Abstract
We present a moderate-resolution spectrum of the peculiar Type II supernova (SN) iPTF14hls taken on day 1153 after discovery. This spectrum reveals the clear signature of shock interaction ...with dense circumstellar material (CSM). We suggest that this CSM interaction may be an important clue for understanding the extremely unusual photometric and spectroscopic evolution seen over the first 600 d of iPTF14hls. The late-time spectrum shows a double-peaked intermediate-width H α line indicative of expansion speeds around 1000 km s−1, with the double-peaked shape hinting at a disc-like geometry in the CSM. If the CSM were highly asymmetric, perhaps in a disc or torus that was ejected from the star 3–6 yr prior to explosion, the CSM interaction could have been overrun and hidden below the SN ejecta photosphere from a wide range of viewing angles. In that case, CSM interaction luminosity would have been thermalized well below the photosphere, potentially sustaining the high luminosity without exhibiting the traditional observational signatures of strong CSM interaction (narrow H α emission and X-rays). Variations in density structure of the CSM could account for the multiple rebrightenings of the light curve. We propose that a canonical 1 × 1051 erg explosion energy with enveloped CSM interaction as seen in some recent SNe, rather than an entirely new explosion mechanism, may be adequate to explain the peculiar evolution of iPTF14hls.
Abstract
Yellow hypergiants (YHGs) are often presumed to represent a transitional post-red supergiant (RSG) phase for stars ∼30–40
M
⊙
. Here we present visual-wavelength echelle spectra of six YHG ...candidates in the Galactic cluster Westerlund 1, and we compare them to known YHGs, IRC +10420 and Hen 3-1979. We find that the six YHG candidates do not exhibit any metal emission lines, nor do they show strong H
α
emission, and as such do not meet the criteria necessary to be classified as YHGs. In conjunction with their moderate luminosities of
log
(
L
/
L
⊙
)
= 4.7–5.4 estimated from optical/infrared photometry, we suggest instead that they are normal yellow supergiants (YSGs) with more modest initial masses around 15–20
M
⊙
. This adds additional support to the hypothesis that Wd1 is a multiage cluster with an older age than previously assumed, and is not a ∼5 Myr old cluster caught at a very specific transitional point when single-star evolution might yield Wolf–Rayet stars, luminous blue variables, RSGs, and YHGs in the same cluster. Nevertheless, the population of YSGs in Wd1 is very unusual, with YSGs outnumbering RSGs, but with both spanning a large luminosity range. Here, we discuss evolutionary scenarios that might have led to the high fraction of YSGs. The number of YSGs and their significant luminosity spread cannot be explained by simple population synthesis models with single or binary stars. Even with multiple ages or a large age spread, the high YSG/RSG ratio remains problematic. We suggest instead that the objects may experience a prolonged YSG phase due to evolution in triple systems.
ABSTRACT
SN 2017hcc was remarkable for being a nearby and strongly polarized superluminous Type IIn supernova (SN). We obtained high-resolution Echelle spectra that we combine with other spectra to ...investigate its line-profile evolution. All epochs reveal narrow P Cygni components from pre-shock circumstellar material (CSM), indicating an axisymmetric outflow from the progenitor of 40–50 km s−1. Broad and intermediate-width components exhibit the classic evolution seen in luminous SNe IIn: symmetric Lorentzian profiles from pre-shock CSM lines broadened by electron scattering at early times, transitioning at late times to multicomponent, irregular profiles coming from the SN ejecta and post-shock shell. As in many SNe IIn, profiles show a progressively increasing blueshift, with a clear flux deficit in red wings of the intermediate and broad velocity components after day 200. This blueshift develops after the continuum luminosity fades, and in the intermediate-width component, persists at late times even after the SN ejecta fade. In SN 2017hcc, the blueshift cannot be explained as occultation by the SN photosphere, pre-shock acceleration of CSM, or a lopsided explosion of CSM. Instead, the blueshift arises from dust formation in the post-shock shell and in the SN ejecta. The effect has a wavelength dependence characteristic of dust, exhibiting an extinction law consistent with large grains. Thus, SN 2017hcc experienced post-shock dust formation and had a mildly bipolar CSM shell, similar to SN 2010jl. Like other superluminous SNe IIn, the progenitor lost around 10 M⊙ due to extreme eruptive mass-loss in the decade before exploding.
