Nitrogen Abundances and the Distance Moduli of the Pleiades and Hyades
aa r X i v : . [ a s t r o - ph . S R ] S e p Nitrogen Abundances and the Distance Moduli of the Pleiadesand Hyades
Blake Miller, Jeremy R. King, Yu Chen
Department of Physics and Astronomy, 118 Kinard Lab, Clemson University, Clemson, SC29634-0978 [email protected] [email protected] [email protected] andAnn M. Boesgaard † Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822 [email protected]
ABSTRACT
Recent reanalyses of
HIPPARCOS parallax data confirm a previously noteddiscrepancy with the Pleiades distance modulus estimated from main-sequencefitting in the color-magnitude diagram. One proposed explanation of this distancemodulus discrepancy is a Pleiades He abundance that is significantly larger thanthe Hyades value. We suggest that, based on our theoretical and observationalunderstanding of Galactic chemical evolution, nitrogen abundances may serveas a proxy for helium abundances of disk stars. Utilizing high-resolution near-UV Keck/HIRES spectroscopy, we determine N abundances in the Pleiades andHyades dwarfs from NH features in the λ ∼ . T eff range, we find thePleiades N abundance (by number) is 0 . ± .
05 dex lower than in the Hyades forstars in a smaller overlapping T eff range around 6000 K; possible systematic errorsin the lower Pleiades N abundance result are estimated to be at the ≤ .
10 dexlevel. Our results indicate [N/Fe] ∼ † Visiting Astronomer, W.M KECK Observatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generous financial support of the W.M. KeckFoundation.
Subject headings:
Star Clusters and Associations – Stars
1. Introduction1.1. The Pleiades Distance Modulus Problem
A provocative early result of the ESA’s
Hipparcos parallax mission was a Pleiades dis-tance modulus (van Leeuwen & Hansen Ruiz 1997; van Leeuwen 1999) 0.30 mag smallerthan that derived from evolutionary models via main-sequence fitting (Pinsonneault et al.1998). Given that metal-poverty explains the small fraction of young nearby disk field dwarfsthat are similarly subluminous in the
Hipparcos -based color-magnitude diagram Soderblom et al.(1998), Pinsonneault et al. (1998) suggested that the parallax versus main-sequence fittingdiscrepancy was caused by small (1 mas) systematic errors in the
Hipparcos
Pleiades par-allaxes. Makarov (2002) recomputed Pleiades parallaxes using the
Hipparcos intermediateastrometry data, finding a shift in the mean parallax that places the inferred distance mod-ulus in substantial agreement with the main-sequence fitting results. Robichon et al. (1999),however, utilize Monte Carlo simulations of the
Hipparcos cluster data, and find that no sys-tematic biases or small angular scale effects are present in the Pleiades
Hipparcos parallaxes.Percival, Salaris & Groenewegen (2005) have cautioned against using lower main-sequence( B − V ) colors, which appear anomalous in the Pleiades (Stauffer et al. 2003), in deriving dis-tances from main-sequence fitting. Excluding these data, Percival, Salaris & Groenewegen(2005) find that various color-magnitude planes consistently yield Pleiades distances 10%larger than parallaxes. Distance determinations of the Pleiades eclipsing binary HD 23642(Southworth, Maxted & Smalley 2005) and the Pleiades binary Atlas Pan, Shao & Kulkarni(2004) are consistent with those determined from cluster main-sequence fitting, and incon-sistent with the smaller distance implied by Hipparcos . The HST/FGS-based parallaxesof 3 Pleiades dwarfs Soderblom et al. (2005) are also in outstanding agreement with themain-sequence fitting-based results, but inconsistent with the
Hipparcos result. 3 –van Leeuwen (2007a) and van Leeuwen (2009) describe a new reduction, with improvedtreatment of small angular scale astrometric correlations noted by Pinsonneault et al. (1998),of the
Hipparcos mission astrometric data that has lead to significantly increased accuracy.The result is a Pleiades distance modulus in accord with the original
Hipparcos result. Indeed,van Leeuwen (2009) cite 3 clusters (Pleiades, Blanco 1, NGC 2516) whose main-sequencestars appear similarly subluminous when compared to those in other clusters.
Grenon (2001) suggests that the discrepancy can be explained by a slightly sub-solarPleiades metallicity, [m/H]= − .
