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# A spectroscopic quadruple as a possible progenitor of sub-Chandrasekhar type Ia supernovae

## Abstract

Binaries have received much attention as possible progenitors of type Ia supernova explosions1, but long-term gravitational effects2,3 in tight triple or quadruple systems could also play a key role in producing type Ia supernovae. Here we report on the properties of a spectroscopic quadruple found within a star cluster: the 2 + 2 hierarchical system HD 744384. Its membership in the open cluster IC 2391 makes it the youngest (43 Myr) spectroscopic quadruple known to us, and among the quadruple systems with the shortest outer orbital period. The eccentricity of the 6 yr outer period is 0.46, and the two inner orbits, with periods of 20.6 d and 4.4 d and eccentricities of 0.36 and 0.15, are not coplanar. Using an innovative combination of ground-based high-resolution spectroscopy5,6,7 and Gaia/Hipparcos astrometry8,9,10,11, we show that this system is undergoing secular interaction, which probably pumped the eccentricity of one of the inner orbits higher than expected for the spectral types of its components. We compute the future evolution of HD 74438 and show that this system is an excellent candidate progenitor of sub-Chandrasekhar type Ia supernovae through white dwarf mergers. Taking into account the contribution of this specific type of type Ia supernova accounts for the chemical evolution of iron-peak elements in the Galaxy better than considering only near Chandrasekhar-mass type Ia supernovae12.

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## Data availability

Supplementary data are provided with this paper: the measured RVs from which all the orbital solutions are computed are available in a separate csv file. In addition, raw spectroscopic data are available in the associated observatory archives:

http://archive.eso.org/scienceportal for ESO/GES and GIRAFFE spectra.

https://ssda.saao.ac.za/ for SALT/HRS spectra. The spectra from the monitoring programme 2018-1-MLT-009 are publicly available except for the 2020-2-MLT-003 programme (three spectra, available from mid-2024 on).

• The HERCULES/UCMJO raw spectra will be made publicly available on the VizieR/CDS repository.

## Code availability

The MSE code used to perform the simulation of the HD 74438 system is available upon request to A.S.H.

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## Acknowledgements

T.M. and S.V.E. are supported by a grant from the Fondation ULB. A.S.H. thanks the Max Planck Society for support through a Max Planck Research Group. T.M. acknowledges M.G. Perderson and T. Van Reeth for investigating the TESS data. T.M. acknowledges A. Tokovinin for explanations about the Multiple Star Catalog. T.Z. and G.T. acknowledge financial support from the Slovenian Research Agency (research core funding P1-0188) and from the European Space Agency (PRODEX Experiment Arrangement C4000127986). G.T. acknowledges support by the Swedish strategic research programme eSSENCE, project grant ‘The New Milky Way’ from the Knut and Alice Wallenberg foundation and grant 2016-03412 from the Swedish Research Council. Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 188.B-3002. These data products have been processed by the Cambridge Astronomy Survey Unit (CASU) at the Institute of Astronomy, University of Cambridge, and by the FLAMES/UVES reduction team at INAF/Osservatorio Astrofisico di Arcetri. These data have been obtained from the GES Data Archive, prepared and hosted by the Wide Field Astronomy Unit, Institute for Astronomy, University of Edinburgh, which is funded by the UK Science and Technology Facilities Council. This work was partly supported by the European Union FP7 programme through ERC grant 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge support from INAF and Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR) in the form of the grant ‘Premiale VLT 2012’. The results presented here have benefited from discussions held during the Gaia–ESO workshops and conferences supported by the ESF (European Science Foundation) through the GREAT Research Network Programme. Some of the observations reported in this paper were obtained with SALT under programmes 2018-1-MLT-009 (principal investigator R.S.) and 2020-2-MLT-03 (principal investigator R. Manick). Polish participation in SALT is funded by grant MNiSW DIR/WK/2016/07. This work has made use of data from the European Space Agency mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work has made use of the VALD database, operated at Uppsala University, the Institute of Astronomy RAS in Moscow and the University of Vienna, the SIMBAD database, operated at CDS, Strasbourg, France, and the VizieR catalogue access tool, CDS, Strasbourg, France (https://doi.org/10.26093/cds/vizier). The original description of the VizieR service was published in ref. 41. This research has made use of Python3 and IPython, and modules NumPy, SciPy and Pandas. All the graphics were generated with Matplotlib except Extended Data Fig. 5.

## Author information

Authors

### Contributions

T.M. initiated the project, performed the follow-up with HRS/SALT and the analysis and interpretation of the data and designed most figures. A.S.H. performed the simulations of the evolution of the quadruple and designed some of the figures. S.V.E. and A.J. contributed to the analysis and interpretation of the results. T.M., S.V.E., A.J. and A.S.H. wrote the manuscript, with input from all authors. M.V.d.S. and D.P. contributed to the analysis of the data. K.P. performed the acquisition and reduction of HERCULES/UCMJO spectra. R.S. contributed to the follow-up with HRS/SALT and to the interpretation of the data. T.Z. and G.T. contributed to the analysis and interpretation of the data as well as to the writing. G.G. and S.R. are the principal investigators of the GES in which the quadruple was discovered. A.G., A.H., G.S. and C.C.W. contributed to the acquisition and reduction of the GES data. All authors provided critical feedback.

### Corresponding author

Correspondence to Thibault Merle.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

## Peer review

### Peer review information

Nature Astronomy thanks Andrei Tokovinin and Tamas Borkovits for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data

### Extended Data Fig. 1 Cross-correlation functions computed from high-resolution spectra of HD 74438 at different epochs as labeled (black lines) and multi-Gaussian fits (red lines).

