Photospheric Temperature Research Articles

Pre-peak Emission in Tidal Disruption Events

The rising part of a tidal disruption event light curve provides unique insight into early emission and the onset of accretion. Various mechanisms are proposed to explain the pre-peak emission, including shocks from debris interaction and reprocessing of disk emission. We study the pre-peak emission and its influence on the gas circularization by a series of gray radiation hydrodynamic simulations with varying black hole mass. We find that, given a super-Eddington fallback rate of 10ṀEdd , the stream–stream collision can occur multiple times and drive strong outflows of up to 9ṀEdd . By dispersing gas to ≳100r s , the outflow can delay gas circularization and leads to sub-Eddington accretion rates during the first few stream–stream collisions. The stream–stream collision shock and circularization shock can sustain a luminosity of ∼1044 erg s−1 for days. The luminosity is generally sub-Eddington and shows a weak correlation with accretion rate at early times. The outflow is optically thick, yielding a reprocessing layer with a size of ∼1014 cm and photospheric temperature of ∼4 × 104 K.

SN 2021wuf: A transitional type Ia supernova with a low-velocity gradient

In this paper, we present an extensive analysis of SN 2021 wuf, a transition between Ia-norm and SN 1991T-like supernovae, which exploded at the periphery of the tidal bridge between the pair galaxy NGC 6500 and NGC 6501, at a redshift of z = 0.01. Our observations, ranging from -21 to +276 days relative to the B-band maximum light, reveal that SN 2021wuf exhibits properties akin to normal SNe Ia, with a peak absolute magnitude of $M_ max $(B)$\ and a post-peak decline rate of $ m_ $(B)$\ The peak bolometric luminosity of this SN is estimated as $1.58\ $\,erg\,$ s^ $, corresponding to a 56Ni mass of $ M_ Ni The spectral features, including high-velocity Si romannumeral2 lambda 6355 lines, a plateau in the Si romannumeral2 lambda 6355 velocity evolution and the nickel-to-iron ratio in the nebular phase, suggest a potential pulsating delayed detonation mechanism. The absence of intermediate-mass elements in the early phase and the high photospheric temperature, as inferred from the line-strength ratio of Si\ romannumeral2 \,lambda 5972 to Si\ romannumeral2 \,lambda 6355 (named as R(Si romannumeral2 )), further support this classification.

Why Observations at Mid-infrared Wavelengths Partially Mitigate M Dwarf Star Host Stellar Activity Contamination in Exoplanet Transmission Spectroscopy

Exoplanet atmosphere transmission spectroscopy for planets orbiting M dwarf stars faces significant challenges due to contamination from stellar magnetic features, i.e., spots and faculae. These features make the stellar surface inhomogeneous and introduce wavelength-dependent signals in the transmission spectrum that complicate its analysis. We identify and explain why using observations at infrared wavelengths greater than a few microns partially mitigates stellar contamination. At these wavelengths the intensity sensitivity to temperature weakens, with two significant consequences. First, the contribution of spots and faculae has a diminished effect because their flux contrast to the quiet-star regions lessens. Second, the star’s spectral features compress in magnitude, an outcome of spectral features being shaped by the star’s photospheric vertical temperature gradient. Both factors are due to the Planck function moving from exponential to linear in temperature toward mid-infrared (mid-IR) wavelengths (the “Rayleigh–Jeans tail”). In contrast to stellar spectra, the depth of the transmission spectroscopy features does not fundamentally vary with wavelength as it is primarily determined by the planet’s atmospheric scale height. The magnitude of reduction in stellar contamination is a factor of a few to several at mid-IR versus near-IR wavelengths, but whether or not this is enough to bypass stellar contamination ultimately depends on the spot coverage area. Nonetheless, the flattening of thermal emission spectral features at IR wavelengths is universal.

