Astrophysical applications of AMO

Atomic, Molecular, and Optical (AMO) physics is a natural field to study astrophysical phenomena, since we receive our information about the universe mainly through electromagnetic waves emitted or absorbed by AMO systems, such as atoms, molecules, or plasmas, that are sensitive to their environment (local density, temperature, magnetic field, etc.).

Our group is involved in the study of the solar wind interacting with various environments in our solar system. The Sun ejects charged particles with speeds of about 400 km/s (roughly 1 million miles per hour) high enough that they can escape its gravity. This stream of plasma is called the solar wind. It is made mostly of electrons and protons, but also of stripped heavier nuclei, such as ⁴He²⁺ (or alpha particle), or C⁶⁺ and O⁸⁺ ions, released from the upper atmosphere of the Sun in directions. Their kinetic energy is usually between 1.5 and 10 keV. The figure beside (from NASA) shows the variability of the solar wind as measured by the Ulysses spacecraft. Although it is always directed away from the Sun, the solar wind speed, density, and composition change. For example, its speed is high (800 km/s) over coronal holes and low (300 km/s) over streamers.

The solar wind flows outward and interact with other bodies in our solar system. Its interaction with Earth magnetic field and atmosphere is responsible for aurora (northern and southern lights), and can lead to geomagnetic storms that affect Earth's magnetic field lines, creating strong currents in power grids. It also explains why plasma tails of comets always point away from the Sun. The solar wind moves supersonically to great distances, filling a region known as the heliosphere, an enormous bubble-like volume surrounded by the interstellar medium (see figure from NASA). This occurs in other star systems as well.

One of the process occuring when a positive ion from the solar wind collides with a neutral atom/molecule (e.g., hydrogen or helium from the heliosphere, or oxigen or nitrogen from a planetary atmosphere, or water from a comet) is the capture of an electron from those targets. The resulting ion has one less positive charge, and is often highly excited. Its de-excitation into more deeply bound levels can lead to the emission of X-ray photons. This can be summarized by

A^q+ + B → A^*(q-1)+ + B⁺,

where the solar wind ion A^q+ has a charge +q before the charge exchange with the neutral atom B, and a charge +(q-1) after the exchange. It is then in an excited state labeled by *.

We performed calculations of the charge exchange cross section σ_ch for fully stripped C⁶⁺ and O⁸⁺ ions with hydrogen atoms. Our goal is to study the X-ray emission from the resulting excited solar wind ion, especially the polarization of the emitted photons. The X-ray polarization gives information about the local environment in which the charge exchange took place, such as the distribution of the solar wind plasma and its velocity, an the local density and distribution of neutral gases. Measuring the polarization of X-rays would act as a probe to know more, not only about the propagation of the solar wind, but also about the environment where charge exchange takes place.

We are interested in knowing if X-ray emission from charge exchange between the solar wind and the heliospheric hydrogen atoms can give information about the heliosphere bubble. We used the density of hydrogen atoms shown beside, which indicates that the density of hydrogen atoms is larger "upwind" (i.e. the direction into which the solar system is moving against the insterstellar gas) than "downwind" (opposite direction), with a "croissant" of higher density at around 200 AU (AU = Astronomical Unit, or the average Earth-Sun distance). The density is lower density closer to the Sun because the solar wind pushes the neutral atoms away. We note that this distribution also fluctuates depending on the Sun activity, with the solar wind pushing more or less the hydrogen atoms outward.

Since the detection of X-rays is obtained using satellite observatories, such as Chandra or XMM-Newton observatories, the signal detected can be from any event along the line-of-sight of the satellite. The intensity of the X-ray emission is obtained by integrating over the line-of-sight (LOS)

,

where N_A(r) is the position dependent density of the solar wind ion, n_H(r) is the position dependent density of heliospheric hydrogen atoms, and v_rel their relative velocity. The polarization of the X-ray photon is determined with respect to the ecliptic plane: it is parallel or perpendicular to it. We can then compute the polarization fraction P as

,

where I_‖ and I_⊥ are the integrated LOS intensity for the parallel and perpendicular polarization, respectively.

We computed P within the ecliptic plane for X-rays induced in charge-exchange collisions of fully stripped solar wind (SW) ions C⁶⁺ and O⁸⁺ with the heliospheric hydrogen atoms. The results were obtained as a function of SW ion velocities, including a range of velocities corresponding to the slow and fast SW, and coronal mass ejections. We found the polarization of charge-exchange X-rays to be between 3% and 8%, depending on the line-of-sight geometry, SW ion velocity, and the selected emission lines. The figure beside shows the angular dependence of the polarization of X-rays emitted in a single de-excitation from the 4p, 5p, and 6p excited states to the 1s state. These results are for C⁶⁺ + H(1s) → C⁵⁺(np) + H⁺, followed by C⁵⁺(np) → C⁵⁺(1s) + γ. The polarization P as a function of the angle of observation (increasing counterclockwise) is expressed as a radius vector, with the axis for π/2 (LOS in the upwind direction) indicated. The blue cones indicate regions within ±10° of the Sun (right side) or directly opposite (left). In this plot, the results for slow (black) and fast (green) SW ions illustrates the variation of P. By measuring the polarization P of X-rays, we can get a diagnostic of the solar wind conditions throughout the heliosphere. We obtained similar results for O⁸⁺.

We are extending this work to other systems, such as X-ray emission from solar wind interacting with water (H₂O) which has been detected from comets. This is done in collaboration with Prof. Vasili Kharchencko .

We are also involved in computing chemical reaction rate relevant to the early universe, such as H₂ + D → HD + H, which gives a molecule with a permanent electric dipole moment crucial to the cooling of large primordial clouds and the formation of the first generation of stars.