The Gaia DR2 View of the Gamma Velorum Cluster: Resolving the 6D Structure

arXiv:1807.03621v2 [astro-ph.SR] 13 Aug 2018 ansqec PS tr icvrdi -asaround X-rays pre- in low-mass, discovered of binary stars group Wolf-Rayet a (PMS) the is sequence cluster main Velorum Gamma The Introduction 1. Pzoe l 00 Je 2000; al. et (Pozzo icvrdtepeec ftoknmtclydsic popu around concentrated distinct spatially kinematically more kinematics, A, two Vel Gamma cluster of tions, the presence investigate the to 201 discovered 2013) al. et al. (Gilmore et Survey Randich Gaia-ESO the from results the used otatwt h infiatyyugraeo 5 of age younger significantly the with contrast n mle naeof age an implies and am e a eodrta am e by tha B Vel suggests Gamma than also older be members may low-mass A Vel observed for Gamma The pattern dispersed. and depletion extended Li more B, Vel Gamma and for otergo fteNC24 lse,lctdaot2dgsout deg 2 about located cluster, 2547 NGC of the of region the to esrcutrwihte xadn fe h omto of formation the after expandend bor then both which γ were cluster populations two denser alt the An a that association. is OB2 formed Vela possibility population the native dispersed mas- of a the regions is around lower-density B formed is in Vel cluster Gamma A Vel denser while Gamma star, a sive that of suggested remnant (2015) bound al. the et Sacco and (2014) td ftergo using but region cloud, the molecular of study same A the in whi di in formed in scenario subclusters a two suggested simulations, the N-body of basis the ain ta.(2017). al. et Damiani uut1,2018 14, August Astronomy 2 e n h xuso frsda a.Mplie l (2015), al. et Mapelli gas. residual of expulsion the and Vel γ γ ac ta.(05 on vdneta ouainBextends B Population that evidence found (2015) al. et Sacco 2 ff 2 itne tr:knmtc n dynamics and kinematics Stars: – distances r lolctda di at located also are Gaia h w ouain yields populations two the xadn.W ugs htGmaVlAadBaetoindepende two words. are B Key and A Vel Gamma that suggest We expanding. am e scoe hnGmaVlBby B Vel Gamma than closer is A Vel Gamma n 0 and eevd... Received aaEOSre bevtoso h on am eou clu Velorum Gamma young the of observations Survey Gaia-ESO am e n ,rsetvl,wt ouainBetne o extended B population with respectively, B, and A Vel Gamma 3 2 1 e ( Vel rn trfraineioe,adaecretymerging. currently are and episodes, star-formation erent e Edig 2009). (Eldridge Vel .Franciosini E. NF-Osraoi srnmc iPlro izadlParl del Piazza Palermo, di Astronomico Osservatorio - INAF srpyisGop el nvriy el,Sta Keele, University, Keele Group, Astrophysics NF-Osraoi srfiiod rer,LroE em 5, Fermi E. Largo Arcetri, di Astrofisico Osservatorio - INAF . R aafrasml fhg-rbblt lse ebr,w members, cluster high-probability of sample a for data DR2 ∼ 091 & 0p ttedsac fVl) ohJe Both Vela). of distance the at pc 10 ± Astrophysics The / 0 cetd... Accepted . 1 a,rsetvl.Ti rnltsit itne f34 of distances into translates This respectively. mas, 016 pncutr n soitos niiul am Velorum Gamma individual: associations: and clusters Open 1 ∼ ff .G Sacco G. G. , ise l 09.Je 2009). al. et ries Gaia ff γ 0Mr(Je Myr 20 rn itne ln h ieo ih,wt h ancluste main the with sight, of line the along distances erent 2 e nteVl B association OB2 Vela the in Vel aucitn.gamvel_Gaia no. manuscript Gaia ̟ A = R iwo h am eou cluster: Velorum Gamma the of view DR2 R aawspromdby performed was data DR1 2 ff . 