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We present 75 near-infrared (NIR; 0.8-2.5 μm) spectra of 34 stripped-envelope core-collapse supernovae (SESNe) obtained by the Carnegie Supernova Project-II (CSP-II), encompassing optical spectroscopic Types IIb, Ib, Ic, and Ic-BL. The spectra range in phase from pre-maximum to 80 days past maximum. This unique data set constitutes the largest NIR spectroscopic sample of SESNe to date. NIR spectroscopy provides observables with additional information that is not available in the optical. Specifically, the NIR contains the strong lines of He i and allows a more detailed look at whether Type Ic supernovae are completely stripped of their outer He layer. The NIR spectra of SESNe have broad similarities, but closer examination through statistical means reveals a strong dichotomy between NIR "He-rich"and "He-poor"SNe. These NIR subgroups correspond almost perfectly to the optical IIb/Ib and Ic/Ic-BL types, respectively. The largest difference between the two groups is observed in the 2 μm region, near the He i λ2.0581 μm line. The division between the two groups is not an arbitrary one along a continuous sequence. Early spectra of He-rich SESNe show much stronger He i λ2.0581 μm absorption compared to the He-poor group, but with a wide range of profile shapes. The same line also provides evidence for trace amounts of He in half of our SNe in the He-poor group., AST-1008343, AST-1613426, AST-1613455, and AST-1613472. C.A. is supported by NASA grant No. 80NSSC19K1717 and NSF grant Nos. AST-1920392 and AST-1911074. J.T. is funded by the consolidated STFC grant No. R276106. L.G. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (MICIU) under the 2019 Ramón y Cajal program RYC2019-027683 and from the Spanish MICIU project PID2020-115253GA-I00. C.G. is supported by a Young Investor Grant (25501) from the VILLUM FONDEN. Time domain research by D.J.S. is also supported by NSF grant Nos. AST-1821987, 1813466, 1908972, & 2108032, and by the Heising-Simons Foundation under grant #2020-1864. M.D.S. is supported by grants from the VILLUM FONDEN (grant No. 28021) and the Independent Research Fund Denmark (IRFD; 8021-00170B).

Lu, J., et al., We present photometric and spectroscopic observations of the 03fg-like Type Ia supernova (SN Ia) ASASSN-15hy from the ultraviolet (UV) to the near-infrared (NIR). ASASSN-15hy shares many of the hallmark characteristics of 03fg-like SNe Ia, previously referred to as "super-Chandrasekhar"SNe Ia. It is bright in the UV and NIR, lacks a clear i-band secondary maximum, shows a strong and persistent C ii feature, and has a low Si ii λ6355 velocity. However, some of its properties are also extreme among the subgroup. ASASSN-15hy is underluminous (M B,peak = 19.14-0.16+0.11 mag), red ((B-V) {B\max }=0.18-0.03+0.01 mag), yet slowly declining (Δm 15(B) = 0.72 ± 0.04 mag). It has the most delayed onset of the i-band maximum of any 03fg-like SN. ASASSN-15hy lacks the prominent H-band break emission feature that is typically present during the first month past maximum in normal SNe Ia. Such events may be a potential problem for high-redshift SN Ia cosmology. ASASSN-15hy may be explained in the context of an explosion of a degenerate core inside a nondegenerate envelope. The explosion impacting the nondegenerate envelope with a large mass provides additional luminosity and low ejecta velocities. An initial deflagration burning phase is critical in reproducing the low 56Ni mass and luminosity, while the large core mass is essential in providing the large diffusion timescales required to produce the broad light curves. The model consists of a rapidly rotating 1.47 M o˙ degenerate core and a 0.8 M o˙ nondegenerate envelope. This "deflagration core-degenerate"scenario may result from the merger between a white dwarf and the degenerate core of an asymptotic giant branch star., The CSP-II has been supported by NSF grants AST-1008343, AST1613426, AST-1613455, AST-1613472, and the Danish Agency for Science and Technology and Innovation through a Sapere Aude Level 2 grant (PI: M.S.) C.A. is supported by the NSF grant AST #1908952. J.L., S.K., M.S., and E.Y.H acknowledge the support provided by the Florida Space Research Program. P.A.H. acknowledges the support by the National Science Foundation grant AST-1715133. L.G. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (MICIU) under the 2019 Ramón y Cajal program RYC2019-027683 and from the Spanish MICIU project PID2020-115253GA-I00. E.B. was supported in part by NASA grant 80NSSC20K0538. M.D.S. is supported by generous grants from Villum FONDEN (13261, 28021) and by a project grant (8021-00170B) from the Independent Research Fund Denmark. Support for T.W.-S.H. was provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51458.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. H.K. was funded by the Academy of Finland projects 324504 and 328898. Support for J.L.P. is provided in part by ANID through the Fondecyt regular grant 1191038 and through the Millennium Science Initiative grant ICN12_009, awarded to The Millennium Institute of Astrophysics, MAS. Time-domain research by D.J.S. is supported by NSF grants AST-1821987, 1813466, & 1908972, and by the Heising-Simons Foundation under grant #2020-1864. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile, and is also based upon observations made with ESO Telescopes at the La Silla or Paranal Observatories under program ID(s) 191. D-0935 and 099.D-0022(A). This work has been partially supported by the Spanish grant PGC2018-095317-B-C21 within the European Funds for Regional Development (FEDER). Finally, this paper also includes observations obtained at the international Gemini Observatory (GN2015A-Q-8, GS-2015A-Q-5), a program of NSFʼs NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. on behalf of the Gemini Observatory partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea).

