Brown, A.

Buccino, A.P.

Cranmer, S.S.

France, K.

Froning, C.S.

Linsky, J.

Mauas, P.

Miguel, Y.

Pineda, J.S.

Rugheimer, S.

Vieytes, M.

Wheatley, P.J.

Wilson, D.J.

Wood, B.E.

Youngblood, A.

Leiden University Scholary Publications

The relative emission from chromospheres and coronae: dependenceon spectral type and ageLinsky, J.; Wood, B.E.; Youngblood, A.; Brown, A.; Froning, C.S.; France, K.; ... ; Wilson, D.J.CitationLinsky, J., Wood, B. E., Youngblood, A., Brown, A., Froning, C. S., France, K., … Wilson, D.J. (2021). The relative emission from chromospheres and coronae: dependence on spectraltype and age. The 20.5th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun(CS20.5) (p. 257). doi:10.5281/zenodo.4567598 Version: Publisher's VersionLicense: Creative Commons CC BY 4.0 licenseDownloaded from: https://hdl.handle.net/1887/3276002 Note: To cite this publication please use the final published version (if applicable).The Relative Emission from Chromospheres andCoronae: Dependence on Spectral Type and AgeJ. Linsky1, B. Wood, A. Youngblood, A. Brown, C. Froning, K. France, A. Buccino, S. Cranmer, P. Mauas, Y. Miguel, J. S. Pineda, S. Rugheimer, M. Vieytes,Peter Wheatley, and D. Wilson1JILA, University of Colorado and NIST, Boulder, CO 80309-0440.1 Perspective• Photochemistry in exoplanet atmospheres is drivenby UV emission from the host star’s chromosphere• X-ray and EUV radiation from a host star’s coronadrives escape from from exoplanet atmospheres.• These two topics would be related if there are sys-tematic trends in the relative emission from the chro-mospheres and coronae of host stars.2 Objectives• Does the ratio of chromospheric to coronal emission(flux ratios and ratios of luminosity to bolometric lu-minosity) depend on stellar parameters? If so, whichparameters?• What do these trends tell us about the relativeamount of magnetic heating in stellar chromospheresand coronae?3 Methods• X-ray fluxes observed by XMM-Newton, Chandra,and ROSAT measure the X-ray emission from stellarcoronae (T=1–10 MK).• Extreme ultraviolet (EUV) radiation (100-912 Å) canbe estimated from the X-ray flux or Lyman-α flux.• The hydrogen Lyman-α line (1216 Å) is the brightestUV emission line in G stars and represents about halfof the total UV emission from M stars. It is a goodtest of the total UV emission from chromospheres(T=4,000–20,000 K). Interstellar absorption has beenremoved to obtain the intrinsic stellar flux.• We analyzed spectra from all available (79) dwarfstars with both reconstructed Lyman-α fluxes and X-ray fluxes: 6 F stars, 18 G stars, 20 K stars, and 35 Mstars many of which have exoplanets.• The Lyman-α data are from various HST observ-ing programs including the MUSCLES and Mega-MUSCLES programs to obtain spectra of M stars.The X-ray fluxes are from the Chandra, XMM-Newton, and ROSAT missions.FIGURE 1: A plot of the reconstructed Lyman-α flux vs. X-ray flux at 1 au.Different symbols represent the fluxes of F, G, and K dwarf stars. The solidlines with different colors are least square fits to the stars in each spectraltype category. The names, rotational periods, and age of the most X-rayactive and least X-ray active K stars are identified. The ⊙ symbol is for theSun at low activity.4 Results: flux/flux comparisons1. F, G, and K stars follow similar trend lines in plots off(X-ray) vs f(Lyman-α). See Figure 1. Young activestars lie near the top of the trend lines and old inactivestars like the Sun lie near the bottom. As stars age,their rotation and activity decrease producing decentalong the same trend lines.2. M stars deviate from the FGK trend lines becausetheir Lyman-α flux ratios are smaller. See Figure 2.3. The relative decrease is Lyman-α emission from Mstars is also shown by plotting the Lyman-α fluxes di-vided by what they would be if M stars followed theFGK trend lines. See Figure 3.FIGURE 2: A plot of the reconstructed Lyman-α flux vs. X-ray flux at 1 aufor a diverse group of dwarf stars. Different symbols represent the fluxes ofdwarf stars of F, G, K, M1–M2, and