Primordial and non-primordial helium

PHYSICS REPORTS (Review Section of Physics Letters) 227, Nos. 1—S (1993) 251—256. North-Holland

PHYSICS REPORTS

Primordial and non-primordial helium B.E.J. Pagel NORDITA, Blegdamsvej 17, 2100 Copenhagen 0, Denmark

Abstract: Existing data on helium, oxygen and nitrogen abundances in HIl, blue compact and irregular galaxies are rediscussed paying special attention to objects with noticeable broad emission lines due to WoIf—Rayet stars, in some ofwhich there is reason to suspect temporary and local enhancements of both helium and nitrogen from stellar winds. The resulting estimate of the primordial helium abundance, Y~= 0.228 ±0.005 (se.), is unchanged from what has been derived previously, but the d Y/dZ slope is still remarkably large, about 4, and in significant disagreement with what had been theoretically predicted for these low-metallicity systems.

1. Introduction It is both an honour and a pleasure to join in the celebration of Willy Fowler’s 80th birthday. Along with many others I owe him a great deal. Back in 1954 in Cambridge (UK), it was he who got me interested in nucleosynthesis and abundances, and his brilliance, his personal warmth and his complete lack of pomposity have been a pleasure and an inspiration throughout the intervening 37 years. Long may it continue! This paper is yet another discussion of helium in extragaiactic HII regions, based in large part on a consolidating paper in the R.A.S. Monthly Notices [1] in which the attempt has been made to tie up a few loose ends left over from previous investigations. The exact value of the primordial helium abundance continues to be an interesting question in the light of new developments such as the discovery of beryllium in a very metal-deficient star [2, 3] and the suggestion of a 17 keY neutrino [4], either of which raises the possibility that there is more to cosmological nucleosynthesis than envisaged in the homogeneous Standard Big Bang (SBBN) model [5,6], which remains very attractive owing to its simplicity but may need correction for inhomogeneities [7]. Furthermore, SBBN together with a reasonable upper limit to primordial D + 3He [5] requires the primordial helium mass fraction Y~to be at least 0.236 [8, 9] and it has been argued that at least if systematic errors are absent an extrapolation of the (He, N) data to zero nitrogen abundance suggests that Y~= 0.220 or less [10]. Consequently it is still of interest to see whether existing estimates of Yp can be improved. The determination of Y~depends in turn on a study of helium abundances in objects with differing abundances of heavier elements (or “metallicities”) due to stellar nucleosynthesis followed by ejection into the interstellar medium and extrapolating to zero metailicity. This extrapolation involves the slope d Y/dZ of an assumed relation between helium and heavier elements, and so the existence or otherwise of such a relation, and the magnitude and possible variability of the slope, therefore become important issues in the determination of Yp as well as being of great significance to the theory of stellar evolution and nucleosynthesis. —

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2. Methods of deriving Yp Helium is the second most abundant element in the visible universe and there is accordingly a vast amount of information about its distribution from optical and radio emission lines in nebulae, optical absorption lines in spectra of hot stars, scale heights of the atmospheres of major planets and the influence of initial helium content on stellar structure and evolution [5, 6, 11—14]. However, most methods are subject to systematic uncertainties that are difficult to quantify, and most objects contain additional helium due to stellar nucleosynthesis so that, while the various data provide excellent evidence for a “floor” to the helium mass fraction somewhere between 20 and 25%, most of them lack the precision required to constrain BBNS theory significantly. However, the required precision of a few per cent in the He/H ratio can be obtained under favourable circumstances from nebular emission lines that are formed predominantly by radiative recombination with theoretically known coefficients that are not very sensitive to temperature or density and there are ionised nebulae (HIl regions) with high surface brightness in irregular, blue compact and Hil galaxies (the latter term denoting galaxies dominated by a bright HII region and discovered in objective-prism surveys), and in the outer regions of some spirals, which have such low abundances of oxygen and other detectable heavy elements that their helium abundance can be expected to be only a little above primordial. At the same time many precautions are needed to secure adequate accuracy: careful spectrophotometry and allowance for complicating physical effects such as undetected neutral helium in the H~region, absorption lines in underlying continua or in the interstellar medium and Earth’s atmosphere and collisional and radiative transfer processes [15]. Peimbert and Torres-Peimbert [16] measured helium abundances in the Magelianic Clouds and noted a small trend for Vto increase with “metallicity” (i.e. the abundance of oxygen and other measurable elements) from IZw18 [17] with 0.02 solar oxygen through IIZw4O [17] and the SMC to the LMC to Orion with about 0.5 solar oxygen, and accordingly proposed a programme to determine Y~from a linear regression of the form Y=Y~+ZdY/dZ,

