arXiv:astro-ph/0202253v1 13 Feb 2002 2 rpittpstuigL using typeset Preprint 2018 1, November version Draft 01 ats sala ao 2001). Mayor & Israelian Santos, 2001; sasgicn cte nteosre ealcte;they ; [Fe/H] observed about the in from scatter range significant a is 1 xr-oa lnt edt aeehne metallicities, period enhanced short stars have of to to stars Compared 2001; ∆[Fe/H] tend parent al. planets neighborhood, et 2001). extra-solar solar solar Gonzalez Mayor the extra 2000; & in Laws period Israelian et & short Santos Santos, Gonzalez 1999; of (Gonzalez 2000; existence star al. parent the the and around planets star the a between 2001). of connection al. possible et unexplained magni- Bruijne band sys- (de coeval V clusters metallicity open a solar as of such nearly hundreds tems and few younger to a for qual- possible than tude) photometric less been comparable (errors it of ity has diagrams been HR recently have construct quite diagrams clusters HR globular only Piotto, quality old constructed, metallicity, (e.g., high low dex Although a coeval, of 1997). of hundredths King few cluster & a globular Cool than given metal- less a the in be that stars must the implies among which size variation in errors licity similar measurement been has in the 6397) NGC to isochrones as (HST) (such model Telescope cases Space in from images Hubble stars from derived in of diagrams dex scatter HR abundances 0.10 The than element less [Fe/H]. shown typically in heavy small, have quite 1981) of is clusters al. homogeneity globular Costa et the Da Buonanno 1999; that al. 1990; et Armandroff Suntzeff Heald (e.g., 1993; & al. studies et photometric Folgheraiter and 1993; 1994) al. et mosky iio,PyisDprmn,Tcno,Ire nttt o Institute Israel Technion, Department, Physics Visitor, eateto hsc n srnm,Uiest fRochest of University Astronomy, and Physics of Department hr r w osbeepaain htacutfor account that explanations possible two are There eetsetocpcsuishv salse na yet as an established have studies spectroscopic Recent Ar- 1992; al. et Sneden (e.g. studies spectroscopic Both .4mgiue.W s hsdvaint siaea pe ii nthe on evolution ∆[Fe/H] limit isochrones metallicities the upper of of an scatter gradient the estimate the for to From limit deviation cluster. upper this isoch this use evolution in We stellar stars model magnitudes. a accuracy. 0.04 about unprecedented stars with non-variable clusters non-binary nearby of diagrams (HR) rvosysetocpcly hssget htsasi pnclus open metallicity. in its stars in that homogeneous suggests This spectroscopically. previously oteiohoe hntelwsatro tr nteHae a b stars. can the polluted the have dia in HR could stars the that of on material star scatter the low high-Z of the of Alternatively position then the isochrone, diagram. in the HR change to large the a on zone in results convection position m metals the and polluted to metallicity insensitive very is their have diagram between which HR the stars on then position the atmosphere, If stars. of zone ≈ SN IPRO EIE RDARMT II H METALLICIT THE LIMIT TO DIAGRAM HR DERIVED HIPPARCOS A USING ipro aalxsadpoe oin aemd tpsil ocon to possible it made have motions proper and Hipparcos ecnie h yohssta rcse soitdwt planet with associated processes that hypothesis the consider We 0 . 2 ± 0 . ,cmae otesn oee,there However, . the to compared 2, A 1. T E tl mltajv 14/09/00 v. emulateapj style X introduction − ≈ 0 . TR NTEHAE R TR POLLUTED? STARS ARE – HYADES THE IN STARS o04(ozlze al. et (Gonzalez 0.4 to 4 rf eso oebr1 2018 1, November version Draft ehooy af 20,Israel 32000, Haifa Technology, f .C Quillen C. A. ABSTRACT r ohse,N 42;[email protected] 14627; NY Rochester, er, . 1 0 hnte r le n ohv mle ovcinzones material convection planetary smaller have rich so metal and older with that are is stars they disk when the gaseous pollutes a sce- into it that driven migration waves to orbital compared density the planetesimals involving of of advantage Trilling ejection involving the One 1996; nario impact Richardson 1998). to & planet al. a Bodenheimer et cause Lin, also transfer (e.g., can migra- and star The orbital planet, a a the into of cause waves system. tion will density solar disk of gaseous driving our proto-planetary via of momentum era angular bodies of massive early more proba- the for sun and during the frequently with more & impacts occurred that (Quillen bly infer We planetesimals of 2000). star-impacting stars disk Holman parent by a the that polluted of mi- is scattering