Discovery of two distinct red clumps in NGC419: a rare snapshot of a cluster at the onset of degeneracy
aa r X i v : . [ a s t r o - ph . S R ] J a n Mon. Not. R. Astron. Soc. , 1–5 (2008) Printed 29 October 2018 (MN L A TEX style file v2.2)
Discovery of two distinct red clumps in NGC 419:a rare snapshot of a cluster at the onset of degeneracy
L´eo Girardi , Stefano Rubele , and Leandro Kerber Osservatorio Astronomico di Padova – INAF, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy Dipartimento di Astronomia, Universit`a di Padova, Vicolo dell’Osservatorio 2, I-35122 Padova, Italy Universidade de S˜ao Paulo, IAG, Rua do Mat˜ao 1226, Cidade Universit´aria, S˜ao Paulo 05508-900, Brazil
Accepted 2008 Dec. 10. Received 2008 Dec. 9; in original form 2008 Nov. 14
ABSTRACT
Colour–magnitude diagrams (CMD) of the SMC star cluster NGC 419, derived fromHST/ACS data, reveal a well-delineated secondary clump located below the classical com-pact red clump typical of intermediate-age populations. We demonstrate that this feature be-longs to the cluster itself, rather than to the underlying SMC field. Then, we use syntheticCMDs to show that it corresponds very well to the secondary clump predicted to appear asa result of He-ignition in stars just massive enough to avoid e − -degeneracy settling in theirH-exhausted cores. The main red clump instead is made of the slightly less massive starswhich passed through e − -degeneracy and ignited He at the tip of the RGB. In other words,NGC 419 is the rare snapshot of a cluster while undergoing the fast transition from classi-cal to degenerate H-exhausted cores. At this particular moment of a cluster’s life, the colourdistance between the main sequence turn-off and the red clump(s) depends sensitively on theamount of convective core overshooting, Λ c . By coupling measurements of this colour sep-aration with fits to the red clump morphology, we are able to estimate simultaneously thecluster mean age ( . +0 . − . Gyr) and overshooting efficiency ( Λ c = 0 . +0 . − . ). Therefore,clusters like NGC 419 may constitute important marks in the age scale of intermediate-agepopulations. After eye inspection of other CMDs derived from HST/ACS data, we suggestthat the same secondary clump may also be present in the LMC clusters NGC 1751, 1783,1806, 1846, 1852, and 1917. Key words:
Stars: evolution – Hertzsprung-Russell (HR) and C-M diagrams
In the last decade, wide field imagers and the Hubble Space Tele-scope (HST) have provided detailed CMDs for the star fields in theMagellanic Clouds. One of the main surprises was the discoverythat the red clump of core-He burners is not a compact feature, butmay present extensions amounting to a few 0.1 mag departing fromits top and bottom parts, as well as a blue extension that connectswith the horizontal branch of the old metal-poor populations.The ground-based observations from Bica et al. (1998) andPiatti et al. (1999) evinced a ∼ . mag extension of the red clumpto fainter magnitudes, spread over wide areas of the outer LMCdisk. Girardi (1999) gave a clear interpretation to this extention –thereafter named secondary red clump – claiming that it is made ofthe stars just massive enough to start burning He in non-degenerateconditions, at ages of ∼ Gyr, whereas the main body of thered clump is made of all the intermediate-age and old stars whichpassed through degenerate cores and the He-core flash at the tip ofthe RGB. The same feature was suggested to be present in the Hip-parcos CMD for the Solar Neighbourhood (Girardi et al. 1998), andprovides an explanation to the vertical extension – amounting toabout . mag in the F W band (see e.g. Holtzman et al. 1997) – of the red clump in the sharp CMDs obtained by the HST forseveral LMC fields.Plots of the red clump magnitude versus age for star clusters– as in Girardi (1999, figs. 3 and 4), and Grocholski & Sarajedini(2002, fig. 6) – seem to confirm the theoretical framework that ledto Girardi’s (1999) prediction of a secondary clump: they clearlyindicate a red clump decreasing in luminosity up to ∼ Gyr, thena jump upwards by about 0.4 mag, and a much slower evolutionthereafter.A handful of faint red clump stars are also found among theradial velocity members of the open clusters NGC 752 and 7789,and possibly also in NGC 2660 and 2204 (Mermilliod et al. 1998;Girardi et al. 2000). The latter authours associated these stars withthe secondary clump feature, and tried to figure out how it couldappear in an object for which the age spread was