This paper describes the rapidly evolving and unusual supernova LSQ13ddu, discovered by the La Silla-QUEST survey. LSQ13ddu displayed a rapid rise of just 4.8$\pm$0.9 d to reach a peak brightness of $-$19.70$\pm$0.02 mag in the $\mathit{LSQgr}$ band. Early spectra of LSQ13ddu showed the presence of weak and narrow He I features arising from interaction with circumstellar material (CSM). These interaction signatures weakened quickly, with broad features consistent with those seen in stripped-envelope SNe becoming dominant around two weeks after maximum. The narrow He I velocities are consistent with the wind velocities of luminous blue variables but its spectra lack the typically seen hydrogen features. The fast and bright early light curve is inconsistent with radioactive $^{56}$Ni powering but can be explained through a combination of CSM interaction and an underlying $^{56}$Ni decay component that dominates the later time behaviour of LSQ13ddu. Based on the strength of the underlying broad features, LSQ13ddu appears deficient in He compared to standard SNe Ib., Comment: 22 pages, 20 figures; accepted for publication in MNRAS

Supernova (SN) 2017cbv in NGC 5643 is one of a handful of type Ia supernovae (SNe~Ia) reported to have excess blue emission at early times. This paper presents extensive $BVRIYJHK_s$-band light curves of SN 2017cbv, covering the phase from $-16$ to $+125$ days relative to $B$-band maximum light. SN 2017cbv reached a $B$-band maximum of 11.710$\pm$0.006~mag, with a post-maximum magnitude decline $\Delta m_{15}(B)$=0.990$\pm$0.013 mag. The supernova suffered no host reddening based on Phillips intrinsic color, Lira-Phillips relation, and the CMAGIC diagram. By employing the CMAGIC distance modulus $\mu=30.58\pm0.05$~mag and assuming $H_0$=72~$\rm km \ s^{-1} \ Mpc^{-1}$, we found that 0.73~\msun $^{56}$Ni was synthesized during the explosion of SN 2017cbv, which is consistent with estimates using reddening-free and distance-free methods via the phases of the secondary maximum of the NIR-band light curves. We also present 14 near-infrared spectra from $-18$ to $+49$~days relative to the $B$-band maximum light, providing constraints on the amount of swept-up hydrogen from the companion star in the context of the single degenerate progenitor scenario. No $Pa{\beta}$ emission feature was detected from our post-maximum NIR spectra, placing a hydrogen mass upper limit of 0.1 $M_{\odot}$. The overall optical/NIR photometric and NIR spectral evolution of SN 2017cbv is similar to that of a normal SN~Ia, even though its early evolution is marked by a flux excess no seen in most other well-observed normal SNe~Ia. We also compare the exquisite light curves of SN 2017cbv with some $M_{ch}$ DDT models and sub-$M_{ch}$ double detonation models.

