A new self-consistent model of circumstellar CO emission for deriving mass-loss rates in red giants. I - The carbon-rich star U Camelopardalis

Details A new self-consistent model of circumstellar CO emission for deriving mass-loss rates in red giants. I - The carbon-rich star U Camelopardalis A new self-consistent model of circumstellar CO emission for deriving mass-loss rates in red giants. I - The carbon-rich star U Camelopardalis

Oct 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990apj...362..652s&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 362, Oct. 20, 1990, p. 652-662.

Astronomy and Astrophysics

Astrophysics

47

Carbon Monoxide, Infrared Spectra, Late Stars, Red Giant Stars, Stellar Mass Ejection, Emission Spectra, Hydrogen, Millimeter Waves, Rotational Spectra, Stellar Models, Infrared: Spectra Stars: Circumstellar Shells Stars: Individual (U Cam) Stars: Late-Type Stars: Mass Loss

Scientific paper

The mass-loss rates of red (super)giant stars can be derived by modeling observations of their circumstellar millimeter-wave emission. We show that models in which the CO emission is not calculated self-consistently with the gas temperature, do not, in general, correctly predict (1) the shapes of the millimeter-wave lines, (2) the J = 2-1 to J = 1-0 intensity ratio, or (3) the radial optical depths of the 4.6 μm vibration-rotation absorption lines toward these stars. Since these three properties are sensitive indicators of the mass-loss rate, we present a new self-consistent model for determining the gas and dust mass-loss rates, the distances to, and the [CO]/[H2] abundance ratio in the circumstellar envelopes of red (super)giant stars. The model is constrained by the shapes and intensities of the CO J = 1-0 and 2-1 lines, and the far-infrared emission observed toward these envelopes by IRA S. The gas heating by dust-gas collisions in our model takes into account the frequency variation of the dust absorptivity and the heating flux, and with the assumption of a power-law relationship between the near- and far-infrared dust absorptivity, is proportional to Q1/20, where Q0 is the far-infrared dust absorptivity. In previous self-consistent models the gas heating by dust scaled as γQ3/2/(αρgrain), where γ was an assumed dust to gas ratio, Q was an "effective" near-infrared dust absorptivity, α was the grain radius, and ρgrain was the grain material density. We find that derived mass-loss rates are (1) very sensitive to the run of kinetic temperature in the envelope, and large errors can result if the kinetic temperature is not calculated self-consistently, (2) modestly sensitive to the inner radius of the circumstellar envelope when the CO is dominantly radiatively excited, and (3) do not scale as the square of the distance to the star.
This model has been applied to the circumstellar CO emission from U Cam, a N5 carbon-rich star, for which we find Mṡ = 2.5-6 × 10-5 Msun yr-1, D = 300-1000 pc, [CO]/[H2] = 2-6 × 10-4, and κ(60 μm), the 60 μm dust emissivity = 100-400 cm2 g-1; the derived ranges in the physical parameters largely reflect uncertainties in the data. The lowest value of MṡV/Lc consistent with the data is 4.5. The gas-to-dust ratio is found to be very large (several thousand) compared to the values inferred for red giant envelopes using gas mass-loss rates derived from models which assume the radial gas temperature variation to be the same as that calculated for the IRC+10216 circumstellar envelope. A possible solution to the puzzle of why the outflow velocity of the U Cam envelope derived from the CO l-0 line is significantly less than that derived from the 2-1 line is found in a hitherto unknown effect: the existence of a2.8 K layer of gas in the outer circumstellar envelope. This effect may make large mass-loss rate stars with high gas-to-dust ratios extremely weak and thus undetectable in the J = l-0 CO line.

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