The vertical dotted line is the mean velocity of IC 2391. Left: HERCULES/UCMJO CCFs (with multiple spectra observed in some of the nights). The time series covers one week and the vertical position of the spectra scales with time. Right: same as left for HRS/SALT CCFs. The time series covers 7 months. The vertical shift of the spectra is arbitrary. Spectral fitting was performed using the two spectra marked with an asterisk and plotted in bold.

### Extended Data Fig. 2 RV solutions using HERCULES/UCMJO and HRS/SALT data points for the brightest AB pair (left) and for the faintest CD pair (right).

The GES data points are not included in the computation of the orbital solutions because taken 5 y before, showing the gravitational effect between the two inner pairs. The uncertainties on the Gaussian mixture model fit to the CCFs provide the error bars on the RVs.

### Extended Data Fig. 3 Orbital parameters of the two inner orbits (A-B and C-D) and the outer one (AB–CD) of the quadruple 2 + 2 system HD 74438.

P is the orbital period, e the eccentricity, ω the periastron argument, T0 the julian date at periastron, v0 the center-of-mass velocity, K1 and K2 the radial velocity amplitudes of the primary and secondary in each orbit. qspec and qdyn are the spectroscopic and dynamical mass ratios. i is the inclination of the orbit on the sky, a the semi-major axis of the orbit around the center of mass, $$\mu _{{{{\mathrm{phot}}}}}^{\prime\prime}$$ the proper motion of the photocentre and Ω the argument of the ascending node.

### Extended Data Fig. 4 Astrophysical parameters of the quadruple system HD 74438 and its components.

Teff is the effective temperature, M, L and R are the mass, the luminosity and the radius in solar units.

### Extended Data Fig. 5 RV solutions of the wide pair AB–CD using center of mass RVs of the AB and CD pairs.

The error bars on the center-of-mass velocities of each pair come from the error propagation of the individual component RVs.

### Extended Data Fig. 6 Orbital solutions from Extended Data Fig. 3 and measured RVs for the four components of the SB4.

Top: GIRAFFE data, middle top: UVES/VLT from GES data, middle bottom: HERCULES/UCMJO data, bottom: HRS/SALT data. Not all the RVs are presented here.

### Extended Data Fig. 7 The two possible orbits of the photocentre of the AB pair around the centre of mass of the AB - CD system (in units of the semi-major axis a″AB-CBphot).

The black solid line corresponds to the orbit of inclination 73.2°, whereas the red solid line corresponds to the orbit with inclination 106.8°. On these orbits, following the direction of orbital motion, the solid circle marks the periastron, the open circle marks the epoch 2015.3 (April 24) when the predicted orbital velocity projected on the plane of the sky (solid-line arrow) matches the Gaia DR2 proper motion (differential with respect to the cluster, and expressed in units of the proper-motion modulus μAB-CDphot) averaged over the Gaia DR2 time span (represented by the thick part of the orbit), the triangle marks the average Gaia DR2 epoch (2015.5), the open square marks the Gaia eDR3 epoch (2016.0). For this epoch, the solid-line arrow marks the predicted orbital motion projected on the plane of the sky, whereas the dashed arrow corresponds to the Gaia eDR3 proper motion (differential with respect to the cluster). The counter-clockwise evolution of the observed proper motion (Gaia DR2 and Gaia eDR3) favours the orbit with 73.2° inclination (black line).

### Extended Data Fig. 8 Probability distributions of merger times in the simulations.

The distribution of times of all merger events is displayed with the blue dotted lines, and the times of the first merger in the system (if applicable) with the red solid line. The distributions of the maximum age reached by the system for the systems in which the maximum wall time was exceeded is shown with the green dashed line.

### Extended Data Fig. 9 Probability distributions of the current mutual inclinations (left panel: Φ AB / AB–CD; right panel: Φ CD / AB–CD) leading to different events in our Monte Carlo simulations (as described in the legends).

The curve labelled ICs corresponds to the distribution of the initial mutual inclinations (encompassing all possible outcomes). This distribution is obtained from the observed values of the individual orbital inclinations on the sky, complemented by flat distributions for the longitudes of the unknown ascending nodes (equation 12).

### Extended Data Fig. 10 One possible future evolution of HD 74438.

Each panel shows an event of interest as labelled at the top, with the time indicated in each subpanel and the system represented schematically in a so-called mobile diagram35, with orbital parameters (semimajor axes a and eccentricities e) and masses (in units of M) indicated. The meaning of the dot colours (representing the stars) is indicated in the legend at the top. The quadruple experiences unstable RLOF, triple CE and merger events, leaving a WD remnant with a sub-Chandrasekhar mass of 1.3 M.

## Supplementary information

### Supplementary Information

Supplementary Notes and Tables 1–4.

### Supplementary Data 1

RV measurements in kilometres per second of (i) each component (A, B, C and D) and (ii) centre of mass of pairs AB and CD, together with their error bars. The associated epochs are given in Gregorian years.

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Merle, T., Hamers, A.S., Van Eck, S. et al. A spectroscopic quadruple as a possible progenitor of sub-Chandrasekhar type Ia supernovae. Nat Astron 6, 681–688 (2022). https://doi.org/10.1038/s41550-022-01664-5

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• DOI: https://doi.org/10.1038/s41550-022-01664-5