Understanding Type Ia Supernova Diversity with PHOENIX

Type Ia Supernovae (SNe Ia) are important astrophysical objects. They produce roughly half of the iron group elements found in the universe, the energy they release drives the evolution of galaxies, and their high intrinsic luminosities allow them to be seen across cosmological distances. Through the width-luminosity relation (often called the Phillips relation), they can be used as ``standardizable candles" to serve as cosmological distance indicators and were instrumental in the discovery of the accelerating expansion rate of the universe. However, despite decades of detailed study, many fundamental questions about these objects remain; including the exact nature of their progenitor systems and the mechanism(s) by which they explode. UV spectra are unique probe of SNe Ia physics and their evolution in throughout the history of the universe, as much of the information about the properties of the progenitor star and the explosion mechanism are encoded in the outer layers of the ejecta; a region that ultraviolet spectra probe at later times than the optical. The ultraviolet properties of SNe Ia are much more diverse than in the optical and near infrared, and may vary with redshift. The ultraviolet properties of SNe Ia have the ability to help us unlock their true nature; but have historically been under-studied due to difficulties in obtaining observations in this wavelength regime. However, recent growth in the data sets of SNe Ia observed with the Hubble Space Telescope and the Neil Gehrels Swift Observatory (Swift), have illuminated the need for detailed modeling of this region in order to perform the differential comparisons necessary to further our understanding of these objects and improve their use as cosmological distance indicators. In Part I, I discuss the theoretical foundations of this work. I briefly review the important aspects of spectral formation in the ultraviolet of SNe Ia, and introduce the two codes SYNOW and PHOENIX) used to generate synthetic spectra of SNe Ia. I apply the spectra generated from these codes to the UV spectra of SN~2011fe and for the first time make line identifications in all the major ultraviolet features near maximum light, including the first ever identifications of C IV and Si IV in a SNe Ia spectrum. Then, using the suite of PHOENIX models, I explore the impact of luminosity variations on the ultraviolet spectra and discuss the connections I find between the ultraviolet and other wavelength regimes. In Part II, I shift focus and discuss how differential comparisons in observational studies can further our understanding of SNe Ia. Chapter 4 details the science cases behind nearly five years of observations using the Astrophysical Research Consortium 3.5-m telescope at Apache Point Observatory, including highlighting instances where my observations contributed to the advancement of our understanding of the underlying physics of all types of supernovae and their progenitors. Chapter 5 focuses on SN 2021fxy, a Type Ia supernovae observed extensively in multiple wavelength regimes by the Precision Observations of Infant Supernovae (POISE) collaboration, for which ultraviolet spectra were obtained with the Hubble Space Telescope. In comparing SN 2021fxy to the broader sample of spectroscopically normal SNe Ia with ultraviolet spectra from HST, I show that mid-ultraviolet flux suppression is a common feature of SNe Ia and discuss possible mechanisms that cuase this flux suppression and how they may be connected to different progenitors and explosion mechanisms. Additionally, I show that SN 2021fxy is substantially similar to another SN Ia with mid-ultraviolet suppression, SN 2017erp, and illustrate how luminosity variations between the two SNe Ia may be responsible for the observed flux differences between them

Direct Analysis of the Broad-Line SN 2019ein: Connection with the Core-Normal SN 2011fe

Type Ia supernovae (SNe Ia) are important cosmological probes and contributors to galactic nucleosynthesis, particularly of the iron group elements. To improve both their reliability as cosmological probes and to understand galactic chemical evolution, it is vital to understand the binary progenitor system and explosion mechanism. The classification of SNe Ia into Branch groups has led to some understanding of the similarities and differences among the varieties of observed SNe Ia. However, partly due to small sample size, little work has been done on the broad-line or 02bo group. We perform direct spectral analysis on the pre-maximum spectra of the broad-line SN 2019ein and compare and contrast it to the core-normal SN~2011fe. Both SN 2019ein and SN 2011fe were first observed spectroscopically within two days of discovery, allowing us to follow the spectroscopic evolution of both supernovae in detail. We find that the optical depths of the primary features of both the CN and BL supernovae are very similar, except that there is a velocity shift between them. We further examine the SN 2002bo-like subclass and show that for nine objects with pre-maximum spectra in the range -6 -- -2 days with respect to B-maximum all the emission peaks of the Si II {\lambda}6355 line of BL are blueshifted pre-maximum, making this a simple classification criterion.Comment: 9 pages, 9 figures, accepted for publication in MNRA