Physics of nova outbursts: Theoretical models of classical nova outbursts with optically thick winds on 1.2 M⊙ and 1.3 M⊙ white dwarfs

Abstract We present time-dependent nova outburst models with optically thick winds for 1.2 and 1.35$\, M_{\odot }$ white dwarfs (WDs) with a mass-accretion rate of $5 \times 10^{-9}\, M_{\odot }$ yr−1 and for a 1.3$\, M_{\odot }$ WD with $2 \times 10^{-9}\, M_{\odot }$ yr−1. The X-ray flash occurs 11 d before the optical peak of the 1.2$\, M_{\odot }$ WD and 2.5 d before the peak of the 1.3$\, M_{\odot }$ WD. The wind mass-loss rate of the 1.2$\, M_{\odot }$ WD (1.3$\, M_{\odot }$ WD) reaches a peak of $6.4 \times 10^{-5}\, M_{\odot }$ yr−1 ($7.4 \times 10^{-5}\, M_{\odot }$ yr−1) at the epoch of the maximum photospheric expansion with the lowest photospheric temperature of log Tph (K) = 4.33 (4.35). The nuclear energy generated during the outburst is lost in the form of radiation (61% for the 1.2$\, M_{\odot }$ WD; 47% for the 1.3$\, M_{\odot }$ WD), gravitational energy of ejecta (39%; 52%), and kinetic energy of the wind (0.28%; 0.29%). We found an empirical relation for fast novae between the time to optical maximum from the outburst tpeak and the expansion timescale τexp. With this relation, we are able to predict the time to optical maximum tpeak from the ignition model (at t = 0) without following a time-consuming nova wind evolution.

The Effect of Starspots on Spectroscopic Age and Mass Estimates of Nonaccreting T Tauri Stars in the Taurus–Auriga Star-forming Region

Accurate age and mass determinations for young pre-main-sequence stars are made challenging by the presence of large-scale starspots. We present results from a near-infrared spectroscopic survey of 10 T-Tauri Stars in Taurus-Auriga that characterize spot-filling factors and temperatures, the resulting effects on temperature and luminosity determinations, and the consequences for inferred stellar masses and ages. We constructed composite models of spotted stars by combining BTSettl-CIFIST synthetic spectra of atmospheres to represent the spots and the photosphere along with continuum emission from a warm inner disk. Using a Markov Chain Monte Carlo algorithm, we find the best-fit spot and photospheric temperatures, spot-filling factors, as well as disk-filling factors. This methodology allowed us to reproduce the 0.75–2.40 μm stellar spectra and molecular feature strengths for all of our targets, disentangling the complicated multicomponent emission. For a subset of stars with multiepoch observations spanning an entire stellar rotation, we correlate the spectral variability and changes in the filling factors with rotational periods observed in K2 and AAVSO photometry. Combining spot-corrected effective temperatures and Gaia distances, we calculate luminosities and use the Stellar Parameters of Tracks with Starspots models to infer spot-corrected masses and ages for our sample of stars. Our method of accounting for spots results in an average increase of 60% in mass and a doubling in age with respect to traditional methods using optical spectra that do not account for the effect of spots.

Cosmological Distance Measurement of Twelve Nearby Supernovae IIP with ROTSE-IIIb

We present cosmological analysis of 12 nearby (z < 0.06) Type IIP supernovae (SNe IIP) observed with the ROTSE-IIIb telescope. To achieve precise photometry, we present a new image-differencing technique that is implemented for the first time on the ROTSE SN photometry pipeline. With this method, we find up to a 20% increase in the detection efficiency and significant reduction in residual rms scatter of the SN lightcurves when compared to the previous pipeline performance. We use the published optical spectra and broadband photometry of well-studied SNe IIP to establish temporal models for ejecta velocity and photospheric temperature evolution for our SNe IIP population. This study yields measurements that are competitive with other methods even when the data are limited to a single epoch during the photospheric phase of SNe IIP. Using the fully reduced ROTSE photometry and optical spectra, we apply these models to the respective photometric epochs for each SN in the ROTSE IIP sample. This facilitates the use of the Expanding Photosphere Method (EPM) to obtain distance estimates to their respective host galaxies. We then perform cosmological parameter fitting using these EPM distances, from which we measure the Hubble constant to be 72.9−4.3+5.7kms−1Mpc−1 , which is consistent with the standard ΛCDM model values derived using other independent techniques.

