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An infrared transient from a star engulfing a planet

Nature volume 617, pages 55–60 (2023)Cite this article

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Abstract

Planets with short orbital periods (roughly under 10 days) are common around stars like the Sun1,2. Stars expand as they evolve and thus we expect their close planetary companions to be engulfed, possibly powering luminous mass ejections from the host star3,4,5. However, this phase has never been directly observed. Here we report observations of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright and long-lived infrared emission. The resulting light curve and spectra share striking similarities with those of red novae6,7—a class of eruptions now confirmed8 to arise from mergers of binary stars. Its exceptionally low optical luminosity (approximately 1035 erg s−1) and radiated energy (approximately 6.5 × 1041 erg) point to the engulfment of a planet of fewer than roughly ten Jupiter masses by its Sun-like host star. We estimate the Galactic rate of such subluminous red novae to be roughly between 0.1 and several per year. Future Galactic plane surveys should routinely identify these, showing the demographics of planetary engulfment and the ultimate fate of planets in the inner Solar System.

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Fig. 1: Discovery location and multicolour light curves of ZTF SLRN-2020.
Fig. 2: Temporal evolution of the spectral energy distribution and integrated luminosity of ZTF SLRN-2020.
Fig. 3: Optical and IR spectra of ZTF SLRN-2020.
Fig. 4: Comparison of ZTF SLRN-2020 outburst with the phase space of luminosity (L90) and timescales (t90) for previously known red novae.

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Data availability

All the data used in this work are provided in Extended data.

Code availability

K.D. will provide python code used to analyse the observations, and any data used to generate figures, on request. The RLOF code used to model the pre-outburst light curve is publicly available.

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Acknowledgements

K.D.’s work was supported by NASA through NASA Hubble Fellowship grant no. HST-HF2-51477.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract no. NAS5-26555. M.M.’s contributions were supported by the National Science Foundation under grant no. 1909203. A.-C.E. acknowledges support by NASA through NASA Hubble Fellowship grant no. HF2-51434 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract no. NAS5-26555. S.R.K. thanks the Heising-Simons Foundation for supporting his research. We thank B. Metzger, T. Matsumoto, M. Soares-Furtado and J. van Roestel for discussions. The discovery of the optical transient was based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility (ZTF) project. ZTF is supported by the National Science Foundation under grant nos. AST-1440341 and AST-2034437, and by a collaboration including Caltech, IPAC, the Weizmann Institute of Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, Trinity College Dublin, the University of Wisconsin at Milwaukee, IN2P3 France, Lawrence Livermore National Laboratories and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC and UW. The ZTF forced-photometry service was funded under Heising-Simons Foundation grant no. 12540303 (PI: Graham). Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership including the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics, and is funded by the Smithsonian Institution and Academia Sinica. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community; we are most fortunate to have had the opportunity to conduct observations from this mountain. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Author information

Authors and Affiliations

  1. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA

    Kishalay De, Deepto Chakrabarty, Anna-Christina Eilers, Erin Kara, Robert Simcoe & Andrew Vanderburg

  2. Center for Astrophysics/Harvard & Smithsonian, Cambridge, MA, USA

    Morgan MacLeod, Charlie Conroy, Abraham Loeb, Nimesh Patel & Richard Teague

  3. Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA, USA

    Viraj Karambelkar, Matthew J. Graham, Lynne A. Hillenbrand, Mansi M. Kasliwal & S. R. Kulkarni

  4. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA

    Jacob E. Jencson

  5. Caltech Optical Observatories, California Institute of Technology, Pasadena, CA, USA

    Richard Dekany & Reed L. Riddle

  6. NSF’s National Optical-Infrared Astronomy Research Laboratory, Tucson, AZ, USA

    Ryan M. Lau & Aaron M. Meisner

  7. Black Hole Initiative, Harvard University, Cambridge, MA, USA

    Abraham Loeb

  8. IPAC, California Institute of Technology, Pasadena, CA, USA

    Frank Masci & Ben Rusholme

  9. Department of Astronomy, University of California, Berkeley, Berkeley, CA, USA

    Michael S. Medford

  10. Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Michael S. Medford

  11. Department of Aerospace, Physics and Space Sciences, Florida Institute of Technology, Melbourne, FL, USA

    Luis Henry Quiroga-Nuñez

  12. National Radio Astronomy Observatory, Array Operations Center, Socorro, NM, USA

    Loránt O. Sjouwerman

  13. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA

    Richard Teague

Authors
  1. Kishalay De
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  2. Morgan MacLeod
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  13. S. R. Kulkarni
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Contributions

K.D. identified the object, initiated follow-up observations, carried out the analysis and wrote the manuscript. M.M. and A.L. led the theoretical interpretation of the transient, created the models presented in this work and wrote the manuscript. V.K., J.E.J., A.-C.E., L.A.H., M.M.K. and R.M.L. assisted with optical/IR follow-up observations, data interpretation and analysis. D.C., C.C., E.K., S.R.K., R.S. and A.V. assisted with interpretation of the data. R.D., M.J.G., F.M., M.S.M., R.L.R. and B.R. are builders of the ZTF observing system and contributed to survey operations during the observations presented here. A.M.M. assisted with analysis of NEOWISE data. N.P. and R.T. assisted with acquisition of SMA data and carried out SMA data analysis. L.H.Q.-N. and L.O.S. assisted with acquistion of VLA data and carried out VLA data analysis. All authors contributed to scientific interpretation.