We present $81$ near-infrared (NIR) spectra of $30$ Type II supernovae (SNe II) from the Carnegie Supernova Project-II (CSP-II), the largest such dataset published to date. We identify a number of NIR features and characterize their evolution over time. The NIR spectroscopic properties of SNe II fall into two distinct groups. This classification is first based on the strength of the He I $\lambda1.083\,\mu$m absorption during the plateau phase; SNe II are either significantly above (spectroscopically strong) or below $50$ angstroms (spectroscopically weak) in pseudo equivalent width. However between the two groups, other properties, such as the timing of CO formation and the presence of Sr II, are also observed. Most surprisingly, the distinct weak and strong NIR spectroscopic classes correspond to SNe II with slow and fast declining light curves, respectively. These two photometric groups match the modern nomenclature of SNe IIP and IIL. Including NIR spectra previously published, 18 out of 19 SNe II follow this slow declining-spectroscopically weak and fast declining-spectroscopically strong correspondence. This is in apparent contradiction to the recent findings in the optical that slow and fast decliners show a continuous distribution of properties. The weak SNe II show a high-velocity component of helium that may be caused by a thermal excitation from a reverse-shock created by the outer ejecta interacting with the red supergiant wind, but the origin of the observed dichotomy is not understood. Further studies are crucial in determining whether the apparent differences in the NIR are due to distinct physical processes or a gap in the current data set., Comment: 32 pages, 17 figures, accepted to ApJ

We present and analyze a near-infrared (NIR) spectrum of the underluminous Type Ia supernova SN 2020qxp/ASASSN-20jq obtained with NIRES at the Keck Observatory, 191 days after B-band maximum. The spectrum is dominated by a number of broad emission features, including the [Fe ii] at 1.644 μm, which is highly asymmetric with a tilted top and a peak redshifted by ≈2000 km s-1. In comparison with 2D non-LTE synthetic spectra computed from 3D simulations of off-center delayed-detonation Chandrasekhar-mass (Mch) white dwarf (WD) models, we find good agreement between the observed lines and the synthetic profiles, and are able to unravel the structure of the progenitor's envelope. We find that the size and tilt of the [Fe ii] 1.644 μm profile (in velocity space) is an effective way to determine the location of an off-center delayed-detonation transition (DDT) and the viewing angle, and it requires a WD with a high central density of ∼4 × 109 g cm-3. We also tentatively identify a stable Ni feature around 1.9 μm characterized by a "pot-belly"profile that is slightly offset with respect to the kinematic center. In the case of SN 2020qxp/ASASSN-20jq, we estimate that the location of the DDT is ∼0.3MWD off center, which gives rise to an asymmetric distribution of the underlying ejecta. We also demonstrate that low-luminosity and high-density WD SN Ia progenitors exhibit a very strong overlap of Ca and 56Ni in physical space. This results in the formation of a prevalent [Ca ii] 0.73 μm emission feature that is sensitive to asymmetry effects. Our findings are discussed within the context of alternative scenarios, including off-center C/O detonations in He-triggered sub-MCh WDs and the direct collision of two WDs. Snapshot programs with Gemini/Keck/Very Large Telescope (VLT)/ELT-class instruments and our spectropolarimetry program are complementary to mid-IR spectra by the James Webb Space Telescope (JWST)., P.H., A.F., B.H., D.C., and E.Y.H. acknowledge support by the National Science Foundation (NSF) grant AST1715133. E.B. was supported in part by NASA grant 80NSSC20K0538. C.A. and B.J.S. are supported by NASA grant 80NSSC19K1717 and NSF grants AST-1920392 and AST-1911074. M.S. is supported by grants from the Villum FONDEN (28021) and the Independent Research Fund Denmark (8021-00170B). M.A.T. acknowledges support from the DOE CSGF through grant DE-SC0019323. L.G. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (MICIU) under the 2019 Ramón y Cajal program RYC2019-027683 and from the Spanish MICIU project PID2020-115253GA-I00. C.R.B., E.Y.H., M. M.P., N.B.S. and P.H. are supported by the NSF grant AST1613472.