A precursor plateau and pre-maximum [O ii] emission in the superluminous SN2019szu: a pulsational pair-instability candidate

ABSTRACT We present a detailed study on SN2019szu, a Type I superluminous supernova at z = 0.213 that displayed unique photometric and spectroscopic properties. Pan-STARRS and ZTF forced photometry show a pre-explosion plateau lasting ∼40 d. Unlike other SLSNe that show decreasing photospheric temperatures with time, the optical colours show an apparent temperature increase from ∼15 000 to ∼20 000 K over the first 70 d, likely caused by an additional pseudo-continuum in the spectrum. Remarkably, the spectrum displays a forbidden emission line (likely attributed to λλ7320,7330) visible 16 d before maximum light, inconsistent with an apparently compact photosphere. This identification is further strengthened by the appearances of [O iii] λλ4959, 5007, and [O iii] λ4363 seen in the spectrum. Comparing with nebular spectral models, we find that the oxygen line fluxes and ratios can be reproduced with ∼0.25 M⊙ of oxygen-rich material with a density of $\sim 10^{-15}\, \rm {g\, cm}^{-3}$. The low density suggests a circumstellar origin, but the early onset of the emission lines requires that this material was ejected within the final months before the terminal explosion, consistent with the timing of the precursor plateau. Interaction with denser material closer to the explosion likely produced the pseudo-continuum bluewards of ∼5500 Å. We suggest that this event is one of the best candidates to date for a pulsational pair-instability ejection, with early pulses providing the low density material needed for the formation of the forbidden emission line, and collisions between the final shells of ejected material producing the pre-explosion plateau.

Early-time Ultraviolet and Optical Hubble Space Telescope Spectroscopy of the Type II Supernova 2022wsp

We report early-time ultraviolet (UV) and optical spectroscopy of the young, nearby Type II supernova (SN) 2022wsp obtained by the Hubble Space Telescope (HST)/STIS at about 10 and 20 days after the explosion. The SN 2022wsp UV spectra are compared to those of other well-observed Type II/IIP SNe, including the recently studied Type IIP SN 2021yja. Both SNe exhibit rapid cooling and similar evolution during early phases, indicating a common behavior among SNe II. Radiative-transfer modeling of the spectra of SN 2022wsp with the TARDIS code indicates a steep radial density profile in the outer layer of the ejecta, a solar metallicity, and a relatively high total extinction of E(B − V) = 0.35 mag. The early-time evolution of the photospheric velocity and temperature derived from the modeling agree with the behavior observed from other previously studied cases. The strong suppression of hydrogen Balmer lines in the spectra suggests interaction with a preexisting circumstellar environment could be occurring at early times. In the SN 2022wsp spectra, the absorption component of the Mg ii P Cygni profile displays a double-trough feature on day +10 that disappears by day +20. The shape is well reproduced by the model without fine-tuning the parameters, suggesting that the secondary blueward dip is a metal transition that originates in the SN ejecta.

Type W and Type 15bn Subgroups of Hydrogen-poor Superluminous Supernovae: Premaximum Diversity, Postmaximum Homogeneity?

In this study, we analyze the postmaximum spectra of a sample of 27 Type I superluminous supernovae (SLSNe-I) in order to search for physical differences between the so-called Type W and Type 15bn subtypes. This paper is a continuation of Könyves-Tóth & Vinkó and Könyves-Tóth. In the former, it was revealed that not all SLSNe-I show the W-shaped absorption feature between 4000 and 5000 Å in the premaximum spectra, and two new SLSN subgroups were disclosed. In the latter, physical differences in the premaximum phases were studied. For completeness, postmaximum data are analyzed in this paper. It is concluded that in terms of photospheric temperature and velocity, Type W and Type 15bn SLSNe-I decrease to a similar value by the postmaximum phases, and their pseudo-nebular spectra are nearly uniform. The relation between the photometric and spectroscopic phases (Φ) between the two subgroups was examined, and it was found that the Φ of Type W SLSNe-I increases monotonically with time, while Type 15bn objects tend to show larger Φ before peak brightness, which evolves slowly. Pseudo-equivalent width (pEW) calculations show that the pEWs of the wavelength range between 4166 and 5266 Å evolve differently in the cases of the two subtypes, while the other parts of the spectra seem to evolve similarly. It was found that the host galaxies of the studied objects do not differ significantly in their star formation rate, morphology, stellar mass, and absolute brightness. The main difference behind the bimodality of Type W and Type 15bn SLSNe-I therefore is in their premaximum evolution.

