Small Molecules Have Big Impacts in Interstellar Clouds

by Anashe Bandari

“One of the key goals, when you think about modern astronomy, considers the life cycle of molecular material,” said Arshia Jacob, an astronomer at Johns Hopkins University. Diffuse atomic gas becomes dense molecular gas, which ultimately forms stars and stellar systems, and continues to evolve over time. Though astronomers understand much of this process, there are a lot of missing pieces.

Jacob is the lead author on a recent paper characterizing the interstellar medium in the Milky Way using SOFIA, the Stratospheric Observatory for Infrared Astronomy, to fill in some of these missing pieces. By studying six hydrides, which are molecules or molecular ions in which one or more hydrogen atoms are bound to a heavier atom through shared electron pairs, Jacob and her collaborators hope to better understand how molecular clouds form and evolve.

Green and red swirls of nebulae are seen over a field of bright blue stars with W3 glowing white. Two spectra are laid over the background image, one green, one red.
W3, one of the 25 Milky Way regions the HyGAL project will study, is seen as the glowing white area in the upper right of this image of the Heart and Soul Nebulae, taken by NASA’s Wide-field Infrared Survey Explorer (WISE). SOFIA looked at the abundances of six hydride molecules in W3, the spectra of two of which are shown in the box at left. Image credit: Nebulae: NASA/JPL-Caltech/UCLA; Spectra: Jacob et al.

Hydrides are useful to astronomers because they are very sensitive tracers of different phases of the interstellar medium, and their chemistry is relatively straightforward. Moreover, hydride observations provide measurements of the amount of material present.

The multi-investigator SOFIA project Hydrides in the Galaxy (HyGAL) uses a diverse selection of hydride molecules, allowing different processes to be monitored while complementing other observations. For example, one of the hydrides studied, argonium, can only form in regions that are almost purely atomic gas, so detecting argonium is indicative of a low molecular content in its surrounding environment. Other hydride molecules can indicate the presence of dense gas, intense cosmic radiation, turbulence, and more.

“Hydrides are small, but we can understand so much from them. Small molecules, big impact,” Jacob said.

In the first stage of the project, the group compared the hydride abundances in three regions of the Milky Way: two star-forming regions, W3(OH) and W3 IRS5, and a young stellar object, NGC 7538 IRS1. Though the average properties of these first three sources are similar, the full HyGAL project plans to study a total of 25 regions. With the remaining 22 sources covering distances from the inner galaxy all the way to the outer galaxy, they expect vastly different results.

“The sources are very different: Some of them are older, some have more chemical enrichment, some are younger and still forming stars,” Jacob said. “All of these will affect the nature of molecules that are formed, like their abundances, for example.”

Moving away from the galactic center, the transitions from atomic to molecular gas change, and the cosmic ray ionization rates vary vastly, which will result in differences in the ratios of molecules present and other properties. This will help astronomers understand the diversity of environments within the Milky Way.

“Imagine you’re moving into a cloud. At each stage, you’re seeing different molecules, reflecting changes in the cloud properties as it gets denser,” Jacob said. “Through this project, we’re filling in the properties of this transition.”

Currently, there have only been a handful of bright sources emitting a broad range of radiation that have been studied in this way, all concentrated in the inner galaxy. The SOFIA data will more than double the existing data, providing additional answers about the structure, dynamics, and chemistry of these clouds and where the dense material comes from.

SOFIA is the only facility presently capable of accessing the frequency range necessary for these observations at the required resolution. The German REceiver Astronomy at Terahertz Frequencies (GREAT) instrument aboard SOFIA allows five frequencies to be monitored simultaneously, each tuned to five of the six hydrides in question to determine the makeup of the cloud sources. These are complemented by studies at radio wavelengths with observatories such as the Karl G. Jansky Very Large Array near Socorro, New Mexico.

“The idea is to give us not only information about the sources themselves, but also information about the different spiral arms they cross, making this truly a study over galactic scales,” Jacob said.

SOFIA is a joint project of NASA and the German Space Agency at DLR. DLR provides the telescope, scheduled aircraft maintenance, and other support for the mission. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science, and mission operations in cooperation with the Universities Space Research Association, headquartered in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart. The aircraft is maintained and operated by NASA’s Armstrong Flight Research Center Building 703, in Palmdale, California. SOFIA achieved full operational capability in 2014, and the mission will conclude no later than Sept. 30, 2022. SOFIA will continue its regular operations until then, including science flights and a deployment to New Zealand this summer.

