###### Abstract

One of the key research themes identified by the Astro2020 decadal survey is
Worlds and Suns in Context. The Advanced X-ray Imaging Satellite (AXIS) is a
proposed NASA APEX mission that will become the prime high-energy instrument
for studying star-planet connections from birth to death. This work explores
the major advances in this broad domain of research that will be enabled by
the AXIS mission, through X-ray observations of stars in clusters spanning a
broad range of ages, flaring M-dwarf stars known to host exoplanets, and young
stars exhibiting accretion interactions with their protoplanetary disks. In
addition, we explore the ability of AXIS to use planetary nebulae, white
dwarfs, and the Solar System to constrain important physical processes from
the microscopic (e.g., charge exchange) to the macroscopic (e.g., stellar wind
interactions with the surrounding interstellar medium). _This White Paper is
part of a series commissioned for the AXIS Probe Concept Mission; additional
AXIS White Papers can be found at theAXIS website with a mission overview
here_.

xx 1 5 The life cycle of stars and their planets from the high energy
perspective Lia Corrales1†, Keivan G. Stassun2†, Tim Cunningham3,4 Girish M.
Duvvuri2, Jeremy J. Drake3, Catherine Espaillat5, Adina D. Feinstein6, Elena
Gallo1, Hans Moritz Günther7, George W. King1, Marina Kounkel8, Carey M.
Lisse9, Rodolfo Montez Jr.3, David A. Principe7, Jesús A. Toalá10, Scott J.
Wolk3, Raven Cilley1, Tansu Daylan11, Margarita Karovska3, Pragati Pradhan12,
Peter J. Wheatley13 and Jun Yang3,7 AXIS Stars & Exoplanets Working Group

###### Contents

  1. 1 The birth of stars and planets
    1. 1.1 Coronal activity, jets, and accretion of young stars
      1. 1.1.1 Magnetic Activity
      2. 1.1.2 Accretion shocks
      3. 1.1.3 Shocks in outflows: jets
    2. 1.2 Case study of stellar jets: DG Tau
      1. 1.2.1 The inner jet of DG Tau
      2. 1.2.2 The outer jet of DG Tau
      3. 1.2.3 Prime targets for AXIS
      4. 1.2.4 Absorption by accretion funnels or the inner disk
      5. 1.2.5 Broad-band absorption
      6. 1.2.6 Detecting absorption signatures from individual elements
    3. 1.3 The effect of stellar activity on planet formation
      1. 1.3.1 Photoevaporation
      2. 1.3.2 Disk Ionization
  2. 2 How stars influence their planets
    1. 2.1 Stellar activity has the power to sculpt planetary systems
    2. 2.2 Stellar activity affects atmospheric retention and habitability
      1. 2.2.1 X-ray studies of the Nearby Solar System Illuminated by its Host Star’s XUV Radiation and Stellar Wind
      2. 2.2.2 Astrospheres: direct observations of stellar wind environments of other worlds
      3. 2.2.3 Spotting coronal mass ejections via coronal dimming
      4. 2.2.4 Reconstructing the high energy environment of terrestrial exoplanets
  3. 3 Deaths of planet-hosting stars
    1. 3.1 The unique plasma environment of Planetary Nebulae
    2. 3.2 The destiny of planetary systems around White Dwarfs

###### List of Figures

  1. 1 Simulated AXIS observation of the Carina star-forming complex
  2. 2 The classical T-Tauri star DG Tau in X-rays
  3. 3 Simulated absorption spectra from planet-forming disks
  4. 4 Illustration of disk photoevaporation due to high-energy stellar radiation
  5. 5 Illustration of angular momentum transport in a protoplanetary disk
  6. 6 Predicted X-ray demographics of planet-hosting stars
  7. 7 Predicted radius evolution of exoplanets subject to photoevaporative mass loss
  8. 8 Illustration of star clusters that AXIS can characterize
  9. 9 Simulated stellar XUV modeling enabled by AXIS
  10. 10 AXIS predictions of hot bubble emission from planetary nebulae
  11. 11 Predicted X-ray flux of metal-polluted white dwarfs

AXIS is poised to become the prime instrument for studying the high-energy
environments of stars and the corresponding effects on their respective
planetary systems. Table 1 summarizes key AXIS qualities, relevant for
studying stars and exoplanets, in comparison to modern X-ray observatories. In
Section 1, we describe the state of the field when it comes to high energy
characterization of protostars, from accretion processes to the effects of
X-rays on planet formation. Section 2 describes the myriad of ways in which
stellar activity, which produces both high-energy irradiation and coronal mass
ejections, can influence the atmospheric properties, demographics, and
habitability of planetary systems, including our own. Finally, Section 3
describes how X-ray observations can help us understand the ultimate fate of
Earth, through the study of planetary nebulae and white dwarfs. AXIS will
provide leaps in understanding in all of these key science areas, tackling the
Astro2020 theme: Worlds and Suns in Context (National Academies of Sciences,
2021).

Figure 1: Simulated 50 ks three-color AXIS observation of point sources in a portion of the Carina star-forming complex. The X-ray image is overlaid with a DSS optical image (white), demonstrating the utility of X-rays piercing the molecular cloud material out of which stars are born. With stars having ages $1-6$ Myr, the entire Carina complex has one of the highest concentrations of massive stars in the Galaxy, in addition to more than 100,000 young, low-mass stars. A Chandra legacy survey of the Carina complex utilized 1.2 Ms over 22 pointings, yielding 14,000 X-ray detections, 90% of which had fewer than 50 counts (Townsley et al., 2011). AXIS can perform even larger surveys in a fraction of the time thanks to its sharp PSF stability and large collecting area across a wide field of view: A single 150 ks AXIS pointing in this portion of the star-forming complex can detect the $\sim 6000$ stars previously detected by Chandra and thousands of new, young, low-mass stars that were previously undetected, easily reaching M3-type dwarfs around the peak of the stellar IMF. Table 1: Key AXIS observatory parameters Property | AXIS | Chandra ACIS | XMM EPIC-pn
---|---|---|---
0.5 keV Eff Area (cm2) | 3300 | 11 | 900
PSF† on-axis (arcsec) | 1.5 | 0.8 | 15
PSF† at $9^{\prime}$ (arcsec) | 1.6 | 7.0 | 18
Field of View (arcmin2) | 452 | 256 | 713
$\Delta E$ (eV) Resolution @ 1 keV | 60 | 100-150 | 80
†As defined by the radius at which 50% of the soft X-ray PSF is captured.

##  1 The birth of stars and planets

The earliest stages of a star’s life, from the initial collapse of a molecular
cloud to the eventual fusion of hydrogen in a stellar core as it joins the
main sequence, are described by a stellar classification system based on the
shape of a star’s infrared spectral energy distribution continuum—be it the
cold broad spectrum of a collapsing cloud, the mixed spectrum of a protostar +
thick heated envelope, or the clearly separated protostar + disk (Lada, 1987).
The class system (Class 0, I, II and III) broadly describes major stages in a
young star’s life which begins at Class 0 when a cold core begins to form as a
star’s natal molecular cloud clump begins to gravitationally collapse.
Molecular clouds have some initial net rotation, and as they gravitationally
contract, conservation of angular momentum results in a protostellar disk
composed of gas and dust with a forming protostar at its center. This Class I
stage is characterized by a protostar, shining via accretion of material from
its surrounding environment (which includes a circumstellar disk and an
envelope or shell formed from its parent molecular cloud). The Class II stage
begins as a young star clears its surrounding environment and is characterized
by an un-embedded, but still accreting, star-disk system. During the Class I
and II stages, light from the central protostar shines on the disk, which can
influence its chemistry and the eventual formation of planets. Eventually, a
young star’s disk is dispersed (removed) leaving any already formed planets
and the central pre-main-sequence star. It remains in this Class III stage
until it joins the main sequence.

This sequence of stellar assembly involves several energetic processes –
accretion, jets/outflows, and magnetic activity – that can produce X-ray
emission, and therefore X-rays are key informants into the detailed physics of
these processes. In turn, the X-rays produced by these stellar processes
become an important source of high-energy insolation of the young planets that
form in the circumstellar environment of the young star.

###  1.1 Coronal activity, jets, and accretion of young stars

Stars form through gravitational fragmentation and collapse of large molecular
clouds. The initial infall is radial, but due to the conservation of angular
momentum, the innermost region spins up as it contracts, and an accretion disk
forms around it. In the earliest stages of star and planet formation, the
central protostar is hidden within the dense cloud and can only be observed in
the IR and longer wavelengths. For more massive stars, the evolution is so
fast that most of the early phases remain invisible. In this summary, we
concentrate on slower-forming stars with masses $<3~{}M_{\odot}$.

As accretion proceeds, the envelope, an outer shell of material from the
parent molecular cloud of a star, becomes thinner. Many young stars have jets
that eject mass and angular momentum and disperse parts of the envelope,
making the star itself visible. This happens before stars reach the main
sequence; the protostars are still contracting, but they already have
convective envelopes and stellar activity. In this stage, protoplanets begin
to form in the accretion disk, while material continues to fall onto the star,
following magnetic field lines that connect the star to the disk. There are
three distinct sources of X-rays during this period that we discuss in the
following sections (see also the review by Schneider et al. (2022)).

