In a close binary system, one star can spend years transferring its material to its companion, like a person gradually giving away everything they own. When the source runs dry, a dense core remains, wrapped in a cloud of its own leftovers. Astronomers have caught exactly this moment in the globular cluster NGC 362: the white dwarf companion of a millisecond pulsar has yet to shed its "cocoon" of material that recently left its surface.
The pulsar NGC362D is a neutron star spinning hundreds of times per second, completing an orbit with its companion every four hours. The distance between them is less than that from Earth to the Moon. Ten years of observations with the Hubble Space Telescope across nine color filters, from ultraviolet to infrared, revealed a single light source at the expected position. Its location matches the pulsar's radio signal to within 0.02 arcseconds.
Comparison with evolutionary models shows that the companion's mass is about 0.18 solar masses, and its surface temperature is about 13,000 kelvin. This means the star has not yet begun to cool; it is in the proto-white-dwarf phase. Mass transfer ceased only about 0.69 billion years ago. In astronomical terms, that is practically yesterday. No other millisecond pulsar has been found with such a young white dwarf companion.
This discovery changes our picture of what these systems look like immediately after mass transfer ends. Previously, it was thought that leftover shells disperse quickly and do not affect observations. Now it is clear that they can persist for hundreds of millions of years and create false signs of a non-degenerate companion. Similar anomalies have been noted in the system PSR J1816+4510 and in the pulsar M2C, suggesting that such transitional phases are not rare.
The cocoon we have seen has not yet dissolved into space. It reminds us that even after a long history of mass transfer, a star does not immediately become clean and understandable. Perhaps systems like this will help explain how millisecond pulsars are born and why some of them look different from what models predict.
The orbital period of the NGC362D system is only 0.17 days, or about 4 hours. In that time, the companion completes a full orbit around the neutron star, and the distance between them is smaller than the distance from Earth to the Moon.
In Robert Wilson's novel 'Spin', there are star systems where residual disks of material around compact objects affect observational properties and create false signatures. Here, a similar idea plays out in real astrophysics: residual material around a young white dwarf can mimic the signatures of a non-degenerate companion.