SOFIA Enables First Clear Look into Star-Forming Region Westerlund 2

Researchers obtained the first clear picture of a step in star formation using the Stratospheric Observatory for Infrared Astronomy, or SOFIA, long-term program called FEEDBACK.

Using the German REceiver for Astronomy at Terahertz Frequencies, or GREAT, in one of its advanced configurations called upGREAT, SOFIA’s FEEDBACK program enabled high-resolution insights into the star-forming region called Westerlund 2 — one of the brightest and most massive star formation regions in the Milky Way. When massive stars begin to form, they emit large quantities of protons, electrons, and heavy atoms, which together are called stellar winds. In extreme cases, the stellar winds can create bubbles of hot plasma and gas in their surrounding clouds.

A color image of the emissions in RCW 49 showing the shell, ridge, inner dust ring, and transition boundary.
A color image of the emissions in RCW 49, the star-forming region of Westerlund 2. Credit: Tiwari et al.

Maitraiyee Tiwari, a postdoctoral researcher, as well as her team at the University of Maryland, analyzed the star cluster Westerlund 2, and found the cluster is surrounded by one of these expanding bubbles of warm gas. They also identified the origin of this bubble, its size, and the energy that drives its expansion. The results were recently published in The Astrophysical Journal.

Tiwari and her team created the detailed picture of Westerlund 2 by measuring the radiation emitted by the star cluster across the entire electromagnetic spectrum, from high-energy X-rays to low-energy radio waves. Previous data in the radio and sub-millimeter wavelengths showed neither the bubble nor how it expanded into the surrounding gas.

The most important measurements of the current study include the far-infrared data on ionized carbon at a wavelength of 157 µm, which can currently only be observed with SOFIA. Due to the expansion movement of the bubble, the wavelength of this line is slightly stretched or compressed, which leads to a so-called red or blue shift — depending on whether the bubble is moving away from the earth or toward it. Based on this wavelength shift, Tiwari and her colleagues were able to determine how the bubble is expanding. Combined with the measurements from the rest of the electromagnetic spectrum, it results in a 3D view of the stellar wind around Westerlund 2.

The researchers also found evidence of the formation of new stars in the envelope region of this bubble. According to their findings, the bubble ruptured about one million years ago, releasing hot plasma and slowing the expansion of its envelope. After another two hundred thousand to three hundred thousand years, however, another particularly bright star — called a Wolf-Rayet star — developed in Westerlund 2, and its strong winds restimulated the bubble, leading the process of expansion and star formation to begin once again.

Stars — albeit less massive ones — will continue to be born in this shell for a very long time.

Researchers aim to understand the primary processes that drive and regulate star formation, and how these processes differ between different star formation regions. So far, it appears that the expansion is always there but can differ in different star formation regions.

The Age of Westerlund 1 Revisited

The status of a nearby cluster of stars, known to be the most massive in the galaxy, is being challenged by new observations. Westerlund 1, thought to contain a total mass of more than 10,000 times that of our Sun, is likely to need its total mass revised downward after astronomers show it could be more than twice as old as previously thought. This hints at a complicated evolutionary history where the ages of the stars provide clues to how massive they can really be.

Hubble image of Westerlund 1
Image of young star cluster Westerlund 1 taken by the Hubble Space Telescope toward the southern constellation of the Altar. Westerlund 1 is home to a variety of the largest and most massive stars known, including red, yellow, and blue supergiants as well as an exotic object known as a magnetar. Westerlund 1 is relatively close-by for a star cluster, at a distance of 15,000 light years, giving astronomers a good laboratory to study the development of massive stars. Credit: ESA/Hubble & NASA

Star clusters are groups of stars smaller than a galaxy, bound together by a shared gravitational pull. They are used by astronomers as distant laboratories to understand how stars evolve. They are formed by giant collapsing clouds of gas, which eventually collapse into smaller clouds that collapse even further. This compression generates extreme pressure, eventually causing the material to ignite and form individual stars.

This means that all stars in a cluster are born at the same time from the same material. Since each prenatal cloud will contain a different amount of material, each star will be born with a different mass. Those that formed from larger clouds, the more massive stars, are those which live the shortest lives – quickly burning through their fuel and dying as energetic explosions known as supernovae. Those with lower masses, similar to our Sun, will live far longer, slowly using their fuel for hundreds of millions of years.

