arXiv:0902.4406v1 [astro-ph.GA] 25 Feb 2009 otiigayugsprsa lse n tlat12 compara- least being at latter and the cluster 2002), clusters, al. super young et (Larsen young surrounding with 6946 NGC a bubble in containing A detected . was diameter external in (Paczy´nskiserved explosions 2000; hypernova Larsen or & Efremov 2002), high- 1998). (Elmegreen, a al. of cloud et Larsen infall HI the star- are stability velocity Alternative mechanisms the collapse. triggering above formation gravitational Lada ISM & ca- neutral against Elmegreen is the bubble criterion SCP05; compressing expanding The of hereafter 1994). a (e.g. pable al. – of front et Whitworth border 2005 ionisation 1977; the or al. wave et along clus- Soria density happen Subsequently, expanding may occur. radially formation may star which formation tered from density, star (ISM) enhance- medium localised local interstellar the a The of produces ment 1996). perturbation Staveley-Smith mechanical & Higdon initial of (Wallin, bubble disk a the within trigger formation the can star (GC) self-propagating cluster of wave globular or a of that passage suggest the assumptions impact and criteria Disk-stability Introduction 1. orsodneto Correspondence edopitrqet to requests offprint Send srnm Astrophysics & Astronomy oebr3 2018 3, November ntepsil eeaino h on asv pncluster open massive young the of generation possible the On rmnn,ioae trfrigbblshv enob- been have bubbles star-forming isolated Prominent, e words. a as Key clusters globular massive that particular. idea the with consistent hsseai scnitn ihtetm-cl fterbac their Conclusions. of time-scale formation. the star with ope for consistent massive young is the scenario of this birth-site the with uncertainties, ontn enus ot,wihcniesteds,sphe disk, the considers which Results. Bolte, & Hernquist Johnston, Methods. DB12adSehno 2 Stephenson and 122 BDSB Context. – accepted –; Received 2 1 prxmtl 24 approximately Aims. PD-Uiesdd oVl oPr´b NVP v Shishim Av. UNIVAP, Para´ıba - Brazil do SP, Vale 12244-000, do Universidade - IP&D P101 S ot lge95190 Brazil A 91501-970, de Departamento Alegre e-mail: Porto Sul, RS, do 15051, Grande CP Rio do Federal Universidade ets h yohssta h otmsiegoua cluste globular massive most the that hypothesis the test We ihter-osrce ri of orbit re-constructed the With asv bet uha lblrcutrpsigthrough passing cluster globular a as such objects massive A aen@fursb,bc@fursb,[email protected] [email protected], [email protected], h risof orbits The oehrwt pncutrfrainascae odensity to associated formation cluster open with Together [email protected] : Galaxy tpesn2adBS 2 by 122 BDSB and 2 Stephenson ± .Bonatto C. : )goua lse:individual: cluster: globular :) y g,myhv rgee h omto fteoe cluste open the of formation the triggered have may ago, Myr 2 ω ..Salerno G.M. aucitn.W100˙AA˙V2 no. manuscript etui tpesn2adBS 2 r optdfrtethre the for computed are 122 BDSB and 2 Stephenson Centauri, ω 1 .Bica E. , etui eso httelts matst sconsistent, is site impact latest the that show we Centauri, ≈ 0 cin pc 600 ω ABSTRACT 1 .Bonatto C. , Centauri; wrsmto nteds,sokwv rpgto n delay and propagation wave shock disk, the in motion kwards lsesBS 2 n tpesn2 ihnuncertainties, Within 2. Stephenson and 122 BDSB clusters n dtoa rgntr foe lses h asv nsin ones massive the clusters, open of progenitors dditional odladhl rvttoa potentials. gravitational halo and roidal 03(iae l 03,rsetvl.2MASS and respectively. 1990) 2003), (Stephenson al. 1990 et (Bica ( in 2003 discovered com- 2007). low- 3 W al. the the disk-crossing et in is (Bica trigger 6397 origin NGC star-formation plex possibility as GC the 584 FSR Another the GC relates 2003). mass for to Cudworth case responsible 6231 & NGC crossing possible be (Rees OC GCs, may A the average, formation. that of on star is Myr 1 some every assumption disk natural the A refer- itational. associa- and OB 2004, is contains Chatterjee example therein). & which ences Cordes possible explo- (Vlemmings, superbubble, A Galaxy tions hypernova Cygnus structures. The and/or impact similar the 2000). the trigger- cloud harbour Larsen be The & may HI Efremov to OCs. (Elmegreen, high-velocity appears sions Galactic 6946 a NGC luminous in of most mechanism the ing to ble 4rdsprins slctda d at located has most 122 is BDSB the Indeed, supergiants, among Galaxy. red the are 14 in clusters known Both OCs 2002). massive al. et (Ortolani aaae l 20)adOtln ta.(02,providing (2002), al. by et rich- confirmed Ortolani suspected of was and age The supergiants an (2001) 1. red al. Fig. in et Nakaya in 2 Stephenson shown of are ness clusters both of www.ipac.caltech.edu/2mass/releases/allsky h C tpesn2adBS 2 were 122 BDSB and 2 Stephenson OCs The grav- essentially are effects triggering the GC, a For Galaxy stronomia nteGalaxy, the in r iui 91-Ubnv,Sa o´ o Campos S˜ao Jos´e Urbanova, dos - 2911 Hifumi, a 1 n .Rodrigues I. and , h iko aaycntigrsa formation. star trigger can galaxy a of disk the ≈ )oe lsesadascain:individual: associations: and clusters open :) sb,[email protected] gs.br, 0Mr n itnefo h u d Sun the from distance a and Myr, 20 ae nsia rs h rsn eut are results present the arms, spiral in waves ω sSehno n DB122. BDSB and 2 Stephenson rs ω Centauri etui hc rse h disk the crossed which Centauri, 2 -opnn oe of model e-component ⊙ ihnimportant within 5 = . p rmthe from kpc 8 images )

