Hubble Space Telescope Imaging Reveals That SN 2015bh Is Much Fainter than Its Progenitor

We present Hubble Space Telescope (HST) imaging of the site of SN 2015bh in the nearby spiral galaxy NGC 2770 taken between 2017 and 2019, nearly four years after the peak of the explosion. In 2017-2018, the transient fades steadily in optical filters before declining more slowly to F814W = -7.1 mag in 2019, ≈4 mag below the level of its eruptive luminous blue variable (LBV) progenitor observed with HST in 2008-2009. The source fades at a constant color of F555W - F814W = 0.4 mag until 2018, similar to SN 2009ip and consistent with a spectrum dominated by interaction of the ejecta with circumstellar material (CSM). A deep optical spectrum obtained in 2021 lacks signatures of ongoing interaction (L Hα 2 1038 erg s-1 for broadened emission 22000 km s-1), but indicates the presence of a nearby H ii region ( 2300 pc). The color evolution of the fading source makes it unlikely that emission from a scattered-light echo or binary OB companion of the progenitor contributes significantly to the flattening of the late-time light curve. The remaining emission in 2019 may plausibly be attributed an evolved/inflated companion or an unresolved ( 23 pc), young stellar cluster. Importantly, the color evolution of SN 2015bh rules out scenarios in which the surviving progenitor is obscured by nascent dust and does not clearly indicate a transition to a hotter, optically faint state. The simplest explanation is that the massive progenitor did not survive. SN 2015bh likely represents a remarkable example of the terminal explosion of a massive star preceded by decades of end-stage eruptive variability. © 2022. The Author(s). Published by the American Astronomical Society.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Hubble Space Telescope Imaging of Luminous Extragalactic Infrared Transients and Variables from the Spitzer Infrared Intensive Transients Survey

The SPitzer InfraRed Intensive Transients Survey (SPIRITS) searched for luminous infrared (IR) transients and variables in nearly 200 nearby galaxies from 2014 to 2019, using the warm Spitzer telescope at 3.6 and 4.5 μm. Among the SPIRITS variables are IR-bright objects that are undetected in ground-based optical surveys. We classify them as (1) transients, (2) periodic variables, and (3) irregular variables. The transients include eSPecially Red Intermediate-luminosity Transient Events (SPRITEs), having maximum luminosities fainter than supernovae, red IR colors, and a wide range of outburst durations (days to years). Here we report deep optical and near-IR imaging with the Hubble Space Telescope (HST) of 21 SPIRITS variables. They were initially considered SPRITE transients, but many eventually proved instead to be periodic or irregular variables as more data were collected. HST images show most of these cool and dusty variables are associated with star-forming regions in late-type galaxies, implying an origin in massive stars. Two SPRITEs lacked optical progenitors in deep preoutburst HST images; however, one was detected during eruption at J and H, indicating a dusty object with an effective temperature of ∼1050 K. One faint SPRITE turned out to be a dusty classical nova. About half the HST targets proved to be periodic variables, with pulsation periods of 670-2160 days; they are likely dusty asymptotic-giant-branch (AGB) stars with masses of ∼5-10 M ⊙. A few of them were warm enough to be detected in deep HST frames, but most are too cool. Out of six irregular variables, two were red supergiants with optical counterparts in HST images; four were too enshrouded for HST detection. © 2022. The Author(s). Published by the American Astronomical Society.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Revealing Efficient Dust Formation at Low Metallicity in Extragalactic Carbon-rich Wolf-Rayet Binaries

We present Spitzer/InfraRed Array Camera observations of dust formation from six extragalactic carbon-rich Wolf-Rayet (WC) binary candidates in low-metallicity (Z ≲ 0.65 Z o˙) environments using multiepoch mid-infrared (IR) imaging data from the SPitzer InfraRed Intensive Transients Survey (SPIRITS). Optical follow-up spectroscopy of SPIRITS 16ln, 19q, 16df, 18hb, and 14apu reveals emission features from C iv λ5801-12 and/or the C iii-iv λ4650/He ii λ4686 blend that are consistent with early-type WC stars. We identify SPIRITS 16ln as the variable mid-IR counterpart of the recently discovered colliding-wind WC4 + O binary candidate, N604-WRXc, located in the subsolar metallicity NGC 604 H ii region in M33. We interpret the mid-IR variability from SPIRITS 16ln as a dust-formation episode in an eccentric colliding-wind WC binary. SPIRITS 19q, 16df, 14apu, and 18hb exhibit absolute [3.6] magnitudes exceeding that of one of the most IR-luminous dust-forming WC systems known, WR 104 (M [3.6] ≲ -12.3). An analysis of dust formation in the mid-IR outburst from SPIRITS 19q reveals a high dust production rate of M o˙ yr-1, which may therefore exceed that of the most efficient dust-forming WC systems known. We demonstrate that efficient dust formation is feasible from early-type WC binaries in the theoretical framework of colliding-wind binary dust formation if the systems host an O-type companion with high mass-loss rates ( M o˙ yr-1). This efficient dust formation from early-type WC binaries highlights their potential role as significant sources of dust in low-metallicity environments. © 2021. The American Astronomical Society. All rights reserved.Immediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Enormous explosion energy of Type IIP SN 2017gmr with bipolar 56Ni ejecta