“By looking at clusters we can see the evolution of stars in action,” says Emma Beasor, a postdoctoral researcher at NOIRLab in Tucson, Arizona, and lead author of a recent paper in The Astrophysical Journal. “We can follow stars of different masses and learn how the mass they are born with affects how they will end their lives.”

One particularly interesting cluster, Westerlund 1, has long been considered the most massive cluster in the local universe, and is thought to contain a total mass of more than 10,000 times that of our Sun. In addition, Westerlund 1 is used as a benchmark object for studies of distant starburst galaxies and for calibrating stellar evolutionary models. Westerlund 1 also contains a high number of rare massive stars, including red, yellow, and blue supergiants, as well as an exotic object known as a magnetar. This unprecedented diversity makes Westerlund 1 truly unique.

The presence of a large number of massive stars means the cluster is young enough that these objects have not yet gone supernova, suggesting that Westerlund 1 formed around four million years ago – a cosmic newborn by astronomical standards. However, new data from NASA’s Stratospheric Observatory for Infrared Astronomy, or SOFIA, has revealed a complicated evolutionary past for Westerlund 1.

“One of the most important parameters we need to measure is the brightness of a star, since this can tell us how old or young the star is,” adds Emma Beasor. “Different stars emit light in different wavebands that we can’t see with our naked eye.”

Red supergiant stars, such as Betelgeuse, emit most of their radiation as infrared light. Using new data from SOFIA, Beasor and collaborators directly measured the brightness of the red supergiants in Westerlund 1, for the first time. In doing so, they found that the stars were all too faint to be a mere four million years old. The observations instead suggest that the red supergiants are more than double that age, around 10 million years old.

This new age estimation hints at a complicated past for Westerlund 1. The presence of the young, hot stars implies an age of around four million years. If the cluster were truly 10 million years old, these stars would all have ended their lives as supernova explosions many years before the red supergiants had time to form. Instead, researchers conclude that Westerlund 1 was not born in a simple, single starburst, but rather via a sustained period of star formation, likely over a period of millions of years. If this is the case, the stars in Westerlund 1 cannot all be as young or as massive as once thought.

Although Westerlund 1 can no longer serve as a benchmark for studies of distant starburst galaxies or stellar evolutionary models, it highlights the complexity of the astrophysical systems we struggle to understand and reminds us how wondrous and surprising the universe can be.

SOFIA to Return from French Polynesia Deployment

The Stratospheric Observatory for Infrared Astronomy, or SOFIA, will return to its base of operations in Palmdale, California, after a four-week deployment in French Polynesia. The SOFIA team completed 13 successful flights from the Fa’a’ā International Airport, where the team observed targets in the Southern Hemisphere.

SOFIA landing in French Polynesia with mountains in the background and tropical trees in the foreground
SOFIA lands at Fa’a’ā International Airport in French Polynesia during deployment. Credit: Robert Simon

The team is returning approximately one month ahead of schedule due to updated COVID-19 precautions. The decision to return SOFIA early to its base of operations aligns with the Centers for Disease Control and Prevention travel guidelines and SOFIA mission partner health and safety protocols. The team is focused on conducting a safe and orderly departure and will return within the next week.

“We are very grateful to the French Polynesian government for their warm welcome and hospitality while SOFIA was in country,” said Naseem Rangwala, SOFIA Project Scientist. “Flying from this Southern Hemisphere base allowed us to complete important observations that contribute to our scientific community’s needs.”

During the deployment, the team observed the concentration of hydride molecules in our Milky Way galaxy and their relation to cosmic rays with the German Receiver at Terahertz Frequencies, or GREAT, instrument. The team also studied star formation, looking at how stellar winds might be triggering or quenching star formation in their surroundings. The opportunity to fly from the Southern Hemisphere also allowed SOFIA to make observations of atomic oxygen in the Earth’s atmosphere. This will help the team of scientists understand the distribution of atomic oxygen in Earth’s atmosphere during different seasons and different parts of the world, contributing to the understanding of climate change.

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.

A New Springboard to the Southern Sky: SOFIA Deploys to French Polynesia

NASA’s Stratospheric Observatory for Infrared Astronomy, or SOFIA, landed at Fa’a’ā International Airport, outside Papeete, Tahiti, French Polynesia, July 19, 2021, to study celestial objects best observed from the Southern Hemisphere. SOFIA regularly deploys to the Southern Hemisphere to observe objects only visible from this part of the world. Typically, SOFIA flies from Christchurch, New Zealand. However, due to COVID-19 travel restrictions, French Polynesia was identified as the right location to continue groundbreaking science to better serve the scientific community.