####  1.1.1 Magnetic Activity

Magnetic fields drive stellar activity through tangling, snapping, and
reconnection events in the protostar’s atmosphere that heat X-ray-emitting
plasmas and accelerate electrons to produce synchrotron radiation. While
seemingly rare, even the youngest (Class 0) stage of a star’s life
demonstrates X-ray emission characteristic of magnetic activity (Grosso et
al., 2020). For other young stars, primarily those at Class I and later,
occasional flares with large loop lengths are observed (Favata et al., 2005);
those flares might occur along the magnetic field lines that connect the star
and the disk. Stars that are still accreting (Class II or CTTS) exhibit X-ray
luminosities that imply less magnetic field activity than later stages of
young stars where the accretion disk has dispersed (Class III or WTTS),
(Preibisch et al., 2005). The physical mechanism for this difference,
thoroughly demonstrated in the Chandra Orion Ultradeep project (COUP Getman et
al., 2005), is still under debate two decades later. Stellar accretion might
suppress the coronal X-ray emission by filling the corona with colder plasma
or disturbing the structure of the magnetic field lines. However, it is still
possible that this is an observational bias because Class II sources are more
obscured and thus the soft X-rays they emit might be less observable. Given
this uncertainty, future large surveys with AXIS that sample time-domain
activity levels for a range of conditions in star-forming clouds (Figure 1)
will provide a clear path for confirming whether or not magnetic activity is
suppressed at earlier stages of star formation. AXIS’s small PSF, which is
stable over a larger field of view than Chandra, enables surveys not just of
the cores of star-forming regions, but also the outer, less dense, regions to
cover a wide variety of conditions.

####  1.1.2 Accretion shocks

High-resolution observations of the He-like triplets of O VII and Ne IX show
that this emission is produced in relatively high-density regions ($n_{e}\sim
10^{12}$ cm-3), which are much higher than in the corona of a main sequence
star ($n_{e}\sim 10^{10}$ cm-3). Instead, they originate in the accretion
shock, where material from a protostar’s circumstellar disk falls onto the
body of the star (Kastner et al., 2002; Brickhouse et al., 2010). As the
material falls onto the star, it is accelerated by gravity and reaches free-
fall velocities of $300-500$ km/s (depending on the mass and radius of the
star). An accretion shock is formed, heating matter to soft X-ray emitting
temperatures ($\sim 1-3\times 10^{6}$ K) when the material impacts the stellar
surface. The triplet spectral features from He-like elements likely arise from
the cooling zone of that shock and from material that has escaped magnetic
confinement, such as winds or outflows with densities higher than typically
encountered in stellar coronae. Understanding the contribution of He-like ions
to the total X-ray flux of young stars is one of the major model ingredients
that will go into interpreting the X-ray flux of young stars observed with
AXIS.

####  1.1.3 Shocks in outflows: jets

Young stars often shed angular momentum through outflows: from slow-moving,
wide-angle disk winds to fast and highly collimated jets launched close to the
star. In some cases, those jets can be traced to several pc from the star.
X-ray emission can be seen from the termination shock where the jet runs into
the interstellar medium (e.g. Bally et al., 2003; Pravdo & Tsuboi, 2005;
Grosso et al., 2006), from inner working surfaces in the shock (Bonito et al.,
2010a), and also from the inner region very close to the launching region
(Güdel et al., 2007; Bonito et al., 2011).

Inner jets, in particular, have been shown to be X-ray emitters in a number of
young stars. X-rays from shocks in outflows that have been resolved by Chandra
are faint and located close, sometimes within an arcsec (10s of au) of the
central star. For younger stars that are still deeply embedded in molecular
clouds, a jet that pierces an opening into the infalling envelope may be
observable even while the central protostar is not, due to obscuration by its
natal cloud. In those cases (e.g. HH 154 Favata et al., 2006), all the
observed X-rays come from jet-induced shocks; the observed emission is
spatially offset from the central source and is so soft that it cannot come
from the embedded star.

Stellar jets provide feedback on the molecular cloud, pierce holes that make
the inner star visible to us, and carry away angular momentum that allows
accretion to proceed. Understanding jets is absolutely crucial to make
progress on several science questions identified in the Astro2020 decadal
survey for “Interstellar Medium and Star and Planet Formation”. In particular,
F-Q2a asks: “What processes are responsible for the observed velocity fields
in molecular clouds?”. To answer this question, we need to quantify the mass
and momentum that stellar jets inject into the clouds. For question F-Q3b:
“How do protostars accrete from envelopes and disks, and what does this imply
for protoplanetary disk transport and structure?”, we need to quantify the
angular momentum removed from the system by winds and outflows and determine
at what radius that occurs. Since X-rays probe the fastest outflow component
of the jet, which are launched deep inside the gravitational well of the star,
possibly even from the inner disk interface or the stellar surface, they are
crucial to getting a complete picture of the stellar jets and their impact on
star formation and molecular clouds.

###  1.2 Case study of stellar jets: DG Tau

Class II sources are typically evolved star-disk systems that have cleared out
their surrounding molecular environment. Observing Class II sources with jets
provides an opportunity to investigate jet-launching mechanisms by revealing
structure closer to the launching region. The best-studied object of this type
is the CTTS DG Tau, which has 450 ks of cumulative observations with Chandra
(Güdel et al., 2005, 2008, 2011), and reveals two spatially distinct
components that are X-ray emitters in the outflow. Proper motion has been
observed from X-ray jets (e.g. Schneider et al., 2012), but for DG Tau this
question is unsettled. Figure 2 shows a 350 ks Chandra image of DG Tau, which
was obtained in a single year. This image resolves this system into three
components: the young stellar corona, an inner jet region, and an outer jet
region.

Figure 2: The CTTS DG Tau in X-rays. The left panel shows a 350 ks Chandra
image combining multiple observations taken in 2010. The image is color-coded
with photons below 1.2 keV shown in red and photons above 1.2 keV in blue.
Three spatially distinct components are visible. The DG Tau corona is
absorbed, and only photons $>1.2$ keV are visible (blue) at this location.
There is a softer inner jet component (red), that is offset about 0.3 arcsec
from the stellar position, but due to the resolution of this image, the two
components overlap. Even with the 1 arcsec resolution of Chandra, the two
components can be spectrally resolved by fitting with two different
temperatures and absorption components (black data with red and blue model
components in the middle panel). There is also an outer jet component that is
several arcsec (650 AU) away from the central star. We expect substructure in
this region, but given the low count rate in Chandra, we only know that it is
spatially extended. This feature will be resolved with AXIS, and a spectrum
can be extracted separately (green). The right panel shows a simulation for a
50 ks AXIS observation of DG Tau based on the models fitted to the Chandra
data. In both the middle and the right panels, the jet (green) spectrum is
binned to 10 counts/bin, and the stellar/inner jet spectrum to 30 counts/bin.
The superior collecting area of AXIS will allow us to fit abundances and
multi-temperature models, search for substructure, and look for proper motion
of the inner and out jet components in a fraction of the observing time
required by Chandra.

####  1.2.1 The inner jet of DG Tau

The inner jet component is barely 0.2” offset from DG Tau itself (30-50 AU),
but its spectrum is much softer than the star because the star is deeply
embedded, but the jet itself is not. This allowed Schneider & Schmitt (2008)
to compare the position of the soft and hard photons and measure the offset,
despite the fact that it is much smaller than the size of the Chandra PSF.
Similar techniques can be employed with AXIS, but thanks to the much larger
collecting area, measurements like this will no longer be limited by the low
count rate. Together with the implementation of sub-pixel resolution
algorithms, such as those developed for Chandra-ACIS, AXIS will thus be able
to probe the evolution of the jet over time through repeat observations of
this target over the course of the mission lifetime.

Different hypotheses have been presented to predict the time revolution of DG
Tau’s inner jet. Bonito et al. (2010b) present a model of a diamond shock, and
Günther et al. (2014) suggests a model where hydrodynamic pressure of the
outer wind component recollimates the inner, faster, less dense region and
focuses a standing recollimation shock. In these scenarios, the shock position
should be mostly steady over time, but the shock intensity should change with
changes in the accretion (and thus mass outflow) rate. Only AXIS has the
collecting area and angular resolution to perform these measurements
repeatedly and for multiple stars.

While DG Tau is so far the only known example where the jet shock is spatially
resolved in imaging, the same spectral shape consisting of a hard, highly
absorbed stellar corona and a much softer, much less absorbed shock component
is also seen in GV Tau and DP Tau (Güdel et al., 2007). It seems likely that
these objects also have X-ray-emitting jets. Unfortunately, with the major
loss of the Chandra ACIS soft energy effective area, those observations are no
longer feasible with Chandra, even for DG Tau, which is the brightest of the
known X-ray sources that exhibit evidence for a corona-jet system. AXIS is the
only Probe-class X-ray telescope under design in the next decade that would be
able to prove or disprove this idea directly through deep imaging.

####  1.2.2 The outer jet of DG Tau

In addition to the inner jet shock, DG Tau also shows an even fainter, outer
region with X-ray emission. This region is clearly resolved by Chandra and
will be clearly resolved by AXIS. Unfortunately, 350 ks of Chandra observation
yield only $\sim 50$ photons from this region. Based on observations in the UV
and optical, where the signal is better and HST can resolve finer details, we
know that there is considerable substructure, including a bow shock with hot
and cold materials. However, the X-rays must come from a different component
in the shock, since neither the temperatures nor the velocities of the
components seen in the IR, optical, and UV are sufficient to explain the
fitted X-ray temperature of 0.2 keV. Assuming that a bow shock or termination
shock is responsible, the mass loss rate seen in the X-rays is about
$10^{-10}M_{\odot}/{\rm yr}$ (Günther et al., 2009), a factor of 1000 below
the mass flux seen in the optical (Hartigan et al., 1995). At the same time,
the velocity must be at least 300 km/s to explain the observed X-ray spectrum.
With AXIS, we will be able to obtain an exposure deep enough to pinpoint the
location of the X-ray emission and associate it with the jet structure. We can
also repeat the observations over the lifetime of the AXIS observatory to
follow the proper motion.

