The average X-ray spectrum of the volume-complete M-, F-, G-, and K-type star sample within 10 pc of the Sun

Abstract

F, G, K and M type stars are the most abundant stellar population in the Milky Way and are expected to contribute to its diffuse X-ray emission. Yet their intrinsic average X-ray spectrum remains poorly constrained due to their faint X-ray luminosities, leaving their collective role in the X-ray background of the Milky Way uncertain. We analysed the volume-complete sample of M- (M0--M6) and FGK-type stars within 10 pc of the Sun using data from eROSITA all-sky survey aboard the Spectrum-Roentgen-Gamma (SRG) mission (eRASS:4). Individual spectra were normalized by exposure and distance and stacked to produce representative averages. The distance-normalized emission measures yield an average X-ray luminosity of $(2.6 \pm0.1)\times 10^{27}$ erg/s for M-type stars, and $(15\pm3)\times 10^{27}$ erg/s for F, G and K-type stars in 0.2--2.0 keV. The average spectra could be well described by a sum of three and two thermal models. Fitted temperatures and abundances remain consistent across M-star subgroups, while early-M stars are surprisingly on average less luminous than mid/late-M types. These results offer new insights into the collective X-ray properties of nearby stars, and provide motivation to explore the link with the unresolved soft X-ray background of the Galaxy.

Figures (13)

• Figure 1: Left: X-ray Luminosity (0.1--2.4 keV) versus Gaia$G_{\mathrm{BP}}$-$G_{\mathrm{RP}}$ colour for the eRASS1-detected M and FGK-type star Merloni2024AA in the western Galactic hemisphere of 10pc- Gaia sample. Filled blue circles: eRASS1 detected M dwarfs ($l \geq 180^\circ$); filled red circles: eRASS1 detected FGK stars ($l \geq 180^\circ$). Notable sources are labeled. Right: eRASS:4 total observed counts in 0.2--2.0 keV versus the distance from the Sun for the 10pc- Gaia sample in western Galactic hemisphere, with detections and non-detections for M dwarfs (blue, cyan) and FGK stars (brown, coral).
• Figure 2: The histogram of the Gaia G apparent (G, filled bars) and absolute ($M_{G}$, outlined bars) magnitudes of the M-type (blue) and FGK-type (red) stars used in this study. The G mag and parallax is taken from Reyle2021AA. Five stars with Gaia saturated magnitudes is not shown here. The vertical dashed line marks the optical loading threshold adpoted in this work is (G >5; J. Robrade, priv. comm.).
• Figure 3: Comparison of the effective area of TM8. The box-plot represents the statistical distribution of the mean effective area over the 0.2--2.0 keV range. The solid horizontal line within each box indicates the average value, while the dashed horizontal line (in the zoomed-in plot) represents the median. The sample includes all 103 M stars and 30 FGK stars used in this study.
• Figure 4: Left panel: Averaged 10 pc M dwarf spectrum with different models. All the models are labeled in the legend, where names follow the XSPEC convention, and the letter G stands for the Gaussian components to fit single emission lines (dotted lines). Right panel: temperature vs. line ratio assuming a single thermal component. The dark red and navy lines show the expected $\textup{O\,viii}/\textup{O\,vii}$ and $\textup{Ne\,x}/\textup{Ne\,ix}$ ratios from AtomDB. Vertical dashed and dotted lines mark ratios derived from Gaussian normalizations in Table \ref{['tab:Mfit']}.
• Figure 5: Model fitting of the averaged X-ray spectra (0.2--2.0 keV) of 103 M dwarf stars using multi-temperature thermal plasma models. The top-left and top-right panels show comparisons between 2-temperature and 3-temperature APEC models (labeled as 2APEC and 3T-APEC), and as variable-abundance counterparts (2T-VAPEC and 3T-VAPEC). The corresponding residuals are also shown. The bottom-left panel displays the spectral fits for 2T-VAPEC and 3T-VAPEC models, highlighting differences in spectral shape and emission features. The bottom-right panel presents a comparative view of residuals amongst all four models to assess the fit quality. Residuals are shown in standard deviations (in sigmas).