Evaluating the Consistency of Cosmological Distances Using Supernova Siblings in the Near-Infrared

The study of supernova siblings, supernovae with the same host galaxy, is an important avenue for understanding and measuring the properties of Type Ia Supernova (SN Ia) light curves (LCs). Thus far, sibling analyses have mainly focused on optical LC data. Considering that LCs in the near-infrared (NIR) are expected to be better standard candles than those in the optical, we carry out the first analysis compiling SN siblings with only NIR data. We perform an extensive literature search of all SN siblings and find six sets of siblings with published NIR photometry. We calibrate each set of siblings ensuring they are on homogeneous photometric systems, fit the LCs with the SALT3-NIR and SNooPy models, and find median absolute differences in $\mu$ values between siblings of 0.248 mag and 0.186 mag, respectively. To evaluate the significance of these differences beyond measurement noise, we run simulations that mimic these LCs and provide an estimate for uncertainty on these median absolute differences of $\sim$0.052 mag, and we find that our analysis supports the existence of intrinsic scatter in the NIR at the 99% level. When comparing the same sets of SN siblings, we observe a median absolute difference in $\mu$ values between siblings of 0.177 mag when using optical data alone as compared to 0.186 mag when using NIR data alone. We attribute this to either limited statistics, poor quality NIR data, or poor reduction of the NIR data; all of which will be improved with the Nancy Grace Roman Space Telescope.Comment: 13 pages, 6 figures. Accepted into Ap

Observations of SN 2017ein Reveal Shock Breakout Emission and A Massive Progenitor Star for a Type Ic Supernova

We present optical and ultraviolet observations of nearby type Ic supernova SN 2017ein as well as detailed analysis of its progenitor properties from both the early-time observations and the prediscovery Hubble Space Telescope (HST) images. The optical light curves started from within one day to $\sim$275 days after explosion, and optical spectra range from $\sim$2 days to $\sim$90 days after explosion. Compared to other normal SNe Ic like SN 2007gr and SN 2013ge, \mbox{SN 2017ein} seems to have more prominent C{\footnotesize II} absorption and higher expansion velocities in early phases, suggestive of relatively lower ejecta mass. The earliest photometry obtained for \mbox{SN 2017ein} show indications of shock cooling. The best-fit obtained by including a shock cooling component gives an estimate of the envelope mass as $\sim$0.02 M$_{\odot}$ and stellar radius as 8$\pm$4 R$_{\odot}$. Examining the pre-explosion images taken with the HST WFPC2, we find that the SN position coincides with a luminous and blue point-like source, with an extinction-corrected absolute magnitude of M$_V$$\sim$$-$8.2 mag and M$_I$$\sim$$-$7.7 mag.Comparisons of the observations to the theoretical models indicate that the counterpart source was either a single WR star or a binary with whose members had high initial masses, or a young compact star cluster. To further distinguish between different scenarios requires revisiting the site of the progenitor with HST after the SN fades away.Comment: 28 pages, 19 figures; accepted for publication in The Astrophysical Journa

Serendipitous Nebular-phase JWST Imaging of SN Ia 2021aefx: Testing the Confinement of 56-Co Decay Energy

We present new 0.3-21 micron photometry of SN 2021aefx in the spiral galaxy NGC 1566 at +357 days after B-band maximum, including the first detection of any SN Ia at >15 micron. These observations follow earlier JWST observations of SN 2021aefx at +255 days after the time of maximum brightness, allowing us to probe the temporal evolution of the emission properties. We measure the fraction of flux emerging at different wavelengths and its temporal evolution. Additionally, the integrated 0.3-14 micron decay rate of $\Delta m_{0.3-14} = 1.35 \pm 0.05$ mag/100 days is higher than the decline rate from the radioactive decay of $^{56}$Co of $\sim 1.2$mag/100 days. The most plausible explanation for this discrepancy is that flux is shifting to >14 micron, and future JWST observations of SNe Ia will be able to directly test this hypothesis. However, models predicting non-radiative energy loss cannot be excluded with the present data.Comment: Accepted for publication in ApJL; 11 pages, 4 figures, 2 tables in two-column AASTEX63 forma

