We report Submillimeter Array (SMA) observations of CO (J=2-1, 3-2 and 6-5) and its isotopologues ( 13 CO J=2-1, C 18 O J=2-1 and C 17 O J=3-2) in the disk around the Herbig Ae star HD 163296 at ∼ 2 ′′ (250 AU) resolution, and interpret these data in the framework of a model that constrains the radial and vertical location of the line emission regions. First, we develop a physically selfconsistent accretion disk model with an exponentially tapered edge that matches the spectral energy distribution and spatially resolved millimeter dust continuum emission. Then, we refine the vertical structure of the model using wide range of excitation conditions sampled by the CO lines, in particular the rarely observed J=6-5 transition. By fitting 13 CO data in this structure, we further constrain the vertical distribution of CO to lie between a lower boundary below which CO freezes out onto dust grains (T 19 K) and an upper boundary above which CO can be photodissociated (the hydrogen column density from the disk surface is 10 21 cm −2 ). The freeze-out at 19 K leads to a significant drop in the gasphase CO column density beyond a radius of ∼155 AU, a "CO snow line" that we directly resolve. By fitting the abundances of all CO isotopologues, we derive isotopic ratios of 12 C/ 13 C, 16 O/ 18 O and 18 O/ 17 O that are consistent with quiescent interstellar gas-phase values. This detailed model of the HD 163296 disk demonstrates the potential of a staged, parametric technique for constructing unified gas and dust structure models and constraining the distribution of molecular abundances using resolved multi-transition, multi-isotope observations.

We present spectra of a sample of Herbig Ae and Be (HAeBe) stars obtained with the Infrared Spectrograph on the Spitzer Space Telescope. All but one of the Herbig stars show emission from polycyclic aromatic hydrocarbons (PAHs) and seven of the spectra show PAH emission, but no silicate emission at 10 µm. The central wavelengths of the 6.2, 7.7-8.2, and 11.3 µm emission features decrease with stellar temperature, indicating that the PAHs are less photo-processed in cooler radiation fields. The apparent low level of photo processing in HAeBe stars, relative to other PAH emission sources, implies that the PAHs are newly exposed to the UV-optical radiation fields from their host stars. HAeBe stars show a variety of PAH emission intensities and ionization fractions, but a narrow range of PAH spectral classifications based on positions of major PAH feature centers. This may indicate that, regardless of their locations relative to the stars, the PAH molecules are altered by the same physical processes in the proto-planetary disks of intermediate-mass stars. Analysis of the mid-IR spectral energy distributions indicates that our sample likely includes both radially flared and more flattened/settled disk systems, but we do not see the expected correlation of overall PAH emission with disk geometry. We suggest that the strength -2of PAH emission from HAeBe stars may depend not only on the degree of radial flaring, but also on the abundance of PAHs in illuminated regions of the disks and possibly on the vertical structure of the inner disk as well.

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We present images and spectra of the Cepheus E (Cep E) region at both optical and infrared wavelengths. Only the brightest region of the southern lobe of the Cep E outflow reveals optical emission, suggesting that the extinction close to the outflow source plays an important rôle in the observed difference between the optical and IR morphologies. Cep E is a unique object since it provides a link between the spectroscopic properties of the optical Herbig-Haro (HH) objects and those of deeply embedded outflows.The observed H 2 infrared lines allow us to determine an excitation temperature of ∼ 2300 K, an Ortho-to-Para ratio of ∼ 3, and an H 2 (1,0)/(2,1) S(1) line ratio of ∼ 9. These results are consistent with the values observed for HH objects with detected NIR emission lines, with shock excitation as the main mechanism for their formation, and also with the values observed for embedded, NIR flows.The optical spectroscopic characteristics of Cep E (HH 377) appear to be similar to the ones of low excitation HH objects. However, the electron density determined from the [SII]6731/6717 line ratio for this object (n e = 4100 cm −3 ), and the [OI]6300/Hα, [SII](6717+6731)/Hα ratios are higher than the values of all of the previously studied low excitation HH objects. This result is likely to be the consequence of an anomalously high environmental density in the HH 377 outflow.The ionization fraction obtained for HH 377 is x e ∼ 1% From this result, together with the observed [OI]6300/Hα line ratio, we conclude that the observed Hα line emission is collisionally excited. From a comparison with shock models, we also conclude that the extinction towards HH 377 is very low. Comparing the observed Hβ and Hα fluxes of HH 377 with model predictions, we determine a shock speed between 15 and 20 km s −1 , although somewhat higher velocities also produce spectra with line ratios that qualitatively agree with the observations of -3 -HH 377.

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