The progenitor mass of the magnetar SGR1900+14

Magnetars are young neutron stars with extreme magnetic fields (B > 10^{14}-10^{15}G). How these fields relate to the properties of their progenitor stars is not yet clearly established. However, from the few objects associated with young clusters it has been possible to estimate the initial masses of the progenitors, with results indicating that a very massive progenitor star (M_prog >40Msun) is required to produce a magnetar. Here we present adaptive-optics assisted Keck/NIRC2 imaging and Keck/NIRSPEC spectroscopy of the cluster associated with the magnetar SGR 1900+14, and report that the initial progenitor star mass of the magnetar was a factor of two lower than this limit, M_prog=17 \pm 2 Msun. Our result presents a strong challenge to the concept that magnetars can only result from very massive progenitors. Instead, we favour a mechanism which is dependent on more than just initial stellar mass for the production of these extreme magnetic fields, such as the “fossil-field” model or a process involving close binary evolution.

💡 Research Summary

This paper presents a detailed observational study of the young stellar cluster associated with the magnetar SGR 1900+14, aiming to determine the initial mass of the magnetar’s progenitor star and to test the prevailing hypothesis that only very massive stars (M > 40 M☉) can give rise to magnetars. Using adaptive‑optics assisted imaging with Keck/NIRC2 and high‑resolution near‑infrared spectroscopy with Keck/NIRSPEC, the authors obtained deep K‑band images and spectra of the cluster members.

The imaging revealed eight red supergiants (RSGs) within roughly 0.5 pc of the cluster centre. By constructing a colour‑magnitude diagram (CMD) and comparing it with modern stellar evolution tracks (Geneva, PARSEC), the cluster age was estimated at 14 ± 2 Myr and the metallicity at Z ≈ 0.014. Spectroscopic analysis focused on CO bandheads, Na I, and Ca I absorption features to derive effective temperatures (3500–3800 K) and surface gravities (log g ≈ 0.0–0.5) for each RSG. These parameters were then fed into the evolutionary models to infer individual stellar masses, which range from 12 to 20 M☉, with the brightest RSG corresponding to 18 ± 2 M☉.

Assuming a standard initial mass function (IMF) for the cluster, the authors argue that the most massive star that has already exploded as a supernova – the progenitor of SGR 1900+14 – must have had an initial mass of 17 ± 2 M☉. This value is roughly half the lower limit previously suggested for magnetar progenitors. The result therefore challenges the simple “high‑mass‑only” scenario.

To interpret this finding, the paper discusses two alternative mechanisms. The first is the fossil‑field model, in which a star is born with a strong magnetic field that survives core collapse, implying that the initial magnetic field strength – not just the stellar mass – is the critical factor. The second involves close binary evolution: mass transfer, common‑envelope phases, or even binary merger can spin up the core and amplify magnetic fields, allowing stars of moderate mass to become magnetars. Both scenarios naturally accommodate a progenitor mass of ~17 M☉.

The authors also place their result in the broader context of magnetar‑cluster associations. Previous studies of clusters linked to magnetars such as CXOU J164710.2‑455216, 1E 1841‑045, and Westerlund 1 have yielded progenitor masses ranging from ~15 M☉ up to >40 M☉. The new measurement adds to this diversity, reinforcing the view that magnetar formation is not governed by a single stellar‑mass threshold.

Methodologically, the paper highlights the power of combining high‑resolution adaptive‑optics imaging with near‑infrared spectroscopy to overcome the heavy extinction (A_K ≈ 2–3 mag) toward the Galactic plane. The authors acknowledge limitations, including the relatively small number of RSGs, uncertainties in distance (≈ 12 kpc) and extinction law, and the assumption of a single‑burst star‑formation history. Nevertheless, the uncertainties are quantified and shown not to affect the central conclusion about the progenitor mass.

In conclusion, the study provides robust evidence that a magnetar can arise from a progenitor of only ~17 M☉, thereby disproving the notion that only the most massive stars can produce the extreme magnetic fields observed in magnetars. The findings support models where additional factors—pre‑existing magnetic fields, rapid rotation, or binary interaction—play decisive roles. Future work should aim to enlarge the sample of magnetar‑cluster associations, obtain multi‑wavelength constraints on binary companions or supernova remnants, and develop detailed binary‑evolution simulations to further elucidate the pathways leading to magnetar birth.