The Gaia-ESO Survey: asymmetric expansion of the Lagoon Nebula cluster NGC 6530 from GES and Gaia DR2

Abstract

The combination of precise radial velocities from multi-object spectroscopy and highly accurate proper motions from Gaia DR2 opens up the possibility for detailed 3D kinematic studies of young star forming regions and clusters. Here, we perform such an analysis by combining Gaia-ESO Survey spectroscopy with Gaia astrometry for ∼900 members of the Lagoon Nebula cluster, NGC 6530. We measure the 3D velocity dispersion of the region to be 5.35 +0.39 −0.34 km s −1 , which is large enough to suggest the region is gravitationally unbound. The velocity ellipsoid is anisotropic, implying that the region is not sufficiently dynamically evolved to achieve isotropy, though the central part of NGC 6530 does exhibit velocity isotropy that suggests sufficient mixing has occurred in this denser part. We find strong evidence that the stellar population is expanding, though this is preferentially occurring in the declination direction and there is very little evidence for expansion in the right ascension direction. This argues against a simple radial expansion pattern, as predicted by models of residual gas expulsion. We discuss these findings in the context of cluster formation, evolution and disruption theories.

The dynamics of the γ Vel cluster and nearby Vela OB2 association

“…We match our AAT targets by position with the Vista Hemishpere Survey catalogue (VHS, McMahon et al 2013) and combine the VHS and 2MASS K-band measures by taking the 2MASS value for K < 12, taking the mean of the two measures for 12 < K < 13, and then using VHS for K > 13. We then use Gaia DR2 G magnitude to calculate the G -K colour for these sources and perform SED fitting using the method outlined in Wright et al (2019) to estimate effective temperatures (T eff ).…”

Kinematic signatures of cluster formation from cool collapse in the Lagoon Nebula cluster NGC 6530

“…The kinematic sample of young stars used for this study was compiled in Wright et al (2019) by combining spectroscopic information (surface gravity indicators, lithium equivalent widths, and Hα emission) from the Gaia-ESO Survey (GES; Gilmore et al 2012;Randich, Gilmore & Gaia-ESO Consortium 2013) with X-ray and infrared-based membership probabilities from Broos et al (2013). This results in a sample of ∼2000 high-probability members, of which ∼900 have reliable astrometry from Gaia (Gaia Collaboration et al 2016) Data Release 2 (DR2; Gaia Collaboration et al 2018) that passed the data selection criteria outlined in Arenou et al (2018) and Lindegren (2018) for astrometric data.…”

“…This results in a sample of ∼2000 high-probability members, of which ∼900 have reliable astrometry from Gaia (Gaia Collaboration et al 2016) Data Release 2 (DR2; Gaia Collaboration et al 2018) that passed the data selection criteria outlined in Arenou et al (2018) and Lindegren (2018) for astrometric data. A parallax cut was then used to exclude the 135 stars more than 2σ from the median parallax of the sample (0.724 mas or 1.33 kpc), and a further 70 stars were removed as proper motion (PM) outliers (>3σ from the median PM in either dimension), providing a sample of 691 highly probable young stars in NGC 6530 with PMs (for more details on the compilation of this sample we refer the reader to Wright et al 2019).…”

“…For both samples, stellar masses for all stars were taken from Wright et al (2019), calculated from comparison of the available photometry to Marigo et al (2017) stellar isochrones. The stars in both samples have stellar masses that vary from ∼100 M for the O4V star HD 164794 down to ∼0.1 M for the faintest stars.…”

“…Map of the spatial distribution of our kinematic sample of young stellar objects (red) and our spatially unbiased 'structural' sample of young stellar objects with M > 0.7 M , both projected on to an inverted Hα image (Drew et al 2014). Wright et al (2019) estimated that the mass function of highly probable members of NGC 6530 exhibited a turnover at around 0.7 M and we use this as the estimated completeness limit of our sample, removing all stars from our structural sample with masses below this and leaving a sample of ∼800 stars. The spatial distribution of both samples is shown in Fig.…”