Double detonations in double white dwarf (WD) binaries undergoing unstable mass transfer have emerged in recent years as one of the most promising Type Ia supernova (SN Ia) progenitor scenarios. One ...potential outcome of this "dynamically driven double-degenerate double-detonation" (D6) scenario is that the companion WD survives the explosion and is flung away with a velocity equal to its >1000 km s−1 pre-SN orbital velocity. We perform a search for these hypervelocity runaway WDs using Gaia's second data release. In this paper, we discuss seven candidates followed up with ground-based instruments. Three sources are likely to be some of the fastest known stars in the Milky Way, with total Galactocentric velocities between 1000 and 3000 km s−1, and are consistent with having previously been companion WDs in pre-SN Ia systems. However, although the radial velocity of one of the stars is >1000 km s−1, the radial velocities of the other two stars are puzzlingly consistent with 0. The combined five-parameter astrometric solutions from Gaia and radial velocities from follow-up spectra yield tentative 6D confirmation of the D6 scenario. The past position of one of these stars places it within a faint, old SN remnant, further strengthening the interpretation of these candidates as hypervelocity runaways from binary systems that underwent SNe Ia.
Abstract
We analyze pre-explosion near- and mid-infrared (IR) imaging of the site of SN 2023ixf in the nearby spiral galaxy M101 and characterize the candidate progenitor star. The star displays ...compelling evidence of variability with a possible period of ≈1000 days and an amplitude of Δ
m
≈ 0.6 mag in extensive monitoring with the Spitzer Space Telescope since 2004, likely indicative of radial pulsations. Variability consistent with this period is also seen in the near-IR
J
and
K
s
bands between 2010 and 2023, up to just 10 days before the explosion. Beyond the periodic variability, we do not find evidence for any IR-bright pre-supernova outbursts in this time period. The IR brightness (
M
K
s
=
−
10.7
mag) and color (
J
−
K
s
= 1.6 mag) of the star suggest a luminous and dusty red supergiant. Modeling of the phase-averaged spectral energy distribution (SED) yields constraints on the stellar temperature (
T
eff
=
3500
−
1400
+
800
K) and luminosity (
log
L
/
L
⊙
=
5.1
±
0.2
). This places the candidate among the most luminous Type II supernova progenitors with direct imaging constraints, with the caveat that many of these rely only on optical measurements. Comparison with stellar evolution models gives an initial mass of
M
init
= 17 ± 4
M
⊙
. We estimate the pre-supernova mass-loss rate of the star between 3 and 19 yr before explosion from the SED modeling at
M
̇
≈
3
×
10
−
5
to 3 × 10
−4
M
⊙
yr
−1
for an assumed wind velocity of
v
w
= 10 km s
−1
, perhaps pointing to enhanced mass loss in a pulsation-driven wind.
We have identified a progenitor candidate in archival Hubble Space Telescope (HST) images for the Type Ic supernova (SN Ic) SN 2017ein in NGC 3938, pinpointing the candidate's location via HST Target ...of Opportunity imaging of the SN itself. This would be the first identification of a stellar-like object as a progenitor candidate for any SN Ic to date. We also present observations of SN 2017ein during the first ∼49 days since explosion. We find that SN 2017ein most resembles the well-studied SN Ic SN 2007gr. We infer that SN 2017ein experienced a total visual extinction of AV 1.0-1.9 mag, predominantly because of dust within the host galaxy. Although the distance is not well known, if this object is the progenitor, it was likely of high initial mass, ∼47-48 M if a single star, or ∼60-80 M if in a binary system. However, we also find that the progenitor candidate could be a very blue and young compact cluster, further implying a very massive (>65 M ) progenitor. Furthermore, the actual progenitor might not be associated with the candidate at all and could be far less massive. From the immediate stellar environment, we find possible evidence for three different populations; if the SN progenitor was a member of the youngest population, this would be consistent with an initial mass of ∼57 M . After it has faded, the SN should be reobserved at high spatial resolution and sensitivity, to determine whether the candidate is indeed the progenitor.
Abstract
We present the densely sampled early light curve of the Type II supernova (SN) 2023ixf, first observed within hours of explosion in the nearby Pinwheel Galaxy (Messier 101; 6.7 Mpc). ...Comparing these data to recently updated models of shock-cooling emission, we find that the progenitor likely had a radius of 410 ± 10
R
⊙
. Our estimate is model dependent but consistent with a red supergiant. These models provide a good fit to the data starting about 1 day after the explosion, despite the fact that the classification spectrum shows signatures of circumstellar material around SN 2023ixf during that time. Photometry during the first day after the explosion, provided almost entirely by amateur astronomers, does not agree with the shock-cooling models or a simple power-law rise fit to data after 1 day. We consider the possible causes of this discrepancy, including precursor activity from the progenitor star, circumstellar interaction, and emission from the shock before or after it breaks out of the stellar surface. The very low luminosity (−11 mag >
M
> −14 mag) and short duration of the initial excess lead us to prefer a scenario related to prolonged emission from the SN shock traveling through the progenitor system.
