopio Nazionale Galileo (TNG), operated on the island of La Palma by the Fundaci Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias; the Liverpool Telescope, operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias with financial support from the UK Science and Technology Facilities Council; the 1.82-m Copernico Telescope and the Schmidt 67/92 cm of INAF-Asiago Observatory; the Catalina Real Time Survey (CRTS) Catalina Sky Survey (CSS) 0.7-m Schmidt Telescope; and the Las Cumbres Observatory (LCO) network. This work is also based in part on archival data obtained with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555; the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA (support was provided by NASA through an award issued by JPL/Caltech); and the Swift telescope.This work has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.SNhunt151 was initially classified as a supernova (SN) impostor (nonterminal outburst of a massive star). It exhibited a slow increase in luminosity, lasting about 450 d, followed by a major brightening that reaches M V ≈ -18 mag. No source is detected to M V ≳ -13 mag in archival images at the position of SNhunt151 before the slow rise. Low-to-mid-resolution optical spectra obtained during the pronounced brightening show very little evolution, being dominated at all times by multicomponent Balmer emission lines, a signature of interaction between the material ejected in the new outburst and the pre-existing circumstellar medium. We also analysed mid-infrared images from the Spitzer Space Telescope, detecting a source at the transient position in 2014 and 2015. Overall, SNhunt151 is spectroscopically a Type IIn SN, somewhat similar to SN 2009ip. However, there are also some differences, such as a slow pre-discovery rise, a relatively broad light-curve peak showing a longer rise time (~50 d), and a slower decline, along with a negligible change in the temperature around the peak (T ≤ 10 4 K). We suggest that SNhunt151 is the result of an outburst, or an SN explosion, within a dense circumstellar nebula, similar to those embedding some luminous blue variables like η Carinae and originating from past mass-loss events. © 2017 The Author(s).https://academic.oup.com/mnras/article/475/2/2614/479530

ASASSN-15no: The Supernova that plays hide-and-seek

We report the results of our follow-up campaign of the peculiar supernova ASASSN-15no, based on optical data covering ~300 days of its evolution. Initially the spectra show a pure blackbody continuum. After few days, the HeI 5876 A transition appears with a P-Cygni profile and an expansion velocity of about 8700 km/s. Fifty days after maximum, the spectrum shows signs typically seen in interacting supernovae. A broad (FWHM~8000 km/s) Halpha becomes more prominent with time until ~150 days after maximum and quickly declines later on. At these phases Halpha starts to show an intermediate component, which together with the blue pseudo-continuum are clues that the ejecta begin to interact with the CSM. The spectra at the latest phases look very similar to the nebular spectra of stripped-envelope SNe. The early part (the first 40 days after maximum) of the bolometric curve, which peaks at a luminosity intermediate between normal and superluminous supernovae, is well reproduced by a model in which the energy budget is essentially coming from ejecta recombination and 56Ni decay. From the model we infer a mass of the ejecta Mej = 2.6 Msun; an initial radius of the photosphere R0 = 2.1 x 10^14 cm; and an explosion energy Eexpl = 0.8 x 10^51 erg. A possible scenario involves a massive and extended H-poor shell lost by the progenitor star a few years before explosion. The shell is hit, heated and accelerated by the supernova ejecta. The accelerated shell+ejecta rapidly dilutes, unveiling the unperturbed supernova spectrum below. The outer ejecta start to interact with a H-poor external CSM lost by the progenitor system about 9 -- 90 years before the explosion.Comment: 11 pages, 4 figures, in press to MNRA

The transitional gap transient AT 2018hso: new insights into the luminous red nova phenomenon

Context. The absolute magnitudes of luminous red novae (LRNe) are intermediate between those of novae and supernovae (SNe), and show a relatively homogeneous spectro-photometric evolution. Although they were thought to derive from core instabilities in single stars, there is growing support for the idea that they are triggered by binary interaction that possibly ends with the merging of the two stars. Aims. AT 2018hso is a new transient showing transitional properties between those of LRNe and the class of intermediate-luminosity red transients (ILRTs) similar to SN 2008S. Through the detailed analysis of the observed parameters, our study supports that it actually belongs to the LRN class and was likely produced by the coalescence of two massive stars. Methods. We obtained ten months of optical and near-infrared photometric monitoring, and 11 epochs of low-resolution optical spectroscopy of AT 2018hso. We compared its observed properties with those of other ILRTs and LRNe. We also inspected the archival Hubble Space Telescope (HST) images obtained about 15 years ago to constrain the progenitor properties. Results. The light curves of AT 2018hso show a first sharp peak (reddening-corrected M-r = -13.93 mag), followed by a broader and shallower second peak that resembles a plateau in the optical bands. The spectra dramatically change with time. Early-time spectra show prominent Balmer emission lines and a weak [Ca II] doublet, which is usually observed in ILRTs. However, the strong decrease in the continuum temperature, the appearance of narrow metal absorption lines, the great change in the H alpha strength and profile, and the emergence of molecular bands support an LRN classification. The possible detection of a M-I similar to -8 mag source at the position of AT 2018hso in HST archive images is consistent with expectations for a pre-merger massive binary, similar to the precursor of the 2015 LRN in M101. Conclusions. We provide reasonable arguments to support an LRN classification for AT 2018hso. This study reveals growing heterogeneity in the observables of LRNe than has been thought previously, which is a challenge for distinguishing between LRNe and ILRTs. This suggests that the entire evolution of gap transients needs to be monitored to avoid misclassifications

SN 2015ba: A type IIP supernova with a long plateau

We present optical photometry and spectroscopy from about a week after explosion to $\sim$272 d of an atypical Type IIP supernova, SN 2015ba, which exploded in the edge-on galaxy IC 1029. SN 2015ba is a luminous event with an absolute V-band magnitude of -17.1$\pm$0.2 mag at 50 d since explosion and has a long plateau lasting for $\sim$123 d. The distance to the SN is estimated to be 34.8$\pm$0.7 Mpc using the expanding photosphere and standard candle methods. High-velocity H-Balmer components constant with time are observed in the late-plateau phase spectra of SN 2015ba, which suggests a possible role of circumstellar interaction at these phases. Both hydrodynamical and analytical modelling suggest a massive progenitor of SN 2015ba with a pre-explosion mass of 24-26 M$_\odot$. However, the nebular spectra of SN 2015ba exhibit insignificant levels of oxygen, which is otherwise expected from a massive progenitor. This might be suggestive of the non-monotonical link between O-core masses and the zero-age main-sequence mass of pre-supernova stars and/or uncertainties in the mixing scenario in the ejecta of supernovae.Comment: 42 pages, 7 pages Appendix, 20 figures, 10 tables, Accepted for publication in MNRAS, 14-June-201