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Register a new userBlue Straggler Stars in Galactic Open Clusters and the effect of field star contamination
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Giovanni Carraro dr
Publication date
2008
fields
Physics
and research's language is
English
Authors
G. Carraro
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We investigate the distribution of Blue Straggler stars in the field of three open star clusters. The main purpose is to highlight the crucial role played by general Galactic disk fore-/back-ground field stars, which are often located in the same region of the Color Magnitude Diagram as Blue Straggler stars. We analyze photometry taken from the literature of 3 open clusters of intermediate/old age rich in Blue Straggler stars, and which are projected in the direction of the Perseus arm, and study their spatial distribution and the Color Magnitude Diagram. As expected, we find that a large portion of the Blue Straggler population in these clusters are simply young field stars belonging to the spiral arm. This result has important consequences on the theories of the formation and statistics of Blue Straggler stars in different population environments: open clusters, globular clusters or dwarf galaxies. As previously emphasized by many authors, a detailed membership analysis is mandatory before comparing the Blue Straggler population in star clusters against theoretical models. Moreover, these sequences of young field stars (blue plumes) are potentially powerful tracers of Galactic structure which require further consideration.
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Aims: This paper presents a new homogeneous catalogue of blue straggler stars (BSS) in Galactic open clusters. Methods: Photometric data for 216 clusters were collected from the literature and 2782 BSS candidates were extracted from 76 of them. Results: We found that the anticorrelation of BSS frequency vs. total magnitude identified in similar studies conducted on Galactic globular clusters extends to the open cluster regime: clusters with smaller total magnitude tend to have higher BSS frequencies. Moreover, a clear correlation between the BSS frequency (obtained normalising the total number of BSS either to the total cluster mass or, for the older clusters, to the total number of clump stars) and the age of the clusters was found. A simple model is developed here to try to explain this last and new result. The model allows us to ascertain the important effect played by mass loss in the evolution of open clusters.
Blue straggler stars are exotic objects present in all stellar environments whose nature and formation channels are still partially unclear. They seem to be particularly abundant in open clusters (OCs), thus offering a unique chance to tackle these problems statistically.We aim to build up a new and homogeneous catalogue of blue straggler stars (BSS) in Galactic OCs using Gaia to provide a more solid assessment of the membership of these stars. We also aim to explore possible relationships of the straggler abundance with the parent clusters structural and dynamical parameters. As a by-product, we also search for possible yellow straggler stars (YSS), which are believed to be stragglers in a more advanced evolution stage. We employed photometry, proper motions, and parallaxes extracted from Gaia DR2 for 408 Galactic star clusters and searched for stragglers within them after performing a careful membership analysis. The number of BBS emerging from our more stringent, selection criteria turns out to be significantly smaller than in previo
Here we discuss the case of the double Blue Straggler Star (BSS) sequence recently detected in the young stellar cluster NGC 2173 in the Large Magellanic Cloud by Li et al (2018). In order to investigate this feature we made use of two HST sets of observations, one (the same one used by Li et al.) probing the cluster central regions, and the other sampling the surrounding field. We demonstrate that when field star decontamination is applied, ~40% of BSS population selected by Li et al. turns out to be composed by field stars interlopers. This contamination mainly affects one of the two sequences, which therefore disappears in the decontaminated colour-magnitude diagram. We analyse the result of tens different decontamination realisations: in none of them we find evidence of a double BSS sequence. Hence we conclude that NGC 2173 harbours a normal single (poorly populated) BSS sequence and that particular care needs to be devoted to the field decontamination process in any study aimed at probing stellar population features or star counts in the LMC clusters.
Li et al. (2018a; Li18a) claimed that the young stellar cluster NGC2173 in the Large Magellanic Cloud (LMC) harbours a bifurcated sequence of blue straggler stars (BSSs), similar to those detected in a few dynamically old globular clusters. However, Dalessandro et al. (2019; D19) re-analyzed the data by taking into account the contamination of the cluster population from LMC field stars, which was completely neglected by L18a. D19 showed that $sim40%$ of the selected BSS sample (and especially the population observed along one of the two sequences) is composed of field star interlopers, concluding that the double BSS sequence is most likely a field contamination artefact. In a recent note Li et al. (2018b) argued that the analysis by D19 is affected by two issues related to (1) the use of different HST instruments/filters in the decontamination procedure, and (2) a presumed overestimate of the number of field stars, which show a central radial segregation. Here we demonstrate that the D19 decontamination procedure is completely unaffected by the use of different HST instruments, and that, the field stars removed by D19 have no significant radial segregation toward the cluster center. Hence, we confirm that the claimed double sequence is just a field contamination artefact.
Blue straggler stars (BSS) are well studied in globular clusters but their systematic study with secure membership determination is lacking in open clusters. We use Gaia DR2 data to determine accurate stellar membership for four intermediate-age open clusters, Melotte 66, NGC 2158, NGC 2506 and NGC 6819, and three old open clusters, Berkeley 39, NGC 188 and NGC 6791, to subsequently study their BSS populations. The BSS radial distributions of five clusters, Melotte 66, NGC 188, NGC 2158, NGC 2506, and NGC 6791, show bimodal distributions, placing them with Family II globular clusters which are of intermediate dynamical ages. The location of minima, $r_mathrm{{min}}$, in the bimodal BSS radial distributions, varies from 1.5$r_c$ to 4.0$r_c$, where $r_c$ is the core radius of the clusters. We find a positive correlation between $r_mathrm{{min}}$ and $N_{mathrm{relax}}$, the ratio of cluster age to the current central relaxation time of the cluster. We further report that this correlation is consistent in its slope, within the errors, to the slope of the globular cluster correlation between the same quantities, but with a slightly higher intercept. This is the first example in open clusters that shows BSS radial distributions as efficient probes of dynamical age. The BSS radial distributions of the remaining two clusters, Berkeley 39 and NGC 6819, are flat. The estimated $N_{mathrm{relax}}$ values of these two clusters, however, indicate that they are dynamically evolved. Berkeley 39 especially has its entire BSS population completely segregated to the inner regions of the cluster.
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