895 1 ise l 04 07,in 2017), 2014, al. et ries .D Je D. R. , eovn h Dstructure 6D the resolving ± 0 . ff 0 a and mas 008 . ise l (2014) al. et ries 5 ± ∼ ff y derived Myr 1 ∼ 8p.W n httetocutr r erycea,adthat and coeval, nearly are clusters two the that find We pc. 38 ries ff ff L 1 rsieS55G ntdKingdom United 5BG, ST5 ordshire ise al. et ries 2 − te othe to etter .Damiani F. , γ Myr, 2 ̟ 2 ABSTRACT B Vel, in n and er- la- on = ch 2; h t mno1 03,Plro Italy Palermo, 90134, 1, amento 02,Foec,Iaye-mail: Italy Florence, 50125, 2 . e eea qaedgesi h eaO2ascain Usin association. OB2 Vela the in degrees square several ver trldt h icvr ftoknmtclydsic pop kinematically-distinct two of discovery the to led ster 608 tcutr oae ln h aeln fsight. of line same the along located clusters nt la itiuinfrtehg-rbblt ebr.Tefi The members. high-probability the for distribution allax f14hg-rbblt ebr fGmaVlAadB by B, and A Vel probability Gamma RV with of stars members selecting high-probability 2 124 al. of et Lindegren (e.g., Je solution Following astrometric the with lems vra rao . q e rudtemsiesa,correspon star, massive the around deg to sq. ing 0.9 of area an over u r lolctda di at located also di are B but and A Vel Gamma that ito ebr a rsmthdwt the with crossmatched was members of list h olo hssuyi oproma perform to is study this of goal The 2. th 4. from Sect. obtained in results given are the sions present we of 2 analysis Sect. in follows: as srmti xesnoise excess astrometric ou Gi olbrto ta.21)i TOPCAT in 2018) al. et Collaboration (Gaia logue aaD2 efudGi oneprsfralojcs ihse with objects, all for counterparts arations Gaia Je t found We in between DR2. used Gaia motions catalogue 2MASS significant the possible of epoch for account to radius popul two the r to the belonging (with pub- on of tions members information probability Velorum (RV) includes Gamma velocity which of dial authors, list these the by from lished start we son, Je by discovered populations two n httetoppltosdi populations two the that find e 1 http: ± E 3 Gaia 5 nti etrw eottedsoey using discovery, the report we letter this In nFg epo h rprmto iga n h par- the and diagram motion proper the plot we 1 Fig. In .Roccatagliata V. , . ditor 0 4 . − + 1 a,wt nrni iprin f0 of dispersions intrinsic with mas, 017 // 1 1 ∼ . . www.starlink.ac.uk 0 0 tr:lt-ye–Sas r-ansqec Stars: – sequence pre-main Stars: – late-type Stars: – < − + nlssadresults and analysis 11 12 6 . . Gaia 5 4 0 P × . ff cad383 and pc RV 4 am e en lsr obndfi of fit combined A closer. being A Vel Gamma r ise l 21) ete xrce subsample a extracted then we (2014), al. et ries ′′ ca h itneo am eou.The Velorum. Gamma of distance the at pc 6 , edsaddfo h ape2 tr with stars 25 sample the from discarded We . B aa n edsuste nSc.3 Conclu- 3. Sect. in them discuss we and data, = 1 . 1 4 − ff [email protected] .Fedele D. , − + / 2 2 > rn itne.Telte sorganized is letter The distances. erent topcat P . . 5 5 − + RV ff a,wihmgtidct prob- indicate might which mas, 1 14 15 rntol nter3 kinematics, 3D their in only not er . . , 2 3 ff A / rntol ieaial,but kinematically, only not er c epciey hwn that showing respectively, pc, .Teeojcsaedistributed are objects These ). ff ise l 21) o hsrea- this For (2014). al. et ries > 1 0 n .Randich S. and , . ril ubr ae1o 5 of 1 page number, Article 75. Gaia ff . am e is B Vel Gamma 038 ise l 21)and (2014) al. et ries ± olwu fthe of follow-up Gaia Gaia 0 . ulations, 1 mas 011