Light-curve modelling for the initial rising phase of rapidly evolving transients powered by continuous outflow

ABSTRACT A wind-driven model is a new framework to model observational properties of transients that are powered by continuous outflow from a central system. While it has been applied to fast blue optical transients (FBOTs), the applicability has been limited to post-peak behaviours due to the steady-state assumptions; non-steady-state physics, e.g. expanding outflow, is important to model the initial rising phase. In this paper, we construct a time-dependent wind-driven model, which can take into account the expanding outflow and the time evolution of the outflow rate. We apply the model to a sample of well-observed FBOTs. FBOTs require high outflow rates (∼30 M⊙ yr−1) and fast velocities (∼0.2–0.3c), with the typical ejecta mass and energy budget of ∼0.2 M⊙ and ∼1052 erg, respectively. The energetic outflow supports the idea that the central engine of FBOTs may be related to a relativistic object, e.g. a black hole. The initial photospheric temperature is 105 –106 K, which suggests that FBOTs will show ultraviolet (UV) or X-ray flash similar to supernova shock breakouts. We discuss future prospects of surveys and follow-up observations of FBOTs in the UV bands. FBOTs are brighter in the UV bands than in the optical bands, and the time-scale is a bit longer than that in optical wavelengths. We suggest that UV telescopes with a wide field of view can play a key role in discovering FBOTs and characterizing their natures.

Correlating Changes in Spot Filling Factors with Stellar Rotation: The Case of LkCa 4

We present a multi-epoch spectroscopic study of LkCa 4, a heavily spotted non-accreting T Tauri star. Using SpeX at NASA’s Infrared Telescope Facility (IRTF), 12 spectra were collected over five consecutive nights, spanning ≈1.5 stellar rotations. Using the IRTF SpeX Spectral Library, we constructed empirical composite models of spotted stars by combining a warmer (photosphere) standard star spectrum with a cooler (spot) standard weighted by the spot filling factor, f spot. The best-fit models spanned two photospheric component temperatures, T phot = 4100 K (K7V) and 4400 K (K5V), and one spot component temperature, T spot = 3060 K (M5V) with an A V of 0.3. We find values of f spot to vary between 0.77 and 0.94 with an average uncertainty of ∼0.04. The variability of f spot is periodic and correlates with its 3.374 day rotational period. Using a mean value for f mean spot to represent the total spot coverage, we calculated spot corrected values for T eff and L ⋆. Placing these values alongside evolutionary models developed for heavily spotted young stars, we infer mass and age ranges of 0.45–0.6 M ⊙ and 0.50–1.25 Myr, respectively. These inferred values represent a twofold increase in the mass and a twofold decrease in the age as compared to standard evolutionary models. Such a result highlights the need for constraining the contributions of cool and warm regions of young stellar atmospheres when estimating T eff and L ⋆ to infer masses and ages as well as the necessity for models to account for the effects of these regions on the early evolution of low-mass stars.