11, indicated by Geneva photometry. However, Stello & Nissen(2001) have used the metallicity-sensitive Stromgren m index to select field stars havingthe same photometric metallicity as the Pleiades to form an empirical Pleiades ZAMS withwhich to conduct main-sequence fitting; Stello & Nissen (2001) deduce a Pleiades distancemodulus in close agreement with previous fitting, but at odds with the Hipparcos paral-laxes. Percival, Salaris & Kilkenny (2003) also carry out empirical main-sequence fittingof the Pleiades using a sample of field stars of known metallicity and high accuracy
Hip-parcos parallaxes, and infer a surprisingly low Pleiades metallicity ([m/H] ∼ − .
4) from a2-color diagram. Assuming this photometric metallicity, they find that the distance frommain-sequence fitting is brought into agreement with the
Hipparcos results.The differences between the Geneva-, 2-color plane-, and Stromgen-based ([Fe/H]=+0 .
08; Eggen 1986) photometric Pleiades metallicity estimates, as well as the simultane-ous claims that metallicities of both − .
11 and − .
40 shift the evolutionary model mainsequences into agreement with the Hipparcos parallaxes, seem remarkable. The implica-tion that a variety of independent modern high-resolution spectroscopic abundance analyses(Boesgaard & Friel 1990; Cayrel, Cayrel de Strobel & Campbell 1988; Wilden et al. 2002;King et al. 2000; Ford, Jeffries & Smalley 2002; Soderblom et al. 2009) have consistentlyoverestimated a near-solar Pleiades metallicity also is remarkable. van Leeuwen (2009) posits that the dichotomy between the
Hipparcos -based H-R dia-grams of the young Pleiades, Blanco 1, and NGC 2516 clusters and older clusters like theHyades and Praesepe is due to “age-dependent luminosity effects” that are neither empiri-cally calibrated out of the theoretical isochrones nor included in the underlying evolutionary 4 –models. Such a luminosity difference between the Hyades/Praesepe and Pleiades main-sequences was inferred on the basis of Stromgren photometry long ago (Crawford & Perry1966), questioned by Eggen (1994), but confirmed by Joner & Taylor (1995). van Leeuwen(2009) note that this so-called “Hyades anomaly” has simply been forgotten, rediscoveredfrom
Hipparcos parallaxes, and relabeled as a Pleiades parallax anomaly.While several origins of a real luminosity difference between the Pleiades and Hyadesmain-sequences exist (Labonte & Rose 1985), a suggestion as old as the Hyades anomalyitself is inter-cluster He differences (Crawford & Stromgren 1966) that yield inter-cluster lu-minosity differences. Pinsonneault et al. (1998) note that a Pleiades He mass fraction of Y = 0 .
37, compared to a solar value Y ⊙ = 0 . − .
28 (Bahcall, Pinsonneault & Wasserburg1995), could explain the 0.3 mag distance modulus discrepancy. Observational evidence forsuch large Y values in young disk stars is muddy Pinsonneault et al. (1998), and the determi-nation of stellar He abundances is fraught with uncertainty and study-to-study differences.A relative comparison of Pleiades He abundances with those in the Hyades, for which the Hipparcos -based and main-sequence fitting-based distance moduli agree, remains elusive:stellar He abundances are best-determined in B stars, which are absent in the Hyades dueto its age.Chemical evolution models (e.g., Carigi & Peimbert 2008) that map the coproduction ofHe, C, O, and Fe in the Galactic disk suggest He proxies could be used to examine the relativePleiades-Hyades Y values in the context of relative cluster C, O, and Fe abundances. WhileO abundances derived from high-excitation O I lines exhibit unsettling large trends with T eff (Schuler et al. 2004, 2006) that complicate an assessment of relative cluster abundances, areliable relative measure comes from the Schuler et al. [O I ]-based determinations for 3 dwarfsin each cluster. These data suggest ∆[O/H]= +0 . ± . − Hyades). TheCarigi & Peimbert (2008) chemical evolution models from Carigi & Peimbert (2008) yield-ing the largest Y difference for this negligible O difference suggest ∆ Y = +0 . ± . − Hyades), significantly smaller than the ∆ Y = +0 .
09 prescribed by Pinsonneault et al.(1998) to resolve the Pleiades’ distance modulus discrepancy.High-excitation C I -based C abundances for cluster stars with T eff ≤ . ± .
02 and − . ± .
03 for the Hyadesand Pleiades (Friel & Boesgaard 1990). Interpreting these relative abundances with theCarigi & Peimbert (2008) models suggests Y (Pleiades) is no larger than Y (Hyades) at the ∼ σ confidence level. The difference (Pleiades − Hyades) in mean cluster F-dwarf Fe abun-dance is ∆[Fe/H]= − . ± .
