Wide-area photometric and astrometric (Gaia DR2) study of the young cluster NGC 6530

Abstract

Context. NGC 6530 is a young cluster, with a complex morphology and star-formation history. We present a statistical study of its global properties, using a new, large list of candidate members down to masses of 0.2 − 0.4M and Gaia DR2 astrometry. Aims. We consider a larger sky region compared to previous studies, to investigate the entire cluster until its periphery, including any diffuse population all around the main cluster. We study the distribution of extinction and age across the different regions, and obtain constraints on the star-formation history. We also study the dynamics of cluster members. Methods. Cluster membership is determined on the basis of literature X-ray data, Hα emission, near-IR and UV excesses from the VPHAS+ and UKIDSS photometric surveys and published near-IR catalogs, and Gaia DR2 astrometry; moreover, we use a method for photometric selection of M-type pre-main-sequence cluster members, which we recently developed and used for other star-formation regions. The list of candidates includes nearly 3700 stars, of which we estimate approximately 2700 genuine NGC 6530 members. Results. Using Gaia parallaxes, the cluster distance is found to be 1325 pc, with errors of 0.5% (statistical) and 8.5% (systematic), in agreement with previous determinations. The cluster morphology and boundaries are established with great confidence, from the agreement between the subsamples of members selected using different criteria. There is no diffuse population of members around the cluster, but there are minor condensations of true members in addition to the two main groups in the cluster core and in the Hourglass nebula. Two such subgroups are spatially associated with the stars 7 Sgr (F2II-III) and HD 164536 (O7.5V). There is a definite pattern of sequential star formation across the cluster, within an age range from less than 0.5 Myr to ∼ 5 Myr. Extinction is spatially non-uniform, with part of the population still embedded or obscured by thick dust. The precise Gaia proper motion data indicate that the NGC 6530 parent cloud collided with the Galactic plane around 4 Myr ago, and we suggest that event as the trigger of the bulk of star formation in NGC 6530. The internal cluster dynamics is also partially resolved by the Gaia data, indicating expansion of the main cluster population with respect to its center.

OB Associations and their origins

“…Zari et al (2018) used this method with the Gaia MG vs. GBP − GRP diagram ( Figure 2) to identify stars younger than 20 Myrs within 500 pc of the Sun, mapping the 3D distribution of the known YSO populations in the solar neighbourhood. Damiani et al (2019) combined this technique with a proper motion selection to map out the stellar population of the Scorpius-Centaurus OB association. This approach can reduce the level of contamination, particularly for groups of stars with distinct proper motions relative to the field star population (such as Sco-Cen), though it could also introduce a kinematic bias that should be considered if the sample is to be used for any structural or kinematic study.…”

“…Due to its proximity and >100 pc size the association spans over 90 • on the sky, extending over the constellations Scorpius, Lupus, Norma, Centaurus, Circinus, Musca, and Crux. The association contains ∼150 B-type stars and (currently) a single O-type star, the runaway O9V star ζ Ophiuchi (Blaauw 1964a;de Geus et al 1989;Brown 1998;de Zeeuw et al 1999;Hoogerwerf et al 2001), as well as a rich population of lower-mass stars (Pecaut & Mamajek 2016;Damiani et al 2019). The total stellar content of the association has been estimated as anywhere from 6,000 (Preibisch & Mamajek 2008) to 13,000 stars (Damiani et al 2019), with a total stellar mass of ∼4,000 M (Wright & Mamajek 2018).…”

“…The runaway star ζ Oph is also shown as a blue circle, with a Gaia DR2 proper motion vector covering 0.5 Myr of motion extending from it. The red contours show the spatial distribution of Gaia-selected pre-main sequence and upper main sequence stars from Damiani et al (2019). The grey dashed rectangles show the historical division (Blaauw 1946) of the association into three subgroups from de .…”

Cause and effects of the massive star formation in Messier 8 East

“…Located in the Sagittarius-Carina arm, near our line of sight toward the Galactic center, it is relatively close to the Sun, at a distance of ∼1.25 kpc (1 corresponding to 0.36 pc) (Damiani et al 2004 andArias et al 2006). Recently, this distance has been confirmed by Gaia parallaxes that yield a value of 1.35 kpc with an uncertainty of 9% (Damiani et al 2019). M8 (Fig.…”

“…We assumed a typical upper limit of ∼2 Myr for the age of class II YSOs. For the open cluster NGC 6530, Damiani et al (2019) used Gaia data to find sequential star formation over an age range from 0.5 to 5 Myr; here we adopted 2 Myr. For the age of Class 0/I YSOs we took ∼0.27 Myr, the average of the ages of Class 0 (∼0.1 Myr) and Class I (∼0.44 Myr) YSOs (Evans et al 2009).…”