Context. Chamaeleon I represents an ideal laboratory to study the cluster
formation in a low-mass environment. Recently, two sub clusters spatially
located in the northern and southern parts of Chamaeleon I were found with
different ages and radial velocities. Aims. In this letter we report new
insights into the structural properties, age, and distance of Chamaeleon I
based on the astrometric parameters from Gaia data-release 2 (DR2). Methods. We
identified 140 sources with a reliable counterpart in the Gaia DR2 archive. We
determined the median distance of the cluster using Gaia parallaxes and fitted
the distribution of parallaxes and proper motions assuming the presence of two
clusters. We derived the probability of each single source of belonging to the
northern or southern sub-clusters, and compared the HR diagram of the most
probable members to pre-main sequences isochrones. Results. The median distance
of Chamaeleon I is ~190 pc. This is about 20 pc larger than the value commonly
adopted in the literature. From a Kolmogorov-Smirnov test of the parallaxes and
proper-motion distributions we conclude that the northern and southern clusters
do not belong to the same parent population. The northern population has a
distance dN = 192.7+/-0.4 pc, while the southern one dS = 186.5+/-0.7 pc. The
two sub-clusters appear coeval, at variance with literature results, and most
of the sources are younger than 3 Myr. The northern cluster is more elongated
and extends towards the southern direction partially overlapping with the more
compact cluster located in the south. A hint of a relative rotation between the
two sub-clusters is also found.Comment: Letter accepted by A&

Franciosini, E.

Randich, S.

Roccatagliata, V.

Sacco, G. G.

English

arXiv

Context. Chamaeleon I represents an ideal laboratory to study the cluster formation in a low-mass environment. Recently, two sub-clusters spatially located in the northern and southern parts of Chamaeleon I were found with different ages and radial velocities. Aims. In this Letter we report new insights into the structural properties, age, and distance of Chamaeleon I based on the astrometric parameters from Gaia data release 2 (DR2).

Methods. We identified 140 sources with a reliable counterpart in the Gaia DR2 archive. We determined the median distance of the cluster using Gaia parallaxes and fitted the distribution of parallaxes and proper motions assuming the presence of two clusters. We derived the probability of each single source of belonging to the northern or southern sub-clusters, and compared the HR diagram of the most probable members to pre-main sequences isochrones.

Results. The median distance of Chamaeleon I is ~190 pc. This is consistent with the revised estimate using the Tycho-Gaia Astrometric Solution, but it is about 20 pc larger than the value commonly adopted in the literature. From a Kolmogorov–Smirnov test of the parallaxes and proper-motion distributions we conclude that the northern and southern clusters do not belong to the same parent population. The northern population has a distance dN = 192.7+0.4−0.4pc, while the southern one dS = 186.5+0.7−0.7pc. The two sub-clusters appear coeval, at variance with literature results, and most of the sources are younger than 3 Myr. The northern cluster is more elongated and extends towards the southern direction partially overlapping with the more compact cluster located in the south. A hint of a relative rotation between the two sub-clusters is also found

