Astronomical Science

STEP: The VST Survey of the SMC and the Magellanic Bridge

Vincenzo Ripepi1 Time Observation survey being carried exploiting the large field of view (FoV) and Michele Cignoni2,3 out at the VLT Survey Telescope. STEP the high resolution of the OmegaCAM Monica Tosi3 will obtain homogeneous photometry instrument on the VLT Survey Telescope Marcella Marconi1 in the g-, r-, i- and Hα-bands over an (VST); see Ripepi et al. (2014) for a Ilaria Musella1 area of 74 square degrees covering the detailed presentation of the survey. The Aniello Grado1 main body of the Small Magellanic STEP survey is the optical complement to Luca Limatola1 Cloud (42 square degrees), the Bridge the VISTA Magellanic Cloud (VMC1) ESO Gisella Clementini3 that connects it to the Large Magellanic Public Survey (Principal Investigator [PI] Enzo Brocato4 Cloud (30 square degrees) and a small M.-R. Cioni), which is collecting Y, J and Michele Cantiello5 part of the Magellanic Stream (2 square Ks near-infrared photometry over an area Massimo Capaccioli6 degrees). Our photometry will allow us of about 184 square degrees covering Enrico Cappellaro7 to detect and measure the magnitudes the LMC, SMC and Bridge. STEP is part Maria-Rosa L. Cioni8,9 of individual stars well below the main of a large international effort aimed at Felice Cusano3 sequence turnoff of the oldest popula- studying in detail stellar populations, Massimo Dall’Ora1 tions. Here we describe the observing structure and evolution of the SMC, and Jay S. Gallagher10 strategy, the photometric techniques, based on photometric and spectroscopic Eva K. Grebel11 and the upcoming data products of the data acquired at the major international Antonella Nota2,12 STEP survey. Preliminary results for facilities (e.g., Hubble Space Telescope Francesco Palla13 the first two fields for which data acqui- [HST] and Very Large Telescope [VLT]). Donatella Romano3 sition is complete are also presented. Gabriella Raimondo5 The colour–magnitude diagram (CMD), Elena Sabbi2 containing stars born over the whole Fedor Getman1 Introduction ­lifetime of the galaxy, is a fossil record of Nicola R. Napolitano1 its SFH. Since the lookback time that Pietro Schipani1 The Local Group dwarf galaxies provide can be safely be investigated is of the Simone Zaggia7 an ideal laboratory for studying and test- order of the evolutionary time of the least ing galaxy formation theories and cos- massive main sequence (MS) star that mology. Their close proximity allows indi- can be resolved, deep CMDs are crucial 1 INAF-Osservatorio Astronomico di vidual stars to be resolved, with accurate to detect Solar-like MS stars (corre- Capodimonte, Naples, Italy photometry and spectroscopy. (see e.g., sponding to the oldest MS turn-off with 2 Space Telescope Science Institute, Tolstoy et al., 2009). Their stellar popula- evolutionary times comparable with an Baltimore, USA tions can be characterised in detail and entire Hubble­ time). Although this is usu- 3 INAF-Osservatorio Astronomico di their star formation histories (SFHs) ally the realm of HST (see e.g., Cignoni et Bologna, Italy derived. The Small Magellanic Cloud al., 2012; 2013), HST’s small FoV does 4 INAF-Osservatorio Astronomico di (SMC) is the closest dwarf galaxy of late not allow a systematic study of the whole Roma, Italy morphological type; hence the best SMC. 5 INAF-Osservatorio Astronomico di location for detailed studies of the prop- ­Teramo, Italy erties of this the most common class of With STEP we aim to investigate the stel- 6 Università Federico II, Naples, Italy galaxies. Its low chemical abundance lar populations of the SMC with CMDs 7 INAF-Osservatorio Astronomico di (Z = 0.004) makes the SMC the best local up to 1–2 magnitudes fainter than the Padova, Italy counterpart to the large majority of dwarf turn-off (TO) of the oldest population (with 8 University of Hertfordshire, Hatfield, irregulars and blue compact galaxies, sufficient photometric quality to reach a United Kingdom whose metallicity distribution is peaked 2 Gyr resolution for stars born 10 Gyr 9 Leibnitz-Institut für Astrophysik Pots- at this mean value. The SMC is also a ago), for a huge area including the entire dam, Germany member of the nearest group of interact- SMC and the Bridge as well. We also 10 University of Wisconsin-Madison, USA ing galaxies. In fact, it is tidally interacting intend to use classical variable stars (e.g., 11 Astronomisches Rechen-Institut, with its neighbours, the Large Magellanic RR Lyrae and Cepheid stars), as popu­ ­Zentrum für Astronomie der Universität Cloud (LMC) and the Milky Way. Investi- lation tracers in the relatively unexplored Heidelberg, Germany gating the signatures of these interactions region of the Bridge, and a spatially 12 European Space Agency, Space Tele- (the Bridge towards the LMC and the ­complete census (up to ~ 1 MA) of pre- scope Science Institute, Baltimore, Magellanic Stream) will allow us to con- main sequence (PMS) objects to investi- USA strain models of galaxy interaction. gate the first stages of star formation. 13 INAF-Osservatorio Astrofisico di Hence the SMC is an ideal benchmark ­Arcetri, Firenze, Italy in the study of the effects of tidal interac- With these broad capabilities, the STEP tions on galaxy evolution. survey will allow us to answer the fol­ lowing open questions: 1) What is the STEP (Small Magellanic Cloud in Time: We are carrying out the first deep and global SFH and age–metallicity relation Evolution of a Prototype interacting homogeneous photometric survey of the (AMR) of the SMC?; 2) Do field and star late-type dwarf galaxy) is a Guaranteed entire SMC body and of the Bridge by cluster components share the same

