发表机构
Université Côte d’Azur; Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université; French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and U. de Chile; Universidad de Concepción; Leibniz Institute for Astrophysics; Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences; European Southern Observatory（蔚蓝海岸大学; 巴黎天文台，巴黎文理研究大学，索邦大学，巴黎西岱大学，塞吉-蓬图瓦兹大学; 中法天文学联合实验室; 康塞普西翁大学; 莱布尼茨天体物理研究所; 波兰科学院尼古拉·哥白尼天文学中心; 欧洲南方天文台）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过首次比较五颗造父变星的同步光学与H波段径向速度，发现V波段振幅比H波段大5%，验证了投影因子几何部分与恒星大气模型预期一致。
AI 中文摘要
Baade-Wesselink方法虽然在理论上适用于通过造父变星校准距离尺度，但目前并未用于宇宙距离阶梯。该方法有多种版本。在所有这些方法中，用于将径向速度转换为脉动速度的投影因子仍然是限制性量。通过首次比较五颗造父变星的同步交叉相关光学和H波段径向速度，我们旨在更好地理解投影因子中与恒星临边昏暗直接相关的几何部分。这五颗造父变星是使用安装在伽利略国家望远镜上的HARPS-N和GIANO-B仪器以GIARPS配置观测的。对每颗恒星的同步径向速度测量应用了双参数线性拟合，以提取两条曲线的振幅比（fVH）以及平均速度之差。我们发现fVH的平均值为1.056，五颗恒星上的均方根为0.016，这意味着V波段径向速度振幅比H波段获得的大约5%。此外，我们发现H波段径向速度曲线平均（即γ速度）比可见光波段蓝移约1 km/s，除了一颗恒星X Cyg，其蓝移为0.4 km/s。在一阶近似下，fVH值直接与两个波段的临边昏暗系数之比（或等效地，几何投影因子之比）相关。发现fVH值与静态恒星大气模型的预期一致。这项工作表明，造父变星在可见光和H波段的差分临边昏暗，以及因此投影因子的几何部分，遵循恒星大气模型的预期。
英文摘要
The Baade-Wesselink method, while theoretically applicable for calibrating the distance scale via Cepheids, is not currently employed in the cosmic distance ladder. There are various versions of this method. In all these approaches, the projection factor used to convert the radial into the pulsation velocity remains the limiting quantity. By comparing for the first time simultaneous cross-correlated optical and H-band radial velocities of five Cepheids, we aim to better understand the geometric part of the projection factor that is directly linked to the limb darkening of the stars. The five Cepheids were observed with the HARPS-N and GIANO-B instruments mounted at the Telescopio Nazionale Galileo in the GIARPS configuration. A two-parameter linear fit was applied to the simultaneous radial velocity measurements of each star in order to extract the amplitude ratio of both curves (fVH) as well as the difference in mean velocities. We find a mean value of fVH of 1.056 with a root mean square over the five stars of 0.016, which means that the V-band radial velocity amplitudes are around 5% larger than the ones obtained in the H band. In addition, we find that the H-band radial velocity curves have an average (i.e., a gamma-velocity) of about 1 km/s blueshifted compared to the visible ones, except for one star, X Cyg, which has a blueshift of 0.4 km/s. At first order, the fVH values are directly linked to the ratio of limb darkening coefficients in both bands (or equivalently the ratio of geometrical projection factors). The fVH values are found to be consistent with expectations from static stellar atmosphere models. This work shows that the differential limb darkening of Cepheids in visible and H bands, and consequently the geometrical part of the projection factor, follows what is expected from stellar atmosphere models.
CommentsAccepted for publication in Astronomy & Astrophysics
