The ultraluminous infrared galaxy Arp 220 has been observed at 0.5" resolution in CO(2-1) and 1 mm continuum using the newly expanded Owens Valley Millimeter Array. The CO and continuum peaks at the double nuclei and the surrounding molecular gas disk are clearly resolved. We find steep velocity gradients across each nucleus (dV ~ 500 km/s within r= 0.3") whose directions are not aligned with each other and with that of the outer gas disk. We conclude that the double nuclei have their own gas disks (r ~ 100 pc). They are counterrotating with respect to each other and embedded in the outer gas disk (r ~ 1 kpc) rotating around the dynamical center of the system. The masses of each nucleus are M_dyn > 2* 10^9 M_sun based on the CO kinematics. Although there is no evidence of an old stellar population in the optical or near infrared spectroscopy of the nuclei (probably due to the much brighter young population), it seems likely that these nuclei were 'seeded' from the pre-merger nuclei in view of their counterrotating gas kinematics. The gas disks probably constitute a significant fraction (~ 50 %) of the mass in each nucleus. The CO and continuum brightness temperatures imply that the nuclear gas disks have high area filling factors (~ 0.5-1) and have extremely high visual extinctions (Av ~ 1000 mag). The molecular gas must be hot (>= 40 K) and dense (>= 10^4-5 cm^-3), given the large mass and small scale-height of the nuclear disks. The continuum data suggest that the large luminosity (be it starburst or AGN) must originate within 100 pc of the two nuclear gas disks which were presumably formed through concentration of gas from the progenitor outer galaxy disks.Comment: 20 pages, 5 figures. Accepted for publication in The Astrophysical Journa

We have studied the temperature distribution and other physical properties of the circumstellar envelope of the prototypical high mass-loss carbon star IRC ]10216 by calculating the thermal balance and the radiative transfer in the envelope self-consistently. Cooling is dominated by CO line emission and adiabatic expansion, and heating by dust-gas collisions throughout most of the envelope. Heating by the grain photoelectric e †ect is important in the outer part of the envelope. The radiative transfer is calculated by using a Monte Carlo method. The mass-loss rate, the CO abundance, the dust-gas momentum transfer efficiency, and the distance to the source are free parameters in our model. These physical parameters are constrained by the comparison of our model results with the observations of various 12CO and 13CO lines. In particular, recent submillimeter-wavelength observations of moderately high excitation transitions, such as the J \ 6 ] 5 line, put very important constraints on the temperature distribution in the inner part of the envelope, and they do not support the presence of very high temperatures (T D 500È1000 K) in the inner part of the envelope (at about 5 ] 1015 cm from the central star) suggested by a previous study. We also Ðnd that a mass-loss rate of 3.25 ] 10~5 yr~1 and a M _ distance of 150 pc provide the best agreement between our model results and observations.

Atomic carbon has been detected in the envelopes of three carbon-rich evolved stars: HD 44179 (=AFGL 915, the 'Red Rectangle'); HD 56126; and, tentatively, the carbon star V Hya. This brings to seven the number of evolved star envelopes in which CI has been detected. Upper limits were found for several other stars, including R CrB. CI was not detected in several oxygen rich post asymptotic giant branch (AGB) stars (OH231.8+4.2, for example), although it is detected in their carbon-rich analogues. Two trends are evident in the data. First, circumstellar envelopes with detectable CI are overwhelmingly carbon rich, suggesting that much of the CI is produced by the dissociation of molecules other than CO. Second, the more evolved the envelope away from the AGB, the higher is the CI/CO ratio. The oxygen-rich supergiant star α Ori remains the only oxygen rich star with a wind containing detectable CI.These data suggest an evolutionary sequence for the CI/CO ratio in cool circumstellar envelopes. This ratio is small (a few %) while the star is on the AGB, and the CI is located in the outer envelope and produced by photodissociation. The ratio increases to about 0.5 as the star evolves away from the AGB because of the dissociation of CO and other carbon-bearing molecules by shocks caused by the fast winds which appear at the end of evolution on the AGB. Finally, the ratio becomes >> 1 as the central star becomes hot enough to photodissociate CO.

We describe models that reproduce the observed near-IR scattered-light images and 13CO (J \ 1 ] 0) line proÐles from the GG Tau circumbinary disk. The observed extent of the scattered-light images requires a 0.13 Ñared circumbinary disk (as determined from millimeter observations), an inner M _ cleared region of 200 AU, and scale height of 16.6 AU at the diskÏs inner edge. To reproduce the brightness distribution, we require extinction of the illuminating starlight prior to scattering within the circumbinary disk. This extinction is obtained by including the e †ects of small circumstellar disks that are coplanar with the circumbinary disk. Further, we Ðnd that the e †ects of geometry and illumination allows the observed scattered-light pattern to be reproduced with a dust grain distribution that Ðts data from other Taurus-Auriga circumstellar environments. This indicates that unless geometries and illuminations are known, great care must be taken when attempting to determine grain properties from analysis of scattered-light images. The observed 13CO line proÐles are reproduced using the same geometry adopted for the near-IR modeling. However, we Ðnd that the 13CO abundance is lower than in the di †use interstellar medium, in agreement with previous investigations indicating CO depletion in circumstellar environments.

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