1 c sn 2 a using , S 2018 ESO R cata- DR2 the g R data, DR2 1 018). gure he p- d- a- a- ′′ e A&A proofs: manuscript no. gamvel_Gaia

Table 1. Result of the maximum-likelihood ﬁt for the two populations 11 GammaVelA GammaVelB ̟ (mas) 2.895 ± 0.008 2.608 ± 0.017 10 σ̟ (mas) 0.038 ± 0.011 0.091 ± 0.016 µα∗ (mas/yr) −6.566 ± 0.043 −5.979 ± 0.074

9 σµα∗ (mas/yr) 0.336 ± 0.032 0.490 ± 0.051 µδ (mas/yr) 9.727 ± 0.047 8.514 ± 0.074 σ (mas/yr) 0.341 ± 0.035 0.513 ± 0.056 8 µδ fA 0.599 ± 0.049 PMDEC (mas/yr) ln Lmax −83.8 7

6 fit we first excluded all objects falling at more than 5σ from the -8 -7 -6 -5 -4 centroid of the total distribution. The fit was performed taking PMRA (mas/yr) into account the full covariance matrix and the intrinsic dispersions of the parallaxes and proper motion components (see e.g. Lindegren et al. 2000); details are given in Appendix A. The re- 15 sulting best-fit values are given in Table 1. Note that we obtain the same results, within the errors, also if we neglect the correlation coefficients, suggesting that correlations are not significant for this cluster. 10 For each star, we also derived astrometric membership prob- abilities PGaia,A and PGaia,B = 1 − PGaia,A, and selected the subsamples of Gamma Vel A and B members with PGaia,A > 0.75 (61 objects) and PGaia,B > 0.75 (46 objects), respectively. In

Stars per bin 5 Fig. 2 we compare the parallaxes and proper motions of these cleaned subsamples with the RVs from Jeffries et al. (2014). The figures show that, while Gamma Vel A is relatively compact and homogeneous, for Gamma Vel B there is a clear trend between 0 the astrometric parameters and the RVs. Since the RVs are inde- 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 pendent from the Gaia data, this suggests that the trend is real Parallax (mas) and not a consequence of correlations between the parameters. In particular, stars with lower RV tend to have larger parallax Fig. 1. Proper motion diagram (top panel) and distribution of parallaxes and total proper motion than those with higher RV. These results (bottom panel) for the members of Gamma Vel A (red) and B (blue) are not affected by the choice of the probability threshold used from Jeffries et al. (2014) with RV membership probability > 0.75. For to select the cleaned sample: using a lower threshold would only comparison, we also plot the members of NGC 2547 B from Sacco et al. slightly increase the scatter, leaving the trends unchanged. (2015, green diamonds and histogram).

3. Discussion shows that there is a very clear separation between the two populations, although with some overlap, not only in the proper mo- 3.1. The distance of Gamma Vel A and B tions, as expected given the difference in kinematics found by The main result of this study is the discovery that Gamma Vel A Jeffries et al. (2014), but also in their parallaxes. In particular, and B are located at different distances along the line of sight. the parallax distribution shows two well defined and separated In particular, we find parallaxes of ̟A = 2.895 ± 0.008 mas for peaks, implying that Gamma Vel A and B are located at different Gamma Vel A and ̟B = 2.608 ± 0.017 mas for Gamma Vel B, distances, with Gamma Vel A being closer than Gamma Vel B. with a difference of ∼ 0.29 mas between the two populations. There also appears to be a larger scatter in Gamma Vel B par- We caution that these results do not take into account possi- allaxes and proper motions, while Gamma Vel A is more con- ble systematic errors or correlations. Gaia DR2 parallaxes and centrated. In the same figure we also plot for comparison the proper motions can be affected by significant spatial correla- members of NGC 2547 B derived by Sacco et al. (2015): these tions that can reach values of 0.04 mas and 0.07 mas/yr over objects tend to have slightly higher values of µα∗, but overlap scales ∼< 1 deg, as well as possible systematic effects that are completely with Gamma Vel B in both parallax and µδ, support- expected to be < 0.1 mas and < 0.1 mas/yr (Lindegren et al. ing the hypothesis that NGC 2547 B and Gamma Vel B are part 2018; Luri et al. 2018). We performed a series of checks on the of the same population. The small differences in parallax and full Gaia sample over the region covered by our dataset, finding proper motions might hint to possible gradients or substructures no evidence for significant spatial variations or correlations that in population B, but our data do not allow to us to draw any con- could affect our results and produce the observed distributions clusion. and trends. While we cannot exclude systematic variations with To quantify the differencesbetween Gamma Vel A and B, we respect to external regions, we can however be confident that the performed a maximum-likelihood fit of the 3D distribution of observed differences between the two populations are real. parallaxes and proper motions using two multivariate gaussian Since the relative errors on parallaxes are below ∼ 10%, we components, one for each population. Since the initial sample can estimate the distances by a simple inversion of the paral- still contains a few clear residual outliers, to better constrain the laxes and calculate their uncertainties using a first order ap-

Article number, page 2 of 5 E. Franciosini et al.: The Gaia DR2 view of the Gamma Velorum cluster: resolving the 6D structure

24 24 24

22 22 22

20 20 20

RV (km/s) 18 RV (km/s) 18 RV (km/s) 18

16 16 16

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 -8 -7 -6 -5 -4 6 7 8 9 10 11 Parallax (mas) PMRA (mas/yr) PMDEC (mas/yr)

Fig. 2. Comparison between the RVs from Jeﬀries et al. (2014) and the Gaia parallaxes and proper motions for stars with PGaia > 0.75. For comparison we also plot the NGC 2457 B members from Sacco et al. (2015). Symbols and colours are the same as in Fig. 1.