03 (Boesgaard & Friel 1990). The Carigi & Peimbert (2008)chemical evolution models suggest this corresponds to ∆ Y = − . ± . While these abundance comparisons are inconsistent with a He abundance differenceable to resolve the Pleiades distance modulus discrepancy, it may be that C, O, and Fe arenot reliable tracers of He. We suggest that N may serve as a more robust proxy for He, andthat relative cluster N abundances should thus be examined.Extant results suggest that [N/Fe] ∼ ∼ ⊙ ≤ M ≤ ⊙ ). Carigi et al. (2005) suggest that obser-vations constrain the contribution of LIMS to N abundances in the solar neighbhorhood to65 − ≥
2. Observational Data
High-resolution ( R ∼ , T eff & − v sin i & −
20 km s − ). Given the moderate trendin [N/H] abundance with T eff we find for the (more numerous) cool Hyades stars, we restrictour attention in the Pleiades to stars with T eff & II
739 and 761).Tables 1 and 2 list the Hyades and Pleiades stars analyzed here. Hyades objects are listedwith van Bueren (1952) designations, while Pleiades stars are listed by their Hertzsprung(1947) identifications. Examples of the spectra can be seen in Figure 1. Tab. 1Tab. 2Fig. 1
3. Abundance Analysis and Results
Stellar parameters taken from the Be abundance studies of Boesgaard & King (2002)and Boesgaard, Armengaud & King (2003) were used to characterize LTE model atmo-spheres interpolated from the grids of Kurucz . The linelist of the 3328 ˚A region was compiledfrom atomic and molecular lines in the Kurucz database , features in the Vienna AtomicLine Database (Kupka et al. 2000), and molecular lines from LIFBASE (Luque & Crosley1999). Our adopted NH dissociation energy adopted is 3.45 eV, intermediate to the canonicalvalue of 3.47 eV (Huber & Herzberg 1979) and the determination of 3.40 ± .
03 eV implied byexperimental measures of various relevant quantities by Ervin & Armentrout (1987). Oscil-lator strengths ( gf -values) were adjusted, typically by ≤ . MOOG package (Sneden 1973) that includes updated bound-free opacity data http://kurucz.cfa.harvard.edu/grids.html http://kurucz.cfa.harvard.edu/linelists.html .
13 and +0 .
00 for the Hyades and Pleiades respectively . Table1 provides the v sin i values adopted from the literature and used to smooth the syntheses inaddition to a Gaussian representing instrumental broadening.We find a small (2-4% of the continuum level) additional continuous veiling is neededto reproduce the depth of the strong non-NH features at 3327.9, 3328.3, 3328.9, 3329.5,3329.9 ˚A for our Hyades stars; this additional veiling, which might signal a slight deficiencyin the bound-free opacity, also substantially improves the line-to-line scatter of the derivedN abundances in a manner not mimiced by small plausible adjustments in the continuumnormalization or smoothing (or both). The additional veiling is added to our synthetic spec-tra by applying an additive constant prior to renormalization. Syntheses and a comparisonwith observed spectra are shown in Figure 1.The abundances in each star are determined from several NH features (or blendedgroup of features) by fitting each individually . The scatter in these in a given star providesa combined measure of random measurement error and continuum fitting uncertainties inthe derived abundances. The abundance results are summarized in Tables 1 and 2. Thefinal three columns contain the mean N abundance (logarithmic by number, on the usualscale where log N ( H ) = 12 . ), the number of features/regions used in determining the mean,and the standard deviation of the individual measurements. The mean logarithmic numberabundance of nitrogen for each star is plotted versus T eff in Figure 2, which reveals a modest0.2 dex trend in the Hyades dwarfs over the 5400-6200 T eff range. The non-parametricSpearman rank correlation coefficient is significant at > .
9% confidence level for the Hyadesdata. Fig. 2We consider three possible sources of this trend. First is a deficiency in continuousopacity– whether truly continuous (e.g., bound-free) opacity or quasi-continuous opacity inthe form of myriad very weak lines unnaccounted for in the linelist. As noted above, we haveguarded against such a deficiency by making small effective enhancements in the assumedveiling to reproduce the depths of strong atomic lines and minimize the scatter in the Nabundances derived from NH features of different strength. The second possibility is that For both the Pleiades and Hyades, we assume [O/H]= +0 .