Corresponding author

Correspondence to Kishalay De.

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Extended data figures and tables

Extended Data Fig. 1 Comparison of the r and i band light curve of ZTF SLRN-2020 (shown as red and yellow circles respectively, as indicated) to visual light curves of Galactic and extragalactic red novae from the literature.

The light curves have been normalized to peak magnitude. For each comparison object, we indicate the photometric band of the archival light curve in the legend. The comparison objects include V1309 Sco8, OGLE-BLG-36017, V838 Mon51, M31-LRN-201552,53, AT 2019zhd54, NGC 3437-OT55, AT 2017jfs56 and AT 2018hso57. Error bars are shown at 1σ confidence and 5σ upper limits are shown as symbols with downward arrows.

Extended Data Fig. 2 Comparison of the g − r (panel (a)) and r − i (panel (b)) observed color evolution of ZTF SLRN-2020 (shown as squares and circles respectively) to a sample of red novae from the literature that have contemporaneous multi-band coverage during the outburst.

The literature objects include some of the objects shown in Extended Data Fig. 1 in addition to AT 2020hat, AT 2020kog58 and AT 2018bwo59. In each panel, we show a black arrow indicating the estimated shift in the color evolution of ZTF SLRN-2020 accounting for the best estimated line-of-sight extinction. Error bars are shown at 1σ confidence.

Extended Data Fig. 3 Comparison of the optical spectrum of ZTF SLRN-2020 to different types of Galactic transients.

The comparison objects include a classical nova (PGIR 19brv60, in magenta), a dwarf nova (U Gem61 in red), a FU-Ori type young star outburst (Gaia 17bi62 in yellow) and two EXor type young star outbursts (ESO-Hα 9963 in green and ASASSN 15qi64 in blue). The continua of some sources have been reddened to match that of ZTF SLRN-2020 for easier visualization.

Extended Data Fig. 4 Identification of spectroscopic features in the NIR spectra of ZTF SLRN-2020 during and after the outburst.

In each panel, the gray/black lines represent the raw/binned spectrum during the outburst ( ≈ 160 d after peak), while the light brown/brown lines show the raw/binned spectrum obtained after the fading of the infrared transient ( ≈ 690 d after peak). We also show a comparison with the M7-III type star HD 108849 (in red). Prominent atomic and molecular absorption features are marked.

Extended Data Fig. 5 Corner plots showing the model-fit parameters of the MCMC DUSTY modeling of the SED of ZTF SLRN-2020 ≈ 120 days after outburst peak.

The median of the posterior distributions and the corresponding 68% confidence intervals are indicated in the labels, while the same quantities are highlighted with vertical bars in the one dimensional histograms.

Extended Data Fig. 6 Corner plots showing the fit parameters of the MCMC DUSTY modeling of the SED of ZTF SLRN-2020 ≈ 320 days after outburst peak.

The labelling follows the same convention as Extended Data Fig. 5. AV is not used as a free parameter in this fit.

Extended Data Fig. 7 Constraints on the distance to ZTF SLRN-2020 using Galactic three dimensional dust extinction maps.

We show the estimated dust extinction as a function of distance for three different extinction maps published in the literature65,66,67. We also show the 90% confidence interval for the estimated foreground extinction to ZTF SLRN-2020 based on our SED modeling as the magenta shaded region. For each dust extinction map, we show the allowed distance interval within the estimated extinction range with shaded vertical bars in the same color.

Extended Data Fig. 8 The progenitor of ZTF SLRN-2020 in the color magnitude diagram.

For different distances along the Galactic disk, we use available three dimensional extinction maps to de-redden the progenitor photometry fluxes. The results are shown as circles for the D0365 map, squares for the M0666 map and as triangles for the G1967 map. We also show the range of absolute magnitudes allowed by the 90% AV confidence region (from the SED modeling) as blue, red and gray shaded regions respectively (see legend). We also plot stellar evolutionary tracks from the MIST database for stars of initial masses ranging from 0.8−3.0 M⊙. The horizontal bar at the bottom shows the estimated 1σ uncertainty in the H − K color.

Extended Data Fig. 9 Evolution of the SED of ZTF SLRN-2020 progenitor prior to the onset of the optical transient.

The orange points show the optical/IR photometry from ≈ 6−12 years before the outburst. The black points show the evolution of the progenitor in the mid-IR starting ≈ 1.7 years before the transient. Solid points indicate detections while hollow points denote 3σ upper limits. For the −30 d and −244 d epochs, we show the best fit Silicate dust emission model fit using the mid-IR photometry (see text), while we show estimated upper limits to the dust emission using the W2 photometry. For the last epoch, we also show the pre-outburst optical brightening of the progenitor, coincident with the mid-IR source detected in NEOWISE. Error bars are shown at 1σ confidence and 5σ upper limits are shown as symbols with downward arrows.

Extended Data Table 1 Archival photometry of the progenitor of ZTF SLRN-2020 along with the time of observation
Full size table
Extended Data Table 2 Spectroscopic follow-up of ZTF SLRN-2020
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Extended Data Table 3 Derived dust parameters from the multi-epoch DUSTY modeling of ZTF SLRN-2020
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Supplementary Information

Supplementary methods, Tables 1 and 2 and references.

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De, K., MacLeod, M., Karambelkar, V. et al. An infrared transient from a star engulfing a planet. Nature 617, 55–60 (2023). https://doi.org/10.1038/s41586-023-05842-x

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