32 The Messenger 157 – September 2014 Figure 1. Map of STEP tiles (the two tiles centred in the direction of the Magellanic Stream are outside of the figure). To highlight the location of the SMC 2 body and of part of the Bridge, black dots indicate the position of known star clusters and associations (according to Bica et al. [2008]). The thick boxes ­correspond to the 1 square degree FoV of the VST tiles. Red boxes represent tiles whose observations are completed, green boxes those with completed

) time series photometry and blue boxes the remain- 0 4_6 ing ones. For comparison, thin grey boxes show

(Deg the VMC tiles, whereas the HST fields are the small ) 3_7 cyan-filled circles (note that the true size of HST .0 fields is significantly smaller). The two tiles analysed 73 in this work (tiles 3_7 and 4_6) are highlighted with

δ + filled red boxes. ( –2 tile and for each filter in the SMC body we obtain a couple of mosaics created by merging five dithered sub-images. We acquire a mosaic of short- and long- –4 exposure times in order to reach faint magnitudes, avoiding saturation for any –10–5 0 5 10 star. The time series images, on the (α–33.0) (Deg) ­contrary, consist of just one shot for each filter. For each tile we also obtain pairs SFH and AMR?; 3) Are there trends in of the RR Lyrae stars), with S/N of 100. of g, i (r, Hα) images during a photometric SFH connected with the interaction his- When summed, these images will allow night, in order to build up lists of second- tory of the SMC?; 4) How did the stellar us to reach g ~ 24 mag with S/N of 10. ary standards for the final photometric component of the Bridge form and what Originally, we planned to image the whole calibration (usually the scientific images is its SFH?; 5) What is the impact of Bridge with time series. However, the are taken in non-photometric conditions, metallicity on PMS accretion and on the overheads proved to be too high and we to increase the probability of execution — global properties of star formation? decided to cover the remaining Bridge see Table 2). fields without time series. STEP tiles are placed so as to maximise the overlap STEP observing strategy with the VMC survey (Cioni et al., 2011). Observations and data reduction

