Nebular Spectroscopy Research Articles

Progenitor mass and ejecta asymmetry of supernova 2023ixf from nebular spectroscopy

Context. Supernova (SN) 2023ixf was discovered in the galaxy M 101 in May 2023. Its proximity provided the scientific community an extremely valuable opportunity to study the characteristics of the SN and its progenitor. A point source detected on archival images and hydrodynamical modeling of the bolometric light curve have been used to constrain the former star’s properties. There is a significant variation in the published results regarding the initial mass of the progenitor. Nebular spectroscopy can be used to enhance our understanding of the SN and its progenitor. Aims. We determined the SN progenitor mass by studying the first published nebular spectrum, taken 259 days after the explosion. Methods. We analyzed the nebular spectrum taken with GMOS at the Gemini North Telescope. We identified typical emission lines, such as [O I], Hα, and [Ca II], among others. Some species’ line profiles show broad and narrow components, indicating two ejecta velocities and an asymmetric ejecta. We inferred the progenitor mass of SN 2023ixf by comparing its spectra with synthetic spectra and by measuring the forbidden oxygen doublet flux. Results. Based on the flux ratio and the direct comparison with spectra models, the progenitor star of SN 2023ixf had a MZAMS between 12 and 15 M⊙. We find that using the [O I] doublet flux provides a less tight constraint on the progenitor mass. Our results agree with those from hydrodynamical modeling of the early light curve and pre-explosion image estimates that point to a relatively low-mass progenitor.

Ground-based and JWST Observations of SN 2022pul. II. Evidence from Nebular Spectroscopy for a Violent Merger in a Peculiar Type Ia Supernova

We present an analysis of ground-based and JWST observations of SN 2022pul, a peculiar “03fg-like” (or “super-Chandrasekhar”) Type Ia supernova (SN Ia), in the nebular phase at 338 days postexplosion. Our combined spectrum continuously covers 0.4–14 μm and includes the first mid-infrared spectrum of a 03fg-like SN Ia. Compared to normal SN Ia 2021aefx, SN 2022pul exhibits a lower mean ionization state, asymmetric emission-line profiles, stronger emission from the intermediate-mass elements (IMEs) argon and calcium, weaker emission from iron-group elements (IGEs), and the first unambiguous detection of neon in a SN Ia. A strong, broad, centrally peaked [Ne ii] line at 12.81 μm was previously predicted as a hallmark of “violent merger” SN Ia models, where dynamical interaction between two sub-M Ch white dwarfs (WDs) causes disruption of the lower-mass WD and detonation of the other. The violent merger scenario was already a leading hypothesis for 03fg-like SNe Ia; in SN 2022pul it can explain the large-scale ejecta asymmetries seen between the IMEs and IGEs and the central location of narrow oxygen and broad neon. We modify extant models to add clumping of the ejecta to reproduce the optical iron emission better, and add mass in the innermost region (<2000 km s−1) to account for the observed narrow [O i] λ λ6300, 6364 emission. A violent WD–WD merger explains many of the observations of SN 2022pul, and our results favor this model interpretation for the subclass of 03fg-like SNe Ia.

An Asymmetric Double-degenerate Type Ia Supernova Explosion with a Surviving Companion Star

We present nebular spectroscopy of SN 2020hvf, a Type Ia supernova (SN Ia) with an early bump in its light curve. SN 2020hvf shares many spectroscopic and photometric similarities to the carbon-rich high-luminosity “03fg-like” SNe Ia. At >240 days after peak brightness, we detect unambiguous emission from [Ca ii] λ λ7291, 7324, which is rarely observed in normal SNe Ia and only seen in peculiar subclasses. SN 2020hvf displays “sawtooth” emission profiles near 7300 Å that cannot be explained with single symmetric velocity components of [Fe ii], [Ni ii], and [Ca ii], indicating an asymmetric explosion. The broad [Ca ii] emission is best modeled by two velocity components offset by 1220 km s−1, which could be caused by ejecta associated with each star in the progenitor system, separated by their orbital velocity. For the first time in an SN Ia, we identify narrow (FWHM = 180 ± 40 km s−1) [Ca ii] emission, which we associate with a wind from a surviving, puffed-up companion star. Few published spectra have sufficient resolution and the signal-to-noise ratio necessary to detect similar narrow [Ca ii] emission; however, we have detected similar line profiles in other 03fg-like SNe Ia. The extremely narrow velocity width of [Ca ii] has only otherwise been observed in SNe Iax at late times. Since this event likely had a double-degenerate “super-Chandrasekhar” mass progenitor system, we suggest that a single white dwarf (WD) was fully disrupted and a wind from a surviving companion WD is producing the observed narrow emission. It is unclear whether this unique progenitor and explosion scenario can explain the diversity of 03fg-like SNe Ia, potentially indicating that multiple progenitor channels contribute to this subclass.

