Spectroscopic membership for the populous 300 Myr-old open cluster NGC 3532

NGC 3532 is an extremely rich open cluster embedded in the Galactic disc, hitherto lacking a comprehensive, documented membership list. We provide membership probabilities from new radial velocity observations of solar-type and low-mass stars in NGC 3532, in part as a prelude to a subsequent study of stellar rotation in the cluster. Using extant optical and infra-red photometry we constructed a preliminary photometric membership catalogue, consisting of 2230 dwarf and turn-off stars. We selected 1060 of these for observation with the AAOmega spectrograph at the Anglo-Australian Telescope and 391 stars for observations with the Hydra-South spectrograph at the Victor Blanco Telescope, obtaining spectroscopic observations over a decade for 145 stars. We measured radial velocities for our targets through cross-correlation with model spectra and standard stars, and supplemented them with radial velocities for 433 additional stars from the literature. We also measured log g, Teff, and [Fe/H] from the AAOmega spectra. Together with proper motions from Gaia DR2 we find 660 exclusive members. The members are distributed across the whole cluster sequence, from giant stars to M dwarfs, making NGC 3532 one of the richest Galactic open clusters known to date, on par with the Pleiades. From further spectroscopic analysis of 153 dwarf members we find the metallicity to be marginally sub-solar, with [Fe/H]=-0.07. Exploiting trigonometric parallax measurements from Gaia DR2 we find a distance of $484^{+35}_{-30}$ pc. Based on the membership we provide an empirical cluster sequence in multiple photometric passbands. A comparison of the photometry of the measured cluster members with several recent model isochrones enables us to confirm the 300 Myr cluster age. However, all of the models evince departures from the cluster sequence in particular regions, especially in the lower mass range. (abridged)Comment: Accepted for publication in A&A. 19 pages, 18 Figures, and 6 Table

Long-term photometry of IC 348 with the Young Exoplanet Transit Initiative network

We present long-term photometric observations of the young open cluster IC 348 with a baseline time-scale of 2.4 yr. Our study was conducted with several telescopes from the Young Exoplanet Transit Initiative (YETI) network in the Bessel R band to find periodic variability of young stars. We identified 87 stars in IC 348 to be periodically variable; 33 of them were unreported before. Additionally, we detected 61 periodic non-members of which 41 are new discoveries. Our wide field of view was the key to those numerous newly found variable stars. The distribution of rotation periods in IC 348 has always been of special interest. We investigate it further with our newly detected periods but we cannot find a statistically significant bimodality. We also report the detection of a close eclipsing binary in IC 348 composed of a low-mass stellar component (M ≳ 0.09 M⊙) and a K0 pre-main-sequence star (M ≈ 2.7 M⊙). Furthermore, we discovered three detached binaries among the background stars in our field of view and confirmed the period of a fourth one

The rotation period distribution of the rich Pleiades-age southern open cluster NGC 2516

Aims. We wish to measure the cool star rotation period distribution for the Pleiades-age rich open cluster NGC 2516 and use it to determine whether cluster-to-cluster variations exist in otherwise identical open clusters. Methods. We obtained 42 d-long time-series CCD photometry of NGC 2516 in the V and Ic filters using the Yale 1 m telescope at CTIO and performed a number of related analyses, including PSF-based time-series photometry. Our data are complemented with additional information from several photometric datasets, literature radial velocities, and Gaia DR2 astrometry. All available data are used to construct an integrated membership list for NGC 2516, containing 844 stars in our ≈1° field of view. Results. We derived 308 rotation periods for late-F to mid-M cluster members from our photometry. We identified an additional 247 periodic M dwarf stars from a prior study as cluster members, and used these to construct a 555-star rotation period distribution for NGC 2516. The colour-period diagram (in multiple colours) has almost no outliers and exhibits the anticipated triangular shape, with a diagonal slow rotator sequence that is preferentially occupied by the warmer stars along with a flat fast rotator sequence that is preferentially populated by the cooler cluster members. We also find a group of extremely slowly rotating M dwarfs (10 d ≲ Prot ≲ 23 d), forming a branch in the colour-period diagram which we call the “extended slow rotator sequence”. This, and other features of the rotational distribution can also be found in the Pleiades, making the colour-period diagrams of the two clusters nearly indistinguishable. A comparison with the well-studied (and similarly aged) open cluster M 35 indicates that the cluster’s rotational distribution is also similarly indistinguishable from that of NGC 2516. Those for the open clusters M 50 and Blanco 1 are similar, but data issues for those clusters make the comparisons somewhat more ambiguous. Nevertheless, we demonstrate the existence of a representative zero-age main sequence rotational distribution and provide a simple colour-independent way to represent it. We perform a detailed comparison of the NGC 2516 rotation period data with a number of recent rotational evolution models. Using X-ray data from the literature, we also construct the first rotation-activity diagram for solar-type stars in NGC 2516, one that we find is essentially indistinguishable from those for the Pleiades and Blanco 1. Conclusions. The two clusters NGC 2516 and Pleiades can be considered twins in terms of stellar rotation and related properties (and M 35, M 50, and Blanco 1 are similar), suggesting that otherwise identical open clusters also have intrinsically similar cool star rotation and activity distributions