In order to address the questions listed The STEP observing strategy is reported The VST (built by the INAF–Osservatorio above, we proposed, and have obtained, in Table 1, whereas Table 2 shows the Astronomico di Capodimonte, Naples, part of the VST Guaranteed Time Obser- constraints for our observations. For each Italy) is a 2.6-metre-wide field optical sur- vation (GTO) allocation by ESO to the vey telescope (Capaccioli & Schipani, ­Italian Istituto Nazionale di Astrofisica Table 1. Observing strategy of the STEP survey 2011) sited on Paranal. The telescope is (INAF) in return for the procurement of (e.g., 5 × 25 s means five dithered exposures of equipped with OmegaCAM, a 1-square- the telescope. With STEP we aim at 25 s each). degree camera built by a consortium of acquiring g, r, i and H photometry for European institutes (Kuijken, 2011). The α Period T (g) T (i) 72 square degrees covering the whole exp exp camera is a 32 CCD, 16 k × 16 k detector 88–90 5 × 25 s; 5 × 25 s; SMC body, the Bridge and 2 square 5 × 520 s 5 × 520 s mosaic with 0.214 arcseconds per pixel degrees of the Magellanic Stream down 91–93 5 × 25 s; 5 × 25 s; scale. to a limiting magnitude (on the AB sys- 10 × 300 s 10 × 300 s tem) of g ~ 24 mag with signal-to-noise Photometric calibration STEP observations started in late 2011 (S/N) of 10 and Hα photometry to ~ 22.5 88–93 1 × 45 s 1 × 45 s during ESO Period 88 and are currently mag with S/N of 5. Time series continuing. Usually, the observations 88–91 1 × 25 s; 1x25 s; are carried out in service mode. The The survey is organised into tiles of 1 × 120s 1 × 180 s ­different colours of the tiles shown in one square degree each, partially over- lapping with each other to allow a ­homogeneous calibration (see Figure 1). Field Seeing Moon Airmass Weather Table 2. Observing constraints. See- In addition, we acquired 24-epoch time (arcseconds) ing is to be interpreted as the full width at half maximum measured on the series photometry of 8 square degrees SMC 1.0–1.1 0.5 1.8 Clear Bridge 1.1 0.5 1.8 Clear image. TS stands for time series while on the Bridge down to g ~ 19.5 mag (i.e., Bridge (TS) 1.4 0.8 1.8 Thin cirrus Phot. Cal. means photometric calibra- reaching fainter than the mean magnitude Phot. Cal. 1.5 0.8 1.8 Photometric tion.

The Messenger 157 – September 2014 33 Astronomical Science Ripepi V. et al., The VST Survey of the SMC and the Magellanic Bridge

tion; 3) preliminary absolute photometric 1*& 1 calibration; 4) relative photometry, relative 1*& and absolute astrometry by means of 2 ( SCAMP ; and 5) image resampling and co-addition through the SWARP3 pack- age. A portion of the final mosaic for tile ,& 4_6 is shown in Figure 2, where several 1*& star clusters and associations are clearly visible.

