Progenitor Scenarios of Supernovae from Local Group Stellar Populations and Supernova Remnants

Supernovae (SNe) are the explosive deaths of stars, but the detailed properties of their progenitors are not well understood. For example, while we know Type Ia SNe are explosions of white dwarfs (WDs), it is unclear whether these WDs generally have a stellar companion or another WD companion. For core-collapse SNe, whose progenitors are stars $>8$ M$_{\odot}$, it is unclear whether stars $>18$ M$_{\odot}$ produce a visible SN, or directly collapse to a black hole in a weak transient event. For my dissertation, I have focused on using high-quality surveys of SN remnants, stellar populations and interstellar medium in our Local Group galaxies to understand the possible progenitor scenarios of SNe. The goal is to make an accurate measurement of the SN delay-time distribution (DTD), which is the rate of SNe versus stellar evolutionary timescale, assuming these stars were all formed in a burst. The DTD can constrain which set of progenitor models are consistent with the population of Type Ia and core-collapse SNe in a survey. To achieve this goal, I have constructed a model of a SN remnant survey in the Local Group that can estimate the visibility times of a SN remnants as a function of their environment. This can be combined with existing high-quality stellar age distribution maps, constructed from resolved stellar populations, to measure the most accurate DTD. I also calculated DTDs of pulsating variables like RR Lyrae and Cepheids, and found a distribution of evolutionary timescales which are not predicted by canonical models. This helped understand possible systematics in the DTD method, and also reassess the progenitor scenarios of astrophysically significant objects like RR Lyrae and Cepheids in the era of big data surveys. Finally, I used direct observations and modeling of radio emission of old SNe/young SN remnants such as SN 1885A and G1.9+0.3 as an alternative, but promising avenue for determining the possible progenitor scenarios of Type Ia SNe

Thirty Years of Radio Observations of Type Ia SN 1972E and SN 1895B: Constraints on Circumstellar Shells

We have imaged over 35 years of archival Very Large Array (VLA) observations of the nearby (d$_{\rm{L}}$ $=$ 3.15 Mpc) Type Ia supernovae SN\,1972E and SN\,1895B between 9 and 121 years post-explosion. No radio emission is detected, constraining the 8.5 GHz luminosities of SN\,1972E and SN\,1895B to be L$_{\nu,8.5\rm{GHz}}$ $<$ 6.0 $\times$ 10$^{23}$ erg s$^{-1}$ Hz$^{-1}$ 45 years post-explosion and L$_{\nu,8.5\rm{GHz}}$ $<$ 8.9 $\times$ 10$^{23}$ erg s$^{-1}$ Hz$^{-1}$ 121 years post-explosion, respectively. These limits imply a clean circumstellar medium (CSM), with $n$ $<$ 0.9 cm$^{-3}$ out to radii of a few $\times$ 10$^{18}$ cm, if the SN blastwave is expanding into uniform density material. Due to the extensive time coverage of our observations, we also constrain the presence of CSM shells surrounding the progenitor of SN\,1972E. We rule out essentially all medium and thick shells with masses of 0.05$-$0.3 M$_\odot$ at radii between $\sim$10$^{17}$ and 10$^{18}$ cm, and thin shells at specific radii with masses down to $\lesssim$0.01 M$_\odot$. These constraints rule out swaths of parameter space for a range of single and double degenerate progenitor scenarios, including recurrent nova, core-degenerate objects, ultra-prompt explosions and white dwarf (WD) mergers with delays of a few hundred years between the onset of merger and explosion. Allowed progenitors include WD-WD systems with a significant ($>$ 10$^{4}$ years) delay from the last episode of common envelope evolution and single degenerate systems undergoing recurrent nova, provided that the recurrence timescale i short and the system has been in the nova phase for $\gtrsim$10$^{4}$ yr, such that a large ($>$ 10$^{18}$ cm) cavity has been evacuated. Future multi-epoch observations of additional intermediate-aged Type Ia SNe will provide a comprehensive view of the large-scale CSM environments around these explosions.Comment: Accepted for publication in the Astrophysical Journa