supposedly verysmall. Indeed, from their discussion it is clear that a single clus-ter age corresponds to a very narrow range of red clump massesand hence to the sampling of either the faint (secondary) or thebright (classical) red clump. The two red clumps could not appeartogether in a single star cluster, unless some other mechanism – e.g. c (cid:13) Girardi, Rubele & Kerber a dispersion in the mass loss along the RGB, or in the efficiency ofovershooting on the main sequence (MS) – were invoked.The reasoning from Girardi et al. (2000) would however nowfail, at least for the star clusters in the Magellanic Clouds. Indeed,now we know that many of them do not represent single ages,but rather a range of ages that can extend up to a few 100 Myr(Mackey et al. 2008; Milone et al. 2008) – or equivalently, to arange of MS turn-off (MSTO) masses of a few 0.05 M ⊙ .By visual inspection of the CMDs for a few SMC star clus-ters studied by Glatt et al. (2008), we came across what was veryevidently a composite structure of main+secondary red clump inNGC 419. The presence of a secondary clump in this cluster wasindeed noticed by Glatt et al. (2008, their sect. 3.6), who howeverhave suggested it to be “a red clump of the old SMC field star pop-ulation”. They also noticed the composite structure in the MS, at-tributing it to an extended period of star formation, similarly to theone claimed by Mackey et al. (2008) for three LMC clusters.In the following, we demonstrate that the composite struc-ture of the red clump in NGC 419 is real and undubiously asso-ciated to the cluster (Sect. 2). We show that it corresponds to thesimultaneous presence of stars which have started burning He un-der non-degenerate and degenerate conditions (Sect. 3). We thenillustrate that this rare occurrence in a cluster allows us to set strin-gent constraints on the cluster age and amount of convective coreovershoooting during the MS evolution (Sect. 4). We then brieflysuggest that NGC 419 is not a unique case, and draw some finalcomments in Sect. 5. We have retrieved from the HST archive the NGC 419 data ob-tained by GO-10396 (PI: J.S. Gallagher). The dataset consists of a740 arcsec area observed with the Advanced Camera for Surveys(ACS) High Resolution Channel (HRC) centered on NGC 419, plusa . × arcsec area observed with the ACS Wide Field Chan-nel (WFC) 37 ′′ offset from the cluster centre. Both datasets werereduced via standard procedures (Sirianni et al. 2005). The HRCdata, given its high spatial resolution, is the most useful for thestudy of the NGC 419 population, whereas the WFC dataset pro-vides the comparison data for interpreting the SMC field.We have performed aperture and PSF photometry on the cal-ibrated HRC image, finding that both provide CMDs very similarto the Glatt et al. (2008) ones. We have then opted for the PSF cat-alogue and cutted it at (sharp F W + sharp F W ) / < . .This quality cut eliminates many outliers from the CMD, especiallyat the faintest magnitudes, but do not affect the morphology of fea-tures at the red clump and MSTO level.The HRC data is plotted in Fig. 1, which shows both the globalCMD and separate panels detailing the red clump and the MSTOregions. The extended nature of the red clump is evident in thefigure . It seems to be formed by a main blob located between F W = 18 . and 18.65, followed by a well-defined faint wingbetween F W = 18 . and 19.0, which we tentatively identifyas the secondary clump. We note that there is no similar wing ex-tending from the top of the red clump. These sequences are about The choice of the F W band for the ordinate is not casual, since F W presents flatter bolometric corrections than F W , over the T eff range of the red clump. This improves the separation in magnitude of theclump substructures, and at the same time keeps the subgiant branch out ofits magnitude range. Figure 1.
The CMD for NGC 419 as derived from the HRC data centeredon the cluster (left panel). The σ error bars, as derived from artificial startests, are drawn at the left. The right panels detail the red clump (top) andMSTO regions (bottom). The overlaid isochrones are from Marigo et al.(2008), for a metallicity Z = 0 . , ages varying from log( t/ yr) = 9 . to . with a constant spacing of 0.05 dex, E F W − F W =0 . , and ( m − M ) F W = 18 . . Notice that these particular isochrones describereverse sequences in the MSTO and red clump regions of the CMD: whereasthe MSTO gets dimmer for increasing ages, the red clump gets brighter. Figure 2.