SN 2018hna: Adding a piece to the puzzles of the explosion of blue supergiants

ABSTRACT We present extensive optical/ultraviolet observations and modelling analysis for the nearby SN 1987A-like peculiar Type II supernova (SN) 2018hna. Both photometry and spectroscopy covered phases extending to &amp;gt;500 d after the explosion, making it one of the best-observed SN II of this subtype. SN 2018hna is obviously bluer than SN 1987A during the photospheric phase, suggesting higher photospheric temperature, which may account for weaker Ba ii λ6142 lines in its spectra. Analysis of early-time temperature evolution suggests a radius of ∼45 R⊙ for the progenitor of SN 2018hna, consistent with a blue supergiant (BSG). By fitting the bolometric light curve with hydrodynamical models, we find that SN 2018hna has an ejecta mass of ∼(13.7–17.7) M⊙, a kinetic energy of ∼(1.0–1.2) × 1051 erg, and a 56Ni mass of about 0.05 M⊙. Moreover, based on standard stellar evolution and the oxygen mass (0.44–0.73 M⊙) deduced from nebular [O i] lines, the progenitor of SN 2018hna is expected to have an initial main-sequence mass &amp;lt;16 M⊙. In principle, such a relatively low-mass star cannot end as a BSG just before core-collapse, except some unique mechanisms are involved, such as rapid rotation, restricted semiconvection, etc. On the other hand, binary scenario may be more favourable, like in the case of SN 1987A. While the much lower oxygen mass inferred for SN 2018hna may imply that its progenitor system also had much lower initial masses than that of SN 1987A.

The IGRINS YSO Survey. III. Stellar Parameters of Pre-main-sequence Stars in Ophiuchus and Upper Scorpius

We used the Immersion GRating Infrared Spectrometer (IGRINS) to determine fundamental parameters for 61 K- and M-type young stellar objects (YSOs) located in the Ophiuchus and Upper Scorpius star-forming regions. We employed synthetic spectra and a Markov chain Monte Carlo approach to fit specific K-band spectral regions and determine the photospheric temperature (T), surface gravity (), magnetic field strength (B), projected rotational velocity (), and K-band veiling (r K ). We determined B for ∼46% of our sample. Stellar parameters were compared to the results from Taurus-Auriga and the TW Hydrae association presented in Paper I of this series. We classified all the YSOs in the IGRINS survey with infrared spectral indices from Two Micron All Sky Survey and Wide-field Infrared Survey Explorer photometry between 2 and 24 μm. We found that Class II YSOs typically have lower and , similar B, and higher K-band veiling than their Class III counterparts. Additionally, we determined the stellar parameters for a sample of K and M field stars also observed with IGRINS. We have identified intrinsic similarities and differences at different evolutionary stages with our homogeneous determination of stellar parameters in the IGRINS YSO survey. Considering as a proxy for age, we found that the Ophiuchus and Taurus samples have a similar age. We also find that Upper Scorpius and TWA YSOs have similar ages, and are more evolved than Ophiuchus/Taurus YSOs.

Probing the tidal disruption event iPTF16axa with cloudy and disc-wind models

ABSTRACT We present both a disc-wind model on the optical/ultraviolet (UV) emission continuum and cloudy modelling on the spectral lines of the tidal disruption event (TDE) intermediate Palomar Transient Factory (iPTF)16axa to understand the disc-wind emission and the properties of the atmosphere that impacts the line luminosity of the TDE. Assuming the optical/UV emission from the wind due to the disc super-Eddington phase, we use the steady structured disc-wind model with a spherical wind with constant velocity to fit the observations on multiple days. The extracted parameters are stellar-mass M⋆ = 6.20 ± 1.19M⊙, disc radiative efficiency log10(η) = −1.22 ± 1.327, wind inner radius rl = (2.013 ± 0.551) × 1014 cm, and velocity vw = 18999.4 ± 1785.1 km s−1. The photosphere temperature for wind emission is ∼2 × 104 K and the disc single blackbody temperature is ∼0.995 × 105 K. We also perform cloudy modelling to explain the observed He and H line luminosities that estimate a wind inner radius rl = 7.07 × 1014 cm and velocity vw = 1.3 × 104 km s−1. The independent analyses of iPTF16axa using cloudy and disc-wind models show comparable results that agree with observations. The cloudy modelling finds that both the super-solar abundance of He and a smaller He II line optical depth is responsible for the enhancement of He II line luminosity over the Hα line luminosity. The super-solar abundance of He II agrees with a relatively large stellar mass and suggests that the disrupted star might have been a red giant.