The unusual Type IIP SN 2017gmr is revisited in order to pinpoint the origin of its anomalous features, including the peculiar light curve after about 100 d. The hydrodynamic modelling suggests the enormous explosion energy of ≈1052 erg. We find that the light curve with the prolonged plateau/tail transition can be reproduced either in the model with a high hydrogen abundance in the inner ejecta and a large amount of radioactive 56Ni, or in the model with an additional central energy source associated with the fallback/magnetar interaction in the propeller regime. The asymmetry of the late H α emission and the reported linear polarization are reproduced by the model of the bipolar 56Ni ejecta. The similar bipolar structure of the oxygen distribution is responsible for the two-horn structure of the [O i] 6360, 6364 Å emission. The bipolar 56Ni structure along with the high explosion energy are indicative of the magneto-rotational explosion. We identify narrow high-velocity absorption features in H α and He i10 830 Å lines with their origin in the fragmented cold dense shell formed due to the outer ejecta deceleration in a confined circumstellar shell. © 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.Immediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

A Comprehensive Optical Search for Pre-explosion Outbursts from the Quiescent Progenitor of SN 2023ixf

We perform a comprehensive search for optical precursor emission at the position of SN 2023ixf using data from the DLT40, ZTF, and ATLAS surveys. By comparing the current data set with precursor outburst hydrodynamical model light curves, we find that the probability of a significant outburst within 5 yr of explosion is low, and the circumstellar material (CSM) ejected during any possible precursor outburst is likely smaller than ∼0.015M ⊙. By comparing to a set of toy models, we find that, if there was a precursor outburst, the duration must have been shorter than ∼100 days for a typical brightness of M r ≃ −9 mag or shorter than 200 days for M r ≃ −8 mag; brighter, longer outbursts would have been discovered. Precursor activity like that observed in the normal Type II SN 2020tlf (M r ≃ −11.5) can be excluded in SN 2023ixf. If the dense CSM inferred by early flash spectroscopy and other studies is related to one or more precursor outbursts, then our observations indicate that any such outburst would have to be faint and only last for days to months, or it occurred more than 5 yr prior to the explosion. Alternatively, any dense, confined CSM may not be due to eruptive mass loss from a single red supergiant progenitor. Taken together, the results of SN 2023ixf and SN 2020tlf indicate that there may be more than one physical mechanism behind the dense CSM inferred around some normal Type II supernovae. © 2023. The Author(s). Published by the American Astronomical Society.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Census of R Coronae Borealis Stars. I. Infrared Light Curves from Palomar Gattini IR

We are undertaking the first systematic infrared (IR) census of R Coronae Borealis (RCB) stars in the Milky Way, beginning with IR light curves from the Palomar Gattini IR (PGIR) survey. The PGIR is a 30 cm J-band telescope with a 25 deg2 camera that is surveying 18,000 deg2 of the northern sky (δ > -28°) at a cadence of 2 days. We present PGIR light curves for 922 RCB candidates selected from a mid-IR color-based catalog. Of these 922, 149 are promising RCB candidates, as they show pulsations or declines similar to RCB stars. The majority of the candidates that are not RCB stars are either long-period variables (LPVs) or RV Tauri stars. We identify IR color-based criteria to better distinguish between RCB stars and LPVs. As part of a pilot spectroscopic run, we obtain NIR spectra for 26 of the 149 promising candidates and spectroscopically confirm 11 new RCB stars. We detect strong He i λ10830 features in the spectra of all RCB stars, likely originating within high-velocity (200-400 km s-1) winds in their atmospheres. Nine of these RCB stars show 12C16O and 12C18O molecular absorption features, suggesting that they are formed through a white dwarf merger. We detect quasiperiodic pulsations in the light curves of five RCB stars. The periods range between 30 and 125 days and likely originate from the strange-mode instability in these stars. Our pilot run results motivate a dedicated IR spectroscopic campaign to classify all RCB candidates. © 2021. The American Astronomical Society. All rights reserved.Immediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Circumstellar Medium Constraints on the Environment of Two Nearby Type Ia Supernovae: SN 2017cbv and SN 2020nlb

