New star clusters discovered towards the Galactic anticentre direction using Gaia DR3 data

Abstract

We report the discovery of 31 new open clusters (OCs) identified in \textit{Gaia}~DR3 data through a systematic search over 220 adjacent $1^\circ\times1^\circ$ fields towards the Galactic anticentre, in the direction of the Perseus arm gap. Eight of them display low-density structures, possibly indicating open cluster remnants properties. The objects were identified and characterized through a combined analysis of photometric, kinematic, and spatial distributions, a methodology successfully applied in our previous works. Their structural properties, mean proper motions, ages, distances and reddening were derived and their centres cross-matched with the available catalogues. The clusters are low-concentrated systems and are mostly located within $3<d<5$ kpc, exhibiting reddening up to $E(B-V)\approx1.5$, and ages from $\sim$20 Myr to 1 Gyr. The new OCs represent a significant increase in the anticentre cluster census: $31\%$ for $3<d<4$ kpc and $12\%$ for $d>4$ kpc. They do not belong to the Perseus arm, but may be associated with the Outer Norma arm. The Gulf of Camelopardalis region appears as an interruption in the Perseus arm, possibly reflecting low star-formation activity, dust obscuration, or that the Milky Way is a flocculent, rather than a grand-design spiral galaxy.

Figures (16)

• Figure 1: Top: schematic drawing of the spiral arms and the local population of known OCs according to. The blue dots represent the population of younger OCs ($log(t) < 7.5$) and the grey ones represents their older counterparts. The 31 newly discovered OCs in this work are also shown as black triangles (young) and red triangles (old). The solid lines represent the spiral arms positions: Perseus (purple), Outer Norma (red), Scutum (yellow) and Sagittarius (green). The Galactic bar and the Sun's location (red star, at 8.34 kpc; Reid et al. (2014)) are also represented. The black solid lines represent the limit in Galactic longitude used in this work. Bottom: The Galactocentric distances versus the distance from the Galactic plane for younger and older OCs inside the region of interest. The Sun's location is also represented by a red star. The approximate location of the arms are also represented: Perseus in purple and Outer Norma in red.
• Figure 2: Spatial coverage of the Galactic fields surveyed in this work. The size of the black squares indicates the surveyed regions. The colours indicate the mean colour index ($G_{BP}-G_{RP}$) of the samples inside each square. The histograms show the local number of stars in bins of Galactic longitude and latitute. The red filled circles indicate the position of the 31 newly discovered OCs in this work. The black filled circles indicate the position of the 225 known OCs detected by our methodology.
• Figure 3: Sequence of steps for OC detection applied to UFMG100 (top), UFMG113 (middle), and Berkeley 66 (bottom). First column: CMD of the entire tile (grey), overplotted with the colour–filtered samples (blue and red). Second column: Density maps of the sky chart (top) and VPDs (middle and bottom) for the colour–filtered subsample. Third column: VPD (top) and sky charts (middle and bottom) for stars filtered by colour and by the selection boxes from the previous step, shown as black filled circles. Fourth column: Histograms of parallaxes and proper motions in right ascension and declination for the final sample of stars filtered by colour, proper motion, and sky position. Modal values are shown by green dashed lines.
• Figure 4: Spatial distribution of the discovered OCs and nearby known OCs. I this figure, the limiting radii are represented by open large circles, the central coordinates by diamonds and OCs members by filled small circles (known OCs) and open blue small circles (new OCs). Left: the discovered OC UFMG108 with its neighbors CMG798 (cyan), CWNU3310 (green), CWNU4361 (red) and King6 (black). Middle: the discovered OCs UFMG98, UFMG100 and UFMG103 together with distant literature OCs (black). Right: the discovered OC UFMG105 and UFMG106 with their respective neighbors CNWU1392 (red) and CNWU1701(black).
• Figure 5: Centre determination for the discovered clusters: UFMG100 (top-left), UFMG104 (top-right), UFMG105 (bottom-left) and UFMG114 (bottom-right). The sample of stars filtered by proper motion e parallax boxes are represented by black opened squares. The entire field population are represented by the grey opened squares. The final centre position is ploted as two red solid lines.