4 they overlap perfectly. By comparing linear fits in the colour- magnitude diagram with the Baraffe et al. (2015) isochrones for the two groups of secure members, it appears that any age difference between the populations is limited to about 3 Myr if 6 the mean population age is 10 Myr, or double this if the mean age is 20 Myr (as suggested by Jeffries et al. 2017). However, the lithium depletion patterns in the two groups, which are

V distance-independent, suggest their ages are more similar than M 8 this (Jeﬀries et al. 2014). A more detailed analysis of the ages of the two populations will be presented in a forthcoming paper (Franciosini et al. in prep.). 10 3.2. The origin of the Gamma Velorum system

12 In the light of our results, we can now review the diﬀerent hy- 1.0 1.5 2.0 2.5 3.0 3.5 pothesis on the properties and the formation of the Gamma Velo- V-I c rum system.

Fig. 3. Absolute V magnitude vs. V − Ic colour for the members of From the parameters given in Table 1, we can derive the Gamma Vel A and B with PGaia > 0.75. VIc photometry is from tangential velocity dispersions of the two populations, finding Jeffries et al. (2009). For comparison, we also plot the NGC 2547 B ∼ 0.6 km s−1 and ∼ 0.8 km s−1 for Gamma Vel A and B, re- members with available VIc photometry from Sacco et al. (2015), as- spectively. These results are puzzling, since the values we find suming the same average distance as Gamma Vel B. Colours are as in for Gamma Vel A and B are respectively larger and smaller Fig. 1. than those obtained from the RVs (0.34 ± 0.16 km s−1 and 1.60 ± 0.37 km s−1, respectively). In the case of Gamma Vel B, the observed dispersions could be different because on the plane proximation (e.g. Bailer-Jones et al. 2018; Luri et al. 2018). As- of the sky we sample a region of the cluster which is smaller than suming conservatively a systematic error of ±0.1 mas, we ob- what we sample along the line of sight, while the origin of the +1.0+12.4 +2.5+15.3 tain dA = 345.4−1.0−11.5 pc and dB = 383.4−2.5−14.2 pc for discrepancy for Gamma Vel A is not clear. Gamma Vel A and B, respectively. Taking into account the aver- Figure 2 clearly shows an anti-correlation between radial ve- age zero point in parallax of ∼ 0.03 mas (Lindegren et al. 2018) locities and parallaxes of Gamma Vel B. This signature is ex- would reduce both distances by ∼ 4 pc. These results therefore pected if the cluster is expanding as suggested by Sacco et al. show that Gamma Vel A is closer than Gamma Vel B by ∼ 38 pc. (2015). They proposed that this expansion was triggered by the Our distance estimate for Gamma Vel A is consistent with formation of γ2 Vel, that expelled the residual gas keeping the +8 2 the distance of 336−7 pc derived for the massive star γ Vel from cluster bound. However, as discussed in the previous section, interferometric observations (North et al. 2007) and with the re- the massive binary and Gamma Vel B are not at the same dis- +39 vised Hipparcos distance of 343−32 pc (van Leeuwen 2007). On tance, therefore the dynamical status of the cluster and its evo- the other hand, our distance estimate for Gamma Vel B is only lution are unrelated with the massive binary. We can speculate marginally consistent with the value of 410 ± 12 pc derived for that the cluster formed in an unbound state, or that its dispersion the Vela OB2 association by de Zeeuw et al. (1999). has been triggered by another massive star that evolved into a In Fig. 3 we plot the colour-magnitude diagram of the Gaia- supernova. On the other hand, the distance of Gamma Vel A is selected sample after correcting for the distances to each popula- consistent with γ2 Vel, therefore, given that the massive binary tion. The figure shows that the sequences of both single and bi- is located at the cluster centre, we can conclude that it belongs nary stars are very clean for both Gamma Vel A and B, and that to Gamma Vel A. It is still not clear why the ages of the cluster

