689 research outputs found
High-velocity features in Type Ia supernova spectra
No full textWe use a sample of 58 low-redshift (z ≤ 0.03) Type Ia supernovae (SNe Ia) having well-sampled light curves and spectra near maximum light to examine the behaviour of high-velocity features (HVFs) in SN Ia spectra. We take advantage of the fact that Si II λ6355 is free of HVFs at maximum light in all SNe Ia, while HVFs are still strong in the Ca II near-infrared feature in many SNe, allowing us to quantify the strength of HVFs by comparing the structure of these two lines. We find that the average HVF strength increases with decreasing light-curve decline rate, and rapidly declining SNe Ia (Δm15(B) ≥ 1.4 mag) show no HVFs in their maximum-light spectra. Comparison of HVF strength to the light-curve colour of the SNe Ia in our sample shows no evidence of correlation. We find a correlation of HVF strength with the velocity of Si II λ6355 at maximum light (vSi), such that SNe Ia with lower vSi have stronger HVFs, while those SNe Ia firmly in the ‘high-velocity’ (i.e. vSi ≥ 12000 km s-1) subclass exhibit no HVFs in their maximum-light spectra. While vSi and Δm15(B) show no correlation in the full sample of SNe Ia, we find a significant correlation between these quantities in the subset of SNe Ia having weak HVFs. In general, we find that slowly declining (low Δm15(B)) SNe Ia, which are more luminous and more energetic than average SNe Ia, tend to produce either high photospheric ejecta velocities (i.e. high vSi) or strong HVFs at maximum light, but not both. Finally, we examine the evolution of HVF strength for a sample of SNe Ia having extensive pre-maximum spectroscopic coverage and find significant diversity of the pre-maximum HVF behaviour
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Spectral Observations and Analyses of Low-Redshift Type Ia Supernovae
No full textThe explosive deaths of stars, known as a supernovae (SNe), have been critical to our understanding of the Universe for centuries. From the first evidence of a changing Universe beyond the Moon (Brahe1573) to the first evidence of the accelerating expansion of the Universe (Riess et al. 1998; Perlmutter et al. 1999), SNe - and often a specific subclass of SNe called Type Ia SNe (SNe Ia) - have been integral to astronomical research. An introduction to SNe, their importance in astronomy, and how we observe them is given in Chapter 1. How SNe Ia explode, what progenitor systems give rise to them, and how different initial conditions affect the observed outcomes of these objects are understood only at a relatively basic level. In other words, a detailed understanding of the physics behind SNe Ia is still lacking. One way astronomers can begin to solve these problems, and others involving SNe Ia, is to obtain and analyze a large, self-consistent dataset of SN Ia observations. This is the goal of the Berkeley SN Ia Program (BSNIP) which comprises the majority of this Thesis.In the second Chapter, I present the full BSNIP sample which consists of 1298 low-redshift (z ≤ 0.2) optical spectra of 582 SNe Ia observed from 1989 through the end of 2008. Many of the SNe have well-calibrated light curves with measured distances as well as spectra which have been corrected for host-galaxy contamination. Most of the data were obtained using the Kast double spectrograph mounted on the Shane 3 m telescope at Lick Observatory with typical wavelength coverage of 3300-10400 Å, which is significantly larger than that of most previously published SN Ia spectral datasets. I also present the BSNIP observing and reduction procedures used during the two decades over which the data were collected. In addition, I describe our spectral classification scheme (using the SuperNova IDentification code, SNID; Blondin & Tonry 2007), utilizing my newly constructed set of SNID spectral templates. These templates allow me to accurately spectroscopically classify the entire BSNIP dataset, and by doing so I am able to reclassify a handful of objects as bona fide SNe Ia and a few other objects as members of some of the peculiar SN Ia subtypes. In fact, the BSNIP dataset includes spectra of nearly 90 spectroscopically peculiar SNe Ia. I also present spectroscopic host-galaxy redshifts of some SNe Ia where these values were previously unknown. The sheer size of the BSNIP dataset and the consistency of the observation and reduction methods makes this sample unique among all other published SN Ia datasets and is complementary in many ways to the large, low-redshift SN Ia spectra presented by Matheson et al. 2008 and Blondin et al. 2011.I present measurements of spectral features of 432 low-redshift (z < 0.1) optical spectra within 20 d of maximum brightness of 261 SNe Ia from the BSNIP sample in the third Chapter. I describe in detail my method of automated, robust spectral feature definition and measurement which expands upon similar previous studies. Using this procedure, I attempt to measure expansion