are orbital 1998), by al. the et caused of (Murray planets planetesimals consequence giant 2000; One al. of et 2001). grating Morbidelli al. or (e.g., abundances et planets belt surface other Gautier asteroid the and changed the the have in of system solar bodies solar our from outer in impacts analyses that abundance cer- is system on There based stars. evidence parent tainly the metallici- systems of planetary metallicities of the the of range increases evolution a subsequent the spanning but stars ties, near frequency 2001). equal al. et (Gilliland caused systems have the planetary would of of which result disruption cluster However a the the be in density instead 2001). stellar may al. high eclipses in observed et than of rarer lack (Gilliland magnitude the neighborhood of Jo- order solar period an the short be planets, could by = planets ([Fe/H] caused the vian Tuc eclipses Because 47 detect cluster may not stars. globular did planets the metallicity gaseous of low study that fairly so- HST is around extra possibility rarer with One be stars parent planets. of lar metallicities enhanced the . 3dx mle ii hnhsbe measured been has than limit smaller a dex, 03 lentvl,gsospaesmyb rdcdwith produced be may planets gaseous Alternatively, 1 , 2 h tnaddvaino ihfidelity high of deviation standard The sdt lc osrit nquantity on constraints place to used e esaefre rmgsta snearly is that gas from formed are ters a eietfidfo comparison a from identified be can s omto a olt h convection the pollute can formation oei h ydscutri about is cluster Hyades the in rone nteH iga eetmt an estimate we diagram HR the in tliiyo h ovcinzn and zone convection the of etallicity rmi ieto perpendicular direction a in gram ftepluino h trby star the of pollution the if tutHertzsprung-Russell struct cte nmtliiisin metallicities in scatter CTE OF SCATTER Y − 0 . 7) 2

(e.g., Laughlin & Adams 1997) allowing a smaller amount they could have caused metallicity enhancements. Fur- of metals to cause a larger surface metallicity enrichment. thermore, nearby open clusters have nearly solar metallic- Because the fraction of mass in the convection zone ities and so are a better match to the properties of parent depends on the stellar mass, higher mass stars (e.g., F stars of extra solar planets than are the stars in globular stars) with smaller convection zones could be more likely clusters which are comparatively extremely metal poor. than lower mass stars with larger convection zones (e.g., K It is difficult to envision any planet formation mecha- stars) to have enhanced metallicities resulting from pollu- nism that would not cause differing amounts of metallic- tion caused by planetary material. However Pinsonneault, ity pollution in different solar systems. We therefore ex- DePoy & Coffee (2001) find no such trend among the par- pect a scatter in the metallicities in individual stars in any ent stars of extra solar planets. Santos, Israelian & Mayor given stellar cluster. We might also expect a reduction in (2001) find that the distribution of metallicities for par- [C/Fe] because the inner solar system is deficient in light ent stars of extrasolar planets is inconsistent with a model elements such as carbon. However despite earlier reports, where these stars have been polluted by the addition of low values of [C/Fe] are not observed in the parent stars high-Z material. of the short period extra solar planets (Gonzalez et al. Except for a narrow region at 6500±300K, known as the 2001). If ice rich cometary material from the outer solar Lithium dip, F stars keep Lithium on their surface nearly system is incorporated into the star on a later timescale, it their entire lifetime (see Balachandran 1995; Soderblom et is possible that light element abundance could be restored. al. 2001). The absence of Li depletion implies that the However this could only occur if material from the outer convection zone does not mix with the interior of the star solar system can survive evaporation. Evaporation rates except within the Lithium dip. For F stars outside the are higher near the high mass and more luminous stars, Lithium dip little mixing takes place and a metallicity however these stars, because of their lower mean densities, enhancement would last nearly the entire lifetime of the are also less likely to cause objects to fragment. Smaller star, however F stars within the dip would not be expected sized bodies because of their larger surface area are less to show metallicity enhancements. Murray et al. (2001); likely to achieve final impact, particularly after a series of Murray & Chaboyer (2001) suggest that F stars near the close approaches which can be caused by resonant trapping Lithium dip, tend to have lower metallicities than F stars (Quillen & Holman 2000). outside the Lithium dip. Spectroscopic studies of F stars in individual open clus- The stellar sample studied by Murray et al. (2001); Mur- ters and moving groups have found that the metallicity ray & Chaboyer (2001) and surveyed for planets draw on scatter is small ∆[Fe/H] . 