500万年前疏散星团M37的色球和日冕活动：日冕剥离的证据？
Chromospheric and Coronal Activity in the 500-Myr-old Open Cluster M37: Evidence for Coronal Stripping?
摘要：
我们提出的光谱调查的结果，以表征色球活动，测量的H $α$发射，在低质量的成员的500万年的疏散星团M37。结合我们的新的测量H $α$光度（$L_{Hα}$）与以前编目的恒星属性，我们确定饱和和不饱和制度的依赖$L_{Hα}$的热光度比，$L_{Hα}/L_{bol}$，对罗斯比数$R_o$。Ro小于0.03 $\pm$ 0.01的所有旋转子收敛到$L_{Hα}/L_{bol}$ =（1.27 $\pm$ 0.02）x 10 $^{-4}$的活动水平。该饱和阈值（$R_{o,sat}$ = 0.03 $\pm$ 0.01）在统计学上小于大多数旋转-活动关系研究中发现的饱和阈值。在不饱和状态下，自转较慢的恒星色球层的活动水平较低，$L_{Hα}/L_{bol}$（$R_o$）遵循指数$β$ = -0.51 $\pm$ 0.02的幂律，比在毕宿星团和天鹰座恒星的$\approx$ 6.5亿年样本中发现的略浅。通过比较这种不饱和的行为，以前发现的日冕活动在M37（通过X射线光度，$L_X$测量），我们证实，色球活动衰减的速度比日冕活动增加$R_o$慢得多。虽然M37成员的$L_{Hα}$和$L_X$与两者的测量值的比较揭示了接近1：1的关系，但是通过比较$L_{Hα}/L_{bol}$和$L_X/L_{bol}$来去除质量依赖性并没有为这种关系提供明确的证据。最后，我们发现，我们的色球的$R_{o,sat}$小于我们的日冕活动指标（$R_{o,sat}$ = 0.03 $\pm$ 0.01对0.09 $\pm$ 0.01）。我们认为这可能是冠状剥离的证据。
英文摘要：
We present the results of a spectroscopic survey to characterize chromospheric activity, as measured by H$α$ emission, in low-mass members of the 500-Myr-old open cluster M37. Combining our new measurements of H$α$ luminosities ($L_{Hα}$) with previously cataloged stellar properties, we identify saturated and unsaturated regimes in the dependence of the $L_{Hα}$-to-bolometric-luminosity ratio, $L_{Hα}/L_{bol}$, on the Rossby number $R_o$. All rotators with Ro smaller than 0.03$\pm$0.01 converge to an activity level of $L_{Hα}/L_{bol}$ = (1.27$\pm$0.02) x 10$^{-4}$. This saturation threshold ($R_{o,sat}$ = 0.03$\pm$0.01) is statistically smaller than that found in most studies of the rotation-activity relation. In the unsaturated regime, slower rotators have lower levels of chromospheric activity, with $L_{Hα}/L_{bol}$($R_o$) following a power-law of index $β$ = -0.51$\pm$0.02, slightly shallower than the one found for a combined $\approx$650-Myr-old sample of Hyades and Praesepe stars. By comparing this unsaturated behavior to that previously found for coronal activity in M37 (as measured via the X-ray luminosity, $L_X$), we confirm that chromospheric activity decays at a much slower rate than coronal activity with increasing $R_o$. While a comparison of $L_{Hα}$ and $L_X$ for M37 members with measurements of both reveals a nearly 1:1 relation, removing the mass-dependencies by comparing instead $L_{Hα}/L_{bol}$ and $L_X/L_{bol}$ does not provide clear evidence for such a relation. Finally, we find that $R_{o,sat}$ is smaller for our chromospheric than for our coronal indicator of activity ($R_{o,sat}$ = 0.03$\pm$0.01 versus 0.09$\pm$0.01). We interpret this as possible evidence for coronal stripping.
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