Star cluster formation history along the minor axis of the Large Magellanic Cloud

Abstract

We analysed Washington CM T 1 photometry of star clusters located along the minor axis of the LMC, from the LMC optical centre up to ∼ 39 degrees outwards to the North-West. The data base was exploited in order to search for new star cluster candidates, to produce cluster CMDs cleaned from field star contamination and to derive age estimates for a statistically complete cluster sample. We confirmed that 146 star cluster candidates are genuine physical systems, and concluded that an overall ∼ 30 per cent of catalogued clusters in the surveyed regions are unlikely to be true physical systems. We did not find any new cluster candidates in the outskirts of the LMC (deprojected distance > ∼ 8 degrees). The derived ages of the studied clusters are in the range 7.2 < log(t yr −1 ) ≤ 9.4, with the sole exception of the globular cluster NGC 1786 (log(t yr −1 ) = 10.10). We also calculated the cluster frequency for each region, from which we confirmed previously proposed outside-in formation scenarios. In addition, we found that the outer LMC fields show a sudden episode of cluster formation (log(t yr −1 ) ∼ 7.8-7.9) that continued until log(t yr −1 ) ∼ 7.3 only in the outermost LMC region. We link these features to the first pericentre passage of the LMC to the MW, which could have triggered cluster formation due to ram pressure interaction between the LMC and MW halo.

Pal 13: its moderately extended low-density halo and its accretion history

“…Nevertheless, we applied a procedure to get rid of field stars that fall inside the defined strip. The method was devised by Piatti & Bica (2012) and used satisfactorily for cleaning CMDs of star clusters projected towards crowded star fields (e.g., Piatti 2017b,c,a, and references therein) and affected by differential reddening (e.g., Piatti 2018b; Piatti et al 2018, and reference therein). It relies on an accurate representation of the star field CMD in terms of its stellar density, luminosity function and color distribution.…”

The surroundings of the Milky Way globular cluster NGC 6809

“…In order to clean the cluster CMD, we followed the recipe used by Piatti & Bica (2012), which was satisfactorily applied elsewhere (e.g. Piatti et al 2018;Piatti & Carballo-Bello 2019) According to that method, we need to define a cluster and a star field areas of equal size. Figure 1 shows a rectangle which delimits both regions, the latter being that outside the rectangle.…”

Extra-tidal structures around the Gaia Sausage candidate globular cluster NGC 6779 (M56)

“…For each reference star field CMD, we generated a sample of boxes (go,(g − r)o) centred on each star, with sizes (∆(g),∆(g − r)) defined in such a way that one of their corners coincides with the closest star in that CMD region. This procedure of representing the reference star field CMD like an assembly of boxes was developed by Piatti & Bica (2012) and successfully used elsewhere (see, e.g, Piatti 2017a,b;Piatti et al 2018). It has the advantage of accurately reproducing the reference star field in terms of stellar density, luminosity function and colour distribution.…”