We are reporting some basic parameters of two newly discovered clusters, VVVX CL 076 and CL 077, recently discovered in the galactic disk area covered by the VISTA Variables in the Via Lactea eXtended (VVVX) ESO Public Survey. The preliminary analysis shows that both clusters are young and relatively close to the Sun.Peer reviewedFinal Published versio

Alegria, S. Ramirez

Alonso-Garcia, J.

Borissova, Jura

Chene, Andre Nicolas

Ivanov, Valentin D.

Kurtev, R.

Lucas, Philip

Medina, Nicolás

Minniti, D.

English

We are reporting some basic parameters of two newly discovered clusters, VVVX CL 076 and CL 077, recently discovered in the galactic disk area covered by the VISTA Variables in the Via Lactea eXtended (VVVX) ESO Public Survey. The preliminary analysis shows that both clusters are young and relatively close to the Sun.Peer reviewe

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BAAA, Vol. 61A, 2019 Asociacio´n Argentina de Astronomı´aR. Gamen, N. Padilla, C. Parisi, F.A. Iglesias & M.A. Sgro´, eds. Bolet´ın de art´ıculos cient´ıficosOpen cluster candidates in the VVVX area: VVVX CL076 andCL077J. Borissova1,2, V.D. Ivanov3, P. Lucas4, R. Kurtev1,2, J. Alonso-Garcia5, S. Ramı´rez Alegr´ıa5, D.Minniti6,2, N. Medina1,2 & A.-N. Chene´71 Instituto de F´ısica y Astronomı´a, Universidad de Valpara´ıso, Chile2 Millennium Institute of Astrophysics, Santiago, Chile3 European Southern Observatory, Garching bei Mu¨nchen, Germany4 Centre for Astrophysics, University of Hertfordshire, UK5 Unidad de Astronomı´a, Facultad de Ciencias Ba´sicas, Universidad de Antofagasta, Chile6 Departamento de F´ısica, Facultad de Ciencias Exactas, Universidad Andre´s Bello, Santiago, Chile7 Gemini Observatory, Northern Operations Center, Hilo, USAContact / jura.borissova@uv.clResumen / Se reportan algunos para´metros ba´sicos de dos cu´mulos estelares, VVVX CL 076 y CL 077, re-cientemente descubiertos en el a´rea del disco gala´cico, cubierta por el relevamiento pu´blico del ESO “VISTAVariables in the Via Lactea eXtended (VVVX)”. El ana´lisis preliminar muestra que ambos cu´mulos son jo´venesy relativamente cercanos al Sol.Abstract / We are reporting some basic parameters of two newly discovered clusters, VVVX CL 076 and CL 077,recently discovered in the galactic disk area covered by the VISTA Variables in the Via Lactea eXtended (VVVX)ESO Public Survey. The preliminary analysis shows that both clusters are young and relatively close to the Sun.Keywords / Galaxy: open clusters and associations — Galaxy: disk — infrared: stars1. IntroductionYoung star clusters are sites of recent star formation andearly evolution in the Milky Way and are often only vis-ible at the infrared wavelengths, being highly obscuredby gas and dust. The VISTA Variables in the Via LacteaeXtended (VVVX) ESO Public Survey is a near-infraredphotometric sky survey that covers nearly 1700 squaredegrees towards the Galactic disk and bulge. It offersan excellent opportunity to search for new, undiscov-ered star clusters in our Galaxy. Recently, Borissovaet al. (2018) reported a list of 120 new star cluster can-didates found by visual inspection of 40 % of the diskarea covered by VVVX. In this paper, we present thenear infrared photometry and Gaia DR2 proper motionsand distance estimates for VVVX CL 076 and VVVXCL 077.Fig. 1 shows the composite JHKS color imagesof the target clusters. The equatorial coordinates ofCL 076 are α(2000) = 273.3545◦, δ(2000) = −18.0921◦and CL 077 is centered around α(2000) = 273.3960◦,δ(2000) = −18.0966◦. Both clusters are projected veryclose to the Galactic plane at galactic latitudes −0.10and −0.14 deg respectively.2. AnalysisTo construct the color-magnitude diagrams, we per-formed PSF photometry of a 2.5 × 2.5 arcmin area sur-rounding the selected candidate in the J , H, and KSbands. We used the DoPHOT photometric routine fol-lowing Alonso-Garc´ıa et al. (2017). The instrumentalmagnitudes were transformed to the standard system.This procedure is described in detail by Alonso-Garc´ıaet al. (2017), Borissova et al. (2011) and Borissova et al.(2014).The most probable cluster members are selected bystatistical decontamination procedure and Gaia DR2proper motion diagrams. To separate the field starsfrom probable cluster members we used the iterativefield-star decontamination algorithm, which divides themagnitude and color ranges into a grid of cells. In eachcell, it estimates the expected number density of clus-ter stars by subtracting the respective field-star num-ber density and, summing over all cells, it obtains atotal number of member stars. The most probablecluster members obtained in this manner are cross-matched with Gaia DR2 catalog (Gaia Collaborationet al., 2018), the proper motion vector diagrams µδ vs.