Research by D.J.S. is supported by NSF grants AST-1821967, AST-1821987, AST-1813708, AST-1813466, and AST-1908972, as well as by the Heising-Simons Foundation under grant #20201864. The CSP-II has been supported by National Science Foundation (NSF) grants AST-1008343, AST-1613426, AST-1613455, and AST-1613472, as well as by the Danish Agency for Science and Technology and Innovation through a Sapere Aude Level 2 grant. E.Y.H. and J.L. also acknowledge the support of the Florida Space Grant Consortium. This work was partially performed at the Aspen Center for Physics, which is supported by NSF grant PHY-1607611. Research by S.V. is supported by NSF grants AST1813176 and AST-2008108. L.G. was funded by the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 839090. This work has been partially supported by the Spanish grant PGC2018-095317-B-C21 within the European Funds for Regional Development (FEDER). M.S. is supported by generous grants from Villum FONDEN (13261, 28021) and by a project grant (8021-00170B) from the Independent Research Fund Denmark. A.V.F. is grateful for financial assistance from the TABASGO Foundation, the Christopher R. Redlich Fund, and the Miller Institute for Basic Research in Science (U.C. Berkeley). Based on observations obtained at the international Gemini Observatory (GN-2015A-Q-8, GS-2015A-Q-5), a program of NSF's NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. on behalf of the Gemini Observatory partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigacion y Desarrollo (Chile), Ministerio de Ciencia, Tecnologia e Innovacion (Argentina), Ministerio da Ciencia, Tecnologia, Inovacoes e Comunicacoes (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). This paper includes data gathered with the Nordic Optical Telescope (PI Stritzinger) at the Observatorio del Roque de los Muchachos, La Palma, Spain. This work is based in part on observations from the Deep Imaging Multi-Object Spectrograph at the Keck II telescope. We are grateful to the staff at the Keck Observatory for their assistance, and we extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA; it was made possible by the generous financial support of the W. M. Keck Foundation. We thank S. Bradley Cenko for assistance with the Keck spectral reductions, as well as Patrick Kelly, WeiKang Zheng, and John Mauerhan for their assistance with the observations. D.J.S. is a visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract 80HQTR19D0030 with the National Aeronautics and Space Administration. Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes 188.D-3003 and 191.D-0935: PESSTO (the Public ESO Spectroscopic Survey for Transient Objects)., Unburned carbon is potentially a powerful probe of Type Ia supernova (SN) explosion mechanisms. We present comprehensive optical and near-infrared (NIR) data on the "transitional" Type Ia SN 2015bp. An early NIR spectrum (t=−9.9 days with respect to B-band maximum) displays a striking C I λ1.0693μm line at 11.9×103~km s−1, distinct from the prominent Mg II λ1.0927μm feature, which weakens toward maximum light. SN 2015bp also displays a clear C II λ6580A notch early (t=−10.9 days) at 13.2×103~km s−1, consistent with our NIR carbon detection. At MB=−18.46, SN 2015bp is less luminous than a normal SN Ia and, along with iPTF13ebh, is the second member of the transitional subclass to display prominent early-time NIR carbon absorption. We find it unlikely that the C I feature is misidentified He I λ1.0830μm because this feature grows weaker toward maximum light, while the helium line produced in some double-detonation models grows stronger at these times. Intrigued by these strong NIR carbon detections, but lacking NIR data for other SNe Ia, we investigated the incidence of optical carbon in the sample of nine transitional SNe Ia with early-time data (t≲−4 days). We find that four display C II λ6580A, while two others show tentative detections, in line with the SN Ia population as a whole. We conclude that at least ∼50% of transitional SNe Ia in our sample do not come from sub-Chandrasekhar mass explosions due to the clear presence of carbon in their NIR and optical spectra., National Science Foundation (NSF) AST-1821967 AST-1821987 AST-1813708 AST-1813466 AST-1908972 PHY-1607611 AST1813176 AST-2008108, Heising-Simons Foundation 20201864, National Science Foundation (NSF) AST-1008343 AST-1613426 AST-1613455 AST-1613472, Danish Agency for Science and Technology and Innovation, Florida Space Grant Consortium, European Commission 839090, Spanish grant within the European Funds for Regional Development (FEDER) PGC2018-095317-B-C21, Villum Foundation 13261 28021, Independent Research Fund Denmark 8021-00170B, TABASGO Foundation, Christopher R. Redlich Fund, Miller Institute for Basic Research in Science (U.C. Berkeley), International Gemini Observatory GN-2015A-Q-8 GS-2015A-Q-5, W.M. Keck Foundation, University of Hawaii 80HQTR19D0030, National Aeronautics & Space Administration (NASA), ESO Telescopes at the La Silla Paranal Observatory 188.D-3003 191.D-0935: PESSTO

