Relation of X-ray activity and rotation in M dwarfs and predicted time-evolution of the X-ray luminosity. (arXiv:2004.02904v1 [astro-ph.SR])

Relation of X-ray activity and rotation in M dwarfs and predicted time-evolution of the X-ray luminosity. (arXiv:2004.02904v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Magaudda_E/0/1/0/all/0/1">E. Magaudda</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stelzer_B/0/1/0/all/0/1">B. Stelzer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Covey_K/0/1/0/all/0/1">K. R. Covey</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Raetz_S/0/1/0/all/0/1">St. Raetz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Matt_S/0/1/0/all/0/1">S. P. Matt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Scholz_A/0/1/0/all/0/1">A.Scholz</a>

We present a sample of 14 M dwarf stars observed with XMM-Newton and Chandra,
for which we also computed rotational periods from Kepler Two-Wheel (K2)
Mission light curves. We compiled X-ray and rotation data from the literature
and homogenized all data sets to provide the largest uniform sample of M dwarfs
(302 stars) for X-ray activity and rotation studies to date. We then fit the
relation between $L_{rm x} – P_{rm rot}$ using three different mass bins to
separate partially and fully convective stars. We found a steeper slope in the
unsaturated regime for fully convective stars and a nonconstant $L_{rm x}$
level in the saturated regime for all masses. In the $L_{rm x}/L_{rm
bol}-R_{rm O}$ space we discovered a remarkable double gap that might be
related to a discontinuous period evolution. Then we combined the evolution of
$P_{rm rot}$ predicted by angular momentum evolution models with our new
results on the empirical $L_{rm x} – P_{rm rot}$ relation to provide an
estimate for the age decay of X-ray luminosity. We compare predictions of this
relationship with the actual X-ray luminosities of M stars with known ages from
100 Myr to a few billion years. We find remarkably good agreement between the
predicted $L_{rm x}$ and the observed values for partially convective stars.
However, for fully convective stars at ages of a few billion years, the
constructed $L_{rm x}-$age relation overpredicts the X-ray luminosity because
the angular momentum evolution model underpredicts the rotation period of these
stars. Finally, we examine the effect of different parameterizations for the
Rossby number ($R_{rm O}$) on the shape of the activity-rotation relation in
$L_{rm x}/L_{rm bol}-R_{rm O}$ space, and we find that the slope in the
unsaturated regime and the location of the break point of the dual power-law
depend sensitively on the choice of $R_{rm O}$.

We present a sample of 14 M dwarf stars observed with XMM-Newton and Chandra,
for which we also computed rotational periods from Kepler Two-Wheel (K2)
Mission light curves. We compiled X-ray and rotation data from the literature
and homogenized all data sets to provide the largest uniform sample of M dwarfs
(302 stars) for X-ray activity and rotation studies to date. We then fit the
relation between $L_{rm x} – P_{rm rot}$ using three different mass bins to
separate partially and fully convective stars. We found a steeper slope in the
unsaturated regime for fully convective stars and a nonconstant $L_{rm x}$
level in the saturated regime for all masses. In the $L_{rm x}/L_{rm
bol}-R_{rm O}$ space we discovered a remarkable double gap that might be
related to a discontinuous period evolution. Then we combined the evolution of
$P_{rm rot}$ predicted by angular momentum evolution models with our new
results on the empirical $L_{rm x} – P_{rm rot}$ relation to provide an
estimate for the age decay of X-ray luminosity. We compare predictions of this
relationship with the actual X-ray luminosities of M stars with known ages from
100 Myr to a few billion years. We find remarkably good agreement between the
predicted $L_{rm x}$ and the observed values for partially convective stars.
However, for fully convective stars at ages of a few billion years, the
constructed $L_{rm x}-$age relation overpredicts the X-ray luminosity because
the angular momentum evolution model underpredicts the rotation period of these
stars. Finally, we examine the effect of different parameterizations for the
Rossby number ($R_{rm O}$) on the shape of the activity-rotation relation in
$L_{rm x}/L_{rm bol}-R_{rm O}$ space, and we find that the slope in the
unsaturated regime and the location of the break point of the dual power-law
depend sensitively on the choice of $R_{rm O}$.

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