Deriving physical parameters of unresolved star clusters. VIII. Limits of aperture photometry for star cluster studies

K. Daugevicius, E. Krisciunas, Erikas Cicenas, Rima Stonkute, V. Vansevičius
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Abstract

Recently, it has been noticed that the discrepancies in the integrated colour indices (CIs) between star clusters and models are mostly due to the projection of bright stars in the apertures. In order to reduce this problem, the method of adaptive aperture photometry has been proposed. This method has been applied to star clusters from the M\,31 Panchromatic $Hubble$ Andromeda Treasury (PHAT) survey, and studies show that the adaptive aperture photometry performs better than the conventional approach. The aim of this study is to determine the best achievable limits on the accuracy and applicability of the aperture photometry method for studying star clusters in the local Universe. We computed a large network of artificial 3D star clusters spanning the parameter space of the M\,31 clusters. We then simulated images of these clusters by projecting each onto a 2D plane from 100 directions. Star cluster images were generated in six passbands to match the PHAT survey. To investigate the limiting accuracy of aperture photometry and the limits of its applicability to star cluster studies, we measured the simulated images and performed parameter determination tests. We demonstrate that star clusters with and without post-main-sequence stars have significant photometric differences. We show that in order to obtain reliable physical parameters of star clusters, the CIs must be measured using an aperture with a radius larger than the cluster’s half-light radius. Furthermore, we demonstrate that the parameter determination of young clusters (sim 10\,Myr) is problematic regardless of the aperture size used. Therefore, it is advisable to determine the parameters of these clusters using colour-magnitude diagram fitting methods, when possible. We also show that the randomness of the viewing angle can lead to a CI uncertainty of up to 0.1\,mag, depending on cluster parameters and aperture size.
推导未解决星团的物理参数。VIII.星团研究中孔径测光的局限性
最近,人们注意到星团和模型之间在综合颜色指数(CIs)上的差异主要是由于亮星在孔径中的投影造成的。为了减少这一问题,有人提出了自适应孔径光度测量法。该方法已被应用于M/31全色仙女座宝库(PHAT)巡天中的星团,研究表明自适应孔径测光法比传统方法表现更好。本研究的目的是确定自适应光圈测光法在研究本宇宙星团方面的精度和适用性的最佳可实现极限。我们计算了一个跨越M/31星团参数空间的大型人造三维星团网络。然后,我们从 100 个方向将每个星团投影到一个二维平面上,模拟出这些星团的图像。星团图像以六个通带生成,以匹配 PHAT 巡天观测。为了研究孔径光度法的极限精度及其在星团研究中的适用极限,我们测量了模拟图像,并进行了参数确定测试。我们证明,有后主序星和无后主序星的星团具有显著的光度差异。我们证明,为了获得星团的可靠物理参数,必须使用半径大于星团半光半径的光圈来测量 CIs。此外,我们还证明,无论使用多大的光圈,年轻星团(sim 10/,Myr)的参数测定都是有问题的。因此,在可能的情况下,最好使用色-星图拟合方法来确定这些星团的参数。我们还发现,视角的随机性会导致高达0.1(mag)的CI不确定性,这取决于星团参数和光圈大小。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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