经典造父变星的脉动色球。钙红外三重态和H $α$谱的变化
Pulsating chromosphere of classical Cepheids. Calcium infrared triplet and H$α$ profile variations
摘要：
最近的研究表明，造父变星的红外辐射在脉动周期内是恒定的，这可能是由于一个约为恒星半径15%的电离气体脉动壳层造成的，这可能归因于造父变星的色球活动。本文的目的是研究造父变星色球层沿脉动周期的动力学结构，并对其大小进行量化。我们目前H $α$和钙近红外三重态（Ca IR）配置文件的变化，使用高分辨率光谱与UVES光谱仪的一个良好的周期覆盖范围从$\approx$ 3至60天的24造父变星的样本。在对谱线轮廓进行定性分析后，我们将范霍夫效应（H $α$和Ca IR之间的速度梯度）量化为造父变星周期的函数。然后，我们使用史瓦西机制（由于冲击波的线加倍）来量化色球的大小。我们发现周期大于$P=10$天的造父变星存在显著的Van Hoof效应，特别是H $α$线的速度梯度比Ca IR延迟达$Δv \approx$ 30 km/s.我们发现长周期造父变星色球的大小至少是恒星半径的$\approx$ 50%，这与先前从红外过量分析中发现的由电离气体制成的壳的尺寸一级一致。最后，对于大多数的长周期造父变星的样本中，我们报告了一个不动的吸收功能的H $α$线，我们属性的拱星信封，围绕色球。分析造父变星的Ca~IR谱线对于消除周期光度关系中红外过剩的偏差具有重要意义，特别是在\textit{Gaia}上的径向速度谱仪（RVS）即将进行的观测中，它可能对色球层敏感。
英文摘要：
It has been shown recently that the infrared emission of Cepheids, which is constant over the pulsation cycle, might be due to a pulsating shell of ionized gas of about 15\% of the stellar radius, which could be attributed to the chromospheric activity of Cepheids. The aim of this paper is to investigate the dynamical structure of the chromosphere of Cepheids along the pulsation cycle and quantify its size. We present H$α$ and Calcium Near InfraRed triplet (Ca IR) profile variations using high-resolution spectroscopy with the UVES spectrograph of a sample of 24 Cepheids with a good period coverage from $\approx$ 3 to 60 days. After a qualitative analysis of the spectral lines profiles, we quantify the Van Hoof effect (velocity gradient between the H$α$ and Ca IR) as a function of the period of the Cepheids. Then, we use the Schwarzschild mechanism (a line doubling due to a shock wave) to quantify the size of the chromosphere. We find a significant Van Hoof effect for Cepheids with period larger than $P=10$ days, in particular H$α$ lines are delayed with a velocity gradient up to $Δv \approx$30 km/s compared to Ca IR. We find that the size of the chromosphere of long-period Cepheids is of at least $\approx$ 50\% of the stellar radius, which is consistent at first order with the size of the shell made of ionized gas previously found from the analysis of infrared excess. Last, for most of the long-period Cepheids in the sample, we report a motionless absorption feature in the H$α$ line that we attribute to a circumstellar envelope that surrounds the chromosphere. Analyzing the Ca~IR lines of Cepheids is of importance to potentially unbias the period-luminosity relation from their infrared excess, particularly in the context of forthcoming observations from the Radial Velocity Spectrometer (RVS) on board \textit{Gaia}, that could be sensitive to their chromosphere.
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