Search

We follow our first paper with an analysis of the ensemble of the extensive preexplosion ground- and space-based infrared observations of the red supergiant (RSG) progenitor candidate for the nearby core-collapse supernova SN 2023ixf in Messier 101, together with optical data prior to the explosion obtained with the Hubble Space Telescope (HST). We have confirmed the association of the progenitor candidate with the supernova (SN), as well as constrained the metallicity at the SN site, based on SN observations with instruments at Gemini-North. The internal host extinction to the SN has also been confirmed from a high-resolution Keck spectrum. We fit the observed spectral energy distribution (SED) for the star, accounting for its intrinsic variability, with dust radiative-transfer modeling, which assumes a silicate-rich dust shell ahead of the underlying stellar photosphere. The star is heavily dust obscured, likely the dustiest progenitor candidate yet encountered. We found median estimates of the star’s effective temperature and luminosity of 2770 K and 9.0 × 10 ^4 L _⊙ , with 68% credible intervals of 2340–3150 K and (7.5–10.9) × 10 ^4 L _⊙ , respectively. The candidate may have a Galactic RSG analog, IRC −10414, with a strikingly similar SED and luminosity. Via comparison with single-star evolutionary models we have constrained the initial mass of the progenitor candidate from 12 M _⊙ to as high as 14 M _⊙ . We have had available to us an extraordinary view of the SN 2023ixf progenitor candidate, which should be further followed up in future years with HST and the James Webb Space Telescope.

ASASSN-21qj is a distant Sun-like star that recently began an episode of deep dimming events after no prior recorded variability. Here we examine archival and newly obtained optical and near-infrared data of this star. The deep aperiodic dimming and absence of previous infrared excess are reminiscent of KIC 8462852 (“Boyajian’s Star”). The observed occultations are consistent with a circumstellar cloud of submicron-sized dust grains composed of amorphous pyroxene, with a minimum mass of 1.50 ± 0.04 × 10 ^−9 M _⊕ derived from the deepest occultations, and a minimum grain size of ${0.29}_{-0.18}^{+0.01}\,\mu {\rm{m}}$ assuming a power-law size distribution. We further identify the first evidence of near-infrared excess in this system from NEOWISE 3.4 and 4.6 μ m observations. The excess emission implies a total circumstellar dust mass of around 10 ^−6 M _⊕ , comparable to the extreme, variable disks associated with terrestrial planet formation around young stars. The quasiperiodic recurrence of deep dips and the inferred dust temperature (ranging from 1800 to 700 K across the span of observations) independently point to an orbital distance of ≃0.2 au for the dust, supporting the occulting material and excess emission being causally linked. The origin of this extended, opaque cloud is surmised to be the breakup of one or more exocometary bodies.

Observational evidence points to a red supergiant (RSG) progenitor for SN 2023ixf. The progenitor candidate has been detected in archival images at wavelengths (≥0.6 μ m) where RSGs typically emit profusely. This object is distinctly variable in the infrared (IR). We characterize the variability using pre-explosion mid-IR (3.6 and 4.5 μ m) Spitzer and ground-based near-IR ( JHK _s ) archival data jointly covering 19 yr. The IR light curves exhibit significant variability with rms amplitudes in the range 0.2–0.4 mag, increasing with decreasing wavelength. From a robust period analysis of the more densely sampled Spitzer data, we measure a period of 1091 ± 71 days. We demonstrate using Gaussian process modeling that this periodicity is also present in the near-IR light curves, thus indicating a common physical origin, which is likely pulsational instability. We use a period–luminosity relation for RSGs to derive a value of M _K = −11.58 ± 0.31 mag. Assuming a late M spectral type, this corresponds to $\mathrm{log}(L/{L}_{\odot })=5.27\pm 0.12$ at T _eff = 3200 K and to $\mathrm{log}(L/{L}_{\odot })=5.37\pm 0.12$ at T _eff = 3500 K. This gives an independent estimate of the progenitor’s luminosity, unaffected by uncertainties in extinction and distance. Assuming the progenitor candidate underwent enhanced dust-driven mass loss during the time of these archival observations, and using an empirical period–luminosity–based mass-loss prescription, we obtain a mass-loss rate of around (2–4) × 10 ^−4 M _⊙ yr ^−1 . Comparing the above luminosity with stellar evolution models, we infer an initial mass for the progenitor candidate of 20 ± 4 M _⊙ , making this one of the most massive progenitors for a Type II SN detected to date.

