Abstracts of Papers on Interstallar extinction
Alfaro E.J., Delgado A.J. (1991, A&A 214, 69)
We analyse the standard indices of B-type stars in uvby photometry to
investigate whether and to what extent these indices could be affected by
systematic trends caused by non-negligible reddening. To perform the
analysis a sample of 17 young open clusters with uvby published photometric
studies, carried out by various authors and using different instrumental
equipment, has been selected. The sample only includes clusters having B-type
members with published MK spectral types.
The results show that the standard values of the c1 index indeed contain
systematic errors, which are larger for larger reddening, and can be expressed
as a linear dependence of the observed reddening slope E(c1)MK / E(b-y)MK on
spectral type. We suggest a simple explanation to account for this
observational evidence, based on reddening dependent coefficients in the
transformation from instrumental to standard quantities in the Stromgren
system. These kinds of tendencies are not so marked for the m1 and (b-y)
standard values as for the c1 index. The main trends shown by the m1 and
(b-y) sequences could be associated with intrinsic peculiarities of reddening
in some clusters included in the sample.
Cardwell J.A., et al. (1989, ApJ 345, 245)
Using the parameterized extinction data of Fitzpatrick and Massa for the
ultraviolet, and various sources for the optical and near-infrared, a
meaningful average extinction law, A(lambda) / A(V), is derived over the
wavelength range 3.5 micron > lambda > 0.125 micron, which is applicable to
both diffuse and dense regions of the interstellar medium. The derived mean
extinction law depends on only one parameter, which is chosen to be
Rv [=A(V) / E(B-V)]. An analytic formula is given for the mean extinction law
which can be used to calculate color excesses or to deredden observations.
The formula closely reproduces the mean extinction laws of Seaton and Savage
and Mathis for values of Rv near 3.1, the value in the diffuse interstellar
medium. However, there are real deviations of individual lines of sight from
the mean law at all values of Rv. These deviations are especially large for
lambda < 0.16 micron.
The mean Rv-dependent extinction law presented here is tentatively extended
beyond the range of the IUE satellite to 0.10 micron on the basis of the four
stars observed in this wavelength range by the Copernicus satellite, plus the
extention of the analytic fits to the IUE spectra by Fitzpatrick and Massa.
In addition, E(0.10 micron-0.13 micron) / A(V) for 12 stars for which the color
data are available is in excellent agreement with E(0.10-0.13) / A(V) derived
from the extension of the Fitzpatrick and Massa analytic fits.
We confirm the result of others that the shape of the extinction law for long
wavelenghts (lambda > 0.7 micron) is independent of Rv to within the errors of
measurements. Because of systematic variations at the V filter, the choice of
normalizing the extinction law to A(V) introduces a spurious dependence of
the longer wavelengths on Rv and normalization to some wavelength longward of
V (e.g, > 0.7 micron) would be more logical. However, normalization at V was
chosen because of the wealth of data there.
From the analytic results presented here, the poor correlation of
[A(0.13 micron) - A(0.17 micron)] with E(B-V), and the much better
correlation of[A(0.22 micron) - A(0.25 micron)] with E(B-V) can be understood
through the specific Rv-dependencies exhibited by these colors.
The existence of the mean extinction law, valid over a large wavelength
interval, suggests that the processes which modify the sizes and compositions
of grains are stochastic in nature and very efficient. Apparently all sizes
are modified simultaneously. When changes in the small grains responsible for
the far-ultraviolet extinction occur, the entire size distribution apparently
varies in a systematic way.
Crawford D.L., Mandwewala N.
(1976, PSAP 88, 917)
Stellar energy fluxes for various spectral types have been combined with
filter transmission curves and several different laws of interstellar
absorption to obtain reddening relations and the ratio of total-to-selective
absorption for the UBV, uvby and Geneva photometric systems. The results
are compatible with observations. Effects due to the bandwidths of the various
systems on the reddening slopes, curvature of the reddening lines, and the
ratio of total-to-selective absorption have to be allowed for in wide-band
Last update: 7 Novembre 1995