LkHα-101 and the Young Cluster in NGC 1579 G. H. Herbig, Sean M. Andrews, and S. E. Dahm Institute for Astronomy, University of Hawaii 2680 Woodlawn Drive, Honolulu, Hawaii 96822, U.S.A. ABSTRACT The central region of the dark cloud L1482 is illuminated by LkHα-101, a heav- 3 ily reddened (AV ≈ 10 mag) high-luminosity (≥ 8 × 10 L⊙) star having an unusual emission-line spectrum plus a featureless continuum. About 35 much fainter (mostly between R = 16 and >21) Hα emitters have been found in the cloud. Their color- magnitude distribution suggests a median age of about 0.5 Myr, with considerable dis- persion. There are also at least 5 bright B-type stars in the cloud, presumably of about the same age; none show the peculiarities expected of HAeBe stars. De-reddened, their apparent V magnitudes lead to a distance of about 700 pc. Radio observations suggest that the optical object LkHα-101 is in fact a hot star surrounded by a small H II region, both inside an optically-thick dust shell. The level of ionization inferred from the shape of the radio continuum corresponds to a Lyman continuum luminosity appropriate for an early B-type ZAMS star. The V − I color is consistent with a heavily reddened star of that type. However, the optical spectrum does not conform to this expectation: the absorption lines of an OB star are not detected. Also, the [O III] lines of an H II region are absent, possibly because those upper levels are collisionally deexcited at high densities. There are several distinct contributors to the optical spectrum of LkHα-101. The Hα emission line is very strong, with wings extending to about ±1700 km s−1, which could be produced by a thin overlying layer of hot electron scatterers. There is no sign of P Cyg-type mass ejection. Lines of Si II are narrower, while the many Fe II lines are still narrower and are double with a splitting of about 20 km s−1. Lines of [Fe II], [O I], [S II] are similarly sharp but are single, at the same velocity as the Fe II average. Work by Tuthill et al. allowed the inference, from K-band interferometry, that the central source is actually a small horseshoe-shaped arc about 0′′. 05 (35 AU) across. A tipped annulus of that size in rotation about a 15 M⊙ star would produce double spectrum lines having about the splitting observed for Fe II. The totality of observational evidence encourages the belief that LkHα-101 is a massive star caught in an early evolutionary state. Subject headings: open clusters and associations: individual (NGC 1579) — stars: in- dividual (LkHα-101) — stars: emission-line — stars: formation — stars: pre-main sequence – 2 – 1. Introduction Current belief is that the formation of a massive star takes place deep in the parent cloud, behind very heavy extinction, and hence will not be optically accessible (Palla & Stahler 1990; Bernasconi & Maeder 1996). But when such a star evolves further to the zero-age main sequence (ZAMS), its radiation and wind will clear out the neighborhood and the star could at some time become optically detectable. We examine here the possibility that LkHα-101, in the nebula NGC 1579, may be such a transition object. NGC 1579 is a clump of bright nebulosity about 2′ across, lying in a dark cloud (L1482) north of the main Taurus-Auriga cloud complex, a line of sight that also passes through the more distant Per OB2 association. Although there are several stars illuminating their own small reflection nebulae nearby, in the era of photography in blue-violet light there was no obvious source of illumination of NGC 1579 itself, nor was any cluster of embedded stars apparent. In 1956 it was found (Herbig 1956) that at the edge of the bright nebulosity there is a very faint, very red star (V ∼ 16) having a powerful Hα emission line and an unusual emission-line spectrum. Subsequent work has shown that the spectra of star and nebula are identical (§4), and that the polarization of the nebula points to a source at that location (Redman et al. 1986), so this is the illuminating source of the nebula. The spectrum of this star, named LkHα-101, has since then been investigated in increasingly greater detail (Herbig 1971; Allen 1972; Thompson et al. 1976; Simon & Cassar 1984; Hamann & Persson 1989, and others). Early radio observations (Brown, Broderick, & Knapp 1976; Cohen 1980; Purton et al. 1982) indicated that there were two principal contributors to the radio continuum at NGC 1579. The first is a point source at the position of LkHα-101, believed to be a hot star plus a very small H II region, both behind an optically-thick dust shell. Later, Hoare et al. (1994) and Hoare & Garrington (1995), from Merlin interferometry, determined mean sizes for this source of 0′′. 