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

Collapsar R-Process Yields Can Reproduce [Eu/Fe] Abundance Scatter in Metal-Poor Stars

It is unclear if neutron star mergers can explain the observed r-process abundances of metal-poor stars. Collapsars, defined here as rotating massive stars whose collapse results in a rapidly accreting disk around a black hole that can launch jets, are a promising alternative. We find that we can produce a self-consistent model in which a population of collapsars with stochastic europium yields synthesizes all of the r-process material in metal-poor ([Fe/H] < -2.5) stars. Our model reproduces the observed scatter and evolution of scatter of [Eu/Fe] abundances. We find that if collapsars are the dominant r-process site for metal-poor stars, r-process synthesis may be linked to supernovae that produce long gamma-ray bursts. Our results also allow for the possibility that core-collapse supernovae beyond those that launch gamma-ray bursts also produce r-process material (e.g., potentially a subset of Type Ic-BL supernovae). Furthermore, we identify collapsar jet properties (isotropic energy, engine luminosity, or engine time) which may trace r-process yield and verify that the amount of r-process yield produced per collapsar in our model (~0.07 Msun) is consistent with other independent estimates. In the future, achieving 0.05 dex precision on distribution scatter or a reliable selection function would further constrain our probe of r-process production. Our model would also hold for another prompt r-process site with a power-law yield, and work is needed to determine if, for example, fast-merging neutron stars can also explain abundance scatter.Comment: 17 pages, 8 figures. Accepted by Ap

Inferring the parallax of Westerlund 1 from Gaia DR2

Westerlund 1 (Wd1) is potentially the largest star cluster in the Galaxy. That designation critically depends upon the distance to the cluster, yet the cluster is highly obscured, making luminosity-based distance estimates difficult. Using {\it Gaia} Data Release 2 (DR2) parallaxes and Bayesian inference, we infer a parallax of $0.35^{+0.07}_{-0.06}$ mas corresponding to a distance of $2.6^{+0.6}_{-0.4}$ kpc. To leverage the combined statistics of all stars in the direction of Wd1, we derive the Bayesian model for a cluster of stars hidden among Galactic field stars; this model includes the parallax zero-point. Previous estimates for the distance to Wd1 ranged from 1.0 to 5.5 kpc, although values around 5 kpc have usually been adopted. The {\it Gaia} DR2 parallaxes reduce the uncertainty from a factor of 3 to 18\% and rules out the most often quoted value of 5 kpc with 99\% confidence. This new distance allows for more accurate mass and age determinations for the stars in Wd1. For example, the previously inferred initial mass at the main-sequence turn-off was around 40 M$_{\odot}$; the new {\it Gaia} DR2 distance shifts this down to about 22 M$_{\odot}$. This has important implications for our understanding of the late stages of stellar evolution, including the initial mass of the magnetar and the LBV in Wd1. Similarly, the new distance suggests that the total cluster mass is about four times lower than previously calculated.Comment: 14 pages, 10 figure

Revised stellar parameters for V471 Tau, a post-common envelope binary in the Hyades

V471 Tau is a post-common-envelope binary consisting of an eclipsing DA white dwarf and a K-type main-sequence star in the Hyades star cluster. We analyzed publicly available photometry and spectroscopy of V471 Tau to revise the stellar and orbital parameters of the system. We used archival K2 photometry, archival Hubble Space Telescope spectroscopy, and published radial-velocity measurements of the K-type star. Employing Gaussian processes to fit for rotational modulation of the system flux by the main-sequence star, we recovered the transits of the white dwarf in front of the main-sequence star for the first time. The transits are shallower than would be expected from purely geometric occultations owing to gravitational microlensing during transit, which places an additional constraint on the white-dwarf mass. Our revised mass and radius for the main-sequence star is consistent with single-star evolutionary models given the age and metallicity of the Hyades. However, as noted previously in the literature, the white dwarf is too massive and too hot to be the result of single-star evolution given the age of the Hyades, and may be the product of a merger scenario. We independently estimate the conditions of the system at the time of common envelope that would result in the measured orbital parameters today.Accepted manuscrip

Variability of Red Supergiants in M31 from the Palomar Transient Factory

Most massive stars end their lives as Red Supergiants (RSGs), a short-lived evolution phase when they are known to pulsate with varying amplitudes. The RSG period-luminosity (PL) relation has been measured in the Milky Way, the Magellanic Clouds and M33 for about 120 stars in total. Using over 1500 epochs of R-band monitoring from the Palomar Transient Factory (PTF) survey over a five-year period, we study the variability of 255 spectroscopically cataloged RSGs in M31. We find that all RGSs brighter than M_K~ -10 mag (log(L/L_sun)>4.8) are variable at dm_R>0.05 mag. Our period analysis finds 63 with significant pulsation periods. Using the periods found and the known values of M_K for these stars, we derive the RSG PL relation in M31 and show that it is consistent with those derived earlier in other galaxies of different metallicities. We also detect, for the first time, a sequence of likely first-overtone pulsations. Comparison to stellar evolution models from MESA confirms the first overtone hypothesis and indicates that the variable stars in this sample have 12 M_sun<M<24 M_sun. As these RSGs are the immediate progenitors to Type II-P core-collapse supernovae (SNe), we also explore the implication of their variability in the initial-mass estimates for SN progenitors based on archival images of the progenitors. We find that this effect is small compared to the present measurement errors.Comment: 17 pages, 10 figure