There are two possible explanations for the X-ray emission in the outer jet.
Either the gas has been heated as it passed through a standing shock close to
DG Tau and is now adiabatically and radiatively cooling, or it is going
through a shock wave at its current location. In the latter case, that shock
wave could be caused by an obstacle such as a previous, slower outflow blob,
or it could be a shock wave traveling along the jet. AXIS observations of DG
Tau can distinguish all of those possibilities. If the gas has previously been
heated and is cooling adiabatically, we expect the jet to have a cone opening
angle. We know that the outer shock is resolved with Chandra, but we have
insufficient signal to constrain its shape. AXIS will deliver an order of
magnitude more counts and an arcsec-scale PSF – enough to detect an opening
angle of 5 degrees at 5 arcsec from the central source. Without expansion, the
density of the emitting material is so high that it cools much faster, and no
X-ray emitting material could reach beyond $\sim 1$ arcsec (140 AU, well past
the system’s proto-Kuiper Belt. Thus, if AXIS finds the outer X-ray jet to be
highly collimated, the jet material must be heated in place by a shock. AXIS
imaging can pinpoint the shock location to see if it is associated with a
known, cooler structure seen in other wavelengths, which the X-ray jet runs
into, as suggested by infrared observations (Maurri et al., 2014). AXIS
spectroscopy (Figure 2) determines the temperature of the plasma, and thus the
shock speed (Chandra indicates about 300 km/s; however, that fit is very
uncertain since it is based on only $\sim 40$ photons). AXIS observations
spaced by a year or more would also reveal the proper motion. At the distance
of DG Tau, 300 km/s corresponds to 0.5 arcsec/year, and with just 50 ks of
AXIS observing the centroid of the jet emission could be determined through
sub-pixel resolution algorithms. A simple comparison between proper motion and
shock speed will show if the shock plows through the moving jet material or if
the material passes a stationary shock wave (e.g., caused by an obstacle or
the magnetic field). In other words, AXIS will finally reveal what physical
mechanism powers X-rays from young stellar jets.

####  1.2.3 Prime targets for AXIS

DG Tau is just one example of a young star with a resolved X-ray jet. We only
discuss it in detail because it has the deepest Chandra exposure of any other
system, and thus we can make the most detailed predictions of what parameters
AXIS will be able to measure. Overall, the AXIS PSF and larger collection area
will allow studies more detailed than what Chandra has done on that single jet
for a larger sample of sources. Table 2 gives examples of young stars with
known X-ray jets showing the variety of conditions and environments that can
be probed with AXIS.

Table 2: Prime targets for observing protostellar jets Object | Type | Chandra ref
---|---|---
DG Tau | CTTS with resolved inner and outer jet emission | see text
HH 168 | Large scale diffuse emission with knots | (Schneider et al., 2009a)
Cepheus A | Inner jets, source itself embedded | (Schneider et al., 2009b)
L 1551 IRS5 (= HH 154) | Inner, stationary jet component and outer knot with proper motion | (Favata et al., 2002; Bally et al., 2003; Schneider et al., 2011)
HH 2 |  | (Pravdo et al., 2001)
RY Tau | Similar to DG Tau | (Skinner et al., 2018)
HD 163296 | Herbig AeBe star, more massive than CTTS | (Swartz et al., 2005; Günther et al., 2013)

Beyond known sources, AXIS will be a discovery engine for new stellar jets.
Its PSF remains concentrated over a much larger field-of-view than Chandra’s
PSF in the 8.3’$\times$8.3’ ACIS FOV. This enables searches for resolved
extended emission over much larger regions, including those with dozens to
hundreds of other young stars with potential jets, in a single observation. At
the same time, the increased effective area collects enough photons to
securely identify jets in star-forming regions and measure jet speeds via
shock temperatures. Currently, the number of jets known in optical and IR
vastly outnumber the known X-ray-detected jets. Given the low soft X-ray
sensitivity of Chandra, we cannot test whether this means that special
conditions are needed to form the fast, X-ray-emitting outflow components or
whether our sensitivity is simply too low to detect them in most cases.

With AXIS, we can answer the question: Does every stellar jet have an inner
fast component? If so, that opens the door to a unified theory of how angular
momentum is removed in the innermost regions of the disk, allowing for ongoing
accretion and disk clearing.

####  1.2.4 Absorption by accretion funnels or the inner disk

In addition to probing hot, X-ray-emitting gas, AXIS can also use X-ray
absorption as a tool to study the environments of planet formation. Soft
X-rays are absorbed by intervening material, such as molecular clouds or
circumstellar disks, changing the shape of the observed spectrum. Although
this absorption is typically quantified as “equivalent hydrogen column
density,” it is actually dominated by photons absorbed by the inner shell
electrons of metals, such as C, O, or N (Wilms et al., 2000).

####  1.2.5 Broad-band absorption

When absorption by individual elements cannot be resolved, it is useful to
assume a set of standard abundances and simply calculate the total gas density
that would provide the correct density of metals. X-rays are absorbed,
regardless of whether the metal is in atomic form, bound in a molecule, or in
a small grain. X-rays are fully absorbed (“gray absorption”) only if the
material is bound in large grains, pebbles, or planets. In this case, the
spectral shape stays the same, but the observed flux is lower. In contrast, in
the optical and IR, only dust grains contribute to the reddening. Thus,
comparing the X-ray absorbing column density and optical reddening gives us a
handle on the gas-to-dust ratio, which we know must vary from the $\sim$100:1
of the ISM and moleuclar clouds to the $\sim$1:1 found in our mature solar
system. Thus this measurement is directly relevant to Astro2020 decadal survey
question F-Q4b: “What is the range of physical environments available for
planet formation?”

From the source at the star itself, line-of-sight X-rays pass through
different environments: The corona, the accretion streams, the disk
atmosphere, the surrounding envelope, the large-scale molecular cloud, and
lastly, the ISM between the star-forming region and us. All of those regions
cumulatively contribute to the X-ray absorbing column density, making the
interpretation of any result ambiguous. One of the most promising avenues is
to study objects with time-variable column densities. For example, in AA Tau,
(Schmitt & Robrade, 2007; Grosso et al., 2007) the accretion funnel passes
through the line-of-sight periodically every 8.5 days, increasing the NH by
about $8\times 10^{21}$ cm-2. Compared to optical/IR data, this number
indicates an elevated gas-to-dust ratio, compatible with the idea that
significant dust evolution happens in the inner disk. For AA Tau we also have
a probe of the outer material. In 2013, the star began to dim by up to 2 mag
in the V band for periods of months or weeks. H2 profiles indicate that the
source dimming is located at $>1$ AU. At this location, the ratio between
X-ray absorbing column density and reddening matches observations in the ISM
(Schneider et al., 2015a), so the grains through which the line of sight
passes have not evolved.

A similar sudden dimming has been observed in CTTS RW Aur A (Schneider et al.,
2015b), a member of the RW Aur binary system, that can be resolved with
Chandra or AXIS. At its maximum, the measured NH increased by more than two
orders of magnitude and interestingly, Günther et al. (2018) find indications
for an extremely high Fe abundance in the emitting plasma at that time. That
iron has to be recently accreted, possibly due to the breakup of an inwardly
migrating differentiated planetesimal [Lisse 2022 REF], or because the grains
migrating inward in the disk were trapped in a pressure trap zone that was
suddenly released as the structure of the inner disk changes (Gárate et al.,
2019) with obvious implications for planet formation in the inner regions of
this system. These two examples show the range of science that detailed X-ray
monitoring of a wide range of young stellar objects can deliver. With its
arcsec-scale PSF over a wide field of view combined with a large collecting
area, AXIS will be able to sample a much larger number of young stars than
Chandra or XMM could and will lead to more objects of the type of AA Tau or RW
Aur A being detected to study, as well as reveal new phenomena in young stars
that we have not yet thought about.

####  1.2.6 Detecting absorption signatures from individual elements

Young stars with planet-forming disks (e.g., Class I and Class II) show a
characteristic increase in absorbing column (NH) as a function of increasing
circumstellar disk inclination (Kastner et al., 2005). For example, the
viewing geometry of an edge-on disk (i.e., a star-disk system whose central
star must shine X-rays through the midplane of the disk before arriving at our
detectors) results in much larger absorption than those viewed face-on. This
trend was identified using a small number of sources whose disk inclinations
were strongly constrained by HST when light from background nebulosity
revealed the disk silhouette.

The Atacama Large Millimeter Array (ALMA) has revolutionized our understanding
of circumstellar disks, in part due to its extreme sensitivity and impressive
angular resolution capabilities. ALMA is sensitive to the long wavelength
thermal dust emission of large mm to cm-sized grains in circumstellar disks,
capable of imaging and spatially resolving hundreds of nearby young stars. It
can also image the disks in molecular hydrogen tracers such as isotopologues
of CO and a plethora of molecular rotational lines revealing that disks are
chemically and morphologically complex (Andrews et al., 2018; Zhang et al.,
2021). Depending on the viewing geometry of the disk to our line of sight,
disk-absorption of X-rays from the central star can be used to constrain the
abundance of oxygen (Figure 3), the 3rd most abundant atom in the galaxy and a
vital element in water, rocks, and simple organics like CO, H2CO, CH3OH—and
thus critical for the chemical evolution of disks, planets, and life as we
know it (Banzatti et al., 2020; Gaidos, 2015; Ercolano et al., 2018).

Figure 3: (Top) Five one-temperature absorbed plasma models differing only in
oxygen abundance of the absorber (colored lines). The oxygen absorption
feature at $\sim 0.55$ keV can be used to strongly constrain the oxygen
content in nearby young planet-forming disks. (Middle and Bottom) A 150 ks
AXIS simulation using a typical flux and absorbing column density for nearby
young stars. The underlying model used to simulate the data (bottom) is an
absorbed multi-temperature thermal plasma with an absorber oxygen abundance of
2 Solar. The large effective area of AXIS will allow a determination of oxygen
abundance for nearby young planet-forming star-disk systems.

In order to constrain the abundance of oxygen using X-ray spectroscopy, it is
very important to collect enough signal at energies between 0.3 keV and 2 keV
where the signature of oxygen absorption is highest. To fully sample the
transition of X-ray absorption starting at 0.55 keV, it is important to
collect a large number of X-ray events in the 0.30–0.65 keV region.
Unfortunately, given the typical x-ray fluxes of nearby young pre-MS stars,
modern X-ray telescopes like XMM-Newton and Chandra lack the sensitivity at
0.30–0.65 keV to confidently detect this specific oxygen absorption feature
given the typical flux of nearby young pre-MS stars. An instrument like AXIS,
with a high soft effective area and stable PSF over a large field of view,
will allow us to identify this characteristic oxygen absorption for many
sources when viewing crowded young stellar clusters.

