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, TYC 2597-735-1. The nebula has two opposing fronts, suggesting a bipolar outflow of material from TYC 2597-735-1. The spectrum of TYC 2597-735-1 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. TYC 2597-735-1 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 TYC 2597-735-1 merged with a lower-mass companion several thousand years ago. TYC 2597-735-1 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.
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All GALEX imaging and grism data of TYC 2597-735-1 and its ultraviolet nebula are publicly available from the Mikulski Archive for Space Telescopes (MAST) in raw and reduced formats (http://galex.stsci.edu/GalexView/ or https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html). All Keck–LRIS and Keck–HIRES data for TYC 2597-735-1 are publicly available from the Keck Observatory Archive (https://koa.ipac.caltech.edu/cgi-bin/KOA/nph-KOAlogin). TYC 2597-735-1 raw photometric light-curve frames, plates and light curves from 1895 to 1985 are publicly available as a part of the DASCH programme (https://projects.iq.harvard.edu/dasch). Data for the more recent photmetry for the light-curve construction is available from the corresponding author on request. All other photometric data for TYC 2597-735-1 were obtained from publicly archived ground- and space-based imaging and surveys, stored on the SIMBAD Astronomical Database (http://simbad.u-strasbg.fr/simbad/) and the NASA/IPAC Infrared Science Archive (https://irsa.ipac.caltech.edu/frontpage/). The relevant data products from the Habitable-zone Planet Finder Spectrograph (HPF) campaign for TYC 2597-735-1 are publicly available at https://github.com/oglebee-chessqueen/BlueRingNebula.git.
We used MESA24 for a portion of our analysis. Although MESA is readily available for public use, we used a custom subroutine and MESA inline code to produce the TYC 2597-735-1 merger evolution model, publicly available at https://github.com/oglebee-chessqueen/BlueRingNebula.git. Use the ATLAS9 pre-set grid of synthetic stellar spectra36 to fit the TYC 2597-735-1 spectral energy distribution to representative stellar spectra. All synthetic stellar spectra are publicly available at https://www.stsci.edu/hst/instrumentation/reference-data-for-calibration-and-tools/astronomical-catalogues/castelli-and-kurucz-atlas. Portions of our analysis used community-developed core Python packages for astronomy, photutils37 and astropy38.
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This research is based on observations made with GALEX, obtained from the MAST data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5–26555. Some of the data presented were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership between the California Institute of Technology, the University of California and NASA. This research made use of the Keck Observatory Archive, which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute, under contract with NASA, and made possible by the financial support of the W. M. Keck Foundation. We recognize and acknowledge the very important cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are fortunate to have the opportunity to conduct observations from this mountain. Some of the data presented were obtained at the Palomar Observatory. This research made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. We thank V. Scowcroft for obtaining Spitzer/IRAC photometry of TYC 2597-735-1. Funding for APASS was provided by the Robert Martin Ayers Sciences Fund. The DASCH data from the Harvard archival plates was partially supported from National Science Foundation (NSF) grants AST-0407380, AST-0909073 and AST-1313370. The American Association of Variable Star Observers has been helpful for finder charts, comparison star magnitudes and recruiting skilled observers, including S. Dufoer, K. Menzies, R. Sabo, G. Stone, R. Tomlin and G. Walker. These results are based on observations obtained with the HPF on the Hobby–Eberly Telescope (HET), which is named in honour of its principal benefactors, William P. Hobby and Robert E. Eberly. These data were obtained during HPF’s engineering and commissioning period. We thank the resident astronomers and telescope operators at the HET for the execution of our observations with HPF. We thank C. Cañas for providing an independent verification of the HPF SERVAL pipeline