30 Jun 2021
arXiv:2106.16223 (Accepted to the The Astrophysical Journal)
Josiah Schwab
We present a set of ultramassive white dwarf models, focused on masses above
$1.3\,M_\odot$. Given the uncertainties about the formation and compositions of
such objects, we construct parameterized model sequences, guided by
evolutionary calculations including both single star and double white dwarf
merger formation channels. We demonstrate that the cooling of objects with
central densities in excess of $10^9\,\rm g\,cm^{-3}$ is dominated by neutrino
cooling via the Urca process in the first $\approx 100$ Myr after formation.
Our models indicate that the recently discovered ultramassive white dwarf ZTF
J190132.9+145808.7 is likely to have experienced this Urca-dominated cooling
regime. We also show that the high densities imply that diffusion is unlikely
to significantly alter the core compositions of these objects before they
crystallize.
07 Jun 2021
Earlier today, I was pleased to
announce
that the MESA stellar evolution
code is now publicly hosted on GitHub.
The MESA stellar evolution code began its life in 2007 as a Subversion
repository hosted on SourceForge.
MESA is somewhat unusual in that a checkout of the code is multiple GB
in size. This is because it versions its key binary assets (e.g.,
equation of state data files) along with the code. In addition, there
are also numerous large text files (containing stellar models) that
can frequently change, but whose diffs are not meaningful.
The SVN proved satisfactory for MESA’s early life, especially as its
development was primarily due to a single person (Bill Paxton).
Around 2016-2017, the developer base had expanded to include several
other regular contributors and SourceForge was becoming increasingly
unreliable.
At that time, we considered moving to git, but solutions for
versioning large binary files with git were in their infancy.
Instead, we migrated the SVN to a paid hosting plan at
Assembla, immediately resolving our performance problems.
Developers who wanted to interface via git (including myself), were
able to do so using git-svn. This was an acceptable
stop-gap measure, but the large binary files meant that the git
repository constructed in this way was in excess of 50 GB and
generally suffered from poor performance when doing complex
operations.
With an understanding that a git migration remained desirable,
sustained effort was applied to reduce the number of large files
tracked in the repository. Some simple improvements were realized by
removing various types of cruft. Others required more involved
structural changes. For example, redesigning the testing
infrastructure so that solutions could be compared between different
machines via a central server removed the necessity to version-control
reference stellar model files.
By 2020, the MESA developer base had continued to grow,
GitLFS was mature, and there was a desire to more
effectively engage with a new generation of users and developers.
With the help of my colleagues, especially Rob Farmer and Bill Wolf,
we put together a plan to migrate MESA to GitHub. In advance of the
migration, documentation was written to provide a
basic introduction to git/GitHub for those who were unfamiliar.
In the end, we chose not to include the complete MESA development
history in the git repository. Instead, the git repository was
initialized to the most recent release (r15140) and a modest number of
patches originally committed to the SVN were applied to the git
repository.
The MESA developers moved to git with minimal interruption in
mid-December 2020. We took about 6 months to get comfortable and
establish and document new workflows, and then finally made the
repository public. This should pave the way for a new era of
collaborative MESA development and interaction with the astrophysics
community.
06 Jan 2021
The Astrophysical Journal, Volume 906, Issue 1, id.53, 14 pp.
Josiah Schwab
We construct evolutionary models of the remnant of the merger of two carbon-oxygen (CO) core white dwarfs (WDs). With total masses in the range 1-2 M⊙, these remnants may either leave behind a single massive WD or undergo a merger-induced collapse to a neutron star (NS). On the way to their final fate, these objects generally experience a ∼10 kyr luminous giant phase, which may be extended if sufficient helium remains to set up a stable shell-burning configuration. The uncertain, but likely significant, mass-loss rate during this phase influences the final remnant mass and fate (WD or NS). We find that the initial CO core composition of the WD is converted to oxygen-neon (ONe) in remnants with final masses ≳1.05 M⊙. This implies that the CO core/ONe core transition in single WDs formed via mergers occurs at a similar mass as in WDs descended from single stars and thus that WD-WD mergers do not naturally provide a route to producing ultramassive CO-core WDs. As the remnant contracts toward a compact configuration, it experiences a "bottleneck" that sets the characteristic total angular momentum that can be retained. This limit predicts that single WDs formed from WD-WD mergers have rotational periods of ≍10-20 minutes on the WD cooling track. Similarly, it predicts remnants that collapse can form NSs with rotational periods ∼10 ms.