0.1 (Friel & Boesgaard 1992, the population of stars in the solar neighborhood and so 1990a,b). These authors also measured no measurable span a large range in ages and metallicities. To try and variation in the [C/Fe] ratio among all stars in all clusters decouple the uncertainty caused by the metallicity scat- studied. These studies would appear to rule out significant ter resulting from the range of stellar ages in the solar metallicity enhancement resulting from planet formation neighborhood from that caused by planet formation we and subsequent planetary evolution in most stars. How- can examine the metallicity scatter in young clusters. We ever the number of F stars observed in each cluster in these can assume that the stars in a given stellar cluster are the studies was not more than a dozen. Establishing cluster same age and were formed from gas that is fairly uniform in membership is not always unambiguous and so the sam- metallicity. Stars in the solar neighborhood are expected ples of stars chosen for detailed spectroscopic study were to have been been born in a variety of environments with not complete. about 10% born in OB associations which become bound open clusters (Roberts 1957). The low eccentricity orbits 2. a limit in the metallicity scatter from the of the solar system planets require that the birth aggre- hyades cluster hr diagram gate of the sun was less than a few thousand stars (Adams & Laughlin 2001), and similar in size to the Trapezium Hipparcos parallaxes and proper motions have made cluster. Open clusters such as the Hyades, with about 400 it possible to construct Hertzprung-Russell diagrams of known members could have been as large as the Trapezium nearby clusters with unprecedented accuracy (de Bruijne when younger. Based on the study of Adams & Laugh- et al. 2001). With improved distances and a sample of 92 lin (2001), we expect that short period extra solar planets high fidelity cluster stars, de Bruijne et al. (2001) have should have a high probability of surviving disruption from constructed an HR diagram with estimated errors in the the passage of nearby stars in open clusters. Despite the log of the luminosity in solar units that average 0.030. The fact that most stars in the solar neighborhood were formed high fidelity sample excludes member beyond 40pc from in smaller, unbound groups, since planetary systems are the cluster center, multiple stars, variable stars and stars likely to survive in open clusters, we can use observations with peculiar or uncertain parallaxes (see section 9.3 of of the stars in open clusters to explore the possibility that (de Bruijne et al. 2001)). Stars were not excluded based significant stellar metallicity enhancements (pollution) are on their position in the HR diagram. likely to occur after a star is formed. If metallicity en- We show in Figure 1 as data points, the difference be- hancements occur, they would occur over a fairly short tween the positions of the stars on the HR diagram and the timescale, ∼ 106 years for the orbital migration scenario 630 Myr evolutionary track at Z=0.024 interpolated from involving density waves in a gaseous disk (Trilling et al. those given as tables by Girardi et al. (2000). The mean 1998), and ∼ 107 years for the migration scenario involving difference between the track and stars is not zero. This scattering of planetesimals (Murray et al. 1998). In open implies that the evolutionary track is not a perfect match clusters, planet systems should survive long enough that to the HR diagram of the cluster. It is impressive that the HR diagram is so good that the evolutionary tracks 3