###  1.3 The effect of stellar activity on planet formation

High-energy radiation from young stars is expected to play a critical role in
energy deposition into the surrounding protoplanetary disk from which planets
form, as X-rays can penetrate through regions of a UVIS opaque, X-ray
optically thin PMS accretion disk and ionize and heat disk molecular gas. For
example, the H13CO+ line strength has been observed to vary, and this behavior
has been linked to changes in X-ray emission, presumably through an X-ray
flare that enhanced the HCO+ abundance in the disk (Cleeves et al., 2017).
More recently, SO, linked to X-ray emission in circumplanetary disks (Law et
al., 2023), has been found to trace an embedded protoplanet in a disk (Young
et al., 2021), opening a new window of study of the chemical properties of the
disk.

Two major areas where AXIS can have an impact on our understanding of
protoplanetary disks and planet formation are photoevaporation and disk
ionization, which we discuss next.

Figure 4: Illustration of disk photoevaporation due to high-energy stellar
radiation (from Armitage (2011)). Relative mass loss rates with radius are
shown for photoevaporation driven by EUV (Font et al., 2004), FUV (Gorti &
Hollenbach, 2009), and X-ray plus EUV radiation (Owen et al., 2010).

####  1.3.1 Photoevaporation

The origin of planets is intricately tied to the evolution of protoplanetary
disks. Significant gas must be present in the inner disk for gas giant planets
to form, so the lifetime of the disk places an upper limit on the timescale
for giant planet formation. The longevity of gas in the disk is dictated by
the rate at which gas is eroded by photoevaporative winds created by high-
energy ionizing radiation from the central star. However, there are no robust
observational constraints on the connection between high-energy radiation and
the dissipation of gas in disks. This lack of observational constraints has
led to uncertainty about the efficiency of photoevaporation during the
critical 1–10 Myr period when planets are expected to form.

High-energy X-ray, EUV, and FUV radiation can ionize the upper disk layers,
causing the gas to become unbound and escape, producing a thermally driven
photoevaporative wind (Figure 4). Models of EUV photoevaporation predict low
mass loss rates of $\sim 10^{-10}M_{\odot}/{\rm yr}$ (Font et al., 2004;
Alexander et al., 2006) while models that include X-ray or FUV emission
achieve high mass loss rates up to $\sim 10^{-8}M_{\odot}/{\rm yr}$ (Gorti &
Hollenbach, 2009; Owen et al., 2010).

Initially, in the first $\sim 10^{5}$ yrs, viscous evolution and radially
inward mass flow dominate disk evolution, leading to a decrease in the mass
accretion rate with time as disk material is depleted onto the star (i.e.,
less gas leads to a slower accretion rate). Once the rate of mass accretion
inward through the disk equals the outward mass loss rate above, though, the
inner disk is no longer able to be replenished and its inventory is rapidly
depleted, leaving behind an inner disk hole (Clarke et al., 2001).

This rapid disk clearing onset is reached later for the EUV photoevaporation
model, since it takes longer for the disk accretion rate to decline to the
lower $\sim 10^{-10}M_{\odot}/{\rm yr}$ level (Hartmann et al., 1998). The
average mass accretion rate of young stars with disks is $\sim
10^{-8}M_{\odot}/{\rm yr}$ (Hartmann et al., 1998), which is the point at
which X-ray- or FUV-driven photoevaporation models are expect to dominate disk
clearing. However, the existence of young stars surrounded by substantial
disks with mass accretion rates $<10{-10}M_{\odot}/{\rm yr}$ indicates that
X-ray or FUV-driven photoevaporation may not always dominate, suggesting that
EUV-driven photoevaporation may play a role (Ingleby et al., 2011); why this
is the case is not understood. The main obstacle in distinguishing between
photoevaporation models is the lack of observational constraints on the high-
energy emission of stars, which AXIS will provide.

Figure 5: Illustration of angular momentum transport due to the MRI in a
protoplanetary disk (from Armitage (2011)). There is a “dead zone” deep in the
disk where X-rays cannot penetrate; this is covered by an “active layer” where
X-rays ionize the disk and accretion occurs. The inner disk within 1 AU is
fully MRI turbulent, as shown in the bottom simulation of density isocontours.

####  1.3.2 Disk Ionization

Accretion in protoplanetary disks is essential for planet formation since the
material in the disk must move inward, and some of this material goes into
forming planets. However, we do not know the source of the angular momentum
transport mechanism that leads to accretion. More than 30 years ago,
magnetorotational instability (MRI) was proposed to explain how turbulent
viscosity could cause accretion in weakly magnetized differentially rotating
disks (Figure 5; Balbus & Hawley (1991)). However, the disk must be
sufficiently ionized for the MRI to be active, and there are likely “dead
zones” in regions of the disk that are not ionized (Gammie, 1996). Cosmic rays
cause too little ionization to produce the MRI (e.g., Cleeves et al., 2013),
but X-rays can ionize disks enough for the MRI to take hold in at least the
uppermost disk layers (Glassgold et al., 1997).

However, corresponding observational support for the MRI is still elusive.
ALMA has measured turbulence levels too low to be consistent with MRI (e.g.,
Flaherty et al., 2015). However, ALMA is not sensitive to the uppermost layers
of the disk and these ALMA-made turbulence measurements may only investigate
the ’dead zone’; meanwhile, the MRI is active in the surface layer of the disk
(see Figure 5).

Given that the MRI is dependent on X-ray ionization, studies of T Tauri stars
with AXIS will better characterize the X-ray emission of young stars and allow
us to connect the protostar’s X-ray luminosity to disk emission lines, which
will probe the effectiveness of the MRI towards angular momentum transport in
protoplanetary disks.

##  2 How stars influence their planets

The structure and evolution of planetary atmospheres are profoundly affected
by the high-energy irradiation and stellar wind environment produced by their
host stars. In addition to the quiescent environment, magnetically driven
flares and coronal mass ejections (CMEs) can impact and alter planet
atmospheres via stripping and charge-exchange interactions, as observed in our
own Solar System (e.g., Mars’ atmospheric stripping and the MAVEN spacecraft’s
results; [REFS]). AXIS observations of stars will, directly and indirectly,
probe all these physical processes, which are key factors for understanding
the observed demographics of exoplanet systems and assessing the habitability
of distant worlds.

The next decade will usher in a new generation of visible and infrared
instrumentation for the detection and characterization of exoplanets. This
ever-growing parameter space demands rapid and reliable estimates of the space
weather environment, for the purposes of identifying prime targets for
atmospheric characterization and habitability studies. When possible, combined
FUV and X-ray observations of planet-hosting stars can be used to derive the
total XUV flux that is primarily responsible for photoevaporation, either
through a piece-wise model reconstruction or via the scaling relation between
the X-ray and the EUV flux (Linsky et al., 2014; King et al., 2018). However,
due to the high levels of extinction that the ISM poses for FUV and EUV
observations, soft X-rays are the only means for probing stellar activity for
the vast majority of stars that are more than 100 parsecs away. Those AXIS
observations can then be used to derive the EUV flux that is primarily
responsible for photoevaporation, either through a piecewise model
reconstruction, or via scaling relation between the X-ray and the EUV flux
(Linsky et al., 2014; King et al., 2018). When AXIS observes planet-hosting
stars for which FUV measurements are available via legacy surveys, the full
spectrum of the stellar EUV emission can be modeled to much higher accuracy.

For all these reasons, AXIS will become the prime observatory for studying the
high-energy properties of stars and their magnetic activity. AXIS will be able
to characterize the space weather environment of over 1800 known planetary
systems. Figure 6 demonstrates the leap in discovery space that AXIS will be
able to achieve when it comes to studying the high-energy properties of
exoplanet-hosting systems. Planet radius is plotted against the X-ray fluxes
expected for 2639 stellar systems with transiting exoplanets, for which enough
information is present in the NASA Exoplanet Archive (Akeson et al., 2013) to
estimate the X-ray luminosity from known age-rotation-activity relations or
from a select sample of X-ray measurements from the literature. The vertical
black line marks the AXIS flux sensitivity limit, which is an order of
magnitude better than the dimmest planet host star detected in a recent
eRosita study (Foster et al., 2022).

Figure 6: As of this writing, there are 5438 confirmed planets cataloged in
the NASA Exoplanet Archive. Of all planets in the catalog, 75% were discovered
via the transit method, and there are $\sim 6100$ planet candidates from TESS
yet to be confirmed. Currently, just under 300 systems have been characterized
by X-ray observations (Foster et al., 2022). AXIS will be able to detect young
M3 dwarfs out to distances of 2 kpc for a $3\sigma$ detection in 20 ks (i.e.,
a flux of $10^{-16}$ erg/s/cm2). This limiting flux will enable AXIS to
measure the X-ray luminosity, and hence the high energy environment, of over
1800 transiting planet-hosting systems across a large range of stellar types.

###  2.1 Stellar activity has the power to sculpt planetary systems

One of the most remarkable results in exoplanet science today has been the
discovery of direct absorption of escaping gas from hot, close-in, short-
period Jupiters and sub-Neptunes (Vidal-Madjar et al., 2003, 2004; Lecavelier
Des Etangs et al., 2010; Ehrenreich et al., 2015; Linsky et al., 2010; Ben-
Jaffel & Ballester, 2013; Fossati et al., 2010; Spake et al., 2018). The
observed mass loss could be attributed to hydrodynamic escape caused by X-ray
and extreme ultraviolet (together, XUV) radiation from the host star (e.g.,
Lammer et al., 2003; Owen & Jackson, 2012). XUV photons are absorbed at higher
planetary altitudes, in lower-density parts of the atmosphere, where they
ionize atoms and molecules. The resulting free electrons heat the gas
collisionally; these processes inflate the gas, potentially leading to the
formation of hydrodynamic outflows (Vidal-Madjar et al., 2003; Yelle, 2004;
Tian et al., 2005; Koskinen et al., 2014). Ongoing atmospheric escape has been
confirmed observationally in a handful of nearby exoplanets through the
detection of escaping hydrogen via transit Lyman alpha spectroscopy and, more
recently, through the detection of metastable helium in the outer atmosphere
(e.g., Oklopčić & Hirata, 2018; Spake et al., 2018; Lampón et al., 2020).

