New star clusters discovered towards the Galactic bulge direction using Gaia DR2
Filipe A. Ferreira, W. J. B. Corradi, F. F. S. Maia, M. S. Angelo, J. F. C. Santos Jr
MMNRAS , 1–6 (2020) Preprint 27 January 2021 Compiled using MNRAS L A TEX style file v3.0
New star clusters discovered towards the Galactic bulgedirection using
Gaia
DR2
F. A. Ferreira (cid:63) , W. J. B. Corradi , , F. F. S. Maia , M. S. Angelo and J. F. C. Santos Jr. Universidade Federal de Minas Gerais, Departamento de Física, Av. Antônio Carlos 6627, 31270-901, Brazil Universidade Federal do Rio de Janeiro, Instituto de Física, 21941-972, Brazil Centro Federal de Educação Tecnológica de Minas Gerais, Av. Monsenhor Luiz de Gonzaga, 103, 37250-000, Brazil Laboratório Nacional de Astrofísica, R. Estados Unidos, 154, 37504-364, Itajubá, MG, Brazil
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
We report the discovery of 34 new open clusters and candidates as a result of asystematic search carried out in 200 adjacent fields of 1 × area projected towardsthe Galactic bulge, using Gaia
DR2 data. The objects were identified and characterizedby a joint analysis of their photometric, kinematic and spatial distribution, whichhas been consistently used and proved to be effective in our previous works. Thediscoveries were validated by cross-referencing the objects position and astrometricparameters with the available literature. Besides their coordinates and astrometricparameters, we also provide sizes, ages, distances and reddening for the discoveredobjects. In particular, 32 clusters are closer than 2 kpc from the Sun, which representsan increment of nearly of objects with astrophysical parameters determined in thenearby inner disk. Although these objects fill an important gap in the open clustersdistribution along the Sagittarius arm, this arm, traced by known clusters, appearsto be interrupted, which may be an artifact due to the incompleteness of the clustercensus.
Key words:
Galaxy: stellar content – open clusters and associations: general –surveys:
Gaia
Finding and characterizing objects projected towards densefields, like the Galactic bulge, has always been a challeng-ing task. However, in the recent years the
Gaia
DR2 cata-logue (Gaia Collaboration et al. 2018), which provides pre-cise astrometric and photometric data for a billion stars inthe whole sky, has allowed a better characterization of openclusters (OCs) (e.g. Rangwal et al. 2019; Angelo et al. 2020;Monteiro et al. 2020) and revolutionized the search for newobjects in the astrometric space, leading to a discovery ofhundreds of new OCs in our Galaxy (Liu & Pang 2019;Castro-Ginard et al. 2020; He et al. 2020).Increasing and completing the database of knownGalactic OCs with well-determined properties is very impor-tant, since clusters have long been used as probes to inves-tigate several Galactic features, such as the disc structureand its scale height (Cantat-Gaudin et al. 2020, hereafterCG+2020), spiral arms and Galactic rotation curve (Dias (cid:63)
E-mail: fi[email protected] et al. 2019), metallicity and abundance gradients (Magriniet al. 2017). Despite this, it has been shown that the nearbyOC census is still incomplete inside 1.8 kpc (Cantat-Gaudinet al. 2019), demanding a continuous search for missing clus-ters.In previous works, we have found 28 new OCs by adopt-ing a methodology involving iterative inspection of propermotion and sky charts after applying a series of filters toenhance cluster/field contrast (Ferreira et al. 2019; Ferreiraet al. 2020, hereafter F+2020). We also characterized thesenewly discovered clusters, thus helping to expand the pa-rameters database of known OCs in the Galaxy.In this work, we have applied the same methodology,extending our search to a region of the sky around the Galac-tic centre hoping to increase the present database of the OCpopulation towards the Galactic bulge.This work is organized as follows: in Sect. 2 we describethe method applied to detect and validate the discovery ofthe new OCs. The analysis is presented in Sect. 3, includingthe membership assessment and the determination of theastrophysical parameters. In Sect. 4, we compare our dis- © 2020 The Authors a r X i v : . [ a s t r o - ph . GA ] J a n F. A. Ferreira et al. coveries with the known OCs and discuss the main results.The concluding remarks are given in Sect. 5.