Accreting supermassive black holes (SMBHs) can exhibit variable emission across the electromagnetic spectrum and over a broad range of timescales. The variability of active galactic nuclei (AGNs) in the ultraviolet and optical is usually at the few tens of per cent level over timescales of hours to weeks. Recently, rare, more dramatic changes to the emission from accreting SMBHs have been observed, including tidal disruption events, 'changing look' AGNs and other extreme variability objects. The physics behind the 're-ignition', enhancement and 'shut-down' of accretion onto SMBHs is not entirely understood. Here we present a rapid increase in ultraviolet-optical emission in the centre of a nearby galaxy, marking the onset of sudden increased accretion onto a SMBH. The optical spectrum of this flare, dubbed AT 2017bgt, exhibits a mix of emission features. Some are typical of luminous, unobscured AGNs, but others are likely driven by Bowen fluorescence - robustly linked here with high-velocity gas in the vicinity of the accreting SMBH. The spectral features and increased ultraviolet flux show little evolution over a period of at least 14 months. This disfavours the tidal disruption of a star as their origin, and instead suggests a longer-term event of intensified accretion. Together with two other recently reported events with similar properties, we define a new class of SMBH-related flares. This has important implications for the classification of different types of enhanced accretion onto SMBHs., Comment: Published in Nature Astronomy

We show how new and upcoming advances in the age of time-domain and multi-wavelength astronomy will open up a new venue to probe the diversity of SN Ia. We discuss this in the context of the ELT (ESO), as well as space based instrument such as James Webb Space Telescope (JWST). As examples we demonstrate how the power of very early observations, within hours to days after the explosion, and very late-time observations, such as light curves and mid-infrared spectra beyond 3 years, can be used to probe the link to progenitors and explosion scenarios. We identify the electron-capture cross sections of Cr, Mn, and Ni/Co as one of the limiting factors we will face in the future.

Indexación: Scopus. E.Y.H. acknowledges the support provided by the National Science Foundation under Grant No. AST-1613472 and by the Florida Space Grant Consortium. L.G. acknowledges support from the FINCA visitor programme. The research work at the Physical Research Laboratory is funded by the Department of Space, Government of India. Facility: Magellan: Baade(FIRE). The enigmatic near-infrared transient VVV-WIT-06 underwent a large-amplitude eruption of unclear origin in 2013 July. Based on its light curve properties and late-time post-outburst spectra, various possibilities have been proposed in the literature for the origin of the object, namely a Type I supernova, a classical nova (CN), or a violent stellar merger event. We show that, of these possibilities, an origin in a CN outburst convincingly explains the observed properties of VVV-WIT-06. We estimate that the absolute K-band magnitude of the nova at maximum was M k = -8.2 ±0.5, its distance d = 13.35 ±2.18 kpc, and the extinction A v = 15.0 ±0.55 mag. © 2018. The American Astronomical Society. All rights reserved. https://iopscience.iop.org/article/10.3847/1538-4357/aae5d3