55 at 18.7 cm and 0′′. 31 at 6 cm. At 10 µm, Danen, Gwinn, & Bloemhof (1995) found it unresolved, and set an upper limit of 0′′. 34 on its FWHM. This is the object to which the optical spectroscopy of LkHα − 101 refers. The second radio continuum component is an extended H II envelope, of diameter of the order of 1′, the whole being embedded in a clumpy H I cloud about 5′ in diameter (Dewdney & Roger 1986). Sharpless (1959) probably included NGC 1579 as S222 in his catalog of H II regions because it is so much brighter on the red Palomar plates than on the blue. However, contrary to that and to expectation from the radio results, the [O III] λλ4957, 5007 lines, characteristic of H II regions, are not present in the spectrum of the nebula or of LkHα-101. Clearly, NGC 1579 is a reflection nebula and owes its redness to interstellar extinction and to the nature of its illuminating source. But why that source does not show the spectrum of a H II region is another matter. If the radio continuum spectrum of the core source in LkHα-101 is the free-free emission of a spherically-symmetric H II region, then the Lyman continuum (Lyc) flux required to maintain that ionization can be obtained (Harris 1976; Brown, Broderick, & Knapp 1976; Knapp et al. 1976). All – 3 – found, on the basis of the properties of OB stars tabulated by Panagia (1973), that the required 1 Lyc flux could be supplied by a single star, if near the ZAMS, of type B0 or B1 . Assuming that the optical continuum is indeed that of a B0.5 ZAMS star, Cohen (1980) found from the narrow-band colors of LkHα-101 that AV = 9.1 ± 0.5 mags. He found also from recombination theory that the Balmer decrement, those lines assumed optically thin, gave AV = 12.5 ± 1 mags. Subsequent investigators (Thompson et al. 1976; McGregor, Persson, & Cohen 1984; Rudy et al. 1991; Kelly, Rieke, & Campbell 1994) using various procedures have found values of AV ranging from 9.7 to 15.8 mags. If one simply assumes that AV = 10 mag. and a distance of 700 pc (obtained later in this paper) then the observed value of V = 15.7 leads to MV = −3.5. This is not incompatible with a normal B0.5 V, given the crudity of this calculation plus the scatter in the values of MV found in the literature for that spectral type: Panagia (1973) gave −3.5; Vacca, Garmany, & Shull (1996) gave −4.1; while Andersen (1991) found −3.2 and −2.9 from two eclipsing binaries. The mass of a B0.5 V according to the latter two sources is 19 and 13 M⊙, respectively. In respect to total luminosity, the match with expectation does not seem so satisfactory. The 4 same authorities (above) give for L of a B0.5 V star (in units of 10 L⊙): Panagia 2.0, Vacca et al. 6.2, Andersen 1.9 and 1.4. Actual integration of the narrow-band photometry of LkHα-101 between 1.3 and 25 µm by Strecker & Ney (1974) and by Simon & Cassar (1984), corrected as above for extinction and distance, gives 0.8 in the same units. However, there is warm dust, presumably illuminated by the star, well away from the core. Harvey, Thronson & Gatley (1979) found that 4 between ∼1 and 160 µm this “extended region” has a total L = 1.2 × 10 L⊙, still somewhat low. The explanation may simply be that most of the radiation of an early B star is in the ultraviolet, and such integrations must miss that fraction that is not re-thermalized by circumstellar dust, or does not appear in the near-infrared free-free continuum. Therefore there is reason to suspect that LkHα-101 may indeed be a star of mass about 15 M⊙ in an interesting phase of its early evolution. We proceed on that assumption. In what follows, we discuss (§2) our optical and near-infrared photometry of the heavily ob- scured cluster of stars surrounding LkHα-101, (§3) the optical spectrum of LkHα-101 at high- resolution, and (§5, 6) the surrounding molecular cloud and its contents. 1 More recent calculations (Vacca, Garmany, & Shull 1996) based on later values of Te and L and improved atmospheric models (Sternberg, Hoffmann & Pauldrach 2003) predict substantially higher Lyc fluxes for OB stars, so a somewhat later B type for LkHα-101 would follow. It is not possible to be more specific until such calculations are extended to types later than B0.5. – 4 – 2. The Star Cluster 2.1. Optical Photometry Figure 1 is a false-color composite of NGC 1579 created from 300 s B, V , and R images of this dataset.