velocities, (pseudo-)equivalent widths (pEWs), spectral feature depths, and fluxes at the center and endpoints of each of nine major spectral feature complexes. A sanity check of the consistency of the measurements is performed using the BSNIP data (as well as a separate spectral dataset). I investigate how velocity and pEW evolve with time and how they correlate with each other. Various spectral classification schemes are employed and quantitative spectral differences among the subclasses are investigated. Several ratios of pEW values are calculated and studied. Furthermore, SNe Ia that show strong evidence for interaction with circumstellar material or an aspherical explosion are found to have the largest near-maximum expansion velocities and pEWs, possibly linking extreme values of spectral observables with specific progenitor or explosion scenarios. A discussion of the relative merits of various classification schemes is presented and I find that purely spectroscopic classification schemes are useful in identifying the most peculiar SNe Ia. However, in almost all spectral parameters investigated the full sample of objects spans a nearly continuous range of values. Comparisons to previously published theoretical models of SNe Ia are made and some of the predictions of these models match the observations presented here. I conclude with a brief discussion of how these measurements and the possible correlations presented will be crucial to future SN surveys.The fourth Chapter of this Thesis presents comparisons of spectral feature measurements to photometric properties of 115 low-redshift (z < 0.1) SNe Ia with optical spectra within 5 d of maximum brightness. The spectral data come from the BSNIP sample described in Chapter 2, and the photometric data come mainly from the Lick Observatory Supernova Search (LOSS) and are published by Ganeshalingam et al. (2010). The spectral measurements come from BSNIP II (Chapter 3 of this Thesis) and the light-curve fits and photometric parameters can be found in Ganeshalingam et al. (in preparation). A variety of previously proposed correlations between spectral and photometric parameters are investigated using the large and self-consistent BSNIP dataset. We also use a combination of light-curve parameters (specifically the SALT2 stretch and color parameters x1 and c) and spectral measurements to calculate distance moduli. The residuals from these models is then compared to the standard model which only uses light-curve stretch and color. The pEW of Si II λ4000 is found to be a good indicator of light-curve width and the pEWs of the Mg II and Fe II complexes are relatively good proxies for color. However, a distance model only using these spectroscopic measurements performs worse than the standard model which uses only light-curve parameters. When using a distance model which combines the ratio of fluxes near ~3600 Å and ~4300 Å with both x1 and c, the Hubble residuals are decreased by 12%, which is found to be significant at the 2.4σ level. The weighted root-mean square of the residuals using this model is 0.130 ± 0.019 mag (as compared to 0.146 ± 0.019 mag when using the same sample with the standard model). This Hubble diagram fit has one of the smallest scatters ever published and at the highest significance ever seen in such a study. Finally, these results are discussed with regard to how they can improve the cosmological accuracy of future, large-scale SN Ia surveys.Finally, I conclude this Thesis with an in-depth study of a quite peculiar SN Ia, not included in the BSNIP sample. Chapter 5 presents and analyzes optical photometry and spectra of the extremely luminous and slowly evolving Type Ia SN 2009dc, and offers evidence that it is a super-Chandrasekhar mass (SC) SN Ia and thus had a SC white dwarf (WD) progenitor. Optical spectra of SN 2007if, a similar object, are also shown. SN 2009dc had one of the most slowly evolving light curves ever observed for a SN Ia, with a rise time of ~23 d and Δm15(B) = 0.72 mag. I calculate a lower limit to the peak bolometric luminosity of ~2.4×1043 erg s-1, though the actual value is likely almost 40% larger. Optical spectra of SNe 2009dc and 2007if obtained near maximum brightness exhibit strong C II features (indicative of a significant amount of unburned material), and the post-maximum spectra are dominated by iron-group elements. All of the spectra of SNe 2009dc and 2007if also show low expansion velocities. However, I see no strong evidence in SN 2009dc for a velocity "plateau" near maximum light like the one seen in SN 2007if (Scalzo et al. 2010). The high luminosity and low expansion velocities of SN 2009dc lead to a derived WD progenitor mass of more than 2 MSun and a 56Ni mass of about 1.4-1.7 MSun. I propose that the host galaxy of SN 2009dc underwent a gravitational interaction with a neighboring galaxy in the relatively recent past. This may have led to a sudden burst of star formation which could have produced the SC WD progenitor of SN 2009dc and likely turned the neighboring galaxy into a "post-starburst galaxy." No published model seems to match the extreme values observed in SN 2009dc, but simulations do show that such massive progenitors can exist (likely as a result of the merger of two WDs) and can possibly explode as SC SNe Ia
