NGC 1647: A young open cluster with a broad main sequence observed with LAMOST

¹ INAF – Osservatorio Astrofisico di Catania, via S. Sofia, 78, 95123 Catania, Italy
² Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, PR China
³ School of Physics and Astronomy, Beijing Normal University, Beijing 100875, PR China
⁴ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁵ INAF – Osservatorio di Astrofisica e Fisica dello Spazio, via P. Gobetti 93/3, 40129 Bologna, Italy

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 February 2026
Accepted: 7 April 2026

Abstract

Aims. In this work we present the results of our analysis of medium-resolution LAMOST spectra of candidate members of the cluster NGC 1647 with the aim of determining the stellar parameters, activity level, lithium abundance, and to study the cluster properties.

Methods. We used the code ROTFIT to determine the atmospheric parameters (T_eff, log g, and [Fe/H]), radial velocity (V_r), and projected rotation velocity (v sin i). Moreover, for solar-type and cooler stars (T_eff≤ 6500 K), we calculated the Hα and Li Iλ6708 net equivalent width by means of the subtraction of non-active photospheric templates. We determined the rotation periods for 160 stars by analyzing the available TESS photometry.

Results. We derived V_r, v sin i, and atmospheric parameters for 341 spectra of 155 stars, plus three additional bright targets with archival UVES spectra. Moreover, we found four double-lined spectroscopic systems for which we provide the radial velocities of the two components. The V_r distribution of the cluster members peaks at −5.3 km s⁻¹ with a dispersion of 1.6 km s⁻¹, while the average metallicity is [Fe/H] = −0.08±0.08 dex, in line with previous determinations, which were based on only a handful stars. From the fitting of the spectral energy distribution of 160 likely members we infer the existence of a differential reddening across the cluster field with an average value of A_V = 1.1 mag. The A_V values show a distinct correlation with the color offset from the lower boundary of the main sequence, as observed in the Gaia color–magnitude diagram; conversely, this offset appears to be uncorrelated with v sin i. These two findings confirm that differential reddening is the primary driver behind the observed extended main-sequence turnoff (eMSTO) in this cluster. The age of NGC 1647, obtained from the lithium abundance, is 203±27 Myr, which is compatible with the values inferred from a gyrochronological approach and the isochrone fitting.