Archivio della Ricerca - Università di Pisa

Astronomy & Astrophysics manuscript no. letter_chaI_lan_ed c©ESO 2018August 27, 2018Letter to the EditorThe double population of Chamaeleon I detected by Gaia DR2V. Roccatagliata1, G. G. Sacco1, E. Franciosini1 and S. Randich1INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: roccatagliata@arcetri.astro.itReceived 17.07.2018; Accepted 13.08.2018ABSTRACTContext. Chamaeleon I represents an ideal laboratory to study the cluster formation in a low-mass environment. Recently, two sub-clusters spatially located in the northern and southern parts of Chamaeleon I were found with different ages and radial velocities.Aims. In this letter we report new insights into the structural properties, age, and distance of Chamaeleon I based on the astrometricparameters from Gaia data-release 2 (DR2).Methods. We identified 140 sources with a reliable counterpart in the Gaia DR2 archive. We determined the median distance of thecluster using Gaia parallaxes and fitted the distribution of parallaxes and proper motions assuming the presence of two clusters. Wederived the probability of each single source of belonging to the northern or southern sub-clusters, and compared the HR diagram ofthe most probable members to pre-main sequences isochrones.Results. The median distance of Chamaeleon I is ∼190 pc. This is consistent with the revised estimate using the Tycho-Gaia Astro-metric Solution, but it is about 20 pc larger than the value commonly adopted in the literature. From a Kolmogorov-Smirnov test ofthe parallaxes and proper-motion distributions we conclude that the northern and southern clusters do not belong to the same parentpopulation. The northern population has a distance dN = 192.7+0.4−0.4 pc, while the southern one dS = 186.5+0.7−0.7 pc. The two sub-clustersappear coeval, at variance with literature results, and most of the sources are younger than 3 Myr. The northern cluster is more elon-gated and extends towards the southern direction partially overlapping with the more compact cluster located in the south. A hint of arelative rotation between the two sub-clusters is also found.Key words. Open clusters and associations: individual: Chamaeleon I - Stars: pre-main sequence - Parallaxes - Proper motions1. IntroductionChamaeleon I is one of the closest low-mass star forming regionswith which it is possible to study all the key processes related tothe formation of a young cluster, as well as the structure of pro-toplanetary disks around young stellar objects.The stellar population of Chamaeleon I has been extensively in-vestigated in the last fifteen years (Feigelson & Lawson 2004;Stelzer et al. 2004; Comerón et al. 2004). The cluster is com-posed of two sub-structures, one northern and one southern. Acomplete study was presented by Luhman (2007) who found thetwo populations to have different ages: 5-6 Myr for the north-ern and 3-4 Myr for the southern sub-cluster, respectively. Thestructure and dynamical properties of Chamaeleon I have beenthoroughly investigated by Sacco et al. (2017), who confirmedthe presence of the two sub-clusters kinematically, with a shiftin velocity of about 1 km/s.The literature value of the distance of Chamaeleon I commonlyadopted is 160 ± 15 pc (Whittet et al. 1997). This value comesfrom the combination of studies that employed different tech-niques. In particular, the extinction analysis of Whittet et al.(1997) constrained the distance in the range between 135 and165 pc, while the weighted average of Hipparcos distances forthree cluster members 1, is 175+20−16 pc (Perryman et al. 1997).Recently, the Hipparcos distance has been revised to 179+11+11−10−10pc by Voirin et al. (2018) using the Tycho-Gaia Astrometric So-lution (TGAS) catalog.Gaia DR2 astrometry (Gaia Collaboration et al. 2018; Lindegren1 HD 97300, HD 97048 and CR Chaet al. 2018) clearly offers a unique opportunity to gather a newview of the region.In this letter we present the parallaxes and proper motions of thecluster members spectroscopically identified by previous works.The analysis of the Gaia DR2 data and the discussion of theclusters kinematics and age are in Sects. 3 and 4, respectively.2. Cluster membership and Gaia DR2 dataOur approach aims to characterize the previously known popula-tions rather than to discover possible new members of the region.For this reason, we compiled a catalog of 244 optical members,combining the observations of Luhman (2007) and Sacco et al.