For close-in planets (within a fraction of an AU), prolonged exposure to
intense and/or flaring stellar radiation can potentially lead to the removal
of a substantial fraction of the atmosphere’s initial light-element (H/He) gas
envelope (e.g., Owen & Wu, 2013; Owen & Lai, 2018; Owen, 2019; Wu, 2019).
Assuming that the incident stellar XUV flux is primarily converted into
expansion work, the instantaneous mass-loss rate from an irradiated atmosphere
can be expected to scale directly with the incident XUV flux and inversely on
the average planetary density (Watson et al., 1981; Erkaev et al., 2007). In
practice, numerical work has shown that the applicability of this “energy-
limited” approximation is fairly limited and that the efficiency of stellar-
driven atmospheric escape depends (strongly) on the planet’s gravity, as well
as the intensity of photoionizing radiation (Lammer et al., 2003; Yelle, 2004;
Tian et al., 2005; Murray-Clay et al., 2009; Owen & Jackson, 2012; Erkaev et
al., 2013; Salz et al., 2016; Caldiroli et al., 2022).

These studies indicate that hydrodynamic escape could play a significant role
in shaping the observed properties of the known hot population of super-Earths
and sub-Neptunes. XUV-driven atmospheric escape is proposed to explain the
“radius valley", an observed dearth of planets with radii of 1.5 to
$2R_{\oplus}$ (Lopez et al., 2012; Fulton et al., 2017; Owen & Wu, 2017), and
potentially the “Neptune desert”, the observed lack of short-period Neptunes
(Mazeh et al., 2016; Owen & Lai, 2018). An understanding of photoevaporative
losses is needed to decipher the full picture of planet formation and
evolution, starting with our understanding of the intrinsic XUV properties of
planet-hosting stars.

The key to reconstructing the XUV exposure histories of exoplanets is X-ray
luminosity measurements of planet host stars, and only the sensitivity and
imaging resolution of AXIS can enable accurate X-ray luminosity measurements
across a large range of ages, for all star types down to M3V (the peak of the
stellar initial mass function). From an instantaneous measurement of a star’s
X-ray luminosity, its planet’s present-day XUV exposure can be calculated
(e.g., Sanz-Forcada et al., 2011; Chadney et al., 2015; King et al., 2018).
Then, with an understanding of the star’s likely X-ray evolution over time,
past and future XUV exposure can be probabilistically determined (e.g., King &
Wheatley, 2021). Combined with theoretical models of atmospheric escape, this
XUV history can be used to predict (or trace) the future (or past) evolution
of individual planets’ atmospheres (e.g., King et al., 2022),Figure 7.

Figure 7: (Left) The predicted radius evolution under photoevaporative escape
models for two planets in the young Praesepe star cluster (King et al., 2022).
Planet K2-100b is near the edge of the Neptune desert, and planet K2-101b is a
sub-Neptune near the large side of the radius-period valley. The circle
markers indicate the current age and radius of these planets. The contours
represent the uncertainty that arises from the uncertainty in the mass
measurement of these planets. Three possible trajectories for the radius
evolution of these planets is predicted, depending on whether the current-day
X-ray luminosity of the host star matches the median $L_{X}/L_{\rm bol}$ value
from the Jackson et al. (2012) relations or deviates from this value within
observed dispersion of stellar X-ray luminosities. (Right) The observed radius
and orbital period of transiting planetary systems listed in the NASA
Exoplanet Archive. The green dots represent planets orbiting stars that are
accessible for X-ray characterization by AXIS. The solid black circles
indicate the current position of the two planets on the left, and their
possible positions on this diagram at age 6 Gyr are plotted with open circles.

Unfortunately, this process is complicated by vast uncertainties in our basic
understanding of how the X-ray luminosity of FGKM-type stars evolves with age.
Young stars typically exhibit $L_{X}/L_{\rm bol}\approx 10^{-3}$, dubbed the
“saturated” regime, until they reach about 100 Myr to 1 Gyr in age (depending
on spectral type), after which the average $L_{X}/L_{\rm bol}$ decays as a
power law with time (e.g., Jackson et al., 2012; Booth et al., 2017; Johnstone
et al., 2021). However, scattered about these average trends is a large
dispersion of an order of magnitude or more in the observed X-ray
luminosities, which represent a single snapshot in time. Modeling the
diversity of lifetime-integrated X-ray exposures of exoplanet atmospheres
requires an understanding of the degree to which this dispersion can be
attributed to intrinsic physical differences among stars or is instead
representative of average temporal variability (e.g., flares) exhibited
randomly by all stars.

With the imaging resolution and collecting area of AXIS, we will be able to
monitor star clusters spanning Myr to Gyr ages to capture X-ray luminosities
(means and dispersions) and high-energy events (flares, coronal mass
ejections, accretion shocks), enabling detailed characterization of their
stellar activity evolution with mass and age. AXIS has the sensitivity to
reach a very wide range of stellar masses (i.e., down to the peak of the
stellar IMF at spectral type around M3) at their faintest (i.e., quiescent
emission at Gyr ages) and the grasp to provide robust statistical
characterization of X-ray luminosities across mass and age. Monitoring these
systems across the nominal mission lifetime of AXIS will sample a
representative range of flare energies from $10^{33}$ to $10^{35}$ erg. These
flare luminosities and occurrence rates will allow for a detailed study of
host stars’ impact on the formation, evolution, and fate of exoplanetary
systems.

The sensitivity of AXIS is such that we can track the relationships of X-ray
luminosity with age for even the dimmest stars near the peak of the stellar
IMF ($\sim$M3 type stars) in old open clusters up to 2.5 kpc away. Detecting
quiescent emission for stars down to M3 will require exposure times $\sim
100-500$ ks (depending on distance and age), and AXIS will repeatedly observe
these fields throughout its lifetime with cadences designed to precisely
measure the stars’ X-ray variability to eliminate the vast stochastic
uncertainty in stellar activity relations. This is of particular importance
for the faint M-dwarf stars, which are the most common stars in the Universe
and the vast majority of terrestrial planet-hosting stars known. Figure 8
shows an all-sky image of stars detected by Gaia (Gaia Collaboration et al.,
2016, 2018), with known stellar clusters and associations color-coded by age
(right). A simulated image of the Carina star-forming complex (left)
demonstrates the superior grasp of AXIS; with a stable $1-2$ arcsec PSF across
a 20 arcmin field-of-view, many thousands of young stars will be resolved.

Figure 8: AXIS surveys of star clusters will provide a comprehensive survey of
stellar X-ray properties and the evolution of activity with age down to the
peak mass of the IMF. (Left) Simulated 50 ks AXIS observation in part of the
Carina star-forming complex overlaid by the DSS optical image. Whereas Chandra
did not detect the lowest-mass stars, or measure reliable hardness ratios of
most stars, after 1.2 Ms and 22 pointings, AXIS will easily detect and obtain
hardness ratios down to M3 dwarfs at the peak of the IMF with just a single
pointing of 150 ks. (Right) AXIS will survey clusters with Myr to Gyr ages to
statistically determine the evolution of stellar activity with age, as a
function of mass. Such a survey has broad implications for the evolution and
habitability of exoplanetary systems.

###  2.2 Stellar activity affects atmospheric retention and habitability

A planet’s affinity for developing life as we know it requires a life-
sustaining atmosphere, the longevity of which depends sensitively on the
energetic radiation from its host star. Solar-type (FGKM) stars steadily
produce XUV light via rotationally driven dynamos (which are mass-dependent
due to the convective zone depth and stellar rotation rate both depending on
stellar mass) that heat stellar chromospheres and coronae to $10^{5}-10^{6}$ K
temperatures, gradually irradiating and eroding planet atmospheres over
stellar lifetimes. More dramatically, powerful impulsive events (X-ray flares)
and the coronal mass ejections (CMEs) associated with stellar magnetic
activity can rapidly strip planetary atmospheres (Vida et al., 2017;
Roettenbacher & Kane, 2017), either rendering a planet barren and
uninhabitable or making room for a more hospitable secondary atmosphere to
accumulate (Owen & Mohanty, 2016). The liquid water planetary habitable zones
for the most common M dwarf stars are within $\sim$0.5 au of the host star,
which is also likely to be highly X-ray active. Understanding the evolution of
stellar X-ray activity, including stellar winds, stellar flaring rates, and
CME properties, as a function of age and mass is thereby required to develop a
complete picture of what planetary atmospheres experience over the course of a
stellar lifetime.

AXIS will contribute significantly to our understanding of habitability in our
own Solar System and beyond. X-ray observations of Solar System planets yield
key insights for the interaction of the Solar wind with planetary atmospheres
(Section 2.2.1). After all, it is hypothesized that the Solar wind is
responsible for stripping Mars of its atmosphere, leading to the ultimate
demise of its vast reservoirs of liquid surface water (Lammer et al., 2003).
With the imaging resolution of AXIS, we can also seek the first detection of
astrospheres (Section 2.2.2) –- the exo-planetary system equivalent of the
heliosphere – for which spectral signatures of charge exchange will provide
direct measurements of the stellar wind flux for planetary systems beyond our
own. Astrospheres, like our heliosphere, are important because they are the
environment that a star carves out of the ISM in which planets grow and
evolve. The slew speed and timing resolution of AXIS will also be key for
evaluating X-ray flares and searching for direct signatures of CMEs through
coronal dimming (Section 2.2.3). And finally, AXIS provides a step up in
sensitivity and spectral resolution that will enable full XUV characterization
of key exoplanet systems: M dwarfs hosting rocky planets in the habitable zone
(Section 2.2.4).