SOFIA Watches a Binary Star System’s Eclipse

by Anashe Bandari

With its observations of a special pair of stars at a special moment in their lives, the Stratospheric Observatory for Infrared Astronomy (SOFIA) is shedding new light on stardust.

Over an interval of 387 days, a giant star in the constellation Aquarius periodically has a dramatic change in its brightness. This is because the star falls into a category called Mira variables, which pulsate over long periods and surround themselves in a shell of dust.

But this isn’t just any Mira variable. The star is one of two in a binary star system known as R Aquarii, where it has a companion white dwarf. The two orbit one another, and the white dwarf crosses in front of the Mira variable every 43.6 years, causing an eclipse from the perspective of a viewer on Earth.

This composite image of R Aquarii resembles a ring of fire over a black field, with a glowing purple “S” flowing through it. Near the center of the image, in the middle of the ring and the “S” wave, is a twinkle of bright white, which is the Mira variable in R Aquarii.
This composite image of R Aquarii resembles a ring of fire over a black field, with a glowing purple “S” flowing through it. Near the center of the image, in the middle of the ring and the “S” wave, is a twinkle of bright white, which is the Mira variable in R Aquarii. The white dwarf is very faint and contributes very little to the optical emission. However, the purple wave is the result of a jet that is powered by the white dwarf accreting dust produced by the Mira variable. The smokey red circles are evidence of explosive events that occurred several hundred years ago. Overlain atop the composite image of R Aquarii is a set of five plots indicating the energy emitted by the system. SOFIA acquired four of the data sets, while the strong purple plot is data from the Infrared Space Observatory from 1996, when R Aquarii’s emission was strongest. The strength also depends on the phase of the binary star system’s eclipse, so it does not increase each successive year, exactly: the flux fell between 2018 and 2019. Credit: NASA/CSC/SAO/STScI/Palomar Observatory/DSS/NSF/NRAO/VLA/LCO/IMACS/MMTF/Sankrit et al.

There’s another thing that’s special about R Aquarii: The periastron, or the point in the orbit where the two stars are closest to each other, happens during the eclipse. This means that as the eclipse occurs – and the pair gets dimmer and dimmer, overall – the white dwarf and the Mira variable get closer and closer together. The white dwarf accretes more and more of the dust surrounding the Mira variable, and, because of this optimal geometry, we get to watch this process occur.

Since 2016, SOFIA, a joint project of NASA and the German Space Agency at DLR, has been monitoring the onset of the eclipse, which started in 2018, with periastron expected to occur in 2023. The flow of dust can be inferred at mid-infrared wavelengths, and SOFIA’s infrared camera, FORCAST, has just the right angular resolution to watch.

By combining what they know about the system – the distance between the two stars, the fact that an eclipse is ongoing, and predictions of how much dust there is – astronomers can figure out the balance between the amount of dust escaping the Mira variable and how much is being accreted by the white dwarf. These are “both very big questions,” said Ravi Sankrit, an astronomer at the Space Telescope Science Institute in Baltimore and first author on a recent paper about SOFIA’s 2018 and 2019 observations of R Aquarii.

“It’s an opportunity to see it in a unique way, because the material that’s being accreted isn’t obscured by the Mira, it’s right out in front,” added Steven Goldman, a scientist with Universities Space Research Association, based at NASA’s Ames Research Center in California’s Silicon Valley. Goldman is a co-author on the paper, which looks at how the onset of the eclipse is beginning to affect the dust surrounding the system.

Since the two stars move from being very far apart to very close to one another, their dust is constantly changing. Continued mid-infrared monitoring is required to fully understand how the dust is affected by the stars’ orbit.

“Binarity, winds, jet formation, mass loss, and accretion are fundamental astrophysics,” Sankrit said. “So, the real excitement here is that you’re getting something that is on a human timescale probing very fundamental aspects of astrophysics.”

The physics Sankrit, Goldman, and their team are uncovering is applicable to more than just R Aquarii. There are hundreds of other similar binaries, and those are just the ones we know of. These other binary systems are likely experiencing the same phenomenon but aren’t oriented correctly for us to be able to see their periastron and the changes in their surrounding dust.

SOFIA is a joint project of NASA and the German Space Agency at DLR. DLR provides the telescope, scheduled aircraft maintenance, and other support for the mission. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science, and mission operations in cooperation with the Universities Space Research Association, headquartered in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart. The aircraft is maintained and operated by NASA’s Armstrong Flight Research Center Building 703, in Palmdale, California. SOFIA achieved full operational capability in 2014, and the mission will conclude no later than Sept. 30, 2022. SOFIA will continue its regular operations until then, including science flights and a deployment to New Zealand this summer.