(2017). From this initial catalog, 206 sources are present in theGaia DR2 archive, but for any further analysis we consider onlythe 140 with an excess source noise less than 1 (as suggested e.g.,in Lindegren et al. 2018). All details oft the relations betweenastrometric excess noise and parallax are given in Appendix A.Assuming that all the cluster members belong to the same popu-lation, it is possible to compute the distance of the entire cluster.Figure 1 shows the distribution of the parallaxes of the clustermembers; the distance commonly adopted in the literature forChamaeleon I is highlighted.The resulting median parallax is 5.248±0.187 mas, where theassociated error is computed as the median absolute deviation(MAD). Since the relative error is lower than 10%, we can cal-culate the distance by inverting the parallax (Luri et al. 2018;Bailer-Jones 2015). The distance of Chamaeleon I is therefore190.5+7.1+3.8−6.5−3.5 pc, which takes into account a conservative system-atic error of 0.1 mas, as discussed by Luri et al. (2018). This dis-Article number, page 1 of 6Article published by EDP Sciences, to be cited as https://doi.org/10.1051/0004-6361/201833890A&A proofs: manuscript no. letter_chaI_lan_edtance is larger than previously assumed in the literature (Whittetet al. 1997), while being marginally consistent with the Hippar-cos distance from three members only.The spatial distribution of the cluster members (after a 3-sigmaclipping on the initial sample) is plotted as a function of the par-allaxes and proper motions in Fig. 2. The two sub-clusters spa-tially identified by Luhman (2007) can be clearly distinguishedin parallax and kinematically.3. AnalysisIn order to quantitatively investigate whether the two sub-clusters are also separated in parallax and kinematics, we se-lected the northern and southern regions using the criterium ofLuhman (2007) based on their declinations; lower than -77◦ forthe northern sub-cluster, and higher than -77◦ for the southernone. We performed a two-sample Kolmogorov-Smirnov test inparallaxes and proper motions. The probabilities that the twosamples belong to the same parent population are 2.39 · 10−12,8.0 · 10−14 and 1.84 · 10−11 in parallax, proper motion in α andδ, respectively. This confirms statistically that the two clustersare spatially separated in parallax and kinematically separated inproper motions.We fitted the distribution of the three astrometric parameters(πi, µα,i, and µδ,i) with a model including two populations, de-scribed by two three-dimensional (3D) multivariate Gaussians(as in Lindegren et al. 2000, 2). To perform our calculations, weused a maximum likelihood approach as in Jeffries et al. (2014)and Franciosini et al. (2018). The likelihood function for eachFig. 1: Histogram of the parallaxes of the Chamaeleon I mem-bers. The solid red line represents the position of the medianparallax. The dotted red vertical line represents the parallax com-monly adopted from the literature. The result of a Gaussian fit isalso shown.star of each population is given byLN/S ,i = (2π)−3/2 |Ci|−1/2 × exp[− 12 (ai − a0)′C−1i (ai − a0)], (1)where Ci is the covariance matrix, |Ci| its determinant (the detailson each term of the matrix are given in Appendix B), and(ai − a0)′ is the transpose of the vectorai − a0 = πi − π0µα,i − µα,0µδ,i − µδ,0 , (2)2 Equations 6-10where π0, µα,0, µδ,0 are the mean values of the cluster.The total likelihood of the double population is therefore givenby:Li = fN LN,i + (1 − fN) LS ,i, (3)where LN and LS are the likelihoods given in Equation 1 forthe northern and southern sub-clusters, fN is the fraction of starsthat belongs to the north component, and fS = (1 − fN) is thefraction of southern stars. This is a reliable assumption for thisregion since our membership is based on different accurate stud-ies. We warn the reader that in other fields with higher contam-inations this assumption cannot be applied since it excludes thepossibility of having interlopers or any type of contamination bynon-members of either of the clusters.The probability for each star of belonging to either the sub-cluster N or S is computed asPN,i = fNLN,iLiPS ,i = (1 − fN)LS ,iLi . (4)Out of the initial 140 sources, we found that 107 have a probabil-ity higher than 80% of belonging to one of the two sub-clusters.The results are listed in Table 1.We highlight that the uncertainties in parallax and proper mo-tions considered in the analysis include only the published er-rors provided in the Gaia DR2 archive, which are derived fromthe formal error in the astrometric processing. Since a simplereceipt to account for systematic error is not yet available, Luriet al. (2018) suggested to discuss a possible influence on the sci-entific results of a systematic error not larger than 0.1 mas inparallax and 0.1 mas/yr for proper motions. We highlight thatthere is no reason to expect a different systematic error betweenthe two clusters, since they are located in the same direction onthe sky and are composed of a homogenous sample of stars inspectral types and magnitudes. For these reasons the systematicerrors do not introduce any effect to our results.Inverting the