As the closest Type Ia supernova in decades, SN 2014J provides a unique opportunity for detailed investigation into observational signatures of the progenitor system and explosion mechanism in addition to burning product distribution. We present a late-time near-infrared spectral series from Gemini-N at $307-466$ days after the explosion. Following the $H$-band evolution probes the distribution of radioactive iron group elements, the extent of mixing, and presence of magnetic fields in the expanding ejecta. Comparing the isolated $1.6440$ $\mu$m [Fe II] emission line with synthetic models shows consistency with a Chandrasekhar-mass white dwarf of $\rho_c=0.7\times10^9$ g cm${}^{-3}$ undergoing a delayed detonation. The ratio of the flux in the neighboring $1.54$ $\mu$m emission feature to the flux in the $1.6440$ $\mu$m feature shows evidence of some limited mixing of stable and radioactive iron group elements in the central regions. Additionally, the evolution of the $1.6440$ $\mu$m line shows an intriguing asymmetry. When measuring line-width of this feature, the data show an increase in line width not seen in the evolution of the synthetic spectra, corresponding to $\approx1{,}000$ km s${}^{-1}$, which could be caused by a localized transition to detonation producing asymmetric ionization in the ejecta. Using the difference in width between the different epochs, an asymmetric component in the central regions, corresponding to approximately the inner $2\times10^{-4}$ of white dwarf mass suggests an off-center ignition of the initial explosion and hence of the kinematic center from the chemical center. Several additional models investigated, including a He detonation and a merger, have difficulty reproducing the features seen these spectra., Comment: 21 pages, 16 figures, accepted in ApJ

This article presents the study of the effects of magnetic fields on non-distributed nuclear burning fronts as a possible solution to a fundamental problem for the thermonuclear explosion of a Chandrasekhar mass ($M_{Ch}$) white dwarf (WD), the currently favored scenario for the majority of Type Ia SNe (SNe~Ia). All existing 3D hydrodynamical simulations predict strong global mixing of the burning products due to Rayleigh-Taylor (RT) instabilities, which is in contradiction with observations. As a first step and to study the flame physics we present a set of computational magneto-hydrodynamic (MHD) models in rectangular flux tubes, resembling a small inner region of a WD. We consider initial magnetic fields up to $10^{12}\,\,\mathrm{G}$ of various orientations. We find an increasing suppression of RT instabilities starting at about $10^9\,\,\mathrm{G}$. The front speed tends to decrease with increasing magnitude up to about $10^{11}\,\,\mathrm{G}$. For even higher fields new small scale finger-like structures develop, which increase the burning speed by a factor of 3 to 4 above the field-free RT-dominated regime. We suggest that the new instability may provide sufficiently accelerated energy production during the distributed burning regime to go over the Chapman-Jougey limit and trigger a detonation. Finally we discuss the possible origins of high magnetic fields during the final stage of the progenitor evolution or the explosion., Accepted by ApJ

We present infrared spectroscopy of the classical nova V339 Delphini, obtained over a $\sim2$ year period. The infrared emission lines were initially symmetrical, with HWHM velocities of 525 km s$^{-1}$. In later ($t\gtrsim77$days, where $t$ is the time from outburst) spectra however, the lines displayed a distinct asymmetry, with a much stronger blue wing, possibly due to obscuration of the receding component by dust. Dust formation commenced at $\sim$ day 34.75 at a condensation temperature of $1480\pm20$K, consistent with graphitic carbon. Thereafter the dust temperature declined with time as $T_{\rm d}\propto{t}^{-0.346}$, also consistent with graphitic carbon. The mass of dust initally rose, as a result of an increase in grain size and/or number, peaked at $\sim$ day 100, and then declined precipitously. This decline was most likely caused by grain shattering due to electrostatic stress after the dust was exposed to X-radiation. An Appendix summarises Planck Means for carbon, and the determination of grain mass and radius for a carbon dust shell., Accepted by MNRAS on 19 December 2016. 19 pages

