by Ann » Fri Jan 22, 2016 12:27 am
neufer wrote:
<<Chief among the uncertainties tied to the classical and type II Cepheid distance scale are: the nature of the period-luminosity relation in various passbands, the impact of metallicity on both the zero-point and slope of those relations, and the effects of photometric contamination (blending) and a changing (typically unknown) extinction law on Cepheid distances. All these topics are actively debated in the literature.
Metallicity could be a problem when it comes to estimating the distance to a galaxy like M101. Although I haven't made an effort to learn anything about the average metallicity of the stars in M101, my impression is that the galaxy is metal-poor. Its yellow population is small, while its blue population is large, bright and widespread. This suggests that M101 has had long periods of relative quiescence in its history, when few stars formed and only low levels of metals were synthesized in stars and mixed with the interstellar medium of M101.
M101 might therefore have a large population of rather metal-poor Cepheids. If metal-poor Cepheids behave differently than metal-rich ones, then the Cepheid-induced distance to M101 might be off. It is even possible that metal-poor SN type Ia might be different than metal-rich ones.
Even so, the combination of large numbers of Cepheids in M101 (there must be!) and a very well-observed SN Ia suggests that the distance to M101 should be relatively well understood.
Ann
[quote="neufer"]
<<Chief among the uncertainties tied to the classical and type II Cepheid distance scale are: the nature of the period-luminosity relation in various passbands, [b][size=110][color=#0040FF]the impact of metallicity[/color][/size][/b] on both the zero-point and slope of those relations, and the effects of photometric contamination (blending) and a changing (typically unknown) extinction law on Cepheid distances. All these topics are actively debated in the literature.
[/quote]
Metallicity could be a problem when it comes to estimating the distance to a galaxy like M101. Although I haven't made an effort to learn anything about the average metallicity of the stars in M101, my impression is that the galaxy is metal-poor. Its yellow population is small, while its blue population is large, bright and widespread. This suggests that M101 has had long periods of relative quiescence in its history, when few stars formed and only low levels of metals were synthesized in stars and mixed with the interstellar medium of M101.
M101 might therefore have a large population of rather metal-poor Cepheids. If metal-poor Cepheids behave differently than metal-rich ones, then the Cepheid-induced distance to M101 might be off. It is even possible that metal-poor SN type Ia might be different than metal-rich ones.
Even so, the combination of large numbers of Cepheids in M101 (there must be!) and a very well-observed SN Ia suggests that the distance to M101 should be relatively well understood.
Ann