Article number, page 3 of 5 A&A proofs: manuscript no. gamvel_Gaia and of the central star are so different. Our results do not support Jeffries, R. D., Naylor, T., Walter, F. M., Pozzo, M. P., & Devey, C. R. 2009, the hypothesisthat Gamma Vel B is part of the low-mass popula- MNRAS, 393, 538 tion of the Vela OB2 association, because the cluster appears to Lindegren, L., Hernández, J., Bombrun, A., et al. 2018, A&A, 616, A2 Lindegren, L., Madsen, S., & Dravins, D. 2000, A&A, 356, 1119 be located at a lower distance. However, a full study of the Vela Luri, X., Brown, A. G. A., Sarro, L. M., et al. 2018, A&A, 616, A9 region is required to address this issue. Mapelli, M., Vallenari, A., Jeffries, R. D., et al. 2015, A&A, 578, A35 Given their large separation, the two clusters are not merg- North, J. R., Tuthill, P. G., Tango, W. J., & Davis, J. 2007, MNRAS, 377, 415 ing as proposed by Mapelli et al. (2015), who assumed a much Pozzo, M., Jeffries, R. D., Naylor, T., et al. 2000, MNRAS, 313, L23 ∼ Randich, S., Gilmore, G., & Gaia-ESO Consortium. 2013, The Messenger, 154, smaller distance between them ( 5 pc) in their simulation of the 47 dynamical interaction between the two systems. Furthermore, Sacco, G. G., Jeffries, R. D., Randich, S., et al. 2015, A&A, 574, L7 their relative velocities along the radial and tangential direction van Leeuwen, F. 2007, A&A, 474, 653 suggests that they did not merge in the past and will not do that in the future. In conclusion our results suggests that Gamma Vel A and Gamma Vel B are two independent clusters seen along the same line of sight. Gamma Vel A is probably bound and formed the massive star γ2 Vel, while Gamma Vel B is expanding.

4. Conclusions In this paper we investigated the properties of the binary cluster Gamma Velorum using the Gaia DR2 astrometric parameters of cluster members pre-selected by spectroscopic observations. Our analysis led to the following main results:

1. We derived the distances of Gamma Vel A and B, finding that they are not cospatial, but Gamma Vel A is closer than Gamma Vel B by ∼ 38 pc. The separation between the two clusters indicates that they are not currently merging, while the comparison with the distance estimates for the massive binary γ2 Vel suggests that this star belongs to Gamma Vel A. 2. We find that Gamma Vel B is expanding.However the origin of this expansion is not clear. 3. We confirm that the two clusters are kinematically separated and are moving apart. In particular, we find a shift in the tangential motion of Gamma Vel B with respect to Gamma Vel A of 0.6 and −1.2masyr−1 (∼ 1 and −2kms−1) along RA and DEC, respectively. 4. The colour-magnitude diagram corrected for the measured distance of each cluster indicates that the two populations are nearly coeval.

Acknowledgements. We thank the anonymous referee for her/his comments, and E. Pancino for useful discussions and suggestions. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), pro- cessed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013).

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Article number, page 4 of 5 E. Franciosini et al.: The Gaia DR2 view of the Gamma Velorum cluster: resolving the 6D structure

Appendix A: Probability distribution of parallaxes and proper motions To derive the mean parallaxes and proper motions of the two populations, we modeled the total probability distribution as the sum of two 3D multivariate gaussian components, one per population, given, for each population and each star, by:

1 1 T −1 L = − xi − x◦ Σ xi − x◦ , i 3/2 −1/2 exp ( ) i ( ) (A.1) (2π) |Σi| " 2 # where

̟i − ̟◦ xi − x◦ = −  µα∗,i µα∗,◦  (A.2)  µδ,i − µδ,◦      and Σi is the sum of the individual covariance matrix and the matrix of intrinsic dispersions:

2 + 2 σ̟,i σ̟,◦ ρ̟µα∗,i σ̟,i σµα∗,i ρ̟µδ,i σ̟,i σµδ,i Σ = ρ σ σ σ2 + σ2 ρ σ σ i  ̟µα∗,i ̟,i µα∗,i µα∗,i µα∗,◦ µα∗µδ,i µα∗,i µδ,i   ρ σ σ ρ σ σ σ2 + σ2   ̟µδ,i ̟,i µδ,i µα∗µδ,i µα∗,i µδ,i µ ,i µδ,◦   δ   (A.3)

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