Let’s make a quick return to the Magellanics after our recent look at WOH G64, a dying star imaged in the Large Magellanic Cloud (see Close-up of an Extragalactic Star). These satellite galaxies of the Milky Way have long proven useful in helping astronomers study the gravitational interactions that shape them, leading to further understanding of galactic structure. But today I want to focus on the star-forming cluster NGC 346, which presents us with something of a conundrum.
Located in the Small Magellanic Cloud some 200,000 light years away, the cluster is massive and particularly lacking in the heavier elements beyond hydrogen and helium. Intensively studied by the Hubble Space Telescope in the mid-2000s, it has become a proxy for much more distant galaxies in the ancient universe, where metals were harder to find. Why, then, did the Hubble data show that while stars in NGC 346 were between 20 and 30 million years old, they were accompanied by planet-forming disks? A new study puts the James Webb Space Telescope onto the question, with intriguing results.
Such disks should have dissipated after a scant 2 to 3 million years. With few heavy elements in the gas surrounding the star, it should be relatively easy for the stellar outflow from the host to blow the disk away. It’s not easy to reconcile that view with planets still forming in a 20 million year old disk that shouldn’t be there in the first place.
Image: This is a James Webb Space Telescope image of NGC 346, a massive star cluster in the Small Magellanic Cloud, a dwarf galaxy that is one of the Milky Way’s nearest neighbors. With its relative lack of elements heavier than helium and hydrogen, the NGC 346 cluster serves as a nearby proxy for studying stellar environments with similar conditions in the early, distant Universe. Ten, small, yellow circles overlaid on the image indicate the positions of the ten stars surveyed in a new study in The Astrophysical Journal. Credit: NASA, ESA, CSA, STScI, O. C. Jones (UK ATC), G. De Marchi (ESTEC), M. Meixner (USRA).
Findings that upset current models are red meat for researchers hunting new theories or adjustments to the old, but the question was whether the disks found in the cluster were actually evidence of accretion, or perhaps some other process that needed investigation. The good news is that JWST has obtained spectra from some of these stars, the first taken of pre-main-sequence stars in a nearby galaxy. The leader of the work, Guido De Marchi (ESA’s European Space Research and Technology Centre, Noordwijk, Netherlands) says the Webb data strongly confirm what Hubble revealed. The conclusion he draws is striking: “…we must rethink how we model planet formation and early evolution in the young Universe.”
That’s a tall order, of course. But with only ten percent of the heavier elements present in the chemical composition of our Sun, NGC 346 forces the question of whether our understanding of how some stars disperse their disks is correct. Perhaps light pressure from the star is more effective when the disk is laden with more metals. Or perhaps in the lack of heavier elements, the star would have to form from a larger cloud of gas in the first place, producing a disk that is more massive and harder to dissipate. Elena Sabbi (Gemini Observatory at NOIRLab in Tucson) puts the matter this way:
“With more matter around the stars, the accretion lasts for a longer time. The discs take ten times longer to disappear. This has implications for how you form a planet, and the type of system architecture that you can have in these different environments.”
Image: This graph shows, on the bottom left in yellow, a spectrum of one of the 10 target stars in this study (and accompanying light from the immediate background environment). Spectral fingerprints of hot atomic helium, cold molecular hydrogen, and hot atomic hydrogen are highlighted. On the top left in magenta is a spectrum slightly offset from the star that includes only light from the background environment. This second spectrum lacks a spectral line of cold molecular hydrogen. On the right one can find the comparison of the top and bottom lines. This comparison shows a large peak in the cold molecular hydrogen coming from the star but not its nebular environment. Also, atomic hydrogen shows a larger peak from the star. This indicates the presence of a protoplanetary disc immediately surrounding the star. The data were taken with the microshutter array on the James Webb Space Telescope’s NIRSpec (Near-Infrared Spectrometer) instrument. Credit: NASA/ESA.
Because the study confirms that the spectral signatures of active accretion and the presence of molecular dust in the material around these stars can be detected, it becomes clear that these narrowband imaging methods can now be applied not only to the Magellanics but galaxies further away. That’s helpful in itself, and raises the prospect of further investigation into whether the core-accretion model for planet formation is fully understood around low-metallicity stars. From the paper:
If disk dissipation in low-metallicity stars were as quick as initially reported (C. Yasui et al. 2009), only very small rocky planets close to the star could form (J. L. Johnson & H. Li 2012).
And this in conclusion, relating the findings to the era known as ‘cosmic noon,’ when star formation in the visible universe would have been at its peak, some 10 to 11 billion years ago:
…our results indicate that, at the low metallicities typical of the early Universe, the disk lifetimes may be longer than what is observed in nearby star-forming regions, thus allowing more time for giant planets to form and grow than in higher-metallicity environments. This may have significant implications for our understanding of the formation of planetary systems in environments similar to those in place at Cosmic Noon.
The paper is De Marchi et al., “Protoplanetary Disks around Sun-like Stars Appear to Live Longer When the Metallicity is Low,” The Astrophysical Journal Vol. 977, No. 2 (16 December 2024), 214 (full text).