Fast and Not-so-Furious: Case Study of the Fast and Faint Type IIb SN 2021bxu

We present photometric and spectroscopic observations and analysis of SN~2021bxu (ATLAS21dov), a low-luminosity, fast-evolving Type IIb supernova (SN). SN~2021bxu is unique, showing a large initial decline in brightness followed by a short plateau phase. With $M_r = -15.93 \pm 0.16\, \mathrm{mag}$ during the plateau, it is at the lower end of the luminosity distribution of stripped-envelope supernovae (SE-SNe) and shows a distinct $\sim$10 day plateau not caused by H- or He-recombination. SN~2021bxu shows line velocities which are at least $\sim1500\,\mathrm{km\,s^{-1}}$ slower than typical SE-SNe. It is photometrically and spectroscopically similar to Type IIb SNe during the photospheric phases of evolution, with similarities to Ca-rich IIb SNe. We find that the bolometric light curve is best described by a composite model of shock interaction between the ejecta and an envelope of extended material, combined with a typical SN~IIb powered by the radioactive decay of $^{56}$Ni. The best-fit parameters for SN~2021bxu include a $^{56}$Ni mass of $M_{\mathrm{Ni}} = 0.029^{+0.004}_{-0.005}\,\mathrm{M_{\odot}}$, an ejecta mass of $M_{\mathrm{ej}} = 0.57^{+0.04}_{-0.03}\,\mathrm{M_{\odot}}$, and an ejecta kinetic energy of $K_{\mathrm{ej}} = 9.3^{+0.7}_{-0.6} \times 10^{49}\, \mathrm{erg}$. From the fits to the properties of the extended material of Ca-rich IIb SNe we find a trend of decreasing envelope radius with increasing envelope mass. SN~2021bxu has $M_{\mathrm{Ni}}$ on the low end compared to SE-SNe and Ca-rich SNe in the literature, demonstrating that SN~2021bxu-like events are rare explosions in extreme areas of parameter space. The progenitor of SN~2021bxu is likely a low mass He star with an extended envelope.Comment: 18 pages, 15 figures, submitted to MNRA

SN 2019ehk: A Double-peaked Ca-rich Transient with Luminous X-Ray Emission and Shock-ionized Spectral Features

We present panchromatic observations and modeling of the Calcium-rich supernova (SN) 2019ehk in the star-forming galaxy M100 (d ≈ 16.2 Mpc) starting 10 hr after explosion and continuing for ~300 days. SN 2019ehk shows a double-peaked optical light curve peaking at t = 3 and 15 days. The first peak is coincident with luminous, rapidly decaying Swift-XRT–discovered X-ray emission (L_x ≈ 10⁴¹ erg s⁻¹ at 3 days; L_x ∝ t⁻³), and a Shane/Kast spectral detection of narrow Hα and He II emission lines (v ≈ 500 km s⁻¹) originating from pre-existent circumstellar material (CSM). We attribute this phenomenology to radiation from shock interaction with extended, dense material surrounding the progenitor star at r (0.1–1) × 10¹⁷ cm. The photometric and spectroscopic properties during the second light-curve peak are consistent with those of Ca-rich transients (rise-time of t_r = 13.4 ± 0.210 days and a peak B-band magnitude of M_B = −15.1 ± 0.200 mag). We find that SN 2019ehk synthesized (3.1 ± 0.11) × 10⁻² M_⊙ of ⁵⁶Ni and ejected M_(ej) = (0.72 ± 0.040) M⊙ total with a kinetic energy E_k = (1.8 ± 0.10) × 10⁵⁰ erg. Finally, deep HST pre-explosion imaging at the SN site constrains the parameter space of viable stellar progenitors to massive stars in the lowest mass bin (~10 M_⊙) in binaries that lost most of their He envelope or white dwarfs (WDs). The explosion and environment properties of SN 2019ehk further restrict the potential WD progenitor systems to low-mass hybrid HeCO WD+CO WD binaries