Examinando por Autor "Karamehmetoglu, E."
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Ítem Forbidden hugs in pandemic times II. The luminous red nova variety: AT 2020hat and AT 2020kog(EDP Sciences, 2021-03-01) Pastorello, A.; Valerin, G.; Fraser, M.; Elias-Rosa, N.; Valenti, S.; Reguitti, A.; Mazzali, P. A.; Amaro, R. C.; Andrews, J. E.; Dong, Y.; Jencson, J.; Lundquist, M.; Reichart, D. E.; Sand, D. J.; Wyatt, S.; Smartt, S. J.; Smith, K. W.; Srivastav, S.; Cai, Y.-Z.; Cappellaro, E.; Holmbo, S.; Fiore, A.; Jones, D.; Kankare, E.; Karamehmetoglu, E.; Lundqvist, P.; Morales-Garoffolo, A.; Reynolds, T. M.; Stritzinger, M. D.; Williams, S. C.; Chambers, K. C.; de Boer, T. J. L.; Huber, M. E.; Rest, A.; Wainscoat, R.We present the results of our monitoring campaigns of the luminous red novae (LRNe) AT 2020hat in NGC 5068 and AT 2020kog in NGC 6106. The two objects were imaged (and detected) before their discovery by routine survey operations. They show a general trend of slow luminosity rise, lasting at least a few months. The subsequent major LRN outbursts were extensively followed in photometry and spectroscopy. The light curves present an initial short-duration peak, followed by a redder plateau phase. AT 2020kog is a moderately luminous event peaking at ∼7 × 1040 erg s-1, while AT 2020hat is almost one order of magnitude fainter than AT 2020kog, although it is still more luminous than V838 Mon. In analogy with other LRNe, the spectra of AT 2020kog change significantly with time. They resemble those of type IIn supernovae at early phases, then they become similar to those of K-type stars during the plateau, and to M-type stars at very late phases. In contrast, AT 2020hat already shows a redder continuum at early epochs, and its spectrum shows the late appearance of molecular bands. A moderate-resolution spectrum of AT 2020hat taken at +37 d after maximum shows a forest of narrow P Cygni lines of metals with velocities of 180 km s-1, along with an Hα emission with a full-width at half-maximum velocity of 250 km s-1. For AT 2020hat, a robust constraint on its quiescent progenitor is provided by archival images of the Hubble Space Telescope. The progenitor is clearly detected as a mid-K type star, with an absolute magnitude of MF606W = -3.33 ± 0.09 mag and a colour of F606W - F814W = 1.14 ± 0.05 mag, which are inconsistent with the expectations from a massive star that could later produce a core-collapse supernova. Although quite peculiar, the two objects nicely match the progenitor versus light curve absolute magnitude correlations discussed in the literature.Ítem Forbidden hugs in pandemic times: I. Luminous red nova at 2019zhd, a new merger in M 31(EDP Sciences, 2021-02-01) Pastorello, A.; Fraser, M.; Valerin, G.; Reguitti, A.; Itagaki, K.; Ochner, P.; Williams, S.C.; Jones, D.; Munday, J.; Smartt, S.J.; Smith, K.W.; Srivastav S., S.; Elias-Rosa, N.; Kankare, E.; Karamehmetoglu, E.; Lundqvist, P.; Mazzali, P. A.; Munari, U.; Stritzinger, M. D.; Tomasella, L.; Anderson, J. P.; Chambers, K. C.; Rest, A.We present the follow-up campaign of the luminous red nova (LRN) AT 2019zhd, the third event of this class observed in M 31. The object was followed by several sky surveys for about five months before the outburst, during which it showed a slow luminosity rise. In this phase, the absolute magnitude ranged from Mr =-2.8 ± 0.2 mag to Mr =-5.6 ± 0.1 mag. Then, over a four to five day period, AT 2019zhd experienced a major brightening, reaching a peak of Mr =-9.61 ± 0.08 mag and an optical luminosity of 1.4 × 1039 erg s-1. After a fast decline, the light curve settled onto a short-duration plateau in the red bands. Although less pronounced, this feature is reminiscent of the second red maximum observed in other LRNe. This phase was followed by a rapid linear