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The Gaia DR2 View of the Gamma Velorum Cluster: Resolving the 6D Structure E A&A 616, L12 (2018) https://doi.org/10.1051/0004-6361/201833815 Astronomy c ESO 2018 & Astrophysics LETTER TO THE EDITOR The Gaia DR2 view of the Gamma Velorum cluster: resolving the 6D structure E. Franciosini1, G. G. Sacco1, R. D. Jeffries2, F. Damiani3, V. Roccatagliata1, D. Fedele1, and S. Randich1 1 INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy e-mail: [email protected] 2 Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG, UK 3 INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy Received 10 July 2018 / Accepted 13 August 2018 ABSTRACT Gaia-ESO Survey observations of the young Gamma Velorum cluster led to the discovery of two kinematically distinct populations, Gamma Vel A and B, respectively, with population B extended over several square degrees in the Vela OB2 association. Using the Gaia DR2 data for a sample of high-probability cluster members, we find that the two populations differ not only kinematically, but are also located at different distances along the line of sight, with the main cluster Gamma Vel A being closer. A combined fit of the two populations yields $A = 2:895 0:008 mas and $B = 2:608 0:017 mas, with intrinsic dispersions of 0:038 0:011 mas ± ± +1:0+12:4 +2:5+15:3 ± and 0:091 0:016 mas, respectively. This translates into distances of 345:4 1:0 11:5 pc and 383:4 2:5 14:2 pc, respectively, showing that Gamma Vel± A is closer than Gamma Vel B by 38 pc. We find that the two− − clusters are nearly− − coeval, and that Gamma Vel B is expanding. We suggest that Gamma Vel A and B∼ are two independent clusters located along the same line of sight. Key words. open clusters and associations: individual: Gamma Velorum – stars: late-type – stars: pre-main sequence – stars: distances – stars: kinematics and dynamics 1. Introduction In this Letter we report the discovery, using Gaia DR2 data, that Gamma Vel A and B differ not only in their three- The Gamma Velorum cluster is a group of low-mass, pre- dimensional (3D) kinematics, but are also located at different main sequence (PMS) stars discovered in X-rays around 2 distances. The Letter is organised as follows. In Sect.2 we the Wolf–Rayet binary γ Vel in the Vela OB2 association present the results obtained from the analysis of Gaia data, and (Pozzo et al. 2000; Jeffries et al. 2009). Jeffries et al.(2014) we discuss them in Sect.3. Conclusions are given in Sect.4. used the results from the Gaia-ESO Survey (Gilmore et al. 2012; Randich et al. 2013) to investigate the cluster kinematics, and discovered the presence of two kinematically distinct popula- 2. Gaia analysis and results tions, Gamma Vel A, more spatially concentrated around γ2 Vel, and Gamma Vel B, more extended and dispersed. The observed The goal of this study is to perform a Gaia follow-up of the two Li depletion pattern for low-mass members also suggests that populations discovered by Jeffries et al.(2014). For this reason, Gamma Vel A may be older than Gamma Vel B by 1 2 Myr, we start from the list of Gamma Velorum members published by and implies an age of 20 Myr (Jeffries et al. 2014,∼ 2017− ), in these authors, which includes information on the radial veloc- contrast with the significantly∼ younger age of 5:5 1 Myr de- ity (RV) probability of belonging to the two populations (with 2 ± rived for γ Vel (Eldridge 2009). PRV;B = 1 PRV;A). These objects are distributed over an area of Sacco et al.(2015) found evidence that Population B extends 0.9 sq. deg− around the massive star, corresponding to 6 6 pc to the region of the NGC 2547 cluster, located about 2 deg south at the distance of Gamma Velorum. The list of members∼ × was of γ2 Vel ( 10 pc at the distance of Vela). Both Jeffries et al. crossmatched with the Gaia DR2 catalogue (Gaia Collaboration ∼ 1 (2014) and Sacco et al.(2015) suggested that Gamma Vel A is 2018) in TOPCAT , using a 200 radius to account for possible the bound remnant of a denser cluster formed around the mas- significant motions between the epochs of the 2MASS cata- sive star, while Gamma Vel B is a dispersed population formed logue used in Jeffries et al.(2014) and Gaia DR2. We found in lower-density regions of the Vela OB2 association. An al- Gaia counterparts for all objects, with separations of <0:400. ternative possibility is that the two populations were both born We discarded 25 stars with astrometric excess noise >1 mas, in a denser cluster which then expanded after the formation of which might indicate problems with the astrometric solution γ2 Vel and the expulsion of residual gas. Mapelli et al.