Premaximum Spectroscopic Diversity of Hydrogen-poor Superluminous Supernovae

We search for the reasons behind the spectroscopic diversity of hydrogen-poor superluminous supernovae (SLSNe-I) in the premaximum phase. Our analysis is a continuation of the paper of Könyves-Tóth & Vinkó, who disclosed two new subtypes of SLSNe-I characterized by the presence/absence of a W-shaped absorption feature in their premaximum spectra between 4000 and 5000 Å (called Type W and Type 15bn, respectively). However, the physical cause of this bimodality is still uncertain. Here we present premaximum spectral synthesis of 27 SLSNe-I with special attention to the photospheric temperature (T phot) and velocity (v phot) evolution. We find that a T phot limit of 12,000 K separates the Type W and Type 15bn SLSNe-I: Type W objects tend to show T phot ≥ 12,000 K, while Type 15bn ones have T phot ≤ 12,000 K. This is consistent with the chemical composition of the studied objects. Another difference between these groups may be found in their ejecta geometry: Type W SLSNe-I may show null polarization, implying spherical symmetry, while the polarization of Type 15bn objects may increase in time. This suggests a two-component model with a spherical outer carbon–oxygen layer and an asymmetric inner layer containing heavier ions. The two subgroups may have different light-curve evolution as well, since six Type W objects show early bumps, unlike Type 15bn SLSNe-I. This feature, however, needs further study, as it is based on only a few objects at present.

1-D dusty photo-ionization models of WR planetary nebulae NGC 2452 and IC 2003

We report dusty photo-ionization models for two Planetary Nebulae NGC 2452 and IC 2003, which have [WR] type central stars, using 1D photo-ionization code Cloudy17.02. We used the medium resolution optical spectra and archival IRAS photometry to constrain our models. The physical size of the ionized nebula derived using accurate distance measurements and absolute Hβ flux available in the literature were used as additional constrains. We examine the importance of photo-electric heating and found that models with and without considering photo-electric heating do not make significant difference for both PNe for the MRN grain size distribution considered in this study. We derive the nebular elemental abundances of these PNe by the empirical method as well as by making dusty photo-ionization models. The values of N/O ratios for both PNe obtained from our models are lower than their respective values arrived using empirical methods. The central stars are assumed to be black bodies and the photospheric temperatures derived respectively for NGC 2452 and IC 2003 from their best fit models are 182 kK and 155 kK and their respective luminosities are 630L⊙ and 1015L⊙. We propose that both the PNe were resulted from low-mass progenitors of mass ⩽2.8 M⊙.

Physics of nova outbursts: A theoretical model of classical nova outbursts with self-consistent wind mass loss

Abstract We present a model for one cycle of a classical nova outburst based on a self-consistent wind mass loss accelerated by the gradient of radiation pressure, i.e., so-called optically thick winds. Evolution models are calculated by a Henyey code for a 1.0 $M_{\odot }$ white dwarf with a mass-accretion rate of 5 × 10−9 $M_{\odot }$ yr−1. The outermost part of the hydrogen-rich envelope is connected to a steadily moving envelope where optically thick winds occur. We confirm that no internal shock waves occur at thermonuclear runaway. The wind mass-loss rate reaches a peak of 1.4 × 10−4 $M_{\odot }$ yr−1 at the epoch of the maximum photospheric expansion, where the photospheric temperature decreases to log Tph (K) = 3.90. Almost all of the accreted mass is lost in the wind. The nuclear energy generated in hydrogen burning is lost in a form of photon emission (64%), gravitational energy (lifting up the wind matter against gravity, 35%), and the kinetic energy of the wind (0.23%). A classical nova should be very bright in a far-UV (100–300 Å) band for one day just after the onset of thermonuclear runaway (∼ 25 d before the optical maximum). In the decay phase of the nova outburst, the envelope structure is very close to that of a steady-state solution.

