Determination of effective temperatures for an extended sample of dwarfs and subdwarfs (F0-K5)
Astron. Astrophys. Suppl. Ser.
Determination of e ective temperatures for an extended sample of dwarfs and subdwarfs (F0-K5)?
A. Alonso
S. Arribas
C. Mart nez-Roger
Instituto de Astrof sica de Canarias
La Laguna (Tenerife)
Spain Electronic mail:
| We have applied the InfraRed Flux Method (IRFM) to a sample of 475 dwarfs and subdwarfs in order to derive their e ective temperatures with a mean accuracy of about 1.5%. We have used the new homogeneous grid of theoretical model atmosphere flux distributions developed by Kurucz (1991, 1993) for the application of the IRFM. The atmospheric parameters of the stars cover, roughly, the ranges: 3500 K Te 8000 K; −3.5 [Fe/H] +0:5; 3.5 log(g) 5. The monocromatic infrared fluxes at the continuum, and the bolometric fluxes are derived using recent results, which satisfy the accuracy requeriments of the work. Photometric calibrations have been revised and applied to estimate metallicities, although direct spectroscopic determinations were preferred when available. The adopted infrared absolute flux calibration, based on direct optical measurements of angular stellar diameters, sets the e ective temperatures determined using the IRFM on the same scale than those obtained by direct methods. We derive three temperatures, TJ , TH and TK , for each star using the monochromatic fluxes at di erent infrared wavelengths in the photometric bands J , H, and K. They show good consistency over 4000 K, and no trend with wavelength may be appreciated. We provide a detailed description of the steps followed for the application of the IRFM, as well as the sources of the errors associated to the di erent inputs of the method, and their transmission into the nal temperatures. We also provide comparison with previous works.
stars; fundamental parameters | stars; Population II | stars; subdwarfs | stars; general
1. Introduction
The stellar e ective temperatures might be, in
principle, determined following a fundamental or direct method
based on the combination of the bolometric fluxes and the
angular diameter measurements according to the
equation:
Te
= ( 4 )1=4 −1=2FB1=o4l;
(1)
where is the Stefan-Boltzmann constant, is the
angular diameter of the star, and FBol is the bolometric flux
measured on the surface of the earth.
However, in practice, we need atmosphere models to
perform secondary corrections to both the angular
diameter measurement (limb darkening) and FBol (interstellar
absorption). The Sun is obviously the only exception, since
its limb darkening can be empirically determined, and its
interstellar absorption is negligible. In any event, the main
di culty a ecting the direct procedure is the limited
number of stars with a reliable measurement of their angular
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?Table 4 is only available in electronic form at CDS Tables 1-3
are also available via ftp 130.79.128.5
diameter (see for instance Table 4 from Mozurkewich et
al. 1991). This problem is remarkably severe for the low
main sequence, as there are no direct measurements of
angular diameter for stars later than F5V, with the above
mentioned exception of the Sun. For this reason, relatively
large uncertainties still remain on the scale of stellar
effective temperatures in the low main sequence, especially
when the e ect of metallicity is considered.
Thus, we are compelled to use semi-direct methods,
which require, as a basis, atmosphere models in addition
to observational information
(see the review by
B¨ohmVitense 1981)
. Among the di erent methods of this type
the Infrared Flux Method
(hereafter IRFM; Blackwell et
al. 1990, and references therein)
seems especially adequate
to analyse the temperatures of F, G, and K stars. The
IRFM has been successfully applied to di erent samples
of population I stars
(e.g. Blackwell et al. 1990; Bell &
Gustafsson 1989; Saxner & Hammarb¨ack 1985)
. However,
the works based on the IRFM devoted to metal poor stars
(Magain 1987; Arribas & Mart nez-Roger 1987, 1989)
are
restricted to rather limited samples.
The present paper is part of a long term programme
aimed to a better de nition of the scale of temperatures
for F, G, and K dwarfs and subdwarfs, using the IRFM. calibration for the infrared flux of Vega, which scales the
This work is relevant to three main topics: (a) analysis of IRFM temperatures to those derived by direct methods,
the global behaviour of atmosphere models
(e.g., Magain is proposed. Thirdly, a method to obtain bolometric fluxes
1987)
, (b) the correct interpretation of the observed HR for metal poor stars was devised in
Alonso et al. (1995
,
diagram
(e.g., Arribas & Mart nez-Roger 1988, 1989)
, and Paper III). Last, but not least, we have used the improved
(c) ne spectroscopic analysis for abundance determina- grid of atmosphere models computed recently by Kurucz
tions of metal poor stars
(e.g., King 1994)
. (1991, 1993).
The general programme has been focussed on the im- This paper provides detailed information on (...truncated)