µα cos δ are created and the stars with obviously dif-ferent proper motion are rejected (Fig. 2, upper panel).The lower panel of Fig. 2 shows the color-magnitude dia-grams for VVVX CL 076 and VVVX CL 077. FollowingZasowski et al. (2013), we then fit the Padova theoreticalisochrones with solar metallicity (Bressan et al., 2012).The best fit for VVVX CL 076 gives an age of 32 Myr,while VVVX CL 077 is most probably 10 Myr old. Theconfidence intervals are ±3 and ±2 Myr, respectively.We used the Gaia RD2 parallaxes as initial values for thedistances, while the mean reddening is determined bythe interactive process of the fitting. The reddening val-Poster contribution - October 2018VVVX CL 076 and CL 077Figure 1: VVVX JHKS composite color images of CL 076(upper panel) and CL 077 (lower panel). The field of viewis 5× 5 arcmin. The North and East vectors are drawn oneach image.ues are E(J−KS) = 1.6±0.2 and E(J−KS) = 1.7±0.2,for CL 076 and CL 077, respectively. The errors repre-sent uncertainty of the fitting procedure with quadrati-cally added errors from photometry and transformationto the standard system. The Gaia DR2 mean reddening,as reported in Borissova et al. (2018) is determined asE(BP −RP ) = 1.4± 0.3 and E(BP −RP ) = 1.6± 0.1,thus the differential reddening is possible, but dipperphotometry and/or spectra of individual cluster mem-bers are necessary to estimate it.We used the Gaia DR2 (Bailer-Jones et al., 2018)distance estimates. The histogram of distance distribu-tion of all stars within 1 arcmin radius of each clusteris shown in Fig. 3. As can be seen, most of the stars arecentered around 3 kpc. If we select only most proba-ble cluster members, CL 076 is placed around 1.6 kpc,while CL 077 stands at 1.8 kpc. The associated errorsFigure 2: Upper panel: The VVVX CL 076 and CL 077proper motion diagrams with sky vectors overplotted. Blueand green circles are most probable CL 076 and CL 077 mem-bers, respectively, while the red circles stand for field stars.The proper motion vectors are scaled by factor of 2 for bettervisibility. Lower panel: The VVVX CL 076 and CL 077 KSvs (J − KS) diagram with most probable cluster members(red large circles) overplotted. The solid red lines are best fitsolar isochrones of 32 and 10 Myr, respectively, taken fromthe Padova database.are around 1 kpc, thus such obtained distances shouldbe taken with caution.The cluster radii were measured by eye on the VVVXKS combined tiles and are estimated in Borissova et al.(2018) as 17′ and 53′. This method was preferred overautomated algorithms, because of the small number ofthe most probable cluster members. The area aroundcluster candidates is smoothed and the density contoursare over-plotted with the lower limit of the contour equalto the density of comparison field.Since these clusters are projected very close to eachother, have similar distances and proper motions, onecan speculate that this is a binary system, formed bytriggering on the same molecular cloud. More data arenecessary to confirm (or reject) this hypothesis.BAAA 61A (2019)Borissova et al.Figure 3: The histograms of distance distribution of CL 076and CL 077. The solid lines are Gaussian distributions of thewhole sample.3. SummaryThe newly discovered clusters projected in the VVVXdisk area are analyzed using VVVX near infrared pho-tometry and Gaia DR2 proper motion and distance esti-mates. The preliminary results show that both clustersare moderately reddened with E(J − KS) = 1.6 ± 0.2and E(J −KS) = 1.7± 0.2. They are relatively nearby:with distances 1.6 and 1.8 kpc from the Sun, and young:with ages of 32 Myr and 10 Myr, for CL 076 and CL 077,respectively.Acknowledgements: We gratefully acknowledge data from theESO Public Survey program ID 198.B-2004 taken with the VISTAtelescope, and products from the Cambridge Astronomical SurveyUnit (CASU). This work has made use of data from the EuropeanSpace Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analy-sis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Support is provided by the Ministry for theEconomy, Development and Tourism, Programa Iniciativa Cien-tica Milenio grant IC120009, awarded to the Millennium Insti-tute of Astrophysics (MAS). ANC’s work is supported by theGemini Observatory. SRA thanks the support by the FONDE-CYT Iniciacio´n project No. 11171025 and the CONICYT + PAI“Concurso Nacional Insercio´n de Capital Humano Avanzado en laAcademia 2017” project PAI 79170089. J.A-G. also acknowledgessupport by FONDECYT Iniciacio´n 11150916. MH acknowledgessupport by the BASAL Center for Astrophysics and AssociatedTechnologies (CATA) through grant PFB-06. We are grateful toanonymous referee for very useful comments, which significantlyimprove the manuscript.ReferencesAlonso-Garc´ıa J., et al., 2017, ApJL, 849, L13Bailer-Jones C.A.L., et al., 2018, AJ, 156, 58Borissova J., et al., 2011, A&A, 532, A131Borissova J., et al., 2014, A&A, 569, A24Borissova J., et al., 2018, MNRAS, 481, 3902Bressan A., et al., 2012, MNRAS, 427, 127Gaia Collaboration, et al., 2018, A&A, 616, A1Zasowski G., et al., 2013, AJ, 146, 64BAAA 61A (2019)

Open cluster candidates in the VVVX area: VVVX CL 076 and CL 077

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Open cluster candidates in the VVVX area: VVVX CL 076 and CL 077

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