Annu. Rev. Astron. Astrophys. 1992. 30: 359-89 Copyright © 1992 by Annual Reviews. All rights reserved

3. ABSOLUTE MAGNITUDE

3.1 Historical Galactic Supernovae

Among the historical Galactic supernovae, four candidate type Ia events (and their remnants) are SN 185 (RCW 86), SN 1006 (PKS 1459-41), Tycho's supernova of 1572 (3C 10), and Kepler's of 1604 (3C 358). To determine their absolute magnitudes we need to know distances, apparent magnitudes, and the interstellar extinction. Distances can be inferred only from the remnants, and apparent magnitudes only from contemporary records of the supernova brightness. The interstellar extinction can be estimated either from the remnants, from the extinction of field stars in the directions of the remnants, or from contemporary descriptions of the supernova colors.

Strom (1988) attempted to put the distances to the four remnants on a self-consistent absolute scale. The known ages and measured angular diameters of the remnants, together with the assumption that they are in the adiabatic phase of their evolution, were first used to determine relative distances to 10% accuracy, and then the scale was fixed to within 20% by considering various determinations of the absolute distances to individual remnants. In Table 3 we list Strom's estimates of the distances and their uncertainties. Distance determinations not considered by Strom (1988) tend to suggest that the remnant distance scale may need to be increased, making the absolute magnitude brighter than in Table 3. Westerlund (1969) determined a distance of 2.5 kpc for the OB association near RCW 86 (but no distance estimates for SN 185 have any basis if, as suggested by Huang & Moriarty-Schieven (1987), RCW 86 is not the remnant of SN 185]. From a new proper-motion study of PKS 1459-41 Long et al (1988) derive a most probable distance range for SN 1006 of 2.1-2.7 kpc, Fesen et al (1989) find a range of 1.5-3.3 kpc on the basis of the background Schweizer-Middleditch star, and Smith et al (1991) obtain 1.4-2.8 kpc from a comparison of proper motions and the shock velocity from the H emission line, all to be compared to 1.4 kpc in Table 3. Smith et al (1991) also obtain a range of 1.5-3.1 kpc for Tycho. From a bow shock model Bandiera (1987) estimates 4.5 ± 1.0 kpc for Kepler, but Blair et al (1991) estimate 2.9 ± 0.4 kpc from the shock emission method. Van den Bergh & Tammann (1991) note that the surprisingly high local supernova frequency that is implied by the historical events could be reduced by increasing the remnant distance scale, again making the absolute magnitudes in Table 3, and their mean, brighter.

In Table 3 we list apparent magnitudes and associated uncertainties from Clark & Stephenson (1977, 1982). Quite discordant estimates can be found in the literature for SN 185 (m_V = -6; Huang & Moriarty-Schieven 1987) and for SN 1006 (m_V = -6; Pskovskii 1978). Other estimates for SN 1572 include m_V = -4.5 ± 0.2 (Pskovskii 1978) and m_V = -4.1 ± 0.15 (de Vaucouleurs 1985), and for SN 1604 m_V = -3.5 ± 0.2 (Pskovskii 1978) and m_V = -2.5 (L88).

For the extinction of SN 185 we use A_V = 1.8 ± 0.3. Leibowitz & Danziger (1983) derived A_V = 1.7 from the Balmer decrement in the RCW 86 remnant and Westerlund (1969) found A_V = 2.0 for the OB association. For SN 1006 we use A_V = 0.3 ± 0.3 on the basis of A_V = 0.46 from the Balmer decrement in the remnant (Lasker 1981) and A_V = 0.28 for the total Galactic extinction in that direction (Burstein & Heiles 1978). The extinction of SN 1572 and SN 1604 can be estimated from their colors, but the result depends on whether they are assumed to have been intrinsically blue like types Ia and II-linear (II-L), with an intrinsic B - V 0.0 at maximum light, or redder like type Ib and Ic. The SN 1572 and SN 1604 light-curve shapes are consistent with all three types (Doggett & Branch 1985). For the intrinsically blue case, we use A_V 2.0 ± 0.3 for SN 1572 [de Vaucouleurs (1985) derived A_V = 1.95 ± 0.1] and A_V = 3.6 ± 0.5 for SN 1604 [Pskovskii (1978) found A_V = 3.81 ± 0.12, Bandiera & van den Bergh (1991) prefer A_V = 3.4 ± 0.9]. If SN 1572 and/or SN 1604 were intrinsically as red as SNe Ib/Ic, which typically are 0.5 mag redder in B - V than SNe Ia (L88, Wheeler & Harkness 1990), then the visual extinction derived from their colors would be about 1.5 mag lower, as indicated in Table 3. Both the high and low values of extinction from the colors of SN 1604 are within the range of independent estimates [e.g. A_V = 2.2 by van den Bergh & Kamper (1977) from foreground field stars, A_V = 3.5 by Danziger & Goss (1980), and A_V = 2.7 ± 0.3 by Blair et al (1991) from the Balmer decrement in the remnant]. Field stars in the direction of SN 1572 suggest that, for a distance of 2.5 kpc, A_V may be in the range 1-3 mag (Brodskaya & Grigor'eva 1962), which favors the higher extinction value for SN 1572.

Our tentative interpretation of the absolute magnitudes in Table 3 is that SN 1572 was either a type Ib (Green 1986, Strom 1988), a type II-L, or some other kind of event that is fainter than type Ia: and that SN 185, SN 1006, and SN 1604 were type Ia. The only absolute magnitude that can possibly be considered to be quantitatively reliable is that of Kepler, M<_V⁰⁰ = -19.7+0.6, and this relies on Kepler having been a highly extinguished type Ia.