Type II-P supernova progenitor star initial masses and SN 2020jfo: direct detection, light-curve properties, nebular spectroscopy, and local environment

ABSTRACT We present optical, ultraviolet, and infrared data of the type II supernova (SN II) 2020jfo at 14.5 Mpc. This wealth of multiwavelength data allows us to compare different metrics commonly used to estimate progenitor masses of SN II for the same object. Using its early light curve, we infer SN 2020jfo had a progenitor radius of ≈700 R⊙, consistent with red supergiants of initial mass MZAMS =11–13 M⊙. The decline in its late-time light curve is best fit by a 56Ni mass of 0.018 ± 0.007 M⊙ consistent with that ejected from SN II-P with ≈13 M⊙ initial mass stars. Early spectra and photometry do not exhibit signs of interaction with circumstellar matter, implying that SN 2020jfo experienced weak mass-loss within the final years prior to explosion. Our spectra at &amp;gt;250 d are best fit by models from 12 M⊙ initial mass stars. We analysed integral field unit spectroscopy of the stellar population near SN 2020jfo, finding its massive star population had a zero age main sequence mass of 9.7$\substack{+2.5\\ -1.3}~{\rm M}_{\odot }$. We identify a single counterpart in pre-explosion imaging and find it has an initial mass of at most $7.2\substack{+1.2\\ -0.6}~{\rm M}_{\odot }$. We conclude that the inconsistency between this mass and indirect mass indicators from SN 2020jfo itself is most likely caused by extinction with AV = 2–3 mag due to matter around the progenitor star, which lowered its observed optical luminosity. As SN 2020jfo did not exhibit extinction at this level or evidence for interaction with circumstellar matter between 1.6 and 450 d from explosion, we conclude that this material was likely confined within ≈3000 R⊙ from the progenitor star.

Inferring the Progenitor Mass–Kinetic Energy Relation of Stripped-envelope Core-collapse Supernovae from Nebular Spectroscopy

The relation between the progenitor mass and the kinetic energy of the explosion is a key toward revealing the explosion mechanism of stripped-envelope core-collapse supernovae (SESNe). Here, we present a method to derive this relation using the nebular spectra of SESNe, based on the correlation between [O i]/[Ca ii], which is an indicator of progenitor mass, and the width of [O i], which measures the expansion velocity of the oxygen-rich material. To explain the correlation, the kinetic energy (E K) is required to be positively correlated with the progenitor mass as represented by the CO core mass (M CO). We demonstrate that SNe IIb/Ib and SNe Ic/Ic-BL follow the same M CO–E K scaling relation, which suggests that helium-rich and helium-deficient SNe share the same explosion mechanism. The M CO–E K relation derived in this work is compared with the ones derived from early phase observations. The results are largely in good agreement. Combined with early phase observations, the method presented in this work provides a chance to scan through the ejecta from the outermost region to the dense inner core, which is important to reveal the global properties of the ejecta and constrain the explosion mechanism of core-collapse SNe.

Inlists for "Inferring the progenitor mass-kinetic energy relation of stripped-envelope core-collapse supernovae from nebular spectroscopy"

Inlists for "Inferring the progenitor mass-kinetic energy relation of stripped-envelope core-collapse supernovae from nebular spectroscopy"

Vesto Slipher, Nebular Spectroscopy, and the Birth of Modern Cosmology, 1912–22

Vesto Slipher, Nebular Spectroscopy, and the Birth of Modern Cosmology, 1912–22

Progenitor and close-in circumstellar medium of type II supernova 2020fqv from high-cadence photometry and ultra-rapid UV spectroscopy

