We present new synthetic broad-band photometric colors for late-type giants based on synthetic spectra calculated with the PHOENIX model atmosphere code .
The grid covers effective temperatures T _ { eff } = 3000 \dots 5000 K , gravities \log g = -0.5 \dots { +3.5 } , and metallicities { [ M / H ] } = +0.5 \dots { -4.0 } .
We show that individual broad-band photometric colors are strongly affected by model parameters such as molecular opacities , gravity , microturbulent velocity , and stellar mass .
Our exploratory 3D modeling of a prototypical late-type giant shows that convection has a noticeable effect on the photometric colors too , as it alters significantly both the vertical and horizontal thermal structures in the outer atmosphere .
The differences between colors calculated with full 3D hydrodynamical and 1D model atmospheres are significant ( e.g. , \Delta ( V - K ) \sim 0.2 mag ) , translating into offsets in effective temperature of up to \sim 70 K. For a sample of 74 late-type giants in the Solar neighborhood , with interferometric effective temperatures and broad-band photometry available in the literature , we compare observed colors with a new PHOENIX grid of synthetic photometric colors , as well as with photometric colors calculated with the MARCS and ATLAS model atmosphere codes .
We find good agreement of the new synthetic colors with observations and published T _ { eff } –color and color–color relations , especially in the T _ { eff } – ( V - K ) , T _ { eff } – ( J - K ) and ( J - K ) – ( V - K ) planes .
Deviations from the observed trends in the T _ { eff } –color planes are generally within \pm 100 K for T _ { eff } = 3500 to 4800 K. Synthetic colors calculated with different stellar atmosphere models agree to \pm 100 K , within a large range of effective temperatures and gravities .
The comparison of the observed and synthetic spectra of late-type giants shows that discrepancies result from the differences both in the strengths of various spectral lines/bands ( especially those of molecular bands , such as TiO , H _ { 2 } O , CO ) and the continuum level .
Finally , we derive several new T _ { eff } – \log g –color relations for late-type giants at solar-metallicity ( valid for T _ { eff } = 3500 to 4800 K ) , based both on the observed effective temperatures and colors of the nearby giants , and synthetic colors produced with PHOENIX , MARCS and ATLAS model atmospheres . Table 2 is available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( 130.79.128.5 ) or via http : //cdsweb.u-strasbg.fr/cgi-bin/qcat ? J/A+A/