decline in all bands. At maximum, the spectra show a blue continuum with prominent Balmer emission lines. The post-maximum spectra show a much redder continuum, resembling that of an intermediate-type star. In this phase, Hα becomes very weak, Hβ is no longer detectable, and a forest of narrow absorption metal lines now dominate the spectrum. The latest spectra, obtained during the post-plateau decline, show a very red continuum (Teff ≈ 3000 K) with broad molecular bands of TiO, similar to those of M-type stars. The long-lasting, slow photometric rise observed before the peak resembles that of LRN V1309 Sco, which was interpreted as the signature of the common-envelope ejection. The subsequent outburst is likely due to the gas outflow following a stellar merging event. The inspection of archival HST images taken 22 years before the LRN discovery reveals a faint red source (MF555W = 0.21 ± 0.14 mag, with F555W-F814W = 2.96 ± 0.12 mag) at the position of AT 2019zhd, which is the most likely quiescent precursor. The source is consistent with expectations for a binary system including a predominant M5-type star.Ítem Investigating the properties of stripped-envelope supernovae; what are the implications for their progenitors?(Monthly Notices of the Royal Astronomical Society, 2019-02-15) Prentice, S. J.; Ashall, C.; James, P. A.; Short, L.; Mazzali, P. A.; Bersier, D.; Crowther, P. A.; Barbarino, C.; Chen, T.-W.; Copperwheat, C. M.; Darnley, M. J.; Denneau, L.; Elias-Rosa, N.; Fraser, M.; Galbany, L.; Gal-Yam, A.; Harmanen, J.; Howell, D. A.; Hosseinzadeh, G.; Inserra, C.; Kankare, E.; Karamehmetoglu, E.; Lamb, G. P.; Limongi, M.; Maguire, K.; McCully, C.; Olivares E., F.; Piascik, A. S.; Pignata, G.; Reichart, D. E.; Rest, A.; Reynolds, T.; Rodríguez, O.; Saario, J. L. O.; Schulze, S.; Smartt, S. J.; Smith, K. W.; Sollerman, J.; Stalder, B.; Sullivan, M.; Taddia, F.; Valenti, S.; Vergani, S. D.; Williams, S. C.; Young, D. R.We present observations and analysis of 18 stripped-envelope supernovae observed during 2013–2018. This sample consists of five H/He-rich SNe, six H-poor/He-rich SNe, three narrow lined SNe Ic, and four broad lined SNe Ic. The peak luminosity and characteristic time-scales of the bolometric light curves are calculated, and the light curves modelled to derive 56Ni and ejecta masses (MNi and Mej). Additionally, the temperature evolution and spectral line velocity curves of each SN are examined. Analysis of the [O I] line in the nebular phase of eight SNe suggests their progenitors had initial masses <20 M . The bolometric light curve properties are examined in combination with those of other SE events from the literature. The resulting data set gives the Mej distribution for 80 SE–SNe, the largest such sample in the literature to date, and shows that SNe Ib have the lowest median Mej, followed by narrow-lined SNe Ic, H/He-rich SNe, broad-lined SNe Ic, and finally gamma-ray burst SNe. SNe Ic-6/7 show the largest spread of Mej ranging from ∼1.2–11 M , considerably greater than any other subtype. For all SE–SNe = 2.8 ± 1.5 M which further strengthens the evidence that SE–SNe arise from low-mass progenitors which are typically <5 M at the time of explosion, again suggesting MZAMS <25 M . The low and lack of clear bimodality in the distribution implies <30 M progenitors and that envelope stripping via binary interaction is the dominant evolutionary pathway of these SNe.Ítem SN 2020wnt: a slow-evolving carbon-rich superluminous supernova with no O II lines and a bumpy light curve(Oxford University Press, 2022-09) Gutiérrez, C.P.; Pastorello, A.; Bersten, M.; Benetti, S.; Orellana, M.; Fiore, A.; Karamehmetoglu, E.; Kravtsov, T.; Reguitti, A.; Reynolds, T.M.; Valerin, G.; Mazzali, P.; Sullivan, M.; Cai, Y.