The PSF photometry on the final mosaics was carried out with Peter Stetson’s /LQGVD\ DAOPHOT/ALLSTAR package. The cata- DUFPLQ logues were matched by means of a ­custom procedure. The final precision of the photometry can be appreciated in Figure 2. g-band VST plate showing the northern and analysed, namely tiles 3_7 and 4_6 Figure 3 (upper panels). It can be seen part of tile 4_6, including the well-known star-­ (see Figure 1). These tiles are repre­ that our sensitivity requirement is met forming region NGC 346 and several other interest- ing clusters and associations, which are labelled. sentative of different environmental con- even in the crowded regions of the SMC ditions in the SMC: tile 3_7 is located in body. A detailed estimate of the com- the Shapley wing, a substructure whose pleteness of our photometry is a funda- ­Figure 1 illustrate the execution status as origin is likely connected to the interac- mental stage in an accurate reconstruc- of Period 92. The total number of hours of tion with the LMC, whereas tile 4_6 is tion of the SFH. To this aim, we have observation allocated up to now to STEP placed in the relatively unperturbed, but devised a custom procedure to add sev- during Periods 88–92 is 182 h; the hours mostly active, northern part of the body eral thousands of artificial stars to our of actual observations in the same period of the SMC. images without generating self-crowding. were about 98.7, with an efficiency (hours The result of the completeness experi- observed/allocated) of about 50%. As a The data reduction was carried out by ments for both tiles is shown in Figure 3 consequence of this rather low observing means of the VSTTube package (Grado (lower panels). It can be seen that 50 % efficiency, the current percentage of com- et al., 2012), which has been specifically completeness is achieved for g ~ 23.5 pletion of the entire survey is about 30%. developed to handle OmegaCAM data. mag in tile 4_6, which is much more The pipeline includes the following steps: crowded than tile 3_7; we reach the same In the following we describe the first two 1) accurate gain homogenisation (flat- completeness level at g ~ 24−24.5 mag tiles that have been completely reduced fielding); 2) image concentration correc- in tile 3_7.

0.5 Figure 3. Upper panels: The photometric errors The colour–magnitude diagram 0.4 Tile 3_7 Tile 4_6 (averaged per bins of magnitude) in g (blue)

and i (magenta) are Figure 4 shows the CMDs of the stars ) 0.3 shown for a sub-frame ag measured in tile 3_7 (left panel) and 4_6 20 × 21 arcminutes in

(m 0.2 (right panel). Since tile 4_6 is located

g,i size, placed in the mid- σ dle of tiles 3_7 and 4_6. in a populous part of the SMC, the corre- 0.1 Lower panels: the com- sponding CMD hosts over four times pleteness in the same more stars than tile 3_7, which is placed 0 regions are shown. The in the relatively low density and peripheral 100 and 50 % levels of 15 20 25 15 20 25 completeness are wing. As a consequence crowding is Mag Mag shown with black solid more severe in tile 4_6 and the corre- and dashed lines, sponding CMD much shallower (but still respectively. 1 1 mag deeper than the oldest MS TO)

) and sparser. To guide the eye, isochrones (% 0.8 of different ages and metallicities (Marigo et al., 2008) are overlaid on the CMDs. ness 0.6 te The metallicity of the youngest isochro- 0.4 nes is assumed to be Z = 0.004, which is consistent with spectroscopic derivations Comple g 0.2 i from H II regions in the SMC and from 0 stellar abundances in very young stars, 18 20 22 24 26 18 20 22 24 26 while the metallicity of the older isochro- MagMag nes is chosen to best fit the red giant

