The complex stellar populations in the background of open clusters in the third Galactic quadrant

Abstract

Multi-color photometry of the stellar populations in five fields in the third Galactic quadrant centred on the clusters NGC 2215, NGC 2354, Haffner 22, Ruprecht 11, and ESO489 SC01 is interpreted in terms of a warped and flared Galactic disk, without resort to an external entity such as the popular Monoceros or Canis Major overdensities. Except for NGC 2215, the clusters are poorly or unstudied previously. The data generate basic parameters for each cluster, including the distribution of stars along the line of sight. We use star counts and photometric analysis, without recourse to Galactic-model-based predictions or interpretations, and confirms earlier results for NGC 2215 and NGC 2354. ESO489 SC01 is not a real cluster, while Haffner 22 is an overlooked cluster aged ∼ 2.5 Gyr. Conclusions for Ruprecht 11 are preliminary, evidence for a cluster being marginal. Fields surrounding the clusters show signatures of young and intermediate-age stellar populations. The young population background to NGC 2354 and Ruprecht 11 lies ∼8-9 kpc from the Sun and ∼1 kpc below the formal Galactic plane, tracing a portion of the Norma-Cygnus arm, challenging Galactic models that adopt a sharp cut-off of the disk 12−14 kpc from the Galactic center. The old population is metal poor with an age of ∼ 2 − −3 Gyr, resembling star clusters like Tombaugh 2 or NGC 2158. It has a large color spread and is difficult to locate precisely. Young and old populations follow a pattern that depends critically on the vertical location of the thin and/or thick disk, and whether or not a particular line of sight intersects one, both, or none.

Galactic Structure in the Outer Disk: The Field in the Line of Sight to the Intermediate-Age open Cluster Tombaugh 1*

“…Two stars slightly depart from this relation, probably due to differential reddening in the FOV. The shape and width of this sequence are comparable with those of the sequences found in the background of NGC2354 (Carraro et al 2016) and in the direction of the Canis Major overdensity (Carraro et al 2008), sequences ascribed to the warped old thin disk, which the line of sight intersects, thus producing the effect of probing a structure confined in distance.…”

“…The reddening law in the third Galactic quadrant has been recently debated in the literature. As discussed by Turner (1976), Moitinho (2001), and Carraro et al (2016), the reddening law in the third Galactic quadrant seems to deviate from the normal one-namely, it is not described by the standard value of 3.1 for…”

“…As thoroughly discussed in Carraro et al (2016), it is quite straightforward to characterize the blue plume, since it would be composed of supposedly young stars, for which a robust handling of the properties is possible with UBV photometry. We discuss the CMD in more detail by plotting stars in different magnitude bins in the classical two-color diagram (TCD) -U B/ -B V , as shown in Figure 5.…”

“…Color-magnitude diagrams (CMDs) of stellar fields in the third quadrant of the Milky Way have repeatedly unraveled the remarkable complexity of stellar populations in the outer Galactic disk. Beyond any doubt, two dominating features appear on top of the main sequence (MS) of the nearby Galactic field: a thick MS having a prominent turnoff (TO) point and made up of intermediate-age stars poor in metals and an almost vertical sequence of young blue stars, known as the blue plume (Carraro et al 2005(Carraro et al , 2016Moitinho et al 2006). This complexity was originally interpreted as the result of the accretion of a satellite onto the Milky Way disk (Bellazzini et al 2004;Martin et al 2004) and of different age and metallicity sequences described as different episodes of star formation in an individual stellar system, the Canis Major dwarf galaxy.…”

On the assessment of the nature of open star clusters and the determination of their basic parameters with limited data

“…In closing, we acknowledge that efforts are being made by several groups (Caetano et al 2015;KroneMartins & Moitinho 2014;Netopil et al 2015;Carraro et al 2016, and references therein) to provide new tools, or re-discover old methods, to derive reliable open cluster parameters. Hopefully, the closed box era of open clusters' fundamental parameter determination will be over soon.…”