Star Formation

Speedrunning Star Formation in the Cygnus X Region

Stars are born in molecular clouds, massive clouds of hydrogen that can contain millions of stellar masses of material. But how do molecular clouds form? There are different theories and models of that process, but the cloud formation is difficult to observe.

A new study is making some headway, and showing how the process occurs more rapidly than thought.

Molecular clouds are an important part of the interstellar medium (ISM) and are embedded in atomic gas, the other main component of the ISM. The third component of the ISM is ionic gas, and all three play roles in star formation.

There are unanswered questions about how molecular hydrogen clouds form from the ISM and then form stars. Molecular hydrogen is notoriously difficult to observe because of its lack of absorption lines in visible, infrared, and UV light. New research shows how one component of the ionized gas in the ISM—ionized carbon (CII)—can be observed to trace how molecular clouds form.

The new research appears in Nature Astronomy. The article is “Ionized carbon as a tracer of the assembly of interstellar clouds,” and the lead author is Nicola Schneider. Schneider is a researcher at the University of Cologne, Germany.

The research focuses on Cygnus X, a massive star-forming region about 4,600 light-years away in the constellation Cygnus. It’s associated with one of the largest molecular hydrogen clouds scientists know of. Studies show that Cygnus X has been forming stars rapidly for the last 10 million years and is still forming them today.

This is an infrared image of the star-forming region Cygnus X. The bright regions contain new stars that are carving bubbles out of the cloud with their outflows and UV radiation. Image Credit: NASA/IPAC/MSX

Stars are born in clouds of molecular hydrogen, but astrophysicists wind the clock back further than that to find their origins. Molecular hydrogen clouds form from reservoirs of atomic hydrogen (HI) in galaxies, though the exact mechanism is not clearly understood. Astrophysicists have developed different models of the mechanism. Some lay out a slow process where gravity, turbulence, and magnetic fields are in equilibrium until disturbed by stellar feedback or spiral arm density. Once disturbed, there’s a slow buildup of density that forms pockets of molecular hydrogen gas. Stars are eventually formed in those pockets.

Other models point to a more rapid, dynamic process. In these models, the large-scale movement of the galaxies themselves triggers the process as warm, tenuous, mostly atomic gas called the warm neutral medium (WNM) transitions to cooler, denser clouds of molecular hydrogen called the cold neutral medium (CNM.) Stellar feedback and supernovae explosions also play a role in driving the gas to greater densities and forming stars. This complicates observations. “It is thus challenging to find the right observational tracers for both the dynamic interaction between gas flows and the thermal and chemical transitions between WNM and CNM,” the authors write in their paper.

The team used observations from SOFIA‘s FEEDBACK program in their work. They compared the distribution of three components of the ISM in Cygnus X: ionized carbon, molecular carbon monoxide and atomic hydrogen. SOFIA’s unique capabilities allowed it to spot faint CII (ionized carbon) radiation from the periphery of the clouds that’s never before been detected. The new research shows that star formation can happen much more rapidly than thought. That rapidity might also explain how massive stars form.

SOFIA (Stratospheric Observatory for Infrared Astronomy) is a converted Boeing 747 that acted as an airborne observatory. It housed a 2.5 m (8.2 ft) diameter infrared telescope. SOFIA was a joint mission between NASA and the German DLR. Its final flight was in September 2022. Image Credit: NASA/DLR

Cygnus X is a vast agglomeration of clouds of luminous gas and dust. Observations of spectral lines of ionized carbon (CII) showed that the clouds have formed there over several million years. In astronomy, that is a very fast process. Not only does this disrupt our understanding of star formation, but it also helps answer a question that slow star formation can’t answer: how do massive stars form if it takes so long?

Massive stars are comparatively rare, but we can still see them in the night sky. This image is of the constellation Orion, and the massive star Rigel is the bright blue star in the lower right corner. It’s about 21 solar masses, and is young; only about 8 million years old. It will eventually explode as a supernova, but not for a long time. Credit: NASA Astronomy Picture of the Day Collection NASA

Massive stars are defined as those 8 times more massive than the Sun. They’re particularly interesting to astrophysicists because they’re so rare: less than 1% of stars in the Milky Way are massive. Several types of feedback impede their formation. Outflows, radiation pressure and magnetic fields are all barriers to stars becoming massive. Massive stars also emit massive amounts of material from the polar jets as they form, further restricting their growth. Astrophysicists have struggled to develop a thorough model that can explain how massive stars form. Since they’re responsible for fusing so many of the heavy elements, scientists are very interested in them.

But by observing the radiation from ionized carbon (CII) on the edges of interstellar gas clouds, this group of researchers has made some headway.

Contrary to previous understanding, the researchers found that the interstellar gas clouds, whose shells are made of molecular hydrogen, are travelling more rapidly than thought, at up to 20 km s-1. “This high speed compresses the gas into denser molecular regions where new, mainly massive stars form. We needed the CII observations to detect this otherwise ‘dark’ gas,” said lead author Schneider.

This figure from the study presents some of the findings. It shows DR21, one of the bright, dense, star-forming molecular clouds in Cygnus X. The large scale cloud structure that DR21 is embedded in is only visible in CII (ionized carbon) emissions (left.) Image Credit: Schneider et al. 2023.

This may be the first time that CII has been used as a tracer to probe how molecular clouds form and give rise to massive stars. But it won’t be the last. “We conclude that the [CII] 158??m line is an excellent tracer to witness the processes involved in cloud interactions and anticipate further detections of this phenomenon in other regions,” the authors write.

The data may be in the archives of the now-defunct SOFIA mission. The FEEDBACK program surveyed multiple regions with a wide range of massive star formation activity. The goal was to “… quantify the relationship between star formation activity and energy injection and the negative and positive feedback processes,” the FEEDBACK website explains.

The researchers are already busy working with the FEEDBACK data. In a press release, lead author Schneider said, “In the list of FEEDBACK sources, there are other gas clouds in different stages of evolution, where we are now looking for the weak CII radiation at the peripheries of the clouds to detect similar interactions as in the Cygnus X region.”

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Evan Gough

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