using a CCF-based method to calculate the radial velocities, which resulted in fully consistent radial velocities to the SERVAL-based radial velocities presented here. The HET is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München and Georg-August Universität Gottingen. The HET collaboration acknowledges support and resources from the Texas Advanced Computing Center. This work was partially supported by funding from the Center for Exoplanets and Habitable Worlds, which is supported by the Pennsylvania State University, the Eberly College of Science and the Pennsylvania Space Grant Consortium. We thank A. Gil de Paz for obtaining the narrow-band-filter Hα imagery, J. Johnson for commissioning TYC 2597-735-1 radial velocity measurements as part of the California Planet Finder programme, and A. Howard for leading Keck–HIRES spectra and performing the primary radial-velocity reduction on all HIRES data. K.H. acknowledges support from a David and Ellen Lee Postdoctoral Fellowship in Experimental Physics at Caltech, and thanks L. Hillenbrand and E. Hamden for discussions about aspects of this work. B.D.M. acknowledges support from the Hubble Space Telescope (number HST-AR-15041.001-A) and the NSF (number 80NSSC18K1708). K.J.S. received support from the NASA Astrophysics Theory Program (NNX17AG28G). G.S. and A.Mo. acknowledge support from NSF grants AST-1006676, AST-1126413, AST-1310885, AST-1517592, AST-1310875 and AST-1907622, the NASA Astrobiology Institute (NNA09DA76A) and PSARC in their pursuit of precision radial velocities in the near-infrared with HPF. We acknowledge support from the Heising-Simons Foundation via grant 2017-0494 and 2019-1177. Computations for this research were performed on the Pennsylvania State University’s Institute for Computational and Data Sciences. G.S. acknowledges support by NASA HQ under the NASA Earth and Space Science Fellowship Program through grant NNX16AO28H, and is a Henry Norris Russell Fellow.
The authors declare no competing interests.
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Extended data figures and tables
From left to right, top to bottom: GALEX far-ultraviolet (FUV), GALEX near-ultraviolet (NUV), DSS-II B band, DSS-II R band, Palomar Hale 200-inch COSMIC Hα narrow band, 2MASS J band, 2MASS H band, 2MASS K band, and WISE 3.4 μm (W1), 4.6 μm (W2), 12 μm (W3) and 22 μm (W4). A reference line for 1 arcmin is included in the GALEX far-ultraviolet image. At a distance of 1.93 kpc, 1 arcmin corresponds to 0.56 pc. Each image covers 10′ × 10′. All images are scaled by asinh(⋅) to accentuate any faint, diffuse emission. The GALEX near-ultraviolet image has been scaled to show the western shock of the blue ring nebula, which makes the brighter near-ultraviolet stars (including TYC 2597-735-1) more enhanced.
a, GALEX low-resolution far-ultraviolet grism imaging reveals that the blue ring nebula emits light only around 1,600 Å. Together with the lack of near-ultraviolet radiation, this points to H2 fluorescence as the main source of emission from blue ring nebula. b, Synthetic models of H2 fluorescence change the distribution of light produced by H2 in the far-ultraviolet, depending on the source of excitation (examples shown are Lyα photon pumping (top spectrum) and electron-impact excitation (bottom spectrum). We convolved high-resolution synthetic H2 fluorescence spectra with the GALEX grism spectral resolution to produce the plots shown. Pumping by Lyα photons (which probably come directly from TYC 2597-735-1) creates peaks in the distribution that are not seen by GALEX near 1,450 Å. Electron-impact fluorescence produces a spectral distribution that better matches where GALEX sees the far-ultraviolet emission being produced.
Extended Data Fig. 3 TYC 2597-735-1 is an outlier when compared with other moderately evolved stars of similar mass.
A large sample of moderately evolved stars39 demonstrates that the effective temperature Teff and surface gravity g of TYC 2597-735-1 are not consistent with the majority of other stars following similar evolutionary tracks. If the present-day observable properties of TYC 2597-735-1 are a consequence of a previous stellar merger, as our MESA models suggest, then we expect that TYC 2597-735-1 is currently puffed up more than usual and will continue to relax over the next thousands of years to better match the trend of evolving stars in Teff–log(g) space (Extended Data Fig. 7).