We study optical light curve(LC) relations of type Ia supernovae(SNe~Ia) for their use in cosmology using high-quality photometry published by the Carnegie-Supernovae-Project (CSP-I). We revisit the classical luminosity-decline-rate ($\Delta m_{15}$) relation and the Lira-relation, as well as investigate the time evolution of the ($B-V$) color and $B(B-V)$, which serves as the basis of the color-stretch relation and Color-MAGnitude-Intercept-Calibrations(CMAGIC). Our analysis is based on explosion and radiation transport simulations for spherically-symmetric delayed-detonation models(DDT) producing normal-bright and subluminous SNe~Ia. Empirical LC-relations can be understood as having the same physical underpinnings: i.e. the opacities, ionization balances in the photosphere, and radioactive energy deposition changing with time from below to above the photosphere. Some 3-4 weeks past maximum, the photosphere recedes to ${}^{56}$Ni-rich layers of similar density structure, leading to a similar color evolution. An important secondary parameter is the central density $\rho_c$ of the WD because at higher densities more electron capture elements are produced at the expense of ${}^{56}$Ni production. This results in a $\Delta m_{15}$ spread of 0.1 mag for normal-bright and 0.7 mag in sub-luminous SNe~Ia and $\approx0.2$ mag in the Lira-relation. We show why color-magnitude diagrams emphasize the transition between physical regimes, and allow to construct templates depend mostly on $\Delta m_{15}$ with little dispersion in both the CSP-I sample and our DDT-models. This allows to separate intrinsic SN~Ia variations from the interstellar reddening characterized by $E(B-V)$ and $R_{B}$. Mixing of different explosion scenarios causes a wide spread in empirical relations which may suggest one dominant scenario., Comment: 49 pages, 17 figures, 3 tables accepted (Original Version: 03/29/2017; revised: 6/30/2017; accepted: 8/4/2017)

Aims. We present a comprehensive dataset of optical and near-infrared photometry and spectroscopy of type Ia supernova (SN) 2016hnk, combined with integral field spectroscopy (IFS) of its host galaxy, MCG -01-06-070, and nearby environment. Our goal with this complete dataset is to understand the nature of this peculiar object. Methods. Properties of the SN local environment are characterized by means of single stellar population synthesis applied to IFS observations taken two years after the SN exploded. We performed detailed analyses of SN photometric data by studying its peculiar light and color curves. SN 2016hnk spectra were compared to other 1991bg-like SNe Ia, 2002es-like SNe Ia, and Ca-rich transients. In addition, we used abundance stratification modeling to identify the various spectral features in the early phase spectral sequence and also compared the dataset to a modified non-LTE model previously produced for the sublumnious SN 1999by. Results. SN 2016hnk is consistent with being a subluminous (MB = −16.7 mag, sBV=0.43 ± 0.03), highly reddened object. The IFS of its host galaxy reveals both a significant amount of dust at the SN location, residual star formation, and a high proportion of old stellar populations in the local environment compared to other locations in the galaxy, which favors an old progenitor for SN 2016hnk. Inspection of a nebular spectrum obtained one year after maximum contains two narrow emission lines attributed to the forbidden [Ca II] λλ7291,7324 doublet with a Doppler shift of 700 km s−1. Based on various observational diagnostics, we argue that the progenitor of SN 2016hnk was likely a near Chandrasekhar-mass (MCh) carbon-oxygen white dwarf that produced 0.108 M⊙ of 56Ni. Our modeling suggests that the narrow [Ca II] features observed in the nebular spectrum are associated with 48Ca from electron capture during the explosion, which is expected to occur only in white dwarfs that explode near or at the MCh limit.

We present near infrared (NIR) spectroscopic and photometric observations of the nearby Type Ia SN 2014J. The seventeen NIR spectra span epochs from +15.3 to +92.5 days after $B$-band maximum light, while the $JHK_s$ photometry include epochs from $-$10 to +71 days. This data is used to constrain the progenitor system of SN 2014J utilizing the Pa$\beta$ line, following recent suggestions that this phase period and the NIR in particular are excellent for constraining the amount of swept up hydrogen-rich material associated with a non-degenerate companion star. We find no evidence for Pa$\beta$ emission lines in our post-maximum spectra, with a rough hydrogen mass limit of $\lesssim$0.1 $M_{\odot}$, which is consistent with previous limits in SN 2014J from late-time optical spectra of the H$\alpha$ line. Nonetheless, the growing dataset of high-quality NIR spectra holds the promise of very useful hydrogen constraints., Comment: 10 pages, 3 Figures, 2 Tables. Accepted for publication in the ApJ Letters

