Infrared excess around nearby red giant branch stars and Reimers law ⋆
Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium
Received: 17 October 2011
Accepted: 30 January 2012
Context. Mass loss is one of the fundamental properties of asymptotic giant branch (AGB) stars, but for stars with initial masses below ~1 M⊙, the mass loss on the first red giant branch (RGB) actually dominates mass loss on the AGB. Nevertheless, mass loss on the RGB is still often parameterised by a simple Reimers law in stellar evolution models.
Aims. We study the infrared excess and mass loss of a sample of nearby RGB stars with reliably measured Hipparcos parallaxes and compare the mass loss to that derived for luminous stars in clusters.
Methods. The spectral energy distributions of a well-defined sample of 54 RGB stars are constructed, and fitted with the dust radiative transfer model DUSTY. The central stars are modelled by MARCS model atmospheres. In a first step, the best-fit MARCS model is derived, basically determining the effective temperature. In a second step, models with a finite dust optical depth are fitted and it is determined whether the reduction in χ2 in such models with one additional free parameter is statistically significant.
Results. Among the 54 stars, 23 stars are found to have a significant infrared excess, which is interpreted as mass loss. The most luminous star with L = 1860 L⊙ is found to undergo mass loss, while none of the 5 stars with L < 262 L⊙ display evidence of mass loss. In the range 265 < L < 1500 L⊙, 22 stars out of 48 experience mass loss, which supports the notion of episodic mass loss. It is the first time that excess emission is found in stars fainter than ~600 L⊙. The dust optical depths are translated into mass-loss rates assuming a typical expansion velocity of 10 km s-1 and a dust-to-gas ratio of 0.005. In this case, fits to the stars with an excess result in log Ṁ (M⊙ yr-1) = (1.4 ± 0.4)log L + (−13.2 ± 1.2) and log Ṁ (M⊙ yr-1) = (0.9 ± 0.3)log (LR/M) + (−13.4 ± 1.3) assuming a mass of 1.1 M⊙ for all objects. We caution that if the expansion velocity and dust-to-gas ratio have different values from those assumed, the constants in the fit will change. If these parameters are also functions of luminosity, then this would affect both the slopes and the offsets. The mass-loss rates are compared to those derived for luminous stars in globular clusters, by fitting both the infrared excess, as in the present paper, and the chromospheric lines. There is excellent agreement between these values and the mass-loss rates derived from the chromospheric activity. There is a systematic difference with the literature mass-loss rates derived from modelling the infrared excess, and this has been traced to technical details on how the DUSTY radiative transfer model is run. If the present results are combined with those from modelling the chromospheric emission lines, we obtain the fits log Ṁ (M⊙ yr-1) = (1.0 ± 0.3)log L + (−12.0 ± 0.9) and log Ṁ (M⊙ yr-1) = (0.6 ± 0.2)log (LR/M) + (−11.9 ± 0.9), and find that the metallicity dependence is weak at best. The predictions of these mass-loss rate formula are tested against the recent RGB mass loss determination in NGC 6791. Using a scaling factor of ~10 ± ~5, both relations can fit this value. That the scaling factor is larger than unity suggests that the expansion velocity and/or dust-to-gas ratio, or even the dust opacities, are different from the values adopted. Angular diameters are presented for the sample. They may serve as calibrators in interferometric observations.
Key words: circumstellar matter / stars: fundamental parameters / stars: mass-loss / planetary systems
Appendix A and Table 4 are available in electronic form at http://www.aanda.org
© ESO, 2012