Hydrogen-Deficient Stars ASP Conference Series, Vol. 391, c 2008 K. Werner and T. Rauch, eds. Hydrogen-Deficient Stars: An Introduction C. Simon Jeffery Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland, UK Abstract. We describe the discovery, classification and statistics of hydrogen- deficient stars in the Galaxy and beyond. The stars are divided into (i) massive / young star evolution, (ii) low-mass supergiants, (iii) hot subdwarfs, (iv) cen- tral stars of planetary nebulae, and (v) white dwarfs. We introduce some of the challenges presented by these stars for understanding stellar structure and evolution. 1. Beginning Our science begins with a young woman from Dundee in Scotland. The brilliant Williamina Fleming had found herself in the employment of Pickering at the Harvard Observatory where she noted that “the spectrum of υ Sgr is remarkable since the hydrogen lines are very faint and of the same intensity as the additional dark lines” (Fleming 1891). Other stars, well-known at the time, later turned out also to have an unusual hydrogen signature; the spectacular light variations of R CrB had been known for a century (Pigott 1797), while Wolf & Rayet (1867) had discovered their emission-line stars some forty years prior. It was fifteen years after Fleming’s discovery that Ludendorff (1906) observed Hγ to be completely absent from the spectrum of R CrB, while arguments about the hydrogen content of Wolf-Rayet stars continued late into the 20th century. Although these early observations pointed to something unusual in the spec- tra of a variety of stars, there was reluctance to accept (or even suggest) that hydrogen might be deficient. Joy & Humason (1923) remarked that the hy- drogen lines were “greatly weakened by partial emission”, while Plaskett (1927) thought they were “due to a supernormal abundance of helium or to the star be- ing an exaggerated form of pseudo-cepheid or giant”. Perhaps the philosophy of the time was best summed up by Payne (1925); “The uniformity of composition of stellar atmospheres appears to be an established fact”. It was at least another decade until quantitative spectral analyses forced the conclusion that in R CrB (Berman 1935) and υ Sgr (Struve & Sherman 1940; Greenstein 1940) “somehow, a very substantial amount of hydrogen had been lost”. It seems self-evident today that there should be stars in which hydrogen has been replaced by helium and other products of nucleosynthesis. At the turn of the last century, helium was virtually unknown. In 1868, Pierre Jules Jansen and Sir Joseph Norman Lockyer had independently discovered a bright yellow line in the solar spectrum. Labelled D3, Lockyer concluded it was caused by an element unknown which he called ηλιoζ (helios). It was not until 1895 that Sir William Ramsay isolated terrestrial helium by treating cleveite with mineral acids. Even 3 4 Jeffery Table 1. Various classes of hydrogen-deficient star, with representative pro- totypes and discoverer / classifier. Prototype Class No. Discovery Population I V1679 Cyg Wolf-Rayet ∼ 230 Wolf & Rayet (1867) υ Sgr H-deficient binary 5 Fleming (1891) σ Ori E Intermediate ∼ 30 Berger (1956) helium˜ B SN 1983N, 1984L SN Ib ∼ 80 Elias et al. (1985) SN 1987M SN Ic ∼ 120 Filippenko et al. (1990) Low-mass supergiants R CrB R CrB ∼ 50 Pigott (1797); Ludendorff (1906) HD182040 H-deficient carbon 5 Curtiss (1916); Rufus (1923) HD124448 Extreme helium B 17 Popper (1942) MV Sgr Hot RCrB 4 Woods (1928); Herbig (1964) FG Sge Born-again 3 Hoffmeister (1944) Hot subdwarfs PG1544+488 sdOD / He-sdB ∼ 50 Green et al. (1986); Heber et al. (1988) BD+75◦325 compact He-sdO ∼ 50 Greenstein & Munc¨ h (1955) BD+37◦1977 low-g He-sdO 5 Wolff et al. (1974) Central stars of planetary nebulae BD+30◦3639 [WC] ∼ 50 Beals (1938); Smith & Aller (1969) A 30 Of-WR(C) 2 Cohen (1977) PG1159–035 O(C) ≡ PG1159 ∼ 40 McGraw et al. (1979) K 1-27 O(He) 4 Henize (1981) White dwarfs HZ 21 DO ∼ 50 Greenstein (1966) L 930-80 DB ∼ 400 Luyten (1952) HZ 43 DC ∼ 360 Humason & Zwicky (1947) DQ ∼ 120 DZ ∼ 80 AM CVn AM CVn binary 21 Greenstein & Matthews (1957) this discovery was serendipitous. Ramsay had been looking for argon but, after removing nitrogen and oxygen, noticed that his sample produced a bright yellow line that matched the solar D3 line. Cleveite is an impure form of uraninite having the composition UO2 with about 10% of the uranium substituted by rare earth elements. Helium is created by the α radiation of the uranium which is trapped (occluded) within the mineral. Lord Rutherford eventually identified the α particle with the He++ nucleus in 1907. The science of hydrogen-deficient stars would have been impossible before these discoveries were made. My brief was to give a talk on the classification and statistics of hydrogen- deficient stars. Forty years ago, not many were known, the