A map of 100° on a side extracted from Gaia DR2 and centered on Alpha Persei reveals two distinct structures—the Alpha Persei star cluster and a conspicuous stellar stream, as widely documented in recent literature. In this work we employ DBSCAN to assess individual stars’ membership and attempt at separating stars belonging to the cluster and to the stream from the general field. In turn, we characterize the stream and investigate its relation with the cluster. The stream population turned out to be significantly older (5 ± 1 Gyr) than the cluster, and to be positioned ∼90 pc away from the cluster, in its background. The stream exhibits a sizeable thickness of ∼180 pc in the direction of the line of view. Finally, the stream harbors a prominent population of white dwarf stars. We estimated an upper limit of the stream mass of ∼6000M ⊙. The stream would therefore be the leftover of a relatively massive old cluster. The surface density map of Alpha Persei indicates the presence of tidal tails. While it is tempting to ascribe their presence to the interaction with the disrupting old star cluster, we prefer to believe, conservatively, they are of Galactic origin.

Представлены результаты исследования широких (примерно 100 × × 100 •) окрестностей рассеянного скопления Альфа Персея (Melotte 20) по данным каталога Gaia DR2. В этой области выделены две структуры. Во-первых, собственно скопление с массой примерно 900 M и приливным радиусом 12.8 пк (4.2 •), имеющее протяженную корону с радиусом примерно 20 •. Во-вторых, «поток» или «волокно» протяженностью примерно 70 •. Этот поток расположен в основном несколько дальше скопления и частично перекрывается с ним. И скопление, и «поток» имеют структурные особенности, свидетельствующие об их приливном взаимодействии. Определены параметры вращения скопления. Обсуждаются различные гипотезы о происхождении «потока».

Due to numerous new discoveries of open star clusters in the last two decades, astronomers need an easy-touse resource to get visual information on the relative position of clusters in the sky. Therefore we propose a new atlas of open star clusters. It is based on a table compiled from the largest modern cluster catalogues. The atlas shows the positions and sizes of 3291 clusters and associations, and consists of two parts. The first contains 108 maps of 12 by 12 degrees with an overlapping of 2 degrees in three strips along the Galactic equator. The second one is an online web application, which shows a square field of an arbitrary size, either in equatorial coordinates or in galactic coordinates by request. The atlas is proposed for the sampling of clusters and cluster stars for further investigation. Another use is the identification of clusters among overdensities in stellar density maps or among stellar groups in images of the sky.

We propose the numerical method for determining the radius of a star cluster using its radial surface density profile. The method realizes the algorithm of an eye estimate but minimizes a subjectivity; its result is in a good agreement with the eye estimate of the radius for the open cluster NGC 2516.