parallaxes, we can derive the distances of the twosub-clusters:dN = 192.7+0.4−0.4 pc dS = 186.5+0.7−0.7 pc.In Figure 3 we show the histograms of the parallax distributionof the 73 most probable northern members, and the 34 southernmembers, where we have highlighted the parallaxes of the twosub-clusters computed from the maximum likelihood estimation(MLE), as well as their spatial distribution. The projected dis-tance between the centers of both clusters along the line of sightis of the same order as their projected separation in the plane ofthe sky and as their spatial extent. This supports the hypothesisthat they both belong to the same physical entity, and it is notonly a chance alignment along the line of sight. We see that thesouthern cluster has a compact structure, while the northern clus-ter, which corresponds to the more distant one, is spatially moreelongated and extends in the direction of the southern cluster.This may reflect the influence of the main filamentary structurepresent in the region, which extends in the north-south directionand has been mapped in C18O by Haikala et al. (2005).4. Discussion and ConclusionIn this section we discuss the age and the kinematic propertiesof the north and south sub-clusters of Chamaeleon I.Article number, page 2 of 6V. Roccatagliata et al.: The double population of Chamaeleon I detected by Gaia DR2Fig. 2: Spatial distribution of sources color coded by parallaxes and proper motions. The horizontal red lines highlight the declinationof -77◦ used by Luhman (2007) to distinguish the two sub-clusters.Table 1: Results from the MLE fit of the two sub-clusters of Chamaeleon I: parallax with its error (π), the parallax dispersion andits error σπ,0 , the proper motions µα and µδ with their errors and the relative dispersions σµα,0 σµδ,0 with their errors. The definitionof the dispersions in parallax and proper motions is given in Appendix B.π σπ,0 µα σµα,0 µδ σµδ,0Cha I North 5.188±0.012 0.060±0.011 -22.069±0.101 0.738±0.063 -0.050±0.115 0.873±0.079Cha I South 5.363±0.021 0.085±0.017 -23.127±0.114 0.571±0.072 1.593±0.238 1.126±0.159frac N 0.638±0.068ln(L)max -271.694.1. The age of Chamaeleon IIn order to investigate whether an age difference is present be-tween the two sub-clusters, we consider the 107 members with aprobability higher than 80% defined in Section 3. We use thelog(Teff)-MJ diagram in order to minimize the effects due toinfrared excesses caused by the presence of protostellar disks.The effective temperatures are compiled either from Luhman(2007) or from Sacco et al. (2017). The absolute J magnitudeof each source has been derived by adopting the mean distancemodule for the northern and southern sub-clusters and correct-ing for AJ (from Luhman 2007). The overplotted isochrones arethe Z=0.013 models from Tognelli et al. (2011) 3. These modelshave a solar metallicity, which is a good approximation for themetallicity of the cluster as found by Spina et al. (2017).As shown in Figure 4, all the sources have ages lower than 5Myr. In particular, apart for a few sources, most of the membersare younger than 3 Myr. While Luhman (2007) found differentages for the two populations (5-6 Myr for the northern one and3-4 Myr for the southern one), we do not find any evidence of anage difference between the two sub-clusters.Our new findings and the differences with respect to the Luh-man (2007) results can be ascribed to two effects: on one hand,Luhman (2007) used the same distance to all sources, adoptinga smaller value than what we find here; on the other hand, hisselection of the two sub-clusters was based only on the spatialdistribution, while in our case we take into account also theirdifferent parallaxes and kinematic properties.3 https://www.astro.ex.ac.uk/people/timn/tau-squared/pisa_details.html4.2. Kinematics properties of the north and southsub-clustersUnder the assumption of an isotropic distribution in a star cluster,we can use the relation of Platais (2012) to derive the velocitydispersion from the proper motion dispersion:σr(km/s) = d (kpc) · 4.37 · σµ(mas/yr), (5)where the σ2µ =σ2µα+σ2µδ2 (McLaughlin et al. 2006).We obtain σr,N = 0.681 ± 0.057 km/s and σr,S = 0.727 ± 0.134km/s, where the uncertainties are computed from the error prop-agation. The velocity dispersions are consistent, within 2 σ, withthe results of Sacco et al. (2017).Given that the northern cluster is in the background, and it ismore redshifted than the closer southern sub-cluster, we con-clude that the two clusters are moving away from each other.In Figure 3 the two arrows represent the proper motions of thetwo sub-clusters with respect to a reference system centered onthe cluster. This confirms that the two sub-clusters are not merg-ing and have a non-zero angular momentum. Combining this re-sult with the differential radial velocity measured by Sacco et al.