Abstract
Stellar winds of cool main-sequence stars are difficult to constrain observationally. One way to measure stellar mass-loss rates is to detect the soft X-ray emission from stellar astrospheres produced by charge exchange between heavy ions of the stellar wind and cold neutrals of the interstellar medium surrounding the stars. Here we report detections of charge-exchange-induced X-ray emission from the extended astrospheres of three main-sequence stars, 70 Ophiuchi, ϵ Eridani and 61 Cygni, based on the analysis of XMM-Newton observations. We estimate the corresponding mass-loss rates to be 66.5 ± 11.1, 15.6 ± 4.4 and 9.6 ± 4.1 times the solar mass-loss rate for 70 Ophiuchi, ϵ Eridani and 61 Cygni, respectively, and compare our results with the alternative ’hydrogen wall’ method. We also place upper limits on the mass-loss rates of several other main-sequence stars. This method has potential utility for determining the mass-loss rates from X-ray observations showing spatial extension beyond a coronal point source.
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Data availability
All data used for this study is publicly available in XMM-Newton data archive at https://nxsa.esac.esa.int/nxsa-web/.
Code availability
We have used the standard tools developed for the data reduction and calibration of XMM-Newton observations, the XMM-Newton Science Analysis System (SAS) (https://www.cosmos.esa.int/web/xmm-newton/sas). We have used the version xmmsas_20201028_0905-19.0.0 of the SAS and followed standard procedures for extraction of spectra of point-like sources as described in https://www.cosmos.esa.int/web/xmm-newton/sas-threads. For science analysis, we applied the X-ray spectral fitting package XSPEC (https://heasarc.gsfc.nasa.gov/xanadu/xspec/) created by the NASA’s High Energy Astrophysics Science Archive Research Center (https://heasarc.gsfc.nasa.gov/). We have used the publicly available Python library Matplotlib for plotting.
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Acknowledgements
K.G.K. and M.G. acknowledge the support by the Austrian Research Promotion Agency (FFG) Project 873671 ‘SmileEarth’. D.K. acknowledges the support by the CNES. J.A.C is supported by Royal Society grant DHF\R1\211068. We are grateful to B. E. Wood for calculating the position angles of several astrospheres.
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K.G.K. conceived the original idea of the paper and performed the majority of data reduction and analysis. M.G. contributed equally with K.G.K. to data interpretation. D.K. contributed with expertise of SWCX emission analysis and modelling in the heliosphere and by extension to astrospheres. J.A.C. provided their expertise for analysis of extended sources and instrumental effects. C.M.L. provided the final insight into the data’s interpretation that helped to refine the mass-loss estimates and put them in context. S.B.S. contributed her expertise on current state-of-the-art of stellar wind modelling and observations. All authors contributed to the text.
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Extended data
Extended Data Fig. 1 Stellar spectra of stars without an astrospheric signal.
Stellar spectra of α Cen (obs ID 0760290301), Procyon (obs ID 0415580201), Prox Cen (obs ID 0801880501), and τ Cet (obs ID 0670380501) fitted with 3T vapec models, with the χ2 shown in the lower panels. The data are presented as mean values ± 1.64σ (90% confidence interval). The number of bins for statistics was 94 (Alpha Cen), 34 (Procyon), 63 (Prox Cen), and 18 (Tau Cet). The black error bars and the black histogram show the spectra and the fit, respectively, obtained with the PN camera, while the green and red error bars and histograms show the spectra and model fits from the MOS1 and MOS2 cameras, respectively.
Extended Data Fig. 2 Spectra of the annuli around the stars without an astrospheric signal.
Spectra of the annuli surrounding α Cen, Procyon, Prox Cen, and τ Cet, for the same observations shown in Extended Data Fig. 1.The data are presented as mean values ± 1.64σ (90% confidence interval). The number of bins for statistics are shown in Table 2. Only PN data are shown for clarity. The black error bars shown the data. The red solid lines shown the vapec+vapec+vapec model with an added Gaussian line at 0.56 keV. Only the red line is visible because the two models with and without the additional Gaussian line overlap thus indicating that no astrospheric CX signal has been detected. The models overlap because the best fit of the data is achieved for a Gaussian line with zero norm, which indicates that adding any additional flux around 0.56 keV in comparison to the one predicted by the stellar model only worsens the fit.
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Kislyakova, K.G., Güdel, M., Koutroumpa, D. et al. X-ray detection of astrospheres around three main-sequence stars and their mass-loss rates. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02222-x
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DOI: https://doi.org/10.1038/s41550-024-02222-x