Measuring Mass-Loss Evolution at the Tip of the Asymptotic Giant Branch

Abstract

Abstract:In the final stages of stellar evolution low-to intermediate-mass stars lose their envelope in increasingly massive stellar winds. Such winds affect the interstellar medium and the galactic chemical evolution as well as the circumstellar envelope where planetary nebulae form subsequently. Characteristics of this mass loss depend on both stellar properties and properties of gas and dust in the wind formation region. In this paper we present an approach towards studies of mass loss using both observations and models, focussing on the stage where the stellar envelope is nearly empty of mass. In a recent study we measure the mass-loss evolution, and other properties, of four planetary nebulae in the Galactic disk. Specifically we use the method of integral field spectroscopy on faint halos, which are found outside the much brighter central parts of a planetary nebula. We begin with a brief comparison between our and other observational methods to determine mass-loss rates in order to illustrate how they differ and complement each other. An advantage of our method is that it measures the gas component directly requiring no assumptions of properties of dust in the wind. Thereafter we present our observational approach in more detail in terms of its validity and its assumptions. In the second part of this paper we discuss capabilities and assumptions of current models of stellar winds. We propose and discuss improvements to such models that will allow meaningful comparisons with our observations. Currently the physically most complete models include too little mass in the model domain to permit a formation of winds with as high mass-loss rates as our observations show.

Emission-line stars in M31 from the SPLASH and PHAT surveys

“…As Miras are AGB stars, they too can be defined as either C or O rich. Models of C-rich Mira variables (C-Miras) are well constrained (Arndt, Fleischer & Sedlmayr 1997;Wachter et al 2002;Sandin, Roth & Schönberner 2010). In addition to having well developed models, this population of stars possesses both the environmental dependence of C stars (e.g.…”

“…C stars have increased opacity due to the presence of CN in their atmospheres, this leads to a substantial increase in their mass loss (e.g. Marigo 2002;Mattsson, Wahlin & Höfner 2010;Sandin, Roth & Schönberner 2010). C-rich stars also tend to be more luminous at redder wavelengths than their non-Crich counterparts (e.g.…”

The evolution of planetary nebulae

“…Our present observational and theoretical knowledge about the AGB wind properties as a function of metallicity is still too meager as to allow a more detailed description of initial configurations in terms of their metal content, although progress, theoretical and observational, is encouraging (Wood et al 1992;van Loon et al 2005;Wachter et al 2008;Sandin 2008;Marshall et al 2004;Mattson et al 2008;Sandin et al 2010a). There are clear indications that the wind speed is somewhat reduced at lower metallicity, but the dependence of the mass-loss rate on metallicity, and on stellar parameters, remains still unclear.…”

A catalogue of integrated H-alpha fluxes for 1,258 Galactic planetary nebulae

“…Many compact high surface brightness PNe have surrounding AGB halos (e.g. Corradi et al 2003;) included in the photometry aperture, but as the typical halo surface brightness is a factor of 10 −3 fainter than the main shell (Corradi et al 2003;Sandin et al 2008;Sandin, Roth & Schönberner 2010), we ignore this contribution. For NGC 7293, a structured AGB halo (Malin 1982;O'Dell 1998;Parker et al 2001;Speck et al 2002;O'Dell, McCullough & Meixner 2004;Meaburn et al 2005) is readily visible on SHASSA.…”