Bumpy Declining Light Curves Are Common in Hydrogen-poor Superluminous Supernovae

Recent work has revealed that the light curves of hydrogen-poor (Type I) superluminous supernovae (SLSNe), thought to be powered by magnetar central engines, do not always follow the smooth decline predicted by a simple magnetar spin-down model. Here we present the first systematic study of the prevalence and properties of “bumps” in the post-peak light curves of 34 SLSNe. We find that the majority (44%–76%) of events cannot be explained by a smooth magnetar model alone. We do not find any difference in supernova properties between events with and without bumps. By fitting a simple Gaussian model to the light-curve residuals, we characterize each bump with an amplitude, temperature, phase, and duration. We find that most bumps correspond with an increase in the photospheric temperature of the ejecta, although we do not see drastic changes in spectroscopic features during the bump. We also find a moderate correlation (ρ ≈ 0.5; p ≈ 0.01) between the phase of the bumps and the rise time, implying that such bumps tend to happen at a certain “evolutionary phase,” (3.7 ± 1.4)t rise. Most bumps are consistent with having diffused from a central source of variable luminosity, although sources further out in the ejecta are not excluded. With this evidence, we explore whether the cause of these bumps is intrinsic to the supernova (e.g., a variable central engine) or extrinsic (e.g., circumstellar interaction). Both cases are plausible, requiring low-level variability in the magnetar input luminosity, small decreases in the ejecta opacity, or a thin circumstellar shell or disk.

Variable thermal energy injection from magnetar spin-down as a possible cause of stripped-envelope supernova light-curve bumps

ABSTRACT Luminosity evolution of some stripped-envelope supernovae such as Type I superluminous supernovae is difficult to explain by the canonical 56Ni nuclear decay heating. A popular alternative heating source is rapid spin-down of strongly magnetized rapidly rotating neutron stars (magnetars). Recent observations have indicated that Type I superluminous supernovae often have bumpy light curves with multiple luminosity peaks. The cause of bumpy light curves is unknown. In this study, we investigate the possibility that the light-curve bumps are caused by variations of the thermal energy injection from magnetar spin-down. We find that a temporal increase in the thermal energy injection can lead to multiple luminosity peaks. The multiple luminosity peaks caused by the variable thermal energy injection is found to be accompanied by significant increase in photospheric temperature, and photospheric radii are not significantly changed. We show that the bumpy light curves of SN 2015bn and SN 2019stc can be reproduced by temporarily increasing magnetar spin-down energy input by a factor of 2–3 for 5–20 d. However, not all the light-curve bumps are accompanied by the clear photospheric temperature increase as predicted by our synthetic models. In particular, the secondary light-curve bump of SN 2019stc is accompanied by a temporal increase in photospheric radii rather than temperature, which is not seen in our synthetic models. We therefore conclude that not all the light-curve bumps observed in luminous supernovae are caused by the variable thermal energy injection from magnetar spin-down and some bumps are likely caused by a different mechanism.

Effects of Winds on the Optical Properties of Type Ib and Ic Supernova Progenitors

Abstract We investigate the effects of winds on the observational properties of Type Ib and Ic supernova (SN Ib/Ic) progenitors using spectral models constructed with the non-LTE stellar atmospheric code CMFGEN. We consider SN Ib/Ic progenitor models of the final mass range of 2.16–9.09 M ⊙ having different surface temperatures and chemical compositions, and calculate the resulting spectra for various wind mass-loss rates and wind terminal velocities. We find that the progenitors having an optically thick wind would become brighter in the optical for a higher mass-loss rate (or a lower wind terminal velocity) because of the formation of the photosphere in the extended wind matter and the contribution from free–free and line emissions from the wind. As a result, for the standard Wolf-Rayet wind mass-loss rate, helium-deficient compact SN Ic progenitors would be brighter in the optical by ∼3 mag compared to the case without the wind effects. We also find that the color dependence on the photospheric temperature is non-monotonic because of the wind effects. Our results imply that inferring the progenitor mass, bolometric luminosity, and effective temperature from the optical observation using the standard stellar evolution model prediction can be misleading. By comparing our fiducial model predictions with the detection limits of the previous SN Ib/Ic progenitor searches, we conclude that a deep search with an optical absolute magnitude larger than ∼−4 is needed to directly identify most of the ordinary SN Ib/Ic progenitors. We discuss implications of our results for the observed SN Ib/Ic progenitor candidates for iPTF13bvn, SN 2019vyr, and SN 2017ein.