SN 2016esw: a luminous Type II supernova observed within the first day after the explosion
No full textWe present photometry, spectroscopy, and host-galaxy integral-field spectroscopy of the Type II supernova (SN) 2016esw in CGCG~229-009 from the first day after the explosion up to 120 days. Its light-curve shape is similar to that of a typical SN II; however, SN 2016esw is near the high-luminosity end of the SN II distribution, with a peak of MmaxV=−18.36 mag. The V-band light curve exhibits a long recombination phase for a SN II (similar to the long-lived plateau of SN 2004et). Considering the well-known relation between the luminosity and the plateau decline rate, SN 2016esw should have a V-band slope of ∼2.10 mag (100 days)−1; however, SN 2016esw has a substantially flatter plateau with a slope of 1.01±0.26 mag (100 days)−1, perhaps indicating that interacting Type II supernovae are not useful for cosmology. At 19.5 days post-explosion, the spectrum presents a boxy Hα emission line with flat absorption profiles, suggesting interaction between the ejecta and circumstellar matter. Finally, based on the spectral properties, SN 2016esw shows similarities with the luminous and interacting SN 2007pk at early epochs, particularly in terms of observable line features and their evolution
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An Explosive Party: Supernovae, Tidal Disruption Events, and Quasi-Periodic Eruptions
No full textAstrophysical transient events, particularly Type Ia supernovae (SNe Ia) and tidal disruption events (TDEs), provide critical insights into stellar systems and black hole activity. This thesis studies polarimetry of SNe Ia and TDEs in an attempt to better understand the geometry and physical processes behind these explosive events. For SN 2019ein, spectropolarimetric data from approximately 11 days before to 10 days after peak brightness revealed a continuum polarization of 0.0--0.3\%. The constant polarization angle indicates axial symmetry in the explosion, consistent across inner and outer ejecta regions. Notably, polarization at the Si II and Ca II lines reached about 1\%, suggesting a largely spherical explosion with localized clumping of intermediate-mass elements. These observations largely disfavor merger-induced models for SN 2019ein. A larger sample of SNe Ia spectropolarimetry observed at Lick Observatory is presented and described. In the study of the TDE AT\,2019qiz, spectropolarimetry showed negligible continuum polarization at peak brightness, suggesting a nearly spherical, optically thick electron scattering photosphere around the black hole. By day 29, polarization increased to around 1\%, indicating a more aspherical interior as the fallback rate decreased and the photosphere receded. This marks the first detailed spectropolarimetric evolution observed in a TDE, and supports the reprocessing model for the source of optical emission in TDEs. The study of AT\,2019qiz is followed by imaging polarimetry of several TDEs from Keck Observatory in an attempt to build a more comprehensive picture of gas geometry in TDEs. Additionally, the thesis explores the newly discovered X-ray quasi-periodic eruptions (QPEs), analyzing archival {\it Hubble Space Telescope} images of the QPE source GSN 069. The data revealed a compact [O III] emission region within a 35 pc radius of the nucleus, along with extended emission up to 2 kpc away. Simulations with the photoionization code \textsc{Cloudy} suggest this emission comes from dense gas, likely ionized by X-rays from a young accretion disk, offering a unique glimpse into the dynamic processes at play in the host galaxies of QPEs
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The Photometric Properties of Nearby Type Ia Supernovae
No full textType Ia supernovae (SNe Ia) are the final brilliant explosion of a carbon-oxygen white dwarf accreting mass from a companion star. At peak brightness, a SN Ia can outshine an entire galaxy of billions of stars. Most SNe Ia have a standardizable luminosity, ideal for use as an extragalactic distance indicator. Measurements of a large sample of SNe Ia over a range of distances enables the estimate of cosmological parameters to help determine the mass-energy content of the Universe (Riess et al. 1998; Perlmutter et al. 1999; Riess et al. 2004; Astier et al. 2006; Riess et al. 2007; Wood-Vasey et al. 2008; Kowalski et al. 2008; Hicken et al. 2009a; Amanullah et al. 2010; Sullivan et al. 2011a; Suzuki et al. 2012).The cosmological application of SNe Ia is predicated upon relationships between the intrinsic luminosity and light-curve properties. Despite the successful measurement of cosmological parameters using