14 based upon the λ I ]-based clusterdwarf results from Schuler et al. (2004) and Schuler et al. (2006). For the Hyades, we assume [C/H]=+0 . I and C features (Schuler, King & The 2006). These features are located at 3325.88, 3326.39/3326.42, 3326.94, 3327.15, 3327.60, 3327.72, 3328.18,3328.24, 3329.75, 3330.28, 3330.38, 3330.45, 3330.50, 3330.64, 3330.81, and 3330.92 ˚A D = 3 .
47 eV Graham & Lew (1978), avalue insignificantly higher than our adopted value.The third possibility is an origin associated with the T eff -dependent abundance trendspreviously observed in Hyades dwarfs. Figures 3, 5, and 10 of Schuler et al. (2006) show a0.5 dex monotonic increase in ∆Fe=[Fe II /H] − [Fe I /H], a 0.6 dex increase in O I -based[O/H] values, and a 0.2 dex increase in [O I ]-based [O/H] values in Hyades dwarfs over the T eff range 6000-4000 K. Whether the trend we see for NH reflects an overdissociation akinto the apparent overexcitation/overionization these other abundances suggest, and what thephysical origin of such effects are, remain unclear; a comparison of Li abundances derivedfrom the λ λ I features in pre-main sequence stars,however, strongly suggests the action of enhanced near UV photoionization in cool veryyoung stars (Bubar et al. 2011). Regardless, we emphasize that the modest trends in Nabundance are not surprising in the context of T eff -dependent trends previously seen in theHyades dwarfs.
4. Discussion
Given the modest T eff trend in the Hyades N abundances, we determine the relativePleiades-Hyades cluster N difference over the same ∼
250 K T eff range spanned by the fourPleiads. For a given cluster, the standard deviation in the mean N abundance over this range, ± .
06 dex for both clusters, empirically estimates internal measurement and relative stellarparameter uncertainties. This per star estimate is also that expected from the maximum T eff uncertainties of Boesgaard & King (2002) and Boesgaard, Armengaud & King (2003) andthe typical mean measurement uncertainties calculated from the last 2 columns of Tables 1and 2. The unweighted mean cluster N abundances computed over the 5940-6180 K rangeare log N (N)= 7 . ± .
03 (uncertainty in the mean) and 7.91 ± .
03 for the Pleiades andHyades, respectively. Weighting the individual abundances by the squared reciprocals of theindividual uncertainties in Tables 1 and 2 yields indistinguishable mean abundances of log N (N)= 7 . ± .
03 (Pleiades) and 7.90 ± .
03 (Hyades).The Pleiades − Hyades difference is then ∆log N (N)= − . ± .
05, indicating thatthe Pleiades N abundance is smaller than that of the Hyades. The mean cluster Fe abun-dances and their uncertainties from Boesgaard & Friel (1990) yield [N/Fe]= 0 . ± .
04 and+0 . ± .
04 for the Hyades and Pleiades respectively, where the quoted errors reflect inter-nal uncertainties in the mean. We also consider three sources of systematic error. First,we note that the solar N abundance was fixed for each feature by slight alterations in the 9 – gf values when calibrating the line list. Thus, the solar-normalized abundances are derivedself-consistently, and are gf -independent; indeed, altering the NH log gf values by ± . ≤ .
01 dex. Second, had we not employedthe veiling corrections for the Hyades stars, then the Hyades-Pleiades abundance differencewould be increased by 0 .
02 dex. Third, systematic differences in dereddened colors at the0 .
02 mag level for ( B − V ) are possible; in this case, the concomitant alteration to the abun-dances through the adopted T eff values is at the ± .
06 dex level. In sum, we gauge possiblesystematic errors in our finding of a Pleiades N abundance that is lower than that in theHyades to be at the 0 .
06 dex level.The simplest conclusions reached here, then, are that a) both the Hyades and Pleiadesresults are consistent with previous conclusions that [N/Fe] ∼ ∼
25% lower than that of the Hyades–thus providingno evidence that the Pleiades He abundance (by mass) is 40% larger than that of the Hyadesif indeed N production in the Galactic disk is a proxy for He production.Another possible systematic effect that must be considered, however, is the influence ofHe abundance on the derived N abundances themselves. That is, we must ask if it is possi-ble that the Pleiades He and N abundances might truly be enhanced relative to the Hyades,but our derived Pleiades N abundance is too low because we have not accounted for such aHe enrichment in our analysis. One effect of such a putative He enhancement would be onthe Pleiades stellar parameters. Equation 4 of Castellani, Degl’Innocenti & Marconi (1999)indicates that an enhancement of ∆ Y = +0 .