On Odd Radio Circles as Supernova Remnants

The origin of arcmin-sized Odd Radio Circles (ORCs) found in modern all-sky radio surveys remain uncertain, with explanations ranging from starburst/AGN-driven shocks to supernova remnants (SNRs) in the low-density ambient medium. Using well-calibrated radio light curve models, we assess the possibility that ORCs are radio SNRs evolving in low ambient densities. Our models imply that ORCs 1-5 and J0624-6948 (near the LMC) as SNRs must be within 200 kpc and 100 kpc from the Sun respectively, given their observed flux densities and angular sizes. To be evolving in the circumgalactic medium of the Milky Way, our models require ORCs 1-5 to be ejecta-dominated SNRs within 50 kpc, evolving in ambient densities of $(0.2-1.2) \times 10^{-3}$ cm$^{-3}$. However, this is statistically unlikely because ORCs 1-5 would have ages $<640$ yrs, much smaller than their expected lifetimes of $\gtrsim$10$^5$ yrs at these densities, and because the low SN rate and steep profile of the stellar halo imply a negligible number of ORC-like SNRs within 50 kpc. The circumgalactic medium SNR scenario for J0624-6948 is more likely (though still low probability) compared to ORCs 1-5, as our models allow J0624-6948 to be $\lesssim$3000 yrs. On the other hand, the interpretation of J0624-6948 as a Sedov-Taylor SNR at 50 kpc (LMC) distance is possible for a wide range of ambient densities ($6 \times 10^{-4} - 0.5$ cm$^{-3}$) and ages $\sim$$(0.2-2.6) \times 10^4$ yr, while also being consistent with the local HI environment.Comment: 9 pages, 4 figures. Submitted to MNRAA

Radio Observations of Six Young Type Ia Supernovae

Type Ia supernovae (SNe Ia) are important cosmological tools, probes of binary star evolution, and contributors to cosmic metal enrichment; yet, a definitive understanding of the binary star systems that produce them remains elusive. In this work we present early-time (first observation within 10 days post-explosion) radio observations of six nearby (within 40 Mpc) SNe Ia taken by the Jansky Very Large Array, which are used to constrain the presence of synchrotron emission from the interaction between ejecta and circumstellar material (CSM). The two motivations for these early-time observations are (1) to constrain the presence of low-density winds and (2) to provide an additional avenue of investigation for those SNe Ia observed to have early-time optical/UV excesses that may be due to CSM interaction. We detect no radio emission from any of our targets. Toward our first aim, these non-detections further increase the sample of SNe Ia that rule out winds from symbiotic binaries and strongly accreting white dwarfs. For the second aim, we present a radiation hydrodynamics simulation to explore radio emission from an SN Ia interacting with a compact shell of CSM, and find that relativistic electrons cannot survive to produce radio emission despite the rapid expansion of the shocked shell after shock breakout. The effects of model assumptions are discussed for both the wind and compact shell conclusions.Comment: (17 pages, 6 figures, accepted to ApJ

The RR Lyrae Delay-Time Distribution: A Novel Perspective on Models of Old Stellar Populations

The delay-time distribution (DTD) is the occurrence rate of a class of objects as a function of time after a hypothetical burst of star formation. DTDs are mainly used as a statistical test of stellar evolution scenarios for supernova progenitors, but they can be applied to many other classes of astronomical objects. We calculate the first DTD for RR Lyrae variables using 29,810 RR Lyrae from the OGLE-IV survey and a map of the stellar-age distribution (SAD) in the Large Magellanic Cloud (LMC). We find that $\sim 46\%$ of the OGLE-IV RR Lyrae are associated with delay-times older than 8 Gyr (main-sequence progenitor masses less than 1 M$_{\odot}$), and consistent with existing constraints on their ages, but surprisingly about $51\%$ of RR Lyrae appear have delay times $1.2-8$ Gyr (main-sequence masses between $1 - 2$ M$_{\odot}$ at LMC metallicity). This intermediate-age signal also persists outside the Bar-region where crowding is less of a concern, and we verified that without this signal, the spatial distribution of the OGLE-IV RR Lyrae is inconsistent with the SAD map of the LMC. Since an intermediate-age RR Lyrae channel is in tension with the lack of RR Lyrae in intermediate-age clusters (noting issues with small-number statistics), and the age-metallicity constraints of LMC stars, our DTD result possibly indicates that systematic uncertainties may still exist in SAD measurements of old-stellar populations, perhaps stemming from the construction methodology or the stellar evolution models used. We described tests to further investigate this issue.Comment: 21 pages, 11 figures. Accepted to Ap