We have used data from the
Gaia
DR2 catalogue (Linde-gren et al. 2018; Evans et al. 2018) to search and characterizestar clusters towards the Galactic centre direction. This cat-alogue provides astrometric (positions, proper motions andparallaxes) and photometric (magnitudes in the G , G BP and G RP bands) data for more than 1.3 billion sources (Gaia Col-laboration et al. 2018). Gaia@AIP (https://gaia.aip.de/) on-line services have been used to extract Gaia
DR2 data withinGalactic coordinates − ◦ ≤ b ≤ ◦ and − ◦ ≤ (cid:96) ≤ ◦ .We applied a first basic filter in our data by keeping onlystars brighter than G = 18 mag, the same filter applied byCantat-Gaudin et al. (2018). This filter removes very faintand less informative sources (with high astrometric uncer-tainties). A second set of filters, which assures quality tothe data by cleaning the sample from contamination due todouble stars, astrometric effects from binary stars and cal-ibration problems, was applied by using equations (1), (2)and (3) from Arenou et al. (2018). In the present work, we have searched for new clusters fol-lowing procedures similar to those described in F+2020,with the whole surveyed area sectioned in tiles of equal area.We restricted our sample to 200 square tiles of 1 × area in Galactic coordinates (Fig. 1).Briefly, for each tile, we determined a mean colour value G BP − G RP and built two smaller subsamples: one with starsbluer than this cut limit and another one with stars redderthan this value. Both subsamples were analysed on VPDs,where we searched for overdensities by inspecting the starsdistribution. Any overdensities found were extracted by ap-plying a box-shaped mask of size 1 mas yr − and searched forclustered structures in sky charts. Any clustered structurewas spatially trimmed by applying a circle-shaped mask of5-10 arcmin radius, depending on its visual size. Finally, weinspected the colour-magnitude diagram (CMD) and par-allax distribution of the resulting samples to confirm thecandidate.The subsamples filtered by proper motion and spatialmasks were used to compute the initial mean values of µ ∗ α , µ δ , (cid:36) , (cid:96) , and b and their dispersion (except for (cid:96) and b ).These quantities are interactively refined during the analysisof confirmed candidates; e.g. the centre and radius are up-dated after construction of the radial density profiles, propermotion and parallaxes are updated after assessing member-ship, etc.In total, we have surveyed a projected area of ∼ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ b ( d e g r ee s ) Figure 1.
Spatial coverage of the Galactic fields surveyed in thiswork. The size of the white squares indicates the surveyed regions.The colours indicate relative stellar densities from the
Gaia
DR2catalogue. The red dots indicate the position of the 34 newlydiscovered OCs in this work. performed an internal match with the initial catalogue, sothis initial number has been reduced to 118 objects (109OCs and 9 GCs).
To identify the detected OCs, we initially matched our cen-tre coordinates with those on the literature. Then we builta reference database by compiling the following catalogues:Röser et al. (2016), Castro-Ginard et al. (2018), Borissovaet al. (2018), Guo et al. (2018), Castro-Ginard et al. (2019),Bica et al. (2019) , Ferreira et al. (2019), Cantat-Gaudinet al. (2019), Torrealba et al. (2019), Bastian, U. (2019),Sim et al. (2019), Liu & Pang (2019),Castro-Ginard et al.(2020), Hao et al. (2020), F+2020, Qin et al. (2020) andHe et al. (2020). We searched for close neighbours within 1 ◦ and considered a newfound candidate when none of the ob-jects share the same astrometric space or when the centresdistance are larger than 5 arcmin for smaller objects and10 arcmin for larger objects. At this point, we identified 84known objects (including OCs and bulge GCs).After this first procedure, we determined the limitingradius for the candidates and repeated the matching proce-dure, but now taking into account the sizes of the objects.So, if the separation of the centres is larger than or equalthe sum of our limiting radius and the literature radius, weassume it as a new cluster, because we do not expect anycontamination. Finally, we also cross-matched our clustersmember lists with the members available in the literature.If the cluster and its neighbours have an area in common inthe sky – for example a halo overlap –, but do not presentany shared members, we assume they are different objects.For the best of our knowledge, the 34 new clusters reportedin this work do not share stars nor are sub-structures of anyknown objects. We used the peak values of proper motions and parallaxescomputed in the Sect. 2 and restricted our sample by a 3D
MNRAS000
MNRAS000 , 1–6 (2020) iscovery of new star clusters Table 1.