(1)

where the heavy-element mass fraction Z is closely related (virtually proportional) to the oxygen abundance: Z ~ 20(0/H). This programme was carried out for HII regions in a mixture of irregular and blue compact galaxies [18] with the results

=

0.233 ±0.005,

dY/dZ

=

3.0 ±1.6,

(2)

which still look substantially correct, but have given rise to various doubts and controversies [6, 19]. In particular, Kunth and Sargent [19] in a study of blue compact and Hil galaxies found no evidence for a dY/dZ slope, but this conclusion was biased by the high weight given on signal: noise grounds to IIZw4O where the yellow helium line ~.5876is cut down by absorption from sodium atoms in the Milky Way [20]. Without IIZw4O their data give dY/dZ = 4.6 ±5.9, large but still insignificant. Values of d Y/dZ greater than about 1 at low metallicities present a challenge to current models of stellar evolution [21].

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3. The newer data For some years I have been engaged, in collaboration with Roberto Terlevich, Jorge Melnick, Ed Simonson and Mike Edmunds, in a programme designed to improve the observational data base for the determination of V~and d Y/cIZ from extragalactic HII regions [1]. We make use of about a dozen suitable objects from the spectrophotometric survey of Terievich et al. [22, 23] supplemented by fresh high-quality observations (especially of 26678) of these and some other objects and by selected data from the literature, paying careful attention to spectrophotometric quality (resolution, signal : noise and linearity) and to the accuracy with which one can correct for the complicating physical factors; in particular, we use only objects in which we are confident that the abundance of neutral helium is negligible. In this way we have compiled a data base of 30 objects, with 15 106(0/H) 240 (or 0.3 solar), for which we have helium mass fractions V with a median standard error of ±0.012 (or 5%) per object. Now it has been clear for quite some time that there is cosmic dispersion in the (Y, Z) relation, the clearest case being that of NGC 5253 [22,24,25] in which the abundances of helium and nitrogen (but probably not of oxygen) are strong functions of where the spectrograph slit is placed. The largest abundance of helium is found in the brightest knot NGC 5253A which also displays a broad emission feature at 24686 due to the presence of Wolf—Rayet stars in the underlying stellar cluster. The visibility of this broad feature in “WR galaxies” is associated with a larger dispersion in the N/O ratio than is found for those low-metallicity HIl regions in irregular and blue compact galaxies in which the feature is definitely undetected (see fig. 1) and some WR galaxies resemble NGC 5253A in having larger abundances of both helium and nitrogen than other HIl regions with the same oxygen abundance. Pagel, Terlevich and Melnick [26] suggested that this effect was caused by a temporary and local pollution of the observed HIl region by products of hydrogenburning ejected in winds from massive stars, analogous to the He, N-rich ring nebulae surrounding some WR stars in the Milky Way [27]. We accordingly plotted helium against nitrogen, as well as oxygen, in the hope of providing a smoother relation. However, this is not an ideal solution,

log~

I -1.0-

4;

—~‘-I P0X4 J

NGC 5253 POXI2O

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117w 40

-

1~pe”

Pox 86

UM439

-1.5

UM461/

/ NGC 4861

7.0

7.5

8.0

8.5

12 + log (0/H) Fig. 1. Plotof log (N/O) against 12 + log (0/H) for WR galaxies or HII regions known to have the broad WR feature (filled circles) and for objects in which that feature is definitely not detected (open circles) in the extragalactic HII region sample of Pagel et al. [1] where references to the abundances and WR classifications can be found.