The Galactic transient V1309 Sco was the result of a merger in a low-mass star system, while V838 Mon was thought to be a similar merger event from a more massive B-type progenitor. In this paper, we ...study a recent optical and infrared (IR) transient discovered in the nearby galaxy NGC 4490 named NGC 4490-OT2011 (NGC 4490-OT hereafter), which appeared similar to these merger events (unobscured progenitor, irregular multi-peaked light curve, increasingly red colour, similar optical spectrum, IR excess at late times), but which had a higher peak luminosity and longer duration in outburst. NGC 4490-OT has less in common with the class of SN 2008S-like transients. A progenitor detected in pre-eruption Hubble Space Telescope (HST) images, combined with upper limits in the IR, requires a luminous and blue progenitor that has faded in late-time HST images. The same source was detected by Spitzer and ground-based data as a luminous IR (2–5 μm) transient, indicating a transition to a self-obscured state qualitatively similar to the evolution seen in other stellar mergers and in luminous blue variables. The post-outburst dust-obscured source is too luminous and too warm at late times to be explained with an IR echo, suggesting that the object survived the event. The luminosity of the enshrouded IR source is similar to that of the progenitor. Compared to proposed merger events, the more massive progenitor of NGC 4490-OT seems to extend a correlation between stellar mass and peak luminosity, and may suggest that both of these correlate with duration. We show that spectra of NGC 4490-OT and V838 Mon also resemble light-echo spectra of η Car, prompting us to speculate that η Car may be an extreme extension of this phenomenon.
ABSTRACT
MCA-1B (also called UIT003) is a luminous hot star in the western outskirts of M33, classified over 20 yr ago with a spectral type of Ofpe/WN9 and identified then as a candidate luminous ...blue variable (LBV). Palomar Transient Factory data reveal that this star brightened in 2010, with a light curve resembling that of the classic LBV star AF And in M31. Other Ofpe/WN9 stars have erupted as LBVs, but MCA-1B was unusual because it remained hot. It showed a WN-type spectrum throughout its eruption, whereas LBVs usually get much cooler. MCA-1B showed an almost four-fold increase in bolometric luminosity and a doubling of its radius, but its temperature stayed ≳29 kK. As it faded, it shifted to even hotter temperatures, exhibiting a WN7/WN8-type spectrum, and doubling its wind speed. MCA-1B is reminiscent of some supernova impostors, and its location resembles the isolated environment of SN 2009ip. It is most similar to HD 5980 (in the Small Magellanic Cloud) and GR 290 (also in M33). Whereas these two LBVs exhibited B-type spectra in eruption, MCA-1B is the first clear case where a Wolf–Rayet (WR) spectrum persisted at all times. Together, MCA-1B, HD 5980, and GR 290 constitute a class of WN-type LBVs, distinct from S Doradus LBVs. They are most interesting in the context of LBVs at low metallicity, a possible post-LBV/WR transition in binaries, and as likely Type Ibn supernova progenitors.
Abstract
We present optical and near-IR photometry and spectroscopy of SN 2013L for the first 4 yr post-explosion. SN 2013L was a moderately luminous (M
r
= −19.0) Type IIn supernova (SN) that showed ...signs of strong shock interaction with the circumstellar medium (CSM). The CSM interaction was equal to or stronger to SN 1988Z for the first 200 d and is observed at all epochs after explosion. Optical spectra revealed multicomponent hydrogen lines appearing by day 33 and persisting and slowly evolving over the next few years. By day 1509, the H α emission was still strong and exhibiting multiple peaks, hinting that the CSM was in a disc or torus around the SN. SN 2013L is part of a growing subset of SNe IIn that shows both strong CSM interaction signatures and the underlying broad lines from the SN ejecta photosphere. The presence of a blue H α emission bump and a lack of a red peak does not appear to be due to dust obscuration since an identical profile is seen in Pa β. Instead this suggests a high concentration of material on the near-side of the SN or a disc inclination of roughly edge-on and hints that SN 2013L was part of a massive interactive binary system. Narrow H α P-Cygni lines that persist through the entirety of the observations measure a progenitor outflow speed of 80–130 km s−1, speeds normally associated with extreme red supergiants, yellow hypergiants, or luminous blue variable winds. This progenitor scenario is also consistent with an inferred progenitor mass-loss rate of 0.3–8.0 × 10−3 M⊙ yr−1.
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