We present deep Chandra X-ray observations of two nearby Type Ia supernovae, SN 2017cbv and SN 2020nlb, which reveal no X-ray emission down to a luminosity LX ≲ 5.3 × 1037 and ≲ 5.4 × 1037 erg s-1 (0.3-10 keV), respectively, at ∼16-18 days after the explosion. With these limits, we constrain the pre-explosion mass-loss rate of the progenitor system to be Ṁ < 7.2 × 10-9 and < 9.7 × 10-9 M⊙ yr-1 for each (at a wind velocity v w = 100 km s-1 and a radius of R ≈ 1016 cm), assuming any X-ray emission would originate from inverse Compton emission from optical photons upscattered by the supernova shock. If the supernova environment was a constant-density medium, we would find a number density limit of nCSM < 36 and < 65 cm-3, respectively. These X-ray limits rule out all plausible symbiotic progenitor systems, as well as large swathes of parameter space associated with the single degenerate scenario, such as mass loss at the outer Lagrange point and accretion winds. We also present late-time optical spectroscopy of SN 2020nlb, and set strong limits on any swept up hydrogen (LHα < 2.7 × 1037 erg s-1) and helium (LHe,λ6678 < 2.7 × 1037 erg s-1) from a nondegenerate companion, corresponding to MH ≲ 0.7-2 × 10-3 M⊙ and MHe ≲ 4 × 10-3 M⊙. Radio observations of SN 2020nlb at 14.6 days after explosion also yield a non-detection, ruling out most plausible symbiotic progenitor systems. While we have doubled the sample of normal Type Ia supernovae with deep X-ray limits, more observations are needed to sample the full range of luminosities and subtypes of these explosions, and set statistical constraints on their circumbinary environments. © 2021. The American Astronomical Society. All rights reserved.Immediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Constraining the Progenitor System of the Type Ia Supernova 2021aefx

We present high-cadence optical and ultraviolet light curves of the normal Type Ia supernova (SN) 2021aefx, which shows an early bump during the first two days of observation. This bump may be a signature of interaction between the exploding white dwarf and a nondegenerate binary companion, or it may be intrinsic to the white dwarf explosion mechanism. In the case of the former, the short duration of the bump implies a relatively compact main-sequence companion star, although this conclusion is viewing-angle dependent. Our best-fit companion-shocking and double-detonation models both overpredict the UV luminosity during the bump, and existing nickel-shell models do not match the strength and timescale of the bump. We also present nebular spectra of SN 2021aefx, which do not show the hydrogen or helium emission expected from a nondegenerate companion, as well as a radio nondetection that rules out all symbiotic progenitor systems and most accretion disk winds. Our analysis places strong but conflicting constraints on the progenitor of SN 2021aefx; no current model can explain all of our observations. © 2022. The Author(s). Published by the American Astronomical Society.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

SN 2016dsg: A Thermonuclear Explosion Involving a Thick Helium Shell

A thermonuclear explosion triggered by a He-shell detonation on a carbon-oxygen white-dwarf core has been predicted to have strong UV line blanketing at early times due to the iron-group elements produced during He-shell burning. We present the photometric and spectroscopic observations of SN 2016dsg, a subluminous peculiar Type I supernova consistent with a thermonuclear explosion involving a thick He shell. With a redshift of 0.04, the i-band peak absolute magnitude is derived to be around −17.5. The object is located far away from its host, an early-type galaxy, suggesting it originated from an old stellar population. The spectra collected after the peak are unusually red, show strong UV line blanketing and weak O i λ7773 absorption lines, and do not evolve significantly over 30 days. An absorption line around 9700-10500 Å is detected in the near-infrared spectrum and is likely from the unburnt He in the ejecta. The spectroscopic evolution is consistent with the thermonuclear explosion models for a sub-Chandrasekhar-mass white dwarf with a thick He shell, while the photometric evolution is not well described by existing models. © 2022. The Author(s). Published by the American Astronomical Society.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]