S. E. Dahm

A SPECTROSCOPIC EXAMINATION OF ACCRETION DIAGNOSTICS FOR NEAR SOLAR MASS STARS IN IC 348

High-resolution optical and moderate-resolution near-infrared spectra were obtained for 40 near solar mass (~2.0-0.5 M⊙) members of the 2-3 Myr old cluster IC 348 in order to examine established accretion diagnostics and the coupling between inner disk gas and hot, micron-sized dust grains inferred from thermal and mid-infrared excesses. The stellar sample was drawn from the cluster census of Luhman in 2003 with membership being confirmed by radial velocity analysis and the presence of strong Li I λ6708 absorption. Of the stars included in this survey, 12 were classified by Lada in 2006 as hosting primordial, optically thick circumstellar disks, 5 as weak or transition disk systems, and 23 as non-excess stars using the measured slope of the stellar spectral energy distribution (SED) through the four Infrared Array Camera channels (3.6-8.0 μm) of Spitzer Space Telescope. Using the velocity width of Hα as an accretion indicator, we find that 11 primordial disk candidates are suspected accretors, suggesting a strong correlation between gaseous inner disks and optically thick dust emission. Of the five weak or transition disk systems observed, two (L21 and L67) exhibit spectroscopic features indicative of accretion. The presence of gas within the inner disk of these systems, which are free of infrared excess emission shortward of ~4.5 μm, may place constraints upon the physical mechanism responsible for inner disk clearing. Mass accretion rates (M) were determined for all suspected accretors using continuum excess measurements near λ6500 and established relationships between He I λ5876, Hα, Ca II λ8542, Paβ, and Brγ line fluxes and accretion luminosity. M values were found to range from log M to –7.2 M⊙ yr^–1, with a median value of –8.1 M⊙ yr^–1. Magnetospheric accretion models of Hα, Paβ, and Brγ emission by Muzerolle et al. and Kurosawa et al. are found to be in relative agreement with observed fluxes and derived M estimates. He I λ10830 with its metastable lower level is confirmed to be a critical indicator of magnetospheric accretion, although deep subcontinuum absorption profiles often associated with its emission are not fully understood. No statistically significant correlation is found between M and the slope of the SED from 3.6 to 8.0 μm or from 8.0 to 24.0 μm; however, the small number of suspected accretors examined does not allow broader conclusions to be drawn.

LkHα 101 and the Young Cluster in NGC 1579

The central region of the dark cloud L1482 is illuminated by LkHα 101, a heavily reddened (AV ≈ 10 mag) high-luminosity (≥8 × 103 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 dispersion. There are also at least five bright B-type stars in the cloud, presumably of about the same age; none show the peculiarities expected of HAeBe stars. Dereddened, 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 zero-age main-sequence 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 Cygni 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], and [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 005 (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.

On the Be and Ae Stars in NGC 6611

The young cluster NGC 6611 is of interest because it has been believed to contain an unusually large number of Be and Ae stars. The cluster is embedded in a very bright H ii region, so that proper subtraction of that background on slit or multiobject fiber‐fed spectra is important. If stellar line emission is present in blue‐violet Balmer lines, it should be more prominent at Hα. We have examined some 40 of these cluster stars on Hα slitless spectrograms, which are not subject to such contamination, and were able to confirm the presence of Hα emission above the continuum level in only four. About 25 much fainter Hα emitters were found in the cluster; these are probably the brightest members of a low‐mass T Tauri population. None are convincingly associated with the famous elephant‐trunk structures seen in projection against the H ii region.