The CMDs for . × arcsec field around NGC 419, asderived from the WFC data after subtracting a circular area of radius 75 ′′ around the cluster. The overlaid isochrones are the same as in Fig. 1, and areplotted for reference only. The bulk of the red clump is below the saturationlimit at F W ∼ . . F W − F W ) than the ridgeline of RGBstars. In addition, the red part of the CMD shows a well delineatedsubgiant branch, and the bump of early asymptotic giant branch(AGB) stars centered at F W = 17 . .There are 55 stars between F W = 18 . and 19.0, 8out of which are red enough to belong to the RGB rather than tothe secondary clump. The main red clump instead contains 341stars. These numbers correspond to the HRC effective area of740 arcsec , centered on NGC 419. We verified that both sets of c (cid:13) , 1–5 wo red clumps in NGC 419 stars distribute all over the HRC image, and share similar distribu-tions of photometric errors and sharpness. So, it is unlikely that thesecondary clump could be an artifact of a given subsample of thedata.Can the field SMC stars account for the 47 stars in the sec-ondary clump, as suggested by Glatt et al. (2008)? To answer thisquestion, we look at the WFC data of Fig. 2, which covers an area33 times larger than the HRC one. It was obtained subtracting fromthe complete WFC catalogue – without applying any quality cut– a circular area of 75 ′′ in radius around NGC 419. The remain-ing area of . × arcsec was considered to be “SMC field”,and contains 150 red clump stars in the F W range between18.1 and 19.0. Therefore, the expected number of these stars inthe 740 arcsec area of HRC is of just 4.5. Moreover, their typicalmagnitudes are closer to those of the main red clump in NGC 419,rather than to the secondary one. We conclude that the field can-not contribute with more than ∼
10% of the 47 stars observed inthe secondary clump, and probably contribute much less.
The bulkof the secondary clump observed in HRC data indeed belongs toNGC 419 . Our interpretation of the red clump structure in NGC 419 is alreadyclear in Fig. 1 and in the discussion of Sect. 1: the fainter secondaryred clump is explained by the core-He burning stars belonging tothe younger isochrones, which have just avoided e − -degeneracybefore igniting He. This feature has been throughfully discussedin Girardi (1999) and Girardi et al. (1998), in the context of galaxyfield populations. It appears naturally in simulations of star-forminggalaxies with moderate-to-high metallicities, provided that the un-derlying stellar models do present a fine resolution in mass (betterthan 0.1 M ⊙ ; see Girardi 1999).In the following, we discuss the specific case of NGC 419 bymeans of newly-computed evolutionary tracks of initial composi-tion ( Z = 0 . , Y = 0 . . The input physics is the same as inBertelli et al. (2008). We have initially adopted a moderate amountof convective overshooting, i.e. Λ c = 0 . , where Λ c is the size ofthe overshooting region across the convective boundary, in pressurescale heigths, following the Bressan et al. (1981, 1993) definitions.For a limited interval of initial masses M i , typically going from M HeF − . M ⊙ to M HeF + 0 . M ⊙ , we follow the evolution upto the thermally pulsing AGB, whereas for smaller masses (down to0.6 M ⊙ ) we have computed only the MS evolution. Stellar tracksare spaced by ∆ M i = 0 . M ⊙ . These tracks are converted to stel-lar isochrones in the ACS/HRC and ACS/WFC Vegamag systemsusing the transformations from Girardi et al. (2008).The isochrones are then fed to the TRILEGAL population syn-thesis code (Girardi et al. 2005) to simulate the photometry of starclusters at the SMC distance. We apply to the TRILEGAL outputthe photometric errors derived from artificial star tests performedon the original HRC images. The results are illustrated in Fig. 3,which shows the expected time evolution of the red clump for acluster for two different cases: either for an almost-instantaneousburst of star formation (with a duration of ∆ log t = 0 . ), andfor a burst spanning a range of ∆ log t = 0 . . The latter casecorresponds roughly to the situation indicated by the MSTO starsin Fig. 1. 20 % of the stars in the simulation are assumed to bedetached binaries with a mass ratio comprised between 0.7 and 1(Woo et al. 2003). Since this latter prescription is rather uncertain,binaries are always marked with a different colour in our plots. It is evident from Fig. 3 that the red clump rapidly transitsfrom a vertically-extended feature, to a much more compact andslightly brighter clump at a mean age t close to 1.25 Gyr. The tran-sition takes place completely in an age interval of just ∼ . Gyrfor the case of an intanstaneous burst, and in about twice this timefor the case of a ∆ log t = 0 . -wide burst. From now on, we willrefer to the mean age of this transition as t HeF .We note that vertically-extended red clumps are present forall ages younger than t HeF , even in the intanstaneous-burst case;however, they are wider than observed in NGC 419, and presentdrop-shaped LFs – i.e. with a sharp cut at the bottom and a moreextended tail at the top. This is not what observed in the red clumpof NGC 419, which presents the maximum of its LF at the top,together with a bump at the faintest magnitudes. A configurationsimilar to NGC 419 is obtained only in the extended-burst case, forages comprised between 1.41 and 1.58 Gyr: indeed, this age rangeis the only one which combines the vertically-extended red clumpof younger ages, with the compact and brighter red clump of olderages, in about the right proportions to explain the observations. Weidentify NGC 419 as belonging to this very limited – and surelyvery