(and associated bolometric corrections and color to effec- Compared to a star which has not accreted additional ma- tive temperature calibrations) can be tested to the level of terial, where would we expect such a star to lie on the HR 0.05 in the log of the luminosity. Nevertheless trends or diagram in the cluster? To know the answer to this ques- smooth deviations from zero on this plot are not important tion we would need to calculate additional stellar evolu- for our study since we are primarily interested in the scat- tionary tracks from stellar models. While the properties of ter of the points. The ages differences in the stars should the radiative zone might be similar because it would have be small compared to the age of the cluster, so other than the same metallicity, and the energy transport through the observational errors, the variations in the stellar metallic- convection should not be strongly affected by the opacity, ities is the primary factor which would cause a scatter in the stellar atmosphere would be redder and since this sets the position of the points on the HR diagram about the the boundary condition for the stellar model, we do not isochrone. expect the star to lie at exactly the same position on the HR diagram. 0.2 We consider two possibilities: 1) The position in the HR diagram for a star which has accreted high-Z material is similar to one that has not. 2) There is a significant differ- ence in the location on the HR diagram and in a direction that would distance the star off the isochrone. If the first 0.1 possibility is true then we suggest that spectroscopic anal- yses can be used to identify candidate stars which have higher metallicities than would be expected from their po- sition on the HR diagram. If such stars are found it would provide strong evidence that they had been polluted by high-Z material after formation, and by processes associ- 0 ated with planet formation. If the second possibility is likely then we emphasize that the scatter from the isochrone observed in the HR diagram can be used to constrain stellar pollution scenarios. Since the level of scatter has a standard deviation about 0.03 dex -0.1 3.9 3.8 3.7 3.6 in [Fe/H] the average star could only have accreted about log(Teff[K]) 0.7 M⊕ of accreted iron from the inner solar system. Only a few stars remain outside the local mean in Fig- Fig. 1– The points show the difference between the ure 1 with deviations greater than 0.05 in the log of the 92 high fidelity clusters Hyades stars identified by de luminosity. However stars with short period giant planets Bruijne et al. (2001) and the Z=0.024, 630 Myr evo- are fairly common. In the solar neighborhood 6-8% of so- lutionary track of Girardi et al. (2000). We use tem- peratures and luminosities derived by de Bruijne et lar type stars have short period giant extra solar planets al. (2001). The scatter from the mean has a stan- (Mayor & Queloz 1995; Marcy, Cochran & Mayor 2000). dard deviation of σ = 0.04 in the log of the luminosity. Of the 92 high fidelity stars we expect 6 ± 2 with signifi- We show as a solid line the difference between coeval evolution tracks that differ by 0.1 dex in [Fe/H]. The cantly higher metallicities. None of the stars is sufficiently dotted line shows the position of the Lithium dip in off the evolutionary track that an enhancement of 0.1-0.2 the Hyades. dex could be allowed. This would suggest that no star in the Hyades is likely to have metallicities enhanced by We see from Figure 1 that the scatter off the evolution the 30M⊕ of rocky material needed to account for the track is less than ∼ 0.05 in the log of the luminosity com- enhanced metallicities of the parent stars of short period pared to the mean local value. However an evolutionary extra solar planets (Murray et al. 2001; Laughlin 2000; track that differs by 0.1 dex in [Fe/H] (shown as the solid Murray & Chaboyer 2001), unless these stars happen to line in Figure 1) is offset by about 0.15 in the log of the lie exactly on the same isochrone as their non polluted luminosity. We therefore estimate that the star to star dif- counterparts. ferences in metallicity could be only as large as 0.03 dex in We have checked that there is no correlation between [Fe/H]. This only a bit larger that that estimated for the the magnitude of the scatter in the HR diagram and the solar neighborhood by Murray et al. (2001), 0.02 dex cor- amount of lithium for the 19 F stars in the high fidelity responding to 0.5M⊕ of accreted iron from the inner solar sample of de Bruijne et al. (2001) that have Lithium system. This limit for the standard deviation of 0.03 dex abundances listed by Boesgaard (1987); Thorburn et al. in [Fe/H], is better than that