a, TYC 2597-735-1 exhibits Hα emission, an unusual trait for evolved stars. The Hα line profile shows variability over short timescales. There is an enhanced blue edge to the emission, a signature of gaseous accretion or disk winds14. The Hα emission suggests that TYC 2597-735-1 is actively accreting matter, possibly from the disk that creates its observed infrared excess emission. b, The Hα bisector velocities at different parts of the Hα emission line profile as a function of time (day; MJD, modified Julian date). Different-coloured diamonds (see legend) represent different flux levels in the line profile probed to determine the bisector value. The points in the line profile plotted show the most dramatic shifts away from the line centre. The line peak bisector is also shown (purple), to demonstrate the day-by-day variability observed in the line profile. The dashed grey line represents no velocity shift from the Hα wavelength centre. Except for the line peak, which fluctuates around about 0 km s−1 shifts, the line profile tends towards negative bisector velocity values, providing evidence that the line profile tends towards a blueshifted enhancement.
All uncertainties are taken as the standard deviation in each data point. a, The best-fitting Keck–HIRES period using the iodine-cell calibration technique. Telluric calibration points show the discrepancy between the two methods. Keck–HIRES radial-velocity (RV) signal suggests a period of about 13.75 days for a companion that produces a radial-velocity amplitude of 196 m s−1. b, The bisector velocity span (BVS) as a function of Keck–HIRES radial-velocity signal shows an anticorrelation trend. c, HET–HPF differential line width (dLW) as a function of radial velocity, highlighting clear variations in the differential line width as a function of radial velocity, which is observed to vary from −250 m s−1 to 250 m s−1 in the HPF radial-velocity data. d, Ca ii infrared triplet (IRT) indices from HET–HPF show strong correlation with differential line width. L1, 8,500 Å; L2, 8,545 Å; L3, 8,665 Å; all line indices are normalized to the average index of L2. e, We show the range of mass the companion could have (assuming the Keck–HIRES iodine-cell radial-velocity signal is the result of a companion), on the basis of its 13.7-day orbital period (a ≈ 0.1 au; blue vertical line). We also show the minimum mass required to eject a collimated, biconical outflow with the velocity of the blue ring nebula (BRN; purple lower limit), owing to the conversion of gravitational energy to kinetic energy as its orbit decays from infinity to a ≈ 0.1 au. We put this hypothetical companion into context with other objects, including Jupiter (MJ; orange line), brown dwarfs (MBD; yellow shaded region) and M stars (0.1M☉; red line). The current radial extent of TYC 2597-735-1 (about 10R☉) is shaded green.
A full description of the process of generating the century-long light curve TYC 2597-735-1 is provided in Supplementary Information. The uncertainty in the binned magnitudes is the root-mean-square scatter divided by the square root of the number of plates used per bin. The rough trend of the light curve of TYC 2597-735-1 shows a total B-mag decay of 0.11–0.12 mag between 1895 and 2015, consistent with 0.09–0.1 mag per century. This falls in the range of predicted secular decay in the MESA models for the case study of the stellar merger history of TYC 2597-735-1.
Extended Data Fig. 7 Evolution of a stellar merger between a 2M⊙ primary star and a 0.1M⊙ companion.
MESA evolutionary models were created to look at how the energy injected into the primary star changes its observed characteristics over time. The coloured lines represent mergers at different evolutionary stages of the primary as it evolves towards the red-giant branch. The horizontal dotted black lines represent the observed parameters for TYC 2597-735-1. This model outcome represents one scenario that helps to justify the history of TYC 2597-735-1 including a stellar merger that created a blue ring nebula 1,000 years later (vertical dashed line).
Extended Data Fig. 8 Demonstration of the velocity line profile fitting to an unblended Fe i line (5,569.6 Å).
Left, rotational velocity fit only (vsin(i); red line). The U-shaped rotational velocity profile alone does not capture the line wings of the Fe i line of TYC 2597-735-1 (black). Middle, macroturbulence velocity fit only (ζ; red line). Although the fit to the line wings is improved, the line core is too narrow. Right, convolved rotational plus macroturbulent velocity profiles provide a better fit to the observed Fe i line (fit, red line; data, black line).
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Hoadley, K., Martin, D.C., Metzger, B.D. et al. A blue ring nebula from a stellar merger several thousand years ago. Nature 587, 387–391 (2020). https://doi.org/10.1038/s41586-020-2893-5