SNe Ia, our understanding of SNe Ia themselves is surprisingly lacking. The SN Ia progenitor system has never been directly observed, making it unclear how many different channels exist to make a SN Ia. The physical nature of the relationship between light-curve parameters and luminosity is also not well understood, and it remains to be seen whether other correlations exist to improve SN Ia distance estimates.The goal of this dissertation is to shed light on the physics of SNe Ia and search for new correlations to improve distance estimates to SNe Ia by analyzing a large sample of well-observed, high-quality SN Ia light curves. I have collected, reduced, and analyzed optical photometric data for 165 nearby SNe Ia as part of the Lick Observatory Supernova Search (LOSS). These data represent a significant contribution to the existing sample of nearby SN Ia light curves.After giving a general overview of SNe Ia in Chapter 1, I present the methods used to obtain and reduce the LOSS data in Chapter 2. In Chapter 3, I use the LOSS data in an analysis of the earliest photometry epochs to understand the explosion physics governing the initial rise of the SN Ia light curve. These early data points also provide a means of testing models to constrain the nature of the binary companion star. I do not find evidence for interaction between SN ejecta and a companion star, ruling out theories requiring a red giant as a companion in most cases. In Chapter 4, I combine the data presented in this thesis with other samples in the literature to place constraints on cosmological parameters. I reject a non-accelerating Universe with 99.999% confidence. In Chapter 5, I present a study of an individual peculiar SN Ia that is unlike any previously published object, bucking the relationships normally observed in SNe Ia. Studying extreme SNe Ia may provide insights into understanding the physics of normal SNe Ia
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Cosmic Fireworks: The Asymmetric Explosions and Fates of Massive Stars
No full textThis thesis investigates the physical properties and geometry of supernovae (SNe) and their immediate environments through an analysis of early-time ultraviolet (UV) spectra and optical spectropolarimetry. The goal is to understand the final stages of massive stellar evolution, including their eventual fates as gravitational-wave sources.Early-time Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) UV spectroscopy of the Type IIP SNe 2021yja and 2022wsp significantly expands the sparse sample of such observations, which are crucial for probing the outermost ejecta and any interaction with circumstellar material (CSM). Radiative transfer modeling revealed SN 2021yja as an energetic event with high nickel production and a lack of CSM interaction signatures. Both SN 2021yja and 2022wsp showed remarkable spectral similarity and required exceptionally steep outer ejecta density profiles (power-law index n > 20) to match observations, providing constraints on the structure of the progenitor's outer layers.Spectropolarimetry of SN 2021yja revealed an unusually high continuum polarization (p ≈ 0.9%) during the early photospheric phase, significantly exceeding typical values for SNe II (p ≤ 0.2%). The constant polarization angle across the continuum and strong lines indicated an axially symmetric, likely bipolar, explosion geometry extending into the outermost layers or interacting with a low-density, aspherical CSM, challenging conventional models of red supergiant envelopes and their immediate surroundings.For the nearby SN 2023ixf, spectropolarimetric observations starting just 1.4 days post-explosion—the earliest ever obtained for any SN—captured a high initial continuum polarization (p ≈ 1%) that rapidly declined to ~0.5% by day +3.5. This decline coincided with the disappearance of narrow CSM interaction “flash features” in the spectra. This temporal correlation provided direct evidence for an aspherical, confined CSM being quickly engulfed by the expanding ejecta, offering a unique probe of the progenitor's immediate environment. State-of-the-art two-dimensional polarized radiative transfer modeling was used to quantitatively interpret these and other spectropolarimetric datasets, pioneering a new approach to extract detailed geometric information about both the explosion and the CSM distribution in core-collapse SNe.Early spectropolarimetry of the Type IIb SN 2024iss revealed negligible interstellar polarization but high and dynamic intrinsic polarization. Both the continuum polarization magnitude and its position angle changed during three weeks after first light, indicating a changing asymmetry axis projected onto the sky.A complementary study constrains the Hubble constant (H0) using gravitational-wave “dark siren” GW190814 and electromagnetically bright GW170817. By statistically correlating gravitational-wave posterior samples with galaxy catalogs and considering systematics such as luminosity cuts (LB/LB*), we derive a joint constraint of H0 = 69+29.0−14.0 km s−1 Mpc−1. This result provides an H0 measurement independent of traditional techniques involving the cosmic distance ladder