singular – age interval. In order to identify the best-fitting age,we compute the χ between data and model, for the red clump re-gion only. The distance modulus is varied until the minimum valueof χ is met for each age t . The results are printed in Fig. 3, andevince the excellent quality of the fit for the 1.41 Gyr-old modelwith ∆ log t = 0 . .Finally, it is worth mentioning that the MS+red clump binariestend to draw a plume departing from the red clump towards bluercolours and brighter magnitudes, which (1) is easily identifiablein observed CMDs because of its colour separation from the redclump, and (2) do not change the drop-shaped form of the LF forthe younger red clumps. Binaries cannot mimick the bimodal LFobserved in the red clump of NGC 419.A question raised by the referee is whether the two red clumpscould be caused by populations with different helium content,abundances of CNO elements, or overshooting efficiency. Althoughnothing can be excluded, so far there are no indications of such ef-fects in clusters as young as NGC 419. Moreover, it is Occam’srazor to refrain us from looking for more complicate alternatives:in fact, our explanation requires only standard physics added to aquite simple distribution of stellar ages – the same one indicated bythe clusters’ MSTO – while keeping all other parameters constant.We recall that the transition between faint and bright red clump issomething that inevitably happens, sooner or later, for every sin-gle stellar population, causing always about the same amount ofbrightening in the red clump over a similar timescale (which is dic-tated mainly by the equation of state of partially-degenerate mat-ter). Therefore, there is no free parameter to be fixed in order toexplain the presence and position of the two red clumps, there isjust the very loose requirement of “a prolonged-enough duration ofthe star formation”. According to the interpretation given in this letter, the red clump inNGC 419 corresponds to stars with a precise internal configurationafter H-exhaustion: their cores have a mass very close to 0.33 M ⊙ ,as their central temperatures approach T c = 10 K. Slighty highercore masses lead to non-degenerate He-ignition. Slightly smallercore masses lead to e − -degeneracy, which halts the core contrac-tion and is followed by the cooling of the central core by plasma c (cid:13) , 1–5 Girardi, Rubele & Kerber
Figure 3.
Models for the evolution of the red clump feature in the CMD as a function of mean population age t , for both a single-busrt population (top panels)and for a composite one with duration of ∆ log t = 0 . (bottom panels), in both cases with the assumption of moderate convective overshooting ( Λ c = 0 . )and Z = 0 . . Single stars are marked in blue, double stars in red. Each panel presents on the top right a box evincing the red clump, and on the top left theluminosity function (LF) for the stars in this box. The best-fitting distance modulus and the associated χ are also displayed. For comparison, the top rightpanel shows the HRC data of NGC 419 on the same scale, after being arbitrarily shifted by 19.0 and 0.09 in magnitude and colour, respectively. The greenvertical lines in all panels mark the median colour of the red clump, and the bluest colour of the MS (see text). Figure 4.
Top panels: The same as in Fig. 3, but now showing the models that best fit the red clump for several values of overshooting efficiency Λ c , and forthe ∆ log t =0 . case only. Although all models reproduce the observed red clump similarly well, they differ very much in their age (from 0.94 to 1.99 Gyr,as Λ c increases from 0.2 to 0.7), and produce different MSTO magnitudes and colours. neutrinos; as a consequence the He-ignition is postponed to a laterstage – namely the RGB tip – at which the core masses have grownup to 0.45 M ⊙ (see Sweigart et al. 1990).These core masses after H-exhaustion do also correspond toa narrow interval of initial masses, comprehending the transitionbetween intermediate- and low-mass stars, M HeF . It has long beenknown that convective core overshooting changes the relation be-tween the initial mass and the H-exhausted core mass, hence di-rectly affecting the value of M HeF , and its corresponding age t HeF (e.g. Bressan et al. 1993). Even if the efficiency Λ c can be con-strained by means of several methods which use either the mor-phology of the CMD or star count ratios (e.g. Woo et al. 2003, andreferences therein), this is still a main source of uncertainty in set-tling the age scale of intermediate-age clusters, and in the theory ofstellar populations in general.Can the NGC 419 giants help us to set constraints on M HeF and t HeF , and hence on Λ c ? Probably yes, considering that higher M HeF values (lower Λ c ) imply bluer MSTOs. Once we identify astar cluster during the particular age t HeF , fixing the position of itsred clump(s) in the CMD, the relative position of the MSTO shoulddepend mainly on overshooting.With this idea in mind, we have computed several set of stellarevolutionary tracks and isochrones, with Λ c varying from 0.2 to0.7. For each one of these sets we identify the age at which, fora ∆ log t = 0 . star formation burst, the red clump morphologyis best reproduced (just as in Fig. 3). Indeed, Fig. 4 shows thatsimilarly good fits of the red clump morphology are obtained forall values of Λ c , but at different ages t HeF . We recall that thesedifferent fits actually represent very similar distributions of the coremass after H-exhaustion .We then measure the colour difference between the medianof the red clump, and the bluest border of the MSTO region de- c (cid:13) , 1–5 wo red clumps in NGC 419 Figure 5.