-Z.; Elias-Rosa, N.; Fraser, M.; Hsiao, E.Y.; Kankare, E.; Kotak, R.; Kuncarayakti, H.; Li, Z.; Mattila, S.; Mo, J.; Moran, S; Ochner, P.; Shahbandeh, M.; Tomasella, L.; Wang, X.; Yan, S.; Zhang, J.; Zhang, T.; Stritzinger, M. D.We present the analysis of SN 2020wnt, an unusual hydrogen-poor superluminous supernova (SLSN-I), at a redshift of 0.032. The light curves of SN 2020wnt are characterized by an early bump lasting ∼5 d, followed by a bright main peak. The SN reaches a peak absolute magnitude of Mmax r = −20.52 ± 0.03 mag at ∼77.5 d from explosion. This magnitude is at the lower end of the luminosity distribution of SLSNe-I, but the rise-time is one of the longest reported to date. Unlike other SLSNe-I, the spectra of SN 2020wnt do not show O II, but strong lines of C II and Si II are detected. Spectroscopically, SN 2020wnt resembles the Type Ic SN 2007gr, but its evolution is significantly slower. Comparing the bolometric light curve to hydrodynamical models, we find that SN 2020wnt luminosity can be explained by radioactive powering. The progenitor of SN 2020wnt is likely a massive and extended star with a pre-SN mass of 80 M and a pre-SN radius of 15 R that experiences a very energetic explosion of 45 × 1051 erg, producing 4 M of 56Ni. In this framework, the first peak results from a post-shock cooling phase for an extended progenitor, and the luminous main peak is due to a large nickel production. These characteristics are compatible with the pair-instability SN scenario. We note, however, that a significant contribution of interaction with circumstellar material cannot be ruled out.Ítem SN 2021fxy: mid-ultraviolet flux suppression is a common feature of Type Ia supernovae(Oxford University Press, 2023-07-01) DerKacy, J.M.; Paugh, S.; Baron, E.; Brown, P.J.; Ashall, C.; Burns, C.R.; Hsiao, E.Y.; Kumar, S.; Lu, J.; Morrell, N.; Phillips, M.M.; Shahbandeh, M.; Shappee, B.J.; Stritzinger, M.D.; Tucker, M.A.; Yarbrough, Z.; Boutsia, K.; Hoeflich, P.; Wang, L.; Galbany, L.; Karamehmetoglu, E.; Krisciunas, K.; Mazzali, P.; Piro, A.L.; Suntzeff, N.B.; Fiore, A.; Gutiérrez, C.P.; Lundqvist, P.; Reguitti, A.We present ultraviolet (UV) to near-infrared (NIR) observations and analysis of the nearby Type Ia supernova SN 2021fxy. Our observations include UV photometry from Swift/UVOT, UV spectroscopy from HST/STIS, and high-cadence optical photometry with the Swope 1-m telescope capturing intranight rises during the early light curve. Early B − V colours show SN 2021fxy is the first 'shallow-silicon' (SS) SN Ia to follow a red-to-blue evolution, compared to other SS objects which show blue colours from the earliest observations. Comparisons to other spectroscopically normal SNe Ia with HST UV spectra reveal SN 2021fxy is one of several SNe Ia with flux suppression in the mid-UV. These SNe also show blueshifted mid-UV spectral features and strong high-velocity Ca II features. One possible origin of this mid-UV suppression is the increased effective opacity in the UV due to increased line blanketing from high velocity material, but differences in the explosion mechanism cannot be ruled out. Among SNe Ia with mid-UV suppression, SNe 2021fxy and 2017erp show substantial similarities in their optical properties despite belonging to different Branch subgroups, and UV flux differences of the same order as those found between SNe 2011fe and 2011by. Differential comparisons to multiple sets of synthetic SN Ia UV spectra reveal this UV flux difference likely originates from a luminosity difference between SNe 2021fxy and 2017erp, and not differing progenitor metallicities as suggested for SNe 2011by and 2011fe. These comparisons illustrate the complicated nature of UV spectral formation, and the need for more UV spectra to determine the physical source of SNe Ia UV diversity. © 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.