####  2.2.1 X-ray studies of the Nearby Solar System Illuminated by its Host
Star’s XUV Radiation and Stellar Wind

AXIS is uniquely poised for solar system science in a manner not possible with
prior X-ray observatories. Numerous solar system objects will be observable by
AXIS during the mission’s lifetime. The instrument’s large effective area will
allow for investigations of temporal variations in X-ray emissions from our
solar system’s planets and local planetesimals, such as asteroids and comets
within 1-2 AU of Earth. This will be the first time a thorough time-domain
survey of solar system objects can be performed in X-rays. Furthermore, the
fantastic spatial resolution will provide high-quality images that may be used
to generate chemical composition maps of neighboring exo-systems. Utilizing
both the spatial and time resolution of AXIS will allow us to probe both the
transportation of elements and chemical reactions in a system due to high-
energy activity, such as solar flare impacts.

Below is a list, in no priority order, giving a number of new Solar System
science programs engendered by AXIS.

  * •

Obtain high-spatial resolution elemental composition maps of the Lunar
surface.

  * •

Observe high-energy particle transfer in Jupiter’s upper atmosphere, and
compare the results with models built on the latest Juno results.

  * •

Follow-up on X-ray detections from the Pluto and Uranus systems to determine
the origin of the signals and probe the Plutonian exosphere.

  * •

Investigate variations in comet emissions during perihelion approach,
including rapid outflow events upon crossing the snow line. Determine if
scattering emission is a significant X-ray emission mechanism for such
systems.

  * •

Search for X-ray emission from new sources, like the Saturnian aurora,
Neptune, and the asteroids.

  * •

Search for and study astrospheres to understand the stellar wind
fluxes,compositions, and local ISM density around other star-planet systems.

For each of these programs, in turn, we will discuss the main science drivers,
expectations with AXIS, and possible limitations that AXIS may encounter.

Lunar Elemental Composition Map

Lunar X-rays were first remotely mapped on the Apollo 15 mission (Adler et
al., 1972), and are dominated by fluorescence line emissions of O, Mg, Al, and
Si from the sunlit side of the Moon (Schmitt et al., 1991; Wargelin et al.,
2004). Subsequent mapping of the elemental composition for the nearside Lunar
surface was performed using well-characterized fluorescence line emissions,
with the current highest surface resolution being 20 km from SELENE (Yokota et
al., 2009). AXIS will have 10 times the spatial resolution, making it capable
of obtaining a 2 km surface resolution on the abundances of O, Mg, Al, Si, and
Fe on the moon using only a 100 ksec exposure. Such a resolution will allow
accurate extrapolation between the remote map of AXIS to the high-resolution
maps of 1-10 km regions performed from lunar orbiters/landers (e.g Lunar
Reconnaissance Orbiter (Chapman, 1994; Chin et al., 2007)). Deeper exposures
will also allow AXIS to detect more exotic elements, such Au and S, that are
not observable with current instruments at sufficient signal-to-noise to be
properly fitted with spectral models.

Propagation of Martian Energetic Neutral Atoms

The diffuse Martian exosphere has been known to emit brightly in X-rays since
the groundbreaking study of Dennerl (2002). Martian X-ray emissions are
dominated by the charge-exchange interaction between solar wind ions and
neutral particles in the atmosphere, generating X-rays and energetic neutral
atoms (ENAs) (Krasnopolsky, 2002; Futaana et al., 2006; Lewkow & Kharchenko,
2014). Although the X-ray emission directly generated from this process has
been detected with XMM and Chandra, thermal X-ray emission from the ENAs has
yet to be remotely observed. The order-of-magnitude increase in effective area
over current missions will permit AXIS to study the time-domain evolution of
ENAs exiting the upper atmosphere after solar-wind ion collisions with Martian
neutrals. Time domain X-ray emission studies, when combined with solar wind
monitor data, will constrain cross-section models for the charge-exchange
interactions, allow for the study of secondary hot atoms cross-sections and
distributions, and probe the transfer of thermal energy from the exosphere to
the Martian surface.

Variability of Jovian Magnetosphere and Exosphere

X-ray emission from Jupiter’s disk, poles, and flux torus has been known since
the pioneering ROSAT observations of Waite et al. (1995). The disk emission is
mainly sourced by the scattering of solar x-ray photons and carries their
variability (Branduardi-Raymont et al., 2007, 2008), while the polar and flux
tube components are coupled to the particles and fields and "weather” of the
Jovian magnetosphere and exosphere (Crary & Bagenal, 1997; Connerney et al.,
1998; Clarke et al., 2002). AXIS will detect Jovian X-ray emissions with a
sensitivity that is $\sim 10\times$ deeper than Chandra, allowing detailed
mapping of the source components. In particular, this will enable measurements
of the emitted Io neutrals through the flux torus, testing models of the
$E\times B$ deposition onto Jupiter’s poles. These results will be correlated
with the latest JUNO/Chandra optical and UV maps of the system. The improved
sensitivity will allow AXIS to also probe these variations at time resolutions
that are not currently possible.

Detection and Classification of Outer Solar System X-rays

Chandra detected X-ray emission from Pluto at a brightness level that was 10x
higher than estimated from standard emission mechanism models (Lisse et al.,
2017). The observed spectrum indicates strong emission of soft X-rays with
elemental abundances corresponding to the surface area ratio of Pluto to
Charon. This suggests that either charge-exchange or fluorescence is
responsible for the emission, but the observed flux is too high for either
mechanism. Follow-up observations performed with XMM have been unable to
confirm these initial findings due to unfavorable geometry and the inherent
low X-ray flux from Pluto. AXIS will be able to detect and resolve spatial
features from Pluto within 50 ks, with a signal-to-noise ratio that would
tightly constrain any proposed emission models. Deeper exposures and follow-up
observations with AXIS will also provide composition and density of the upper
Plutonian atmosphere as a function of time, possibly also probing the transfer
of particles between Pluto and Charon.

X-rays have potentially been detected with low signal-to-noise ratios from
Uranus (Dunn et al., 2021). Saturn’s aurora and Neptune have yet to be
detected in the X-ray, but their measurement could tell us a lot about the
charge exchange precipitation mechanism thought to dominate X-rays from the
Jovian aurora, as well as the albedo for scattering of X-rays in dense
atmospheres.

Evolution of X-ray Emissions from Comets

As a comet approaches perihelion, it absorbs solar energy and ejects neutral
material from its surface outward via sublimation and localized jet-streams
(Wegmann et al., 2004; A’Hearn et al., 2011). The neutral ejecta forms a
diffuse cometary atmosphere that generates X-rays from charge exchange
interactions between highly-charged, heavy solar wind ions ( 0.1% of all solar
wind ions) and neutral gas in the comet’s extended coma atmosphere. Cometary
emissions are demonstrated to be useful for indirectly probing the charge
state and solar wind composition of minor ions (i.e., those heavier than He)
via fitting of comet spectra at CCD-resolution (Lisse et al., 2005; Bodewits
et al., 2007; Snios et al., 2016).

The increased observation window provided by AXIS will allow a deeper study of
solar wind composition via cometary observations at a wider variety of
heliospheric latitudes and longitudes. New research on cometary emissions
suggests that coherent scattering of solar X-rays by cometary dust and ice
particles may also contribute significantly to the total emission intensity at
energies greater than 1 keV (Krasnopolsky, 1997; Snios et al., 2014), though
deep exposures of comets with sufficient signal-to-noise spectra are required
before conclusive remarks can be made on the matter. AXIS will also permit a
comprehensive investigation of the presence of scattered solar X-ray emission
from comets at energies greater than 1 keV, a current topic of debate in the
community that is limited by the lack of signal-to-noise available with
current instruments.

Current limitations in X-ray instruments restrict cometary observations to be
exclusively during a comet’s brightest activity period during their perihelion
approach, allowing for only $1-2$ visits per comet. AXIS will resolve cometary
emissions at distances $2-3$ times farther away than presently possible,
allowing one to study the evolution X-ray emissions from comets as they pass
through the snow line and begin to rapidly sublimate water from their surface.
This period of activity is also when outflows of dust/ice grains of volatiles
that have accumulated on the comet’s surface during its passage through the
outer Solar System are ejected and interact directly with the ever-increasing
solar wind flux. Probing this short-time domain of high-energy chemistry is an
exciting prospect for AXIS.

Asteroidal X-rays

X-rays have yet to be detected from asteroids, but like the lunar surface,
should emit due to scattering and fluorescence of solar X-ray photons. Deep
exposure observations of nearby asteroids with AXIS may provide the first
detection of X-ray emissions from asteroids. Measurement of the asteroidal
versus lunar albedo for scattering will inform resource usage of these bodies,
as well as help solve the problem of X-ray scattering from cometary dust coma,
described above.

####  2.2.2 Astrospheres: direct observations of stellar wind environments of
other worlds

The winds of late-type stars are tenuous and very difficult to observe
directly. This is a major problem for understanding the energetic stellar wind
environments of planets, and also for understanding wind-driven angular
momentum loss, stellar rotational evolution and the evolution of stellar
magnetic activity. AXIS will be able to address this through the detection of
X-ray charge-induced emission resulting from the interaction of stellar wind
ions with inflowing ISM neutrals Wargelin & Drake (2001).

Like the heliosphere of our own Solar System, all stars carve out a bubble, or
“astrosphere”, in the ISM via their outflowing stellar winds. As they travel
through the galaxy, stars are surrounded by this bubble of charged gas and
magnetic fields, rounded with a soft shock at the front towards the ram
direction and trailing into a long tail behind. It is within the protective
astrosphere that the host star dominates the local environment and planetary
systems. In general, astrospheres are most easily studied at their boundaries
(astropauses) where hot stellar wind ions interact with shocks in the cold
neutral ISM via highly efficient charge exchange. The high spatial resolution
and imaging capability of AXIS makes it plausible to observe astropause X-rays
from young star winds, which are expected to generate ion fluences in excess
of $10^{7}$ protons/cm2-s-ster-MeV (scaled from the X-ray flux of the sun).
Astropause charge exchange will give rise to a non-thermal low-energy (300-
600 eV) signal in addition to the thermal signature of the star. If a nearby
star (within $\sim 25$ pc) is surrounded by an astrosphere of order 100 AU,
the emission region will exceed 1.5 arcsec and be visible to AXIS as a
slightly extended source with softer, line-dominated X-rays in the surrounding
halo. An attempt to detect astrospheric charge exchange emission from Proxima
Centauri using Chandra was unsuccessful due to limited sensitivity (Wargelin &
Drake, 2002), although a useful upper limit to the wind mass loss rate was
obtained.