SOFIA takes off from its base of operations at NASA’s Armstrong Flight Research Center’s Building 703 in Palmdale, California.
SOFIA takes off from its base of operations at NASA’s Armstrong Flight Research Center’s Building 703 in Palmdale, California, on its way to Fa’a’ā, French Polynesia. Credit: NASA/Carla Thomas

SOFIA will operate from French Polynesia to observe highly important objects that cannot be seen from the Northern Hemisphere. An advantage of winter is that the water vapor in the Earth’s upper atmosphere is much lower during the months of July through September at Southern Hemisphere latitudes, compared to the Northern Hemisphere’s summer months.

SOFIA landing in French Polynesia
SOFIA landing at Fa’a’ā International Airport, outside Papeete, Tahiti, French Polynesia, on July 19, 2021. Credit: DSI/Florian Beherens

Among the many objects best seen from Southern Hemisphere latitudes, SOFIA will be looking at the central-most regions of our galaxy, the Milky Way.

SOFIA in French Polynesia
SOFIA arrives at Fa’a’ā International Airport, outside Papeete, Tahiti, French Polynesia, on July 19, 2021. Credit: NASA/J. Spooner

For the next eight weeks, 20 flights are planned with the German Receiver at Terahertz Frequencies, or GREAT, instrument, which is operated by the Max Planck Institute of Radio Astronomy in Bonn and the University of Cologne, both in Germany. Additionally, the SOFIA team plans to use the High-resolution Airborne Wideband Camera-Plus, or HAWC+, to observe important astronomical targets in the southern skies.

During the GREAT flight series, the team will search for gases that can reveal the presence of cosmic rays, highly energetic charged particles that stream through our Milky Way galaxy. When a hydrogen atom combines with another element, such as argon or oxygen, simple molecules called hydrides are formed, some of which can be used to find cosmic rays. While cosmic rays can be detected directly within our solar system, astronomers know much less about their presence elsewhere in space. By measuring the concentration of hydride molecules, SOFIA’s observations will help researchers understand how common cosmic rays are in different parts of our galaxy, providing clues about the origin of these mysterious particles.

SOFIA will also make new measurements of atomic oxygen in Earth’s atmosphere, building on the success of previous observations. SOFIA is the only observatory capable of measuring atomic oxygen in this region of Earth’s atmosphere and these observations are important for understanding climate change. Atomic oxygen, a particular form of unbonded oxygen, plays an important role in cooling the upper atmosphere and, therefore, is used to estimate temperatures in this region. Climate models predict that increasing greenhouse gases will raise temperatures in the lower atmosphere yet decrease temperatures in the mesosphere. These continued observations will allow a team of German scientists to monitor the seasonal and latitudinal variability of atomic oxygen, leading to a more accurate understanding of the relationship between the lower and upper atmosphere.

Noctilucent or "night shining" clouds forming in the mesosphere as seen from the International Space Station
Noctilucent or “night shining” clouds forming in the mesosphere as seen from the International Space Station on May 29, 2016. These clouds form between 47 to 53 miles (76 to 85 kilometers) above Earth’s surface, near the boundary of the mesosphere and thermosphere, a region known as the mesopause. SOFIA will make new observations of atomic oxygen in Earth’s atmosphere, building on the success of previous observations. Credit: ESA/NASA/Tim Peake

At the end of August, HAWC+ will be installed and flown for an expected total of 12 flights. The scheduled flights include the first observations of the Study of Interstellar Magnetic Polarization: a Legacy Investigation of Filaments. This new program focuses on the observation of high-gas-density, thread-shaped regions called filaments, where most stars form. The scientific team will make maps of star-forming regions relatively close to Earth. These maps will help constrain the relative importance of self-gravity, turbulence, and magnetic fields. The most exciting aspect of this project is the wide variety of spatial scales that will be observed. At the end of this program, the scientific team will better understand the role of magnetic fields in star formation, from large star-forming regions down to the scale of planet-forming disks.

The SOFIA team also plans to observe the galactic center using HAWC+ to understand the magnetic fields in our own galaxy. This work will complement the previous successful SOFIA Legacy Program which mapped much of the Milky Way using another SOFIA instrument. These multiple views of our galaxy will help scientists better understand the role of magnetic fields in star formation and in the regions of the galaxy closest to the central supermassive black hole.

