My student's paper "The unusual 3D distribution of NaCl around the AGB star IK Tau" is now free to read on arXiv:
https://arxiv.org/abs/2302.06221 and has been accepted for publication in the journal Astronomy & Astrophysics.
The unusual 3D distribution of NaCl around the AGB star IK Tau
NaCl is a diatomic molecule with a large dipole moment, which allows for its
detection even at relatively small abundances. It has been detected towards
several evolved stars, among which is the AGB star IK Tau, around which it is
distributed in several clumps that lie off-center from the star. We aim to
study the three-dimensional distribution of NaCl around the AGB star IK Tau,
and to obtain the abundance of NaCl relative to H$_2$ for each of the clumps.
First, a new value for the maximum expansion velocity is determined. The
observed ALMA channel maps are then deprojected to create a three-dimensional
model of the distribution of NaCl. This model is then used as input for the
radiative transfer modelling code magritte, which is used to obtain the NaCl
abundances of each of the clumps by comparing the observations with the results
of the magritte simulations. Additionally, the rotational temperature of the
clumps is determined using population diagrams. We derive an updated value for
the maximum expansion velocity of IK Tau $\upsilon_\mathrm{exp}$ = 28.4 km/s. A
spiral-like shape can be discerned in our three-dimensional distribution model
of the NaCl. This spiral lies more or less in the plane of the sky. The
distribution is also flatter in the line-of-sight direction than in the plane
of the sky. We find clump abundances between $9 \times 10^{-9}$ and $5 \times
10^{-8}$ relative to H$_2$, where the relative abundance is typically lower for
clumps closer to the star. For the first time, we used deprojection to
understand the three-dimensional environment of an AGB star and calculated the
fractional abundance of NaCl in clumps surrounding the star.
To briefly explain the title: an AGB star is a star similar to our Sun but in a late/dying phase of its life/death. AGB stands for "asymptotic giant branch" which is a silly name that comes from where these stars appear on the HR-diagram (
https://astronomy.swin.edu.au/cosmos/h/hertzsprung-russell+diagram).
Hertzsprung-Russell Diagram | COSMOS
AGB stars throw off their outer layers over a time period of around 100,000 years. Which is short on a cosmic scale, but long on a human scale, especially when compared with supernovae, which are the explosions that more massive stars (more than 10 times the mass of our Sun) go through.
These sorts of stars, of which IK Tau is one, can throw off about an Earth-mass worth of gas every year. The gas is mostly molecular and some of it goes on to form dust. AGB stars are actually an important source of cosmic dust.
Anyway, for this particular project, which started off as a Masters thesis, we wanted to look at the unusual clumpy distribution of NaCl. Yes, that's like table salt, except around these stars it's in molecular form, not the kind of crystal you might put on your dinner.
What was unusual about the salt distribution? Well we usually expect molecules to form in round-ish (spherical) blobs centred on the star. But that's not what we saw for salt around IK Tau.
Instead of a round-ish blob centred on the star, we saw several blobs at different distances from the star and we wanted to understand why.
Step 1 was to create a 3D reconstruction of the salt distribution. That's what you can see in this animation. The little yellow dot in the centre is the star, to scale, and the coloured blobs are the salt.
The star looks small here, but it is actually bigger than the size of the Earth's orbit around the Sun (2 or 3 au in radius)! It started out not much larger than the Sun but in death has expanded a lot! The clouds of NaCl look giant but they are actually only seen in the inner part of what we call the circumstellar envelope. These circumstellar envelopes are really big — up to tens of thousands of au.
(an "au" is the distance between the Earth and the Sun and stands for astronomical unit)
So when we get molecular emission forming so unevenly in very specific directions (as opposed to being seen almost in all directions but just being a bit blobby) that means something has to have caused the uneven formation.
A possible explanation is that the salt forms in the region where the gas being ejected from the star clashes with something else — like a large planet or a small, cool star — orbiting the dying star.
Taïssa Danilovich