We analyze orbital solutions for 48 massive multiple-star systems in the Cygnus OB2 association, 23 of which are newly presented here, to find that the observed distribution of orbital periods is ...approximately uniform in log P for P < 45 days, but it is not scale-free. We identify six stars, all supergiants, that exhibit aperiodic velocity variations of ~30 km ssup -1 attributed to atmospheric fluctuations.
We present the spectroscopic evolution of AT 2017gfo, the optical counterpart of the first binary neutron star (BNS) merger detected by LIGO and Virgo, GW170817. While models have long predicted that ...a BNS merger could produce a kilonova (KN), we have not been able to definitively test these models until now. From one day to four days after the merger, we took five spectra of AT 2017gfo before it faded away, which was possible because it was at a distance of only 39.5 Mpc in the galaxy NGC 4993. The spectra evolve from blue (∼6400 K) to red (∼3500 K) over the three days we observed. The spectra are relatively featureless-some weak features exist in our latest spectrum, but they are likely due to the host galaxy. However, a simple blackbody is not sufficient to explain our data: another source of luminosity or opacity is necessary. Predictions from simulations of KNe qualitatively match the observed spectroscopic evolution after two days past the merger, but underpredict the blue flux in our earliest spectrum. From our best-fit models, we infer that AT 2017gfo had an ejecta mass of 0.03 M , high ejecta velocities of 0.3c, and a low mass fraction ∼10−4 of high-opacity lanthanides and actinides. One possible explanation for the early excess of blue flux is that the outer ejecta is lanthanide-poor, while the inner ejecta has a higher abundance of high-opacity material. With the discovery and follow-up of this unique transient, combining gravitational-wave and electromagnetic astronomy, we have arrived in the multi-messenger era.
Abstract
The diversity of Type II supernovae (SNe II) is thought to be driven mainly by differences in their progenitor’s hydrogen-rich (H-rich) envelope mass, with SNe IIP having long plateaus (∼100 ...days) and the most massive H-rich envelopes. However, it is an ongoing mystery why SNe II with short plateaus (tens of days) are rarely seen. Here, we present optical/near-infrared photometric and spectroscopic observations of luminous Type II short-plateau SNe 2006Y, 2006ai, and 2016egz. Their plateaus of about 50–70 days and luminous optical peaks (≲−18.4 mag) indicate significant pre-explosion mass loss resulting in partially stripped H-rich envelopes and early circumstellar material (CSM) interaction. We compute a large grid of
MESA
+
STELLA
single-star progenitor and light-curve models with various progenitor zero-age main-sequence (ZAMS) masses, mass-loss efficiencies, explosion energies,
56
Ni masses, and CSM densities. Our model grid shows a continuous population of SNe IIP–IIL–IIb-like light-curve morphology in descending order of H-rich envelope mass. With large
56
Ni masses (≳0.05
M
⊙
), short-plateau SNe II lie in a confined parameter space as a transitional class between SNe IIL and IIb. For SNe 2006Y, 2006ai, and 2016egz, our findings suggest high-mass red supergiant (RSG) progenitors (
M
ZAMS
≃ 18–22
M
⊙
) with small H-rich envelope masses (
) that have experienced enhanced mass loss (
) for the last few decades before the explosion. If high-mass RSGs result in rare short-plateau SNe II, then these events might ease some of the apparent underrepresentation of higher-luminosity RSGs in observed SN II progenitor samples.
Abstract
The nondetection of companion stars in SN Ia progenitor systems lends support to the notion of double-degenerate systems and explosions triggered by the merging of two white dwarfs. This ...very asymmetric process should lead to a conspicuous polarimetric signature. By contrast, observations consistently find very low continuum polarization as the signatures from the explosion process largely dominate over the pre-explosion configuration within several days. Critical information about the interaction of the ejecta with a companion and any circumstellar matter is encoded in the early polarization spectra. In this study, we obtain spectropolarimetry of SN 2018gv with the ESO Very Large Telescope at −13.6 days relative to the
B
-band maximum light, or ∼5 days after the estimated explosion—the earliest spectropolarimetric observations to date of any SN Ia. These early observations still show a low continuum polarization (≲0.2%) and moderate line polarization (0.30% ± 0.04% for the prominent Si
ii
λ
6355 feature and 0.85% ± 0.04% for the high-velocity Ca component). The high degree of spherical symmetry implied by the low-line and continuum polarization at this early epoch is consistent with explosion models of delayed detonations and is inconsistent with the merger-induced explosion scenario. The dense UV and optical photometry and optical spectroscopy within the first ∼100 days after the maximum light indicate that SN 2018gv is a normal SN Ia with similar spectrophotometric behavior to SN 2011fe.
Abstract
AT 2020mot is a typical UV/optical tidal disruption event (TDE) with no radio or X-ray signatures in a quiescent host. We find an
i
-band excess and rebrightening along the decline of the ...light curve which could be due to two consecutive dust echoes from the TDE. We model our observations following van Velzen et al. and find that the near-infrared light curve can be explained by concentric rings of thin dust within ∼0.1 pc of a ∼6 × 10
6
M
⊙
supermassive black hole (SMBH), among the smallest scales at which dust has been inferred near SMBHs. We find dust covering factors of order
f
c
≤ 2%, much lower than found for dusty tori of active galactic nuclei. These results highlight the potential of TDEs for uncovering the environments around black holes when including near-infrared observations in high-cadence transient studies.