Derived astrophysical parameters for the studied clusters. Uncertainties are also presented, except in the case of µ ∗ α , µ δ and (cid:36) , where 1-sigma dispersions are reported instead. ID RA DEC r lim E(B − V) (m − M) log t µ ∗ α µ δ (cid:36) ( deg ) ( deg ) ( arcsec ) ( mag ) ( mag ) ( mas/yr ) ( mas/yr ) ( mas ) UFMG63 263.400 -25.010 654.2 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± box in the astrometric space: 1 mas yr − around the meanproper motion values and 1 mas around the mean parallax.Then we used the centre coordinates as a first guess tobuild the Radial Density Profiles (RDPs). The RDPs wereused to estimate the size of our OC candidates, the localbackground density and to refine the centres previously com-puted. See F+2020 for details. We defined the limiting radius( r lim ) as the radius beyond which stellar densities start dofluctuate around a nearly constant value. Since our cluster candidates are projected against dense stel-lar fields, in order to derive stars membership likelihoods,we applied a routine (fully described in Angelo et al. 2019)that evaluates statistically the overdensity of cluster stars incomparison to those in a nearby field in the 3D astrometricspace ( µ ∗ α , µ δ , (cid:36) ). We call the region centred in the cluster’scoordinates and restricted by its r lim as the cluster region.The comparison field is restricted by a ring-like region withinner radius r lim +3 (cid:48) . The outer radius is such that the fieldarea is equal to 3 times the cluster region. A detailed de-scription of the procedure can be find in F+2020. We em-ployed PARSEC-COLIBRI models (Marigo et al. 2017) toperform isochrone fittings on the CMDs of the decontami-nated cluster samples to determine age, distance and red-dening. To do so, we visually inspected the match betweenthe model and the cluster stars loci in specific evolutionaryregions (main sequence, the turnoff, and the giant clump).To convert E ( G BP − G RP ) to E ( B − V ) , we adopted a red- dening law (Cardelli et al. 1989; O’Donnell 1994). Fig. 2presents a sequence of panels showing our results for thecluster UFMG94: the best-fitting isochrone to the decon-taminated CMD, the cluster members spatial distributionand a diagram showing the cluster members distribution inproper motion in declination in function of their parallaxes.The resulting values for the centre coordinate, size,mean astrometric values, colour excess, distance module andage for all the clusters are presented in the Table 1. This ta-ble is also available electronically. We have crossmatched ourmemberlist catalogue ids with Gaia
EDR3 (Gaia Collabo-ration et al. 2020), applied the quality filters that ensuresastrometric valid solutions and made the zero point paral-laxes correction. The OCs 3d astrometric spaces ( µ ∗ α , µ δ and (cid:36) ) are concentrated and present similar mean proper mo-tions, paralaxes and dispersions with respect to Gaia
DR2,i.e., the OC candidates presented a concentrated structureand seems not to be random fluctuations in
Gaia
DR2 data.
According to the literature (Dias et al. 2002; Kharchenkoet al. 2013; Ferreira et al. 2020; Sim et al. 2019; Liu & Pang2019; Castro-Ginard et al. 2020; Cantat-Gaudin & Anders2020; He et al. 2020), there exist at least 206 OCs with de-termined parameters projected towards the Galactic centredirection. Our method was capable to recover 109 OCs inthis region, 34 of them are reported as new objects, which
MNRAS , 1–6 (2020)
F. A. Ferreira et al. BP -G RP )181614121086 G candidato42 (cid:95) (cid:155) cos (cid:98) ( O )-33.5-33.4-33.3-33.2 (cid:98) ( O ) -0.5 0.0 0.5 1.0 1.5 µ (cid:95) cos (cid:98) (mas.yr -1 )0.50.60.70.80.91.01.1 p l x ( m a s ) Figure 2.