Shifting the focus of Type Ia supernova (SN Ia) cosmology to the near-infrared (NIR) is a promising way to significantly reduce the systematic errors, as the strategy minimizes our reliance on the empirical width-luminosity relation and uncertain dust laws. Observations in the NIR are also crucial for our understanding of the origins and evolution of these events, further improving their cosmological utility. Any future experiments in the rest-frame NIR will require knowledge of the SN Ia NIR spectroscopic diversity, which is currently based on a small sample of observed spectra. Along with the accompanying paper, Phillips et al. (2018), we introduce the Carnegie Supernova Project-II (CSP-II), to follow up nearby SNe Ia in both the optical and the NIR. In particular, this paper focuses on the CSP-II NIR spectroscopy program, describing the survey strategy, instrumental setups, data reduction, sample characteristics, and future analyses on the data set. In collaboration with the Harvard-Smithsonian Center for Astrophysics (CfA) Supernova Group, we obtained 661 NIR spectra of 157 SNe Ia. Within this sample, 451 NIR spectra of 90 SNe Ia have corresponding CSP-II follow-up light curves. Such a sample will allow detailed studies of the NIR spectroscopic properties of SNe Ia, providing a different perspective on the properties of the unburned material, radioactive and stable nickel produced, progenitor magnetic fields, and searches for possible signatures of companion stars., 20 pages, 7 figures, accepted for publication in PASP

We present nebular phase optical and near-infrared spectroscopy of the Type Ia supernova (SN) 2017cbv. The early light curves of SN~2017cbv showed a prominent blue bump in the $U$, $B$ and $g$ bands lasting for $\sim$5 d. One interpretation of the early light curve was that the excess blue light was due to shocking of the SN ejecta against a nondegenerate companion star -- a signature of the single degenerate scenario. If this is the correct interpretation, the interaction between the SN ejecta and the companion star could result in significant H$\alpha$ (or helium) emission at late times, possibly along with other species, depending on the companion star and its orbital separation. A search for H$\alpha$ emission in our +302 d spectrum yields a nondetection, with a $L_{H\alpha}$$, Comment: 11 pages, 3 figures, 2 tables. ApJ accepted

We present late-time ($200-400$ days) near-infrared spectral evolution for the Type Ia supernova SN 2005df. The spectra show numerous strong emission features of [CoII], [CoIII], and [FeII] throughout the $0.8-1.8$\mu m region. As the spectrum ages, the cobalt features fade as would be expected from the decay of $^{56}$Co to $^{56}$Fe. We show that the strong and isolated [FeII] emission line at $1.644$\mu m provides a unique tool to analyze near-infrared spectra of Type Ia supernovae. Normalization of spectra to this line allows separation of features produced by stable versus unstable isotopes of iron group elements. We develop a new method of determining the initial central density, $\rho_c$, and the magnetic field, $B$, of the white dwarf using the width of the $1.644$\mu m line. The line width is sensitive because of electron capture in the early stages of burning, which increases as a function of density. The sensitivity of the line width to $B$ increase with time and the effects of the magnetic field shift towards later times with decreasing $\rho_c$. The initial central density for SN 2005df is measured as $\rho_c=0.9(\pm0.2)$ (in $10^9$g/cm$^3$), which corresponds to a white dwarf close to the Chandrasekhar mass ($\rm M_{Ch}$) with $\rm M_{WD}=1.313(\pm0.034)$M$_{\odot}$ and systematic error less than $0.04$M$_{\odot}$. Within $\rm M_{Ch}$ explosions, however, the central density found for SN 2005df is very low for a H-accretor, possibly suggesting a helium star companion or a tidally-disrupted white dwarf companion. As an alternative, we suggest mixing of the central region. We find some support for high initial magnetic fields of strength $10^6$G for SN 2005df, however, $0$G cannot be ruled out because of noise in the spectra combined with low $\rho_c$., Comment: 13 pages, 14 figures, submitted to ApJ