ABSTRACT We present observations of SN 2020fqv, a Virgo-cluster type II core-collapse supernova (CCSN) with a high temporal resolution light curve from the Transiting Exoplanet Survey Satellite (TESS) covering the time of explosion; ultraviolet (UV) spectroscopy from the Hubble Space Telescope (HST) starting 3.3 d post-explosion; ground-based spectroscopic observations starting 1.1 d post-explosion; along with extensive photometric observations. Massive stars have complicated mass-loss histories leading up to their death as CCSNe, creating circumstellar medium (CSM) with which the SNe interact. Observations during the first few days post-explosion can provide important information about the mass-loss rate during the late stages of stellar evolution. Model fits to the quasi-bolometric light curve of SN 2020fqv reveal 0.23 M⊙ of CSM confined within 1450 R⊙ (1014 cm) from its progenitor star. Early spectra (&amp;lt;4 d post-explosion), both from HST and ground-based observatories, show emission features from high-ionization metal species from the outer, optically thin part of this CSM. We find that the CSM is consistent with an eruption caused by the injection of ∼5 × 1046 erg into the stellar envelope ∼300 d pre-explosion, potentially from a nuclear burning instability at the onset of oxygen burning. Light-curve fitting, nebular spectroscopy, and pre-explosion HST imaging consistently point to a red supergiant (RSG) progenitor with $M_{\rm ZAMS}\approx 13.5\!-\!15 \, \mathrm{M}_{\odot }$, typical for SN II progenitor stars. This finding demonstrates that a typical RSG, like the progenitor of SN 2020fqv, has a complicated mass-loss history immediately before core collapse.

Direct Evidence of Two-component Ejecta in Supernova 2016gkg from Nebular Spectroscopy*

Abstract Spectral observations of the type-IIb supernova (SN) 2016gkg at 300–800 days are reported. The spectra show nebular characteristics, revealing emission from the progenitor star’s metal-rich core and providing clues to the kinematics and physical conditions of the explosion. The nebular spectra are dominated by emission lines of [O i] λλ6300, 6364 and [Ca ii] λλ7292, 7324. Other notable, albeit weaker, emission lines include Mg I] λ4571, [Fe ii] λ7155, O I λ7774, Ca II triplet, and a broad, boxy feature at the location of Hα. Unlike in other stripped-envelope SNe, the [O i] doublet is clearly resolved due to the presence of strong narrow components. The doublet shows an unprecedented emission line profile consisting of at least three components for each [O i]λ6300, 6364 line: a broad component (width ∼2000 km s−1), and a pair of narrow blue and red components (width ∼300 km s−1) mirrored against the rest velocity. The narrow component appears also in other lines, and is conspicuous in [O i]. This indicates the presence of multiple distinct kinematic components of material at low and high velocities. The low-velocity components are likely to be produced by a dense, slow-moving emitting region near the center, while the broad components are emitted over a larger volume. These observations suggest an asymmetric explosion, supporting the idea of two-component ejecta that influence the resulting late-time spectra and light curves. SN 2016gkg thus presents striking evidence for significant asymmetry in a standard-energy SN explosion. The presence of material at low velocity, which is not predicted in 1D simulations, emphasizes the importance of multidimensional explosion modeling of SNe.

Neutron-Capture Element Abundances in Planetary Nebulae

Nebular spectroscopy is a valuable tool for assessing the production of heavy elements by slow neutron(n)-capture nucleosynthesis (the s-process). Several transitions of n-capture elements have been identified in planetary nebulae (PNe) in the last few years, with the aid of sensitive, high-resolution, near-infrared spectrometers. Combined with optical spectroscopy, the newly discovered near-infrared lines enable more accurate abundance determinations than previously possible, and provide access to elements that had not previously been studied in PNe or their progenitors. Neutron-capture elements have also been detected in PNe in the Sagittarius Dwarf galaxy and in the Magellanic Clouds. In this brief review, I discuss developments in observational studies of s-process enrichments in PNe, with an emphasis on the last five years, and note some open questions and preliminary trends.