Sequence of panels showing our results for the cluster UFMG94. Left: example of PARSEC-COLIBRI isochrone fitted (solidline) over the cleaned CMD. We overplotted the corresponding binary sequence (dashed lines) by deducing 0.75 mag from the G magnitudevalues. Middle: and example of the cluster members spatial distribution. Right: (cid:36) versus µ α cos δ plot for stars in the cluster area (thatis, r ≤ r lim ). In each panel, the symbol colours were assigned according to the membership scale represented by the colour bar in therightmost plot. represents an increment of ∼ of the known OCs withderived parameters in this same direction.According to the mentioned catalogues, there exist 82OCs with known distance values within 2 kpc. Our new OCssample represents a fractional increment of ∼ up to1.1 kpc, 38% up to 1.5 kpc and 39% up to 2 kpc. Accord-ing to the catalogues, 118 OCs have astrometric parametersfrom Gaia
DR2 data in this region, in this way, our findingsrepresent an increment of about ∼ of this sample. Theincrement represented by our newly found OCs is shown inthe Table 2.Recently, CG+2020 built a large and homogeneous OCcatalogue constituted by objects characterized with Gaia
DR2 data, which includes known OCs and newly foundOCs. Their catalogue contains 60 OCs within the regioncovered in this work, which means that our sample providesa significant increase of objects with distance, colour excessand age in the investigated region. Fig. 4 shows OCs fromCG+2020 (orange and red square) and our newly discoveredsample (cyan and red triangles) in the Galactic plane. Fig.3 shows that most of our new findings present ages between . < log t < . .The top panel of the Fig. 4 shows how young OCs(blue squares), with ages limited by 50 Myr (correspond-ing to log t < . ), delineate the local Galactic structure asspiral arms. We note that our findings are located in theSagittarius arm, with the exception of two objects (UFMG87 and UFMG91).The bottom panel of the Fig. 4 shows how our findingscontributed to the local OCs population in the Sagittariusarm. However, even with the addition of objects presentedin F+2020, we still note a lack of objects at the position ( X, Y ) ∼ (1000 , − pc, suggesting that either the armis discontinuous at this position or we may still be missingyoung clusters at this particular direction. Table 2.
The impact of the newly discovered OCs in this workover the local OC population.Sample fraction increaseTotal /
206 17% with
Gaia data /
118 29% with astrophysical parameters /
129 26% with d < kpc /
82 39% with d < . kpc /
57 38% with d < . kpc /
32 19%
Figure 3.
Blue: OC sample from the homogeneous OC cata-logue constituted by objects characterized with
Gaia
DR2 datapresented in CG+2020 and restricted by the Galactic coordinates − ◦ ≤ (cid:96) ≤ ◦ and − ◦ ≤ b ≤ ◦ . Red: The OC sample found inthis work. We discovered and derived astrophysical parameters for 34OCs projected in the Galactic bulge direction using
Gaia
DR2 data. Our methodology, combined with the high preci-sion of
Gaia
DR2 astrometric and photometric data, allowedus to find the OCs in the astrometric space. The investigated
MNRAS000
MNRAS000 , 1–6 (2020) iscovery of new star clusters ¡ ¡ ¡ ¡ ¡ ¡ ¡ Y ( p c ) CG + 2020 (all)CG + 2020 (young)this workthis work (all) ¡ ¡ ¡ ¡ Y ( p c ) CG + 2020 (all)CG + 2020 (young)this workthis work (young)F + 2020 (all)F + 2020 (young)
Figure 4.
Top: OCs presented in CG+2020 (orange and redsquares) and in the present work (cyan and blue triangles). Bot-tom: zoom in showing also objects from F+2020 (yellow and pinkupside down triangles).
OCs in this work are mainly located within 2 kpc from theSun and are projected in the Galactic bulge direction. Theirages are comprised between ∼
10 Myr and ∼ . and . .When restricting the distance range to 2 kpc, our OCssample represents an increment of ∼ at this regime ofdistances. In the same way, our findings represent an incre-ment of about ∼ of OCs that have been characterizedby means of Gaia
DR2 data. Although these objects fill animportant gap in the open clusters distribution along theSagittarius arm, this arm, traced by known clusters, ap-pears to be interrupted. Even though such discontinuitieshave been observed in other galaxies, it could also be anartifact due to the incompleteness of the cluster census.
ACKNOWLEDGEMENTS
We thank the agencies FAPEMIG, CNPq and CAPES (fi-nance code 001). This research has made use of the VizieRcatalogue access tool, CDS, Strasbourg, France and hasmade use of data from the European Space Agency (ESA)mission
Gaia ( ), pro-cessed by the Gaia
Data Processing and Analysis Con-sortium (DPAC, ). Funding for the DPAC has been pro- vided by national institutions, in particular the institutionsparticipating in the
Gaia
Multilateral Agreement. This re-search has made use of TOPCAT (Taylor 2005).
DATA AVAILABILITY
The data underlying this article is publicly available (GaiaDR2) or is available in the article.
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