Abstract
Type Ibn supernovae (SNe) are a rare class of stellar explosions whose progenitor systems are not yet well determined. We present and analyze observations of the Type Ibn SN 2019kbj, and ...model its light curve in order to constrain its progenitor and explosion parameters. SN 2019kbj shows roughly constant temperature during the first month after peak, indicating a power source (likely circumstellar material interaction) that keeps the continuum emission hot at ∼15,000 K. Indeed, we find that the radioactive decay of
56
Ni is disfavored as the sole power source of the bolometric light curve. A radioactive decay + circumstellar material (CSM) interaction model, on the other hand, does reproduce the bolometric emission well. The fits prefer a uniform-density CSM shell rather than CSM due to a steady mass-loss wind, similar to what is seen in other Type Ibn SNe. The uniform-density CSM shell model requires ∼0.1
M
⊙
of
56
Ni and ∼1
M
⊙
total ejecta mass to reproduce the light curve. SN 2019kbj differs in this manner from another Type Ibn SN with derived physical parameters, SN 2019uo, for which an order of magnitude lower
56
Ni mass and larger ejecta mass were derived. This points toward a possible diversity in SN Ibn progenitor systems and explosions.
Abstract
We present the photometry of 16 91T/99aa-like Type Ia Supernovae (SNe Ia) observed by the Las Cumbres Observatory. We also use an additional set of 21 91T/99aa-like SNe Ia and 87 normal SNe ...Ia from the literature for an analysis of the standardizability of the luminosity of 91T/99aa-like SNe. We find that 91T/99aa-like SNe are 0.2 mag brighter than normal SNe Ia, even when fully corrected by the light-curve shapes and colors. The weighted rms of the 91T/99aa-like SNe (with
z
CMB
> 0.01) Hubble residuals is 0.25 ± 0.03 mag, suggesting that 91T/99aa-like SNe are also excellent relative distance indicators to ±12%. We compare the Hubble residuals with the pseudo-equivalent width (pEW) of Si
ii
λλ
6355 around the date of maximum brightness. We find that there is a broken linear correlation between those two measurements for our sample including both 91T/99aa-like and normal SNe Ia. As the
pEW
max
(Si
ii
λλ
6355) increases, the Hubble residual increases when
pEW
max
(Si
ii
λλ
6355) < 55.6 Å. However, the Hubble residual stays constant beyond this. Given that 91T/99aa-like SNe possess shallower Si
ii
lines than normal SNe Ia, the linear correlation at
pEW
max
(Si
ii
λλ
6355) < 55.6 Å can account for the overall discrepancy of Hubble residuals derived from the two subgroups. Such a systematic effect needs to be taken into account when using SNe Ia to measure luminosity distances.
Abstract
We present photometric and spectroscopic data of SN 2018lab, a low-luminosity Type IIP supernova (LLSN) with a
V
-band peak luminosity of −15.1 ± 0.1 mag. SN 2018lab was discovered by the ...Distance Less Than 40 Mpc (DLT40) SN survey only 0.73 days post-explosion, as determined by observations from the Transiting Exoplanet Survey Satellite (TESS). TESS observations of SN 2018lab yield a densely sampled, fast-rising, early-time light curve likely powered by ejecta–circumstellar medium (CSM) interaction. The blueshifted, broadened flash feature in the earliest spectra (<2 days) of SN 2018lab provides further evidence for ejecta–CSM interaction. The early emission features in the spectra of SN 2018lab are well described by models of a red supergiant progenitor with an extended envelope and a close-in CSM. As one of the few LLSNe with observed flash features, SN 2018lab highlights the need for more early spectra to explain the diversity of the flash feature morphology of Type II SNe.
Abstract
SN 2018aoz is a Type Ia SN with a
B
-band plateau and excess emission in infant-phase light curves ≲1 day after the first light, evidencing an over-density of surface iron-peak elements as ...shown in our previous study. Here, we advance the constraints on the nature and origin of SN 2018aoz based on its evolution until the nebular phase. Near-peak spectroscopic features show that the SN is intermediate between two subtypes of normal Type Ia: core normal and broad line. The excess emission may be attributable to the radioactive decay of surface iron-peak elements as well as the interaction of ejecta with either the binary companion or a small torus of circumstellar material. Nebular-phase limits on H
α
and He
i
favor a white dwarf companion, consistent with the small companion size constrained by the low early SN luminosity, while the absence of O
i
and He
i
disfavors a violent merger of the progenitor. Of the two main explosion mechanisms proposed to explain the distribution of surface iron-peak elements in SN 2018aoz, the asymmetric Chandrasekhar-mass explosion is less consistent with the progenitor constraints and the observed blueshifts of nebular-phase Fe
ii
and Ni
ii
. The helium-shell double-detonation explosion is compatible with the observed lack of C spectral features, but current 1D models are incompatible with the infant-phase excess emission,
B
max
–
V
max
color, and weak strength of nebular-phase Ca
ii
. Although the explosion processes of SN 2018aoz still need to be more precisely understood, the same processes could produce a significant fraction of Type Ia SNe that appear to be normal after ∼1 day.
We present photometric and spectroscopic observations of the type Ibn supernova (SN) 2019uo, the second ever SN Ibn with flash ionization (He ii, C iii, N iii) features in its early spectra. SN ...2019uo displays a rapid post-peak luminosity decline of 0.1 mag day−1 similar to most of the SNe Ibn, but is fainter ( mag) than a typical SN Ibn and shows a color evolution that places it between SNe Ib and the most extreme SNe Ibn. SN 2019uo shows P-cygni He i features in the early spectra which gradually evolve and become emission dominated post peak. It also shows faster evolution in line velocities as compared to most other members of the type Ibn subclass. The bolometric light curve is fairly well described by a 56Ni + circumstellar interaction model.
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