Extension of the effective temperature scale of giants to types later than M6

Abstract

Abstract. Effectivetemperaturesforninegiantstarsarederivedfromrecentdiameterdeterminationsat2.2µmwiththeFLUORbeam combiner on the IOTA interferometer. A good overlapwith previous studies for stars earlier than M6 is verified. Thepresentpaperextendsthetemperaturescaleofgiantstarsbasedon direct measurements to types between M6 and M8.Key words: techniques: interferometric – stars: atmospheres –fundamental parameters – late-type – infrared: stars1. IntroductionTheeffectivetemperatureofastarcharacterizestheradiometricproperties of the atmosphere in a purely formal but very usefulway. Effective temperatures may not in general be determinedfrom either photometric colors or from atomic and molecularexcitation, since the effective temperature is a global propertyand not a local property. Where a computed model atmosphereis available for a star, the effective temperature of the modelwill be meaningful to the extent that the model contains therequired physics and boundary values. A more “direct” deter-mination of the effective temperature, possible with little or nomodeldependence,maybeobtained,accordingtothedefinitionof effective temperature, from two observables - the apparentbolometric flux and the apparent angular diameter.Steady improvements in techniques of optical interferome-try make it possible to conduct increasingly focused studies ofstellareffectivetemperature.Herewereportanextensionofthecalibration of the effective temperatures of M giants to spec-tral types as late as M8. These will be useful as benchmarks inevaluating model atmospheres for very cool stars, and in manyapplications, such as correlating model luminosity functionswith observed colors for composite systems.