(2017), this represents a hint of rotation of the two sub-clusters.This is a new and puzzling result. Indeed, in young high-massclusters rotation has been theoretically predicted by Mapelli(2017), and it has been observed, for example, in the high-mass star forming region R136 in the Large Magellanic Cloud(Hénault-Brunet et al. 2012). However, in simulations with sim-ilar total mass to low-mass environments, such as ChamaeleonI, Mapelli (2017) did not find a clear signature of rotation as inhigh-mass environments.A more detailed analysis of the cluster dynamics will be pre-sented in an upcoming paper, together with an updated census ofthe members using Gaia DR2 data.Acknowledgements. This project has received funding from the EuropeanUnion’s Horizon 2020 research and innovation programme under the MarieArticle number, page 3 of 6A&A proofs: manuscript no. letter_chaI_lan_edFig. 3: Left: Parallax distribution of the most probable members (P≥80%) of the northern (in blue) and southern (in magenta) sub-clusters. In each panel the MLE parallaxes for both clusters are shown. Right: Spatial distribution of the northern and the southernsub-cluster (as in the left panel). The red arrows represent the differential proper motions in α and δ with respect to a mean propermotion between the northern and southern cluster.Fig. 4: log(Teff) - MJ Diagram of the most probable members (P≥80%). The color code is as in Figure 3. The solid lines are thepre-main sequence isochrones at different ages between 1 and 10 Myr. .Sklodowska-Curie grant agreement No 664931. This work has made use ofdata from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Anal-ysis 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.ReferencesBailer-Jones, C. A. L. 2015, PASP, 127, 994Comerón, F., Reipurth, B., Henry, A., & Fernández, M. 2004, A&A, 417, 583Feigelson, E. D. & Lawson, W. A. 2004, ApJ, 614, 267Franciosini, E., Sacco, G. G., Jeffries, R. D., et al. 2018, A&A, 616, L12Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al. 2018, ArXiv e-printsHaikala, L. K., Harju, J., Mattila, K., & Toriseva, M. 2005, A&A, 431, 149Hénault-Brunet, V., Gieles, M., Evans, C. J., et al. 2012, A&A, 545, L1Jeffries, R. D., Jackson, R. J., Cottaar, M., et al. 2014, A&A, 563, A94Lindegren, L., Hernandez, J., Bombrun, A., et al. 2018, ArXiv e-printsArticle number, page 4 of 6V. Roccatagliata et al.: The double population of Chamaeleon I detected by Gaia DR2Lindegren, L., Madsen, S., & Dravins, D. 2000, A&A, 356, 1119Luhman, K. L. 2007, ApJS, 173, 104Luri, X., Brown, A. G. A., Sarro, L. M., et al. 2018, ArXiv e-printsMapelli, M. 2017, MNRAS, 467, 3255McLaughlin, D. E., Anderson, J., Meylan, G., et al. 2006, ApJS, 166, 249Perryman, M. A. C., Lindegren, L., Kovalevsky, J., et al. 1997, A&A, 323, L49Platais, I. 2012, Star clusters, 360Sacco, G. G., Spina, L., Randich, S., et al. 2017, Astronomy & Astrophysics,601, A97Spina, L., Randich, S., Magrini, L., et al. 2017, A&A, 601, A70Stelzer, B., Micela, G., & Neuhäuser, R. 2004, A&A, 423, 1029Tognelli, E., Prada Moroni, P. G., & Degl’Innocenti, S. 2011, A&A, 533, A109Voirin, J., Manara, C. F., & Prusti, T. 2018, A&A, 610, A64Whittet, D. C. B., Prusti, T., Franco, G. A. P., et al. 1997, A&A, 327, 1194Article number, page 5 of 6A&A proofs: manuscript no. letter_chaI_lan_edAppendix A: Selection dataIn this appendix we show the distribution of all the parallaxesand parallax errors of the 206 sources with a Gaia counterpart.In the last lower panel we see the effect of selecting only thesources with excess errors lower than 1, and we notice that inthis way almost all the sources with higher error in parallax areautomatically excluded from our analysis.Appendix B: Probability density functionThe covariance matrix Ci of Equation 1 corresponds toCi =Ci,11 Ci,12 Ci,13Ci,21 Ci,22 Ci,23Ci,31 Ci,32 Ci,33 . (B.1)Following Lindegren et al. (2000) each term of the covariancematrix corresponds to:[Ci]11= σ2π,i + σ2π,0[Ci]22= σ2µα,i + σ2µα,0[Ci]33= σ2µδ,i + σ2µδ,0[Ci]12=[Ci]21= σπ,i · σµα,i · ρ (π, µα)[Ci]13=[Ci]31= σπ,i · σµδ,i · ρ (π, µδ)[Ci]23=[Ci]32= σµα,i · σµδ,i · ρ (µα, µδ),(B.2)where ρ (π, µα), ρ (π, µδ), ρ (µα, µα) are the correlation coeffi-cients4, σπ,i, σµα,i and σµδ,i are the errors associated to eachmeasurement2, while σπ,0, σµα,0 and σµδ,0 are the intrinsic disper-sions of π, µα and µδ obtained from the Maximum LikelihoodEstimation of the probability given in Eqs. 3 and 1.4 from the Gaia archiveFig. A.1: Upper two panels: distribution of the parallaxes anderrors in parallax as a function of the astrometric excess noise.The vertical red line corresponds to the excess noise = 1. Bottompanel: parallaxes vs. the error in parallaxes. Sources excludedby our analysis are highlighted with red circles.Article number, page 6 of 6