previously estimated by Friel (1993)). The observed scatter, even though it seems to & Boesgaard (1990a) from less than a dozen F stars which peak near the Lithium dip, does not seem to be obviously was roughly 0.1 dex. This low level of scatter in the metal- related to the mixing below the convection zone. We find licities suggests that the gas from which the stars form is no relation between the scatter off the isochrone and the quite homogenous in metallicity. Because massive stars effective temperature of the star as would be expected from become supernovae well before low mass stars are finished a pollution model which is sensitive to the percentage of accreting this limit also suggests that nearby supernovae mass within the convection zone. are not capable of substantially enriching the ISM in their own nearby star forming regions. Stellar isochrones are computed assuming that stars do 3. discussion not accrete additional high-Z material after formation. 4

In this letter we have used the high quality HR diagram (approximately 0.1 dex) (King et al. 2000; Friel & Boes- of the Hyades to place limits on the scatter in the metallic- gaard 1990a). ities of this cluster. We find that the scatter in [Fe/H] has Despite the fact that the ISM (along 1kpc line of sight standard deviation approximately 0.03 dex. This estimate absorption lines) appears to have a uniform metallicity is an improvement upon previous spectroscopic estimates (Meyer, Jura & Cardelli 1998), the metallicities of mov- and suggests that open clusters are formed from gas that ing groups differ significantly (Friel & Boesgaard 1990a,b), is homogenous in metallicity. and the youngest ones seem to have increasingly subsolar We have introduced the possibility that the extremely values. There also are extremely metal rich stars some of low scatter from a predicted isochrone can be used to con- which are not young. The high metallicity stars are kine- strain stellar pollution scenarios which involve the addition matically related, they tend to be in moving groups or of high-Z material to the star from processes associated streams (Soubiran 1999). This suggests that high metal- with planet formation. This can be done if licity stars are formed in clusters that are uniform in metal- tracks are calculated which include the addition of high-Z licity just like lower metallicity stars. This may rule out metals after formation to see where stars lie on the HR di- a scenario where even a few percent of stars have their agram. If polluted stars lie off the isochrone for unpolluted metallicity increased significantly by the affects of plane- stars then the scatter of stars in the HR diagram can be tary processes and would instead support a model where used to constrain the extent of stellar pollution. Otherwise higher metallicity stars are more likely to harbor short comparisons between spectroscopic measured metallicities period planets. However there is good evidence that one and those estimated from the mean properties of the clus- parent star of an extra solar planet has engulfed planetary ter can be used to identify stars which are likely to have material; the detection of Lithium 6 in HD 82943 (Israelian been polluted by high-Z material. et al. 2001). Further study of uniform populations such as Since few stars in young clusters have had comprehen- are found in young clusters may be able to determine the sive metallicity analyses, subsequent studies could be ex- relative statistical importance of both scenarios. panded and verified, and including more stars with a larger range of mass. Care should be taken that stars with dis- crepant metallicities are not thrown out of the sample and This work could not have been carried out without assumed to be non-cluster members. Currently there is no helpful discussions with Don Garnett, Caty Pilachowski, observational evidence that metallicities of different mass Jonathan Lunine, Ari Laor, Patrick Young, Dave Arnett, stars in a given cluster differ. Comparison of spectroscop- Adam Burrows, Nadya Gorlova, and Mike Meyer. I thank ically measured metallicities in the Hyades, and Joss de Bruijne for providing me with information on the NGC 2264 between G and F stars find that the metallic- high fidelity Hyades sample. A. C. Q. gratefully acknowl- ities are the same within the observational uncertainties edges hospitality and support as a Visitor at the Technion.

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