Infrared excess around nearby RGB stars and Reimers law

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 dom inates mass loss on the AGB. Nevertheless, mass loss on the RGB is still often parameterised by a simple Reimers la w in stellar evolution models. Aims. To study the infrared excess and mass loss of a sample of nearb y RGB stars with reliably measured Hipparcos parallaxes and compare the mass loss to that derived for luminous stars in cl usters. Methods. The spectral energy distributions of a well-defined sample o f 54 RGB stars are constructed, and fitted with the dust radiat ive transfer model DUSTY. The central stars are modelled by MARC S model atmospheres. In a first step, the best-fit MARCS model i s derived, basically determining the e ff ctive temperature. In a second step, models with a finite dus t optical depth are fitted and it is determined whether the reduction in χ2 in such models with one additional free parameter is statist ically significant. Results. Among the 54 stars, 23 stars are found to have a significant inf rared excess, which is interpreted as mass loss. The most luminous star withL = 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 n otion of episodic mass loss. It is the first time that excess emission is found in star s f inter than∼600 L⊙. The dust optical depths are translated into massloss ratesassuming a typical expansion velocity of 10 km s −1 and a dust-to-gas ratio of 0.005. In this case, fits to the star s with an excess result in loġ M (M⊙ yr−1) = (1.4 ± 0.4) logL + (−13.2 ± 1.2) and logṀ (M⊙ yr−1) = (0.9 ± 0.3) log(L R/M) + (−13.4 ± 1.3) assuming a mass of 1.1 M ⊙ for all objects. We caution that if the expansion velocity an d dust-to-gas ratio have di fferent values from those assumed, the constants in the fit will change. If these p arameters are also functions of luminosity, then this would affect both the slopes and the o ffsets. The mass-loss rates are compared to those derived for l uminous stars in globular clusters, by fitting both the infrared excess, as in the present paper, and the chromos pheric lines. There is excellent agreement between these va lu s nd the mass-loss rates derived from the chromospheric activity. T here is a systematic di fference 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 mod el is run. If the present results are combined with those from modellin g the chromospheric emission lines, we obtain the fits log Ṁ (M⊙ yr) = (1.0±0.3) logL+(−12.0±0.9) and logṀ (M⊙ yr) = (0.6±0.2) log(L R/M)+(−11.9±0.9), and find that the metallicity dependence is weak at best. The predictions of these mass-loss rate form ula are tested against the recent RGB mass loss determinatio n in NGC 6791. Using a scaling factor of ∼8 ± ∼5, both relations can fit this value. That the scaling factor i s larger than unity suggests that the expansion velocity and /or dust-to-gas ratio, or even the dust opacities, are di ff rent from the values adopted. Angular diameters are presented for the sample. They may serve as calibrators in in terferometric observations.