Detection and separation of the two flux sources will only be possible with
the high resolution and large collection area capabilities of AXIS. Such
observations will place constraints on wind mass loss rates and will probe the
space weather of exoplanets.

####  2.2.3 Spotting coronal mass ejections via coronal dimming

Magnetic reconnection events in the stellar atmosphere can lead to the
ejection of stellar plasmas known as coronal mass ejections (CMEs) and flare
radiation emission. Fast (minute- to hour-long) stellar flares are readily
observed at optical wavelengths, recently made possible by the successful
launch of transiting exoplanet surveys such as Kepler/K2 (Davenport, 2016;
Ilin et al., 2019, 2021) and the Transiting Exoplanet Survey Satellite (TESS)
(Howard et al., 2019; Feinstein et al., 2020; Günther et al., 2020). However,
multi-wavelength monitoring campaigns utilizing X-ray observations show that
X-ray flares are not always correlated with the UV and optical (Osten et al.,
2005), and empirically establishing the relationship between flares observed
in XUV and coronal mass ejections remains elusive for stars other than the Sun
(Aarnio et al., 2011).

Nonetheless, multi-wavelength campaigns have revealed key insights into flare-
affiliated mechanisms, such as coronal dimming (Hudson et al., 1996; Sterling
& Hudson, 1997; Veronig et al., 2021; Loyd et al., 2022), the Orrall-Zirker
effect (Orrall & Zirker, 1976; Woodgate et al., 1992; Robinson et al., 1993),
and the Neupert effect (Neupert, 1968; Dennis & Zarro, 1993; Tristan et al.,
2023). The Orrall-Zirker effect is expressed as an enhancement in Ly-$\alpha$
emission during flares as the result of non-thermal proton beams, and is
observable in the UV only. However, coronal dimming and the Neupert Effect can
both be observed in the UV and X-ray. Unfortunately, the strong absorption of
UV light by the interstellar medium makes observations of such flares nearly
impossible beyond the local Milky Way, whereas AXIS will provide access to
stellar flares out to several kpc.

Not every X-ray flare is associated with a CME, and determining whether a
planet is subject to high-energy irradiation versus high-energy particles is
essential for assessing surface habitability. Coronal dimming provides the
means to determine whether an X-ray flare is also associated with a CME event:
when the CME ejects coronal plasma, it leaves behind a low-density coronal
hole, reducing the X-ray emission from that area. Evidence of coronal dimming
can be found in light curves when there is a post-flare decrease in the
overall flux of the star. Coronal dimming can also be traced by looking for
emission features from $T\sim 10^{6}$ K plasmas, which roughly corresponds to
coronal temperatures (Dissauer et al., 2018). More recent studies have
systematically searched EUV and X-ray observations with the Extreme Ultra-
Violet Explorer, XMM-Newton, and Chandra for coronal dimming events. To
achieve this goal, there needs to be sufficient pre- and post-flare
observations to be able to say if the dimming post-flare is statistically
significant.

Of the available data searched, 21 dimmings were identified in 13 different
stars (Veronig et al., 2021). This has yielded exciting insight into a growing
field of looking for evidence of CMEs; however, this first-look analysis
explores a small sample size. The overall occurrence rate of CMEs associated
with flares has yet to be determined for other stars. Understanding this
occurrence rate not only has implications for stellar astrophysics, but also
for planet atmospheric retention and chemistry (Segura et al., 2010;
Youngblood et al., 2017; Tilley et al., 2019; Chen et al., 2021).

Surveys of flare-active stars with AXIS will yield significant insights into
coronal physics. The high spatial and energy resolution, broad energy
coverage, and efficient readout times will allow for detailed studies of
specific emission lines which trace the corona during flares and CMEs, which
last on the order of minutes to hours. We will be able to resolve lines such
as highly ionized Fe, Mg, O, and Si, and in doing so we can isolate and study
how specific species behave in the presence of extremely energetic outbursts
on other stars. The large field-of-view will facilitate broad, statistical
studies of X-ray flares, allowing us to constrain X-ray flare rates and
energies and occurrence rates of CMEs, as compared to the large optical
surveys that have been carried out thus far.

Table 3: Prime targets for high energy characterization Object | 0.5-2 keV flux | System notes
---|---|---
| (erg/cm2/s) |
TRAPPIST-1 | $4\times 10^{-14}$ | Seven Earth-sized planets orbiting an M7.5 dwarf;
|  | three are in the habitable zone; all seven are being observed by JWST
LHS 1140 | $1\times 10^{-14}$ | M4.5 dwarf with one Earth-like planet (planet c) and one super-Earth (planet b);
|  | planet b targeted for characterization by JWST
K2-18 | ${}^{\dagger}1\times 10^{-14}$ | M2.5 dwarf with a cool super-Earth (planet b);
|  | targeted for characterization by JWST
TOI-1468 | ${}^{\dagger}1\times 10^{-14}$ | M3.0 dwarf with two moderately large rocky planets
†Estimated from stellar activity relations versus age or rotation period

####  2.2.4 Reconstructing the high energy environment of terrestrial
exoplanets

M stars are the most abundant and long-lived stars in the Universe, as well as
the smallest, making them the easiest systems in which to find potentially
habitable terrestrial planets. Unfortunately, they are also very faint ($\sim
0.01L_{\odot}$), making them the most difficult stars to characterize with
modern X-ray observatories. The soft X-ray sensitivity of AXIS provides a
factor of five improvement over the current abilities of XMM-Newton, and even
more so than Chandra, which will make AXIS the premier instrument for studying
the high-energy environments of exoplanetary systems.

Using only 0.75% of the nominal AXIS guaranteed exoplanet time allocation
mission, we will obtain exquisite high signal-to-noise X-ray spectra and light
curves of four high-impact terrestrial planetary systems (Table 3). Each
system (Table 3) hosts a planet that has been identified as the most ideal
terrestrial planets for atmospheric characterization through either
transmission or emission spectroscopy (TESS Atmospheric Characterization
Working Group), and almost all are being explored with JWST Cycle 1
observations. A monitoring campaign of 250 ks per target over the course of
the AXIS mission operation will be able to fully characterize the high-energy
environment of these planets by investigating the properties of their host
stars.

Figure 9:  (Left) Simulated 250 ks spectra for the M dwarf TRAPPIST-1
(Wheatley et al., 2017) (dark purple). AXIS will provide dramatically better
spectral resolution and more signal-to-noise than the same exposure time with
the XMM-Newton EPIC-pn (light purple). This will provide strong constraints on
the differential emission measure (DEM) reconstruction required to fully
understand the stellar atmosphere and activity level. (Right, modified from
King et al. (2018)) The EUV to X-ray scaling relation inferred from Solar data
is overlaid with other proposed stellar scaling relations (King et al., 2018;
Johnstone et al., 2021; Sanz-Forcada, 2022) and the measured XUV properties of
three M dwarfs (red, yellow, and gree; see Chadney et al. (2015)). The purple
data points show the inferred TRAPPIST-1 EUV/X-ray luminosity ratio obtained
via DEM reconstruction with a single X-ray luminosity data point (e.g., from
current XMM data, light purple) versus a 250 ks observation with AXIS (dark
purple). Vastly different solutions for total EUV flux are obtained when using
a single X-ray luminosity as a data point versus datasets with resolved line
features. The soft X-ray sensitivity and energy resolution of AXIS enhances
the accuracy and lowers the EUV flux uncertainty from a factor of two to just
30%, constraining EUV to X-ray scaling relationships and profoundly affecting
our ability to predict surface habitability of terrestrial worlds.

Temperature structure and emission measures of stellar coronae

The sensitivity and spectral resolution of AXIS will provide an exquisite,
high signal-to-noise spectrum from the integrated exposure of these systems.
Figure 9 shows how a 300 ks AXIS observation, with our nominal 60 eV
resolution at 1 keV, can easily distinguish among different stellar coronae
models. Such a spectrum will enable a full differential emission measure (DEM)
solution which reconstructs the coronal plasma distribution with temperature
and provides the most reliable prediction for the EUV environment of the
terrestrial planet. Using the legacy quality UV dataset from the mega-MUSCLES
survey for TRAPPIST-1 (Loyd et al., 2016; Wilson et al., 2021) with the AXIS
data will produce a significant leap in accuracy of determining the EUV
radiation field properties of TRAPPIST-1, reducing model uncertainties by a
factor of 3 and providing a sound XUV basis for understanding whether or not a
terrestrial-like atmosphere on TRAPPIST-1 b-h could be retained, and assessing
the atmospheric chemistry and habitability of those worlds (Khodachenko et
al., 2007; Lammer et al., 2007; Bolmont et al., 2017; Vida et al., 2017;
Wheatley et al., 2017; Johnstone et al., 2019) for such planets.

Metal abundance profiles of stellar coronae

Few measurements of coronal abundances are available, and many of the existing
studies are at low signal-to-noise ratio. Poppenhaeger et al. (2021) describe
how EUV-driven atmospheric escape is sensitive to the coronal iron abundance,
but these claims are based on very limited data. In addition to improving
predictions of unobserved EUV flux (described above), measuring metal
abundances in stellar coronae will resolve open questions about how magnetic
fields vary across different spectral types (e.g. Laming, 2015). Figure 9
shows the strongest metal lines that are clearly visible in the simulated AXIS
spectra.