c S 2018 ESO ⊙ kpc 6 = s 2 G.M. Salerno et al.: Massive open clusters generated by ω Centauri?

Table 1. Present-day cluster positions

Cluster ℓ b α(J2000) δ(J2000) ◦ ◦ ◦ ′ ′′ ( ) ( ) (h:m:s) ( , , ) (1) (2) (3) (4) (5) ω Centauri 309.10 +14.97 13:26:46 −47:28:37 BDSB 122 26.84 +0.65 18:37:58 −6:53:00 Stephenson 2 26.18 −0.06 18:39:20 −6:01:44

′ ′ Fig. 1. 5 5 2MASS KS images of Stephenson 2 (left) and × from the catalogues of Bica et al. (2003) and Dutra et al. BDSB 122 (right). Figure orientation: North to the top and (2003) have already been studied in detail (e.g. Soares et al. East to the left. 2008; Ortolani et al. 2008; Hanson & Bubnick 2008). We trace backwards in time the orbits of ω Centauri and Sun that of the OC Stephenson 2 (and consequently, also that of 10 BDSB122 + Stephenson2 BDSB 122) in the disk, testing an impact hypothesis for the ωCen − present position origin of these two massive OCs. Orbit integrations in the 8 Galactic potential using as constraints the GC space veloc- ity have been widely applied to 54 GCs (e.g. Dinescu et al. 6 2003; Allen, Moreno & Pichardo 2008). This paper is structured as follows. In Sect. 2 we study 4 the past orbit of ω Centauri. In Sect. 2.1, the past orbits of Stephenson 2 and ω Centauri are compared looking for 2 Outer Arm spatial and time coincidence. Conclusions are in Sect. 3.

(kpc) GC

GC 0

y Bar

Local Arm 2. ω Centauri as a projectile −2 6 ω Centauri, the most massive Galactic GC (4 10 M⊙- × −4 Nakaya et al. 2001), has a metallicity spread and a flat den- Norma sity distribution typical of a nucleus captured −6 by the Galaxy (Bekki & Freeman 2003). Thus, irrespective −Crux of the existence of young massive clusters, somehow associ-

Perseus ated with the impact site, the orbit of ω Centauri under the −8 Sagittarius−Carina Galactic potential is worth studying, especially to look for −16 −14 −12 −10 −8 −6 −4 −2 0 2 4 consequences of the last disk passage. Evidence of a similar x (kpc) GC disk impact and a star-forming event has been observed in the spiral galaxy NGC 4559 with Hubble Space Telescope Fig. 2. The present-day positions (and uncertainties) of (HST), XMM-Newton, and ground-based (SCP05) data. Stephenson 2, BDSB 122, and ω Centauri overplotted on a The age of the star-forming complex is < 30Myr, with a schematic projection of the Galaxy as seen from the North ring-like distribution. It appears to be an∼ expanding wave pole, with 7.2 kpc as the Sun’s distance to the Galactic cen- of star formation, triggered by an initial density pertur- tre. Main structures are identified. bation. The most likely triggering mechanism was a col- lision with a satellite dwarf galaxy crossing through the