Composite image of Centaurus A. Magnetic fields from SOFIA/HAWC+ are shown as streamlines over an image of the galaxy taken at visible and submillimeter wavelengths by the European Southern Observatory and Atacama Pathfinder Experiment, X-ray wavelengths from the Chandra X-Ray observatory and infrared from the Spitzer Space Telescope. HAWC+ will study magnetic fields in our own Milky Way galaxy. Credit: Optical: European Southern Observatory (ESO) Wide Field Imager; Submillimeter: Max Planck Institute for Radio Astronomy/ESO/Atacama Pathfinder Experiment (APEX)/A.Weiss et al.; X-ray and Infrared: NASA/Chandra/R. Kraft; JPL-Caltech/J. Keene; SOFIA

The process of operating from a temporary base, called a deployment, involves relocating the SOFIA aircraft and observing instruments, storage and cooling equipment, and, of course, moving critical personnel for several weeks and sometimes months at a time. It requires an active partnership and high level of coordination with local authorities, airport facilities, and workforce, as well as local vendors.

Detailed flight plans for the southern deployment series are posted on the SOFIA Science Center website.

Media inquiries regarding SOFIA’s southern deployment should be sent to the NASA Ames newsroom.

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 Witnesses Rare Accretion Flare on Massive Protostar

Infrared data obtained by SOFIA were crucial for studying an outburst from a massive protostar in the iconic Cat’s Paw Nebula that is now glowing at 50,000 times the luminosity of the Sun.

Infrared image of the Cat's Paw Nebula with inset showing high-mass protostar pre- and post-outburst
The Cat’s Paw Nebula (NGC 6334), imaged here by NASA’s Spitzer Space Telescope using the IRAC instrument, is a star-forming region of the Milky Way galaxy. The dark filament running through the middle of the nebula is a particularly dense region of gas and dust. The inset shows the region of a newly visible high-mass protostar with pre- and post-outburst luminosity imaged by the Cerro Tololo Inter-American Observatory and NASA’s Stratospheric Observatory for Infrared Astronomy, respectively. Credits: Cat’s Paw Nebula: NASA/JPL-Caltech; Left inset: De Buizer et al. 2000; Right inset: Hunter et al. 2021

Even though the birth of stars is hidden from the view of even the most powerful optical telescopes, longer wavelength infrared and millimeter light can pierce through the tons of obscuring gas and dust. These observations reveal the environments where massive stars are forming, which enables astronomers to finally compare the physics governing these lesser-known processes with those that are observationally well established for low-mass stars like our Sun.

Stars form via the gradual, continuous accretion of matter from a surrounding disk. But this steady process is occasionally interrupted as a massive clump from the disk falls onto the forming star, causing a tremendous outburst of energy that can last from several months to hundreds of years. Such outbursts have been seen in dozens of low-mass protostars during the past 50 years.

While monitoring NGC 6334 I, a well-studied protostar cluster in the Cat’s Paw Nebula, researchers discovered a millimeter outburst from a massive protostar with the Atacama Large Millimeter/submillimeter Array, or ALMA. Unlike some of its companions, this particular protostar is so deeply embedded that it was not even detectable in the infrared prior to the outburst.

Using the Stratospheric Observatory for Infrared Astronomy, or SOFIA, the region was revisited after the discovery of the millimeter outburst. Observations by SOFIA’s FORCAST and HAWC+ instruments revealed that infrared emission from the protostar had also increased considerably. Not only could the protostar be seen in the infrared, but it was now the brightest infrared source in the entire cluster.

“The most interesting aspects of this outburst are the extreme luminosity and longevity,” said James De Buizer, a Universities Space Research Association senior scientist for SOFIA based at Ames and coauthor on the study. “This event now exceeds all other accretion outbursts in massive protostars by a factor of about three in both energy output and duration.”

Because the radiation generated from an accretion event emerges mainly in the infrared, SOFIA data are crucial for deriving the total luminosity of the young star and the fundamental parameters of the outburst. Combining the SOFIA and ALMA data allowed astronomers to test predictions of how massive disks fragment. It also helped them rule out alternative causes of the outburst, like a stellar merger, or less likely explanations for the protostar’s sudden appearance, like changes in gas and dust clouds along the telescope’s line of sight.

“Since the matter distribution surrounding the star is clumpy, fragments occasionally fall onto the growing star,” said Todd Hunter, an astronomer at the National Radio Astronomy Observatory, or NRAO, in Charlottesville, Virginia, and lead author on the paper. “In this case, it may have even triggered a temporary change in the size and temperature of the protostar.”

