104 research outputs found
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 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 6.0 10 erg s Hz 45
years post-explosion and L 8.9 10 erg
s Hz 121 years post-explosion, respectively. These limits imply a
clean circumstellar medium (CSM), with 0.9 cm out to radii of a
few 10 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
0.050.3 M at radii between 10 and 10 cm, and
thin shells at specific radii with masses down to 0.01 M.
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
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
10 yr, such that a large ( 10 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 mas corresponding to a
distance of 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; the new {\it Gaia} DR2 distance shifts this down to
about 22 M. 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
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