4 gas-rich outer disk of NGC4559, which appears to be Sun, an estimated mass of 2 4 10 M⊙, and an age of − × the dwarf galaxy visible a few arcsec NW of the com- 7 12Myr (Figer et al. 2006). Stephenson2 has 26 red su- plex. The picture is reminiscent of a scaled-down version − +1.9 pergiants, is located at d⊙ =5.8−0.8 kpc from the Sun, has of the Cartwheel galaxy (Struck-Marcell & Higdon 1993; 4 an estimated mass of 4 10 M⊙, and an age of 12 17Myr Struck-Marcell et al. 1996). (Davies et al. 2007). Their× distances from the Sun− are the As another example, proper motions (PMs) and ra- same, within uncertainties, and their projected separation dial velocity suggest that the GC NGC6397 crossed on the sky is 100pc. The designation Stephenson2 was the Galactic disk 5 Myr ago, which possibly triggered originally given≈ by Ortolani et al. (2002), and also adopted the formation of the OC NGC 6231 (Rees & Cudworth by Dias et al. (2002, and updates). Stephenson 2 and 2003), thus lending support to the present scenario BDSB122 are clearly in the red supergiant (RSG) phase (Wallin, Higdon & Staveley-Smith 1996). NGC 6397 and (Bica, Santos Jr. & Alloin 1990). Davies et al. (2007) refer NGC6231 are closely projected on the sky (∆ℓ 5◦, to these clusters as RSGC 1 and RSGC 2, respectively. ∆ b 13◦). However, in the case of ω Centauri as≈ possi- The positions (and uncertainties) of both clusters, to- ble generator≈ of BDSB 122 and Stephenson 2, the GC is gether with ω Centauri (NGC5139), are shown in Fig. 2 su- now widely apart from the pair of massive OCs (∆ℓ 77◦, perimposed on a schematic view of the (based on ∆ b 15◦). Thus, PM and are fundamental≈ Momany et al. 2006 and Drimmel & Spergel 2001). The lo- constraints≈ for the analysis of ω Centauri, and impact solu- cus of BDSB 122 and Stephenson 2 is slightly more internal tions require a detailed integration of the orbit across the than the Scutum-Crux arm. Several other young clusters Galactic potential. G.M. Salerno et al.: Massive open clusters generated by ω Centauri? 3

8 4

ω Cen impact Steph2 d=5.05 kpc 6 3 Steph2 d=5.83 kpc Steph2 d=7.74 kpc

4 2

2 1

0 0 Y (kpc) Y (kpc)

−2 −1

−4 −2

−6 −3

−8 −4 −8 −6 −4 −2 0 2 4 6 8 −8 −7 −6 −5 −4 −3 −2 −1 0 X (kpc) X (kpc) 4 4 ω Cen impact ω Cen impact Steph2 d=5.83 kpc Steph2 d=5.83 kpc 3 Steph2 d=5.05 kpc 3 Steph2 d=5.05 kpc Steph2 d=7.74 kpc Steph2 d=7.74 kpc

2 2

1 1

0 0 Z (kpc) Z (kpc)

−1 −1

−2 −2

−3 −3

−4 −4 −8 −7 −6 −5 −4 −3 −2 −1 0 −8 −7 −6 −5 −4 −3 −2 −1 0 X (kpc) Y (kpc)

Fig. 3. Top-left panel: Galactocentric XY-plane projection of the ω Centauri orbit over the past 2 Gyr. Top-right: The past 30Myr orbit of ω Centauri (solid line) for RGC =7.2 kpc. Additional neighbouring orbits (dotted) are, from bottom to top: JHB96 (-10%), JHB96 (+10%), FSC96. The impact site on the disk is shown by the empty square. Arrows indicate orbit direction. Orbits of Stephenson 2 for the assumed distance from the Sun (and uncertainties) are shown. The corresponding XZ and YZ projections are in the bottom panels. Empty symbols over the Stephenson 2 orbits indicate its possible positions 24 Myr ago. The Sun at its present position (asterisk) and the Galactic Centre (filled circle) are shown.