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SD

~ 10

N —

IOOY

—

t-~

SD

N~~(0

(0

IOSD

u~ .-~NO ~ ti?tzv—

.—e)

I

i0~ 0/H 28

N 5253

N5455

N5~I 22

-

461

I 0

I 4

8

12

106

16

20

24

N/H

Fig. 2. Regressions ofhelium against oxygen and nitrogen abundances for objects with definite detections ofWoIf—Rayet features (filled circles) and objects in which such features are undetected or doubtful (open circles). Maximum-likelyhood regression lines are shown for the latter category, with alternatives equivalent to ±hr limits. Larger and smaller circles represent higher and lower weights, respectively, and a few typical error bars are shown [1].

because the excess nitrogen due to winds from massive stars is a “secondary” nucleosynthesis product [28], the abundance of which (relative to excess helium) increases with the metailicity, whereas the constant N/0 in (what we take to be) unpolluted objects suggests that the underlying nitrogen content, resulting from the general chemical evolution of the parent galaxy, is primary, so that there cannot simultaneously be linear relationships between helium and nitrogen from both sources. Furthermore, subsequent observations across NGC 5253 [1,24, 25] showed that there is no very detailed correlation between helium and nitrogen abundances in different parts of the nebula. In our new survey [1], therefore, we have tried to avoid including polluted objects altogether, by omitting the definite WR galaxies, but including those whose “WR” status is uncertain; these add weight without noticeably biasing the results. The resulting relations between helium and nitrogen and oxygen are shown in fig. 2. 4. Discussion Figure 2 shows that the 19 objects in which WR features are absent or doubtful define an excellent linear relation between helium and oxygen within the metallicity range considered. This is given by V=

0.2279 + 122(0/H) ±0.0057 ± 42(s.e.)

(3)

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255

with an rms deviation of only 0.006 which suggests that our errors may have been overestimated. Separate solutions for 9 objects in which WR features are definitely undetected and for 10 in which they are doubtful give similar results though with less weight. The relation between helium and nitrogen shows a noticeable departure from linearity in the sense of becoming flatter with increasing nitrogen abundance, which is due to an increase in N/0 with 0/H at the larger nitrogen abundances in our range; this increase does not show up in fig. 1 because the objects with larger N/H are all in the “doubtful” WR category. The tight cluster of 16 objects (with WR features either doubtful or definitely undetected) having 10~N/H 66 gives the solution V=

0.2273 + 3080(N/H) ±0.0064 ±1500(s.e.),

(4)

i.e. almost exactly the same value of V~as from the regression with oxygen, leading to an estimate of ±0.005 for its standard error. It is also clear from this that there is no support for suggestions of a still steeper slope at the lowest nitrogen abundances leading to V~ 0.220 [10], once the offending definite WR objects are removed. Systematic errors can readily bridge the gap between our V~and the minimum value of 0.236 required for consistency of SBBN theory [8, 9], but as I have argued before a value greater than 0.240 is very unlikely. In the other direction, one should take note of a ver~’low helium abundance, V = 0.21 ±0.01 found recently for an HII galaxy from the Second Byurakan Survey SBS 0335-052 [29], which was not included in our analysis; clearly a refinement of this number would be of great interest. (After the meeting, a paper appeared giving fresh data on this object and claiming a low helium abundance in one component [32], but this result is clearly wrong because of underlying absorption at 24471 and, when this is allowed for, both components fit eq. (3).) While the above regression against oxygen is very similar to previous results, the coefficient of (0/H) in eq. (3), corresponding to dV/dZ = 6, is still very large despite removal of the bias due to WR galaxies which may have affected previous estimates thereof. There are still two additional sources of bias to be removed, namely underestimates of oxygen abundance due to neglect of temperature fluctuations and of a depletion (expected to be of the order of 15%) from the gas phase due to formation of dust; and radiative transfer effects from the dust causing the helium abundance to be overestimated as a result of the assumption of hydrogen recombination according to case B [30]. Allowance for these effects reduces our estimate of d V/dZ to 4.0 ±1.3, which is still very large compared to a theoretical prediction, based on mass-loss rates assumed to be proportional to that d V/dZ ~ 0.6 for these low-metallicity objects [21]. The same theory also underestimates Z°~5, the incidence of WR galaxies at metallicities corresponding to the SMC and below [31] and probably underestimates the mass loss rate at low metallicities; but many other factors such as the initial mass function, the detailed behaviour of intermediate-mass stars ejecting helium in planetary nebulae and effects of rotation and close binary evolution may well be involved.

