Kavli Affiliate: Troy A. Porter
| First 5 Authors: Shmuel Bialy, Shmuel Bialy, , ,
| Summary:
Stars and planets form within cold, dark molecular clouds. In these dense
regions, where starlight cannot penetrate, cosmic rays (CRs) are the dominant
source of ionization — driving interstellar chemistry(Dalgarno (2006, PNAS,
103, 12269)), setting the gas temperature(Goldsmith et al. (1969, ApJ, 158,
173)), and enabling coupling to magnetic fields(McKee & Ostriker (2007, ARA&A,
45, 565; arXiv:0707.3514)). Together, these effects regulate the collapse of
clouds and the onset of star formation. Despite this importance, the cosmic-ray
ionization rate, $zeta$, has never been measured directly. Instead, this
fundamental parameter has been loosely inferred from indirect chemical tracers
and uncertain assumptions, leading to published values that span nearly two
orders of magnitude and limiting our understanding of star formation physics.
Here, we report the first direct detection of CR-excited vibrational H$_2$
emission, using textitJames Webb Space Telescope (JWST) observations of the
starless core Barnard 68 (B68). The observed emission pattern matches
theoretical predictions for CR excitation precisely, confirming a decades-old
theoretical proposal long considered observationally inaccessible. This result
enables direct measurement of $zeta$, effectively turning molecular clouds
into natural, light-year-sized, cosmic-ray detectors. It opens a transformative
observational window into the origin, propagation, and role of cosmic rays in
star formation and galaxy evolution.
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