Abstract
Stellar mergers are a brief but common phase in the evolution of binary star systems1,2. These events have many astrophysical implications; for example, they may lead to the creation of atypical stars (such as magnetic stars3, blue stragglers4 and rapid rotators5), they play an important part in our interpretation of stellar populations6 and they represent formation channels of compact-object mergers7. Although a handful of stellar mergers have been observed directly8,9, the central remnants of these events were shrouded by an opaque shell of dust and molecules10, making it impossible to observe their final state (for example, as a single merged star or a tighter, surviving binary11). Here we report observations of an unusual, ring-shaped ultraviolet ('blue') nebula and the star at its centre, TYC2597-735-1. The nebula has two opposing fronts, suggesting a bipolar outflow of material from TYC2597-735-1. The spectrum of TYC2597-735-1 and its proximity to the Galactic plane suggest that it is an old star, yet it has abnormally low surface gravity and a detectable long-term luminosity decay, which is uncharacteristic for its evolutionary stage. TYC2597-735-1 also exhibits Hα emission, radial-velocity variations, enhanced ultraviolet radiation and excess infrared emission-signatures of dusty circumstellar disks12, stellar activity13 and accretion14. Combined with stellar evolution models, the observations suggest that TYC2597-735-1 merged with a lower-mass companion several thousand years ago. TYC2597-735-1 provides a look at an unobstructed stellar merger at an evolutionary stage between its dynamic onset and the theorized final equilibrium state, enabling the direct study of the merging process.
Highlights
The “blue” ring nebula (Figure 1(a)) is a rare far-ultraviolet emitting object discovered by the Galaxy Evolution Explorer[15]
We find that a lowmass companion (Mc ∼ 0.1M ) reasonably reproduces TYC 2597-735-1’s effective temperature, luminosity, and surface gravity at a post-merger age of tage ≈ 1, 000 years, accounting for TYC 2597-735-1’s position in Teff -logg space (ED Figure 3)
We find the de-projected surface rotation velocity of TYC 2597-735-1 to be ≈ 25 km/s, larger than expected for a star which has just evolved off the main sequence (v < 10 km/s)[30]
Summary
We derive a minimum dust mass in the present-day circumstellar disk Mdust 5 × 10−9M. Keck/HIRES Observations High-resolution stellar spectra of TYC 2597-735-1 were obtained at optical wavelengths using the Keck High Resolution Echelle Spectrometer (HIRES) with the C2 Decker and iodine cell calibration[68]. This age is a factor of ∼ 2 smaller than that estimated for the BRN of 2,000 years We note that both the primary star’s effective temperature and surface gravity 1,000 years after the merger energy injection adequately match the present-day derived values measured from optical spectroscopy and photometry of TYC 2597-735-1, which explains why TYC 2597-7351’s stellar properties are slightly skewed away from the bulk of moderately-evolved stars in the Teff -logg plane[39] (ED Figure 3). If every merger produced a BRN, the expected BRN fraction would be fBRN
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