The persistence of planetary systems after their host stars evolve into their post-main-sequence phase is poorly constrained by observations. Many young white dwarf systems exhibit infrared excess emission and/or spectral absorption lines associated with a reservoir of dust (or planetesimals) and its accretion. However, most white dwarfs are too cool to sufficiently heat any circumstellar dust to detectable levels of emission. The Helix Nebula (NGC 7293) is a young, nearby planetary nebula; observations at mid- and far-infrared wavelengths have revealed excess emission associated with its central white dwarf (WD 2226-210). The origin of this excess is ambiguous. It could be a remnant planetesimal belt, a cloud of comets, or the remnants of material shed during the post-asymptotic giant branch (post-AGB) phase. Here we combine infrared (Stratospheric Observatory for Infrared Astronomy, Spitzer, Herschel) and millimeter (Atacama Large Millimeter/submillimeter Array) observations of the system to determine the origin of this excess using multiwavelength imaging and radiative transfer modeling. We find the data are incompatible with a compact remnant planetesimal belt or post-AGB disk, and conclude the dust most likely originates from deposition by a cometary cloud. The measured dust mass, and lifetime of the constituent grains, implies disruption of several thousand Hale–Bopp equivalent comets per year to fuel the observed excess emission around the Helix Nebula’s white dwarf.

We present near-IR imaging polarimetry of five classical FU Ori-type objects (FU Ori, V1057 Cyg, V1515 Cyg, V1735 Cyg, Z CMa) with an ∼0.″1 resolution observed using HiCIAO+AO188 at the Subaru Telescope. We observed scattered light associated with circumstellar dust around four of them (i.e., all but V1515 Cyg). Their polarized intensity distribution shows a variety of morphologies with arms, tails or streams, spikes, and fragmented distributions, many of which were reported in our previous paper. The morphologies of these reflection nebulae significantly differ from many other normal young stellar objects (Class I–II objects). These structures are attributed to gravitationally unstable disks, trails of clump ejections, dust blown by a wind or a jet, and a stellar companion. We can consistently explain our results with the scenario that their accretion outbursts (FUor outbursts) are triggered by gravitationally fragmenting disks, and with the hypothesis that many low-mass young stellar objects experience such outbursts. [ABSTRACT FROM AUTHOR]

We analyze results from the first 18 months of monthly submillimeter monitoring of eight star-forming regions in the JCMT Transient Survey. In our search for stochastic variability in 1643 bright peaks, only the previously identified source, EC 53, shows behavior well above the expected measurement uncertainty. Another four sources—two disks and two protostars—show moderately enhanced standard deviations in brightness, as expected for stochastic variables. For the two protostars, this apparent variability is the result of single epochs that are much brighter than the mean. In our search for secular brightness variations that are linear in time, we measure the fractional brightness change per year for 150 bright peaks, 50 of which are protostellar. The ensemble distribution of slopes is well fit by a normal distribution with σ ∼ 0.023. Most sources are not rapidly brightening or fading at submillimeter wavelengths. Comparison against time-randomized realizations shows that the width of the distribution is dominated by the uncertainty in the individual brightness measurements of the sources. A toy model for secular variability reveals that an underlying Gaussian distribution of linear fractional brightness change σ = 0.005 would be unobservable in the present sample, whereas an underlying distribution with σ = 0.02 is ruled out. Five protostellar sources, 10% of the protostellar sample, are found to have robust secular measures deviating from a constant flux. The sensitivity to secular brightness variations will improve significantly with a sample over a longer time duration, with an improvement by factor of two expected by the conclusion of our 36 month survey. [ABSTRACT FROM AUTHOR]

Investigating variability at the earliest stages of low-mass star formation is fundamental in understanding how a protostar assembles mass. While many simulations of protostellar disks predict non-steady accretion onto protostars, deeper investigation requires robust observational constraints on the frequency and amplitude of variability events characterized across the observable SED. In this study, we develop methods to robustly analyze repeated observations of an area of the sky for submillimeter variability in order to determine constraints on the magnitude and frequency of deeply embedded protostars. We compare 850 μm JCMT Transient Survey data with archival JCMT Gould Belt Survey data to investigate variability over 2–4 year timescales. Out of 175 bright, independent emission sources identified in the overlapping fields, we find seven variable candidates, five of which we classify as Strong, and the remaining two we classify as Extended to indicate that the latter are associated with larger-scale structure. For the Strong variable candidates, we find an average fractional peak brightness change per year of , with a standard deviation of . In total, 7% of the protostars associated with 850 μm emission in our sample show signs of variability. Four of the five Strong sources are associated with a known protostar. The remaining source is a good follow-up target for an object that is anticipated to contain an enshrouded, deeply embedded protostar. In addition, we estimate the 850 μm periodicity of the submillimeter variable source, EC 53, to be 567 ± 32 days, based on the archival Gould Belt Survey data. [ABSTRACT FROM AUTHOR]