Rotation periods for cool stars in the open cluster NGC 3532

Context. A very rich cluster intermediate in age between the Pleiades (150 Myr) and the Hyades (600 Myr) is needed to probe the rotational evolution, especially the transition between fast and slow rotation that occurs between the two ages. Aims. We study the rich 300 Myr-old open cluster NGC 3532 to probe this important transition and to provide constraints on angular momentum loss. Measuring the rotation periods builds on our prior work of providing spectroscopic membership information for the cluster, and it supports the chromospheric activity measurements of cluster stars that we provide in a companion paper. Methods. Using 42 d-long photometric time series observations obtained with the Yale 1 m telescope at CTIO, we measured rotation periods for members of NGC 3532 and compared them with the predictions of angular momentum evolution models. Results. We directly measured 176 photometric rotation periods for the cluster members. An additional 113 photometric rotation periods were identified using activity information, described fully in the companion paper, resulting in a total sample containing 279 rotation periods for FGKM stars in NGC 3532. The colour-period diagram constructed from this rich data set shows a well-populated and structured slow rotator sequence, and a fast rotator sequence evolved beyond zero-age main sequence age whose stars are in transition from fast to slow rotation. The slow rotator sequence itself is split into slightly slower and faster rotators, a feature we trace to photometric binary status. We also identify an extended slow rotator sequence extending to P ∼ 32 d, apparently the analogue of the one we previously identified in NGC 2516. We compare our period distribution to rotational isochrones in colour-period space and find that all considered models have certain shortcomings. Using more detailed spin-down models, we evolve the rotation periods of the younger NGC 2516 forward in time and find that the spindown of the models is too aggressive with respect to the slow rotators. In contrast, stars on the evolved fast rotator sequence are not spun down strongly enough by these models. Our observations suggest a shorter crossing time for the rotational gap, one we estimate to be ∼80 Myr for early-K dwarfs

A detailed understanding of the rotation-activity relationship using the 300 Myr old open cluster NGC 3532

Context. The coeval stars of young open clusters provide insights into the formation of the rotation-activity relationship that elude studies of multi-age field populations. Aims. We measure the chromospheric activity of cool stars in the 300 Myr old open cluster NGC 3532 in concert with their rotation periods to study the mass-dependent morphology of activity for this transitional coeval population. Methods. Using multi-object spectra of the Ca I

A rotational age for the open cluster NGC 2281

Context. Cool star rotation periods have become an important tool in determining ages of open clusters. Aims. We aim to estimate the age of the open cluster NGC 2281 based on the rotational properties of its low-mass members. Previous age estimates for this open cluster range from 275 Myr to 630 Myr. Methods. Based on an eight month-long photometric time series obtained at the 1.2 m robotic STELLA telescope in Tenerife, we measured rotation periods for 126 cool star members (70% of the observed members) of NGC 2281. Results. The large set of rotation periods allows us to construct a rich colour-period diagram for NGC 2281 with very few outliers above the slow rotator sequence. We identify an evolved fast rotator sequence which can be used to accurately age date the open cluster relative to other open clusters. Comparisons with M 37 and M 48 show that all three open clusters are roughly coeval, and we estimate the age of NGC 2281 to be 435 ± 50 Myr. Through comparisons with the younger NGC 3532 and the older Praesepe, we determine the spin down rates of mid-K and early-M fast rotators to be significantly lower than for early-K stars. We suspect that the spin down of early-K fast rotators might be governed by an additional mass dependence. Conclusions. Finally, we show the path towards an empirical description of the evolved fast rotator sequences in open clusters