A hybrid envelope-stripping mechanism for massive stars from supernova nebular spectroscopy

When nuclear fuel in the core of a massive star with a zero-age main-sequence mass $M_{\rm ZAMS} \gtrsim 8M_\odot$ is exhausted, the central part of the iron or magnesium core collapses and forms a neutron star or a black hole. At the same time, the material above the collapsing core is rapidly ejected, leading to a stripped-envelope supernova (SESN) explosion if the outer hydrogen envelope of the star was removed before its explosion. The envelope is presumably stripped either via strong stellar winds or due to mass transfer to a companion star in a close binary orbit. It is not clear which process is dominant, and whether different mechanisms are at work for different classes of SESNe; type IIb, Ib, and Ic SNe in order of increasing degree of envelope stripping. In this work, a new analysis of late-time nebular spectra of SESNe is presented, which is more sensitive to differences in the core structure than early-phase spectral analysis. The results show that the progenitors of SNe IIb and Ib are indistinguishable except for the residual amount of hydrogen envelope while the progenitors of SNe Ic are not only deficient in hydrogen and helium, but are also distinctly more massive than SNe IIb and Ib. These findings strongly suggest that more than one mechanism is responsible for the removal of the outer hydrogen envelope and the deeper helium layer, with the former most likely due to binary interaction, and the latter involving a mass-dependent process such as strong stellar winds or episodic pre-explosion mass ejection.

Nebular Spectroscopy of Kepler’s Brightest Supernova

Abstract We present late-time (∼240–260 days after peak brightness) optical photometry and nebular (+236 and +264 days) spectroscopy of SN 2018oh, the brightest supernova (SN) Ia observed by the Kepler telescope. The Kepler/K2 30 minute cadence observations started days before explosion and continued past peak brightness. For several days after explosion, SN 2018oh had blue “excess” flux in addition to a normal SN rise. The flux excess can be explained by the interaction between the SN and a Roche-lobe filling non-degenerate companion star. Such a scenario should also strip material from the companion star that would emit once the SN ejecta become optically thin, imprinting relatively narrow emission features in its nebular spectrum. We search our nebular spectra for signs of this interaction, including close examination of wavelengths of hydrogen and helium transitions, finding no significant narrow emission. We place upper limits on the luminosity of these features of 2.6, 2.9 and 2.1 × 1037 erg s−1 for Hα, He i λ5875, and He i λ6678, respectively. Assuming a simple model for the amount of swept-up material, we estimate upper mass limits for hydrogen of 5.4 × 10−4 M ⊙ and helium of 4.7 × 10−4 M ⊙. Such stringent limits are unexpected for the companion-interaction scenario consistent with the early data. No known model can explain the excess flux, its blue color, and the lack of late-time narrow emission features.

Nebular spectroscopy of SN 2014J: Detection of stable nickel in near-infrared spectra

We present near-infrared (NIR) spectroscopy of the nearby supernova 2014J obtained ∼450 d after explosion. We detect the [Ni II] 1.939 μm line in the spectra indicating the presence of stable 58Ni in the ejecta. The stable nickel is not centrally concentrated but rather distributed as the iron. The spectra are dominated by forbidden [Fe II] and [Co II] lines. We used lines, in the NIR spectra, arising from the same upper energy levels to place constraints on the extinction from host galaxy dust. We find that that our data are in agreement with the high AV and low RV found in earlier studies from data near maximum light. Using a 56Ni mass prior from near maximum light γ-ray observations, we find 0.053 ± 0.018 M⊙ of stable nickel to be present in the ejecta. We find that the iron group features are redshifted from the host galaxy rest frame by ∼600 km s−1.

Nebular Spectroscopy of the “Blue Bump” Type Ia Supernova 2017cbv

Abstract We present nebular phase optical and near-infrared spectroscopy of the Type Ia supernova (SN) 2017cbv. The early light curves of SN 2017cbv showed a prominent blue bump in the U, B, and g bands lasting for ∼5 days. One interpretation of the early light curve is that the excess blue light is due to shocking of the SN ejecta against a nondegenerate companion star—a signature of the single degenerate scenario. If this is the correct interpretation, the interaction between the SN ejecta and the companion star could result in significant Hα (or helium) emission at late times, possibly along with other species, depending on the companion star and its orbital separation. A search for Hα emission in our +302 d spectrum yields a nondetection, with a L Hα &lt; 8.0 × 1035 erg s−1 (given an assumed distance of D = 12.3 Mpc), which we verified by implanting simulated Hα emission into our data. We make a quantitative comparison to models of swept-up material stripped from a nondegenerate companion star and limit the mass of hydrogen that might remain undetected to M H &lt; 1 × 10−4 M ⊙. A similar analysis of helium star related lines yields a M He &lt; 5 × 10−4 M ⊙. Taken at face value, these results argue against a nondegenerate H- or He-rich companion in Roche lobe overflow as the progenitor of SN 2017cbv. Alternatively, there could be weaknesses in the envelope-stripping and radiative transfer models necessary to interpret the strong H and He flux limits.