These observations show the importance of continuous access to the infrared to enable time-domain studies of the important accretion stage of massive star formation. Moreover, the new observations provide strong evidence of episodic accretion in young massive stars. Presumably such accretion bursts, while rare for an individual object, often occur somewhere in the galaxy, due to the large number of protostars in this phase.

“Without SOFIA, accurate measurements of the mass and luminosity of this event and future events would not be possible,” added co-author Crystal Brogan, also an astronomer at NRAO.

These new findings confirm that the formation of high-mass stars can be considered a scaled-up version of the process by which low-mass stars, like our Sun, are born. The main differences are that massive stars would form with larger disks, higher accretion rates, and on much shorter time scales (around 100,000 years instead of several million years).

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.

Massive, Young Star Bursts Into View

Infrared image of the Cat's Paw Nebula with inset showing high-mass protostar pre- and post-outburst
The Cat’s Paw Nebula (NGC 6334), imaged here by NASA’s Spitzer Space Telescope using the IRAC instrument, is a star-forming region of the Milky Way galaxy. The dark filament running through the middle of the nebula is a particularly dense region of gas and dust. The inset shows the region of a newly visible high-mass protostar with pre- and post-outburst luminosity imaged by the Cerro Tololo Inter-American Observatory and NASA’s Stratospheric Observatory for Infrared Astronomy, respectively. Credits: Cat’s Paw Nebula: NASA/JPL-Caltech; Left inset: De Buizer et al. 2000; Right inset: Hunter et al. 2021

A young, high-mass star recently burst into view in a corner of the Cat’s Paw Nebula, a star-forming region of the Milky Way galaxy. The nascent star was previously invisible, hidden by tons of obscuring gas and dust. Now, it is the brightest source of infrared light in the entire cluster of young stars, and shines with the light of 50,000 Suns. NASA’s telescope on an airplane, the Stratospheric Observatory for Infrared Astronomy, or SOFIA, studied the star’s outburst – the brightest and longest-lasting of its kind ever observed.

The observations helped determine the cause of the flare. Stars form via gradual, continuous accretion, or accumulation of matter drawn in by gravity from a surrounding disk. But this steady process is occasionally interrupted as a massive clump from the disk falls onto the forming star, causing a tremendous outburst of energy. That energy is mainly in the form of infrared light, which SOFIA can observe. This image by NASA’s retired Spitzer Space Telescope shows the location within the nebula of the newly visible, high-mass protostar. The inset compares its pre- and post-outburst luminosity. The latter was detected by SOFIA and shows it is 16 times brighter than before. The new findings also confirm that the way high-mass stars are born is comparable to the formation of low-mass stars, like our Sun.

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 Delivers First Complete Map of Ionized Carbon in the Fireworks Galaxy

Calibration for young distant galaxies is now possible

NGC 6946 the Fireworks Galaxy
NGC 6946, the Fireworks Galaxy. Image credit: ESA/Hubble & NASA, A. Leroy, K.S. Long

Using the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS), developed by the University of Stuttgart and installed aboard the flying observatory SOFIA, a team led by Frank Bigiel of the Argelander Institute for Astronomy, or AIfA, at the University of Bonn, Germany, has completed a far-infrared map of the spiral galaxy NGC 6946 revealing the distribution of ionized carbon in this galaxy. These data help to estimate the star-formation rate not only in the nearby universe but also in distant galaxies of the early universe. Continue reading “SOFIA Delivers First Complete Map of Ionized Carbon in the Fireworks Galaxy”

Science Result: Surprisingly Young Nebula Hints at Formation of Stars in the Early Universe

Astronomers are still trying to understand how stars and galaxies formed in the early universe. Now, scientists have new clues from a glowing nebula filled with clouds of hot gas and dust, known as RCW 120. Data from NASA’s Stratospheric Observatory for Infrared Astronomy, or SOFIA, suggest that this nebula may be representative of how stars formed in the early universe.

Composite image of the nebula RCW 120
Composite image of the nebula RCW 120. The ring-shaped clouds around the nebula were detected by the Spitzer Space Telescope. SOFIA measured the glowing gas shown in red and blue to study the nebula’s expansion speed and determine its age. The blue gas represents gas expanding in the direction toward Earth and the red away from Earth. The expansion is triggering the birth of stellar neighbors at breakneck speeds – and revealing the nebula is younger than previously believed. Credit: NASA/JPL-Caltech/SOFIA

Continue reading “Science Result: Surprisingly Young Nebula Hints at Formation of Stars in the Early Universe”