On the absorption of light from the brightest stars in M 31

The classical Q-method for the brightest OB stars, the period–luminosity (P–L) relation for the Cepheids and the maximum magnitude–rate of decline (MMRD) relation for novae were used to study the absorption variations in M 31. An extinction map with 34 arcsec resolution was constructed. This resolution corresponds to a characteristic size of about 100 pc at which the colour excesses correlate most strongly. The mean colour excesses are E(B – V) = 0.51 ±0.27 mag from the OB stars, 0.21 ± 0.09 mag from Cepheids and 0.14 ± 0.10 mag from novae. The colour excess shows no big radial dependence within 12 kpc from the centre. The distributions of the colour excesses determined from OB stars (located predominantly in spiral arm regions) in six bins along the radius were compared with distributions generated via Monte Carlo simulation to estimate the optical thickness of the disk at different galactocentric distances. The total optical thickness of the disk is τ_B = 11.5 near the bulge and decreases to 2 in the outermost parts (R/R ₂₅ = 0.62, where R ₂₅ is the isophote diameter at B = 25 mag arcsec⁻²). We obtain a radial dust scale factor R _D = 4.8 ± 0.4 kpc. The constant ratio of the dust to OB star scale factors in the Z-direction was found to be (Z _D/Z _S = 1.25 ± 0.25). We derived vertical scale factors Z _S = 76 ± 36 pc for the OB stars and Z _D = 95±45 pc for the dust. The visible OB stars lay on a protruding conical surface with an angle of about 2° between the base plane and the surface of the cone as a sequence of the optical thickness gradient toward the centre in the spiral arm regions.

Key Words

• M 31,
• the light extinction,
• the dust distribution,
• the optical thickness