distinctions between them were poorly understood (Dinger 1970), and it was possible to compile cat- alogues for conference proceedings (Drilling & Hill 1986; Jeffery et al. 1996). Hydrogen-Deficient Stars 5 Table 2. Major surveys yielding hydrogen-deficient stars Survey Reference HZ Humason & Zwicky (1947) FFeige Feige (1958) FB Faint Blue Stars Greenstein & Sargent (1974) PG Palomar-Green Green et al. (1986) BPS HK Objective-Prism Beers et al. (1992) HS Hamburg-Schmidt Hagen et al. (1995) HE Hamburg-ESO Wisotzki et al. (1996) EC Edinburgh-Cape Stobie et al. (1997) SDSS Sloan Digital Sky Stoughton et al. (2002) With the benefit of hindsight and on-line databases, it is now easier to explore history and identify the early landmarks (Table 1). Meanwhile, large-scale spec- troscopic surveys have produced a torrent of new stars with hydrogen-deficient spectra (Table 2). The number of classes has increased, classification has be- come a serious issue, and statistics are changing monthly. Here we aim to give a gentle introduction to the history and growing diversity of our stars, as well as to point to some of the topics explored elsewhere in these proceedings. 2. Population I and Massive Hydrogen-Deficient Stars The following classes of hydrogen-deficient star are associated with the evolution of massive or upper-main-sequence stars. Wolf-Rayet Stars. Wolf-Rayet (WR) stars were identified as peculiar from the bright bands seen on the continuous spectra of three “small” stars in Cygnus (Wolf & Rayet 1867) which in retrospect were identified as emission lines due to ionised atoms including helium. By 1894, some 55 WR stars were known, most of which had been discovered (of course) by Fleming (Campbell 1894). Remarkably, there was debate as to whether these stars were either pre-main sequence or highly-evolved stars through the 1960’s, while disagreement on whether they were H-deficient persisted into the 1980’s. WR stars are found solely in spiral arms, OB associations and young clus- ters, and hence are associated with massive star evolution. They can be clearly divided into two sequences, one showing nitrogen-rich spectra, the other being carbon-rich. Numerically, some 230 WR stars are known in the Galaxy, 159 hav- ing mV < 15 (van der Hucht 2001). Hydrogen has been detected in about half of those analyzed in detail. They are not exclusive to metal-rich environments since about 100 have been found in the Large Magellanic Cloud, and a dozen in the Small Magellanic Cloud. He-rich B Stars or Intermediate Helium Stars. σ Orionis is a tightly bound group of young OB stars, with a total mass of some 100 M⊙ or more. Berger (1956) noted the spectrum of star E to show a series of excessively strong lines of neutral helium, extending to a continuum discontinuity on the ultraviolet side of the Balmer series limit. Subsequent investigation has shown σ Ori E and 6 Jeffery stars like it to be chemically peculiar main-sequence B stars. Some 24 are listed in the catalogue of Drilling & Hill (1986), which may be further divided into fast and slow rotators. Measurements of the helium/hydrogen ratio show that the apparent helium abundances vary with a period of 1–10 d. In σ Ori E, the helium anomaly is associated with a dipole magnetic field of some 104 G inclined ∼ 90◦ to the rotation axis; the magnetic caps are metal-poor. It is generally agreed that the surface abundance distribution is governed by diffusion, where both radiative and magnetic fields act selectively to concentrate particular ele- ments in the line-forming region at specific locations on the stellar surface (e.g. Hunger & Groote 1999). H-Deficient or υ Sgr Binaries. Despite its unusual spectrum (Fleming 1891), it was large radial velocity variations that really drew attention to υ Sgr (Camp- bell 1899). It remained unique until Bidelman (1950) recognized its similarity to HD30353 = KS Per. Two similar stars (V426 Car and V1037 Sco) were dis- covered during a survey of OB+ stars (Drilling 1980). BI Lyn was recognized as the fifth “hydrogen-deficient binary” when it was accidentally observed in a survey of subdwarf B binaries (Jeffery & Aznar Cuadrado 2001). Remarkably, V426 Car had first been noted as a faint star with a spectrum “of the fifth type” (meaning it shows emission lines) in the former constellation of Argo by none other than Fleming (1892) herself. The chief characteristics of the class are strong helium lines on a metallic spectrum, and radial velocity variations of several tens of km s−1. The orbital periods range from circa 50 d to 360 d. Most of these hydrogen-deficient binaries show emission at Hα and Hβ and also evidence of pulsations. From high-resolution ultraviolet spectra, Dudley & Jeffery (1990) showed that υ Sgr is a double-lined system, and hence that the primary has a mass 5 ∼ 3M⊙ and luminosity ∼ 10 L⊙.