Correlations of Coronal X-Ray Emission with Activity, Mass, and Age of the Nearby K and M Dwarfs

Fleming, T. A.; Schmitt, J. H. M. M.; Giampapa, M. S.

doi:10.1086/176150

Cited by 133 publications

(138 citation statements)

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“…Our initial results suggest that increased activity in the most active M dwarfs manifests itself as a stronger magnetic field, rather than a higher filling factor, in contrast to the behavior seen for less active stars with earlier spectral types. ͉B͉ f is slightly higher on EV Lac than on Gliese 729, which is as expected, since EV Lac rotates more rapidly than Gliese 729, and has a larger X-ray flux (Fleming et al 1995) and H␣ equivalent width (seen in our data). We have used the Zeeman-sensitive Fe I line at 8468.40 Å to measure magnetic fields on the active M dwarfs EV Lac and Gliese 729.…”

Section: Discussionsupporting

confidence: 87%

Detection of Strong Magnetic Fields on M Dwarfs

Johns–Krull

Valenti

1996

The Astrophysical Journal

232

242

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We have detected Zeeman splitting of the Fe I line ( g eff ϭ 2.5) at 8468.40 Å in the very active M4.5 Ve stars Gliese 729 and Gliese 873 (EV Lac). High-resolution (R ϭ 120,000), low-noise (10.5%) spectra show clear Zeeman-shifted components from which we infer field strengths of 2-4 kG, independent of uncertainties in model atmospheres. Similar observations of a sequence (M0 V-M5 V ) of low-activity M dwarfs demonstrate that the wing components in the 8468.40 Å line are not due to the ubiquitous TiO lines in the vicinity. This strongly suggests that discrepancies in the shape of the magnetically sensitive iron line are due to magnetic fields rather than differences in photospheric temperature. By fitting the ratio of active to inactive line profiles with simple one-component models, we estimate that 50 H 13% of the photosphere of EV Lac is covered by 3.8 H 0.5 kG magnetic fields, while 50 H 13% of Gliese 729 is covered by 2.6 H 0.3 kG fields. This confirms earlier reports of large magnetic fields on M dwarf flare stars. We see evidence for a distribution of magnetic field strength, spatially across the surface and/or with depth. The twofold increase in field strength relative to G and K dwarfs is predicted by flux tube equilibrium arguments, but the 1 kG difference in the measured field strengths for these two stars suggests that field strengths may also grow with activity as the photospheric filling factor approaches unity.

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Section: Discussionsupporting

confidence: 87%

Detection of Strong Magnetic Fields on M Dwarfs

Johns–Krull

Valenti

1996

The Astrophysical Journal

232

242

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“…Our goal is to establish the gross X-ray spectral properties of nearby post-T Tauri stars. Such data, when compared with similar results already available for T Tauri stars embedded in molecular clouds (e.g., Neuhauser et al 1995) and for main-sequence field stars (e.g., Fleming, Schmitt, & Giampapa 1995), should offer clues to the evolutionary status of stars in nearby associations and, more generally, to the mechanism(s) responsible for the bright X-ray emission that appears ubiquitous among young (age 100 Myr), solar-mass stars. Here we present an analysis of ROSAT Position Sensitive Proportional Counter (PSPC) data available for the known members of the TWA, the TA, and the bPMG.…”

Section: Introductionsupporting

confidence: 77%

“…The last sample consists of the K and M dwarfs studied by Fleming et al (1995;D < 7 pc, agese1 Gyr) and all single F and G stars within 14 pc for which data are available in the BSC. 5 Aside from 1 UMa, which likely is a member of the Ursa Major moving group (age 300 Myr; e.g., Messina & Guinan 2002), the stars in the nearby F-G sample likely have ages of at least 2 Gyr (e.g., Habing et al 2001).…”

Section: Comparison With T Tauri Stars the Hyades And Fieldmentioning

confidence: 99%

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ROSATX‐Ray Spectral Properties of Nearby Young Associations: TW Hydrae, Tucana‐Horologium, and the β Pictoris Moving Group

Kastner

Crigger

Rich

et al. 2003

ApJ

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We present archival ROSAT data for three recently identified, nearby (D < 70 pc), young ($10-40 Myr) stellar associations: the TW Hydrae association, the Tucana-Horologium association, and the Pictoris moving group. The distributions of ROSAT X-ray hardness ratios (HR1, HR2) for these three groups, whose membership is dominated by low-mass, weak-lined T Tauri stars, are tightly clustered and very similar to one another. The value of HR1 for TW Hya itself-the only bona fide classical T Tauri star in any of the nearby groups-is clearly anomalous among these nearby young stars. We compare the hardness ratio distributions of stars in the three nearby groups with those of T Tauri stars, the Hyades, and main-sequence dwarfs in the field. This comparison demonstrates that the X-ray spectra of F through M stars soften with age and that F and G stars evolve more rapidly in X-ray spectral hardness than do K and M stars. It is as yet unclear whether this trend can be attributed to age-dependent changes in the intrinsic X-ray spectra of stars of type F and later, to a decrease in the column density of circumstellar gas (e.g., in residual protoplanetary disks), or to the diminishing contributions of star-disk interactions to X-ray emission. Regardless, these results demonstrate that analysis of archival ROSAT X-ray spectral data can help both to identify nearby, young associations and to ascertain the X-ray emission properties of members of known associations.

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“…In spite of these theoretical predictions X-ray and Ho activity has been observed on stars with masses below the fully convective boundary, corresponding to spectral type rv M3 (Fleming et al 1995, Gizis et al 2000. Recent Ho observations of ultracool field dwarfs indicate, however, a decline of activity setting in near the substellar limit at spectral type M9 (Mohanty & Basri 2002, see also Basri this volume).…”

Section: Introductionmentioning

confidence: 99%