2 2 2 2.1. Orbit computation (Hernquist 1990), and φhalo(R) = vhalo ln (R + d ), 11 10 where Mdisk = 1 10 M⊙, Mspher = 3.4 10 M⊙, −1× × The present three-component mass-distribution model vhalo = 128kms , R and z are the cylindrical coordi- of the Galaxy follows that employed in the study nates, and the scale lengths a = 6.5 kpc, b = 0.26 kpc, of a high-velocity black hole on a Galactic-halo or- c = 0.7kpc, and d = 12.0 kpc. Table 1 shows the Galactic bit in the solar neighbourhood (Mirabel et al. 2001, and Equatorial coordinates of the three clusters. Following and references therein). In short, we use the three- Mizutani, Chiba & Sakamoto (2003), the relevant pa- component model of Johnston, Hernquist & Bolte rameters for computing the motion of ω Centauri are (1996) – hereafter JHB96 – in which the disk, the distance from the Sun d⊙ = 5.3 0.5kpc, the PM −1 spheroidal, and halo gravitational potentials are de- components (mas yr ) µα cos(δ) = ± 5.08 0.35 and 2 − ± 2 √ 2 2 µδ = 3.57 0.34, and finally the heliocentric radial scribed as φdisk(R,z)= GMdisk/qR + a + z + b −1 −  − r ± GMspher velocity V = 232.5 0.7kms . (Miyamoto & Nagai 1975), φspher(R) = ± − R+c 4 G.M. Salerno et al.: Massive open clusters generated by ω Centauri?

ω Cen impact (7.6 kpc) ω Cen impact (7.6 kpc) ω Cen impact (7.2 kpc) ω Cen impact (7.2 kpc) Steph2 d=5.05 kpc Steph2 d=5.05 kpc 0 Steph2 d=5.83 kpc 0 Steph2 d=5.83 kpc Steph2 d=7.74 kpc Steph2 d=7.74 kpc Steph2 d=5.05 kpc Steph2 d=5.05 kpc Steph2 d=5.83 kpc Steph2 d=5.83 kpc Steph2 d=7.74 kpc Steph2 d=7.74 kpc

−1 −1 Y (kpc) Y (kpc)

−2 −2

−3 −3 −5 −4 −3 −2 −5 −4 −3 −2 X (kpc) X (kpc)

Fig. 4. Close-up of the impact site. Left panel: Orbits computed with RGC =7.2 kpc (empty symbols) and RGC =7.6kpc (filled symbols). Right: Same as left panel but including error distribution for the ω Centauri impact site and Stephenson 2 proto-cluster position. A random selection of impact sites (open circles) is shown within the ω Centauri error ellipsoid.

The models were computed with RGC = 7.2kpc tainties (Sect. 1). The interesting fact is that the orbit of (Bica et al. 2006) as the distance of the Sun to the Galactic Stephenson 2 passes close to the impact site of ω Centauri at centre. The Galactic velocities of ω Centauri are U = a comparable time, within uncertainties (see below). Since 54.3 9.5kms−1, V = 44.2 8.2kms−1, and W = Stephenson 2 and BDSB 122 have almost the same position 1.3± 13.0kms−1. Alternatively,− ± we also computed or- (within uncertainties), the same conclusions hold for the − ± bits with RGC = 7.6 kpc, obtained by Eisenhauer et al. latter cluster. The XZ and YZ-plane projections (bottom (2005). It should be noted that recently, by means of sta- panels) show that ω Centauri emerged at 45◦ from the tistical parallax of central , Trippe et al. (2008) found plane to its present position. ≈ RGC = 8.07 0.32 kpc, while Ghez et al. (2008), with the ± orbit of one star close to the black hole, found RGC = To probe orbital uncertainties owing to the adopted 8.0 0.6kpc or RGC = 8.4 0.4kpc, under different as- potential, we also employed the potential model of sumptions.± Cluster distances± are heliocentric, which do not Flynn, Sommer-Larsen & Christensen (1996) – hereafter depend on RGC; on the other hand, the value of RGC has FSC96 – and tested consequences of variations of 10% in some effect on the potentials, which thus, affects orbit com- the input parameters of JHB96. The results are shown± in putation. Since the difference between the adopted value of Fig. 3 (top-right panel), from which we conclude that orbit RGC and the more recent ones is not exceeding, the value variations due to the adopted potential are much smaller of RGC should not influence much the present results. than our error ellipsoid (Fig. 4, right panel). Based on the rotation curves of Brand & Blitz (1993) Close-ups of the ω Centauri impact site and the back- and Russeil (2003), and an estimate with the galaxy mass traced positions of Stephenson 2 are shown in Fig. 4 (left model described above (Mirabel et al. 2001), we derived −1 panel) for a Sun’s distance to the Galactic centre of 7.2 kpc Vc = 214 4kms as the orbital velocity of Stephenson 2. and 7.6 kpc. It is clear that the latter value is not critical The nearly± flat Galactic rotation curve at the Stephenson 2 for the encounter. The right panel shows the error ellip- position allows to adopt this circular velocity also for the or- soid of several impact site simulations computed by vary- bits corresponding to distance uncertainties (Sect. 1). The ing initial conditions according to the errors in the differ- orbit of ω Centauri, computed back over 2 Gyr, is compara- ent relevant input quantities. The ellipsoid reflects varia- ble to that derived by Mizutani, Chiba & Sakamoto (2003), tions implied by velocity uncertainties in the PM, radial in particular the Rosette pattern seen projected on the XY velocity and present position of ω Centauri along the line of plane (Fig. 3). The simulation shows that ω Centauri hit sight in the (U,V,W) velocities. The impact obtained with the disk as recently as 24 2Myr ago. Because of such a a Galactocentric distance R = 7.6 kpc is also shown. short time, fossil remains of± this event may be detectable GC The disk-orbit of Stephenson2 crosses the ω Centauri ellip- in the disk nowadays. soid error distribution. The range in impact site to proto- Figure 3 (top-left panel) shows the Galactic XY-plane cluster separations contains distances smaller than 1 kpc, projection of the last 2 Gyr orbital motion of ω Centauri. with an average separation of 500 pc (intersection≈ area in In the remaining panels we focus on the last 30 Myr of the Fig. 4, right panel). Larger separations∼ would require pro- motion of ω Centauri and Stephenson 2. For Stephenson 2 hibitive expansion velocities, despite the fact that we are we consider the different orbits that result from the dealing with an encounter in a denser, central part of the adopted distance from the Sun and corresponding uncer- disk, while in NGC 4559, the event was external. G.M. Salerno et al.: Massive open clusters generated by ω Centauri? 5

