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tu144
2011-Apr-17, 10:05 AM
What does it mean a metallicity slope of 0.25 mag dex^-1 ?

tusenfem
2011-Apr-17, 10:53 AM
dex seems to be a unit of abundance

googling [unit dex] one can find
dex
a logarithmic unit being used in astronomy. Originally, dex was a convenient function defined by dex (x) = 10x. But the notation is now being used after the exponent in expressions such as -.043 dex, meaning 10-.043. Thus 1 dex equals a factor of 10, making the dex identical to the bel. The name "dex" is a contraction of "decimal exponent."

Hornblower
2011-Apr-17, 10:55 AM
What does it mean a metallicity slope of 0.25 mag dex^-1 ?

Metallicity of what?
What is mag?
What is dex?

tu144
2011-Apr-17, 01:55 PM
Full quote is:

Based on the Cepheid metallicity calibration of Sakai et al. (2004) (with adopted LMC and solar values for 12 + log(O/H) of 8.50 and 8.70, respectively; and a metallicity slope of 0.25 mag dex^-1), we estimate the metallicity correction in transforming from an LMC to a Galactic-based Cepheid zero point to be 0.25 x0.2 = 0.05 mag, with a residual scatter of about +- 0.07 mag.
Freedman, Madore (2010) "Hubble Constant", 3.1.3 Metalicity

I get the dex now, but it doesnt make sense for me in context of 0.25mag/dex.
What does it mean that I have 0.25mag/dex?
I have no idea mags of what and dex of what.

ngc3314
2011-Apr-17, 05:45 PM
in an astronomical context, "magnitude" by itself almost always means the measure of brightness on a magnitude scale. So in this case, evidence suggests that the Cepheid mean luminosity by 0.25 magnitude (10^(0.4*0.25) in luminosity) for each factor of 10 change in metal abundance.