Confidence regions in the t vs. Λ c plane. The continuous red linefollow the locus of minimum χ values derived from the fitting of the redclump morphology (see also Fig. 4). The continuous blue line describe themodels which perfectly fit the colour difference between the red clump andMSTO. Dashed lines present the estimated 70% confidence limits. fined by the 98% percentile of the star counts above an absolutemagnitude of 2.5. This quantity is little sensitive to the fraction ofbinaries, and our simulations indicate that it can be measured witha σ error of 0.006 mag. Fig. 5 shows this quantity in the t vs. Λ c plane, together with the estimated 70% confidence level region ofthe best-fitting model to the red clump morphology. The best si-multaneous fit of the two quantities is obtained for Λ c = 0 . +0 . − . and t = 1 . +0 . − . Gyr (with random errors only).Importantly, these t and Λ c determinations are largely freefrom uncertainties in the cluster distance and reddening. However,they may be slightly affected by other factors like the assumed frac-tion of binaries, the mixing-length parameter, and the detailed starformation history of NGC 419. More detailed analysis (in prepa-ration) will aim to reduce these uncertainties by using informationfrom the complete CMD, and better exploring the parameter space.A few preliminary conclusions can be advanced here: (1) Modelsfor ∆ log t values of 0.1 and 0.2 do also provide good fits of thered clump morphology, with their best-fitting ages differing by lessthan 6% from those we find for ∆ log t = 0 . ; these age differ-ences are comparable to the above-mentioned random errors. How-ever, such models have to be excluded because they clearly providea worst description of the MSTO region of the CMD. (2) On theother hand, models with a fraction of binaries as small as 10% tendto provide slightly better fits of the red clump LF, at essentially thesame ages as those found with 20% of binaries, and with just amodest impact in the morphology of the MSTO. NGC 419 can be definitely added to the list of star clusters with asecondary red clump, together with the Milky Way open clustersNGC 752 and 7789, and possibly also NGC 2660 and 2204, whichwere already discussed by Girardi et al. (2000). This time, however,we are facing a very populous cluster which presents a CMD richof details, from its lower MS up to the AGB carbon star sequence(see Fig. 1, and Frogel et al. 1990). The secondary red clump itselfis very well populated and its detection can hardly be controversial.This fine CMD feature provides strong constraints to the core massreached by its MS stars. All these aspects make of NGC 419 anexcellent tool for calibrating stellar evolution models, as well asthe age sequence of intermediate-age populations.Can we identify additional star clusters in the Magellanic Clouds, having the same secondary clump feature? Probably yes,since these galaxies contain dozens of populous clusters with agesaround 1 Gyr, and the presence of multiple turn-offs is a commonfeature among them (Milone et al. 2008). Indeed, from a rapid eyeinspection of published CMDs obtained with HST/ACS, we noticethat dual (main+secondary) red clump structures seem to be presentalso in the LMC clusters NGC 1751, 1783, 1806, 1846, 1852, and1917 – see figs. 7, 8, 9, 16 and 17 in Milone et al. (2008), and fig. 1in Mackey et al. (2008). All these clusters have multiple turn-offs,with the youngest one being at F W ∼ . , which is compara-ble with the NGC 419 one if we consider the ∼ . mag differencein the SMC–LMC distance moduli. A subsequent paper will exam-ine these clusters in close detail, in the perspective of deriving morestringent constraints to stellar evolutionary models. Cluster funda-mental parameters such as the age, distance and reddening, will bere-evaluated as well. ACKNOWLEDGMENTS
The data presented in this paper were obtained from the Multi-mission Archive at the Space Telescope Science Institute (MAST).STScI is operated by the Association of Universities for Researchin Astronomy, Inc., under NASA contract NAS5-26555. We thankA. Bressan, G. Bertelli and M. Clemens for the useful comments,and support from INAF/PRIN07 CRA 1.06.10.03, contract ASI-INAF I/016/07/0, and the Brazilian agencies CNPq and FAPESP.
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