##  3 Deaths of planet-hosting stars

In five billion years, our Sun will enter the red giant phase, shedding its
atmosphere and dramatically altering all planets in the Solar System as it
makes its way to the endpoint of Solar evolution, a white dwarf. To understand
the ultimate fate of Earth, we seek to characterize the plasma environment of
planetary nebulae and look for evidence of planetary system remnants around
other white dwarfs.

###  3.1 The unique plasma environment of Planetary Nebulae

Planetary nebulae (PNe) are proven astrophysical laboratories for a range of
physical processes (shocks, ionization, photoevaporation, dust destruction,
binary interactions, etc.; see (Zijlstra, 2015; Kwitter & Henry, 2022)).
Chandra and XMM-Newton observations of PNe hint at a new suite of physical
processes that only AXIS, with its high-spatial resolution and increased
sensitivity, is poised to answer. X-ray emission from PNe contains multiple
components, with the central soft and hard X-ray-emitting point sources
(Montez et al., 2010, 2015, and references therein) surrounded by nebular
cavities filled with hot and diffuse X-ray emitting gas called hot bubbles
(Kastner et al., 2012; Ruiz et al., 2013). The soft X-ray emission from the
central star is likely due to self-shocking winds, while the hard X-ray
emission stems from known and suspected spun-up binary companions (Montez et
al., 2015). The diffuse hot bubble emission (filling a nebular cavity
typically $5-60^{\prime\prime}$ on the sky) is intricately tied to the
formation of the PN.

During the final stages of stellar evolution, high mass loss from the slow
wind of an Asymptotic Giant Branch (AGB) star is swept into the nebular shell
by a new fast wind that reaches speeds of several 1000 km/s. This wind-wind
interaction generates an adiabatically-shocked region with plasma temperatures
that should scale with the square of the fast wind speed. However, Chandra and
XMM-Newton observations show diffuse hot bubble temperatures in the 1-3
$\times 10^{6}$ K range instead of the $>10^{7}$ K predicted by their fast
wind properties. AXIS will thus enable a new understanding of the physical
processes required to explain the temperature discrepancy.

Specifically, numerical simulations suggest that clump and filament forming
instabilities mix nebular material with that of the hot bubble, producing the
observed soft X-ray emission with plasma temperatures of $T_{X}\approx 10^{6}$
K (Stute & Sahai, 2006; Toalá & Arthur, 2018) and enhancing thermal
conductivity (Soker, 1994; Steffen et al., 2008; Toalá & Arthur, 2016). Study
of these processes is limited due to the faint ($L_{X}\approx 10^{31}$ erg/s,
see Figure 10) emission from PNe. In fact, the Chandra Planetary Nebula Survey
(ChanPlaNS; Kastner et al., 2012; Freeman et al., 2014; Montez et al., 2015)
is likely biased towards only the brightest PNe hot bubbles, which are
predicted to arise from more massive progenitor stars and Wolf-Rayet-like
central stars. Figure 10 demonstrates that AXIS will be able to probe hot
bubble emission from lower initial mass stars for the first time as well as
more distant PNe. With AXIS we will acquire high signal-to-noise images of
over 100 PNe, $\sim 80$ of have been observed with weak or no signal from
Chandra. Such observations will allow us to constrain the full suite of plasma
properties: temperatures, chemical abundances, and overlying absorption with
unprecedented precision. Typically hot bubble emission with Chandra and XMM-
Newton seldom exceeds a hundred counts; AXIS will be able to capture tens of
thousands of photons from exposures as low as 10 ks. Additionally, for
numerous PNe, the signal will allow us to spatially map these properties
across the hot bubbles, as well as clump and filament structures.

Figure 10: AXIS predictions of hot bubble emission from planetary nebulae
(PNe) assuming a distance of 1.5 kpc (the ChanPlaNS distance limit). Three
initial mass tracks are depicted from Steffen et al. (2008) predictions for
thermal conduction in the hot bubble. The central star evolves from low
$T_{\rm eff}$ to high $T_{\rm eff}$ over thousands of years. The symbols on
the tracks are color-coded by the number of counts detected in 10 ks of an
AXIS exposure. For comparison, the hot bubble emission from the ChanPlaNS
sample is shown as black squares (Montez et al., in prep).

###  3.2 The destiny of planetary systems around White Dwarfs

It is well established that $25-50\%$ of white dwarfs exhibit metal pollution
in their atmosphere, and the primary source of this pollution is thought to be
the remnants of evolved exoplanetary systems (Zuckerman et al., 2003, 2010;
Koester et al., 2014). As such, these debris-accreting white dwarfs provide
unique constraints on the bulk composition of exoplanetary material (Zuckerman
et al., 2007; Harrison et al., 2018; Hollands et al., 2018), that serve as a
key input into models of planet formation. Following the evolution and
ultimate death of the host star through AGB evolution, planetary bodies (e.g.,
planets, moons, asteroids) entering the Roche radius are tidally disrupted,
resulting in a disc of rocky debris orbiting the stellar remnant (Jura, 2003).
As evidence of this, $2-6\%$ of polluted white dwarfs host a dusty disc
(Manser et al., 2020), as inferred from an excess of infrared emission
attributed to dust particle interactions in the disc (Zuckerman & Becklin,
1987; Farihi et al., 2012). This circumstellar debris disc is believed to last
for $\sim 1$ Myr, over which time the white dwarf photosphere is replenished
with $Z>3$ atoms (Girven et al., 2012; Cunningham et al., 2021). Optical and
UV spectroscopy enables the measurement of metal abundances, from which
accretion rates, abundance ratios, and disc lifetimes can be inferred, but the
scientific interpretation also requires accurate white dwarf atmospheric
models. X-ray observations directly measure the accretion rate, providing an
independent constraint for these models.

The recent discovery of X-rays from G29–38 confirmed for the first time the
ongoing accretion of planetary debris, providing the first independent
measurement of the accretion rate onto a metal-polluted white dwarf
(Cunningham et al., 2022). This object was discovered via a 105 ks Chandra
ACIS-S observation, which confirmed it as a source of soft X-rays with a
plasma temperature of $0.5\pm 0.2$ keV. Now that this new class of X-ray
source has been discovered, it is essential to characterize the fundamental
properties of these debris-accreting systems. AXIS will be 150 times more
sensitive to this class of objects than Chandra, providing unprecedented
ability to study the endpoints of stellar systems like our own. Firstly, a
detailed follow-up of the prototypical metal-polluted white dwarf G29-38
(which will remain the stand-out target for such studies largely owing to its
proximity to Earth, 17 pc), is warranted. A 100 ks observation with AXIS will
yield $\sim$1000 counts, enabling spectral features to be resolved and
precision determinations of the metal abundance ratios (i.e., Mg, Si, Fe, O).

AXIS observations will bring about a much needed connection between mid-
infrared Spitzer/JWST spectroscopy (where G29–38 is a GTO target) and HST/Keck
UVIS spectroscopy. Infrared spectroscopy allows a determination of the
mineralogical composition of the surrounding debris disk supplying the
accreting pollutants (Reach et al., 2009). This accretion replenishes the
photosphere for which UV and optical spectroscopy allows the determination of
photospheric abundances (e.g., Xu et al., 2014). A spectral analysis of X-ray
observations bridges the gap between these two regimes of observations. But
even with the precision of JWST, there will be degeneracies in the
mineralogical fitting. High signal-to-noise spectra of X-ray line emission,
enabled by the soft X-ray sensitivity and energy resolution of AXIS, can break
that degeneracy.

Figure 11: Predicted X-ray flux in the $0.3-7.0$ keV band for the sample of
hydrogen atmosphere metal-polluted white dwarfs (DAZ) with a measured Ca
abundance (Farihi, 2016, Williams et al., in prep). The X-ray flux is computed
as the inferred accretion rate from optical/UV spectroscopy over the distance
squared, the measured Chandra X-ray flux for G29—38 (error bars). Also
indicated are estimated detection thresholds for a 115 ks exposure using
Chandra, XMM-Newton, and AXIS. Thirty eight stars fall above the AXIS
detection threshold, allowing accretion rates to be independently measured
across the full range of effective temperatures.

Beyond a detailed follow-up of the closest (and currently the only known)
metal-polluted white dwarf X-ray source, AXIS will enable a survey of metal-
polluted white dwarfs. Figure 11 shows that a single survey of 115 ks
observations should yield $30-40$ new X-ray sources. This will provide
independent accretion rate measurements across a broad range of white dwarf
cooling ages, allowing one to test atmospheric models throughout white dwarf
evolution. In particular, there are two key uncertainties in the atmospheric
models of white dwarfs: convective overshoot (Freytag et al., 1996; Kupka et
al., 2018; Cunningham et al., 2019) and thermohaline mixing (Bauer & Bildsten,
2018, 2019; Wachlin et al., 2022). These mixing processes are of key
importance across the full range of stellar evolution, and white dwarfs offer
a unique window in which to test them. Both processes predict an increase in
the time-averaged accretion rates from UV/optical spectroscopy because of the
larger convection zone masses and longer sinking timescales. Overshoot
predominantly affects DA white dwarfs cooler than 18000 K, with average
accretion rates predicted to be around a factor 5 higher than models that omit
overshoot (Cunningham et al., 2019), while thermohaline mixing predicts
accretion rates up to 3 orders of magnitude higher for H-atmosphere white
dwarfs warmer than 15000 K (Bauer & Bildsten, 2019). The increased accretion
rates in these two distinct regimes can be tested with an AXIS survey of
isolated white dwarfs with exposure times ranging from 10–200 ks for each
white dwarf. Furthermore, differences in the mean time-integrated accretion
rates of H-dominated and He-dominated atmosphere white dwarfs have been
observed with spectroscopic observations (see Fig. 10 of (Farihi, 2016)).
There is no evolutionary reason that such differences should exist, and a
survey of independent X-ray observations will reveal whether this is due to
systematic uncertainties in the atmospheric models or if it truly is a new
physical phenomenon.

###### Acknowledgements.

This white paper represents the joint efforts and communications among the
community of scientists making up the AXIS Stars & Exoplanets Science Working
Group to present the unique new contributions the AXIS mission will bring to
X-ray science.

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yes

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