A Chandra X-Ray Census of the Interacting Binaries in Old Open Clusters—Collinder 261

Abstract

We present the first X-ray study of Collinder 261 (Cr 261), which at an age of 7 Gyr is one of the oldest open clusters known in the Galaxy. Our observation with the Chandra X-ray Observatory is aimed at uncovering the close interacting binaries in Cr 261, and reaches a limiting X-ray luminosity of L X ≈ 4 × 10 29 erg s −1 (0.3-7 keV) for stars in the cluster. We detect 107 sources within the cluster half-mass radius r h , and we estimate that among the sources with L X 10 30 erg s −1 , ∼26 are associated with the cluster. We identify a mix of active binaries and candidate active binaries, candidate cataclysmic variables, and stars that have "straggled" from the main locus of Cr 261 in the colour-magnitude diagram. Based on a deep optical source catalogue of the field, we estimate that Cr 261 has an approximate mass of 6500 M ⊙ , roughly the same as the old open cluster NGC 6791. The X-ray emissivity of Cr 261 is similar to that of other old open clusters, supporting the trend that they are more luminous in X-rays per unit mass than old populations of higher (globular clusters) and lower (the local neighbourhood) stellar density. This implies that the dynamical destruction of binaries in the densest environments is not solely responsible for the observed differences in X-ray emissivity.

The X-ray emissivity of low-density stellar populations

“…Using Chandra, Vats & van den Berg (2017) found a total of 151 X-ray sources in Collinder 261, to a limiting luminosity of 4 × 10 29 ergs/s (0.3-7 keV). Thirty-three of these sources are active binaries and ten to eleven are CVs.…”

“…Thirty-three of these sources are active binaries and ten to eleven are CVs. From Vats & van den Berg (2017)'s X-ray membership identification, the total X-ray luminosity of point source members of Cr 261 can be estimated as (8.6 ± 1.0) × 10 31 ergs/s (0.5-2 keV). The Chandra spectrum from the half-mass radius of Collinder 261, using the double subtraction method, gives a total L X (0.5-2)= 1.1 ± 0.4 × 10 32 erg/s, in agreement.…”

“…The key to our analysis is the careful collation of data on a variety of stellar populations. Recent work has added significantly to our understanding of X-ray emissivity in old open clusters (Gosnell et al 2012;van den Berg et al 2013;Vats & van den Berg 2017;Vats et al 2018) and dwarf ellipticals (Ge et al 2015). To understand the nature of X-ray sources Figure 1.…”

“…Verbunt (2001) suggested that the elevated X-ray luminosity per unit mass observed in low-density open clusters could be the result of significant mass-loss (see also van den Berg et al 2013;Ge et al 2015;Vats & van den Berg 2017). In the simplest scenario, the total cluster mass is the dominant cluster parameter that decides the number of binaries, with the primordial scaling between binary numbers and total cluster mass being roughly linear (e.g.…”

X-ray sources in Galactic globular clusters and old open clusters

“…Gyr) open clusters had already been studied with ROSAT (e.g , Belloni & Tagliaferri 1998,. but Chandra and also XMM-Newton have brought the more distant-and often older-open clusters within reach (e.g Gosnell et al 2012, Vats & van den Berg 2017, Vats et al 2018…”