Broad-band photometric colors and effective temperature calibrations for late-type giants

Abstract

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 . . . 5000 K, gravities log g = −0.5 . . . +3.5, and metallicities [M/H] = +0.5 . . . −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., ∆(V −K) ∼ 0.2 mag), translating into offsets in effective temperature of up to ∼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 theDeviations from the observed trends in the T eff -color planes are generally within ±100 K for T eff = 3500 to 4800 K. Synthetic colors calculated with different stellar atmosphere models agree to ±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.

Red supergiants around the obscured open cluster Stephenson 2

“…This is more easily seen in Fig. 8, which shows the 9 Theoretical magnitudes calculated from PHOENIX models and calibrated against observational data suggest that all late-M giants have (J − K) 0 ∼ 1.3 (Kučinskas et al 2005). The calibration of Groenewegen (2004), based on older models, gives (J − K) 0 = 1.37 for dustfree M7 stars.…”

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars

“…Line formation in the 1D model atmospheres: influence of the mixing-length parameter α MLT According to the classical Schwarzschild criterion, 1D LHD models of the red giant studied here are convectively stable in the outer photospheric layers (log τ Ross < ∼ 1), irrespective of the mixing length parameter, α MLT (Kučinskas et al 2005;Ludwig & Kučinskas 2012). Since line formation depths typically never 3 Including the indirect temperature dependence via electron pressure.…”

“…In one of the early attempts, Kučinskas et al (2005) have found that convection may noticeably alter the thermal structure of the red giant atmosphere which, in turn, may affect the spectral energy distribution of the emerging radiation field. This may yield a difference of ∼0.2-0.3 mag in broad-band photometric colors predicted by the 3D hydrodynamical and classical 1D models.…”

A third cluster of red supergiants in the vicinity of the massive cluster RSGC3

“…To derive the colour excesses for RSGs, an estimation of Kučinskas et al (2005) there is no explicit relation between spectral type and (J − K) 0 , as the authors classify their stars on a temperature scale. We transform these into spectral types using the derived temperatures Groenewegen (2004) and Levesque et al (2005) where available, and using the data from Cenarro et al (2001) everywhere else (marked with an asterisk in Table A.1).…”