Using a Hipparcos Derived HR Diagram to Limit the Metallicity
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Draft version November 1, 2018 A Preprint typeset using L TEX style emulateapj v. 14/09/00 USING A HIPPARCOS DERIVED HR DIAGRAM TO LIMIT THE METALLICITY SCATTER OF STARS IN THE HYADES – ARE STARS POLLUTED? A. C. Quillen1,2 Draft version November 1, 2018 ABSTRACT Hipparcos parallaxes and proper motions have made it possible to construct Hertzsprung-Russell (HR) diagrams of nearby clusters with unprecedented accuracy. The standard deviation of high fidelity non-binary non-variable stars about a model stellar evolution isochrone in the Hyades cluster is about 0.04 magnitudes. We use this deviation to estimate an upper limit on the scatter in metallicities in stars in this cluster. From the gradient of the isochrones evolution in the HR diagram we estimate an upper limit for the scatter of metallicities ∆[Fe/H]. 0.03 dex, a smaller limit than has been measured previously spectroscopically. This suggests that stars in open clusters are formed from gas that is nearly homogeneous in its metallicity. We consider the hypothesis that processes associated with planet formation can pollute the convection zone of stars. If the position on the HR diagram is insensitive to the metallicity of the convection zone and atmosphere, then stars which have very polluted convection zones can be identified from a comparison between their metallicity and position on the HR diagram. Alternatively if the pollution of the star by metals results in a large change in the position of the star on the HR diagram in a direction perpendicular to the isochrone, then the low scatter of stars in the Hyades can be used to place constraints on quantity of high-Z material that could have polluted the stars. 1. introduction the enhanced metallicities of parent stars with extra so- lar planets. One possibility is that gaseous planets may Both spectroscopic studies (e.g. Sneden et al. 1992; Ar- be rarer around fairly low metallicity stars. Because the mosky et al. 1994) and photometric studies (e.g., Suntzeff − . 1993; Folgheraiter et al. 1993; Heald et al. 1999; Da Costa HST study of the globular cluster 47 Tuc ([Fe/H] = 0 7) did not detect eclipses caused by planets, short period Jo- & Armandroff 1990; Buonanno et al. 1981) have shown that the homogeneity of heavy element abundances in vian planets could be an order of magnitude rarer than in globular clusters is quite small, typically less than 0.10 dex the solar neighborhood (Gilliland et al. 2001). However the lack of observed eclipses may instead be a result of the in [Fe/H]. The scatter of stars from model isochrones in HR diagrams derived from Hubble Space Telescope (HST) high stellar density in the cluster which would have caused images in cases (such as NGC 6397) has been similar in size the disruption of planetary systems (Gilliland et al. 2001). Alternatively, gaseous planets may be produced with to the measurement errors which implies that the metal- licity variation among the stars in a given globular cluster equal frequency near stars spanning a range of metallici- must be less than a few hundredths of a dex (e.g., Piotto, ties, but the subsequent evolution of the planetary systems increases the metallicities of the parent stars. There is cer- Cool & King 1997). Although high quality HR diagrams of coeval, low metallicity, old globular clusters have been tainly evidence based on abundance analyses in our solar arXiv:astro-ph/0202253v1 13 Feb 2002 constructed, only quite recently has it been possible to system that impacts from bodies in the asteroid belt or outer solar system have changed the surface abundances construct HR diagrams of comparable photometric qual- ity (errors less than a few hundreds of a V band magni- of the earth and other planets (e.g., Morbidelli et al. 2000; tude) for younger and nearly solar metallicity coeval sys- Gautier et al. 2001). One consequence of the orbital mi- grating of giant planets caused by scattering of a disk of tems such as open clusters (de Bruijne et al. 2001). Recent spectroscopic studies have established an as yet planetesimals (Murray et al. 1998), is that the parent stars are polluted by star-impacting planetesimals (Quillen & unexplained possible connection between the metallicity Holman 2000). We infer that impacts with the sun proba- of a star and the existence of short period extra solar planets around the parent star (Gonzalez 1999; Santos et bly occurred more frequently and for more massive bodies during the early era of our solar system. The transfer al. 2000; Gonzalez & Laws 2000; Gonzalez et al. 2001; of angular momentum via driving of density waves into a Santos, Israelian & Mayor 2001). Compared to stars in the solar neighborhood, parent stars of short period proto-planetary gaseous disk will cause the orbital migra- tion of a planet, and can also cause a planet to impact the extra-solar planets tend to have enhanced metallicities, star (e.g., Lin, Bodenheimer & Richardson 1996; Trilling ∆[Fe/H] ≈ 0.2±0.2, compared to the sun. However, there is a significant scatter in the observed metallicities; they et al. 1998). One advantage of the orbital migration sce- nario involving ejection of planetesimals compared to that range from about [Fe/H]≈ −0.4 to 0.4 (Gonzalez et al. involving density waves driven into a gaseous disk is that 2001; Santos, Israelian & Mayor 2001). There are two possible explanations that account for it pollutes the stars with metal rich planetary material when they are older and so have smaller convection zones 1 Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627; [email protected] 2 Visitor, Physics Department, Technion, Israel Institute of Technology, Haifa 32000, Israel 1 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.