The double population of Chamaeleon I detected by Gaia DR2

Context. Chamaeleon I represents an ideal laboratory to study the cluster formation in a low-mass environment. Recently, two sub-clusters spatially located in the northern and southern parts of Chamaeleon I were found with different ages and radial velocities.  Aims: In this Letter we report new insights into the structural properties, age, and distance of Chamaeleon I based on the astrometric parameters from Gaia data release 2 (DR2).  Methods: We identified 140 sources with a reliable counterpart in the Gaia DR2 archive. We determined the median distance of the cluster using Gaia parallaxes and fitted the distribution of parallaxes and proper motions assuming the presence of two clusters. We derived the probability of each single source of belonging to the northern or southern sub-clusters, and compared the HR diagram of the most probable members to pre-main sequences isochrones.  Results: The median distance of Chamaeleon I is 190 pc. This is consistent with the revised estimate using the Tycho-Gaia Astrometric Solution, but it is about 20 pc larger than the value commonly adopted in the literature. From a Kolmogorov-Smirnov test of the parallaxes and proper-motion distributions we conclude that the northern and southern clusters do not belong to the same parent population. The northern population has a distance dN = 192.7+0.4-0.4pc, while the southern one dS = 186.5+0.7-0.7pc. The two sub-clusters appear coeval, at variance with literature results, and most of the sources are younger than 3 Myr. The northern cluster is more elongated and extends towards the southern direction partially overlapping with the more compact cluster located in the south. A hint of a relative rotation between the two sub-clusters is also found

SACCO, GIUSEPPE GERMANO

FRANCIOSINI, Elena

RANDICH, Maria Sofia

OA@INAF - Istituto Nazionale di Astrofisica

Astronomy and Astrophysics

Context. Chamaeleon I represents an ideal laboratory to study the cluster formation in a low-mass environment. Recently, two sub-clusters spatially located in the northern and southern parts of Chamaeleon I were found with different ages and radial velocities. Aims. In this Letter we report new insights into the structural properties, age, and distance of Chamaeleon I based on the astrometric parameters from Gaia data release 2 (DR2).
				Methods. We identified 140 sources with a reliable counterpart in the Gaia DR2 archive. We determined the median distance of the cluster using Gaia parallaxes and fitted the distribution of parallaxes and proper motions assuming the presence of two clusters. We derived the probability of each single source of belonging to the northern or southern sub-clusters, and compared the HR diagram of the most probable members to pre-main sequences isochrones.
				Results. The median distance of Chamaeleon I is ~190 pc. This is consistent with the revised estimate using the Tycho-Gaia Astrometric Solution, but it is about 20 pc larger than the value commonly adopted in the literature. From a Kolmogorov–Smirnov test of the parallaxes and proper-motion distributions we conclude that the northern and southern clusters do not belong to the same parent population. The northern population has a distance $ d_\mathrm N=192.7_{-0.4}^{+0.4}\text{pc} $, while the southern one $ \textstyle d_\mathrm S=186.5_{-0.7}^{+0.7}\text{pc} $. The two sub-clusters appear coeval, at variance with literature results, and most of the sources are younger than 3 Myr. The northern cluster is more elongated and extends towards the southern direction partially overlapping with the more compact cluster located in the south. A hint of a relative rotation between the two sub-clusters is also found

V. Roccatagliata

G. G. Sacco

E. Franciosini

S. Randich

EDP Sciences OAI-PMH repository (1.2.0)

The double population of Chamaeleon I detected by

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The double population of Chamaeleon I detected by Gaia DR2

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