Determination of dynamical ages of open clusters through the A+ parameter – I

Abstract

The sedimentation level of blue straggler stars (BSS) has been shown to be a great tool to investigate the dynamical states of globular clusters (GCs). The area enclosed between the cumulative radial distributions of BSS and a reference population up to the half-mass radius of the clusters, $A^+_{\mathrm{rh}}$, is known to be a measure of the sedimentation of BSS in GCs. In this work, we calculate $A^+_{\mathrm{rh}}$ for 11 open clusters (OCs) using a combined list of main-sequence turn-off stars, sub-giant branch stars, and red-giant branch stars as reference population. The BSS, the reference populations, and the cluster members are identified using the proper motions and parallaxes from the Gaia DR2 data. In a subset of clusters, the BSS are confirmed cluster members on the basis of radial velocity information available in the literature. Using the Pearson and Spearman rank correlation coefficients, we find weak correlations between the estimated values of $A^+_{\mathrm{rh}}$ and other markers of dynamical ages of the clusters, i.e. the number of central relaxations a cluster has experienced since its formation, and the structural parameters of the clusters. Based on statistical tests, we find that these correlations are similar to the corresponding correlations among the less evolved GCs, albeit within large errors.

UOCS VII. Blue Straggler Populations of Open Cluster NGC 7789 with UVIT/AstroSat

“…We detect all the 12 BSS members from Nine et al (2020) in the NUV filters N245M and N263M, but do not detect 2 BSS in the NUV filter N279N and 3 BSS in the F169M filter. In addition, we detect 4 BSS candidates from Rao et al (2021) in all the UVIT filters, that are marked on the Figure 1. The G-band magnitudes of the BSS range from ∼1 magnitude below the turnoff point to ∼2 magnitude above the turnoff point.…”

“…To characterize our UVIT detected sources further, we cross-matched them with the cluster members from Rao et al (2021) and with the cluster members from Nine et al (2020) by using a search radius of 1.5 for the NUV filters and 2 for the filter F169M. The information of the total number of members identified as counterparts in each UVIT filter is given in the Table 1.…”

“…Upon comparing our 16 BSS candidates with the BSS candidates identified by Rain, Ahumada, & Carraro (2021), we find 9 BSS to be common. 4 BSS candidates identified by Nine et al (2020) and 3 BSS candidates identified by Rao et al (2021) are missing in Rain, Ahumada, & Carraro (2021). Most of their remaining 7 BSS candidates are located beyond 28 from the cluster center, the radius of the cluster as per Rao et al (2021).…”

“…We first compiled the photometric data of these sources in NUV from GALEX (Martin et al 2005) Fitzpatrick (1999) and Indebetouw et al (2005) by providing the mean value of extinction A V = 0.868 to VOSA. This mean extinction in the V band was obtained from the mean value of A G of the Gaia DR2 cluster members (Rao et al 2021) using the correlation available between the extinctions from Wang & Chen (2019). VOSA calculates synthetic photometry for selected filters for the chosen theoretical models which are then compared with the extinction corrected values of the fluxes of the sources using a chi-square test.…”

“…The information of the total number of members identified as counterparts in each UVIT filter is given in the Table 1. The Figure 1 shows the Gaia DR2 colormagnitude diagram (CMD) of 1056 sources detected in the NUV filter N245M that are having a counterpart in the cluster members identified by Rao et al (2021). Among these, 497 sources are commonly identified as cluster members by Nine et al (2020).…”