Carbon-rich giants in the HR diagram and their luminosity function

Abstract

Abstract. The luminosity function (LF) of nearly 300 Galactic carbon giants is derived. Adding BaII giants and various related objects, about 370 objects are located in the RGB and AGB portions of the theoretical HR diagram. As intermediate steps, (1) bolometric corrections are calibrated against selected intrinsic color indices; (2) the diagram of photometric coefficients k 1/2 vs. astrometric true parallaxes are interpreted in terms of ranges of photospheric radii for every photometric group; (3) coefficients C R and C L for bias-free evaluation of mean photospheric radii and mean luminosities are computed. The LF of Galactic carbon giants exhibits two maxima corresponding to the HC-stars of the thick disk and to the CV-stars of the old thin disk respectively. It is discussed and compared to those of carbon stars in the Magellanic Clouds and Galactic bulge. The HC-part is similar to the LF of the Galactic bulge, reinforcing the idea that the Bulge and the thick disk are part of the same dynamical component. The CV-part looks similar to the LF of the Large Magellanic Cloud (LMC), but the former is wider due to the substantial errors on HIPPARCOS parallaxes. The obtained mean luminosities increase with increasing radii and decreasing effective temperatures, along the HC-CV sequence of photometric groups, except for HC0, the earliest one. This trend illustrates the RGB-and AGB-tracks of low-and intermediate-mass stars for a range in metallicities. From a comparison with theoretical tracks in the HR diagram, the initial masses M i range from about 0.8 to 4.0 M for carbon giants, with possibly larger masses for a few extreme objects. A large range of metallicities is likely, from metal-poor HC-stars classified as CH stars on the grounds of their spectra (a spheroidal component), to near-solar compositions of many CV-stars. Technetium-rich carbon giants are brighter than the lower limit M bol −3.6 ± 0.4 and centered at M bol −4.7 3) on average. Most RCB variables and HdC stars range from M bol −1 to −4 against −0.2 to −2.4 for those of the three population II Cepheids in the sample. The former stars show the largest luminosities ( M bol < ∼ −4 at the highest effective temperatures (6500-7500 K), close to the M bol −5 value for the hot LMC RCB-stars (W Men and HV 5637). A full discussion of the results is postponed to a companion paper on pulsation modes and pulsation masses of carbon-rich long period variables (LPVs; Paper IV, present issue).

The chemical composition of carbon stars. The R-type stars

“…They also derive bolometric magnitudes for R type stars ∼1−2 mag brighter than those obtained by Knapp et al (2001). We believe that the parallaxes by Bergeat et al (2002a) are more accurate since the parallaxes by Knapp et al (2001) might undergo a bias when K > ∼ 8, whose effect is to overestimate the M K (see their Fig. 2).…”

“…Differences in the photometry found in the literature for a given star are typically below ∼0.2 mag for the majority of early-R stars, but it can be higher for late-R stars due to their known photometric variability (see Table 1). All the photometric magnitudes were corrected for interstellar extinction using the A J values derived by Bergeat et al (2002a) and the relations by Cardelli et al (1989). For the stars HIP 39 118, HIP 62 944 and HIP 69 089, not included in the Bergeat et al (2002a) sample, we use the galactic interstellar extinction model by Arenou et al (1992) and the parallaxes by Knapp et al (2001).…”

Synthetic photometry for carbon-rich giants

“…4.1.4), the hydrostatic COMARCS models -a representative sample of which is also shown in the figure -are able to describe the bulk of the Mendoza objects which are not significantly affected by dynamic effects. The mean relation of bolometric corrections given by Bergeat et al (2002;their (1)), which rests on blackbody-fits to photometric data, extends to even larger colour indices. As also noted by Kerschbaum et al (see the discussion in their Sect.…”

“…The observational results of Mendoza & Johnson (1965) are already given in the Johnson system, no conversion is needed therefore. The fit to the values of an extended sample of C-type giants provided by Bergeat et al (2002;their …”

LMC S63: a historical reappraisal of the outburst behaviour of a deeply eclipsing Magellanic symbiotic star

“…The red giant of LMC S63 then lies on the evolutionary track of a low metallicity (Z=0.008) 1.9 M ⊙ star in the HR diagram given in Fig. 9 of Bergeat et al (2002a).…”