PHOTOIONIZATION HEATING OF NOVA EJECTA BY THE POST-OUTBURST SUPERSOFT SOURCE

The expanding ejecta from a classical nova remains hot enough ($\sim10^{4}\, {\rm K}$) to be detected in thermal radio emission for up to years after the cessation of mass loss triggered by a thermonuclear instability on the underlying white dwarf (WD). Nebular spectroscopy of nova remnants confirms the hot temperatures observed in radio observations. During this same period, the unstable thermonuclear burning transitions to a prolonged period of stable burning of the remnant hydrogen-rich envelope, causing the WD to become, temporarily, a super-soft X-ray source. We show that photoionization heating of the expanding ejecta by the hot WD maintains the observed nearly constant temperature of $(1-4)\times10^4\mathrm{~K}$ for up to a year before an eventual decline in temperature due to either the cessation of the supersoft phase or the onset of a predominantly adiabatic expansion. We simulate the expanding ejecta using a one-zone model as well as the Cloudy spectral synthesis code, both incorporating the time-dependent WD effective temperatures for a range of masses from $0.60\ M_{\odot}$ to $1.10\ M_{\odot}$. We show that the duration of the nearly isothermal phase depends most strongly on the velocity and mass of the ejecta and that the ejecta temperature depends on the WD's effective temperature, and hence its mass.

Nebular spectroscopy of the nearby Type IIb supernova 2011dh

We present nebular spectra of the nearby Type IIb supernova (SN) 2011dh taken between 201 and 678 days after core collapse. At these late times, SN 2011dh exhibits strong emission lines including a very broad and persistent H{\alpha} feature. New models of the nebular spectra confirm that the progenitor of SN 2011dh was a low-mass giant (M ~ 13 - 15 M_sun) that ejected ~0.07 M_sun of 56Ni and ~0.27 M_sun of oxygen at the time of explosion, consistent with the recent disappearance of a candidate yellow supergiant progenitor. We show that light from the SN location is dominated by the fading SN at very late times (~2 yr) and not, for example, by a binary companion or a background source. We present evidence for interaction between the expanding SN blastwave and a circumstellar medium at late times and show that the SN is likely powered by positron deposition by ~1 yr after explosion. We also examine the geometry of the ejecta and show that SN 2011dh's nebular line profiles indicate a globally spherical explosion with aspherical components or clumps.

The nebular spectra of the Type Ia supernova 1991bg: further evidence of a non-standard explosion

Type Ia supernovae (SNe Ia) come in a large range of luminosities, as determined mostly by the amount of 56Ni produced in the explosion. Nevertheless, they can be normalized and used as standard candles, which suggests that they share a similar origin. The thermonuclear explosion of a Chandrasekhar-mass (MCh) white dwarf accreting mass from a main sequence or red giant companion (the single degenerate scenario) is a favourite configuration, but the presence of SNe Ia that result from the merging of two white dwarfs of total mass exceeding MCh is supported by rate studies. SNe of the spectroscopically peculiar 1991bg class are the least luminous SNe Ia. They produce ∼0.1 M⊙ of 56Ni, which is difficult to reconcile with hydrodynamic explosion models. Here, the properties of the inner ejecta of SN 1991bg are investigated by means of synthetic nebular spectroscopy. In order to reproduce the transformation of the spectra from broader, [Fe ii]+[Fe iii] lines at day ∼120 to narrow, [Fe iii] lines at day ≳210, the innermost region must deviate significantly in density from the prediction of MCh models. In particular, a substantially lower density is required in the innermost ≈3000 km s−1 in order to provide the needed increase of ionization with time. This leads to a mass deficit of ∼0.15 M⊙ in the region inside ≈3000 km s−1 with respect to MCh models, and points to a different type of explosion. Early-time studies require a low explosion kinetic energy and lack of burning products in the outer layers. When combined with the results from this paper, the merger scenario may be a viable candidate for 1991bg-like SNe.