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Trash to Treasure: Extracting Cosmological Utility from Sparsely Observed Type Ia Supernovae
No full textType Ia supernovae (SNe Ia) are magnificent explosions in the Cosmos that are thought to result from the thermonuclear runaway of white dwarf stars in multistar systems (see, e.g., Jha et al. 2019, for a recent review). Though the exact details of the progenitor system(s) and explosion mechanism(s) remain elusive, SNe Ia have proven themselves to be immensely valuable in shaping our understanding of the physical laws that govern the evolution of the Universe (i.e., physical cosmology). This value is manifested chiefly in two empirical facts: (i) SNe Ia are incredibly luminous (reaching the equivalent of several billion Suns), and (ii) the relatively similar peak luminosities that all "normal" SNe Ia reach can be further homogenized by exploiting a correlation with the rate of photometric evolution (e.g., Phillips 1993). Together, these facts make SNe Ia excellent extragalactic distance indicators, and their use as such led to the discovery of the accelerating expansion of the Universe (Riess et al. 1998; Perlmutter et al. 1999). Through this, the current cosmological paradigm came into favor — the so-called ΛCDM model, where the Universe consists primarily of repulsive dark energy (of which a leading candidate is Einstein’s cosmological constant, Λ) and cold dark matter (CDM).In this thesis, I present a comprehensive study that follows the entire SN Ia cosmology lifecyle, from data acquisition to cosmological analysis (albeit of a different flavor than those mentioned above). While these “bookends” provide natural segmentation points in this thesis, there is a third, intermediate segment which serves to present a complementary method for SN Ia distance measurement that is far less data intensive than conventional approaches. In this way, the segments are hierarchical, each depending on its predecessor and enabling its successor.After appropriately setting the stage in Chapter 1, I delve into the first segment (data acquisition) with Chapter 2, a data release and analysis of 93 multipassband SN Ia light curves collected between 2005 and 2018, and Chapter 3, a complementary release of 637 low-redshift SN Ia optical spectra from a similar time interval. In both, I describe open-source software I developed for data processing and analysis purposes, and make — in addition to the data themselves — useful, value-added data products (e.g., fitted parameters from light curves) available to the community. When combined with prior releases, the Berkeley SN Ia sample now reaches nearly 2000 optical spectra and more than 250 multiband light curves, all observed and processed with the utmost care for quality and internal consistency.This large, homogeneous sample proves critical for the second segment of this thesis, in which I ultimately develop and validate the aforementioned technique — the snapshot distance method (SDM) — for estimating the distance to an SN Ia from sparse observations. As a prerequisite to the SDM, I develop, in Chapter 4, an open-source software package called deepSIP that is capable of determining the phase and light-curve shape of an SN Ia — both of which conventionally require a well-sampled light curve — from a single optical spectrum. At its heart, deepSIP consists of a set of three convolutional neural networks trained on a significant fraction of all publicly available SN Ia optical data (including those presented in the first segment of this thesis), with judicious augmentation steps included to promote telescope agnosticism and model robustness. The impressive performance of deepSIP enables the SDM, which, as I demonstrate in Chapter 5, is capable of deriving an SN Ia distance estimate from as little as one optical spectrum and one epoch of 2+ passband photometry with notable precision over a wide range of SN Ia parameters.This leads, finally, into the last segment of this thesis (cosmological analysis), where I use the SDM to turn trash (i.e., SN Ia observations that were previously unusable owing to data sparsity) into treasure (i.e., reliable distance estimates to be used in a cosmological study). In particular, in Chapter 6, I combine a novel sample of 137 SDM-resurrected SNe Ia with a large literature sample of SNe Ia and SNe II to measure peculiar velocities and set leading (from an SN-only perspective) constraints on the cosmological parameter combination fσ8 and the nature of bulk flows in the local Universe. Moreover, the methods by which I perform this analysis establish a reproducible and extensible blueprint for future such analyses as large-scale surveys come online and unleash an unprecedented data volume
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