5. Conclusion While the overall agreement between SBBN theory and what we can deduce about primordial abundances is generallyvery good, there are still some small discrepancies and hints that there may be more to Big Bang nucleosynthesis than meets the eye. The above discussion is respectfully dedicated to Willy Fowler who has done more than most both for the modern development of

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SBBN theory in the 1960’s and more recently in the quest for alternatives that may prove to be still more interesting. References [1] B.E.J. Pagel, E.A. Simonson, Ri. Terlevich and MG. Edmunds, Mon. Not. R. Astron. Soc. 255 (1992) 325. [2] G. Gilmore, B. Edvardsson and P.E. Nissen, Astrophys. i. 378 (1991) 17. [3] 5G. Ryan, i.E. Norris, MS. Bessell and C.P. Deliyannis, Astrophys. i. 388 (1991) 184. [4] A. Hime and AN. ielley, Phys. Lett. B 257 (1991) 441. [5]J. Yang, MS. Turner, G. Steigman, D.N. Schramm and K. Olive, Astrophys. J. 281 (1984) 493. [6] AM. Boesgaard and G. Steigman, Ann. Rev. Astron. Astrophys. 23 (1985) 319. [7] H. Kurki-Suonio, R.A. Matzner, K.A. Olive and D.N. Schramm, Astrophys. i. 353 (1990) 406. [8] L.M. Krauss and P. Romanelli, Astrophys. i. 358 (1990) 47. [9] T.P. Walker, G. Steigman, D.N. Schramm, K.A. Olive and H-S. Kang, Astrophys. J. 376 (1991) 393. [10] G.M. Fuller, RN. Boyd and iD. Kalen, Astrophys. J. Lett. 371 (1991) Lii. [11] B.E.J. Pagel, in: The Big Bang and Element Creation, ed. D. Lynden-Bell, Phil. Trans. R. Soc. London A 307 (1982) 19. [12] PA. Shaver, D. Kunth and K. Kjär, eds Primordial Helium (ESO, Garching, 1983). [13] B.E.J. Pagel, in: A unified View of the Macro- and the Micro-Cosmos, First International School on Astro-Particle Physics, Erice, eds A. de Rujula, DV. Nanopoulos and PA. Shaver (World Scientific, Singapore, 1987) p. 399. [14] B.E.J. Pagel, Phys. Scripta T 36 (1991) 7. [15] K. Davidson and T.D. Kinman, Astrophys. J. Suppl. 58 (1985) 321. [16] M. Peimbert and S. Torres-Peimbert, Astrophys. i. 203 (1976) 581. [17] L. Searle and W.L.W. Sargent, Astrophys. i. 173 (1972) 25. [18] J. Lequeux, M. Peimbert, iF. Rayo, A. Serrano and S. Torres-Peimbert, Astron. Astrophys. 80 (1979) 155. [19] D. Kunth and W.L.W. Sargent, Astrophys. J. 273 (1983) 81. [20] H.B. French, Astrophys. J. 240 (1980) 41. [21] cf. A. Maeder, Astron. Astrophys. 264 (1992) 105. [22] A. Campbell, R.J. Terlevich and J. Melnick, Mon. Not. R. Astron. Soc. 223 (1986) 811. [23] Ri. Terlevich, J. Melnick, J. Masegosa and M. Moles, Astron. Astrophys. Suppl. 91(1991) 285. [24] i. Walsh and J.-R. Roy, Astrophys. i. Lett. 319 (1987) L57. [25] J. Walsh and i.-R. Roy, Mon. Not. R. Astron. Soc. 239 (1989) 297. [26] B.E.J. Pagel, Ri. Terlevich and i. Melnick, Pub. Astron. Soc. Pacific 98 (1986) 1005. [27] C. Esteban, J.M. Vilchez, Li. Smith and A. Manchado, Astron. Astrophys. 244 (1991) 205. [28] B.EJ. Pagel, in: Starbursts and Galaxy Evolution, eds T. Montmerle and i.T.T. Van (Editions Frontières, Paris, 1987) p. 227. [29] E. Terlevich, R. Terlevich, E. Skillman, A. Stepanian and V. Lipovetskii, in: 31st Herstmonceux Conference: Elements and the Cosmos, eds MG. Edmunds and R.J. Terlevich (Cambridge Univ. Press, Cambridge, 1992) p. 21. [30] S.A. Cota and G. Ferland, Astrophys. J. 326 (1988) 889. [31] Ph. Arnault, D. Kunth and H. Schild, Astron. Astrophys. 224 (1989) 73. [32] J. Melnick, M. Heydari-Maladeri and P. Leisy, Astron. Astrophys. 253 (1992) 16.