34 The Messenger 157 – September 2014 4,2

16  18 g 20  !+ 5 Myr, z = 0.004 50 Myr, z = 0.004 22 100 Myr, z = 0.004 F  1" 12 300 Myr, z = 0.004 %" 500 Myr, z = 0.004 24 3 Gyr, z = 0.001 5 Gyr, z = 0.001  12 Gyr, z = 0.001 2&! –0.5 0.5 1.5 2.5 –0.5 0.5 1.5 2.5 g–i g–i  Figure 4. CMD of tile 3_7 (left panel) and 4_6 (right panel) with overlaid stellar isochrones from Marigo et al. (2008): for metal abundance Z = 0.004, ages 5 Myr (green continuous line), 50 Myr (red dashed l        line), 100 Myr (blue continuous line), 300 Myr (pink FmH dashed line) and 500 Myr (cyan continuous line); for Z = 0.001, ages 3 Gyr (black dashed line), 5 Gyr objects at the transition between the RC Figure 5. For tile 3_7, the CMD regions used to iden- (orange continuous line) and 12 Gyr (dashed red and BL phases, hence stars with ages tify Upper Main Sequence (UMS), Blue Loop (BL), line). The assumed distance modulus and reddening Red Clump (RC), Sub Giant Branch (SGB), reddened E(B - V) are 18.9 and 0.08 mag for tile 3_7 (left between 500 Myr and 1 Gyr, our conclu- RC stars (RS) and Field Contamination (FC) sources panel), 18.9 and 0.04 mag for tile 4_6 (right panel) sion is that the region of tile 3_7 has been are illustrated. respectively. relatively more active than for tile 4_6 at these epochs. If we take into account the fact that tile 3_7 is in the low-density The spatial distribution branch (RGB). The assumed distance wing, at the frontier of the Bridge con- modulus is (m - M)0 = 18.90 mag, necting the SMC to the LMC, it is tempt- The spatial distribution of stars in differ- while the assumed reddening values, ing to relate the higher activity in tile 3_7 ent evolutionary stages yields important E(B - V) = 0.08 and 0.04 mag for tiles with an SMC/LMC interaction. Moreover, information on the star formation pro- 3_7 and 4_6, respectively, are chosen the CMD of tile 4_6 shows a prominent cesses over the region. We have counted to provide the best fit. RGB bump (physically caused by the stars in different age regions of the CMD H-burning shell crossing the chemical (see Figure 5), sampling the upper main In terms of stellar populations, the mere discontinuity left over by the convective sequence (UMS; red box), blue loop (BL; presence in both fields of an extended envelope), just above the RC, a feature pink box), red clump (RC; green box), and MS, a well-populated blue loop (BL) and absent in tile 3_7. sub-giant branch (SGB; blue box). Field red clump (RC) phase, as well as a wide contamination (FC; consisting of interlop- RGB, is a clear indication of common From a theoretical point of view, the evi- ers from the Milky Way and background prolonged activity in the SMC. Indeed, dence of an RGB bump brighter than galaxies) and highly reddened RC stars RGB and RC stars are stellar evidence of the RC is a clear indication that interme- (RS) are sampled within the cyan and yel- activity prior to 1 Gyr ago, while BL and diate and old star formation took place low boxes, respectively. In order to illus- bright MS stars are tracers of activity a at relatively low metallicity (Z = 0.001 or trate the power of this approach, Figure 6 few hundreds of Myr and a few Myr ago, less) in tile 4_6. However its apparent shows the spatial distributions of these respectively. However, both CMDs show lack in tile 3_7 CMD could be just due to different groups of stars for tile 4_6. an apparent lack of horizontal branch the lower number of stars and requires (HB) stars, suggesting that both the further exploration with a synthetic CMD It is clear that tile 4_6 harbours many regions, and presumably the entire SMC, approach in order to be corroborated. On clusters and associations, probably formed a minor fraction of their stars at the other hand, the populous RC protru- because of the generally high activity of epochs older than 10 Gyr ago. sion in the CMD of tile 3_7 represents a this region. The stellar density of older significant difference from tile 4_6, since populations (RC and SGB, middle left There are interesting differences between the latter is globally much more popu- and bottom left panels, respectively) the CMDs for tiles 3_7 and 4_6. First of lated. All these differences have to be increases smoothly towards the SMC all, the morphology of the RC is rather investigated taking into proper account centre (in this case, the lower-right corner elliptical in tile 4_6 CMD, while it shows a the larger crowding of tile 4_6, and the of tile 4_6), while the density of younger clear protrusion towards brighter mag­ corresponding larger photometric errors populations (UMS and BL, top left and nitudes in tile 3_7. Considering that the and blending effects. top right panels, respectively) is very RC protrusion is likely populated by irregular and dominated by inhomoge­