Synchrotron emission from double-peaked radio light curves of the symbiotic recurrent nova V3890 Sagitarii

We present radio observations of the symbiotic recurrent nova V3890 Sagitarii following the 2019 August eruption obtained with the MeerKAT radio telescope at 1.28 GHz and Karl G. Janksy Very Large Array (VLA) at 1.26 to 5 GHz. The radio light curves span from day 1 to 540 days after eruption and are dominated by synchrotron emission produced by the expanding nova ejecta interacting with the dense wind from an evolved companion in the binary system. The radio emission is detected early on (day 6) and increases rapidly to a peak on day 15. The radio luminosity increases due to a decrease in the opacity of the circumstellar material in front of the shocked material and fades as the density of the surrounding medium decreases and the velocity of the shock decelerates. Modelling the light curve provides an estimated mass-loss rate of $M_{\textrm {wind}} \approx 10^{-8} {\textrm {M}}_\odot~{\textrm {yr}}^{-1}$ from the red giant star and ejecta mass in the range of $M_{\textrm {ej}}=10^{-5}-10^{-6}~{\textrm {M}}_\odot$from the surface of the white dwarf. V3890 Sgr likely hosts a massive white dwarf similar to other symbiotic recurrent novae, thus considered a candidate for supernovae type Ia (SNe Ia) progenitor. However, its radio flux densities compared to upper limits for SNe Ia have ruled it out as a progenitor for SN 2011fe

Quantifying the energy balance between the turbulent ionised gas and young stars

We investigate the ionised gas morphology, excitation properties, and kinematics in 19 nearby star-forming galaxies from the PHANGS-MUSE survey. We directly compare the kinetic energy of expanding superbubbles and the turbulent motions in the interstellar medium with the mechanical energy deposited by massive stars in the form of winds and supernovae, with the aim to answer whether the stellar feedback is responsible for the observed turbulent motions and to quantify the fraction of mechanical energy retained in the superbubbles. Based on the distribution of the flux and velocity dispersion in the H$\alpha$ line, we select 1484 regions of locally elevated velocity dispersion ($\sigma$(H$\alpha$)>45 km/s), including at least 171 expanding superbubbles. We analyse these regions and relate their properties to those of the young stellar associations and star clusters identified in PHANGS-HST data. We find a good correlation between the kinetic energy of the ionised gas and the total mechanical energy input from supernovae and stellar winds from the stellar associations, with a typical efficiency of 10-20%. The contribution of mechanical energy by the supernovae alone is not sufficient to explain the measured kinetic energy of the ionised gas, which implies that pre-supernova feedback in the form of radiation/thermal pressure and winds is necessary. We find that the gas kinetic energy decreases with metallicity for our sample covering Z=0.5-1.0 Zsun, reflecting the lower impact of stellar feedback. For the sample of superbubbles, we find that about 40% of the young stellar associations are preferentially located in their rims. We also find a slightly higher (by ~15%) fraction of the youngest (1-2.5 Myr) stellar associations in the rims of the superbubbles than in the centres, and the opposite for older associations, which implies possible propagation or triggering of star formation.Comment: 31 pages (including 5 pages in appendix), 19 figures, the abstract is abridged; submitted to A&A (in mid May; awaiting report

Quasars That Have Transitioned from Radio-quiet to Radio-loud on Decadal Timescales Revealed by VLASS and FIRST

We have performed a search over 3440 deg² of Epoch 1 (2017–2019) of the Very Large Array Sky Survey to identify unobscured quasars in the optical (0.2 2500%) but roughly steady fluxes over a few months at 3 GHz are inconsistent with extrinsic variability due to propagation effects, thus favoring an intrinsic origin. We conclude that our sources are powerful quasars hosting compact/young jets. This challenges the generally accepted idea that "radio-loudness" is a property of the quasar/AGN population that remains fixed on human timescales. Our study suggests that frequent episodes of short-lived AGN jets that do not necessarily grow to large scales may be common at high redshift. We speculate that intermittent but powerful jets on subgalactic scales could interact with the interstellar medium, possibly driving feedback capable of influencing galaxy evolution

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