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Variability of H$α$ chromospheric activity of solar-like stars revealed by the time-domain data of LAMOST Medium-Resolution Spectroscopic Survey

Han He, Ali Luo, Haotong Zhang, Song Wang

Abstract

The variability of H$α$ chromospheric activity of solar-like stars is investigated by using the time-domain data of LAMOST Medium-Resolution Spectroscopic Survey (MRS). We use $R_\mathrm{Hα}$ index (ratio of H$α$ luminosity to bolometric luminosity) to measure the H$α$ activity intensity of a spectrum, and utilize the median of the $R_\mathrm{Hα}$ values of multiple observations ($R_\mathrm{Hα}^\mathrm{median}$) as the representative activity intensity of a stellar source. The H$α$ variability of a stellar source is indicated by the extent of $R_\mathrm{Hα}$ fluctuation ($R_\mathrm{Hα}^\mathrm{EXT}$) of multiple observations. Our sample shows that $R_\mathrm{Hα}^\mathrm{EXT}$ of solar-like stars is about one order of magnitude smaller than $R_\mathrm{Hα}^\mathrm{median}$. The distribution of $\log R_\mathrm{Hα}^\mathrm{EXT}$ versus $\log R_\mathrm{Hα}^\mathrm{median}$ reveals the distinct behaviors between the stellar source categories with lower ($\log R_\mathrm{Hα}^\mathrm{median} < -4.85$) and higher ($\log R_\mathrm{Hα}^\mathrm{median} > -4.85$) activity intensity. For the former stellar source category, the top envelope of the distribution first increases and then decreases with $\log R_\mathrm{Hα}^\mathrm{median}$; while for the latter category, the top envelope of the distribution is largely along a positive correlation line. In addition, for the stellar sources with lower activity intensity, the large-$\log R_\mathrm{Hα}^\mathrm{EXT}$ objects near the top envelope of the $\log R_\mathrm{Hα}^\mathrm{EXT}$ versus $\log R_\mathrm{Hα}^\mathrm{median}$ distribution tend to have long-term and regular variations of H$α$ activity; while for the stellar sources with higher activity intensity, the H$α$ variations are more likely to be random fluctuations.

Variability of H$α$ chromospheric activity of solar-like stars revealed by the time-domain data of LAMOST Medium-Resolution Spectroscopic Survey

Abstract

The variability of H chromospheric activity of solar-like stars is investigated by using the time-domain data of LAMOST Medium-Resolution Spectroscopic Survey (MRS). We use index (ratio of H luminosity to bolometric luminosity) to measure the H activity intensity of a spectrum, and utilize the median of the values of multiple observations () as the representative activity intensity of a stellar source. The H variability of a stellar source is indicated by the extent of fluctuation () of multiple observations. Our sample shows that of solar-like stars is about one order of magnitude smaller than . The distribution of versus reveals the distinct behaviors between the stellar source categories with lower () and higher () activity intensity. For the former stellar source category, the top envelope of the distribution first increases and then decreases with ; while for the latter category, the top envelope of the distribution is largely along a positive correlation line. In addition, for the stellar sources with lower activity intensity, the large- objects near the top envelope of the versus distribution tend to have long-term and regular variations of H activity; while for the stellar sources with higher activity intensity, the H variations are more likely to be random fluctuations.

Paper Structure

This paper contains 34 sections, 6 equations, 19 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Time-domain data of LAMOST-MRS
  3. Sample selection
  4. Screening conditions for MRS spectra
  5. Solar-like condition
  6. Signal-to-noise ratio condition
  7. Removing low-quality spectra
  8. Removing spectra with large projected rotational velocities
  9. Removing spectra contaminated by the H II regions
  10. Screening conditions for stellar sources
  11. Time-domain condition
  12. Condition for consistency of stellar atmospheric parameters
  13. Condition for consistency of H$\alpha$ profiles
  14. Removing sources in the MRS catalog of close binaries
  15. Statistical properties of the selected MRS time-domain sample of solar-like stars
  16. ...and 19 more sections

Figures (19)

  • Figure 1: Example of the MRS time-domain spectra of H$\alpha$ line of solar-like stars. The four columns show the MRS time-domain H$\alpha$ spectra of four example stellar sources, respectively, with the activity levels of the stellar sources increasing from left to right. The uid and the number of MRS spectra of each stellar source are printed. The different spectra of the same stellar source are displayed in a variety of colors for ease of distinction. In each column, the top row shows the MRS H$\alpha$ spectra normalized by the continuum, and the bottom row shows the relative flux of the H$\alpha$ spectra with respect to the reference spectrum (the spectrum with the lowest H$\alpha$ central flux; displayed in gray). The wavelength shift caused by radial velocity is corrected. The time spans of the time-domain spectra of the four stellar sources are all greater than 1,000 days. The two dash-dot lines in each panel enclose a 3 Å-wide wavelength range around the H$\alpha$ line center, within which the variations of the H$\alpha$ profiles caused by stellar activity mainly happen.
  • Figure 2: Observing timeline for all the spectra in the MRS time-domain sample of solar-like stars employed in this work. The bottom axis shows the observation time using the modified Julian day ($\mathrm{MJD} = \mathrm{JD} - 2400000.5$), and the top axis shows the UTC time of the observations. The vertical dotted lines (corresponding to August 1st 00:00 UTC of each calendar year) separate the five observing years of MRS. The width of each bin in the diagram is 25 days.
  • Figure 3: Distribution histogram of $N_\mathrm{obs}$ for all the stellar sources in the MRS time-domain sample of solar-like stars employed in this work.
  • Figure 4: Distribution histogram of $T_\mathrm{span}$ for all the stellar sources in the MRS time-domain sample of solar-like stars employed in this work. The vertical dashed line corresponds to $T_\mathrm{span} = 200$ days, which divides the full sample of stellar sources into two groups ($T_\mathrm{span} < 200$ days and $T_\mathrm{span} > 200$ days).
  • Figure 5: Distribution of $R_\mathrm{H\alpha}$ index with stellar effective temperature for (a) the spectra and (b) the stellar sources (using $R_\mathrm{H\alpha}^\mathrm{median}$) in the MRS time-domain sample of solar-like stars employed in this work. In panel (a), the $R_\mathrm{H\alpha}$ values from the multiple observations of the same stellar source are connected by a vertical line segment, and the different line segments (stellar sources) are displayed in a variety of colors for ease of distinction. The error bar ($\pm \delta R_\mathrm{H\alpha}$) in the bottom right corner of each panel shows the median of the uncertainties of the $R_\mathrm{H\alpha}$ values (about $1.4 \times 10^{-7}$).
  • ...and 14 more figures