The Messenger 157 – September 2014 35 Astronomical Science Ripepi V. et al., The VST Survey of the SMC and the Magellanic Bridge

neities. Indeed, most UMS stars are UMS Blue loop 0.2 0.02 found aggregated in clusters/associations c8 –72.0 c9 –72.0 (identified clusters are indicated with c7 labels c1 to c18 in the maps in Figure 6), c15 –72.2 c17 c6 c18 –72.2 with the most prominent ones corre- c13 c10 c5 c16

sponding to NGC 346 (c15 in the map) C C –72.4 c14 0.1 –72.4 0.01 and NGC 371 (c9). DE DE c2 –72.6 c4 –72.6 We point out that the distribution of c3 RC stars shows clusters as well (c10, c11, –72.8 c1 –72.8 c11 c12), but that none of them turns out to have a counterpart in the UMS map (Fig- 16.5 15.5 16.5 15.5 ure 6). A simple explanation is that all RA RA agglomerates found in the latter are Red clump Reddened RC stars younger than 100 Myr, and hence, too 0.2 0.050 young to host RC stars, while clusters –72.0 –72.0 detected only in the RC maps are neces- c12 sarily too old to still have UMS stars in –72.2 –72.2 existence. It is also worth noting that c10 among the clusters visible using RC stars, C –72.4 0.1 C –72.4 0.025 namely c10, c11 and c12, only c10 and DE DE c12 are seen also in the SGB map. Indeed, –72.6 –72.6 the c11 cluster, the well-known NGC 419, is about 1 Gyr old, and hence, the tran­ –72.8 c11 –72.8 sition between the MS and the RGB phase, i.e. the SGB phase, is poorly pop- 16.5 15.5 16.5 15.5 ulated (the Hertzsprung Gap). There is RA RA an excellent correspondence between SGB Field contamination the literature ages and the evolutionary 0.10 0.10 phase adopted to detect the different –72.0 –72.0 c1–c18 structures, testifying to the relia- c12 bility of this approach. Finally, the map –72.2 –72.2 of reddened RC stars (middle right panel c10 in Figure 6) suggests a very patchy red- C –72.4 0.15 C –72.4 0.15 dening distribution. DE DE –72.6 –72.6

Star clusters –72.8 –72.8

Stellar clusters and associations are 16.5 15.5 16.5 15.5 RA RA among the main targets of the STEP ­survey. The known objects will be char- cluster, carefully studied on the basis Figure 6. The spatial distributions of the stellar pop- acterised in detail, and new clusters and of HST data and showing a dispersion in ulations selected from the CMD of Figure 5 are shown. The maps are produced by calculating a 2D associations, possibly missed by previ- age of about 0.15 dex (e.g., Glatt et al. histogram of stellar positions and then smoothing ous studies, will be searched for. In the 2009). It is extremely crowded and the the result with a Gaussian kernel. Labels c1 to c18 previous section we have shown how central regions are almost inaccessible indicate clusters identified by eye. several clusters can be detected in tile with the VST. IC 1624 is a younger and 4_6 through the analysis of the spatial looser cluster with respect to NGC 419. distribution. However these objects are We characterised the two clusters by cluster’s formation and evolution due to only a small part of the clusters/associa- measuring the structural parameters internal dynamical processes and the tions content of tile 4_6, and the compi­ through the analysis of their surface interaction with the galactic environment. lation by Bica et al. (2008) lists 114 clus- brightness profiles and estimating their The analytical function most suitable to ters and associations in tile 4_6 alone. ages. describe the SBPs of young Magellanic Cloud star clusters is that suggested In order to get an idea of STEP’s capabili- The number densities and surface bright- by Elson, Fall & Freeman (1987, EFF). We ties for cluster studies, we have analysed ness profiles (SBPs) are useful tools to used these models in our analysis. in more detail two systems located in tile study the properties of star clusters in 4_6: NGC 419 and IC 1624. NGC 419 is a different galactic environments. These The SBPs of NGC 419 and IC 1624 are well-known intermediate-age (~ 1 Gyr) profiles contain information about the shown in the upper panels of Figure 7;