10 (that needed to explain the Pleiades-Hyadesdistance modulus discrepancy) would lead to a 0.05 mag reduction in ( B − V ) colors ofsolar metallicity M V = 6 Pleiades dwarfs. The dwarfs we compare here are brighter, and anestimate of the color sensitivity for them can be made using the reciprocal and reciprocity the-orems to find (cid:0) ∂c∂Y (cid:1) T (where c is ( B − V ) color, T is the effective temperature, and Y is the Hemass fractions) by taking: (cid:0) ∂M V ∂Y (cid:1) c from equation 3 of Castellani, Degl’Innocenti & Marconi(1999), (cid:16) ∂T∂M V (cid:17) c from Figure 4 of Castellani, Degl’Innocenti & Marconi (1999), and (cid:0) ∂c∂T (cid:1) Y from the calibration of Saxner & Hammarback (1985). The result is the same as above, witha +0 .
10 increase in He mass fraction leading to a ( B − V ) color bluer by 0.05 mag.The result of such a helium-induced color shift would be to overestimate the Pleiades T eff values by ∼
170 K and to underestimate the log g values by ∼ .
02 dex. Compensatingfor these parameter errors, including the effects on [Fe/H] and the feedback of metallicityon the derived N abundance, would lower our N abundances by 0 . − .
18. Thus, any suchHe-induced parameter effects act in the opposite way needed to mask a proposed truly higherPleiades N abundance. 10 –A second effect of a putative higher Pleiades He abundance is that on the (model)photospheric structure. We have rerun our analyses using ATLAS12 model atmospheres withthe standard solar He abundance and He abundances enhanced 33% and 75% by number.For stars with T eff = 4800 K, the He-enhanced atmospheres yield N abundances lowered by0.04 dex and 0.08 dex compared to the solar He atmospheres; for stars with T eff = 6000 K,the reductions are 0.03 and 0.07 dex. Just as for the parameter-based effects, He-inducedatmospheric structure effects act in the opposite way needed to mask a proposed truly higherPleiades N abundance.
5. Summary
Our analysis of high-resolution and -S/N near-UV spectroscopy yields a Pleiades N num-ber abundance that is the same as or up to 25 ±
9% lower than in the Hyades. This result isconsistent with previous abundance work suggesting that [N/Fe] ratios of local Galactic diskstars are solar over a range of [Fe/H]. If, as we argue, N production serves as a reliable proxyfor He production in the Galactic disk, then our results provide no evidence for a Pleiades Heabundance larger than that of the Hyades. This conclusion is consistent with those reachedfrom the relative Pleiades-Hyades C, O, and Fe abundances in the context of our currentunderstanding of Galactic chemical evolution. This conclusion is also robust against theeffects of an unrealized but truly higher Pleiades He abundance on model atmospheric struc-ture and our stellar parameters. If the Pleiades distance modulus discrepancy and Hyadesanomaly are not due to unrealized systematic parallax and photometric measurement errors,then our results suggests their physical explanation is not associated with He abundance.This work was supported by NSF grants AST 02-39518 and AST 09-08342 to J.R.K.,and AST 05-05899 to A.M.B.
Facility:
KECK. 11 –
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This preprint was prepared with the AAS L A TEX macros v5.2.
15 –Table 1. Hyades Atmospheric Parameters a and AbundancesStar T eff log g ξ v sin i h log N (N) i N σ (vB) (K) cgs (km s − ) (km s − ) (dex)9 5538 4.44 1.06 3.4 d b b b b b c c c b b c b b b d c a a Boesgaard & King (2002) b Paulson et al. (2003) c Glebocki & Stawikowski (2000) d Estimated as part of our analysis from non-Nitrogen features 16 –Table 2. Pleiades Atmospheric Parameters a and AbundancesStar T eff log g ξ v sin i h log N (N) i N σ (H II ) (K) cgs (km s − ) (km s − ) (dex)948 5960 4.39 1.36 <
12 7.83 16 0.081794 5940 4.39 1.35 12 7.72 11 0.121856 6150 4.37 1.54 16 7.75 7 0.053179 6180 4.37 1.56 < a Boesgaard, Armengaud & King (2003)
Fig. 1.— Our observed spectra (solid points) of the Hyades dwarf vB 92 and the Pleiadesdwarf H II
948 are shown with synthetic spectra of varying N abundance; A (N) indicatesthe logarithmic number abundance of nitrogen on the usual scale where that of hydrogen, A (H), is defined as 12. 17 – Fig. 2.— The mean logarithmic number abundance of N for each star is plotted versus T effeff