M3–M6 spectral types. The solid lineswith different colors are least square fits to the stars in each spectral typecategory. The fluxes of cool M dwarfs are ploted with the star names andspectral types. The data for the metal-poor subdwarf Kapteyn’s star andthe very cool M8 V star TRAPPIST-1 (black) are not included in the leastsquares linear fits. The ⊙ symbol is for the Sun at low activity.FIGURE 3: Ratios of the reconstructed Lyman-α flux, fLyα , to the Lyman-α flux predicted if the M stars followed the G star trend line, fexp . Thestars are indentified and color coded red for M0 V to M2.5 V stars or bluefor M3 V to M8 V stars. The larger circles indicate later spectral type. Thehorizontal dashed line is where the M stars should lie if their Lyman-α fluxfollowed the G star trend line for their X-ray flux.FIGURE 4: Plot of L(X-ray)/L(bol) and L(Lyman-α)/L(bol) vs Teff for starsyounger than 450 Myr. The solid red line is a least squares linear fitL(Lyman-α)/L(bol data. The solid blue line is the 10−3 saturation level.The ages (in Myr) are given for four stars.5 Results: Comparing L(X-ray)/L(bol)with L(Lyman–α/L(bol)1. For stars younger than 450 Myr, L(Lyman-α)/L(bol)increases to cooler stars reaching saturation (10−3)near T=3200 K. See Figure 4. This indicates that anincreasing fraction of L(bol) is the energy source forchromospheric heating in the cooler stars.2. For stars younger than 450 Myr, L(X-ray)/L(bol) jumpsto saturation near T=5200 K. Ages in Myr are indi-cated. Thus for K and M stars, L(X-ray) exceedsL(Lyman-α but the difference decreases to coolerstars.3. For stars older than 4 Gyr, L(Lyman-α)/L(bol) islarger than L(X-ray)/L(bol), but the ratio decreasesfrom a factor of 100 at T=6,000 K to a factor of 3 nearT=2,400 K. See Figure 5. Thus, L(Lyman-α)/L(X-ray) decreases rapidly to cooler old stars,FIGURE 5: Plot of L(X-ray)/L(bol) and L(Lyman-α)/L(bol) vs Teff for starsolder than 4 Gyr. The solid lines are least squares linear fits to L(Lyman-α)/L(bol) (red) and L(X-ray)/L(bol) (blue) data. The circled symbols are forthe GJ 176.6 Conclusions• There are correlations between the high energy emis-sion from coronae that drives escape from exo-planet atmospheres and the UV emission from chro-mospheres that photo-dissociates molecules in exo-planet atmospheres.• The correlations depend on both age and Teff .• For dwarf stars of all ages, chromospheric emissionas measured by L(Lyman-α) increases systemati-cally as Teff decreases.• For young stars (t < 450 Myr), X-ray saturation oc-curs for stars with Teff < 5, 200 K. Thus for youngstars cooler than 5,200 K, the L(X-ray)/L(Lyman-α)ratio increases rapidly to the cooler stars.• Older stars (t > 4 Gyr) behave differently. L(X-ray)/L(Lyman-α) increases rapidly to the cooler stars.• These correlations provide important constraints ontheories of magnetic heating on stellar chromo-spheres and coronae.7 What does this mean?Emission from the corona and chromosphere requires mag-netic heating in these layers. Our result that for M stars theweakness of chromospheric vs coronal emission implies thatthe relative amount of heating at chromospheric temperaturesis decreasing towards the cooler stars. We can either call theM stars chromospheric weak or coronal strong compared tohotter stars. This result should stimulate theoretical studiesof magnetic heating in M dwarf stars. The change in internalstructure as stars become fully convective near spectral typeM3.5 V and thus do not have a tachocline may be important.This poster is a summary of the paper Linsky et al. (ApJ902, 3 (2020)). Also arXiv 2012.11738. The first author can becontacted at jlinsky@jila.colorado.edu. This work is supportedby grants HST-GO-12475, 12596, 13650, 14640, and 15071from STScI.3

The relative emission from chromospheres and coronae: dependence on spectral type and age

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The relative emission from chromospheres and coronae: dependence on spectral type and age

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