36 The Messenger 157 – September 2014 13 Figure 7. Left panels: Black dots show the observed radial stellar surface brightness profile around the NGC 419 14 r < 85 arcsec 18 ­centres of the clusters NGC 419 and IC 1624. The 15 solid red line shows the profile fitting from the EFF 19 16 law, while the dashed line represents the fitting

) radius. Right panels: The CMDs of the same clusters 17 –2

c within the fitting radius (dots); the red solid and 20 18 se dashed lines in the upper-right panel (NGC 419) rc 19 show the isochrones for 900, 1000 and 1100 Myr, g

g a respectively. The red solid line in the lower-right 21 20

ma panel (IC 1624) shows the isochrone for 170 Myr.

g ( 21 22 22 23 23 24 24 25 110100 –1 012 r (arcsec) (g–i)

13 IC 1624 14 r < 39 arcsec 18 15

19 16

)μ 17 –2 20 18 csec 19 g g ar 21 20 ma

g ( 21

μ 22 22 23 23 24 24 25 110100 –1 0 1 2 r (arcsec) (g–i) in each panel the vertical dashed line To be more quantitative, we adopted the ESO community with the deepest indicates the “fitting radius”, i.e., the limit the isochrones from Marigo et al. (2008) homogeneous survey of the whole SMC within which the fitting procedure is with proper assumptions about redden- ever performed. ­carried out. The profiles in Figure 7 were ing and distance modulus to fit the obtained by: 1) estimating the cluster observed CMDs (red solid lines in Figure centre; 2) dividing the profile in annuli of 7). As a result, for NGC 419, the magni- References different sizes; 3) averaging the counts tude difference between RC and bulk of Bica, E. et al. 2008, MNRAS, 389, 678 in each annulus. Each panel shows the the MS-TO suggests an age between Capaccioli, M. & Schipani, P. 2011, The Messenger, EFF law that best fits the data. We veri- 900 and 1100 Myr, but the large photo- 146, 2 fied that the results for both clusters are metric errors prevent any definitive con- Cignoni, M. et al. 2013, ApJ, 775, 83 in agreement with similar analyses, con- clusion about a genuine age spread. Cignoni, M. et al. 2012, ApJ, 754, 130 Cioni, M. R. L. et al. 2011, The Messenger, 144, 25 firming the reliability of our approach. The situation is completely different in Elson, R. A. W., Fall, S. M. & Freeman, K. C. 1987, IC 1624. The 170 Myr old isochrone fits ApJ, 323, 54 To estimate the ages of the two clusters, all the main evolutionary phases very Glatt, K. et al. 2009, AJ, 138, 1403 we first plotted their CMDs within the well, including the MS-TO, the red enve- Grado, A. et al. 2012, MemSAIT, 19, 362 Marigo, P. et al. 2008, A&A, 482, 883 ­fitting radius defined above (Figure 7 lope of the BL and the average luminosity Kuijken, K. 2011, The Messenger, 146, 8 lower panels). The CMD of NGC 419 is of the loop. The results for the ages of Ripepi, V. et al. 2014, MNRAS, 442, 1897 rather scattered, due to the high crowd- both clusters are, again, in very good Tolstoy, L., Hill, V. & Tosi, M. 2009, ARAA, 47, 371 ing, which boosts dramatically photo­ agreement with literature results. Similar metric errors and incompleteness towards work will be carried out for all the clus- Links the cluster centre. However, the CMD ters de­tected in the context of the STEP of IC 1624 is rather loose because of the survey. 1 VMC Survey: http://star.herts.ac.uk/~mcioni/vmc/ 2 global paucity of stars in this cluster. In SCAMP package: http://www.astromatic.net/soft- ware/scamp terms of age, there is a clear difference We are looking forward to a timely com- 3 SWARP package: http://www.astromatic.net/soft- between the two clusters, with IC 1624 pletion of all the data acquisition in order ware/swarp being much younger than NGC 419. to exploit STEP at its best and provide

The Messenger 157 – September 2014 37