(2015), on (e.g. Lindegren et al. 2018). Following Jeffries et al.(2014), we the basis of N-body simulations, suggested a scenario in which then extracted a subsample of 124 high-probability members of the two subclusters formed in the same molecular cloud, but Gamma Vel A and B, by selecting stars with RV probability in different star-formation episodes, and are currently merging. >0:75. A study of the region using Gaia DR1 data was performed by Damiani et al.(2017). 1 http://www.starlink.ac.uk/topcat/ Article published by EDP Sciences L12, page 1 of4 A&A proofs:A&Amanuscript 616, L12 no. (2018) gamvel_Gaia_final Table 1. ResultResult of of the the maximum-likelihood maximum-likelihood fit fit for for the the two two populations. populations. 11 GammaVelAGamma Vel A GammaVelB Gamma Vel B ̟ (mas) 2.895 0.008 2.608 0.017 10 $ (mas) 2:895± 0:008 2:608± 0:017 σ̟ (mas) 0.038 ±0.011 0.091 ±0.016 σ$ (mas) 0:038± 0:011 0:091± 0:016 µα (mas/yr)1 6.566 ± 0.043 5.979 ± 0.074 µα ∗(mas yr− ) − 6:566± 0:043 − 5:979± 0:074 9 σ∗µα (mas/yr)1 0−.336 ±0.032 0−.490 ±0.051 σ ∗(mas yr ) 0:336± 0:032 0:490± 0:051 µµα (mas/yr)− 9.727 0.047 8.514 0.074 δ ∗ 1 ± ± µσδ (mas(mas yr/−yr)) 0 9.341:727± 00.035:047 0 8.513:514± 00.056:074 8 µδ 1 ±± ±± σfµAδ (mas yr− )0 0.599:341 00.049:035 0:513 0:056 PMDEC (mas/yr) f 0:599±± 0:049 ± lnA Lmax 83±.8 7 ln L − 83:8 max − 6 centroid of the total distribution. The fit was performed taking -8 -7 -6 -5 -4 into account the full covariance matrix and the intrinsic disper- PMRA (mas/yr) sionsstill contains of the parallaxes a few clear and residual proper outliers, motion to components better constrain (see e. theg. Lindegrenfit we first etexcluded al. 2000); all detailsobjects are falling given at in more Appendix than 5σ A.from The there- sultingcentroid best-fit of the values total distribution. are given in The Table fit 1. was We performed note that we taking also 15 obtaininto account the same the results, full covariance within the matrix errors, and if we the neglect intrinsic the disper-corre- lationsions of coe thefficients, parallaxes suggesting and proper that correlations motion components are not significant (see e.g. forLindegren this cluster. et al. 2000); details are given in AppendixA. The re- sultingFor best-fit each star, values we also are givenderived in astrometric Table1. We membership note that we prob- also obtain the same results, within the errors, if we neglect the corre- 10 abilities PGaia,A and PGaia,B = 1 PGaia,A, and selected the sub- lation coefficients, suggesting that− correlations are not significant samples of Gamma Vel A and B members with PGaia,A > 0.75 for this cluster. (61 objects) and PGaia,B > 0.75 (46 objects), respectively. In Fig.For 2 we each compare star, we the also parallaxes derived astrometric and proper membership motions of prob-these Stars per bin abilities PGaia;A and PGaia;B = 1 PGaia;A, and selected the sub- 5 cleaned subsamples with the RVs− from Jeffries et al. (2014).The figuressamples show of Gamma that, while Vel Gamma A and B Vel members A is relatively with PGaia compact;A > 0 and:75 homogeneous,(61 objects) and forP GammaGaia;B > Vel0:75 B there (46 objects), is a clear respectively. trend between In theFig. astrometric2 we compare parameters the parallaxes and the and RVs. proper Since motions the RVs ofare these inde- cleaned subsamples with the RVs from Jeffries et al.(2014). The 0 pendent from the Gaia data, this suggests that the trend is real figures show that, while Gamma Vel A is relatively compact and 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 and not a consequence of correlations between the parameters. Parallax (mas) Inhomogeneous, particular, stars for Gamma with lower Vel RV B there tend is to a have clear larger trendbetween parallax andthe astrometric total proper parameters motion than and those the with RVs. higher Since theRV. RVs These are resu inde-lts Fig. 1. 1.ProperProper motion motion diagram diagram (top (top panel panel) and) distributionand distribution of parallaxes of par- arependent not a fromffected the byGaia the choicedata, this of the suggests probability that the threshold trend is used real (allaxesbottom ( panelbottom) for panel the) members for the members of Gamma of VelGamma A (red) Vel and A (red) B (blue) and toand select not a the consequence cleaned sample: of correlations using a lower between threshold the parameters.
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