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 photometry.

Last update: 7 Novembre 1995