Atomic data for neutron-capture elements

We present total and final-state resolved charge transfer (CT) rate coefficients for low-charge Ge, Se, Br, Kr, Rb, and Xe ions reacting with neutral hydrogen over the temperature range 10^2--10^6 K. Each of these elements has been detected in ionized astrophysical nebulae, particularly planetary nebulae. CT rate coefficients are a key ingredient for the ionization equilibrium solutions needed to determine total elemental abundances from those of the observed ions. A multi-channel Landau Zener approach was used to compute rate coefficients for projectile ions with charges q=2-5, and for singly-charged ions the Demkov approximation was utilized. Our results for five-times ionized species are lower limits, due to the incompleteness of level energies in the NIST database. In addition, we computed rate coefficients for charge transfer ionization reactions between the neutral species of the above six elements and ionized hydrogen. The resulting total and state-resolved CT rate coefficients are tabulated and available at the CDS. In tandem with our concurrent investigations of other important atomic processes in photoionized nebulae, this work will enable robust investigations of neutron-capture element abundances and nucleosynthesis via nebular spectroscopy.

Advances in atomic data for neutron-capture elements

AbstractNeutron(n)-capture elements (atomic number Z &gt; 30), which can be produced in planetary nebula (PN) progenitor stars via s-process nucleosynthesis, have been detected in nearly 100 PNe. This demonstrates that nebular spectroscopy is a potentially powerful tool for studying the production and chemical evolution of trans-iron elements. However, significant challenges must be addressed before this goal can be achieved. One of the most substantial hurdles is the lack of atomic data for n-capture elements, particularly that needed to solve for their ionization equilibrium (and hence to convert ionic abundances to elemental abundances). To address this need, we have computed photoionization cross sections and radiative and dielectronic recombination rate coefficients for the first six ions of Se and Kr. The calculations were benchmarked against experimental photoionization cross section measurements. In addition, we computed charge transfer (CT) rate coefficients for ions of six n-capture elements. These efforts will enable the accurate determination of nebular Se and Kr abundances, allowing robust investigations of s-process enrichments in PNe.

New atomic data for trans-iron elements and their application to abundance determinations in planetary nebulae1This review is part of a Special Issue on the 10th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas.

Investigations of neutron(n)-capture element nucleosynthesis and chemical evolution have largely been based on stellar spectroscopy. However, the recent detection of these elements in several planetary nebulae (PNe) indicates that nebular spectroscopy is a promising new tool for such studies. In PNe, n-capture element abundance determinations reveal details of s-process nucleosynthesis and convective mixing in evolved low-mass stars, as well as the chemical evolution of elements that cannot be detected in stellar spectra. Only one or two ions of a given trans-iron element can typically be detected in individual nebulae. Elemental abundance determinations thus require corrections for the abundances of unobserved ions. Such corrections rely on the availability of atomic data for processes that control the ionization equilibrium of nebulae (e.g., photoionization cross sections and rate coefficients for various recombination processes). Until recently, these data were unknown for virtually all n-capture element ions. For the first six ions of Se, Kr, and Xe — the three most widely detected n-capture elements in PNe — we are calculating photoionization cross sections and radiative and dielectronic recombination rate coefficients using the multi-configuration Breit–Pauli atomic structure code AUTOSTRUCTURE. Charge transfer rate coefficients are being determined with a multichannel Landau–Zener code. To calibrate these calculations, we have measured absolute photoionization cross sections of Se and Xe ions at the Advanced Light Source synchrotron radiation facility. These atomic data can be incorporated into photoionization codes, which we will use to derive ionization corrections (hence abundances) for Se, Kr, and Xe in ionized nebulae. Using Monte Carlo simulations, we will investigate the effects of atomic data uncertainties on the derived abundances, illuminating the systems and atomic processes that require further analysis. These results are critical for honing nebular spectroscopy into a more effective tool for investigating the production and chemical evolution of trans-iron elements in the Universe.