Merce Crosas

Physical Parameters of the IRC +10216 Circumstellar Envelope: New Constraints from Submillimeter Observations

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 effect 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 ~ 500-1000 K) in the inner part of the envelope (at about 5 × 1015 cm from the central star) suggested by a previous study. We also find that a mass-loss rate of 3.25 × 10-5 M☉ yr-1 and a distance of 150 pc provide the best agreement between our model results and observations.

Atomic Carbon in the Envelopes of Carbon-rich Post-Asymptotic Giant Branch Stars

Atomic carbon has been detected in the envelopes of three carbon-rich evolved stars: HD 44179 (\AFGL 915, the ii Red Rectangle ˇˇ), HD 56126, and, tentatively, the carbon star V Hya. This brings to seven the number of evolved star envelopes in which C I has been detected. Upper limits were found for several other stars, including R CrB. C I was not detected in several oxygen-rich postasymptotic giant branch (AGB) stars (OH 231.8)4.2, for example), although it is detected in their carbon-rich analogs. Two trends are evident in the data. First, circumstellar envelopes with detectable C I are overwhelmingly carbon-rich, suggesting that much of the C I is produced by the dissociation of molecules other than CO. Second, the more evolved the envelope away from the AGB, the higher the C I/CO ratio. The oxygen- rich supergiant star a Ori remains the only oxygen-rich star with a wind containing detectable C I. These data suggest an evolutionary sequence for the C I/CO ratio in cool circumstellar envelopes. This ratio is small (a few percent) while the star is on the AGB, and the C I 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. Subject headings: circumstellar matterstars: abundancesstars: AGB and post-AGB ¨ stars: evolution

GG Tauri's Circumbinary Disk: Models for Near-Infrared Scattered-Light Images and 13CO (J=1→0) Line Profiles

We describe models that reproduce the observed near-IR scattered-light images and 13CO (J=1→0) line profiles from the GG Tau circumbinary disk. The observed extent of the scattered-light images requires a 0.13 M☉ flared circumbinary disk (as determined from millimeter observations), an inner cleared region of 200 AU, and scale height of 16.6 AU at the disks 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 effects of small circumstellar disks that are coplanar with the circumbinary disk. Further, we find that the effects of geometry and illumination allows the observed scattered-light pattern to be reproduced with a dust grain distribution that fits 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 profiles are reproduced using the same geometry adopted for the near-IR modeling. However, we find that the 13CO abundance is lower than in the diffuse interstellar medium, in agreement with previous investigations indicating CO depletion in circumstellar environments.

BLR chemistry redux

We show that at column densities higher than 1023 cm−2, the formation of H2 is enhanced significantly by the presence of dust. The radiative association of H(n=1) and H*(n=2) does not contribute much to increasing the abundance of H2 in the neutral zone. However, H2 formation via associative ionization still needs to be investigated, and several excited states of hydrogen may be important in this process.

Disks in the Arp220 nuclei: Modeling high resolution line and continuum observations

Abstract We model molecular and near-infrared emission from the central region of the ultra-luminous infrared galaxy Arp220. We use high angular resolution CO data taken with the Owens Valley millimeter array and NIR images from HST/NICMOS to compare with Monte Carlo line radiative transfer and scattering models. The models suggest that each one of the two nuclei of Arp220 has a dusty molecular disk with a mass of about 10 9 M ⊙ , which is in Keplerian rotation.