For the GC-induced formation hypothesis to be valid, the expanding bubble scenario such as that in NGC 4559 the time-scales associated with the onset of star for- would apply. The latter case is more probable, since two mation (after impact), duration of star formation and clusters have been formed. the cluster age, should be compatible with the disk- Levy (2000) performed 2D hydrodynamic simulations crossing age. Following Putte & Cropper (2009), the first to study the impact of GCs on the Galactic disk in the time scale in not well known, ranging from virtually presence of available gas. They found that the moving GC instantaneous, i.e. negligible as compared to the clus- causes a shock wave in the gas that propagates through the ter age, to 15 Myr (L´epine & Duvert 1994) and 30 Myr disk in a kpc scale, thus producing star formation. More (Wallin, Higdon & Staveley-Smith 1996). The star forma- recently, Putte & Cropper (2009) simulated in detail the tion time-scale may be short, 2 105 yr, as suggested by effects of GC impacts on the disk that basically confirm ≈ × McKee & Tan (2002) for stars more massive than 8 M⊙. Wallin, Higdon & Staveley-Smith (1996) and Levy (2000), Given that the ages of Stephenson 2 and BDSB 122 are even in the absence of gas at the impact site. They found within 12-17 Myr and 7-12 Myr, respectively, ω Centauri, focussing of disk material with compression on a scale of which crossed the disk 24 Myr ago, may have triggered 10 pc, that may subsequently attract gas leading to star their formation only if the≈ star-formation onset occurred in formation.∼ The compression increases with the GC mass. less than 15 Myr, which is within the accepted range. In At this point an interesting question arises. For a rate the case of≈ NGC4559 these time-scales combined are less of 1GC impact per Myr, it can be expected a high prob- than 30 Myr (Soria et al. 2005). ability∼ of occurring one GC impact within 1-2 kpc of any The∼ above clues suggest an interesting event, that the location within the inner Galaxy in about 10 Myr. However, most recent crossing of ω Centauri through the disk oc- the star-formation efficiency of these events appears to be curred very close to the sites where two massive OCs were low, according to Putte & Cropper (2009). Indeed, of the formed. Both Stephenson 2 and BDSB 122 are somewhat 54 GCs with accurate proper motions studied by them, only younger than the age of the impact, and the differences of three appear to be associated to young OCs. Possibly, con- a few Myrs are consistent with the shock propagation and ditions like GC mass and impact site properties, such as subsequent star formation. The overall evidence gathered the availability of molecular gas, temperature and density, in the present analysis supports ω Centauri as the origin of constrain the star-formation efficiency. this localised star formation in the Galaxy, which harbours Evidence drawn from the present work suggests that two of the more massive known OCs. GCs can be progenitors of massive OCs. In particular we fo- This work suggests a scenario where the disk passage cused on ω Centauri. Density-wave shocks are possibly not of GCs can generate OCs, massive ones in particular, as the sole responsible for the formation of the more massive indicated by the orbit of ω Centauri and its impact site. OCs, a possibility to be further explored, both theoretically As a consequence, not all OC formation is induced by the and observationally. spiral density wave mechanism in spiral arms. Acknowledgements 3. Summary and conclusions We thank the anonymous referee for suggestions. 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